Pilar Espinet-González

Why can’t I measure the external quantum efficiency of the Ge subcell of my multijunction solar cell?

The measurement of the external quantum efficiency (EQE) of low bandgap subcells in a multijunction solar cell can be sometimes problematic. In particular, this paper describes a set of cases where the EQE of a Ge subcell in a conventional GaInP/GaInAs/Ge triple-junction solar cell cannot be fully measured. We describe the way to identify each case by tracing the I-V curve under the same light-bias conditions applied for the EQE measurement, together with the strategies that could be implemented to attain the best possible measurement of the EQE of the Ge subcell.

Evaluation of the reliability of commercial concentrator triple-junction solar cells by means of accelerated life tests (ALT)

A temperature accelerated life test on commercial concentrator lattice-matched GaInP/GaInAs/Ge triple-junction solar cells has been carried out. The solar cells have been tested at three different temperatures: 119, 126 and 164 °C and the nominal photo-current condition (820 X) has been emulated by injecting current in darkness. All the solar cells have presented catastrophic failures. The failure distributions at the three tested temperatures have been fitted to an Arrhenius-Weibull model. An Arrhenius activation energy of 1.58 eV was determined from the fit. The main reliability functions and parameters (reliability function, instantaneous failure rate, mean time to failure, warranty time) of these solar cells at the nominal working temperature (80 °C) have been obtained. The warranty time obtained for a failure population of 5 % has been 69 years. Thus, a long-term warranty could be offered for these particular solar cells working at 820 X, 8 hours per day at 80 °C.

On the use of I–V curves as a diagnosis tool for proper external quantum efficiency measurements of multijunction solar cells

External quantum efficiency measurement of multijunction solar cells is not an easy task. In this paper we propose to trace the I-V curve of the multijunction device under the same light bias conditions intended to be applied for the EQE measurement as an effective way to minimize artifacts and determine the optimum light and voltage bias conditions for the measurement. In this way, the analysis of the I-V curve will help to determine the proper voltage bias needed (if any), as well as to distinguish whether the external quantum efficiency measurement is being affected by shunt problems, early breakdown or luminescent coupling. This is of special relevance in order to determine the origin of the measurement artifact affecting the external quantum efficiency measurement of MJSCs.