Douglas M. Bishop

Reducing the interfacial defect density of CZTSSe solar cells by Mn substitution

Cation disorder which arises from the size and chemical environment similarity of Cu and Zn is the limiting factor in Cu2ZnSnSxSe4−x (CZTSSe) performance. Cation substitution is one effective way to solve this issue, however, the most commonly reported substitutes, Ag and Cd, are not ideal as they detract from the earth-abundant and non-toxic motivation of CZTSSe. Mn is a promising candidate in comparison with other candidates (e.g. Fe, Ni or Co), because of its oxidation state stability and larger ionic size mismatch with Cu. In this study, Cu2MnxZn1−xSn(S,Se)4 (CMZTSSe) thin film solar cells were prepared by chemical spray pyrolysis and a subsequent selenization process. We study the influence of Mn substitution on the morphological, structural, optical, electrical and device properties. A distinct phase transformation from CZTSSe kesterite to C(M,Z)TSSe stannite is observed at 20% Mn substitution. A high amount of Mn substitution (x ≥ 0.6) is shown to increase the carrier density significantly which introduces more defects and non-radiative carrier recombination as shown by quenched photoluminescence intensity. Consequently, reduction in device performance is observed for these samples. The highest power conversion efficiency is achieved at x ≈ 0.05 with η = 7.59%, Voc = 0.43 V, Jsc = 28.9 mA cm−2 and FF = 61.03%. The improved open circuit voltage (Voc) and fill factor (FF) are attributed to the improved shunt resistance and carrier transport due to low defect density especially at the CdS/CMZTSSe interface. Finally, based on our electrical characterization, a few suggestions to improve the efficiency are proposed.

Ultrathin high band gap solar cells with improved efficiencies from the world’s oldest photovoltaic material

Selenium was used in the first solid state solar cell in 1883 and gave early insights into the photoelectric effect that inspired Einstein’s Nobel Prize work; however, the latest efficiency milestone of 5.0% was more than 30 years ago. The recent surge of interest towards high-band gap absorbers for tandem applications led us to reconsider this attractive 1.95 eV material. Here, we show completely redesigned selenium devices with improved back and front interfaces optimized through combinatorial studies and demonstrate record open-circuit voltage (VOC) of 970 mV and efficiency of 6.5% under 1 Sun. In addition, Se devices are air-stable, non-toxic, and extremely simple to fabricate. The absorber layer is only 100 nm thick, and can be processed at 200 ˚C, allowing temperature compatibility with most bottom substrates or sub-cells. We analyze device limitations and find significant potential for further improvement making selenium an attractive high-band-gap absorber for multi-junction device applications.Wide band gap semiconductors are important for the development of tandem photovoltaics. By introducing buffer layers at the front and rear side of solar cells based on selenium; Todorov et al., reduce interface recombination losses to achieve photoconversion efficiencies of 6.5%.

Modification of defects and potential fluctuations in slow-cooled and quenched Cu2ZnSnSe4 single crystals

Recent literature reports have shown the ability to manipulate Cu-Zn cation ordering for Cu2ZnSnSe4 (CZTSe) via low temperature treatments. Theoretical arguments suggest that one of the major roadblocks to higher VOC—significant band tailing—could be improved with increased cation order; however, few direct measurements have been reported and significant device improvements have not yet been realized. This report investigates electrical properties, defects, and devices from quenched and slow-cooled single crystals of CZTSe. The extent of disorder was characterized by Raman spectroscopy as well as x-ray diffraction, where the change in Cu-Zn order can be detected by a changing c/a ratio. Quenched samples show higher acceptor concentrations, lower hole mobilities, and a lower-energy photoluminescence (PL) peak than crystals cooled at slower rates, consistent with a reduction in the bandgap. In addition, samples quenched at the highest temperatures showed lower PL yield consistent with higher quantities of d...

Passivation and thickness control of highly efficient kesterite solar cells

Kesterite Cu2ZnSn(SxSe1−x)4 (CZTSSe) is an attractive photovoltaic absorber material because of its tunable bandgap, earth abundance, and low toxicity. However, efficiency and open circuit voltage remain significantly below theoretical limits. We recently showed that back-contact engineering with MoO3/Au on exfoliated vapor-deposited kesterite solar cells can improve device performance. Here, we demonstrate more promising results, which translate into high power conversion efficiencies of up to 12.2% for solution-deposited CZTSe with thicknesses as low as 1.1 μm. Time-resolved terahertz spectroscopy of exfoliated films showed significantly faster recombination at the back surface than at the front. When atomic layer deposited Al2O3 was used to passivate the exposed back surface of exfoliated films, front and back surfaces showed nearly identical recombination dynamics. After thermally depositing high work function MoO3 and reflective Au as the back contact on the Al2O3-passivated absorber, we obtained devices with efficiencies of up to 11.6%. Applying the same strategy of exfoliating working devices and engineering the back contact resulted in efficiencies of up to 12.2% for passivation with a 10 nm layer of Se instead of Al2O3. Further development of such passivation and back-contact engineering approaches may lead to higher efficiency devices with absorber thicknesses below 1 μm.

Optimization of Silver-alloying for improved photovoltaic properties of CZTSSe

Cu2ZnSn(SxSe1-x)4 (CZTSSe) photovoltaic devices are primarily limited by low open-circuit voltages, believed to be caused in part by disorder on the Cu/Zn sublattice. We show that band tailing in the absorber can be addressed by alloying Ag into CZTSSe to replace Cu. We fabricate thin films across the full alloy range of pure-Cu to pure-Ag and show that as Ag is alloyed into the CZTSe system, the gap between the PL and optical bandgap shrinks from ~110 meV for pure CZTSe, to 0 meV for the pure Ag compound. Improved device efficiencies up to 10.2% are achieved with low Ag-ACZTSe samples. Unlike CZTSSe champion devices the current record ACZTSe device appears to be limited by the interface rather than bulk defects. To achieve optimal efficiency, the annealing conditions were modified to accommodate the significantly lower melting point of the Ag-containing compound. The range of optimal temperatures across the Cu-Ag-alloy range are shared to accelerate improved device efficiencies.