Design Strategies for High-efficiency Kesterite Solar cells and their Application in Integrated Devices with a Lithium-Ion Battery

Abstract

As climate change due to carbon emissions and global warming emerges seriously, the need for renewable energy generation has risen. Among renewable energy sources, solar cells are the future main energy resources with their availability and high efficiency. There are various types of solar cells depending on materials, and research is necessary for the purpose of developing an optimal solar cell that satisfies high-efficiency, stability, and low-cost. Kesterite solar cells have been evaluated as a next-generation solar cell thanks to their low-cost elemental constitution and film type configuration and many studies have been conducted based on them. Since their first power conversion efficiency (PCE) was reported in 2006, the rapid development of PCE has been achieved, and high-quality studies based on the synthesis of the absorber materials for improving efficiency have been continuously investigated. Current issues that the kesterite solar cell faces are low open-circuit voltage and fill factor (FF). Due to the synthetic instability of the absorption layer, many secondary phases remain inside, and the quality of the film is still poor. During the synthesis process, high temperature over the 500 degrees should be accompanied and it leads to decomposing of absorber which is induced by the reaction between the absorber and back contact electrode. As a result, undesired secondary phases would remain inside the absorber layer, especially the bottom part of them. This problem could be dealt with by modifying the annealing process or introducing an appropriate intermediate layer to prevent reaction occurred. However, one of the strategies that can be proposed for improving the PCE of kesterite solar cells is introducing excellent transparent electrodes (TEs). By embedding the TEs with superb optoelectronic properties, it is possible to enhance the photo harvesting and lower the sheet resistance. As a result, PCE can be increased by upgrading the collected short circuit current and the FF. In this work, the following studies have been conducted as strategies to improve the PCE of kesterite solar cells. First, I have demonstrated the improved electrical performance for kesterite solar cells by introducing an intermediate layer between the absorber and the back contact layer. Second, the increased PCE has been demonstrated on the kesterite solar cells prepared along with Mg- and Ga-doped ZnO(MGZO). Lastly, I have implemented a miniature type energy storage system in which the kesterite solar cell is connected to a lithium-ion battery. The details of these three studies are as follows. First, the PCE of the kesterite solar cells was improved by introducing the graphene oxide intermediate layer. During the high pressure and temperature process, the absorber layer could react with Mo back contact to cause its decomposition. It, eventually lead to left behind unwanted chalcogen-based binary and ternary secondary phases. These phases play a role to degrade the film quality. The generated secondary phases prohibited the photo-generation and migration of carriers and caused recombination thus, the solar cell PCE degenerated. In order to suppress such a harmful reaction, a synthesis process was performed by introducing the graphene oxide as the intermediate layer. As a result, the formation of secondary phases such as ZnSe and SnSe2 was successfully prevented and it was demonstrated through chemical and crystallographic analysis. Besides, the role of a buffer for the graphene oxide which is capable of successfully mitigating the interference effect that may appear under the absorber layer was confirmed and it was demonstrated through optical electric field distribution generated in the absorber layer. The improved PCE of 11.2% was achieved in the kesterite solar cell with the graphene oxide intermediate layer. Second, in order to improve the PCE of kesterite solar cells, a TE with excellent optical and electrical properties has been developed. The TE developed in this study can be used not only in inorganic solar cells such as kesterite, but also in various solar cells, sensors, LEDs, and smart windows. Currently, the most widely used TE is InSnO2 (ITO). However, due to the scarcity of In, the need for alternative materials has emerged. In this study, Ga-and Mg-doped ZnO was studied in order to successfully replace the existing ITO and improve the optoelectronic properties. Through the optimization of the doping concentration, a transmittance of 93% and a metal scale sheet resistance of 3 Ω/sq at a wavelength of 500-600 nm were demonstrated. The figure of merit confirmed the top-class value of 1400 when compared with the existing TE material. Previously reported TE materials exhibit the high FOM by containing noble metals, or with an approach in which a specific process such as a metal stacking or a network configuration is requested. However, the MGZO TE which is demonstrated in this study can be deposited simply and reproducibly by a sputtering process and shows excellent optoelectronic properties despite being formed as a single film. In the flexibility test, which is an essential indicator to evaluate next level TE, the stable bending performance was confirmed under the conditions bending radius of up to 11 mm and 500 cycles. The MGZO TE was used on the kesterite solar cell to investigate the improvement of electrical characteristics. Compared to the conventional AZO-applied solar cell, the improved PCE of 9.61% was confirmed in the MGZO-applied solar cell. Furthermore, I demonstrated here that a specific range of average wavelengths should be introduced to compare and evaluate TEs using FOM through additional experiments more accurately. Third, a miniature type energy storage system for charging lithium-ion batteries using kesterite solar cells was implemented. The purpose of the charging test is to demonstrate a system that is able to fully charge the battery and does not generate overvoltage even during a long-term charging cycle. The kesterite solar cells were used for stable charging for many cycles. By synchronizing the charging voltage of the solar cell and the lithium-ion battery, a system was constructed that does not accompany the flammable risk due to overvoltage even after long-time charging. In addition, the lithium-ion battery can be fully charged through carefully adjusted voltage. The lithium metal as the anode and Li4Ti5O12 (LTO) as the cathode were used for lithium-ion battery configuration. The effect of heat transferred from the solar cell to battery during charging on the stability of lithium metal was also investigated. In the charging test, the Li-LTO coin-cell was fully charged a 1C. Even though the charging was conducted for 3 hours under the condition of 1C, the coin-cell operated without any problems. In the operation for 20 cycles, solar cells and lithium-ion cells showed stable cycle retention for the PCE and specific capacity, respectively. The heat transferred from the solar cell leads to the growth of smaller lithium nuclei, which makes the Li plating uniform, thereby improving the long-term stability of the Li metal anode. In the 50 cycle operation, more stable cycle retention appeared in the coin-cell charged through the solar cell.