School of Electrical, Computer and Energy Engineering
PhD Final Oral Defense
Towards high-efficiency thin-film solar cells: from theoretical analysis to experimental exploration
by
Shi Liu
08/04/2015
11:00 am
ERC 490
Committee:
Dr. Yong-Hang Zhang (chair)
Dr. Shane R. Johnson
Dr. Dragica Vasileska
Dr. Hongbin Yu
Abstract
GaAs single-junction solar cells have been studied extensively in recent years, and have reached over 28% efficiency. Further improvement requires an optically thick but physically thin absorber to provide not only large short-circuit current, but also high open-circuit voltage. By detailed simulation using a semi-analytical model, it is concluded that ultra-thin GaAs solar cells with hundreds of nanometers thickness, and the combination of textured surfaces and reflective back surfaces can potentially offer efficiencies greater than 30% under one sun AM1.5G solar spectrum. Moreover, the impacts of non-Lambertian scattering on the cell’s optical properties and device performance are studied.
The most practical structure is realized by integrating the ultra-thin GaAs solar cell with a AlInP back scattering layer coated with a highly reflective Au mirror. The applied MgF2/ZnS double-layer anti-reflection coating for the device is optimized using transfer matrix method. A power conversion efficiency of 19.1% is achieved experimentally in a 300 nm thick GaAs solar cell with AlInP/Au reflective back scattering, and the device performance is analyzed using a modified semi-analytical model with Phong distribution implemented to account for non-Lambertian scattering. A Phong exponent m of ~12 is determined by fitting both simulated short-circuit current density and external quantum efficiency to their experimental values. Additionally, a non-radiative lifetime of ~130 ns and a specific series resistivity of 1.2 Ω·cm2 are determined to account for the 1 V open-circuit voltage and the 77.8 % fill factor.
Thin-film CdTe solar cells have also attracted a lot of attention as a result of continuous improvements in their device performance, which recently achieved 21.0 % using poly-crystalline CdTe. To address the issue of the lower efficiency record compared to detailed-balance limit, single-crystalline Cd(Zn)Te/MgCdTe double heterostructures (DH) grown on InSb (100) substrates are studied.
A CdZnTe/MgCdTe DH consisting of a 3 µm thick Cd0.9946Zn0.0054Te middle layer, which is lattice-matched to an InSb substrate, has been grown using molecular beam epitaxy. A long carrier lifetime of 3.4×102 ns has been demonstrated at room temperature, which is approximately three times as long as that of a CdTe/MgCdTe DH with identical layer thickness. This substantial improvement is due to the reduction in misfit dislocation density in the CdZnTe alloy. These findings indicate that CdZnTe lattice-matched to InSb has great potential as applied to high-efficiency solar cells as well as virtual substrates for high-performance large-area HgCdTe focal plane arrays.
The recombination lifetimes and interface recombination velocities of several sets of CdTe/MgxCd1-xTe DH samples with different CdTe layer thicknesses, MgCdTe layer thicknesses and Mg compositions are examined using time-resolved photoluminescence measurements. A long carrier lifetime of 0.83 µs and a low interface recombination velocity (IRV) of 47 ± 20 cm/s have been measured for the CdTe/Mg0.46Cd0.54Te (15 nm) DHs. Longer lifetimes of up to 3.2 µs and smaller IRV of below 5 cm/s are obtained in the DH samples with thicker MgCdTe barriers. These values are very close to the best reported numbers for GaAs/AlGaAs DHs. The impact of carrier escape due to thermionic emission over the barrier on the interface recombination velocity is also studied.