Determination of defect states and surface photovoltage in PTB7:PC71BM based bulk heterojunction solar cells

Defects formed during the device processing acts as recombination centers and shows adverse effects on the photovoltaic performance of the organic solar cells (OSCs). In this manuscript, a study on the effect of defect states on the characteristics of bulk heterojunction (BHJ) solar cells, consisted of unannealed or thermally annealed active layer (ActL) of PTB7:PC71BM blend, have been performed by using the illumination intensity-dependent current-voltage and frequency dependent impedance spectroscopy measurements. Our results indicate that the photovoltaic performance of devices based on annealed ActL is superior than the bench dried ActL due to thermal annealing induced improvement in the thin film morphology. The measured low-frequency capacitance-voltage (C–V) characteristics under varying illumination intensities show strong dependency on the photoexcitation however, the change in the C–V characteristics is found to be quite different for unannealed and annealed devices. The C–V characteristics are analyzed using the drift-diffusion model to extract the effective built-in voltage (Vbi) and the surface photovoltage as a function of illumination intensity. The capacitance-frequency characteristics under illumination reveal that the change in the capacitance is associated with the photo-induced carrier occupation in the intermixture donor-acceptor phases, which act as the defect states in such devices. Further, it is modeled to quantify the defect states and to demonstrate the importance of thermal annealing for improving the OSCs device performance.