The design and fabrication of high efficiency thin film CdS/Cu2S solar cells

Thin film photovoltaic cells of CdS/Cu₂S which exhibit conversion efficiencies in excess of 9% have been designed and fabricated. Specific cell designs are prepared from an analysis of optical and electronic loss mechanisms operative in the cell. Material and engineering modifications to the fabrication process are then made to minimize specific energy conversion losses. The present cell design consists of five thin film layers which are sequentially prepared on a copper substrate 35 μm thick. In addition to the material control required for each component layer, the electrical, chemical, mechanical and topological compatibilities at the interfaces between each adjoining layer must be assured to achieve the desired cell performance. Our present analysis shows that a fully optimized solar cell based on a CdS/Cu₂S junction will have a practical conversion efficiency limit of about 11%. It is anticipated that practical conversion efficiencies of 14–15% can be achieved utilizing a (CdZn)S/Cu₂S junction designed to produce the maximum open-circuit voltage possible using Cu₂S as the absorbing layer. Present cell results which incorporate this design are presented.     

Doping profiles in CdTe/CdS thin film solar cells

CdS/CdTe thin film solar cells showing comparable properties as commercial cells have been prepared by close space sublimation (CSS) in our own laboratory. We characterised the cells by capacitance-voltage profiling (C-V), thermal admittance spectroscopy (TAS), and thermally stimulated capacitance measurements (TSCAP). The doping profiles of the CdTe layer obtained by C-V measurements confirm the well known rise in dopant concentration with increasing depth if the usual evaluation procedure is employed. However, the TAS and TSCAP measurements reveal deep acceptors in the CdTe layer with a large concentration exceeding that of the shallow dopants. Under these conditions, C-V measurements are shown to yield an apparently rising dopant concentration even for homogeneous doping. A combined simulation of doping profiles measured at different temperatures using a fixed and uniform shallow and deep doping fits well to measured doping concentration. These results indicate how to get reliable information on the shallow dopant concentration.     

Kinetics of the dry formation of Cu2S in Cu2S/CdS solar cells

Atomic absorption spectroscopy and electron spectroscopy for chemical analysis were used to study the growth of Cu₂S by the dry barrier process in Cu₂S/CdS solar cells at various reaction temperatures and for various thicknesses of the evaporated CuCl layer. Coulometric titration was used to measure the stoichiometry of the Cu₂S. It was shown that for reaction temperatures lower than 140°C the diffusion of cadmium in Cu₂S limits the Cu₂S growth, whereas at higher temperatures the cadmium-copper exchange reaction rate is the limiting factor.     

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.     

Unravelling complex nature of CdS/CdTe based thin film solar cells

Thin film solar cells based on CdS/CdTe hetero-structure has shown a drastic improvement changing from 16.5 to 22.1% efficiency during a short period of time from ~2013 to ~2016. This has happened in the industrial environment and the open research in this field has stagnated over a period of two decades prior to ~2013. Most of the issues of this hetero-structure were not clear to the photovoltaic (PV) community and research efforts should be directed to unravel its complex nature. Issues related to materials, post-growth treatment, chemical etching prior to metallisation and associated device physics are the main areas needing deeper understanding in order to further develop this device. After a comprehensive research programme in both academia and in industry on these materials, surfaces and interfaces and fully fabricated devices over a period of over three decades by the main author, the current knowledge as understood today, on all above mentioned complex issues are presented in this paper. Full understanding of this structure will enable PV developers to further improve the conversion efficiency beyond 22.1% for CdS/CdTe based solar cells.     

Electroconversion of CdS to CuxS for thin film solar cells

Electroconversion of vacuum-evaporated CdS films to cuprous sulphide (Cu_xS) for the fabrication of thin film CdS/Cu_xS solar cells was investigated. The Cu_xS films so formed were characterized by electrochemical and electron diffraction methods. The dependence of the stoichiometry x of the Cu_xS on conversion parameters such as the concentration, pH and temperature of the electrolyte (CuSO₄) and the cathode (CdS) current density was established. A 0.4 M concentration bath with the pH value maintained at 2.5 was found to be most suitable. For this bath, optimum combinations of the bath temperature and cathode current density yielding the most stoichiometric Cu_xS (x?1.99) were found to be in the ranges 55–65°C and 2.5-5.0 mA cm^-2 respectively. The films converted under the optimum conditions exhibit predominantly a chalcocite structure. The observed kinetics of the conversion process was understood in terms of a proposed two-step reaction at the cathode, involving a chemical ion exchange and an electrochemical reduction reaction.     

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.     

2.16 Technology of large area Cu2SCdS solar cells

Solar energy can be converted in different ways: the most attractive application is the direct conversion of solar radiation to electricity. For large scale application large area photovoltaic devices are necessary. Highly efficient solar cells can be produced on the basis of thin semiconducting films. An automized pilot line production for CdS layers, which are evaporated under high vacuum conditions, and their subsequent treatment to get an encapsulated thin film solar cell, are described. Investigation methods and a theoretical model of the heterojunction are reported.     

Variations of interlayer structure in Cu x S/CdS bilayer thin film with annealing of CdS: an ellipsometric study

Irregularities at the interface in Cu _x S/CdS thin films can be controlled by annealing CdS film prior to chemiplating. The interlayer formed on CdS films annealed at 200°C is comparatively smooth. In CdS films annealed at higher temperatures, the interlayer is rather thick and the CdS intrusions into this layer are thin. An ellipsometric technique is used for this study and the effective medium theory which is utilized to interpret the results is based on the difference in reaction rate in the grains as well as grain boundaries during chemiplating.     

Optoelectronic characteristics of Cu(In,Ga)(S,Se)2 thin film solar cells obtained from varied chalcogenization processes

We investigated industrially produced chalcopyrite solar cells based on the absorber modifications Cu(In,Ga)Se₂ and Cu(In,Ga)(S,Se)₂ in order to study the nature of the experimentally verified efficiency improvement, mainly caused by an increased open circuit voltage. We show that the introduction of sulfur during the absorber formation via rapid thermal processing leads to a substantial lowering of the surface doping concentration and widening of the space charge region (SCR). Temperature dependent diode analysis revealed a reduction of the SCR recombination in (Se,S) devices which would lead to a larger splitting of quasi-Fermi levels and hence to an increased open circuit voltage as compared to neat Cu(In,Ga)Se₂ devices.     

Reactive sputtering of thin Cu2S films for application in solar cells

Thin layers of cuprous sulphide were deposited by reactive r.f. sputtering; the target was pure copper and the sputtering gas was an ArH₂S mixture. We describe here how the composition of the films and their stoichiometry can be derived accurately both from X-ray diffraction and the optical reflection and transmission spectra. Measurement of the electrical resistivity can be used as a quick qualitative identification method.The application of these characterization methods to our sputtered films indicates that the crucial parameters to be controlled are the total pressure of the sputtering gas and, in particular, the partial pressure of the H₂S. Too low partial pressures of H₂S result in the presence of copper precipitations in the Cu₂S film, whereas too high H₂S partial pressures result in the presence of copper-deficient Cu_xS phases; there is an intermediate range of H₂S partial pressures in which pure chalcocite films (Cu₂S) are obtained. When these films were sputtered onto evaporated CdS layers, we obtained Cu₂S/CdS solar cells with a total area efficiency of above 4%.     

Surface sulfurization studies of thin film Cu(InGa)Se2 solar cells

Sulfurization of copper indium gallium diselenide (CIGS) thin films solar cell absorber has been attempted to enhance the open circuit voltage of the device by increasing the band gap of the absorber near the interface. The homogeneous co-evaporated Cu(InGa)Se₂ thin film was used for sulfurization. The sulfurization was studied in hydrogen sulfide and mixture of hydrogen sulfide and hydrogen selenide gases with the inclusion of oxygen. The structural and compositional properties of the absorber layers were investigated by XRD, EDS and AES analysis. The device results were characterized using current-voltage (I-V) and quantum efficiency. Sulfurization in hydrogen sulfide gas forms a fully converted sulfide layer at the top of the absorber layer, which in turn forms a barrier for the current collection; the device results also support this observation. Sulfurization in mixture of hydrogen sulfide and hydrogen selenide gases forms a wide band gap Cu(InGa)(SeS)₂ layer at the surface by partial replacement of Se by S. The device does not show an increase in open circuit voltage. This may be attributed to the diffusion of Ga away from the surface with the inclusion of sulfur at the surface, which counters the beneficial effect of sulfur at the surface.     

Evaporation of CuCl and CuCl2 for the fabrication of Cu2S/CdS thin film solar cells

The so-called dry method for the fabrication of Cu₂S/CdS thin film solar cells uses a thin CuCl film,in order to convert the surface layer of the CdS film into Cu₂S. Usually, the CuCl film is deposited by evaporation of freshly prepared CuCl powder. In the present paper we investigate the influence of contamination of this evaporation material by CuCl₂ and by CuCl₂·2H₂O. The experiments show that about 99% of these contaminants decompose into CuCl during the evaporation process.     

色素增感太阳电池多孔TiO2薄膜研究

采用溶胶-凝胶法制备复合硫化镉的二氧化钛纳米多孔薄膜;用差热分析仪、X射线分析仪、紫外-可见光分光光度计对薄膜的热处理制度、吸光度以及电池的性能进行了研究.结果表明在550℃下热处理,可以形成结晶良好,吸光度较好的TiO2薄膜;试制了太阳能电池,测定了电池性能.     

The effect of impurities on the doping and VOC of CdTe/CdS thin film solar cells

The effect of introducing impurities in CdTe, namely antimony (Sb) and oxygen (O), on the net carrier concentration in CdS/CdTe solar cells and on their open-circuit voltage (V_OC) has been investigated. Oxygen was introduced in the CdTe films during the deposition of this layer by the close-spaced sublimation process. The total pressure was held constant at 1330 Pa (N₂ and O₂). The amount of oxygen was varied by varying its partial pressure. Antimony was introduced into CdTe using a post-deposition diffusion process. Following the deposition of CdTe a thin film (a few nm) of Sb was deposited onto the CdTe surface and subsequently heat-treated to cause in-diffusion of Sb. The temperature and time during the diffusion process were varied in the range of 300-525 °C and 20-160 min respectively. In both instances it was possible to vary (increase) the doping concentration in CdTe. The increase in doping was accompanied by an increase in V_OC. However, in all instances the doping in CdTe reached a maximum value, beyond which further increases were not possible leading to saturation in V_OC. The highest V_OC measured was similar to state-of-the-art values in the range of 800-830 mV, and the highest doping concentration measured was in the 10¹⁶ cm^− 3 range.     

C-V calculations in CdS/CdTe thin films solar cells

Polycrystalline thin film CdS/CdTe heterojunction solar cells are important candidates for large scale photovoltaic applications. In this work we use a C-V (capacitance vs. voltage) theoretical method for the determination of the interface charge density σ and band discontinuity ΔEv of the CdS/CdTe heterojunction. The methodology is based on three cardinal equations: i) line up of the bands relative to the common Fermi level (at equilibrium) or the quasi-Fermi level (when voltage is applied), ii) charge neutrality and iii) the total capacitance of the heterostructure. We used CdS/CdTe solar cells, grown in our laboratory by the chemical bath deposition (for CdS film) and the close space vapor transport (for CdTe film) techniques. The interface parameters σ, and ΔEv are determined from C-V fitting between the calculated and the measured curve. The methodology presented in this study is general and can be applied to semiconductor-semiconductor and semimetal-semiconductor heterojunctions.     

Characterisation of Cu(In,Ga)Se2-based thin film solar cells on polyimide

Thin films of Cu(In,Ga)Se₂ (CIGS) were deposited at temperatures below 450 °C on polyimide (PI) substrates coated with Mo in a roll-to-roll set up by a combination of co-evaporation and ion-beam techniques. Flexible solar cells ITO/i-ZnO/CdS/CIGS/Mo/PI with and without Na incorporation were then fabricated. The films and solar cells were examined by: X-ray fluorescence spectroscopy (XRF) and Auger electron spectroscopy (AES), to determine the elemental composition, as well as by X-ray diffraction for structure- and scanning electron microscopy (SEM) for morphology-analysis. Photoluminescence (PL) and PL-excitation (PLE) at temperatures from 4.2 to 78 K were also used to estimate the band-gap energy of CIGS, examine the electronic properties and defect nature. The aim of this study was to correlate the incorporation of Na with optical and structural parameters of the CIGS layers as well as with the solar cell performance.     

Annealing effects on the chemical deposited CdS films and the electrical properties of CdS/CdTe solar cells

CdS layers grown by chemical bath deposition (CBD) are annealed in the oxygen and argon-hydrogen atmosphere respectively. It has been found that the open circuit voltage of the CdS/CdTe solar cell increases when the CBD CdS is annealed with oxygen before the deposition of CdTe by close spaced sublimation (CSS), while the performance of the solar cell decreases when the CBD CdS is annealed with argon-hydrogen. Electronic properties of the CdS films are investigated using X-ray photo-electron spectroscopy (XPS), which indicates that the Fermi level is shifting closer to the conduction band after annealing in the oxygen and consequently a higher open circuit voltage of the solar cell can be obtained.