3.1.2. Metal-free compounds
Our group developed a dibenzo[f,h]thieno[3,4-b]quinoxaline-based dye, TQTFA, for the ternary system, TQTFA/P3HT/PC71BM [PC71BM=([6,6]-phenyl C71 butyric acid methyl ester), Figure 11].24 The matched energy levels of TQTFA, PC71BM, and P3HT allow photogenerated excitons to dissociate into electrons and holes, which are then driven toward the cathode and anode, respectively. Therefore, both the broader absorption profile and energetic cascade between components result in improved Jsc and Voc. The best performing device exhibited a power conversion efficiency of 4.50 %. The efficiency was increased by almost 15 % compared to the device without TQTFA.
Low-band-gap polymers based on 1,4-diketo-3,6-dithienylpyrrolo[3,4-c]pyrrole (DPP) have attracted considerable interest in the field of solar cells because of their useful optical properties and their mechanical and thermal stabilities.25–27 A small HOMO/LUMO energy gap makes DPP a good candidate for ternary solar cell applications. In 2008, Nguyen et al. developed a small band gap compound, SMD1, based on the DPP core (Figure 12) and incorporated it as the third component in a P3HT/PC71BM BHJ solar cell.28
Absorption spectra of SMD1, P3HT, and PC71BM in the film state are shown in Figure 13. The three components show complementary absorptions and thus are ideal for ternary solar cell applications. Increasing the SMD1 content in the P3HT/PC71BM active layer resulted in increased absorption in the 650–800 nm region. However, the efficiency of the annealed cell decreased with increasing SMD1 content. The decreased efficiency correlated with a rougher active layer surface. Cells fabricated from solutions containing SMD1 (2 mg mL−1), P3HT (10 mg mL−1), and PC71BM (10 mg mL−1) were annealed for 30 s at 130 °C before producing their maximum power conversion efficiency (Jsc=8.6 mA cm−2; Voc=0.63 V; η=3.21 %).
Figure 13. Normalized absorption spectra for thin films of PC71BM (solid), P3HT (dashed), and SMD1 (dotted). Reprinted with permission from Ref. 28. Copyright 2008, American Institute of Physics.
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Yang et al. used DMPA-DTDPP (Figure 14), a DPP-based compound, as an additive for the P3HT/PCBM BHJ solar cell.29 The device incorporating 5 wt % DMPA-DTDPP exhibited the highest efficiency at 3.37 %, representing an approximate 12 % increase relative to the reference cell without additives. This improvement may have resulted from the ternary cell’s more efficient light harvesting and reduced surface roughness characteristics compared to the P3HT/PCBM binary cell.
Sharma et al. also reported a new low-band-gap small molecule DPP-CN (Figure 15), a NIR absorber, as an additive for the P3HT/PC71BM BHJ solar cell.30 The light harvesting property and cascade energy levels resulted in an improved PCE. After optimization, the highest efficiency of 4.7 % was achieved with the device incorporating 10 wt % DPP-CN, which was higher than the reference cell without additives (η=3.23 %).
Sharma et al. reported the use of a low-band-gap material, BTD-TNP (λabs in a thin film=650 nm) as the additive in copolymer P (λabs in a thin film=534 nm)/PCBM system (Figure 16).31 The P/PCBM/BTD-TNP ternary system, with a 1:1:1 (w/w/w composite ratio, provided a Jsc of 5.8 mA cm−2, a Voc of 0.81 V, and an overall efficiency of 2.6 %. The cell efficiency (η=1.27 %) was twice that of the annealed P/PCBM copolymer (1:1, w/w) system. Furthermore, thermal annealing improved the device efficiency, indicating that the excitons dissociated more effectively into charge carriers in the presence of BTD-TNP. The incident photon-to-current efficiency (IPCE) spectrum of the device in their study shows two strong bands originating from copolymer P and BTD-TNP, indicating the contribution of both copolymer P and BTD-TNP to light harvesting. The authors concluded that BTD-TNP acts as a photosensitizer and also facilitates efficient charge separation by providing a path for copolymer P excitons to migrate toward the P:PCBM interface.
The same authors also developed the low-band-gap compound Se-SM and a new phenylenevinylene copolymer, P2, for use in ternary BHJ systems (Figure 17).32 The Se-SM film showed a broad absorption band with λmax at 640 nm. By comparison, λmax for the absorption band of copolymer P2 is roughly 426 nm. Although binary photovoltaic devices operate at low efficiencies (copolymer P2/PCBM=0.52 %, Se-SM/PCBM=1.30 %), the ternary devices showed significant improvement. For example, after thermal annealing, the overall efficiency of P2/Se-SM/PCBM (1:0.5:1 w/w/w) was approximately 3.16 %. Again, this performance enhancement arises from increased light harvesting and more effective charge separation.
SM is a sensitizer comprising a thienothiadiazole core and two peripheral cyanovinylene 4-nitrophenyl units (Figure 18); it was used as the third component in a P3HT/PCBM cell.33 Incorporation of SM resulted in increases in Jsc and Voc, and thus the overall efficiency increased to approximately 3.69 % compared to the efficiency of the pristine cell (2.9 %). Thermal annealing provided further increases in efficiency, to 4.1 %. These gains were attributed to a broader absorption spectrum, which produced increased Jsc values, and to greater resolution of energy levels, which yielded the increased Voc values (Figure 18).
There have been attempts to develop additives with wider HOMO/LUMO band gaps. The Coumarin 6 dye (Figure 19) possesses a larger band gap than P3HT; it was used by Ismail et al. in 2010 as the third component in a P3HT/PCBM BHJ solar cell.34 The dye exhibits good light-harvesting properties at wavelengths comparable to those of P3HT and an energy cascade that is compatible with P3HT and PCBM for both LUMO and HOMO energy levels.34 However, both the photocurrent and cell efficiency decrease with increasing Coumarin 6 content. The authors attributed this outcome to the inability of Coumarin 6 to convey excitons and charge carriers through the ternary composite.
Kwon et al. reported the use of HMBI (Figure 20), comprised of electron-donating carbazole and electron-accepting diketone moieties, as the additive for solution-processed BHJ organic solar cells.35 P3HT has a broad absorption band between 400 and 550 nm, whereas HMBI absorbs at shorter wavelengths, between 350 and 400 nm (Figure 20). The best performing cell consisted of 3 wt % HMBI dopant and produced the best efficiency of 2.9 %, surpassing that of the pristine P3HT/PCBM device at 2.6 %. In addition to broadening the OPV device’s absorption spectrum, energy transfer from HMBI to P3HT provided more effective charge separation at the P3HT/PCBM interface. An optimized P3HT/HMBI ratio is important for charge transport. At high HMBI concentrations, a large number of dispersed HMBI molecules hinder charge transport, resulting in deteriorated cell performance, as evidenced from AFM images.35
Figure 20. Structure of HMBI and UV/Vis absorption spectra of P3HT, PCBM, and HMBI (top) and the associated energy level diagram (bottom). Reprinted with permission from Ref. 35. Copyright 2011, Elsevier.
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3.2. Polymer/polymer/PCBM-based ternary solar cells
Compared to the additives based on small molecules, polymeric types generally have broader absorption profiles and better atmospheric stability.36 Hayashi et al. investigated the effect of the MEH-PPV on the performance of the P3HT/PCBM BHJ solar cell.37 Introducing MEH-PPV into the active layer yielded an increase in Voc from 0.38 to 0.50 V. This might have resulted from a reduction in the HOMO energy level after introduction of the MEH-PPV polymer. The best power conversion efficiency under 1.5 AM simulated solar illumination (100 mW cm−2) was 1.12 %, slightly better than that of the pristine cell (0.85 %).
Tai et al. synthesized a new ambipolar polymer poly[2,3-bis(thiophen-2-yl)-acrylonitrile-9,90-dioctyl-fluorene] (FLC8, shown in Figure 21), with HOMO (−5.68 eV) and LUMO (−3.55 eV) energy levels lying between those of PCBM and P3HT, respectively.38 The ambipolar polymer facilitated polaron transport with comparable hole and electron mobility (approx. 10−4 cm2 V−1 s−1). After preparation of the ternary P3HT/PCBM/FLC8 composite (1:0.8:0.05 by weight), the PCE of the ternary solar cell (η=2.93 %) increased by approximately 30 % compared to the reference cell without FLC8 (η=2.26 %). The observed PCE enhancement resulted from improvements in both Jsc and FF. The increased Jsc was attributed to the more effective charge separation at the donor/acceptor (P3HT/FLC8 and FLC8/PCBM) interfaces because of a favorable energy cascade. By contrast, the improvement in FF resulted from fast charge transfer arising from FLC8s ambipolarity. Reductions in charge recombination resulted in an increased shunt resistance (Rsh).
Egbe at al. developed two poly(p-phenylene ethynylene)-alt-poly(p-phenylene vinylene-based conjugated polymers, DO-PThE1-PPV2 (D1) and MEH-PThE1-PPV2 (D2) (Figure 22) with the same conjugated backbone but with different types and volume fractions of alkoxy side chains.39 Ternary OPVs with a D1/D2/PCBM composite ratio of 0.5:0.5:3 (w/w/w) and binary OPVs with a D1/PCBM or D2/PCBM composite ratio of 1:3 (w/w) were fabricated for comparison. Voc, Jsc, and cell efficiency in the ternary system were higher than those of either binary system. AFM images of the three composite films showed that the surface roughness of the ternary system (0.5 nm) was between those of D1 (10.7 nm) and D2 (0.3 nm). The side-chain volume fraction affected the active layer’s nanomorphology, and combining D1 and D2 films could be used to optimize the phase separation to improve charge separation. Additionally, hole mobility in the D1–D2 film was 2.6×10−4 cm2 V−1 s−1, which is higher than that of D1 (1.8×10−5 cm2 V−1 s−1) and D2 (2×10−6 cm2 V−1 s−1) films, as measured by hole mobility charge extraction using a linearly increasing voltage (CELIV). CELIV results showed that the D1–D2 film formed a new intermolecular arrangement, favorable for a higher charge carrier mobility compared to the individual D1 and D2 polymers. Consequently, the most efficient ternary cell exhibited a PCE of 2.0 %.
Electron-deficient benzothiadiazole is often used for the synthesis of polymers that absorb at longer wavelengths.40, 41 Kim et al. fabricated a ternary system based on a soluble fullerenes and two conjugated polymers, P3HT and the benzothiadiazole compound F8BT (Figure 23), as P3HT/PCBM/F8BT at a ratio of 1:0.6:1.4 (w/w/w) under different thermal annealing conditions.42 Cell efficiency was optimized at an annealing temperature of 130 °C. At 1.94 %, the efficiency of the P3HT/PCBM/F8BT ternary cell with a composition ratio of 1:0.6:1.4 (w/w/w) was higher than that of the P3HT/PCBM binary system (1.38 %) with a composition ratio of 1:2. However, an optimized binary system comprising P3HT/PCBM with a 1:1 ratio operated at 3.16 % cell efficiency; it is possible that the low carrier mobility of F8BT resulted in a low cell photocurrent.
Koppe et al. used NIR-absorbing PCPDTBT (Figure 24) as an additive in the P3HT/PCBM system.43 This offered two advantages: i) PCPDTBT absorbs well in the NIR region, and ii) the LUMO (P3HT, −2.9 eV; PCPDTBT, −3.55 eV; PCBM, −4.3 eV) and HOMO cascade (P3HT, −5.1 eV; PCPDTBT, −5.3 eV; PCBM, −6.0 eV) arising from the three components allow facile charge separation between P3HT and PCBM, PCPDTBT and PCBM, and between P3HT and PCPDTBT.43 The IPCE spectrum confirmed the PCPDTBT contribution to the photocurrent. Integrative mode of time-of-flight (TOF) measurements suggested that P3HT was the major hole transporter and that PCBM was the exclusive electron transporter. Voc, Jsc, and the efficiency of the ternary cells increased with increasing PCPDTBT content, up to 20 wt %, although FF did not share this trend.
Recently, Machui et al. pointed out the possibility to obtain important insights into morphological behavior in the P3HT/PCPDTBT/PCBM system through wide-angle X-ray scattering (GiWAXS), differential scanning calorimetric measurements (DSC), and space-charge limited current (SCLC) transport analysis.44 They concluded that: i) an increase in the PCPDTBT amount will decrease the PCBM crystallinity; ii) an effect of the PCPDTBT amount can be found on the electron-only device, whereas there is no influence on the hole-only device; and iii) lower Jsc and FF were attributed to the reduced PCBM crystallinity.
Addition of a series of BT4T polymers (Figure 25) to the P3HT/PCBM blend broadened the absorption spectrum of the active layer and also improved the molecular ordering of P3HT, albeit at the expense of decreased P3HT absorption.45 By comparison, crystalline BT4T-12 in the P3HT/BT4T-12 composite showed a decrease in carrier transport at the grain boundaries as well as a reduced FF and device performance. Thus, the P3HT/BT4T/PCBM ternary system operated at 2.52 % efficiency, and exhibited better Voc, Jsc, and FF than the binary system without BT4T.
Ternary systems using two benzothiadiazole-based polymers as sensitizers were reported by separate research groups.46, 47 The structures of the dyes and their photovoltaic parameters are listed in Table 1. The complementary absorption and miscibility characteristics of PDIDTDTBT and PTDIDTTBT resulted in an improved performance of the resulting ternary cell compared to that of the binary cell using a single polymer.46 By comparison, the ternary systems formed with PCDTBT, PFDTBT, and PCBM were less efficient than the binary system using a single sensitizer. The authors attributed the degraded performance to similar absorption profiles and the incompatibility of the two dyes.47
Table 1. Chemical structure and their OPV devices performance.
|Compounds||Weight ratio copolymer/PCBM||JSC [mA cm−2]||VOC [V]||FF||η [%]|
|PTDIDTTBT|| || || || || |
|PDIDTCTBT|| || || || || |
| || || || || || |
| || || || || || |
| || || || || || |
The structure of PCPDTBT is shown in Figure 24; Hsu et al. used two new D–A copolymers, P1 and P5 (Figure 26), as additives in a PCPDTBT/PC71BM active layer.48 The PCPDTBT/P1/PC71BM ternary system, with a composite ratio of 1:1:4, exhibited a higher efficiency (2.5 %) than the 1:2 PCPDTBT/PC71BM binary system (1.4 %) or the 1:2 binary composite P1/PC71BM (2.0 %). By contrast, the 1:1:4 PCPDTBT/P5/PC71BM ternary system showed a performance (1.9 %) inferior to the 1:2 P5/PC71BM binary system (2.2 %). The superior performance of the ternary cell of P1 was credited to the better compatibility of P1 with PCPDTBT because the two dyes had the same donor moieties. Additionally, the ternary blend had a broader spectral coverage.
Sharma et al. prepared two alternating copolymers with phenylenevinylene blocks, PB and P3 (Figure 27 a), for solar cell applications.49 P, PB, and PCBM have compatible energy alignments (Figure 27 b), resulting in efficient charge separation in the device. The generated electrons travel through the PCBM layer to the cathode, and the holes migrate through both P3 and PB to the anode. The PCE of the as-cast 1:1:1 P/PB/PCBM ternary cell reached 2.56 %, which was higher than that in either the as-cast P3/PCBM (1.15 %) or the as-cast PB/PCBM cell (1.57 %). After annealing, the PCE of the ternary cell was further increased to 3.48 %. Thermal treatment triggered reorganization of P3 and PB in the composite to form more crystalline structures.
You et al. reported BHJ solar cells consisting of two donor polymers (Figure 28) and PCBM.50 The polymer pairs, TAZ and DTBT or DTffBT and DTPyT, have similar conjugated skeletons and side chains. Consequently, it is possible to mix the two components in different ratios without phase separation occurring. The ternary cells always performed better than the binary cells, although the efficiency varied with the dye ratio. For example, i) a ternary device consisting of TAZ/DTBT in a 3:7 weight ratio showed a PCE value of 5.88 %, which was better than that of either TAZ (η=4.06 %) or DTBT (η=4.39 %) binary devices in a 1:1 ratio with PCBM; and ii) the 1:1 ternary device consisting of DTffBt and DTPyT had a higher PCE (7.02 %) than the binary devices based on either DTffBt (η=6.26 %) or DTPyT (η=6.30 %) in a 1:1 ratio with PCBM. Neither the energy transfer nor the charge transfer between different donor materials played an important role in the cell performance. Instead, the authors proposed that the excitons generated in an individual donor polymer migrate to the respective polymer/PCBM interface, where they dissociate into free electrons and holes. The electrons are then transported through PCBM to the cathode. Ideally, the EQE spectra are approximately the sum of the individual “sub-cells.” In their study, You et al. used polymers with complementary absorption characteristics (Figure 28) to investigate the weight ratio effect on OPV device performance.
Thompson et al. also investigated the ternary solar cells containing two P3HT analogues as donor polymers, that is, high-band-gap poly(3-hexylthiophene-co-3-(2-ethylhexyl)thiophene) (P3HT75-co-EHT25) and low-band-gap poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-DPP-10 %), with PCBM as an acceptor.51 They varied the ratio of the three components and found that Voc increased as the amount of P3HT75-co-EHT25 increased. The overall polymer/fullerene ratio of the ternary-blend BHJ solar cells was individually optimized at each polymer/polymer ratio. As a result, the ternary-blend BHJ solar cells showed power conversion eﬃciencies up to 5.51 %, exceeding those of the corresponding binary blends (3.16 and 5.07 %). This concept may be applied to multiple donor systems. The authors indicated that a ternary system that used one donor and two acceptors (vide infra) could be also an example cell.52
3.3. Small molecule/small molecule/PCBM-based ternary solar cells
The bisazo-based small-molecule dyes, D1 and D2 (Figure 29), with long wavelength absorptions (λmax=608 nm and 580 nm, respectively) developed by Sharma et al. were used as donors in OPV devices.53 After thermal annealing, the efficiency of the binary cells based on D1/PCBM (1:1, w/w) and D2/PCBM (1:1, w/w) reached 2.62 and 2.10 %, respectively. The improved cell performance of D1/PCBM was attributed to a higher hole mobility and a smaller band gap. The 1:1:1 D1/D2/PCBM ternary cell was fabricated to take advantage of the complementary absorption bands of the two dyes and the favorable energy levels among D1, D2, and PCBM. Device efficiency reached 3.61 % after thermal annealing. Photoluminescence (PL)-quenching studies suggest that the photoinduced charge-transfer effect is faster in ternary systems than it is in binary systems.
Roncali et al. developed the BODIPY compounds 1 and 2 shown in Figure 30 with complementary absorption bands for the use in ternary OPV devices.54 The IPCE spectra of the ternary cell based on 1/2/PCBM in a 1:1:2 ratio showed the contribution of both dyes to the photocurrent (Figure 31). The device had the following performance parameters: Jsc=4.70 mA cm−2, Voc=0.866 V, FF=0.42, and η=1.70 %. Both Jsc and efficiency were improved in comparison to those of binary cells based on 1/PCBM (Jsc=4.43 mA cm−2, η=1.17 %) or 2/PCBM (Jsc=4.14 mA cm−2, η=1.34 %).55
Figure 31. IPCE spectra of the BHJ cell based on 1/2/PCBM in a 1:1:2 weight ratio. Reprinted with permission from Ref. 54. Copyright 2009, The Royal Society of Chemistry.
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3.4. Polymer/PCBM/fullerene derivative-based ternary solar cells
In addition to using two donors and one acceptor (normally PCBM or PC71BM) as the active layer to broaden the device’s absorption spectrum and increase the photocurrent, ternary devices using one donor and two acceptors as the active layer have been assembled to increase Jsc and Voc. Chen et al. reported the fabrication of ternary systems based on P3HT, PCBM, and a surfactant based on C60-derivatives tethered with a thiophene-containing segment (PCBTE, PCBBTE, or PCBTTE; Figure 32).56 The PCBTTE compound was the most effective additive among these because of its compatibility with P3HT, as determined from surface energy analysis and DSC. A ternary cell based on a 1:0.8 mix of P3HT/PCBM blended with 5 % PCBTTE operated at 4.37 % efficiency. This was higher than the efficiency (3.85 %) of the pristine binary system. The added PCBTTE was found to improve the ordering of P3HT polymer chains, which proved beneficial for light harvesting and charge transport. Moreover, the introduction of the PCBTTE additive reduced the aggregation of PCBM that occurred during the annealing process, which resulted in an improved thermal stability of the cell, as evidenced by the TEM image (Figure 32).56
Thompson et al. reported a ternary system using P3HT as the donor and two functional fullerenes, PCBM and indene-C60 bis-adduct (ICBA), as the acceptors. The chemical structures and energy levels of PCBM and ICBA are shown in Figure 33.52 Photovoltaic properties were investigated for various PCBM and ICBA compositions. The results indicate that Voc for the ternary BHJ solar cells could be tuned between the limiting Voc values of the corresponding binary solar cells without significant perturbation of Jsc or FF. Although the efficiency of the ternary systems did not surpass that of the binary system, this study demonstrated that the photovoltaic parameters of ternary blends are not necessarily constrained by the same factors or constrained by the same limitations as those in binary blends.
Chan et al. developed a C60-containing block copolymer (C60-BCP) as the additive in P3HT/C60 solar cells (Figure 34).57 The ternary system based on P3HT/C60 with a 1:0.5 weight ratio and 20 wt % C60-BCP exhibited a 2.56 % cell efficiency, a fivefold increase over that of the 1:0.5 ratio P3HT/C60 (0.48 %). Additionally, the C60-BCP facilitated the formation of a self-organized nanostructured P3HT domain and reduced interfacial tensions between P3HT and C60, thus increasing the compatibility within the active layer. Because of the poor solubility of C60 in common organic solvents, polymer/C60 blends generally require difficult post-treatments. Use of C60-BCP as an additive to encourage self-organization and active-layer compatibility greatly increases the utility and cost-effectiveness of C60 and has the potential for application in other ternary systems.