Raman scattering analysis of SiH bond stretching modes in hydrogenated microcrystalline silicon for use in thin-film photovoltaics

The presence of a pair of peaks in the high wavenumber infrared (IR) absorption region of hydrogenated microcrystalline silicon (μc-Si:H) has been recently proposed as a strong indicator of poor quality material that is prone to oxidation and is therefore unsuitable for thin-film, photovoltaic applications. In this work, we show that these peaks located at 2083 and 2100 cm⁻¹ are also present in the Raman scattering spectra of μc-Si:H and therefore can be directly measured on substrates that are suitable for solar cells. We present results for material grown by matrix-distributed electron-cyclotron resonance (MD-ECR) plasma-enhanced chemical vapour deposition (PECVD) on both crystalline silicon and borosilicate glass substrates. The narrow, twinned peaks detected by Raman disappear with time—presumably due to oxidation—although a broad peak at 2100 cm⁻¹ remains.     

Polycrystalline silicon film deposited by ICP-CVD

We studied the deposition of polycrystalline silicon (poly-Si) using SiH₄/SiH₂Cl₂/H₂ mixtures by inductively coupled plasma chemical vapor deposition. The deposition rate and crystalline quality were improved by increasing RF power. The poly-Si film deposited with the [SiH₂Cl₂]/[SiH₄] ratio of 2 and the RF power of 1500 W exhibited the deposition rate of 4.2 Å/s, the polycrystalline volume fraction of 88%, the Raman FWHM of 7 cm⁻¹, and the TEM grain size of 1200 Å. The solar cell made of this material exhibited a conversion efficiency of 3.14%.     

Performance improvements of crystalline silicon by iterative gettering process for short duration and with the use of porous silicon as sacrificial layer

In this work, we demonstrate that an efficient purification method of silicon wafers where iterative sequences were used. Each sequence consists of forming porous silicon (PS) on both sides of the samples, followed by thermal annealing in an infrared furnace under N₂/SiCl₄ ambient. Improvements of the electronic parameters were obtained by optimizing the heat treatments temperatures and the number and duration of the iteration sequences. Best results were obtained for temperatures below 980 °C and for three sequences of 20 min each one. After three sequences the mobility of the majority carrier improved from 94 cm² V⁻¹ s⁻¹ (for untreated wafer) to about 374 cm² V⁻¹ s⁻¹. The observed results were explained taking into account the transport properties of the impurities in the porous media and their concentration at the walls at each iteration. It was found that short iterative sequences give almost the same results than one long sequence duration. Silicon solar cells based on iterative gettered silicon wafers exhibit an increase in the short-circuit current and the open-circuit voltage. This fact seems to be important to ameliorate solar grade silicon (SGS) based solar cells performances.     

Epi-n-IZO thin films/1 0 0 Si, GaAs and InP by L-MBE––a novel feasibility study for SIS type solar cells

High quality epitaxial indium zinc oxide (heavily indium oxide doped) (epi-n-IZO) thin films were optimized by laser-molecular beam epitaxy (L-MBE) i.e., pulsed laser deposition (PLD) technique for fabricating novel iso- and hetero-semiconductor–insulator–semiconductor (SIS) type solar cells using Johnson Matthey “specpure”- grade 90% In₂O₃ mixed 10% ZnO (as commercial indium tin oxide (ITO) composition) pellets. The effects of substrate temperatures, substrates and heavy indium oxide incorporation on IZO thin film growth, opto-electronic properties with 1 0 0 silicon (Si), gallium arsenide (GaAs) and indium phosphide (InP) wafers were studied. As well as the feasibility of developing some novel models of iso- and hetero-SIS type solar cells using epi-IZO thin films as transparent conducting oxides (TCOs) and 1 0 0 oriented Si, GaAs and InP wafers as base substrates was also studied simultaneously. The optimized films were highly oriented, uniform, single crystalline approachment, nano-crystalline, anti-reflective (AR) and epitaxially lattice matched with 1 0 0 Si, GaAs and InP wafers without any buffer layers. The optical transmission T (max) 95% is broader and absolute rivals that of other TCOs such as ITO. The highest conductivity observed is σ=0.47×10³ Ω⁻¹ cm⁻¹ (n-type), carrier density n=0.168×10²⁰ cm⁻³ and mobility μ=123 cm²/V s. From opto-electronic characterizations, the solar cell characteristics and feasibilities of fabricating respective epi-n-TCO/1 0 0 wafer SIS type solar cells were confirmed. Also, the essential parameters of these cells were calculated and tabulated. We hope that these data be helpful either as a scientific or technical basis in semiconductor processing.     

A polymer gel electrolyte composed of a poly(ethylene oxide) copolymer and the influence of its composition on the dynamics and performance of dye-sensitized solar cells

A polymer gel electrolyte composed of a poly(ethylene oxide) derivative, poly(ethylene oxide-co-2-(2-methoxyethoxy) ethyl glycidyl ether), mixed with gamma-butyrolactone (GBL), LiI and I₂ is employed in dye sensitized solar cells (DSSC). The electrolyte is characterized by conductivity experiments, Raman spectroscopy and thermal analysis. The influence of the electrolyte composition on the kinetics of DSSC is also investigated by transient absorption spectroscopy (TAS). The electrolyte containing 70 wt.% of GBL and 20 wt.% of LiI presents the highest conductivity (1.9 × 10⁻³ S cm⁻¹). An efficiency of 4.4% is achieved using this composition. The increase in I_SC as a function of GBL can be attributed an increase in the mobility of the iodide (polyiodide) species. The increase in the yield of the intermediate species, I₂⁻, originating in the regeneration reaction, is confirmed by TAS. However, the charge recombination process is faster at this composition and a decrease in the V_oc is observed. Photovoltage decay experiments confirm an acceleration in charge recombination for the DSSC assembled with the electrolyte containing more GBL. Raman investigations show that in this electrolyte the I₅⁻/I₃⁻ ratio is higher. Theoretical calculations also indicate that the I₅⁻ species is a better electron acceptor.     

Structural properties of phosphorous-doped polycrystalline silicon

The structural properties and hydrogen bonding of undoped and phosphorous doped polycrystalline silicon produced by step-by-step laser dehydrogenation and crystallization technique were investigated using Raman spectroscopy and hydrogen effusion measurements. At low laser fluences, E_L, a two-layer system is created. This is accompanied by the change in hydrogen bonding. The intensity of the Si–H vibration mode at 2000 decreases faster than the one at 2100 cm⁻¹. This is even more pronounced in phosphorous-doped specimens. The laser crystallization results in an increase of the hydrogen binding energy by approximately 0.2–0.3 eV compared to the amorphous starting materials.     

Interface characterisation of ZnSe/CuGaSe2 heterojunction

The ZnSe/CuGaSe₂ heterojunctions were fabricated by flash evaporation technique of CuGaSe₂ onto the (110) surface of ZnSe crystals. CuGaSe₂ layers had thickness 2–4 μm and showed a hole concentration up to (1.5–18.0)×10¹⁸ cm⁻³ and mobility μ4–24 cm² V⁻¹ s⁻¹ at 300 K. The charge carrier concentration in ZnSe crystals at 300 K was n=5.6×10¹⁶ cm⁻³ and their mobility μ=300 cm² V⁻¹ s⁻¹. The investigated ZnSe/CuGaSe₂ heterojunctions have at the interface an intermediate layer with a thickness of 450–750 Å and a linear graded band gap as well as an i-ZnSe compensated layer with a thickness of 1–2 μm and resistivity ρ10⁸–10⁹ Ω cm. The i-ZnSe layer is highly compensated due to the presence of Cu acceptor impurities. In this layer the Fermi level position E_c−F₀690 meV and a trap level position E_t−F₀17 meV were determined. The total trap concentration in the i-ZnSe layer is N_t5×10¹⁴ cm⁻³. The mean free path of excited charge carriers in the graded band gap region was calculated as λ55 Å. On the basis of experimental data analysis of electrophysical properties of both ZnSe/CuGaSe₂ heterojunctions and constituent materials the energetic band diagram of the investigated heterostructures is proposed. The current transport mechanism through ZnSe/CuGaSe₂ heterojunctions is consequently elucidated.     

Application of scanning mid-IR-laser microscopy for characterization of semiconductor materials for photovoltaics

Scanning mid-IR-laser microscopy was previously demonstrated as an effective tool for the characterization of different semiconductor crystals. Now the technique has been successfully applied for the investigation of CZ Si_xGe_1−x – a promising material for photovoltaics – and multicrystalline silicon for solar cells. In addition, this technique was shown to be appropriate for the imaging of polishing-induced defects as well as huge defects such as “pin holes”. Besides, previously unexplained “anomalous” (cubic power) dependence of signal of the scanning mid-IR-laser microscope in the optical-beam-induced light scattering mode on the photoexcitation power obtained for mechanically polished samples has now been attributed to the excess carrier scattering on charged linear defects, likely dislocation lines. The conclusion is made in the article that scanning mid-IR-laser microscopy may serve as a very effective tool for defect investigations in materials for modern photovoltaics.     

Promotion of microcrystallization by argon in moderately hydrogen diluted silane plasma

Using argon as a diluent of SiH₄, undoped hydrogenated microcrystalline silicon (μc-Si:H) films, having σ_D10⁻⁵ S cm⁻¹, were prepared at a very high deposition rate of 36 Å/min. Micrograins were identified with several well-defined crystallographic orientations. The effect of variation of Ar-dilution on the electrical and structural properties of Si:H films were studied systematically. Addition of H₂ to the Ar-diluted SiH₄ plasma improved the network structure by eliminating defects, introducing structural reorientation and grain growth, although, reducing the deposition rate. Accordingly, highly conducting (σ_D10⁻³S cm⁻¹) undoped μc-Si:H film was achieved utilizing energy released by de-excitation of metastable state of Ar (denoted as Ar*), in association with network modulation by atomic hydrogen in the plasma.     

Low-temperature deposition of polycrystalline silicon thin films by hot-wire CVD

Polycrystalline silicon films have been prepared by hot-wire chemical vapor deposition (HWCVD) at a relatively low substrate temperature of 430°C. The material properties have been optimized for photovoltaic applications by varying the hydrogen dilution of the silane feedstock gas, the gas pressure and the wire temperature. The optimized material has 95% crystalline volume fraction and an average grain size of 70 nm. The grains have a preferential orientation along the (2 2 0) direction. The optical band gap calculated from optical absorption by photothermal deflection spectroscopy (PDS) showed a value of 1.1 eV, equal to crystalline silicon. An activation energy of 0.54 eV for the electrical transport confirmed the intrinsic nature of the films. The material has a low dangling bond-defect density of 10¹⁷ cm³. A photo conductivity of 1.9 × 10⁻⁵ Ω⁻¹cm⁻¹ and a photoresponse (σ_ph/σ_d) of 1.4 × 10² were achieved. A high minority-carrier diffusion length of 334 nm as measured by the steady-state photocarrier grating technique (SSPG) and a large majority-carrier mobility-lifetime (μτ) product of 7.1 × 10⁻⁷cm²V⁻¹ from steady-state photoconductivity measurement ensure that the poly-Si : H films possess device quality. A single junction n---i---p cell made in the configuration n⁺-c-Si/i-poly-Si: H/p-μc-Si : H/ITO yielded 3.15% efficiency under 100 mW/cm² AM 1.5 illumination.     

Characterization of the 75% Gd0.8Sr0.2CoO3−δ/25% Ce0.9Gd0.1O2−δ composite cathode system for use in intermediate temperature solid oxide fuel cells

The composite cathode system is examined for suitability on a Ce_0.9Gd_0.1O_2−δ electrolyte based solid oxide fuel cell at intermediate temperatures (500–700 °C). The cathode is characterized for electronic conductivity and area specific charge transfer resistance. This cathode system is chosen for its excellent thermal expansion match to the electrolyte, its relatively high conductivity (115 S cm⁻¹ at 700 °C), and its low activation energy for oxygen reduction (99 kJ mol⁻¹). It is found that the decrease of sintering temperature of the composite cathode system produces a significant decrease in charge transfer resistances to as low as 0.25 Ω cm². The conductivity of the cathode systems is between 40 and 88 S cm⁻¹ for open porosities of 30–40%.     

Synthesis of CuInS2 by chemical route: optical characterization

CuInS₂ powder was prepared by wet chemical route. The chalcopyrite structure of the powder was revealed by XRD studies. Raman measurements of the powder sample indicated four prominent peaks at 292, 305, 340 and 472 cm⁻¹. The possible origin of the 305 cm⁻¹ peak was investigated and was found to be some local vibration in the structure. The peaks at 292 and 340 cm⁻¹ were ascribed to A₁ and B₂ modes, respectively. The peak at 472 cm⁻¹ which was due to the formation of SO₄⁻² ion at lower pH value of the precursor solution could be eliminated by using pH>11.0. Photoluminescence (PL) studies of the CuInS₂ powder indicated two distinct peaks at 1.49 and 1.42 eV. Post deposition annealing treatment in H₂ atmosphere revealed the formation of excess sulphur vacancy leading to the peak at 1.42 eV in the PL spectra while O₂ annealing of the powder created a deep defect level at 1.10 eV. Thick CuInS₂ films were prepared by Doctor's blade technique. Optical transmittance studies of these films indicated direct allowed transition at 1.5 eV.     

Reduction of defects of polycrystalline silicon thin films by heat treatment with high-pressure H2O vapor

An improvement of electrical properties of pulsed laser crystalllized silicon films was achieved by simple heat treatment with high-pressure H₂O vapor. The electrical conductivity of 7.4×10¹⁷ cm⁻³ phosphorus-doped 50-nm-thick pulsed laser crystallized silicon films was markedly increased from 1.6×10⁻⁵ S/cm (as crystallized) to 2 S/cm by heat treatment at 270°C for 3 h with 1.25×10⁶ Pa H₂O vapor because of reduction of density of defect states localized at grain boundaries. Spin density was reduced from 1.7×10¹⁸ cm⁻³ (as crystallized) to 1.2×10¹⁷ cm⁻³ by heat treatment at 310°C for 3 h with 1.25×10⁶ Pa H₂O vapor.     

Electrical conduction studies of hot wall deposited CdSexTe1−x thin films

CdSe_xTe_1−x thin films of different compositions have been deposited on cleaned glass substrates using the hot wall deposition technique under conditions very close to thermodynamical equilibrium with minimum loss of material. The electrical conductivity of the deposited films has been studied as a function of temperature. All the films showed a transition from phonon-assisted hopping conduction through the impurity band to grain-boundary-limited conduction in the conduction/valence band at temperature around 325 K. The conductivity has been found to vary with composition; it varied from 0.0027 to 0.0198 Ω⁻¹ cm⁻¹ when x changed from 0 to 1. The activation energies of the films of different compositions determined at 225 and 400 K have been observed to lie in the range 0.0031–0.0098 and 0.0285–0.0750 eV, respectively. The Hall-effect studies carried out on the deposited films revealed that the nature of conductivity (p or n-type) was dependent on film composition; films with composition x=0 and 0.15 have been found to be p-type and the ones with composition x=0.4, 0.6, 0.7, 0.85 and 1 have been observed to exhibit n-type conductivity. The carrier concentration has been determined and is of the order of 10¹⁷ cm⁻³. The majority of carrier mobilities of the films have been observed to vary from 0.032 to 0.183 cm² V⁻¹ s⁻¹ depending on film composition. The study of the mobility of the charge carriers with temperature in the range of 300–450 K showed that the mobility increased with power of temperature indicating that the type of scattering mechanism in the studied temperature range is the ionized impurity scattering mechanism.     

Performance of p-type silicon-oxide windows in amorphous silicon solar cell

a-SiO_x films have been prepared using silane and pure oxygen as reactive gases in plasma CVD system. Diborane was introduced as a doping gas to obtain p-type conduction silicon oxide. Infrared absorption spectra show the incorporation of Si–O stretch mode around 1000 cm⁻¹. The optical bandgap increases with the oxygen to silane gas ratio, while the electrical conductivity decreases. Hydrogenated amorphous silicon solar cells have been fabricated using p-type a-SiO_x with around 1.85 eV optical bandgap and conductivity greater than 10⁻⁷ S/cm. The measured current–voltage characteristics of the solar cells under 100 mW/cm² artificial light are V_oc=0.84 V, J_sc=14.7 mA/cm², FF=0.635 with a conversion efficiency of 7.84%.     

Nanocrystalline silicon film grown by LEPECVD for photovoltaic applications

This work deals with the characterization of nanocrystalline (nc) silicon films, grown using the plasma enhanced chemical vapour deposition (PECVD) process based on a low-voltage–high-current arc discharge plasma named LEPECVD (low-energy PECVD).The structural, electrical and chemical properties of the LEPECVD grown films have been studied as a function of the deposition parameters (substrate temperature, growth rate, silane dilution). The results show that the films consist of elongated nanocrystals along the 1 1 1direction, embedded in an amorphous matrix. The crystallite size along the 1 1 1 direction is in the range of 9-20 nm. The volume fraction of crystallinity (χ_c) varies between 51% and 78%, depending on preparation conditions. Conductivity values of the order of 10⁻⁶ Ω⁻¹ cm⁻¹ for the layers were measured, making the material suitable for the p–i–n junction application.     

The obstructed diffusion of the I3 ion in mesoscopic TiO2 membranes

The diffusional permeability of I⁻₃ ion in acetonitrile in free standing TiO₂ membrane with a porosity of 55% was examined. The apparent diffusion coefficient, D_app at 25°C of the ion was found to be 3.4×10⁻⁶ cm^{2 −1}, an order of magnitude smaller than the free diffusion at the same temperature. The temperature dependency of D_app was measured in the range 0–30°C and analysed in terms of the Walden product. The diffusional activation energy was found to be 13.5 kJ/mol. The parameters of interest for the efficiency of mesoscopic wet solar cells are discussed. A back of an envelope calculation shows that although the obstructed diffusion coefficient of the I⁻₃ ion was an order of magnitude smaller than the free diffusion the diffusional flux is still sufficient to meet a current density of 50 mA cm⁻². At incident photon flux of 1 kW m⁻² and at a photopotential of 0.6 V this would correspond to a solar energy efficiency of approximately 30%.     

Effect of 8 MeV electron irradiation on electrical properties of CuInSe2 thin films

Radiation damages due to 8 MeV electron irradiation in electrical properties of CuInSe₂ thin films have been investigated. The n-type CuInSe₂ films in which the carrier concentration was about 3×10¹⁶ cm⁻³, were epitaxially grown on a GaAs(0 0 1) substrate by RF diode sputtering. No significant change in the electrical properties was observed under the electron fluence <3×10¹⁶ e cm⁻². As the electron fluence exceeded 10¹⁷ e cm⁻², both the carrier concentration and Hall mobility slightly decreased. The carrier removal rate was estimated to be about 0.8 cm⁻¹, which is slightly lower than that of III–V compound materials.     

Towards high-efficiency thin-film silicon solar cells with the “micromorph” concept

Tandem solar cells with a microcrystalline silicon bottom cell (1 eV gap) and an amorphous-silicon top cell (1.7 eV gap) have recently been introduced by the authors; they were designated as “micromorph” tandem cells. As of now, stabilised efficiencies of 11.2% have been achieved for micromorph tandem cells, whereas a 10.7% cell is confirmed by ISE Freiburg. Micromorph cells show a rather low relative temperature coefficient of 0.27%/K. Applying the grain-boundary trapping model so far developed for CVD polysilicon to hydrogenated microcrystalline silicon deposited by VHF plasma, an upper limit for the average defect density of around 2 × 10¹⁶/cm³ could be deduced; this fact suggests a rather effective hydrogen passivation of the grain-boundaries. First TEM investigations on μc-Si : H p-i-n cells support earlier findings of a pronounced columnar grain structure. Using Ar dilution, deposition rates of up to 9 Å/s for microcrystalline silicon could be achieved.     

Theoretical maximal efficiency for a silicon solar cell with a two infrared-photon absorption in an inserted sub-structure

One of the possible optimized device designs far silicon solar cell photocurrent enhancement, consists of a cell having an inserted sub-structure with extrinsic gap levels. A middle-gap impurity and defect level band may actually allow a two infrared photon absorption. The junction near local defect layer design (Li et al., 1992) was assumed to enhance the sub-band-gap light absorption but it also enhances the recombination mechanisms strongly. Kuznicki (1993) has proposed another design with an L-H interface insertion at the edges of a continuous sub-structure to avoid extra recombination. The maximal photocurrent due to an additional infrared absorption calculated in this way is smaller than ΔI_ph = 16.8 mA cm⁻². In the case when the widths of the absorbing sub-structure are negligible compared to the width of the emitter, the simulated maximal efficiency can vary from 30.87 mW cm⁻² to 40.51 mW cm⁻².