Angela N. Fioretti

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.

Synthesis, structure, and optoelectronic properties of II–IV–V2 materials

II–IV–V2 materials offer the promise of enhanced functionality in optoelectronic devices due to their rich ternary chemistry. In this review, we consider the potential for new optoelectronic devices based on nitride, phosphide, and arsenide II–IV–V2 materials. As ternary analogs to the III–V materials, these compounds share many of the attractive features that have made the III–Vs the basis of modern optoelectronic devices (e.g. high mobility, strong optical absorption). Control of cation order parameter in the II–IV–V2 materials can produce significant changes in optoelectronic properties at fixed chemical composition, including decoupling band gap from lattice parameter. Recent progress has begun to resolve outstanding questions concerning the structure, dopability, and optical properties of the II–IV–V2 materials. Remaining research challenges include growth optimization and integration into heterostructures and devices.

Understanding and control of bipolar self-doping in copper nitride

Semiconductor materials that can be doped both n-type and p-type are desirable for diode-based applications and transistor technology. Copper nitride (Cu3N) is a metastable semiconductor with a solar-relevant bandgap that has been reported to exhibit bipolar doping behavior. However, deeper understanding and better control of the mechanism behind this behavior in Cu3N is currently lacking in the literature. In this work, we use combinatorial growth with a temperature gradient to demonstrate both conduction types of phase-pure, sputter-deposited Cu3N thin films. Room temperature Hall effect and Seebeck effect measurements show n-type Cu3N with an electron density of 1017 cm-3 for low growth temperature (≈ 35 °C) and p-type with a hole density between 1015 cm-3 and 1016 cm-3 for elevated growth temperatures (50 °C to 120 °C). Mobility for both types of Cu3N was ≈ 0.1 cm2/Vs to 1 cm2/V. Additionally, temperature-dependent Hall effect measurements indicate that ionized defects are an important scattering mechanism in p-type films. By combining X-ray absorption spectroscopy and first-principles defect theory, we determined that VCu defects form preferentially in p-type Cu3N while Cui defects form preferentially in n-type Cu3N; suggesting that Cu3N is a compensated semiconductor with conductivity type resulting from a balance between donor and acceptor defects. Based on these theoretical and experimental results, we propose a kinetic defect formation mechanism for bipolar doping in Cu3N, that is also supported by positron annihilation experiments. Overall, the results of this work highlight the importance of kinetic processes in the defect physics of metastable materials, and provide a framework that can be applied when considering the properties of such materials in general.

Effects of low temperature annealing on the transport properties of zinc tin nitride

ZnSnN2 has recently garnered increasing interest as a potential solar absorber material due to its direct bandgap that is predicted to be tunable from 1.0-2.1 eV based on cation disorder. One important challenge to the further development of this material for photovoltaics (PV) is to reliably synthesize films with carrier density ≤1017 electrons cm-3. In this work, we perform a systematic annealing study on compositionally-graded Zn-Sn-N thin films to determine the effects on carrier density and transport of such post-growth treatment. We find that annealing up to 6 hr under an activated nitrogen atmosphere results in a reduction in carrier density by ~80% for zinc-rich films, and by ~50% for stoichiometric films. However, we also find that annealing reduces mobility as a function of increasing annealing time. This result suggests that initial film disorder hampers the benefits to film quality that should have been gained through annealing. This finding highlights that carefully managed initial growth conditions will be necessary to obtain PV-quality ZnSnN2 absorber films.

Nitride layer screening as carrier-selective contacts for silicon heterojunction solar cells

A three-architecture method for screening new materials’ viability as carrier-selective contacts for silicon solar cells is presented. Test-structure solar cells were fabricated with a standard silicon heterojunction contact for the front side, and one of three treatments on the back: bare silicon, intrinsic amorphous silicon (i-aSi:H), or n-type amorphous silicon on an i-aSi:H passivation layer. Then, the candidate contact material of interest was deposited on the back of each test structure and the cells were finished with evaporated Al. By analysing the current-voltage characteristics of each type of architecture, the carrier selectivity and surface passivation quality of novel contact materials or material stacks can be rapidly screened. To demonstrate the utility of this method, we present a preliminary investigation of nitride compounds as electron-selective contacts in silicon solar cells, in particular zinc tin nitride (ZnSnN2), which is naturally n-type and has favourable band alignments with c-Si. ZnSnN2 was deposited by reactive sputtering as an electron contact on each test structure. No passivation was observed, but decent electron-selectivity was observed when in direct contact with the silicon wafer. When combined with intrinsic amorphous silicon, poor performances were obtained with poor selectivity and the occurrence of an S-shape, likely due to insufficient selectivity of the ZTN. Similar results were obtained for 2 nm and 20 nm thick layers, indicating selectivity did not result from the Al over-layer, but was in fact due to the ZTN itself. Overall, this work shows the three-architecture screening method is a useful tool for assessing novel contact materials for silicon heterojunction solar cells.A three-architecture method for screening new materials’ viability as carrier-selective contacts for silicon solar cells is presented. Test-structure solar cells were fabricated with a standard silicon heterojunction contact for the front side, and one of three treatments on the back: bare silicon, intrinsic amorphous silicon (i-aSi:H), or n-type amorphous silicon on an i-aSi:H passivation layer. Then, the candidate contact material of interest was deposited on the back of each test structure and the cells were finished with evaporated Al. By analysing the current-voltage characteristics of each type of architecture, the carrier selectivity and surface passivation quality of novel contact materials or material stacks can be rapidly screened. To demonstrate the utility of this method, we present a preliminary investigation of nitride compounds as electron-selective contacts in silicon solar cells, in particular zinc tin nitride (ZnSnN2), which is naturally n-type and has favourable band alignments with c-S...

Exciton photoluminescence and benign defect complex formation in zinc tin nitride

Emerging photovoltaic materials need to prove their viability by demonstrating excellent electronic properties. In ternary and multinary semiconductors, disorder and off-stoichiometry often cause defects that limit the potential for high-efficiency solar cells. Here we report on Zn-rich ZnSnN2 (Zn/(Zn + Sn) = 0.67) photoluminescence, high-resolution X-ray diffraction, and electronic structure calculations based on Monte-Carlo structural models. The mutual compensation of Zn excess and O incorporation affords a desirable reduction of the otherwise degenerate n-type doping, but also leads to a strongly off-stoichiometric and disordered atomic structure. It is therefore remarkable that we observe only near-edge photoluminescence from well-resolved excitons and shallow donors and acceptors. Based on first principles calculations, this result is explained by the mutual passivation of ZnSn and ON defects that renders both electronically benign. The calculated bandgaps range between 1.4 and 1.8 eV, depending on the degree of non-equilibrium disorder. The experimentally determined value of 1.5 eV in post-deposition annealed samples falls within this interval, indicating that further bandgap engineering by disorder control should be feasible via appropriate annealing protocols.