A comparative study of the chemical composition, crystal structure, carrier concentration, mobility, and the energy gap at room temperature of the selenides and tellurides of normal Cu- III-VI2 and ordered defect Cu-III3-VI5and Cu-III5-VI8 compounds, grown by Bridgman techniques, is made. It is established that these compounds have a chalcopyrite-related structure with the exception of CuIn5Se8 and CuGa5Te8 that have hexagonal and cubic structure, respectively. Smaller volume of the unit cell and lower concentration of the charge carrier of the 1:3:5 and 1:5:8 phases, as compared to the 1:1:2, are attributed to the presence of the ordered vacancies. A new mechanism of the scattering of the charge carriers with the donor-acceptor defect pair (DADP) is suggested to explain the low value of mobility in these compounds. The anomaly observed in the band gap is explained in terms of the relative shift of the conduction band minimum and valence band maximum due to the donor-acceptor defect pairs and cation vacancies, respectively.

We report on a study where the properties of films obtained by RF-magnetron sputtering and by Selenization of elemental precursor layers are analysed by Raman scattering, x-ray diffraction and optical measurements. Three routes were followed to prepare CIS films on glass. CIS type-I was prepared by selenization at various temperatures, CIS type-II was prepared by RF-magnetron sputtering on room temperature substrate followed by annealing at 450°C in air for 10 min and CIS type-III was prepared by RF-magnetron sputtering on a substrate at a temperature ranging from 200°C up to 500°C with a post-deposition annealing in the same conditions as for CIS type-II. Correlating the results from x-ray diffraction with the Raman scattering and optical measurements it was possible to establish unequivocally the formation of CIS with the chalcopyrite structure for CIS type-I at 400°C. Through the same procedure it was possible to establish a way to produce CIS type-II with the chalcopyrite structure. A high density of defects was inferred from the transmission results. Finally the growth dynamics of CIS type-III was studied. It was observed a structural/compositional transition around substrate temperature of 300°C. It was observed that all the films had a sphalerite structure even for the highest substrate temperatures. It was establish the need for a post-deposition annealing to obtain CIS type-III with the chalcopyrite structure. The Raman scattering was found to be a very sensitive technique that allowed us to uncover a difference in the CIS type-I and II with the chalcopyrite structure.

Nanoporous metal oxide thin films are currently attracting interest for a wide range of electronic applications, including sensors and dye-sensitised photovoltaic cells. However, limited, and poorly controlled, film fabrication routes represent a key factor impeding the development of such devices. To date, device applications have largely been limited to sol-gelfabricated nanocrystalline films of titanium dioxide (TiO2). Such studies have recently been extended to the application of an alternative film fabrication technique, notably that of chemical bath deposition (CBD), for the growth of zinc oxide (ZnO). One interesting feature of CBDfabricated films of ZnO is that, under specific conditions of supersaturation, highly reticulated layers may be obtained, such an observation suggesting that control of morphology is possible. Thick nanoporous films of ZnO have been deposited, upon conducting glass substrates, under both acidic and alkaline conditions, from a solution containing the metal ion, added acid or base, and either a chelating agent, such as ethylenediamine or triethanolamine, or a buffer, hexamethylenetetraamine (HMT). The deposition rate is controlled by systematic adjustment of both temperature and pH, together with the nature, and relative concentration, of the reactants in the solution (chelating agent or metal ion). The material properties of the resulting films have been characterised through the use of a range of techniques, including Scanning Electron Microscopy (SEM), X-Ray Powder Diffraction (XRD) and Energy Dispersive Analysis by XRays (EDAX), and the results are discussed within the context of the suitability of the deposited films for incorporation within nanocrystalline devices.

An apparatus developed recently at Colorado State University utilizes a diffractionlimited optics system and a high-resolution translation system to measure the laser-induced photocurrent at resolutions of 1 μm and equivalent incident laser intensities of 1 sun (100 mW/cm2). Multiple lasers in the 635-830 nm range can be easily selected by changing the fiberoptic connectors. The spot profile and location are unchanged when different lasers are selected. In addition, a laser temperature tuned through the 825-857 nm range allows measurement of local variations in the quantum efficiency near the CdTe band gap, which track intermixing of CdS/CdTe. This capability extends the previous analysis, which includes the separation of series resistance and shunting effects. The effect of post-deposition processing and elevated temperature stress on local variations in electrical and optical parameters, especially using the near-bandgap wavelengths, are examined using a series of samples fabricated at NREL.

A novel, non-vacuum and a patented process developed by ISET for manufacturing terrestrial CIGS solar cells was successfully applied to fabricate CIGS solar cells on flexible 50 μm thick Mo foil for space power applications. Solar cells of area 5.0 cm2 were fabricated with their AM0 efficiency approaching 9.0% with an estimated power density of 250 watts/kg. With further improvements it is possible to fabricate CIGS solar cells with AM0 efficiencies approaching 13%. This process can be used to fabricate low cost, light-weight CIGS solar cells on a variety of flexible substrates much lighter than Mo foil to achieve power densities approaching 1.0 kW/kg.

We have developed an electrodeposition bath based on a buffer solution so that the stability of the electrodeposition process is enhanced and no metal oxides or hydroxides precipitate out of solution. The buffer-solution-based bath also deposits more gallium in the precursor films. Asdeposited precursors are stoichiometric or slightly Cu-rich CuIn1−XGaXSe2 (CIGS). Only a minimal amount of indium was added to the electrodeposited precursor films by physical vapor deposition to obtain a 9.4%-efficient device.

The surface morphology of chemical bath deposited In(OH)xSy thin films has been studied using SEM and TEM. Different surface morphologies have been obtained depending on the deposition conditions (reactant concentrations, pH and bath temperature) as a consequence of the variation of both the homogeneous nucleation and particle-growth rates with them. The In(OH)3 colloids present in the bulk solution and on the substrate surface play an important role in the growth mechanism since they promote the In(OH)xSy formation. It has been found that the growth mechanism of the films is the socalled particle-by-particle or colloidal growth.

Intermixing at the CdS/CdTe interface is considered crucial for the performance of CdTe-based solar cells. High-efficiency devices have been fabricated in spite of large lattice mismatch between CdS and CdTe. Intermixing and formation of CdSxTe1−x alloys in present-generation devices result from high-temperature CdTe deposition and post-deposition CdCl2 treatment. In this paper, we present our work on preparation of CdSxTe1−x-alloy powders and films. In this process, the CdS and CdTe powders are sized and mixed, isostatically pressed into a bar, sealed into closed-quartz ampoules, annealed at temperatures >1000°C, and water quenched. X-ray diffraction (XRD) analysis showed that they contained only distinct phases on S-rich and Te-rich sides of the phase diagram. Recently, two new batches of CdSxTe1−x material, produced by an improved process have been obtained (x =0.71 and x= 0.35 in CdSxTe1−x alloy). Initial XRD results from the films deposited by close spaced sublimation (CSS) using these powders reveal that both samples of bulk powders (prior to CSS deposition) are single-phase and demonstrate CdS-like characteristics (hexagonal). We have deposited CdSxTe1−x-alloy films using these powders. These alloy powders evaporate more congruently as opposed to mixed powders of CdS and CdTe. We will also present results on the structural properties of thin CdSxTe1−x alloy films deposited using these powders by CSS.

Most of the low resistance back contacts formed on high efficiency CdS/CdTe solar cells involve copper either in elemental form (such as Cu/Au back contacts) or as dopant in other material (such as Cu-doped ZnTe). But copper is also suspected to be a cause of degradation of devices in long-term stability tests due to its high diffusion coefficient in polycrystalline CdTe. In this paper, we present results on the development of nitrogen-doped ZnTe back contacts for CdS/CdTe solar cells. Reproducible N-doped p-ZnTe films were prepared using reactive RF magnetron sputtering with Ar/N2 gas mixtures. The conductivity of the doped ZnTe films was more than five orders of magnitude higher than that of intrinsic films. We find that annealing in air can further increase the conductivity. Efficiencies near 10% have been achieved with a ZnTe:N/Ni back contact.

ZnO films were grown by radical-source molecular beam epitaxy (RS-MBE) on sapphire and glass substrates, and they were characterized in terms of Hall mobility and optical transmission. Undoped ZnO films exhibit a low intrinsic defect density and optical properties close to bulk ZnO. By Ga doping, a resistance ρ as low as 2×10−4 Ωcm could be achieved. Balancing high conductivity and low transmission losses due to free carrier absorption in the infrared, the optimum was obtained for ρ=3.4×10−4Ωcm, electron mobility μe=37 cm2/Vs and an average transmission T of 96% in the wavelength range 400-1100 nm. Polycrystalline growth on glass yields slightly reduced but still good film quality (μe=30 cm2/Vs, T=90%). By the incorporation of Mg, conducting Mg0.3Zn0.7O films with an increased band gap up to ∼ 4eV were realized.

The role of Cu in CdTe/CdS solar cell instability remains the subject of much debate. The investigation of a range of ‘Cu’-contacted CdTe/CdS cells, which had received various thermal stress treatments, is described. Cells that were stressed in air exhibit strong current-voltage (I-V) rollover and junction degradation. No such degradation was observed for ‘Cu’-contacted cells that had been stressed in dry-N2 atmosphere. Cu is found to diffuse rapidly through the cell structure during back contact annealing and to accumulate in the CdS layer. With stress, significant levels of Cu dope the grain bulk, producing (with Cl) high resistance, photoconducting CdS. This behavior is independent of stress atmosphere and is, therefore, unlikely to (initially) be a dominating mechanism for cell degradation. Our results suggest simple air oxidation of the back contact interface to be a likely origin of I-V rollover in CdTe/CdS cells.

ZnO films were grown directly on epitaxial CuInSe2 (001) (CIS) by radical-source molecular beam epitaxy (RS-MBE). The substrate-film interdiffusion was investigated dependent on the ZnO growth temperature. Secondary Ion Mass Spectroscopy (SIMS) profiles indicate the mutual temperature-activated diffusion of Zn and In at a growth temperature of 440°C which is absent at 250°C. Zn indiffusion into the CIS substrate leads to characteristic changes in the photoluminescence (PL) properties, whereas the In outdiffusion into the growing ZnO film causes an increased carrier concentration.

Buffer layers are commonly used in the optimization of thin-film solar cells. For CuInSe2-and CdTe-based solar cells, multilayer transparent conductors (TCOs, e.g., ZnO or SnO2) are generally used in conjunction with a CdS heterojunction layer. Optimum cell performance is usually found when the TCO layer in contact with the CdS is very resistive or almost insulating. In addition to affecting the open-circuit voltage of a cell, it is commonly reported that buffer layers affect stress-induced degradation and transient phenomena in CdTe- and CuInSe2-based solar cells. In amorphous silicon solar cells, light-induced degradation has a recoverable and a nonrecoverable component too, and the details of the mechanism may depend on the p-type contact layer. Because of the similarity of transients and degradation in dissimilar material systems, it is suggested that degradation and recovery are driven by carriers rather than by diffusing atomic species. The question that must be addressed is why, not how, species diffuse and atomic configurations relax differently in the presence of excess carriers. In this paper, I suggest that the operating conditions of a cell can change the carrier transport properties. Often, excess carriers may enhance the conductance in localized regions (“filaments”) and buffer layers; limiting current flow into such filaments may therefore control the rate and amount of degradation (or recovery).

According to our model for the current transport in CuInS2/CdS/ZnO solar cells presented previously, the dominant recombination under illumination occurs at the CuInS2/CdS interface. In this contribution we expand this model for cells where the absorber has been prepared in different sequential processes by using results from jV curve measurements performed at varied temperature and illumination. We find that in contrast to CuInSe2 and CuGaSe2 solar cells from the Ångström Solar Center the CuInS2 cells exhibit a qualitative change in the dominant recombination mechanism between the dark and under illumination. The dominant recombination mechanism under illumination appears to be affected by absorber preparation parameters.

In previous work [1,2], we modeled the cross-over of the I-V curves of thin film CdS/CdTe solar cells in terms of an electron (minority carrier) current in the vicinity of the back contact. In this work, we focus on the necessary extension of this analytical model based on a series of measurement results. Especially the wavelength and voltage dependence of the current at forward bias is illustrated in these measurements. The various possible causes for this kind of behavior are discussed and modeled. The extensions to the previous model, needed to describe the voltage and wavelength dependent behavior of I-V curves of real CdTe/CdS solar cells, are proposed.

The effect of initial film composition and substrate in the sulfurization of CuInSe2 was investigated. CuInSe2 films deposited on either soda-lime glass (SL) or Corning 7059® borosilicate glass (7059) substrates were reacted in flowing H2S for times from 1 to 8 hours. Films with Cu-rich composition, Cu/In > 1, reacted for 1 hour had nearly all the Se replaced by S. For Cu-poor films the incorporation of S was significantly reduced. In addition, in Cu-poor films on SL glass CuInS2 and NaInS2 were found at the film surface. These phases were not detected in films on 7059 substrates or in Cu-rich films. A phenomenological model is proposed to explain the formation of segregated surface phases in Cu-poor films on SL substrates.

An essential processing step in CdTe/CdS polycrystalline solar cells is heat treatment in CdCl2. We present photoluminescence results from single crystals of CdTe that have been exposed to CdCl2 treatments at 387 C similar to those used in actual cell fabrication. Using sub band gap excitation from a tunable diode laser, we probe states in the interior of the crystal. We show that high-purity (99.998 percent) CdCl2 treatment results in the appearance of a 1.45 eV donor-acceptor transition that is likely due to a Cl-Cu center. Low purity (99.7 percent) CdCl2treatment results in the appearance of the 1.45 eV line and a 1.555 eV Cu-related emission. These results indicate that the CdCl2 treatment has an effect on the interior of CdTe grains, in addition to its already well established effect on grain boundaries in polycrystalline CdS/CdTe devices. They also imply that CdCl2 treatment may result in the incorporation of Cu into the CdTe grains. The results will be related to the effects of CdCl2 on polycrystalline CdS/CdTe devices that have been observed by other groups.

This work is supported by NREL

CuInSe2 films were prepared by the three-stage process using the evaporation of In2Se3, Cu2Se and Se. In the third stage, the depth of Cu-poor surface region was precisely adjusted by the control of the evaporation rate of In2Se3 and Se. The interface between Cu-poor surface region and stoichiometric CuInSe2 film was characterized using AES, microEDAX, and RBS. The 9.59% efficiency was achieved in the 0.16 cm2 area cell. The efficiency is the same level of the CIS cell prepared using the element coevaporation, opening up the application of binary compounds for highly efficient Cu(In,Ga)Se2 cells.

We report on the development of Mo/CdTe/CdS/indium-tin-oxide, thin-film solar cells grown by radio-frequency magnetron sputtering. This is an inverted configuration compared to the conventional glass/tin-oxide/CdS/CdTe/metal cells. Molybdenum was chosen as a substrate because its thermal expansion coefficient and the work function are close to those of CdTe. We have achieved AM1.5 conversion efficiencies of 7.8 percent on 0.05 cm2 area devices. Our best cells had a nitrogen-doped ZnTe layer between the molybdenum and the CdTe for a somewhat improved back contact. However, we observe a significant rollover in the IV curve in forward current that indicates a back-diode effect. This implies the need for improvement of the electronic properties of the molybdenum - CdTe and possibly CdS - ITO interfaces.

The experimental technique in the photocurrent-capacitance method of estimating minority diffusion lengths, as applied to CuInSe2-based photovoltaic cells, has been improved, principally by replacing the earlier chopped light and monochromator technique by one employing a steady strong stable light source and a long pass optical filter. The filter requires an absorption edge at an appropriate wavelength to provide the penetrating light at sufficient intensity, which has been found to be 1.1 μm for CuInSe2 cells and 1.0 μ for Cu(In,Ga)Se2 cells. In addition, the parallel capacitance is determined with reverse bias under the same illumination as that during the photocurrent measurements and at a suitable frequency, such as 10 kHz. With these changes, the technique has been found to be more tolerant to cell imperfections and reproducible estimates of diffusion length have been obtained on CuInSe2 and Cu(In,Ga)Se2 cells.