Performance enhancement of fullerene-based solar cells by light processing

Abstract

A key challenge to the commercialization of organic bulk heterojunction solar cells is the achievement of morphological stability, particularly under thermal stress conditions. Here we show that a low-level light exposure processing step during fabrication of blend polymer:PC 60 BM solar cells can result in a 10-fold increase in device thermal stability and, under certain conditions, enhanced device performance. The enhanced stability is linked to the light-induced oligomerization of PC 60 BM that effectively hinders their diffusion and crystallization in the blend. We thus suggest that light processing may be a promising, general and cost-effective strategy to optimize fullerene-based solar cell performance. The low level of light exposure required suggests not only that this may be an easily implementable strategy to enhance performance, but also that light-induced PC 60 BM oligomerization may have inadvertently influenced previous studies of organic solar cell device behaviour.

Increased thermal stabilization of polymer photovoltaic cells with oligomeric PCBM

“…Fullerenes tend to diffuse through layers to form large mm-scale crystals or to accumulate at active layer-electrode interfaces, 12 both occurring notably when heated above the glass transition temperature and potentially leading to reductions in device efficiencies. 11,13,14 While photo-induced reactions between PCBM molecules may lead to morphological stability, 15,16 they can also give rise to dimers that act as charge traps and reduce short circuit current ( J sc ) values. 17 PCBM is also difficult to cast as a thin film, resulting in rough surfaces of large aggregates.…”

Nanoscale mapping by electron energy-loss spectroscopy reveals evolution of organic solar cell contact selectivity

“…3 Performance of OPVs is strongly dependent on the donor/acceptor morphology 4 and variations of the internal arrangement of donor and acceptor molecules is considered to be the main source of efficiency decrease limiting their potential commercialization. 5 Numerous studies have focused on the importance of the bulk morphology properties of the organic layer. 6,7 The use of additives 8 , solvent annealing 9 and thermal treatments 10 can enhance the crystallinity and would increase the device efficiency.…”

“…16 Indeed, morphology of the active layer in BHJ solar cells is frequently the result of a meta-stable configuration and several vertical segregation profiles can be formed. 17 Thermodynamically stable morphology blends can be obtained by using high transition temperature (T g ) polymers 18 or by cross-linking the organic phase 19 . In general terms one of the issues that still require intensive work is the understanding of specific mechanisms behind active layer/outer contact interface chemical and morphological evolution and its relationship to device performance.…”

Impact of solution phase behaviour and external fields on thin film morphology: PCBM and RRa-P3HT model system

“…The impact of light exposure in hindering fullerene crystallisation within a polymer matrix has been previously reported and linked to a fullerene oligomerization process (in the absence of oxygen). 19 Our Figure 10. Optical micrographs of 3:7 RRa-P3HT:PCBM thin films cast from a 30 and 20 mg/ml PCBM in CB solution prepared at room temperature on substrates exposed to UVO for 15 and 60 min respectively.…”