Band-gap engineering in CuIn(Se,S)2 absorbers for solar cells

Thin films based on CuInSe₂ have become very successful as absorber layers for solar cells. It is only in the recent past that gallium (Ga) and sulfur (S) were incorporated into CuInSe₂ in order to increase the energy band gap of the film to an optimum value with the ultimate aim of producing more efficient devices. This paper focuses on the incorporation of S into partly selenized CuInSe₂ films in order to produce CuIn(Se,S)₂ films with varying S/Se+S ratios, resulting in different band-gap energies. This was achieved by varying the conditions when selenizing Cu/In alloys in H₂Se/Ar, and then exposing these various partly selenized films to H₂S/Ar under identical conditions.     

Exergetic optimization of a solar photovoltaic thermal (PV/T) air collector

In this paper, an exergetic optimization has been developed to determine the optimal performance and design parameters of a solar photovoltaic thermal (PV/T) air collector. A detailed energy and exergy analysis has been carried out to calculate the thermal and electrical parameters, exergy components, and exergy efficiency of a typical PV/T air collector. The thermal and electrical parameters of a PV/T air collector include solar cell temperature, back surface temperature, outlet air temperature, open‐circuit voltage, short‐circuit current, maximum power point voltage, maximum power point current, etc. An improved electrical model has been used to estimate the electrical parameters of a PV/T air collector. Furthermore, a new equation for the exergy efficiency of a PV/T air collector has been derived in terms of design and climatic parameters. A computer simulation program has been also developed to calculate the thermal and electrical parameters of a PV/T air collector. The results of numerical simulation are in good agreement with the experimental measurements noted in the previous literature. Moreover, the simulation results obtained in this paper are more precise than the one given by the previous literature, and the new exergy efficiency obtained in this paper is in good agreement with the one given by the previous literature. Finally, exergetic optimization has been carried out under given climatic, operating, and design parameters. The optimized values of inlet air velocity, duct length, and the maximum exergy efficiency have been found. Parametric studies have been also carried out. Copyright © 2010 John Wiley & Sons, Ltd.     

Efficient and novel Sb2S3 based solar cells with chitosan/poly(ethylene glycol)/electrolyte blend

We present here a novel solar cell made of ITO/composite p‐doped Sb₂S₃ + P3HT + PANI+TiO₂/amorphous Sb₂S₃ + P3HT + PANI + TiO₂/n‐doped Sb₂S₃ + P3HT + PANI + TiO₂/solid carrier/aluminum as counter electrode. With spraying technique, the layers were deposited and the thickness of films was 1 μm. A new solid carrier of electrolyte was a blend consisted of chitosan (low MW), polyethylene glycol and electrolyte. X‐ray diffraction was recorded to confirm the amorphous nature of the blend. Information about the surface appearance and roughness of a solid carrier dry and soaked in the electrolyte was given by atomic force microscopy. The solar cell was examined at very low and low light intensity (5% and 35% of sun, respectively), and at standard test conditions (100% of sun) using different light sources. The whole cell surface was 7.5 cm² while the illuminated part was 3 cm². Obtained results expressed for the illuminated surface showed the highest efficiency of 23.1% at 5% of sun while the efficiency of the cell was 2.9% at 35% of sun and only 0.75% at intensity of 100% of sun.     

Analysis of Cu(In,Ga)(S,Se)2 thin-film solar cells by means of electron microscopy

The present work gives an overview of how electron microscopy and its related techniques are used to analyze individual layers and their interfaces in Cu(In,Ga)(S,Se)₂ thin-film solar cells. Imaging of samples can be performed at scales of down to the (sub)angstroms range. At similar spatial resolutions, information on composition can be gathered by means of energy-dispersive X-ray spectroscopy (EDX) and on spatial distributions of electrostatic Coulomb potentials in the specimen by applying electron holography. Microstructural and compositional properties as well as charge-carrier collection and radiative recombination behavior of the individual layers are accessible by use of electron backscatter diffraction, EDX, electron-beam-induced current (EBIC) and cathodoluminescence measurements, available in scanning electron microscopy. The present contribution gives an overview of the various scanning and transmission electron microscopy techniques applied on Cu(In,Ga)(S,Se)₂ thin-film solar cells, examples from case studies, and also demonstrates how these techniques may be combined in order to improve the analysis. Particularly, EBIC results show a reduced charge-carrier collection at Cu(In,Ga)Se₂ grain boundaries, while no indication was found for a charge accumulation at the grain boundaries by electron holography.     

Improved Cu(In,Ga)(S,Se)2 thin film solar cells by surface sulfurization

Surface sulfurization was developed as a technique for fabricating efficient ZnO : Al/CdS/graded Cu(In,Ga)(S,Se)₂/ Mo/glass solar cells. Prior to the sulfurization, single-graded Cu(In,Ga)Se₂ (CIGS) films were deposited by a multi-stage process. The sulfurization of CIGS films was carried out using a H₂S---Ar mixture at elevated temperatures. The crystallographic and compositional properties of the absorber layers were investigated by XRD, SEM and AES analyses. After sulfurization, sulfur atoms were substituted for selenium atoms at the surface layer of CIGS films to form a Cu(In,Ga)(S,Se)₂ absorber layer. The diffusion of sulfur depends strongly on the grain structure of CIGS film. The cell efficiency of the 8–11% range before sulfurization was improved dramatically to 14.3% with V_oc = 528 mV, J_sc = 39.9 mA/cm² and FF = 0.68 after the sulfurization process.     

Photoelectrochemical studies on [MnMoO2(NCS)(Ox)3(H2O)2] (Ox=8‐quinolinol): a novel system for solar energy conversion

Photoelectrochemical studies were carried out on [MnMoO₂(NCS)(Ox)₃(H₂O)₂] {Ox=8‐quinolinol} complex in aqueous dimethylformamide medium in a Honda cell. The system developed a maximum potential of 335 mV when exposed to visible light at 30°C and was found to be reversible. The photogalvanic behaviour has been further investigated by varying the pH, temperature and photosensitizers. When a temperature difference between the illuminated and dark half‐cells was maintained, the system generated 410 mV at 60°C. A solid‐state galvanic cell, developed using the complex mixed with tetraethylammonium perchlorate (TEAP), showed a maximum voltage of 25 mV. A sandwich galvanic cell, constructed from transparent tin oxide‐glass/complex/platinum, developed a maximum photovoltage of 88 mV when irradiated with a tungsten halogen lamp. Copyright © 2000 John Wiley & Sons, Ltd.     

Study of one-step electrodeposition condition for preparation of Culn(Se, S)2 thin films

Some condition parameters for the one-step electrodeposition to prepare the quaternary CuIn(Se, S)₂ thin films were studied. It was found that filtering out the precipitation in the source solution after several hours from the preparation of the solution was good to obtain a given composition quaternary films. The copper to indium ratio of the films changed drastically depending on the applied potential. It became easy to control the film composition in a relatively wide range of the applied potential by decreasing selenium concentration in the source solution.     

Pore-size effect on photovoltaic performance of dye-sensitized solar cells composed of mesoporous anatase-titania

The effect of the pore size of mesoporous anatase-TiO₂ on the photovoltaic performance of dye-sensitized solar cells (DSSCs) is investigated. The mesoporous TiO₂ particles are synthesized by two different methods using a soft template of tri-block copolymer and a hard template of mesoporous ZnO/Zn(OH)₂-composite. These methods produce the same high surface area (S_BET ∼ 210 m² g⁻¹) but different pore sizes of 6.8 and 3.0 nm, respectively. With the mesoporous TiO₂ having larger pores, the photo-conversion efficiency (η) is increased significantly to 6.71%, compared with 5.62% that is typically achieved using P25 TiO₂ nanopowders. By comparison, only half the performance (3.05%) has been observed with mesoporous TiO₂ that has small pores. Mesoporous TiO₂ with suitable pore sizes (∼6.8 nm) makes the most of its high surface area and thereby allows a high uptake of dye to enhance the current density. In contrast, the low efficiency of mesoporous TiO₂ with small pores is attributed to the low uptake of dye due to the smaller pore size (∼3.0 nm), which blocks the diffusion and adsorption of dye molecules through the pores.     

Near-band-edge photoluminescnce in Cu(In,Ga)Se2 solar cells

Photoluminescence (PL) and PL decay characteristics of the near-band-edge (NBE) PL at room temperature have been studied on the Cu(In,Ga)Se₂ (CIGS) solar cells. The carrier recombination process has been discussed with emphasis on the photovoltaic properties of the solar cell. It has been found that: (i) PL intensity of the CIGS solar cells is much stronger than that in the corresponding CIGS thin films, (ii) the PL decay time of the cell is longer than that of the CIGS film, and (iii) the PL decay time of the CIGS solar cell exhibits strong dependence on the PL excitation intensity. In the CIGS solar cell, intense PL is obtained under the open circuit condition (oc), in contrast to the very low PL yield under the short circuit (sc) condition. The PL decay time under the sc condition is much shorter than that under the oc condition. Excitation intensity dependence of PL intensity and the PL decay time have been studied, and they are discussed with relation to the photo-voltage due to the PL excitation light. PL and injection EL under the external DC bias have been studied. The mapping image of NBE-PL intensity has been compared with that of the laser beam induced current (LBIC), and the PL intensity image reflects the photovoltaic properties of the CIGS solar cells. We demonstrated that NBE-PL of the CIGS solar cell reflects the photovoltaic effect, and it can be utilized as a powerful characterization method.     

A superstrate solar cell based on In2(Se,S)3 and CuIn(Se,S)2 thin films fabricated by electrodeposition combined with annealing

Stacked thin films composed of In₂(Se,S)₃ and CuIn(Se,S)₂ layers were grown on a fluorine-doped tin oxide (FTO)-coated glass substrate using electrodeposition of the corresponding selenide (In₂Se₃ and CuInSe₂) precursors followed by annealing in H₂S flow (5% in Ar). Structural characterizations of both layers revealed that the resulting film quality strongly depended on annealing conditions of both CuIn(Se,S)₂ and In₂(Se,S)₃ layers: a compact and uniform film was obtained by annealing both layers at 400 °C. Performance of Au/CuIn(Se,S)₂/In₂(Se,S)₃/FTO superstrate-type solar cells also followed these structural characteristics, i.e., a preliminary conversion efficiency of 2.9% was obtained on the device based on 400 °C-annealed In₂(Se,S)₃ and CuIn(Se,S)₂ layers.     

Issues on the chalcopyrite/defect-chalcopyrite junction model for high-efficiency Cu(In,Ga)Se2 solar cells

Considering the chalcopyrite/defect-chalcopyrite junction model for Cu(In_1−xGa_x)Se₂-based devices and our previously reported findings for the Cu(In_1−xGa_x)₃Se₅ defect chalcopyrites, we have postulated that uniform high-Ga-content photovoltaic structures (with x > 0.35) do not yield acceptable device performance due to the electrical and structural differences between both types of materials (chalcopyrite and defect-chalcopyrite).In this contribution, the structural properties of the surface region of Ga containing absorber materials have been studied by grazing incidence X-ray diffraction. We find that there are significant differences between surface and bulk. A structural model is proposed for the growth of the chalcopyrite/defect-chalcopyrite junction relative to its Ga content. And we demonstrate that closely lattice matched high-Ga-content structures (x > 0.35) can produce solar cells withv acceptable performances. The high-voltage and low-current electrical outputs from high Ga structures are very desirable in module fabrication because overall resistive losses can be substantially reduced.     

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.     

Cu(In,Ga)Se2 solar cells on stainless steel substrates covered with insulating layers

We have developed the flexible Cu(In,Ga)Se₂ (CIGS) solar cells on the stainless steel substrates with the insulating layer for the fabrication of the integrated module. The CIGS films have strong adhesion to the Mo films with insulating layers. An efficiency of 12.3% was achieved by the flexible CIGS solar cell with a structure of ITO/ZnO/CdS/CIGS/Mo/SiO₂/stainless steel. The insertion of the SiO₂ insulating layer did not have an influence on the formation of the CIGS film and solar cell performances.     

The effect of NaF on Cu(In, Ga)Se2 thin film solar cells

This work investigates NaF, on Mo coated sodium barrier glass, as a sodium precursor for the growth of Cu(In, Ga)Se₂ for thin film solar cells. These precursor layers are investigated by X-ray photoelectron spectroscopy (XPS) before and after annealing, and after exposure to selenium. XPS is also performed on the Cu(In, Ga)Se₂ layer, deposited with NaF. The influence of the NaF on the absorber growth is studied by Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD). The electrical properties are investigated by cell fabrication and characterization. Cell results are comparable when NaF or out-diffusion of sodium from the soda lime glass are used.     

The improved photovoltaic response of commercial monocrystalline Si solar cell under natural and artificial light by using water flow lens (WFL) system

The performances of a photovoltaic system based on high‐efficiency commercial monocrystalline Si solar cell associated with the water flow lens (WFL) system are investigated. This system enables the cooling of the surface of the cell, indirectly cooling the surrounding, and, on the other hand, it allows us to investigate, depending on the position of the cell and the WFL system, the influence of larger and smaller intensities of the light with the inevitable change in the spectrum. All of these effects are very important and can greatly contribute to the better photovoltaic performance of the used cells. Indoor characterization at higher and lower light intensities is performed using both different spectra and intensity of the light. The obtained results show that at low/lower light intensity, spectra are more dominant than the intensity of light itself and that the used WFL system always improves the photovoltaic response leading to a higher efficiency of the tested solar cell. It was found that the ratios of the short circuit current (I_sc) and the input light energy (P_input) are 4.42 and 8.96 without and with the use of the WFL system in the measurements, respectively. The same Si solar cell is also tested in outdoor condition, but this time using the WFL system to concentrate sunlight to produce a larger amount of power and water flow for cooling the surface of the solar cell. Again, a higher efficiency (an increase from 25.7% to 33.5%) by using the WFL system was obtained.     

Near-IR dye-sensitized solar cells using a new type of ruthenium complexes having 2,6-bis(quinolin-2-yl)pyridine derivatives

New ruthenium(II)-polypyridyl complexes 1a (X=H) and 1b (X=Cl) having 2,6-bis(4-carboxyquinolin-2-yl)pyridine derivatives were synthesized as a sensitizer for dye-sensitized solar cells (DSCs), and their photophysical and photochemical properties were characterized. Both of the complexes showed broad electronic absorption bands in the near-IR region, which were assigned to the metal-to-ligand charge transfer (MLCT) transitions. On the other hand, the photovoltaic performance of the DSCs sensitized with them were different from each other. The DSC sensitized with 1a exhibited higher IPCE value than that of the one sensitized with 1b. The substituent effects on the ligand on photovoltaic performance of the DSCs were examined.     

Glass/ITO/In(O,S)/CuIn(S,Se)2 solar cell with conductive polymer window layer

Hybrid solar cells based on the combination of conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) doped with polystyrenesulfonate (PSS) and inorganic semiconductor CuIn(S,Se)₂ (CISSe) were investigated. The CuInSe₂ (CISe) absorber layers were electrodeposited on ITO covered glasses from aqueous solutions with various ratios of elements. The ITO/In(O,S)/CISSe photovoltaic (PV) junctions were prepared by the sulfurization of ITO/CISe precursors at 450 °C in the H₂S atmosphere.The PEDOT–PSS layer of p-type is considered an alternative to the traditional window top layer on the CISSe absorber layer in the cell structure. The polymer deposition was performed by help of the spin-casting technique. PV properties of the prepared ITO/In(O,S)/CISSe and ITO/In(O,S)/CISSe/PEDOT–PSS structures were studied, with emphasis on the role of conductive polymer layer in the cell structure.     

Life cycle cost analysis of single slope hybrid (PV/T) active solar still

This paper presents the life cycle cost analysis of the single slope passive and hybrid photovoltaic (PV/T) active solar stills, based on the annual performance at 0.05 m water depth. Effects of various parameters, namely interest rate, life of the system and the maintenance cost have been taken into account. The comparative cost of distilled water produced from passive solar still (Rs. 0.70/kg) is found to be less than hybrid (PV/T) active solar still (Rs. 1.93/kg) for 30 years life time of the systems. The payback periods of the passive and hybrid (PV/T) active solar still are estimated to be in the range of 1.1–6.2 years and 3.3–23.9 years, respectively, based on selling price of distilled water in the range of Rs. 10/kg to Rs. 2/kg. The energy payback time (EPBT) has been estimated as 2.9 and 4.7 years, respectively.     

Impact of Zn1-xMgxO:Al transparent electrode for buffer-less Cu(In,Ga)Se2 solar cells

Buffer layers such as CdS and ZnS are used in high efficiency Cu(In,Ga)Se₂ (CIGS) thin film solar cells. Eliminating buffer layer is attractive to realize low-cost thin film solar cells by reducing fabrication process. However, the elimination of the buffer layers leads to shunting due to the interface recombination between transparent conductive oxide (TCO) and CIGS layers. To reduce the interface recombination, the control of conduction band offset (CBO) is effective. In this study, we fabricated Zn_1−xMg_xO:Al (ZMO:Al) as the TCO for the CBO control. ZMO:Al was prepared by co-sputtering of ZnO:Al₂O₃ (ZnO:Al) and MgO:Al₂O₃ targets. ZMO:Al shows high transmittance in visible region and the band gap energy widen with the addition of Mg to ZnO:Al. Buffer-less CIGS solar cells with an Al/NiCr/TCO/CIGS/Mo/soda-lime glass structure using ZMO:Al and ZnO:Al were fabricated. For comparison, ZnO/CdS buffered cell was also fabricated. Current density-voltage characteristics of the devices showed the cell with ZMO:Al film achieved higher efficiency compared to the buffer-less cell with ZnO:Al. This result suggested that the control of CBO is important to reduce interface recombination between TCO layer and CIGS absorber.     

Increase in photovoltaic performances of dye-sensitized solar cells—Modification of interface between TiO2 nano-porous layers and F-doped SnO2 layers

In order to improve the physical and chemical contacts between a porous TiO₂ layer and an F-doped SnO₂ transparent conductive layer (FTO), the surface of the FTO layer is polished. After polishing, the surface roughness decreased. However, light transmittance and sheet resistance did not vary largely. The short circuit current (J_sc) and efficiencies increased after the FTO was polished. It was found that the interfacial charge transfer between a TiO₂ layer and an FTO layer decreased by impedance measurement, which suggests that contacts between an FTO and a TiO₂ layer are improved because of the flatted surfaces or removal of electrical impurities. We propose one of the industrially important phenomena that surface polishing of FTO is one of the ways to increase photovoltaic performances for DSCs.