Thin films of cadmium telluride (CdTe) electrodeposited from aqueous solutions have been used as the absorber in solar cells of the type glass/conducting transparent oxide/n-type window layer/p-type absorber layer/back contact. The window layer for these cells has been Cd_xZn_1-xS (usually x = 1) made by a variety of means eg. compound evaporation, spray pyrolysis, electroless deposition, electrodeposition from aqueous and aprotic solvents using either direct or periodic pulse potentials. Recent results have demonstrated the consequences of using various conducting transparent oxides as both the front contact and the window layer. In this paper, the preparation and properties of the thin films and some cells fabricated from them are presented to illustrate that efficient solar cells can be made with a variety of window layers.

Based on weak bond dangling bond conversion, a complete quantitative solution for the distribution of gap states in a-Si:H in equilibrium and general nonequilibrium conditions can be derived. In this picture the density of states distribution, apart from the structural equilibration between Si-Si-bonds, Si-H-bonds, and Si-dangling bonds and a Gaussian spread of available defect energies, is completely governed by the concentrations of free carriers and the resulting electronic occupation functions in the gap. The model is shown to account for the vast majority of experimental data regarding equilibrium and so called saturated defect densities, such as their dependence on temperature, illumination intensity, band gap, hydrogen content and slope of band tails. Moreover experimental data of photoconductivity, μr-products of electrons and holes versus temperature and light intensity as well as versus position of Fermi level in intrinsic and moderately doped a-Si:H films have been predicted more accurately than by any standard defect model.

CuInS₂ single crystaLs of moderate size have been produced by the gradient freeze technique under different sulfur pressures. Results from EDX show that with higher sulfur pressures the cracking along the ingot decreases, but at 2 bar these cracks are still present. XRD shows that CuS is present at these cracks and is due to the loss of In₂S into the gas phase. Despite this loss of In₂S PL reveals the material to be In-rich.

Though second phases consisting of Cu_2-xSe or In₂O₃ are known to occur in CuInSe₂ deposited by various methods and on a variety of substrates, establishing whether these second phases are present in CdS/CuInSe₂ heterojunction solar cells has been complicated by the additional material layers in the solar cells. Hence the effect of second phases on the electrical properties and device performance has not been specified in detail in previous studies of this structure. In this paper the authors investigate the use of x-ray diffraction to determine whether second phases are present in CdS/CuInSe₂ solar cells prepared by chemical spray pyrolysis. On phase in the CuInSe₂ of the solar cells. An attempt to remove Cu_2-xSe by chemically selective etches failed, perhaps because the diffraction peak attributed to Cu_2-xSe actually had a different cause. Future studies will attempt to determine this cause. If a second phase is the cause, both removing it or preventing its formation will be studied in an effort to determine its effect on solar cell efficiency.

Spray pyrolysis was used to fabricate n-Si(1OO)/SiO₂/SnO₂ heterojunctions in view of photoconversion of solar energy. The effects of the temperature T_f at which the Sn0₂ film is formed (400-500°C) are particularly addressed. Direct imaging of the interfacial Si0₂ layer by HRTEM is presented, showing the relatively abrupt character of the interfaces. It is shown that (i) the open-circuit photopotential V_oc strongly decreases when T_f is incresased (ii) at a given T_f, V_oc presents a maximum as a function of the interfacial Si0₂ thickness. These results are discussed in terms of charge accumulation and current limitation at the Si/Si0₂ boundary.

In this paper the operation principles of a new folded ultra-thin-layer solar cell are being discussed. The cell was introduced recently by Graetzel and coworkers. The relationship between photocurrent and luminescence in this cell is derived for the simplest kinetic scheme appropriate for this type of solar cell. It involves three stages, i.e. four levels. In our simpel model these stages are connected by rate constants. We show picosecond time-resolved measurements of the luminescence decay curve and of the luminescence spectrum of the triplet state of the adsorbed trinuclear-Ruthenium dye molecules. Picosecond time-resolution of these signals is essential for distinguishing between relevant and irrelevant luminescence signals emitted from the cell. Moderately fast electron injection from the triplet state of this dye with 172 ps time constant yields very efficient conversion of absorbed photons to injected electrons. The time-response of the photocurrent is determined by the filling and emptying of traps in the depletion layer. We discuss the potential of this cell for the photovoltaic solar energy conversion.

A connection is made between the luminescence or radiative recombination in an absorber material and the current voltage characteristics of a quantum converter of light. A relationship between luminescence and voltage is derived, using detailed balance and the chemical potential of the excitation, which is similar to that obtained using the techniques of Shockley and Queisser or R. T. Ross. This model relates the absorptivity and photoluminescence efficiency of the light absorber to the I V curve. In this way both thermodynamic properties, or voltage, and the kinetics, or charge transfer and current, can be combined in order to optimize materials and configurations. The model is applied to dye sensitized Ti0₂ solar cells, and compared with preliminary experimental data for Ru based charge transfer dyes and inorganic compounds. The luminescence model is found to be applicable to dye sensitized converters, as well as to standard silicon solar cells, light detectors, and LEDs.

Optical coatings are used to increase the efficiency, extend the life, improve the electrophysical characteristics and stability of solar energy converters based on various physical principles, including semiconductor solar cells. When solar cells are placed on the exterior of collectors in photothermal systems, and generate both electric and thermal power, the optical coating applied to their surfaces gives them highly unusual selective properties, namely, reduced reflection of solar radiation (and high transparency in this part of the spectrum), which leads to and higher integrated solar absorption coefficient, enhanced infrared reflection which ensures that the thermal emission coefficient is as low as possible. Solar cells then not only generate electric power but, at the same time, covered by these coatings, act as selective optical surfaces for solar collectors.

Thin polycrystalline films of iron disulfide have been grown on different substrates by chemical vapour deposition. The films were characterized using optical absorption and TEM. RBS and EDAX analysis has been used to explore the chemical stoichiometry. XRD and FTIR allowed the identification of both FeS₂ phases pyrite and marcasite. A novel method for sensitization of highly porous Ti0₂ elecrodes with ultra thin (10-20 nm) polycrystalline films of FeS₂ (pyrite) is presented. Photoelectrochemical solar cell using the above electrode generated high photovoltage of up to 600mV compared with single crystalline electrode (200 mV). In this device the semiconductor with a small band gap and high absorption coefficient (FeS₂ pyrite; E_G = 0.9 eV; a = 6 x 10⁵ cm^-1) absorbs the light and injects electrons into the conduction band the wide band gap semiconductor (Ti0₂ anatase; E_G = 3.2 eV). Regeneration of holes is taking place by electron transfer from redox system in the electrolyte.

Photovoltaic devices based on sprayed films of Al doped CdS and electrolytically deposited poly(3-methylthiophene) (PMeT) yield short circuit current densities and open circuit voltages in the range 2 - 3 mA cm^-2 and 0.3 - 0.4 V in white light. Synthesis parameters for both CdS and PMeT do control the physical properties of the films and the performance of the cell.

Photoeffects in phthalocyanine and perylene thin films and phthalocyanine/perylene hetero- p/n- and perylene/Au Schottky-cells were investigated in wavelength, temperature, and intensity dependent measurements. It is concluded that the charge carrier generation occurs only in a small region near or at a p/n- or Schottky type contact whereas the photoconductivity is a true bulk effect. Both types of photoeffects experience their own recombination processes leading to different intensity dependences.

Iron pyrite thin films have been obtained by flash evaporation. Properties of the films are modified by annealing them in a sulfur atmosphere. Here we present some results on the influence of the annealing temperature (250 degree(s)C <EQ T_s <EQ 450 degree(s)C) on the optical properties of two groups of samples with thickness approximately equals 0.35 micrometers and approximately equals 1.00 micrometers , respectively. Sulfuration temperature has a clear influence on the optical absorption of the films. Two maxima at energies approximately equals 4.0 eV are observed in the reflection spectra, similar to those found with single crystals but the peaks intensities vary with T_s. Results are discussed by considering the enhanced stoichiometry and crystallinity of the films.

In this contribution the material properties of polycrystalline (Cu,Ag)(In,Ga)(S,Se)₂ thin films and their suitability for heterojunction solar cells are discussed. The copperchalcogenides play an important role in the formation of these films. However, a deteriorating impact of these secondary phases on the cell performance exists. Methods to overcome this dilemma are presented. The role of the morphology and of the chemistry of grain boundaries is illustrated in the CuGaSe₂ and CuIn(S,Se)₂ system. Important parameters for the design of a heterojunction solar cell are discussed. During the course of this work solar cells with efficiencies exceeding 14% could be obtained.

Acidic aqueous electrolyte/semiconductor contacts have been used to measure basic material properties of thin film polycrystalline CuGaSe₂ and to modify its composition (growth of a Zn containing interface layer or removal of excess Cu₂Se). Although simple models are applied to a complex situation for the characterization, the comparison of different techniques (capacitance and photocurrent collection) and trends versus film composition can enlighten our view of the properties of the semiconducting films. Interfacial chemical modifications, with excellent control, can be performed with this kind of approach and represent an alternative to pure chemical bath deposition for the buffer layers involved in the CuGaSe₂/Buffer/ZnO devices, as well as an alternative to the cyanide etching of the undesirable Cu₂Se from the Cu-rich CuGaSe₂ films, that posses otherwise high quality properties. A first feedback onto solid state devices is investigated and results can be considered as promising.

Thin film solar cells require a structural control in nanometer dimensions. The only techniques currently available to investigate and control thin film preparation techniques in real space are based on scanning probe techniques as developed by Rohrer and Binning in 1981. However, the investigation of surface roughness on the ten to hundred nanometer range has proven to be especially complicated due to the convolution of tip and surface structures depending on the relative size dimensions of both the tip and the surface features. Special care is necessary to avoid multiple tip imaging as the major source of errors. Examples given in this paper include investigations of nanometer clusters of metals as used for semiconductor surface modifications in photoelectrochemical solar cells and of amorphous hydrocarbon films investigated as possible new materials for photovoltaics or as selective absorbers for solar thermal applications. More detailed investigations of molecular thin films for organic solar cells are presented. Here, besides information about the film structure, crystal growth mechanisms of organic crystalilites prepared by evaporation techniques also were investigated using scanning tunneling microscopy (STM) and scanning electron microscopy (SEM) techniques.

An efficient maximum power point tracker (MPPT) has been developed and can be used with a photovoltaic (PV) array and a load which requires lower voltage than the PV array voltage to be operated. The MPPT makes the PV array to operate at maximum power point (MPP) under all insolation and temperature, which ensures the maximum amount of available PV power to be delivered to the load. The performance of the MPPT has been studied under different insolation levels.

Porous, polycristalline TiO₂ and Nb₂O₅ photoelectrodes were prepared by a sol- gel process. Small CdS and PbS particles were then deposited onto the electrodes by a wet- chemical method. The particles had diameters of a few nm and showed strong quantization effects. Photoelectrochemical measurements showed a very efficient sensitization of the oxide substrates by the semiconductor particles. The investigation showed that electrons were injected from the conduction band of the photoexcited semiconductor particles.

Fine powders and sublimed films (15 micrometers thick) of pure chloroaluminum phthalocyanine (ClAlPc) undergo both, chemical reaction and structural modification, when immersed in aqueous solutions containing anions (I₃^-/I^- or Br^- at various acid pH values). The x-ray diffraction diagrams of powder and films of untreated ClAlPc exhibit a strong characteristic peak corresponding to a d-spacing of 3.29 angstrom. A 48 h. immersion of ClAlPc (powder or film) resulted in the appearance of a new peak at 3.45 angstrom in the diffraction diagram. The peak intensity at d equals 3.45 angstrom is only 11% (powder) or 47% (film) of the main peak intensity at d equals 3.29 angstrom. It clearly shows that the crystal structure of ClAlPc has only been partially modified in both cases. However, the modification is more complete for the film than for the powder. Chemical analyses by neutron activation were performed on fine powders of ClAlPc before and after immersion. For instance ClAlPc absorbs 7.6% by weight of iodine after 24 h of immersion in I₃^-/I^- (0.005 M/0.4 M) redox electrolyte at pH 1.0. The decrease in the chlorine content of untreated ClAlPc from 6.1% to 3.7% after immersion could be explained in terms of an hydrolysis reaction of some ClAlPc. These results lead us to a model where by the surface of the ClAlPc crystallites would be hydrolyzed to HOAlPc and heavily doped with anions taken up from the aqueous solution. These transformations improve the photoactivity of the material.

The CdS(Se)-electrolyte interface provides a relatively efficient means of converting optical energy into electricity. A [Cd(Fe(CN)₆]^2-/1- overlayer is demonstrated to stabilize these semiconductors against photodecomposition processes. In the presence of such cyanometalate layers, the kinetics of interfacial charge transfer are greatly dependent on the nature of intercalated alkali cations. These cations interact not only with the overlayer but also with the semiconductor itself. The effect of specific adsorption by soft cations such as Cs⁺ and Hg²⁺ has also been investigated. n- CdS/[Cd(Fe(CN)₆]^2-/1- interfaces allow for the addition of electroactive species to the electrolyte which provide opportunities to generate useful products using solar energy.

We discuss here our most recent results with the characterization of epitaxial deposits of various phthalolcyanine dyes formed by vacuum deposition (O/I-MBE) or solution deposition on the surface of metal dichalcogenide semiconductors, such as SnS₂. Surface electron diffraction techniques used during the vacuum deposition process help to verify the type and extent of long range ordering of these dyes. SnS₂ semiconductor substrates allow for the photoelectrochemical characterization of the dye layers, starting with the deposition of submonolayer amounts of material. High quantum yields per absorbed photon are seen for ultrathin films of InPc-Cl, VOPc, and CuPc on SnS₂, and the photocurrent spectra suggest similar ordering at the monolayer level, even though multilayer structures are quite different. Ordered Pc thin films are also obtained for a new class of liquid crystalline phthalocyanines (LC-Pc), where the hydrocarbon side chains are attached to the Pc ring by amide linkages. Deposition of ultrathin films of these materials produces photocurrent spectra which are quite similar to those obtained for low coverages of the vacuum deposited Pc. Photocurrent spectra on SnS₂ show that the first monolayer of material may have a completely different surface structure than the bulk of the multilayer LC-Pc thin film. The nature of dye/dye' interfaces and their effect on exciton dissociation events has also been explored using vacuum deposited materials. Superlattices of Pcs were formed by vacuum deposition, where the active dye was sandwiched between various spacer molecules with thicknesses down to a few molecular layers. Transient photocurrent yield spectra from such assemblies suggests that exciton dissociation events in such materials can be confined to within a few molecular layers of the dye/dye' interface.

In our work with the phthalocyanines and perylenes, we have formulated a hierarchy of placement of dyes in p-n heterojunction devices to optimize the short circuit current density. Computer modeling of Schottky barrier cells, with parameters fit to experimental results and incorporating field-dependent carrier generation, were used to optimize the power efficiency. The model predicts an optimum carrier concentration density and suggests different hierarchies for utilization of Foerster radiationless energy transfer. Synthesis and purification of materials is also discussed. In terms of purity, most materials used in the literature are shown to have been quite below solar grade. A newly devised purification technique is introduced. A hydration mechanism is shown to exist for chloroaluminum phthalocyanine, previously thought immune to hydration. The latter mechanism had been mistaken before for a simple phase transformation and can be induced by various different treatments with organic solvents in which chloroaluminum phthalocyanine is not soluble. Testing of p-n and Schottky barrier cells is also discussed. The different capacitance vs. voltage (C-V) spectroscopies are compared, and the case for the small signal method is argued over the triangular voltage sweep. Several cautions on the interpretation of the C-V curves are noted.

ClAlPc thin films were grown on SnO₂ substrates at various sublimation rates ranging from 100 to 10,000 angstroms min^-1. ClAlPc films were also grown at 2000 angstroms min^-1 and heat treated at 300 degree(s)C for various periods of time going from 1 to 60 min. All films were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoelectrochemistry in I₃^-/I^- at pH 3. All sublimed ClAlPc films were porous. The sublimation rate only affects the diameter of the rod shaped `crystallites' characterizing the morphology of the material. The short-circuit photocurrent increases with the sublimation rate. It is in direct relation with the total area of the solid-electrolyte interface available for the collection of the photogenerated charges. Selected area diffraction patterns of all as-sublimed films display the characteristic triclinic structure of ClAlPc single crystal. The polycrystalline phase which is responsible for the diffraction pattern is only a minor component of the solid phase. The major part of the film is mostly amorphous. In contact with I₃^-/I^-, there is an uptake of iodine into both amorphous and polycrystalline regions of as-sublimed films. A short (1 min) heat treatment of as-sublimed ClAlPc only alters the structure of the crystalline region of ClAlPc. The new structure is also triclinic but is impermeable to iodine uptake. Longer heat treatments are required to transform part of the amorphous phase of the film into the new triclinic structure. In I₃^-/I^-, the new triclinic structure of ClAlPc is about one order of magnitude less photoactive than as-sublimed ClAlPc.

Photoreduction of CO₂ to formic acid (HCO₂^-) and a small quantity of carbon monoxide (CO) can be achieved in N,N-dimethylformamide by using oligo(p- phenylenes) (OPP-n) as a photocatalyst and triethylamine (TEA) as a sacrificial electron donor under > 290 nm irradiation. Among OPP-n, p-terphenyl (OPP-3) and p-quaterphenyl (OPP-4) show high photocatalytic activity for the formation of HCO₂^-, in which the apparent quantum yields of HCO₂^- formation for OPP-3 and OPP-4 are 0.072 and 0.084, respectively. The laser flash photolysis and pulse radiolysis studies reveal that the photocatalysis initially start from the reductive quenching of the singlet state of OPP-n by TEA followed by the formation of the radical anion of OPP-n, OPP-n^-, resulting in direct electron transfer from OPP-n^- to CO₂ molecules.

Photoelectrochemical solar cells can be made on the basis of majority and minority carrier devices. Since the forward dark current limits the photovoltage, the kinetics of the charge transfer across the interface has been studied in detail. It is shown that the exchange current, and consequently the forward current, can be kinetically or diffusion controlled, depending on the charge transfer rate at the interface. In the case of a majority carrier device, the forward current is usually much smaller than that predicted from the thermionic emission model, leading to higher photovoltages in a photoelectrochemical cell. It is further shown that the forward current of a minority carrier device is mainly governed by the injection and recombination of minority carriers. Here the conversion efficiency is limited by the quality of the semiconductor, similarly as in pure solid state devices.

Very sensitive near infrared (NIR) emission and visible/NIR cw dye laser based transient absorption techniques are employed to explore elementary steps of redox reactions at semiconductor materials. The focus is on chemistry relevant to the conversion of abundant long wavelength solar photons (red and NIR) into electricity in photoelectrochemical cells. Direct reduction of excited O₂(¹(Delta) ), a long-lived NIR energy carrier, has been demonstrated at a silicon electrode, opening up the possibility of direct conversion of the chemically stored 1 eV quantum of O₂(¹(Delta) ) into electrical energy in a singlet O₂ driven regenerative cell. In a parallel direction, the rise of the one electron oxidation intermediate I₂- produced upon photo-oxidation of iodide at dye sensitized as well as bare TiO₂ colloidal particles has been observed for the first time by monitoring the ²(pi) _g $IMP ²(Sigma) _u⁺ absorption in the 700 - 800 nm region. Elucidation of the detailed steps of halide-to-halogen oxidation at the semiconductor/solution interface is important for progress in NIR photon energy storage and conversion as the intrinsically modest driving forces of such processes require careful optimization of the efficiency of each reaction step.

The structure of multilayered TiO₂ films formed by nano-structured colloidal particles was studied by scanning electron microscopy, x-ray powder diffraction, and other methods. The structure of the surface and inner layers of the films are discussed in relation to the methods of preparation and subsequent treatment of the films.