We report on capacitance spectroscopy measurements on Cu(In,Ga)Se2 based solar cells where the gallium content was varied systematically. In all samples we found, depending on the measurement conditions, a minority carrier and/or a majority carrier signal. The minority trap can be attributed to the prominent donor-like state N1 or â already discussed in literature. However, the origin of the majority trap remains unclear.

The transparent conducting CuInO2 thin films were prepared by radio frequency (RF) reactive sputtering and post growth annealing. A study of structural, optical, and electrical properties was performed on the films. The crystalline phase in the films was identified to be the delafossite structure. The optical properties, such as the wavelength dependence of the transmittance and the band gap energy, were determined. The average transmittance is 70% in the wavelength range of 400-1100 nm and the band gap is ˜3.7 eV. The temperaturedependence of electrical conductivity in the CuInO2 delafossite thin films was measured from 70 to 400K. The resistivity, carrier density, and mobility of the thin films at 300K were 1.8x101 Δcm, 1.6x1019 cm-3 and 2x10-1 cm2/Vs, respectively. Hall coefficient indicated that the CuInO2 thin films are n-type conductors. The electrical conductivity showed semiconducting type at room temperature.

A simple statistical model describes measured absorption and photoluminescence data of Cu(In1-x, Gax)Se2 thin films. The broadening of the transition peak in the absorption spectra stems from band gap fluctuations. The extent of the spatial inhomogeneities as expressed in the standard deviation Eg μ reaches a maximum of Eg μ = 90 meV for films with equal amounts of indium and gallium, indicating alloy disorder as one possible source of the band gap fluctuations. The fluctuations observed lead to a decrease δVOC of the maximum possible open-circuit voltage VOC of almost 150 mV. However, the experimentally measured, low VOC of solar cells with high gallium content cannot be explained by band gap fluctuations alone. Consequently, our analysis suggests that the dominant recombination process in Cu(In1-x, Gax)Se2 thin film solar cells with high gallium content is not governed by the band gap energy, but is more likely due to deep levels within the forbidden gap.

AgGa5Se8 and Ag(In1-xGax)Se2 thin films with different Ag/Ga atomic ratios have been deposited on the corning 1737 glass substrates by molecular beam epitaxy (MBE) system. This crystallographic property of AgGa5Se8 thin films has been investigated by x-ray diffraction and rietveld analysis. These films had the tetragonal structure with the space group of P-42m, regardless of Ag/Ga atomic ratio. The lattice parameters and the optical band gap energy decreased with an increase in the Ag/Ga atomic ratio. Thus, the structural and optical properties of these AgGa5Se8 thin films were controlled by the Ag/Ga atomic ratio.

CdTe thin film solar cells still suffer from problems related to back contacts which provide stable ohmic contacts without electrical losses. In previous studies metal/CdTe contacts have shown significant limitations. A promising option for the realization of ohmic back contacts is the use of a highly p-doped ZnTe interlayer on top of the CdTe absorber, in particular since metal/ZnTe contacts have shown very good electrical properties. In this work we studied the electronic und chemical properties of nitrogen doped p-ZnTe films. p-ZnTe:N films were prepared by using reactive RF magnetron sputtering with N2/Ar gas mixtures or by thermal evaporation with an additional nitrogen plasma source. Samples and their contacts have been prepared in DAISY-SOL (DArmstadt Integrated SYstem for SOLar energy research) which combines a full vacuum production with an in-situ photoelectron spectroscopy (XPS/UPS) analysis. The results of XPS/UPS investigations and electrical measurements will be discussed in comparison to previous results on metal/CdTe contacts. As ZnTe forms an interlayer in the CdTe thin film solar cell, the ZnTe/CdTe interface properties must be also taken into consideration. Our experiments have shown in agreement to previous studies that the valence band offset is nearly ideal for hole transport across the interface. Also electrical measurements have been carried out to investigate the metal/ZnTe/CdTe layer sequences in their contact properties.

CuInS2 films were prepared on Si(111) by MOMBE using standard MBE Cu and In sources and Ditertbutyldisulfide (TBDS) as an organic sulfur precursor. The films were analyzed in situ by XPS, LEED and UPS. Deposition at 250°C yields chalcopyrite films [1] with admixtures of CuSi2 and CuIn alloys. Deposition at higher temperatures up to 550°C was used to clarify the feasibility of the process. High quality LEED diffraction patterns show epitaxial growth but CuSi2 precipitations are still observed for deposition on Si(111). A buffer layer of indiumsulfide showed no considerable effect on the interface morphology. The obtained LEED pattern are in accordance with the epitaxial relation Si{111}||CuInS2{112}. Even for temperatures as high as 550°C no incorporation of Carbon or residual hydrocarbons in the film or adsorbed at the surface were detected.

We have equipped our transmission electron microscope (accelerating voltage up to 300 kV) with a cathodoluminescence (CL) system that covers a wavelength range of 180 — 1800 nm and temperatures from 10 K upwards. This contribution shows how this system can be utilized to study the initial damage process due to electron irradiation in Cu(In,Ga)Se2 thin solar films. This damage leads essentially to atomic defects that cannot structurally be imaged in the transmission electron microscope, but influence the luminescence spectra. We analyse in-situ the spectral evolutions with electron dose of Cu(In1-xGax)Se2 with [Ga]/([Ga]+[In]) ratio x ranging from x=0 to x=1 and interpret the defect formation kinetics with a first model. The obtained results indicate that the films with equal Ga and In concentration are the least radiation sensitive. The voltage dependence of the damage rate indicates that the damage arises essentially due to displacement by electron knock-on (in the voltage range 150 — 300 kV).

We investigated radiation-induced defects in CIGS solar cells with a solar-cell simulator to analyze the spectral response of the irradiated cells. The damage constant of the minority-carrier diffusion length of the cells irradiated with 1 MeV protons was determined to be 3.5 ×10-5. This analysis led to the relation between the defect introduction rate and proton energy, and was obtained using the same method, as was the defect annealing rate. This result agreed well with that estimated from an analysis of changes in short-circuit current degradation.

Photoluminescence (PL) spectra of as grown Ga-rich, and Ga-rich plus Ge-doped and annealed CCSVT (Chemical Close-Spaced Vapor Transport) — CuGaSe2 thin films have been investigated. Visible (514.5 nm) and Ultra- Violet (351.1 nm) excitation energies of the laser have been used in order to determine intrinsic and extrinsic defects created due to the implantation as well as separating near surface from bulk recombination.

Both visible and UV-PL spectra of the undoped films show the well known luminescence of Ga-rich CuGaSe2, which can be described by the widely accepted model of fluctuating potentials. Unlike the visible- and UV-PL emissions of Ge- implanted and annealed films differ strongly. Obviously, Ge-implantation in combination with the thermal treatment results in an extrinsic doping of the material producing so far unknown states in the CuGaSe2 band gap. Comparing the visible- and the UV-PL spectra we found an accumulation of these extrinsic doping levels in the near-surface-region of the films.

We use two-dimensional simulations to explore if charged columnar grain boundaries (GBs) in Cu(In,Ga)Se2 (CIGS) solar cells increase device performance. Although the simulations confirm that charged GBs can increase photocurrent by forming minority-carrier collection channels, this generally occurs at the expense of overall efficiency. Furthermore, improvements in photocurrent require significant GB minority collection beyond a diffusion length of the space-charge region. This collection from deep within the base can be detected by quantum efficiency (QE) spectra, electron-beam-induced current (EBIC) experiments, near-field scanning optical microscopy (NSOM), and fast photoluminescence decay. Simulations of all these experiments indicate that GB charge sufficient to significantly increase photocurrent collection is generally inconsistent with actual observations.

For preparation of ZnO/CdS/Cu(In,Ga)Se2 solar cells, physical vapor deposition (PVD) was employed to deposit CdS buffer layers in ultrahigh vacuum on Se-decapped absorber surfaces, thus realizing an all ‘dry' fabrication process of the device. An 14.1% total area and 14.5% active area efficient ZnO/CdS/Cu(In,Ga)Se2 solar cell under AM1.5 conditions was achieved after annealing the as-prepared solar cells in air. Kelvin probe force microscopy (KPFM) measurements were carried out in-situ to monitor the initial growth of the CdS buffer layer on the absorber, as well as its electronic properties, in particular, the work function. It was observed that the PVD-CdS growth is initially inhibited at the absorber grain boundaries. Quantum efficiency measurements allowed us to suppose that during the initial growth stage a passivation of the grain boundaries occurs. The latter explains the higher short-circuit currents of the cells with PVD-CdS compared to their references with CdS grown by chemical bath deposition (CBD). The beneficial effect of the annealing seems to originate from a formation of a region with higher band gap than that of the absorber bulk and inverted conductivity type at the absorber surface, close to the CdS/Cu(In,Ga)Se2 interface, leading to a dramatic change in the electronic transport properties and finally, to a significant enhancement of the open-circuit voltage. Annealing of the ZnO/PVD-CdS/Cu(In,Ga)Se2 solar cells provides formation of PVDCdS/ Cu(In,Ga)Se2 interface with properties similar to that of reference samples with CBD-CdS.

For laboratory scale devices the transfer of the Cu(In,Ga)Se2 (CIGSe) thin film technology from a rigid float glass substrate to flexible titanium foil is achieved. Here we will highlight the effect of the rough, flexible substrate on the electrical and optical characteristics of the transparent window layers of such devices. The roughness of the substrate may give rise to changes in sheet resistance of the transparent front contact and in the reflectivity of the completed device. However, the devices deposited onto the kind of titanium foil substrate used in this work are smooth enough not to affect the sheet resistance; nevertheless optical effects are enough to affect the process window for an antireflective coating. With the use of an antireflective coating, a maximum efficiency of 17.4% could be reached for 0.5 cm2 area devices on glass.

Analytical TEM techniques have been used to characterize the structure and chemistry of chalcopyrite solar cell devices produced by elemental evaporation using a bi-layer process at Ts=400°C and Ts=550°C. Dislocations, stacking faults, twins and voids exist in both materials but at a reduced density at the higher deposition temperature. The higher processing temperature also improved the density and structure of the grain boundaries and created a less rough surface. The CdS layer coats the surface conformally, although the roughness impacts the structure. CdS fills the grooves in the surface and infiltrates lower density grain boundaries to significant depths. The chemistry of the grain boundaries does not appear to be significantly impacted by the presence of the Cd and S.

Surface etching of CdTe under illumination in aqueous aniline-based solutions has been investigated. Analysis of treated samples shows the formation of crystalline Te on the CdTe surface. Analysis of bath and treatment conditions indicated the presence of aniline, Cl- ions, dissolved O2, consistent illumination and careful control of pH as critical for successful surface etching. A photocatalytic mechanism is proposed for the reaction, where CdTe is etched by H2O2 or similar, generated through reduction of dissolved O2 by conduction band electrons and complemented by oxidation of aniline. The generated Te is stable in the bath and the treatment provides reproducible surfaces. CdTe/CdS devices of 12% conversion efficiency were processed with 30 min aniline treatments. The toxicity of aniline is a concern and criteria for possible substitutes are discussed.

The photosensitivity and electro-reflection spectra of CdS/CdTe heterojunctions annealed up to one hour in presence of CdC12 have been studied. The lifetime of charge carriers, generated in the interface layer, has been determined.

The SnO2/CdS/CdTe/Ni thin films solar cells, deposited by close-spaced sublimation (CSS) technique are photosensitive in the 1.5-2.4 eV spectral region. The spectral distribution of photocurrent is not essentially modified by the increase of the annealing time. A slight decrease of the photocurrent in the 1.5-1.7 eV energy range is observed and the photocurrent slightly increases in the 1.7-2.4 eV region. The recombination of the nonequilibrium charge carriers in the interface layer takes place via recombination levels having a lifetime value, which decreases from 46 μs down to 30 μs along with photon energy increase from 1.55 eV to 2.4 eV respectively (T=293K). The correlation between the light penetration depth in component layers, estimated from the absorption spectral distribution and the photon energies was established.

Detailed analysis of drive level capacitance, capacitance voltage characteristics and capacitance transient was carried out to elucidate the influence of N1 defects on carrier concentration profiles. The type of hysteresis observed in CV curves proves that N1 traps originate from close-to-interface minority carrier traps. We show that in the presence of these states one must be careful while interpreting DLCP data. It was found that due to the incomplete charging of N1 defects, low frequency drive level profiles may give erroneous defects densities.

Current transient responses to voltage and illumination steps are investigated to elucidate the mechanisms involved in carrier transport in CdS/CdTe cells. For most cells, the response to a dark, forward-bias step after a long dark soak at zero bias is a current growth curve For one such cell, the magnitude of the transient is ≈ 22% of the starting value with half of the growth occurring within ≈ 10 sec, the other half requiring 1000's of seconds. The effect is completely reversible and a mirror-image decay curve at zero bias after dark bias-on equilibration can be measured. Similarly, a complex of growth and decay curves are observed on application of illumination steps with constant bias. Similar transients have been observed by McMahon [1] and del Cueto et al. [2]. This paper is a survey of these effects in cells from 3 different fabricators.

These transients, with varying magnitudes and directions, were seen in all the cells studied. In general, the better the cell, the smaller the magnitudes of the transients. They range from changes by factors of 10 for pathological cells to subtle fast transients (1-2%) in excellent cells.

Beside the important implications these transients have for accurate measurements of cell efficiency and stability, they provide clues about the carrier transport mechanisms. One of the mechanisms proposed involves the occupation of deep donor traps with small hole cross sections, changing their recombination kinetics. The second hypothesis involves the modulation of the junction barrier profile by changing the charge on deep acceptors and donors by carrier trapping, leading to a change in the effective junction barrier height. A third involves defect mutation such as that of [Cui] donors into [VCd- Cu] acceptor complexes, depending on the position of the quasi-Fermi levels.

We report a multi-element study of impurities in CdS window layers by dynamic and quantitative SIMS. Two CdS/TCO/glass samples, grown separately using nominally the same conditions, were considered. In203:F grown on soda lime glass was used as TCO followed by a sputtered CdS layer. One of the samples was subsequently used as a substrate for growth of CdTe by CSS. SIMS was carried out on both samples, and O, Na, Si, Cl, Sb, In, Zn, Sn, Pb, Cu, Te, S and Cd were depth profiled. It was shown that before CdTe growth, most of the impurity elements showed flat levels in the CdS ranging from 2-3×1020 cm-3 for Zn and O to 2-3×1017 cm-3 for Na, Cl, Sb and Te. Si was found to segregate at the CdS/TCO interface with a maximum level of 1018 cm-3. However, following CdTe growth, the impurities in the CdS layer showed higher concentrations and different profile shape compared to those before CdTe growth. Some of the impurities also showed a diffusion-like profile following the CdTe growth as compared to before. Possible explanations of these changes are discussed in terms of the purity of the starting materials and the growth environments, as well as the diffusion from the TCO and glass.

It has long been recognized that the introduction of a narrow band of states in a semiconductor band gap could be used to achieve improved power conversion efficiency in semiconductor-based solar cells. The intermediate band would serve as a “stepping stone” for photons of different energy to excite electrons from the valence to the conduction band. An important advantage of this design is that it requires formation of only a single p-n junction, which is a crucial simplification in comparison to multijunction solar cells. A detailed balance analysis predicts a limiting efficiency of more than 50% for an optimized, single intermediate band solar cell. This is higher than the efficiency of an optimized two junction solar cell. Using ion beam implantation and pulsed laser melting we have synthesized Zn1-yMnyOxTe1-x alloys with x<0.03. These highly mismatched alloys have a unique electronic structure with a narrow oxygen-derived intermediate band. The width and the location of the band is described by the Band Anticrossing model and can be varied by controlling the oxygen content. This provides a unique opportunity to optimize the absorption of solar photons for best solar cell performance. We have carried out systematic studies of the effects of the intermediate band on the optical and electrical properties of Zn1-yMnyOxTe1-x alloys. We observe an extension of the photovoltaic response towards lower photon energies, which is a clear indication of optical transitions from the valence to the intermediate band.

This paper presents the influence of the solution chemistry of chemical bath deposition (pH and complexing agents) on the performance of CuIn(S,Se)2 cells after an initial CN treatment. It is shown that it is possible to modify the deposition conditions of the CdS by increasing the pH of the solution and by replacing the complexing agent (ammonia) by citrate ions. Both NH3 based and citrate based process give very homogenous and covering thin films. However, in the case of the citrate based process a decrease of open circuit voltage (Voc) and fill factor (FF) and thus of the cell efficiencies is observed. This points out that the main role of the buffer layer is not only related to the specific properties of the CdS itself but also to the near surface modifications of the CuIn(S,Se)2 caused by the presence of the complexing agent in the bath.