Chemically deposited CdS by an ammonia-free process for solar cells window layers

Chemically deposited CdS window layers were studied on two different transparent conductive substrates, namely indium tin oxide (ITO) and fluorine doped tin oxide (FTO), to determine the influence of their properties on CdS/CdTe solar cells performance. Three types of CdS films obtained from different chemical bath deposition (CBD) processes were studied. The three CBD processes employed sodium citrate as the complexing agent in partial or full substitution of ammonia. The CdS films were studied by X-ray diffraction, optical transmission spectroscopy and atomic force microscopy. CdS/CdTe devices were completed by depositing 3 μm thick CdTe absorbent layers by means of the close-spaced vapor transport technique (CSVT). Evaporated Cu-Au was used as the back contact in all the solar cells. Dark and under illumination J-V characteristic and quantum efficiency measurements were done on the CdS/CdTe devices to determine their conversion efficiency and spectral response. The efficiency of the cells depended on the window layer and on the transparent contact with values between 5.7% and 8.7%.     

Optical characterization of In2S3 solar cell buffer layers grown by chemical bath and physical vapor deposition

In this paper, we study the optical properties of indium sulfide thin films to establish the best conditions to obtain a good solar cell buffer layer. The In₂S₃ buffer layers have been prepared by chemical bath deposition (CBD) and thermal evaporation (PVD). Optical behavior differences have been found between CBD and PVD In₂S₃ thin films that have been explained as due to structural, morphological and compositional differences observed in the films prepared by both methods. The resultant refractive index difference has to be attributed to the lower density of the CBD films, which can be related to the presence of oxygen. Its higher refractive index makes PVD film better suited to reduce overall reflectance in a typical CIGS solar cell.     

Investigations on electron beam evaporated Cu(In0.85Ga0.15)Se2 thin film solar cells

CIGS bulk with composition of CuIn_0.85Ga_0.15Se₂ was synthesized by direct reaction of elemental copper, indium, gallium and selenium. CIGS thin films were then deposited onto well-cleaned glass substrates using the prepared bulk alloy by electron beam deposition method. The structural properties of the deposited films were studied using X-ray diffraction technique. The as-deposited CIGS films were found to be amorphous. On annealing, the films crystallized with a tetragonal chalcopyrite structure. An intermediate Cu-rich phase precipitated at 200 °C and dissociated at higher annealing temperatures. Average grain size calculated from the XRD spectra indicated that the films had a nano-crystalline structure and was further corroborated by AFM analysis of the sample surface. The chemical constituents present in the deposited CIGS films were identified using energy dispersive X-ray analysis. CIGS based solar cells were then fabricated on molybdenum and ITO coated glass substrates and the efficiencies have been evaluated.     

Electro deposited In2S3 buffer layers for CuInS2 solar cells

We report the electro deposition of In₂S₃ buffer layers for CuInS₂ solar cells. All materials and deposition conditions were selected taking into account environmental, economic and technological aspects of a potential transfer to large volume industrial production. Different bath compositions and electro deposition parameters were studied. The obtained films exhibited complete substrate coverage, confirmed by SEM and XPS. In/S ratio close to 2/3 was obtained. XPS measurements detected the presence of indium hydroxide, transforming into oxide upon anneal at 200 °C. Maximum photoelectric conversion efficiency of 7.1% was obtained, limited mainly by a low fill factor (51%). Further process optimization is expected to lead to efficiencies comparable to CdS buffers. So far, open-circuit voltages as high as 660 mV were demonstrated.     

CuGaSe2 solar cells prepared by MOVPE

Metal organic vapor-phase epitaxy (MOVPE) is used to prepare epitaxial reference films and solar cells based on CuGaSe₂. Room temperature Hall measurements are performed on epitaxial CuGaSe₂. Conductivities up to 0.7 (Ω cm)⁻¹ were obtained. Highest mobilities of 270 cm²/Vs are observed for near stoichiometric slightly Ga-rich films. Net charge carrier concentration is higher in the Cu-rich grown films than in the Ga-rich films. Solar cells with epitaxial absorber are prepared that reach efficiencies of 3.3%. First polycrystalline solar cells are grown on Mo/glass at reduced substrate temperatures. Under AM1.5 illumination open-circuit voltages up to 740 mV and efficiencies of 2.0% are obtained.     

Spectral beam splitting technology for increased conversion efficiency in solar concentrating systems: a review

Solar concentrating systems that employ one or more quantum receivers may realize improved energy utilization and higher electric conversion efficiency by incorporating spectral beam splitting technology. Such techniques were investigated in thermophotovoltaic conversion, introduced in the early 1960s, and in concentrating PV devices using cells of different band-gap materials, proposed as early as 1955. One major application was found in systems combining quantum and thermal receivers. This article presents a review of the various solar hybrid beam splitting systems proposed in the literature and the different spectrum splitting strategies employed.     

A novel UV-mediated low-temperature sintering of TiO2 for dye-sensitized solar cells

Titania pastes were fired at 450 °C in oxygen to give white titania that was used to prepare dye-sensitized solar cells (DSSC). Titania fired at lower temperature and/or under inert atmosphere have brown stripes and cells made from these stripes had no measurable efficiency. When the titania paste was screen printed and then heated and simultaneously irradiated with UV light, white stripes were obtained. Improved efficiency was noted for PV cells made from pastes heated at lower temperature under irradiation vs. cells made from low-temperature heated paste but without irradiation. UV irradiation appears to facilitate clean oxidation of residual organic materials in the titania precursor pastes. The best cells in our study made with our titania paste treated at 450 °C in oxygen had the following characteristics: efficiency=3.45%; V_oc=630 mV; J_sc=8.5 mA/cm²; and a fill factor=0.64.     

The limiting efficiency of band gap graded solar cells

Two fundamental mechanisms limit the maximum attainable efficiency of solar cells, namely the radiative recombination and Auger recombination. We show in this paper that proper band gap grading of the solar cell localizes the Auger recombination around the metallurgical junction. Two beneficial effects result from this Auger recombination localization; first the cell is less sensitive to the surface conditions, and second, the previous estimates for the limiting efficiency of solar cells by Shockley, Tiedje, and Green are revised upwardly. We calculate the optimum bandgap grading profile for several real material systems, including GaInAsP lattice matched to InP, and a-SiGe on a-Si substrate.     

Influence of scattering layers on efficiency of dye-sensitized solar cells

Thin titaniumdioxide (TiO₂) semiconductor layer with different scattering layers was investigated in dye-sensitized solar cells (DSSC). Since the cost of the photoactive dye in the DSSC is relatively high, it is reasonable to assume that the price of the dye could be one of the decisive factors in determining the price of the DSSC modules. Use of a thin layer of nanocrystalline TiO₂ would imply reduction in the amount of dye coverage, however, lower amount of dye in the thin films would imply fewer electron generation upon illumination. Thus, it becomes necessary to include a light scattering layer such that the lower photon conversion due to thin layer could be compensated. In the present study up to 80% increase in current density was observed due to inclusion of scattering layers. Reflectance and transmittance measurements were employed in order to study the optical properties of these scattering layers. The scattering layers, which are considered here, are TiO₂-Rutile, zirconiumdioxide (ZrO₂), and layers consisting of these two in various proportions. With a 4 μm thin titanium dioxide semiconductor layer as photo electrode and an additional light scattering layer (consisting of TiO₂-Rutile and ZrO₂ in a ratio of 1:3), efficiencies of 6.8% were achieved.     

Energy conversion efficiency of solar cells coated with fluorescent coloring agent

A coating of fluorescent coloring agent (FCA) on the solar cells gives 30% increase in the energy conversion efficiency of the solar cell. This increase is attributable to the reduction of the reflection of incident light. The reflectances show low values at the excitation wavelengths, where the incident light is absorbed to excite the FCA. The fluorescence quantum yield for a dried FCA was much larger than that for FCAs dissolved in paint thinner.     

Radiation response of InP/Si and InGaP/GaAs space solar cells

An analysis of the radiation response of state-of-the-art InP/Si, InGaP, and dual junction (DJ) InGaP/GaAs space solar cells under both electron and proton irradiated is presented. The degradation data are modeled using the theory of displacement damage dose. For each technology, a characteristic curve which describes the cell degradation in any radiation environment is determined, and the characteristic curves are used to compare the radiation resistance of the different technologies on an absolute scale. The radiation data are used as input to a code which predicts the end-of-life (EOL) performance of a solar panel in earth orbit. The results show that in orbits outside the earth's radiation belts, the high-efficiency DJ InGaP/GaAs solar panels provide the highest EOL specific power. However, in orbits which pass through the belts, the radiation hard InP/Si panels provide the highest specific power by as much as 30%.     

Characterization of the PSG/Si interface of H3PO4 doping process for solar cells

The precipitation of P in the emitter region of H₃PO₄ spray doped silicon for solar cell applications has been investigated by electron microscopy, X-ray microanalysis and electrical measurements after annealing for two different times. P, Si and O concentration profiles show that the composition of the phosphorous silicate glass (PSG) is in agreement with a solid solution of P₂O₅ in SiO₂ and that P concentration is peaked at the PSG/Si interface. TEM observations have shown for the shorter annealing the formation of a 20 nm thick defect layer at the silicon surface; this layer evolves into a network of large rod-like monoclinic (or orthorhombic) SiP precipitates, which extend in depth up to about 100 nm for the longer treatment. The SiP crystal structure and the habit planes are the same as previously reported in literature. No deeper defect that could interact with the junction located at about 300 nm has been detected. Although the SiP precipitation takes place entirely at the Si surface, it is not significantly affected by the orientation of the crystals and by the texturing process. The amounts of both electrically active and inactive P obtained by the H₃PO₄ spray technique have been compared with the ones obtained by the conventional POCl₃ technique. The former process presents a larger amount of inactive dopant, a finding that is in keeping with the microstructural and microanalytical observations. Instead the amount of active P is similar in the two cases, a result attributed to the precipitation and clustering phenomena of the excess dopant.     

Fill factor losses in ZnO/CdS/CuGaSe2 single-crystal solar cells

Current–voltage characteristics of ZnO/CdS/CuGaSe₂ single-crystal solar cells with solar conversion efficiency values of η=3.5%, 6.0%, 6.7% and 9.7% were analyzed using the single diode equation. The effect of each of the achieved parameters on the fill factor was calculated. The calculations revealed that the fill factor reduction due to the series resistance remained below Δff=4.4% under illumination, while this effect would have been much higher if the illumination had not reduced the series resistance markedly. The calculation furthermore revealed that the fill factor reduction due to the shunt resistance remained below Δff=3.6% under illumination. This effect would have been negligible if the illumination had not also reduced the shunt resistance in all studied cells. The increase of the saturation current density under illumination has brought about considerably high fill factor losses (at least Δff=8.3%) in all studied cells. Already the dark saturation current density and the diode ideality factor in such cells have been found to be much higher than the ones in the cells based on CuInSe₂. This seems to be the most substantial restriction to the fill factor, and thus to the performance, of solar cells based on CuGaSe₂. An explanation for this different behavior seems to lie in the different band structures of these cells.     

Electrodeposition of CuInSe2 layers using a two-electrode system for applications in multi-layer graded bandgap solar cells

In order to develop low-cost, large area multi-layer graded bandgap solar cell structures, CuInSe₂ layers were grown using a simplified two-electrode system. Atomic force microscopy (AFM) shows the layers grown consist of nano- and micro- size particles. Photoelectrochemical (PEC) cell, X-ray photoelectron spectroscopy (XPS) and X-ray fluorescence (XRF) measurements confirm that it is possible to grow CuInSe₂ layers with p-, i- and n-type electrical conduction, as pre-determined for applications in multi-layer device structures. XRF, XPS and PEC measurements show that Cu-richness provides p-type conduction and In-richness provides n-type conduction in electrodeposited CuInSe₂ layers. It is also possible to grow materials with different bandgaps in the range 1.00–1.90 eV. The combination of these two properties allows growth of multi-layer structures and preliminary work on these devices show good rectifying properties and exhibit photovoltaic activity. These new developments will be presented in this paper.     

Fabrication of multilayer TiO2 thin films for dye-sensitized solar cells with high conversion efficiency by electrophoresis deposition

This research coats a commercial TiO₂ nanoparticle Degussa P25 with good roundness and size uniformity on an indium tin oxide (ITO) glass substrate and to be photoelectrical electrode by electrophoresis deposition. It combined with dye N719, electrolyte I^-/ and counter-electrode of Pt layer to produce dye-sensitized solar cells (DSSCs). Through the electrophoretic technique, a multilayer film of an appropriate thickness is deposited in the suspension containing TiO₂ nanoparticles and isopropanol. In this process, electric current, voltage, and the number of deposition cycles are well controlled to obtain a single TiO₂ film of around 3.3 μm thick. Stacking is then performed to obtain a multilayer-typed TiO₂ film of around 12 μm thick. As the sintering temperature reaches 400 °C, the prepared multilayer TiO₂ film with a good compactness can increase the dye adsorption capability of the thin film and enhance its adsorption percentage. In addition, the heat treatment will transfer a portion of the rutile crystalline into the anatase crystalline, resulting in better material properties for DSSCs application. DSSCs produced are exposed to metal halide lamp and their energy conversion efficiency is measured. The I-V curve of the produced DSSCs shows that it has an excellent energy conversion efficiency of 6.9%.     

CuInS2/In2S3 thin film solar cell using spray pyrolysis technique having 9.5% efficiency

Copper indium sulfide (CuInS₂)/In₂S₃ solar cells were fabricated using spray pyrolysis method and high short circuit current density and moderate open circuit voltage were obtained by adjusting the condition of deposition and thickness of both the layers. Consequently, a relatively high efficiency of 9.5% (active area) was obtained without any anti-reflection coating. The cell structure was ITO/CuInS₂/In₂S₃/Ag. We avoided the usual cyanide etching and CdS buffer layer, both toxic, for the fabrication of the cell.     

Fabrication of Cu2ZnSnS4 films by sulfurization of Cu/ZnSn/Cu precursor layers in sulfur atmosphere for solar cells

Cu₂ZnSnS₄ (CZTS) absorbers were grown by sulfurization of Cu/ZnSn/Cu precursors in sulfur atmosphere. The reaction mechanism of CZTS formation from the precursor was analyzed using XRD and Raman spectroscopy. The films with a single phase CZTS were formed at 560 and 580 °C by sulfurization for 30 min. The film grown at 560 °C showed bi-layer morphology with grooved large grains on the top and dense small grains near the bottom of the film. On the other hand, the film grown at 580 °C showed large grains with grooves that are extended from surface top to bottom of the film. The solar cell fabricated with the CZTS film grown at 560 °C showed the best conversion efficiency of 4.59% for 0.44 cm² with V_oc=0.545 V, J_sc=15.44 mA/cm², and FF=54.6. We found that further improvement of the microstructure of CZTS films can increase the efficiency of CZTS solar cells.     

A new approach to high-efficiency solar cells by band gap grading in Cu(In,Ga)Se2 chalcopyrite semiconductors

High efficiencies in Cu(In,Ga)(S,Se)₂ solar cells result from alloying CuInSe₂ base material with the corresponding Ga- or S-containing compound. Compositional grading is one important issue in these devices. To obtain high efficiencies a reconstructed Cu-depleted absorber surface is essential. We consider this Cu/In grading non-intentional, process related and present a model which explains its importance. Another approach to improve performance is controlled intentional band gap grading via Ga/In and S/Se grading during the deposition. We show that appropriate grading can improve current and voltage of the device simultaneously. The key objective is to design a larger band gap for recombination and a lower band gap for absorption to energetically separate the mechanisms of carrier recombination and current generation.     

Modelling the quantum efficiency of cadmium telluride solar cells

A simple analytical model is presented describing the quantum efficiency of cadmium telluride (CdTe) solar cells. This model is based on a consistent set of parameters that were extracted from electrical and optical measurements. These measurements also reveal the CdTe solar cells to mainly rely on carrier generation as well as carrier collection within the space-charge region. Recombination in this part of the cell is hence taken into account. As a result, quantum efficiency spectra can be closely fitted by an expression that includes the lifetime of the minority carriers and the width of the space-charge region as free variables. The comparison of the calculated quantum efficiency curves with the experimental ones gives fundamental insight into the specific operation of CdTe solar cells.     

MS2 (M = W, Mo) photosensitive thin films for solar cells

Photosensitive textured WS₂ and MoS₂ films can be obtained by the techniques of reactive sputtering and solid state reaction, as long as the substrates used are each coated with a 10–20 nm Ni layer. When MS₂ (M = W, Mo) layers are deposited onto these substrates and then annealed for half an hour at 1073k in an argon atmosphere, textured films crystallized in the 2H-MS₂ structure are obtained, with their c crystallite axes perpendicular to the plane of the substrate. The films are nearly stoichiometric. The crystallinity enhancement of the films can be attributed to an improvement in the crystallization process related to liquid NiS phases present at the grain boundaries during annealing. Residual phases (Ni_xS_y; Ni;…) are distributed at the grain boundaries and do not strongly disturb the properties of the WS₂ and MoS₂ films. The optical absorption spectra are similar to those of single crystals, and the high photosensitivity of the films is attributed to a grain size enhancement by the NiS phase.