Emitter/absorber interface of CdTe solar cells

Journal Article · Fri Jun 17 00:00:00 EDT 2016 · Journal of Applied Physics

DOI:https://doi.org/10.1063/1.4953820· OSTI ID:1257658

Song, Tao ^[1]; Kanevce, Ana ^[2]; Sites, James R. ^[1]

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1. Physics Department, Colorado State University, Fort Collins, Colorado 80523, USA
2. National Renewable Energy Laboratory, Golden, Colorado 80401, USA

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_C ≤ 0.3 eV) can help maintain good cell efficiency in spite of high interface defect density, much like with Cu(In,Ga)Se₂ (CIGS) cells. The basic principle is that positive ΔE_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_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_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_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_C. As a result, these materials are predicted to yield higher voltages and would therefore be better candidates for the CdTe-cell emitter.

• Research Organization: National Renewable Energy Laboratory (NREL), Golden, CO (United States)
• Sponsoring Organization: USDOE Office of Energy Efficiency and Renewable Energy (EERE), Renewable Power Office. Solar Energy Technologies Office; U.S. Department of Energy SunShot program
• Grant/Contract Number: AC36-08GO28308; UGA-0-41027-18
• OSTI ID: 1257658
• Alternate ID(s): OSTI ID: 1257863; OSTI ID: 1270791
• Report Number(s): NREL/JA-5K00-66287; JAPIAU; 10.1063/1.4953820
• Journal Information: Journal of Applied Physics, Journal Name: Journal of Applied Physics Vol. 119 Journal Issue: 23; ISSN 0021-8979
• Publisher: American Institute of Physics (AIP)
• Country of Publication: United States
• Language: English

Related Subjects

14 SOLAR ENERGY
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
CdTe
numerical modeling
interface recombination
semiconductors
electronic transport
electronic bandstructure
computer simulation
photoconductivity
charge recombination
interface defects
heterostructures
electrical properties and parameters
solar cells