Melissa Johnson

Calculation of the lattice dynamics and Raman spectra of copper zinc tin chalcogenides and comparison to experiments

The electronic structure, lattice dynamics, and Raman spectra of the kesterite, stannite, and pre-mixed Cu-Au (PMCA) structures of Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) were calculated using density functional theory (DFT). Differences in longitudinal and transverse optical (LO-TO) splitting in kesterite, stannite, and PMCA structures can be used to differentiate them. The Γ-point phonon frequencies, which give rise to Raman scattering, exhibit small but measurable shifts, for these three structures. Experimentally measured Raman scattering from CZTS and CZTSe thin films were examined in light of DFT calculations and deconvoluted to explain subtle shifts and asymmetric line shapes often observed in CZTS and CZTSe Raman spectra. Raman spectroscopy in conjunction with ab initio calculations can be used to differentiate between kesterite, stannite, and PMCA structures of CZTS and CZTSe.

Alkali-metal-enhanced grain growth in Cu2ZnSnS4 thin films

The highest efficiency solar cells based on copper zinc tin sulfide (CZTS), a promising photovoltaic material comprised of earth abundant elements, are built on soda lime glass (SLG), a substrate which contains many impurities, including Na and K. These impurities may diffuse into CZTS films during processing and affect film structure and properties. We have investigated the effects of these impurities on the microstructure of CZTS films synthesized by ex situ sulfidation of Cu–Zn–Sn alloy films co-sputtered on SLG, Pyrex, and quartz. CZTS films synthesized on SLG were found to have significantly larger grains than films grown on the other substrates. Furthermore, we show that by including a bare additional piece of SLG in the sulfidation ampoule, the grain size of films grown on nominally impurity-free quartz increases from 100s of nm to greater than 1 μm. This demonstrates conclusively that impurities in SLG volatilize in S-containing atmospheres and incorporate into nearby CZTS films synthesized on other substrates. Impurity concentrations in these CZTS films were examined using depth profiling with time-of-flight secondary ion mass spectrometry (TOF-SIMS). Of all the impurities present in SLG, the TOF-SIMS experiments implicated Na, K, and Ca as possible elements responsible for the enhanced grain growth. To investigate the effects of these impurities individually, we introduced very small and controllable amounts of Na, K, or Ca into the sulfidation ampoule during CZTS synthesis. Impurity amounts as low as 10−6 moles of Na or 10−7 moles of K resulted in a dramatic increase in grain size, from 100s of nm to several microns, for films deposited on quartz, while Ca loading had no visible effect on the final microstructure. Based on this vapor transport mechanism, we thus demonstrate an approach for delivering precisely controlled amounts of specific impurities into CZTS films on arbitrary substrates to facilitate large-grain growth.

Crossover From Nanoscopic Intergranular Hopping to Conventional Charge Transport in Pyrite Thin Films

Pyrite FeS2 is receiving a resurgence of interest as a uniquely attractive thin film solar absorber based on abundant, low-cost, nontoxic elements. Here we address, via ex situ sulfidation synthesis, the long-standing problem of understanding conduction and doping in FeS2 films, an elusive prerequisite to successful solar cells. We find that an abrupt improvement in crystallinity at intermediate sulfidation temperatures is accompanied by unanticipated crossovers from intergranular hopping to conventional transport, and, remarkably, from hole-like to electron-like Hall coefficients. The hopping is found to occur between a small volume fraction of conductive nanoscopic sulfur-deficient grain cores (beneath our X-ray diffraction detection limits), embedded in nominally stoichiometric FeS2. In addition to placing constraints on the conditions under which useful properties can be obtained from FeS2 synthesized in diffusion-limited situations, these results also emphasize that FeS2 films are not universally p-type. Indeed, with no knowledge of the active transport mechanism we demonstrate that the Hall coefficient alone is insufficient to determine the sign of the carriers. These results elucidate the possible transport mechanisms in thin film FeS2 in addition to their influence on the deduced carrier type, an enabling advancement with respect to understanding and controlling doping in pyrite films.

Phase Stability and Stoichiometry in Thin Film Iron Pyrite: Impact on Electronic Transport Properties.

The use of pyrite FeS2 as an earth-abundant, low-cost, nontoxic thin film photovoltaic hinges on improved understanding and control of certain physical and chemical properties. Phase stability, phase purity, stoichiometry, and defects, are central in this respect, as they are frequently implicated in poor solar cell performance. Here, phase-pure polycrystalline pyrite FeS2 films, synthesized by ex situ sulfidation, are subject to systematic reduction by vacuum annealing (to 550 °C) to assess phase stability, stoichiometry evolution, and their impact on transport. Bulk probes reveal the onset of pyrrhotite (Fe(1-δ)S) around 400 °C, rapidly evolving into the majority phase by 425 °C. This is supported by X-ray photoelectron spectroscopy on (001) crystals, revealing surface Fe(1-δ)S formation as low as 160 °C, with rapid growth near 400 °C. The impact on transport is dramatic, with Fe(1-δ)S minority phases leading to a crossover from diffusive transport to hopping (due to conductive Fe(1-δ)S nanoregions in an FeS2 matrix), followed by metallicity when Fe(1-δ)S dominates. Notably, the crossover to hopping leads to an inversion of the sign, and a large decrease in magnitude of the Hall coefficient. By tracking resistivity, magnetotransport, magnetization, and structural/chemical parameters vs annealing, we provide a detailed picture of the evolution in properties with stoichiometry. A strong propensity for S-deficient minority phase formation is found, with no wide window where S vacancies control the FeS2 carrier density. These findings have important implications for FeS2 solar cell development, emphasizing the need for (a) nanoscale chemical homogeneity, and (b) caution in interpreting carrier types and densities.

Reactive sputter deposition of pyrite structure transition metal disulfide thin films: Microstructure, transport, and magnetism

Transition metal disulfides crystallizing in the pyrite structure (e.g., TMS2, with TM = Fe, Co, Ni, and Cu) are a class of materials that display a remarkably diverse array of functional properties. These properties include highly spin-polarized ferromagnetism (in Co1−xFexS2), superconductivity (in CuS2), an antiferromagnetic Mott insulating ground state (in NiS2), and semiconduction with close to optimal parameters for solar absorber applications (in FeS2). Exploitation of these properties in heterostructured devices requires the development of reliable and reproducible methods for the deposition of high quality pyrite structure thin films. In this manuscript, we report on the suitability of reactive sputter deposition from metallic targets in an Ar/H2S environment as a method to achieve exactly this. Optimization of deposition temperature, Ar/H2S pressure ratio, and total working gas pressure, assisted by plasma optical emission spectroscopy, reveals significant windows over which deposition of single-...

Evaluating spinel ferrites MFe2O4 (M = Cu, Mg, Zn) as photoanodes for solar water oxidation: prospects and limitations

The search for ideal semiconductors for photoelectrochemical solar fuel conversion has recently recognized the spinel ferrites as promising candidates due to their optoelectronic tunability together with superb chemical stability. However, a systematic understanding of the main material factors limiting their performance is currently lacking. Herein, nanostructured thin-film electrodes of three representative spinels, namely CuFe2O4 (CFO), MgFe2O4 (MFO) and ZnFe2O4 (ZFO), are prepared by a solution-based approach and their photoelectrochemical (PEC) properties are comprehensively characterized. Annealing post-treatments together with the deposition of NiFeOx overlayers are found to improve the native n-type response, although a dominant bulk recombination (especially in MFO) limits the saturation photocurrents (below 0.4 mA cm−2 at 1.23 V vs. RHE). Likewise, prominent Fermi level pinning due to surface states at around 0.9 V vs. RHE in all cases appears to limit the photovoltage (to ca. 300 mV). Rapid-scan voltammetry is used to gain insight into the surface states and the operation of the overlayer. Interestingly, the NiFeOx is ineffective at mitigating Fermi level pinning, but clearly participates as an electrocatalyst to improve the overall performance. Generally, these results evidence the potential and current intrinsic limitations of the spinel ferrites—establishing a roadmap for the optimization of these materials as photoanodes for solar water oxidation.

Defect Mitigation of Solution-Processed 2D WSe2 Nanoflakes for Solar-to-Hydrogen Conversion

Few-atomic-layer nanoflakes of liquid-phase exfoliated semiconducting transition metal dichalcogenides (TMDs) hold promise for large-area, high-performance, low-cost solar energy conversion, but their performance is limited by recombination at defect sites. Herein, we examine the role of defects on the performance of WSe2 thin film photocathodes for solar H2 production by applying two separate treatments, a pre-exfoliation annealing and a post-deposition surfactant attachment, designed to target intraflake and edge defects, respectively. Analysis by TEM, XRD, XPS, photoluminescence, and impedance spectroscopy are used to characterize the effects of the treatments and photoelectrochemical (PEC) measurements using an optimized Pt-Cu cocatalyst (found to offer improved robustness compared to Pt) are used to quantify the performance of photocathodes (ca. 11 nm thick) consisting of 100-1000 nm nanoflakes. Surfactant treatment results in an increased photocurrent attributed to edge site passivation. The pre-annealing treatment alone, while clearly altering the crystallinity of pre-exfoliated powders, does not significantly affect the photocurrent. However, applying both defect treatments affords a considerable improvement that represents a new benchmark for the performance of solution-processed WSe2: solar photocurrents for H2 evolution up to 4.0 mA cm-2 and internal quantum efficiency over 60% (740 nm illumination). These results also show that charge recombination at flake edges dominates performance in bare TMD nanoflakes, but when the edge defects are passivated, internal defects become important and can be reduced by pre-annealing.

Robust Hierarchically Structured Biphasic Ambipolar Oxide Photoelectrodes for Light-Driven Chemical Regulation and Switchable Logic Applications

Tunable ambipolar photoelectrochemical behavior emerges from microdomains of nanostructured p-type CuFeO2 and n-type Fe2 O3 that arise from a single facile solution-processed thin film. The switchable operation of this system is controlled by chemical, optical, or electronic inputs with a uniquely high photocurrent response (on the order of 1 mA cm-2 ), suitable for robust practical application as an oxygen photoregulator.

Substrate and temperature dependence of the formation of the Earth abundant solar absorber Cu2ZnSnS4 by ex situ sulfidation of cosputtered Cu-Zn-Sn films

Copper zinc tin sulfide (CZTS) thin films were synthesized by ex situ sulfidation of Cu-Zn-Sn metal alloy precursor films cosputtered from Cu, Cu-Zn, and Cu-Sn targets onto five different substrate materials: single crystal quartz, fused quartz, sapphire, Pyrex, and soda lime glass (SLG). Cosputtered precursor films, which were found to consist of Cu, Zn, and Sn metals and Cu6.26Sn5 ordered alloys, were sulfidized between 100 and 600 °C, corresponding to an S pressure range of 0.051–36 Torr. While CZTS forms at temperatures as low as 300 °C on all substrates, the films phase composition is dominated by binary metal sulfides between 300 and 400 °C. Significant phase composition variations among films synthesized on different substrates begin to emerge at 400 °C. Films grown on SLG are nearly phase pure CZTS by 500 °C, with small amounts of ZnS. In contrast, films deposited on all other substrates persistently contain significant amounts of impurity phases such as SnS2 and Cu4Sn7S16 until the sulfidation temperature is increased to 600 °C. Significant grain growth also begins between 500 and 600 °C. At 600 °C, CZTS films synthesized on SLG were found to have significantly larger grains than films grown on any of the other substrates. These results demonstrate that CZTS phase purity and grain size, properties that may affect solar cell performance, are affected by impurity diffusion from the SLG substrate, further emphasizing the importance of selecting appropriate substrates.Copper zinc tin sulfide (CZTS) thin films were synthesized by ex situ sulfidation of Cu-Zn-Sn metal alloy precursor films cosputtered from Cu, Cu-Zn, and Cu-Sn targets onto five different substrate materials: single crystal quartz, fused quartz, sapphire, Pyrex, and soda lime glass (SLG). Cosputtered precursor films, which were found to consist of Cu, Zn, and Sn metals and Cu6.26Sn5 ordered alloys, were sulfidized between 100 and 600 °C, corresponding to an S pressure range of 0.051–36 Torr. While CZTS forms at temperatures as low as 300 °C on all substrates, the films phase composition is dominated by binary metal sulfides between 300 and 400 °C. Significant phase composition variations among films synthesized on different substrates begin to emerge at 400 °C. Films grown on SLG are nearly phase pure CZTS by 500 °C, with small amounts of ZnS. In contrast, films deposited on all other substrates persistently contain significant amounts of impurity phases such as SnS2 and Cu4Sn7S16 until the sulfidation t...

Semiconducting alternating multi-block copolymers via a di-functionalized macromonomer approach

The development of fully-conjugated semiconducting block-copolymers is an important goal for organic electronics, but to date has been almost exclusively limited to materials containing poly(3-alkylthiophenes). Here we present the prototype of a class of fully-conjugated semiconducting block copolymers (prepared using a versatile route based on conjugated macromonomers and a cross-coupling polycondensation) that exhibit hole mobility in field effect transistors of the order of 0.1 cm2 V−1 s−1 and nanoscopic phase domain separation.