The electrical conversion and storage of solar energy is a crucial world target in the long-term scenario. Therefore, the use and integration of photovoltaic (PV) technologies into different electrochemical processes for obtaining solar fuels have been strongly developed in the last years. Following this approach, the integration of PV and other energy storage systems such as batteries is a logical approach that pursues simplification of capture and storage of the solar energy through direct conversion into (electro)chemical energy. These so-called solar batteries offer the advantage of carrying out in a single device, a process normally done in several steps in two independent units, which result expensive, bulky and inefficient. Among several other configurations, the application of this concept to redox flow batteries has attracted attention considering their advantages, including decoupling of energy and power and large-scale development.

Despite the obvious interest in these systems, the direct influence of the redox potential (i.e., selected redox pair) into the operation point of the PV, constitutes an important challenge in the design of such devices. Therefore, some studies have focused on metal oxides and/or on PV tandem configurations, for instance CdS/DSSC, being applied to organic redox pairs and/or to vanadium redox flow batteries (VRFB) reaching limited state of charge (SoC).

In this work, we report the adaptation and integration of thin film PV to VRFB in a single device. Copper Indium Gallium Selenide (CIGS) modules have been adapted by the interconnection of CIGS commercial cells deposited on flexible metallic substrates. This way, three and four-cell modules leading to different open circuit voltages (OCV) and i-V performances were integrated in VRFB with modified carbon felt (CF) electrodes. Two kinds of batteries reaching different cell voltages were evaluated.

A very close dependence between the VRFB cell potential and the photocurrent of the photovoltaic system has been observed, ultimately influencing the overall capacity of the battery. In particular, there is a clear difference between using 3 or 4 CIGS cells, due to the different operation points. Therefore, it is obvious that adapting of the PV is necessary before its integration. Ultimately, the four-cell module has provided enough photovoltage for carrying out the unbiased photo-assisted charge of the battery up to SoC 100%, with acceptable charge time and overall round-trip energy conversion efficiency of around 4%.