Intensity-dependent transient photocurrent of organic bulk heterojunction solar cells

An understanding of the behaviors of photo-generated charge carriers (CCs) has a crucial meaning for establishing a reliable model to describe the operating concept of photovoltaic devices. One of the most-widely used techniques to characterize transport behavior of CCs is a transient photocurrent measurement using a short laser pulse excitation. However, conventional transient photocurrent measurements, often referred to as the time-of-flight method, have a drawback due to the demand for relatively thicker active layers. This is mainly to observe a clear transit time for the CCs; however, some uncertainties can arise when the properties of a thicker active layer are adapted to the properties of conventionally thin active layer of an optimized device configuration. Therefore, we studied two models, one based on a polythiophene derivative and the other based on a narrow band-gap polymer derivative of an organic bulk heterojunction system, by using a transient photocurren t method with a conventional active layer thickness. This comparative study has shown clearly different transient photocurrent behaviors between the two systems when the excitation intensity, as well as the applied electric field, is varied.