Inter‐Fullerene Electronic Coupling Controls the Efficiency of Photoinduced Charge Generation in Organic Bulk Heterojunctions

Abstract

energy and (ii) equally efficient collection of those charges. The former relies on appropriate choice of donor and acceptor materials that balance minimal reorganization energy with optimally aligned energy levels, while the latter principally relies on high hole and electron mobilities in their respective phases. [1] Over the past decade, significant research efforts have been devoted to gaining a better fundamental understanding of the relationship between molecular properties and the photophysics of interfacial and bulk free charge generation and transport in OPV material systems, which has directly led to increases in the performance of these materials by rational design. [2][3][4][5][6] In the case of polymer:fullerene OPVs, the polymer acts as the dominant light absorber and also directs film morphology, which are two crucial tasks for an OPV material. Naturally, a greater amount of effort has gone into understanding how structural, electronic, and physicochemical properties of the polymer affect charge generation photophysics than has been expended for the fullerene. Nonetheless, these systems are inherently bipartite, and, as we show in this report, the properties of the fullerene acceptor should be as carefully considered as those of the polymer for maximizing free carrier generation.We are particularly interested in the question of how local electronic coupling between individual fullerene molecules affects photoinduced charge generation (PCG). Some of the present authors have addressed this question previously, modulating aggregation of either the polymer or the fullerene to control PCG. [7] Here, the electronically coupled donor aggregates (polythiophene crystals) are always present. We investigate the importance of local coupling between fullerenes in high-loading OPV blends containing more complicated bulk heterojunction (BHJ) structures and interfaces, i.e., -those relevant to working devices. The observed high-frequency electron mobility in the fullerene phase is taken as a qualitative measure of the local electronic coupling strength between fullerenes.From an experimental standpoint, this question has been extremely difficult to answer in bulk heterojunction structures because of the large number of variables contributing to PCG Photoinduced charge generation (PCG) dynamics are notoriously difficult to correlate with specific molecular properties in device relevant polymer:fullerene organic photovoltaic blend films due to the highly complex nature of the solid state blend morphology. Here, this study uses six judiciously selected trifluoromethylfullerenes blended with the prototypical polymer poly(3-hexylthiophene) and measure the PCG dynamics in 50 fs-500 ns time scales with time-resolved microwave conductivity and femtosecond transient absorption spectroscopy. The isomeric purity and thorough chemical characterization of the fullerenes used in this study allow for a detailed correlation between molecular properties, driving force, local intermolecular electronic coupling and, ultimately, the ...

Single-layered organic photovoltaics with double cascading charge transport pathways: 18% efficiencies

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Single-layered organic photovoltaics with double cascading charge transport pathways: 18 % efficiencies

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