Analysis of electrical transients in the fourth quadrant of thin films photovoltaics: The case of organic bulk heterojunction solar cell

Abstract

Current-voltage (J-V) measurements under illumination are essential for studying solar cells. They directly provide fundamental parameters of the solar cell, as the short-circuit current (JSC), the open-circuit voltage (VOC), and the fill-factor (FF); but a deeper investigation of transport phenomena along the fourth quadrant is hampered by the tight linear scale, limited between zero to ~1 V. In this work, we present transient measurements carried out in an organic solar cell (ITO/PEDOT:PSS/PTB7-Th:PC71BM/Ca/Al), which allows to analyze J-V curves, by extending the scale from a linear to a logarithmic scale. For this, we used a circuit similar to that used in transient measurements, by replacing the voltage source by a variable load resistance RL (from 50 Ω to 1 MΩ). A voltage transient ΔV(t) and its time decay τ are measured over each RL, and τ-RL curves were obtained, keeping the device under different intensities of sunlight. Each τ-RL curve is divided in three distinct zones: one in which the extraction dominates the photocurrent; the second where a competition between extraction and non-geminate recombination is established; and the third, dominated by recombination and diffusion. An equivalent circuit is used to analyze the τ-RL curves, in which the diode resistance plays a relevant role, especially in the region close to VOC. Not only recombination mechanisms can be better analyzed through this new approach, but also the distinction between drift and diffusion transports.