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For successful long-term deployment and operation of kesterites Cu2ZnSn(S x Se1-x )4 (CZTSSe) as light-absorber materials for photovoltaics, device stability and recovery in kesterite solar cells are investigated. A low-temperature heat treatment is applied to overcome the poor charge extraction that developed in the natural aging process. It is suggested that defect states at aged CZTSSe/CdS heterojunctions were reduced, while apparent doping density in the CZTSSe absorber increased due to Cd/Zn interdiffusion at the heterojunction during the annealing process. In situ annealing experiments in a transmission electron microscope were used to investigate the elemental diffusion at the CZTSSe/CdS heterojunction. This study reveals the critical role of heat treatment to enhance the absorber/Mo back contact, improve the quality of the absorber/buffer heterojunction, and recover the device performance in aged kesterite thin-film solar cells. Ministry of Education (MOE) Published version This work was supported by the Engineering and Physical Sciences Research Council (EPSRC) (EP/T005491/1 and EP/S023836/1). The authors also appreciate the support from the North East Centre for Energy Materials (NECEM) (EP/ R021503/1) and the British Council Newton Fund Institutional Links Grant (CRP01286). This research was funded by the Ministry of Education, Singapore Tier 2 MOE2019-T2-2- 066.

Aluminium-doped zinc oxide (AZO) is a potential low-cost alternative to indium tin oxide (ITO) for application in optoelectronic devices as a transparent conducting thin film. Typically, AZO thin films are deposited using expensive, high vacuum equipment with high energy cost and materials wastage. In this study, slot-die coating was used as an inexpensive alternative to vacuum deposition to form AZO nanoparticle thin films under ambient laboratory conditions. The films were characterised structurally, optically and electrically and compared with a commercially obtained AZO film fabricated using physical vapour deposition (PVD). Structural characterisation of the nanoparticle film shows uniform coverage across the substrate with increased crystal quality following annealing in Ar up to 500 $$^{\circ }$$ ∘ C. The optical properties of the nanoparticle film exhibit a wider band gap than the PVD film, while the high density of grain boundary defects between the nanoparticles inhibits sheet conductivity. Graphical abstract

Cu 2ZnSn(S,Se) 4 (CZTSSe) is a promising material for thin-film photovoltaics, however, the open-circuit voltage ( V OC) deficit of CZTSSe prevents the device performance from exceeding 13% conversion efficiency. CZTSSe is a heavily compensated material that is rich in point defects and prone to the formation of secondary phases. The landscape of these defects is complex and some mitigation is possible by employing non-stoichiometric conditions. Another route used to reduce the effects of undesirable defects is the doping and alloying of the material to suppress certain defects and improve crystallization, such as with germanium. The majority of works deposit Ge adjacent to a stacked metallic precursor deposited by physical vapour deposition before annealing in a selenium rich atmosphere. Here, we use an established hot-injection process to synthesise Cu 2ZnSnS 4 nanocrystals of a pre-determined composition, which are subsequently doped with Ge during selenisation to aid recrystallisation and reduce the effects of Sn species. Through Ge incorporation, we demonstrate structural changes with a negligible change in the energy bandgap but substantial increases in the crystallinity and grain morphology, which are associated with a Ge-Se growth mechanism, and gains in both the V OC and conversion efficiency. We use surface energy-filtered photoelectron emission microscopy (EF-PEEM) to map the surface work function terrains and show an improved electronic landscape, which we attribute to a reduction in the segregation of low local effective work function (LEWF) Sn(II) chalcogenide phases.

This paper is motivated by the potential advantages of higher doping and lower contact barriers in CdTe photovoltaic devices that may be realized by using n-type rather than the conventional p-type solar absorber layers. We present post-growth doping trials for indium in thin polycrystalline CdTe films using the diffusion of indium metal with indium chloride. Chemical concentrations of indium up to 1019 cm−3 were achieved and the films were verified as n-type by hard x-ray photoemission. Post-growth chlorine treatment (or InCl3) was found to compensate the n-doping. Trial structures comprising CdS/CdTe:In verified that the doped absorber structures performed as expected both before and after chloride treatment, but it is recognized that this is not an optimum combination. Hence, in order to identify how the advantages of n-type absorbers might be fully realized in future work, we also report simulations of a range of p–n junction combinations with n-CdTe, a number of which have the potential for high V oc.

Alternative n-type buffer layer such as In 2 S 3 has been proposed as a Cd-free alternative in kesterite Cu 2 ZnSn ( S , Se ) 4 (CZTSSe) solar cells. In this study, optical and electronic characterization techniques together with device analysis and simulation were used to assess nanoparticle-based CZTSSe absorbers and solar cells with CdS and In 2 S 3 buffers. Photoluminescence spectroscopy indicated that CZTSSe absorbers with In 2 S 3 buffer had a lower density of detrimental non-radiative defects and a higher concentration of copper vacancies V Cu +, responsible for p-type conductivity in CZTSSe, in comparison to the absorber with CdS buffer. Capacitance–voltage (C–V) measurements revealed that the In 2 S 3 buffer-based CZTSSe devices had a three times higher apparent doping density and a consequently narrower space charge region than devices with a CdS layer. This resulted in poorer collection of photo-generated charge carriers in the near-IR region despite a more favorable band alignment as determined by x-ray photoelectron and inverse photoelectron spectroscopy. The presence of interfacial defect states in In 2 S 3 devices as determined by C–V and biased quantum efficiency measurements is also responsible for the loss in open-circuit voltage compared with reference devices with CdS.

Cu2ZnSnS4 nanocrystals are annealed in a Se-rich atmosphere inside a transmission electron microscope. During the heating phase, a complete S-Se exchange reaction occurs while the cation sublattice and morphology of the nanocrystals are preserved. At the annealing temperature, growth of large Cu2ZnSnSe4 grains with increased cation ordering is observed in real-time. This yields an annealing protocol which is transferred to an industrially-similar solar cell fabrication process resulting in a 33% increase in the device open circuit voltage. The approach can be applied to improve the performance of any photovoltaic technology that requires annealing because of the criticality of the process step.

© 2019 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 × 10 16 cm −3 were achieved at low growth temperature of 390 °C. No As segregation could be detected at grain boundaries, even for 10 19 As cm −3 . A shallow acceptor level (+0.1 eV) due to As Te 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.

Sodium is a key dopant in thin film photovoltaic cells with reported benefits including promotion of grain growth, passivation of grain boundaries and increased carrier concentration in chalcopyrite and kesterite based solar cells. Research-grade devices fabricated in substrate configuration often rely on diffusion of Na from a soda lime glass substrate into the photovoltaic absorber layer during high temperature processing. However, for samples on flexible substrates such as foils and plastics, this is not available and requires alternative approaches. In this work, we fabricate Earth-abundant $$\hbox {Cu}_2\mathrm{ZnSn(S,Se)}_4$$ thin film solar cells from nanoparticle inks on flexible molybdenum substrates and demonstrate a simple, low-cost route to incorporating Na in solution thereby making it compatible with large area, high volume manufacturing. The technique is verified to improve the device efficiency relative to a reference flexible device built on molybdenum foil.

International audience; Kesterite solar cells based on chalcogenide Cu2ZnSn(S,Se)4 (CZTSSe) are a viable approach to thin film photovoltaics, utilising Earth-abundant, non-toxic elements. CZTSSe films produced from nanoparticle inks offer a cost-effective solution-based method of fabrication. However, improving efficiency in these devices has proved challenging, in part due to the presence of detrimental complex defects within the bulk of the CZTSSe absorber. In this study, the behaviour of nanoparticle-based CZTSSe absorbers and solar cells made from relatively low and high quality grade chemicals is investigated with a view to improving cost-effectiveness of the ink-based fabrication process. Photoluminescence spectroscopy revealed the presence of similar shallow acceptor plus shallow donor states in both low and high purity precursor absorbers. We demonstrate a relationship between the average depth of energy band-edge potential fluctuations and absorber quality where the higher grade chemical precursor-based absorber outperforms the lower purity version. In addition, the low purity precursor absorber had a higher total defect density resulting in a 10 meV increase in the average electrostatic potential fluctuations. Deep level transient spectroscopy in solar devices indicated the presence of detrimental deep defect states in both types of absorber. Notwithstanding the high purity precursor absorber with lower defect density, the power conversion efficiencies of both types of CZTSSe solar cells were similar (~5%), implying an issue other than defects in the absorber bulk inhibits device performance as evidenced by quantum efficiency analysis and current–voltage measurements.

The use of formamide (FA) as an alternative capping ligand to oleylamine (OLA) during the preparation of Cu2ZnSnS4 (CZTS) nanoparticle inks was investigated. Photoluminescence properties of the selenised CZTS absorber layers revealed quasi-donor acceptor pairs (QDAP) as the main transition for carrier recombination with similar electrostatic potential fluctuations in both type of ligands. In OLA layers the transition shifts from QDAP to band-impurity as temperature rises. Solar cell devices fabricated from FA Cu2ZnSn(Se,S)4 were shunted due to the high porosity of the film while in contrast, the combination of FA + OLA absorber yielded reduced back contact barrier height, lower series resistance and higher fill factor and efficiency compared to OLA devices. This is a direct result of the removal of the fine grain carbon rich layer from the back contact interface usually present in films prepared with OLA only.

Tin sulphide (SnS) thin films deposited by thermal evaporation on glass substrates are studied for different substrate temperatures. The increase in substrate temperature results in the increase of the crystallite size and change in orientation of the films. The crystal structure of the film is that of SnS only and for temperatures ≤ 300oC the films are of random orientation, whereas for higher temperatures the films become (040) oriented. The variation of Sn/S composition was accompanied by a reduction in optical energy bandgap from 1.47 to 1.31 eV as the substrate temperature increases. The Urbach energy was found stable at 0.169 ± 0.002 eV for temperature up to 350oC. Photoluminescence emission was observed only for films exhibiting stoichiometric properties and shows that a precise control of the film composition is critical to fabricate devices while an increase in grain size will be essential to achieve high efficiency.

Ultra-thin CdTe photovoltaic solar cells with an absorber thickness of 0.5 μm were produced by metal organic chemical vapour deposition onto indium tin oxide coated boroaluminosilicate glass. A wide band gap Cd1−xZnxS alloy window layer was employed to improve spectral response in the blue region of the solar spectrum. X-ray photoelectron spectroscopy, X-ray diffraction and scanning electron microscopy were used to monitor changes in the chemical composition and microstructure of the Cd1−xZnxS/CdTe solar cell after varying the post-deposition CdCl2 activation treatment time and annealing temperature. The CdCl2 treatment leached Zn from the Cd1−xZnxS layer causing a redshift in the spectral response onset of window absorption. S diffusion occurred across the Cd1−xZnxS/CdTe interface, which was more pronounced as the CdCl2 treatment was increased. A CdTe1−ySy alloy was formed at the interface, which thickened with CdCl2 treatment time. Small concentrations of S (up to 2 at.%) were observed throughout the CdTe layer as the degree of CdCl2 treatment was increased. Greater S diffusion across the Cd1−xZnxS/CdTe interface caused the device open-circuit voltage (Voc) to increase. The higher Voc is attributed to enhanced strain relaxation and associated reduction of defects in the interface region as well as the increase in CdTe grain size.

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 (similar to 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.

Earth abundant Cu

The role of sodium (Na) in cadmium telluride (CdTe) thin film photovoltaic absorbers deposited on a metal-coated substrate is investigated. Introducing Na during the growth of the structure influences the morphological and crystallographic properties of the CdTe layer grown by metal-organic chemical vapour deposition (MOCVD). It is observed that the introduction of Na between the metal and CdTe layers induces a slight randomisation via promotion of (400) and (220) orientations. It is shown that the inclusion of Na between the substrate and the Mo back metal contact enlarges the CdTe grains following a CdCl2 treatment by 50% but weakens the adhesion to the substrate. The introduction of Na between the Mo back contact and CdTe layer promotes the formation of large faceted grains for the as-grown films with an average grain size ten times larger than in the case of Na free deposition while maintaining good adhesion to the substrate. There is no further grain growth following CdCl2 treatment however the CdTe grain size is still double that of the Na-free samples.

Ultrathin CdTe photovoltaic solar cells were produced by metal organic chemical vapour deposition in a single horizontally configured growth chamber. Solar cell activation was investigated by varying the duration of the CdCl2 layer deposition and 420°C thermal anneal to promote Cl diffusion into the CdTe. Thicker CdCl2 layers used in activation treatment resulted in a greater degree of sulphur interdiffusion, up to 2 at.-%, into the CdTe layer. The thicker CdCl2 activation layer was necessary to lower the reverse saturation current density for obtaining optimum experimental photovoltaic (PV) device performances. Modelling of the PV performances with equivalent solar cell structure for optimised devices using solar cell capacitance simulation software resulted in an overestimated open circuit voltage (Voc). The simulations showed that reduced acceptor states at the CdTe interface with the intermixed region resulted in the largest decrease in Voc when considering large back surface recombination velocities.

In a constant effort to capture effectively more of the spectral range from the sun, multi-junction cells are being investigated. In this context, the marriage of thin film and dye-sensitized solar cells (DSC) PV technologies may be able to offer greater efficiency whilst maintaining the benefits of each individual technology. DSC devices offer advantages in the nature of both the metal oxide photo-electrode and dye absorption bands, which can be tuned to vary the optical performance of this part of a tandem device, while CdTe cells absorb the majority of light above their band-gap in only a few microns of thickness. The key challenge is to assess the optical losses with the goal of reaching a net gain in photocurrent and consequently increased conversion efficiency. This study reports on the influence of optical losses from various parts of the stacked tandem structure using UV-VIS spectrometry and EQE measurements. A net gain in photocurrent was achieved from a model developed for the DSC/CdTe mechanically stacked tandem structure.

This paper details the AM0 conversion efficiency of a metal–organic chemical vapor phase deposition thin-film cadmium telluride (CdTe) solar cell deposited onto a cerium-doped cover glass (100 μm). An AM0 best cell conversion efficiency of 12.4% (0.25-cm2 contact area) is reported. An AM0 mean efficiency of 12.1% over eight cells demonstrated good spatial uniformity. Excellent adhesion of the cell structure to the cover glass was observed with an adhesive strength of 38 MPa being measured before cohesive failure of the test adhesive. The device structure on cover glass was also subject to severe thermal shock cycling of +80 °C to −196 °C, showing no signs of delamination and no deterioration of the photovoltaic (PV) performance.

Ultra-thin Cd(₁ ₋ ₓ)Zn(ₓ)S/CdTe devices were produced by atmospheric pressure metal organic chemical vapour deposition (AP-MOCVD) with varying CdTe absorber thicknesses ranging from 1.0 to 0.2 mm and compared to baseline cells with total CdTe thickness of 2.25μ. The ultra-thin CdTe layers (≤1 μm) were intentionally doped with As to induce p-type conductivity in the absorber. Cell performance reduced with CdTe thickness, with the magnitude of photo-current generation loss becoming more significant for the very thin CdTe layers. The decline in cell performance was lower than the optically limited performance relating to a decrease in shunt resistance, Rsh, especially for the thinnest cells due to areas of incomplete CdTe coverage and large presence of pin-holes leading to micro-shorts. Incorporation of Zn into the CdS window layer improved cell performance for all devices except when 0.2 μm thick CdTe was used. This improvement was markedly in the blue region owing to enhanced optical transparency of the window layer. External quantum efficiency (EQE) measurements showed a red-shift of the window layer absorption edge due to leaching out of Zn during the CdCl₂ treatment. Reduction of the CdCl₂ deposition time was demonstrated to recover the blue response of the ultra-thin cells.

Ultra-thin photovoltaic (PV) devices were produced by atmospheric pressure metal organic chemical vapour deposition (AP-MOCVD) incorporating a highly absorbing intermediate sulphurised FeS x layer into a CdS/CdTe structure. X-ray diffraction (XRD) confirmed a transitional phase change to pyrite FeS 2 after post growth sulphur (S) annealing of the FeS x layer between 400 °C and 500 °C. Devices using a superstrate configuration incorporating a sulphurised or non-sulphurised FeS x layer were compared to p–n devices with only a CdS/CdTe structure. Devices with sulphurised FeS x layers performed least efficiently, even though pyrite fractions were present. Rutherford back scattering (RBS) confirmed deterioration of the CdS/FeS x interface due to S inter-diffusion during the annealing process.

An assessment of Cd0.9Zn0.1S window layer thickness and its impact on photoresponse uniformity in CdTe thin film photovoltaic (PV) devices is presented. A triple-wavelength laser beam induced current (LBIC) system provided a spatially resolved photocurrent mapping technique. Three diode lasers; λ = 405, 658, 810 nm gave photon absorption and carrier generation characteristics over the spectral range of the Cd0.9Zn0.1S/ CdTe devices. Two contrasting device structures were grown by metal-organic chemical vapour deposition (MOCVD), where the uniformity of the Cd0.9Zn0.1S window layer was known to vary: 1. uniform 240 nm Cd0.9Zn0.1S/ 2 μm CdTe and 2. a poorly nucleated 40 - 300 nm Cd0.9Zn0.1S/ 2 μm CdTe. Calculated photon penetration depths, δp allowed for the separation of identified defects within the device cross-section. Cd0.9Zn0.1S pin holes were identified in 240 nm Cd0.9Zn0.1S/ 2 μm CdTe where 405 nm photon ‘punch-through’ into the absorber material was observed. These pin holes also led to a localised reduction in photoresponse at λ = 658 and 810 nm. In the device structure where the Cd0.9Zn0.1S window layer thickness was known to vary from 40 to 300 nm, CdTe pin holes were identified where localised ∼ 50 μm regions of reduced photoresponse, at all wavelengths were observed. Local variations in both Cd0.9Zn0.1S and CdTe thickness were also identified where variable absorption led to a distribution of LBIC photoresponse. It was demonstrated that reduced photoresponse uniformity at all incident wavelengths was related to reduced device shunt resistance, Rsh and open-circuit voltage, Voc.

This paper reports important developments achieved with CdTe thin-film photovoltaic devices produced using metalorganic chemical vapour deposition at atmospheric pressure. In particular, attention was paid to understand the enhancements in solar cell conversion efficiency, to develop the cell design, and assess scalability towards modules. Improvements in the device performance were achieved by optimising the high-transparency window layer (Cd0.3Zn0.7S) and a device-activation anneal. These increased the fill factor and open-circuit voltage to 77 ± 1% and 785 ± 7 mV, respectively, compared with 69 ± 3% and 710 ± 10 mV for previous baseline devices with no anneal and thicker Cd0.3Zn0.7S. The enhancement in these parameters is associated with the two fold to three fold increase in the net acceptor density of CdTe upon air annealing and a decrease in the back contact barrier height from 0.24 ± 0.01 to 0.16 ± 0.02 eV. The optimum thickness of the window layer for maximum photocurrent was 150 nm. The cell size was scaled from 0.25 to 2 cm2 in order to assess its impact on the device series resistance and fill factor. Finally, micro-module devices utilising series-connected 2-cm2 sub-cells were fabricated using a combination of laser and mechanical scribing techniques. An initial module-to-cell efficiency ratio of 0.9 was demonstrated for a six-cell module with the use of the improved device structure and processing. Prospects for CdTe photovoltaic modules grown by metalorganic chemical vapour deposition are commented on.

A L8 matrix of experiments has been carried out to investigate various deposition parameters of Cadmium telluride (CdTe)/Cadmium sulphide (CdS) solar cells grown using metal organic chemical vapour deposition (MOCVD). The results of the matrix were analysed using a criterion to establish which growth parameters are significant and warrant further investigation. The most significant parameters were CdTe growth temperature and in situ arsenic doping of the CdTe absorber layer. Characterisation of the device structures showed that CdTe grain enlargement from 1 μm to ∼3 μm occurred at the higher CdTe deposition temperature of 390 °C. SIMS depth profiles verified that arsenic concentrations, within the device structures, of 2.5 × 1018 and 2 × 1019 atoms cm−3 were achieved for the different partial pressures. A model for the behaviour of arsenic in polycrystalline CdTe material, based on only partial passivation of grain boundaries and saturation of the grain boundaries at As concentrations above >2.5 × 1018 atoms cm−3 is presented and discussed.

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 (~620–690 mV). Post-deposition annealing in air for 30 min 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 180 min, 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 800 mV and a best cell efficiency of 15.3%.

The microstructure of 4?13 ?m thick CdTe absorber layers in CdTe/CdS/ITO/glass solar cell structures grown by metal-organic chemical vapour deposition (MOCVD) at 350 ?C has been studied. The crystalline texture, lattice parameter and grain size were measured as a function of thickness for the as-grown layers, and as a function of annealing temperature and time for annealing in both nitrogen (N2) and cadmium chloride (CdCl2) environments. The average grain sizes developed with thickness as r (?m) = 0.050x ? 0.10 (4 < x < 12 ?m), and this behaviour is contrasted with that for close-spaced sublimation material grown at 500 ?C. Annealing in both ambients promoted grain growth (with Rayleigh grain size distribution functions and Burke?Turnbull exponents being n = 7 at 440 ?C and ~4 at 400 ?C), a development of the grown-in preferred orientation from [1?1?1] to [2?1?1], and relief of the grown-in compressive stress. A growth mechanism by which development of the [2?1?1] preferred orientation may accompany grain growth is described. It is concluded that MOCVD growth at temperatures higher than 350 ?C used here will be required to produce the larger grain sizes required for photovoltaic applications.

Four different types of solar cells prepared in different laboratories have been characterized by impedance spectroscopy (IS): thin-film CdS/CdTe devices, an extremely thin absorber (eta) solar cell made with microporous TiO2/In(OH)xSy/PbS/PEDOT, an eta-solar cell of nanowire ZnO/CdSe/CuSCN, and a solid-state dye-sensitized solar cell (DSSC) with Spiro-OMeTAD as the transparent hole conductor. A negative capacitance behavior has been observed in all of them at high forward bias, independent of material type (organic and inorganic), configuration, and geometry of the cells studied. The experiments suggest a universality of the underlying phenomenon giving rise to this effect in a broad range of solar cell devices. An equivalent circuit model is suggested to explain the impedance and capacitance spectra, with an inductive recombination pathway that is activated at forward bias. The deleterious effect of negative capacitance on the device performance is discussed, by comparison of the results obtained for a conventional monocrystalline Si solar cell showing the positive chemical capacitance expected in the ideal IS model of a solar cell.

Yttrium fluoride (YF3) is a material with good potential as a single-layer anti-reflection (AR) coating deposited onto glass substrates. YF3 is a possible candidate to replace more studied fluorides such as MgF2 or ThF4 the latter being radioactive. For thin-film photovoltaic solar cells, depositing such layers could be a cost-effective way of improving the transmission of light to the p–n junction. However, long-term stability of these AR coatings is an important issue to be considered. This paper is concerned with the residual stress of YF3 single layers deposited onto glass substrates, being a potential failure mechanism. The measured stress values are correlated to the structure of the layers, using ex situ characterization techniques, such as X-ray diffraction (XRD) and atomic force microscopy (AFM), in an attempt to explain the observed changes. Three substrate temperatures were investigated during deposition, namely 25, 115, and 210 °C. Intrinsic stress of up to 197 MPa has been observed in amorphous YF3 single layers deposited by electron beam evaporation, at ambient temperature. At a substrate temperature of 210 °C, the intrinsic stress decreased to 67 MPa in the YF3 layer, developing an orthorhombic structure. Adsorptive stress is an important issue encountered in YF3 layers, directly related to a low packing density. The in situ stress measurements were carried out using a novel optical approach with a laser-fiber system, briefly introduced here. The residual stress values measured with this novel optical system were compared to two ex situ stress measurement techniques. The optical performance of the YF3 single layers was also assessed using a spectrometer ex situ and in situ by the interferometry capability of the novel in situ monitoring device.

To demonstrate the potential for building integrated photovoltaics (BIPV) incorporation of thin-film photovoltaics, commercially available quantum dots (QDs) have been deposited, as part of a poly(methyl methacrylate) (PMMA) composite film, on a cadmium telluride (CdTe) micro-module. This resulted in an increase in photocurrent generation through the luminescent down-shifting (LDS) process. The optical properties of these films were characterized through UV–vis spectroscopy. The impact of the film on the micro-module was studied through current–voltage (I–V) and external quantum efficiency measurements. Further layers were added to the initial single-layer LDS film, however no additional improvement to the micro-module were observed. Additionally, a range of emission wavelengths have been explored. The majority of these films, when tested on a CdTe device, were shown to improve the photocurrent generation whilst also visually displaying the vivid colour palette provided by quantum confined materials. The future feasibility of using QD based LDS films for large scale BIPV-based power generation has also been discussed.

MOCVD has been associated with batch processing of III–V opto-electronic devices for decades, with epitaxial structures deposited on up to 200 mm diameter wafers. Recent development in thin film PV has seen the gap in conversion efficiencies closing in on that of the commonly found multicrystalline Si wafer based PV. To further improve the conversion efficiency of thin film PV towards the theoretical limits of single junction solar cells requires a technique such as MOCVD with scalability potential. Preliminary results on the development of a chamberless inline process are reported for up to 15 cm wide float glass, progressively coating each layer in the CdTe solar cell as the heated substrate passes under each coating head in turn and entirely at atmospheric pressure. Emphasis is made on ensuring that the chamberless coating heads can be operated safely using a combination of nitrogen curtain flows and a balanced exhaust pressure system. Results are also presented on the exclusion of oxygen and moisture from the coating area, achieved using the same gas flow isolation process. This paper also reviews the achievements made to-date in the transfer of the high efficiency batch MOCVD produced CdTe solar cell to the chamberless inline process demonstrating device quality thin films deposition.

This work describes progress towards achieving a flexible, high specific power and low-cost photovoltaic (PV) for emerging large area space applications. The study reports the highest conversion efficiency of 15.3% AM1.5G for a CdTe device on ultra-thin cerium-doped cover glass, the standard protective material for extra-terrestrial PVs. The deposition technique used for all of the semiconductor layers comprising the device structure was atmospheric pressure metal organic chemical vapour deposition. Improvements to the device structure over those previously reported led to a Voc of 788 mV and a relatively low series resistance of 3.3 Ω·cm2. These were largely achieved by the introduction of a post-growth air anneal and a refinement of the front contact bus bars, respectively. The aluminium-doped zinc oxide transparent conductive oxide, being the first layer applied to the cover glass, was subject to thermal shock cycling +80 to (-) 196°C to test the adhesion under the extreme conditions likely to be encountered for space application. Scotch Tape testing and sheet resistance measurements before and after the thermal shock testing demonstrated that the aluminium-doped zinc oxide remained well adhered to the cover glass and its electrical performance unchanged.

This paper details the preliminary findings of a study to achieve a durable thin-film CdTe photovoltaic (PV) device structure on ultrathin space-qualified cover glass. An aluminum-doped zinc oxide (AZO) transparent conducting oxide was deposited directly onto the cover glass using metalorganic chemical vapor deposition (MOCVD). The AZO demonstrated low sheet resistance of 10 Ω/□ and high optical transparency of 85% as well as excellent adherence and environmental stability. Preliminary deposition of PV layers onto the AZO on cover glass, by MOCVD, showed the possibility of such a structure, yielding a device conversion efficiency of 7.2%. High series resistance (10 Ω cm2) and low V oc (586 mV) were identified as the limiting factors when compared with the authors’ platform process on indium tin oxide-coated aluminosilicate. The coverage of the Cd1−x Zn x S window layer along with the front contacting of the device were shown to be the major causes of the low efficiency. Further deposition of AZO/CdTe employing an oxygen plasma cleaning step to the cover glass and evaporated gold front contacts significantly improved the device performance. With a highest conversion efficiency of 10.2%, series resistance improved to 4.4 Ω cm2, open-circuit voltage (V oc) up to 667 mV, and good adhesion, this represents the first demonstration of direct deposition of CdTe solar cells onto 100-μm-thick space-qualified cover glass.

In an effort to find an alternative way of incorporating quantum dot (QD) based luminescent down-shifting (LDS) films into photovoltaic (PV) devices, transparent (at long wavelength) zinc oxide (ZnO) films have been grown by chemical bath deposition (CBD) to form novel encapsulation matrices for QD based LDS films. The ZnO films appear to be stoichiometric (Zn:O) and meso-porous as characterised by energy dispersive x-ray analysis and scanning electron microscopy. QDs have been loaded into the films for varying lengths of time (24–72 h) resulting in measureable fluorescence upon excitation with a blue LED.

The electrical properties of CdTe/CdS solar cells grown by metal organic chemical vapor deposition were investigated by a technique of impedance measurements under varied intensity of AM1.5 illumination. A generalized impedance model was developed and applied to a series of CdTe/CdS cells with variations in structure and doping. The light measurements were compared to the conventional ac measurements in dark under varied dc bias, using the same methodology for equivalent circuit analysis in both cases. Detailed information on the properties of the device structure was obtained, including the properties of the main p-n junction under light, minority carrier lifetime, back contact, as well as the effect of the blocking ZnO layer incorporated between the transparent conductor and CdS layers. In particular, the comparison between samples with different chemical concentrations of As has shown that the total device impedance and the series resistance are strongly increased at lower As densities, resulting in the lower collection current and efficiencies. At the same time the minority carrier lifetime was found to be one order of magnitude larger for the lowest value of As density, when compared to the optimized devices.

Utilisation of wide bandgap Cd1−x Zn xS alloys as an alternative to the CdS window layer is an attractive route to enhance the performance of CdTe thin film solar cells. For successful implementation, however, it is vital to control the composition and properties of Cd1−x Zn xS through device fabrication processes involving the relatively high-temperature CdTe deposition and CdCl2 activation steps. In this study, cross-sectional scanning transmission electron microscopy and depth profiling methods were employed to investigate chemical and structural changes in CdTe/Cd1−x Zn xS/CdS superstrate device structures deposited on an ITO/boro-aluminosilicate substrate. Comparison of three devices in different states of completion—fully processed (CdCl2 activated), annealed only (without CdCl2 activation), and a control (without CdCl2 activation or anneal)—revealed cation diffusion phenomena within the window layer, their effects closely coupled to the CdCl2 treatment. As a result, the initial Cd1−x Zn xS/CdS bilayer structure was observed to unify into a single Cd1−x Zn xS layer with an increased Cd/Zn atomic ratio; these changes defining the properties and performance of the Cd1−x Zn xS/CdTe device.