Reducing the efficiency–stability–cost gap of organic photovoltaics with highly efficient and stable small molecule acceptor ternary solar cells

Abstract

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...

Impact of backbone fluorination on nanoscale morphology and excitonic coupling in polythiophenes

“…Furthermore, it has been shown that fluorination often leads to enhanced thermal and oxidative stability (9)(10)(11). Given a combination of these desirable properties, fluorine-containing CPs have led to most of the best-performing polymer-based solar cells to date (12)(13)(14)(15).…”

Organic Photovoltaics over Three Decades

“…[253] Ternary all-polymer systems with 9% PCE are reported. Here the role of the two rather similar acceptors is to influence the aggregation of the acceptors, a strategy that has been successful.…”

“…They have been integrated as acceptors in polymer blends earlier, [257] but have never given the same high PCEs now reported. [253] These are formally ternary blends, with two different acceptors. It is with the continued fine-tuning of chemical structure of these, through judicious choice of the location of substituents, that the balance between aggregation and dispersion can be found.…”

A New Electron Acceptor with meta‐Alkoxyphenyl Side Chain for Fullerene‐Free Polymer Solar Cells with 9.3% Efficiency

“…Zhan's group first reported IDTT‐based acceptor (ITIC), when blended with PTB7‐Th, the device delivered a PCE of 6.8% 23. Recently, using ITIC as electron acceptor, nonfullerene PSCs have reached high performance of 9–12% 24, 25, 26, 27, 28, 29. These promising results have attracted tremendous interests to investigate IEIC and ITIC derivatives as electron acceptors.…”