Study of phase equilibria and defect chemistry of the Cu-Zn-Sn-S system from first-principles and computational thermodynamics towards photovoltaic applications

Open Access
Author:
Guan, Pinwen
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
December 04, 2017
Committee Members:
  • Zi-Kui Liu, Dissertation Advisor
  • Zi-Kui Liu, Committee Chair
  • Roman Engel-Herbert, Committee Member
  • Ismaila Dabo, Committee Member
  • John B. Asbury, Outside Member
  • Vincent Crespi, Committee Member
Keywords:
  • Thermodynamics
  • DFT
  • CALPHAD
  • Solar materials
  • Defect
  • Semiconductors
Abstract:
Recently, there has been an increasing interest in the photovoltaic materials Cu2ZnSnS4 (CZTS) due to earth-abundancy and non-toxicity of its constituent elements. Its alloy with Se, Cu2ZnSn(S,Se)4 (CZTSSe), has reached a conversion efficiency of 12.7%. Numerous experimental studies have revealed that the phase constitutions and defect chemistry have very important influences on the conversion efficiency. To further improve the performance, a thorough understanding of the phase equilibria and defect chemistry of the Cu-Zn-Sn-S system is therefore urgently needed. In this dissertation, the phase equilibria and defect chemistry of the Cu-Zn-Sn-S system are modeled using a methodology integrating the CALculation of PHAse Diagram (CALPHAD) method and first-principles calculations. The S-Se system is also modeled due to its important role in CZTSSe. The first-principles phonon calculations based on the quasi-harmonic approximation and the Debye model are used to obtain the Gibbs energies of the solid phases. The central quantities in defect chemistry, the formation energies of point defects, are calculated based on the supercell approach with proper finite-size corrections for charged defects. The data obtained from first-principles calculations are used to estimate the parameters in the CALPHAD model, and finally generate a self-consistent thermodynamic description of the studied system in agreement with available experimental data in the literature. Based on the thermodynamic model, the phase-defect diagrams of the Cu-Zn-Sn-S system are plotted, showing the phase equilibria and defect concentrations simultaneously under different conditions. It is observed that for reducing secondary phases and harmful point defects, (i) the composition should be towards ZnS-SnS2 side; (ii) the excess sulfur should not be in a large amount; (iii) the temperature and the pressure should not be too high or too low and optimal values exist. A large portion of effort in this dissertation is devoted to the methodology development, which can be divided into three categories: first-principles calculations, CALPHAD method, and their integration. For the first-principles calculations, the configuration-based estimation (CBE) method is developed to calculate the formation entropies of point defects efficiently, which avoids the time-consuming phonon calculations of the defected supercell with low symmetry. In addition, a hybrid method is developed to calculate the thermodynamic properties of the crystal with a large unit cell efficiently, where the acoustic phonons are approximated using a Debye-like model and the optical phonons are explicitly calculated in terms of vibrational frequencies. For the CALPHAD method, a physical model of thermal vacancies is proposed, which not only solves the long-standing controversy within the CALPHAD community, but also generates vapor-solid phase diagram in agreement with the experimental data. For the integration of the first-principles calculations and the CALPHAD method, a sublattice model that can treat perfect lattice sites, isolated point defects, point defect complexes and the charge carriers simultaneously is devised, with the model parameters being computable from the first-principles calculations. The above methodology development enables the modeling of complicated multi-component semiconductor systems, which is demonstrated in the Cu-Zn-Sn-S system.