Asymmetric Electron Acceptors for High‐Efficiency and Low‐Energy‐Loss Organic Photovoltaics

Abstract

Low energy loss and efficient charge separation under small driving forces are the prerequisites for realizing high power conversion efficiency (PCE) in organic photovoltaics (OPVs). Here, a new molecular design of nonfullerene acceptors (NFAs) is proposed to address above two issues simultaneously by introducing asymmetric terminals. Two NFAs, BTP‐S1 and BTP‐S2, are constructed by introducing halogenated indandione (A1) and 3‐dicyanomethylene‐1‐indanone (A2) as two different conjugated terminals on the central fused core (D), wherein they share the same backbone as well‐known NFA Y6, but at different terminals. Such asymmetric NFAs with A1‐D‐A2 structure exhibit superior photovoltaic properties when blended with polymer donor PM6. Energy loss analysis reveals that asymmetric molecule BTP‐S2 with six chlorine atoms attached at the terminals enables the corresponding devices to give an outstanding electroluminescence quantum efficiency of 2.3 × 10−2%, one order of magnitude higher than devices based on symmetric Y6 (4.4 × 10−3%), thus significantly lowering the nonradiative loss and energy loss of the corresponding devices. Besides, asymmetric BTP‐S1 and BTP‐S2 with multiple halogen atoms at the terminals exhibit fast hole transfer to the donor PM6. As a result, OPVs based on the PM6:BTP‐S2 blend realize a PCE of 16.37%, higher than that (15.79%) of PM6:Y6‐based OPVs. A further optimization of the ternary blend (PM6:Y6:BTP‐S2) results in a best PCE of 17.43%, which is among the highest efficiencies for single‐junction OPVs. This work provides an effective approach to simultaneously lower the energy loss and promote the charge separation of OPVs by molecular design strategy.

Understanding the Effect of End Group Halogenation in Tuning Miscibility and Morphology of High‐Performance Small Molecular Acceptors

“…[ 1–6 ] Nonfullerene small molecular acceptors (SMAs) [ 7–29 ] are the main driving force for the recent development of the field, with power conversion efficiencies (PCEs) over 17% reported by different teams. [ 30–43 ] SMAs have many attractive properties including their strong and tunable absorption, the easily adjustable energy levels and the enhanced chemical and device stability. Most of these excellent features are enabled by the flexible and feasible synthesis of the SMAs that leads to numerous judiciously designed molecular structures.…”

Wide bandgap polymer donors for high efficiency non-fullerene acceptor based organic solar cells

“…Similar instances of high J SC (high EQE) despite incredibly low or zero T HOMO or T LUMO have been reported elsewhere. 65,[246][247][248][249][250] Voltage loss (V loss ), calculated from the difference between optical bandgap of the solar cell active layer and V OC of the device (V loss = E g /q -V OC ), is the key concern for obtaining the maximum performance from a given pair of WBG polymer donor and NFA. 248,250 This voltage loss (also referred to as energy loss, E loss = eV loss ) can be minimized by increasing the difference between E LUMO of the acceptor and E HOMO of the donor as the V OC of OSCs is proportional to this difference.…”

Influences of Quinoid Structures on Stability and Photovoltaic Performance of Nonfullerene Acceptors

“…[ 30–35 ] The power conversion efficiencies (PCEs) of Y‐series NFA–based OSCs have been boosted to over 17% by material optimization, [ 36–38 ] or using ternary blends. [ 39–41 ] Thereby, Y‐series NFAs have become the best performing electron acceptors for OSCs to date.…”