Hot Wire CVD for thin film triple junction cells and for ultrafast deposition of the SiN passivation layer on polycrystalline Si solar cells

We present recent progress on hot-wire deposited thin film solar cells and applications of silicon nitride. The cell efficiency reached for μc-Si:H n-i-p solar cells on textured Ag/ZnO presently is 8.5%, in line with the state-of-the-art level for μc-Si:H n-i-p's for any method of deposition. Such cells, used in triple junction cells together with hot-wire deposited proto-Si:H and plasma-deposited SiGe:H, have reached 10.5% efficiency. The single junction μc-Si:H n-i-p cell is entirely stable under prolonged light soaking. The triple junction cell, including protocrystalline i-layers, is within 3% stable, due to the limited thicknesses of the two top cells. The application of SiN_x:H at a deposition rate of 3 nm/s to polycrystalline Si wafer solar cells has led to cells with 15.7% efficiency. We have also achieved record high deposition rates of 7.3 nm/s for transparent and dense SiN_x;H. Hot-wire SiN_x:H is likely to be the first large commercial application of the Hot Wire CVD (Cat-CVD) technology.     

Ultrafast deposition of silicon nitride and semiconductor silicon thin films by hot wire chemical vapor deposition

The technology of Hot Wire Chemical Vapor Deposition (HWCVD) or Catalytic Chemical Vapor Deposition (Cat-CVD) has made great progress during the last couple of years. This review discusses examples of significant progress. Specifically, silicon nitride deposition by HWCVD (HW-SiN_x) is highlighted, as well as thin film silicon single junction and multijunction junction solar cells. The application of HW-SiN_x at a deposition rate of 3 nm/s to polycrystalline Si wafer solar cells has led to cells with 15.7% efficiency and preliminary tests of our transparent and dense material obtained at record high deposition rates of 7.3 nm/s yielded 14.9% efficiency. We also present recent progress on Hot-Wire deposited thin film solar cells. The cell efficiency reached for (nanocrystalline) nc-Si:H n-i-p solar cells on textured Ag/ZnO presently is 8.6%. Such cells, used in triple junction cells together with Hot-Wire deposited proto-Si:H and plasma-deposited SiGe:H, have reached 10.9% efficiency. Further, in our research on utilizing the HWCVD technology for roll-to-roll production of flexible thin film solar cells we recently achieved experimental laboratory scale tandem modules with HWCVD active layers with initial efficiencies of 7.4% at an aperture area of 25 cm².     

Development of microcrystalline silicon carbide window layers by hot-wire CVD and their applications in microcrystalline silicon thin film solar cells

Microcrystalline silicon carbide (μc-SiC:H) thin films in stoichiometric form were deposited from the gas mixture of monomethylsilane (MMS) and hydrogen by Hot-Wire Chemical Vapor Deposition (HWCVD). These films are highly conductive n-type. The optical gap E₀₄ is about 3.0-3.2 eV. Such μc-SiC:H window layers were successfully applied in n-side illuminated n-i-p microcrystalline silicon thin film solar cells. By increasing the absorber layer thickness from 1 to 2.5 μm, the short circuit current density (j_SC) increases from 23 to 26 mA/cm² with Ag back contacts. By applying highly reflective ZnO/Ag back contacts, j_SC = 29.6 mA/cm² and η = 9.6% were achieved in a cell with a 2-μm-thick absorber layer.     

Microcrystalline silicon carbide thin films grown by HWCVD at different filament temperatures and their application in n-i-p microcrystalline silicon solar cells

To optimize the performance of microcrystalline silicon carbide (µc-SiC:H) window layers in n-i-p type microcrystalline silicon (µc-Si:H) solar cells, the influence of the rhenium filament temperature in the hot wire chemical vapor deposition process on the properties of µc-SiC:H films and corresponding solar cells were studied. The filament temperature T_F has a strong effect on the structure and optical properties of µc-SiC:H films. Using these µc-SiC:H films prepared in the range of T_F = 1800-2000 °C as window layers in n-side illuminated µc-Si:H solar cells, cell efficiencies of above 8.0% were achieved with 1 µm thick µc-Si:H absorber layer and Ag back reflector.     

Improvement of μc-Si:H n-i-p cell efficiency with an i-layer made by hot-wire CVD by reverse H2-profiling

The technique of maintaining a proper crystalline ratio in microcrystalline silicon (μc-Si:H) layers along the thickness direction by decreasing the H₂ dilution ratio during deposition (H₂ profiling) was introduced by several laboratories while optimizing either n-i-p or p-i-n μc-Si:H cells made by PECVD. With this technique a great increase in the energy conversion efficiency was obtained. Compared to the PECVD technique, the unique characteristics of HWCVD, such as the catalytic reactions, the absence of ion bombardment, the substrate heating by the filaments and filament aging effects, necessitate a different strategy for device optimization. We report in this paper the result of our method of using a reverse H₂ profiling technique, i.e. increasing the H₂ dilution ratio instead of decreasing it, to improve the performance of μc-Si:H n-i-p cells with an i-layer made by HWCVD. The principle behind this technique is thought to be a compensation effect for the influence of progressing silicidation of the filaments during the growth of μc-Si:H, if the filament current is held constant during growth. The dependence of the material crystallinity on thickness with and without H₂ profiling is discussed and solar cell J-V parameters are presented. Thus far, the best efficiency of μc-Si:H n-i-p cells made on a stainless steel substrate with an Ag/ZnO textured back reflector made in house has been improved to 8.5%, which is the highest known efficiency obtained for n-i-p cells with a hot-wire μc-Si:H i-layer.     

Analysis of single junction a-Si:H solar cells grown on different TCO’s

In consequence of previous investigation of individual transparent conductive oxide (TCO) and absorber layers a study was carried out on hydrogenated amorphous silicon (a-Si:H) solar cells with diluted intrinsic a-Si:H absorber layers deposited on glass substrates covered with different TCO films. The TCO film forms the front contact of the super-strata solar cell and has to exhibit good electrical (high conductivity) and optical (high transmittance) properties. In this paper we focused our attention on the influence of using different TCO’s as a front contact in solar cells with structure as follows: Corning glass substrate/TCO (800, 950 nm)/p-type μc-Si:H (∼5 nm)/p-type a-Si:H (10 nm)/a-SiC:H buffer layer (∼5 nm)/intrinsic a-Si:H absorber layer with dilution R = [H₂]/[SiH₄] = 20 (300 nm)/n-type a-Si:H layer (20 nm)/Ag + Al back contact (100 + 200 nm). Diode sputtered ZnO:Ga, textured and non-textured ZnO:Al [3] and commercially fabricated ASAHI (SnO₂:F) U-type TCO’s have been used. The morphology and structure of ZnO films were altered by reactive ion etching (RIE) and post-deposition annealing.It can be concluded that the single junction a-Si:H solar cells with ZnO:Al films achieved comparable parameters as those prepared with commercially fabricated ASAHI U-type TCO’s.     

Amorphous silicon thin film solar cells deposited entirely by hot-wire chemical vapour deposition at low temperature (< 150 °C)

Amorphous silicon n-i-p solar cells have been fabricated entirely by Hot-Wire Chemical Vapour Deposition (HW-CVD) at low process temperature < 150 °C. A textured-Ag/ZnO back reflector deposited on Corning 1737F by rf magnetron sputtering was used as the substrate. Doped layers with very good conductivity and a very less defective intrinsic a-Si:H layer were used for the cell fabrication. A double n-layer (µc-Si:H/a-Si:H) and µc-Si:H p-layer were used for the cell. In this paper, we report the characterization of these layers and the integration of these layers in a solar cell fabricated at low temperature. An initial efficiency of 4.62% has been achieved for the n-i-p cell deposited at temperatures below 150 °C over glass/Ag/ZnO textured back reflector.     

Hot-wire CVD deposited n-type μc-Si films for μc-Si/c-Si heterojunction solar cell applications

Phosphorous-doped microcrystalline silicon (μc-Si) films were prepared using hot-wire chemical vapor deposition (HWCVD). Structural, electrical and optical properties of these thin films were systematically studied as a function of PH₃ gas mixture ratio. We report recent results for p-type crystalline silicon-based heterojunction (HJ) solar cells using the HWCVD n-μc-Si film to form an n-p junction. The surface morphology of the crystalline Si substrate after hydrogen treatment was examined using atomic force microscopy. A transfer length method was used to modify the indium-tin-oxide (ITO) deposition parameters in order to reduce front ITO/n-μc-Si contact resistance. In our best solar cell sample (1 cm²) without any buffer layer, the conversion efficiency of 15.1% has been achieved with an open circuit voltage of 0.615 V, fill factor of 0.71 and short circuit current density of 34.6 mA/cm² under 100 mW/cm² condition. The spectral response of this cell will also be discussed.     

Nanostructured solar cell by spray pyrolysis: Effect of titania barrier layer on the cell performance

ZnO nanostructured solar cells with CuInS₂ absorber layer were prepared by chemical spray method. In order to increase chemical stability of ZnO nanorods against dissolution in the next steps of the cell preparation, and reduce the electrical shorts between the front and back contacts, an amorphous TiO₂ layer was deposited on ZnO nanorods by ALD or sol-gel spray technique. The thicknesses of the layer (≤ 5 nm by spray and ≤ 1 nm by ALD), which did not impede the collection of carriers, were determined. TiO₂ thicknesses above these optimal values led to s-shaped I-V curves, causing the decrease in solar cell efficiency from 2.2 to 0.7% due to the formation of an additional junction blocking charge carrier transport in the device under forward bias. Nanostructured cells suffered from somewhat higher interface recombination but showed still two times higher current densities (~ 10 mA/cm²) than the planar devices did.     

Band gap profiles of intrinsic amorphous silicon germanium films and their application to amorphous silicon germanium heterojunction solar cells

Intrinsic amorphous silicon germanium (i-a-SiGe:H) films with V, U and VU shape band gap profiles for amorphous silicon germanium (a-SiGe:H) heterojunction solar cells were fabricated. The band gap profiles of i-a-SiGe:H were prepared by varying the GeH₄ and H₂ flow rates during the deposition process. The use of i-a-SiGe:H with band gap profile in an absorber layer for a-SiGe:H heterojunction solar cells was investigated. The solar cell using a VU shape band gap profile shows a higher efficiency compared to other shapes. The highest efficiency obtained for an a-SiGe:H heterojunction solar cell using the VU shape band gap profile technique was 9.4% (V_oc = 0.79 V, J_sc = 19.0 mA/cm² and FF = 0.63).     

Process control of high rate microcrystalline silicon based solar cell deposition by optical emission spectroscopy

Silicon thin-film solar cells based on microcrystalline silicon (μc-Si:H) were prepared in a 30 × 30 cm² plasma-enhanced chemical vapor deposition reactor using 13.56 or 40.68 MHz plasma excitation frequency. Plasma emission was recorded by optical emission spectroscopy during μc-Si:H absorber layer deposition at deposition rates between 0.5 and 2.5 nm/s. The time course of SiH^? and H_β emission indicated strong drifts in the process conditions particularly at low total gas flows. By actively controlling the SiH₄ gas flow, the observed process drifts were successfully suppressed resulting in a more homogeneous i-layer crystallinity along the growth direction. In a deposition regime with efficient usage of the process gas, the μc-Si:H solar cell efficiency was enhanced from 7.9 % up to 8.8 % by applying process control.     

Influence of process steps on the performance of microcrystalline silicon thin film transistors

Bottom gate microcrystalline silicon thin film transistors (μc-Si TFT) have been realized with two types of films: μc-Si(1) and μc-Si(2) with crystalline fraction of 80% and close to 100% respectively. On these TFTs we applied two types of passivation (SiN_x and resist). μc-Si TFTs with resist as a passivation layer present a low leakage current of about 2.10^− 12 A for V_G = − 10 and V_D = 0.1V an ON to OFF current ratio of 10⁶, a threshold voltage of 7 V, a linear mobility of 0.1 cm²/V s, and a sub-threshold voltage of 0.9 V/dec. Microcrystalline silicon TFTs with SiN_x as a passivation present a new phenomenon: a parasitic current for negative gate voltage (− 15 V) causes a bump and changes the shape of the sub-threshold region. This excess current can be explained by and oxygen contamination at the back interface.     

Application of pulsed laser deposited zinc oxide films to thin film transistor device

Transparent zinc oxide (ZnO) thin films were deposited on various substrates using a pulsed laser deposition (PLD) technique. During the PLD, oxygen pressure and substrate temperature were varied in order to find an optimal preparation condition of ZnO for thin film transistor (TFT) application. Dependence of optical, electrical and crystalline properties on the deposition conditions was investigated. The ZnO thin films were then deposited on SiN/c-Si layer structures in order to fabricate a TFT device. The pulsed laser deposited ZnO films showed a remarkable TFT performance: field effect mobility (μ_FE) of 2.4-12.85 cm²/V s and ratio of on and off current (R_on/off) in 2-6 order range. Influence of ZnO preparation conditions on the resulting TFT performance was discussed.     

Dye-sensitized solar cells based on oriented ZnO nanowire-covered TiO2 nanoparticle composite film electrodes

The oriented ZnO nanowire-covered TiO₂ nanoparticle composite film electrodes were fabricated by screen-printed TiO₂ nanoparticle layer on conducting glass and low-temperature hydrothermal growth of ZnO nanowires. The film morphology, composition and crystalline structure were confirmed by field-emission scanning electron microscopy, energy dispersive X-ray spectra and X-ray diffraction patterns respectively. Dye-sensitized solar cells based on the composite electrode gained short-circuit current density of 8.04 mA/cm², open-circuit photovoltage of 0.67 V, fill factor of 0.40, and overall conversion efficiency of 2.15%.     

Synthesis and optical characterization of n-ZnO and p-Cu2ZnSnS4 nanocrystalline thin films for low cost solar cells

High quality ZnO/Cu₂ZnSnS₄ thin films as a window/absorber layers were successfully synthesized via spin coating the sol-gel precursor of each composition without using any vacuum facilities. In this study, the impact of annealing temperature (400 °C, 3 h) on the ZnO window layer and different thickness (3 and 5 layers) of the Cu₂ZnSnS₄ (CZTS) absorber layer were investigated. X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), scanning electron microscope (SEM) and UV–vis–NIR spectroscopy were used for the structural, compositional, morphological and optical absorption analysis of each layer. ZnO exhibits wurtzite hexagonal crystal structure with particle size equals to 8.60 and 28.59 nm for fresh and annealed films, respectively. Micro-strain and dislocations density decreased with the annealing temperature. X-ray diffraction patterns for CZTS films show small peak at (112) according to the kesterite structure with particle size in nano-scale for the two thicknesses. ZnO films demonstrated direct optical band gap of 3.23 and 3.21 eV for fresh and annealed films, respectively. CZTS films (3 and 5 layers) also have direct optical band with optimum value (1.51 eV) for thickness of 5 layers. The J-V characteristics of the CZTS-based thin film solar cells (CZTS/ZnO/ZnO:Ag) were measured under air mass AM 1.5 and 100 mW/cm² illumination. The values of the short circuit current (J_sc), open circuit voltage (V_oc) and fill factor (FF) also have been obtained.     

Applications of microcrystalline hydrogenated cubic silicon carbide for amorphous silicon thin film solar cells

We demonstrated the fabrication of n-i-p type amorphous silicon (a-Si:H) thin film solar cells using phosphorus doped microcrystalline cubic silicon carbide (μc-3C-SiC:H) films as a window layer. The Hot-wire CVD method and a covering technique of titanium dioxide TiO₂ on TCO was utilized for the cell fabrication. The cell configuration is TCO/TiO₂/n-type μc-3C-SiC:H/intrinsic a-Si:H/p-type μc- SiC_x (a-SiC_x:H including μc-Si:H phase)/Al. Approximately 4.5% efficiency with a V_oc of 0.953 V was obtained for AM-1.5 light irradiation. We also prepared a cell with the undoped a-Si_1−xC_x:H film as a buffer layer to improve the n/i interface. A maximum V_oc of 0.966 V was obtained.     

The charge-transfer property and the performance of dye-sensitized solar cells of nitrogen doped zinc oxide

In this study two methods, namely the solution and annealing methods, were used to prepare nitrogen-doped ZnO. The X-ray photoelectron spectroscopy (XPS) was performed to identify the composition and chemical states of N-doped ZnO. The N doping by the solution method was found to effectively decrease the acceptor effects. Surface photovoltage measurements (SPS) revealed a redshift of the threshold wavelength for the N-doped ZnO. And the recombination of photoinduced electron–hole pairs in this semiconductor material was obviously suppressed. The N-doped ZnO (solution method) exhibits the best performances among all the materials, even superior to N-doped ZnO (annealing method). Its J_sc and η values (9.35 mA/cm² and 2.64%) have enhanced by several times compared with un-doped ZnO (J_sc, 2.85 mA/cm²; η, 0.67%). The overall conversion efficiency of ZnO-based dye-sensitized solar cells was successfully improved by the N doping.     

Rapid Thermal Annealing of ZnO Nanocrystalline Films for Dye-Sensitized Solar Cells

Nanocrystalline ZnO thin films were prepared by the sol–gel method and annealed at 600 °C by conventional (CTA) and rapid thermal annealing (RTA) processes on fluorine-doped tin oxide (FTO)-coated glass substrates for application as the work electrode for a dye-sensitized solar cell (DSSC). ZnO films were crystallized using a conventional furnace and the proposed RTA process at annealing rates of 5 °C/min and 600 °C/min, respectively. The ZnO thin films were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM) analyses. Based on the results, the ZnO thin films crystallized by the RTA process presented better crystallization than films crystallized in a conventional furnace. The ZnO films crystallized by RTA showed higher porosity and surface area than those prepared by CTA. The results show that the short-circuit photocurrent (J _sc) and open-circuit voltage (V _oc) values increased from 4.38 mA/cm² and 0.55 V for the DSSC with the CTA-derived ZnO films to 5.88 mA/cm² and 0.61 V, respectively, for the DSSC containing the RTA-derived ZnO films.     

CuIn1 − xGaxSe2 photovoltaic devices for tandem solar cell application

CuIn_1 − xGa_xSe₂ (CIGS) solar cells show a good spectral response in a wide range of the solar spectrum and the bandgap of CIGS can be adjusted from 1.0 eV to 1.7 eV by increasing the gallium-to-indium ratio of the absorber. While the bandgaps of Ga-rich CIGS or CGS devices make them suitable for top or intermediate cells, the In rich CIGS or CIS devices are well suited to be used as bottom cells in tandem solar cells. The photocurrent can be adapted to the desired value for current matching in tandem cells by changing the composition of CIGS which influences the absorption characteristics. Therefore, CIGS layers with different [Ga]/[In + Ga] ratios were grown on Mo and ZnO:Al coated glass substrates. The grain size, composition of the layers, and morphology strongly depend on the Ga content. Layers with Ga rich composition exhibit smaller grain size and poor photovoltaic performance. The current densities of CIGS solar cells on ZnO:Al/glass varied from 29 mA cm^− 2 to 13 mA cm^− 2 depending on the Ga content, and 13.5% efficient cells were achieved using a low temperature process (450 °C). However, Ga-rich solar cells exhibit lower transmission than dye sensitized solar cells (DSC). Prospects of tandem solar cells combining a DSC with CIGS are presented.     

Phosphorus‐Rich Colloidal Cobalt Diphosphide (CoP2) Nanocrystals for Electrochemical and Photoelectrochemical Hydrogen Evolution

Developing earth‐abundant and efficient electrocatalysts for photoelectrochemical water splitting is critical to realizing a high‐performance solar‐to‐hydrogen energy conversion process. Herein, phosphorus‐rich colloidal cobalt diphosphide nanocrystals (CoP₂ NCs) are synthesized via hot injection. The CoP₂ NCs show a Pt‐like hydrogen evolution reaction (HER) electrocatalytic activity in acidic solution with a small overpotential of 39 mV to achieve ?10 mA cm^?2 and a very low Tafel slope of 32 mV dec^?1. Density functional theory (DFT) calculations reveal that the high P content both physically separates Co atoms to prevent H from over binding to multiple Co atoms, while simultaneously stabilizing H adsorbed to single Co atoms. The catalytic performance of the CoP₂ NCs is further demonstrated in a metal–insulator–semiconductor photoelectrochemical device consisting of bottom p‐Si light absorber, atomic layer deposition Al–ZnO passivation layers, and the CoP₂ cocatalyst. The p‐Si/AZO/TiO₂/CoP₂ photocathode shows a photocurrent density of ?16.7 mA cm^?2 at 0 V versus reversible hydrogen electrode (RHE) and an output photovoltage of 0.54 V. The high performance and stability are attributed to the junction between p‐Si and AZO, the corrosion‐resistance of the pinhole‐free TiO₂ protective layer, and the fast HER kinetics of the CoP₂ NCs.