Three terminal Si-Si:Ge monolitic tandem solar cells
Date
2011
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Stand-alone transparent Si solar cell and low band gap Si:Ge solar cell can be applied together in a high performance multi-junction solar cell system to absorb the middle and long wavelength light. In this system, optics needs to be designed carefully for the back side of transparent Si solar cell to optimize the long wavelength light transmission. Photon losses cannot be eliminated in the stand-alone system. In this thesis a 3-terminal Si-Si:Ge monolithic tandem solar cell consisting of a top silicon solar cell and a bottom Si:Ge solar cell sharing a common base, which can take advantage of both the silicon solar cell and Si:Ge solar cells and overcome the current mismatch effect and photon losses between Si solar cell and Si:Ge solar cell is designed, modeled and fabricated. It can replace independent Si solar cell and Si:Ge solar cell in a multi-junction solar cell system. Modeling of 3 terminal Si-Si:Ge monolithic tandem solar cell is done to find out its maximum efficiency. A modified PERL cell is put on top of the modeled structure because it's the world record silicon solar cell. Its thickness is modified to analyze the effects of near band edge absorption. There are three cases for the bottom Si:Ge solar cell under modified PERL cell, the best fabricated device, product of the best measured parameters and the model of Si:Ge solar cell14. Modeling shows that 3 terminal Si-Si:Ge solar cell outperforms PERL cell in most cases, and a thicker Si device leads to higher efficiency. The efficiency of 3 terminal Si-Si:Ge solar cell with 800um silicon substrate is 10% relative and 2.5% absolute higher than PERL cell at 500 suns. Three groups of solar cells are fabricated. Performance of Si:Ge solar cells in this structure improve significantly due to better Si:Ge epitaxial layers growth conditions. The Si:Ge solar cell achieved with a Jsc of 3.26mA/cm2, Voc of 189mV below the silicon filter under one sun without light trapping. Silicon solar cells in this structure have fine performances, the best one which has a POCl3 diffused emitter has a Jsc 22.9mA/cm2, Voc 598.5mV and FF 77.3%. This work demonstrates that three terminal Si-Si:Ge monolithic tandem solar cell can achieve higher potential efficiency than standalone transparent silicon solar cells and Si:Ge solar cells and avoid current mismatching which exists in two terminal multi-junction solar cells.