Diffusion length and lifetime determination in p-n junction solar cells and diodes by forward-biased capacitance measurements

Abstract
A new method is described and illustrated for determining the value and the temperature dependence of the minority-carrier diffusion length and lifetime in the base region of p-n junction solar cells and diodes. The method applies to devices after the p-n junction is formed, and thus includes the influence of the junction fabrication on the diffusion length and lifetime. The method requires only forward-biased capacitance measurements at the device terminals. It combines the dependencies of the low-frequency and high-frequency capacitance on forward bias in such a way as to yield the component of the capacitance associated with the minority carriers in the quasi-neutral base region. From this quasi-neutral-base capacitance the minority-carrier diffusion length and lifetime are then deduced. The accuracy of this method is estimated to be ±5 percent based on the accuracy of the capacitance bridges and of the measured temperature. To illustrate the method and its accuracy, it is applied to a set of silicon p-n junction diodes having base doping concentrations ranging from about 1014to 1018cm-3, and the values of the diffusion lengths determined by the capacitance method are compared with those obtained using the current response to X-ray excitation. Excellent agreement is seen. In contrast, the open-circuit-voltage-decay method yields values of the diffusion length that differ appreciably from those determined by the capacitance and X-ray methods. The reasons for the relative inaccuracy of the open-circuit-voltage-decay method applied to silicon devices are discussed. The practical limitations of the capacitance method are indicated, and an extension of the method is discussed that makes it applicable to devices having highly doped base regions and surface (emitter) layers. In such devices, the capacitance of the quasi-neutral emitter region can contribute appreciably to the total forward-biased capacitance, and the method can yield information about the material properties of the degenerately doped surface layer. The material properties determined are the phenomenological emitter lifetime and a measure of the energy bandgap narrowing in the emitter.

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