Wide-Bandgap Selenium-Based Solar Cells for Tandem Device Applications

The work and research conducted during this PhD project was motivated by the concept of a photoelectrochemical water splitting device. A photoelectrochemical device offers the direct conversion of solar energy into stored chemical energy in the form of chemical bonds. This may bridge the gap between intermittent solar power and the requirement for renewable energy storage within one single device. The most promising approach for photo water splitting (photoelectrolysis) is the monolithic tandem that requires two different semiconductor absorbers that may each capture different parts of the solar spectrum. The silicon solar cell dominates the photovoltaics market and is a suitable bottom-cell of the tandem. A suitable top-cell contender is yet to be fully determined. Selenium was the world’s first photovoltaic material to be discovered but was, due to its large bandgap of ≈1.9 eV, outshone by silicon. The large bandgap of trigonal selenium makes it inefficient as a single-junction solar cell, but highly suitable as a wide-bandgap top-cell absorber. The development of a monolithically grown Se-Si tandem is further motivated by the low melting point of selenium (220 °C) that allows for low temperature fabrication, which both minimises damage to the silicon and potentially allows for cheap and scalable manufacturing.

Thus efforts were made to develop efficient and reproducible top-cell seleniumbased heterojunction photovoltaic devices. Following in the footsteps of literature a champion device efficiency of 6.4% was achieved which lies merely 0.1% below the world record of 6.5% from 2017. For incorporation in a tandem Se-Si device, both sides of the top-cell must be transparent so that light with energies below the Se top-cell bandgap is transmitted to the silicon bottom-cell. This requires both sides of the device to be semi-transparent, which was achieved by replacing the typical gold metal contact with an In₂O₃:Sn transparent thin film. This work led to the first reported bifacial selenium single-junction solar cell from which a champion device could demonstrate a state of the art power conversion efficiency of 5.2% when illuminating through the n-type contact and 2.7% when illuminating through the p-type contact. This difference in performance is attributed to present low charge carrier lifetimes and mobilities in selenium which suggest inverting the typical device structure when incorporating it into a tandem device. The few inversion attempts of this work were not successful so further future investigations are highly encouraged. Regardless, an optimum selenium thickness was found to be around 300-500 nm for both illumination directions.

Even without the inversion of the selenium top-cell, attempts were still made to develop Se-Si tandem photovoltaic devices, that culminated in the first ever reported Se-Si tandem with a power conversion efficiency of 2.2%. The main reasons for the rather poor performance are yet to be identified and requires further research.

Finally, various studies were performed on selenium in order to identify and estimate some of its charge carrier transport limitations. Time-resolved terahertz spectroscopy reveals a low photoexcited charge carrier lifetime in the order of 3 ns and a low charge carrier mobility of around 5 cm²/Vs that results in a charge carrier diffusion length of around 200 nm. Low photoluminescence signals of selenium were only obtainable below 50 K, that along with the low lifetime are attributed to the existence of defects in its bandgap. From capacitance-voltage profiling of a single-junction device, an acceptor density of selenium is extrapolated to be around 9 × 10¹⁵ cm⁻³ as well as a depletion region width of around 200 nm. These findings coincide well with the measured selenium thickness optimum of around 300-500nm. Some electronic structure calculations are presented which indicate charge carrier transport limitations due to large effective carrier masses and density of states of selenium. Ultimately, calculations and simulations are performed and compared with the findings of the work to estimate the main present device limitations.