18% Efficiency organic solar cells

Optimizing the molecular structures of organic photovoltaic (OPV) materials is one of the most effective methods to boost power conversion efficiencies (PCEs). For an excellent molecular system with a certain conjugated skeleton, fine tuning the alky chains is of considerable significance to fully explore its photovoltaic potential. In this work, the optimization of alkyl chains is performed on a chlorinated nonfullerene acceptor (NFA) named BTP-4Cl-BO (a Y6 derivative) and very impressive photovoltaic parameters in OPV cells are obtained. To get more ordered intermolecular packing, the n-undecyl is shortened at the edge of BTP-eC11 to n-nonyl and n-heptyl. As a result, the NFAs of BTP-eC9 and BTP-eC7 are synthesized. The BTP-eC7 shows relatively poor solubility and thus limits its application in device fabrication. Fortunately, the BTP-eC9 possesses good solubility and, at the same time, enhanced electron transport property than BTP-eC11. Significantly, due to the simultaneously enhanced short-circuit current density and fill factor, the BTP-eC9-based single-junction OPV cells record a maximum PCE of 17.8% and get a certified value of 17.3%. These results demonstrate that minimizing the alkyl chains to get suitable solubility and enhanced intermolecular packing has a great potential in further improving its photovoltaic performance.

Single-Junction Organic Photovoltaic Cells with Approaching 18% Efficiency.

Molecular design of non-fullerene acceptors is of vital importance for high-efficiency organic solar cells. The branched alkyl chain modification is often regarded as a counter-intuitive approach, as it may introduce an undesirable steric hindrance that reduces charge transport in non-fullerene acceptors. Here we show the design and synthesis of a highly efficient non-fullerene acceptor family by substituting the beta position of the thiophene unit on a Y6-based dithienothiophen[3,2-b]-pyrrolobenzothiadiazole core with branched alkyl chains. It was found that such a modification to a different alkyl chain length could completely change the molecular packing behaviour of non-fullerene acceptors, leading to improved structural order and charge transport in thin films. An unprecedented efficiency of 18.32% (certified value of 17.9%) with a fill factor of 81.5% is achieved for single-junction organic solar cells. This work reveals the importance of the branched alkyl chain topology in tuning the molecular packing and blend morphology, which leads to improved organic photovoltaic performance. Molecular design of acceptor and donor molecules has enabled major progress in organic photovoltaics. Li et al. show that branched alkyl chains in non-fullerene acceptors allow favourable morphology in the active layer, enabling a certified device efficiency of 17.9%.

Non-fullerene acceptors with branched side chains and improved molecular packing to exceed 18% efficiency in organic solar cells

Improving power conversion efficiency (PCE) is important for broadening the applications of organic photovoltaic (OPV) cells. Here, a maximum PCE of 19.0% (certified value of 18.7%) is achieved in single-junction OPV cells by combining material design with a ternary blending strategy. An active layer comprising a new wide-bandgap polymer donor named PBQx-TF and a new low-bandgap non-fullerene acceptor (NFA) named eC9-2Cl is rationally designed. With optimized light utilization, the resulting binary cell exhibits a good PCE of 17.7%. An NFA F-BTA3 is then added to the active layer as a third component to simultaneously improve the photovoltaic parameters. The improved light unitization, cascaded energy level alignment, and enhanced intermolecular packing result in open-circuit voltage of 0.879 V, short-circuit current density of 26.7 mA cm-2 , and fill factor of 0.809. This study demonstrates that further improvement of PCEs of high-performance OPV cells requires fine tuning of the electronic structures and morphologies of the active layers.

Single-Junction Organic Photovoltaic Cell with 19% Efficiency

Solar cell efficiency tables (version 57)

A medium-bandgap acceptor IBCT based on an 2-(1-oxo-1,2-dihydro-3H-benzo[b]cyclopenta[d]thiophen-3-ylidene)malononitrile end unit was developed. IBCT has an optical bandgap of 1.65 eV and afforded a power conversion efficiency of 11.26% and an open-circuit voltage of 1.02 V in single-junction organic solar cells when blending with a wide-bandgap copolymer donor L1. The L1:IBCT solar cell was further used as the front cell in tandem solar cells, yielding a 15.25% efficiency. IBCT is among the best medium-bandgap acceptors.

An efficient medium-bandgap nonfullerene acceptor for organic solar cells

Systematic side-chain engineering on nonacyclic acceptor–donor–acceptor (A–D–A) nonfullerene acceptors, NNFA[n, m], was carried out. “n” and “m” stand for the number of alkyl carbon atoms in the in-plane and out-of-plane side chains, respectively. Five acceptors, NNFA[0, 6], NNFA[6, 3], NNFA[6, 6], NNFA[12, 3] and NNFA[12, 6], were prepared and applied in organic solar cells by blending with a wide-bandgap copolymer donor (FTAZ). The alkyl chains substantially affect the NNFAs’ solubility and photovoltaic performance. The solubility varies from 23 mg mL−1 to 226 mg mL−1 in chloroform when changing the total alkyl carbons (2n + 4m). If the total alkyl carbons are equal, the NNFA with longer out-of-plane alkyl chains (higher “m”) shows higher solubility than that with longer in-plane alkyl chains (higher “n”). NNFA[6, 6] and NNFA[12, 3] with medium solubility (∼100 mg mL−1) present suitable miscibility with FTAZ, and afford more favorable morphology and higher device performance than other NNFAs. FTAZ:NNFA[12, 3] solar cells gave the highest power conversion efficiency of 10.81%.

Understanding the side-chain effects on A–D–A acceptors: in-plane and out-of-plane

The performance of a wide-band gap copolymer donor PDTPO-BDTT in nonfullerene solar cells was investigated. These solar cells presented broad photoresponse and high short-circuit current density. PDTPO-BDTT:IT-4F and PDTPO-BDTT:NNFA-4F solar cells with more efficient photoluminescence quenching and better film morphology gave decent power conversion efficiencies of 10.96 and 10.04%, respectively, which are much higher than those of the previously reported PDTPO-BDTT:fullerene solar cells.

https://pubs.acs.org/doi/pdf/10.1021/acsomega.9b01363

Qishi Liu

Papers

An efficient medium-bandgap nonfullerene acceptor for organic solar cells

Understanding the side-chain effects on A–D–A acceptors: in-plane and out-of-plane

A Wide-Band Gap Copolymer Donor for Efficient Fullerene-Free Solar Cells.