Backbone Conformation Tuning of Carboxylate-Functionalized Wide Band Gap Polymers for Efficient Non-Fullerene Organic Solar Cells

Single-component organic solar cells-Perspective on the importance of chemical precision in conjugated block copolymers

Organic photovoltaics (OPV) present a promising thin-film solar cell technology with particular benefits in terms of weight, aesthetics, transparency, and cost. However, despite being studied intensively since the mid 90's, OPV has not entered the mass consumer market yet. Although the efficiency gap with other thin-film photovoltaics has largely been overcome, active layer stability and performance reproducibility issues have not been fully resolved. State-of-the-art OPV devices employ a physical mixture of electron donor and acceptor molecules in a bulk heterojunction active layer. These blends are prone to morphological changes, leading to performance losses over time. On the other hand, in "single-component" organic solar cells, the donor and acceptor constituents are chemically connected within a single material, preventing demixing and thereby enhancing device stability. Novel single-component materials affording reasonably high solar cell efficiencies and improved lifetimes have recently emerged. In particular, the combination of donor and acceptor structures in conjugated block copolymers (CBCs) presents an exciting approach. Nevertheless, the current CBCs are poorly defined from a structural point of view, while synthetic protocols remain unoptimized. More controlled synthesis followed by proper structural analysis of CBCs is, however, essential to develop rational structure-property-device relations and to drive the field forward. In this perspective, we provide a short overview of the state-of-the-art in single-component organic solar cells prepared from CBCs, reflect on their troublesome characterization and the importance of chemical precision in these structures, give some recommendations, and discuss the potential impact of these aspects on the field.

Carboxylate-Containing Wide-Bandgap Polymers for High-Voltage Non-Fullerene Organic Solar Cells

As one of the polymer modification strategies, carboxylate functionalization has proved effective in downshifting the energy levels and enhancing polymer crystallinity and aggregation. However, high-performance carboxylate-containing polymers are still limited for organic solar cells (OSCs), especially with open-circuit voltage (V_OC) above 1.0 V. Herein, we utilize two carboxylate-functionalized wide-band gap (WBG) donor polymers (TTC-F and TTC-Cl) to pair with two WBG electron acceptors (BTA5 and F-BTA5) for high-voltage OSCs. Due to the deeper molecular energy levels, chlorinated polymer TTC-Cl shows higher V_OC than fluorinated polymer TTC-F. Furthermore, because of the stronger aggregation in the film, the TTC-Cl-based devices attain suppressed energetic disorders and trap-assisted recombination, decreasing voltage loss and J_SC loss. Finally, the TTC-Cl: F-BTA5 blend achieves a higher V_OC of 1.17 V and an excellent PCE of 10.98%, one of the best results for high-voltage carboxylate-containing polymers. In addition, the TTC-Cl: BTA5 combination demonstrates the highest V_OC of 1.25 V with an ultralow nonradiative energy loss of 0.17 eV. Our results indicate that the carboxylate-containing polymer donors have significant application potential for high-voltage OSCs due to reduced energy loss and improved charge transport and dissociation. Furthermore, the matched absorption spectra with the indoor light sources and low voltage loss promote these material combinations to construct high-performance indoor photovoltaics.

Concurrently Improved Jsc, Fill Factor, and Stability in a Ternary Organic Solar Cell Enabled by a C-Shaped Non-fullerene Acceptor and Its Structurally Similar Third Component

A ternary strategy is recognized as a promising approach that enjoys both the simplicity of fabrication conditions and potential to improve performance in organic solar cells. Herein, a C-shaped narrow band gap non-fullerene acceptor GL1 with a C_2v symmetry based on a new core was designed and synthesized. A power conversion efficiency (PCE) of 11.43% was achieved by employing PBDB-T:GL1 as an active layer to fabricate photovoltaic devices. To further promote photovoltaic performance, following a similar-structure prescreen principle, a middle band gap acceptor F-2Cl with the same backbone shape, side-chain distribution, and dipole moment orientation as GL1 was introduced as the guest acceptor into the active layer. Thus, benefiting from the collaboration of complementary absorption, cascade energy levels, and well-modified microstructure of the active layer, a 13.17% PCE was obtained with concurrently elevated J_sc, fill factor, and stability for the optimized ternary device. This work presents a successful example of prescreening the third component to simplify the workload for a high-performance ternary device.

Isomeric Effect of Wide Bandgap Polymer Donors with High Crystallinity to Achieve Efficient Polymer Solar Cells

Two highly crystalline polymer donors (PBTz4T2C-a, PBTz4T2C-b) with isomers (4T2C-a, 4T2C-b) are synthesized and applied in polymer solar cells. The developed polymers possess proper energy levels and complementary absorption with an efficient electron acceptor IT2F. It is interesting that the photophysical properties, crystallinity, and active layer morphology characteristic can be significantly changed by just slightly regulating the substitution position of the carboxylate groups. A series of simulation calculations of the two isomers are conducted in the geometry and electronic properties to explore the difference induced by the position adjustment of carboxylate groups. The results decipher that 4T2C-b moiety features much stronger intramolecular noncovalent S⋯O interactions compared to that of 4T2C-a, implying a higher coplanarity and much stronger crystallinity, and leading to excessive phase separation in PBTz4T2C-b:IT2F blend film. In contrast, PBTz4T2C-a with 4T2C-a moiety exhibits suitable crystallinity with a lower the highest occupied molecular orbital level, higher film absorption coefficient, and charge mobilities, resulting in a much higher power conversion efficiency of 11.02%. This research demonstrates that the molecular conformation is of great importance to be considered for developing high-performance polymer donors.

Increased conjugated backbone twisting to improve carbonylated-functionalized polymer photovoltaic performance

Two conjugated polymers containing different linkers were synthesized to study their photovoltaic performances.

Synthesis of aryl-substituted thieno[3,2-b]thiophene derivatives and their use for N,S-heterotetracene construction

Fiesselmann thiophene synthesis was applied for the convenient construction of thieno[3,2-b]thiophene derivatives. Thus, new 5- or 6-aryl-3-hydroxythieno[3,2-b]thiophene-2-carboxylates were obtained by condensation of 5- or 4-aryl-3-chlorothiophene-2-carboxylates, respectively, with methyl thioglycolate in the presence of potassium tert-butoxide. The saponification of the resulting esters, with decarboxylation of the intermediating acids, gave the corresponding thieno[3,2-b]thiophen-3(2H)-ones. The latter ketones were used to synthesize new N,S-heterotetracenes, namely 9H-thieno[2',3':4,5]thieno[3,2-b]indoles by their treatment with arylhydrazines in accordance with the Fischer indolization reaction.

Improved Efficiency in All-Small-Molecule Organic Solar Cells with Ternary Blend of Nonfullerene Acceptor and Chlorinated and Nonchlorinated Donors

Ternary nonfullerene all-small-molecule organic solar cells (NFSM-OSCs) were developed by incorporating a nonfullerene acceptor (IDIC) and two structurally similar small molecular donors (SM and SM-Cl), where SM-Cl is a novel small molecular donor derived from the reported molecular donor SM. When doping 10% SM-Cl in the SM:IDIC binary system, the power conversion efficiency (PCE) of the ternary solar cell was dramatically increased from 9.39 to 10.29%. Characterization studies indicated that the two donors tend to form an alloy state, which effectively down-shifted the highest occupied molecular orbital (HOMO) energy level of the donor, thus promoting a higher open-circuit voltage. Interestingly, incorporating a third component (SM-Cl) with a lower crystallinity was proven to facilitate the demixing between donors and acceptors, which was contrary to the traditional findings of enhanced phase separation through the incorporation of highly crystalline molecule. Although the morphological modulation has always been a bottleneck issue in NFSM-OSCs, the findings in this work indicated that the modulation on crystallinity deviation between donors and acceptors could be an effective method to further improve the performance of NFSM-OSCs, providing a new perspective on NFSM-OSCs.

Regulation of Molecular Packing and Blend Morphology by Finely Tuning Molecular Conformation for High-Performance Nonfullerene Polymer Solar Cells

The asymmetric thienobenzodithiophene (TBD) structure is first systematically compared with the benzo[1,2-b:4,5-b']dithiophene (BDT) and dithieno[2,3-d:2',3'-d']benzo[1,2-b:4,5-b']dithiophene (DTBDT) units in donor-acceptor (D-A) copolymers and applied as the central core in small molecule acceptors (SMAs). Specific polymers including PBDT-BZ, PTBD-BZ, and PDTBDT-BZ with different macromolecular conformations are synthesized and then matched with four elaborately designed acceptor-donor-acceptor (A-D-A) SMAs with structures comparable to their donor counterparts. The resulting polymer solar cell performance trends are dramatically different from each other and highly material-dependent, and the active layer morphology is largely governed by polymer conformation. Because of its more linear backbone, the PTBD-BZ film has higher crystallinity and more ordered and denser π-π stacking than those of the PBDT-BZ and PDTBDT-BZ films. Thus, PTBD-BZ shows excellent compatibility with and strong independence on the SMAs with varied structures, and PTBD-BZ-based cells deliver high power conversion efficiency (PCE) of 10-12.5%, whereas low PCE is obtained by cells based on PDTBDT-BZ because of its zigzag conformation. Overall, this study reveals control of molecular conformation as a useful approach to modulate the photovoltaic properties of conjugated polymers.

Backbone Coplanarity Tuning of 1,4-Di(3-alkoxy-2-thienyl)-2,5-difluorophenylene-Based Wide Bandgap Polymers for Efficient Organic Solar Cells Processed from Nonhalogenated Solvent

Halogenated solvents are prevailingly used in the fabrication of nonfullerene organic solar cells (NF-OSCs) at the current stage, imposing significant restraints on their practical applications. By copolymerizing phthalimide or thieno[3,4-c]pyrrole-4,6-dione (TPD) with 1,4-di(3-alkoxy-2-thienyl)-2,5-difluorophenylene (DOTFP), which features intramolecular noncovalent interactions, the backbone planarity of the resulting DOTFP-based polymers can be effectively tuned, yielding distinct solubilities, aggregation characters, and chain packing properties. Polymer DOTFP-PhI with a more twisted backbone showed a lower degree of aggregation in solution but an increased film crystallinity than polymer DOTFP-TPD. An organic thin-film transistor and NF-OSC based on DOTFP-PhI, processed with a nonhalogenated solvent, exhibited a high hole mobility up to 1.20 cm² V^-1 s^-1 and a promising power conversion efficiency up to 10.65%, respectively. The results demonstrate that DOTFP is a promising building block for constructing wide bandgap polymers and backbone coplanarity tuning is an effective strategy to develop high-performance organic semiconductors processable with a nonhalogenated solvent.

Double Acceptor Block-Containing Copolymers with Deep HOMO Levels for Organic Solar Cells: Adjusting Carboxylate Substituent Position for Planarity

A deep highest occupied molecular orbital (HOMO) level is a prerequisite for polymer donor material to boost the organic solar cells (OSCs) performance by achieving high open circuit voltage ( V_oc). Abandoning the traditional concept of donor-acceptor (D-A) structure, two copolymers PBTZ-4TC and PBTZ-C4T based on acceptor₁-π-acceptor₂ (A₁-π-A₂) architecture, where thiophene as the bridge, the difluorinated benzotriazole (BTZ) as A₁ unit alternating copolymerized with 4,4'-dicarboxylate-substituted difluorotetrathiophene (4TC) and 3,3'-dicarboxylate-substituted difluorotetrathiophene (C4T) as A₂, respectively, are developed. Because of the double acceptor blocks with high electron affinity, both A₁-π-A₂ type copolymers possess the lower HOMO levels of 5.52-5.56 eV, which are lower than most D-A type donors. Polymer PBTZ-4TC and PBTZ-C4T have the same backbone but only differ with the position of carboxylate substituent on the A₂ unit. Intriguingly, subtle optimizing the position of the carboxylate-substitute causes a significantly difference on the properties of the A₁-π-A₂ type copolymers. PBTZ-C4T with more planar geometry is demonstrated with better light absorption, higher crystallinity, more pronounced temperature-dependent aggregation effect, and favorable bulk heterojunction morphology but with slightly higher HOMO level and more emission energy loss relative to the PBTZ-4TC. The PBTZ-C4T device exhibits the higher power conversion efficiency (PCE) of 9.34% than the PBTZ-4TC-based one (8.75%). These results reveal that concept of A₁-π-A₂ type copolymers not only can afford more flexibility in tuning the energy levels to achieve the deep HOMO levels but also can provide a facial strategy to greatly enrich the types of polymer donors for high-performance OSCs.

Influence of an ester directing-group on defect formation in the synthesis of conjugated polymers via direct arylation polymerization (DArP) using sustainable solvents

We report the application of green solvents in DArP and the structure-dependent β-defect formation due to an ester directing group.