Overcoming the voltage losses caused by the acceptor‐based interlayer in laminated indoor OPVs

Harvesting indoor light to power electronic devices for the Internet of Things has become an application scenario for emerging photovoltaics, especially utilizing organic photovoltaics (OPVs). Combined liquid‐ and solid‐state processing, such as printing and lamination used in industry for developing indoor OPVs, also provides a new opportunity to investigate the device structure, which is otherwise hardly possible based on the conventional approach due to solvent orthogonality. This study investigates the impact of fullerene‐based acceptor interlayer on the performance of conjugated polymer–fullerene‐based laminated OPVs for indoor applications. We observe open‐circuit voltage (VOC) loss across the interface despite this arrangement being presumed to be ideal for optimal device performance. Incorporating insulating organic components such as polyethyleneimine (PEI) or polystyrene (PS) into fullerene interlayers decreases the work function of the cathode, leading to better energy level alignment with the active layer (AL) and reducing the VOC loss across the interface. Neutron reflectivity studies further uncover two different mechanisms behind the VOC increase upon the incorporation of these insulating organic components. The self‐organized PEI layer could hinder the transfer of holes from the AL to the acceptor interlayer, while the gradient distribution of the PS‐incorporated fullerene interlayer eliminates the thermalization losses. This work highlights the importance of structural dynamics near the extraction interfaces in OPVs and provides experimental demonstrations of interface investigation between solution‐processed cathodic fullerene layer and bulk heterojunction AL.


| INTRODUCTION
Solution-processed organic solar cells are a promising solar cell technology with the potential for scalable production using wet deposition techniques such as printing and coating leading to a low environmental impact. 1 However, the combination of high power-conversion efficiencies and long lifetime require more understanding and development to compete with established technologies.A niche market for organic photovoltaics (OPVs), lately gaining much attention also in academic publications, [2][3][4] is indoor applications to power, for example, connected sensors and electronic shelf labels.Under indoor climate, the main degradation challenges associated with elevated temperature and light-induced "burn-in" is not a limiting factor for a lifetime.
The interfaces in OPV devices, starting from the substrate-electrode interface to those within the active layer (AL), play a crucial role in determining the efficiency of the device. 5,6Among these interfaces, two types, in particular, receive significant attention in typical heterojunction OPVs, namely, the semiconductor-semiconductor interface between the donor and acceptor 7,8 and the semiconductor-metal interface between the AL and electrodes. 9,102][13] The bulk heterojunction (BHJ) approach, which involves interpenetration of the donor and acceptor, is a commonly used technique in OPVs.However, it is challenging to predict the final morphology of the film from the solution. 14An ideal arrangement of donor and acceptor, characterized by highly ordered domains of donor and acceptor, with the donor predominantly at the anode electrode and the acceptor predominantly at the cathode electrode, has been suggested by simulations 15,16 (Figure 1A).
As depicted in Figure 1A, the ideal AL arrangement in OPVs requires careful consideration of multiple parameters to achieve high performance.One of the critical parameters is the charge transfer (CT) dynamics at the donor-acceptor interface, where the energy of the CT state (E CT ) governs charge carrier separation and recombination, thereby impacting the maximum opencircuit potential. 17,18Additionally, the effective charge generation is facilitated by a domain width that is two times the size of the exciton diffusion length. 19The ordered arrangement of donor and acceptor domains also ensures noninterrupted transport through the pure phase. 15,20Furthermore, the energy alignment at the semiconductor-metal interface, which influences the open-circuit voltage (V OC ) and device performance, is dictated by the electrical, chemical, optical, and filmforming properties of the transport layers. 9,21,22There have been numerous studies on the nonradiative recombination losses at the electrode-AL interface, including surface recombination, energy level pinning, and thermalization losses. 21,23,24hile the ideal AL arrangement implies positive effects of pure donor (acceptor) compositions at the anode (cathode) side, recent studies demonstrated a counterintuitive approach to the ideal arrangement by adding a pure acceptor phase on the "wrong" anode electrode.Incorporation of the acceptor phase on the anode electrode enhances device V OC ; however, the mechanism behind the increased V OC is still under debate.Ding and Forrest 25 demonstrated that the V OC loss that comes from a static dipole at the active region-anode interface can be reduced by inserting a tunnel-thin acceptor layer on the anode.Kotadiya et al. 26 reported that the acceptor incorporation on the anode leads to Ohmic contact formation that suppresses surface recombination losses, while Pranav et al. 27 suggested that the suppression of the surface recombination is due to an enhanced built-in potential.Although these studies come in handy in understanding the interfacial energy losses and enhancing V OC output, they are limited by the thickness of the inserted tunnel-thin interlayer.As shown in these reports, when interlayer thickness exceeds the threshold of tunneling, charge transport will be hindered.Moreover, there are hardly any studies on the influence of the structural order of the acceptor-based interlayers on the energy loss of OPVs.A comprehensive understanding of the ideal arrangement near the electrode interface and strategies to mitigate energy losses are necessary for maximizing the potential of OPVs.This study aims to increase the understanding of energy losses at the AL/cathode interface.We focus on BHJ ALs and interlayers based on fullerenes, where we investigate the impact of the fullerene-based acceptor interlayer on the performance of conjugated polymer-fullerene-based OPVs for indoor applications (Figure 1B).We focus on solution processing of bulk thick (nontunneling) acceptor-based interlayers, as the insensitivity of interlayers to film thickness is of critical importance for industrial upscaling. 28For many years, efforts have been made to achieve consistent work batchto-batch variations, purification, and reproducibility in the context of fullerene-based OPV upscaling.While nonfullerene acceptors (NFAs) have pushed the powerconversion efficiency to a high value approaching 30% (at 2000 lx, warm light-emitting diode [LED]), 29 such efforts for upscaling are still in their early stages with respect to NFAs, motivating us to use fullerenes as a case of study here, especially considering that our focus is for industrial applications.NFA-based interlayers are interesting for future case studies but are outside the scope of this manuscript.We employ an industrially applicable lamination technique, 30 which enables an investigation of the interface between solution-processed cathode [6,6]-phenyl-C 61 -butyric acid methyl ester (PC 61 BM) and polymer:PC 61 BM-based OPVs, deposited from nonorthogonal solvents.Although the utilization of the interlayer PC 61 BM would intuitively give an optimal device performance, we found that the PC 61 BM interlayer leads to a reduced V OC .To address this challenge, we successfully incorporated insulating organic elements, specifically polyethyleneimine (PEI) and polystyrene (PS), into the PC 61 BM interlayers.This integration yielded two distinct benefits: a lowered work function (W f ) of the cathode and the potential repulsion of minority carriers for the PC 61 BM:PEI interlayer, as well as an enhanced energetic alignment between the interlayer and the AL for the PC 61 BM:PS interlayer, improving overall device performance.Neutron reflectivity (NR) studies further uncover that the fundamental mechanisms of decreased W f are different in both cases: in the case of PEI incorporation, a tunnel-thin layer of self-arranged PEI on top of PC 61 BM could potentially eliminate hole transfer from the BHJ AL blend to the acceptor interlayer, leading to a reduced hole population on the highest occupied molecular orbital (HOMO) states, hence reducing the recombination losses; in the case of PS incorporation, the structural order of the film is improved, creating a gradient distribution of the PC 61 BM:PS mixture, hence decreasing the thermalization losses due to the acceptor interlayer.Our work provides one of the experimental demonstrations of interface investigation across the solution-processed cathodic fullerene layer and BHJ, highlighting the importance of understanding the morphological arrangement of the interlayers on the device V OC .Our finding is also aligned with recent development in perovskite solar cells, where fullerene is frequently used as an interlayer. 31 was obtained from Heraeus GmbH.The pristine PH1000 was mixed with ethylene glycol (from Sigma-Aldrich) and Capstone FS-30 surfactant (Dupont) in a volume ratio of 93.5:6:0.5 (PH1000:EG:FS-30) and further diluted in deionized water in a 2:1 ratio (v:v, ink:water).A measure of 125 μm heat-stabilized polyethylene terephthalate (PET) Melinex ST505 (Tekra) was utilized as a substrate.

| Device fabrication and electrical characterization
Fabrication of laminated OPV devices started from the slot-die coating (Solar X3; FOM Technologies) PED-OT:PSS on top of PET foils, followed by baking at 130°C for 15 min.The cathode side of the device was completed by spin-coating 32 nm SnO 2 and annealing at 115°C for 2 min in air.For reference AL/AL laminated devices, ALs were spin-coated on both cathode and anode stacks and were annealed at 80°C for 2 min in the glovebox.PBTZTstat-BDTT and PC 61 BM were blended in a 1:1.5 (w:w) ratio in o-xylene:methylnaphthalene (85:15, v:v) at 45 g/L total concentration.PTQ10:PC 61 BM ink was prepared 1:1.5 (w:w) ratio in o-xylene:DPE (85:15, v:v) at 40 g/L total concentration. 33PM7:PC 61 BM ink was prepared 1:1.5 (w:w) ratio in o-xylene:DPE (85:15, v:v) at 45 g/L total concentration.For AL/acceptor-based interlayer laminated devices, PC 61 BM, PC 61 BM:PEI, or PC 61 BM:PS solutions were spin-coated on the cathode stack on top of SnO 2 at 2000 r/min for 30 s and annealed at 80°C for 2 min in the glovebox.ALs were coated on the anode stack.The resulting stacks were prepatterned with a scalpel to isolate three active areas per substrate.The active area is subject to subjective errors of ±0.05 cm 2 .The cathode and anode stacks were laminated in the air using a roll laminator (GSS DH-650S; Graphical Solutions Scandinavia AB) at a roll temperature of 115°C and a force of ∼50 N (measured with a force sensor FlexiForce A201; Tekscan).In the end, the laminated devices were placed between two glass slides to provide mechanical support for easier handling.Additionally, silver paint (Agar AGG302) was applied to the exposed PEDOT:PSS contacts.It is important to note that the devices were not encapsulated.
The current-density-voltage (J-V) curves were obtained by using a Keithley 2400 SourceMeter while the devices were under cool LED irradiation at a temperature ranging from 20°C to 25°C.The LED source's emission spectra and irradiance (Supporting Information: Figure S1) were measured using a high-precision fiber optics spectrometer (QE-Pro; Ocean Optics) and a Hamamatsu silicon photodiode S1133-01.Integrating the corresponding emission spectrum obtained 34 from the specific device location, the illuminance, power density, and current density were calculated as 552 lx, 148 µW/ cm 2 , and 67 µA/cm 2 , respectively.

| Photoelectron spectroscopy (ultraviolet photoelectron spectroscopy [UPS])
The substrates used for UPS and NR were cleaned via sonication in detergent, followed by sequential washing in deionized water, acetone, and 2-propanol.The films (18-30 nm) of fullerene-based interlayers were spincoated and annealed in the glovebox according to the previously mentioned fabrication methods.The samples were transferred to the load lock chamber of the ultrahigh vacuum system.The UPS experiments were performed in a home-designed spectrometer.The excitation source was monochromatic He I radiation with a photon energy of 21.22 eV.The W F was derived from the secondary electron cutoff and the vertical ionization potential from the frontier edge of the occupied density of states with an error margin of 0.05 eV.All the measurements were executed with a base pressure lower than 1 × 10 −9 mbar.

| External quantum efficiency (EQE)
The Solar Cell Spectral Response Measurement System QE-R3011 from Enli Technology Co., Ltd. was employed for EQE.To obtain the EQE EL (external quantum efficiency-electroluminescence) values, an in-house built system was utilized.This system consisted of a Hamamatsu silicon photodiode 1010B, a Keithley 2400 SourceMeter for voltage supply and recording injected currents, and a Keithley 485 picoammeter to measure the intensity of emitted light.

| Fourier transform photocurrent spectroscopy-EQE (FTPS-EQE)
For FTPS-EQE measurements, a Bruker Optics Vertex 70 spectrometer was employed.The spectrometer was equipped with a quartz tungsten halogen lamp, a quartz beam splitter, and an external detector option.To amplify the photocurrent produced when the photovoltaic devices were illuminated with modulated light from a Fourier transform infrared spectroscopy (FTIR), a lownoise current amplifier (SR570) was used.The output voltage from the current amplifier was then fed back into the external detector port of the FTIR, enabling the FTIR's software to collect the photocurrent spectrum.

| NR
The NR experiments were carried out using the reflectometer MORPHEUS located at SINQ (Paul Scherer Institut).
During NR experiments, a neutron beam was directed at the sample surface under a small angle (θ), reflected at each composition interface, and detected.The samples were mounted vertically, and the reflected beam was detected using a He-3 detector.The intensity of the reflected beam was measured as a function of the scattering vector normal to the surface Q z = 4π(sin θ)/λ, with λ being the neutron wavelength.The wavelength used in the experiments was 4.8 Å.Samples were measured from 0°to 3°in 2θ.

| Atomic force microscopy (AFM)
AFM measurements were performed with a dimension 3100 system using antimony-doped silicon cantilevers in tapping mode NR.

| RESULTS AND DISCUSSION
Our study was conducted on polymer:fullerene BHJ AL PBTZT-stat-BDTT:PC 61 BM-(Supporting Information: Figure S2) based OPV devices, characterized under typical indoor illumination.The devices were fabricated via the lamination method on flexible PET substrates.Cathode and anode stacks were processed independently, and placed in between two counter-rotating rolls to join them using lamination under certain pressure and temperature, shown in Figure 2A (see Supporting Information for further details).The anode side of the stack includes a slot-die-coated highly conductive PED-OT:PSS layer, which works as the electrode and holetransport layer, and a spin-coated BHJ AL.The cathode stack consists of a slot-die-coated PEDOT:PSS layer as the electrode, a spin-coated SnO 2 followed by a spin-coated BHJ AL (Figure 2C).These laminated semitransparent devices show a fill factor (FF) of 0.75, short-circuit current density J SC of 33 µA/cm 2 , and V OC of 0.62 V under 552 lx illumination conditions (equivalent to an irradiance of 148 µW/cm 2 , see details in Supporting Information: Figure S1).
To approach what could be considered a naive "ideal" morphology model (Figure 1A), we now want to add a pristine electron-accepting PC 61 BM layer between the PBTZT-stat-BDTT:PC 61 BM AL and the SnO 2 interlayer to investigate whether the pure phase of the acceptor layer deposited on the favorable electrode would lead to optimal performance.The above-mentioned lamination technique provides the possibility to study the interface of the solution-processed BHJ AL and cathodic fullerene layer, without the risk of dissolving the underlying layer.Since the BHJ AL has a favorable pristine acceptor phase as an interlayer close to it, one would assume that in ideal scenario energy levels are well aligned for achieving a high device performance (Figure 2B).
Figure 2D shows the J-V characteristics of BHJ AL/AL laminated devices compared to BHJ AL/cathodic PC 61 BM laminated devices under 552 lx light illumination.Surprisingly, the pure fullerene acceptor phase between the AL and SnO 2 to the cathode stack shifts the (J-V) characteristics strongly to lower V OC and FF, whereas J SC has increased moderately.The V OC difference between devices with BHJ AL/AL lamination and BHJ AL/cathodic PC 61 BM lamination is 120 ± 20 mV.Since the V OC loss occurs when the AL is changed to the acceptor interlayer, we evaluate the origin of the V OC loss at the cathode stack by examining alternative interlayers.By altering the first interlayer in the cathode stack from SnO 2 to ZnO as well as changing the cathodic acceptor-based interlayer PC 61 BM layer to PC 71 BM layer (Supporting Information: Figure S3A,B), we observe similar V OC losses.Therefore, we conclude that the V OC loss likely arises from the interface between the AL and PC 61 BM interlayer.

| Elimination of V OC losses by the utilization of PC 61 BM:PEI and PC 61 BM:PS cathodic acceptor-based interlayers
Studies on perovskite solar cells with fullerenes as an interlayer have shown that fullerenes can result in nonradiative recombination losses.It was shown that the critical loss process in high-performing p-i-n-type solar cells is a surface-mediated process that occurs in the first monolayer of fullerenes. 35There have been studies where PCBM interlayers were modified with insulating molecules (e.g., PEI or PS) to achieve high-quality pinhole-free films, resulting in passivation of the trap states at the perovskite surface and a reduction of nonradiative recombination losses. 31,32These studies provide a strategy to mitigate the V OC losses across the perovskite-interlayer interface.
Inspired by the developments in perovskite solar cells, we have implemented these strategies to investigate the effects of these insulating molecules mixed with PC 61 BM on the performance of the laminated OPVs.Devices with anode stacks with BHJ AL laminated to cathode stacks with PC 61 BM:PEI (9 vol% PEI) show an increased V OC of 0.62 V and FF of 0.68, and interlayer PC 61 BM:PS (11 vol% PS) display an improved V OC of 0.60 V and FF of 0.74, compared to 0.49 V V OC and 0.60 FF for pristine PC 61 BM-based interlayer (Figure 2D).Modified cathodic PC 61 BM interlayer-based devices exhibit similar V OC as BHJ AL/AL laminated devices (Table 1).These results strongly suggest that thermalization loss at the pure phase of PC 61 BM on the cathode electrode could result in the loss of V OC .By diluting the cathodic PC 61 BM with either PEI or PS, we speculate that the thermalization losses are eliminated and conditions for a better energetic alignment are created.To investigate the sensitivity of PS and PEI to PC 61 BM vol%, three stoichiometries were evaluated, all with a similar increase of V OC compared to pure PC 61 BM cathode (Supporting Information: Figures S4 and S5).
To examine the generality of the energy loss across the interface of polymer:PC 61 BM and interlayer PC 61 BM, we have investigated two more donor materials, PTQ10 and PM7 with PC 61 BM as the acceptor.The molecular structures of the materials used in the investigation are shown in Supporting Information: Figure S6A,B.We find that the previously observed voltage loss when PC 61 BM was used as an interlayer and performance enhancement with PC 61 BM interlayer is applicable to all the tested donor materials (Supporting Information: Figure S7A,B).
To gain further insights on V OC loss in these interlayers, we utilized FTPS-EQE and EQE EL and analyzed the contributions of radiative and nonradiative charge recombination (Supporting Information: Figure S8).Additionally, we determined the energy gap (E g ) by examining the derivative of the EQE edge, 36,37 which was found to be 1.81 eV despite the variations in the interlayer.The radiative loss, calculated using the formula V k T e J J = ( ) ln(( ) + 1) OC,rad B ph 0 , remains constant (1.00 eV) despite changes in the interlayer.This suggests that radiative recombination makes negligible contributions to the difference of the V OC loss.The nonradiative loss, calculated using the formula OC,nonrad B ln(EQE ) EL , yields a value of 0.41 V for the PC 61 BM interlayer, while PC 61 BM:PEI and PC 61 BM:PS interlayer-based devices show a value of 0.36 V, largely suppressing the V OC loss compared to pristine PC 61 BM interlayer-based devices.In summary, our findings indicate that V OC loss in fullerene-based interlayers is not primarily influenced by radiative recombination, but rather by nonradiative recombination losses.Introducing insulating molecules into the PC 61 BM interlayer effectively mitigates these nonradiative losses at the interfaces between the absorbing layer and cathodic acceptor interlayers.

| Interfacial energy loss dependence on energy levels of the device
To investigate why the mixture of insulating molecules with PC 61 BM can improve the device V OC compared to the pristine PC 61 BM, UPS measurement was performed on fullerene-based interface layers (Supporting Information: Figure S9).It was found that the W f of the pristine PC 61 BM varies from 4.40 to 4.45 eV depending on the processing solvent (Figure 3A).The energy diagram of the devices with and without modification of PC 61 BM interlayer is illustrated in Figure 3B.The addition of PEI into PC 61 BM lowers the W f to 4.33 eV, while PC 61 BM:PS also shows a reduced W f of 4.27 eV.One would assume that the energy level alignment was intuitively well matched from the pristine PC 61 BM interface layer to polymer:PC 61 BM-based AL.However, we observed a significant V OC loss as shown in Figure 2D.The reduced W f due to the addition of either PEI or PS to PC 61 BM resulted in an improved V OC compared to pristine PC 61 BM-based devices.For PC 61 BM:PEI interlayer compared to pristine PC 61 BM, we see a V OC difference of 0.13 V and a W f difference of 0.12 eV, while for PC 61 BM:PS interlayer compared to pristine PC 61 BM, the difference in V OC is 0.11 V and in W f is 0.13 eV, resulting in a change in W f and V OC of similar magnitudes.This indicates that reduced W f leads to a better energetic alignment across the AL and acceptorbased interlayer, increasing the device V OC . 38,39Possible mechanisms for the reduced W f of the insulating molecule incorporated acceptor interlayers are discussed in the following section.

| Interfacial energy loss dependence on the vertical distribution of the acceptor-based interlayers
Since the addition of PEI or PS in PC 61 BM film helps to reduce the V OC loss, we are motivated to understand the mechanism behind the device V OC increase and W f reduction.Therefore, we move forward to investigate the interaction between PC 61 BM and PEI or PC 61 BM and PS, especially considering that microstructure-related aspects like vertical phase separation could be very important.In the field of OPVs, NR has been employed as a technique for exploring the vertical stratification of BHJ ALs. 40,41In this study, we have employed NR as a means of investigating the influence of interlayer's vertical arrangement on the performance of OPV devices.Upon the addition of PEI to PC 61 BM, the surface of the film (air interface) is significantly enriched with a layer of ~5 nm PEI with an SLD of 2 × 10 −6 Å −2 , depicting strong surface segregation of PEI from PC 61 BM (Figure 4B).This is different from pristine PC 61 BM (Figure 4A), which is composed of one layer with a thickness of 17 nm.We find that PC 61 BM on top of from the surface energy and chemical structure difference.With the NR results on the vertical phase separation, we can now rationalize why the PEI addition enhances the V OC .The reduced V OC observed in pure acceptor interlayer devices we tentatively ascribe to the ambipolar nature of PC 61 BM, 42,43 which facilitates the transfer of both electrons and holes.This characteristic increases the likelihood of hole (minority carrier) back-transfer from the BHJ polymer HOMO to the HOMO of the acceptor interlayer, leading to recombination losses in the pure cathodic PC 61 BM phase.Conversely, the presence of self-arranged PEI between the AL and acceptor interlayer potentially eliminates hole transfer from the AL blend to the pure acceptor interlayer, resulting in an increased V OC and decreased W f .The increased PC 61 BM content has a strong influence on the device V OC and FF , which could be translated to longer extraction times and more likelihood of recombination events (Supporting Information: Figure S12).
In the case of PS-added PC 61 BM, the vertical phase separation is different from PEI-added PC 61 BM, although we also observed W f reduction from the UPS measurements.In this case, a gradient film is observed on top of the SnO 2 layer.To quantify the vertical distribution of the components, one can the volume fraction of PC 61 BM and PS.A mass conversion equation was used to calculate ϕ PCBM and ϕ PS : PS where ϕ ϕ = 1 − PS PCBM is the volume fraction of PS.The PC 61 BM:PS gradient film exhibits three regions with PC 61 BM ratios of 53 vol%, 68 vol%, and 40 vol% at the SnO 2 interface, in the middle part, and at the air interface (Figure 4D).Overall, PC 61 BM:PS mixture at the bottom, PC 61 BM enrichment in the middle, and PS enrichment at the top surface area interpreted by the NR fitting.Therefore, the PS addition has a different mechanism for improving the device V OC compared to the self-assembled layer of PC 61 BM:PEI as stated above.The mixture between PS and PC 61 BM enhances the structural ordering PC 61 BM, resulting in a narrower width of the LUMO, and thereby decreasing the thermalization losses.This reduction in the thermalization losses leads to a decrease in the negative pinning energy and a subsequent decrease in the W f at the contact.
Based on the observed changes in vertical arrangement, W f , and pinning energy, it is speculated that the electron affinity (EA) of PC 61 BM interlayers also undergoes modifications when using insulating molecules.This phenomenon can be attributed to two potential reasons.First, the introduction of a PS "wetting layer" at the interface between PC 61 BM and the cathode may result in a decoupling effect. 10This implies that the screening of electrons in the PC 61 BM LUMO by the cathode is reduced, making the interface EA more closely resemble the bulk EA.Consequently, thermalization losses decrease.
The second possibility is that the introduction of PS induces changes in the film morphology, thereby affecting the stacking of PC 61 BM.This alteration may lead to a sharper edge in the LUMO density of states at the interface.Consequently, the density of states does not extend as far into the energy gap, resulting in reduced thermalization losses.Both scenarios ultimately contribute to an improved energy level matching at the interface of the BHJ AL-cathodic acceptor interlayer, as discussed in our manuscript.
Our findings demonstrate the potential to significantly reduce V OC loss across the AL and cathodic interlayer interface.This paves the way for the development of materials with improved interface properties and decreased energy losses.These results open new avenues for the design and optimization of indoor OPVs for industrial applications.Further investigations and optimizations are warranted to fully explore the potential of these advancements in enhancing device performance and efficiency.

| CONCLUSIONS
In conclusion, inspired by the ideal morphology of OPV devices, we have investigated the effects of inserting pristine PC 61 BM as an interlayer between the AL and the cathode electrode enabled by the industrially viable method, lamination.We find that the addition of the pure fullerene acceptor phase strongly decreases the V OC of OPV devices.Our investigation shows that the V OC loss arises from the interface between the AL and the PC 61 BM interlayer.The V OC loss is mitigated upon the addition of insulating molecules (e.g., PEI or PS) into PC 61 BM.In spite of similar efforts in improving the V OC , UPS and NR measurements elucidate that PEI and PS have fundamentally different mechanisms for decreasing the V OC losses.Upon adding PEI to OPV devices, a selfassembled tunnel layer forms spontaneously between the AL and PC 61 BM interlayers.This layer may reduce hole transfer from the BHJ AL to the acceptor blend, thereby potentially eliminating the hole population at the acceptor interlayer's HOMO and reducing recombination losses.The change in the W f and the vertical arrangement of PC 61 BM with PS insulating matrix can lead to reduced thermalization losses, facilitating an improved energetic alignment across the interface.Our results highlight the importance of the rational design of interlayer materials for industrial development of OPVs.This work underscores the critical role of deliberate interlayer material design in driving the industrial advancement of indoor OPVs.The insights gained from this study provide valuable guidance for researchers and engineers working toward the practical implementation of efficient and commercially viable OPV devices.
Schematic cross-section of morphologies of active layers.(A) "Ideal morphology" model.(B) Bulk heterojunction/ acceptor-based interlayer investigated in the work.CT, charge transfer.
Work function values of the [6,6]-phenyl-C61-butyric acid methyl ester (PC 61 BM)-based interlayers.(B) Energy diagram of the layers used for the laminated organic photovoltaics device: work function (W f ) values of PC 61 BMPC61BM:PEI, PC 61 BM polyethyleneimine; PC61BM:PS, PC 61 BM polystyrene acceptor interlayers and SnO 2 interlayer, and highest unoccupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the active layer materials in the bulk heterojunction (BHJ).
NR was performed to determine component distribution normal to the surface, enabling us to see the influence of PEI or PS on PC 61 BM.The PC 61 BM:PEI in CF or PC 61 BM:PS in oxylene were deposited on SnO 2 /SiO x /Si substrate to ensure the corresponding cathode interface properties of AL/ cathodic PC 61 BM laminated devices.The experimentally measured reflectivity curves were fitted using GenX (details in Supporting Information: Figure S10) to obtain the neutron scattering length density (SLD) distributions.The corresponding SLD depth profiles derived from fitting are shown in Figure 4A-C.Fitted SLDs of SnO 2 , PC 61 BM from o-xylene, PEI, and PS were found to be 3 × 10 −6 Å −2 , 3.8 × 10 −6 Å −2 , 2 × 10 −6 Å −2 , and 1.2 × 10 −6 Å −2 , respectively.Note that the SLD of the interface between the Si substrate and SiO x is shown by the abrupt increase of SLD located at 0 Å in the depth profile.On top of the SiO x , the thickness of the SnO 2 layer ranged from 320 to 450 Å.Since the gradual change in SLD from one layer to the next represents the roughness of the film, the comparison of Figure 4A-C indicates that the roughness of the PC 61 BM layer at the air interface is decreased upon the addition of PEI or PS, consistent with the results obtained using AFM in Supporting Information: Figure S11.

F
SnO 2 is characterized by an SLD of 4.1 × 10 −6 Å −2 , implying that the blend has a composition of around 23 nm pure PC 61 BM.The SLD value of pure phase PC 61 BM in the PC 61 BM:PEI blend shows a different value from the SLD of the pristine PC 61 BM (Figure 4A) due to the processing from different solvents.The results indicate total segregation of PEI from PC 61 BM.We believe that the phase segregation of PEI on the top surface originates I G U R E 4 Corresponding scattering length density (SLD) depth profiles of (A) [6,6]-phenyl-C61-butyric acid methyl ester (PC 61 BM), (B) PC 61 BM polyethyleneimine (PC 61 BM:PEI), (C) PC 61 BM polystyrene (PC 61 BM:PS), and (D) PS volume fraction distribution on the SnO 2 interlayer, calculated from (C).