Pulsed Laser Deposition of Cu2ZnSnS4 Solar Cells: Alternative Routes and Optimization

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Abstract

Cu2ZnSnS4 (CZTS) is a promising p-type solar cell absorber material that consists of earthabundant and non-toxic elements. It has a high absorption coefficient and an ideal bandgap of 1.5 eV for single-junction solar cells. This work has been focused on improving all aspects of CZTS solar cells made of Pulsed Laser Deposition (PLD). PLD is a versatile, non-equilibrium technique for depositing complex materials. The studies are divided into PLD improvements, CZTS absorber properties, device performance and CZTS/Si tandem devices. PLD is a technique that in principle is simple to operate, but many of the subprocesses are difficult to control. We have found that the deposition of oxide and sulfide precursors depends in a similar way on the laser fluence. The deposition can be carried out with high reproducibility, lower deposition time and more homogeneous films over a large area. We have in previous studies done by our group deposited Cu-poor precursor films from a stoichiometric target at low fluence. By changing the stoichiometric target to a Cu-poor/Zn-rich target it was possible to deposit Cu-poor films with shorter deposition time and better reproducibility at high fluence (2 J/cm2 ). Decreasing the spot-size also enabled us to obtain a more spherical-shaped plasma
and deposit larger-area samples with a smaller thickness gradient compared to the previously used more forward-directed plasma onto the substrate. When the deposition conditions were set, various targets could be exchanged to deposit precursors of the desired composition. A number of fundamental properties of CZTS have been investigated as well, ranging from synthesis from oxide precursors, Ag alloying and Ba alloying of CZTS and Cu/Zn ordering of CZTS. UPS and XPS depth profile measurements were performed at Ag alloyed CZTS absorbers to investigate the band structure at the interface between the buffer layer of CdS and the Ag alloyed CZTS. From the data, we have estimated that the Ag alloying reduced the conduction band offset (CBO) at the interface. Ba alloyed CZTS absorbers have been synthesized and we have observed differences in the Raman and photoluminescence spectra relative to the pure CZTS. However, the efficiency improvement was negligible compared to pure CZTS. Without establishing the baseline process it was difficult to draw conclusions. We have also studied the formation mechanism of CZTS from oxide precursors. The SnO2 phase was the last one to convert to sulfide and SO2 gas formed a bubble-like structure during the oxide route process, and it was possible to obtain high-quality CZTS with large grains with an oxide route. We have increased the in-house PLD efficiency from 2.6% to 5.4% by using the oxide route. This achievement was only marginally better than the previous PLD record cell of 5.2%, which was sulfurized and annealed at UNSW, Sydney, Australia. However, if we use anti-reflective coating as in 5.2% solar cell we can still increase the efficiency. On the device level, the post-annealing process has been studied extensively. It is a heat treatment process that is common for kesterite solar cells and a process that involves many layers and different effects on the layers. We have tried a combination of pressure and temperature at the different stages of the fabrication process to rule out the possible interpretations. At least two major processes contribute to the enhancements after the postannealing, the quality of the buffer layer CdS and the formation of a high bandgap barrier layer that reduces non-radiative recombination. The first tandem device based on a Si-bottom cell with a top layer of CZTS deposited by PLD has been made. We have compared the oxide and sulfide routes for synthesizing thin films of CZTS on Si. The oxide route showed the most promising result because it limited the sulfur exposure on the Si bottom part. Moreover, the TiN barrier was partly oxidized into TiON, which is a better diffusion barrier against Cu and a more transparent layer. However, it also has a higher resistance and a lower fill factor (FF). When Si with the bandgap of 1.1 eV is used as a bottom cell for a tandem device, ideally the top cell should have a gap of 1.7 eV. By synthesizing more Cu/Zn ordered CZTS absorbers, we have increased the bandgap from 1.5 to 1.6 eV to better match the CZTS/Si tandem device.
Original languageEnglish
PublisherTechnical University of Denmark
Number of pages150
Publication statusPublished - 2020

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