Lynda Kaye Johnson

Studies of the fine-grain sub-layer in the printed CZTSSe photovoltaic devices

Cu2ZnSn(S, Se)4 (CZTSSe) is a promising low-cost and earth-abundant photovoltaic (PV) absorber material. Using binary and ternary chalcogenide nanoparticles as precursors, we have developed a chemical solution route to produce CZTSSe PV devices with the device efficiency as high as 8.8%. The cross-sectional view of the CZTSSe film shows an interesting bilayer microstructure which consists of an upper micrometer-sized polycrystalline (large-grain) layer and a lower layer with a dense, smooth bottom amorphous (fine-grain) morphology. In this paper, we present the composition and properties of the layers and our understanding on the fine-grain layers impact on the device performance. Based on the observed optoelectronic properties, a numerical model for the CZTSSe-based PV device is developed, and the simulation results show that the fine-grain layer does not affect the devices performance. An experimental design of CZTSSe PV devices with different thicknesses of the fine-grain layers has also confirmed our findings.

A simple solution-based route to high-efficiency CZTSSe thin-film solar cells

Solution-based Cu2ZnSn(S,Se)4 (CZTSSe) thin film deposition routes focusing on chemistries with attractive scalability and handling characteristics, while delivering high efficiency devices are promising technologies for low-cost solar cells. In this paper we describe a new approach for the fabrication of CZTSSe thin films using inks comprised of mixtures of binary and ternary metal-chalcogenide nanocrystals dispersed in simple organic solvents. The resulting blended inks can be coated under ambient conditions to give precursor films which are in turn converted to device-quality CZTSSe absorbers via thermal annealing in a selenium atmosphere. Using this approach we have demonstrated CZTSSe solar cells with total area efficiencies in excess of 8.5% (or 9.6% per active area) under 1 Sun AM1.5 illumination.

Characterization of CZTSSe photovoltaic device with an atomic layer-deposited passivation layer

We describe a CZTSSe (Cu2ZnSn(S1−x,Sex)4) photovoltaic (PV) device with an ALD (atomic layer deposition) coated buffer dielectric layer for CZTSSe surface passivation. An ALD buffer layer, such as TiO2, can be applied in order to reduce the interface recombination and improve the devices open-circuit voltage. Detailed characterization data including current-voltage, admittance spectroscopy, and capacitance profiling are presented in order to compare the performance of PV devices with and without the ALD layer.

Characterization and understanding of performance losses in a highly efficient solution-processed CZTSSe thin-film solar cell

We present results on the characterization of a highly efficient CZTSSe solar cell fabricated using a solution-based process, aiming to gain a better understanding of its efficiency-limiting causes. Under red light illumination, we observed a red-kink in the current-density versus voltage (J-V) curve, likely due to a persistent photoconductivity in the buffer layer. Temperature-dependent J-V analysis suggests that interface recombination is the dominant loss mechanism. Defect analysis using admittance spectroscopy (AS) shows a single bulk defect level at ~63 meV and may be attributed to copper vacancy (VCu). The carrier concentration of the device determined using drive-level capacitance profiling (DLCP) is ~2.5×1016 cm-3.

Effect of the selenization process on structural and device properties of nanoparticle-derived CZTSSe thin films

Cu2ZnSn(S,Se)4 thin films and solar cells were fabricated using binary and ternary nanoparticle precursor films selenized using three different processes. Cross-sectional SEM images of the resulting CZTSSe solar cells do not show significant difference in microstructure. Detail analyses of the CZTSSe solar cells using current-voltage (J-V), external quantum efficiency (EQE), temperature-dependent J-V, and admittance spectroscopy (AS) were conducted to investigate the effect of the selenization process on the optical and electrical properties of the absorber.

Impact of humidity exposure during device fabrication on CZTSSe solar cell performance

The effect of exposing Cu2ZnSn(S,Se)4 films to ambient humidity during various stages of device fabrication was studied. Cu2ZnSn(S,Se)4 films were made via solution-based processing of binary and ternary metal-chalcogenide nanoparticle precursors. Following the formation of precursor films, storage conditions between successive steps for device completion were varied. The performance of finished devices showed little sensitivity to overnight storage conditions with the exception of exposure to high relative humidity prior to the p-n junction formation, which resulted in catastrophic efficiency loss. The time evolution of the moisture degradation was exponential, with a characteristic time of approximately 2 hours.

Characterization of printed CZTSSe films for photovoltaic applications

Cu2ZnSn(S, Se)4 (CZTSSe) is a promising alternative absorber material for thin-film photovoltaic applications because of its earth-abundant constituents, tunable band gap, and high optical absorption coefficient. Using binary and ternary chalcogenide nanoparticles as precursors we have developed a chemical route to produce high efficiency CZTSSe photovoltaic (PV) devices via solution based methods. The printed CZTSSe films show an interesting microstructure consisting of an upper micrometer-sized polycrystalline layer (large-grain layer) and a bottom fine-grain layer. In this paper, we present our results on characterization of the layers including composition, electronic and optical properties. Based on the observed properties we develop a numerical model for the CZTSSe PV device and present the simulation results. We anticipate that the combination of detailed characterization and device model will help us better understand the limitations of our current devices and indicate potential improvement paths.