Fully Transparent Photovoltaic Devices Based on ZnO_1-xS_x Absorbers Compatible with 3D Design—Study of the Impact of the MoO_x Hole Transport Layer

The development of Transparent Photovoltaic (TPV) devices has gained significant attention in recent years due to their potential for seamless integration into Building-Integrated Photovoltaics (BIPV). BIPV has been identified as a key enabling technology for the development of "Near Zero Energy Buildings" (NZEB), achieved through the integration of a new generation of PV modules capable of replacing traditional architectural elements. TPV devices are also of interest in other applications, such as Product-Integrated Photovoltaics (PiPV), and any scenarios where aesthetics are important. TPV devices have the potential to revolutionize photovoltaic technology by enabling on-site generation while minimizing visual impact. However, a major challenge in the development of TPV technology, as well as for many PV second generation technologies, has been the low efficiency achieved, which limits their overall power output.

One of the most promising approaches to achieve transparent PV is to utilize the ultraviolet region of the solar spectra, while avoiding absorption in the range of the photopic response of the human eye, thus attaining complete transparency. One promising approach to achieve high transparency on solar cells is by using wide bandgap materials with bandgap energy (E_g) above 3.0 eV. However, the choice of absorber material is critical, particularly for UV-selective solar cells, due to the lower amount of photons in this spectral region, making it a must to have a good spectral match with the bandgap and the UV onset to maximize photon absorption. In this way, ZnO_1-xS_x has been proposed as an ideal UV absorber thanks to a composition-tunable bandgap that can shift from 3.2 eV (pure ZnO) down to 2.7 eV by anionic substitution of oxygen by sulfur (x=0.5). It is noteworthy that ZnO_1-xS_x is a material composed of earth-abundant raw elements, compatible with scalable fabrication processes and synthesized at low temperatures, which potentially allows minimizing the carbon footprint, economic costs and energy expenditure associated with device manufacturing.

In this work we present the first fully oxide based solar cells with structure SLG/FTO/MoO_x/ZnO_1-xS_x/ZnO/AZO/AZO. Being the first full ALD compatible process which enables high process reproducibility and conformity for the coverage of 3D structures. Moreover, the maximum process temperature below 200 ^oC making it compatible with polymeric substrates. Additionally, we explore a novel approach to significantly increase the open-circuit voltage (V_oc) of the ZnOS TPV solar cells by the fine tuning of the MoO_x HTL, achieving V_oc records for these devices close to the 800 mV. The Average Photopic Transmittance (APT) of the complete structure has a value of 73%, showing indeed excellent transparency. Optical, structural and electrical characterization of the absorber and devices will be presented (Spectrophotometry, Raman, SEM, J-V under AM1.5G and LED illumination).