Jeffrey P. Bosco

Interface stoichiometry control to improve device voltage and modify band alignment in ZnO/Cu2O heterojunction solar cells

The interface stoichiometry of cuprous oxide (Cu2O) was controlled by adjusting the O2 and Zn partial pressures during ZnO sputter deposition and measured by high-resolution X-ray photoelectron spectroscopy of ultrathin (<3 nm) ZnO films on Cu2O. Open-circuit voltage measurements for ZnO/Cu2O heterojunctions under AM1.5 illumination were measured and it was found that a stoichiometric interface can achieve the voltage entitlement dictated by the band alignment, whereas the non-stoichiometric interface showed large open-circuit voltage deficits. These results highlight not only the need for stoichiometric interfaces in Cu2O devices, but also a reproducible experimental method for achieving stoichiometric interfaces that could be applied to any potential heterojunction partner. Additionally, valence-band offset measurements indicated changing the interface stoichiometry shifted the band alignment between Cu2O and ZnO, which accounts for the variation in previously reported band offset values.

Band alignment of epitaxial ZnS/Zn3P2 heterojunctions

The energy-band alignment of epitaxial zb-ZnS(001)/α-Zn_(3)P_(2)(001) heterojunctions has been determined by measurement of shifts in the phosphorus 2p and sulfur 2p core-level binding energies for various thicknesses (0.6–2.2 nm) of ZnS grown by molecular beam epitaxy on Zn_(3)P_(2). In addition, the position of the valence-band maximum for bulk ZnS and Zn3P2 films was estimated using density functional theory calculations of the valence-band density-of-states. The heterojunction was observed to be type I, with a valence-band offset, ΔE_V, of −1.19 ± 0.07 eV, which is significantly different from the type II alignment based on electron affinities that is predicted by Anderson theory. n^(+)-ZnS/p-Zn_(3)P_(2) heterojunctions demonstrated open-circuit voltages of >750 mV, indicating passivation of the Zn_(3)P_(2) surface due to the introduction of the ZnS overlayer. Carrier transport across the heterojunction devices was inhibited by the large conduction-band offset, which resulted in short-circuit current densities of <0.1 mA cm^(−2) under 1 Sun simulated illumination. Hence, constraints on the current density will likely limit the direct application of the ZnS/Zn_(3)P_(2) heterojunction to photovoltaics, whereas metal-insulator-semiconductor structures that utilize an intrinsic ZnS insulating layer appear promising.

Performance enhancement of a graphene-zinc phosphide solar cell using the electric field-effect.

The optical transparency and high electron mobility of graphene make it an attractive material for photovoltaics. We present a field-effect solar cell using graphene to form a tunable junction barrier with an Earth-abundant and low cost zinc phosphide (Zn3P2) thin-film light absorber. Adding a semitransparent top electrostatic gate allows for tuning of the graphene Fermi level and hence the energy barrier at the graphene-Zn3P2 junction, going from an ohmic contact at negative gate voltages to a rectifying barrier at positive gate voltages. We perform current and capacitance measurements at different gate voltages in order to demonstrate the control of the energy barrier and depletion width in the zinc phosphide. Our photovoltaic measurements show that the efficiency conversion is increased 2-fold when we increase the gate voltage and the junction barrier to maximize the photovoltaic response. At an optimal gate voltage of +2 V, we obtain an open-circuit voltage of V oc = 0.53 V and an efficiency of 1.9% under AM 1.5 1-sun solar illumination. This work demonstrates that the field effect can be used to modulate and optimize the response of photovoltaic devices incorporating graphene.

Energy-band alignment of II-VI/Zn3P2 heterojunctions from x-ray photoemission spectroscopy

The energy-band alignments for zb-ZnSe(001)/α-Zn_(3)P_2(001), w-CdS(0001)/α-Zn_(3)P_2(001), and w-ZnO(0001)/α-Zn_(3)P_2(001) heterojunctions have been determined using high-resolution x-ray photoelectron spectroscopy via the Kraut method. Ab initio hybrid density functional theory calculations of the valence-band density of states were used to determine the energy differences between the core level and valence-band maximum for each of the bulk materials. The ZnSe/Zn_(3)P_2 heterojunction had a small conduction-band offset, ΔEC, of −0.03 ± 0.11 eV, demonstrating a nearly ideal energy-band alignment for use in thin-film photovoltaic devices. The CdS/Zn_(3)P_2 heterojunction was also type-II but had a larger conduction-band offset of ΔEC = −0.76 ± 0.10 eV. A type-III alignment was observed for the ZnO/Zn_(3)P_2 heterojunction, with ΔEC = −1.61 ± 0.16 eV indicating the formation of a tunnel junction at the oxide–phosphide interface. The data also provide insight into the role of the II-VI/Zn_(3)P_2 band alignment in the reported performance of Zn_(3)P_2 heterojunction solar cells.

Passivation of Zn3P2 substrates by aqueous chemical etching and air oxidation

Surface recombination velocities measured by time-resolved photoluminescence and compositions of Zn_(3)P_2 surfaces measured by x-ray photoelectron spectroscopy (XPS) have been correlated for a series of wet chemical etches of Zn_(3)P_2 substrates. Zn_(3)P_2 substrates that were etched with Br_2 in methanol exhibited surface recombination velocity values of 2.8 × 10^4 cm s^(−1), whereas substrates that were further treated by aqueous HF–H_(2)O_2 exhibited surface recombination velocity values of 1.0 × 10^4 cm s^(−1). Zn_(3)P_2 substrates that were etched with Br_2 in methanol and exposed to air for 1 week exhibited surface recombination velocity values of 1.8 × 10^3 cm s^(−1), as well as improved ideality in metal/insulator/semiconductor devices.

Molecular beam epitaxy of n-type ZnS: A wide band gap emitter for heterojunction PV devices

Low-temperature epitaxy of zinc-blende ZnS films on GaAs(001) substrates was demonstrated by compound-source molecular beam epitaxy. The epitaxial relationship between the film and substrate was determined by reflection high-energy electron diffraction, high-resolution X-ray diffraction, and selected area electron diffraction measurements to be ZnS(001)∥GaAs(001). Strain-relaxation of the ZnS lattice occurred within the first 300 nm of film growth. The resistivity of the films could be tuned through the incorporation of an Al impurity dopant. The lowest thin-film resistivity achieved was 0.003 Ω-cm, with corresponding electron carrier concentration and mobility of roughly 4.5×1019 cm-3 and 46 cm2 V-1 s-1, respectively. Al, Ag, and In metals were found to make good ohmic contact to heavily doped ZnS films, whereas ITO and AZO transparent conductive oxides did not. Applications to novel PV devices incorporating low electron affinity absorbers are discussed.