CdTe solar cell degradation studies with the use of CdS as the window material

We present in this work the degradation effects with time in thin film CdTe/CdS solar cells, where the CdS and CdTe layers are deposited by chemical bath deposition (CBD) and close space vapor transport (CSVT), respectively. The CdS thin films were grown from different baths by varying the S/Cd ratio. The variation of the S/Cd ratio allowed us to control the morphology and the density of defects, thus giving rise to better quality CBD CdS films. Depending on the S/Cd ratio an improvement of the morphology and capacitance signal was observed, these factors have also an influence on the open-circuit voltage, short-circuit current density, fill factor and conversion efficiency of the solar cell. The variation with time of these parameters in our devices was tracked during a period of 3 years measured directly on the exposed back contact regions (CdTe/Cu/Au). A discussion on the deterioration of the photovoltaic (PV) performance of the solar cells is presented in correlation with the local environmental conditions. This particular environment has contamination, and represents another type of stress for standard PV operations. These conditions reduce the mean life time of solar cells beyond short periods; this can be of interest for PV community.     

Processing effects on the structure of CdTe, CdS and SnO2 thin films

Thin films of CdTe and CdTe/CdS and SnO₂ used for heterojunction solar cells were deposited on glass substrates. The effects resulting from the processing with thermal heat and CdCl₂ treatments are investigated. The optical properties are determined by photoluminescence (PL) and transmission spectra. The compositional changes within the CdTe film structures are studied by 2 MeV ⁴He⁺ beam using the Rutherford backscattering (RBS) technique. The optical and the RBS data are then correlated to the evolution of high-efficiency solar cells.     

Thermally and light-activated current in ZnO/CdS/CuGaSe2 single crystal solar cells

Current–voltage characteristics of ZnO/CdS/CuGaSe₂ single crystal solar cells which have gone through repetitive annealing treatment were analysed using the two-diode equation. The fitting analysis revealed that current transport in these cells is governed by two competing transport mechanisms relating strongly to interface states and that both mechanisms are thermally and light-activated. The activation energy values for both mechanisms were calculated from the temperature dependence of the parameters describing each of them. Furthermore, it was found that the calculated activation energy values depend on temperature and illumination intensity. A qualitative explanation for this dependence is given.     

Annealing effect of Cu2Te---Au contact to evaporated CdTe film on photovoltaic properties of CdS/CdTe solar cell

The annealing effect of an evaporated Cu₂Te---Au contact to CdTe film on the photovoltaic properties of thin-film CdS/vacuum-evaporated CdTe solar cells has been investigated. V_oc and J_sc for the cells with Cu₂Te---Au contact increased greatly with increasing annealing temperature and showed a maximum value at around 250. Photovoltaic properties of the cell with Cu₂Te---Au contact were improved by annealing in a greater extent than those of the cell with Te---Au or Te---Cu contacts. The cells with Cu₂Te---Au contact exhibit a higher conversion efficiency, comparing with the cells with Te---Au or Te---Cu contacts. The cell with Cu₂Te---Au contact showed a conversion efficiency of 10%. Cu₂Te---Au contact acts as the best pseudoohmic contact on vacuum evaporated CdTe film.     

CdTe contacts for CdTe/CdS solar cells: effect of Cu thickness, surface preparation and recontacting on device performance and stability

Device performance of thin film CdTe/CdS solar cells having different methods for fabricating the primary back contact are presented. Wet and dry methods for forming the primary contact (Cu₂Te) were evaluated with Cu layers from 0 to 15 nm. Extensive analysis of J–V curves is presented, including effects of temperature, intensity and accelerated stress. A procedure for recontacting stress-degraded cells allowed separation of contact and junction degradation modes. The junction recombination is shown to be a Shockley–Read–Hall mechanism. Stress increases the recombination current density J₀ by 2–3 orders of magnitude, resulting in a loss in V_oc of 100–200 mV which is not restored with recontacting. Rollover is eliminated by recontacting the device while fill factor is partially restored with recontacting. For devices with a Cu layer, no significant differences in illuminated solar cell performance between the wet and dry process were observed before or after stress, but there were large differences in the dark J–V related to a blocking contact. To first order, unstressed devices without Cu contact layers behave similar to stressed devices with Cu; lower V_oc, higher resistance, and appearance of a blocking contact.     

Efficient near-infrared downconversion in GdVO4:Dy phosphors for enhancing the photo-response of solar cells

Near-infrared (NIR) downconversion (DC) has been observed in a Dy³⁺-doped GdVO₄ phosphor, where one ultraviolet-blue photon can be split efficiently into two NIR photons. Underlying mechanism for the process of NIR-DC is analyzed in terms of absorption spectrum, static and dynamic photoemission and monitored excitation spectra. Internal quantum efficiency is obtained up to 111% on the basis of experimental and theoretical calculation results. This enables the phosphor promising in significant enhancement of spectral response of silicon solar cells, particularly in the range of 200-500 nm.     

Electronic structure and optical properties of CuGaS2 and CuInS2 solar cell materials

We report energy bands, density of states and optical properties of CuGaS₂ and CuInS₂ chalcopyrites. The electronic structure has been computed using linear combination of atomic orbitals (LCAO) scheme within density functional theory (DFT) and full-potential linearised augmented plane wave method. The energy bands, density of states, components of dielectric tensors and absorption coefficients are compared with the available data. It is seen that the present LCAO-DFT calculations reproduce the electronic properties of both the chalcopyrites in a reasonable way. The optical properties show more absorption of solar radiations for CuGaS₂ chalcopyrite, depicting its more usefulness in the solar cells.     

Analysis of illumination-intensity-dependent j–V characteristics of ZnO/CdS/CuGaSe2 single crystal solar cells

Current–voltage characteristics of ZnO/CdS/CuGaSe₂ single crystal solar cells measured at room temperature are investigated depending on illumination intensity. The characteristics can be described using the two-diode model, indicating two current transport mechanisms acting in the cells. The first and dominant mechanism is recombination of carriers at the interface between CdS and CuGaSe₂. The second one is recombination in the depletion region, which has been found to have a small effect on the solar cell photovoltaic performance. Both the diode ideality factor and the saturation current density of the dominant diode increase under illumination. A model based on interface recombination can explain these results. This model allows the estimation of diffusion voltage, capture cross-section of holes at the interface and mobility of electrons in the CdS layer.     

Examination of blocking current-voltage behaviour through defect chalcopyrite layer in ZnO/CdS/Cu(In,Ga)Se2/Mo solar cell

Blocking current-voltage behaviour of ZnO/CdS/Cu(In,Ga)Se₂/Mo solar cells, which is either temperature- or light-conditioned, is examined using a comprehensive numerical device simulator. Effects of defect states in the defect-chalcopyrite layer and at the CdS/defect-chalcopyrite interface are investigated. Acceptor-like defect states either in a defect-chalcopyrite layer or at the CdS/defect-chalcopyrite interface cause different trapping under red light or white light. This results in different potential profiles throughout the structure, which determine the changeable I−V behaviour under forward bias. Simulation results show that these acceptor-like defect states can also control the temperature-conditioned blocking I−V behaviour.     

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.     

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.     

Fe3O4 nanoparticles induced magnetic field effect on efficiency enhancement of P3HT:PCBM bulk heterojunction polymer solar cells

Fe₃O₄ magnetic nanoparticles (mean size of about 10 nm) capped by surfactant oleic acid (OA) were incorporated into P3HT:PCBM BHJ-PSCs by doping in the P3HT:PCBM photoactive layer for the first time. The PCE of the OA-Fe₃O₄:P3HT:PCBM BHJ-PSC device is enhanced by ∼18% at the optimum OA-Fe₃O₄ NPs doping ratio of 1%. The role of the magnetic property of Fe₃O₄ NPs on the PCE of OA-Fe₃O₄:P3HT:PCBM devices was studied, confirming the exclusive contribution of the Fe₃O₄ NPs to the observed enhancement of PCE. The enhancement of the PCE of the OA-Fe₃O₄:P3HT:PCBM BHJ-PSC device is found to be primarily due to the increase of short-circuit current (J_sc) by ∼14%, which is attributed to the magnetic field effect originated from the superparamagnetism of Fe₃O₄ NPs, resulting in the increase of the population of triplet excitons. Finally, the effect of Fe₃O₄ NPs on the enhancement of PCE of OA-Fe₃O₄:P3HT:PCBM device is further investigated by comparing different means of doping in P3HT:PCBM or PEDOT:PSS layer, confirming that such an effect can be achieved only when Fe₃O₄ NPs are doped in the P3HT:PCBM photoactive layer.     

The synthesis of ZnO/SrTiO3 composite for high-efficiency photocatalytic hydrogen and electricity conversion

Heterogeneous ZnO–SrTiO₃ nanocomposites were synthesized via a facile hydrothermal method. The highest H₂ production rate, 1317.44 μmol g^?1 within 5 h under solar-light irradiation was achieved for the Zn/Sr ratio of 9:1 of the ZnO–SrTiO₃. More interestingly though the ZnO–SrTiO₃ is a semiconducting system, it could be used as an electrolyte in the low temperature solid oxide fuel cell without electronic conducting short circuiting problem. This device displayed an open circuit voltage of 1.14 V and reached the maximum power density of 564 mW cm^?2 at 550 °C. These results are attributed to the enhanced separation of the charge-hole pairs by the heterogeneous structure of ZnO–SrTiO₃ nanocomposites, which possess obvious both heterogeneously ionic and semiconduction. This new discovery indicates a good promising candidate for both solar and hydrogen energy conversions.     

Chemical stability of sputtered Mo/Sb2Te3 and Ni/Sb2Te3 layers in view of stable back contacts for CdTe/CdS thin film solar cells

Sb–Te phases sputtered from an Sb₂Te₃/Sb/Te target in a substrate temperature range from 293 to 523 K are characterised by X-ray diffraction (XRD) and Hall measurements. A simple thermodynamic model is introduced for estimating the chemical stability of the Ni/Sb₂Te₃ and the Mo/Sb₂Te₃ interface. These data and the results of kinetic test reactions for sputtered Ni/Sb₂Te₃, Ni/Sb–Te, Mo/Sb₂Te₃ and Mo/Sb–Te layers are compared using XRD measurements. Metal/Sb₂Te₃ thin film double-layer systems are used as a model for an innovative back contact for CdTe/CdS thin film solar cells offering an improved long-term stability.     

Photovoltaic effects of a:C/C60/Si (p–i–n) solar cell structures

An attempt to improve the efficiency of the heterojunction p-a:C/n-Si has been made by introducing the highly insulating C₆₀ layer between the semiconductor layers of the cell structure. The conductivity of the implanted films is found to increase during the implantation process and it is attributed to the complete disintegration of the fullerene molecules. The efficiency of this structure is found to be 0.1% under AM 1.5 conditions which is ten times higher than the cell fabricated using the boron ion implanted fullerene without the insulating layer.     

Photocatalytic degradation of organic dyes with Er:YAlO3/ZnO composite under solar light

The Er³⁺:YAlO₃/ZnO composite, a new photocatalyst that could effectively utilize visible light, was prepared by ultrasonic dispersion and liquids boiling method in this work. In succession, the Er³⁺:YAlO₃/ZnO composite, Er³⁺:YAlO₃ particle and pure ZnO powder were characterized by X-ray diffraction (XRD). The Acid Red B dye as a model compound was degraded under solar light irradiation to evaluate the photocatalytic activity of the Er³⁺:YAlO₃/ZnO composite. In addition, the effects of Er³⁺:YAlO₃ content, heat-treated temperature and heat-treated time on photocatalytic activity of Er³⁺:YAlO₃/ZnO composite were reviewed through the degradation of Acid Red B dye under solar light. Otherwise, the effects of initial concentration, Er³⁺:YAlO₃/ZnO amount, solution acidity and solar light irradiation time on the photocatalytic degradation of Acid Red B dye were investigated in detail. It was found that the photocatalytic activity of Er³⁺:YAlO₃/ZnO composite is much higher than that of pure ZnO powder for the similar system. Perhaps, the use of this Er³⁺:YAlO₃/ZnO composite may provide a new way to take advantage of ZnO in sewage treatment aspects using solar energy.     

Influence of growth and microstructure of chemical deposited CdS films on the electrical properties of CdS/CdTe solar cells

Thin film n-CdS/p-CdTe solar cells were prepared from chemical bath deposited CdS and electrodeposited CdTe layers. The microstructural and some electrical properties of these layers were studied and connection with photovoltaic performance of the cells was shown. Especially, adherence of the CdS films and the quality of heterojunction interface manifesting themselves in the value of the open-circuit voltage U_oc depend on the cadmium precursor used for CdS deposition and on whether pH buffered conditions were applied or not. The number of CdS layers in the cells needed to obtain U_oc of about 700 mV is connected with the CdS deposition conditions.     

Influence of 1-methyl-3-propylimidazolium iodide on I3/I redox behavior and photovoltaic performance of dye-sensitized solar cells

In this paper, we investigated redox behavior of I⁻ and I₃⁻ in 3-methoxypropionitrile (MePN) with different concentrations of 1-methyl-3-propylimidazolium iodide (MPII) and iodine by cyclic voltammetry and electrochemical impedance spectroscopy. It was found that the apparent diffusion coefficient (D) values of triiodide and iodide ions, the serial resistance (R_s) and the charge-transfer resistance (R_ct) decreased slightly with increase of the concentration of I₃⁻ in MePN containing 1.4 M MPII. Moreover, the R_ct and D values of triiodide and iodide ions affection on dye-sensitized solar cells (DSCs) should be considered as a whole. The DSCs with the electrolyte (1.4 M MPII, 0.1 M LiI, 0.1 M I₂, 0.5 M TBP, in MePN) gave short circuit photocurrent density (J_sc) of 14.44 mA/cm², open circuit voltage (V_oc) of 0.72 V, and fill factor (FF) of 0.69, corresponding to the photoelectric conversion efficiency (η) of 7.17% under one Sun (AM1.5).     

Influence of Alq3/Au cathode on stability and efficiency of a layered organic solar cell in air

A relatively long lifetime organic solar cell, containing a thin tris-8-hydroxy-quinolato aluminum (Alq₃) layer under an Au cathode, is described. Half-lifetime of the cell in the darkness is over 7 weeks and 30% of the initial power conversion efficiency (η) is obtained after 18 weeks. This represents a substantial increase in lifetime compared to that of the unencapsulated Al-cathode cells. Efficiency is enhanced by 60 times compared to a cell that does not contain Alq₃, being 0.60%. The proposed role of Alq₃/Au cathode in this cell is discussed in detail.     

High performance rare earth oxides LnOx (Ln = La, Ce, Nd, Sm and Dy)-modified Pt/SiO2 catalysts for CO oxidation in the presence of H2

The catalytic activities of low-temperature CO oxidation and preferential CO oxidation (PROX) in rich-H₂ stream over the Pt/SiO₂ and rare earth oxides modified Pt/SiO₂ catalysts have been investigated. The addition of rare earth oxide additives to the Pt/SiO₂ catalyst can promote the catalytic performance remarkably. The catalytic activities for PROX are dependent on the basicities of these oxides. The higher basic strength leads to higher activity. For Pt–CeO₂/SiO₂, the catalytic activity depends strongly on the content of CeO₂ and on the mole ratio of CO/O₂. XRD and H₂-temperature-programmed reduction (TPR) results show that the additives not only improve the dispersion of metallic Pt on the support surface, but also increase the reducibility of PtOx in Pt–CeO₂/SiO₂. The formation of the Pt–Ce alloy decreases the electron back-donation from the Pt 5d orbital to the 2σ* orbital of CO, and consequently suppresses CO–Pt bonding, resulting in the lowered CO coverage.