Microcrystalline silicon thin-film solar cells prepared at low temperature using PECVD

The low-temperature deposition of μc-Si:H has been found to be effective to suppress the formation of oxygen-related donors that cause a reduction in open-circuit voltage (V_oc) due to shunt leakage. We demonstrate the improvement of V_oc by lowering the deposition temperature down to 140°C. A high efficiency of 8.9% was obtained using an Aasahi-U substrate. Furthermore, by optimizing textured structures on ZnO transparent conductive oxide substrates, an efficiency of 9.4% was obtained. In addition, relatively high efficiency of 8.1% was achieved using VHF (60 MHz) plasma at a deposition rate of 12 Å s⁻¹. Thus, this low-temperature deposition technique for μc-Si:H is promising for obtaining both high efficiency and high-rate deposition technique for μc-Si:H solar cells.     

Fast deposition of microcrystalline silicon films with preferred (2 2 0) crystallographic texture using the high-density microwave plasma

A novel high-density and low-temperature microwave discharge utilizing a spoke antenna has been applied for the fast deposition of microcrystalline silicon (μc-Si:H) films with preferred (2 2 0) orientation. Systematic deposition studies were performed from pure and H₂-diluted SiH₄ systems with microwave power, total pressure, H₂ dilution ratio and substrate temperature as variables, combined with plasma diagnostics using optical emission spectroscopy and Langmuir probe techniques. The effects of deposition parameters on the film crystallinity, crystal orientation and defect density are demonstrated.     

New materials and deposition techniques for highly efficient silicon thin film solar cells

This paper reviews recent efforts to provide the scientific and technological basis for cost-effective and highly efficient thin film solar modules based on amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon. Textured ZnO:Al films prepared by sputtering and wet chemical etching were applied to design optimised light-trapping schemes. Necessary prerequisite was the detailed knowledge of the relationship between film growth, structural properties and surface morphology obtained after etching. High rate deposition using plasma enhanced chemical vapour deposition at 13.56 MHz plasma excitation frequency was developed for μc-Si:H solar cells yielding efficiencies of 8.1% and 7.5% at deposition rates of 5 and 9 Å/s, respectively. These μc-Si:H solar cells were successfully up-scaled to a substrate area of 30×30 cm² and applied in a-Si:H/μc-Si:H tandem cells showing initial test cell efficiencies up to 11.9%.     

Application of real-time spectroscopic ellipsometry for characterizing the structure and optical properties of microcrystalline component layers of amorphous semiconductor solar cells

Over the past few years, we have applied real-time spectroscopic ellipsometry (RTSE) to probe hydrogenated amorphous silicon (a-Si:H)-based solar cell fabrication on the research scale. From RTSE measurements, the microstructural development of the component layers of the cell can be characterized with sub-monolayer sensitivity, including the time evolution of (i) the bulk layer thickness which provide the deposition rates, and (ii) the surface roughness layer thickness which provide insights into precursor surface diffusion. In the same analysis, RTSE also yields the optical properties of the growing films, including the dielectric functions and optical gaps. Results reported earlier have been confined to p-i-n and n-i-p cells consisting solely of amorphous layers, because such layers are found to grow homogeneously, making data analysis relatively straightforward. In this study, we report the first results of an analysis of RTSE data collected during the deposition of an n-type microcrystalline silicon (μc-Si:H) component layer in an a-Si:H p-i-n solar cell. Such an analysis is more difficult owing to (i) the modification of the underlying i-layer by the H₂-rich plasma used in doped μc-Si:H growth and (ii) the more complex morphological development of μc-Si:H, including surface roughening during growth.     

Silicon-based thin-film solar cells fabricated near the phase boundary by VHF PECVD technique

The light-soaked and annealing behaviors for silicon (Si)-based thin-film single-junction solar cells fabricated near the phase boundary using a very-high-frequency plasma-enhanced chemical vapor deposition (VHF PECVD) technique are investigated. The hydrogen dilution ratio is changed in order to achieve wide band gap hydrogenated amorphous Si (a-Si:H) and narrow band gap hydrogenated microcrystalline Si (μc-Si:H) absorbers. Just below the a-Si:H-to-μc-Si:H transition, highly hydrogen-diluted a-Si:H solar cells with a good stability against light-soaking and fast annealing behavior are obtained. In contrast, the solar cell fabricated at the onset of the μc-Si:H growth is very unstable and its annealing behavior is slow. In the case of μc-Si:H solar cells with the crystal volume fraction of 43–53%, they show the lowest light-induced degradation among the fabricated solar cells. However, it is very difficult to recover the degraded μc-Si:H solar cells via thermal annealing.     

High-pressure condition of SiH4+Ar+H2 plasma for deposition of hydrogenated nanocrystalline silicon film

The characteristics of 13.56-MHz discharged SiH₄+Ar+H₂ plasma at high pressure (2–8 Torr), used for the deposition of hydrogenated nanocrystalline silicon (nc-Si:H) films in a capacitively coupled symmetric PECVD system, has been investigated. Plasma parameters such as average electron density, sheath field and bulk field are extracted from equivalent circuit model of the plasma using outputs (current, voltage and phase) of RF V–I probe under different pressure conditions. The conditions of growth in terms of plasma parameters are correlated with properties of the hydrogenated nanocrystalline silicon films characterized by Raman, AFM and dc conductivity. The film deposited at 4 Torr of pressure, where relatively low sheath/bulk field ratio is observed, exhibits high crystallinity and conductivity. The crystalline volume fraction of the films estimated from the Raman spectra is found to vary from 23% to 79%, and the trend of variation is similar to the RF real plasma impedance data.     

A-Si:H buffer in a-SiGe:H solar cells

Profiled a-SiGe:H-buffer layers between the doped and the absorption layers of amorphous silicon germanium (a-SiGe:H) solar cells are routinely used to avoid bandgap discontinuities and high-defect densities at the p/i- and i/n interface. Here, we present a much simpler approach replacing the profiled a-SiGe:H-buffer layers at both interfaces by a-Si:H-buffer layers. It is demonstrated that for a-SiGe:H solar cells (thickness of the E_G=1.5 eV part is 54 nm) these structures yield similar open circuit voltage V_OC and fill factor (FF) compared to the bandgap profiled layer at the same short circuit current density j_SC. The influence of thickness, optical bandgap and position of the buffer layers on the solar cell performance is investigated.     

Single-crystalline silicon solar cell with pp heterojunction of c-Si substrate and μc-Si : H film

Annealing effects of the single-crystalline silicon solar cells with hydrogenated microcrystaline silicon (μc-Si : H) film were studied to improve the conversion efficiency. Boron-doped (p⁺) μc-Si : H film was deposited in a RF plasma enhanced chemical vapor deposition system (RF plasma CVD) on the rear surface of the cell. With the optimized annealing conditions for the substrate, the conversion efficiency of 21.4% (AM1.5, 25°C, 100 mW/cm²) was obtained for 5 × 5 cm² area single crystalline-solar cell.     

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.     

Amorphous silicon films with high deposition rate prepared using argon and hydrogen diluted silane for stable solar cells

Hydrogenated amorphous silicon films with high deposition rate (4–5 Å/s) and reduced Staebler–Wronski effect are prepared using a mixture of silane (SiH₄), hydrogen and argon. The films show an improvement in short and medium range order. The structural, transport and stability studies on the films are done using X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman scattering studies, electrical conductivity and diffusion length measurement. Presence of both atomic hydrogen and Ar^* in the plasma causes breaking of weak SiSi bonds and subsequent reconstruction of strong bonds resulting in improvement of short and medium range order. The improved structural order enhances the stability of these films against light soaking. High deposition rate is due to the lesser etching of growing surface compared to the case of only hydrogen diluted silane.     

A new light trapping TCO for nc-Si:H solar cells

A new transparent conducting light trapping structure with no free carrier absorption for solar cells is described. Indium oxide doped with molybdenum (IMO), prepared by the hollow cathode sputtering technique, exhibits high charge-carrier mobility up to 80 cm²/V s. No free-carrier absorption in the near infrared region has been found in the IMO. The superior long-wavelength transparency, however, is not sufficient for thin film Si solar cell applications. To obtain the highest possible short circuit current, the TCO needs to possess additional light trapping structure. Anisotropic etching of fiber texture oriented ZnO has been shown to result in an effective light trapping structure. Here we propose a bilayer structure that consists of light trapping-intrinsic ZnO and IMO (the ZnO/IMO bilayer). Both layers show low free-carrier absorption up to the wavelength of 1200 nm. We demonstrate the use of such a transparent conducting light trapping oxide (TCLO) in nanocrystalline (nc-Si:H) solar cells fabricated by a single chamber, batch-type PECVD process. Incorporation of such a transparent conducting light trapping bilayer can increase solar cell short-circuit current density (J_sc) by >30% compared to flat bilayers.     

Thickness dependence of microcrystalline silicon solar cell properties

This paper addresses the performance of pin and nip solar cells with microcrystalline silicon (μc-Si:H) absorber layers of different thickness. Despite the reverse deposition sequence, the behavior of both types of solar cells is found to be similar. Thicker absorber layers yield higher short-circuit currents, which can be fully attributed to an enhanced optical absorption. Open-circuit voltage V_OC and fill factor FF decrease with increasing thickness, showing limitations of the bulk material. As a result of these two contrary effects the efficiency η varies only weakly for absorber layers of 1 to 4 μm thickness, yielding maximum values up to 8.1 %. For a-Si:H/μc-Si:H stacked solar cells an initial efficiency of 12% has been obtained.     

Plasma deposition of thin film silicon: kinetics monitored by optical emission spectroscopy

The optical emission spectroscopy technique is used to characterise the temporal behaviour of a pure silane plasma in the first 90 s after ignition of a static closed-chamber very high frequency glow discharge. Special interest is drawn to the formation of microcrystalline silicon (μc-Si:H) in absence of any hydrogen feedstock gas dilution. The kinetics of the emission lines of SiH^* and H_α is reported. The deposited films are characterised by photothermal deflection spectroscopy, Fourier transform infra red (FT-IR) absorption and show typical microcrystalline fingerprints; for the first time, such material is used as absorber layer in n–i–p type solar cell devices.     

Electronic transport properties of the μc-(Si,Ge) alloys prepared by ECR

Some μc-(Si,Ge):H alloys have been grown using low-pressure, reactive ECR plasma deposition with high H dilution and subtle (sub ppm) B-doping. Incorporating these high-quality materials into devices leads to low-gap μc-(Si,Ge) solar cells with acceptable performance. This justifies a detailed investigation of the electronic transport properties of μc-(Si,Ge):H alloys by employing the microwave photomixing technique.From the measurements of the electric field dependence of the drift mobility and lifetime, we have found strong evidence for the existence of long-range potential fluctuations in μc-(Si,Ge):H alloys. We determine the depth and range of the potential fluctuations, and subsequently the charged defect density, as a function of the deposition rate. It was found that the film transport properties do not degrade or enhance monotonically with increasing deposition rate; there exists a valley point where the strongest potential fluctuations occur as a result of a significant increase in the charged defect density. Beyond this point, the film quality increases again. The evidence indicates that it is the long-range potential fluctuations that result in the deterioration of the transport properties of μc-(Si,Ge):H alloys. Specifically, it is the increase in the depth, and a decrease in the length of the potential fluctuations, which lead to a decrease in the mobility, and consequently in the photoconductivity. Our present results demonstrate that aside from the increase of charged scattering centers, compositional disorder in the alloys play an important role with the build-up of the potential fluctuations.     

Amorphous solar cells, the micromorph concept and the role of VHF-GD deposition technique

During the last two decades, the Institute of Microtechnology (IMT) has contributed in two important fields to future thin-film silicon solar cell processing and design:

(1) In 1987, IMT introduced the so-called “very high frequency glow discharge (VHF-GD)” technique, a method that leads to a considerable enhancement in the deposition rate of amorphous and microcrystalline silicon layers. As a direct consequence of reduced plasma impedances at higher plasma excitation frequencies, silane dissociation is enhanced and the maximum energy of ions bombarding the growing surface is reduced. Due to softer ion bombardment on the growing surface, the VHF process also favours the formation of microcrystalline silicon. Based on these beneficial properties of VHF plasmas, for the growth of thin silicon films, plasma excitation frequencies f_exc in the range 30–300 MHz, i.e. clearly higher than the standard 13.56 MHz, are indeed scheduled to play an important role in future production equipment.

(2) In 1994, IMT pioneered a novel thin-film solar cell, the microcrystalline silicon solar cell. This new type of thin-film absorber material––a form of crystalline silicon––opens up the way for a new concept, the so-called “micromorph” tandem solar cell concept. This term stands for the combination of a microcrystalline silicon bottom cell and an amorphous silicon top cell. Thanks to the lower band gap and to the stability of microcrystalline silicon solar cells, a better use of the full solar spectrum is possible, leading, thereby, to higher efficiencies than those obtained with solar cells based solely on amorphous silicon.

Both the VHF-GD deposition technique and the “micromorph” tandem solar cell concept are considered to be essential for future thin-film PV modules, as they bear the potential for combining high-efficiency devices with low-cost manufacturing processes.     

Performance of double junction a-Si solar cells by using ZnO:Al films with different electrical and optical properties at the n/metal interface

High-quality ZnO:Al films have been prepared by using RF-magnetron-sputtering method with resistivity ranging from 10⁻¹ to 10⁻⁴ Ω cm and transmittance above 90% in visible region. We have fabricated small area (1 cm²) double junction (a-Si/a-Si) solar cells using ZnO/Al and ZnO/Ag as back contact. The conversion efficiency of double junction a-Si solar cell increases from 9.9% to 10.9% by using ZnO/Al back contact and to 11.4% by using ZnO/Ag as back contact. Effect of variation of thickness of i-layer on performance of the cell has also been studied.     

Quantum efficiency and admittance spectroscopy on Cu(In,Ga)Se2 solar cells

We investigate the electronic transport properties of Cu(In,Ga)Se₂ solar cells by means of quantum efficiency and temperature dependent admittance spectroscopy. A simple evaluation scheme of quantum efficiency data is introduced which accounts for recombinatoric losses in the Us buffer layer and in the Cu(In,Ga)Se₂ absorber. By admittance spectroscopy, we find that the controlled incorporation of Na into the absorber material leads to a shallow acceptor state at about 75 meV above the valence band.     

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.     

Doping of a-SiCX:H films including μc-Si:H by hot-wire CVD and their application as a wide gap window for heterojunction solar cells

Impurity doping using B₂H₆ gas using hot-wire CVD has been tried for hydrogenated amorphous silicon-carbon (a-SiC_X:H) alloy films including hydrogenated microcrystalline silicon (μc-Si:H) with carbon content, C/(Si+C), of about 28%. The dark- and photoconductivities of B-doped samples are larger than those of undoped samples. The activation energy for dark conductivity of the B-doped sample with the doping gas ratio, B₂H₆/(SiH₄+CH₄), of about 0.054% is 0.17 eV. This value is smaller than that of the undoped sample. The P-doped samples also show larger dark- and photoconductivities and smaller activation energy than the undoped samples.     

Porous TiO2 thin films synthesized by a spray pyrolysis deposition (SPD) technique and their application to dye-sensitized solar cells

Titanium dioxide (TiO₂) thin films were synthesized on glass substrates from titanium(IV)oxy acetylacetonate 2-butanol solution by a spray pyrolysis deposition (SPD) technique. The films consisted of TiO₂ leaflets and showed the oriented growth along the (2 0 0) direction. The surface area of the film was successfully increased by adding a small amount of aluminum(III) acetylacetonate (AA) in the source solution. This is because AA sublimates easily during the film formation to leave many pores within the film. A dye-sensitized solar cell was constructed with the TiO₂ film which was deposited on the fluorine-doped tin(IV) oxide layer by the SPD technique. The conversion efficiency of the cell was effectively enhanced as high as 3.2% at AA content of 0.6 at% in the source solution, attributing to the fact that the amount of a dye anchored on the surface of TiO₂ layer was the highest at this AA content. Although the conversion efficiency is relatively low, this finding leads to the possibility of an industrial production of a dye-sensitized solar cell in the near future.