The positions of electronic band edges are one important metric for determining a material's capability to function in a solar energy conversion device that produces fuels from sunlight. In particular, the position of the valence band maximum (conduction band minimum) must lie lower (higher) in energy than the oxidation (reduction) reaction free energy in order for these reactions to be thermodynamically favorable. We present first principles quantum mechanics calculations of the band edge positions in five transition metal oxides and discuss the feasibility of using these materials in photoelectrochemical cells that produce fuels, including hydrogen, methane, methanol, and formic acid. The band gap center is determined within the framework of DFT+U theory. The valence band maximum (conduction band minimum) is found by subtracting (adding) half of the quasiparticle gap obtained from a non-self-consistent GW calculation. The calculations are validated against experimental data where possible; results for several materials including manganese(ii) oxide, iron(ii) oxide, iron(iii) oxide, copper(i) oxide and nickel(ii) oxide are presented.

Long-term sustainable solar energy conversion relies on identifying economical and versatile semiconductor materials with appropriate band structures for photovoltaic and photocatalytic applications (e.g., band gaps of ∼ 1.5-2.0 eV). Nickel oxide (NiO) is an inexpensive yet highly promising candidate. Its charge-transfer character may lead to longer carrier lifetimes needed for higher efficiencies, and its conduction band edge is suitable for driving hydrogen evolution via water-splitting. However, NiO's large band gap (∼ 4 eV) severely limits its use in practical applications. Our first-principles quantum mechanics calculations show band gaps dramatically decrease to ∼ 2.0 eV when NiO is alloyed with Li2O. We show that Lix Ni1-x O alloys (with x=0.125 and 0.25) are p-type semiconductors, contain states with no impurity levels in the gap and maintain NiO's desirable charge-transfer character. Lastly, we show that the alloys have potential for photoelectrochemical applications, with band edges well-placed for photocatalytic hydrogen production and CO2 reduction, as well as in tandem dye-sensitized solar cells as a photocathode.

Quantum computers have the potential to advance material design and drug discovery by performing costly electronic structure calculations. A critical aspect of this application requires optimizing the limited resources of the quantum hardware. Here, we experimentally demonstrate an end-to-end pipeline that focuses on minimizing quantum resources while maintaining accuracy. Using density matrix embedding theory as a problem decomposition technique, and an ion-trap quantum computer, we simulate a ring of 10 hydrogen atoms without freezing any electrons. The originally 20-qubit system is decomposed into 10 two-qubit problems, making it amenable to currently available hardware. Combining this decomposition with a qubit coupled cluster circuit ansatz, circuit optimization, and density matrix purification, we accurately reproduce the potential energy curve in agreement with the full configuration interaction energy in the minimal basis set. Our experimental results are an early demonstration of the potential for problem decomposition to accurately simulate large molecules on quantum hardware.

Solar cells based on single pn junctions, employing single-gap semiconductors can ideally achieve efficiencies as high as 34%. Developing solar cells based on intermediate-band semiconductors (IBSCs), which can absorb light across multiple band gaps, is a possible way to defy this theoretical limit and achieve efficiencies as high as 60%. Here, we use first principles quantum mechanics methods and introduce CoO and Co0.25Ni0.75O as possible IBSCs. We show that the conduction band in both of these materials is divided into two distinct bands separated by a band gap. We further show that the lower conduction band (i.e., the intermediate band) is wider in Co0.25Ni0.75O compared with CoO. This should enhance light absorption from the valence band edge to the intermediate band, making Co0.25Ni0.75O more appropriate for use as an IBSC. Our findings provide the basis for future attempts to partially populate the intermediate band and to reduce the lower band gap in Co0.25Ni0.75O in order to enhance the potential of this material for use in IBSC solar cell technologies. Furthermore, with proper identification of heterojunctions and dopants, CoO and Co0.25Ni0.75O could be used in multi-color light emitting diode and laser technologies.

We use two different ab initio quantum mechanics methods, complete active space self-consistent field theory applied to electrostatically embedded clusters and periodic many-body G0W0 calculations, to reanalyze the states formed in nickel(II) oxide upon electron addition and ionization. In agreement with interpretations of earlier measurements, we find that the valence and conduction band edges consist of oxygen and nickel states, respectively. However, contrary to conventional wisdom, we find that the oxygen states of the valence band edge are localized whereas the nickel states at the conduction band edge are delocalized. We argue that these characteristics may lead to low electron–hole recombination and relatively efficient electron transport, which, coupled with band gap engineering, could produce higher solar energy conversion efficiency compared to that of other transition-metal oxides. Both methods find a photoemission/inverse-photoemission gap of 3.6–3.9 eV, in good agreement with the experimental range, lending credence to our analysis of the electronic structure of NiO.