Theoretical and experimental studies on open circuit voltage decay lifetime and series resistance in solar cells and other P-N junction devices

Abstract

Open-Circuit Voltage Decay (OCVD) is a simple and widely used technique for determining the minority carrier lifetime (??) in p–n junction devices. In this work, theoretical expressions for both Forward Current-Induced Voltage Decay (ICVD) and Photo-Voltage Decay (PVD) in a base-dominated Back Surface Field (BSF) solar cell of arbitrary thickness have been derived. These expressions are applicable to the general case for any value of the effective surface recombination velocity (S) at the back contact of the base. Two specific cases are analyzed in detail: Effective BSF cell where S = 0 Conventional cell where S ? ?For S = 0 and small W (where W is the base thickness and L is the diffusion length), the reciprocal slope of the ICVD or PVD plot equals the minority carrier lifetime in the base. For S ? ?, the reciprocal slope corresponds to a different limiting case. These results are valid for voltage decay following a long-duration pulse. The behavior of PVD after a short light pulse (PVD-SP) and for arbitrary pulse durations is also discussed. Experimental results of PVD for both conventional and BSF cells are presented and found to be consistent with theory. In particular, the theoretical prediction of the wavelength dependence of the PVD slope is experimentally verified in conventional cells. Additional effects such as injection level and temperature on ICVD are analyzed. An exact theory for OCVD is developed, accounting for carriers stored in the emitter. The results differ significantly from those for a base-dominated diode due to heavy doping effects in the emitter. It is shown that the quasi-static approximation is highly accurate in the emitter except for very short times, but is invalid in the base. Earlier theories based on the charge-control model are shown to be inaccurate. A new method is suggested for obtaining a more accurate value of ?? from observed OCVD data. Experimental results on BSF solar cells fabricated in the Solid State Physics Laboratory agree well with the proposed theory. Finally, a new method for measuring the series resistance of a solar cell is developed. Detailed experimental investigations using this method are reported. The PVD technique is applied for the first time to determine series resistance in Si solar cells. The method is shown to be simple and accurate. However, it is noted that all existing methods, including this one, do not yield a unique value of series resistance because the emitter resistance is distributed and cannot be represented by a single lumped parameter. Analytical expressions for the output characteristics of the solar cell, accounting for distributed resistance, are derived.