Abstract
Space charge effects can significantly degrade charge collection in organic photovoltaics (OPVs), especially in thick‐film devices. The two main causes of space charge are doping and imbalanced transport. Although these are completely different phenomena, they lead to the same voltage dependence of the photocurrent, making them difficult to distinguish. Herein, a method is introduced on how the build‐up of space charge due to imbalanced transport can be monitored in a real operating organic solar cell. The method is based on the reconstruction of quantum efficiency spectra and requires only optical input parameters that are straightforward to measure. This makes it suitable for the screening of new OPV materials. Furthermore, numerical and analytical means are derived to predict the impact of imbalanced transport on charge collection. It is shown that when charge recombination is sufficiently reduced, balanced transport is not a necessary condition for efficient thick‐film OPVs.
Highlights
Introduction quasineutral regionIn the quasineutral region, the internal fieldThe efficiency of organic photovoltaics (OPVs) has increased from about 10% to over 16% within only a few years.[1,2] The is screened, and charge transport is conducted by diffusion only
We present a simple and robust method on how the build-up of space charge due to imbalanced transport can be monitored in a real operating device
We show that charge collection becomes a function of the position and the collection zone varies according to an analytical model for imbalanced transport
Summary
We begin with an analytical model for a device limited by imbalanced transport. As shown by Goodman and Rose,[24] and later adapted to OPVs by Mihailetchi et al.,[21] the. Given the limited diffusion length in organic semiconductors, it is intuitively clear that many of the carriers photogenerated in the field-free region will undergo recombination instead of getting collected at the electrodes To quantify these losses, we consider the spatially resolved charge-collection probability f cðxÞ.[22,28] The collection probability serves as a weighting factor for the generation rate to predict the photocurrent, jph 1⁄4 q∫ f cðxÞGðxÞdx. The extra photocurrent due to the peak is normalized to the case without background illumination, i.e., when the device is not disturbed by space charge effects This way, the model mimics the white-light bias EQE measurements described later, and other experiments in which a fixed carrier population is probed by applying a small perturbation, such as transient photocurrent and transient photovoltage. We will introduce a method that is much more simple and requires only standard equipment available in most OPV laboratories
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