Cu₂Zn(Sn,Ge)(S,Se)₄ Wide Bandgap Kesterite for Indoor Photovoltaic Application

Indoor photovoltaic (IPV) cells have the potential to power distributed and remote sensors, actuators, and communication devices, enabling the widespread implementation of the Internet of Things (IoT). Commercial (c-Si, CIGS, CdTe) and emerging (perovskite, organic solar cells) photovoltaic technologies face several challenges for indoor applications, including cost, toxicity, stability, and/or spectral mismatch. In contrast, kesterite materials are composed of earth-abundant, non-toxic elements and exhibit excellent stability, with a wide bandgap tunability between 1.0 eV up to 2.1 eV. This technology has recently achieved certified efficiencies of 14.9% under AM1.5G for the narrow band gap (Se-rich; 1.1 eV) compound and 13.1% for wider bandgap (1.5 eV), demonstrating its high efficiency potential.<br/>In this work, we present the first complete theoretical and experimental study of the behavior of kesterite solar cells under indoor illumination conditions, with different high efficiency devices. First, we will show that the most relevant and mature photovoltaic technology, that is c-Si, expectedly drops the performance in indoor conditions due to its narrow band gap. Hence, the use of wide band gap materials is required for a good performance in indoor conditions. We will show that, under indoor conditions, the wide band gap (S-pure; 1.5 eV) can achieve efficiencies above 20%.<br/>The experimental results demonstrate the excellent optoelectronic properties under low injection conditions, showing that the efficiency of narrow bandgap (12% efficiency in AM1.5G conditions) is practically unchanged with the AM1.5G light intensity down to 0.2 suns. To further support the high potential of kesterite materials for low injection applications we studied the narrow bandgap device performance under simulated indoor conditions by red shifting the typically used indoor spectra, corresponding to the blackbody emission at 2700K, 3000K, 4000K and 5000K. These spectra can be achieved using a calibrated LED based solar simulator. As predicted from the numerical simulation results, the devices demonstrate an outstanding efficiency over 18% for spectrally shifted conditions, and over 13% at 2700K illumination conditions for wide bandgap kesterite (Eg = 1.7 eV) (Figure 1).<br/>These results motivate the development of efficient kesterite solar cells with a wider bandgap, specifically tailored for IPV applications. The indoor performance of the wide bandgap kesterite material has been studied using the same experimental procedure, revealing a performance of above 13% under simulated indoor conditions, consistently with the numerical simulation. The charge carrier extraction is analysed with spectral response (External quantum efficiency, EQE) and the changes induced by indoor light are characterized by performing EQE under dark and indoor illumination conditions. Furthermore, to prevent interface recombination and drastically improve the IPV performance two strategies will be presented: (i) Ge alloying to further widen the bandgap and minimize the spectral mismatch, and (ii) device engineering by using passivation interlayers (e.g. Al₂O₃), adjusting the thermal post deposition treatment conditions or employing other electron-selective contacts (e.g. ZnSnO).<br/>Therefore, the pathway for achieving efficiencies over 20% for indoor kesterite solar cells will be presented. These original ideas will set the stage for affordable, bio-safe, and durable indoor solar cells. It will also provide a technical approach for the comprehensive design of other emerging PV technologies.