Non-Fullerene Electron Acceptors for Use in Organic Solar Cells

Abstract

ConspectusThe active layer in a solution processed organic photovoltaic device comprises a light absorbing electron donor semiconductor, typically a polymer, and an electron accepting fullerene acceptor. Although there has been huge effort targeted to optimize the absorbing, energetic, and transport properties of the donor material, fullerenes remain as the exclusive electron acceptor in all high performance devices. Very recently, some new non-fullerene acceptors have been demonstrated to outperform fullerenes in comparative devices. This Account describes this progress, discussing molecular design considerations and the structure–property relationships that are emerging.The motivation to replace fullerene acceptors stems from their synthetic inflexibility, leading to constraints in manipulating frontier energy levels, as well as poor absorption in the solar spectrum range, and an inherent tendency to undergo postfabrication crystallization, resulting in device instability. New acceptors have to address these limitations, providing tunable absorption with high extinction coefficients, thus contributing to device photocurrent. The ability to vary and optimize the lowest unoccupied molecular orbital (LUMO) energy level for a specific donor polymer is also an important requirement, ensuring minimal energy loss on electron transfer and as high an internal voltage as possible. Initially perylene diimide acceptors were evaluated as promising acceptor materials. These electron deficient aromatic molecules can exhibit good electron transport, facilitated by close packed herringbone crystal motifs, and their energy levels can be synthetically tuned. The principal drawback of this class of materials, their tendency to crystallize on too large a length scale for an optimal heterojunction nanostructure, has been shown to be overcome through introduction of conformation twisting through steric effects. This has been primarily achieved by coupling two units together, forming dimers with a large intramolecular twist, which suppresses both nucleation and crystal growth. The generic design concept of rotationally symmetrical aromatic small molecules with extended π orbital delocalization, including polyaromatic hydrocarbons, phthalocyanines, etc., has also provided some excellent small molecule acceptors. In most cases, additional electron withdrawing functionality, such as imide or ester groups, can be incorporated to stabilize the LUMO and improve properties. New calamitic acceptors have been developed, where molecular orbital hybridization of electron rich and poor segments can be judiciously employed to precisely control energy levels. Conformation and intermolecular associations can be controlled by peripheral functionalization leading to optimization of crystallization length scales. In particular, the use of rhodanine end groups, coupled electronically through short bridged aromatic chains, has been a successful strategy, with promising device efficiencies attributed to high lying LUMO energy levels and subsequently large...

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

“…PBDTTT-EFT (PCE10) has recently attracted attention in polymer:NFA solar cells due to its high efficiency with fullerene derivatives and its absorption in the low energy region in the spectrum. 32 In order to validate that using two NFAs can yield state of the art photovoltaic performances, we used a low band gap polymer PCE10 with the same NFA acceptors in ternary solar cells.…”

“…PBDTTT-EFT (PCE10) has recently attracted attention in polymer:NFA solar cells due to its high efficiency with fullerene derivatives and its absorption in the low energy region in the spectrum. 32 In order to validate that using two NFAs can yield state of the art photovoltaic performances, we used a low band gap polymer PCE10 with the same NFA acceptors in ternary solar cells.The un-optimized preliminary results showed that a PCE of 11.0 ± 0.4% is achievable with a Voc of 1.03 V, a high Jsc of 17.3 mA/cm 2 and a decent FF of 0.61 using 1:0.5:0.5 PCE10:IDTBR:IDFBR ratio without the need for any processing additive or heat treatment (Table 1). These results showed that the potential of high performing polymers such as PCE10 can be boosted with A1:A2 approach using NFAs.…”

Environmentally-friendly solvent processed fullerene-free organic solar cells enabled by screening halogen-free solvent additives

“…Bulk heterojunction (BHJ) OSC, comprising a p-type semiconductor as the electron donor and an n-type semiconductor as the electron acceptor has been developed rapidly over the years [2,7,9,[17][18][19]. Great efforts have been made to improve the power conversion effi-ciencies (PCEs) of OSCs by utilizing novel materials and new processing methods over the past decades [1,[20][21][22][23][24][25][26][27][28][29][30][31].…”

A Near‐Infrared Photoactive Morphology Modifier Leads to Significant Current Improvement and Energy Loss Mitigation for Ternary Organic Solar Cells

“…Bulk‐heterojunction organic solar cells (OSCs) have received great progresses in recent years with the development of new photovoltaic materials and device engineering 1, 2, 3, 4, 5, 6, 7. In particular with the advantages of strong light‐harvesting capability and ease energy level tunability,8, 9, 10, 11, 12, 13, 14 OSCs employing nonfullerene acceptors have made breakthroughs of power conversion efficiencies (PCEs) over 13% 15, 16, 17, 18.…”