Interfacial Strain Engineering in Wide-Bandgap GeS Thin Films for Photovoltaics

Abstract

Wide-bandgap semiconductors exhibiting a bandgap of ∼1.7–1.9 eV have generated great interest recently due to their important applications in tandem solar cells as top cells and emerging indoor photovoltaics. However, concerns about the stability and toxicity especially in indoor application limit the choice of these materials. Here we report a new member of this family, germanium monosulfide (GeS); this material displays a wide bandgap of 1.7 eV, nontoxic and earth-abundant constituents, and high stability. We find that the little success of GeS solar cells to date is primarily attributed to the challenge in fabricating high-quality polycrystalline GeS films, wherein the high thermal expansion coefficient (α = 3.1 × 10–5 K–1) combined with high crystallization temperature (375 °C) of GeS induces large tensile strain in the GeS film that peels off GeS from the substrate. By introducing a high-α buffer layer between GeS and substrate, we achieve a high-quality polycrystalline GeS thin film that compactly adheres to substrate with no voids. Solar cells fabricated by these GeS films show a power conversion efficiency of 1.36% under AM 1.5G illumination (100 mW cm–2). The unencapsulated devices are stable when stored in ambient atmosphere for 1500 h. Their efficiencies further increase to 3.6% under indoor illumination of 1000 lux.

Solution-processed Ge(ii)-based chalcogenide thin films with tunable bandgaps for photovoltaics

“…2c), in good agreement with the expected binding energy in GeS. 34 No other possible Ge chemical states such as Ge 4+ in GeS 2 and GeO 2 are found in the XPS spectrum considering the perfect Gaussian-Lorentzian peak tting of Ge 3d. Similarly, S exhibits the expected valence state of S 2À in GeS (Fig.…”

“…2a shows the amorphous state of the as-deposited films, indicating that the deposition at 160 °C does not induce crystallization considering the high crystallization temperature of GeS (375 °C). 34 The as-deposited amorphous GeS films exhibit a uniform and smooth surface morphology measured from scanning electron microscopy (SEM) characterization (Fig. S4 † ).…”

“…3b ), which is thick enough to absorb much of the incident sunlight due to the large absorption coefficient of GeS (greater than 10 5 cm −1 ). 34 The thickness of GeS films can be easily regulated by varying the deposition time, where the 1.5 h, 3 h and 6 h deposition processes lead to an increased thickness from 212 nm through 463 nm to 765 nm, respectively (Fig. S8 † ).…”

“…33 GeS thin-lm solar cells have demonstrated great potential for indoor photovoltaics arising from their wide bandgap of 1.7 eV that matches well with the emission spectra of commonly used indoor light sources ranging from 400 to 700 nm. [34][35][36] Furthermore, the capacity to continuously tune the bandgap of Ge-based monochalcogenides by alloying GeS with GeSe allows the realization of optimal-bandgap single-junction solar cells and multi-junction cells that boost the ultimate limit of the photovoltaic efficiency from 33% (single-junction) to 42% (tandem). 32 However, all current GeS and GeSe thin lms have consistently been fabricated by vacuum-assisted vapor deposition that suffers from high-cost vacuum apparatus and a lack of physical exibility.…”

Surface-Defect States in Photovoltaic Absorber GeSe

“…Germanium monoselenide (GeSe) has recently drawn increasing attention in solar cell applications due to its appropriate bandgap (about 1.14 eV) for single-junction solar cells, , high absorption coefficient (>10 5 cm –1 at visible light), , large carrier mobility (about 128.6 cm 2 V –1 s –1 ), , high dielectric constant (>15, which greatly screens the impact of charged defects), and easy sublimation feature enabling the deposition of high-quality films through an industrial close-space sublimation (CSS) method. ,, This simple binary compound possesses earth-abundant and nontoxic constituents, only one fixed orthorhombic phase at room temperature, and high stability in ambient atmosphere. − In particular, GeSe has a perovskite-like antibonding valence band maximum arising from the Ge-4s and Se-4p coupling, thereby enabling the bulk-defect-tolerant properties. − …”