To obtain high quality ZnO thin films for use as transparent electrodes of amorphous silicon solar cells, hydrogen treatment of the films using photo-chemical vapor deposition was performed for the first time. The as-deposited ZnO thin film was irradiated by UV light during the flow of hydrogen molecules in the presence of photo-sensitizers of mercury. As the treatment time increased, resistivity decreased from 1 × 10−2Ωcm to 2 × 10−3Ωcm. Moreover, haze ratio increased from 20% to 48%. Hydrogen radicals were thought to be playing various roles on the neighborhood of the surface region and the grain boundary region. This new trial gave us new understanding into the relation between hydrogen and ZnO. Moreover, these results could be applied to the process of amorphous silicon solar cells and a possible increase of efficiency is expected.

SnO2:F is a widely used transparent conductor and commercially available in a multilayer structure as Tech glass. Current applications include photovoltaics, electrochromics and displays. Optical design of these and other applications requires knowledge of the optical constants, in some cases, over the whole solar spectrum. Various optical property measurements were performed including variable angle spectroscopic ellipsometry, and spectral transmittance and reflectance measurements. This material is deposited in several steps and has a fairly complex structure. The measured data were fit to models based on this structure to obtain the optical indices. Atomic force microscopy confirmed the optically modeled surface roughness.

The dark conductivity σ of poorly conducting In2O3-x films can be enhanced by orders of magnitude by blue or UV light exposure at any temperature between 4.2K and 300K. The resulting high σ state of amorphous films reverts back to its original low σ state upon heating to 400K in an inert gas and can be repeated many times. In crystalline films the high σ state is stable to higher temperatures where annealing leads to some outdiffusion of oxygen and hence to a conductivity increase which can only be reversed by external re-oxidation. The photoinduced increase in dark conductivity is accompanied by an increase in photoconductivity at hv = 0.6 or 2eV by a factor of 107 at 4.2K.

Acousto-electric current saturation has been observed in thin, polycrystalline Al-doped zinc oxide films grown on fused quartz. The films, grown by laser ablation, are c-axis textured with high angle grain boundaries between the grains. After annealing at 600°C in 0.1 mtorr oxygen, the films exhibit a current saturation at a current density of 2 105 A/cm2 and electric fields of 5 103 V/cm. However, under constant field the current density falls and the current saturation behavior is not maintained. Current saturation at the same current density can, however, be restored by increasing the electric field. Similarly, the appearance and disappearance of the current saturation behavior can be reversibly controlled by annealing in different oxygen partial pressures at 200°C. The degradation phenomena is attributed to the high acoustic flux enhancing the diffusion of oxygen along the grain boundaries where oxygen can alter the grain boundary potential barrier and hence the electric field in the grains.

Production requirements for transparent conductors for thin film photovoltaics are reviewed, with particular reference to magnetron sputtered zinc oxide. On-going optimization processes for ZnO:AI prepared on 0.43 m2 substrates by bipolar, reactive sputtering from ZnO:A12O3 targets and by reactive sputtering from a Zn:Al target are described. Optical emission from the plasma is used to study the state of the sputtering target. Attention is drawn to the spatial dependence of deposition conditions under the cathode.

We are examining various spinel-structured thin films (e.g., Cd2SnO4, Zn2SnO4) to develop higher-quality transparent conducting oxides (TCO) than more conventional materials such as indium tin oxide. Here, we report on cadmium indate (CdIn2O4, CIO), which is another member of this family. Thin films of CIO were deposited by radio-frequency (RF) magnetron sputtering, from an oxide target, onto borosilicate glass substrates. The variables included the substrate temperature, sputtering gas composition, and pressure. Film properties were measured before and after heat treatment. Characterization involved Hall effect measurements, optical and infrared spectrophotometry, X-ray diffraction, and atomic-force microscopy. Film resistivities as low as 2.3x10-4Ω cm were achieved for a film thickness of 0.55 μm. The transmittance was 90% in the visible region of the spectrum, without correction for substrate losses and without an anti-reflection coating. The plasma resonance occurred at longer wavelengths than for other materials and this, with a bandgap of approximately 3.1 eV, presents a wide window for optical transmittance. The highest mobility was 54 cm2 V-s-1 and the highest carrier concentration was 7.5x1020 cm-3.

A novel, proprietary, and general approach to depositing thin films and coatings, The novel Jet Vapor Deposition TM (JVDTM) process is described, in which single or multiple “jets in low vacuum” are coupled with “mobile substrates” to generate a wide range of multicomponent, multilayer and “host-guest” thin films and coatings. Highly transparent and conducting aluminum-doped ZnO (ZnO:A1) thin films have been deposited at room temperature on glass slide, flexible polymer and thin film PV coated substrates by the JVDTM process. The thin films obtained from the optimized operating conditions have an average transmittance of about 85% in the visible range and electrical resistivities of 7 to 10×10-4 Ω-cm. Proper control of substrate bias, dopant content, deposition rate, and oxygen to metal ratio are the keys to depositing low resistivity and high transparency thin films.

Transparent Conducting Oxide films are essential for all kind of thin film solar cells. Doped Zinc Oxide (ZnO) in particular has several advantages when compared to other TCOs. It is stable in a hydrogen plasma and it acts as a diffusion barrier with respect to impurities from the substrate. In this work results on textured ZnO films deposited by RF magnetron sputtering are presented. The dependence of the film properties on the RF power, the substrate temperature and the water-vapor partial pressure added to the argon atmosphere has been investigated. We have found that high quality textured aluminum doped ZnO (ZnO:AI) films can be grown at low temperatures (150 °C) using the sputtering technique. Textured surfaces show two types of surface morphology; columnar and granular, depending on the deposition conditions. The ZnO:AI films seem to undergo a structural transition from hexagonal (mostly columnar appearance) to cubic structure (mostly granular appearance).

Despite the early discovery of the Photovoltaic effect by Bechquerel almost 107 years ago, its commercial value had never been seriously considered because of its high cost in production and low in energy conversion efficiency. A familiar predicament has restrained demand for photovoltaic application: People won't use them until they are affordable, but they won't get affordable until there is a mass market for them. As has happened in other expensive markets, businesses are desperate to crack the conundrum. Since the oil shortage in the 70s, crystalline solar cell efficiency has increased from 8% to over 13% to date on 100 mm2 commercial silicon cells by refining the process with advanced device technologies. Together with increased production volume, the cost per watt has been reduced from over $ 8 in the 80's to as low as $3 to day. However, the cost is still considered too high to compete with fossil fuel energy. Further cost reduction is necessary by improving the cell and module conversion efficiency. The increase in cell efficiency and the process to achieve the goal, however, have to follow the golden rule of economics that the operational cost is a fraction of the profit in return. It is shown numerically in this paper that Anti Reflection (AR) coating on silicon solar cells by Atmospheric Pressure Chemical Vapor Deposition (APCVD) technique for large volume production could have over 650% profit return.

Owing to its self-limiting growth mechanism the Atomic Layer Epitaxy (ALE) technique is capable of growing uniform high quality thin films on large area substrates. Therefore, ALE is an attractive choice for depositing transparent electrically conducting films for large area applications, such as solar cells and flat panel displays. In this paper studies on ALE growth of In2O3 and ZnO based transparent conducting thin films will be presented. In2O3, In2O3:Sn and In 2O3:F films were grown at 500 °C and their lowest resistivities were about 3 x 10-3, 2 x 10-3 and 6 x 10-4 Ωcm, respectively. Low temperature (120 - 350 °C) ALE deposition processes were developed for ZnO and ZnO:AI films, the latter having resistivities as low as 8 x 10-4 Ωcm. A straightforward scale-up of the ZnO process from 5 x 5 to 30 x 30 cm 2 substrate size was also demonstrated.

Solar cells based on amorphous silicon (a-Si:H) exhibit a decreased fill factor if ZnO is used as front electrode instead of SnO2. This is due to a poor electric contact between the ZnO and the p-type a-SiC:H(B) layer. To gain a deeper understanding of the chemical and electronic properties of the ZnO/p interface, in-situ XPS measurements were applied to thin a-SiC:H(B) films deposited on ZnO. The effects of CO2 and H2 plasma pretreatments on clean ZnO surfaces and the influence of deposition conditions on the ZnO/a-SiC:H interface were investigated. Upon H2 plasma treatment the formation of SiOx by chemical transport of Si from the reactor walls is observed. Furthermore, a shift of all core levels towards higher binding energies indicates the formation of an accumulation layer. CO2 plasma treatments show no effects on ZnO. Depth profiling across the ZnO/a-SiC:H interface indicates SiO2 formation on ZnO. The depth profile of ZnO related core levels exhibits two features: a reduction of the ZnO at the interface, and, after longer sputter times, a core level shift towards higher binding energy due to an hydrogen induced accumulation layer in the n-type ZnO. The latter causes a depletion of the p-layer resulting in an enhanced series resistance and diminished fill factor. To reduce the depletion layer thin highly conductive microcrystalline layers were introduced, increasing the fill factor up to 74%.

The electrical conductance and optical absorption coefficient of microcrystalline indium oxide (c – In2 O 3-x ) can be changed reversibly by several orders of magnitude by photoreduction and reoxidation. Photoreduction is achieved by exposure to ultraviolet light hv ≥ 3.5eV in vacuum or an inert gas. The effects are similar to those previously observed in amorphous In2 O3-x

The optical properties of microcrystalline silicon (µc-Si) were estimated using the EMA (Effective Medium Approximation) method. This modeling was based on two-phase mixture, amorphous silicon (a-Si) and crystalline silicon (c-Si) with volume fractions of fa and fc respectively. From this modeling, it could be possible to understand thatµc-Si has lower light absorption characteristics than a-Si over all solar spectrum by considering hydrogen involvement in embedded a-Si part of iic-Si and crystalline volume fraction. Also, it is proposed that p-type pe- Si is superior to n-type tic-Si because of its high optical gap of Eo4 and its low absorption coefficient spectrum shape.

The current cost of electricity generated by PV power is still extremely high with respect to utility power and there remain questions as to whether PV power can ever be competitive with electricity generated by fossil fuels. A previous paper by the authors supplied an objective framework for considering this important question and introduced analytical tools to define and project the technical/economic status of PV power through the year 2010. In this paper, we update this previous work in the context of the recent announcements by Amoco/Enron Solar of projected sales of PV power in 1997 at rates considerably less than the US average.

We report the use of semiconductor septum electrochemical photovoltaic (SC-SEP) cell, modeled after green plant photosynthesis, in an application of solar energy conversion. A SC-SEP cell consists of a thin film of CdSe deposited on a metallic substrate (Ti, Ni, or stainless steel) as a bipolar septum electrode separating two aqueous solutions. It has been established that, upon irradiation of the septum electrode with visible light of the solar spectrum, photogenerated electron-hole pairs are separated under the influence of electric field. On the side of the septum electrode facing the light, the most powerful oxidant (holes) will oxidize species in the bathing solution, while the most powerful reducing agent (electrons) present on the dark side will carry out reduction of heavy metal ions or evolve hydrogen from aqueous solutions including seawater. Specifically, we report the use of photon energy generated electrons and ‘holes’ for the cleavage water to hydrogen and/or valuable chemicals in SC-SEP cells.

We report on the preparation and characterization of semiconductor devices based on porous nano-crystalline TiO2 films. We present experimental results on photoconduction to elucidate the role of trap states in this material. We then analyse the behavior of a SnO2/TiO2/Pt Schottkydiode and finally present spectral photoresponse data obtained on a hetero-junction between TiO2 films and PbS quantum dots. We find that the porous films hold some promise for electronic applications, in which fast reponse times are not required.

The potential use of porous silicon as an antireflective coating on solar cells has recently been recognized. This study investigates the effect of current density, anodization time, and surface conditions on the reflectance of porous silicon which was fabricated by anodizing (100) float zone single crystal Si wafers. The wafers were coated on one side with Al prior to anodization, and a HFbased solution was used as the electrolyte. Current densities of 5 – 100 mA/cm2 were used to anodize both polished and unpolished wafers over time intervals ranging from 2sec - 30 minutes. Reflectance properties were tested over the 400 - 1100 nm range, and minimum reflectances of 3 – 5% were achieved. The reflectance of the best porous Si sample normalized with respect to the sun's spectrum compares favorably with the reflectance of a double layer ZnS/ MgF2 with prior texturing.

The recrystallization of CdTe powders in different fluxes was studied and the possibility to grow p- and n - type CdTe monograin powders of high conductivity was shown. It was determined that very important technological factors for creating p - type conductivity in CdTe are the cooling rate and the presence of Te in flux. NaCl content in CdCl2 flux was found to inhibit the crystal growth and to allow for doping CdTe with Na that acts as an effective acceptor impurity in CdTe. The region of most effective added Na concentrations was found.

Dual beam (bias and probe) transient Surface Photovoltage (SPV) measurements were made on undoped Semi-Insulating (SI) GaAs over an extended temperature range. Above 270 K, SPV recovery transients following a bias pulse were shown to reflect near surface conductivity changes; these are in turn controlled by surface/interface state thermal emission. Owing to the absence of a strong surface electric field in this material, the emitted carriers are not immediately removed from the near surface region. The recapturing of the emitted carriers is shown to be responsible for non-exponential conductivity and reciprocal-SPV transients. This behavior is considered to be characteristic of relaxation-type semiconductors with near-surface ungated structures. Below 150 K, the photoinduced transition of EL2 from its ground to metastable state EL2& was shown to change the effective electron and hole mobilities and augment the SPV signals immediately following the bias pulse. Thermally induced EL2& recovery above 120 K decreases the SPV signal from its maximum. This decay transient was analyzed and the decay rate fitted to a single exponential. An activation energy of 0.32 eV and a pre-exponential constant of 1. 9× 1012 s−1 were obtained, and attributed to the thermal recovery rate for EL2&.

We have measured the photovoltaic spectra at 300K for a PIN GaAs/AlGaAs structure containing five coupled wells (50A/28A) grown by molecular beam epitaxy (MBE). The spectra were obtained in the energy range from 1.40 eV to 1.60 eV. This is the region in which optical transitions between the sub-band valence and conduction states are possible. Five direct optical transitions are allowed for this structure. These transitions are normally difficult to measure at room temperature because of broadening, nevertheless, some of the allowed transitions were observed from the photovoltaic spectra and agreed with calculations. We have previously shown that measurements made using electroreflectance (ER) agree with these results. However, with ER, three possible transitions were observed but only one with certainty, possibly because of interference caused by adjacent line spectra interaction. This interference appears to be less pronounced in the photovoltaic spectra, which aids in the identification of transitions.