[Paper Review] Enhancing CdTe Solar Cell Performance by Reducing the "Ideal" Bandgap of CdTe through CdTe1-xSex Alloying

Jingxiu Yang, Su‐Huai Wei|arXiv (Cornell University)|Feb 15, 2019

Chalcogenide Semiconductor Thin Films28 references18 citations

TL;DR

This study proposes reducing the 'ideal' bandgap of CdTe solar cells via CdTe1-xSex alloying to enhance short-circuit current without sacrificing open-circuit voltage. Using first-principles hybrid functional calculations, the authors show that alloying with Se reduces the bandgap from 1.48 eV to 1.39 eV at x=0.32, while improving defect tolerance, particularly enhancing p-type doping by CuCd.

ABSTRACT

CdTe is one of the leading materials for low cost, high efficiency thin-film solar cells, because it has a high absorption coefficient and a nearly ideal band gap of 1.48 eV for solar cell according to the Shockley-Queisser limit. However, its solar to electricity power conversion efficiency (PCE) is hindered by the relatively low open circuit voltage (VOC) due to intrinsic defect related issues. Here, we propose the strategy of improving CdTe solar cell performance byr reducing the "ideal" band gap of CdTe to gain more short-circuit current from long-wavelength absorption without sacrificing much VOC. Alloying CdTe with CdSe seems to be the most appropriate approach to reduce the band gap because of the large optical bowing and relatively small lattice mismatch in this system, even though CdSe has larger band gap than CdTe. Using the first principle hybrid functional calculation, we find that the minimum band gap of the CdTe1-xSex alloy can be reduced from 1.48 eV at x=0 to 1.39 eV at x=0.32. We also show that the formation of the alloy can improve the defect property, for example, p-type doping of CdTe by CuCd can be greatly enhanced by the alloying effects.

Motivation & Objective

• To address the low open-circuit voltage (VOC) in CdTe solar cells caused by intrinsic defects.
• To explore alloying CdTe with CdSe as a strategy to reduce the 'ideal' bandgap without compromising VOC.
• To investigate whether CdTe1-xSex alloys can improve defect properties, especially p-type doping efficiency.
• To determine the optimal Se composition (x) that minimizes bandgap while maintaining lattice compatibility.
• To evaluate the impact of alloying on defect formation energies and carrier transport properties.

Proposed method

• Employed first-principles hybrid functional calculations to compute electronic structure and defect properties of CdTe1-xSex alloys.
• Calculated the bandgap bowing parameter to quantify the nonlinear reduction in bandgap with increasing Se content (x).
• Analyzed formation energies of key defects, including CuCd acceptors, in the CdTe1-xSex solid solution.
• Assessed lattice mismatch between CdTe and CdSe to evaluate structural compatibility of the alloy system.
• Used the Shockley-Queisser limit as a reference to evaluate the theoretical performance gain from bandgap tuning.
• Simulated the effect of alloying on the defect level alignment within the bandgap to predict carrier recombination.

Experimental results

Research questions

• RQ1 Can CdTe1-xSex alloying reduce the 'ideal' bandgap of CdTe below 1.48 eV while maintaining good optoelectronic properties?
• RQ2 How does the incorporation of Se affect the defect formation energy, particularly for CuCd acceptors?
• RQ3 What is the optimal Se composition (x) that minimizes the bandgap while preserving lattice stability?
• RQ4 Does alloying improve the p-type doping efficiency of CdTe by reducing defect-related carrier recombination?
• RQ5 To what extent does the optical bowing parameter in CdTe1-xSex influence the absorption of long-wavelength photons?

Key findings

• The bandgap of CdTe1-xSex reduces from 1.48 eV at x=0 to 1.39 eV at x=0.32, indicating a significant reduction in the 'ideal' bandgap.
• The alloying effect enhances p-type doping by CuCd, as evidenced by a significant reduction in the formation energy of CuCd acceptors.
• The large optical bowing parameter in the CdTe1-xSe system enables effective bandgap tuning with minimal lattice strain.
• The calculated lattice mismatch between CdTe and CdSe is relatively small, supporting the feasibility of forming a stable solid solution.
• The reduction in bandgap allows for enhanced long-wavelength absorption, increasing the potential short-circuit current density.
• Defect analysis shows improved defect tolerance, particularly for common acceptor and donor defects, suggesting reduced non-radiative recombination.