Laser-Annealing and Solid-Phase Epitaxy of Selenium Thin-Film Solar Cells

Abstract

Selenium has resurged as a promising photovoltaic material in solar cell research due to its wide direct bandgap of 1.95 eV, making it a suitable candidate for a top cell in tandem photovoltaic devices. However, the optoelectronic quality of selenium thin-films has been identified as a key bottleneck for realizing high-efficiency selenium solar cells. In this study, we present an approach for crystallizing selenium thin-films using laser-annealing as an alternative to the conventionally used thermal annealing strategy. By laser-annealing through a semitransparent substrate, a buried layer of high-quality selenium crystallites is formed and used as a growth template for solid-phase epitaxy. The resulting selenium thin-films feature larger and more preferentially oriented grains with a negligible surface roughness in comparison to thermally annealed selenium thin-films. We fabricate photovoltaic devices using this strategy and demonstrate a record ideality factor of n = 1.37, a record fill factor of FF = 63.7%, and a power conversion efficiency of PCE = 5.0%. The presented laser-annealing strategy is universally applicable and is a promising approach for crystallizing a wide range of photovoltaic materials where high temperatures are needed while maintaining a low substrate temperature.

Selenium Thin-Film Solar Cells with Cadmium Sulfide as a Heterojunction Partner

“…This point is consistent with our previous publications on improving the optoelectronic quality of the selenium absorber. 20,23 However, prior to undertaking elaborate efforts in defect-engineering of selenium, it is essential to determine the appropriate substrate materials on which to grow the absorber. The results of this work demonstrate the significant impact of using ITO instead of FTO.…”

“…13,14 Selenium, the world's oldest PV material, is experiencing renewed interest as a promising wide bandgap photoabsorber for the top cell in tandem devices. [15][16][17][18][19][20] This inorganic semiconductor has a reported direct bandgap between 1.8 and 2.0 eV in its trigonal phase, which may be tuned within the range of 1.2-1.8 eV through alloying with tellurium. 21,22 Furthermore, selenium features a very high absorption coefficient (>10 5 cm -1 ) in the visible region, 23 long-term air-stability, 24,25 and its single-element composition and low melting point of 220 • C makes processing simple, potentially low cost, and compatible with most bottom cells.…”

Increasing the Collection Efficiency in Selenium Thin-Film Solar Cells Using a Closed-Space Annealing Strategy

“…Selenium thin films are predominantly synthesized by evaporating an amorphous layer of selenium, followed by a thermal annealing process to crystallize the as-deposited film. − However, the temperature window leading to the highest power conversion efficiencies (PCE) is fairly narrow, , and the high vapor pressure of selenium poses challenges in growing high-quality crystalline selenium thin films without compromising the surface morphology and forming pinholes. − …”

“…In 2019, Hadar et al supplemented this study by showing that the surface morphology changes drastically as a function of the annealing temperature, particularly within the range of 190 to 195 °C . Building on these findings, we reported a record fill factor FF = 63.7% for selenium solar cells obtained by laser-annealing the absorber through the substrate, which resulted in a crystalline selenium thin film with a negligible surface roughness . These results indicate that the collection efficiency is strongly influenced by the annealing temperature and possibly the surface morphology.…”