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 (ΔE2) of OSCs. As a result, PMHJ OSCs based on D18/BS3TSe‐4F achieve a PCE of 18.48%. By incorporating another mid‐bandgap acceptor Y6‐O into D18/BS3TSe‐4F to form a ternary PMHJ, a higher open‐circuit voltage (VOC) can be achieved to realize an impressive PCE of 19.03%. The findings of using pseudosymmetric electron acceptors in enhancing device efficiency provides an effective way to develop highly efficient acceptor materials for OSCs.

Achieving 19% Power Conversion Efficiency in Planar‐Mixed Heterojunction Organic Solar Cells Using a Pseudosymmetric Electron Acceptor

Organic photovoltaics (OPVs) have progressed steadily through three stages of photoactive materials development: (i) use of poly(3-hexylthiophene) and fullerene-based acceptors (FAs) for optimizing bulk heterojunctions; (ii) development of new donors to better match with FAs; (iii) development of non-fullerene acceptors (NFAs). The development and application of NFAs with an A-D-A configuration (where A = acceptor and D = donor) has enabled devices to have efficient charge generation and small energy losses (Eloss < 0.6 eV), resulting in substantially higher power conversion efficiencies (PCEs) than FA-based devices. The discovery of Y6-type acceptors (Y6 = 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]-thiadiazolo[3,4-e]-thieno[2″,3″:4',5']thieno-[2',3':4,5]pyrrolo-[3,2-g]thieno-[2',3':4,5]thieno-[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) with an A-DA' D-A configuration has further propelled the PCEs to go beyond 15% due to smaller Eloss values (∼0.5 eV) and higher external quantum efficiencies. Subsequently, the PCEs of Y6-series single-junction devices have increased to >19% and may soon approach 20%. This review provides an update of recent progress of OPV in the following aspects: developments of novel NFAs and donors, understanding of the structure-property relationships and underlying mechanisms of state-of-the-art OPVs, and tasks underpinning the commercialization of OPVs, such as device stability, module development, potential applications, and high-throughput manufacturing. Finally, an outlook and prospects section summarizes the remaining challenges for the further development of OPV technology.

Renewed Prospects for Organic Photovoltaics.

The power conversion efficiency (PCE) of organic solar cells (OSCs) is increasing continuously, however, commercialization is far from being achieved due to the very high synthetic cost of materials and toxic solvents. Poly(3-hexylthiophene) (P3HT) is the cheapest donor, however, its PCE has remained relatively low for a long time. Recently, a PCE of over 9% has been reported. This study was performed with the aim of predicting the performance of P3HT based organic solar cells through statistical data fit, bypassing the complexity of OSC devices. For this purpose, we have used machine learning to explore data from previously reported devices. Molecular descriptors were used to train machine learning models. We have identified the descriptors that have a positive impact on the PCE. Various machine learning models are used to classify non-fullerene acceptors (NFAs) on the basis of their PCEs. We have also developed various regression models to predict the PCE. The support vector machine showed the best predictive capability. Machine learning models were also trained to predict the energy levels. Over 3000 NFAs were designed using high performing and easily synthesizable building blocks. For virtual screening of new molecules, energy levels were predicted through the already trained model. Acceptors with suitable energy levels matching with those of P3HT were selected. The PCEs of these selected acceptors were also predicted. 87 acceptors with >7.5% PCE were selected. Green solvents were selected on the basis of Hansen solubility parameters predicted using machine learning. Green solvent-containing organic solar cells have better scope of commercialization due to their environment-friendly nature. This study will pave the way for the cheap and fast design of materials for efficient P3HT-based organic solar cells. This study will help to select potential candidates and speed up the breakthroughs.

A time and resource efficient machine learning assisted design of non-fullerene small molecule acceptors for P3HT-based organic solar cells and green solvent selection

Wide Bandgap Polymer with Narrow Photon Harvesting in Visible Light Range Enables Efficient Semitransparent Organic Photovoltaics

The ternary strategy has been proved to be an efficient approach to improve the power conversion efficiency (PCE) of organic solar cells (OSCs). However, little attention has been paid to deriving the general design principle for selecting an appropriate third component. Herein, we proposed a facile strategy for third-component selection in non-fullerene acceptor-based ternary OSCs. By sharing the same central unit with the host acceptor and a single fluorinated end group, the designed non-fullerene acceptor can successfully function as the third component in ternary OSCs. Following this design principle, we synthesized the BTP-F acceptor, and then incorporated it into the PM6:BTP-eC9 blend. Encouragingly, the optimized ternary OSC exhibited a high PCE of 18.45%, which is among the highest efficiency values reported for OSCs so far. In addition, the PM6:BTP-eC9:BTP-F ternary OSC displayed superior stability compared to the host system. Systematic characterizations reveal that the introduction of BTP-F into the ternary blend increases the charge transport, improves the active-layer morphology and reduces non-radiative recombination, therefore leading to a simultaneously enhanced short-circuit current, fill factor and open-circuit voltage. Furthermore, the Y6-F and L8-BO-F acceptors have been also synthesized as the third components in ternary OSCs. Compared with the binary devices, the ternary devices all exhibited improved PCEs. These results confirm the general application of the strategy we proposed, which provides a new way to further improve the efficiency of ternary OSCs.

A facile strategy for third-component selection in non-fullerene acceptor-based ternary organic solar cells

How to manipulate the phase separation and molecular arrangement to meet the need of efficient charge generation and extraction remains a long-standing challenge in all-small-molecule organic solar cells (ASM-OSCs). Herein, a small molecule acceptor Y7 as a morphology modulator was incorporated into a B1:BO-4Cl matrix to fabricate ternary ASM-OSCs. Y7 possesses excellent compatibility with the acceptor BO-4Cl but poor compatibility with the donor B1. The two acceptors prefer to form an alloy-like structure in ternary blends due to their good compatibility, which is conducive to fine-tuning the molecular arrangement for facilitating charge extraction. The inferior compatibility originating from the strong intermolecular interaction between Y7 and B1 can provide a driving force to manipulate the phase separation between the donor and acceptor for gaining well-formed nanofibrous and bi-continuous interpenetrating networks, leading to efficient charge separation, transport and collection in ternary blends. The ternary ASM-OSCs with 10 wt% Y7 in acceptors achieve a top-ranked power conversion efficiency of 16.28% with a holistic improvement of short-circuit current density, open-circuit voltage and fill factor. This work opens a new avenue to optimize the morphology for further boosting the performance of OSCs.

Rational compatibility in a ternary matrix enables all-small-molecule organic solar cells with over 16% efficiency

A dissymmetric backbone and selenophene substitution on the central core was used for the synthesis of symmetric or dissymmetric A-DA'D-A type non-fullerene small molecular acceptors (NF-SMAs) with different numbers of selenophene. From S-YSS-Cl to A-WSSe-Cl and to S-WSeSe-Cl, a gradually red-shifted absorption and a gradually larger electron mobility and crystallinity in neat thin film was observed. A-WSSe-Cl and S-WSeSe-Cl exhibit stronger and tighter intermolecular π-π stacking interactions, extra S⋅⋅⋅N non-covalent intermolecular interactions from central benzothiadiazole, better ordered 3D interpenetrating charge-transfer networks in comparison with thiophene-based S-YSS-Cl. The dissymmetric A-WSSe-Cl-based device has a PCE of 17.51 %, which is the highest value for selenophene-based NF-SMAs in binary polymer solar cells. The combination of dissymmetric core and precise replacement of selenophene on the central core is effective to improve Jsc and FF without sacrificing Voc .

A Synergistic Strategy of Manipulating the Number of Selenophene Units and Dissymmetric Central Core of Small Molecular Acceptors Enables Polymer Solar Cells with 17.5 % Efficiency.

Hai-rui Bai

Papers

Rational compatibility in a ternary matrix enables all-small-molecule organic solar cells with over 16% efficiency

A Synergistic Strategy of Manipulating the Number of Selenophene Units and Dissymmetric Central Core of Small Molecular Acceptors Enables Polymer Solar Cells with 17.5 % Efficiency.

Two-Pronged Effect of Warm Solution and Solvent-Vapor Annealing for Efficient and Stable All-Small-Molecule Organic Solar Cells