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An automated current-voltage curve fitting method has been developed for thermophotovoltaic (TPV) cells. TPV cells are similar to solar cells except TPV cells utilize the infrared portion of a hot radiator spectrum instead of the solar spectrum. The method is automatic, accurate, fast, and simple. The fit is based on a two diode model with one diode having n1=1 and the second diode having n2=2. The program extracts saturation currents for the n1=1 and n2=2 terms, series resistance, shunt resistance, light-generated current, breakdown current, and breakdown voltage. Only one curve with forward bias data and optional reverse bias data is necessary as input. Two curves can be inputted if one curve has forward bias data and the other has reverse bias data. The model has successfully fitted both ternary and quaternary cells with a wide range of cell parameters. Differences between the fit and the data are generally smaller than 0.5% for short-circuit current, open-circuit voltage, fill factor, and maximum power. The curve-fitting methodology is also expected to be applicable to solar cells. Introduction Thermophotovoltaic (TPV) energy conversion is the direct conversion of infrared radiation into electricity by means of a photodiode (1, 2). The current-voltage (I-V) curves generated by TPV diodes (cells) are very similar to solar cell I-V curves, although TPV cells are often tested under reverse bias as well as forward bias conditions. To the authors’ knowledge, there is so far no published literature specifically regarding the curve-fitting of I-V curves generated by TPV cells. The present paper describes a novel curve-fitting method that is expected to be applicable to solar cells as well as TPV cells. There are several I-V curve fitting methods reported for solar cells. An early paper (3) fits the IV curve to a single exponential including series and shunt resistances. Other investigators have focused on determining series resistance (4-7), fitting I-V curves to a single exponential model (8-12), or fitting to a two exponential model (13-17). Many of the models in the literature fit five parameters simultaneously using a least squares approach to minimize the error. The model in this paper (TRIVET) is different in that it determines five cell parameters in a particular well-defined order, and then determines the reverse bias parameters. One advantage of determining parameters sequentially rather than simultaneously is that the effect of shunt resistance can be separated from the effects of idealities and saturation or recombination currents. Also, the initial determination of the saturation and recombination currents is independent of the series resistance. The present approach is simple and highly accurate, and has been used extensively at Lockheed Martin to successfully fit TPV IV curves. It has been found experimentally that TPV cells with InGaAsSb epilayers grown lattice matched on GaSb substrates can be well-fit to a single exponential model, whereas TPV cells with InGaAs epilayers grown lattice mismatched on InP substrates are better fit with a two exponential model. In this paper the topic is limited to a two exponential model plus an additional optional exponential term for the reverse bias condition. This two exponential model can be effectively used for either lattice matched InGaAsSb or lattice mismatched InGaAs cells. Curve-fitting Method Fitting of the TPV I-V curves is accomplished using Eq. 1. The n1=1 term and the n2=2 terms account for the nonlinearity in the ln(ISC) vs. VOC plots, especially at low current levels. ( ) ( ) 1 . Eq e I 1 e I 1 e I R IR V I I B ) IR V (

Accurate Method for Forward and Reverse Bias Curve Fitting of TPV I-V Data