Single-Junction Organic Solar Cell with over 15% Efficiency Using Fused-Ring Acceptor with Electron-Deficient Core

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

Energy loss within organic solar cells (OSCs) is undesirable as it reduces cell efficiency1–4. In particular, non-radiative recombination loss3 and energetic disorder5, which are closely related to the tail states below the band edge and the overall photon energy loss, need to be minimized to improve cell performance. Here, we report how the use of a small-molecule acceptor with torsion-free molecular conformation can achieve a very low degree of energetic disorder and mitigate energy loss in OSCs. The resulting single-junction OSC has an energy loss due to non-radiative recombination of just 0.17 eV and a high power conversion efficiency of up to 16.54% (certified as 15.89% by the National Renewable Energy Laboratory). The findings take studies of organic photovoltaics deeper into a new regime, beyond the limits of energetic disorder and large energy offset for charge generation. An organic solar cell designed with minimal energetic disorder exhibits very low energy loss due to non-radiative recombination and highly efficient operation.

High-efficiency organic solar cells with low non-radiative recombination loss and low energetic disorder

With the recent emergence of a new class of high-performance nonfullerene acceptors (NFAs), organic solar cells (OSCs) have entered a new phase of research featuring high power conversion efficienc...

New Phase for Organic Solar Cell Research: Emergence of Y-Series Electron Acceptors and Their Perspectives

Recent advances in nonfullerene acceptors (NFAs) have enabled the rapid increase in power conversion efficiencies (PCEs) of organic photovoltaic (OPV) cells. However, this progress is achieved using highly toxic solvents, which are not suitable for the scalable large-area processing method, becoming one of the biggest factors hindering the mass production and commercial applications of OPVs. Therefore, it is of great importance to get good eco-compatible processability when designing efficient OPV materials. Here, to achieve high efficiency and good processability of the NFAs in eco-compatible solvents, the flexible alkyl chains of the highly efficient NFA BTP-4F-8 (also known as Y6) are modified and BTP-4F-12 is synthesized. Combining with the polymer donor PBDB-TF, BTP-4F-12 shows the best PCE of 16.4%. Importantly, when the polymer donor PBDB-TF is replaced by T1 with better solubility, various eco-compatible solvents can be applied to fabricate OPV cells. Finally, over 14% efficiency is obtained with tetrahydrofuran (THF) as the processing solvent for 1.07 cm2 OPV cells by the blade-coating method. These results indicate that the simple modification of the side chain can be used to tune the processability of active layer materials and thus make it more applicable for the mass production with environmentally benign solvents.

Eco-Compatible Solvent-Processed Organic Photovoltaic Cells with Over 16% Efficiency.

A new electron-rich central building block, 5,5,12,12-tetrakis(4-hexylphenyl)-indacenobis-(dithieno[3,2-b:2',3'-d]pyrrol) (INP), and two derivative nonfullerene acceptors (INPIC and INPIC-4F) are designed and synthesized. The two molecules reveal broad (600-900 nm) and strong absorption due to the satisfactory electron-donating ability of INP. Compared with its counterpart INPIC, fluorinated nonfullerene acceptor INPIC-4F exhibits a stronger near-infrared absorption with a narrower optical bandgap of 1.39 eV, an improved crystallinity with higher electron mobility, and down-shifted highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. Organic solar cells (OSCs) based on INPIC-4F exhibit a high power conversion efficiency (PCE) of 13.13% and a relatively low energy loss of 0.54 eV, which is among the highest efficiencies reported for binary OSCs in the literature. The results demonstrate the great potential of the new INP as an electron-donating building block for constructing high-performance nonfullerene acceptors for OSCs.

Dithieno[3,2-b:2',3'-d]pyrrol Fused Nonfullerene Acceptors Enabling Over 13% Efficiency for Organic Solar Cells.

Conformation Locking on Fused-Ring Electron Acceptor for High-Performance Nonfullerene Organic Solar Cells

In this work, we present a structurally defined blue-shift non-fullerene electron acceptor (ITCT) matching with PTB7-Th as the electron donor for high-performance fullerene-free polymer solar cells (PSCs). The PTB7-Th/ITCT blend based bulk-heterojunction (BHJ) devices give an appreciable PCE of 8.23% and a relatively high VOC of 0.96 V without any optimization. Upon device processing with thermal annealing and the addition of 0.1% DIO additive, the active layer achieves a more ordered and crystalline morphology. Consequently, a high PCE of 10.42% has been achieved with a VOC of 0.95 V, a JSC of 15.13 mA cm−2 and a high FF of 0.72. To the best of our knowledge, the PCE of 10.42% is among the highest values reported in the literature to date for non-fullerene PSCs using PTB7-Th as the electron donor.

High performance non-fullerene polymer solar cells based on PTB7-Th as the electron donor with 10.42% efficiency

The present invention discloses a conjugate small molecule electron acceptor material in a polymer solar cell blending active layer, and a preparation method thereof. The acceptor material uses a hepta-condensed ring unit as a core; and an end is an electron-withdrawing unit A-D-A conjugated small molecule. The method is mild and easy in synthesis steps; synthetic raw materials are cheap and easyto obtain; and the overall preparation is cheap in cost. The material has good solubility and film forming property; an ultraviolet-visible light absorption spectrum indicates that an optical band gapof the acceptor material is moderate (1.6-1.7 eV); the conjugate small molecule electron acceptor material has good visible light absorption at 600-750 nm and has a moderate LUMO energy level (-3.93eV); organic solar cells with ITCT as the acceptor material exhibit extremely high photoelectric conversion efficiency (10.28%); and the conjugate small molecule electron acceptor material has great potential in high-efficiency non-fullerene solar cells.

Jia Sun

Papers

Dithieno[3,2-b:2',3'-d]pyrrol Fused Nonfullerene Acceptors Enabling Over 13% Efficiency for Organic Solar Cells.

Conformation Locking on Fused-Ring Electron Acceptor for High-Performance Nonfullerene Organic Solar Cells

High performance non-fullerene polymer solar cells based on PTB7-Th as the electron donor with 10.42% efficiency

Conjugate small molecule electron acceptor material in polymer solar cell blending active layer, and preparation method thereof