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.

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...

ABSTRACT:We report the synthesis and polymerization of a novel thieno[3,2-b]thiophene-diketopyrrolopyrrolebased monomer. Copolymerization with thiophene afforded a polymer with a maximum hole mobility of 1.95 cm 2 V -1 s -1 , which is the highest mobility from a polymer-based OFET reported to date. Bulk-heterojunction solar cells comprising this polymer and PC 71 BM gave a power conversion efficiency of 5.4%. T here is considerable interest in the synthesis of narrow-bandgap conjugated polymers for use in organic photovoltaic (OPV) and organic field-effect transistor (OFET) devices. Their solution processability and mechanical properties allow access to a new generation of cheap and flexible transistors and solar devices. Current state of the art polymeric materials have allowed for the fabrication of OFETs with mobilities of ∼1 cm 2 V -1 s -1 1 and OPV devices with power conversion efficiencies (PCEs) of over 7%.2 Diketopyrrolopyrrole (DPP)-based copolymers have emerged as extremely attractive materials for both thin-film transistors and solar cell devices in recent years. The DPP core's electrondeficient nature has been exploited for the synthesis of extremely narrow band gap donor-acceptor-type materials that are wellsuited for use in OPVs with high PCEs reported from both small molecules and polymers.3-6 Furthermore, the planarity of the DPP skeleton and its ability to accept hydrogen bonds (and other types of electrostatic interactions) result in copolymers that encourage π-π stacking. Typically, these DPP-based copolymers are prepared via either Suzuki or Stille coupling of the 3,6-bis(5-bromothiophen-2-yl)-2,5-dialkylpyrrolo[3,4-c]pyrrole-1,4-(2H,5H)-dione core with the desired comonomer, which as a result means that the electron-deficient DPP unit is always flanked by two thiophenes. Variation of the comonomer has yielded polymers with extremely attractive properties for both OPV and OFET devices. For instance, copolymerization with thiophene derivatives or benzothiadiazole resulted in polymers with impressive ambipolar charge-carrier mobilities. 7,8 Recently, a copolymer of 3,6-bis(5-bromothiophen-2-yl)-N,N 0 -bis(2-octyl-1-dodecyl)-1,4-dioxopyrrolo[3,4-c]pyrrole and 2,5-bis(trimethylstannyl)thieno[3,2-b]thiophene showed an impressive hole mobility of ∼0.9 cm 2 V -1 s -1 . 9 In terms of materials for OPVs, copolymerization of the same DPP monomer (albeit having a slightly different solubilizing alkyl chain) with phenylene-1,4-diboronic acid bispinacol ester afforded a polymer that was used to fabricate OPV devices with efficiencies of ∼5.5%. 10Despite the significant number of reports on DPP-based copolymers, there have been very few studies on the effect of modifying the 3,6-bis(5-bromothiophen-2-yl)-2,5-dialkylpyrrolo-[3,4-c]pyrrole-1,4-(2H,5H)-dione monomer. One very recent example was the replacement of the flanking thiophenes by furans, which resulted in polymers with high PCEs in solar cell devices. 11 We were interested in increasing the intermolecular association of DPP-based copolymers by repla...

High-performance, solution-processed transistors fabricated from semiconducting polymers containing indacenodithiohene repeat units are described. The bridging functions on the backbone contribute to suppressing large-scale crystallization in thin films. However, charge carrier mobilities of up to 1 cm(2)/(V s) for a benzothiadiazole copolymer were reported and, coupled with both ambient stability and long-wavelength absorption, make this family of polymers particularly attractive for application in next-generation organic optoelectronics.