[Show abstract][Hide abstract] ABSTRACT: Materials research with a focus on enhancing the minority-carrier lifetime of
the light-absorbing semiconductor is key to advancing solar energy technology
for both early-stage and mature material platforms alike. Tin sulfide (SnS) is
an absorber material with several clear advantages for manufacturing and
deployment, but the record power conversion efficiency remains below 5%. We
report measurements of bulk and interface minority-carrier recombination rates
in SnS thin films using optical-pump, terahertz (THz)-probe transient
photoconductivity (TPC) measurements. Post-growth thermal annealing in H_2S gas
increases the minority-carrier lifetime, and oxidation of the surface reduces
the surface recombination velocity. However, the minority-carrier lifetime
remains below 100 ps for all tested combinations of growth technique and
post-growth processing. Significant improvement in SnS solar cell performance
will hinge on finding and mitigating as-yet-unknown recombination-active
defects. We describe in detail our methodology for TPC experiments, and we
share our data analysis routines as freely-available software.
Full-text · Article · Jan 2016 · Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: Doping activity in both beta-phase (β-) and amorphous (a-) Sn-doped gallium oxide (Ga2O3:Sn) is investigated by X-ray absorption spectroscopy (XAS). A single crystal of β-Ga2O3:Sn grown using edge-defined film-fed growth at 1725°C is compared with amorphous Ga2O3:Sn films deposited at low temperature (<300°C). Our XAS analyses indicate that activated Sn dopant atoms in conductive single crystal β-Ga2O3:Sn are present as Sn4+, preferentially substituting for Ga at the octahedral site, as predicted by theoretical calculations. In contrast, inactive Sn atoms in resistive a-Ga2O3:Sn are present in either +2 or +4 charge states depending on growth conditions. These observations suggest the importance of growing Ga2O3:Sn at high temperature to obtain a crystalline phase and controlling the oxidation state of Sn during growth to achieve dopant activation.
Preview · Article · Dec 2015 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: We use electronic transport and atom probe tomography to study ZnO:Al / SiO2
/ Si Schottky junctions on lightly-doped n- and p-type Si. We vary the carrier
concentration in the the ZnO:Al films by two orders of magnitude but the
Schottky barrier height remains constant, consistent with Fermi level pinning
seen in metal / Si junctions. Atom probe tomography shows that Al segregates to
the interface, so that the ZnO:Al at the junction is likely to be metallic even
when the bulk of the ZnO:Al film is semiconducting. We hypothesize that Fermi
level pinning is connected to the insulator-metal transition in doped ZnO, and
that controlling this transition may be key to un-pinning the Fermi level in
oxide / Si Schottky junctions.
[Show abstract][Hide abstract] ABSTRACT: An outstanding challenge in the development of novel functional materials for optoelectronic devices is identifying suitable charge-carrier contact layers. Herein, we simulate the photovoltaic device performance of various n-type contact material pairings with tin(II) sulfide (SnS), a p-type absorber. The performance of the contacting material, and resulting device efficiency, depend most strongly on two variables: conduction band offset between absorber and contact layer, and doping concentration within the contact layer. By generating a 2D contour plot of device efficiency as a function of these two variables, we create a performance-space plot for contacting layers on a given absorber material. For a simulated high-lifetime SnS absorber, this 2D performance-space illustrates two maxima, one local and one global. The local maximum occurs over a wide range of contact-layer doping concentrations (below 1016 cm−3), but only a narrow range of conduction band offsets (0 to −0.1 eV), and is highly sensitive to interface recombination. This first maximum is ideal for early-stage absorber research because it is more robust to low bulk-minority-carrier lifetime and pinholes (shunts), enabling device efficiencies approaching half the Shockley-Queisser limit, greater than 16%. The global maximum is achieved with contact-layer doping concentrations greater than 1018 cm−3, but for a wider range of band offsets (−0.1 to 0.2 eV), and is insensitive to interface recombination. This second maximum is ideal for high-quality films because it is more robust to interface recombination, enabling device efficiencies approaching the Shockley-Queisser limit, greater than 20%. Band offset measurements using X-ray photoelectron spectroscopy and carrier concentration approximated from resistivity measurements are used to characterize the zinc oxysulfide contacting layers in recent record-efficiency SnS
devices. Simulations representative of these present-day devices suggest that record efficiency SnS
devices are optimized for the second local maximum, due to low absorber lifetime and relatively well passivated interfaces. By employing contact layers with higher carrier concentrations and lower electron affinities, a higher efficiency ceiling can be enabled.
[Show abstract][Hide abstract] ABSTRACT: We discuss techniques for performing continuous measurements across a wide range of pressure–field–temperature phase space, combining the milli-Kelvin temperatures of a helium dilution refrigerator with the giga-Pascal pressures of a diamond anvil cell and the Tesla magnetic fields of a superconducting magnet. With a view towards minimizing remnant magnetic fields and background magnetic susceptibility, we characterize high-strength superalloy materials for the pressure cell assembly, which allows high fidelity measurements of low-field phenomena such as superconductivity below 100 mK at pressures above 10 GPa. In situ tunability and measurement of the pressure permit experiments over a wide range of pressure, while at the same time making possible precise steps across abrupt phase transitions such as those from insulator to metal.
[Show abstract][Hide abstract] ABSTRACT: Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent thermal evaporation, and it absorbs visible light strongly. However, for a long time the record power conversion efficiency of SnS solar cells remained below 2%. Recently we demonstrated new certified record efficiencies of 4.36% using SnS deposited by atomic layer deposition, and 3.88% using thermal evaporation. Here the fabrication procedure for these record solar cells is described, and the statistical distribution of the fabrication process is reported. The standard deviation of efficiency measured on a single substrate is typically over 0.5%. All steps including substrate selection and cleaning, Mo sputtering for the rear contact (cathode), SnS deposition, annealing, surface passivation, Zn(O,S) buffer layer selection and deposition, transparent conductor (anode) deposition, and metallization are described. On each substrate we fabricate 11 individual devices, each with active area 0.25 cm(2). Further, a system for high throughput measurements of current-voltage curves under simulated solar light, and external quantum efficiency measurement with variable light bias is described. With this system we are able to measure full data sets on all 11 devices in an automated manner and in minimal time. These results illustrate the value of studying large sample sets, rather than focusing narrowly on the highest performing devices. Large data sets help us to distinguish and remedy individual loss mechanisms affecting our devices.
Preview · Article · May 2015 · Journal of Visualized Experiments
[Show abstract][Hide abstract] ABSTRACT: We quantify the effects of growth temperature on material and device
properties of thermally evaporated SnS
thin-films and test structures. Grain size, Hall mobility, and majority-carrier concentration monotonically increase with growth temperature. However, the charge collection as measured by the long-wavelength contribution to short-circuit current exhibits a non-monotonic behavior: the collection decreases with increased growth temperature from 150 °C to 240 °C and then recovers at 285 °C. Fits to the experimental internal quantum efficiency using an opto-electronic model indicate that the non-monotonic behavior of charge-carrier collection can be explained by a transition from drift- to diffusion-assisted components of carrier collection. The results show a promising increase in the extracted minority-carrier diffusion length at the highest growth temperature of 285 °C. These findings illustrate how coupled mechanisms can affect early stage device development, highlighting the critical role of direct materials property measurements and simulation.
Full-text · Article · May 2015 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: Secondary phase segregation is hypothesized to have detrimental impacts on Cu2ZnSnS4 (CZTS) thin-film solar cells. In this study, we demonstrate the potential of using kinetic stabilization to inhibit phase decomposition in CZTS. By growing CZTS films at low temperature, we achieve a kinetically stabilized alloy with an expanded solid solution window in the pseudoternary CuS-ZnS-SnS phase diagram. Using X-ray absorption spectroscopy, we study the structural evolution and stability of this metastable alloy upon annealing. For near-stoichiometric samples, we observe a continuous emergence of short-range order toward crystalline CZTS that is nearly complete after a 1-min anneal at 450 °C. For Zn-rich samples, we detect precipitation of ZnS upon annealing, which suggests that the excess Zn exists as cation antisite defects in metastable CZTS.
No preview · Article · Jan 2015 · IEEE Journal of Photovoltaics
[Show abstract][Hide abstract] ABSTRACT: The demand for low-cost and scalable renewable energy continues to spur research in thin-film photovoltaics (PV), in particular those with chalcogenide semiconductor absorbers, which can combine high power conversion efficiencies (PCE) with efficient materials utilization. Materials utilization is becoming increasingly efficient in PV manufacturing, and as a result capital equipment comprises an ever larger fraction of module manufacturing costs. Streamlined and high-throughput deposition systems are more easily realized when the physics of materials deposition works in one's favor. CdTe is a prominent example because the nearly equivalent vapor pressures of Cd and Te facilitate congruent evaporation, assisting feedstock purification to suppress deleterious second-phase particles and point defects. The biggest current loss mechanism is the 19% of the incident light that is absorbed in the SnS layer but not collected as current.
Full-text · Article · Nov 2014 · Advanced Materials
[Show abstract][Hide abstract] ABSTRACT: Tin (II) sulfide is a promising earth-abundant thin-film solar absorber material due to its strong absorption and near-optimal bandgap. We demonstrate phase-pure evaporation of SnS in a CdTe-like manufacturing process, achieving phase-pure SnS thin-films through thermal evaporation of SnS powder. We investigate the effects of SnS film thickness and growth rate on film morphology and correlate results with device performance. Working devices are achieved with SnS film thicknesses as low as 370 nm and growth rates of up to 50 Å/s, with efficiencies ranging from 1.1% to 2.6% in as-grown films.
[Show abstract][Hide abstract] ABSTRACT: We preform device simulations of a tin sulfide (SnS) device stack using SCAPS to define a path to 10% efficient devices. We determine and constrain a baseline device model using recent experimental results on one of our 3.9% efficient cells. Through a multistep fitting process, we find a conduction band cliff of -0.2 eV between SnS and Zn(O,S) to be limiting the open circuit voltage (VOC). To move towards a higher efficiency, we can optimize the buffer layer band alignment. Improvement of the SnS lifetime to >1 ns is necessary to reach 10% efficiency. Additionally, absorber-buffer interface recombination must be suppressed, either by reducing recombination activity of defects or creating a strong inversion layer at the interface.
[Show abstract][Hide abstract] ABSTRACT: Tin sulfide is regarded as a possible earth-abundant alternative for chalcogenide thin film photovoltaics. The material has strong absorption in the visible wavelength region and the possibility for high carrier mobility. We review recent progress for SnS solar cell efficiencies. Annealing in H2S gas and surface passivation of SnS are thought to be two key components that increase efficiency of SnS devices. An efficiency of η = 3.88%  was achieved via thermal evaporation, a manufacturing-friendly deposition method.
[Show abstract][Hide abstract] ABSTRACT: 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.
No preview · Article · May 2014 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: We have investigated the evolution of work function in epitaxial correlated perovskite SmNiO3 (SNO) thin films spanning the metal–insulator transition (MIT) by Kelvin probe force microscopy (KPFM). Combining contact-mode atomic force microscopy, KPFM and electrostatic force microscopy (EFM), we present charge writing processes associated with point defect engineering in SNO thin films. Surface potential tuning in two-terminal devices is demonstrated and compared to thermal control by proximity to the phase transition boundary. The charge distribution, retention, and diffusion on SNO were systematically examined. Local compositional changes by AFM-tip induced electric fields are shown to be a viable approach to spatially engineer electronic properties of correlated oxides towards eventual applications in electronics.
[Show abstract][Hide abstract] ABSTRACT: For most metals, increasing temperature (T) or disorder will quicken electron
scattering. This hypothesis informs the Drude model of electronic conductivity.
However, for so-called bad metals this predicts scattering times so short as to
conflict with Heisenberg's uncertainty principle. Here we introduce the
rare-earth nickelates (RNiO_3, R = rare earth) as a class of bad metals. We
study SmNiO_3 thin films using infrared spectroscopy while varying T and
disorder. We show that the interaction between lattice distortions and Ni-O
bond covalence explains both the bad metal conduction and the insulator-metal
transition in the nickelates by shifting spectral weight over the large energy
scale established by the Ni-O orbital interaction, thus enabling very low
\sigma while preserving the Drude model and without violating the uncertainty
[Show abstract][Hide abstract] ABSTRACT: The rare-earth nickelates (LnNiO3, Ln = lanthanide) are interesting from both fundamental and applied perspectives, but synthesis remains a bottleneck to research due to their thermodynamic instability. Here we report the synthesis of SmNiO3 thin films on oxidized silicon wafers by physical vapor deposition followed by high pressure oxygen annealing at intermediate temperatures. The high pressure annealed films show an insulator–metal transition characteristic of bulk samples. Our experimental observations then allow us to estimate bounds on the phase stability regime, which are particularly useful given the dearth of direct thermodynamic data available for LnNiO3. We examine the limitations of these thermodynamic analyses applied to ultra-thin films. The stabilization of SmNiO3 on a canonical semiconductor template creates opportunities to study the utility of the above room temperature insulator–metal transition (at TIM = 400 K) in electronic devices.
[Show abstract][Hide abstract] ABSTRACT: The rare-earth nickelates (RNiO3) exhibit interesting phenomena such as
unusual antiferromagnetic order at wavevector q = (1/2, 0, 1/2) and a tunable
insulator-metal transition that are subjects of active research. Here we
present temperature-dependent transport measurements of the resistivity,
magnetoresistance, Seebeck coefficient, and Hall coefficient (RH) of epitaxial
SmNiO3 thin films with varying oxygen stoichiometry. We find that from room
temperature through the high temperature insulator-metal transition, the Hall
coefficient is hole-like and the Seebeck coefficient is electron-like. At low
temperature the N\'eel transition induces a crossover in the sign of RH to
electron-like, similar to the effects of spin density wave formation in
metallic systems but here arising in an insulating phase ~200 K below the
insulator-metal transition. We propose that antiferromagnetism can be
stabilized by bandstructure even in insulating phases of correlated oxides,
such as RNiO3, that fall between the limits of strong and weak electron