A History and Perspective of Non-Fullerene Electron Acceptors for Organic Solar Cells

Asymmetric Nonfullerene Small Molecule Acceptors for Organic Solar Cells

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.

Benzothiadiazole-based non-fullerene acceptors

2D Side‐Chain Engineered Asymmetric Acceptors Enabling Over 14% Efficiency and 75% Fill Factor Stable Organic Solar Cells

Asymmetric simple unfused acceptor enabling over 12% efficiency organic solar cells

Side-chain engineering on nonfullerene acceptors (NFAs) is crucial for modulating their solubility and crystallinity as well as packing behaviours in active layers to pursue high-performance organic solar cells (OSCs). High weight ratios of side chains are generally used by NFAs for the desired device efficiencies. Side-chain economy has seldom been discussed despite increased cost and difficulties in synthesis when optimizing the molecular design. Herein, we introduce 7H-dibenzo[c,g]carbazole (DCB) as an electron-donating core to design unfused-ring acceptors (UFAs) with a dramatically low weight ratio of side chains. DCB-4F has thus been designed and compared with the carbazole cored analogue (CB-4F). The unique conformation of the DCB core endows DCB-4F with higher solubility (8.2 mg mL−1 in chloroform) compared to CB-4F (2.2 mg mL−1) when using the same side chains. Featuring a lowest unoccupied molecular orbital (LUMO) level of −3.86 eV and an optical bandgap of 1.55 eV, the DCB-4F film exhibits an absorption profile (maximum 667 nm) complementary to polymer donor PM6. The PM6:DCB-4F as-cast OSCs deliver a power conversion efficiency (PCE) of 9.56% with a high open-circuit voltage (VOC) of 1.00 V. By adding 10 wt% PC71BM into the casting solutions, a greatly improved PCE of 11.17% is readily achieved, which is one of the highest PCEs for as-cast single-junction UFA-based devices. The PM6:DCB-4F based blends show homogeneous nano-fiberous morphology and higher hydrophobicity. The design of conformation-tuned NFAs using sterically hindered DCB-like cores is promising to achieve highly efficient as-cast OSCs.

An unfused-ring acceptor with high side-chain economy enabling 11.17% as-cast organic solar cells

Organic solar cells (OSCs) and perovskite solar cells (PVSCs) are emerging photovoltaic technologies with solution-processability and performance that can feasibly be tuned via the molecular design of key organic photovoltaic materials (OPMs). With the innovation of fused-ring electron acceptors (FREAs), the power conversion efficiency (PCE) values of OSCs have been pushed as high as 18%. Meanwhile, molecular hole-transport materials (HTMs) have demonstrated promising advantages compared to the widely used material spiro-OMeTAD, allowing PCEs of over 20% to be realized in dopant-free PVSCs. In this review, we provide updates on progress related to the design and use of dithieno[3,2-b:2′,3′-d]pyrrole (DTP)-based OPMs as symmetric/asymmetric FREAs and dopant-free HTMs for obtaining record-high power conversion efficiencies and stable solar cells since 2016, with the objective of giving insightful views into material design and device construction strategies to boost photovoltaic performance. Strategies relating to judicious molecular designs are demonstrated for fine-tuning the electronic structures, crystallinity, thermal properties, and molecular orientations of DTP-based OPMs. Structure–property correlations are discussed in detail. New device engineering and processing strategies tailored to DTP-OPM-based solar cells are emphasized. Finally, we briefly summarize original designs of DTP-derived OPMs to address the challenging issues facing emerging solar cells and highlight key considerations relating to the development of new efficient OPMs.

Jinru Cao

Papers

2D Side‐Chain Engineered Asymmetric Acceptors Enabling Over 14% Efficiency and 75% Fill Factor Stable Organic Solar Cells

Asymmetric simple unfused acceptor enabling over 12% efficiency organic solar cells

An unfused-ring acceptor with high side-chain economy enabling 11.17% as-cast organic solar cells

The design of dithieno[3,2-b:2′,3′-d]pyrrole organic photovoltaic materials for high-efficiency organic/perovskite solar cells

Over 15.5% efficiency organic solar cells with triple sidechain engineered ITIC