Temperature Dependence of Electrical Characterization in n + - CdS/p - CdTe Thin Film Solar Cells - Study of Shallow/Dee
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Temperature Dependence of Electrical Characterization in n+ - CdS/ p - CdTe Thin Film Solar Cells – Study of Shallow/Deep Defects Poonam Rani Kharangarh, George E Georgiou, and Ken K Chin Department of Physics and Apollo CdTe Solar Energy Center, New Jersey Institute of Technology (NJIT), University Heights, Newark ,New Jersey, 07102, USA ABSTRACT For CdTe there is no real distinction between defects and impurities exists when nonshallow dopants are used. These dopants act as beneficial impurities or detrimental carrier trapping centers. Unlike Si, the common assumption that the trap energy level Et is around the middle of the band-gap Ei, is not valid for thin film CdTe. Trap energy levels in CdTe band-gap can distributed with wide range of energy levels above EF. To identify the real role of traps and dopants that limit the solar cell efficiency, a series of samples were investigated in thin film n+CdS/p-CdTe solar cell, made with evaporated Cu as a primary back contact. It is well known that process temperatures and defect distribution are highly related. This work investigates these shallow level impurities by using temperature dependent current-voltage (I-V-T) and temperature dependent capacitance-voltage (C-V-T) measurements. I-V-T and C-V-T measurements indicate that a large concentration of defects is located in the depletion region. It further suggests that while modest amounts of Cu enhance the cell performance by improving the back contact to CdTe, the high temperature (greater than ~100°C) process condition degrade device quality and reduce the solar cell efficiency. This is possibly because of the well-established Cu diffusion from the back contact into CdTe. Hence, measurements were performed at lower temperatures (T = 150K to 350K). The observed traps are due to the thermal ionization of impurity centers located in the depletion region of p-CdTe/n+-CdS junction. For our n+-CdS/p-CdTe thin film solar cells, hole traps were observed that are verified by both the measurement techniques. These levels are identical to the observed trap levels by other characterization techniques. INTRODUCTION In today’s practical photovoltaic devices, several physical mechanisms that describe the nature of defects may interfere with the data interpretation when different measurement techniques such as Deep Level Transient Spectroscopy (DLTS), Temperature-dependent Current-Voltage (I-V-T), Frequency Dependent Admittance Spectroscopy (AS) and Photo Induced Current Transient Spectroscopy (PICTS) are used. Such challenges arise due to the violation of basic assumptions such as single - junction, ohmic contact, highly conductive absorbers layer, and measurement in reverse bias. A lot of research work has been done in order to increase thin film solar cells efficiency. The doping, especially the shallow one, is essential for the function of the device and for comprehension and interpretation of measurements. I-V-T and C–V-T measurements are very basic and wide spread method to determine the shallow or deep defects, because it us
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