In the last decade, transparent amorphous oxide semiconductors (TAOS) have become an essential component of many electronics, from ultra high resolution displays to solar cells. However, these disordered oxides typically rely on expensive component metals like indium to provide sufficient charge carrier conduction, and their optoelectronic properties are not as predictable and well-described as those of traditional, crystalline semiconductors. Herein we report on our comprehensive study of the amorphous zinc-tin-oxide (a-ZTO) system for use as an indium-free, n-type TAOS. Using a combination of high-throughput co-deposition growth, high resolution spectral mapping, and atomistic calculations, we explain the development of disorder-related subgap states in SnO2-like a-ZTO and optical bandgap reduction in ZnO-like a-ZTO. In addition, we report on a composition-induced electronic and structural transition in ZnO-like a-ZTO resulting in an exceptionally high figure of merit, comparable to that of amorphous indium-gallium-zinc-oxide. Our results accelerate the development of a-ZTO and similar systems as indium-free TAOS materials.

Cu2Sn1-xGexS3 is a p-type semiconductor alloy currently investigated for use as an absorber layer in thin film solar cells. The aim of this study is to investigate the properties of this alloy in thin film form in order to establish relationships between group IV composition and structural, vibrational and opto-electronic properties. Seven single phase Cu2Sn1-xGexS3 films are prepared from x = 0 to 1, showing a uniform distribution of Ge and Sn laterally and in depth. The films all show a monoclinic crystal structure. The lattice parameters are extracted using Le Bail refinement and show a linear decrease with increasing Ge content. Using density-functional theory with hybrid functionals, we calculate the Raman active phonon frequencies of Cu2SnS3 and Cu2GeS3. For the alloyed compounds, we use a virtual atom approximation. The shift of the main Raman peak from x = 0 to x =1 can be explained as being half due to the change in atomic masses and half being due to the different bond strength. The bandgaps of the alloys are extracted from photoluminescence measurements and increase linearly from about 0.90 to 1.56 eV with increasing Ge. The net acceptor density of all films is around 10(18)cm(-3). These analyses have established that the alloy forms a solid solution over the entire composition range meaning that intentional band gap grading should be possible for future absorber layers. The linear variation of the unit cell parameters and the band gap with group IV content allows composition determination by scattering or optical measurements. Further research is required to reduce the doping density by two orders of magnitude in order to improve the current collection within a solar cell device structure. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The band alignment of Ta2O5/GaN has been measured experimentally. The HCl cleaning has been found to be effective in removing oxygen from the GaN surface and was used prior to deposition of the Ta2O5 films by radio frequency magnetron sputtering. Variable angle spectroscopic ellipsometry was performed to measure the thickness, optical constants and band gap of GaN and Ta2O5/GaN samples. The valence band offset of Ta2O5/GaN was measured by X-ray photoelectron spectroscopy using Kraut's method, and found to be 0.70±0.25eV. The results provide experimental evidence of an earlier theoretical study of higher effective barrier for holes than for electrons in the Ta2O5/GaN material system.

The electronic and atomic structures of amorphous transparent tin oxides have been investigated by a combination of X-ray spectroscopy and atomistic calculations. Crystalline SnO is a promising p-type transparent oxide semiconductor due to a complex lone-pair hybridization that affords both optical transparency despite a small electronic band gap and spherical s-orbital character at the valence band edge. We find that both of these desirable properties (transparency and s-orbital valence band character) are retained upon amorphization despite the disruption of the layered lone-pair states by structural disorder. We explain the anomalously large band gap widening necessary to maintain transparency in terms of lone-pair stabilization via atomic clustering. Our understanding of this mechanism suggests that continuous hole conduction pathways along extended lone pair clusters should be possible under certain stoichiometries. Moreover, these findings should be applicable to other lone-pair active semiconductors.

A general modelling methodology has been developed to evaluate the effects of chemical interdiffusion and misfit dislocations on the performance of heterojunction solar cells made from highly mismatched materials. Results for the exemplar materials system CdS-CdTe are contrary to the widely held belief that such interdiffusion is beneficial to photovoltaic performance. In the model, recombination is presumed to take place at the cores of misfit dislocations, with the distribution of these dislocations in the interdiffused layer being calculated so as to minimise the total energy (an incidental result shows that the total number of dislocations is independent of the diffusion profile). The model takes calculated chemical profiles, optical absorption, and dislocation distributions from which the photovoltaic performance and recombination losses are evaluated. It was shown that for the realistic case in which the interdiffused region does not extend beyond the space charge region, the photovoltage losses dominate over any photocurrent gains. Methods to engineer mixed junctions that may increase solar conversion efficiency are discussed.

CdTe thin-film solar cells are now the main industrially established alternative to silicon-based photovoltaics. These cells remain reliant on the so-called chloride activation step in order to achieve high conversion efficiencies. Here, by comparison of effective and ineffective chloride treatments, we show the main role of the chloride process to be the modification of grain boundaries through chlorine accumulation, which leads an increase in the carrier lifetime. It is also demonstrated that while improvements in fill factor and short circuit current may be achieved through use of the ineffective chlorides, or indeed simple air annealing, voltage improvement is linked directly to chlorine incorporation at the grain boundaries. This suggests that focus on improved or more controlled grain boundary treatments may provide a route to achieving higher cell voltages and thus efficiencies.

6,. brief review of the scientific status of thin film solar cells is presented, and the generic research challenges identified. A study of the control of grain size in the sublimation growth of CdTe is described in which the pressure of an ambient gas was used to control the nucleation density and hence grain size. Grain diameter varied with pressure according to D (!lm) = 0.027 (±a.Oll) x P (torr) + 0.90 (±0.31) for pressures up to 200 torr. It was found that PV devices made using large-grained CdTe increased in efficiency up to the point where the grain boundary resistances were no longer limiting. The electrical performance of CdS/ITO and CdS/Sn02 junctions were investigCltedfor both as-grown and oxygen annealed CdS surfaces. The latter induced rectification, regardless of the substrate type, this being attributed to the formation of a CdO/CdS junction. Attempts to influence the performance of CdTe/CdS devices grown on oxidized and reduced CdS are described.

The electrical properties of bi-layer Ta2O5/Al2O3 and Nb2O5/Al2O3 metal-insulator-insulator-metal nanostructures as rectifiers have been investigated. The ultra-thin (1-6 nm) insulator layers were deposited by atomic-layer deposition or rf magnetron sputtering with Al as metal contacts. Variable angle spectroscopic ellipsometry was performed to extract the optical properties and band gap of narrow band gap insulator layers while the surface roughness of the metal contacts was measured by atomic force microscopy. Superior low voltage large signal and small signal nonlinearities such as asymmetry of 18 at 0.35 V, rate of change of non-linearity of 7.5 V-1, and responsivity of 9 A/W at 0.2 V were observed from the current-voltage characteristics. A sharp increase in current at similar to 2 V on Ta2O5/Al2O3 device can be ascribed to resonant tunneling. (C) 2015 Elsevier B.V. All rights reserved.

A vapour-phase reaction process has been used to deposit smooth and uniform CH3NH3PbI3 perovskite material to enable the measurement of its optical dispersion relations, n and k, by ellipsometry. Fitting was achieved with a combination of Tauc–Lorenz, critical point parabolic band (CPPB) and harmonic oscillators. We have used the dispersion relations in an all-optical model of new planar device architectures in order to establish design rules for future materials choices to maximize the short-circuit current (Jsc) performance. For 500nm of MAPI with no window layer, the maximum performance expected from the model is Jsc=21.63mAcm−2. The ability of thin layers (in the range 20–60nm) of a range of window layer materials (TiO2, WO3, ZnO, Nb2O5, CdS, and Cd0.4 Zn0.6S) to enhance the short-circuit current of the devices was investigated. The performance of the oxides showed interference behaviour, with the first maxima in their Jsc curves exceeding the value achievable without a window layer. However, after the first maximum, the performance generally fell off with increasing thickness. The only material to stay greater than the no-window condition for the entire investigated range is WO3. The highest performance (Jsc of 22.47mAcm−2) was obtained with 59nm of WO3, with that of TiO2, ZnO, and Nb2O5 being marginally lower. Parasitic absorption in CdS window layers caused the Jsc to decrease for all non-zero thicknesses – it gives no interference enhancement and its use cannot be recommended on optical grounds. Use of the wider gap alloy Cd0.4Zn0.6S gave higher currents than did CdS but its performance was not so high as for the oxides. Observations are made on the practicalities of fabricating the target structures in the fabrication of practical PV devices.

The CdCl 2 treatment is a key step in CdTe solar cell fabrication. However, despite its near ubiquitous use, the process is nonideal as CdCl 2 is both expensive and potentially hazardous to utilize in processing. In this paper, we report on the development of a NH 4 Cl replacement to the CdCl 2 process, which is a low-cost noncarcinogenic alternative. Comparative cells were fabricated and compared via C -V, J-V, scanning electron microscopy, and external quantum efficiency analysis. Further process optimization led to device efficiencies of up to 11.5%, achieved using this new process, with V OC values of up to 832 mV, which is relatively high.

Transparent conducting oxides (TCOs), with high optical transparency (≥85%) and low electrical resistivity (10−4 Ω·cm) are used in a wide variety of commercial devices. There is growing interest in replacing conventional TCOs such as indium tin oxide with lower cost, earth abundant materials. In the current study, we dope Zr into thin ZnO films grown by atomic layer deposition (ALD) to target properties of an efficient TCO. The effects of doping (0–10 at.% Zr) were investigated for ~100 nm thick films and the effect of thickness on the properties was investigated for 50–250 nm thick films. The addition of Zr4+ ions acting as electron donors showed reduced resistivity (1.44 × 10−3 Ω·cm), increased carrier density (3.81 × 1020 cm−3), and increased optical gap (3.5 eV) with 4.8 at.% doping. The increase of film thickness to 250 nm reduced the electron carrier/photon scattering leading to a further reduction of resistivity to 7.5 × 10−4 Ω·cm and an average optical transparency in the visible/near infrared (IR) range up to 91%. The improved n-type properties of ZnO: Zr films are promising for TCO applications after reaching the targets for high carrier density (>1020 cm−3), low resistivity in the order of 10−4 Ω·cm and high optical transparency (≥85%).

Ellipsometry was used to measure the amplitude ratio and phase difference of light undergoing a phase shift as it interacts with a thin film of organic–inorganic hybrid perovskite CH3NH3PbI3 (MAPI) deposited onto a (100) silicon wafer. The refractive index and extinction coefficient was extracted from a multi-oscillator model fit to the ellipsometry data, as a function of wavelength, from 300 to 1500nm.

The CdCl2 treatment is a key step in CdTe solar cell fabrication. However, despite its near ubiquitous use, the process is nonideal as CdCl2 is both expensive and potentially hazardous to utilize in processing. In this paper, we report on the development of a NH4 Cl replacement to the CdCl2 process, which is a low-cost noncarcinogenic alternative. Comparative cells were fabricated and compared via C-V, J-V, scanning electron microscopy, and external quantum efficiency analysis. Further process optimization led to device efficiencies of up to 11.5%, achieved using this new process, with V-OC values of up to 832 mV, which is relatively high.

A combinatorial methodology has been adopted to determine the optimum composition of a Cd(1-x)ZnxS window layer for CdTe solar cells. The methodology generated a large, self consistent dataset which permitted an unambiguous relationship between x, conversion efficiency and related cell parameters to be determined. An optimum composition of x = 0.57 was shown to maximise cell efficiency. Analysis of J - V curves, measured over 72 separate cells show that both short circuit current, J(SC), and fill factor, FF, values increase with respect to x over the range 0.1 - 0.5 7. EQE measurements show that further increases in J(SC) value are limited by the band gap of the highly resistive transparent (HRT) ZnO layer. The methodology demonstrates a rapid route, compared to conventional experiments, to the further optimisation of CdTe solar cells.

CuSbS2 is emerging as a novel absorber for sustainable photovoltaics. We fabricated CuSbS2 thin films by the sulfurization of sputtered metallic layers. Characterization of the morphological, optical, electrical and surface properties is presented. Overpressure of inert gas during sulfurization reduced antimony loss. The largely single-phase films exhibit optimal characteristics for PV applications, including a band gap of about 1.5 eV and p-type conductivity. The carrier concentrations were ~1017 cm-3 and the mobility was generally about 10 cm2 V-1 s-1. Surface property measurements are dominated by Sb2O3 and a secondary phase may be responsible for PL emission above the optical gap.

MgCl 2 is shown to be a cheap and non-toxic replacement for the costly and environmentally unfriendly salt CdCl 2 that has long been used as the ‘activation’ step in the production of cadmium telluride solar cells.

ABSTRACTA major limitation of the cross‐section electron beam‐induced current method—the use of roughly fractured surfaces to provide the cross‐section—has been overcome with the use of focused ion beam microscope sample preparation. Using this method, it was possible to undertake a study of the relation between junction position and the corresponding external quantum efficiency (EQE) curves. For the case of cadmium telluride (CdTe) cells, it was demonstrated that the EQE curve shape that indicates a buried CdTe homojunction only arises if the junction is buried by more than 0.9 µm from the heterointerface. This highlights the limitations of interpreting EQE curve shape to determine junction position. It was also shown that extended postgrowth annealing degrades the cadmium sulfide by Kirkendall voiding, and this leads to efficiency loss. © 2014 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons, Ltd.

A combinatorial methodology, developed for the rapid optimisation of sputtered transparent conducting oxides, was applied to Si doped ZnO. A wide range of compositions have been explored over a single sample to determine an optimum composition, with respect to the minimisation of resistivity, of x = 0.65% wt. SiO2. A fundamental investigation of the conduction band non-parabolicity yields values of me0=0.35m0 and C = 0.3 eV−1 for the conduction band minimum effective mass and the non-parabolicity factor, respectively. The variation of extracted band gap values with respect to dopant concentration provided an estimate of the magnitude of re-normalization effects. A model is proposed to describe the carrier transport behaviour for a degenerate polycrystalline semiconductor by accounting for the tunnelling of carriers through grain boundaries.

The processes of the noncatalytic synthesis of structures with CdTe nanowires by magnetron sputtering deposition are studied. It is shown that the deposition of magnetron sputtered CdTe onto substrates covered by a porous SiO2 layer can result in CdTe nanowires formation. The porosity of SiO2 layers with thicknesses from 2 to 15 nm fabricated by magnetron sputtering deposition is estimated.