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.
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 external quantum efficiencies of CdTe solar cells fabricated by the atmospheric pressure metal organic chemical vapour deposition (AP–MOCVD) method have been measured with one and two dye doped ...luminescence down-shifting (LDS) layers on top. Excitation energy transfer between the dyes is used to extend the absorption ability of the LDS layer to λ=350nm and increase the external quantum efficiency (EQE) of the cells for wavelengths λ<540nm. The observed increase in the EQE corresponds to a rise in the short circuit current density of 1.88mA/cm2 under AM1.5G illumination spectra, which is equivalent to a 10% relative solar cell efficiency increase. A simple model is presented, which accounts for the absorption and photon collection efficiencies of the LDS layer.
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
The role of the window layer in Cd1−xZnxS/CdTe photovoltaic (PV) device structures was investigated using a triple wavelength laser beam induced current (LBIC) technique. Full pn structures were ...deposited using metal organic chemical vapour deposition (MOCVD). The triple wavelength LBIC technique allowed for the diagnostic analysis of carrier collection uniformity at 3 wavelength dependent photon penetration depths, δp. An assessment of the Cd1−xZnxS window layer film thickness and surface coverage showed a correlation between lateral device performance and window layer thickness distribution. Islands of thick Cd1−xZnxS were shown to increase localised blue absorption, correlating to isolated regions of increased red and IR photoresponse. Overall device open-circuit voltage, Voc is shown to demonstrate a linear dependence on thick (∼300nm) Cd1−xZnxS surface coverage. Voc and shunt resistance Rsh losses attributed to the window layer were found to jointly contribute to the overall cell efficiency losses. A non-uniform pin-hole distribution, leading to Rsh loss, was found to be independent of the overall Cd1−xZnxS thickness distribution; thus the triple wavelength LBIC measurement was able to identify causes for the observed losses in both Voc and Rsh.
► Carrier collection uniformity in CdTe solar cells measured using an LBIC system. ► Cd1−xZnxS/CdTe solar cells grown using MOCVD. ► Cd1−xZnxS surface coverage and thickness uniformity is examined. ► Device Voc found to be heavily dependent on Cd1−xZnxS surface coverage. ► Complex pin-hole clustering found to impact Rsh of all cells examined.
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