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.     

Efficiency simulations of thin film chalcogenide photovoltaic cells for different indoor lighting conditions

Photovoltaic (PV) energy is an efficient natural energy source for outdoor applications. However, for indoor applications, the efficiency of PV cells is much lower. Typically, the light intensity under artificial lighting conditions is less than 10 W/m² as compared to 100-1000 W/m² under outdoor conditions. Moreover, the spectrum is different from the outdoor solar spectrum. In this context, the question arises whether thin film chalcogenide photovoltaic cells are suitable for indoor use. This paper contributes to answering that question by comparing the power output of different thin film chalcogenide solar cells with the classical crystalline silicon cell as reference. The comparisons are done by efficiency simulation based on the quantum efficiencies of the solar cells and the light spectra of typical artificial light sources i.e. an LED lamp, a “warm” and a “cool” fluorescent tube and a common incandescent and halogen lamp, which are compared to the outdoor AM 1.5 spectrum as reference.     

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.     

Effects of high-temperature annealing on ultra-thin CdTe solar cells

High-temperature annealing (HTA), a process step prior to vapor cadmium chloride (VCC) treatment, has been found to be useful for improving the crystallinity of CdTe films and the efficiency of ultra-thin CdTe solar cells. Scanning electron microscopy, optical absorption, photoluminescence measurements and analyses on photoluminescence results using spectral deconvolution reveal that the additional HTA step produces substantial grain growth and reduces grain boundary defects. It also reduces excessive sulfur diffusion across the junction that can occur during the VCC treatment. The HTA step helps to produce pinhole-free CdTe films and reduce electrical shorts in ultra-thin CdTe solar cells. An efficiency of about 11.6% has been demonstrated for ultra-thin CdS/CdTe solar cells processed with HTA step.     

Initial and degraded performance of thin film CdTe solar cell devices as a function of copper at the back contact

Copper performs an important role in obtaining high-performance thin-film CdTe solar cell devices. Both initial performance and performance after stress depends strongly on the total copper content at the back-contact, the Cd to Te ratio on the backside, the etching process, and the way the copper is activated. With regard to getting high open circuit voltage a small amount of Cu seems sufficient upon the right anneal treatment. However, regarding open circuit voltage degradation for stressed devices there seems to be an optimum amount of Cu. Te-enrichment does not seem to have a big impact on device stability.     

Investigation of the excitonic luminescence band of CdTe solar cells by photoluminescence and photoluminescence excitation spectroscopy

The excitonic luminescence band of polycrystalline cadmium telluride layers has been investigated by Photoluminescence (PL) and Photoluminescence excitation spectroscopy (PLE). CdTe was deposited by means of close space sublimation and the samples were activated by different chlorine containing compounds, i.e. cadmium chloride, hydrochloric acid, and sodium chloride as well as by simple air activation or received no post deposition treatment. In the PL spectra, four different peaks within the excitonic luminescence band were resolved. These include the free-exciton peak and two transitions of excitons bound to defects. Furthermore, free excitons and band to band transitions were detected by means of PLE. The PL and PLE spectra are discussed with respect to the post deposition treatments.     

The formation of different phases of CuxTe and their effects on CdTe/CdS solar cells

Material studies and device applications of Cu_xTe in an NREL-developed CdTe solar cell structured as glass/Cd₂SnO₄/ZnSnO_x/CdS/CdTe are presented. The Cu_xTe primary back contact was formed by evaporating a Cu layer with various thicknesses at room temperature on HNO₃/H₃PO₄ (NP) solution etched CdTe layer. A post-annealing was then followed. The structural evolution and electrical properties of Cu_xTe were investigated. Cu/Te ratio and post-annealing temperature are two processing parameters in this study. The Cu_xTe phases are mainly controlled by the Cu/Te ratio. After a post-annealing at a low temperature, such as 100 °C, no Cu_xTe phase transformation from its as-deposited phase was observed. A post-annealing treatment at a higher temperature, such as 250 °C, can reveal the stoichiometric Cu_xTe phases based on the Cu/Te ratio used in the devices. But a post-annealing at a further higher temperature, such as 400 °C, resulted in a complicated Cu_xTe phase appearance. CuTe, Cu_1.4Te, and Cu₂Te are three major phases detected by X-ray diffraction (XRD) for different Cu thickness application annealed at 250 °C. Application of Cu thicker than 60 nm degrades open-circuit voltage (V_oc) and shunting resistance (R_sh), but increases series resistance (R_s). The correlation between device performance and the Cu_xTe back contact illustrates that the process used for forming the Cu₂Te back contact failed to produce good fill factor (FF) and also introduced higher barrier height. The best device was observed for a back contact with a mixed Cu_1.4Te and CuTe phases.     

Cu-related recombination in CdS/CdTe solar cells

Cu used in the back contact of CdS/CdTe solar cells is known to improve contact behavior and open-circuit voltage. A study of devices made with varying Cu amounts confirmed these observations. However, Cu was also found to be deleterious to current collection. Time-resolved photoluminescence measurements of CdTe devices show that carrier lifetime decreased with increased Cu concentration. Drive-level-capacitance-profiling and low-temperature photoluminescence suggest this decrease in lifetime was associated with increased recombination center density introduced by Cu in the CdTe layer. The resulting impact of increased Cu on device performance was a voltage-dependent collection of photogenerated carriers that reduced fill-factor.     

Ordered polycrystalline thin films for high performance CdTe/CdS solar cells

An ordered polycrystalline approach is proposed to overcome fundamental problems associated with random polycrystalline thin films, namely grain boundaries and inhomogeneity. The approach consists of two main steps: (1) the deposition of a patterned growth mask and (2) the selective-area deposition of the ordered polycrystals. The ordered polycrystalline approach was investigated using the CdTe/CdS material system. Experimental results demonstrate that SiO₂ and Si₃N₄ are effective growth masks and that temperature is a dominant parameter for selective-area deposition. PL and XRD characterization indicates that the ordered polycrystalline technique has the potential for improving the crystal quality and order of polycrystalline CdTe thin films. The approach appears to be fairly general and could be applied to other material systems.     

Study of the physical properties of Bi doped CdTe thin films deposited by close space vapour transport

Bi doped cadmium telluride (CdTe:Bi) thin films were grown on glass-substrates by the close space vapour transport method. CdTe:Bi crystals grown by the vertical Bridgman method, varying the nominal Bi concentration in the range between 1 × 10¹⁷ and 8 × 10¹⁸ cm^− 3, were used in powder form for CdTe:Bi thin film deposition. Dark conductivity and photoconductivity measurements in the 90-300 K temperature range and determination by photoacoustic spectroscopy of the optical-absorption coefficient of the films in the 1.0 to 2.4 eV spectral region were carried out. The influence of Bi doping levels upon the intergrain barrier height and other associated grain boundary parameters of the polycrystalline CdTe:Bi thin films were determined from electrical, optical and morphological characterization.     

Influence of electrodeposition potential and heat treatment on structural properties of CdTe films

A modified Butler-Volmer model is developed to predict the potential of perfect stoichiometry (PPS) for electrodeposition. CdTe thin films are deposited in an acidic solution; their electrodeposition mechanism is investigated using cyclic voltammetry. Calculated and experimental PPS values exhibit good agreement. At PPS, well-connected granular CdTe thin films can be deposited; these are predicted to be intrinsic, and slightly p-type due to cadmium vacancies. Type conversion occurs only because of defect redistribution and local defect reactions after annealing; the converted n-type layer exhibits lower resistivity and higher mobility. A film annealed at 350 °C exhibits excellent crystallization.     

Effect of back-contact barrier on thin-film CdTe solar cells

The presence of a back-contact barrier affects the current–voltage characteristics of thin-film CdS/CdTe/metal solar cells primarily by impeding hole transport, a current-limiting effect commonly referred to as “rollover.” In this work, the CdS/CdTe solar cell with a CdTe/metal back-contact barrier is modeled by two opposite polarity diodes in series. Analytic simulations are fitted to the measured current–voltage curve, the voltage distribution between the two diodes is shown under different conditions, and the back-contact barrier height is extracted. Room-temperature barrier heights exceeding 0.5 eV will generally result in significant fill-factor reduction.     

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%.     

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.     

Microstructural features of cadmium telluride photovoltaic thin film devices

In order to study the microstructure of cadmium telluride (CdTe) photovoltaic thin film solar cells, manufactured by an in-line manufacturing process, Scanning Electron Microscopy characterization (SEM) and X-ray diffraction (XRD) characterization were performed. SEM measurement showed that no substantial changes in the grain structure of CdTe layers occurred during the Cadmium Chloride (CdCl₂) treatment. No change in the cubic CdTe lattice parameter “a” was observed for the CdCl₂ treated sample. It is inferred that the primary effect of the CdCl₂ treatment in the devices studied is the passivation of grain boundaries and bulk defects. XRD studies show a loss of preferred orientation (as determined from the peak ratios) of planes during the copper compound treatment indicating recrystallization of the grains due to the Cu treatment. Also the Cu treated sample showed decrease in value of the lattice parameter “a”.     

Physical properties of Bi doped CdTe thin films grown by CSVT and their influence on the CdS/CdTe solar cells PV-properties

The physical properties of Bi doped CdTe films, grown on glass substrates by the Closed Space Transport Vapour (CSVT) method, from different Bi doped CdTe powders are presented. The CdTe:Bi films were characterized using Photoluminescence, Hall effect, X-Ray diffraction, SEM and Photoconductivity measurements. Moreover, CdS/CdTe:Bi solar cells were made and their characteristics like short circuit current density (J_sc), open circuit voltage (V_OC), fill factor (FF) and efficiency (η) were determined. These devices were fabricated from Bi doped CdTe layers deposited on CdS with the same growth conditions than those used for the single CdTe:Bi layers. A correlation between the CdS/CdTe:Bi solar cell characteristics and the physical properties of the Bi doped CdTe thin films are presented and discussed.     

Some physical properties of Sn-doped CdO thin films prepared by chemical bath deposition

Undoped and Sn-doped CdO thin films were prepared by the chemical bath deposition method by means of a procedure that improves the deposition efficiency. All as-grown films were crystallized in the cubic structure of cadmium peroxide (CdO₂) and transformed into CdO with a cubic structure after an annealing process. The as-grown films have a high resistivity (> 10⁶ Ω cm) and an optical bandgap around 3.6 eV. Undoped CdO displays an optical bandgap around 2.32–2.54 eV and has an electrical conductivity of 8 × 10^− 4 Ω cm. The Sn incorporation into CdO produces a blue shift in the optical bandgap (from 2.55 to 2.84 eV) and a decrease in the electrical conductivity.The deposition procedure described here gives colloid-free surface thin films as indicated by the surface morphology analysis.     

CdTe photoelectrochemical solar cells—chemical modification of surfaces

Chemical modification of bothn andp type CdTe has been found to improve the performance and stability of PEC solar cells. The surfaces, modified by Ru³⁺, have been examined by a variety of techniques. Modification results in enhanced barrier height at the surface due to the formation of a passivating oxide layer.     

Effects of substrate temperature and post-deposition anneal on properties of evaporated cadmium telluride films

The effects of substrate temperature and post-deposition heat treatment steps on the morphology, structural, optical and electrical properties of thin film CdTe layers grown by vacuum evaporation were investigated. Scanning electron microscopy and X-ray diffraction (XRD) techniques were employed to study the structural changes. It was observed that the grain sizes and morphologies of as-deposited layers were similar for substrate temperatures of − 173 °C and − 73 °C. However, CdTe films produced at a substrate temperature of 27 °C had substantially larger grain size and clearly facetted morphology. Annealing at 200-400 °C in air did not cause any appreciable grain growth in any of the films irrespective of their growth temperature. However, annealing at 400 °C reduced faceting in all cases and initiated fusing between grains. XRD studies showed that this behavior after annealing at 400 °C coincided with an onset of a degree of randomization in the originally strong (111) texture of the as-grown layers. Optical band gap measurements showed sharpening of the band-edge upon annealing at 400 °C and a band gap value in the range of 1.46-1.49 eV. Resistivity measurements indicated that annealing at 400 °C in air forms a highly resistive compensated CdTe film. All results point to 400 °C to be a critical annealing temperature at which optical, structural and electrical properties of CdTe layers start to change.