Controlled quasi-diffuse light characterisation of photoelectrochemical devices and low concentration photovoltaic optics using an integrating sphere

Abstract Testing photoelectrochemical (PEC) and photovoltaic devices under realistic operating conditions is a critical phase of system development. However, most studies still focus on standard AM 1.5 G tests at one sun (1000 W m ⁻² ) irradiance with direct collimated light beams, which does not capture outdoor operation under broad angular illumination from diffuse skies and low solar elevation. Outdoor field trials are difficult to benchmark reproducibly due to the unpredictable nature of outdoor environments. Herein, we demonstrate a simple, laboratory-based method for generating controlled, angularly broad ‘quasi-diffuse’ illumination by coupling a continuous solar simulator to an integrating sphere. By positioning a PEC device at the integrating sphere exit aperture, we produce a controlled quasi-diffuse illumination geometry and demonstrate its use in two case studies involving WO ₃ |BiVO ₄ |NiFeOOH photoanodes: one with a planar architecture and another coupled with a crossed-compound parabolic concentrator (CCPC) with wide-angle acceptance, designed to harvest diffuse and off-normal light. After normalisation to one-sun equivalent irradiance, the non-concentrating (planar) configuration performed similarly under direct and quasi-diffuse irradiance, whereas the CCPC-coupled configuration retained 26.3% of the direct-illumination photocurrent under quasi-diffuse conditions, consistent with acceptance-angle limited concentration. Whilst demonstrated here for non-concentrating and CCPC-coupled WO ₃ |BiVO ₄ |NiFeOOH photoanodes, this method is in principle extendable to other compact planar photo-absorbers and low concentration non-imaging optics, provided that their dimensions are compatible with the exit aperture of the integrating sphere and their operation remains meaningful at the reduced irradiance available after sphere losses. This work provides a practical, controllable and transferable framework for quasi-diffuse light characterisation that complements AM 1.5 G measurements and can be benchmarked across different research teams. It offers a simple way to assess diffuse light tolerance and to validate ray tracing models using readily available laboratory equipment, before embarking on more challenging field trials.