One-dimensional nanostructures, such as nanorod (NR) arrays, are expected to improve the photovoltaic (PV) response of solar cells with an ultrathin absorber due to an increased areal (junction) ...density and light trapping. We report on the deposition of CdS and CdTe:As semiconductor thin films on ZnO NR arrays by means of metalorganic chemical vapour deposition (MOCVD). The change in optical properties of the ZnO NRs upon the growth of CdS shell was monitored and compared to the simulated data, which confirmed the presence of strong light scattering effects in the visible and near infrared regions. The PV performance of nanostructured vs. planar CdS/CdTe solar cells (grown using the material from the same MOCVD run) showed similar conversion efficiencies (~ 4%), despite the current density being lower for the nanostructured cell due to its thicker CdS window. A clear improvement in the quantum efficiency was however observed in the near infrared region, resulting from the light trapping by the ZnO/CdS core-shell NR structure. We also showed that reduction of surface defects and use of high absorber carrier density would boost the efficiency beyond that of planar CdTe solar cells. The reported device performance and the direct observation of light trapping are promising towards optimisation of extremely-thin-absorber CdTe PV devices.
•Extremely thin absorber CdTe thin film solar cells fabricated using ZnO nanorod array films.•Conformal growth of CdS and CdTe shells on ZnO nanorods achieved by MOCVD.•Optical properties and light trapping were studied experimentally and theoretically.•Over 4% conversion efficiency reported using less than 0.5µm CdTe absorber.
Doping of CdTe using Group-V elements (As, P, and Sb) has gained interest in pursuit of increasing the cell voltage of CdTe thin film solar devices. Studies on bulk CdTe crystals have shown that much ...higher acceptor concentration than the traditional copper treatment is possible with As, P or Sb, enabled by high process temperature and/or rapid thermal quenching under Cd overpressure. We report a comprehensive study on in-situ As doping of poly-crystalline CdTe solar cells by MOCVD, whereby high acceptor densities, approaching 3 × 1016 cm−3 were achieved at low growth temperature of 390 °C. No As segregation could be detected at grain boundaries, even for 1019 As cm−3. A shallow acceptor level (+0.1 eV) due to AsTe substitutional doping and deep-level defects were observed at elevated As concentrations. Devices with variable As doping were analysed. Narrowing of the depletion layer, enhancement of bulk recombination, and reduction in device current and red response, albeit a small near infrared gain due to optical gap reduction, were observed at high concentrations. Device modelling indicated that the properties of the n-type window layer and associated interfacial recombination velocity are highly critical when the absorber doping is relatively high, demonstrating a route for obtaining high cell voltage.
The acceptor concentration of the CdTe absorber layer in CdZnS/CdTe thin film solar cells tuned up to 3 × 1016/cm3 via in-situ arsenic doping, and its influence on photovoltaic properties and the defect structure established. No grain-boundary segregation observed even at the highest arsenic concentrations. Devices become sensitive to window layer properties at high acceptor concentrations. Modelling suggests ways to suppress the interfacial recombination to achieve remarkable Voc values. Display omitted
•In-situ As doping of poly-crystalline CdTe films by MOCVD resulted in high acceptor densities up to 3 × 1016 cm−3.•The photovoltaic properties and defect structure of CdTe:As absorbers identified.•Jsc reduces slightly due to bulk recombination due to narrowing of depletion layer.•Voc increases but becomes sensitive to interface recombination at high As concentrations.•Modelling shows ways to suppress interface recombination and boost the Voc.
We report on the effects of using an atomic layer deposited ZnO transparent buffer layer with > 106 Ω cm resistivity on the performance of CdZnS/CdTe solar cells grown by metalorganic chemical vapour ...deposition (MOCVD). The buffer film thickness is adjusted by optical modelling to suppress the reflection losses at the front contact. A clear improvement, up to 1.8% in conversion efficiency, was obtained in comparison to reference devices without the ZnO buffer layer, thanks to the enhancement of the current density (Jsc) and fill factor (FF). Device spectral response showed improved collection for most of the visible region. Reflectance measurements confirmed that the ZnO film reduced the optical reflectance around the transparent front contact. This effect permitted light management through the front contact leading to an improvement of the Jsc and hence the photovoltaic conversion efficiency. These results are intriguing since the literature on CdTe solar cells did not previously report improvement to the photocurrent and device response through controlling the highly-resistive transparent buffer layer.
The light J-V curves (left) and EQE spectrum (right) of CdTe solar cells with and without a ZnO HRT layer.
PV enhancement of CdTe thin film solar cells using a 50 nm ALD ZnO buffer film reported. Reduced reflection at the front contact region contributed to the photocurrent significantly, boosting the conversion efficiency up to 1.8%. Optical engineering of the transparent front contact may further improve anti-reflection property and PV performance of CdTe thin film solar cells. Display omitted
•ALD grown ZnO films used in CdTe thin film solar cells as highly-resistive-transparent (HRT) buffer to impart AR effect.•BBoth the Jsc and FF improved significantly•Jsc gain originates mainly from reduced reflection at the front contact as predicted by the optical model.•Up to 1.8% net gain in conversion efficiency obtained using the ZnO HRT layer.
Recent developments in CdTe solar cell technology have included the incorporation of ternary alloy Cd(Se,Te) in the devices. CdTe absorber band gap grading due to Se alloying contributes to current ...density enhancement and can result in device performance improvement. Here we report Cd(Se,Te) polycrystalline thin films grown by a chamberless inline atmospheric pressure metal organic chemical vapour deposition technique, with subsequent incorporation in CdTe solar cells. The compositional dependence of the crystal structure and optical properties of Cd(Se,Te) are examined. Selenium graded Cd(Se,Te)/CdTe absorber structure in devices are demonstrated using either a single CdSe layer or CdSe/Cd(Se,Te) bilayer (with or without As doping in the Cd(Se,Te) layer). Cross-sectional TEM/EDS, photoluminescence spectra and secondary ion mass spectroscopy analysis confirmed the formation of a graded Se profile toward the back contact with a diffusion length of ~1.5 μm and revealed back-diffusion of Group V (As) dopants from the CdTe layer into Cd(Se,Te) grains. Due to the strong Se/Te interdiffusion, CdSe in the Se bilayer configuration was unable to form an n-type emitter layer in processed devices. In situ As doping of the Cd(Se,Te) layer benefited the device junction quality with current density reaching 28.3 mA/cm2. The results provide useful insights for the optimisation of Cd(Se,Te)/CdTe solar cells.
Compositional control of optical band gap in Cd(Se,Te) thin films (0 ≤ x ≤ 1) (left) and the Se profile in finished Cd(Se,Te) devices (right) fabricated by MOCVD. Display omitted
•Cd(Se,Te) thin films (0 ≤ x ≤ 1) deposited with linear compositional control using a chamber-less MOCVD process.•CdSe-only and CdSe/CdSe0.2Te0.8 bilayers utilised for Cd(Se,Te) grading in devices.•In-situ and ex-situ group V (arsenic) doping of the graded Cd(Se,Te) obtained.•Device performance improved for thinner CdSe layers and in-situ doped bilayers.•Good control of Se profile achieved via CdSe and CdSe/CdSe0.2Te0.8 properties.
Nickel (Ni) and cobalt (Co) nanowire arrays (NWs) grown by electrodeposition in porous nano-templates are studied by the ferromagnetic resonance (FMR) technique at room temperature (RT) by comparing ...the effects of template type (alumina and polycarbonate) and the deposition substrate (i.e., metallic back contact). The line-width and resonance field of the FMR spectra strongly depends on the orientation of the applied field direction. A model is developed to analyze the spectra in order to extract the magnetic parameters such as g-values, spin–spin relaxation times (T2) and uniaxial anisotropy parameters. The experimental FMR spectra and their resonance field values were fitted using the imaginary part of magnetic susceptibility and a dispersion relation of magnetization, including the Bloch–Bloembergen type damping term. The easy axes of magnetization for all Ni and Co NWs were found to be perpendicular to the wire-axis. Surface spin modes have been observed only when pure Au was used as substrate. A discussion will be provided to explain the observed differences in terms of the anisotropic behavior and magnetic parameters of the NWs for different substrates and growth templates.
Thin film CdTe solar cells were produced by MOCVD, at atmospheric pressure, under a hydrogen atmosphere (i.e. oxygen-free). Window layer alloying with zinc (forming Cd1−xZnxS) and extrinsic p-type ...doping with arsenic (giving CdTe:As) have been used to improve photovoltaic solar cell performances, but as-grown MOCVD-CdTe PV cells are still typically characterised by low Voc (~620–690mV). Post-deposition annealing in air for 30min at low temperature (170°C) prior to evaporation of the back contacts led to significant increases in Voc and FF. XPS measurements revealed back surface oxidation, resulting in formation of Te–O species. This was also the case for a device aged under ambient laboratory conditions. Extended annealing in air of a fresh device, for up to 180min, continued to improve both Voc and FF. At longer annealing times the Voc remained relatively stable whilst the FF started to deteriorate. External quantum efficiency showed loss of photocurrent generation after excessive oxidation prior to back contact metallisation. Controlled back surface oxidation resulted in Voc values exceeding 800mV and a best cell efficiency of 15.3%.
•Low temperature thermal treatment in air improves a CdTe solar cell PV performances by enhancing the Voc and FF.•X-ray photoelectron spectroscopy revealed an increase in oxygen at the back surface associated with Te–O binding energies.•The CdTe solar cells remained stable over a broad exposure period of oxidation treatments until FF eventually deteriorated.
The role of CdCl2 activation in the production of high quality CdTe-based photovoltaic devices remains a subject of much debate. In this study, CdTe-based cells produced in three independent ...laboratories using different device fabrication technologies are investigated before and after CdCl2 activation with regard to structural changes (recrystallisation and grain growth) and sulphur out-diffusion. Using scanning transmission electron microscopy (STEM) and x-ray diffraction it is demonstrated that CdCl2 activation of the investigated cells produces no statistical structural changes to the CdTe. Additionally, energy dispersive spectrometry (EDS) performed in the STEM on the same samples illustrates that the change in sulphur diffusion following activation is more limited than expected from previous studies; no change is detectable when the thermal budget for CdTe deposition is significantly greater than that for activation. This suggests that the efficiency enhancement during CdCl2 treatment is not due to sulphur out-diffusion. Lastly, cathodoluminescence microscopy is used to demonstrate in two dimensions how sulphur diffuses into a model sample and the results are found to be consistent with STEM–EDS. Some spectroscopic evidence for enhanced sulphur diffusion along grain boundaries is also observed.
•Chlorine activation was found to cause little additional sulphur diffusion in cells.•No recrystallisation or grain growth was observed in CdTe following Cl-activation.•Cathodoluminescence was used to observe sulphur diffusion in 2D.•Cells made by a range of methods had very similar sulphur diffusion profiles.
Ultra-thin CdTe:As/Cd1-xZnxS photovoltaic solar cells with an absorber thickness of 0.5 μm were deposited by metal-organic chemical vapour deposition on indium tin oxide coated boro-aluminosilicate ...substrates. The Zn precursor concentration was varied to compensate for Zn leaching effects after CdCl2 activation treatment. Analysis of the solar cell composition and structure by X-ray photoelectron spectroscopy depth profiling and X-ray diffraction showed that higher concentrations of Zn in the Cd1-xZnxS window layer resulted in suppression of S diffusion across the CdTe/Cd1-xZnxS interface after CdCl2 activation treatment. Excessive Zn content in the Cd1-xZnxS alloy preserved the spectral response in the blue region of the solar spectrum, but increased series resistance for the solar cells. A modest increase in the Zn content of the Cd1-xZnxS alloy together with a post-deposition air anneal resulted in an improved blue response and an enhanced open circuit voltage and fill factor. This device yielded a mean efficiency of 8.3% over 8 cells (0.25 cm2 cell area) and best cell efficiency of 8.8%.
•CdCl2 anneal treatment resulted in S diffusing to the back contact.•High Zn levels created mixed cubic/hexagonal structure at the p-n junction.•Increased Zn in Cd1-xZnxS supressed S diffusion into CdTe.•Device Voc was enhanced overall with an additional back surface air anneal.
Cd1−xZnxS and CdTe:As thin films were deposited using a recently developed chamberless inline process via metalorganic chemical vapour deposition (MOCVD) at atmospheric pressure and assessed for ...fabrication of CdTe photovoltaic (PV) solar cells. Initially, CdS and Cd1−xZnxS coatings were applied onto 15×15cm2 float glass substrates, characterised for their optical properties, and then used as the window layer in CdTe solar cells which were completed in a conventional MOCVD (batch) reactor. Such devices provided best conversion efficiency of 13.6% for Cd0.36Zn0.64S and 10% for CdS which compare favourably to the existing baseline MOCVD (batch reactor) devices. Next, sequential deposition of Cd0.36Zn0.64S and CdTe:As films was realised by the chamberless inline process. The chemical composition of a 1μm CdTe:As/150nm Cd0.36Zn0.64S bi-layer was observed via secondary ions mass spectroscopy, which showed that the key elements are uniformly distributed and the As doping level is suitable for CdTe device applications. CdTe solar cells formed using this structure provided a best efficiency of 11.8% which is promising for a reduced absorber thickness of 1.25μm. The chamberless inline process is non-vacuum, flexible to implement and inherits from the legacy of MOCVD towards doping/alloying and low temperature operation. Thus, MOCVD enabled by the chamberless inline process is shown to be an attractive route for thin film PV applications.
•CdS, CdZnS and CdTe thin films grown by a chamberless inline process•The inline films assessed for fabricating CdTe solar cells•13.6% conversion efficiency obtained for CdZnS/CdTe cells
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