Department of Electrical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
IMEC, Leuven B-3001, Belgium.
Nat Commun. 2014;5:3245. doi: 10.1038/ncomms4245.
Charge transfer (CT) states at a donor-acceptor heterojunction have a key role in the charge photogeneration process of organic solar cells, however, the mechanism by which these states dissociate efficiently into free carriers remains unclear. Here we explore the nature of these states in small molecule-fullerene bulk heterojunction photovoltaics with varying fullerene fraction and find that the CT energy scales with dielectric constant at high fullerene loading but that there is a threshold C60 crystallite size of ~4 nm below which the spatial extent of these states is reduced. Electroabsorption measurements indicate an increase in CT polarizability when C60 crystallite size exceeds this threshold, and that this change is correlated with increased charge separation yield supported by CT photoluminescence transients. These results support a model of charge separation via delocalized CT states independent of excess heterojunction offset driving energy and indicate that local fullerene crystallinity is critical to the charge separation process.
在有机太阳能电池的电荷光生过程中，给体-受体杂化结的电荷转移 (CT) 态起着关键作用，然而，这些态有效地离解为自由载流子的机制仍不清楚。在这里，我们研究了具有不同富勒烯分数的小分子-富勒烯体异质结光伏中的这些态的性质，发现 CT 能量在高富勒烯负载下与介电常数相关，但在富勒烯微晶尺寸低于约 4nm 时，这些态的空间范围减小。电吸收测量表明，当 C60 微晶尺寸超过此阈值时，CT 极化率增加，并且这种变化与 CT 光致发光瞬变支持的增加的电荷分离产率相关。这些结果支持了通过离域 CT 态进行电荷分离的模型，该模型与过量异质结偏移驱动力无关，并表明局部富勒烯结晶度对电荷分离过程至关重要。
