Haowei Peng

Evaluation of photovoltaic materials within the Cu-Sn-S family

Next-generation thin film solar cell technologies require earth abundant photovoltaic absorber materials. Here we demonstrate an alternative approach to design of such materials, evaluating candidates grouped by constituent elements rather than underlying crystal structures. As an example, we evaluate thermodynamic stability, electrical transport, electronic structure, optical and defect properties of Cu-Sn-S candidates using complementary theory and experiment. We conclude that Cu2SnS3 avoids many issues associated with the properties of Cu4SnS4, Cu4Sn7S16, and other Cu-Sn-S materials. This example demonstrates how this element-specific approach quickly identifies potential problems with less promising candidates and helps focusing on the more promising solar cell absorbers.

Electronic and thermoelectric analysis of phases in the In2O3(ZnO)k system

The high-temperature electrical conductivity and thermopower of several compounds in the In2O3(ZnO)k system (k=3, 5, 7, and 9) were measured, and the band structures of the k=1, 2, and 3 structures were predicted based on first-principles calculations. These phases exhibit highly dispersed conduction bands consistent with transparent conducting oxide behavior. Jonker plots (Seebeck coefficient versus natural logarithm of conductivity) were used to obtain the product of the density of states and mobility for these phases, which were related to the maximum achievable power factor (thermopower squared times conductivity) for each phase by Ioffe analysis (maximum power factor versus Jonker plot intercept). With the exception of the k=9 phase, all other phases were found to have maximum predicted power factors comparable to other thermoelectric oxides if suitably doped.

A computational framework for automation of point defect calculations

Abstract A complete and rigorously validated open-source Python framework to automate point defect calculations using density functional theory has been developed. The framework provides an effective and efficient method for defect structure generation, and creation of simple yet customizable workflows to analyze defect calculations. The package provides the capability to compute widely-accepted correction schemes to overcome finite-size effects, including (1) potential alignment, (2) image-charge correction, and (3) band filling correction to shallow defects. Using Si, ZnO and In2O3 as test examples, we demonstrate the package capabilities and validate the methodology.

Novel phase diagram behavior and materials design in heterostructural semiconductor alloys

Theoretically predicted metastable phases are realized in thin-film synthesis of Mn1−xZnxO and Sn1−xCaxS alloys. Structure and composition control the behavior of materials. Isostructural alloying is historically an extremely successful approach for tuning materials properties, but it is often limited by binodal and spinodal decomposition, which correspond to the thermodynamic solubility limit and the stability against composition fluctuations, respectively. We show that heterostructural alloys can exhibit a markedly increased range of metastable alloy compositions between the binodal and spinodal lines, thereby opening up a vast phase space for novel homogeneous single-phase alloys. We distinguish two types of heterostructural alloys, that is, those between commensurate and incommensurate phases. Because of the structural transition around the critical composition, the properties change in a highly nonlinear or even discontinuous fashion, providing a mechanism for materials design that does not exist in conventional isostructural alloys. The novel phase diagram behavior follows from standard alloy models using mixing enthalpies from first-principles calculations. Thin-film deposition demonstrates the viability of the synthesis of these metastable single-phase domains and validates the computationally predicted phase separation mechanism above the upper temperature bound of the nonequilibrium single-phase region.

Defect mechanisms in the In2O3(ZnO)k system (k = 3, 5, 7, 9)

The defect chemistry of several compounds in the In2O3(ZnO)k series (k = 3, 5, 7, and 9) was investigated in bulk specimens by analysis of the dependence of their conductivity on the oxygen partial pressure. The resulting Brouwer slopes were inconsistent with a doubly charged oxygen vacancy defect model, and varied with the phase. The k = 3 phase had behavior similar to donor-doped In2O3, and the behavior of the other phases resembled that of donor-doped ZnO. The donor in both cases is proposed to be In occupying Zn sites. First principles calculations of the formation energy of intrinsic defects in this system support the proposed models. The present work expands prior theoretical analysis to include acceptor defects, such as cation vacancies (VZn, VIn) and oxygen interstitials (Oi).

Pathway to oxide photovoltaics via band-structure engineering of SnO

All-oxide photovoltaics could open rapidly scalable manufacturing routes, if only oxide materials with suitable electronic and optical properties were developed. SnO has exceptional doping and transport properties among oxides, but suffers from a strongly indirect band gap. Here, we address this shortcoming by band-structure engineering through isovalent but heterostructural alloying with divalent cations (Mg, Ca, Sr, and Zn). Using first-principles calculations, we show that suitable band gaps and optical properties close to that of direct semiconductors are achievable, while the comparatively small effective masses are preserved in the alloys. Initial thin film synthesis and characterization support the feasibility of the approach.

Characterization of defects in copper antimony disulfide

Copper antimony disulfide (CuSbS2) with the chalcostibite structure is a promising photovoltaic (PV) absorber material with several excellent measured optoelectronic properties, such as a solar matched band gap and tunable hole concentration. However, much less is known from an experimental perspective about defects in CuSbS2, even though the defects are critical for solar cell performance. Here, we explore the defect properties in CuSbS2 thin film materials and photovoltaic devices using photoluminescence and capacitance-based spectroscopies, as well as first principles theoretical calculations. We measured three electrically and optically active acceptor defects in CuSbS2, and assigned them to the copper vacancies, sulfur vacancies, and/or copper on antimony antisites by comparison with theoretical calculations. Their activation energies, concentrations, and capture cross sections have been determined and compared to other chalcogenide absorber materials. These fundamental parameters should enable more accurate simulations of CuSbS2 PV devices, paving the way for future improvements in CuSbS2 solar cell efficiencies.

KAg11(VO4)4 as a candidate p-type transparent conducting oxide

For a material to be a good p-type transparent conducting oxide (TCO), it must simultaneously satisfy several design principles regarding its bulk and defect phase thermochemistry, its optical absorption spectrum, and its electric transport properties. Recently, we predicted Ag3VO4 to be p-type but with low conductivity and an optical band gap not large enough for transparency. To improve on the transport and optical properties of Ag3VO4, we searched an extended material space including quaternary compounds based on Ag, V, O, and an additional atom for a new candidate p-type TCO. From this set of quaternary materials, we selected KAg11(VO4)4, a known oxide with a crystal structure related to that of Ag3VO4. Notably, one could expect a possible enhancement of the concentration of hole producing Ag-vacancy defects in KAg11(VO4)4 due to its different local geometries of Ag atoms (2- and 3-fold coordinated) with respect to the 4-fold coordinated Ag atoms in Ag3VO4. By performing first-principles calculations, we found that KAg11(VO4)4 is an intrinsic p-type conductor and can be synthesized under conditions similar to those predicted for the synthesis of Ag3VO4. However, we predict that the intrinsic hole content in KAg11(VO4)4 is similar to that in Ag3VO4 even though KAg11(VO4)4 contains 2- and 3-fold coordinated Ag, hole producing sites with a lower defect formation energy than the 4-fold coordinated one. Our calculation demonstrates that the advantage from lower coordination number of the Ag atom in KAg11(VO4)4 can be offset by the change in the range of Ag chemical potential in which synthesis is allowed due to the oxide phases that Ag forms with K and that energetically compete with KAg11(VO4)4.

Addendum to “Convergence of density and hybrid functional defect calculations for compound semiconductors”

This paper reported first-principles supercell calculations for the formation enthalpy H of numerous charged defects in several semiconductors and insulators. The individual energy contributions leading to the final result for H , as given in Table IV of the original paper, were not included in the original submission. This Addendum provides detailed data allowing the interested reader to retrace the steps that lead to the final result, accessible through the Supplemental Material [1].