Pronounced Backbone Coplanarization by π-Extension in a Sterically Hindered Conjugated Polymer System Leads to Higher Photovoltaic Performance in Non-Fullerene Solar Cells

Recent Advances of Benzodithiophene-Based Donor Materials for Organic Solar Cells

Recently, the power conversion efficiency (PCE) of organic solar cells (OSCs) has increased dramatically, making a big step toward the industrial application of OSCs. Among numerous OSCs, benzodithiophene (BDT)-based OSCs stand out in achieving efficient PCE. Notably, single-junction OSCs using BDT-based polymers as donor materials have completed a PCE of over 19%, indicating a dramatic potential for preparing high-performance large-scale OSCs. This paper reviews the recent progress of OSCs based on BDT polymer donor materials (PDMs). The development of BDT-based OSCs is concisely summarized. Meanwhile, the relationship between the structure of PDMs and the performance of OSCs is further described in this review. Besides, the development and prospect of single junction OSCs are also discussed.

18.6% Efficiency All-Polymer Solar Cells Enabled by a Wide Bandgap Polymer Donor Based on Benzo[1,2-d:4,5-d']bisthiazole

The limited selection of wide bandgap polymer donors for all-polymer solar cells (all-PSCs) is a bottleneck problem restricting their further development and remains poorly studied. Herein, a new wide bandgap polymer, namely PBBTz-Cl, is designed and synthesized by bridging the benzobisthiazole acceptor block and chlorinated benzodithiophene donor block with thiophene units for application as an electron donor in all-PSCs. PBBTz-Cl not only possesses wide bandgap and deep energy levels but also displays strong absorption, high-planar structure, and good crystallinity, making it a promising candidate for application as a polymer donor in organic solar cells. When paired with the narrow bandgap polymer acceptor PY-IT, a fibril-like morphology forms, which facilitates exciton dissociation and charge transport, contributing to a power conversion efficiency (PCE) of 17.15% of the corresponding all-PSCs. Moreover, when introducing another crystalline polymer acceptor BTP-2T2F into the PBBTz-Cl:PY-IT host blend, the absorption ditch in the range of 600-750 nm is filled, and the blend morphology is further optimized with the trap density reducing. As a result, the ternary blend all-PSCs achieve a significantly improved PCE of 18.60%, which is among the highest values for all-PSCs to date.

Halogen-Free π $\upi$ -Conjugated Polymers Based on Thienobenzobisthiazole for Efficient Nonfullerene Organic Solar Cells: Rational Design for Achieving High Backbone Order and High Solubility

In π $\upi$ -conjugated polymers, a highly ordered backbone structure and solubility are always in a trade-off relationship that must be overcome to realize highly efficient and solution-processable organic photovoltaics (OPVs). Here, it is shown that a π $\upi$ -conjugated polymer based on a novel thiazole-fused ring, thieno[2',3':5,6]benzo[1,2-d:4,3-d']bisthiazole (TBTz) achieves both high backbone order and high solubility due to the structural feature of TBTz such as the noncovalent interlocking of the thiazole moiety, the rigid and bent-shaped structure, and the fused alkylthiophene ring. Furthermore, based on the electron-deficient nature of these thiazole-fused rings, the polymer exhibits deep HOMO energy levels, which lead to high open-circuit voltages (V_OC s) in OPV cells, even without halogen substituents that are commonly introduced into high-performance polymers. As a result, when the polymer is combined with a typical nonfullerene acceptor Y6, power conversion efficiencies of reaching 16% and V_OC s of more than 0.84 V are observed, both of which are among the top values reported so far for "halogen-free" polymers. This study will serve as an important reference for designing π $\upi$ -conjugated polymers to achieve highly efficient and solution-processable OPVs.

Machine Learning-Assisted Polymer Design for Improving the Performance of Non-Fullerene Organic Solar Cells

Despite the progress in machine learning (ML) in terms of prediction of power conversion efficiency (PCE) in organic photovoltaics (OPV), the effectiveness of ML in practical applications is still lacking owing to the complex structure-property relationship. Therefore, verifying the potential of ML through experiments can amplify the use of ML models. Herein, we developed a new series of π-conjugated polymers comprising benzodithiophene and thiazolothiazole with fluorination and alkylthio chains (PBDTTzBO, PFSBDTTzBO, and PFBDTTzBO) for non-fullerene (NF) acceptors based on our random-forest ML model for OPVs. Notably, the order of the ML-predicted PCEs of these polymers with IT-4F (9.93, 11.35, and 11.47%) was in good agreement with their experimental PCEs (5.24, 7.35, and 10.30%). In contrast, an inverse correlation was observed between the predicted (9.20, 12.29, and 12.20%) and experimental (11.98, 1.57, and 6.53%) PCEs with Y6. Both the findings are interpreted in terms of surface morphology, transient photoconductivity, charge carrier mobility, polymer orientation, and miscibility, quantified by the Flory-Huggins parameters. Herein, we present an ML-assisted polymer design for high-performance non-fullerene organic photovoltaics (NFOPVs) and elucidate the importance of the subtle alterations in the morphology of NFOPVs.

Naphthobisthiadiazole-Based π-Conjugated Polymers for Nonfullerene Solar Cells: Suppressing Intermolecular Interaction Improves Photovoltaic Performance

Naphthobisthiadiazole has been known as a promising building unit of π-conjugated polymers for organic photovoltaics (OPVs). Here, we synthesized new NTz-based polymers that were combined with a benzodithiophene (BDT) unit having alkylthienyl substituents in the polymer backbone, named PNTzBDT, and PNTzBDT-F and PNTzBDT-Cl with fluorine and chlorine groups in the substituents, respectively. The polymers had significantly improved solubility than the previously reported NTz-based polymer (PNTz4T), most likely due to the torsion of the alkylthienyl substituents with respect to the BDT moiety, which suppresses the intermolecular interaction between the backbones. Despite the lower intermolecular interaction and thereby lower crystallinity, these polymers, in particular PNTzBDT and PNTzBDT-F, exhibited higher photovoltaic performances, with power conversion efficiencies as high as 13.3%, than PNTz4T in the cells that used Y6 as the acceptor material. The improved performance was ascribed to the enhanced miscibility of the polymers with the nonfullerene acceptor due to the increased solubility, which in addition led to the better charge generation and reduced charge recombination. These results indicate that NTz-based π-conjugated polymers have high potential for nonfullerene-based OPVs.