Simple and fast fabrication of a-Si:H/c-Si hetero-junction solar cells by dual-chamber hot wire chemical vapor deposition

One of the fabrication issues in hetero-junction crystalline Si solar cells is the overhead time between the deposition steps of the top and bottom surfaces, because flipping of the progressing wafer is necessary to process the both sides of the wafer. To reduce the overall processing time by reducing the overhead time, we propose a dual-chamber deposition system, where thin films on the top and bottom surfaces of the Si wafer are simultaneously deposited. We have evaluated the proposed deposition system by demonstrating fabricated hetero-junction crystalline Si solar cells, which were compared with solar cells fabricated by a conventional plasma-enhanced chemical deposition system. We have obtained the power conversion efficiency of 15.5% from solar cells fabricated by our dual-chamber system; and additional analyses confirmed that the proposed dual-chamber system is, in principle, competitive with conventional systems in terms of the fabricated solar cell performance. This novel concept for the fabrication of a hetero-junction crystalline Si solar cell is expected to lay an important foundation in the future thin film crystalline Si based photovoltaic industry.     

Fabrication and characterisations of n-CdS/p-PbS heterojunction solar cells using microwave-assisted chemical bath deposition

n-CdS/p-PbS heterojunction solar cells were prepared via microwave-assisted chemical bath deposition method. A cadmium sulfide (CdS) window layer (340 nm thickness) was deposited on an indium tin oxide (ITO) glass. A lead sulfide (PbS) absorber layer (985–1380 nm thickness) with different molar concentrations (0.02, 0.05, 0.075, and 0.1 M) was then grown on ITO/CdS to fabricate a p–n junction. The effects of changing molar concentration of the absorber layer on structural and optical properties of the corresponding PbS thin films and solar cells were investigated. The optical band gap of the films decreased as the molarity increased. The photovoltaic properties (J–V characteristics, short circuit current, open circuit voltage, fill factor, and efficiency) of the CdS/PbS heterostructure cells were examined under 30 mW/cm² solar radiation. Interestingly, changing molar concentration improved the photovoltaic cells performances, the solar cell exhibited its highest efficiency (1.68%) at 0.1 M molar concentration.     

Fabrication of CdTe solar cells by laser-driven physical vapor deposition

Polycrystalline cadmium sulfide-cadmium telluride heterojunction solar cells were fabricated for the first time using a laser-driven physical vapor deposition method. An XeCl excimer laser was used to deposit both of the II–VI semiconductor layers in a single vacuum chamber from pressed powder targets. Results are presented from optical absorption, Raman scattering, X-ray diffraction, and electrical characterization of the films. Solar cells were fabricated by deposition onto SnO₂-coated glass with top contacts produced by gold evaporation. Device performance was evaluated from the spectral quantum efficiency and current-voltage measurements in the dark and with air mass 1.5 solar illumination.     

Hot wire chemical vapor deposition of Si containing materials for solar cells

A review of the hot wire chemical vapor deposition (HWCVD) of Si-containing materials for solar cell applications is given. A short history of the technique is given, starting from the early 1970s up to the present time. This is followed by a summary of radical detection and gas phase interaction results aimed towards achieving a basic understanding of this process. Next, issues particular to HWCVD growth are presented. These deal mainly with the filament, and include different methods of mounting filaments, filament contamination issues, filament alloying and its effect on both filament lifetime and film properties, and substrate heating by the filament. Differences between PECVD and HWCVD growth are then summarized, and this is followed by examples of research results indicating unique film properties. Included in these examples are works on amorphous silicon, microcrystalline silicon, silicon nitride, and a new technique for deposition of large grained poly Si by utilizing the etching of silicon by atomic hydrogen produced by the filament. Finally, the future prospects of HWCVD are briefly discussed.     

Plasma enhanced chemical vapor deposition of excellent a-Si:H passivation layers for a-Si:H/c-Si heterojunction solar cells at high pressure and high power

The intrinsic a-Si:H passivation layer inserted between the doped a-Si:H layer and the c-Si substrate is very crucial for improving the performance of the a-Si:H/c-Si heterojunction (SHJ) solar cell. The passivation performance of the a-Si:H layer is strongly dependent on its microstructure. Usually, the compact a-Si:H deposited near the transition from the amorphous phase to the nanocrystalline phase by plasma enhanced chemical vapor deposition (PECVD) can provide excellent passivation. However, at the low deposition pressure and low deposition power, such an a-Si:H layer can be only prepared in a narrow region. The deposition condition must be controlled very carefully. In this paper, intrinsic a-Si:H layers were prepared on n-type Cz c-Si substrates by 27.12 MHz PECVD at a high deposition pressure and high deposition power. The corresponding passivation performance on c-Si was investigated by minority carrier lifetime measurement. It was found that an excellent a-Si:H passivation layer could be obtained in a very wide deposition pressure and power region. Such wide process window would be very beneficial for improving the uniformity and the yield for the solar cell fabrication. The a-Si:H layer microstructure was further investigated by Raman and Fourier transform infrared (FTIR) spectroscopy characterization. The correlation between the microstructure and the passivation performance was revealed. According to the above findings, the a-Si:H passivation performance was optimized more elaborately. Finally, a large-area SHJ solar cell with an efficiency of 22.25% was fabricated on the commercial 156 mm pseudo-square n-type Cz c-Si substrate with the opencircuit voltage (V_oc) of up to 0.732 V.     

Photovoltaic water electrolysis using the sputter-deposited a-Si/c-Si solar cells

The solar cell which is employed for photovoltaic water electrolysis was fabricated by sputter-depositing a-Si:H on Si wafer. The electrolysis was conducted by connecting the series-connected solar cell externally to the hydrogen and the oxygen evolution electrodes. The conversion efficiency of 3.0% from the solar to hydrogen chemical energy was obtained and can be expected to be improved further by optimizing the system.     

High-efficiency μc-Si/c-Si heterojunction solar cells

P-type microcrystalline silicon (μc-Si (p)) on n-type crystalline silicon (c-Si(n)) heterojunction solar cells is investigated. Thin boron-doped μc-Si layers are deposited by plasma-enhanced chemical vapor deposition on CZ-Si and the V_oc of μc-Si/c-Si heterojunction solar cells is higher than that produced by a conventional thermal diffusion process. Under the appropriate conditions, the structure of thin μc-Si films on (1 0 0), (1 1 0), and (1 1 1) CZ-Si is ordered, so high V_oc of 0.579 V is achieved for 2×2 cm² μc-Si/multi-crystalline silicon (mc-Si) solar cells. The epitaxial-like growth is important in the fabrication of high-efficiency μc-Si/mc-Si heterojunction solar cells.     

PbS nanocrystal solar cells fabricated using microwave-assisted chemical bath deposition

n-CdS/p-PbS heterojunction solar cell was fabricated using microwave-assisted Chemical Bath Deposition (CBD). The CdS window layer (340 nm thickness) was deposited on ITO-glass. The PbS absorber layer (685–1250 nm thickness) with different molar concentration (0.02, 0.05, 0.075 and 0.1 M) was then grown on ITO/CdS to fabricate the p–n junction. X-ray diffraction analysis confirms the formation of pure and nanocrystalline CdS and PbS phases with a preferred orientation depending on molarity; (111) or (200). Scanning electron microscopy observations show a uniform surface morphology with gatherings. UV–Vis spectrophotometer and FTIR was used to estimate the optical properties. Optical measurements gave an energy gap of 2.6 eV for CdS whereas that for PbS thin films were found to vary in a narrow range 0.40–0.47 eV, depending on the molar concentration. The photovoltaic properties under 30 mW/cm² solar radiation including J–V characteristics, short-circuit current (I_sc), open-circuit voltage (V_oc), fill factor (ff), efficiency (η) of CdS/PbS heterojunction cells have been as well examined. The results show that changing the molar concentration improved the performances of the fabricated photovoltaic cells; a high efficiency was observed at 0.1 M. However, high series resistance and poor crystallinity of PbS lead to low efficiency at lower molarity.     

Reduction of amorphous incubation layer by HCl addition during deposition of microcrystalline silicon by hot-wire chemical vapor deposition

The amorphous incubation layer, which is formed in the initial growth stage of hydrogenated microcrystalline silicon (μc-Si:H) thin film deposited at low temperature, is harmful to the electric properties of film. In this study, the effect of the addition of HCl gas on the reduction of such an amorphous incubation layer was investigated during the silicon deposition on a glass substrate at 220 °C by hot-wire chemical vapor deposition process using the Raman spectroscopy, the X-ray diffraction and the field-emission scanning electron microscopy. In the initial stage of deposition where the silicon film deposited without HCl addition consisted almost entirely of the amorphous incubation layer; highly crystalline silicon films could be deposited with HCl addition. As the flow rate of HCl increased, the crystallinity of silicon films increased but the film growth rate decreased. The surface morphology of films prepared with HCl addition became smoother with smaller grain size than that prepared without HCl.     

Synthesis of highly conductive boron-doped p-type hydrogenated microcrystalline silicon (μc-Si:H) by a hot-wire chemical vapor deposition (HWCVD) technique

Boron-doped hydrogenated microcrystalline silicon (μc-Si:H) films were prepared using hot-wire chemical vapor deposition (HWCVD) technique. Structural, electrical and optical properties of these thin films were systematically studied as a function of B₂H₆ gas (diborane) phase ratio (Variation in B₂H₆ gas phase ratio, dopant gas being diluted in hydrogen, affected the film properties through variation in doping level and hydrogen dilution). Characterization of these films from low angle X-ray diffraction and Raman spectroscopy revealed that the high conductive film consists of mixed phase of microcrystalline silicon embedded in an amorphous network. Even a small increase in hydrogen dilution showed marked effect on film microstructure. At the optimized deposition conditions, films with high dark conductivity (0.08 (Ω cm)⁻¹) with low charge carrier activation energy (0.025 eV) and low optical absorption coefficient with high optical band gap (2.0 eV) were obtained. At these deposition conditions, however, the growth rate was small (6 Å/s) and hydrogen content was large (9 at%).     

CdO/Cu2O solar cells by chemical deposition

CdO and Cu₂O thin films have been grown on glass substrates by chemical deposition method. Optical transmittances of the CdO and Cu₂O thin films have been measured as 60–70% and 3–8%, respectively in 400–900 nm range at room temperature. Bandgaps of the CdO and Cu₂O thin films were calculated as 2.3 and 2.1 eV respectively from the optical transmission curves. The X-ray diffraction spectra showed that films are polycrystalline. Their resistivity, as measured by Van der Pauw method yielded 10⁻²–10⁻³ Ω cm for CdO and approximately 10³ Ω cm for Cu₂O. CdO/Cu₂O solar cells were made by using CdO and Cu₂O thin films. Open circuit voltages and short circuit currents of these solar cells were measured by silver paste contacts and were found to be between 1–8 mV and 1–4 μA.     

Effect of HCl addition on the properties of p-type silicon thin films during hot-wire chemical vapor deposition

The effect of HCl addition on the structural, electrical, and optical properties of p-type silicon films, prepared by hot-wire chemical vapor deposition (HWCVD), was investigated. As the ratio of HCl to SiH₄ increased, the amount of amorphous silicon decreased and the crystalline volume fraction increased in the deposited film. To investigate the effect of HCl addition on the deposition behavior in the initial stage, the transmission electron microscope (TEM) grid membrane was exposed for 10 s using a shutter above the grid membrane during HWCVD and the grid membrane was observed by TEM. When HCl was not added, a continuous film was observed on a grid membrane, consisting of crystalline nanoparticles embedded in an amorphous matrix whereas when HCl is added, isolated individual crystalline nanoparticles without amorphous silicon were observed. The HCl addition increased the dark conductivity of films by about 3 orders of magnitude but decreased the optical band gap slightly.     

A new route for fabricating CdO/c-Si heterojunction solar cells

CdO/c-Si solar cells have been made by depositing CdO thin films on p-type monocrystalline silicon substrate by means of the rapid thermal oxidation (RTO) technique using a halogen lamp at 350 °C/45 s in static air. Results on structural, optical, and electrical properties of grown CdO films are reported. The electrical and photovoltaic properties of CdO/Si solar cells are examined. Under AM1 illumination condition, the cell shows an open circuit voltage (V_OC) of 500 mV, a short circuit current density (J_SC) of 27.5 mA/cm², a fill factor (FF) of 60%, and a conversion efficiency (η) of 8.84% without using frontal grid contacts and/or post-deposition annealing. Furthermore, the stability of solar cells characteristics is tested.     

Enhancement in the performance of dye-sensitized solar cells containing ZnO-covered TiO2 electrodes prepared by thermal chemical vapor deposition

A ZnO-covered TiO₂ (denoted as ZnO/TiO₂) film was prepared by incorporating a small quantity of particulate ZnO in a TiO₂ matrix by thermal chemical vapor deposition. When used in a dye-sensitized solar cell, an enhancement was observed in both short-circuit photocurrent (J_sc) and open-circuit voltage (V_oc) by 12% and 17%, respectively, relative to those of a cell containing a bare TiO₂ film. The observed J_sc enhancement is attributed to the increase in the surface area of the ZnO/TiO₂ film, and the V_oc enhancement to the formation of a potential barrier by ZnO at TiO₂/electrolyte interface. The films were characterized by FE-SEM, EDX, and XRD.     

High-density inductively coupled plasma chemical vapor deposition of silicon nitride for solar cell application

The silicon nitride films were deposited by means of high-density inductively coupled plasma chemical vapor deposition in a planar coil reactor. The process gases used were pure nitrogen and a mixture of silane and helium. Passivated by silicon nitride, solar cells show efficiency above 13%. Strong H-atom release from the growing SiN film and Si–N bond healing are responsible for the improved electrical and passivation properties of SiN film. This paper presents the optimal refractive index of SiN for single layer antireflection coating as well as double layer antireflection coating in solar cell applications.     

Control of μc-Si/c-Si interface layer structure for surface passivation of Si solar cells

Outstanding passivation properties for p-type crystalline silicon surfaces were obtained by using very thin n-type microcrystalline silicon (μc-Si) layers with a controlled interface structure. The n-type μc-Si layers were deposited by the RF PE-CVD method with an insertion of an ultra-thin oxide (UTO) layer or an n-type amorphous silicon (a-Si : H) interface layer. The effective surface recombination velocity (SRV) obtained was very small and comparable to that obtained using thermal oxides prepared at 1000°C. The structural studies by HRTEM and Raman measurements suggest that the presence of UTO produces a very thin a-Si : H layer under the μc-Si. A crystal lattice discontinuity caused by these interface layers is the key to a small SRV.     

Microdoping compensation of microcrystalline silicon obtained by hot-wire chemical vapour deposition

Undoped hydrogenated microcrystalline silicon was obtained by hot-wire chemical vapour deposition at different silane-to-hydrogen ratios and low temperature (<300°C). As well as technological aspects of the deposition process, we report structural, optical and electrical characterisations of the samples that were used as the active layer for preliminary p–i–n solar cells. Raman spectroscopy indicates that changing the hydrogen dilution can vary the crystalline fraction. From electrical measurements an unwanted n-type character is deduced for this undoped material. This effect could be due to a contaminant, probably oxygen, which is also observed in capacitance–voltage measurements on Schottky structures. The negative effect of contaminants on the device was dramatic and a compensated p–i–n structure was also deposited to enhance the cell performance.     

Numerical simulation of the growth characteristics of laser chemical vapor deposition of silicon carbide

A thermal model, which involves heat transfer in substrate and gases, mass transfer in gases, and chemical reaction on the top surface of the substrate, is set up to simulate the Laser Chemical Vapor Deposition (LCVD) process of Silicon Carbide (SiC) by a finite volume method. Methyltrichlorosilane (MTS; SiCl₃CH₃) and hydrogen (H₂) are chosen as precursor and carrier gas, respectively. A designed set of model cases is executed for both stationary and moving laser beams. For the cases of stationary laser beam, the shape of the SiC deposits is higher and wider with increasing laser power. For the cases of moving laser beam, a narrow strip of SiC deposits is formed along the laser scanning path. Due to the low sticking coefficient of SiC deposits at high temperature, the volcano-like defects occur on the top center of the SiC deposits for both stationary and moving laser beams.