On efficiency of earth-abundant chalcogenide photovoltaic materials buffered with CdS: the limiting effect of band alignment

Abstract

Earth-abundant and environmentally-friendly Cu 2 -II-IV-VI 4 (II=Sr, Ba; IV=Ge, Sn; VI=S,Se) are considered materials for the absorber layers in thin film solar cells. Attempts to understand and improve optoelectronic properties of these newly emerged absorbers resulted in an efficiency of 5.2% in less than two years. However, the energy band alignment at the buffer/absorber interface has not been studied yet; an information which is of crucial importance for designing high performance devices. Therefore, current study focuses on the band offsets between these materials and the CdS buffer. Using first-principles calculations, band discontinuities are calculated at the buffer/absorber interface. The results yield a type-II band alignment between all Cu 2 -II-IV-VI 4 absorbers and CdS, hence a negative DE c . Adoption of a negative DE c (cliff-like conduction band offset) at the buffer/ absorber interface, however, gives rise to low open circuit voltage and high interface-related recombinations. Therefore, it is necessary to search for an alternative buffer material that forms a type-I band alignment with these absorbers, where the conduction band minimum and the valence band maximum are both localized on the absorber side.

Spiers Memorial Lecture: Next generation chalcogenide-based absorbers for thin-film solar cells

“…, short bulk minority carrier lifetime), but also by the usage of the standard CdS buffer layer, which leads to a high degree of interface recombination and further limitation of V oc . This finding also agrees with theoretical investigation of band alignment between CBTS (as well as other Cu 2 –II–IV–X 4 compounds) and CdS using first-principles calculations, 131 which shows that heterojunctions between CBTS and CdS form undesirable large negative CBO values.…”

Inorganic Lead-Free B-γ-CsSnI3 Perovskite Solar Cells Using Diverse Electron-Transporting Materials: A Simulation Study

“…Besides, the valence band offset at the ETL/CsSnI 3 interface can generate a hole barrier to prevent the hole in CsSnI 3 from moving to ETLs. However, as the electron affinity of SnO 2 is too large for the PSC with SnO 2 , there are cliff-like band offsets in the conduction band at the SnO 2 /CsSnI 3 interface (Δ E c = χ­(CsSnI 3 ) – χ­(SnO 2 ) = 3.62 – 4.5 = −0.88 eV, Δ E v = Δ E c + E g (CsSnI 3 ) – E g (SnO 2 ) = −0.88 + 1.3 – 3.5 = −3.08 eV), which can give rise to high interface-related recombination …”

“…The PSCs with SnO 2 exhibit the lowest efficiencies in all of the simulated PSCs. It is attributed to their cliff-like band offsets at the SnO 2 /CsSnI 3 interface (see Figure ), which give rise to high interface-related recombination and low fill factor (FF) . When the thickness of absorbers (CsSnI 3 ) increases, recombination in the absorbers and reverse saturated current increases.…”

“…It is attributed to their cliff-like band offsets at the SnO 2 /CsSnI 3 interface (see Figure 2), which give rise to high interface-related recombination and low fill factor (FF). 23 When the thickness of absorbers (CsSnI 3 ) increases, recombination in the absorbers and reverse saturated current increases. Therefore, V oc decreases with increasing CsSnI 3 thickness.…”

Recent Progress on Cu2BaSn(SxSe1–x)4: From Material to Solar Cell Applications

“…However, Ghorbani et al predicted that the CdS/CBTS is not the best p-n junction due to the difficulty in the formation of beneficial spike-like band energy. [110] Recombination is always suppressed by the moderate spike-like offset (0.0-0.3 eV) where the conduction band of the absorber is below that of the buffer layer. However, cliff-like offset band alignment can be formed where the conduction band of the absorber is above that of the buffer layer.…”