Principal Investigators
at MIT: Professor L.C. Kimerling and Dr. J. Michel
Principal Investigator
at Masdar Institute: Assistant Professor Mahieddine Emziane
Description and Objectives
This collaborative research project focuses on the following three major research projects in thin-film and high efficiency photovoltaic technology:
- High-efficiency thin-film Si solar cells with novel light trapping;
- Ge-based thin-film thermophotovoltaic (TPV) cells with photon recirculation;
- AlGaAs/Si/Ge tandem solar cells for ultra-high efficiency energy conversion.
It addresses the key issues with novel device designs and process integration concepts, and encompasses different approaches, strategies, designs and implementations for achieving very high-performance solar cells based on thin-film structures of selected semiconductor materials.
Approach
1. To apply a new back-reflector concept to very thin crystalline, polycrystalline and amorphous Si solar cells. According to systematic simulations, by using optimized back reflector design, the relative efficiency enhancement can be as high as 53% for a 2 mm thick photovoltaic cell. We expect therefore the thin-film solar cell efficiency to greatly improve by using this backside reflector design.
2. To develop monolithic TPV cells on a Ge epitaxial layer on Si wafers. As a next step towards a fully functioning TPV cell, we will develop a suitable Bragg reflector on top of the Ge PV cell. The reflectance for the below-gap radiation will be maximized while the reflectance for the above-gap radiation will be minimized. This will lead to a photon recirculation that enhances the overall device performance.
3. To design and implement a tandem cell innovation based on (AlGaAs)m/Si/Ge structures. Since we can vary the AlGaAs composition, multiple AlGaAs layers (m) with different compositions can be used in the tandem cell. Our simulation shows that the efficiency of the tandem cell with m=2 can be as high as 58 % under AM1.5G and should increase with m up to the theoretical limit for the collection of wavelengths shorter than 1600 nm. The AlGaAs will be epitaxially grown of the font of the Si wafer. The Ge buffer layer will accommodate the 4% lattice mismatch between AlGaAs and Si. Ge will also be used as part of the tandem cell on the backside of the wafer for near IR photocurrent generation.
Progress
1. For comparison, thin-film solar cells with and without light trapping scheme were fabricated. The Si film thickness was varied to evaluate the dependence of the light trapping effect on the film thickness. Preliminary measurements show excellent IV characteristics from these cells. This indicates that this new concept can be applied to commercial thin-film PV cells, lower the production cost and make solar electricity generation cost-competitive, and sustainable through the use of small volume of naturally abundant silicon.
2. We are investigating different ways to reduce the dark current of the Ge TPV cell. We also are designing the Bragg reflector (which is a photonic bandgap filter) to be integrated later with the optimized Ge TPV cell.
3. We started the modeling analysis of a tandem cell involving only two cells initially: Si cell on Ge cell, and this work is in progress.
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Towards Very High-Performance Thin Film Photovoltaic Cells: Designs and Implementations