In this article structural properties as well as morphological aspects of CuIn(S,Se)2 thin film solar cell absorbers, produced by annealing of electroplated precursors, are discussed. Real-time X-ray diffraction (XRD) experiments while precursor annealing have shown, that a reduced amount of electrodeposited selenium is the key parameter to realize a chalcopyrite formation mechanism similar to the one known for sputtered stacked elemental layer (SEL) precursors. Absorber layers processed from precursors produced by simultaneous electrodeposition of copper, indium and selenium show a preferable absorber morphology with an average grain size on the micrometer scale when the electrochemically deposited amount of selenium is reduced to [Se] / [In] = 0.1. The amount of selenium, missing for the formation of a stoichiometric chalcopyrite, was deposited in a second process step prior to precursor annealing. Solar cells produced from these absorbers show light conversion efficiencies up to 10%.

This work investigates the mechanical properties of ITO on PEN substrates as a function of processing conditions, including rf power, substrate temperature, and substrate treatment. The best mechanical performance is obtained from high substrate temperature and low rf power. Plasma treatment gases also influence mechanical properties, with mixture of nitrogen and hydrogen gases producing the best results. This work provides an initial understanding of the impact of sputter process conditions on film's mechanical performance.

ZnO thin films have been prepared by thermal oxidation of metal zinc films with different porosity. High and low porosity metal zinc films are prepared by DC magnetron sputtering and capillaritron ion beam sputtering, respectively. Near bandgap UV emission of ZnO thin films prepared from thermal oxidation of the low porosity film exhibit a maximum PL intensity after thermal oxidation at 410°C, while ZnO films prepared by thermal oxidation of the high porosity films exhibit a maximum PL intensity at oxidation temperature of 900°C. SEM micrographs indicate that ZnO prepared by thermal oxidation of low porosity films have a smooth surface morphology after thermal oxidation at 410°C, while ZnO prepared by thermal oxidation of high porosity zinc films exhibit a grain size of ∼ 800 nm after thermal oxidation at 1000°C.

We present numerical calculations of the optical absorption characteristics of graded Indium Gallium Nitride Schottky Barriers, and study their implications for photovoltaic and photodetector applications. We consider the two cases of digital and continuous grading because of their different effects on the collection of photo-generated carriers due to band discontinuities. Composition grading can achieve desired spectral response between the ranges of 0.7 eV and 3.43 eV afforded by the Indium Gallium Nitride alloy system. The presence of spontaneous and piezoelectric polarizations in this material system adds bulk and/or interface bound charges in graded layers. This has a non-trivial effect on the band profile seen by the photo-generated carriers. The layer thicknesses needed for optimal absorption characteristics are well above the theoretical critical thickness limits reported in the literature for abrupt heterojunctions. However, experimental data about critical thicknesses is scarce, especially for graded compositions. Therefore, we calculate the characteristics of the Schottky barrier for the case of spontaneous polarization only and also for the case of both spontaneous and piezoelectric polarization assuming no relaxation. The low or even negative Schottky barrier heights at low Gallium composition necessitates the use of high Gallium composition layers next to the metal, in order to suppress the excessive dark currents

ZnMgO was fabricated by MOCVD and the films were applied to a buffer layer of Cu(InGa)Se2 solar cells.

A nove growth method for Cu(InGa)Se2 was described in which cracked Se was utilized.

We have theoretically evaluated the phase stability and electronic structure of a Zn1-xMgxO solid solution. The enthalpies of formation for both the wurtzite and rocksalt phases of Zn1-xMgxO were calculated using a plane-wave pseudopotential method within the density functional formalism. For 0 < x < 0.5, the calculated enthalpies of formation (&[Delta]H) for the wurtzite phases were lower than those for the rocksalt phases. On the other hand, for x > 0.75, the &[Delta]H values for the wurtzite phase were larger than those for the rocksalt phases. This indicates that the wurtzite phase is more stable for a Zn1-xMgxO solid solution with 0 < x < 0.5, while the rocksalt phase is more stable for a solid solution with x > 0.75. The band gaps of a wurtzite Zn1-xMgxO solid solution increase with increasing Mg content. MgO substitution on Zn1-xMgxO largely affects the conduction band leaving the valence band nearly unchanged. The conduction band minimum (CBM) shifted to higher energy with increasing Mg content. These theoretical results qualitatively agree with the experimental results for Zn1-xMgxO thin films fabricated by RF magnetron co-sputtering.

A set of 24 identically made CdS/CdTe devices were subjected to accelerated lifetime testing (ALT) under open-circuit bias, 1 sun illumination, and temperatures of 60, 80, 100, and 120 °C. A total of 6 identical devices were tested for statistical purposes at each temperature. Current density-voltage (JV) measurements were made on stressed cells for up to 2000+ hours. Device performance parameters were calculated as a function of temperature and stress time using discrete element circuit models. Forward current behavior was represented by two parallel diodes to simulate recombination currents in the quasi-neutral (QNR) and space-charge (SCR) regions. Backcontact behavior was studied using a parallel combination blocking diode and shunt conductance. A systematic pattern of degradation was apparent with increased stress temperature. At 60 °C, degradation associated with the CdTe/backcontact dominates. At temperatures above 80 °C, greater losses in fill factor (FF) and open-circuit voltage (Voc) were observed. Recombination current modeling of JV data attributes this to increased space-charge recombination. Calculated diffusion lengths based upon an Arrhenius-derived activation energy of 0.63 eV in this temperature-range suggests Cu diffusion into the SCR is mechanistically responsible for the observed increased recombination, and decreased Voc and FF. At lower temperatures (60 to 80 ºC), degradation was considerably slower with a measured activation energy of 2.9 eV.

Appropriate interpretation of quantum-efficiency (QE) measurements made on non-ideal solar cells often reveal subtle features of the photodiodes. QE measurements on CIGS and CdTe thin-film solar cells with CdS buffer layers demonstrate some of these features, and in each case we identify the electrical processes responsible. One well-known complication in cells with CdS buffer layers is “photodoping”, where blue light exposure increases the n-type CdS carrier density. The resulting modification of the p-n junction alters the carrier collection for all wavelengths and can lead to misinterpretation of results. The use of a white, DC-bias light during QE measurements generally solves this problem, but it has not been clear what bias light level is sufficient for accurate results. Measurements with varying intensities of DC-bias light show that complications due to CdS photoconductivity are generally mitigated near 0.05 sun intensity. Other factors which can cause misinterpretation of QE measurements include the presence of secondary barriers, and photoconductivity in the absorber layer. Numerical simulations of band profiles under various light-bias conditions are consistent with the experimental data. The analysis is used to suggest a standard set of measurement conditions for reliable QE analysis.

Photocurrent spectral distribution and relaxation in CdS/CdTe heterojunctions Sergiu Vatavu, Petru Gasin, Iuliana Caraman. Thin film CdS/CdTe solar cells have proved their importance in use as solar energy converters achieving 16.5 % efficiency. Thin film CdS/CdTe heterojunctions have been deposited by close spaced sublimation method (CSS), onto SnO2 covered glass plates (2õ2 cm2). The thickness of CdS and CdTe thin films was 0.3-1.6 m and 2.3-6.6 m respectively. For to enhance CdS/CdTe heterojunctions photoelectrical parameters and photosensitivity, the deposition procedure was followed by annealing in presence of CdCl2 at 420C for 15-60 min. Ni has been used as back contact to CdTe. After annealing, the photoelectrical parameters at 100 mW/ñm2 are: Uoc=0.73 V, Isc=21.7 mA/cm2, ff=0.43.

The analysis of the photosensitivity for samples with different thicknesses of the component layers in the 78-293K temperature range showed, that the increase of the photoconductivity for photon energies ħw>1.9 eV is determined by the generation-recombination processes in CdSxTe1-x interface layer, and in the 1.42 eV-1.85 eV region, by light absorption mechanism in CdTe.

At direct biases of the CdS/CdTe heterojunctions (2Uoc), the photocurrent spectral distribution is determined by the generation-recombination processes in CdS layer. The annealing of the heterojunction in presence of CdCl2 results in photocurrent increase more than one order of magnitude. CdS/CdTe at reverse biases, having a constant sensitivity in visible and near infrared spectral region, can be used in photometry as radiation detector.

The analysis of the absorption spectra and the photocurrent spectral distribution at direct biases resulted in determination of the holes mean free path in CdTe layer, being equal to 1.05 mm.

The ratio of the ambipolar diffusion coefficient and surface recombination velocity for CdS/CdTe interface is 0.3 m and the diffusion length is 0.6 m at 78K. The same parameters for Ni/CdTe interface are: 0.025 m and 0.05 m respectively.

The phococurrent relaxation curves have been studied for illumination throught both sides of the heterojunctions at different biases, temperatures and wavelength, determining different penetration depths and materials excitation. Nonequilibrium charge carriers lifetime has been determined. The thermal annealing of the CdS/CdTe heterojunctions in presence of CdCl2 determine the nonequilibrium charghe carrier lifetime increase by more than one order of magnitude from 22 s up to 280 s.

For industrial CdTe solar cell production, thin, dense and pinhole-free films have to be deposited at high rates. Using a close-to-industrial close space sublimation (CSS) process, we deposited CdTe films at different temperatures and rates. The morphology of these films was analyzed by scanning electron microscopy (SEM) and x-ray diffraction (XRD). The measurements show the size, size distribution and orientation of the grains to strongly depend on deposition temperature and rate. A suitable combination of these parameters yields large grains of about 3 microns diameter even for deposition temperatures below 750 K and at high deposition rates of 3 microns per minute. The interplay of CdTe layer morphology and solar cell performance is elucidated employing a TCO-CdS-CdTe-Mo film sequence for complete solar cells.

Self-assembled monolayers of fluorinated thiols have been used as a means of surface conditioning to modify the work function of polycrystalline, wide-gap, chalcopyrite thin films. The molecular dipole, characteristic of such polar molecules, could be transferred to the surface of the semiconductor. Self-arrangement and orientation of the molecules upon adsorption ensured a net dipole contribution that was observed by means of ultra-violet photoemission spectroscopy. Such an approach offers a simple way for interface engineering, with a potential impact on the design of compound-specific band alignments of chalcopyrite-based devices. Molecular mechanics calculations of the expected molecular geometries complemented this work.

We have measured the electrical, optical and morphological properties of as-grown Hg1-xCdxTe films prepared by r.f. sputtering. The Hg1-xCdxTe films were grown at substrate temperatures ranging from 25°C to 150°C. Films grown at temperatures lower than or equal to 70°C were highly resistive (≥105 Φ-cm) and were not measurable by our Hall apparatus. Optical transmission data show that the band gap of these films ranged from 0.8 eV-1.5 eV, satisfying the optimum band gap criteria. Plan view and cross sectional SEM studies indicate that the films grown at 85°C and 100°C have larger grains with compact grain boundaries and these films typically yield the best cell performance. Spectroscopic ellipsometry studies are being used to estimate the band-gap as a function of substrate temperature.

We have also studied the effect of various back contacts and have fabricated complete solar cells. We found that Cu-Au serves as the best ohmic back contact to CdS/HgCdTe solar cell. Our preliminary results of J-V analyses on the complete solar cell show that the efficiency is mainly limited by the short-circuit current. Electrical-bias-dependent QE measurements indicate voltage-dependent current collection mainly in the long wavelength region. Further optimization of growth parameters and CdCl2 treatment needs to be carried out to improve the cell performance.

Metastabilities in the electrical characteristics of CIGS devices are commonly observed phenomena originating from persistent changes of shallow and deep levels distributions within the absorber. We examine characteristic changes induced by voltage bias and light together with their relaxation behavior and interpret them as the consequences of a negative-U type of centers predicted by theoretical calculations of Lany and Zunger. It is shown how the properties of these centers justify a model of p+ layer explaining specific features of light and dark current-voltage characteristics. The discussion showing the impact of various charge distributions on carrier transport is presented. The arguments are provided, that centers responsible for metastable effects are also to blame for majority of photovoltaic losses exhibited in various devices.

Due to its basic physical and chemical properties CdTe has become a favored base material for thin film solar cells using robust, high-throughput manu-facturing procedures. The technology shows significant potential for attaining cost levels of <1 $/Wp. Close-Spaced Sublimation (CSS) is the fastest and simplest deposition process for both semiconductors used, CdTe and CdS, permitting in-line production at high linear speed of about 1 m/min.. The individual manufacturing steps for integrated modules are explained in view of their incorporation in the production line. ANTEC Solar GmbH has entered production of CdTe thin film modules on a scale of 100,000 m2 per annum using CSS as the deposition procedure for the semiconductor films and high-rate in-line sputtering for transparent and opaque contacts. Standard module size is 60 × 120 cm2. The production line is operative since 2002, without major problems. This article recounts the efforts to master lab processes for application in production.

Cu(In,Ga)Se2 (CIGS) photovoltaic cells require a highly conducting and transparent top electrode for optimum device performance. ZnO thin films doped with 2 wt.% Al (ZnO:Al) are commonly used to ensure sufficient conductivity while providing acceptable transparency to the active absorber layers. Deposition of transparent conducting oxide (TCO) films on CIGS cells often is performed at room temperature in the manufacturing process because of production advantages. However, material properties and reproducibility may be poorer at room temperature than at higher temperatures. Maximum mobilities of 2 wt.%-doped ZnO:Al grown at room temperature in pure Ar are typically ∼20-25 cm2V-1s-1. Relatively high carrier concentration is therefore required to achieve the desired conductivity. This high carrier concentration produces low infrared transmittance due to increased free-carrier absorption.

World-record CIGS cells produced at the National Renewable Energy Laboratory (NREL) are known to tolerate photolithographic processing temperatures of ∼100°C, though significant changes in device performance occur beyond 200°C. In this study, we investigate whether ZnO:Al films with superior material properties can be produced at the elevated temperatures consistent with CIGS processing parameters. We examine undoped ZnO and ZnO:Al with doping levels of 0.5, 1, and 2 wt.% Al2O3 for growth at substrate temperatures from 25° to 360°C using radio-frequency magnetron sputtering. For films grown in a 100% Ar ambient, optimal electrical and optical properties are achieved at ∼150°-200°C. Controlled incorporation of H2 in the Ar sputtering ambient at 200°C increases mobility to 48 cm2V-1s-1 for undoped ZnO and 36 cm2V-1s-1 for 0.5 wt.%-doped ZnO:Al. Both values are considerably higher than the 25 cm2V-1s-1 of 2 wt.%-doped ZnO:Al deposited at 200°C in 100% Ar. We have explored whether these higher-mobility films can be exploited in the design of high-quality CIGS devices produced at NREL. Preliminary results show similar open-circuit voltage and only slightly lower short-circuit current compared to devices utilizing the standard 2 wt.%-doped ZnO:Al deposited at room temperature. This suggests that higher-performance devices may result once the TCO thickness is optimized for the higher mobility.

We have grown device-grade Cu(In,Ga)Se2 (CIGS) thin films using a RF-cracked Se-radical beam source. A unique combination of film properties: smooth surface, large grain size and high photovoltaic performance are shown. A competitive energy conversion efficiency of 17 % has been demonstrated from a solar cell fabricated using a CIGS absorber grown with a Se-radical source. In addition to the unique combination of film properties and high photovoltaic performance, a significant improvement in the use of Se source material in comparison with the conventional Se-evaporative sources has been demonstrated.

Non-equilibrium ion implantation of Ge in p-type polycrystalline thin film CuGaSe2 (CGSe) prepared by Chemical Close-spaced Vapor Transport (CCSVT) has been performed with the goal to achieve n-type doping of this chalcopyrite semiconductor. Using Electron Spin Resonance (ESR) it is shown that Ge implantation induces a paramagnetic specie at g = 2.003. A model is proposed that assigns the ESR signal to electrons trapped by donor states that are electrically inactive. Moreover, UV photoluminescence of Ge implanted films has evidenced a new peak emission at 1.47 eV, which is resolved as a radiative recombination of a hole bound to the native copper vacancy and an electron bound to a deep donor with an ionization energy of ED =360±10 meV.

We have investigated the formation of Cu(In,Ga)Se2 thin films by real-time X-ray diffraction (XRD) experiments while annealing differently deposited and composed stacked elemental layer (SEL) precursors.

The in-situ measurements during the selenization of bi-layered Cu/In precursors reveal, that the semiconductor formation process is similar for precursors with thermally evaporated or sputtered indium. In both cases, the formation of binary copper and indium selenides is observed at temperatures around the melting point of selenium. After subsequent selenium transfer reactions, the chalcopyrite CuInSe2 is formed from the educt phases Cu2-xSe and InSe.

The addition of gallium leads to the formation of the intermetallic precursor phase Cu9Ga4, which reduces the overall amount of copper and gallium selenides at process temperatures above 500 K. This causes an ongoing selenization in the indium selenium subsystem, which results in the formation of CuInSe2 from the educt phases Cu2-xSe and the selenium richest indium selenide g-In2Se3.

The dependence of deposition rate of CdTe film on deposition pressure and RF power has been studied. The optimum argon pressure was found to be 5 mTorr while the growth rate as a function of power is slightly superlinear. A deposition rate of 140 nm/min has been achieved which is five times faster than for our conventional deposition conditions. Films were characterized by means of X-ray diffraction (XRD) and Atomic Force Microscopy (AFM). Pressure has been found to have a greater effect on film morphology than magnetron sputter power. Working devices have been made with 2-micron layers of CdTe deposited at different powers and 10 mTorr of argon pressure. The effect of different temperatures of CdCl2 post-deposition treatment on cell performance was studied. Efficiency of 11.8% with a 2-micron CdTe layer deposited at 133 nm/min was achieved.

To validate p-type semiconducting behavior in Ag-doped ZnSe, single-crystal films were grown on GaAs(100) substrates using an evaporation method with ZnSe and Ag2Se powder sources. The heteroepitaxial relationship between ZnSe(100) and GaAs(100) was observed using X-ray diffraction and transmission electron microscopy; secondary phases containing silver or silver selenide were not detected. A film doped with Ag at 1 1020 atm·cm-3 had a conductivity of 1.5 x 10-5 S·cm-1. The hot-probe test indicated p-type polarity, with a clear and reproducible rectifying characteristic demonstrated by forming a ZnSe:Ag/p-GaAs:Zn junction. The work function of a ZnSe:Ag film measured by ultraviolet photoelectron spectroscopy was 6.3 eV. The ZnSe:Ag film is suitable as an injection layer in widegap semiconductor devices and organic light-emitting diodes.