Dynamic behavior of photogenerated charge carriers in diketopyrrolopyrrole-linked tetrabenzoporphyrin-based bulk heterojunction thin films probed with time-resolved terahertz spectroscopy

Abstract

Understanding of charge carrier dynamics on a picosecond time scale is very important for generation and relaxation of charge carriers in organic photovoltaic solar cells. Here, we study the dynamics of photogenerated charge carriers of diketopyrrolopyrrole-linked tetrabenzoporphyrin (C4-DPP-BP) bulk heterojunction (BHJ) thin films mixed with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) by subpicosecond time-resolved terahertz (THz) spectroscopy. For the excitation at 800 nm, the decay time constants of the time-resolved THz signals are 0.5 ps and 14 ps that result from the relaxation of bound polaron pair. Furthermore, the results reveal that the amplitude of the subpicosecond decay in the time-resolved THz signal is sensitive to the fluence of the optical excitation pulse. It is considered that this decaying component at higher fluence originates from polaron pair annihilation. We also discuss the difference in carrier dynamics between neat C4-DPP-BP and the C4-DPP-BP based BHJ thin films to understand dynamic behavior of the local charge carriers.