Solution-processed organic photovoltaics (OPV) offer the attractive prospect of low-cost, light-weight and environmentally benign solar energy production. The highest efficiency OPV at present use low-bandgap donor polymers, many of which suffer from problems with stability and synthetic scalability. They also rely on fullerene-based acceptors, which themselves have issues with cost, stability and limited spectral absorption. Here we present a new non-fullerene acceptor that has been specifically designed to give improved performance alongside the wide bandgap donor poly(3-hexylthiophene), a polymer with significantly better prospects for commercial OPV due to its relative scalability and stability. Thanks to the well-matched optoelectronic and morphological properties of these materials, efficiencies of 6.4% are achieved which is the highest reported for fullerene-free P3HT devices. In addition, dramatically improved air stability is demonstrated relative to other high-efficiency OPV, showing the excellent potential of this new material combination for future technological applications.

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Technological deployment of organic photovoltaic modules requires improvements in device light-conversion efficiency and stability while keeping material costs low. Here we demonstrate highly efficient and stable solar cells using a ternary approach, wherein two non-fullerene acceptors are combined with both a scalable and affordable donor polymer, poly(3-hexylthiophene) (P3HT), and a high efficiency, low band-gap polymer in a single-layer bulk-heterojunction devices. The addition of a strongly absorbing small molecule acceptor into a P3HT-based non-fullerene blend increases the device efficiency up to 7.7 ± 0.1% without any solvent additives. The improvement is assigned to changes in microstructure that reduces charge recombination and increases the photovoltage, and to improved light harvesting across the visible region. The stability of P3HT-based devices in ambient conditions is also significantly improved relative to polymer:fullerene devices. Combined with a low band gap donor polymer (PBDTTT-EFT, also known as PCE10), the two mixed acceptors also lead to solar cells with 11.0 ± 0.4% efficiency and a high open-circuit voltage of 1.03 ± 0.01V.Currently, the materials used in organic photovoltaics (OPV) are dominated by fullerene acceptors in combination with low band gap donor polymers which typically require complex and multi-step syntheses. [1][2][3][4][5] However, the commercialization of OPV requires the availability of inexpensive materials in large quantities such as poly(3-hexylthiophene) (P3HT). P3HT is readily scalable via flow or micro-reactor synthesis, even using 'green' solvents, whilst retaining a high degree of control over molecular weight and regioregularity. 6 The P3HT:60PCBM blend exhibits one of the most robust microstructures within OPV. [7][8][9] However, it has a limited open-circuit voltage (Voc) and short-circuit current (Jsc) in photovoltaic devices. 10 We have recently shown that solar cells using an alternative small molecule non-fullerene acceptor (NFA), IDTBR, when mixed with P3HT, can achieve power conversion efficiencies of up to 6.4%. 11 These results have revived interest in the use of P3HT for high performing devices and non-fullerene acceptors. [12][13][14][15][16][17][18] The combination of stability, cost and performance for P3HT:NFA devices, make them a compelling choice for commercialization of OPV compared to devices using fullerenes, for which the high costs and energy involved are prohibitive for large scale production.Recently, multi-component heterojunctions (ternary or more) have emerged as a promising strategy to overcome the power conversion efficiency (PCE) bottleneck associated with binary bulk-heterojunction (BHJ) solar cells. 3,4,[19][20][21][22][23]24 However, simultaneous increase in the Voc, Jsc and FF is a challenge in the ternary approach because of the trade-off between photocurrent and voltage. 23,25,26 Reports show ternary blends using fullerene acceptors, where the Voc is increased using a second acceptor (A2) with a higher electron affin...

Fullerenes have formed an integral part of high performance organic solar cells over the last 20 years, however their inherent limitations in terms of synthetic flexibility, cost and stability have acted as a motivation to develop replacements; the so-called non-fullerene electron acceptors. A rapid evolution of such materials has taken place over the last few years, yielding a number of promising candidates that can exceed the device performance of fullerenes and provide opportunities to improve upon the stability and processability of organic solar cells. In this review we explore the structure-property relationships of a library of non-fullerene acceptors, highlighting the important chemical modifications that have led to progress in the field and provide an outlook for future innovations in electron acceptors for use in organic photovoltaics.