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

A low temperature processable tin oxide interlayer via amine-modification for efficient and stable organic solar cells

14.55% efficiency PBDB-T ternary organic solar cells enabled by two alloy-forming acceptors featuring distinct structural orders

Charge density modulation on a thieno[2′′,3′′:5′,6′]-s-indaceno[2′,1′:4,5]dithieno[3,2-b:2′,3′-d]pyrrole (IPT) core has been conducted for a systematic study of its impact on the electronic structure, molecular packing and photovoltaic performance of asymmetric fused-ring acceptors (FRAs). Herein, a series of IPT-based FRAs (i.e.IN-4F, INO-4F, IPT-4F and IPCl-4F) are designed by incorporating 2-ethylhexyl, 2-ethylhexyloxy, hydrogen and chloro side chains, respectively, onto the IPT core. Enhancing the electron-withdrawing ability of the side-chains decreases the optical bandgap but lowers the lowest unoccupied molecular orbital (LUMO) level, which yields a trade-off between JSC and VOC in organic solar cells (OSCs). Furthermore, the FRAs exhibit tunable miscibility and crystallinity, reflected in the FF and JSC values of the OSCs. By pairing with polymer donor PM6, IPT-4F based devices achieve the highest PCE of 14.62% with a balanced VOC of 0.88 V and JSC of 22.15 mA cm−2 and a high FF of 75.01%. Our research demonstrates that electronic density modulation on asymmetric FRAs is an effective way to systematically optimize the device parameters in pursuit of high performance OSCs.

Charge Density Modulation on Asymmetric Fused-Ring Acceptors for High-Efficiency Photovoltaic Solar Cells

Efficient thick film non-fullerene organic solar cells enabled by using a strong temperature-dependent aggregative wide bandgap polymer

In this study, a porous inorganic/organic (ZnO/PEIE, where PEIE is polyethylenimine ethoxylated) (P-ZnO) hybrid material has been developed and adopted in the inverted organic solar cells (OSCs). The P-ZnO serving as the electron transport layer (ETL) not only presents an ameliorative work function, but also forms the cratered surface with increased ohmic contact area, revealing suppressed charge recombination and enhanced charge extraction in devices. Particularly, P-ZnO-based OSCs show improved light trapping in the active layer compared with ZnO-based ones. The universality of P-ZnO serving as ETL for efficient OSCs is verified on three photovoltaic systems of PBDB-T/DTPPSe-2F, PM6/Y6, and PTB7-Th/PC71BM. The enhancements of 8% in power conversion efficiency (PCE) can be achieved in the state-of-the-art OSCs based on PBDB-T/DTPPSe-2F, PM6/Y6, and PTB7-Th/PC71BM, delivering PCEs of 14.78%, 16.57%, and 9.85%, respectively. Furthermore, a promising PCE of 14.13% under air-processed condition can be achieved for PZnO/PBDB-T/DTPPSe-2F-based OSC, which is among the highest efficiencies reported for air-processed OSCs in the literature. And the P-ZnO/PBDB-T/DTPPSe-2F-based device also presents superior long-term storage stability whether in nitrogen or ambient air-condition without encapsulation, which can maintain over 85% of its initial efficiency. Our results demonstrate the great potential of the porous hybrid PZnO as ETL for constructing high-performance and air-stable OSCs.

/pdf/highly-efficient-organic-solar-cells-enabled-by-a-porous-zno-krvepgg1tq.pdf

Hongtao Wang

Papers

A low temperature processable tin oxide interlayer via amine-modification for efficient and stable organic solar cells

14.55% efficiency PBDB-T ternary organic solar cells enabled by two alloy-forming acceptors featuring distinct structural orders

Charge Density Modulation on Asymmetric Fused-Ring Acceptors for High-Efficiency Photovoltaic Solar Cells

Efficient thick film non-fullerene organic solar cells enabled by using a strong temperature-dependent aggregative wide bandgap polymer

Highly efficient organic solar cells enabled by a porous ZnO/PEIE electron transport layer with enhanced light trapping