Improved InGaP solar cells grown by molecular beam epitaxy by use of tellurium doping

Li, Brian D

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https://hdl.handle.net/2142/115756 Copy

Description

Title

Improved InGaP solar cells grown by molecular beam epitaxy by use of tellurium doping

Author(s)

Li, Brian D

Issue Date

2022-04-29

Director of Research (if dissertation) or Advisor (if thesis)

Lee, Minjoo

Department of Study

Electrical & Computer Eng

Discipline

Electrical & Computer Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• Solar cells
• phovotoltaics
• III-V
• molecular beam epitaxy
• doping

Abstract

Solar energy based on photovoltaic (PV) cells, or solar cells, is one of the fast-growing sectors of renewable energy. To expand the use of PV, it is vital to improve cell efficiency, which enables reduced cost in $/Watt. By far the highest efficiency solar cells are so-called multi-junction solar cells (MJSCs) consisting of multiple stacked “sub-cells” made from different III-V materials. These cells have achieved record efficiencies of 39.2% under 1-sun, much greater than the 26.7% record achieved with silicon (Si), the material that currently dominates the PV market. One of the most important sub-cells for MJSC’s is made from (Al)InGaP, a phosphide material with a high bandgap energy of 1.9-2.2 eV that is present in almost all record III-V MJSCs. In this thesis, we study the effects of n-type dopant on 1.9 eV In0.49Ga0.51P (hereafter InGaP) solar cells grown by molecular beam epitaxy (MBE). The 2 dopants studied here are the commonly used Si and the less studied dopant tellurium (Te). The first part of this thesis studies the growth conditions necessary to achieve high Te doping in 1.9 eV InGaP. In the literature, a well-established challenge of Te doping is surface segregation, in which the Te atom stays on the growth surface rather than incorporating into the bulk crystal during growth. In our MBE-grown InGaP, significant surface segregation was observed with the nominal substrate growth temperature Tsub = 460 °C. Ultimately, high Te incorporation and a uniform doping profile were achieved with 2 key growth changes. The first important growth change was to reduce the substrate temperature to Tsub = 420 °C. The second major change was to perform a Te “pre-dose”, in which the Te dopant source was deposited onto the growth surface for several minutes prior to InGaP growth, thus building up a significant population of surface Te. The well-doped InGaP:Te sample was compared to InGaP:Si under photoluminescence (PL), and both samples were found to have similar peak PL intensity, indicating comparable material quality. In the second part of this thesis, InGaP solar cells were grown with both Si doping and Te doping in the n-type absorber layer, known as the emitter. The solar cells were compared under both as-grown (AG) conditions, in which there is no post-growth annealing, and after rapid-thermal annealing (RTA), in which the cells are heated to 800-1000 °C to improve material quality. First, for AG cells, there was a significant improvement in light-generated carrier collection, as measured by external quantum efficiency (EQE). The EQE-derived value for short-circuit current density (Jsc) improved from 9.60 to 10.15 mA/cm2, a 5.4% relative improvement. Based on modeling, this result was directly attributable to a ~4× higher minority carrier lifetime in the Te-doped emitter, which indicates superior optical quality. Therefore, InGaP:Te potentially has lower point defect density compared to InGaP:Si in the AG material. However, after RTA, solar cells with Si-doped emitter had consistently higher efficiencies than Te-doped cells. Based on diode analysis and doping measurements, the worse efficiency of RTA’d Te-doped cells may be due to poor material quality in the space-charge region of the p-n junction or due to Te deactivation/diffusion out of the emitter. Despite the challenges of RTA, Te doping holds promising applications for future work on phosphide-based solar cells.

Graduation Semester

2022-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Brian Li

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