Characterization of fundamental parameters of front junction amorphous/crystalline silicon heterojunction solar cells using various electrical methods

Date
2010
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University of Delaware
Abstract
Silicon heterojunction solar cells promise low cost processing and high efficiency cells because they can be processed at low temperatures and have very well passivated surfaces. With a thin intrinsic layer between amorphous doped layers and crystalline silicon, these cells have demonstrated open-circuit voltages greater than 720mV and efficiencies over 22%. In the thesis, electrical measurements are used to characterize the behavior of the front heterojunction silicon solar cells. Doping density, excess defect density, and free-carrier density can be obtained from measurements from the capacitance system using different methods. Impedance spectroscopy utilizes a simple RC parallel circuit model to get parallel capacitance, parallel resistance, and series resistance from a complex impedance plot. Minority carrier lifetime on a completed solar cell is calculated from the Nyquist plot. JV-T measurements are used to get the barrier height that carriers have to overcome to be collected along with some diode parameters. Internal photoemission method is used to get estimated values of valence and conduction band-offset for samples with different structures. The goal of this thesis is to use different methods to determine parameters that relate to open-circuit voltage and fill factor. Our focus is on the effect of the a-Si layer: either none, or 5nm i-layer in front, back, or both sides. This i-layer strongly affects the and FF. The open-circuit voltage correlates with the lifetime determined from impedance spectroscopy.
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