Implied Voltage and Current Characterization in Organic Solar Cells using Transient Photoluminescence

Photoluminescence (PL) measurements of solar cells under operating conditions provide a powerful tool for determining their upper performance limits. In organic solar cells, however, PL typically consists of overlapping contributions from radiative free charge carrier recombination and non-dissociated radiative exciton recombination, with the latter often obscuring the former. As a result, steady state PL measurements only capture the combined effects of both radiative recombination processes. To overcome this limitation, an adaptation of time-resolved photoluminescence (tr-PL) is introduced. For this method, an electric voltage is applied in addition to the light pulse. Upon shutoff of the laser, the solar cell is also disconnected from the voltage source and the PL decay under open circuit condition is recorded. By exploiting the significantly different lifetimes of non-dissociated excitons (pico- to nanoseconds) and free charge carriers (microseconds), the PL contributions of each species can be identified via fitting and extrapolation of the decays. Hence, the free charge carrier PL and non-dissociated exciton PL can be determined for a range of applied voltages. Using a state-of-the-art D18:Y6 organic solar cell (power conversion efficiency: 16.2%) we demonstrate that the implied voltage (i.e., the quasi-Fermi level separation in the absorber layer) can be determined from the free charge carrier PL under operation conditions revealing the effects of transport losses both in steady-state and time-resolved. By leveraging the measured current, a PL-based pseudo current-voltage curve is constructed, showing an implied efficiency (free of transport losses) of 18.1%. Unlike conventional methods to obtain a pseudo current-voltage curve (e.g. intensity dependent VOC measurements), this approach eliminates the need for assumptions about the photogenerated current density. In fact, when combined with electroluminescence measurements, the method enables a quantification of the photogenerated current density in the voltage range between the maximum power point and VOC, revealing a 5% reduction compared to the short-circuit current density. Analysis of the PL originating from non-dissociated excitons suggests field-dependent exciton dissociation to be the cause of the measured voltage dependence of the photogenerated current. Therefore, this novel method represents a significant advancement in the characterization of organic solar cells, specifically for understanding exciton dissociation dynamics in low offset organic absorber systems.