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Title: Emitter/absorber interface of CdTe solar cells

Abstract

The performance of CdTe solar cells can be very sensitive to the emitter/absorber interface, especially for high-efficiency cells with high bulk lifetime. Performance losses from acceptor-type interface defects can be significant when interface defect states are located near mid-gap energies. Numerical simulations show that the emitter/absorber band alignment, the emitter doping and thickness, and the defect properties of the interface (i.e., defect density, defect type, and defect energy) can all play significant roles in the interface recombination. In particular, a type I heterojunction with small conduction-band offset (0.1 eV ≤ ΔE{sub C} ≤ 0.3 eV) can help maintain good cell efficiency in spite of high interface defect density, much like with Cu(In,Ga)Se{sub 2} (CIGS) cells. The basic principle is that positive ΔE{sub C}, often referred to as a “spike,” creates an absorber inversion and hence a large hole barrier adjacent to the interface. As a result, the electron-hole recombination is suppressed due to an insufficient hole supply at the interface. A large spike (ΔE{sub C} ≥ 0.4 eV), however, can impede electron transport and lead to a reduction of photocurrent and fill-factor. In contrast to the spike, a “cliff” (ΔE{sub C} < 0 eV) allows high hole concentration in the vicinity of the interface, which will assist interface recombination and result inmore » a reduced open-circuit voltage. Another way to mitigate performance losses due to interface defects is to use a thin and highly doped emitter, which can invert the absorber and form a large hole barrier at the interface. CdS is the most common emitter material used in CdTe solar cells, but the CdS/CdTe interface is in the cliff category and is not favorable from the band-offset perspective. The ΔE{sub C} of other n-type emitter choices, such as (Mg,Zn)O, Cd(S,O), or (Cd,Mg)Te, can be tuned by varying the elemental ratio for an optimal positive value of ΔE{sub C}. These materials are predicted to yield higher voltages and would therefore be better candidates for the CdTe-cell emitter.« less

Authors:
;  [1];  [2]
  1. Physics Department, Colorado State University, Fort Collins, Colorado 80523 (United States)
  2. National Renewable Energy Laboratory, Golden, Colorado 80401 (United States)
Publication Date:
OSTI Identifier:
22596806
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Applied Physics; Journal Volume: 119; Journal Issue: 23; Other Information: (c) 2016 Author(s); Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; 75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; CADMIUM SULFIDES; CADMIUM TELLURIDE SOLAR CELLS; CADMIUM TELLURIDES; COMPUTERIZED SIMULATION; DEFECTS; DENSITY; DOPED MATERIALS; ELECTRIC POTENTIAL; ELECTRONS; FILL FACTORS; HETEROJUNCTIONS; HOLES; INTERFACES; PERFORMANCE; PHOTOCURRENTS; RECOMBINATION; THICKNESS

Citation Formats

Song, Tao, E-mail: tsong241@gmail.com, Sites, James R., and Kanevce, Ana. Emitter/absorber interface of CdTe solar cells. United States: N. p., 2016. Web. doi:10.1063/1.4953820.
Song, Tao, E-mail: tsong241@gmail.com, Sites, James R., & Kanevce, Ana. Emitter/absorber interface of CdTe solar cells. United States. doi:10.1063/1.4953820.
Song, Tao, E-mail: tsong241@gmail.com, Sites, James R., and Kanevce, Ana. Tue . "Emitter/absorber interface of CdTe solar cells". United States. doi:10.1063/1.4953820.
@article{osti_22596806,
title = {Emitter/absorber interface of CdTe solar cells},
author = {Song, Tao, E-mail: tsong241@gmail.com and Sites, James R. and Kanevce, Ana},
abstractNote = {The performance of CdTe solar cells can be very sensitive to the emitter/absorber interface, especially for high-efficiency cells with high bulk lifetime. Performance losses from acceptor-type interface defects can be significant when interface defect states are located near mid-gap energies. Numerical simulations show that the emitter/absorber band alignment, the emitter doping and thickness, and the defect properties of the interface (i.e., defect density, defect type, and defect energy) can all play significant roles in the interface recombination. In particular, a type I heterojunction with small conduction-band offset (0.1 eV ≤ ΔE{sub C} ≤ 0.3 eV) can help maintain good cell efficiency in spite of high interface defect density, much like with Cu(In,Ga)Se{sub 2} (CIGS) cells. The basic principle is that positive ΔE{sub C}, often referred to as a “spike,” creates an absorber inversion and hence a large hole barrier adjacent to the interface. As a result, the electron-hole recombination is suppressed due to an insufficient hole supply at the interface. A large spike (ΔE{sub C} ≥ 0.4 eV), however, can impede electron transport and lead to a reduction of photocurrent and fill-factor. In contrast to the spike, a “cliff” (ΔE{sub C} < 0 eV) allows high hole concentration in the vicinity of the interface, which will assist interface recombination and result in a reduced open-circuit voltage. Another way to mitigate performance losses due to interface defects is to use a thin and highly doped emitter, which can invert the absorber and form a large hole barrier at the interface. CdS is the most common emitter material used in CdTe solar cells, but the CdS/CdTe interface is in the cliff category and is not favorable from the band-offset perspective. The ΔE{sub C} of other n-type emitter choices, such as (Mg,Zn)O, Cd(S,O), or (Cd,Mg)Te, can be tuned by varying the elemental ratio for an optimal positive value of ΔE{sub C}. These materials are predicted to yield higher voltages and would therefore be better candidates for the CdTe-cell emitter.},
doi = {10.1063/1.4953820},
journal = {Journal of Applied Physics},
number = 23,
volume = 119,
place = {United States},
year = {Tue Jun 21 00:00:00 EDT 2016},
month = {Tue Jun 21 00:00:00 EDT 2016}
}