CuInSe2 monograin powders (MGP) were synthesized from Cu-In alloys of different Cu/In concentration ratios and elemental Se in liquid phase of flux material in evacuated quartz ampoules. The surface morphology, phase structure, and composition of the powder crystals were analyzed by scanning electron microscopy, X-ray diffraction, and energy-dispersive X-ray analysis respectively. Bulk composition was analyzed polarographically. Photoluminescence spectra were measured at 9 K. It was found that the composition of MGP material (Cu/In concentration ratio) can be controlled by the concentration ratio of precursor Cu-In alloys. Single phase CuInSe2 growth is realisable between 0.7<Cu/In<1 at the growth temperature of 1000 K. Photoluminescence spectra of near-stoichiometric materials had one dominant peak at 0.93 eV, which is typical to In-rich CuInSe2. Samples with high In content exhibited two broad bands with peak positions at 0.86 and 0.93 eV.

Rapid thermal annealing (RTA), with fast ramp up and down rates, was performed on several Cu(In,Ga)Se2 (CIGS) films and solar cells under various peak annealing temperatures and holding times. The XRD, SEM, Hall- effect, photo J-V, and quantum efficiency (Q-E) measurements were made on CIGS films and cells before and after RTA treatments to study the effects of RTA on the CIGS film properties and cell performance. The results show that RTA treatments under optimal annealing condition can provide significant improvements in the electrical properties (resistivity, carrier concentration, and mobility) of CIGS films and cell performance while preserving the film composition and microstructure morphology.

The reasons why the open circuit voltage (Voc) of high-x CuIn1-xGaxSe2 (CIGS)/ZnO solar cells remain low are discussed. Here it is shown that the Voc ceiling can be interpreted simply on the basis of a model that the valence-band energy (Ev) of CIGS is almost immovable irrespective of x. When the conduction-band energy (Ec) of ZnO is lower than that of high-x CIGS (DEc<0), the built-in potential (Vbi) of a CIGS/ZnO junction is equivalent to the flat-band potential (Vbi) that arises from the separation between the Fermi energies of the two materials. If the Ev (and therefore the Fermi energy) of p-type CIGS is constant with increasing x, the Vbi and Voc that follows the Vbi remain unchanged since the Fermi energy of ZnO is constant. This unchangeable Voc reduces the conversion efficiency of high-x CIGS cells in cooperation with reduced photocurrents due to a larger bandgap. A positive offset, ΔEc>o gives rise to a photoelectrons barrier in the conduction-band that partially cancels Voc, thus the Voc of a low-x CIGS cell is governed by the Ec of CIGS. Based upon this concept, a material selection guideline is given for the windows and transparent electrodes appropriate for high-x CIGS absorbers-based solar cells.

Electrical, optical and structural properties of Ag/CdTe structures exposed to thermal (in dark) and photoannealing (under illumination) have been studied. The effective diffusion coefficie nt of Ag in CdTe films have been estimated from resistance versus duration of annealing curves. In the range of 280-420°C the effective coefficient of thermal diffusion (Dt) and photodiffusion (Dph) are described as Dt= 1.9x105exp (-1.60/kT) and Dph =8.7x103exp(-1.36/kT). The acceleration of Ag diffusion under illumination was tentatively attributed to photoionization of Ag increasing the interstitial flux of silver. Ag/CdTe structures exposed to annealing were characterized by X-ray diffraction (XRD), I-V, C-V, conductivity-temperature and optical transmission measurements. In XRD patterns of annealed Ag/CdTe structures, besides the intensive (111) peak of cubic CdTe, the weak peaks of Ag2Te phase are also present. The temperature dependence of conductivity of annealed Ag/CdTe structures showed the energy levels 0.13 eV.

Indium sulfide buffer layers deposited by the Spray-Ion Layer Gas Reaction (Spray-ILGAR) technique have recently been used with Cu(In,Ga)(S,Se)2 absorbers giving cells with an efficiency equal to the cadmium sulfide references. In this paper we show the first results from cells prepared with Cu(In,Ga)Se2 absorbers (sulfur free). These cells reach an efficiency of 13.1% which remains slightly below the efficiency of the cadmium sulfide reference. However, temperature dependant current-voltage measurements reveal that the activation energy of the dominant recombination mechanism remains unchanged from the cadmium sulfide buffered cells indicating that recombination remains within the space charge region.

The formation of MoSe2 has been studied on polycrystalline Mo layers and on Mo single crystals in dependence of the Mo orientation, the Na concentration, and also as a function of the Se source and the substrate temperatures. The Mo substrates were selenized by evaporation of Se. The samples were analyzed by means of X-ray diffraction, Rutherford backscattering spectrometry, elastic recoil detection analysis, and by conventional and high-resolution transmission electron microscopy. It was found that the crystal structure and orientation of the MoSe2 layer change with increasing substrate temperature. However, the texture of MoSe2 does not depend on the orientation of the Mo substrate. It was also found that the MoSe2 growth is significantly influenced by the Na concentration at substrate temperatures of 450°C and 580°C.

CIGS (Cu(In,Ga)Se2) thin-film solar modules on glass substrates are currently on the verge of commercialization. Entirely new application areas could be accessed with CIGS modules fabricated on thin and flexible non-glass substrates. Additionally, the roll-to-roll manufacturing of such flexible CIGS modules promises to be a low-cost production method. Different external Na supply methods and a vacuum-deposited buffer were investigated in this contribution, a sample of the challenges we face when modifying the standard, industrial CIGS module production process to the particular requirements of flexible substrates. Both metal foil substrates and polymer films are considered. Our excellent best results of above 14 % for single cells on titanium, more than 11% on polyimide, and around 7 % for modules on both substrates indicate our progress in developing flexible CIGS.

The CELLO (solar cell local characterization) technique, which allows to measure all parameters of a solar cell locally with high spatial resolution, is applied for the first time for thin film solar cells based on Cu(In,Ga)Se2 (CIGS) and micro-crystalline Si (μcSi). Results for local effective serial resistance, photocurrent and current/voltage at the working point will be presented, demonstrating the applicability of CELLO to thin film solar cells. Based on the measurements, quantitative predictions for the potential of these cells will be made.

Measurements of p-type Cu(InGa)Se2 (CIGS) using deep-level transient spectroscopy (DLTS) show peaks associated with minority-carrier traps, even though data were collected using reverse bias conditions not favorable to injecting minority-carrier electrons. These DLTS peaks occur in the temperature range of 50 to 150 K for the rate windows used and correspond to electron traps having activation energies usually in the range of 0.1 to 0.2 eV for alloys of CIS, CGS, and CIGS. The peak values also depend on the number of traps filled. For short filling times of 10 μs to 100 μs, a small peak appears. As the DLTS filling pulse width increases, the peak increases in response to more traps being filled, but it also broadens and shifts to lower temperature suggesting that a possible series of trap levels, perhaps forming a defect band, are present. The peaks usually saturate in a timeframe of seconds. These filling times are sufficient for electrons to fill traps near the interface from the n-type side of the device due to a thermionic emission current. Admittance spectroscopy data for the same samples are shown for comparison.

Analysis of CuIn1-x Gax Se2-y Sy (CIGSS) absorber and molybdenum back contact layer was carried out to understand the changes in the microstructure of CIGSS layer as a function of the deposition conditions and the nature of stress in the underlying Mo film. All the depositions were carried out on 10 cm x 10 cm glass substrates. Compressive and tensile stressed molybdenum films were prepared with combinations of deposition parameters; power and pressure. CIGSS absorber layer was prepared by depositing metallic precursors using DC magnetron sputtering followed by selenization and sulfurization. Molybdenum layer deposited at 300 W and 3 x 10 Torr pressure produced compressive stress with compact, well adherent and lower sheet resistance as compared to the tensile stressed film deposited at 200 W and 5 x 10 Torr. The crystallinity of the CIGSS film was found not to depend on the stress in the underlying molybdenum film. However, the adhesion at the Mo/CIGSS as well as gallium profile at the Mo/CIGSS interface were affected by the stress.

We report the results of our studies on the optical and electronic structure of a wide range of polycrystalline thin-film CuIn1-xGaxSe2 (CIGS) alloys. The composition range includes CIS and nearly stoichiometric (slightly Cu-poor) (24.3±0.3 at.% Cu) CIGS with x values located around the value that has the best efficiency (x ∼ 0.28). Relative to nearly stoichiometric CIS and CIGS, we find a reduction in the absorption strength in the spectral range 1-3 eV. This reduction can be explained in terms of the predominance of Cu 3d and Se 4p states at the valence band maximum (VBM). In addition, Cu-poor CIS and CIGS materials show an increase in bandgap because the p-d repulsive interaction in Cu-poor CIGS is less than that in nearly stoichiometric CIGS. High efficiency is discussed in terms of optical properties.

ZnSe has been shown to be a promising alternative buffer in CuInS2 thin film solar cells. Here we present for the first time photoemission measurements to determine the band alignment at the ZnSe/CuInS2 interface. Epitaxial CuInS2 is used as a substrate. ZnSe is deposited in varying thicknesses by MOCVD. X-ray photoelectron spectra are measured with an Mg laboratory source and with synchrotron radiation. A valence band offset of 0.4+/-0.1eV is obtained.

CdSe thin films were prepared using unipolar current pulse electrodeposition technique on fluorine doped SnO2 coated glass plates. The electrodeposition bath consisted of aqueous solution including CdSO4, SeO2 and pH was adjusted using H2SO4. Growth of CdSe was accomplished under wide range of variables like current density, pulse on and off time in order to elucidate the effect on crystalline structure, grain size and optical properties. A low current density and increased off time favored grain orientation and crystallinity. Increased current density and on time favored grain size reduction.

The development of novel solar absorbers and device configurations that incorporate only materials which are cost effective, abundant, and non-toxic may be required for widespread deployment of photovoltaics. Cu3BiS3 (Eg=1.2 eV) has been previously reported to be a suitable solar absorber for use in thin film photovoltaic devices. We have developed a physical vapor deposition synthesis for Cu3BiS3, and will employ combinatorial methods to identify novel device configurations in an effort to produce a device exhibiting sufficient efficiency to capture the interest of the photovoltaics community.

In this paper we describe synchrotron based, state-of-the-art spectroscopic methods for the analysis of surfaces and interfaces in thin film photovoltaic devices, their merits and their limitations. Using results obtained with the “CISSY” end station at the BESSY synchrotron in Berlin, Germany, we show how surface sensitive Synchrotron excited X-ray Photoelectron Spectroscopy (SXPS) and Soft X-ray Emission Spectroscopy (SXES), which yields compositional and chemical depth information in the ten to hundred nm scale, have increased our knowledge of the chemistry of surfaces and buried interfaces of these systems.

Ag(In1-xGax)Se2 thin films have been deposited on Mo-coated soda-lime glass substrates by the three-stage process using a molecular beam epitaxy (MBE) system. We found a remarkable decrease in the substrate temperature during the 2nd stage in which the film composition changes to a Ag excess. A single phase chalcopyrite AIGS thin film with a slightly Ag poor composition was obtained by using the temperature monitoring composition method. The cell performance of the AIGS thin film solar cell was found to strongly depend on the Ga/(In+Ga) and Ag/(In+Ga) atomic ratios.

A high efficiency wide-gap (Eg=1.7eV) Ag(In0.2Ga0.8)Se2 thin film solar cell with a total-area efficiency of 9.3% (10.2% active area efficiency), Voc = 949mV, Jsc = 17.0 mA/cm2, FF = 0.577, and total area = 0.42 cm2 was achieved. The junction formation mechanism of AIGS devices is discussed based on electron beam induced current (EBIC) and scanning capacitance microscopy (SCM) analyses.

Device-grade CuGaSe2 (CGSe) and CuInS2 (CIS) thin films for photovoltaic applications have been subjected to dry surface treatments based on In-S and Ga-S by means of chemical vapor deposition (CVD), carried out in an open-tube system. Film properties have been monitored from time and temperature processing series. Improved PV performance has been demonstrated from devices based on treated CGSe compared to those based on reference samples. Numerical simulations have been performed, pointing out the conditions such surface treatments should fulfil in order to improve the performance of devices based on wide gap absorbers.

A comparison has been made of MOCVD grown CdTe/CdS solar cells processed either by ex situ annealing with CdCl2 or doping with arsenic, in situ, together with various optional anneals. A materials comparison was made of both routes using Jsc measurements on arrays of gold contacts to the CdTe. The Jsc increased from around 1 mA cm-2 for undoped and unannealed layers to a range of 25-30 mA cm-2 for CdCl2 annealed layers. In situ arsenic doping resulted in Jsc values up to 18 mA cm-2. The annealing characteristics were very different for these films, compared with the CdCl2 annealed films, with annealing at 500°C dramatically reducing the Jsc. Only annealing under nitrogen at 400°C produced an improvement in Jsc and further evidence from SIMS analysis suggests that hydrogen passivation of the arsenic dopant may have a significant effect on the dopant activity.

We have studied the photoluminescence of CdTe crystals doped with two elements, Cu and Cl, that are frequently used in CdTe-based solar cells. Ions were implanted into high-quality single crystals of CdTe at the Toledo Heavy Ion Accelerator Lab in our Department. We used a standard Monte Carlo simulation program to plan an implant dosage at three different energies. The lattice damage was removed by thermal annealing in an inert atmosphere using a proximity cap to avoid surface deterioration. The PL spectra at 40K were obtained at 488 nm or 752nm to match the absorption depth with the implant profile. Using implant densities typically of 1016, 1017, and 1018 /cm3, and laser excitation power densities ranging over several orders of magnitude, we have identified band-to-band transitions, free-to-bound transitions, bound-exciton lines, and donor-acceptor pair transitions related to these species.

The influence of mechanical pressure along the direction across the interface of n-InSe-p-GaSe heterojunctions on saturation photo-e.m.f. and short-circuit current is investigated. It is shown that at the InSe/GaSe optical contacts subjected to a pressure P = 35-40 kPa an increase of the open-circuit voltage nearly twice and short-circuit current more than by a factor of five in comparison to the initial samples takes place. It makes possible to predict a possibility of considerable increasing photoconversion efficiency of such structures.