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In organic solar cells (OSCs), a thick active layer usually yields a higher photocurrent with broader optical absorption than a thin active layer. In fact, a ∼300 nm thick active layer is more compatible with large-area processing methods and theoretically should be a better spot for efficiency optimization. However, the bottleneck of developing high-efficiency thick-film OSCs is the loss in fill factor (FF). The origin of the FF loss is not clearly understood, and there a direct method to identify photoactive materials for high-efficiency thick-film OSCs is lacking. Here, we demonstrate that the mobility field-dependent coefficient is an important parameter directly determining the FF loss in thick-film OSCs. Simulation results based on the drift-diffusion model reveal that a mobility field-dependent coefficient smaller than 10

It is revealed that the re-entrant phase behavior of the conjugated polymer:small molecule systems is indeed a result of thermodynamics. The composition difference (Δϕ) between liquidus and binodal reflects the crystalline–amorphous interaction.

For polymer solar cells (PSCs), the mixture of polymer donors and small-molecule acceptors (SMAs) is fine-tuned to realize a favorable kinetically trapped morphology and thus a commercially viable device efficiency. However, the thermodynamic relaxation of the mixed domains within the blend raises concerns related to the long-term operational stability of the devices, especially in the record-holding Y-series SMAs. Here, a new class of dimeric Y6-based SMAs tethered with differential flexible spacers is reported to regulate their aggregation and relaxation behavior. In their polymer blends with PM6, it is found that they favor an improved structural order relative to that of Y6 counterpart. Most importantly, the tethered SMAs show large glass transition temperatures to suppress the thermodynamic relaxation in mixed domains. For the high-performing dimeric blend, an unprecedented open circuit voltage of 0.87 V is realized with a conversion efficiency of 17.85%, while those of regular Y6-base devices only reach 0.84 V and 16.93%, respectively. Most importantly, the dimer-based device possesses substantially reduced burn-in efficiency loss, retaining more than 80% of the initial efficiency after operating at the maximum power point under continuous illumination for 700 h. The tethering approach provides a new direction to develop PSCs with high efficiency and excellent operating stability.

Summary As the power conversion efficiency of organic photovoltaic has been dramatically improved to over 18%, achieving long-term stability is now crucial for applications of this promising photovoltaic technology. Among the high-efficiency systems, most are using BTP-4F and its analogs as acceptors. Herein, we determine the thermal transition temperatures (Tg) of seven BTP analogs to develop a structure-Tg framework. Our results point out an unresolved molecular design conundrum on how to simultaneously achieve high performance and intrinsic stability with BTP-based acceptors. We also show that PC71BM has miscibility above the percolation threshold in PM6 and can maintain local charge percolation and improved stability in ternary devices. However, PC71BM is not miscible with BTP-C3-4F and unfavorable vertical gradients that develop during aging still degrade performance. This points to a second thermodynamic conundrum. A compound with differential miscibility in the donor polymer can only impact percolation, and a compound with differential miscibility with the BTP only impacts diffusion.

Rapid increase in the power conversion efficiency of organic solar cells (OSCs) has been achieved with the development of non-fullerene small-molecule acceptors (NF-SMAs). Although the morphological stability of these NF-SMA devices critically affects their intrinsic lifetime, their fundamental intermolecular interactions and how they govern property–function relations and morphological stability of OSCs remain elusive. Here, we discover that the diffusion of an NF-SMA into the donor polymer exhibits Arrhenius behaviour and that the activation energy Ea scales linearly with the enthalpic interaction parameters χH between the polymer and the NF-SMA. Consequently, the thermodynamically most unstable, hypo-miscible systems (high χ) are the most kinetically stabilized. We relate the differences in Ea to measured and selectively simulated molecular self-interaction properties of the constituent materials and develop quantitative property–function relations that link thermal and mechanical characteristics of the NF-SMA and polymer to predict relative diffusion properties and thus morphological stability. Studies on the morphology stability of polymer donor–small-molecule acceptor blends relevant to solar cell stability reveal relationships between their intermolecular interactions and the thermodynamic, kinetic, thermal and mechanical properties.

Donor polymer number-average molar mass (Mn) has long been known to influence organic photovoltaic (OPV) performance via changes in both the polymer properties and the resulting bulk heterojunction...

Semitransparent organic photovoltaic cells (ST-OPVs) are emerging as a solution for solar energy harvesting on building facades, rooftops, and windows. However, the trade-off between power-conversion efficiency (PCE) and the average photopic transmission (APT) in color-neutral devices limits their utility as attractive, power-generating windows. A color-neutral ST-OPV is demonstrated by using a transparent indium tin oxide (ITO) anode along with a narrow energy gap nonfullerene acceptor near-infrared (NIR) absorbing cell and outcoupling (OC) coatings on the exit surface. The device exhibits PCE = 8.1 ± 0.3% and APT = 43.3 ± 1.2% that combine to achieve a light-utilization efficiency of LUE = 3.5 ± 0.1%. Commission Internationale d’eclairage chromaticity coordinates of (0.38, 0.39), a color-rendering index of 86, and a correlated color temperature of 4,143 K are obtained for simulated AM1.5 illumination transmitted through the cell. Using an ultrathin metal anode in place of ITO, we demonstrate a slightly green-tinted ST-OPV with PCE = 10.8 ± 0.5% and APT = 45.7 ± 2.1% yielding LUE = 5.0 ± 0.3% These results indicate that ST-OPVs can combine both efficiency and color neutrality in a single device.

Summary Multi-component organic solar cells (OSCs) comprising more than two donor and acceptor materials have attracted significant research attention, as they can offer broader and better absorption, hence increasing solar cell performance. However, the morphology of multi-component OSCs is exceptionally complicated and challenging to control. Here, we develop a highly efficient (near 17.7%) quaternary OSC (q-OSC) using two polymer donors (namely PM6 and PTQ10) along with a fullerene (PC71BM) and a non-fullerene acceptor (N3). Our quaternary system demonstrates a new type of “rivers and streams” functional hierarchical (multi-length scale) morphology, where small domains of PTQ10 and PC71BM act as separators that spatially separate PM6 and N3, which effectively suppressed charge recombination, enhanced hole transport, and balanced charge transportation. These improvements in the quaternary system contribute to the increased internal quantum efficiency (IQE) and, thus, lead to an excellent JSC and device performance, which surpass their respective binary and ternary OSCs.

Conjugated polymers have proven to be an important class of materials for flexible and stretchable electronics. To ensure long-term thermal and mechanical stability of associated devices, there is ...

Core engineering of small molecule acceptors (SMAs) is crucially important for enhancing device efficiency for nonfullerene organic solar cells (NF-OSCs). The most commonly used SMAs (e.g., ITIC) utilize indacenodithieno[3,2-b]thiophene (IDTT) as the central core, which has restricted their absorption ranges due to the weak electron-donating ability and short conjugation length. Here, we fused two electron-rich units, namely cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) and dithieno[3,2-b:2′,3′-d]pyran (DTPR), into the cores for constructing low-bandgap SMAs. The resulting CPDT-4Cl and DTPR-4Cl molecules exhibit extended nonacyclic central cores and strengthened intramolecular transfer (ICT) effect, resulting in red-shifted absorption (up to ∼950 nm) and up-shifted HOMO levels compared with IDTT-4Cl. Consequently, the NF-OSCs based on PTB7-Th:CPDT-4Cl and PTB7-Th:DTPR-4Cl achieved higher PCEs of 12.15% and 10.75%, respectively, than those of the PTB7-Th:IDTT-4Cl ones (7.70%). Notably, high short-circuit current densities (JSC) of 23–25 mA cm−2 were obtained by the CPDT-4Cl and DTPR-4Cl-based devices, indicating the great potential of the electron-donating CPDT and DTPR as promising building blocks to construct high-performance low-bandgap SMAs.

Summary The field of organic solar cells has seen rapid developments after the report of a high-efficiency (15.7%) small molecule acceptor (SMA) named Y6. In this paper, we design and synthesize a family of SMAs with an aromatic backbone identical to that of Y6 but with different alkyl chains to investigate the influence of alkyl chains on the properties and performance of the SMAs. First, we show that it is beneficial to use branched alkyl chains on the nitrogen atoms of the pyrrole motif of the Y6. In addition, the branching position of the alkyl chains also has a major influence on material and device properties. The SMA with 3rd-position branched alkyl chains (named N3) exhibits optimal solubility and electronic and morphological properties, thus yielding the best performance. Further device optimization using a ternary strategy allows us to achieve a high efficiency of 16.74% (and a certified efficiency of 16.42%).

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 a specific substitu...

Summary Although non-fullerene small molecular acceptors (NF-SMAs) are dominating current research in organic solar cells (OSCs), measurements of thermodynamics drivers and kinetic factors determining their morphological stability are lacking. Here, we delineate and measure such factors in crystallizable NF-SMA blends and discuss four model systems with respect to their meta-stability and degree of vitrification. We determine for the first time the amorphous-amorphous phase diagram in an NF-SMA system and show that its deep quench depth can result in severe burn-in degradation. We estimate the relative phase behavior of four other materials systems. Additionally, we derive room-temperature diffusion coefficients and conclude that the morphology needs to be stabilized by vitrification corresponding to diffusion constants below 10−22 cm2/s. Our results show that to achieve stability via rational molecular design, the thermodynamics, glass transition temperature, diffusion properties, and related structure-function relations need to be more extensively studied and understood.

Fluorination of the donor polymer or non-fullerene acceptor (NFA) in an organic photovoltaic device is an effective method to improve device efficiency. Although there have been many studies on donor polymer fluorination, blends containing both a fluorinated donor and fluorinated NFA have rarely been reported. In this study, we use two donor polymers (4′-FT-HTAZ and 4′-FT-FTAZ) and two NFAs (ITIC-Th and ITIC-Th1) with different amounts of fluorine (from 2F to 6F) to investigate how the degree of fluorination in a blend impacts device performance. We find that fluorinating the NFA leads to a higher short-circuit current density (Jsc) and fill factor (FF), however, the open-circuit voltage (Voc) is decreased due to a depressed lowest unoccupied molecular orbital (LUMO) level. Adding additional fluorine to the donor polymer does not have a large effect on the Jsc or FF, but it does lead to an improved Voc. By fluorinating the NFA and having more fluorine on the donor polymer, we obtain both a high Jsc and Voc simultaneously, leading to a power conversion efficiency over 10% in the case of 4′-FT-FTAZ:ITIC-Th1, which has the most amount of fluorine (6F).

Newly developed fused-ring electron acceptors (FREAs) have proven to be an effective class of materials for extending the absorption window and boosting the efficiency of organic photovoltaics (OPVs). While numerous acceptors have been developed, there is surprisingly little structural diversity among high performance FREAs in literature. Of the high efficiency electron acceptors reported, the vast majority utilize derivatives of 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (INCN) as the acceptor moiety. It has been postulated that the high electron mobility exhibited by FREA molecules with INCN end groups is a result of close π-π stacking between the neighboring planar INCN groups, forming an effective charge transport pathway between molecules. To explore this as a design rationale for electron acceptors, we synthesized a new fused-ring electron acceptor, IDTCF, which has methyl substituents out of plane to the conjugated acceptor backbone. These methyl groups hinder packing and expand the π-π stacking distance by ∼1 Å, but have little impact on the optical or electrochemical properties of the individual FREA molecule. The extra steric hindrance from the out of plane methyl substituents restricts packing and results in large amounts of geminate recombination, thus degrading the device performance. Our results show that intermolecular interactions (especially π-π stacking between end groups) play a crucial role in performance of FREAs. We demonstrated that the planarity of the acceptor unit is of paramount importance as even minor deviations in end group distance are enough to disrupt crystallinity and cripple device performance.

A major challenge hindering the further development of all-polymer solar cells (all-PSCs) employing polymerized small-molecule acceptors is the relatively low fill factor (FF) due to the difficulty in controlling the active-layer morphology. The issues typically arise from oversized phase separation resulting from the thermodynamically unfavorable mixing between two macromolecular species, and disordered molecular orientation/packing of highly anisotropic polymer chains. Herein, a facile top-down controlling strategy to engineer the morphology of all-polymer blends is developed by leveraging the layer-by-layer (LBL) deposition. Optimal intermixing of polymer components can be achieved in the two-step process by tuning the bottom-layer polymer swelling during top-layer deposition. Consequently, both the molecular orientation/packing of the bottom layer and the molecular ordering of the top layer can be optimized with a suitable top-layer processing solvent. A favorable morphology with gradient vertical composition distribution for efficient charge transport and extraction is therefore realized, affording a high all-PSC efficiency of 17.0% with a FF of 76.1%. The derived devices also possess excellent long-term thermal stability and can retain90% of their initial efficiencies after being annealed at 65 °C for 1300 h. These results validate the distinct advantages of employing an LBL processing protocol to fabricate high-performance all-PSCs.

A record power conversion efficiency (PCE) of over 19% is realized in planar-mixed heterojunction (PMHJ) organic solar cells (OSCs) by adopting the asymmetric selenium substitution strategy in making a pseudosymmetric electron acceptor, BS3TSe-4F. The combined molecular asymmetry with more polarizable selenium substitution increases the dielectric constant of the D18/BS3TSe-4F blend, helping lower the exciton binding energy. On the other hand, dimer packing in BS3TSe-4F is facilitated to enable free charge generation, helping more efficient exciton dissociation and lowering the radiative recombination loss (ΔE

Solution-processed organic solar cells (OSCs) are a promising candidate for next-generation photovoltaic technologies. However, the short exciton diffusion length of the bulk heterojunction active layer in OSCs strongly hampers the full potential to be realized in these bulk heterojunction OSCs. Herein, we report high-performance OSCs with a pseudo-bilayer architecture, which possesses longer exciton diffusion length benefited from higher film crystallinity. This feature ensures the synergistic advantages of efficient exciton dissociation and charge transport in OSCs with pseudo-bilayer architecture, enabling a higher power conversion efficiency (17.42%) to be achieved compared to those with bulk heterojunction architecture (16.44%) due to higher short-circuit current density and fill factor. A certified efficiency of 16.31% is also achieved for the ternary OSC with a pseudo-bilayer active layer. Our results demonstrate the excellent potential for pseudo-bilayer architecture to be used for future OSC applications., The so-called pseudo-bilayer (PB) organic solar cell (OSC) device architecture can promote enhanced exciton dissociation and charge transport, leading to improved device performance. Here, the authors report high-efficiency OSCs that features a PB architecture and optimized ternary system.

Organic solar cells based on D18:Y6 recently exhibited a record power conversion efficiency of over 18%. We have initially studied the molecular packing and thermodynamic properties of three D18:NFA blends by employing synchrotron X-ray techniques, secondary ion mass spectrometry and UV-vis miscibility measurements. The D18 polymer exhibits strong chain extension in films, which is beneficial to charge transport. Miscibility and other characterizations explain the disparate performance of three systems and the processing procedures. Our contribution reveals several unique property–performance relations of D18-based photovoltaic devices and help guide design or fabrication of yet higher efficiency organic solar cells.

Organic solar cells (OSCs) based on D18:Y6 have recently exhibited a record power conversion efficiency of over 18%. The initial work is extended and the device performance of D18-based OSCs is compared with three non-fullerene acceptors, Y6, IT-4F, and IEICO-4Cl, and their molecular packing characteristics and miscibility are studied. The D18 polymer shows unusually strong chain extension and excellent backbone ordering in all films, which likely contributes to the excellent hole-transporting properties. Thermodynamic characterization indicates a room-temperature miscibility for D18:Y6 and D18:IT-4F near the percolation threshold. This corresponds to an ideal quench depth and explains the use of solvent vapor annealing rather than thermal annealing. In contrast, D18:IEICO-4Cl is a low-miscibility system with a deep quench depth during casting and poor morphology control and low performance. A failure of ternary blends with PC71 BM is likely due to the near-ideal miscibility of Y6 to begin with and indicates that strategies for developing successful ternary or quaternary solar cells are likely very different for D18 than for other high-performing donors. This work reveals several unique property-performance relations of D18-based photovoltaic devices and helps guide design or fabrication of yet higher efficiency OSCs.

Developing high-performance donor polymers is important for nonfullerene organic solar cells (NF-OSCs), as state-of-the-art nonfullerene acceptors can only perform well if they are coupled with a matching donor with suitable energy levels. However, there are very limited choices of donor polymers for NF-OSCs, and the most commonly used ones are polymers named PM6 and PM7, which suffer from several problems. First, the performance of these polymers (particularly PM7) relies on precise control of their molecular weights. Also, their optimal morphology is extremely sensitive to any structural modification. In this work, a family of donor polymers is developed based on a random polymerization strategy. These polymers can achieve well-controlled morphology and high-performance with a variety of chemical structures and molecular weights. The polymer donors are D-A1-D-A2-type random copolymers in which the D and A1 units are monomers originating from PM6 or PM7, while the A2 unit comprises an electron-deficient core flanked by two thiophene rings with branched alkyl chains. Consequently, multiple cases of highly efficient NF-OSCs are achieved with efficiencies between 16.0% and 17.1%. As the electron-deficient cores can be changed to many other structural units, the strategy can easily expand the choices of high-performance donor polymers for NF-OSCs.

Some fundamental questions in the organic solar cell (OSC) community are related to the role of bulk and interfacial morphology on key processes such as charge generation, recombination, and extraction that dictate power conversion efficiencies (PCEs). The challenges with answering these questions arise due to the difficulty in accurately controlling, as well as comprehensively characterizing the morphology in bulk-heterojunction (BHJ) OSC blends. In this work, large variations in the interfacial and bulk morphologies of different low molecular weight fraction (LMWF) PM6:Y6 blends were detected despite the blends being fabricated from ostensibly the same building blocks. A drop in PCE from ∼15% to ∼5% was observed when the concentration of LMWFs of the PM6 polymer was increased from 1% to 52%. The drop in PCEs is found to be due to the lowering of the short-circuit current density (JSC) and fill-factor (FF) values as a result of compromised charge generation efficiencies, increased bulk trap densities, reduced charge transport, and inefficient charge extraction. The origin of the high device performance in the 1% LMWF blend is rationalized by the favorable bulk and interfacial morphological features, resolved from four techniques at sub-nanometer to sub-micrometer length scales. First, the closer donor:acceptor (D:A) interactions, smaller D and A domains, and increased D:A interfacial area facilitate ultrafast electron and hole transfer at the D:A interface. Second, the better long-range ordering and optimal phase separation of the D:A regions lead to superior charge transport and extraction.

Molecular packing, crystallinity, and texture of semiconducting polymers are often critical to performance. Although frame-works exist to quantify the ordering, interpretations are often just qualitative, resulting in imprecise and liberal use of terminology. Here, we reemphasize the continuity of the degree of molecular ordering and advocate that a more nuanced and consistent terminology is used with regards to crystallinity, semicyrstallinity, paracrystallinity, crystallite/aggregate, and related characteristics. We are motivated in part by our own imprecise and inconsistent use of terminology and the need to have a primer or tutorial reference to teach new group members. We show that a deeper understanding can be achieved by combining grazing-incidence wide-angle X-ray scattering and differential scanning calorimetry. We classify a broad range of representative polymers into four proposed categories based on the quantitative analysis of molecular order based on the paracrystalline disorder parameter (g). A small database is presented for over 10 representative conjugated and insulating polymers ranging from amorphous to semicrystalline. Finally, we outline the challenges to rationally design perfect polymer crystals and propose a new molecular design approach that envisions conceptual molecular grafting that is akin to strained and unstrained hetero-epitaxy in classic (compound) semiconductors thin film growth., Comment: Review article, 37 pages, 9 figures

Side-chain engineering of conjugated polymer donor materials is an important way for improving photovoltaic performances of polymer solar cells (PSCs). On the basis of the polymer J61 synthesized in our group, here, we design and synthesize three new 2D-conjugated polymers J62, J63, and J64 with different types of side chains to further investigate the effect of side chain on their physicochemical and photovoltaic properties. With the narrow bandgap n-type organic semiconductor (n-OS) ITIC as acceptor, the optimized PSCs based on polymer donor of J62 with linear octyl, J63 with linear unsaturated hexylene, and J64 with cyclohexane side chains display power conversion efficiency (PCE) of 10.81%, 8.13%, and 8.59%, respectively. After thermal treatment at 200 °C for 2 h on the active layer,the PCE of the PSC based on J63 still keeps 92% of the original value, which verifies that the cross-linking of the polymer can improve the thermal stability of PSCs. Morphological studies show that the active layer based ...

The effects of fused-ring regiochemistry on the physicochemical and photovoltaic properties of n-type organic semiconductor (n-OS) acceptors are investigated. Two n-OS isomers TPTC and TPTIC were prepared with different oxygen positions in the central fused-ring unit of the acceptor molecules: oxygen is connected with benzene in TPTC and it is connected with two thiophenes in TPTIC. It is found that TPTC tends to cause excessive self-aggregation with several different packing motifs or polymorphs, while TPTIC with compact alkyl chains forms well-defined crystals. The electron mobility of TPTC, which is measured by the space-charge-limited current (SCLC) method, is much lower than that of TPTIC. When blending these acceptors with the polymer PTQ10, excessive self-aggregation of TPTC leads to large phase separation and exhibits little change after thermal annealing treatment, while the intermolecular interaction in TPTIC is appropriate to achieve suitable phase separation in its blend films with PTQ10, and the stacking of both crystallites was obviously improved after thermal annealing. Thus the PSCs with TPTIC as the acceptor show a much higher power conversion efficiency (PCE) of 10.42%, in comparison with that (1.97%) of the device with TPTC as the acceptor. These results indicate that the regiochemistry of the n-OS acceptors greatly influences the aggregation behavior of the molecules, which strongly affects the performance of the PSCs, and the structure–property relationship of the materials with the regiochemistry could guide the development of high performance n-OS acceptors.

Three n-OS acceptors with Eg values of

Fluorinated conjugated polymers leading to enhanced photovoltaic device performance has been widely observed in a variety of donor–acceptor copolymers; however, almost all these polymers have fluorine substituents on the acceptor unit. Building upon our previously reported PBnDT-FTAZ, a fluorinated donor–acceptor conjugated polymer with impressive device performance, we set this study to explore the effect of adding the fluorine substituents onto the flanking thiophene units between the donor unit (BnDT) and the acceptor unit (TAZ). We developed new synthetic approaches to control the position of the fluorination (3′ or 4′) on the thiophene unit, and synthesized four additional PBnDT-TAZ polymers incorporating the fluorine-substituted-thiophene (FT) units, 3′-FT-HTAZ, 4′-FT-HTAZ, 3′-FT-FTAZ, and 4′-FT-FTAZ. We discover that relocating the fluorine substituents from the acceptor to the flanking thiophene units have a negligible impact on the device characteristics (short circuit current, open circuit volta...

Newly developed fused-ring electron acceptors (FREAs) have proven to be an effective class of materials for extending the absorption window and boosting the efficiency of organic photovoltaics (OPVs). While numerous FREA small molecules have been developed, there is surprisingly little structural diversity among high performance FREAs in literature. For example, of the high efficiency electron acceptors reported, the vast majority utilize derivatives of 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (INCN) as the acceptor moiety. It has been postulated that the high electron mobility exhibited by FREA molecules with INCN end groups is a result of close π-π stacking between the neighboring planar INCN groups, forming an effective charge transport pathway between molecules. To explore this as a design rationale for electron acceptors, we synthesized a new fused-ring electron acceptor, IDTCF, which has methyl substituents out of plane to the conjugated acceptor backbone. These methyl groups hinder packing and expand the π-π stacking distance by ~ 1 A, but this change doesn’t affect the optical or electrochemical properties of the individual acceptor molecule. Overall, our results show that intermolecular interactions (especially π-π stacking between end groups) play a crucial role in performance of FREAs. We demonstrated that the planarity of the acceptor unit is of paramount importance as even minor deviations in end group distance are enough to disrupt crystallinity and cripple device performance.

Developing effective methods to make efficient bulk-heterojunction polymer solar cells at roll-to-roll relevant active layer thickness is of significant importance. We investigate the effect of fullerene content in polymer:fullerene blends on the fill factor (FF) and on the performance of thick-film solar cells for four different donor polymers PTB7-Th, PDPP-TPT, BDT-FBT-2T, and poly[5,5′-bis(2-butyloctyl)-(2,2′-bithiophene)-4,4′-dicarboxylate-alt-5,5′-2,2′-bithiophene] (PDCBT). At a few hundreds of nanometers thickness, increased FFs are observed in all cases and improved overall device performances are obtained except for PDCBT upon increasing fullerene content in blend films. This fullerene content effect was studied in more detail by electrical and morphological characterization. The results suggest enhanced electron mobility and suppressed bimolecular recombination upon increasing fullerene content in thick polymer:fullerene blend films, which are the result of larger fullerene aggregates and improved interconnectivity of the fullerene phases that provide continuous percolating pathways for electron transport in thick films. These findings are important because an effective and straightforward method that enables fabricating efficient thick-film polymer solar cells is desirable for large-scale manufacturing via roll-to-roll processing and for multijunction devices.

International audience; The environmental impact of solution processed organic solar cells (OSCs) can be mitigated by introducing so-called green solvents during the fabrication processes. However, the effects of such green solvents on the molecular-level structures and optoelectronic properties lack in-depth characterization. Here, insights into the structure–processing–property correlation of a PPDT2FBT:PC61BM bulk-heterojunction (BHJ) system processed from a green solvent, ortho-xylene (o-XY), is investigated in comparison with the same blend processed from a traditional halogenated solvent, chlorobenzene (CB). The BHJ blends are characterized with various techniques probing at difference length scales, and an increased donor:acceptor (D:A) interfacial area as well as well-mixed features in the bulk morphologies of the active layer are observed for the o-XY processed BHJ blend. Furthermore, molecular-level differences in the D–A intermolecular interactions at the BHJ interfaces in o-XY and CB cast films are elucidated by 2-dimensional solid-state nuclear magnetic resonance (ssNMR) measurements and analysis. These results are consistent with the device properties, suggesting that the green-solvent-processed devices have longer charge carrier lifetimes and faster charge carrier extraction. The optimized PPDT2FBT:PC61BM devices processed from o-XY can achieve a noteworthy higher power conversion efficiency (PCE) owing to a higher short-circuit current density and fill factor.

With power conversion efficiency now over 17%, a long operational lifetime is essential for the successful application of organic solar cells. However, most non-fullerene acceptors can crystallize and destroy devices, yet the fundamental underlying thermodynamic and kinetic aspects of acceptor crystallization have received limited attention. Here, room-temperature (RT) diffusion coefficients of 3.4 × 10-23 and 2.0 × 10-22 are measured for ITIC-2Cl and ITIC-2F, two state-of-the-art non-fullerene acceptors. The low coefficients are enough to provide for kinetic stabilization of the morphology against demixing at RT. Additionally profound differences in crystallization characteristics are discovered between ITIC-2F and ITIC-2Cl. The differences as observed by secondary-ion mass spectrometry, differential scanning calorimetry (DSC), grazing-incidence wide-angle X-ray scattering, and microscopy can be related directly to device degradation and are attributed to the significantly different nucleation and growth rates, with a difference in the growth rate of a factor of 12 at RT. ITIC-4F and ITIC-4Cl exhibit similar characteristics. The results reveal the importance of diffusion coefficients and melting enthalpies in controlling the growth rates, and that differences in halogenation can drastically change crystallization kinetics and device stability. It is furthermore delineated how low nucleation density and large growth rates can be inferred from DSC and microscopy experiments which could be used to guide molecular design for stability.

Inner side-chain engineering on Y6 has been proven successful in improving short-circuit current density (JSC) through fine-tuning aggregated structures of acceptors. However, it fails in tuning the lowest unoccupied molecular orbital level (LUMO) and open-circuit voltage (VOC). In this paper, we turn to focus on engineering the outer side chains on the flanking thienothiophene units with 4-hexylphenyl (PhC6) and 6-phenylhexyl (C6Ph) chains. Use of PhC6 enhances the steric effect between the attached phenyl and the ending group, which in combination with the additional conjugation effect provided by the linking phenyl leads to upshifted energy levels and increased VOC as a result. Again, substitution with the bulkier PhC6 unprecedentedly improves film-morphology with reduced paracrystalline disorder and long period and increased root-mean-square composition variations as well, leading to increased electron and hole mobilities and suppressed monomolecular recombination with JSC and fill-factor (FF) simultaneously enhanced. The PM6:BTP-PhC6-based devices yield a higher efficiency value of 16.7% than the PM6:BTP-C6Ph-based one (15.5%). Therefore, this study shows a conceptual advance in materials design towards reducing the conflict between VOC and JSC in binary blended organic solar cells, which can be achieved by introducing bulkier chains to twist the backbone and simultaneously enhance the packing order.

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.

In bulk-heterojunction polymer solar cells (PSC), the molecular-level mixing between conjugated polymer donors and small-molecule acceptors plays a crucial role in obtaining a desirable morphology and good device stability. It has been recently shown that the thermodynamic limit of this mixing can be quantified by the liquidus miscibility, the composition of the small-molecule acceptor in amorphous phases in the presence of small-molecule crystals, and then converted to the Flory- Huggins interaction parameter χ. This conversion maps out the amorphous miscibility. Moreover, the quantitative relations between χ and the fill factor of PSC devices were established recently. However, the commonly used measurement of this liquidus miscibility, scanning transmission X-ray microscopy, is not easily and readily accessible. Here, we delineate a method based on common visible light microscopy and ultraviolet-visible absorption spectroscopy to replace the X-ray measurements. To demonstrate the feasibility of this technique and methodology, a variety of conjugated polymers (PffBT4T-C 9 C 13 , PDPP3T PBDT-TS1, PTB7-Th, and FTAZ) and their miscibility with fullerenes or nonfullerene small molecules (PC 71 BM, PC 61 BM, and EH-IDTBR) are characterized. The establishment of this methodology will pave the way to a wider use of the liquidus miscibility and the critical miscibility-function relations to optimize the device performance and obtain good stability in PSCs and other devices.

Donor polymer fluorination has proven to be an effective method to improve the power conversion efficiency of fullerene-based polymer solar cells (PSCs). However, this fluorine effect has not been well-studied in systems containing new, non-fullerene acceptors (NFAs). Here, we investigate the impact of donor polymer fluorination in NFA-based solar cells by fabricating devices with either a fluorinated conjugated polymer (FTAZ) or its non-fluorinated counterpart (HTAZ) as the donor polymer and a small molecule NFA (ITIC) as the acceptor. We found that, similar to fullerene-based devices, fluorination leads to an increased open circuit voltage (Voc) from the lowered HOMO level and improved fill factor (FF) from the higher charge carrier mobility. More importantly, donor polymer fluorination in this NFA-based system also led to a large increase in short circuit current (Jsc), which stems from the improved charge transport and extraction in the fluorinated device. This study demonstrates that fluorination is also advantageous in NFA-based PSCs and may improve performance to a higher extent than in fullerene-based PSCs. In the context of other recent reports on demonstrating higher photovoltaic device efficiencies with fluorinated materials, fluorination appears to be a valuable strategy in the design and synthesis of future donors and acceptors for PSCs.