Characterization of multijunction concentrator solar cells

Abstract

This chapter summarizes the state of the art in instruments and methods for the characterization of concentrator multijunction (MJ) solar cells (MJSC). The current-voltage characteristic (I-V curve) under illumination and the spectral response or quantum efficiency (QE) are the main properties of a solar cell. The measurement of the I-V curve as a function of light concentration provides the most relevant information on cell performance such as peak efficiency or series resistance losses. Concentrator solar simulators should fulfill strict spatial uniformity and spectral requirements to provide accurate measurements. Precise spectral tunability is required to reduce errors in the measurement of latest-generation cell architectures, especially those not based on a germanium bottom cell with an excess of current. The main types of concentrator solar simulators are described, and advice on appropriate reference sensors and measurement precautions is provided. Spectral characterization under controlled simulator conditions is essential to analyze MJ cell behavior under the ever-varying spectral conditions found in real operation. The relevance of the QE is that it allows calculating the photocurrent generated by each subcell under a particular spectral irradiance and diagnosing possible mal- functioning of the different regions of the cell. Cell electroluminescence (EL) is also discussed as a useful tool for assessing defects. Spatially resolved EL can be used to analyze solar cell internal defects, sheet resistivity, or thermal inhomogeneity under steady concentrated light. Typical characterization set-ups, test procedures, and potential instrumentation issues are discussed for QE and EL.