Structure and Electronic Properties of Quaternary Chalcogenide Semiconductors from First Principles

The development of computer technology and the understanding of ab initio theoretical formalisms gave birth to computational materials science. Scientists thus have a powerful tool to keep exploring for new materials. At the same time, the necessity and desire to seek for new energy resource are undiminished. As solar energy plays an increasingly important role in environmentally friendly energy, researchers hope to find novel materials to replace silicon, which has been used in the field for decades. Utilizing computational materials science with the help of powerful supercomputers and well-developed approximate approaches to pursue material exploration is the focus of this thesis. There have been some commercially successful photovoltaic absorbers other than silicon, including CdTe and Cu (In,Ga)(S,Se)2. However, the low abundances of element Te and In, or the toxicity of Cd hinder terawatt deployment. To find substances taking the place of these toxic or rare elements, the kesterite Cu2ZnSn(S,Se)4 (CZTSSe), which substitutes In and Ga with Zn and Sn, is one alternative pathway, though the record achievable photovoltaic efficiency only reached 12.6% due to the antisite disorder enabled by the chemical and ionic size similarity of Zn and Cu. One solution to reduce antisite disorder is to replace Zn by Ba in Cu2BaSn(S,Se)4 (CBTSSe). Not only are the toxicity and rarity of the constituent elements avoided, but the coordination of Ba is also changed from 4-fold to 8-fold. The structure is therefore thought to become more resistant to the formation of structural defects. Since then, further substitutions of element in all four sites were pursued by exploring I2-II-IV-X4 (I = Li, Cu, Ag; II = Zn, Cd, Mn, Fe, Ba, Sr, Pb; IV = Si, Ge, Sn; X = S, Se), quaternary chalcogenides semiconductor material. At the outset of this thesis, the electronic, optical and structural properties were computed and analyzed using density functional theory for several compounds from the I2-II-IV-X4 family, which were successfully synthesized by colleagues from the experimental group of Dr. David Mitzi, including: Ag2BaSiS4 (indirect gap: 2.2 eV), Ag2PbSiS4 (indirect gap: 1.9 eV), Cu2PbGeS4 (indirect gap: 1.55 eV), Cu2SrSiS4 (direct gap: 3.4 eV), Ag2SrSiS4 (indirect gap: 2.08 eV) and Ag2SrGeS4 (indirect gap: 1.73 eV). Among them, Ag2PbSiS4, with direct band gap at about 2 eV (larger than optimal for single junction PV) and strong optical response within the visible spectrum, could be useful for tandem junction solar cells; though Cu2PbGeS4 shows strongest optical response, the indirect nature of bandgap is not ideal for application in PV devices; the high degree of band curvature (prospects of low effective masses and high mobilities) renders Ag2SrSiS4 and Ag2SrGeS4 suitable as PV buffer layer materials, and the inversion symmetry breaking enables them candidates for non-linear optical (NLO) applications. Broadening the I2-II-IV-X4 quaternary chalcogenide stoichiometry, this thesis next studies the quaternary chalcogenides in cubic phases of different stoichiometries. The study of I-II-IV-X (II = Sr, Pb; IV = Si, Ge, Sn; X = S, Se) involves solving partial site occupancies from the experimental geometry and electronic properties analysis to study the effect from geometric structure to band structure. Through an examination of the Ag-Pb-Si-S and Ag-Sr-Sn-S prototype element systems, they form in a cubic structure with vacancy-disordered Ag sites in stoichiometry of 2:3:2:8 instead of an orthorhombic structure with stoichiometry of 2:1:1:4. Ag2Pb3Si2S8 (direct gap: 1.95 eV), Ag2Sr3Si2S8 (direct gap: 2.66 eV), Ag2Sr3Si2S8 (direct gap: 2.87 eV), Ag2Sr3Ge2S8 (direct gap: 1.95 eV) and Ag2Sr3Ge2Se8 (direct gap: 1.90 eV) all have noncentrosymmetric crystal structure, indicating potential for frequency doubling devices. Ag2Pb3Si2S8 and Ag2Sr3Ge2Se8 have band gaps within a reasonable regime for light-assisted water-splitting applications. The third thrust of the thesis focuses on the I2-I’-V-X4. Ag2(NH4)AsS4, which has been synthesized previously and has an experimental band gap of 2.05 eV, is researched by molecular dynamics to understand the local configuration of ammonium and to quantify the effect of ammonium movement on the electronic properties. Ammonium is found to rotate freely within the cage of eight S atoms, connected with band gap fluctuations up to approximately 0.3 eV. The final chapter of the thesis focuses on divalent Eu, a rare-earth element with suitable ionic radius to form quaternary chalcogenides as I2-Eu-IV-X4 (I = Li, Cu, Ag; IV = Si, Ge, Sn; X = S, Se). This project uses the hybrid density functional HSE06 to determine an appropriate choice of the exchange parameter ? to accurately describe the highly correlated f orbitals of Eu, a key prerequisite to understand the accuracy and limit of this class of theory for predicting energy band structures in these f electron containing systems. While higher α values are needed to capture the f band position in literature ARPES benchmark data for EuS, a lower range of ? values, about 0.3, appears to empirically cover diffuse reflectance spectroscopy (DRS) determined band gaps in I2-II-IV-X4 compounds, consistent with the case of Cu2EuSnSe4. Comprehensive prediction of properties across I2-Eu-IV-X4 chalcogenides reveals a significant dependence on I-site composition. It is found that Ag-based compounds have indirect band gaps and are most stable in the I222 and 〖Ama2〗^†(APGS) space groups. In contrast, Li-based compounds favor the I4 ̅2m space group and have a direct gap. Though the bandgap transition in Li-based compounds is direct, the predicted band gap values are too large for most photovoltaic applications. Cu-based compounds energetically prefer the Ama2 and P31 space groups and contain the most promising direct bandgap materials for photovoltaics. From the band structure, and subsequent experimental results, Cu2EuSnSe4 (direct band gap: 1.53 eV (Ama2)) emerges as a promising candidate for photovoltaic application due to its direct band gap and strong optical response within the visible light range.