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CdTe semiconductor is an absorbent material used in “tandem” photovoltaic solar cells. This material is commonly deposited by thermal evaporation presenting electrical resistivity values about of 105 W·cm to 109 W·cm. CdTe is applied in thin solar cells as p-type layer which is in contact with metal back electrode in solar cells. In the CdTe/metal junction a Schottky barrier exits; and small number of charge carriers have enough energy to get over the barrier and cross to the metal back contact. To solve part of this problem, nanostructured Te thin films were used as intermediate layers between CdTe and metal contact. Te layers whit different physical properties were deposited on CdS/CdTe structure by thermal evaporation employing different growth parameters. The electrical parameters of CdTe solar cells were influenced by p+ Te regions. p+ Te regions used as intermediate layer with large deposition time increases the FF and VOC values from 30% to 60% and 560 mV to 730 mV respectively. Also, the electrical resistivity is reduced from 106 W·cm to 103 W·cm. In this sense, Te region implemented as nanostructure allows to reduce the series resistance from 99 W to 20 W and increases the shunt resistance from 1445 W to 4424 W; Te region as thin films demonstrated not be adequate.

Recently, advances in the CdTe technology made CdTe a great candidate for commercial thin film solar cells because it is used in infrared windows for optoelectronic devices such as photovoltaic ones. Nowadays, solar cells with nanometric thicknesses are increasingly required due to an enhanced behavior effect. It is common to observe interdiffusion phenomena during several technological processes that are required for the fabrication of photovoltaic devices. It is very important to note that this interdiffusion could strongly affect the final efficiency of devices. In this work, the study of two materials as an interlayer between the CdTe thin film and metallic contact is presented. Te and ZnTe were deposited on the CdTe surface by using a close space sublimation technique. The electrical properties of the devices were evaluated after two and four years to observe the electrical behavior of Te and ZnTe layers implemented as part of the back-contact.

In this work, SnS thin films were deposited by employing the Close Spaced Vapor Transport (CSVT) technique under air atmosphere. Single-phase, p-type SnS thin films were synthesized by varying the final pressure in the chamber and its effect on the properties of SnS were studied. The pressure impact on the directional preferred orientation (DPO) of grains is presented for the first time. The analysis of different pressure values on deposited film properties was performed by X-ray diffraction analysis, Raman spectroscopy, scanning electron microscopy, atomic force microscopy, optical measurements and electric characterization techniques. Keywords: SnS thin films, CSVT, Chamber pressure variations, Physical properties

CdTe semiconductor thin film as absorbent material needs to be recrystallized, in order to improve the electrical and crystalline properties of this material. CdCl 2 thermal treatment was used to reduce the defects between CdS and CdTe thin films that helps to obtain the physical properties require. On the other hand, CdCl 2 treatment leads a special blue color in the CdTe surface. The electrical properties of CdTe are influenced for this coloration and confirm a close correlation between the quality of interfaces and its photovoltaic efficiency on CdTe solar cells. A systematic characterization was made to understand the blue color origin. This study revealed TeO X on the CdTe surface.

CdS polycrystalline thin films have been used as a window layer in CdTe, Cu(In,Ga)Se2 and Cu2ZnSnS4 solar cells; the specific optical and electrical properties of the CdS-partner play an important role in the photovoltaic device performance, so that one of the problems to be solved is to reduce the pin-hole density and intergrain cavities on the CdS surface. SnO2:F/soda-lime glass substrates (FTO) were treated with HCl, Ar and O2. CdS films were deposited on FTO substrates by using the chemical bath deposition technique and then CdS thin films were thermally annealed on CdCl2, Ar atmospheres and muffle at atmospheric ambient. Optical and morphological measurements revealed that the HCl treatment improves the CdS growth because the defect levels were reduced while the grain size is increased. The processed CdS-thin films were applied on CdTe solar cells and the photovoltaic efficiency was about 12% when the CdS thin films were thermally annealed in muffle at atmospheric ambient.

CdS polycrystalline thin films have been used as window layer in solar cells; the quality of the CdS-partner plays an important role in the photovoltaic device performance. CdS thin films were deposited using Chemical Bath Deposition (CBD) and photoassisted-CBD technique. The SnO 2 :F substrates used in this work were treated with HCl (0.1 M) and some of themwere annealed in different atmospheres (Ar and O 2 ). The properties of CdS thin films were influenced by the HCl treatment, position and the movement of substrates inside the reactor glass. CdS samples were deposited in areas of 4 cm 2 , 50 cm 2 and 100 cm 2 ; obtaining thin films with thickness of 35 - 300 nm with good crystalline quality and uniform morphology. All samplespresented optical transmittance around of 85 - 91% and average gap energy of 2.5 eV.

"CdTe thin films solar cells are currently produced using a layer sequence of glass/FTO/CdS/CdTe/metal contact (Cu/Ag), these films are deposited by two different techniques, chemical bath deposition (CBD) and close space vapour transport (CSVT). In order to reach reasonable conversion efficiencies, the device has to be thermally treated in a hydrated-CdCl 2 atmosphere. This study was carried out using X-ray diffraction (XRD), photoluminescence, SEM-EDS, four probe method and SIMS profiling of CdTe. These analyses confirm the presence of hydrated CdCl 2 and CdTe phases on CdTe surface and shown a good recrystallization morphology helping to the carriers mobility along the structure. Using the thermal treatment was possible to reduce the resistivity of CdTe thin film; it is a result to the Cl migration along the CdTe solar cell structure, reducing the defects between CdS and CdTe thin films. A strong CdTe thin film recrystallization was observed by the implementation of a hydrated-CdCl 2 treatment doing to this a good candidate to CdTe solar cells process."

In this Part 2 of this series of articles, the procedure proposed in Part 1, namely a new parameter extraction technique of the shunt resistance (R sh ) and saturation current (I sat ) of a current-voltage (I–V) measurement of a solar cell, within the one-diode model, is applied to CdS-CdTe and CIGS-CdS solar cells. First, the Cheung method is used to obtain the series resistance (R s ) and the ideality factor n. Afterwards, procedures A and B proposed in Part 1 are used to obtain R sh and I sat . The procedure is compared with two other commonly used procedures. Better accuracy on the simulated I–V curves used with the parameters extracted by our method is obtained. Also, the integral percentage errors from the simulated I–V curves using the method proposed in this study are one order of magnitude smaller compared with the integral percentage errors using the other two methods.

AgInSnS 4 thin films were prepared by adding a tin salt to the starting solution used for preparing chalcopyrite AgInS 2 thin films by spray pyrolysis The AgInSnS 4 films were grown at substrate temperatures in the 300–400 °C range, using an alcoholic solution comprised of silver acetate, indium chloride, tin chloride and thiourea. The tin chloride content in the starting solution was gradually varied in terms of the molar ratio x = [Sn]/([S] + [Ag]) from 0 to 0.5 to obtain Sn-doped chalcopyrite AgInS 2 ( x 4 ( x = 0.2–0.4). X-ray diffraction studies indicated that AgInSnS 4 has a cubic spinel-like structure with lattice parameter of 10.77 A. All AgInSnS 4 thin films exhibited p-type conduction, and their room temperature conductivity ranged from 10 − 1 to 10 − 2 S/cm. The conductivity versus 1/T plots for this material showed an Arrhenius-like behavior, from which two activation energies of E a1 = 0.23–0.40 eV and E a2 = 0.07–0.20 eV were determined. These results suggest that the grain boundary scattering and the ionization of shallow acceptors dominate the charge carrier transport in the sprayed AgInSnS 4 thin films. The AgInSnS 4 absorption spectrum revealed an energy gap around E g = 1.89 eV, which was associated to direct-allowed transitions. To our knowledge, the quaternary compound has been prepared for the first time using spray pyrolysis.

article i nfo Available online 8 November 2008 In this work we report the study of the thermal and optical properties of polycrystalline CdS thin films deposited by the gradient recrystallization and growth technique. CdS films were grown on pyrex glass substrates. These studies were carried out using an open photoacoustic cell made out of an electret microphone. From X-ray diffraction, atomic force microscope and photoluminescence measurements we observed polycrystalline CdS films with good morphology and crystalline quality. We obtained a thermal diffusivity coefficient of our samples with values ranging from 3.15 to 3.89×10 −2 cm

Chalcopyrite AgInS 2 :Sn thin films were prepared by spray pyrolysis technique for a constant ratio of [Ag]/[In]=1.5 and different SnCl 4 concentrations (0.05, 0.1 and 0.2 ml) in the spray solution obtaining x =[SnCl 4 ]/[Ag]+[In]=0.01, 0.02, 0.04. All films were deposited at substrate temperature of 375 °C. The deposited film for which x =0.02 exhibited p-type conductivity, having band-gap energies of 1.87 and 2.01 eV. Photoluminescence (PL) studies reveal several PL bands located at 1.45, 1.68, 1.70, 1.80 and ∼1.88 eV at 10 K. Each one of these PL structures are related to different defects, the 1.45 eV emission is related to indium vacancies, 1.80 eV emission to interstitial silver (Ag in ) or Indium in sites of silver (Ag In ), whereas, the other emissions (1.70 and 1.88 eV) are related with a donor–acceptor pair recombination and free to bound transition, respectively, due to sulphur vacancies. Sn in excess modifies the emission bands located at 1.70 and 1.88 eV; we found that Sn reduces sulphur vacancies and PL spectra are dominated by acceptor impurities.

Chalcopyrite (ch) and orthorhombic (o) AgInS2 thin films were prepared by spray pyrolysis using a ratio of [Ag] / [In] = 1.5 and 0.83 respectively. AgInS2 polycrystalline material was annealing in a sulphur atmosphere at 400 °C for 2 h. The estimated optical gap energies were 1.87 and 2.01 eV for ch–AgInS2 and 1.98 eV for o–AgInS2. All the deposited films exhibited n-type conductivity. Photoluminescence (PL) studies reveal in both phases several PL bands. In ch–AgInS2 the PL bands were observed to be centered at 1.45, 1.7 and 1.88 eV at 10 K and an excitation intensity of 10 W cm− 2. The 1.45 eV emission is related with indium vacancies whereas the other emissions (1.70 and 1.88 eV) are related with a donor–acceptor pair recombination and free to bound transition respectively. A new PL band was observed in the annealed sample, this band was centered at 2.02 eV at 10 K and it is related to the transition between a closed level to the conduction band and the splitting valence band (0.15 eV). PL bands in o–AgInS2 samples were observed at 1.45 and 1.524 eV at 10 K and are related with a free to bound transition. Finally o–AgInS2 shows two emission bands located at 1.45 and 1.59 eV, the o–AgInS2 annealed sample in a sulphur atmosphere showed a new PL band located at 2.01 eV at 10 K, this band is related with an energy transition between a level near the conduction band to the splitting valence band (0.063 eV).

AgInSnxS2−x (x = 0–0.2) polycrystalline thin films were prepared by the spray pyrolysis technique. The samples were deposited on glass substrates at temperatures of 375 and 400 °C from alcoholic solutions comprising silver acetate, indium chloride, thiourea and tin chloride. All deposited films crystallized in the chalcopyrite structure of AgInS2. A p-type conductivity was detected in the Sn-doped samples deposited at 375 °C, otherwise they are n-type. The optical properties of AgInSnxS2−x (x

The structural, electrical and optical properties of Na-doped CuInS2 thin films grown by spray pyrolysis were studied. These films crystallized in the sphalerite structure of CuInS2, and showed to contain traces of indium sulfide and CuIn5S8 as impurity phases. All films were In-rich and showed p-type conductivity. The film conductivity was strongly affected by Na-doping, which decreased from 10−2 to 10−5 S/cm by increasing the [Na]/[Cu] ratio from 0.005 to 0.03 in the spray solution. The band gap energy was observed to increase, from 1.4 to 1.45 eV, with increasing the [Na]/[Cu] ratio. Our results suggested that Na could be an effective acceptor impurity in sprayed CuInS2.

AgInS 2 thin films were prepared by spray pyrolysis of an alcoholic solution of silver acetate, indium chloride and thiourea. The samples were grown at substrate temperatures of 375 and 400 °C from a spray solution in which the silver to indium molar ratio ([Ag]/[In]) was varied in the range 0.5–1.625. The AgInS 2 chalcopyrite phase and its orthorhombic modification were found to be predominant ones in the films grown from solutions with [Ag]/[In]=0.87–1.6 and 0.87, respectively. The Ag 2 S, In 2 S 3 and AgIn 5 S 8 secondary phases were found to present in sprayed films. Estimated optical gap energies were 1.88 and 2.03 eV for chalcopyrite AgInS 2 and 1.98 eV for orthorhombic AgInS 2 . All deposited films exhibited n-type conduction and a room temperature resistivity in the range 10 3 –10 6 Ω m.

Chemical bath deposition (CBD) is an inexpensive and low temperature method (25-90 deg. C) that allows to deposit large area semiconductor thin films. However, the extent of the desired heterogeneous reaction upon the substrate surface is limited first by the competing homogeneous reaction, which is responsible for colloidal particles formation in the bulk solution, and second, by the material deposition on the CBD reactor walls. Therefore, the CBD method exhibits low efficiency in terms of profiting the whole amount of starting materials. The present work describes a procedure to deposit ZnO thin films by CBD in an efficient way, since it offers the possibility to minimize both the undesirable homogeneous reaction in the bulk solution and the material deposition on the CBD reactor walls. In a first stage, zinc peroxide (ZnO{sub 2}) crystallizing with cubic structure is obtained. This compound shows a good average transparency (90%) and an optical bandgap of 4.2 eV. After an annealing process, the ZnO{sub 2} suffers a transformation toward polycrystalline ZnO with hexagonal structure and 3.25 eV of optical bandgap. The surface morphology of the films, analyzed by atomic force microscope (AFM), reveals three-dimensional growth features as well as no colloidal particles upon the surface,more » therefore indicating the predominance of the heterogeneous reaction during the growth.« less

The use of a high resistivity transparent oxides (HRT) onto low resistivity transparent oxides (TCO) has been found to improve the performance of CdS/CdTe solar cells. In this work is presented and discussed a study about the use of a commercial conductive glass with SnO2:F as TCO and SnO2 as resistive transparent oxide as front contact in CdS/CdTe solar cells. The SnO2 was deposited by pneumatic spray pyrolysis and its electrical and optical properties were controlled and optimized by varying the substrate temperature and deposition time with the application of a post thermal treatment in O2 atmosphere. The transfer length method (TLM) was implemented to characterize electrically the SnO2:F/SnO2 as well the SnO2/CdS and SnO2:F/SnO2/CdS interfaces through the resistivity and specific contact resistivity measurements and their dependence on O2 treatment were analyzed. The use of the TCO/buffer layer on the CdS/CdTe solar cell in combination with the thermal treatment proposed in this work for TCO and HRT reduced the front contact/CdS contact resistance in a 10 factor and the solar cell efficiency was improved from 6.4% to 12.2%.

Cu(In, Ga)Se2 (CIGS) and Cu2ZnSnS4 (CZTS) semiconductors are direct band gap materials; when these types of material are used in solar cells, they provide efficiencies of 22.1% and 12.6%, respectively. Most traditional fabrication methods involve expensive vacuum processes including co-evaporation and sputtering techniques, where films and doping are conducted separately. On the other hand, the chemical bath deposition (CBD) technique allows an in situ process. Cu-doped CdS thin films working as a buffer layer on solar cells provide good performing devices and they may be deposited by low cost techniques such as chemical methods. In this work, Cu-doped CdS thin films were deposited using the CBD technique on SnO2:F (FTO) substrates. The elemental analysis and mapping reconstruction were conducted by EDXS. Morphological, optical and electrical properties were studied, and they revealed that Cu doping modified the CdS structure, band-gap value and the electrical properties. Cu-doped CdS films show high resistivity compared to the non-doped CdS. The appropriate parameters of Cu-doped CdS films were determined to obtain an adequate window or buffer layer on CIGS and CZTS photovoltaic solar cells.

In this work, a typical superstrate structure of a CdS/CdTe solar cell has been developed, incorporating a Te p+ region in the CdTe surface as a back contact. The Te region was grown as a nanostructured thin film material on the CdTe surface by employing the close space vapor transport (CSVT) technique. The incorporation of a nanostructured Te p+ region to the typical CdS/CdTe structure increased the efficiency from 4.5%–12%. A strong dependence on the electrical characteristics of the CdTe/metal junction on the morphological properties of the Te p+ region was observed, and an appreciable reduction in the Cu diffusion into the solar cell was measured by secondary ion mass spectrometry (SIMS). Thus, we demonstrate that a nanostructured Te p+ region is the best option to improve the performance of the back contact.

We present in this work the processing and characterization of two semiconductor thin films (ZnS and Zn x Cd 1− x O) grown by chemical bath deposition (CBD), and used as heterojunction partners on CSVT-CdTe films. The photovoltaic properties of the Zn x Cd 1− x O/CdTe heterojunction are reported here for the first time. The analysis of the basic properties of the films was carried out by standard optical and electrical characterization techniques. The heterojunction systems were studied by means of I–V characteristics, spectral response and quasi-static C–V measurements. Short-circuit currents of 8.9 and 22 mA/cm 2 , open-circuit voltages of 0.489 V and 0.324 V, fill factors of 0.29 and 0.42 and conversion efficiencies of 1.26 and 3.12% were obtained for SnO 2 /Zn 0.9 Cd 0.1 /CdTe and SnO 2 /ZnS/CdTe, respectively, under normalized 100 mW/cm 2 illumination (AM1).

AgInS2 and AgInS2:Sn semiconductors are currently interesting because of their potential applications like solar cells. Earlier results show that electrical and optical properties of AgInS2 appear to be dominated by donor-like intrinsic defects because it usually displays n-type conductivity. Chalcopyrite AgInS2 and AgInS2:Sn thin films were prepared by spray pyrolysis. It was noted that the Sn-doping produces no appreciable effect on the structural neither the optical properties. The remarkable difference between undoped and some Sn-doped samples lies in the conductivity type. Photoluminescence (PL) studies reveal that Sn reduces sulphur vacancies; electron paramagnetic resonance (EPR) studies in Sn-doped samples, show two superimposed signals close to the g ~ 2 region. One of them was assigned to punctual defects in the lattice due to substitution of Sn instead of In or S sites. This promotes a conductivity increase in this material; the second one shows a hyperfine structure which implies a nuclear spin l = 9/2 assigned to In in the lattice. Both signals were used to determine relative Sn quantities which substituting In or S determining the conductivity type.

Extensive research has been done during the last two decades on cadmium sulfide (CdS) thin films, mainly due to their application to large area electronic devices such as thin film field-effect transistors (Schon et al., 2001) and solar cells (Romeo et al., 2004). For the latter case, chemical bath deposited (CBD) CdS thin films have been used extensively in the processing of CdTe and Cu(In,Ga)Se2 solar cells, because it is a very simple and inexpensive technique to scale up to deposit CdS thin films for mass production processes and because among other n-type semiconductor materials, it has been found that CdS is the most promising heterojunction partner for these well-known polycrystalline photovoltaic materials. Semiconducting n-type CdS thin films have been widely used as a window layer in solar cells; the quality of the CdS-partner plays an important role into the PV device performance. Usually the deposition of the CdS thin films by CBD is carried out using an alkaline aqueous solution (high pH) composed mainly of some sort of Cd compounds (chloride, nitrate, sulfate salts, etc), thiourea as the sulfide source and ammonia as the complexing agent, which helps to prevent the undesirable homogeneous precipitation by forming complexes with Cd ions, slowing down thus the surface reaction on the substrate. CdS films have to fulfill some important criteria to be used for solar cell applications; they have to be adherent to the substrate and free of pinholes or other physical imperfections. Moreover, due to the requirements imposed to the thickness of the CdS films for the solar cells, it seems to be a function of the relative physical perfection of the film. The better structured CdS films and the fewer flaws present, the thinner the film can be, requirement very important for the processing of Cu(In,Ga)Se2 based thin film solar cells, thickness ~ 30 50 nm. In such case, the growth of the thin CdS film is known to occur via ion by ion reaction, resulting thus into the growth of dense and homogeneous films with mixed cubic/hexagonal lattice structure (Shafarman and Stolt, 2003). The reason to choose the CBD method to prepare the CdS layers was due to the fact that CBD forms a very compact film that covers the TCO layer, in the case of the CdTe devices and the Cu(In,Ga)Se2 layer without pinholes. Moreover, the CdS layer in a hetero-junction solar cell must also be highly transparent and form a chemical stable interface with the

In this work, we are reporting the results of the deposition and characterization of Cu(InGa)Se 2 thin films prepared by thermal co-evaporation and the results of the photovoltaic devices made with these Cu(In, Ga)Se 2 thin films, using as the window material partner a CdS layer prepared from two different chemical bath solution recipes. Results have shown that high quality polycrystalline films have been obtained, which have very uniform morphology with grain sizes of tenths of a micrometer and a chemical composition very close to the stoichiometry of the Cu(In, Ga)Se 2 . After processing the Cu(In, Ga)Se 2 thin film samples into solar cells, the highest conversion efficiency achieved was ∼ 11 %

"Polycrystalline Cu(In,Ga)Se2 (CIGS) solar cells are attractive because low cost techniques can be used to obtain high efficiency thin film photovoltaic devices. Several research groups around the world have developed CIGS/CdS solar cells with efficiencies larger than 15% [1] using evaporation, making it an attractive and reliable technique for thin film deposition. Our PVD system is provided with MBE-type Knudsen cells to deposit CIGS thin films on glass/Molibdenum (Mo) substrates. The deposition conditions for each metal source have been established by doing a deposition profile of temperature data vs. growth rate by co-evaporation to obtain CIGS thin film for solar cells. Characterization of the co-evaporated CIGS thin films was performed by X-ray diffraction (X-RD), scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) techniques. Good quality polycrystalline films were obtained as shown by X-RD patterns. SEM micrographs show films having a very uniform appearance with large grain sizes (»1 ¹m). Photoluminescence (PL) studies on CIGS samples with different Ga and Cu concentrations (Ga/Ga+In) = 0.25 and 0.34 and (Cu/In+Ga) = 0.83, 0.88 and 0.94) have been performed. The EDS results have shown that is possible to control very precisely the CIGS thin film composition using these Knudsen cells. Film thicknesses of »3-4 ¹m, were measured with an Ambios profilemeter XP 100 stylus type. A conversion efficiency of 10.9 % has been achieved for solar cells made from the co-evaporated absorbers."