Chalcopyrite Cu(In,Ga)S2 is a promising absorber in thin film solar cells, although the comparable high band gaps so far do not correspond to equivalent high open circuit voltages. We have performed photoluminescence studies on CdS passivated absorber layers deposited on Mo coated soda lime glass. From spectrally and spatially resolved (≤ 1μm) room temperature photoluminescence measurements we have extracted the local splitting of quasi- Fermi levels (EFn-EFp) and local absorption (A(ω)) particularly in the sub bandgap-regime via Planck's generalized law. We observe a substantial negative correlation coefficient between the local sub bandgap/defect absorption and the local (EFn-EFp), which we interpret in terms of the recombination of photogenerated minority carriers (here electrons) via sub bandgap states/deep defects. Moreover we have correlated local PL yields with corresponding values at neighbor sites versus distance (increment analysis). As we find lateral correlation distances in the vicinity of average grain sizes we conclude grains with PL yield and according different splitting of (EFn-EFp) to be independent from one another and be laterally distributed randomly.

High efficiency photovoltaic devices are normally fabricated on single crystalline substrates. These single crystalline substrates are expensive and volume production for widespread usage has not been realistic. To date, large volume production of solar cells is on less expensive non-crystalline substrates such as glass. Typically the films grown on glass are polycrystalline with less than ideal efficiency. It was proposed that a dramatic gain in the efficiency may be achieved if one uses a biaxially oriented buffer layer on glass to grow biaxial semiconductor films to fabricate solar devices compared to that of films grown directly on glass. Biaxial films are not exactly single crystal but have strongly preferred crystallographic orientations in both the out-of-plane and in-plane directions. Typically the misorientation between grains can be small (within a few degrees) and may possess low carrier recombination rate. In this paper we shall discuss growth techniques that would allow one to produce biaxial buffer layers on glass. A specific strategy using an atomic shadowing mechanism in an oblique angle deposition configuration that allows one to grow biaxial buffer layers such as CaF2 on glass substrate will be discussed in detail. Results of heteroepitaxy of semiconductor materials such as CdTe and Ge on these biaxial buffer/glass substrates characterized by x-ray pole figure, reflection high energy electron diffraction (RHEED) pole figure and transmission electron microscopy (TEM) will be presented.

Pulsed-laser-deposited ZnO thin films were exposed to a 1.5 MeV helium ion beam to study the changes in radiative and non-radiative recombination. We first measured photoluminescence (PL) spectra at 4.2 K excited with the 325 nm line of a HeCd laser. The as-deposited films showed a donor-bound exciton peak at 3.3567 eV attributed to Zn interstitials. After irradiation the donor-bound-exciton dominated PL spectra shifted to acceptor-bound behaviour with a signal at 3.3519 eV, tentatively attributed to Li or Na acceptors. In contrast to the approximately 30 % decrease of the PL signal near the band edge, we observed a strong concomitant enhancement of the green/orange PL band, located between 2.1 eV and 2.8 eV, by a factor of over 4. Candidates for those transitions are Li impurities and/or O vacancies. For comparison, the steady-state photocurrent decreased strongly in the irradiated region, which can also be attributed to increased non-radiative recombination through oxygen-related defects.

Scanning capacitance microscopy (SCM) often shows a change in contrast at grain boundaries [1-3]. The origins of this contrast and the efficacy of SCM as a tool to identify band bending at grain boundaries in pc-Si and mc-Si are discussed. Contrast at these grain boundaries could be influenced by different oxide growth rates or by defect states at the oxide interface. In order to determine the influence of such mechanisms on the SCM signal, such effects must be modeled; we show that a simple one-dimensional model agrees well with more detailed models of SCM signal strength and indicates, for example, that very small changes in oxide thickness measurably affect the SCM signal. In our experimental data, the uniformity and quality of the oxide layer are confirmed, and increased contrast consistent with depletion regions is still observed at higher order grain boundaries as identified by electron backscattering diffraction, including 9 and 27a. Scans of the SCM signal as a function of dc probe voltage allow such regions to be more quantitatively investigated.

Systematic first-principles calculations on transition metal (TM) impurities of the 3d series in Si have been performed. The equilibrium sites, migration energies, electrically-active gap levels, charge and spin states are predicted. While the properties of the isolated interstitials are experimentally well-known, much less experimental information is available about the consequences of their interactions with vacancy-like defects. We discuss here the properties of isolated interstitial Ti, Fe, and Ni, their interactions with vacancies and divacancies, the properties of the resulting substitutional impurities, and of the TM-divacancy {V-TM-V} complexes. In equilibrium, interstitial Ti, Fe, and Ni do not become substitutional, but a number of processing steps commonly used in PV manufacturing introduce highly mobile vacancies into the bulk. These vacancies strongly interact with interstitial TMs. At the substitutional site, Ti, Fe, and Ni have very different electrical properties than at the tetrahedral interstitial site. In particular, the electrical activity (and stable spin state) of Ti and Fe are greatly reduced, suggesting that the passivation by vacancies plays an unrecognized role during a variety of high-temperature processes.

We analyze the dependence of the interface defect density Dit in amorphous/crystalline (a-Si:H/c-Si) heterostructures on the microscopic properties of the ultrathin (10nm) undoped a-Si:H films. It is shown that the hydrogen bonding configuration, probed by infrared spectroscopy, determines the a-Si:H network disorder, which in turn governs the annealing behavior of these structures upon a short thermal treatment at moderate temperatures (T ≤ 200°C). While the as-deposited Dit seems to be determined by the local structure at the interface, the final state of the annealed samples is determined by the bulk a-Si:H network strain as reflected in the valence band tail slope. Thus it appears valid to treat the equilibrated a-Si:H/c-Si interface as a 2D-layer that has the projected defect properties of the 3D a-Si:H bulk.

Metamorphic triple-junction solar cells can currently attain efficiencies as high as 41.1%. Using additional junctions could lead to efficiencies above 50%, but require the development of a wide bandgap (2.0-2.2eV) material to act as the top layer. In this work we demonstrate wide bandgap InyGa1-yP grown on GaAsxP1-x via solid source molecular beam epitaxy. Unoptimized tensile GaAsxP1-x buffers grown on GaAs exhibit asymmetric strain relaxation, along with formation of faceted trenches 100-300 nm deep in the [01-1] direction. Smaller grading step size and higher substrate temperatures minimizes the facet trench density and results in symmetric strain relaxation. In comparison, compressively-strained graded GaAsxP1-x buffers on GaP show nearly-complete strain relaxation of the top layers and no evidence of trenches. We subsequently grew InyGa1-yP layers on the GaAsxP1-x buffers. Photoluminescence and transmission electron microscopy measurements show no indication of phase separation or CuPt ordering. Taken in combination with the low threading dislocation densities obtained, MBE-grown InyGa1-yP layers are promising candidates for future use as the top junction of a multi-junction solar cell.

Photoluminescence (PL) spectroscopy of nanocrystalline TiO2 using ultraviolet light excitation reveals a range of intra-bandgap defect states which emit at visible wavelengths. In this study we use 350 nm excitation to probe the luminescent defect states of TiO2 nanotubes fabricated by anodization of titanium. The nanotubes show a broad visible luminescence from 450 to 700 nm with a peak at 520-550 nm or 2.4-2.3 eV. The intensity of nanotube PL is orders of magnitude lower than that of nanoparticulate anatase and P25 (mixed-phase anatase/rutile) films of comparable thickness. Similar to the PL of the nanoparticles, the nanotube PL is increased by vacuum annealing. The nature of the nanotube defects is investigated through shifts in the intensity and shape of the PL spectra in hole or electron scavenging environments. We find the PL intensity of the nanotubes to be less dependent on environment than that of conventional TiO2 nanoparticles. We conclude that there are two inter-related reasons for decreased intensity and decreased environmental dependence of PL from TiO2 nanotubes as compared to nanoparticles: decreased density of defect states and improved carrier transport.

Cu2ZnSnS4 (CZTS) is a promising alternative for Cu(In,Ga)Se2 (CIGS) absorber layers in thin film solar cells and is comprised of commodity elements which will enable scale-up of chalcopyrite panel production unhindered by elemental supplies and costs. Various CZTS synthesis methods, especially sulfurization of stacked metal or metal sulfide layers, are being studied and have led to cell efficiencies up to 6.7% [1]. Here we report our studies of CZTS thin film synthesis via room temperature sputtering from a single CZTS target and co-sputtering from Cu2S, ZnS and SnS2 binary targets, both followed by sulfurization between 500 C - 600 C using either elemental sulfur vapor or in-situ generated H2S. Sputtering from sulfur-containing targets is designed to increase the sulfur content in the precursor films to promote stoichiometry. We report on the effects of processing including deposition on soda-lime and borosilicate glasses and deposition of Na-containing layers on film morphology (AFM/SEM), composition (EDS), phase (XRD), grain size (XRD/EBSD), grain boundary structure (EBSD), optical (spectroscopic ellipsometry) and electrical properties. Processing conditions producing desirable Zn-rich/Cu-poor films are identified [1]. The formation of MoSe2 at Mo/CIGS interface is believed to promote Ohmic contacts, but in CZTS we associate excessive formation of frangible MoS2 with film delamination from Mo/borosilicate glass substrates. Strategies for preventing delamination including adhesion layers are investigated and discussed. P-N junctions are formed with CdS/ZnO using chemical bath deposition and sputtering, and I-V characteristics are reported. Schottky junctions are formed and C-V measurements are used to determine the doping in the CZTS absorber layers.[1] H. Katagiri, et al., MRS Symp. Proc. 1165 1165-M04-01 (2009).

Hydrogen is commonly introduced into silicon solar cells to reduce the deleterious effects of defects and to increase cell efficiency. When hydrogen is introduced into multicrystalline Si that is often used for the fabrication of solar cells, the H atoms become trapped by carbon impurities to produce defect structures known at H2*(C). These defects act as both a source and a sink for hydrogen in H-related defect reactions. IR spectroscopy has been used to determine what H- and C-related defects are formed in multicrystalline Si when the carbon concentration is varied. A process that is used by industry to introduce hydrogen into Si solar cells is the post-deposition annealing of a hydrogen-rich SiNx layer. The H2*(C) defects provide a strategy for estimating the concentration and penetration depth of the hydrogen that is introduced by this method.

Forming a chemically stable low-resistance back contact for CdTe thin film solar cells is critically important to the cell performance. This paper reports theoretical study of the effects of the back contact material, Sb2Te3, on the performance of the CdTe solar cells. First-principles calculations show that Sb impurities in p-type CdTe are donors and can diffuse with low diffusion barrier. There properties are clearly detrimental to the solar cell performance. The Sb segregation into the grain boundaries may be required to explain the good efficiencies for the CdTe solar cells with Sb2Te3 back contacts.

The photovoltaic market is currently dominated by multicrystalline silicon. However, this material is characterized by intrinsic structural heterogeneity due to point defects, dislocations and grain boundaries. In order to improve the cell performance the control of the electrical properties of the grain boundaries and dislocations is required. The laser beam induced current technique allows the estimation of the variations of the charge capture rates due to the different trapping centers, and is a powerful tool for the characterization of multicrystalline silicon solar cells. Nevertheless, one has to control the reflected light in order to obtain a reliable estimation of the electrical parameters.

Cu2ZnSnS4 (CZTS) is an alternative material to Cu(In,Ga)Se2 (CIGSe) for use in thin film photovoltaic absorber layers composed solely of commodity elements [1,2]. Thus, if similar material quality and performance can be realized, its use would allow scale-up of terrestrial thin film photovoltaic production unhindered by material price or supply constraints. Here we report on our research on the deposition of CZTS by RF sputtering from a single CZTS target and co-sputtering from multiple binary sources on Mo-coated glass. We find some samples delaminate during post-sputtering furnace annealing in S vapor. Samples on borosilicate glass (BSG) delaminate much more frequently than those on soda-lime glass (SLG). We investigate the influences of the formation of frangible phases such as MoS2 at the CZTS/Mo interface and residual and thermal mismatch stress on delamination. We implicate fracture in a layer of MoS2 as the mechanism of delamination between the Mo and CZTS layers using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Wafer curvature measurements show significant (˜400 MPa) deposition stress for minimally optimized Mo deposition; however nearly stress-free Mo layers with good adhesion can be deposited using a multi-step Mo deposition recipe. Co-sputtering CZTS adds 100 MPa of stress on both BSG and SLG, however delamination is nearly absent for samples deposited on low-stress Mo layers. We investigate metallic diffusion barrier layers to prevent the formation of MoS2 at the interface. Lastly we discuss the importance of removing Mo oxide by sputter etching before CZTS deposition and its effects on adhesion and series resistance.

The in situ X-ray reciprocal space mapping (in situ RSM) of symmetric diffraction measurements during lattice-mismatched InGaAs/GaAs(001) growth were performed to investigate the strain relaxation mechanisms. The evolution of the residual strain and crystal quality were obtained as a function of InGaAs film thickness. Based on the results, the correlation between the strain relaxation and the dislocations during the film growth were evaluated. As a result, film thickness ranges with different relaxation mechanisms were classified, and dominant dislocation behavior in each phase were deduced. From the data obtained in in situ measurements, the quantitative strain relaxation models were proposed based on a dislocation kinetic model developed by Dodson and Tsao. Good agreement between the in situ data and the model ensured the validity of the dominant dislocation behavior deduced from the present study.

GaTe and GaTe:In single crystals were grown from high purity Ga (7N) and zone refined Te (>7N) precursor materials. InSe thin films were deposited by thermal evaporation onto the sulfur passivated GaTe:In substrates at various substrate temperatures from 450K-550K to fabricate p-GaTe:In/n-InSe heterojunction solar cells. Scanning electron microscopy (SEM), X-ray diffraction (XRD), electron probe microanalysis (EPMA), and X-ray photoelectron spectroscopy (XPS) were used to characterize GaTe:In crystals and InSe thin film surfaces. The current-voltage characteristics of p-GaTe:In/n-InSe solar cells were measured under dark and under illumination of 75mW/cm2. Dark J-V measurements showed that the reverse saturation current density (J0) decreased from 3.8 x 10-6 A/cm2 to 1.5 x 10-9 A/cm2 and the ideality factor was reduced from 2.04 to 1.15 as a result of surface passivation. Under illumination of 75 mW/cm2, the open-circuit voltage (Voc) increased from 0.54V to 0.68V and short-circuit current density (Jsc) increased from 7.19 mA/cm2 to 8.65 mA/cm2 for solar cells with surface passivated GaTe:In substrates, leading to an increased solar cell efficiency of 5.03%. EPMA measurements revealed that the InSe thin films deposited at 550 K on GaTe:In substrates were near stoichiometric with enhanced grain size contributing also to better solar cell performance.

The visible photoluminescence of nanocrystalline TiO2 is examined in the presence of surface binding agents and as a function of vacuum annealing in order to probe the molecular nature of surface defects. The photoluminesence (PL) of bulk crystals of anatase TiO2 from (101) and (001) planes is also reported in order to test the hypothesis that electron and hole traps are spatially isolated on different crystal planes. We find that a number of hole scavengers are capable of quenching the PL associated with trapped electrons, while the ability of oxygen to quench PL through electron scavenging varies with the nature of the sample. We conclude that hole scavengers exert their influence on the PL through reaction with valence band holes rather than with spatially isolated trapped holes. Scavenging of electrons by O2, on the other hand, depends on adsorption at oxygen vacancies and varies with TiO2 sample.

CuInSe2 (CIS) is commercially processed using energy intensive vacuum processes such as sputtering and thermal evaporation followed by thermal annealing. In order to reduce the cost of fabricating CIS photovoltaic absorber layers we need fast and cheap processing methods. We have investigated the use of non-vacuum electrochemical deposition (ED) followed by ultra violet pulsed laser annealing (UV-PLA). We report here on the results of ns pulsed KrF irradiation of ED CIS films and ED CIS films which were first annealed in a Se atmosphere.

Hydrogen is commonly used to remove (or at least reduce) the electrical activity of numerous defects and impurities in Si. Although hydrogenation works quite well for many defects, it has generally been unsuccessful with transition metal (TM) impurities. A number of {TM,Hn} complexes have been detected using optical or electrical techniques. Even though the gap levels of the isolated TM shift upon hydrogenation, many {TM,Hn} complexes remain electrically active. The nature of the complexes responsible for specific DLTS lines is generally not known, and the number of H interstitials in a given complex is assumed. We have performed systematic first-principles calculations involving Ti-H and Ni-H interactions in Si, assuming both interstitial and substitutional sites for the TM. The equilibrium configurations, binding energies, and approximate gap levels of all the {Ti,Hn} and {Ni,Hn} complexes are calculated.