Vladan Stevanović

Hybrid Organic–Inorganic Perovskites (HOIPs): Opportunities and Challenges

The conclusions reached by a diverse group of scientists who attended an intense 2-day workshop on hybrid organic-inorganic perovskites are presented, including their thoughts on the most burning fundamental and practical questions regarding this unique class of materials, and their suggestions on various approaches to resolve these issues.

Defect Tolerant Semiconductors for Solar Energy Conversion.

Defect tolerance is the tendency of a semiconductor to keep its properties despite the presence of crystallographic defects. Scientific understanding of the origin of defect tolerance is currently missing. Here we show that semiconductors with antibonding states at the top of the valence band are likely to be tolerant to defects. Theoretical calculations demonstrate that Cu3N with antibonding valence band maximum has shallow intrinsic defects and no surface states, in contrast to GaN with bonding valence band maximum. Experimental measurements indicate shallow native donors and acceptors in Cu3N thin films, leading to 10(16)-10(17) cm(-3) doping with either electrons or holes depending on the growth conditions. The experimentally measured bipolar doping and the solar-matched optical absorption onset (1.4 eV) make Cu3N a promising candidate absorber for photovoltaic and photoelectrochemical solar cells, despite the calculated indirect fundamental band gap (1.0 eV). These conclusions can be extended to other materials with antibonding character of the valence band, defining a class of defect-tolerant semiconductors for solar energy conversion applications.

Methylammonium Bismuth Iodide as a Lead-Free, Stable Hybrid Organic-Inorganic Solar Absorber.

Methylammonium lead halide (MAPbX3 ) perovskites exhibit exceptional carrier transport properties. But their commercial deployment as solar absorbers is currently limited by their intrinsic instability in the presence of humidity and their lead content. Guided by our theoretical predictions, we explored the potential of methylammonium bismuth iodide (MBI) as a solar absorber through detailed materials characterization. We synthesized phase-pure MBI by solution and vapor processing. In contrast to MAPbX3, MBI is air stable, forming a surface layer that does not increase the recombination rate. We found that MBI luminesces at room temperature, with the vapor-processed films exhibiting superior photoluminescence (PL) decay times that are promising for photovoltaic applications. The thermodynamic, electronic, and structural features of MBI that are amenable to these properties are also present in other hybrid ternary bismuth halide compounds. Through MBI, we demonstrate a lead-free and stable alternative to MAPbX3 that has a similar electronic structure and nanosecond lifetimes.

Assessing capability of semiconductors to split water using ionization potentials and electron affinities only.

We show in this article that the position of semiconductor band edges relative to the water reduction and oxidation levels can be reliably predicted from the ionization potentials (IP) and electron affinities (AE) only. Using a set of 17 materials, including transition metal compounds, we show that accurate surface dependent IPs and EAs of semiconductors can be computed by combining density functional theory and many-body GW calculations. From the extensive comparison of calculated IPs and EAs with available experimental data, both from photoemission and electrochemical measurements, we show that it is possible to sort candidate materials solely from IPs and EAs thereby eliminating explicit treatment of semiconductor/water interfaces. We find that at pH values corresponding to the point of zero charge there is on average a 0.5 eV shift of IPs and EAs closer to the vacuum due to the dipoles formed at material/water interfaces.

Material descriptors for predicting thermoelectric performance

In the context of materials design and high-throughput computational searches for new thermoelectric materials, the need to compute electron and phonon transport properties renders direct assessment of the thermoelectric figure of merit (zT) for large numbers of compounds untenable. Here we develop a semi-empirical approach rooted in first-principles calculations that allows relatively simple computational assessment of the intrinsic bulk material properties which govern zT. These include carrier mobility, effective mass, and lattice thermal conductivity, which combine to form a semi-empirical metric (descriptor) termed βSE. We assess the predictive power of βSE against a range of known thermoelectric materials, as well as demonstrate its use in high-throughput screening for promising candidate materials.

Non-equilibrium deposition of phase pure Cu2O thin films at reduced growth temperature

Cuprous oxide (Cu2O) is actively studied as a prototypical material for energy conversion and electronic applications. Here we reduce the growth temperature of phase pure Cu2O thin films to 300 °C by intentionally controlling solely the kinetic parameter (total chamber pressure, Ptot) at fixed thermodynamic condition (0.25 mTorr pO2). A strong non-monotonic effect of Ptot on Cu-O phase formation is found using high-throughput combinatorial-pulsed laser deposition. This discovery creates new opportunities for the growth of Cu2O devices with low thermal budget and illustrates the importance of kinetic effects for the synthesis of metastable materials with useful properties.

Variations of ionization potential and electron affinity as a function of surface orientation: The case of orthorhombic SnS

We investigated the dependence of absolute SnS band-edge energies on surface orientation using density functional theory and GW method for all surfaces with Miller indices −3≤h,k,l≤3 and found variations as large as 0.9 eV as a function of (hkl). Variations of this magnitude may affect significantly the performance of photovoltaic devices based on polycrystalline SnS thin-films and, in particular, may contribute to the relatively low measured open circuit voltage of SnS solar cells. X-ray diffraction measurements confirm that our thermally evaporated SnS films exhibit a wide distribution of different grain orientations, and the results of Kelvin force microscopy support the theoretically predicted variations of the absolute band-edge energies.

Oxide enthalpy of formation and band gap energy as accurate descriptors of oxygen vacancy formation energetics

Despite the fundamental role oxygen vacancy formation energies play in a broad range of important energy applications, their relationships with the intrinsic bulk properties of solid oxides remain elusive. Our study of oxygen vacancy formation in La1−xSrxBO3 perovskites (BCr, Mn, Fe, Co, and Ni) conducted using modern, electronic structure theory and solid-state defect models demonstrates that a combination of two fundamental and intrinsic materials properties, the oxide enthalpy of formation and the minimum band gap energy, accurately correlate with oxygen vacancy formation energies. The energy to form a single, neutral oxygen vacancy decreases with both the oxide enthalpy of formation and the band gap energy in agreement with the relation of the former to metal–oxygen bond strengths and of the latter to the energy of the oxygen vacancy electron density redistribution. These findings extend our understanding of the nature of oxygen vacancy formation in complex oxides and provide a fundamental method for predicting oxygen vacancy formation energies using purely intrinsic bulk properties.

Intrinsic Material Properties Dictating Oxygen Vacancy Formation Energetics in Metal Oxides

Oxygen vacancies (V(O)) in oxides are extensively used to manipulate vital material properties. Although methods to predict defect formation energies have advanced significantly, an understanding of the intrinsic material properties that govern defect energetics lags. We use first-principles calculations to study the connection between intrinsic (bulk) material properties and the energy to form a single, charge neutral oxygen vacancy (E(V)). We investigate 45 binary and ternary oxides and find that a simple model which combines (i) the oxide enthalpy of formation (ΔH(f)), (ii) the midgap energy relative to the O 2p band center (E(O 2p) + (1/2)E(g)), and (iii) atomic electronegativities reproduces calculated E(V) within ∼0.2 eV. This result provides both valuable insights into the key properties influencing E(V) and a direct method to predict E(V). We then predict the E(V) of ∼1800 oxides and validate the predictive nature of our approach against direct defect calculations for a subset of 18 randomly selected materials.

Synthesis and optical band gaps of alloyed Si–Ge type II clathrates

Inorganic type II clathrates are low density, semiconducting allotropes of group IV elements with the potential for optoelectronic applications. This class of materials is predicted to have direct or nearly-direct band gaps, and, when Si and Ge are alloyed in the clathrate structure, the band gap is tunable in the range of 0.8–1.8 eV. In this work, we demonstrate for the first time the synthesis of alloyed Si–Ge type II clathrates. Within this alloy system, we find an amorphous region which is likely due to a miscibility gap. The optical absorptance spectra of the crystalline clathrate samples show the predicted band gap tuning with Ge content, and calculations find that the Si type II clathrate has a strong absorption coefficient for the direct interband transition. The findings in this work lay the foundation for the future use of type II clathrates in optoelectronic applications.