This PDF file contains the front matter associated with SPIE Proceedings Volume XXXX, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Our efforts focus on developing a method to produce hydrogenated nanocrystalline silicon (nc-Si:H) with larger crystallites to enhance carrier transport properties. A new PECVD methodology, called double pulsed PECVD (DPPECVD), employs alternating low frequency and high frequency discharge sub-cycles to sequentially grow and etch the evolving film, respectively. This confers enhanced process control compared to conventional methods, and provides a pathway to achieve our goal of enhanced carrier mobility. Preliminary results demonstrate nc-Si:H films possessing grains as large as 29 nm, with (220) preferred orientation, which is suitable for solar cell applications. Reactions between plasma species in a SiF₄:H₂:SiH₄ glow discharge, which expectedly contribute to evolution of large grains, are also discussed. Our findings suggest the double pulse strategy is a valuable method for manipulating the microstructural evolution of PECVD grown thin film materials.

We present measured optical and electrical characteristics of guided-mode resonant hydrogenated amorphous silicon (a-Si:H) thin-film solar cells. Nano-patterns with a 300-nm period and a 50-nm grating depth are fabricated on glass substrates, followed by the deposition of a transparent conducting oxide layer as a top contact. About 320-nm thick p-i-n thin-film a-Si:H solar cell is deposited on indium-tin-oxide coated glass substrate, followed by the deposition of a bottom contact layer. Compared to a planar reference solar cell, ~40% integrated absorption enhancement is observed for the 450–750-nm wavelength range. Short circuit current density of the nano-patterned solar cell is obtained as 14.1mA/cm², which is ~50% improvement over the reference planar cell.

The plasma enhanced chemical vapor deposition technology (PECVD) is one of the commonly used deposition techniques for amorphous and microcrystalline silicon thin film and solar cell fabrication. The VHF-PECVD process developed at Dresden University of Technology enables the homogenous dynamic deposition of a-Si:H and μc-Si:H layers at high deposition rates. The most important features of this deposition system are linear plasma sources (500x100mm) operated at very high excitation frequencies (81.36 MHz – 140 MHz) in combination with moving substrates. The advantage of such plasma concept is the homogeneous deposition on large area substrates. The higher excitation frequency leads to an increase in electron density and to an enhancement of silane dissociation in the bulk plasma. Furthermore, the lower ion-bombardment energy reduces defect creation in the layers. In this work the influence of excitation frequencies (81.36 – 140 MHz) on the amorphous silicon deposition process will be analyzed. First, a study of the plasma homogeneity of a-Si:H was carried out. At the 140 MHz excitation frequency a homogeneous deposition of amorphous silicon with deposition rates of 1.56 nm/s has been achieved, compared to 0.66 nm/s at 81.36 MHz and the same process pressure. Furthermore, the influence of higher excitation frequencies on the material and structural properties of intrinsic layers has been analyzed and will be discussed. Finally, p-i-n solar cell structures with successful implemented intrinsic absorber layers deposited at higher excitation frequencies at different deposition parameters were fabricated and will be presented.

Low temperature plasma processes provide a toolbox for etching, texturing and deposition of a wide range of materials. Here we present a bottom up approach to grow epitaxial crystalline silicon films (epi-Si) by standard RFPECVD at temperatures below 200°C. Booth structural and electronic properties of the epitaxial layers are investigated. Proof of high crystalline quality is deduced from spectroscopic ellipsometry and HRTEM measurements. Moreover, we build heterojunction solar cells with intrinsic epitaxial absorber thickness in the range of a few microns, grown at 175°C on highly doped (100) substrates, in the wafer equivalent approach. Achievement of a fill factor as high as 80 % is a proof that excellent quality of epitaxial layers can be produced at such low temperatures. While 8.5 % conversion efficiency has already been achieved for a 3.4 μm epitaxial silicon absorber, the possibility of reaching 15 % conversion efficiency with few microns epi-Si is discussed based on a detailed opto-electrical modeling of current devices.

A unique aspect of the photovoltaic (PV) business is that the products being manufactured have such a long operating life. PV modules are being guaranteed to operate with limited performance degradation for between 10 and 25 years meaning that even the most mundane elements of the product must, obviously, last beyond that life span.

We report investigation of SnS van der Waals epitaxies (vdWEs) grown by molecular beam epitaxy (MBE) technique. Experimental results demonstrate an indirect bandgap of ~1 eV and a direct bandgap of ~1.25 eV. Substantial improvement in the crystallinity for the SnS thin films is accomplished by using graphene as the buffer layer. Using this novel growth technique we observed significant lowering in the rocking curve FWHM of the SnS films. Crystallite size in the range of 2-3 μm is observed which represents a significant improvement over the existing results. The absorption coefficient, α, is found to be of the order of 10⁴ cm^-1 which demonstrates sharp cutoff as a function of energy for films grown using graphene buffer layers indicating low concentration of localized states in the bandgap. Hole mobility as high as 81 cm²V^-1s^-1 is observed for SnS films on graphene/GaAs(100) substrates. The improvements in the physical properties of the films are attributed to the unique layered structure and chemically saturated bonds at the SnS/graphene interface. As a result, the interaction between the SnS thin films and the graphene buffer layer is dominated by a weak vdW force and structural defects at the interface, such as dangling bonds or dislocations, are substantially reduced.

Depending on the semiconductor material, the luminescence lifetime of semiconductor wafers can vary over a broad range from microseconds for Si-wafers down to sub-nanoseconds for III/V and II/VI based thin film or organic materials. The lifetime of a given wafer sample depends on the free charge carrier dynamics and can therefore be affected by several parameters. An important example is the influence of bulk or surface defects [1], thus the lifetime is a possible indicator for wafer quality. On dye-sensitized solar cells, lifetime measurements are also useful to characterize the energy transfer process from the sensitizer to the conduction band [2]. We have developed a setup for time-resolved photoluminescence measurements (TRPL) based on pulsed diode lasers and time-correlated single photon counting (TCSPC) with highly sensitive single photon detectors. Depending on the detector type, the instrument response function (IRF) can be as short as 100 ps and the laser pulse rate can be adapted to the luminescence lifetime of the material. The resolvable lifetimes extend from approx. 50 ps up to several hundred microseconds. The electronics can also be integrated into a microscope based setup for imaging with a lateral resolution down to the sub-μm range [3] as well as testing the lifetime behaviour at different injection levels. We will show measurement results of the system on an GaAsP-based Quantum Well.

Variable temperature Hall measurements are extremely important when investigating the electronic transport properties of materials. Materials of interest for solar cell applications are typically characterized by very small charge carrier mobilities that are difficult, if not impossible to measure using traditional DC field techniques^1-3. For these materials special techniques like AC field Hall are required to reliably measure the small Hall voltages and low mobilities.

ZnSe thin films were prepared by, simple low cost, closed space sublimation method (CSS). The silver doping was achieved by ion exchange process, i.e. immersing the films in low concentrated silver nitrate solution for different time periods and flowed by heated treatment in vacuum. The effect of silver concentration on the optical properties , such as refractive index, absorption coefficient and optical band gap, have been calculated from the normal transmission spectra in UV, Visible and NIR region. The structure of the films was studied by X-ray diffraction. The EDS attached to SEM was used to determine the composition of the films. The electrical resistivity, at room temperature, was also measured and it was reduced considerably as silver concentration increased.

In recent years, resumed intensive studies of heterojunctions, in connection with the possibility of using them on the most economical and relatively efficient photodetectors. In this economy is determined by the structure of production technology and the value of the source material. Thus there is a substitution reaction on the surface of the films Cd_1-xZn_xS formed a second layer of Cu₂S, whose thickness is determined by the time of deposition. When light 1,45⋅10⁴ lux photocells studied Cu₂S-Cd_1-xZn_xS and Cu₂Se-Cd_1-xZn_xSe generated voltage 0,5÷0,6V and in 0,45-0,58V, current J_sc 15 ÷ 20 mA/sm² and 12-15mA/cm² and efficiency were = 11%. and 8%. respectively. It is established that with increasing content of Zn in the base material-circuit voltage U_os photocell increases and short-circuit current decreases. Using as a base material of solid solutions of Cd_1-xZn_xS and Cd_1-xZn_xSe causes an increase in the potential barrier at the contact.

Cu(In_1-x,Ga_x)S₂ was studied using photoreflectance spectroscopy. In this study, efforts are devoted to optimizing PR set-up for measuring CIGS grown by electrodeposition: issues such as photoluminescence perturbation, high roughness and scattering are addressed. Dual frequency photoreflectance, where both probe and pump beams are modulated, is proposed here to over come the poor signal to noise ratio. Considering the low electric field regime, material parameters are extracted by employing the third derivative functional form of dielectric functions to fit data. The reliability of the technique is finally tested by measuring PR spectra on a specific 15 x 15 cm² wafer and explanations of PR line-shape evolution on this wafer are discussed.

Thin film samples of (Cu,Ga)InSe₂ (CIGS) were prepared by DC magnetron sputtering and the selenisation process onto soda lime glass substrates. All samples had the same deposition conditions, and the optimal sputtering thickness of samples with one CuGa/In pair and two CuGa/In pairs are also the same. After sample deposition, X-ray diffraction (XRD), scanning electron microscope (SEM) and Hall effect measurements were used to characterize the properties of these samples. From XRD measurement results, excepting an extra small CuSe peak existing in the samples with two CuGa/In pairs, the XRD peaks of all samples are perfectly matched with the phase diagram of CuGa_0.3In_0.7Se₂ material. It was also found that the grain sizes of the samples with one CuGa/In pair are larger than those with two CuGa/In pairs from SEM images. All these observations on samples with two CuGa/In pairs can be attributed to the fact that the less In incorporation in CIGS films, which it has been proven that the sample with low In-to-CuGa ratio has stronger CuSe peak from XRD result. Furthermore, the p-type carrier characteristics can be observed for all samples from Hall measurement results. The carrier mobility and concentration of the samples with one CuGa/In pair can be achieved as high as 15.28 cm²/Vs and as low as 1.50×10¹⁶ cm^-3, respectively, while the carrier mobility and concentration of the ones with two CuGa/In pairs can be achieved as 6.4 cm²/Vs and 6.27×10¹⁷ cm^-3, respectively. The results of superior electrical properties of samples with one CuGa/In pair agree well with the observations form XRD and SEM results. In the final, the optimal value of In-to-CuGa ratio during CuGa/In layers deposition in this study is 0.625.

An overview of different transparent conductors is given. In addition, atmospheric pressure CVD of ZnO resulted in conductivities below 1 mΩ cm for a temperature of 480°C, whereas at a process temperature of 200°C a value of 2 mΩ cm was obtained. Also atmospheric pressure spatial ALD was used to make conductive ZnO. Furthermore, the properties of transparent conductive oxides (TCO) can be enhanced by application of metallic grids. This way, sheet resistances of below 0.1 Ω/sq and transmittances above 85 % can be achieved. Modeling indicates that the performance of thin film cells can be enhanced by18% using a grid/TCO combination. Light scattering is a vital element of thin film solar cells and both texturization and multimaterial approaches for advanced light management such as plasmonics are discussed.

For silicon rich oxide (SRO) thin films prepared by sputtering deposition, we have found a trustable structure model to describe the optical properties in the whole region of wavelength. In the process of optical characterization the ellipsometer measures the phase change (Psi and Delta) of light reflected from the thin films, which are then fitted with appropriate structural models using Ellipsometry analysis software WVASE32. We have found that the a-Si/SiO single-layer model can give accurate matches between calculated results and experimental results for the whole measured spectrum. The transmittance data generated from the single-layer model is consistent with the experimental results measured by spectrophotometer. Finally the model is applied to predict the optical behavior of multilayer samples, reasons are sought to explain the difference between the calculated data and the experimental data.

Thin film solar cells are novel technologies receiving particular attention for their potentials to produce less expensive and more environmentally friendly renewable electricity from the sun light. This paper reports on different techniques that have been considered to improve the light harvesting, from the plasmonic resonance of noble metal nanoparticles, to the nanoparticles of a different semiconductor with different refractive index and band gap, to the anti-reflection surface textures as moth-eye-like shapes. Results of novel simulations solving the Maxwell’s equations are compared to previous simulations.