Abstract
The typical structure of high efficiency Cu(InGa)Se2 (CIGS)-based thin film solar cells is substrate/Mo/CIGS/CdS/i-ZnO/ZnO:Al(AZO) where the sun light comes through the transparent conducting oxide (i.e., i-ZnO/AZO) side. In this study, the thickness of an intrinsic zinc oxide (i-ZnO) layer was optimized by considering the surface roughness of CIGS light absorbers. The i-ZnO layers with different thicknesses from 30 to 170 nm were deposited via sputtering. The optical properties, microstructures, and morphologies of the i-ZnO thin films with different thicknesses were characterized, and their effects on the CIGS solar cell device properties were explored. Two types of CIGS absorbers prepared by three-stage co-evaporation and two-step sulfurization after the selenization (SAS) processes showed a difference in the preferred crystal orientation, morphology, and surface roughness. During the subsequent post-processing for the fabrication of the glass/Mo/CIGS/CdS/i-ZnO/AZO device, the change in the i-ZnO thickness influenced the performance of the CIGS devices. For the three-stage co-evaporated CIGS cell, the increase in the thickness of the i-ZnO layer from 30 to 90 nm improved the shunt resistance (RSH), open circuit voltage, and fill factor (FF), as well as the conversion efficiency (10.1% to 11.8%). A further increas of the i-ZnO thickness to 170 nm, deteriorated the device performance parameters, which suggests that 90 nm is close to the optimum thickness of i-ZnO. Conversely, the device with a two-step SAS processed CIGS absorber showed smaller values of the overall RSH (130–371 Ω cm2) than that of the device with a three-stage co-evaporated CIGS absorber (530–1127 Ω cm2) ranging from 30 nm to 170 nm of i-ZnO thickness. Therefore, the value of the shunt resistance was monotonically increased with the i-ZnO thickness ranging from 30 to 170 nm, which improved the FF and conversion efficiency (6.96% to 8.87%).
Highlights
Among the renewable and sustainable energy sources, solar electricity has attracted considerable attention, and significant progress has been achieved over the last few decades
The results revealed that the change in the intrinsic zinc oxide (i-ZnO) thickness affected the performance of the CIGS cells
The two types of CIGS absorbers prepared by three-stage co-evaporation and two-step sulfurization after the selenization (SAS)
Summary
Among the renewable and sustainable energy sources, solar electricity has attracted considerable attention, and significant progress has been achieved over the last few decades. The chalcopyrite CIGS thin-film solar cells have demonstrated excellent properties among photovoltaic technologies, such as a high electric conversion efficiency (23.35%) [3], roll-to-roll flexibility [4,5], and excellent outdoor performance stability because the CIGS thin-film solar cells are deemed the most promising photovoltaic technology for future energy [6,7]. Even though diverse fabrication processes of CIGS light absorbers have been successful with efficiencies close to 20%, the most successful processes with greater than 20% electrical conversion efficiency are three-stage co-evaporation The CIGS absorber prepared by a three-stage co-evaporation process exhibited a smooth and dense surface morphology, while the CIGS obtained from a two-step metallization-selenization process exhibited high surface roughness [10]
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