Intensity-modulated spectroscopy on loaded organic photovoltaic cells

We configured a generic digital lock-in amplifier as a light intensity-modulated spectrometer for photovoltaic (PV) cells for intensity-modulated spectroscopy (IMS) up to 250 kHz. We performed IMS on a state-of-the-art bulk heterojunction (BHJ) organic PV (OPV) cell and introduced a new mode of IMS wherein PV cells work under finite load, including maximum power point (MPP). Quantitative analysis supported by equivalent circuit simulations establishes MPP-IMS as favourable alternative to the commonly used intensity-modulated photovoltage/ photocurrent spectroscopy (IMVS/IMPS) modes. Using IMS under finite load, we identify a high frequency feature that is invisible in both IMPS and IMVS. The feature is ageing-related and becomes more prominent after long-term storage. We propose an extended equivalent circuit model that locates the origin of this feature at the BHJ itself, and link it to diffusion of indium ions etched from the transparent electrode by the hole extracting PEDOT:PSS. Finally, we introduce a method to determine BHJ capacitance by IMS without absolute calibration of light intensity.