The role of transparent conducting oxides in metal organic chemical vapour deposition of CdTe/CdS Photovoltaic solar cells

A systematic study is made between the relationship of Cd_0.9Zn_0.1S/CdTe photovoltaic (PV) device properties for three different commercial transparent conducting oxide (TCO) materials and some experimental CdO to determine the role of the TCO in device performance. The resistance contribution from the TCO was measured after depositing the gold contact architectures directly onto the TCOs. These were compared with the Cd_0.9Zn_0.1S/CdTe device properties using the same contact arrangements. Series resistance for the commercial TCOs correlated with their sheet resistance and gave good agreement with the PV device series resistance for the indium tin oxide (ITO) and fluorine doped tin oxide (FTO) 15 Ω/Sq. superstrates. The devices on the thicker FTO 7 Ω/sq superstrates were dominated by a low shunt resistance, which was attributed to the rough surface morphology causing micro-shorts. The device layers on the CdO substrate delaminated but devices were successfully made for ultra-thin CdTe (0.8 μm thick) and compared favourably with the comparable device on ITO. From the measurements on these TCOs it was possible to deduce the back contact resistance and gave an average value of 2 Ω.cm². The correlation of fill factor with series resistance has been compared with the predictions of a 1-D device model and shows excellent agreement. For high efficiency devices the combined series resistance from the TCO and back contact need to be less than 1 Ω.cm².     

In situ deposition of cadmium chloride films using MOCVD for CdTe solar cells

This paper reports on the first deposition of cadmium chloride (CdCl₂) films by metal organic chemical vapour deposition (MOCVD). As the p-n junction can be deposited by MOCVD, the in situ CdCl₂ treatment of the device allows for containment of the whole process. MOCVD allows a high level of control over material properties and excellent repeatability. Deposition of CdCl₂, on glass and silicon, at different II:VII precursor ratios and substrate temperatures are reported. The precursors used are dimethylcadmium and tertiarybutylchloride or n-hexylchloride, respectively for the cadmium and chlorine species. Results are presented on the surface morphology and layer structure. CdCl₂ was in its hydrate form once exposed to ambient air. Preliminary results on the effects of in situ CdCl₂ treatment on MOCVD CdS/CdTe:As devices are reported and compared with untreated devices, using current-voltage characterisation. The CdCl₂ treatment successfully resulted in MOCVD devices having open-circuit voltage higher than 600 mV and fill factor above 50%.     

ZnxCd1 − xS as a heterojunction partner for CuIn1 − xGaxS2 thin film solar cells

Zinc cadmium sulfide (Zn_xCd_1 − xS) heterojunction partner layer prepared with chemical bath deposition (CBD) has exhibited better blue photon response and higher current densities due to its higher bandgap than that of conventional cadmium sulfide (CdS) layer for CuIn_1 − xGa_xS₂ (CIGS2) solar cells. CIGS2/Zn_xCd_1 − xS devices have also shown higher open circuit voltage, V_oc indicating improved junction properties. A conduction band offset has been observed by J-V curves at various temperatures indicating that still higher V_oc can be obtained by optimizing the conduction band offset. This contribution discusses the effect of variation of parameters such as concentration of compounds, pH of solution and deposition time during CBD on device properties and composition and crystallinity of film. Efficiencies comparable to CIGS2/CdS devices have been achieved for CIGS2/Zn_xCd_1 − xS devices.     

Dependence of efficiency of thin-film CdS/CdTe solar cell on parameters of absorber layer and barrier structure

Dependences of the open-circuit voltage, short-circuit current, fill factor, and efficiency of a CdS/CdTe solar cell on the resistivity and thickness of the p-CdTe absorber layer, the noncompensated acceptor concentration N_a-N_d, and carrier lifetime τ in CdTe, are investigated, and optimization of these parameters in order to improve the solar cell efficiency is performed. It has been shown that the observed low efficiency of CdS/CdTe solar cells is caused by the too short electron lifetime in the range of 10^− 10-10^− 9 s and too thin (3-5 µm) CdTe layer currently used for fabrication of CdTe/CdS solar cells. To achieve an efficiency of 28-30%, the resistivity and thickness of the CdTe absorber layer, the noncompensated acceptor concentration, and carrier lifetime should be ∼ 0.1 Ω·cm, ≥ 20-30 µm, ≥ 10¹⁶ cm^− 3, and ≥ 10^− 6 s, respectively.     

Study of electrical and optical properties of Cd1 − x Zn x S thin films

Thin films of Cd_0.9Zn_0.1S and CdS were prepared by thermal evaporation under vacuum of 10^–6 Torr and with deposition rate of 60 nm/min. X ray diffraction studies confirm the hexagonal structure of both CdS and Cd_0.9Zn_0.1S films. The effect of heat treatments with or without CdCl₂ enhances the grain size growth and improves the crystalline of the films. Moreover, the activation energy is decreased by heat treatment with or without CdCl₂ for all thin films. The optical absorption coefficient of Cd_0.9Zn_0.1S thin films were determined from measured transmittance and reflectance in the wavelength range of 300 to 2500 nm. The optical absorption spectra reveal the existence of direct energy gap for these films. It was found that the optical energy gap decreases upon annealing or CdCl₂ treatments.     

Structure and optical properties of polycrystalline Cd1−xZnxTe thin films prepared by simultaneous evaporation

Cd_1–xZn_xTe films were deposited by simultaneous evaporation of CdTe and ZnTe. These Cd_1–xZn_xTe films were of cubic phase, and strongly (1 1 1) oriented as deposited. Predominant direct optical transitions were observed and the band gap varied with zinc content in a non-linear way. A structure development of CdS/CdTe/ZnTe : Cu solar cells with a Cd_1–xZn_xTe buffer layer was proposed.     

Indium sulfide buffer/CIGSSe interface engineering: Improved cell performance by the addition of zinc sulfide

Indium sulfide buffer layers deposited by the spray-ion layer gas reaction (Spray-ILGAR) technique are a viable alternative to the traditional cadmium sulfide buffer layer in thin film solar cells. In the present work we report on the results of manipulating the absorber/buffer interface between the chalcopyrite Cu(In,Ga)(S,Se)₂ absorber (CIGSSe) and the indium sulfide buffer. It is shown that the deposition of a small amount of zinc sulfide at the absorber/buffer interface can be used to increase the open circuit voltage. A small but significant increase of 20 mV (up to 580 mV), as compared to the pure indium sulfide buffered cells is possible leading to an increase in the overall efficiency.     

Cu(In,Ga)Se2 solar cells with double layered buffers grown by chemical bath deposition

In based mixture In_x(OH,S)_y buffer layers deposited by chemical bath deposition technique are a viable alternative to the traditional cadmium sulfide buffer layer in thin film solar cells. We report on the results of manipulating the absorber/buffer interface between the chalcopyrite Cu(In,Ga)Se₂ (CIGS) absorber and CdS or ZnS buffer by addition of a thin In based mixture layer. It is shown that the presence of thin In_x(OH,S)_y at the CIGS absorber/CdS or ZnS buffer interfaces greatly improve the solar cell performances. The performances of CIGS cells using dual buffer layers composed of In_x(OH,S)_y/CdS or In_x(OH,S)_y/ZnS increased by 22.4% and 51.6%, as compared to the single and standard CdS or ZnS buffered cells, respectively.     

Carrier density and mobility in CdxZn1−xS chemically sprayed films

The electronic and optical properties of Cd_xZn_1?xS films (0.9 >x 0.1) fabricated using the chemical spraying method have been investigated. Thermoelectric and photothermoelectric effects were used to determine the carrier density and the mobility. It was observed that, although the carrier density increased somewhat on addition of zinc, the decrease in mobility was quite significant. The low mobility observed in these Cd_xZn_1?xS films was relatively insensitive to illumination and heat treatment. This latter behaviour suggested that the addition of zinc (10% or more) to CdS films would cause a genuine reduction in the mobility. This differs from the low mobility observed for resistive CdS films in which the high mobility state could be recovered by heat treatment.     

Dependence of carrier lifetime on Cu-contacting temperature and ZnTe:Cu thickness in CdS/CdTe thin film solar cells

Cu diffusion from a ZnTe:Cu contact interface can increase the net acceptor concentration in the CdTe layer of a CdS/CdTe photovoltaic solar cell. This reduces the space-charge width (W_d) of the junction and enhances current collection and open-circuit voltage. Here we study the effect of Cu concentration in the CdTe layer on carrier lifetime (τ) using time-resolved photoluminescence measurements of ZnTe:Cu/Ti-contacted CdTe devices. Measurements show that if the ZnTe:Cu layer thickness remains constant and contact temperature is varied, τ increases significantly above its as-deposited value when the contacting temperature is in a range that has been shown to yield high-performance devices (~ 280° to ~ 320 °C). However, when the contacting temperature is maintained near an optimum value and the ZnTe:Cu thickness is varied, τ decreases with ZnTe:Cu thickness.     

Studies of key technologies for large area CdTe thin film solar cells

The structure and main manufacturing technologies of CdTe film solar cells of large area are reviewed. Among the technologies, some have been developed for application in a pilot manufacturing line. The high resistant SnO₂ (HRT) thin films have been fabricated by PECVD. The effects of annealing on the structure and properties have been studied. A surface etching process of CdTe in low temperature and lower concentration of nitric acid has been developed. The Cd_1 − xZn_xTe ternary compound films have been studied. In order to improve the back contact layer, Cd_0.4Zn_0.6Te layer with 1.8 eV band gap as a substitute for ZnTe layer is introduced in CdTe cells. The effects of the technologies on performance of CdTe cells and feasibility of application in the modules are discussed.     

Effect of CdCl2 annealing treatment on thin CdS films prepared by chemical bath deposition

In order to study the CdS recrystallization mechanism, a comparative study was carried out on thin films prepared by chemical bath deposition. The CdS films were annealed in air with or without a CdCl₂ coating layer. In-situ Raman spectra obtained during the annealing showed that both the air- and the CdCl₂-annealing did not cause rearrangement of the neighboring atoms in the CdS clusters below ~ 300 °C. CdS thin film was partially oxidated to CdO and CdSO₄ on the cluster surface when annealed in air. The oxides and the sulfur stoichiometric deficiency prevented the clusters to coalesce at higher temperatures. Coating thin CdS film with a thin CdCl₂ layer protected it from oxidation during annealing in air and promoted formation of Cl_S and V_Cd point defects in CdS. The anti-oxidation was attributed to the incorporation of a significant amount of Cl into CdS to form the Cl_S, which prevented the oxygen in-diffusion and chemical bonding during the annealing. The anti-oxidation at the CdS nano-crystalline surface and the point defects formed in the CdS promoted coalescence of the neighboring clusters without the need of long-range redistribution of the atoms. Large CdS grains with good crystalline quality formed through recrystallization during the CdCl₂ heat treatment, which provided the solid basis for the subsequent CdTe growth and high efficient CdS/CdTe solar cell fabrication.     

Metal-organic chemical vapor deposition of ultra-thin photovoltaic devices using a pyrite based p-i-n structure

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 FeS₂ 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.     

Flexible cadmium telluride/cadmium sulphide thin film solar cells on mica substrate

Polycrystalline thin film II–VI compound semiconductors of cadmium sulfide (CdS) and cadmium telluride (CdTe) are the leading materials for the development of cost effective and reliable photovoltaic systems. The two important properties of these materials are its nearness to the ideal band gap for photovoltaic conversion efficiency and they have high optical absorption coefficients. Usually thin film solar cells are made by hetero-junction of p-type CdTe with n-type CdS partner window layer. In this article, we have deposited CdTe films on mica substrates using thermal evaporation technique and CdTe/CdS junction were developed by depositing a thin layer of CdS on to the CdTe substrate from chemical bath deposition method. The device was characterized by current voltage and photocurrent spectroscopy technique prior to the deposition of the transparent conducting layer. The devices were annealed in air at different temperatures and found that the device annealed at 673?K had better photovoltaic parameters. The efficiency of a typical device under 50?mW?cm^?2 illumination was estimated as 4%.     

Study of the physical properties of PLD grown cobalt doped nanocrystalline Zn0.9Cd0.1S thin films

We report a systematic study of structural, optical, and magnetic measurements on Zn_0.9Cd_0.1S:yCo films in the concentration range of 0.005 ≤ y ≤ 0.05 M using pulsed laser deposition technique. Structure, composition analysis, and optical measurements revealed that Cobalt is incorporated into the lattice, as Co²⁺ substituting Zn²⁺ ions, forming a solid solution with cubic structure instead of Cobalt precipitates. Low temperature magnetization measurements reveal a paramagnetic behavior. UV–vis measurements showed a red shift with respect to undoped sample in the energy band gap with increasing Cobalt concentration. Photoluminescence measurements shows ~ 300 times increase in intensity by Cobalt doping in Zn_0.9Cd_0.1S matrix.     

Effects of CdCl2 in CdTe on the properties of sintered CdS/CdTe solar cells

Sintered CdS films on glass substrates with low electrical resistivity and high optical transmittance have been prepared by a coating and sintering method. All-polycrystalline CdS/CdTe solar cells with different microstructures and properties of the CdTe layer were fabricated by coating a number of CdTe slurries, which consisted of cadmium and tellurium powders, an appropriate amount of propylene glycol and various amounts of CdCl₂, on the sintered CdS films and by sintering the glass-CdS-(Cd + Te) composites at various temperatures. The presence of more than 5 wt% of CdCl₂ in the (Cd + Te) layer enhances the sintering of the CdTe film and the junction formation by a liquid-phase sintering mechanism. A low sintering temperature results in poor densification of the CdTe layer and the CdS-CdTe interface, whereas a high sintering temperature results in a deeply buried homojunction. The optimum temperature for the sintering of the CdTe layer and for junction formation decreases with increasing amount of CdCl₂. All-polycrystalline CdS/CdTe solar cells with an efficiency of 10.2% under solar irradiation have been fabricated by a coating and sintering method using cadmium and tellurium powders for the CdTe layer.     

CdTe thin film solar cells with reduced CdS film thickness

A study was performed to reduce the CdS film thickness in CdTe thin film solar cells to minimize losses in quantum efficiency. Using close space sublimation deposition for CdS and CdTe a maximum efficiency of ~ 9.5% was obtained with the standard CdS film thickness of ~ 160 nm. Reduction of the film CdS thickness to less than 100 nm leads to poor cell performance with ~ 5% efficiency, mainly due to a lower open circuit voltage. An alternative approach has been tested to reduce the CdS film thickness (~ 80 nm) by depositing a CdS double layer. The first CdS layer was deposited at high substrate temperature in the range of 520-540 °C and the second CdS layer was deposited at low substrate temperature of ~ 250 °C. The cell prepared using a CdS double layer show better performance with cell efficiency over 10%. Quantum efficiency measurement confirmed that the improvement in the device performance is due to the reduction in CdS film thickness. The effect of double layer structure on cell performance is also observed with chemical bath deposited CdS using fluorine doped SnO₂ as substrate.     

Transmission electron microscopy study of dislocations and interfaces in CdTe solar cells

We report on our transmission electron microscopy study of dislocations and interfaces in CdTe solar cells. The atomic structure of dislocations formed inside CdTe grains have been determined by atomic-resolution transmission electron microscopy. We discuss the electronic properties of the dislocations and explore the effects of oxygen on the interdiffusion at CdS/CdTe interface. We find that the presence of oxygen in either CdS or CdTe suppresses the interdiffusion at the CdS/CdTe interface. We have further investigated interdiffusion at the CdS/Zn₂SnO₄ interface. We find that Zn diffuses into CdS from Zn₂SnO₄ and Cd diffuses into Zn₂SnO₄ from CdS. The possible effects of the interdiffusion are discussed. Finally, we have examined the distribution of intentionally introduced Cu at the CdTe/CdS junction, and we find that Cu is distributed uniformly in the CdS layer.     

Morphology, optical, and photoelectrochemical properties of electrodeposited nanocrystalline ZnO films sensitized with Cd x Zn1−x S nanoparticles

Nanocrystalline ITO/ZnO films formed by porous zinc oxide microplatelets 1–3 μm in size and 100–200 nm in thickness, which consist of 30–50 nm ZnO crystallites, were sensitized to visible light by Cd _x Zn_1?x S nanocrystals deposited using the method of successive ionic layer adsorption and reaction (SILAR). The composition of Cd _x Zn_1?x S nanocrystals as well as the dependence between molar Cd(II) fraction in the films and the ratio of cadmium and zinc nitrate concentrations in solutions used for the SILAR procedure were determined by a combination of electron, Raman, and energy-dispersive X-ray spectroscopies. The photovoltage observed at illumination of the ITO/ZnO/Cd _x Zn_1?x S heterostructures by white light (λ >400 nm) in aqueous Na₂S solution increases with a decrease of Cd(II) content proportionally to an increment in the conduction band potential of the Cd _x Zn_1?x S nanocrystals. The photocurrent density normalized to the light absorbance of the ITO/ZnO/Cd _x Zn_1?x S films increases by a factor of around four when the conduction band potential of Cd _x Zn_1?x S nanocrystals grows by 220 mV as a result of Cd(II) fraction changing from 1.0 to 0.62–0.67. The results show that Cd _x Zn_1?x S solid solutions are more advantageous sensitizers for the short-wavelength part of the sensitivity window of the liquid-junction solar cells (400–450 nm) than conventionally used cadmium sulfide.     

Electrodepositions of CdS,ZnS and Cd1-xZnxS films

Thin films of CdS, ZnS and Cd_1-xZn_xS, (0< x<1) have been electrodeposited from acidic bath using CdSO₄, ZnSO₄ and Na₂S₂O₃ at pH between 2 to 2.5 onto different substrates. The structural and optical properties of these films have been studied. It was found that CdS, ZnS and Cd_1-x Zn_x S film could be electrodeposited from acidic bath. The XRD patterns showed that the films consist of mixed phases of CdS and ZnS with presence of free Cd, Zn and S. The optical properties showed that bandgaps of CdS and ZnS are 2.4 and 3.55 eV respectively. The bandgaps of Cd_1-x Zn_x S films varied between 2.4 to 3.55 eV, depending upon Zn content in the film.