Recent advances of carbon nanotubes in perovskite solar cells

Perovskite solar cells (PSCs) have exhibited tremendous potential in photovoltaic fields owing to their appreciable performance and simple fabrication. Nevertheless, device performances are still required to be further improved before commercial applications. As one‐dimensional materials, carbon nanotubes (CNTs) have been utilized to regulate stability and efficiency of PSCs because of their excellent chemical stability, flexibility, as well as tunable optical and electrical characteristics. In this review, we comprehensively summarize various functions of CNTs in PSCs, such as transparent electrodes, hole/electron‐transport layers, counter electrodes, perovskite additives, and interlayers. Additionally, applications of CNTs toward the advancement of flexible and semitransparent PSCs are provided. Finally, we preview the challenges and research interests of using CNTs in high‐efficiency and stable perovskite devices.

F I G U R E 1 Device structures: (A) traditional n-i-p mesoscopic, (B) n-i-p planar, and (C) p-i-n planar perovskite solar cells.CE, counter electrode; ETL, electron-transport layer; HTL, hole-transport layer; TE, transparent electrode.electrode (TE).As shown in Figure 1, structures of PSCs can be divided into three types: the traditional n-i-p mesoscopic, n-i-p planar, and p-i-n planar devices.The first type of device has a common structure of F-doped tin oxide (FTO)/compact titanium dioxide (c-TiO 2 )/ mesoporous TiO 2 (m-TiO 2 )/perovskite/2,2′,7,7′-tetrakis (N,N-di-p-methoxyphenylamine)−9,9′-spirobifluorene (Spiro-OMeTAD)/gold (Au) with a champion PCE of 24.8%, 4 while the second n-i-p planar device possesses an architecture of FTO/tin dioxide (SnO 2 )/perovskite/Spiro-OMeTAD/Au with a best efficiency of 25.8%. 5Lately, the PCE of the inverted p-i-n PSC has achieved 25.5% with an architecture of indium tin oxide (ITO)/[2- (3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (MeO-2PACZ)/perovskite/lithium fluoride (LiF)/C 60 / bathocuproine/Ag. 6Although these simple device structures give PSCs with appreciable efficiencies and convenient fabrication, there are still many challenges that inhibit PSCs to meet the desired requirements of practical applications, such as the fragility and increased cost of ITO, the instability of the dopants in Spiro-OMeTAD, the cost of metal CE materials as well as their complicated thermal evaporation technique.8][9] To mitigate these questions, researchers have exploited low-cost and stable carbon-based HTLs and electrodes.In 2013, Han's group first employed the carbon CE in PSCs and obtained an efficiency of 6.64%. 109][20][21][22][23][24] Interestingly, CNTs can be divided by the number of wall layers into three types, including singlewall CNTs (SWCNTs), double-wall CNTs (DWCNTs), as well as multi-wall CNTs (MWCNTs).Among them, SWC-NTs show the most unique photoelectric properties, such as high conductivity (>10 7 S cm −1 ), 25 exceptional carrier mobility (>10 5 cm 2 V −1 s −1 ), 26,27 and chirality-dependent conductive and optical characteristics. 28,29SWCNTs have three different typical structures according to their chiral angles (θ).For example, when values of θ are 0 • and 30 • , SWCNTs are referred to as zigzag and armchair tubes, respectively.All other tubes (0 • < θ < 30 • ) are known as being chiral. 30Furthermore, based on different electronic types, SWCNTs exhibit metallic and semiconducting behaviors.The electronic and optical properties of SWC-NTs are mostly affected by the chiral angle and chiral index at which it is wrapped with regard to the monolayer graphene. 31,32DWCNTs have high optical conductivity and lie between SWCNTs and MWCNTs, especially, those that exhibit better dispersibility than SWCNTs, contributing to solution-processed applications. 33Several technologies, including arc discharge, 34,35 laser ablation, 36,37 as well as chemical vapor deposition (CVD), 17,38 have been employed to fabricate CNTs.Among these methods, CVD is considered as the predominant method owing to its relatively low cost, elevated productivity, and good controllability.Additionally, CNTs can be self-assembled toward differentshaped macrostructures, for instance, films and fibers, which have been made available for planar and fibershaped photovoltaic devices. 18,39n this review, we thoroughly describe unique advantages of CNTs in PSCs.Based on the brief chronology of the development of CNTs in PSCs (Figure 2), we first outline and discuss specific utilizations of CNTs in different functional layers, including the TE, HTL, ETL, CE, interlayer and additive in perovskite, highlighting their roles in increasing the efficiency and stability of PSCs.Then, we concentrate on the utilization of CNTs in flexible, foldable, and textile, as well as semitransparent PSCs (ST-PSCs).Finally, we provide an outlook on

CNTs as transparent electrodes
The TE needs high optical transparency to ensure that more sunlight reaches the perovskite layer and high electrical conductivity to collect holes/electrons.In this respect, ITO and FTO are the wide TEs in PSCs because of their outstanding transparency and low sheet resistance (R S ).Nevertheless, the high cost, complicated fabrication process, and fragile nature would be big concerns.The CNT films not only have excellent optical and conductive properties but also show high mechanical stability, abundant raw materials, and easy preparation, and are considered to be alternative materials for TEs.][42][43][44] Furthermore, they synthesized a carbon-welded isolated SWCNT network, composed of ∼85% isolated tubes. 45As a result, this SWCNT film exhibited excellent performance (R s of 25 Ω sq.−1 at 90% T).
Regarding the utilization of CNT-based TEs in PSCs, Jeon et al. 46 fabricated ITO-free p-i-n planar devices with SWCNT networks (Figure 3A) and obtained an efficiency of 2.71%.This low performance was attributed to the incompatibility between SWCNTs and HTL, as well as the low transparency and conductivity of the pristine SWCNT film.Afterwards, they adopted nitric acid (HNO 3 ) to not only change the SWCNT surface properties from hydrophobic to hydrophilic but also act as p-type dopant to improve the conductivity and optical transparency (Figure 3B), leading to an increased PCE of 6.32%.However, this doping effect was unstable because of the evaporation of HNO 3 molecules from the SWCNT surface in ambient environment. 47In comparison, metal oxides showed stronger doping stability.Among them, MoO 3 was used as the common p-type dopant because of its high work function and chemical inertness.It was found that device performance was impacted by the thickness of the MoO 3 layer on SWCNTs.As shown in Figure 3C, the 2nm-thick MoO 3 layer enabled SWCNT-based device with a better performance compared to the 6-nm-thick MoO 3 layer, which might be ascribed to the superior energy level 0.72 13    alignment from former doped SWCNTs. 48In addition, fabricating high-crystallinity, large-diameter, and smallbundle SWCNTs is recognized as an important way to obtain high-performance TEs. 41Zhang et al. 49 chose a SWCNT network with an average diameter of ∼2.2 nm and isolated and small-bundle nanotubes as TEs to prepare pi-n PSCs.Their outstanding optoelectrical properties led to a record PCE of 19% in ITO-free CNT-based devices.Despite DWCNTs possess a higher optical density than SWCNTs, their higher solubility and mechanical strength enable solution-processed transparent conductive films. 50,51Similar to SWCNTs, doping is a significant technology to improve conductivities of DWCNTs.Zhang et al. 52 employed HNO 3 -doped DWCNT films with an R s of 35 Ω sq.−1 at 90% T in a p-i-n device and yielded a PCE of 17.4%.Nevertheless, the doping behavior in DWCNTs was rather distinct from that in SWCNTs because the inner wall of the DWCNT exhibited less susceptibility to dopants and the surrounding surfactants limited doping effect.For example, when using the trifluoromethanesulfonic acid (TFMS) to dope DWCNTs, electron transfer occurred mainly from outer walls of DWCNTs to TFMS rather than from the inner walls.Besides, the doping impact achieved near zero, while the distance of the outer wall from TFMS was 4 Å (Figure 3D), which demonstrated that surfactants completely hindered the doping effect. 53Fortunately, the weaker doping effect for DWCNTs caused a better energy alignment compared to SWCNTs (Figure 3E).In addition, the DWCNT film showed a smoother morphology, contributing to forming a better contact with the upper layer.Accordingly, the TFMS-doped DWCNT-based device delivered a PCE of 17.2%.In order to further lower sheet resistances of solution-processed DWCNT films, Shawky et al. 33 adopted calcination to effectively remove insulated surfactants without damaging the DWCNTs (Figure 3F).More importantly, an enhanced p-doping effectiveness formed on the surfactant-removed DWCNTs.The device fabricated with this DWCNT film exhibited a PCE of 17.7% without hysteresis.These results suggest that using stable and effective dopants to alter transparent conductive properties of CNT TEs is the key to obtaining high-efficiency CNT-based devices.
Apart from doping technologies, the incorporation of conducting polymers into CNT networks to form hybrid films is a workable method to enhance conductivities of CNT films.Fan et al. 54 directly introduced poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) in CNT networks by spin-coating methods, leading to an R s value of 75 Ω sq.−1 at 78% T. Hu et al. 55 adopted sulfonated CNTs (SCNTs) as secondary polymerization templates to in situ synthesize PEDOT:PSS.The PEDOT:PSS:SCNT hybrid film with a thickness of 70 nm showed over 3500 S cm −1 at ∼83% T and was integrated in an n-i-p PSC, revealing an efficiency of 13.31%.
PSCs fabricated with CNT TEs have obtained a PCE of 19%, demonstrating great potential for CNT films in PSC devices.However, most of CNT films still possessed lower conductivities than ITO/FTO electrodes.Although doping is a reliable strategy to enhance CNT conductivity while adjusting the Fermi level, some issues of dopants should be considered, such as the doping effectivity, chemical stability, and optical absorption.Additionally, composite films of CNTs and conductive polymers also show the practicability in PSCs.Briefly, substantially more work is required to further enhance transparent conductive abilities of CNT films.

CNTs as hole-transport layers
7][58][59] Therefore, a good HTL should possess appropriate energy level alignment, high hole mobility, and good thermal stability. 60The Spiro-OMeTAD and PEDOT:PSS are the most popularly utilized as effective HTLs in n-i-p and p-i-n perovskite devices, respectively.2][63] Recently, CNTs have been regarded as promising HTLs in perovskite devices because of their advantages of superior stability, low cost, and excellent hydrophobicity.Li et al. 64 applied a free-standing CNT film coated with poly(methylmethacrylate) (PMMA) polymer as an HTL in PSCs and obtained an efficiency of 5.82%.It was found that PMMA shrinkage could improve the contact between CNTs and perovskite, resulting in a reduction in contact resistance.Surprisingly, the device derived an exceptionally high V OC with a value of 1.45 V, which was much higher than that of the PSC with Spiro-OMeTAD as HTL.However, the CNT layer was usually regarded as a conductor and hardly exhibited substantial charge carrier selectivity. 65Incorporating holetransport materials into CNT films to fabricate hybrid HTLs is a good method to resolve this issue.Habisreutinger et al. 66 reported two-layer HTLs prepared by SWC-NTs wrapped with poly(3-hexylthiophene) (P3HT) and PMMA in n-i-p devices (Figure 4A).The functionalization of P3HT altered electronic properties of SWCNTs and enhanced their p-type natures.Meanwhile, the PMMA was employed on the surface of the P3HT/SWCNTs acting as an encapsulant to protect the perovskite layer from oxygen and moisture (Figure 4B).Finally, the device showed an efficiency of 14.2% as well as improved stability.Similarly, Wang et al. 65 reported a hole conductive protecting layer based on the SWCNT/graphene oxide (GO)/PMMA thin film.Because of the enhanced hole-transport selectivity of SWCNT/GO, the mesoscopic n-i-p structured device delivered a PCE of 11.7%, which could match that of devices with Spiro-OMeTAD.Furthermore, the PCE of the SWCNT/GO/PMMA device could be enhanced to 13.3% via optimizing the preparation technology of the perovskite layer.
Although PMMA can serve as a protective layer and interfacial enhancer to boost the stability and efficiency of PSCs, its insulating property cannot optimize the hole-transport ability of CNTs.Therefore, choosing hole conductive polymers to replace PMMA can further increase photovoltaic performances of devices.Habisreutinger et al. 67 utilized two-layer HTLs composed of P3HT/SWCNTs and undoped Spiro-OMeTAD in n-i-p devices.The obtained PCE of 15.4% was much higher than that of the PSC with only undoped Spiro-OMeTAD (6.8%) (Figure 4C).The photoinduced absorption measurements indicated that most of photogenerated holes were transferred from perovskite to SWCNTs (Figure 4D).The existence of SWCNTs in the two-layer HTL accelerated the charge extraction rate, resulting in a higher photovoltaic performance.In the follow-up study, they used SnO 2 and FA 0.83 MA 0.17 Pb(I 0.83 Br 0.17 ) 3 as an electron-accepting layer and light absorber layer (Figure 4E), respectively, to further optimize the PCE to 18.8%, surpassing the device with undoped Spiro-OMeTAD, even those with Li-doped Spiro-OMeTAD as HTLs (Figure 4F). 68Mazzotta et al. 69 adopted a low-cost non-conjugated polymer (ethylene vinyl acetate, EVA) to stably disperse SWCNTs and MWC-NTs in organic solvents, resulting in high-quality EVAfunctionalized SWCNT and MWCNT conductive films, respectively.Then, two films were utilized to replace P3HT/CNTs in two-layer HTLs of PSCs, achieving PCEs of 16.8% and 17.1%, respectively.
Besides above two-layer HTLs, the composite of CNTs and other hole-transport materials was directly used as the HTL in efficient and stable PSCs.Yoon et al. 70 developed novel hybrid HTLs composed of SWCNTs and PEDOT:PSS to prepare low-temperature-processed p-i-n planar devices.The SWCNT dispersion was first deposited onto the ITO substrate to fabricate SWCNT films.Subsequently, PEDOT:PSS was prepared on SWCNTs to improve the hole selectivity.It was found that the optical loss of the SWCNTs/PEDOT:PSS hybrid film was negligible, which was attributed to the wide bandgap of SWCNTs.Moreover, randomly distributed SWCNTs partly penetrated into perovskites through PEDOT:PSS, contributing to an effec-tive hole extraction and transport, which was elucidated by the steady-state photoluminescence (PL) spectra.The fabricated device yielded a PCE of 12.5%, outperforming the device with only PEDOT:PSS.Lu et al. 71 synthesized a novel binaphthylamine-based hole-transport molecule to functionalize MWCNTs by surface absorption, which looked like that some small molecules were trapped in a sturdy chain.The non-encapsulated p-i-n PSC gave an excellent stability and an efficiency of 17.2%, superior to that of the PSC with PEDOT:PSS-based HTL (14.69%).Additionally, a hybrid material consisting of CNTs and NiO x was demonstrated as an efficient HTL.Ryu et al. 72 added CNTs into a NiO x precursor solution to prepare a hybrid HTL, which showed enhanced conductivity (Figure 4G) and improved hole extraction properties without reducing optical and surface morphological features of NiO x .Accordingly, the optimized p-i-n planar device exhibited a PCE of 16.9% (Figure 4H).These results indicated that the hybrid HTLs have a promise in efficient, stable, and low-temperature-processed planar PSCs.
As discussed above, CNT-based HTLs show a promising application in PSCs.In order to realize high-efficient PSCs, their conductivities and hole mobilities should be further enhanced to boost the photocurrent and FF.In addition, their unsubstantial hole selectivity compared with conventional HTLs should be considered.Functionalizing carbon nanomaterials and constructing hybrid films with conventional hole-transport materials are reliable strategies to improve photovoltaic performance of CNT-based PSCs.

CNTs as additives in perovskite layers
Solution-prepared perovskite films usually possess abundant grain boundaries that provide many defects/traps, resulting in severe charge recombination.4][75] Therefore, controlling over morphology and crystallinity of perovskite layers is of key importance to realize high-performance PSCs. 76Many technologies have been applied to control defect densities and improve grain sizes of perovskites, such as introducing ionic liquids, [77][78][79] Lewis acids/bases, [80][81][82] and inorganic/organic salts [83][84][85] in solution-processed techniques.Because of the unique structure, excellent optoelectronic properties and chemical inertness, researchers aim to embed CNTs into the perovskite layer to enhance the device performance.
Cheng et al. 86 added MWCNTs into perovskite layers, which could serve as charge-transport channels between perovskite grains.Moreover, MWCNTs accelerated the hole collection of carbon electrodes.The fabricated PSC achieved a PCE of 11.6%, which was up by ∼15% compared to the PSC without MWCNTs.Zhang et al. 87 studied influences of the functionalized MWCNTs on the perovskite crystallization.It was found that sulfonate CNTs (s-CNTs) could enlarge micron-sized grains in the perovskite film and fill grain boundaries, but the pristine CNTs were unable to realize these effects.As shown in Figure 5A, the methylammonium iodide (MAI) molecules easily gathered around s-CNTs owing to interactions between sulfonic acid and PbI 2 , resulting in crystallization of perovskites.Further heat treatment would boost the perovskite grain growth, while s-CNTs were maintained in grain boundaries.The fabricated PSC with s-CNTs delivered a PCE of 15.1%, outperforming the device with pristine CNTs (10.3%).Wang et al. 88 utilized amino-functionalized CNTs (CNT-NH 2 ) and methylammonium chloride (MACl) as additions to manipulate the crystallinity and crystal orientation of perovskites.The MACl addition was conducive to crystal grain growth by forming the intermediate phase.Meanwhile, CNT-NH 2 improved the grain size because the -NH 2 functional groups exhibited a strong affinity for Pb 2+ , leading to extra heterogenous nucleation sites for perovskite growth.In addition, interactions between functional groups in CNT-NH 2 and surface atoms of perovskites might decrease surface energies of (110) crystal facets, regulating the perovskite crystal orientation.Due to the improved crystallinity, crystal orientation and grain size, the MA 0.85 FA 0.15 PbI 3 film with MACl and CNT-NH 2 exhibited low recombination and excellent charge extraction and transport.Accordingly, the device based on this excellent perovskite obtained a PCE of 21.05% and showed exceptional reproducibility and stability.Recently, Li et al. 89 also demonstrated that CNT-NH 2 can act as growth template to facilitate crystal grain growth of perovskite films and carrier transfer channels to accelerate charge extraction and transport.
SWCNTs were also adopted as additives in the perovskite layer.However, SWCNTs face a difficult dispersion in the solvent compared to MWCNTs. 33To mitigate the issue, appropriate surfactants need to be developed to improve their solubilities.Maruyama's group demonstrated that semiconducting SWCNTs (s-SWCNTs) could act as the perovskite crystal growth templates, and chargetransport channels, improving the PCE of PSCs to 19.5% from 18.1%. 90In their study, the sodium deoxycholate (DOC) was used as a surfactant to help s-SWCNTs well disperse in water.Meanwhile, the carbonyl groups in DOC could passivate grain boundaries via forming Lewis adducts (Figure 5B).The perovskite grains grown on the s-SWCNT was confirmed by the TEM imaging and the fast Fourier transform analysis (Figure 5C).Although performance enhancement had been achieved, the insulating feature of DOC limited the device FF.In further work, they employed a novel surfactant, 4,6-di(anthracen-9-yl)−1,3phenylene bis(dimethylcarbamate) (DPB), with the appropriate energy level and superior mobility to replace DOC (Figure 5D).It was found that the DPB could improve the s-SWCNT solubility in N,N-dimethylformamide (DMF), which is a conventional solvent for perovskite precursors.The DPB-attached s-SWCNTs promoted the growth of large-size perovskite crystals (Figure 5E,F), improved the passivation effect, as well as lessened the charge trap.Accordingly, an efficiency of 20.7% was obtained, exceeding the PSC with DOC-attached SWCNTs.
High-performance PSCs are usually based on perovskites with lead (Pb) element, but their toxicity is one of the main obstacles toward commercialization.More importantly, Pb may leak from perovskite films, resulting in the degeneration of PSCs. 91Although researchers try to replace Pb with tin (Sn), the Sn-based PSCs show much lower efficiency than Pb-based devices. 92,93Therefore, it is essential to manage Pb leakage.Recently, Wang et al. 94 utilized the poly(acrylic acid) (PAA) covalently functionalized CNTs (CNT-PAA) as additives in the perovskite film to suppress Pb leakage and achieve a champion PCE of 21.8%.It was found that a dense layer of PAA grafted on the surface of the CNT framework could provide abundant carboxyl groups, which were helpful for promoting perovskite growth and immobilizing the Pb ions (Figure 5G).Moreover, the CNT-PAA could reduce unintentional defect states and act as charge-transport channels in perovskite layer.To study the effects of the PAA molecular weight on photovoltaic performance of PSCs, different weights of PAA were employed to combine with CNTs (named CNT-PAA-S and CNT-PAA-L for 22.4% and 35.1% weight content of PAA, respectively).As shown in Figure 5H, the CNT-PAA-L additive enabled PSCs with higher photovoltaic parameters compared to the CNT-PAA-S additive, which was ascribed to more functional locations on the longer side chains of the former.The lead leakages of devices were measured by detecting lead concentration from the flame atomic absorption spectrometer (Figure 5I).The Pb 2+ concentration leaked from the control device was 9.5 ppm after 600 min, which was markedly higher than those of 4.4 and 2.9 ppm from devices with CNT-PAA-S and CNT-PAA-L, respectively.This revealed that the existence of CNT-PAA could dramatically suppress lead leakage.Especially, the device with CNT-PAA-L realized a sequestration efficiency of ∼70%.The excellent lead fixation originated from the strong chelation between Pb 2+ and CNT-PAA, achieving the insoluble CNT-PAA-Pb adducts.
From this section, it is found that functionalized CNTs can serve as crystal growth templates in the precursor solution to prepare large-size or strongly orientated perovskite films.Meanwhile, abundant functional groups from functionalized CNTs are helpful for lessening charge traps, passivating defects, as well as suppressing Pb leakages.Moreover, CNTs existed in perovskite grain boundaries contribute to facilitating charge transport.These effects would enable PSCs with efficient, stable, and environmental-friendly properties.

CNTs in electron-transport layers
ETLs can extract photogenerated electrons from active layers and transport them to TEs or CEs while blocking holes. 95Therefore, an effective ETL needs to meet several requirements: a suitable energy level for effective electron collection, excellent electron mobility, and high chemical stability. 96,97Especially, the ETL needs high optical transmittance when employed in n-i-p PSCs to allow enough light reach the perovskite light absorber.The n-type semiconductive metal oxide nanoparticles, including TiO 2 , [98][99][100] ZnO, 101,102 and SnO 2 , 103,104 are popular ETL materials in conventional PSCs.Nevertheless, disordered nanoparticles in ETLs usually increase the rate of charge recombination due to random transport paths and multitudinous grain boundaries, leading to low device performance. 105Moreover, when forming nanoparticle films, the higher annealing temperature is limited in flexible devices fabricated on plastic substrates.][108] CNTs as one-dimensional materials have high electron mobilities, and can be used as conducting channels in nanoparticle films.Although CNTs show a p-type character in air due to oxygen molecules absorbed on their surfaces, it was previously found that CNTs could enhance the electron mobility and directionality of the TiO 2 nanofibers. 109,110Batmunkh et al. 111 introduced SWC-NTs in TiO 2 porous films to prepare n-i-p PSC devices (Figure 6A).It was found that 0.1 wt% SWCNTs largely increased the electron transport and reduced the carrier recombination, leading to an enhancement in J SC from 19.0 to 21.6 mA cm −2 .Moreover, SWCNTs shifted the conduction band minimum of TiO 2 to form a suitable band energy alignment, which increased V OC to 1.002 V from 0.986 V.As a result, a PCE of 16.11% was obtained.More-over, SWCNTs could suppress hysteretic J-V behaviors and improve stabilities of PSCs.However, when further increasing the concentration of SWCNTs, large charge recombination was observed because abundant junctions served as recombination points.In addition, since the utilized SWCNTs were composed of metallic-SWCNTs (m-SWCNTs) and semiconducting-SWCNTs (s-SWCNTs), the mechanism of these materials as composite ETLs to realize the performance enhancement in PSCs was still unclear.To understand the effects of SWCNT electronic types, Bati et al. 112 prepared s-SWCNTs and m-SWCNTs and integrated them into the TiO 2 ETLs.As shown in Figure 6B, the PSC fabricated with 2:1 s-:m-SWCNTs gave the best efficiency of 19.4%.The devices with no SWCNTs, only s-SWCNTs, only m-SWCNTs, 1:2 and 1:1 (s-:m-SWCNTs) delivered PCEs of 17.04%, 18.10%, 17.77%, 17.21%, and 18.09%, respectively.Obviously, the presence of SWCNTs could effectively enhance PCEs of devices.However, the device with only m-SWCNTs showed a reduced J SC compared to other PSCs, which was attributed to the insufficient built-in potentials of m-SWCNT/TiO 2 junctions and the accelerated charge recombination process near the Femi level of m-SWCNTs.To further study effects of conductive types of SWCNTs on the charge-transfer kinetics and interfacial carrier extractions, time-resolved PL characterizations were provided.The results showed that ETL with (2:1) s-:m-SWCNTs/TiO 2 gave a shortest decay time, which agreed with the prior findings in J-V curves.Density functional theory calculations confirmed that excited electrons in s-SWCNT could transfer to the TiO 2 surface and that this transport could be accelerated by m-SWCNTs in mixed SWCNTs (Figure 6C,D).Interestingly, the existence of m-SWCNTs in mixed SWCNTs effectively suppressed the degradation speed because of their lower environmental sensitivities, contributing to device lifetimes.These results suggested that the mixed SWCNTs were expected to fabricate efficient and stable perovskite devices.
To investigate interfacial performance of PSCs, Macdonald et al. 113 employed spectroscopic signatures, including PL, time-correlated single-photon counting (TCSPC), and femtosecond transient absorption spectroscopy (fs-TAS) to estimate the charge transfer between the composite ETL (CNT-TiO 2 ) and perovskite layers.It was found that TiO 2 /MAPbI 3 showed a higher quenching efficiency than CNT-TiO 2 /MAPbI 3 in the PL spectroscopy (Figure 6E), indicating a high-efficiency charge transfer from MAPbI 3 to TiO 2 instead of CNT-TiO 2 , which was in contrast to their previous report. 111They attributed this to the different MAPbI 3 deposition methods employed in two studies.The TCSPC spectra are exhibited in Figure 6F and were fitted to obtain the surface component (τ 1 ) and bulk component (τ 2 ), which could represent trap-assisted processes and free charge recombination, respectively. 114For the τ 1 value, TiO 2 /MAPbI 3 and CNT-TiO 2 /MAPbI 3 showed decay times of 0.58 and 0.73 ns, respectively, indicating that CNTs were unable to improve charge transfer, which was in accordance with PL spectra.The τ 2 for the CNT-TiO 2 /MAPbI 3 displayed a value of 97 ns, higher than that of the TiO 2 /MAPbI 3 (60 ns).The longer lifetime (τ 2 ) revealed a lower rate of bimolecular recombination in the CNT-TiO 2 /MAPbI 3 .The fs-TAS and dynamics of the decay kinetics for TiO 2 -MAPbI 3 and CNT-TiO 2 /MAPbI 3 layers are compared in Figure 6G.The decay for TiO 2 -MAPbI 3 (5.7 ns) was quicker than that for CNT-TiO 2 /MAPbI 3 (8.6 ns), further confirming quicker charge transfer for the former.To sum up, CNTs could not enhance the interfacial charge transfer but effectively suppressed trap states, decreasing the recombination in the device.Interestingly, the presence of CNTs boosted recombination resistances while diminishing chemical capacitances.Based on these results, the fabricated n-i-p device with the CNT-TiO 2 composite ETL yielded a PCE of 20.4%, exceeding that of the control device without CNTs (18.4%).
Besides SWCNTs, MWCNTs were also used in TiO 2based ETLs.Mohammed 115 utilized MWCNTs as additives in TiO 2 ETLs to modify n-i-p devices and achieved an efficiency of 21.4%.The Fermi level was changed to −4.03 eV from −4.27 eV for the ETL after adding MWC-NTs (Figure 6H), resulting in a better energy level at the MWCNT-TiO 2 /perovskite interface, which increased the V OC to 1.085 V from 0.845 V.Moreover, MWCNTs enabled the perovskite film with bigger grains and higher crystallinity, leading to an improved light absorbance.Amini et al. 116 adopted MWCNT-graphene-TiO 2 hybrid films (Figure 6I) as ETLs and constructed n-i-p PSCs.Compared to devices with bare TiO 2 , CNT-TiO 2 , and graphene-TiO 2 ETLs, the CNT-graphene-TiO 2 -based device delivered a larger J SC of 24.8 mA cm −2 and a higher PCE of 13.97%.These results were attributed to the combination of excellent electron extraction abilities of MWCNTs and large specific surface areas from graphene films, which were demonstrated by PL and photovoltage decay measurements.Therefore, the simultaneous existence of CNTs and graphene, instead of using them individually, in an ETL system would be able to bring more significant charge collection in PSCs.
8][119] To address these challenges, Mohammed and Shekargoftar 120 added CNTs into the ZnO ETL to prepare n-i-p PSCs.They revealed that the degradation of perovskite films was induced by localized positive ions of hydroxyl groups (-OH) on the ZnO surface.CNTs could effectively alter the ZnO surface and protect perovskites from hydroxyl agents, resulting in the high stability and low J-V hysteresis.Moreover, highquality perovskite film and enhanced charge extraction properties were achieved after adding CNTs.Accordingly, the PCE was raised to 18.79% from 15.05%.Tang et al. 121 achieved high-efficiency and hysteresis-free n-i-p devices by developing CNT-SnO 2 hybrid ETLs.CNTs were first modified via oxidization processes and then mixed with SnCl 4 .The mixed solution was deposited on the ITO substrate and the CNT-SnO 2 hybrid ETL was formed after annealing.It was found that CNTs effectively reduced the resistance and trap-state density of SnO 2 , leading to a high electron extraction certified by PL and electrochemical impedance spectroscopy (EIS) measurements.Indeed, the device with CNTs yielded a PCE of 20.33%, outperforming the referenced PSC (17.90%).
These results revealed that CNTs could work as effective additives in metal oxide ETLs to enhance electron transport and reduce defect densities.Nevertheless, few researches directly use CNT films (or modified CNT films) as ETLs in PSCs, possibly because the as-prepared CNT film shows a p-type characteristic in air environment owing to absorbed oxygen molecules.Therefore, the development of stable n-type CNT films is expected for CNT-based ETLs in PSCs.

CNTs as counter electrodes
CEs in PSCs are adopted to collect electrons/holes and transfer them to the external circuit.Metal electrodes, such as Au and Ag, are conventional CEs in highperformance perovskite devices due to their excellent conductivities. 7Nevertheless, high cost and complicated deposition using the thermal evaporation technique limit their large-scale applications.Most seriously, the potential metal migration and ion diffusion would lead to the perovskite film deterioration. 9,13To overcome these dif-ficulties, researchers aim to develop carbon electrodes with low cost, outstanding conductivity, and chemical inertness, which are becoming promising replacements for metal-based CEs. 122,123Interestingly, carbon electrodes can replace both metal electrodes and HTLs in PSCs (named HTL-free PSCs), contributing to simplifying device structures and further cutting costs. 124,125NTs have been recognized as one of the most potential carbon CEs because of their exceptional conductivity, stability, and hydrophobicity, as well as tunable work function.In 2014, Li et al. 126 first applied a CNT film to replace the Au electrode and fabricated HTL-free ni-p PSCs (Figure 7A).As shown in Figure 7B, the PSC with CNTs showed a PCE of 6.87%, outperforming the device with Au electrode.The enhancement mainly originated from the larger V OC and J SC , which was attributed to better hole extraction of CNTs than Au, resulting in a reduced interface recombination.However, the larger sheet resistance of the CNT layer caused the lower FF.It was found that adding Spiro-OMeTAD into the CNT film could enhance the hole extraction, further improving the PCE to 9.90%.Aitola et al. 127 also demonstrated that introducing Spiro-OMeTAD into SWCNT networks could effectively reduce interfacial charge recombination, but the electrical property of the SWCNT CE was not improved.
To optimize the conductivity, Luo et al. 128 developed a cross-stacked super-aligned CNT (CSCNT) sheet (Figure 7C), and then introduced it into HTL-free n-ip devices.As shown in Figure 7D, the device with 50 CNT layers (CSCNT-50) gave a PCE of 8.65% due to the highest conductivity.However, it was unable to further enhance PCEs by increasing the number of CNT layers to 75, which might be ascribed to the long-distance transfer of photogenerated holes because the CSCNT-75 layer was far away from the FTO side.In further study, they utilized the CSCNT film coated with SnO 2 to work as a CE in an inverted PSC (Figure 7E). 129It was found that values of J SC , V OC , and FF were much increased after introducing SnO 2 , leading to a high efficiency of 14.3% (Figure 7F).This was ascribed to the decreased carrier recombination because of the effective electron selectivity from SnO 2 .Moreover, the SnO 2 @CSCNT-based PSC showed excellent performance under high moisture, thermal stress, or persistent illumination.However, the cross-stacking structure caused some interspaces between CNTs, leading to nonuniform and rough surfaces.Moreover, these interspaces might produce conductive blind areas in CNT films and could not prevent moisture from infiltrating.To address these two issues, Tian et al. 130 introduced graphene in cross-stacking CNT layers to form printable free-standing graphene/CNT layers that could be easily transferred onto the Spiro-OMeTAD/perovskite/TiO 2 /FTO surface, acting as the CEs and moisture-blocking layers for devices (Figure 7G).It was found that the hybrid film with one-layer graphene and eight-layer CNTs (graphene-1/CNT-8) exhibited a sheet resistance of 88 Ω sq.−1 , which was much lower than that of the CNT-8 film without graphene (199 Ω sq.−1 ).The sheet resistance could be further reduced to 54 Ω sq.−1 after introducing four-layer graphene (graphene-4).Accordingly, the optimum PCE of 15.36% was obtained for the device with the hybrid graphene-4/CNT-8 film.In contrast, the PSC with only CNTs exhibited a PCE of 8.96%.This enhancement originated from high conductivities of hybrid layers and low series resistances of entire device.Furthermore, the PSCs with graphene-4/CNT-8 layers showed exceptional stability and could maintain 86% of its initial value after storage in a high-humidity environment for about 500 h, which was ascribed to the effective humidity blocking of hybrid films (Figure 7H).Li et al. successfully fabricated the efficient mesoscopic n-i-p PSCs using the SWCNT/graphite/carbon black composite CEs.Owing to the enhanced conductivity and hole collection efficiency, the device delivered a PCE of 14.7% with a V OC up to 1 V. Thus, these results indicate that further elevating electric conductivities of CNT CEs is necessary.
A mismatched energy level between the pristine CNT and perovskite layers causes inferior hole extraction and low V OC for PSCs. 131Doping has been considered to be an effective method to regulate Femi levels and conductivities of CNT films. 132,133Luo et al. 128 adopted iodine vapor at high temperature to develop an iodine-doped CNT sheet.Because the iodine doping efficiently enhanced carrier concentration and electrical conductivity of the CNT sheet, the PSC yielded a PCE of 10.54%, outperforming the pristine device (9.34%).Zheng et al. 131 8A).The band alignments of relevant functional layers are displayed in Figure 8B.Because of the suitable E F and enhanced conductivity, holes could be effectively collected by B-MWCNTs.Accordingly, the PSC with B-MWCNTs yielded an efficiency of 14.6% (Figure 8C), which could be further increased to 15.23% by introducing a thin insulating layer (Al 2 O 3 ) on m-TiO 2 .To enhance interface contacts between B-MWCNTs and perovskite, Yang et al. 134 provided an easy and low-cost ultrasound spray technology.Uniform and smooth B-MWCNT films were fabricated on perovskite layers.In addition, preparing NiO nanoparticles between B-MWCNTs and perovskite made the device achieve an optimum PCE of 15.80%.Chen et al. 135 mixed CNTs and acid solution (HNO 3 and H 2 SO 4 ) together and then heated the mixture at 108 • C for 4 h to realize functionalized CNT electrodes (HA-CNT).It was found that oxygen groups could not only modify the work function of CNTs to achieve a better energy level alignment but also could enhance the CNT/perovskite interface, resulting in an improved hole extraction and reduced charge transfer resistance.
Differentiating from high-temperature doping (functionalization) methods, surface charge transfer dopants, such as inorganic and organic acids, 136,137 can dope the CNTs through simple physical adsorption at room temperature. 138Accordingly, the structural integrity of CNTs would be preserved, ensuring better electronic performance.However, doping CNT-based top electrodes in PSCs is a challenging task due to damage from acid solutions for photoactive layers underneath.To address this question, Lee et al. 139 developed an ex situ vapor-assisted doping technology, in which the freestanding CNT sheet was first doped by the TFMS vapor, and then transferred onto the FACsPbI 3 layer without damaging the device (Figure 8D).The sheet resistance of the CNT electrode was reduced by 21.3% and the work function was boosted to 4.96 eV from 4.75 eV after 30 s of TFMS doping, leading to reduced potential losses in terms of hole extraction and transport (Figure 8E).Accordingly, the CNT-based device delivered a PCE of 17.6% (Figure 8F) and showed much more stable performance than Agbased device in atmosphere (60 ± 5 • C, relative humidity of 50 ± 10%).Nevertheless, the long-exposure doping might induce reactions between TFMS and 4-tert-butylpyridine (t-BP) in Spiro-OMeTAD, limiting the doping effectivity.Additionally, the doping time was difficult to precisely regulate, leading to a low reproducibility.To address these limitations, Jeon et al. 140 dispersed TFMS in a non-polar o-dichlorobenzene solvent and directly coated it onto CNT back electrodes without impairing the perovskite film, while minimizing the reaction with t-BP.By combining the optimized CNT density and Spiro-OMeTAD concentration, the CNT-based PSC realized a champion efficiency of 18.8%, exceeding that of the reference device with Au (18.4%).
Interestingly, beyond conventional operating environments, PSCs fabricated by CNT CEs were also explored in high-temperature conditions.Aitola et al. 141 suggested that n-i-p PSCs with SWCNT-Spiro-OMeTAD composite electrodes had superior long-term stability at elevated temperatures.Although the SWCNT-based PSC exhibited a lower initial PCE (15.0%) than that of the Au-based device (18.4%), the former possessed a much better thermal stability.In detail, the SWCNT-based PSC exhibited only a moderate linear PCE loss (the slope is −0.005% h −1 ) for 140 h at 60 • C in N 2 atmosphere.In contrast, the Au-based PSC displayed an exponential decay, losing 20% of the remaining PCE in only 8 h of aging time.This was attributed to the metal migration-caused deterioration in the device under elevated temperature.However, if the PSC operates at an exceedingly high temperature of over 100 • C, the organic-inorganic hybrid perovskite and organic HTL are inapplicable due to their instabilities under thermal stress. 9,142,143At this point, Dong et al. employed an inorganic CsPbI 2 Br perovskite and CNTs as absorber and carbon electrode, respectively.Such HTLfree n-i-p device delivered an efficiency of 11.31% and held above 80% of initial efficiency after heating at 200 • C for 45 h, suggesting an outstanding thermal stability under high-temperature environment.More importantly, its PCE temperature coefficient was superior to those of traditional Si and CuInGaSe devices, and was comparable to those of GaAs devices, owing to the superior V OC and FF temperature coefficients but inferior J SC temperature coefficients.This work may enable CNT-based PSCs with prospective utilization in extreme conditions including near-space and desert environments.
Based on the above discussion, it can be found that CNT-based CEs exhibit large application potential in high-stability and low-cost PSCs.Although PCEs of CNTbased devices are still lower than those of conventional PSCs, several methods can be adopted to upgrade the photovoltaic performance, including increasing CNT conductivity, tuning their work functions, and enhancing CNT/perovskite interface.Interestingly, the excellent thermal and chemical stabilities make CNTs a highly competitive choice in high-temperature PSCs.

CNTs as interlayers
The typical device architecture of a PSC is composed of layer-by-layer conformations, resulting in numerous interfaces.5][146] Interlayers are usually employed between perovskite and HTLs or electrodes to facilitate charge transport and suppress interfacial recombination. 147,148oreover, the interlayer can dramatically enhance stability, which is helpful for commercial applications of PSCs. 149arious interfacial materials, such as metal oxides, 150 carbon nanomaterials, 151,152 and polymers, 147,149 have been used to optimize interfaces in PSCs.Among these interlayers, CNTs show a promise because of their onedimensional structure, large specific surface area and good electrical properties.Ihly et al. 153 found that highly enriched s-SWCNTs could enhance hole extraction and lower back-transfer and recombination for PSCs.Based on this character, they used the (6,5) s-SWCNT film as the interlayer between MAPbI 3 and Spiro-OMeTAD to achieve the efficient and long-lived charge separation.It was found that the PSC with the 5-nm-thick (6,5) s-SWCNT interlayer exhibited the optimum photovoltaic performance, which was mainly ascribed to the largest improvements in J SC and FF.Recently, Wang et al. 154 suggested that defective MWCNTs (D-MWCNTs) could tune charge transfer kinetics regarding HTL and the interface between HTL and graphene electrode.Thanks to electrostatic dipole moment interactions between oxygen groups of D-MWCNTs and Spiro-OMeTAD (Figure 9A), the energy level of HTL was modified, leading to quicker charge transfer although electron redistributions and onedimensional channels (Figure 9B).Simultaneously, seamless connections between HTLs and carbon electrodes were created because D-MWCNTs induced interface coupling with graphene at nano scale.Accordingly, the device delivered an excellent PCE of 22.07% (Figure 9C) and presented a remarkable operational stability.
Functionalized CNTs not only show better surface nature but can also provide crystal growth sites for perovskites due to their abundant functional groups, which are helpful for upgrading the interface contact between CNTs and perovskites.However, it is a major challenge to use a spin-coating deposition method to fabricate a compact layer of CNTs on the perovskite layer without causing decomposition.To address this issue, Tiong et al. 155 developed a simple and efficient method to in situ form a thin octadecylamine-functionalized SWCNT (ODA-SWCNT) layer on perovskites in n-i-p devices.The ODA-SWCNTs exhibited the improved hydrophobicity and a good dispersion in the chlorobenzene.By depositing ODA-SWCNTs onto the wet perovskite surface, it was found that the grain sizes of perovskites were dramatically increased.This important morphology transformation of the perovskite film was ascribed to effects of carbonyl groups on ODA-SWCNTs, which had strong affinity toward Pb 2+ ions and caused the nucleation and growth of perovskite grains on SWCNTs.Besides, the SWCNT network could restrict evaporation rates of polar solvents, for instance, DMF and dimethylsulfoxide (DMSO), to reduce the nucleation densities of perovskite grains, leading to the growth of larger grains.In addition, as shown in Figure 9D, solvent vapors locked in perovskite layers could create conditions of dynamically near-equilibrium between SWCNTs and perovskite during annealing process.As a result, micrometer-scale perovskite grains grew vertically on the substrate.Ultrafast transient absorption (TA) measurements were adopted to investigate photogenerated charge dynamics, particularly charge transfer dynamics in the perovskite layer with and without the ODA-SWCNT film (Figure 9E).Clearly, the faster charge transfer and longer carrier lifetime were obtained after introducing the ODA-SWCNTs, which was ascribed to the increased grain sizes and decreased grain boundaries in perovskite layers.This advantageous morphology in turn significantly improved J SC of PSCs and nearly totally eliminated the hysteresis.The best PCE of 16.1% was obtained for devices adopting (FA 0.83 MA 0.17 ) 0.95 Cs 0.05 Pb(I 0.83 Br 0.17 ) 3 as light absorbers.Additionally, the device demonstrated excellent stability under conditions of high humidity because of the enhanced hydrophobicity of ODA-SWCNTs.
Designing a nanocarbon-based compound interlayers with both outstanding conductivities and large specific surface areas is another significant method to improve device performance.As shown in Figure 9F, Li et al. 156 prepared CNT@graphene (CNT@G) core-shell hybrids through in situ growth of graphene on CNT cores using plasma-enhanced CVD, which were adopted as the interlayer between Spiro-OMeTAD and Au.This hybrid displayed the strong capability to mechanically "hold" Spiro-OMeTAD molecules because of π-π interactions between Spiro-OMeTAD and nanocarbons (Figure 9G), resulting in an outstanding morphological thermal stability.Interestingly, the CNT@G/Spiro-OMeTAD layer not only effectively blocked iodide diffusion in perovskites but also prevented moisture from the humid environment.Moreover, the energy level of the CNT@G layer aligned well between Spiro-OMeTAD and Au (Figure 9H).Holes could be selected from Spiro-OMeTAD to the CNT@G nanohybrid and then were transferred more quickly to Au because of high conductivities of nanohybrid interlayers.Accordingly, the device with CNT@G delivered a PCE of 19.56%, as well as a good thermal stability and water stability.
It is noteworthy that HTL-free carbon-based PSCs feature high stabilities and low cost.However, most of paintable carbon pastes are prepared on the perovskite layer although the doctor-blading process, resulting in inferior perovskite/carbon interfaces because of many interfacial cracks or gaps.To mitigate this issue, Ryu et al. 157 dripped the MWCNT dispersion when spinning perovskite precursor solution to make MWCNTs penetrate into both perovskites and carbon electrodes (Figure 9I).Thus, MWCNTs could work as charge transfer channels.
The PL spectra were adopted to study the hole transfer behavior from perovskites to carbon electrodes.As shown in Figure 9J, it was found that the presence of CNTs could effectively reduce PL intensity.Especially, the perovskite layer prepared by MWCNT dripping methods showed greater decrease in PL intensity compared with the post-MWCNTs formed by the spin-coating technology.This indicated that MWCNTs were helpful for facilitating hole transfer.In addition, the characteristic frequency peak of the device with MWCNT dripping was shifted to the lowest frequency in EIS Bode plots (Figure 9K), suggesting the longest carrier lifetime.As a result, the penetrated MWCNTs boosted the interfacial contact between perovskite and carbon, resulting in enhancements in V OC and FF, and a PCE of 13.57% (Figure 9L).Similarly, Wang et al. 158 employed SWCNTs to enhance the perovskite/carbon interface, and selected a more common MAPbI 3 as the light harvester.Compared to MWCNTs, the SWCNT bridges exhibited higher charge-transport abilities and uniform networks, causing a superior interfacial hole extractions and transfer abilities.Hence, the device gained a PCE of 15.73% and an outstanding stability.Yang et al. 159 adopted polyethyleneimine-functionalized CNTs (PEI/CNTs) as bifunctional bridges in all inorganic CsPbI 3based devices.The PEI/CNT bridge reduced interface resistance and passivated surface trap states of perovskite due to plentiful amine functional groups on PEI/CNTs.This effort induced a PCE of 10.55% with an FF of 0.71, exceeding the device without PEI/CNTs (a PCE of 7.41% with an FF of 0.56).
Interfacial layers are essential for determining the efficiencies and stabilities of PSCs.CNTs have been successfully employed as interlayers at perovskite/HTL, HTL/metal electrode, and perovskite/carbon electrode interfaces.Their one-dimensional structure can work as charge channels to effectively promote hole transfer.In addition, the incorporation of other materials, such as twodimensional graphene and polymers, into CNTs to form hybrid layers is a superior method to further enhance charge collection efficiency.It is noteworthy that functionalized CNTs with suitable groups can provide supplemental functions, such as reducing surface defects/traps, tuning perovskite morphology, and blocking moisture, which are helpful for efficient and stable devices.

CNTS IN FLEXIBLE PEROVSKITE SOLAR CELLS
Because of their solution-processed, thin-film, and lightweight nature, PSCs have attracted much attention in flexible electronic devices and wearable equipment. 160,161dditionally, the preparation of flexible PSCs (f-PSCs) is compatible with industrial roll-to-roll (R2R) printing processes, which could enable large-scale production and push commercial applications of perovskite devices in future. 162,163At present, the PCE of lab-scale f-PSCs has reached more than 23%. 164,165However, since most high-efficiency f-PSCs are generally fabricated with ITO electrodes, their fragilities and costs are major issues hindering the commercialization of f-PSCs.Numerous electrodes with the high transparent conductivity and flexibility, such as metal nanowires, 166 carbon nanomaterials, 49,167 and conductive polymers, 168 have been developed to substitute ITO electrodes in f-PSCs.Among these electrodes, one-dimensional CNTs with conjugated double bonds endow outstanding charge-transport characteristics and extraordinary mechanical properties.More importantly, CNTs can be used to fabricate both film-shaped and fiber-shaped electrodes, which enable them to meet different-shaped f-PSCs.
Wang et al. 169 prepared a perovskite absorber on a Ti metal foil substrate with TiO 2 nanotube arrays, and transferred CNT networks to serve as hole collectors as well as top TEs.This kind of f-PSC exhibited a PCE of 8.31% and demonstrated good flexibility.In contrast, Jeon et al. 46 directly transferred aerosol SWCNT films onto polyethylene terephthalate (PET) substrates and fabricated SWCNT/PEDOT:PSS/CH 3 NH 3 PbI 3 /PC 61 BM/Al devices.It was found that HNO 3 doping enabled the SWCNT-based device with a PCE of 5.38%, which reached 60% of that of the ITO-based device (9.05%).Subsequently, the stable MoO 3 doping was adopted in SWCNT-based f-PSCs, leading to a higher PCE of 11%. 48Interestingly, flexible PSCs fabricated with SWCNTs and graphene electrodes are compared in Figure 10A.Although SWCNT-based devices showed lower efficiency than graphene-based devices, the former exhibited higher mechanical stability because of entangled configurations of nanotube networks.To achieve entirely solution-processable f-PSCs, Jeon et al. 170 replaced metal and ITO electrodes with SWCNT films to construct all-SWCNT-electrode flexible devices.Economic modeling suggested that the material cost of the novel architecture was only 33% that of conventional cells.However, this all-CNT-electrodes f-PSC only obtained a PCE of 7.32% and an inferior FF.These relatively low values originated from the poor transparent conductivities of CNT films compared to conventional metal/ITO electrodes.Zhang et al. 49 fabricated flexible inverted PSCs using high-quality and small-bundle SWCNT films with excellent transparent conductive properties, delivering PCE values of 18% (Figure 10B) and 15.8% on flexible substrates with active areas of 0.09 and 1 cm 2 , respectively.The SWCNT-based PSC was certified to maintain ∼85% of the initial PCE after bending 1000 times at a radius of 6 mm.Even when the radius was decreased to 4 mm, the device was still capable of holding over ∼80% of the initial PCE (Figure 10C).Moreover, owing to the hydrophobic nature of SWCNTs, the unencapsulated f-PSC could retain more than 80% of the initial PCE value after 1 month, while ITO-based f-PSCs only maintained a value of 50%.More significantly, the SWCNT cost was only ∼$40 per square meter (50 Ω sq.−1 @85% T), much cheaper than commercial ITO electrodes (∼$900 per square meter for 15 Ω sq.−1 @85% T).These advantages suggest the large application potential of SWCNT-based electrodes in flexible solar cells.
Except for TEs, CNTs were also employed as CEs in f-PSCs because of their stable chemical stabilities, good electrical conductivities, and lower costs.Luo et al. indicated an inverted n-i-p f-PSC with SnO 2 -coated CSCNT cathode.The device displayed V OC , J SC , and FF values of 19.2 mA cm −2 , 0.91 V, and 0.60, respectively, resulting in a PCE of over 10%.However, the efficiency decreased from original 10.3% to 8.2% after 300 bending times at a bending radius of 4 mm.This reduction was ascribed to the increased series resistance of the ITO/polyethylene naphthalate (PEN) substrate, perovskite cracking and interface delamination in the flexible device after continuous bending.In their further study, the ITO electrode was substituted by graphene.Meanwhile, the back electrode was also fabricated with cross-stacking CNTs (Figure 10D). 171The all-carbon-based f-PSC performed outstanding mechanical flexibility compared with that of the reference PSC with ITO and Au electrodes at different bending radii (Figure 10E).Especially at a radius of 2.2 mm, the carbonbased device still held 85% of the original PCE, but the ITO-based f-PSC was almost damaged.In addition, after bending 1500 times at a radius of 4 mm, the carbonbased device maintained durability superior to that of the ITO-based f-PSC (Figure 10F).More importantly, the all-carbon-f-PSC exhibited better stability than the reference f-PSCs with Au and Ag electrodes under over 1000 h light soaking (Figure 10G).This was attributed to the excellent chemical stability and hydrophobicity of carbon electrodes, which effectively blocked moisture ingress at high temperatures.
Recently, an increase in the demand for super flexible electronics, such as foldable and textile devices, has motivated efforts to focus on electrodes with extraordinary mechanical stability. 172,173The foldable device is distinguished from the ordinary flexible device because the former has to withstand extreme mechanical stresses.The CNT film possesses remarkable mechanical resilience, and has been considered as a promising foldable electrode. 174,175Yoon et al. 176 first reported a foldable PSC with the SWCNT-polyimide (SWCNT-PI) nanocomposite electrode (Figure 10H,I).Since SWCNTs were embedded into the PI polymer, the composite electrode indicated an exceptionally smooth morphology and flexibility.More importantly, the high glass transition temperature of PI could make the SWCNT-PI electrode withstand the high-temperature heat treatment when using MoO x as the p-type dopant, leading to an enhanced conductivity and hole transfer efficiency.The foldable PSCs made by MoO xdoped SWCNT-PI electrodes delivered a PCE of 15.2% and could withstand more than 10 000 "folding" cycles at a 0.5 mm folding radius (Figure 10J); nevertheless, the referenced ITO-based device was unable to withstand even one folding cycle.The excellent mechanical resilience was ascribed to SWCNTs embedded in PI and the ultrathin SWCNT-PI film (7 μm).However, the PI material reduced the optical transmittance of the pristine SWCNT film in the spectral range from 400 to 600 nm, which should be considered in high-performance foldable PSCs in the future.
Wearable devices stand for a novel and significant trend in current electronics.In particular, electronic textiles are highly desired in diverse fields, including health care, sports, or military applications. 177Thus, it is essential to incorporate formable and wearable power systems in accordance with the users' body shape into textile electronic devices.The fiber-shaped PSC was regarded as a promising candidate for energy supplies of wearable electronic devices because of both high efficiency and all solid state. 172,178Peng's group first reported a fiber-shaped PSC in 2014. 179As shown in Figure 11A, the CH 3 NH 3 PbI 3 absorber was dip-coated on a stainless steel fiber anode with c-TiO 2 , m-TiO 2 , and Spiro-OMeTAD layers, subsequently, CNT sheets were winded and acted as cathodes.This allsolid-state fiber-shaped device displayed a PCE of 3.3% (Figure 11B) and was successfully woven into PSC textiles (Figure 11C), indicating great potential in wearable applications.In further work, 180 they developed an elastic fiber-shaped PSC by employing stretchable aligned CNT fibers and spring-like modified Ti wires as two electrodes (Figure 11D).This new fiber-shaped PSC revealed a stable PCE under both stretching and bending (Figure 11E).Moreover, the textile device woven by the elastic fibershaped PSCs also exhibited excellent flexibility and elastic abilities (Figure 11F).However, perovskite layers prepared via a dip-coating method showed inferior qualities, leading to low photovoltaic performances.To mitigate the issue, the cathodic deposition solution process was developed to fabricate high-coverage and uniform perovskite layers on curved surfaces of metal wires. 181During the fabrication process, the aligned TiO 2 nanotubes were first prepared on Ti wire surface for ETL, and then spongestructured PbO films were coated on TiO 2 nanotubes by cathodic deposition, which could react with hydroiodic acid and form crystal PbI 2 .At last, the CH 3 NH 3 PbI 3 nanocrystals were prepared by the reaction of PbI 2 and CH 3 NH 3 I.The obtained perovskite film exhibited high quality and uniformity compared to that fabricated via the dip-coating technology.Additionally, conductivities of CNT sheets were increased by introducing silver through a thermal deposition.Based on these efforts, the efficiency of the fiber-shaped PSC was enhanced to a value of 7.1%.To improve crystal sizes of perovskite film, Peng's group designed a strip-shaped PEN/ITO electrode to serve as transparent conductive substrate. 182This flat interface was helpful for growing perovskite microcrystals.Meanwhile, the CNT film fabricated from the spinnable CNT array was utilized as both the electrode and HTL.Accordingly, a PCE of 9.49% was obtained.Although photovoltaic performances were improved, larger-size perovskite crystals would lower flexible stabilities of f-PSCs because of their cracks during bending or twisting.Therefore, the trade-off between photovoltaic performance and flexibility for the perovskite layer has to be considered.
The above reported fiber-shaped PSCs are made of metal wires and CNT electrodes.Nevertheless, the metal wire electrode may suffer from the low surface area and potential metal migration.Similar to all-carbon-electrodebased planar PSCs, 170,171  performance stability.The PSC with super flexibility could be wrapped on a capillary tube with a radius of 0.3 mm (Figure 11I), revealing excellent performance for textile electronics.However, due to relatively lower conductivities of CNT fibers, this photovoltaic performance was much lower than that of fiber-shaped devices with metal wires. 182s shown above, CNT electrodes have been largely developed in flexible PSCs.The CNT films enable planar f-PSCs to achieve high flexibility and portability because of their excellent mechanical stability and low density, which make them a promising electrode material to replace conventional ITO/FTO.It is worth mentioning that constructing all-carbon-electrode f-PSCs, briefly, in which the CNT or graphene film works as a TE and the CNT, graphene, or carbon paste film acts as a back electrode in the device, will further reduce processing costs through compatibility with continuous R2R process.Additionally, CNTs have exhibited impressive applications in the fields of foldable and fiber-shaped PSCs, indicating their broad prospects and huge potential in extreme mechanical and textile electronics.

CNTS IN SEMITRANSPARENT PEROVSKITE SOLAR CELLS
ST-PSCs have attracted increasingly attention because they can not only generate electrical power by using solar light but also can allow visible transparency.4][185][186][187] Considering the optical transmission of devices, the TEs have become an essential component of ST-PSCs.In recent years, ITO and FTO have been widely utilized for transparent bottom electrodes owing to their good transparency and conductivity.However, their depositions usually need an ultrahigh vacuum condition, as well as a high-temperature annealing process for crystallization, which introduces physical damage to the sublayer when they work as top TEs. 188,189Although employing a buffer layer can effectively suppress this damage, it would bring about additional optical loss. 190,191NT-based transparent conductive films exhibit distinctively high optical transmittance. 45More importantly, they can be introduced in PSCs although solution-process and direct dry-transfer technologies. 53,139These features make them applicable to both top and bottom TEs.Li et al. 126 first fabricated FTO/c-TiO 2 /m-TiO 2 /CH 3 NH 3 PbI 3 /CNT structured ST-PSCs.The CNT film was synthetized by the FCCVD technology and transferred onto the perovskite surface without a vacuum environment at room temperature.Nevertheless, the lower transparent conductivity and charge selectivity of the pristine CNT electrode led to a poor bifaciality, namely, the PCE of 3.88% obtained from the CNT side is much lower than that obtained from the FTO side (6.29%).Fortunately, the better coverage and charge collection were realized by incorporating Spiro-OMeTAD with CNT film, and the PCE could be further improved.Lee et al. 192 also demonstrated the ST-PSCs with CNT films combined with Spiro-OMeTAD as transparent top electrodes (Figure 12A,B).It was found that the CNT film with 80% T at 550 nm was most suitable for fabricating ST-PSCs, which enabled the device to have high transmittance in 750-900 nm wavelengths.The MoO x layer was adopted as the dopant to further increase photovoltaic performance of CNT-based ST-PSCs because of its p-type doping and stability. 193Accordingly, a PCE of 18.80% was achieved, surpassing that of the referenced ITO/MoO x -based cells (15.96%) (Figure 12C).This was attributed to the enhanced electron-blocking ability originated from high Spiro-OMeTAD concentration in CNT-based devices.On the other hand, ITO/MoO x -based devices suffered from damages when sputtering ITO electrodes on both Spiro-OMeTAD and MoO x layers.Although the utilized CNT film showed a higher optical transmittance in the infrared range than the ITO electrode, the semi-transmittance of the entire device was slightly higher for ITO/MoO x -based ST-PSCs (Figure 12D) owing to effects of total internal reflections of different layers in devices.However, the higher PCE enabled CNT-based ST-PCSs with advantages to construct tandem photovoltaic devices with silicon bottom cells.Accordingly, a PCE of 24.42% was obtained for the four-terminal perovskite-silicon tandem device.Despite the Spiro-OMeTAD can effectively enhance the charge selectivity of CNTs, the device stability is still inadequate to meet needs of commercial applications due to hydroscopic dopants in the Spiro-OMeTAD HTL. 56Thus, stable dopant-free HTLs are highly required to boost overall performance of CNT-based devices.Zhang et al. utilized the dopant-free poly(triarylamine) (PTAA) as HTL in SWCNT-based ST-PSCs (Figure 12E).It was found that PTAA tightly wrapped around SWCNTs through π-π interactions, resulting in a high-efficiency hole transport in devices, which was demonstrated by EIS and microwave photoconductivity decay spectra.Accordingly, a PCE of 16.76% was obtained.Moreover, the dopant-free feature of PTAA avoided the humidity-orientated deterioration of perovskites, leading to a better stability than the PSC with the doped Spiro-OMeTAD (Figure 12F).The PTAA-wrapped SWCNT electrode was also used to prepare perovskite-silicon tandem cells.As shown in Figure 12G, the tandem cell constructed by a bottom silicon device and a PTAA-wrapped SWCNT-based ST-PSC yielded a PCE of 24.12%.This value was slightly higher than the PCE of 23.12% for the tandem device with Spiro-OMeTAD, but better stabilities could further stimulate overall performance of SWCNT-based ST-PSCs and tandem devices.Apart from introducing additional hole-transport/electron-blocking materials into SWCNT networks, passivating surface defects of perovskite is an effective method to reduce charge recombination. 194Elakshar et al. 195 reported a passivation technology using MAI as a reagent, which converted PbI 2 impurities in deposited Cs 0.12 FA 0.88 PbI 3 layers to pure perovskite phases, resulting in decreased surface defects and enhanced adhesion of SWCNTs to absorber layers.The device with the architecture of ITO/SnO 2 /PCBA/Cs 0.12 FA 0.88 PbI 3 /SWCNT delivered a PCE of 16.3% with a V OC of 0.981 V, a J SC of 21.6 mA cm −2 , and an FF of 77% when it was illuminated from the ITO side.However, the device illuminated from SWCNTs side yielded a PCE of 6.9% with a V OC of 1.007 V, a J SC of 10.6 mA cm −2 , and an FF of 65%.Obviously, the largest differences lie in values of J SC and FF, which might be related to the thickness and conductivity of the SWCNT electrode, as well as relatively low charge extraction ability duo to lacking HTLs.
Zhang et al. 196 developed a novel technology to prepare CNT-based ST-PSCs, in which CNTs/ITO (FTO) were adopted as the transparent top electrodes.Briefly, the front semi-device was prepared via sequentially depositing the SnO 2 , perovskite, Spiro-OMeTAD and CNT (MWCNT or SWCNT) layers on an FTO substrate.Meanwhile, a rear electrode was fabricated by spraying CNTs on another FTO or ITO substrate.The final device exhibited in Figure 12H was obtained via stacking the semi-device and rear electrode under a slight pressure (2.5 kPa).It was found that the monofacial PCE of the device with a 4-μmthick MWCNT film was 14.4%; however, when integrating MWCNT with the FTO electrode, even when the thickness of the MWCNT film was only 400 nm, the device delivered a higher monofacial PCE of 18.7%.This was ascribed to lessened series resistances of entire devices when using MWCNT/FTO integrated electrodes.Unfortunately, only a 0.6% PCE was obtained when this device was illuminated from the rear side due to poor transmit-tance of the 400-nm-thick MWCNT film (Figure 12I).To improve the transmittance of the rear-side electrode, the small-diameter SWCNT and ITO electrodes were adopted.The SWCNT-PSC delivered PCEs of 21.4% and 16.8% from front illumination and rear illumination, respectively, and also reached a bifacial power density of 24.0 mW cm −2 for natural reflection and 34.1 mW cm −2 for artificial reflection (Figure 12J), revealing promising prospect in bifacial solar cell applications.Additionally, the SWCNT-PSC was employed as top subcell to construct four-terminal perovskite/CuInSe 2 tandem devices, and achieved a PCE of 27.1%.
From this section, ST-PSCs have been realized by replacing traditional metal electrodes with CNTs.Mild fabrication technologies of CNT films could effectively avoid additional damage to the perovskite and HTL.Compared to MWCNTs, SWCNTs could enable ST-PSCs to exhibit higher bifaciality due to their small diameters and low light absorption.Moreover, the SWCNT-based ST-PSCs have been studied in tandem and bifacial solar cells, suggesting their special applications compared with conventional PSCs.It is noteworthy that enhancing the charge extraction and transport efficiency of SWCNTs is still the key to obtaining efficient ST-PSCs.

CONCLUSION AND PERSPECTIVES
PSCs have exhibited a speedy advancement in just a few years and they are now close to photovoltaic performances of traditional Si solar cells.However, the costly functional layers and long-term stabilities of PSCs are still critical challenges when devices face the commercialization.CNTs have shown significant potentialities to reduce the cost while enhancing the stability of PSCs due to their abundance, chemical inertness, and hydrophobicity.In addition, the excellent electronic conductivity, high optical transmittance as well as simple fabrication enable their utilization in bottom and top TEs, which will be essential for flexible and ST-PSCs.Nonetheless, photovoltaic performances of CNT-based PSCs demand further enhancement to make them compare favorably with state-of-the-art PSCs.
The exact function of CNTs in various structures ranges from TEs, charge-transport layers (CTLs), including HTLs and ETLs, as well as CEs, perovskite additives to interlayers.First, the reported CNT TE in PSCs still showed lower performance than traditional ITO/FTO electrodes.Although doping has been supposed to be an effective technology to increase conductivities of CNT films, there are some issues of dopants that should be considered, such as the doping effectivity, chemical stability, and optical absorption.Additionally, hybrid TEs based on CNTs and conductive polymers also show the practicability in PSCs.Although solution-processed CNT films exhibit smooth surface morphology, the residual surfactant needs to be addressed because its electrical insulation may decrease the carrier mobility of CNTs.Moreover, more attention should be given to the influence of the heterojunction formed between the CNT TE and CTL, which might cause barrier resistance leading to a reduction in the FF. 197econd, CNTs have exhibited capability of the notable electrical conductivity in CTLs, yet many CNT films composed of metallic and semiconducting CNTs display conductor characteristics, leading to relatively low charge selection efficiencies compared with common HTLs/ETLs.Tuning the energy level alignment by doping methods and compositing with other semiconductor materials will be viable strategies to reduce energy loss and enhance charge extraction.Owing to large specific surface areas, CNTs usually absorb oxygen molecules in air and show p-type features, which make them more suitable for acting as HTLs.Regarding ETLs, CNTs mainly work as beneficial additives in common inorganic ETLs, such as TiO 2 , SnO 2 , and ZnO layers, to enhance their electron transfer properties and reduce defect densities.However, few reports directly use CNT films (or modified CNT films) as ETLs in PSCs or ETL-free devices.Developing stable n-type CNT films in air should be considered seriously.In addition, the charge transfer mechanism at the CNT/perovskite interface should be comprehensively considered and characterized as much as possible.In particular, further insight into effects of CNT electronic types in extracting and transporting charges would be very valuable to realize high-performance PSCs.Third, CNTs were successfully demonstrated as potential CEs for boosting lifetimes of PSCs.However, to date, the photovoltaic performances of PSCs based on CNT CEs are still lagging behind those of Au-based devices.Enhancing interface engineering and reducing sheet resistances are required to facilitate the charge extraction, collection, and transport.Constructing perovskite phase junctions, such as perovskite-perovskite homojunction and heterojunction, 198,199 should be considered to further boost the charge separation and photovoltaic performance of PSCs or CTL-free PSCs based on CNT CEs.More broadly, the development of CNTbased inorganic PSCs is helpful for expanding applications of PSCs into extreme conditions including near-space and desert environments.Fourth, with regard to additives in the perovskite, functionalized CNTs can serve as a template to prepare the large-size or strongly oriented perovskite grains, resulting in longer charge lifetimes and lessened defect densities in perovskite films.In particular, s-SWCNTs with a direct bandgap of up to 2 eV and high conductivities along the tube axis display capabilities in the field of additives.However, because of strong van der Waals interactions, SWCNTs face difficulty in realizing high solubility in DMF and DMSO solvents.Hence, the design of some polymers with molecular structures or special functional groups is urgently demanded to wrap or graft on the surface of SWCNTs, aiming to improve their solubility while achieving additional functions, such as optimizing qualities of perovskite layers and suppressing the lead exposure risk.This effort would be a promising direction to obtain efficient, stable, and eco-friendly PSCs.Finally, CNTs have been successfully used as interlayers at perovskite/HTL, HTL/metal electrode, and perovskite/carbon electrode interfaces.Their one-dimensional structure can work as charge channels to effectively promote interfacial hole transfer.It is noteworthy that fabricating hybrid or functionalized CNT films with other materials can further improve charge dynamics and stabilities of PSCs, but these materials should be attentively chose and designed to ensure practical enhancements in PSC performance.Moreover, the potential of the CNT-based interlayer in perovskite/ETL should also be explored to reduce defect densities of both perovskite and common ETLs.
While applications of CNTs in flexible and semitransparent perovskite devices have become more fashionable due to their unreplaceable characteristics, the key to constructing novel CNT-based PSCs deserves careful consideration.In f-PSCs, CNT films have exhibited huge advantages in enhancing the mechanical stabilities of devices.However, the CNT-based f-PSC delivered a lower initial PCE than that of ITO/FTO-based f-PSCs.To further boost the PCE, the transmittance and conductivity of the CNT-based TE should be considered, as they directly influence J SC and FF of f-PSCs.Also, considerably more effort is needed to optimize the roughness and hydrophilic properties of CNT films to achieve high-quality upper constituent layers.The investigations on scalable fabrication of CNT-based f-PSC although R2R technology are still inadequate.On the one hand, high-performance, large-scale, and continuous CNT transparent conductive films need to be further exploited.On the other hand, how to prepare high-quality perovskites on flexible substrates is the main research direction.Additionally, it is advantageous to continue investigating CNT-based foldable and fiber-shaped PSCs, extending their applications in extreme mechanical and textile electronics.Regarding semitransparent devices, CNT films can act as both top and bottom TEs in ST-PSCs.Especially in top electrodes, mild fabrication technologies of CNT films could effectively avoid additional damage to the perovskite and CTL.However, the ST-PSC revealed low bifaciality when using CNT-based TEs.To obtain efficient bifacial photovoltaics, further improving transparent conductive property of the CNT electrode and preparing high-efficiency ST-PSCs are of great importance but very challenging.
Anyway, considerable follow-up efforts should be made in several aspects, including CNTs and perovskite materials, device structures, and interface engineering, to further promote photovoltaic performance and long-term stability of CNT-based PSCs, making them competitive in photovoltaic fields.
selected boron (B) doping to boost the hole extraction and transport of MWCNT electrodes in HTL-free n-i-p devices.The B atoms were introduced into MWCNTs (called B-MWCNTs) through thermal annealing of MWCNTs and H 3 BO 3 under an Ar atmosphere at 1000 • C. It was found that the doping process efficiently reduced defects and improved the graphitization of the MWCNTs, which was demonstrated by Raman and X-ray photoelectron spectroscopy spectra.More importantly, B-doping enabled the E F of B-MWCNT to lie at −4.55 eV; in contrast, the E F of the pristine MWCNTs and the reference thermally treated MWCNTs without B-doping (T-MWCNTs) were −4.46 and −4.43 eV, respectively.This downshift in E F indicated that the B atom in B-MWCNTs acted as an acceptor, which efficiently facilitated the interfacial charge extraction and transport (Figure

F
I G U R E 1 0 (A) Schematic showing perovskite solar cells (f-PSCs) using graphene and single-wall carbon nanotube (SWCNT).Reproduced with permission from Ref. 48.Copyright 2017, American Chemical Society.(B) J-V characteristics of the champion SWCNT-based f-PSC.(C) Bending test of f-PSCs based on indium tin oxide (ITO) and SWCNT electrodes under the bending radii of 4, 6, and 12 mm.Reproduced with permission from Ref. 49.Copyright 2021, Wiley-VCH.(D) Device architecture of the all-carbon-electrode-based f-PSC.(E) The power conversion efficiency (PCE) stability of f-PSCs with increasing bending radius after 200 bending times.(F) Bending endurances of the ITO/PEN-based and all-carbon-electrode-based devices as a function of bending cycles.(G) The PCE stability with and without all-carbon-electrodes.Reproduced with permission from Ref. 171.Copyright 2018, Wiley-VCH.(H) The 3D illustration and (I) optical image of the foldable PSCs.(J) Folding durability of the MoO x /SWCNT-polyimide (PI)-based device (R = 0.5 mm).Reproduced with permission from Ref. 176.Copyright 2021, Wiley-VCH.BCP, bathocuproine; PEDOT:PSS, poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate).
double-twisted fibrous PSCs with CNT fibers can effectively avoid these issues because of high surface areas and excellent chemical stabilities of CNTs.As shown in Figure 11G, Li et al. demonstrated the double-twisted PSC with CNT fibers.The CNT fiber anode was sequentially covered by layers of c-TiO 2 , m-TiO 2 , CH 3 NH 3 PbI 3-x Cl x , and P3HT/SWCNT/Ag nanowires.Afterward, double-twisted structure devices were prepared by twisting pristine CNT fibers, which delivered a PCE of 3.03% (Figure 11H) and stable F I G U R E 1 1 (A) Structure diagram and (B) the J-V characteristic of the fiber-shaped device.(C) Photograph of a textile.Reproduced with permission from Ref. 179.Copyright 2014, Wiley-VCH.(D) Schematic showing the elastic perovskite solar cell (PSC).(E) J-V characteristics of three PSCs with series or parallel before and after stretching.(F) Photograph showing an elastic PSC textile.Reproduced with permission from Ref. 180.Copyright 2015, Royal Society of Chemistry.(G) The structure and (H) J-V characteristic of the double-twisted fibrous device.(I) The optical photo showing the double-twisted fiber-shaped PSC wrapped on the capillary tube.Reproduced with permission from Ref. 202.Copyright 2015, Wiley-VCH.CNT, carbon nanotube; P3HT, poly(3-hexylthiophene); PCE, power conversion efficiency; SWNT, single-wall nanotube.
This work was financially supported by the National Natural Science Foundation of China (52192610, 62274127, and 62304163), National Key Research and Development Program of China (2021YFA0715600, 2021YFA0717700, and 2018YFB2202900), Natural Science Basic Research Program of Shaanxi (2023-JC-QN-0471), Qinchuangyuan Cited High-level Innovation and Entrepreneurship Tal-