Abstract
Semiconductor multijunction solar cell is a cutting-edge photovoltaic technology aimed at developing a frontier solution to the cleanenergy demand and environmental problem. Due to theefficient photoabsorption and energy conversion in the visible and near-infrared spectral ranges of the solar spectrum, the multijunction solar cell structures have shown an unprecedented application potential by demonstrating a solar conversion efficiency of over 44%. Among various multijunction solar cell structural designs, the GaxIn1-xP/GaAs double-junction tandem structure is considered as the most fundamental building block for developing the industry-standard triple- and even more junction photovoltaic cells with super high efficiency. Therefore, obtaining a better and more in-depth understanding of physical properties of the GaxIn1-xP/GaAs double-junction tandem device structure, especially some fundamental optoelectronic processes in the individual structural layer, including photoexcitation, transport and the mid-way recombination of charge carriers, is crucial for further improving the energy conversion efficiency. In this thesis, the mid-way radiative recombination, diffusiontransport, localizationmechanism, and photocurrent spectra of charge carriers in the GaxIn1-xP/GaAs double-junction tandem solar cellsgrown on GaAs substrates with different misorientation angleswereinvestigatedin detail.
Our main findings are summarized as below. Efficientradiative recombination of carriers in the GaxIn1-xP/GaAs double-junction tandem solar cell samples was demonstrated by using electroluminescence (EL) and photoluminescence (PL) techniques. The radiative recombination intensity was shown to be dependent on the intrinsic material-related parameters such as the doping concentration, growth thickness and the substrate misorientation angle both experimentally and theoretically. The radiative recombination was thus revealed to be an important loss channel of carriers in the GaxIn1-xP/GaAs double-junction tandem solar cells.
Super strong transverse diffusion of minority carriers in the top GaxIn1-xP subcell was foundby the micro-EL image surveying. Theoretical simulation on the experimental data shows that the minority carrier diffusion length is as long as ~93 μmat a forward bias of 2.75 V, which is ~30 times longer than that of unbiased GaxIn1-xP epilayer. Origin of this super transverse diffusion wasargued, and itsinfluence on device performance wasalso discussed.
Significant correlations of carrier localization and luminescence behaviors with the substrate misorientationangle in the top GaxIn1-xP subcells wereunveiled by excitation intensity- and temperature-dependent PL. The large difference in potential energy profile of GaxIn1-xP layers,caused by the different degreesof atomic ordering,wasargued to interpret the observed PL distinctions.
Vertical transport and photoresponse mechanismsof charge carriers in the GaxIn1-xP/GaAs double-junction tandem solar cells were studied by temperature- and reverse bias-dependent photocurrent (PC) spectroscopy. Both the temperature and reverse bias were shown to have significant impact on the device photoresponse, in particular on the photoresponse due to the absorption of photons with energy above the bandgap of GaAs and GaxIn1-xP, namely the supra-bandgap photoresponse. A model was proposed to simulate the observed temperature- and reverse-bias dependence of the supra-bandgap photoresponse.