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The finale of a trilogy: Comparing terpolymers and ternary blends with structurally similar backbones for use in organic bulk heterojunction solar cells
Abstract
Building on our previous works that compared the efficacy of terpolymers vs. ternary blends in improving the performance of bulk heterojunction organic solar cells, the final piece of this series of studies focuses on comparing terpolymer and ternary blends constructed with two polymers with structurally similar backbones (monoCNTAZ and FTAZ) yet markedly different open circuit voltage (Voc) values. Terpolymers and ternary blends of five different ratios were studied and the results demonstrate that while the overall performance of both the systems is similar, the ternary blends exhibit higher short circuit current (Jsc) values, while the terpolymers exhibit higher Voc values. Investigation of the charge transfer state using low-energy external quantum efficiency (EQE) indicates that the ternary blends are governed by a parallel-like mechanism, while the terpolymer does not follow this mechanism. The key morphological difference between the systems, as elucidated by resonance soft X-ray scattering (RSoXS), is the slightly smaller size (∼60 nm) of domains in the ternary blends compared to that of the terpolymer (∼80 nm), which may affect exciton harvesting in the terpolymer system and lead to lower Jsc values. In addition, a lower driving force for the formation of charge transfer (CT) state is also likely to contribute to the lower Jsc values in the terpolymer system. All together, the data show that structurally similar (perhaps even miscible) polymers still exhibit key differences in performance when paired in terpolymers vs. ternary blends and allow us to further illuminate the underlying mechanisms of such complex systems.
... Compared to the widely studied fluorine substitution, the cyano functional group is a stronger electron-withdrawing functional group due to the inductive and resonance effects; thus, adding the cyano substituent to conjugated polymer backbones can decrease the highest occupied molecular orbital (HOMO) energy level of the polymer by a larger degree and shift the absorption of the polymers to a longer wavelength, which benefits the V OC and J SC of the solar cells, respectively. 17,18 Currently, cyano units are widely used in nonfullerene acceptors end groups; however, cyano-substituted polymers with high performance have not been widely reported. 19−23 In our previous study, Li et al. developed a benzotriazole (TAZ)-acceptor-moiety-based polymer with two cyano units on the TAZ unit (CNTAZ, renamed diCNTAZ herein for clarity) 21 and achieved an efficiency of 6.0%. ...
... In order to get a comprehensive understanding of the effect of the cyano substitution on polymers, in this study, we synthesized a series of polymers with varying amounts of cyano groups on the TAZ unit: HTAZ (0), monoCNTAZ 17 (1), and diCNTAZ (2). Along with understanding the optical, electrochemical, and photovoltaic characteristics, we also explored changes in morphology, charge transfer state energy, charge carrier density and lifetime, and nongeminate recombination rate as the number of cyano substituents changes. ...
The design of polymer structures has played a vital role in improving the efficiency of organic solar cells (OSCs). A common approach to increase solar cell efficiency is to add specific substituent to the polymer backbone; yet the cyano substituent, a strong electron-withdrawing group, has not received much attention as such substituents with conjugated polymers. In this study, we systematically explore the impact of cyano-substitution on conjugated polymers by varying the amount of cyano groups, from zero to two per repeat unit, on the benzotriazole acceptor moiety of our polymers. We find that cyano substitution effectively decreases the energy levels of the polymer, leading to high VOC values, however, the impact of additional cyano groups offer diminishing returns. Moreover, cyano substitution also decreases the band gap of the polymers, inducing a shift of polymer absorption to longer wavelength. Along with the optical and electrochemical differences, the cyano-effect was also thoroughly characterized by changes in morphology, charge transfer state energy, charge carrier density, lifetime, and non-geminate recombination rate. The champion polymer (efficiency of 8.6 %) has a single cyano substituent, monoCNTAZ, which affords high VOC, JSC, and FF due to deep energy level, red-shifted absorption and efficient charge transport properties respectively. However, further additions of cyano groups degrade the performance. Overall, this work highlights the benefits and limitations of cyano functionalization on OSC polymers.
... Several other TOSCs have since been attributed to the parallel-like model [70][71][72][73][74][75][76][77] , supported by investigations of morphology and electronic behaviour. However, the examples of TOSCs assigned to a parallel-like model are still a considerable minority, and therefore the generality and validity of the model are difficult to judge. ...
In ternary organic solar cells (TOSCs), three different components are mixed to form the photoactive layer, opening up opportunities to boost the power conversion efficiency-for example, by broadening the absorption range, improving the blend morphology or tuning the exciton splitting and charge extraction. Because of these possibilities, ternary systems are among the best performing OSCs and will have a crucial role in the future of organic photovoltaics. Owing to the interplay of three different components, the mechanisms in TOSCs are complex. Multiple models for those mechanisms currently exist, which differ mainly in the description of the composition dependence of the open-circuit voltage. However, these models are not defined precisely, they are based on narrow presuppositions and they frequently contradict each other. Moreover, although the state of knowledge has evolved since the development of models, new TOSCs are still assigned to them. This Review describes the existing models and concepts, highlights their inconsistencies and summarizes newer results on electronic and morphological properties of TOSCs. Subsequently, the conventional models are revisited in the light of these new insights, with the aim of pointing out existing gaps and providing the stimulus for challenging old concepts. Sections
... Several other TOSCs have since been attributed to the parallel-like model [70][71][72][73][74][75][76][77] , supported by investigations of morphology and electronic behaviour. However, the examples of TOSCs assigned to a parallel-like model are still a considerable minority, and therefore the generality and validity of the model are difficult to judge. ...
... In a recent study, random terpolymers based on fluorinated benzotriazole, cyano benzotriazole and BDT monomers were reported and properties were compared with a ternary blend of two alternating polymers and PC 71 BM (Figure 8; Kelly et al., 2018). The random terpolymers are prepared from CNTAZ and FTAZ in 9:1 P81, 7:3 P82, 1:1 P83, 3:7 P84, and 1:9 P85 feed ratios, respectively. ...
The synthesis of donor (D)-acceptor (A) polymers using structurally elaborated monomers is an active research field. Some of the challenges with the use of alternating D-A polymers for photovoltaic applications are the relatively narrow absorption widths, the presence an absorption valleys in the visible region, unoptimized molecular energy levels and even lack of compatibility of the polymers with the common acceptors. The synthesis and characterization of polymers consisting of multiple chromophores (random and regular terpolymers) with complementing properties is currently gaining momentum in order to delicately optimize properties of polymers. A random terpolymer can either be of a system composed of one donor and two acceptors [(D-A1)-ran-(D-A2)] or a one acceptor and two donor segments [(D1-A)-ran-(D2-A)] incorporated in the polymer backbone. By varying the composition of the monomers in the feed of the polymerization reaction, the properties of the resulting terpolymers can be carefully optimized. Using this strategy, many materials with desired properties have been developed and power conversion efficiency (PCE) surpassing 14% in a single layer bulk heterojunction (BHJ) solar cell device have been reported. This review summarizes the most recent advances made in the development of electron donor terpolymers for organic photovoltaics (OPVs). The properties of the terpolymers are compared with their respective reference polymer.
... The evolution of the V OC , following the increasing concentration of D2 or A2, indicates that the ternary system is likely working as a series connection of OSCs. 40,41 The highest PCE is achieved with ratio 1 : 1 : 0.2 of the D1 : A1 and third compound (D2 or A2) in ternary OSCs. All OSCs were found to retain their PCE after 300 hours being stored in the dark and in the dry air of Calgary, Alberta. ...
This report demonstrates that the open circuit voltage (V OC) and power conversion efficiency (PCE) of polymer:fullerene (PBDB-T:PC 60 BM) organic solar cells can be simultaneously enhanced by introducing a molecular p-conjugated donor or acceptor as a third component, thus affording high performance ternary blend devices. The use of a PDI-based acceptor ''tPDI 2 N-hex'' or benzothioxanthene-based donor ''tBTI-IDT'' as a third component leads to V OC increases of 60 to 150 mV without significant loss in power conversion efficiency (PCE). The best solar cell PCE of 8.8% was achieved by doping the PBDB-T:PC 60 BM system with 10% by weight of tPDI 2 N-hex.
Terpolymer fabrication is an effective methodology for molecular engineering and generating high-performance organic photovoltaic materials to construct highly efficient polymer solar cells. Modification of the polymer PM6 by incorporating a third component results in the formation of a ternary copolymer is reported to outperform PM6 in achieving enhanced device performances. However, one of the major challenges in constructing high-performance terpolymers is to counter the molecular disorder caused by the backbone entropy induced by the third moiety. In this work, double B←N bridged bipyridine (BNBP) was used as the third component, which possessed strong out-of-plane electrostatic dipole owing to the saddle-shaped B←N fused ring structure. The out-of-plane dipole moment introduced in the modified PM6 terpolymer could be used as a means for tuning and optimizing the nanostructures of the blended films. The prepared PM6-BNBP-4 blend polymer with 4% of the benzodithiophene dione monomers replaced by BNBP resulted in excellent power conversion efficiency of 19.13%. This work demonstrated that the out-of-plane electrostatic dipole moment in saddle-shaped molecules is valuable for achieving high-performance organic photovoltaic donor materials. This article is protected by copyright. All rights reserved.
A new terpolymer acceptor is presented, comprising various ratios of the same dithienothienopyrrolobenzothiadiazole (BTP) core with different side chains—alkoxy side chains (BTPO‐IC) and alkyl side chains (BTP‐IC)—and thiophene units, for use in all‐polymer organic photovoltaics. Devices incorporating binary blends of this terpolymer and the polymer PM6 as the active layer displayed open‐circuit voltages (VOC) that increase linearly upon increasing the molar ratio of BTPO‐IC. For example, the optimized device incorporating PM6:PY‐0.2OBO (i.e., with 20 mol% of BTPO‐IC) (1:1.2 wt.%) blend, with the smallest domain sizes but largest coherence length and combined face‐on and edge‐on orientation fractions among all blends, have a champion power conversion efficiency (PCE) of 16.7% (VOC = 0.97 V; JSC = 25.2 mA cm⁻²; FF = 0.68), whereas the device containing a similar blend ratio of the PM6:PY‐OD:PY‐OBO ternary blend (1:0.96:0.24 wt.%) displayed a PCE of 8.6% (VOC = 0.969 V; JSC = 18.7 mA cm⁻²; FF = 0.48). The device with PM6:PY‐0.2OBO displays better thermal stability than the devices with PM6: PY‐OD or PY‐OBO. Thus, employing terpolymer acceptors with differently functionalized side‐chain units can be an effective approach for simultaneously optimizing the aggregation domain and enhancing the PCEs and thermal stabilities of all‐polymer devices.
Inverted small molecule organic photovoltaics (SMPVs) were fabricated with BTR as a donor, the narrow band gap material Y11 as an acceptor and NC70BA fullerene derivatives with a broad band gap as the third component materials. When 20 wt% Y11 was replaced by NC70BA, a higher PCE of 13.92% was achieved, which is attributed to simultaneous enhancement of the JSC of 22.16 mA cm⁻², the VOC of 0.905 V and the FF of 69.4%. The hollow spherical structure of NC70BA may lead to the connection of Y11 and BTR molecules to form more continuous electron transport channels. Shallow lowest unoccupied molecular orbital (LUMO) energy levels form a cascade of LUMO energy levels among the used materials. In addition, the molecular arrangement and crystallinity in the photoactive layer can be markedly adjusted by incorporating NC70BA into BTR:Y11 binary films. The well-optimized phase separation and molecular arrangement in ternary photoactive layers can well support an enhanced JSC and FF for ternary SMPVs. Thus, the photovoltaic performance of SMPVs can be well improved by employing a ternary strategy with NC70BA as the third component materials.
Alkylthienyl decorated two-dimensional (2D) conjugated electron-donating (D) unit such as 4,8-bis(2-alkylthienyl)benzo[1,2-b:4,5-b’]dithiophene has been widely used for designing highly efficient photovoltaic materials. However, the thienyl side chains are less involved in the backbone Frontier molecular orbitals (FMOs). In this paper, we replace the side-chains from tri(n-propyl)silylthienyl to tri(n-propyl)silylethynyl and report two alkynyl-decorated 2D conjugated narrow-bandgap (~1.4 eV) nonfullerene acceptors, named ISI-4F and ISI-4Cl. In a striking contrast to the thienyl-linked counterpart, the ethylnyl is significantly involved in the backbone FMOs of the new acceptor molecules, enhancing the absorptivity and reducing the energy levels. With PM6 as the donor polymer, 12.5% and 12.1% efficiencies are obtained. The VOC are as high as 0.87 – 0.88 V. The tetrachlorinated acceptor supplies a larger fill factor (67.9% vs. 62.2%) while a smaller short-circuit current-density (22.8 vs. 20.5 mA/cm2) than the tetrafluorinated counterpart does. When introduced as acceptor guests of PM6:Y6, 16.1% and 15.9% efficiencies are achieved. These results demonstrate the trialkylsilylalkynyl can be potential side-chain candidates for designing highly efficient acceptor materials.
With the recent remarkable advances in the efficiency of organic solar cells, the need to distill key structure–property relationships for semiconducting materials cannot be understated. The fundamental design criteria based on these structure–property relationships will help realize low-cost, scalable, and high-efficiency materials. In this study, we systematically explore the impact of a variety of functional groups, including nitrogen heteroatoms, fluorine substituents, and cyano groups, on benzotriazole (TAZ)-based acceptor moieties that are incorporated into the conjugated polymers. Specifically, a pyridine heterocycle was used to replace the benzene unit of TAZ, leading to the PyTAZ polymer, and a cyano substituent was added to the benzene of the TAZ unit, resulting in the CNTAZ polymer. The PyTAZ polymer suffers from low mobility and poor exciton harvesting, driven by large and excessively pure domains when blended with PCBM. The inclusion of fluorine substituents, placed strategically along the polymer backbone, can mitigate these issues, as shown with 4FT–PyTAZ. However, when this same approach is used for the cyano-functionalized polymer (CNTAZ), the resulting polymer (4FT–CNTAZ) is overfunctionalized and suffers from impure domains and recombination issues. The cyano group has a larger impact on the TAZ core compared to the nitrogen heteroatom due to the strong electron-withdrawing strength of the cyano group. Because of this, further functionalization of the cyano-based polymers has less fruitful impact on the polymer properties and results in deterioration of the solar cell efficiency. Overall, this work highlights some of the benefits, thresholds, and limitations for functionalization of conjugated polymers for organic solar cells.
Current research indicates that exciton dissociation into free charge carriers can be achieved in material combinations with the highest occupied molecular orbital (HOMO) offset lowered to 0 eV in non‐fullerene organic solar cells. However, the quantitative relationship between the HOMO offset and exciton dissociation has not been established because of the difficulty in achieving continuously tunable HOMO offsets. Here, the binary blends of PTQ10:ZITI‐S and PTQ10:ZITI‐N are combined to form the positive and negative HOMO offsets of 0.20 and −0.07 eV, respectively. While the PTQ10:ZITI‐S binary blend delivers a decent power conversion efficiency (PCE) of 10.69% with a short‐circuit current (Jsc) of 16.94 mA cm−2, the PTQ10:ZITI‐N with the negative offset shows a much lower PCE of 7.06% mainly because of the low Jsc of 12.03 mA cm−2. Because the tunable HOMO levels can be realized in organic semiconducting alloys based on ZITI‐N and ZITI‐S acceptors, the transformation of the HOMO energy offset from negative to positive values is achieved in the PTQ10:ZITIN:ZITI‐S ternary blends, delivering much‐improved PCEs up to 13.26% with a significant, 74% enhancement of Jsc to 20.93 mA cm−2. With detailed investigations, the study reveals that the minimum HOMO offset of ≈40 meV is required to achieve the most‐efficient exciton dissociation and photovoltaic performance. Exciton dissociation into free charges is the most important optoelectronic process in organic solar cells, which is driven by the energy‐level offset of electron‐donor and ‐acceptor materials. With the continuously tunable HOMO level achieved by organic semiconducting alloys, the minimum HOMO offset of ≈40 meV is shown to be necessary to achieve the most efficient exciton dissociation and photovoltaic performance.
While a large number of terpolymers have been developed for polymer solar cells (PSCs), very few studies have directly focused on the rational selection of the third component to balance the miscibility and crystallinity for forming the de-sired morphology, and universal terpolymer strategies for preparing different donor/acceptor systems are lacking. Herein, we employ a new strategy involving the integration of benzodithiophene (BDT) and thieno[2,3-f]benzofuran (TBF) seg-ments to construct a series of terpolymer donors, and a profound influence on the crystallinity and miscibility of the blend films as well as on the ultimate device performance is observed. Incorporating highly crystalline TBF segments into a low crystalline BDT-based alternating copolymer can not only increase the order of the microstructure, conserve favorable face-on orientation and promote the formation of proper phase-separation features but also generate high exciton dissociation and suppress charge recombination. This strategy was successfully applied in the reported J52 system and provided a remarkable two-fold boost in performance. Finally, competitive PCEs of 11.9%, 12.4% and 12.2% accompanied by high FFs of 73%, 71% and 76% were recorded for TBFCl50-FTAZ:ITIC-, TBFCl50-BDD:ITIC- and TBFCl50-BDD:IDIC-C4Ph-based devices, respectively, via the above terpolymer strategy. Thus, our discovery provides a promising and innovative method for finely controlling the microstructure of heterojunctions for designing high-performance terpolymers.
The development of conjugated alternating donor–acceptor (D–A) copolymers with various electron‐rich and electron‐deficient units in polymer backbones has boosted the power conversion efficiency (PCE) over 17% for polymer solar cells (PSCs) over the past two decades. However, further enhancements in PCEs for PSCs are still imperative to compensate their imperfect stability for fulfilling practical applications. Meanwhile development of these alternating D–A copolymers is highly demanding in creative design and syntheses of novel D and/or A monomers. In this regard, when being possible to adopt an existing monomer unit as a third component from its libraries, either a D′ unit or an A′ moiety, to the parent D–A type polymer backbones to afford conjugated D–A terpolymers, it will give a facile and cost‐effective method to improve their light absorption and tune energy levels and also interchain packing synergistically. Moreover, the rationally controlled stoichiometry for these components in such terpolymers also provides access for further fine‐tuning these factors, thus resulting in high‐performance PSCs. Herein, based on their unique features, the recent progress of conjugated D–A terpolymers for efficient PSCs is reviewed and it is discussed how these factors influence their photovoltaic performance, for providing useful guidelines to design new terpolymers toward high‐efficiency PSCs.
Crystallizable, high-mobility conjugated polymers have been employed as secondary donor materials in ternary polymer solar cells in order to improve device efficiency by broadening their spectral response range and enhancing charge dissociation and transport. Here, contrasting effects of two crystallizable polymers, namely, PffBT4T-2OD and PDPP2TBT, in determining the efficiency improvements in PTB7-Th:PC71BM host blends are demonstrated. A notable power conversion efficiency of 11% can be obtained by introducing 10% PffBT4T-2OD (relative to PTB7-Th), while the efficiency of PDPP2TBT-incorporated ternary devices decreases dramatically despite an enhancement in hole mobility and light absorption. Blend morphology studies suggest that both PffBT4T-2OD and PDPP2TBT are well dissolved within the host PTB7-Th phase and facilitate an increased degree of phase separation between polymer and fullerene domains. While negligible charge transfer is determined in binary blends of each polymer mixture, effective energy transfer is identified from PffBT4T-2OD to PTB7-Th that contributes to an improvement in ternary blend device efficiency. In contrast, energy transfer from PTB7-Th to PDPP2TBT worsens the efficiency of the ternary device due to inefficient charge dissociation between PDPP2TBT and PC71BM.
Ternary organic solar cells (OSCs) are very attractive for further enhancing the power conversion efficiencies (PCEs) of binary ones but still with a single active layer. However, improving the PCEs is still challenging because a ternary cell with one more component is more complicated on phase separation behavior. If the two donors or two acceptors have similar chemical structures, good miscibility can be expected to reduce the try-and-error work. Herein, we report ternary devices based on two small molecule donors with the same backbone but different substituents. While the both binary devices show PCEs about 9%, the PCE of the ternary cells is enhanced to 10.17% with improved fill factor and short-circuit current values and external quantum efficiencies almost in the whole absorption wavelength region from 440 to 850 nm. The same backbone enables the donors miscible at molecular level, and the donor with a higher HOMO level plays hole relay process to facilitate the charge transportation in the ternary devices. Since side-chain engineering has been well performed to tune the active materials’ energy levels in OSCs, our results suggest that their ternary systems are promising for further improving the binary cells’ performance although their absorptions are not complementary.
Organic/polymer semiconductors provide unique possibilities and flexibility in tailoring their optoelectronic properties to match specific application demands. One of the key factors contributing to the rapid and continuous progress of organic photovoltaics (OPVs) is the control and optimization of photoactive-layer morphology. The impact of morphology on photovoltaic parameters has been widely observed. However, the highly complex and multilength-scale morphology often formed in efficient OPV devices consisting of compositionally similar components impose obstacles to conventional morphological characterizations. In contrast, due to the high compositional and orientational sensitivity, resonant soft X-ray scattering (R-SoXS), and related techniques lead to tremendous progress of characterization and comprehension regarding the complex mesoscale morphology in OPVs. R-SoXS is capable of quantifying the domain characteristics, and polarized soft X-ray scattering (P-SoXS) provides quantitative information on orientational ordering. These morphological parameters strongly correlate the fill factor (FF), open-circuit voltage (Voc), as well as short-circuit current (Jsc) in a wider range of OPV devices, including recent record-efficiency polymer:fullerene solar cells and 12%-efficiency fullerene-free OPVs. This progress report will delineate the soft X-ray scattering methodology and its future challenges to characterize and understand functional organic materials and provide a non-exhaustive overview of R-SoXS characterization and its implication to date.
The oligomer-type small molecule PDT2FBT-ID is applied in a polymer/small molecule/fullerene ternary system. The PTB7-Th/PDT2FBT-ID/PC71BM ternary active layer shows complementary absorption spectra, enhanced face-on orientation and energy transfer property. Comparing to binary system, the FF of ternary system is improved from 66.72% to 76.14% and the Jsc is improved from 17.90 mA cm−2 to 18.92 mA cm−2. The maximum 11.1% PCE is obtained in ternary system with common inverted devices structure. An alloy-like domain structure and energy transfer between the two donor materials are found coexist in the ternary system. Based on the combined model, the variation of morphology and Voc was discussed in detail and compared with previous publications. More interpretation for the selection of third compound in high performance ternary organic solar cells was provided and this facile design strategy could also be widely used in other systems.
In recent years the concept of ternary blend bulk heterojunction (BHJ) solar cells based on organic semiconductors has been widely used to achieve a better match to the solar irradiance spectrum, and power conversion efficiencies beyond 10% have been reported. However, the fill factor of organic solar cells is still limited by the competition between recombination and extraction of free charges. Here, we design advanced material composites leading to a high fill factor of 77% in ternary blends, thus demonstrating how the recombination thresholds can be overcome. Extending beyond the typical sensitization concept, we add a highly ordered polymer that, in addition to enhanced absorption, overcomes limits predicted by classical recombination models. An effective charge transfer from the disordered host system onto the highly ordered sensitizer effectively avoids traps of the host matrix and features an almost ideal recombination behaviour.
The power conversion efficiency (PCE) of organic solar cells has been constantly refreshed in the past ten years from 4% up to 11% due to the contribution from the chemists on novel materials and the physicists on device engineering. For practical applications, a single bulk heterojunction structure may be the best candidate due to the cell with a high PCE, easy fabrication and low cost. Recently, ternary solar cells have attracted much attention due to enhanced photon harvesting by using absorption spectral or energy level complementary materials as the second donor or acceptor based on a single bulk heterojunction structure. For better promoting the development of ternary solar cells, we summarize the recent progress of ternary solar cells and try our best to concise out the scientific issues in preparing high performance ternary solar cells.
In the past few years, ternary organic solar cells (OSCs) featuring multiple donor or acceptor materials in the active layer have emerged as a promising structure to simultaneously improve all solar cell parameters compared with traditional binary OSCs. Power conversion efficiencies around 10% have been achieved for conjugated polymers in a ternary structure, showing the great potential of ternary systems. In this review, we summarize progress in developing ternary OSCs and discuss many of the designs, chemistries and mechanisms that have been investigated. We conclude by highlighting the challenges and future directions for further development in the field of ternary blend OSCs.
The performance of organic solar cells is substantially enhanced by introducing high-mobility conjugate polymers as an additive for the first time. The most pronounced effect is observed in one type of devices with the average power conversion efficiencies increased from 8.75% to 10.08% after the addition of a high-mobility polymer for only 0.5 weight percent in the active layers, which is mainly attributed to the increased hole mobility and carrier lifetime in the devices. Besides the hole mobility, the energy band structure of the additive is also found to be critical to the enhancement of the device performance. This work demonstrates a novel approach for improving the efficiencies of organic solar cells.
Broadening the absorption bandwidth of polymer solar cells by incorporating multiple absorber donors into the bulk-heterojunction active layer is an attractive means of resolving the narrow absorption of organic semiconductors. However, this leads to a much more complicated system, and previous efforts have met with only limited success. Here, several dual-donor and multi-donor bulk-heterojunction polymer solar cells based on a pool of materials with different absorption ranges and preferred molecular structures were studied. The study shows clearly that compatible polymer donors can coexist harmoniously, but the mixing of incompatible polymers can lead to severe molecular disorder and limit device performance. These results provide guidance for the general use of multiple-donor bulk heterojunctions to overcome the absorption limitation and achieve both high performance and fabrication simplicity for organic solar cells. P olymer photovoltaic cells have shown great potential as a means to harvest solar energy in a highly processable and cost-effective manner 1–5. Typical organic solar cells consist of a mixture of a polymer (or organic small molecule) donor and a C 60 derivative acceptor as the photoactive layer. In bulk-heterojunction (BHJ) organic solar cells, the absorbed incident photons generate tightly bound electron–hole pairs, which then dissociate into electrons and holes at the nearby donor/acceptor interface. The electrons and holes are then transported to their respective electrodes 6–8
Abstract. In ternary bulk heterojunction solar cells based on a semiconducting biphenyl-dithienyldiketopyrrolopyrrole copolymer donor and two different fullerene acceptors that distinctly differ in electron affinity, the open-circuit voltage is found to depend in a slightly sublinear fashion on the relative ratio of the two fullerenes in the blend. Similar effects have previously been observed and have been attributed to the formation of an alloyed fullerene phase possessing electronic levels that are the weighted average of the two components. By analyzing the contribution of the charge transfer (CT)-state absorption to the external quantum efficiency of the ternary blend solar cells as a function of composition, we find no evidence for a CT state formed between the polymer and an alloyed fullerene phase. Rather, the results are consistent with the presence of two distinct CT states, one for each polymer–fullerene combination. The two-state CT model does not, however, explain the sublinear behavior of the open-circuit voltage as a function of the blend composition.
To mimic the performance of the tandem solar cells, ternary blend solar cells with a single active layer of P3HT:PCPDTBT:PC61BM were cast from chlorobenzene and thermally annealed. By varying blending ratio, thermal annealing time and P3HT molecular weight, the device performance was enhanced relative to the binary references. To understand this, the morphology of the active layer was studied using hard and soft x-ray scattering methods in concert with bright field and energy resolved transmission electron microscopies. We found that the phase separation of the amorphous PCPDTBT and P3HT guided the formation of P3HT fibrils, resulting in a unique multi-length-scale morphology. This morphology consisted of bundles of well-defined P3HT fibrils, forming a network, imbedded in an amorphous mixture of the PCBM, PCPDTBT and P3HT. The two polymers acted independently in their specific photoactive ranges, and the sensitization of PCPDTBT benefited the cascade charge transfer. This multi-length-scale morphology was linked to the improved device performance of P3HT: PCPDTBT:PC61BM and the photophysics of the active layer.
The open-circuit voltage (Voc) of polymer:fullerene bulk heterojunction solar cells is determined by the interfacial charge-transfer (CT) states between polymer and fullerene. Fourier-transform photocurrent spectroscopy and electroluminescence spectra of several polymer:fullerene blends are used to extract the relevant interfacial molecular parameters. An analytical expression linking these properties to Voc is deduced and shown to be valid for photovoltaic devices comprising three commonly used conjugated polymers blended with the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Voc is proportional to the energy of the CT states ECT. The energetic loss qΔV between ECT and qVoc vanishes when approaching 0 K. It depends linearly on T and logarithmically on illumination intensity. Furthermore qΔV can be reduced by decreasing the electronic coupling between polymer and fullerene or by reducing the nonradiative recombination rate. For the investigated devices we find a loss qΔV of ∼0.6 eV at room temperature and under solar illumination conditions, of which ∼0.25 eV is due to radiative recombination via the CT state and ∼0.35 eV is due to nonradiative recombination.
Organic solar cells (OSCs) have advantages like lightweight , flexibility, colorfulness and solution processability [1]. The active layer of OSCs generally contains two organic semiconductors: an electron donor and an electron acceptor. The donor and acceptor make nanoscale phase separation to allow efficient exciton dissoci-ation and also form a three-dimensional (3D) passage to rapidly transfer free charge carriers to respective electrodes [2]. However, such binary system usually shows insufficient coverage of solar irradiation spectrum due to the narrow optical absorption of organic compounds [3]. Recently, ternary OSCs containing three absorption-complementary materials (e.g., two donors and one acceptor, or one donor and two acceptors) have attracted great attention. Ternary solar cells harvest more sunlight and demonstrate better performance than binary solar cells in some cases [3]. Polymer:fullerene:nonfullerene solar cells combine the advantages of fullerene acceptors (high electron mobility) and non-fullerene acceptors (strong visible or near-infrared (NIR) absorption), and achieved over 10% power conversion efficiencies (PCEs) [4,5]. Recently, we reported a highly efficient low-bandgap nonfullerene acceptor (CO i 8DFIC) with strong NIR absorption. PTB7-Th:CO i 8DFIC (1:1) binary cells gave 26.12 mA cm À2 short-circuit current density (J sc) and 12.16% PCE [6]. Here, we report highly efficient ternary cells based on PTB7-Th, CO i 8DFIC and PC 71 BM (Fig. 1a). Fullerene improves electron transport in the active layer and enhances external quantum efficiency (EQE), leading to high J sc and fill factor (FF). A PCE of 14.08% was achieved. The absorption spectra for PTB7-Th, CO i 8DFIC and PC 71 BM films are shown in Fig. 1b. PC 71 BM absorbs short-wavelength light, which is complementary to PTB7-Th and CO i 8DFIC. The lowest unoccupied molecular orbital levels (LUMO) for PTB7-Th (À3.12 eV), PC 71 BM (À3.67 eV) and CO i 8DFIC (À3.88 eV) show a stepwise alignment (Fig. 1c), suggesting that PC 71 BM can facilitate electron transfer from PTB7-Th to CO i 8DFIC [6,7]. Solar cells with a structure of ITO/ZnO/D:A 1 :A 2 /MoO 3 /Ag were made, where D is PTB7-Th, A 1 is CO i 8DFIC and A 2 is PC 71 BM. The weight ratio between D and A 1 + A 2 was fixed to 1:1.5, while the content of A 2 in acceptors gradually increased from 0% to 100% (Table S1 online) [8]. Initially, PTB7-Th:CO i 8DFIC (1:1.5) binary cells gave a PCE of 10.48%, with an open-circuit voltage (V oc) of 0.69 V, a J sc of 23.84 mA cm À2 and a FF of 63.8%. After adding small amount of fullerene (A 2) into the blend, J sc and FF increased dramatically. When D:A 1 :A 2 ratio (w:w:w) was 1:1.05:0.45, the ternary cells gave a PCE of 14.08%, with a V oc of 0.70 V, a J sc of 28.20 mA cm À2 and a FF of 71.0%. To the best of our knowledge, this is the first report demonstrating that the PCE for organic solar cells exceeds 14%. Further increasing fullerene content, V oc slightly increased, while J sc and FF decreased, leading to reduced PCEs. PTB7-Th:PC 71 BM (1:1.5) binary cells gave a PCE of 7.36%, with a V oc of 0.75 V, a J sc of 16.21 mA cm À2 and a FF of 60.2%. The performance for ternary cells (D:A 1 :A 2 = 1:1.05:0.45) is sensitive to the active layer thickness and additive content (Tables S2, S3 online). The optimal thickness for the active layer and the optimal 1,8-diiodooctane (DIO) content are 108 nm and 1 vol%, respectively. The J-V curves and the corresponding EQE spectra for the binary and the best ternary solar cells are shown in Fig. 1d, e. Compared with PTB7-Th:CO i 8DFIC cells, the ternary cells show enhanced EQE at 300-1,050 nm, consisting with the high J sc. The integrated current densities from EQE spectra of PTB7-Th:CO i 8DFIC and the ternary cells are 22.75 and 26.92 mA cm À2 , respectively. The EQE enhancement for the ternary cells might result from enhanced light absorption and efficient generation and transport of free charge carriers. The absorption spectra for the binary and ternary blend films are shown in Fig. S1 (online). Compared with
Despite the potential of ternary polymer solar cells (PSCs) to improve photocurrents, ternary architecture is not widely utilized for PSCs because its application has been shown to reduce fill factor (FF). In this paper, a novel technique is reported for achieving highly efficient ternary PSCs without this characteristic sharp decrease in FF by matching the highest occupied molecular orbital (HOMO) energy levels of two donor polymers. Our ternary device—made from a blend of wide-bandgap poly[4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene-alt-2,5-dioctyl-4,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione) (PBDT-DPPD) polymer, narrow-bandgap poly[4,8-bis[5-(2-ethylhexyl)-2-thienyl]benzo[1,2-b:4,5-b′]dithiophene-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2- 6-diyl)] (PTB7-Th) polymer, and [6,6]-phenyl C70-butyric acid methyl ester (PC70BM)—exhibits a maximum power conversion efficiency of 10.42% with an open-circuit voltage of 0.80 V, a short-circuit current of 17.61 mA cm⁻², and an FF of 0.74. In addition, this concept is extended to quaternary PSCs made by using three different donor polymers with similar HOMO levels. Interestingly, the quaternary PSCs also yield a good FF (≈0.70)—similar to those of corresponding binary PSCs. This study confirms that the HOMO levels of the polymers used on the photoactive layer of PSCs are a crucial determinant of a high FF.
Mixing different compounds to improve functionality is one of the pillars of the organic electronics field. Here, the degree to which the charge transport properties of the constituent materials are simply additive when materials are mixed is quantified. It is demonstrated that in bulk heterojunction organic solar cells hole mobility in the donor phase depends critically on the choice of the acceptor material, which may alter the energetic disorder of the donor. The same holds for electron mobility and disorder in the acceptor. The associated mobility differences can exceed an order of magnitude compared to pristine materials. Quantifying these effects by a state-filling model for the open-circuit voltage (VOC) of ternary Donor:Acceptor1:Acceptor2 (D:A1:A2) organic solar cells leads to a physically transparent description of the surprising, nearly linear tunability of the VOC with A1:A2 weight ratio. It is predicted that in binary OPV systems, suitably chosen donor and acceptor materials can improve the device power conversion efficiency (PCE) by several percentage points, for example from 11% to 13.5% for a hypothetical state-of-the-art organic solar cell, highlighting the importance of this design rule.
Recent studies demonstrated that with proper selection of chemically compatible constituents, the open-circuit voltage (Voc) of ternary-blend solar cells can be tuned across the composition window of the active layer. In this study, we probed the limit of the offset between the lowest unoccupied molecular orbital (LUMO) energy levels of the two acceptors in ternary blends containing one donor and two acceptors. We demonstrate, for the first time, that ternary-blend active layers with two acceptors having energy-level difference between their LUMO levels exceeding 0.4 eV can still result in solar cells exhibiting composition-dependent open-circuit voltage (Voc). Our results suggest strong electronic interactions between the acceptors, with the electron wavefunction delocalized over multiple molecules. These findings have broadened the library of possible candidates for active layers of ternary-blend solar cells with tunable Voc and established guidelines for the design of next-generation of materials for efficient performance of such devices.
Using small molecule donor (SMD) semiconductors in organic photovoltaics (OPVs) has historically afforded lower power conversion efficiencies (PCEs) than their polymeric counterparts. The PCE difference is attributed to shorter conjugated backbones, resulting in reduced intermolecular interactions. Here, a new pair of SMDs is synthesized based on the diketopyrrolopyrrole–benzodithiophene–diketopyrrolopyrrole (BDT-DPP2) skeleton, but having fluorinated and fluorine-free aromatic side-chain substituents. Ternary OPVs having varied ratios of the two SMDs with PC61BM as the acceptor exhibit tunable open-circuit voltages (Vocs) between 0.833 V and 0.944 V, due to a fluorination-induced shift in energy levels and the electronic “alloy” formed from the miscibility of the two SMDs. A 15% increase in PCE is observed at the optimal ternary SMD ratio, with the short-circuit current density (Jsc) significantly increased to 9.18 mA/cm2. The origin of Jsc enhancement is analyzed via charge generation, transport, and diffuse reflectance measurements, and is attributed to increased optical absorption from a maximum in film crystallinity at this SMD ratio, observed by grazing incidence wide-angle X-ray scattering.
In recent years, ternary solar cells have become an important photovoltaic-technology since they combine the merits of both single junction and tandem solar cells with their processing ease, mechanical flexibility, and lightweight. To date, their efficiency has reached over 12%, which paved the way for commercialization of organic solar cells. In this review, we first present general principles of ternary solar cells, followed by a comprehensive review of recent advances in ternary systems including the D1:D2:A system and D:A1:A2 system. In the end, we summarize the fundamentals and provide a prospect on organic ternary solar cells.
Non-fullerene acceptors (NFAs) are becoming a serious contender to fullerene-based electron acceptors in organic photovoltaics, due to their structural versatility and easily tunable optical and electronic properties. However, NFA-based solar cells often have a decreased short-circuit current (Jsc) and fill factor (FF) compared to their fullerene-based counterparts. Here, we investigate the fundamental causes of this decrease in the performance of solar cells using a non-fullerene acceptor (SF-PDI2) paired with two polymer donors, FTAZ and PyCNTAZ, compared with their fullerene-based counterparts. Through a number of experimental techniques and morphological studies, we show that the SF-PDI2-based solar cells suffer from insufficient charge generation, transport, and collection when compared with the PCBM-based solar cells. The SF-PDI2-based solar cells show increased bimolecular recombination, which, together with other recombination loss mechanisms in these cells, causes a significant decrease in their Jsc and FF. Notably, the less pure domains, low electron mobility (on the order of 10⁻⁵ cm² V⁻¹ s⁻¹), and imbalanced mobility (in regard to the hole mobility) further explain the low FF. On the other hand, the higher open-circuit voltage (Voc) in the SF-PDI2 devices is mainly due to the increase in the CT state energy. It is worth mentioning that the PyCNTAZ-based devices show an ultralow charge separation energy (ΔECS), close to 0 eV. Our results demonstrate that further increasing the mobility (both of electrons and holes) in these NFA-based solar cells would be a viable approach to further enhance the efficiency of these new types of solar cells, ideally, without losing the high Voc of such cells.
We present a novel ternary organic solar cell with uncommonly thick active layer (>300 nm), featuring thickness invariant charge carrier recombination and delivering 11% power conversion efficiency (PCE). The ternary blend was used to demonstrate photovoltaic modules of high technological relevance both on glass and flexible substrates, yielding 8.2% and 6.8% PCE, respectively.
Fluorinating conjugated polymers is a proven strategy for creating high performance materials in polymer solar cells, yet few studies investigated the importance of the fluorination method. We compare the performance of three fluorinated systems: a poly(benzodithieno-dithienyltriazole) (PBnDT-XTAZ) random copolymer where 50% of the acceptor units are difluorinated, PBnDT-mFTAZ where every acceptor unit is monofluorinated, and a 1:1 physical blend of the difluorinated and nonfluorinated polymer. All systems have the same degree of fluorination (50%), yet via different methods (chemically vs. physically, random vs. regular). We show that these three systems have equivalent photovoltaic behavior: ~5.2% efficiency with a short circuit current (Jsc) at ~11 mA cm-2, an open circuit voltage (Voc) at 0.77 V, and a fill factor (FF) of ~60%. Further investigation of these three systems demonstrate that the charge generation, charge extraction, and charge transfer state are essentially identical for the three studied systems. Transmission electron microscopy shows no significant differences in the morphologies. All these data illustrate that not only is it possible to improve performance via regular or random fluorination, but also by physical addition via a ternary blend. Thus, our results demonstrate the versatility of incorporating fluorine in the active layer of polymer solar cells to enhance device performance.
Ternary organic solar cells (OSCs), with a simple structure, can be easily adopted as sub‐cells in a tandem design, thereby further enhancing the power conversion efficiency (PCE). Considering the potential to surpass the theoretical PCE limit in OSCs, we incorporated a benzo[1,2‐b;4,5‐b']dithiophene‐based small molecule into a poly(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b;4,5‐b']dithiophene‐2,6‐diyl‐alt‐(4‐(2‐ethylhexyl)‐3‐fluorothieno[3,4‐b]thiophene‐)‐2‐carboxylate‐2‐6‐diyl)):[6,6]‐phenyl‐C71‐butyric acid methyl ester host system. A hitherto unrealized PCE of 12.1% was achieved at the optimized composition of the ternary blend. The ternary blend surprisingly had a face‐on and edge‐on coexistent texture, which is far better than that of the face‐on orientated host film. To the best of our knowledge, this intriguing result refutes for the first time a general paradigm that high‐performance OSCs are unambiguously linked to face‐on structures. Therefore, our study provides a new platform for refining the theoretical underpinning of multiple blending OSCs.
The compositional dependence of the open-circuit voltage (Voc) in ternary blend bulk heterojunction (BHJ) solar cells is correlated with the miscibility of polymers, which may be influenced by a number of attributes, including crystallinity, the random copolymer effect, or surface energy. Four ternary blend systems featuring poly(3-hexylthiophene-co-3-(2-ethylhexyl)thiophene) (P3HT75-co-EHT25), poly(3-hexylthiophene-co-(hexyl-3-carboxylate)), herein referred to as poly(3-hexylthiophene-co-3-hexylesterthiophene) (P3HT50-co-3HET50), poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-DPP-10%), and an analog of P3HTT-DPP-10% with 40% of 3-hexylthiophene exchanged for 2-(2-methoxyethoxy)ethyl)thiophen-2-yl (3MEO-T) (featuring an electronically decoupled oligoether side-chain), referred to as P3HTTDPP-MEO40% are explored in this work. All four polymers are semicrystalline, rich in rr-P3HT content, and perform well in binary devices with PC61BM. Except for P3HTTDPP-MEO40%, all polymers exhibit similar surface energies (~21-22 mN/m). P3HTTDPP-MEO40% exhibits an elevated surface energy of around 26 mN/m. As a result, despite the similar optoelectronic properties and binary solar cell performance of P3HTTDPP-MEO40% compared to P3HTT-DPP-10%, the former exhibits a pinned Voc in two different sets of ternary blend devices. This is a stark contrast to previous rr-P3HT-based systems and demonstrates that surface energy, and its influence on miscibility, plays a critical role in the formation of organic alloys and can supersede the influence of crystallinity, the random copolymer effect, similar backbone structures, and HOMO/LUMO considerations. Therefore, we confirm surface energy compatibility as a figure-of-merit for predicting the compositional dependence of the Voc in ternary blend solar cells and highlight the importance of polymer miscibility in organic alloy formation.
Highly efficient electron extraction is achieved by using photoconductive cathode interlayer in inverted ternary organic solar cells (OSCs) where a near-IR absorbing porphyrin molecule is used as the sensitizer. The OSCs show improved device performance when the ratio of the two donors varies in a large region and the maximum power conversion efficiency up to 11.03% is demonstrated.
This study demonstrates high-performance, ternary-blend polymer solar cells by modifying a binary blend bulk heterojunction (PPDT2FBT:PC71BM) with the addition of a ternary component, PPDT2CNBT. PPDT2CNBT is designed to have complementary absorption and deeper frontier energy levels compared to PPDT2FBT, while being based on the same polymeric backbone. A power conversion efficiency of 9.46% is achieved via improvements in both short-circuit current density (JSC) and open-circuit voltage (VOC). Interestingly, the VOC increases with increasing the PPDT2CNBT content in ternary blends. In-depth studies using ultraviolet photoelectron spectroscopy and transient absorption spectroscopy indicate that the two polymers are not electronically homogeneous and function as discrete light harvesting species. The structural similarity between PPDT2CNBT and PPDT2FBT allows the merits of a ternary system to be fully utilized to enhance both JSC and VOC without detriment to fill-factor via minimized disruption of semi-crystalline morphology of binary PPDT2FBT:PC71BM blend. Further, by careful analysis, charge carrier transport in this ternary blend is clearly verified to follow parallel-like behavior.
A regularly alternating terpolymer and a random terpolymer were synthesized from the constituent units of two donor–acceptor polymers with complementary absorption. They were then compared to a physical blend of these two donor–acceptor polymers in order to investigate the best means of extending the light absorption range in bulk heterojunction (BHJ) solar cells. While all three methods broadened the light absorption, the physical blend provided the best improvement in power conversion efficiency (4.10% vs 3.63% and 2.67% for the random and regular terpolymers, respectively). This is due to the increase in aggregation in the physical blend, as demonstrated in the UV–vis spectra, which likely leads to higher local mobility and less recombination. This study shows that in order to effectively increase the light absorption (and therefore performance) of a polymer:fullerene based BHJ solar cell, a terpolymer must retain a structure which allows sufficient aggregation.
Organic bulk heterojunction solar cells based on ternary blends of two donor absorbers and one
acceptor are investigated by experiments and modeling. The commonly observed continuous tunability
of the open circuit voltage VOC with the donor1 : donor2 ratio can quantitatively be explained as quasi-
Fermi level splitting due to photocreated charges filling a joint density of states that is broadened by
Gaussian disorder. On this basis, a predictive model for the power conversion efficiency that accounts
for the composition-dependent absorption and the shape of the current–voltage characteristic curve is
developed. When all other parameters, most notably the fill factor, are constant, we find that for stateof-
the-art absorbers, having a broad and strong absorption spectrum, ternary blends offer no advantage
over binary ones. For absorbers with a more narrow absorption spectrum ternary blends of donors with
complementary absorption spectra, offer modest improvements over binary ones. In contrast, when,
upon blending, transport and/or recombination kinetics are improved, leading to an increased fill factor,
ternaries may offer significant advantages over binaries.
Triazole based structural units have been widely used to construct conjugated polymers for optoelectronic applications; yet the design and synthesis of such units have been limited to just a few known examples. We report a general yet versatile synthetic approach toward a diverse set of triazole based conjugated molecules bearing various electron accepting abilities. The structural differences of as-synthesized three new triazole acceptors have a significant impact on the optoelectronic properties of conjugated polymers incorporating these triazoles. Bulk heterojunction solar cells based on one of these new polymers, PyCNTAZ, feature a high open circuit voltage of ∼1 V and a notable efficiency of 8.4% with an active layer thickness around 300 nm.
Ternary-blend bulk-heterojunction solar cells have provided a unique opportunity for tuning the open-circuit voltage (Voc) as the “effective” highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) energy levels shift with active-layer composition. Grazing-incidence X-ray diffraction (GIXD) measurements performed on such ternary-blend thin films reveal evidence that the two polymer donors interact intimately; their ionization potentials are thus reflections of the blend compositions. In ternary-blend thin films in which the two polymer donors do not interact physically, the polymer donors each retain their molecular electronic character; solar cells constructed with these ternary blends thus exhibit Vocs that are pinned to the energy level difference between the highest of the two lying HOMO and the LUMO of the electron acceptor. These observations are consistent with the organic alloy model proposed earlier. Quantification of the square of the square-root differences of the surface energies of the components provides a proxy for the Flory–Huggins interaction parameter for polymer donor pairs in these ternary-blend systems. Of the three ternary-blend systems examined herein, this quantity has to be below 0.094 in order for ternary-blend solar cells to exhibit tunable Voc.
The use ternary organic components is currently being pursued to enhance the power conversion efficiency of bulk heterojunction solar cells by expanding the spectral range of light absorption. Here, we report a ternary blend polymer solar cell containing two donor polymers, poly-3-oxothieno[3,4-d]isothiazole-1,1-dioxide/benzodithiophene (PID2), polythieno[3,4-b]-thiophene/benzodithiophene (PTB7) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as an acceptor. The resulting ternary solar cell delivered a power conversion efficiency of 822% with a short-circuit current density J(sc) of 16.8 mA cm(-2), an open-circuit voltage V-oc of 0.72 V and a fill factor of 68.7%. In addition to extended light absorption, we show that J(sc) is improved through improved charge separation and transport and decreased charge recombination, resulting from the cascade energy levels and optimized device morphology of the ternary system. This work indicates that ternary blend solar cells have the potential to surpass high-performance binary polymer solar cells after further device engineering and optimization.
Developing novel materials and device architectures to further enhance the efficiency of polymer solar cells requires a fundamental understanding of the impact of chemical structures on photovoltaic properties. Given that device characteristics depend on many parameters, deriving structure-property relationships has been very challenging. Here we report that a single parameter, hole mobility, determines the fill factor of several hundred nanometer thick bulk heterojunction photovoltaic devices based on a series of copolymers with varying amount of fluorine substitution. We attribute the steady increase of hole mobility with fluorine content to changes in polymer molecular ordering. Importantly, all other parameters, including the efficiency of free charge generation and the coefficient of nongeminate recombination, are nearly identical. Our work emphasizes the need to achieve high mobility in combination with strongly suppressed charge recombination for the thick devices required by mass production technologies.
Ternary organic photovoltaic donor:acceptor blend active materials composed of three distinct species possess remarkable advantages over neat semiconductors and binary donor:acceptor blends. A blended semiconductor is a foreign concept for inorganic semiconductors, whereas electronically disparate organic semiconductors can be mixed while mutually enhancing the properties of each. This feature allows ternary semiconductors to realize many of the advantages of tandem solar cells in a single layer.
Device performance is recognized to be generally sensitive to morphology in bulk heterojunction solar cells. Through the use of quantitative morphological measurements, it is demonstrated that devices based on benzodithiophene and fluorinated benzotriazole moieties constitute an exception to this design rule and exhibit a range of morphologies that yield similar high performance. In particular, the fill factor (FF) remains above 65% even with factor of two changes in domain size and factor of two changes in relative domain purity. Devices with active layer thicknesses of 250 nm are employed, which are capable of increasing optical absorption to produce high photocurrent. The general insensitivity to both morphology and thickness is likely related to the measured low equilibrium miscibility of fullerene in the polymer of 3-4%. The materials and processes investigated therefore provide insights into functional material design that yield increased processing latitude and may be more amenable to roll-to-roll processing.
Polymer:fullerene solar cells are demonstrated with power conversion efficiencies over 7% with blends of PBDTTPD and PC61BM. These devices achieve open-circuit voltages (Voc) of 0.945 V and internal quantum efficiencies of 88%, making them an ideal candidate for the large bandgap junction in tandem solar cells. Voc’s above 1.0 V are obtained when the polymer is blended with multiadduct fullerenes; however, the photocurrent and fill factor are greatly reduced. In PBDTTPD blends with multiadduct fullerene ICBA, fullerene emission is observed in the photoluminescence and electroluminescence spectra, indicating that excitons are recombining on ICBA. Voltage-dependent, steady state and time-resolved photoluminescence measurements indicate that energy transfer occurs from PBDTTPD to ICBA and that back hole transfer from ICBA to PBDTTPD is inefficient. By analyzing the absorption and emission spectra from fullerene and charge transfer excitons, we estimate a driving free energy of –0.14 ± 0.06 eV is required for efficient hole transfer. These results suggest that the driving force for hole transfer may be too small for efficient current generation in polymer:fullerene solar cells with Voc values above 1.0 V and that non-fullerene acceptor materials with large optical gaps (>1.7 eV) may be required to achieve both near unity internal quantum efficiencies and values of Voc exceeding 1.0 V.
Ternary solar cells enjoy both an increased light absorption width, and an easy fabrication process associated with their simple structures. Significant progress has been made for such solar cells with demonstrated efficiencies over 7%; however, their fundamental working principles are still under investigation. This Perspective is intended to offer our insights on the three major governing mechanisms in these intriguing ternary solar cells: charge transfer, energy transfer, and parallel-linkage. Through careful analysis of exemplary cases, we summarize the advantages and limitations of these three major mechanisms and suggest future research directions. For example, incorporating additional singlet fission or upconversion materials into the energy transfer dominant ternary solar cells has the potential to break the theoretical efficiency limit in single junction organic solar cells. Clearly, a feedback loop between fundamental understanding and materials selection is in urgent need to accelerate the efficiency improvement of these ternary solar cells.
The evolution of the open-circuit voltage (Voc) with composition in ternary blend bulk heterojunction (BHJ) solar cells is correlated with the miscibility of the polymers. Ternary blends based on poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) and poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-DPP-10%) with phenyl-C61-butyric acid methyl ester (PC61BM) acceptor were investigated. The Voc is pinned to the lower value of the P3HTT-DPP-10%:PC61BM binary blend even up to 95% PCDTBT in the polymer fraction. This is in stark contrast to the previously investigated system based on P3HTT-DPP-10%, poly(3-hexylthiophene-co-3-(2-ethylhexyl)thiophene) (P3HT75-co-EHT25) and PC61BM, where the Voc varied regularly across the full composition range, as explained by an organic alloy model, implying strong physical and electronic interaction between the polymers. Photocurrent spectral response (PSR) and external quantum efficiency (EQE) measurements indicate that the present system does not exhibit the hallmarks of alloy formation. Measured values of the surface energies of the polymers support miscibility of P3HTT-DPP-10% with P3HT75-co-EHT25 but not with PCDTBT. Surface energy is proposed as a figure of merit for predicting alloy formation and compositional dependence of the Voc in ternary blend solar cells and miscibility between polymers is proposed as a necessary attribute for polymer pairs that will display alloy behavior.
Recently, researchers have paid a great deal of attention to the research and development of organic solar cells, leading to a breakthrough of over 10% power conversion efficiency. Though impressive, further development is required to ensure a bright industrial future for organic photovoltaics. Relatively narrow spectral overlap of organic polymer absorption bands within the solar spectrum is one of the major limitations of organic solar cells. Among different strategies that are in progress to tackle this restriction, the novel concept of ternary organic solar cells is a promising candidate to extend the absorption spectra of large bandgap polymers to the near IR region and to enhance light harvesting in single bulk-heterojunction solar cells. In this contribution, we review the recent developments in organic ternary solar cell research based on various types of sensitizers. In addition, the aspects of miscibility, morphology complexity, charge transfer dynamics as well as carrier transport in ternary organic composites are addressed.
Ternary blend bulk heterojunction organic solar cells comprising either a polythiophene donor and two fullerene acceptors or two polythiophene donors and a fullerene acceptor are shown to have unique electronic properties. Measurements of the photocurrent spectral response and the open-circuit voltage show that the HOMO and LUMO levels change continuously with composition in the respective two-component acceptor or donor pair, consistent with the formation of an organic alloy. However, optical absorption of the exciton states retains the individual molecular properties of the two materials across the blend composition. This difference is attributed to the highly localized molecular nature of the exciton, and the more delocalized intermolecular nature of electrons and holes that reflect the average composition of the alloy. As established here, the combination of molecular excitations that can harvest a wide range of photon energies, and electronic alloy states that can adjust the open-circuit voltage provide the underlying basis of ternary blends as a platform for highly efficient next generation organic solar cells.
Here we demonstrate a conceptually new approach, the parallel-like bulk heterojunction (PBHJ), which maintains the simple device configuration and low-cost processing of single-junction BHJ cells while inheriting the major benefit of incorporating multiple polymers in tandem cells. In this PBHJ, free charge carriers travel through their corresponding donor-polymer-linked channels and fullerene-enriched domain to the electrodes, equivalent to a parallel-like connection. The short-circuit current (J(sc)) of the PBHJ solar cell is nearly identical to the sum of those of the individual "subcells", while the open-circuit voltage (V(oc)) is between those of the "subcells". Preliminary optimization of the PBHJ devices gives improvements of up to 40% in J(sc) and 30% in overall efficiency (η) in comparison with single-junction BHJ devices.
Recent research advances on conjugated polymers for photovoltaic devices have focused on creating low band gap materials, but a suitable band gap is only one of many performance criteria required for a successful conjugated polymer. This work focuses on the design of two medium band gap (~2.0 eV) copolymers for use in photovoltaic cells which are designed to possess a high hole mobility and low highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. The resulting fluorinated polymer PBnDT-FTAZ exhibits efficiencies above 7% when blended with [6,6]-phenyl C(61)-butyric acid methyl ester in a typical bulk heterojunction, and efficiencies above 6% are still maintained at an active layer thicknesses of 1 μm. PBnDT-FTAZ outperforms poly(3-hexylthiophene), the current medium band gap polymer of choice, and thus is a viable candidate for use in highly efficient tandem cells. PBnDT-FTAZ also highlights other performance criteria which contribute to high photovoltaic efficiency, besides a low band gap.
The development of predictive models relating materials' structures to photovoltaic device performance is crucial for the optimization of organic solar cells. Several factors can favor dissociation of interfacial charge-transfer states, including a large overall free energy loss driving charge separation, large domain sizes, and strong macroscopic electric fields. However, in all these cases, modulating these parameters to enhance charge photogeneration may have a negative impact upon other aspects of device performance. Similarly, increasing the domain size to favor CT-state dissociation can reduce the efficiency of exciton diffusion to the interface, while increasing the internal electric field by reducing the photoactive layer thickness would reduce light absorption. Following Onsager theory, charge photogeneration should be favored by reducing the Coulomb binding energy of the CT states. The introduction of insulator layers at the charge separation interface and the use of redox relays both enhance the performance of dye-sensitized solar cells.