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The objective of this work is to increase the open circuit voltage of CuGaSe2(CGS)-based solar cells without decreasing their efficiency. For that, the interface between the p-type CGS absorber and the n-type CdS/ZnO window layer is compared using three different recipes for the growth of the buffer layer. Results show the importance of the adaptation of the CdS buffer layer to the CuGaSe2 absorber film. A maximum open circuit voltage of 922 mV is achieved for the devices when using 60ºC as the chemical bath temperature and a low thiourea concentration. Drive-level capacitance profiling, external quantum efficiency and temperature dependent current-voltage measurements reveal a better quality of the CdS/CuGaSe2 interface for this buffer layer deposition conditions. Factors such as the larger depletion region width and the lower doping level, reducing the tunnelling component, are pointed out as responsible of the higher Voc.
In2S3 thin films appear to be a promising candidate for photovoltaic applications due to its stability, wide band gap and photosensitivity. The optical band gap value of Indium sulfide (In2S3) thin films reported in the literature varies from 2.0 eV to 2.4 eV following their synthesis process. These values are too small for an application as buffer layer in solar cells. In Present work, we report that incorporation of Sn using [SnCl4.5H2O] could increase the bandgap to wider ranges. In2S3/Sn thin films were deposited on soda lime glass substrate using chemical spray pyrolysis (CSP) technique. The spraying solution contained indium chloride (InCl3), thiourea [CS(NH2)2] and [SnCl4.5H2O]. Studies were done on films prepared using different Sn/In ratios. Depth profile using x-ray photoelectron spectroscopy showed that incorporation of Sn increased the concentration of oxygen in the samples. Band gap of the films increased with increase in Sn/In ratios. Depending on the ratios, bandgap could be varied from 2.72 eV to 3.78 eV. At lower mixing levels wide band gap low resistive In2S3/Sn films could be obtained which is highly useful for buffer layer applications. Low resistive buffer layer will decrease the series resistance of the cell and wider band gap will improve light transmission in the blue wavelength, both these factors help in increasing the short circuit current of the photovoltaic cell. Samples having higher Sn/In ratios showed wider band gap (up to3.78 eV).Though the samples had very high bandgap and high resistivity these samples were highly photosensitive [(IL-ID)/ID = 11,000]. The results proved that tin incorporation modified the band gap and electrical properties of the In2S3/Sn films favorably over wider ranges making it highly suitable for different optoelectronic applications.
The pentenary chalcopyrite compound Cu(InGa)(SeS)2 provides several potential advantages over Cu(InGa)Se2 as the absorber layer in thin film solar cells, especially with wide bandgap alloys. The effects of S addition to the quaternary alloy are investigated with films deposited by elemental thermal co-evaporation and by the reaction of metallic precursors in hydride gases. With co-evaporated films the addition of S complicates the control of composition through the film. The incorporation of the chalcogen species Se and S depend on the relative Cu in the film and, for films with excess Cu, on the relative group III composition. For the precursor reaction process the addition of S by the inclusion of H2S gas in the reaction enables control of the relative Ga concentration and bandgap of the film. With both processes the incorporation of S during deposition also effects the morphology and grain size. The co-evaporated films have smaller grains with S while the reacted films have larger grains which may be due to the higher TSS the S reaction enables.
Junction capacitance methods were used to examine a matched pair of CuInGaSe2 (CIGS) thin film solar cells, one with Na incorporated into the absorber and the other with a diffusion barrier to inhibit the Na incorporation from the soda-lime glass. Typical cells showed a 50% increase in efficiency with the addition of Na. Forward biased admittance spectroscopy revealed a large defect density located near the CdS/CIGS heterojunction in the reduced Na samples not present in the higher Na samples. This may be responsible for the lower Voc, contributing to the loss in efficiency when Na is not added. Drive-level capacitance profiles revealed free carrier densities of 3×1014cm-3 and 1.1×1014cm-3 for the higher and reduced Na samples, respectively. Transient photocapacitance spectra indicated a slight improvement in absorber properties with the addition of Na, but not enough to account for the large loss in efficiency.
Thin films of CuIn1-xAlxSe2 have been produced by the selenisation of magnetron sputtered Cu/In/Al precursor layers using elemental selenium and the chemical and physical properties of the layers have been determined for different conditions of synthesis. For optimum conditions of synthesis it was found possible to produce single phase films with the chalcopyrite structure. These films were pinhole free, had good adhesion and were conformal to the substrate. The films had uniform depth profiles as determined using the MiniSIMS. The layers were highly photoactive, indicating that they have the potential to be used to fabricate thin film photovoltaic solar cell devices.
Using molecular beams, polycrystalline thin CuInS2 (CIS) films of different thicknesses were grown on Si substrates covered with a sputtered Mo-buffer layer. Systematic photoluminescence and photoreflectance measurements were performed to investigate the influence of strain - introduced during growth - on the optical properties. The transition energy of the free A-exciton (FXA) decreases with increasing tensile strain caused by (i) increasing thickness of the Mo buffer layer and (ii) decreasing thickness of the CIS layer. Furthermore, the energetic splittings between FXA, FXB, and FXC increase with increasing tensile strain. When combined with X-ray diffraction data, the oscillator strengths of the excitonic transitions yield information on the strain distribution within the films.
Highly transparent, conductive ZnO:Al doped films have been deposited by a non-vacuum spray deposition method. At substrate temperatures above 400C we attain resistivites of 5x10-3Ohmcm and free charge carrier concentrations of 10-20cm-3. ZnO film growth and quality are sensitive to the precursor solution. For a non-vacuum process the properties of the films are excellent.
The challenge is to lower the deposition temperature to a maximum of 250C to be useful for Cu(In, Ga)(S, Se)2 solar cells and yet maintain the ZnO film quality and conductivity. As the deposition temperature decreases the resistivity of the ZnO drastically increases yet is conducting enough to be used undoped as the intrinsic ZnO layer. This is particularly relevant as the deposition technique is readily up-scalable to roll to roll coating processes.
In electron-backscatter diffraction, crystalline orientation maps are formed while the electron beam of an SEM scans the sample surface. EBSD requires a flat sample to avoid shadowing of the electrons from the detector by surface features. In this work, we investigate the preparation of CdTe samples deposited by close-spaced sublimation for EBSD analysis. Untreated samples were rough, resulting in areas with no EBSD signal. We processed the samples by polishing and ion-beam milling. Polishing produced flat samples, but low-quality EBDS data, because the top surface of the samples had poor crystallinity. In contrast, ion-beam milling proved to be suitable for producing flat samples with minimal surface damage, yielding good EBSD data. We also analyzed the samples with atomic force microscopy, and correlated the quality of the EBSD data with sample roughness. The EBSD data showed that the CdTe films were randomly oriented and had columnar growth and a high density of <111> twin boundaries.
Indium tin oxide (ITO) has drawn a great deal of attention due to its potential use as transparent electrodes in organic light emitting diode (OLED) and photovoltaic applications. This work focuses on understanding the role of impurity defects on the electrical conduction and transmittance of ITO. Thin films of ITO with high carrier concentration have been deposited onto polyethylene napthalate (PEN) substrates by electron-beam deposition without introduction of oxygen into the chamber. The influence of air anneals on the electrical and optical properties of ITO/PEN samples can is evaluated in terms of the oxygen content and is explained in terms of changes in the free electron concentrations. Rutherford backscattering spectrometry and X-ray photoelectron spectroscopy analysis were used to determine the oxygen content in the film. A Hall effect measurement is used to determine the dependence of electrical properties on oxygen content. The electrical properties of the ITO films such as carrier concentration, electrical mobility, and resistivity abruptly changes after annealing in the air atmospheres. In addition, optical transmittance is improved from 7 to 71 % and optical band gap changes from 3.18 to 3.25 eV after heat treatment. The optical band gap narrowing behavior is because of impurity band and heavy carrier concentration. Metal impurity clusters form in the films as a result of oxygen deficiency and also generate defects and/or impurity states in the band gap and produces an optical band gap shift by merging of these impurity states and conduction band.
The linear polarisation of luminescence light allows conclusions on the symmetry of defects in semiconductors with non-cubic symmetry, like chalcopyrites, for which three shallow acceptors have been identified by photoluminescence. The polarisation dependent photoluminescence allows to determine the symmetry of the defects relative to the c-axis of the crystal. A simple geometrical model implies that chalcogen sites show a predominant direction perpendicular to the c-axis, while metal sites show a predominant direction parallel to the c-axis. Since all three shallow acceptors show polarization parallel to the c-axis, it can be concluded that they are situated on a metal site
Conductive tin-oxide (SnO2) film is doped by group V or VII elements. Of all possible dopants, fluorine provides n-type SnO2 with the best electronic and optical properties. However, the commonly used fluorine dopant, bromotrifluoromethane (CBrF3), is a greenhouse gas. Thus, an alternative fluorine source is needed. In this work, we compared CIF3 as a fluorine dopant to CBrF3. With CBrF3 dopant, optimized carrier concentration and electron mobility values can reach to mid 1020 cm-3 and over 40 cm2/V-s, respectively. After carrier concentration saturates, the electronic mobility continues to improve with an increase of CBrF3 dopant concentration. As a comparison, to achieve similar carrier concentration, far less CIF3 dopant is required. However, the electron mobility is lower (<30 cm2/V-s) and does not improve with an increase of dopant concentration. The low electron mobility increases the optical absorption, especially of long wavelengthes. Considering CdTe/CdS solar cell efficiency, the device with a CIF3-doped SnO2 window layer provides the lower photocurrent.
We present a multi-element study of impurities in CdTe/CdS photovoltaic cells, aimed at understanding their origins and impact on devices. Our investigation was based on calibrated secondary ion mass spectrometry (SIMS) depth profiling, with Na, Zn, Sn, O, Sb, Si, Cl, In, Cu and Pb being studied. The solar cell structures were grown by sputtering and close-space sublimation, and some of them were further processed (CdCl2 and Br2-methanol) for the purpose of comparison. Using source materials of different purity allowed us to establish the origins of impurities. We found that some elements increased in concentration upon processing, namely Cl (x100), Na (x10), and In (x1.5). The concentrations of Si, Cu, Zn, Sn and Sb found in a processed device were largely unchanged - and are similar to those found in a high purity single crystal CdTe reference sample.
The primary routes for increasing CdS/CdTe solar cell efficiency involve increasing free carrier density, reducing bulk and interface recombination, and/or reducing back contact barrier height. This paper focuses on the role of the back contact barrier in increasing cell efficiency. Measurement of barrier height and back surface recombination are outlined and three CdTe/MX/M back contact prototypes, each with particular strengths, are discussed to bring out important issues.
InP/Si engineered substrates formed by wafer bonding and layer transfer have the potential to significantly reduce the cost and weight of III-V compound semiconductor solar cells. InP/Si substrates were prepared by He implantation of InP prior to bonding to a thermally oxidized Si substrate and annealing to exfoliate an InP thin film. Following thinning to remove damage caused by the implantation and exfoliation from the surface of the InP transferred film, InGaAs solar cells lattice-matched to bulk InP were grown on those substrates using metal-organic chemical vapor deposition. The photovoltaic current-voltage characteristics of the InGaAs cells fabricated on the wafer-bonded InP/Si substrates were comparable to those synthesized on commercially available epi-ready InP substrates, thus providing an initial demonstration of wafer-bonded InP/Si substrates as an alternative to bulk InP substrates for solar cell applications.
In an effort to overcome the lack of a suitable metal as an ohmic back contact for CdTe solar cells, a study was carried out on the potential for using a highly arsenic (As) doped CdTe layer with metallization. The deposition of full CdTe/CdS devices, including the highly doped CdTe:As and the CdCl2 treatment, were carried out by metal organic chemical vapour deposition (MOCVD), in an all-in-one process with no etching being necessary. They were characterized and compared to control devices prepared using conventional bromine-methanol back contact etching. SIMS and C-V profiling results indicated that arsenic concentrations of up to 1.5 × 1019 at·cm-3 were incorporated in the CdTe. Current-voltage (J-V) characteristics showed strong improvements, particularly in the open-circuit voltage (Voc) and series resistance (Rs): With a 250 nm thick doped layer, the series resistance was reduced from 9.8 Ω·cm2 to 1.6 Ω·cm2 for a contact area of 0.25 cm2; the J-V curves displayed no rollover, while the Voc increased by up to 70 mV (~ 12 % rise). Preliminary XRD data show that there may be an As2Te3 layer at the CdTe surface which could be contributing to the low barrier height of this contact.
We report and electronic and optical characterization of three wide-bandgap alloys of the Cu(InxGa1-x)(SeyS1-y)2 pentenary material system. Devices were characterized using admittance spectroscopy as well as drive-level capacitance profiling. The devices showed activated defect behavior typical of thin-film solar cell devices. Optical characterizations were carried out with Transient Photocapacitance and Transient Photocurrent spectroscopies. These data showed broad exponential bandtails with large Urbach energies, indicative of a moderately high degree of compositional and/or structural disorder. The temperature dependence of the TPC spectra was examined in detail and we were able to observe the thermal emission of electrons from defects into the conduction band. The emission energy of these features corresponds well with the measured optical threshold and the known bandgap of the cells. Thus we infer an upper bound of about 50meV for the lattice relaxation energy following the optical transition into the defect.
The impact of the conduction band offset (CBO) between window/Cu(In,Ga)Se2 (CIGS) layers on the light soaking (LS) effect in CIGS solar cells has been studied with continuous CBO control using a (Zn,Mg)O (ZMO) window layer. Two types of CIGS solar cells with different window/buffer/absorber layers configurations were fabricated, i.e., ZMO/CIGS (without buffer layer) and ZMO/CdS/CIGS structures. The CBO values between ZMO and CIGS layers were controlled to -0.15~0.25 eV. Plus and minus signs of CBO indicate the conduction band minimums of ZMO above and below that of CIGS, respectively. Current-voltage (J-V) characteristics of the solar cells with different LS durations revealed that a positive CBO value higher than 0.16 eV induces J-V curve distortion, i.e., LS effect, and all the J-V characteristics stabilized in 30 min. The degrees of the LS effect were dominated by the CBO value between ZMO and CIGS layers in the both structure regardless of the existence of CdS buffer layers. These results indicate that the LS effect is dominated by the highest barrier for photo-generated electrons in the conduction band diagram, i.e., the CBO between ZMO and CIGS layers, and quantitatively the LS effect emerges the CBO value higher than 0.16 eV.
CdS/CdTe solar cells with 2.3 μm CdTe were prepared with four different types of p+/n+ transparent back contacts (TBCs) - ZnTe:Cu/ZnO:Al, ZnTe:N/ZnO:Al, ZnTe:Cu/ITO, and ZnTe:N/ITO. ZnTe:N/ITO was found to give the best results. This back contact was then used to make cells of lesser (1.8 μm and 0.7 μm) CdTe thickness, in those cases giving good performance up to 9.1% efficiency. Bifacial J-V was performed on all cells with the optimum ZnTe:N/ITO back contact.
This paper analyses the photovoltaic parameters of the most promising CdS/CdTe solar cells for large application prepared by close space sublimation (CSS) and high vacuum evaporation (HVE). CdS/CdTe solar cells that have an efficiency of~10 % have been studied by current-voltage, capacitance-voltage and quantum efficiency measurements. The current-voltage characteristics show that the addition of small amounts of Cu or Te to back contacts by thermal evaporation improves contact properties by p+-doping the CdTe surface and creating a pseudo-ohmic contact. The cell deposited by CSS seriously suffers in FF compared to the cells prepared by HVE. For both types of the cells above mentioned measurements reveal that the efficiency of CdS/CdTe solar cells fabricated by CSS is limited by a light-dependent shunt resistance and by a high series resistance, but concerning of the cells fabricated by HVE - by the formation of a thick layer of CdS1-xTex at the interface.
The electronic and materials properties of two series of wide-bandgap solar cells with Cu-poor CuGaSe2 (CGS) absorbers have been studied, to better understand limitations on the device performance. One series of samples displayed distinct lateral non-uniformities in Cu/Ga ratio, Na content, and thickness, likely due to a limited supply of Se during CGS growth. The second series of samples appeared uniform. The most prominent electronic difference was the presence of a distinct band of near-interface defect states in the more non-uniform set of samples. The device performance did not appear to be limited by defects in the bulk CGS film until the defect density was larger than 2×1016 cm-3. Instead, interface recombination appears to be a significant factor limiting Voc in both sets of samples.