CIGS TFT and inhomogeneity effects on CIGS solar cells
Date Issued
2016
Date
2016
Author(s)
Zhu, Xiaobo
Abstract
Cu(Ga, In)Se2 (CIGS) thin film transistors and the inhomogeneity effects on CIGS solar modules are investigated. CIGS is one of the best candidate materials for thin film solar cell due to its strong light absorption as well as its relatively high mobility. It also has the advantage to reduce production cost for photovoltaics devices, and nowadays, the fabrication of CIGS solar cell is mature and commercial. But some problems like uniformity, morphology, and yield still affect the quality of CIGS solar cell and reduce its market in the competition with other kinds of solar cell like Si based solar cell, CdTe solar cell and CZTS solar cell. Besides the application in thin film solar cell, CIGS also has the potential to have a role in other fields, like light sensor, telecommunication, and thin film transistor (TFT). By solving the problems occurred in the fabrication of CIGS solar cell and achieving its application in other fields will promote the value of CIGS. In the first part of this dissertation, CIGS TFT is investigated. The fabricated CIGS TFT achieves a saturation mobility of ~1.8 cm2/V-s, and the on-off ratio over 3 orders of magnitude for the first time. We use a special ring pattern to simplify the fabrication process and avoided the problems occurred in the traditional TFTs. In the structure of CIGS TFT, Al2O3 is deposited by atomic layer deposition (ALD) on CIGS film as the dielectric layer. With the help of Al2O3 layer, the on current is high due to its high dielectric constant, and moreover, the channel can be passivated by Al2O3 layer, so that defects on the interface of CIGS/Al2O3 decrease, and eventually increase the saturation mobility. We also have applied thin-down process on the CIGS thin film, which was prepared for solar cell fabrication, to meet the required conditions for CIGS TFT. The characteristics of CIGS TFT are investigated consequently, and the improvement of saturation mobility after thin down process is found. This might be due to the different qualities of different layers in CIGS thin film, and is justified by the measurements of photoluminescence (PL) and X-ray diffraction (XRD). Moreover, the performances of CIGS TFTs with different content in CIGS films are investigated. TFT with high Cu/(Ga+In) ratio tends to have both high carrier concentration and saturation mobility. Sodium atoms might play important roles in these properties. In the second part of this dissertation, the inhomogeneity effects on CIGS solar modules are investigated. We firstly demonstrate the impact of residual strain on the CIGS solar module with Ga content fluctuation using first principle calculations. The simulation results show that the inhomogeneity effect is magnified by residual strain due to the enhanced band gap fluctuations. Then the 3D simulation results of CIGS solar cells are obtained with the residual strain effect incorporated. The parameters used in the model for the simulation are the same to those used in the commercial CIGS solar cell. A module consists of 3 cells in series, while each cell is divided into 3 sections. Intracell inhomo, intercell inhomo, and combined intracell+intercell inhomo are considered within a module, and the effects of both Ga content and thickness fluctuation are investigated. Among these three distributions, intracell+intercell inhomo is closest to the real distribution of inhomogeneity in the fabrication of CIGS solar module. In reality, Ga content fluctuation is serious and thickness fluctuation can be well controlled. The simulation results show that in terms of Ga content fluctuation, Intracell fluctuation causes VOC degradation, and intercell fluctuation causes JSC and fill factor degradations, and in terms of thickness fluctuation, VOC remains the same due to the constant Ga content, JSC is degraded due to the increase of excess carrier recombination, and the tendency of fill factor degradation is similar to Ga content fluctuation.
Subjects
Cu(In, Ga)Se2
Thin film transistors
Thin down process
saturation mobility
Ga content
Inhomogeneity effects
residual strain
thickness fluctuation
SDGs
Type
thesis
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