Promising window layer of thin film Si solar cell with p–i–n structure prepared by using SiH2Cl2

The fabrication process for a-Si:H solar cells with p–i–n structure contains a problem of damage to the SnO₂ substrate particularly at higher process temperatures. We have reported that the suppression of darkening and wide optical gap (E_opt) are obtained by using SiH₂Cl₂ instead of SiH₄ as a source gas (Mater. Res. Soc. Symp. Proc. 609 (2000), in press). In this paper, p-type a-Si:H:(Cl) was investigated. Comparable E_opt and dark conductivity (σ_dark) to those of conventional a-SiC:H were obtained. Solar cells using this a-Si:H:(Cl) show higher current density (J_sc) and higher collection efficiency in all wavelength regions as compared to a p-layer not using chlorine processes. The newly developed p-layer has been applied to solar cells with p–i–n structure fabricated at higher substrate temperatures (T_s). Although the a-Si:H material deposited at higher substrate temperatures has been reported as being more stable against light soaking (21st IEEE PVSC Proceeding, Florida, USA, 1990, p. 1656), the high temperature processing is difficult to apply to the a-Si:H p–i–n structure because of the significant darkening of SnO₂ at higher T_s. With an a-Si:H:(Cl) buffer layer, a-Si:H solar cells can be fabricated at higher T_s (300°C) with reasonable cell performance. The best stabilized efficiency was 7.5% obtained at a T_s of 250°C.     

Deposition of highly photoconductive wide band gap a-SiOx:H thin films at a high temperature without H2-dilution

Electrical and optical characterisation of hydrogenated amorphous silicon–oxygen alloy thin films (a-SiO_x:H, x<2) grown in a single chamber radio frequency plasma enhanced chemical vapour deposition (PECVD) system at a high substrate temperature of 300 °C is presented. The samples were investigated by Fourier transform infrared spectroscopy (FTIR), optical transmission, the constant photocurrent method (CPM), conductivity and steady-state photoconductivity measurements. With increasing oxygen concentration, the Tauc gap increases from 1.69 to 2.73 eV. The sample with an oxygen concentration of 26.2 at% and a reasonably high bandgap of 2.18 eV shows photoconductivity comparable to that of pure a-Si:H films. The Urbach parameter (E₀) increases almost linearly with oxygen concentration whereas the dangling bond defect density is found to be saturating at a value of about 7.1×10¹⁶ cm⁻³. One of the highly alloyed samples with exhibited a detectable photosensitivity.     

High-rate deposition of hydrogenated amorphous silicon films using inductively coupled silane plasma

Inductively coupled plasma (ICP) generated at 13.56 MHz has been employed for high-rate deposition of device-quality hydrogenated amorphous silicon (a-Si:H). It has been shown that an increase in the flow rate of a monosilane gas enhances the generation rate of deposition precursors, while the ion flux decreases and becomes saturated. The defect density reaches the minimum at a deposition rate of 2.3 nm/s. It has also been demonstrated that even at deposition rates around 4 nm/s, a-Si:H deposited at 150°C exhibits a subgap defect density lower than 6×10¹⁶ cm⁻³ after 12 h AM1 (100 mW/cm²) light soaking.     

Microcrystalline-phase p-type a-Si:O:H windows prepared by Cat-CVD

Different amounts of oxygen, boron-doped hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (μc-Si:H) deposition were carried out using catalytic chemical-vapor deposition (Cat-CVD) process. Pure silane (SiH₄), hydrogen (H₂), oxygen (O₂), and diluted diborane (B₂H₆) gases were used at the deposition pressure of 0.1–0.5 Torr. The tungsten catalyst temperature (T_fil) was varied from 1700 to 2100 °C. Sample transmittance measurement shows an optical-band gap (E_gopt) variation from 1.45 to 2.1 eV X-ray diffraction (XRD) spectra have revealed silicon microcrystalline phases for the samples prepared at the temperature greater than T_fil1900 °C. For the used silicon oxide deposition conditions, no strong tungsten filament degradation was observed after a number of sample preparations.     

Optimization of process parameters in a large-area hot-wire CVD reactor for the deposition of amorphous silicon (a-Si:H) for solar cell application with highly uniform material quality

Scale-up of a-Si:H-based thin film applications such as solar cells, entirely or partly prepared by hot-wire chemical vapor deposition (HWCVD), requires research on the deposition process in a large-area HWCVD system. The influence of gas supply and filament geometry on thickness uniformity has already been reported, but their influence on material quality is systematically studied for the first time. The optimization of deposition parameters for obtaining best material quality in our large-area HWCVD system resulted in an optimum filament temperature, T_fil≈1600°C, pressure, p=8 mTorr and silane flow, F(SiH₄)=100 sccm, keeping the substrate temperature at T_S=200°C. A special gas supply (gas shower with tiny holes of uniform size) and a filament grid, consisting of six filaments with an interfilament distance, d_fil=4 cm were used. The optimum filament-to-substrate distance was found to be d_fil–S=8.4 cm. While studying the influence of different d_fil and gas supply configurations on the material quality, the above-mentioned setup and parameters yield best results for both uniformity and material quality. With the setup mentioned, we could achieve device quality a-Si:H films with a thickness uniformity of ±2.5% on a circular area of 20 cm in diameter. The material, grown at a deposition rate of r_d≈4 Å/s, was characterized on nine positions of the 30 cm×30 cm substrate area, and revealed reasonable uniformity of the opto-electronic properties, e.g photosensitivity, σ_Ph/σ_D=(2.46±0.7)×10⁵, microstructure factor, R=0.17±0.05, defect densities, N_d(PDS)=(2.06±0.6)×10¹⁷ cm⁻³ and N_d(CPM)=(2.05±0.5)×10¹⁶ cm⁻³ (film properties are given as mean values and standard deviations). Finally, we fabricated pin solar cells, with the i-layer deposited on small-area p-substrates distributed over an area of 20 cm×20 cm in this large-area deposition system, and achieved high uniformity of the cell parameters with initial efficiencies of η=(6.1±0.2)% on the 20 cm×20 cm area.     

Evolution of microstructure and phase in amorphous, protocrystalline, and microcrystalline silicon studied by real time spectroscopic ellipsometry

Real time spectroscopic ellipsometry has been applied to develop deposition phase diagrams that can guide the fabrication of hydrogenated silicon (Si:H) thin films at low temperatures (<300°C) for highest performance electronic devices such as solar cells. The simplest phase diagrams incorporate a single transition from the amorphous growth regime to the mixed-phase (amorphous+microcrystalline) growth regime versus accumulated film thickness [the a→(a+μc) transition]. These phase diagrams have shown that optimization of amorphous silicon (a-Si:H) intrinsic layers by RF plasma-enhanced chemical vapor deposition (PECVD) at low rates is achieved using the maximum possible flow ratio of H₂ to SiH₄ that can be sustained while avoiding the a→(a+μc) transition. More recent studies have suggested that a similar strategy is appropriate for optimization of p-type Si:H thin films. The simple phase diagrams can be extended to include in addition the thickness at which a roughening transition is detected in the amorphous film growth regime. It is proposed that optimization of a-Si:H in higher rate RF PECVD processes further requires the maximum possible thickness onset for this roughening transition.     

Effect of annealing on the electrical, optical and structural properties of hydrogenated amorphous silicon films deposited in an asymmetric R.F. plasma CVD system at room temperature

This paper reports the effect of annealing on hydrogenated amorphous silicon films (a-Si : H) deposited by r.f. self-bias technique on cathode in an asymmetric r.f. plasma CVD system at room temperature. Detailed study of the variation of the dark and photoconductivity (σ_D and σ_ph) as a function of temperature and light intensity, surface morphology, hydrogen evolution, optical absorption, subgap absorption and related parameters, thermal and structural disorder on the optical-absorption edge, IR vibrational modes and bonded hydrogen content have been carried out on unannealed and annealed samples at different temperatures (T_a) from 100°C to 550°C. It is found that the values of σ_ph increase and that of Urbach energy (E_o), subgap defect density (N_d) and the polyhydride to monohydride ratio decrease upto T_a=250°C and beyond 250°C the values of σ_ph decrease and that of E_o, N_d and the polyhydride to monohydride ratio increase. The best opto-electronic properties with much improved σ_ph and σ_ph/σ_D and dominant monohydride bonding are obtained after annealing the room temperature deposited film at 250°C for 1 h. The σ_D data obeys a Meyer Neldel rule in annealed a-Si : H films. The value of optical band gap is found to be related to the E_o and the hydrogen content. The Urbach energy (E_o) which is a measure of the disorder is the sum of structural and thermal disorder. The structural disorder part decreases with the annealing temperature upto 300°C and thereafter it increases. The curves of optical absorption coefficient versus photon energy at different T_a converge to a common point.     

Widegap a-Si:H films prepared at low substrate temperature

Wide bandgap hydrogenated amorphous silicon (a-Si:H) films have been prepared by the PECVD method at a low substrate temperature (80°C) controlling the incorporation of hydrogen (bonded with silicon) into the film. Optimizing the deposition parameters viz. hydrogen dilution, rf power, a-Si:H film with E_g ∼ 1.90 eV and σ_ph ≥ 10⁻⁴ Scm⁻¹ has been developed. This film exhibited better optoelectronic properties compared to a-SiC:H of similar optical gap. The quantum efficiency measurement on the Schottky barrier solar cell structure showed a definite enhancement of blue response. Surface reaction as well as structural relaxation under suitable deposition condition have been claimed to be responsible for the development of such material.     

Low-temperature preparation of boron-doped nanocrystalline SiC:H films using mercury-sensitized photo-CVD technique

We investigated the dependence of hydrogenated boron-doped nanocrystalline silicon-carbide (p-nc-SiC:H) film characteristics against the substrate temperature (T_sub) by the transmission electron micrograph, Raman spectrum, and dark conductivity measurements. High quality of nanocrystalline growth at the low temperature of 120°C shows that the mercury-sensitized photo-assisted chemical vapor deposition (photo-CVD) technique is promising for a low-temperature fabrication of thin film solar cells onto flexible plastic substrates.     

Application of titanium containing amorphous hydrogenated carbon films (a-C:H/Ti) as optical selective solar absorber coatings

A combined PVD/PECVD process for the vacuum deposition of titanium containing amorphous hydrogenated carbon films is described. Elemental compositions of the deposited films have been determined by in situ core level photoelectron spectroscopy (XPS). The long-term stability of the plasma process has been demonstrated. Target poisening has not been observed. We have fabricated optical selective surfaces by the deposition of a-C:H/Ti multilayers onto aluminum substrates. Eventhough we have not optimized layer thicknesses and stoichiometries so far, the experimental results are promising: solar absorptance α_S of 0.876 and thermal emittance _100°C of 0.061 have been achieved yielding an optical selectivity sα_S/_100°C of 14.4. Accelerated aging tests of these coatings have demonstrated their aging stability: the service lifetime is predicted to amount to more than 25 years. Raman spectroscopy has been used to monitor changes in the structure of the aged coatings. Degradation mechanisms are being discussed.     

More stable low gap a-Si:H layers deposited by PE-CVD at moderately high temperature with hydrogen dilution

In the present work, several series with variation of deposition parameters such as hydrogen dilution ratio, VHF-power and plasma excitation frequency f_exc have been extensively analyzed. Compared with “conventional” more-stable layers obtained at 200–250°C and high H₂ dilution ratios of about 10, it was observed that electrical transport properties after light-induced degradation of layers deposited at “moderately high” temperatures (300–350°C) are equivalent but required lower H₂ dilution ratios (between 2 and 4). As a consequence, the deposition rate of more stable layers obtained at moderately high temperatures is increased by a factor of 2. Moreover, optical gaps of a-Si:H deposited at 300–350°C are significantly lower (by approx. 10 meV); furthermore, they decrease with f_exc.     

Low temperature deposition of high open-circuit voltage (>1.0 V) p-i-n type amorphous silicon solar cells

P-i-n type hydrogenated amorphous silicon (a-Si:H) solar cells were deposited by the radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) process at a low substrate temperature of 125 °C, which is compatible with low-cost poly (ethylene terephthalate) (PET) plastic substrates. Wide band gap (E_opt>1.88 eV) intrinsic a-Si:H films were achieved before the onset of the microcrystalline regime by changing the hydrogen dilution ratios. On the other hand, the structural, optical and electrical properties of p-type hydrogenated amorphous silicon carbide (p-a-SiC:H) window layers have been optimized at 125 °C. High quality p-a-SiC:H film with high optical band gap (E₀₄=2.02 eV) and high conductivity (σ_d=1.0×10⁻⁷ S/cm) was deposited at ‘low-power regime’ under low silane flow rates and high H₂ dilution conditions. With the combination of wide band gap p-a-SiC:H window layers and intrinsic a-Si:H layers, a high V_oc of 1.01 V (efficiency=5.51%, FF=0.72, J_sc=7.58 mA/cm²) was obtained for single junction a-Si:H p-i-n solar cell at a low temperature of 125 °C. Finally, flexible a-Si:H solar cell on PET substrate with efficiency of 4.60% (V_oc=0.98 V, FF=0.69, J_sc=6.82 mA/cm²) was obtained.     

Preparation of In2O3:F thin films grown by spray pyrolysis technique

Transparent conducting fluorine doped indium oxide (In₂O₃:F) thin films have been deposited on Corning 7059 glass substrates by the spray pyrolysis technique. The structural, electrical, and optical properties of these films were investigated as a function of substrate temperature. The X-ray diffraction pattern of the films deposited at lower substrate temperature (T_s=300 °C) showed no peaks of In₂O₃:F. In the useful range for deposition (i.e. 425–600 °C), the orientation of the films was predominantly [400]. For the 4500 Å thick In₂O₃:F deposited with an F content of 10-wt%, the minimum sheet resistance was 120 Ω and average transmission in the visible wavelength rang (400–700 nm) was 88%.     

A hybrid solar-assisted adsorption cooling unit for vaccine storage

A concept of a hybrid adsorption cooling unit for vaccine storage utilizing solar energy as a main power supply and a gas burner as an alternative power supply has been developed. The components of the cooling unit have been designed to work under the weathering conditions of Burkina Faso, West coast of Africa according to the requirements of the World Health Organization. For the first adsorber, which is driven by a gas burner, zeolite-13X has been selected. For the second adsorber to be driven by solar energy selective water sorbent SWS-2L has been applied. Water is selected as a refrigerant for both adsorbents. Theoretical investigations of the expected performance of the designed cooling unit have shown a coefficient of performance (COP) of 0.28 for the solar-operated system based on the heat input to the adsorption unit, at the design conditions of T_evap=−5 °C, T_con=55 °C, T_ads=38 °C, T_des(max)=122 °C. For the gas-heated system, also a COP of 0.28 has been estimated at the design conditions of T_evap=−5 °C, T_con=55 °C, T_ads=38 °C, T_des(max)=280 °C. The variations of COP, cooling capacity and the heating power required to operate both systems have been estimated for a broad range of desorption temperatures. It turns out that the SWS-2L/water system is much more sensitive to the operating conditions than the zeolite-13X/water system. The obtained results should serve in designing both control and heating components of the cooling unit.     

Sprayed CeO2 thin films for electrochromic applications

Cerium dioxide (CeO₂) thin films were prepared by spray pyrolysis using hydrated cerium chloride (CeCl₃·7H₂O) as source compound. The films prepared at substrate temperatures below 300°C were amorphous, while those prepared at optimal conditions (T_s=500°C,s=5 ml/min) were polycrystalline, cubic in structure, preferentially oriented along the (2 0 0) direction and exhibited a transmittance value greater than 80% in the visible range. The cyclic voltammetry study showed that films of CeO₂ deposited on ITO pre-coated glass substrates were capable of charge insertion/extraction when immersed in an electrolyte of propylene carbonate with 1 M LiClO₄.These films also remained fully transparent after Li+ intercalation/deintercalation.     

Electrical and photoluminescence properties of evaporated CuIn1−xGaxTe2 thin films

Polycrystalline thin films of CuIn_1−xGa_xTe₂ have been deposited by flash evaporation on Corning glass 7059 substrates at T_s=200°C. Hall and resistivity measurements have been carried out down to 77 K. These films are p-type and the variation of the resistivity may be linked to defects, disorder of the material or grain boundaries. The PL spectra of these films after annealing in argon atmosphere at T_a=450°C have showed a broad band emission between 0.98 and 1.12 eV in which the main peak appears at 1.05 eV (at 4.2 K).     

Study of the solid phase crystallization behavior of amorphous sputtered silicon by X-ray diffraction and electrical measurements

In situ X-ray diffraction (XRD) measurements have been used to study the amorphous-to-crystalline transformation in hydrogenated amorphous silicon (a-Si:H) thin films deposited by DC-Magnetron Sputtering at 300°C. The a-Si:H layers of 2.85 μm thickness were solid phase crystallized (SPC) and the crystallization kinetic was studied from in situ XRD measurements and also by in situ electrical conductance measurements during isothermal annealing at 630°C. The apparition and the evolution of the (1 1 1) peak in the XRD spectra during the annealing of the layer permit to follow the SPC kinetic which is the same as the electrical conductance kinetic (G=f(t)) performed in the same annealing conditions as in the XRD experiment. Several isothermal annealings at different temperatures permit to extract the characteristic parameters of the crystallization from the G=f(t) evolutions. These parameters are the thermally activated crystallization characteristic time and its activation energy.     

Plasma-enhanced chemical vapour-deposited silicon nitride films; The effect of annealing on optical properties and etch rates

The optical properties and etch rates of silicon nitride (SiN_x:H) deposited by plasma-enhanced chemical vapour deposition (PECVD) and their correlation with bond concentrations have been studied. By varying the silane-to-total gas ratio, films with refractive index (n) between 1.92 and 3.00 were deposited. Higher n films had increased absorption and decreased etch rates. Annealing the samples at different temperatures revealed that all films were thermally stable up to 750 °C, above which all experienced a rise in n, attributed mainly to mass densification. The etch rate correlated well the N–H bond concentration for both annealed and as-deposited films.     

Titanium-containing amorphous hydrogenated silicon carbon films (a-Si:C:H/Ti) for durable solar absorber coatings

By the incorporation of silicon into titanium-containing amorphous hydrogenated carbon films (a-C:H/Ti), the lifetime stability at 250°C in air can be strongly enhanced. A combined PVD/PECVD process for the vacuum deposition of these titanium-containing amorphous hydrogenated silicon carbon films (a-Si:C:H/Ti) is described. Elemental compositions of the deposited films have been determined by in situ core-level photoelectron spectroscopy (XPS). Optical constants for these films have been determined in the wavelength range from 400 to 2500 nm by means of spectrophotometry. Single layers of a-Si:C:H/Ti and a-C:H/Ti deposited on aluminum and copper substrates have been subjected to comparative aging tests. At 250°C in air, the stability of the a-Si:C:H/Ti films is significantly higher than that of the a-C:H/Ti films. If the silicon content is not too high, the aging properties under humid conditions do not suffer a lot from the incorporation of silicon. However, if the silicon content is clearly higher than the carbon content, the humidity resistance will decrease. For an absorber coating for flat plate solar collectors, the optimized silicon content is expected to be in the range where the high-temperature stability in air is already improved, and where the humidity resistance is still good. For vacuum collectors, a higher silicon content might be advantageous.     

Equations for estimating global solar radiation in data sparse regions

The knowledge of the amount of solar radiation in an area/region is very essential in the field of Solar Energy Physics. In this work two equations are put forward for estimating global solar radiation from common climate variables in data sparse regions. The first is the Hargreaves equation, R_s=0.16R_aT_d^0.5 where R_a is the extraterrestrial solar radiation and T_d is the temperature difference (maximum minus minimum), while the second is the Angstrom equation, R_s=R_a(0.28+0.39n/N) where n and N are the measured sunshine hours and the maximum daylight duration respectively. The global solar radiation estimated by the two equations for three sites, Owerri (5°28′N, 7°2′E), Umudike (5°29′N, 7°33′E) and Ilorin (8°32′N, 4°46′E), located in different climate zones of in Nigeria, West Africa, are in agreement with those of earlier workers and that from Photovoltaic Geographic Information System (PVGIS) project. The implication of this in solar photovoltaic applications has been stressed.