Time resolved spectroscopy of GaxIn1-xP-GaAs double-junction solar cell

Abstract

The efficiency of multi-junction (MJ) solar cells can be significantly enhanced with the wider coverage of the solar spectrum and higher conversion. Time resolved optical spectroscopy is a powerful tool for optical study in semiconductor industry and for the measurement of time constants of physical processes such as absorption or emission of a given material. In this work, we report on the measurement of the time-resolved spectroscopy of the GaxIn1-xPGaAs double-junction photovoltaic structures under different conditions of temperature and reverse bias voltage by using laser pulses as the excitation source and attempt to obtain some important time constants of carriers, such as, transit time of carriers in this kind of complex double-junction solar cells. The measurements of these time-resolved events can provide important information about the performance of the GaxIn1-xP-GaAs double-junction photovoltaic structures and thus let us know how to improve the efficiency of the tandem solar cells. The device system for the study of time resolved photocurrent was the GaxIn1-xP-GaAs double-junction photovoltaic structure grown with metalorganic chemical vapour deposition technique. The excitation laser pulses were provided by a Nd:YAG pulse laser with the wavelength of 532 nm and pulse width of 10 ns. Time-resolved photocurrent signal of the GaxIn1-xP-GaAs double-junction photovoltaic structure was measured and recorded by a Boxcar (SRS250). In order to obtain the efficiency and accuracy in luminescence measurement, the Boxcar is interfaced to a computer and a developed software that allows synchronous scan control of the gate delay time in the boxcar integrator is provided. The effects of temperature and reverse bias voltages changing were evaluated by photocurrent, carried out using a pulsed laser with 532 nm as an excitation source. Compared to the photocurrent measured under 10K, significant reduction of the rising time was observed in the photocurrent spectra. The rising time is cut down remarkably with the growth of the reverse bias voltage applied. The spectra shift can be explained in terms of conductivity in the solar cell structure: as the temperature and the bias voltage increases, the relaxation time of electrons is reduced.