Polycrystalline silicon thin films and solar cells prepared by rapid thermal CVD

Polycrystalline silicon (poly-Si) films ( 10 μm) were grown from dichlorosilane by a rapid thermal chemical vapor deposition (RTCVD) technique, with a growth rate up to 100 Å/s at the substrate temperature (T_s) of 1030°C. The average grain size and carrier mobility of the films were found to be dependent on the substrate temperature and material. By using the poly-Si films, the first model pn⁺ junction solar cell without anti-reflecting (AR) coating has been prepared on an unpolished heavily phosphorus-doped Si wafer, with an energy conversion efficiency of 4.54% (AM 1.5, 100 mW/cm², 1 cm²).     

Poly-Si thin films by metal-induced growth for photovoltaic applications

Poly-Si films were produced using a metal-induced growth technique by sputtering from an n-type Si target onto a 50 nm thick Co seed-layer at 625°C. Silicon grew heteroepitaxially on the CoSi₂ layer formed due to the reaction between the sputtered Si atoms and Co at the beginning stage of deposition. A 5 μm thick Si film with grain features up to 1 μm was produced on the thin and flexible tungsten substrate by using a two-step sputtering method. The films also have a natural texture structure on the surface that is strongly recommended in thin-film solar cells in order to obtain high current density by increasing incident light trapping. After post-sputtering annealing at 700°C, the measured minority carrier lifetime for poly-Si film was 1.33 μs which shows the film to be suitable for photovoltaic applications. To explore the photovoltaic applications by using MIG poly-Si films, Au/n-Si Schottky photodiodes were fabricated due to the process simplicity. The effects of different parameters, which include film doping density, active-layer thickness, Si film surface conditions and hydrogenation, were studied. It was found that with the increasing of doping density, the open-circuit voltage (V_oc) increased while short-circuit current density (J_sc) decreased. Increasing the poly-Si active-layer thickness tended to improve the light absorption with an increased J_sc, but the V_oc was decreased due to a higher value of reverse saturation current. Because the metal/semiconductor interface condition facilitates the carrier transport in Schottky devices, the earlier study of modifying the Si surface by polishing showed an improved V_oc. The overall photo response was further improved by plasma hydrogenation.     

Dependence of the recombination in thin-film Si solar cells grown by ion-assisted deposition on the crystallographic orientation of the substrate

One promising strategy for achieving high-quality polycrystalline silicon thin-film solar cells on glass is based on low-temperature ion-assisted deposition for epitaxial thickening of a thin, large-grained seeding layer on glass. The crystal growth on the seeding layer is influenced by various factors, amongst which the crystal orientation of the grains plays a substantial role. In this paper we investigate how the electronic properties of solar cells grown on “ideal” seeding layers (Si wafers) are influenced by the crystallographic orientation of the substrate. The Si wafers are heavily doped p-type, ensuring that their contribution to the photogenerated current is small. The films grown on (1 0 0)-oriented Si substrates have a very low density of structural defects, while the films grown on (1 1 1)-oriented Si substrates display a high density of twin defects. The electronic properties of the thin-film solar cells were investigated by means of open-circuit voltage measurements as a function of the incident light intensity. The (1 0 0)-oriented diodes consistently exhibit a higher V_oc than the (1 1 1)-oriented diodes throughout the entire illumination range from 10⁻³ to 10³ Suns. We determine 7 μm as the bulk minority carrier diffusion length of the as-grown (1 0 0)-oriented Si film. A lower bound of 3 μm was found for the bulk minority carrier diffusion length in the as-grown (1 1 1)-oriented Si film. The performances of both types of solar cells were improved by hydrogenation in an ammonia plasma. At voltages around the 1-Sun maximum power point the improvement is due to a reduction of non-ideal current mechanisms. The diffusion length of the (1 0 0) diode remains unaffected by hydrogenation while the lower bound of the diffusion length of the (1 1 1) diode improves to 10 μm.     

High-efficiency drift-field thin-film silicon solar cells grown on electronically inactive substrates

A drift-field in the base region of a solar cell can enhance the effective minority-carrier diffusion length, thus increasing the long-wavelength spectral response and energy-conversion efficiency. Silicon thin-films of 20–32 μm thickness as a cell base layer were grown by liquid-phase epitaxy (LPE) on electronically inactive heavily doped p⁺⁺-type CZ silicon substrates. Growth was performed from In/Ga solutions, and in a purified Ar/4%H₂ forming gas ambient, rather than pure H₂. The Ga dopant concentration was tailored throughout the p-type film to create a drift-field in the base layer of the solar cell. An independently confirmed efficiency of 16.4% was achieved on such an LPE drift-field thin-film silicon solar cell with a total cell area of 4.11 cm². Substrate thinning, combined with light trapping which is encouraged by the textured front surface and a highly reflective aluminium rear surface, is demonstrated to improve the long-wavelength response and thus, increase cell efficiency by a factor of up to 23.7% when thinned to a total cell thickness of 30 μm.     

Fabrication of poly-crystalline silicon films using plasma spray method

We have developed a new fabrication technique of poly-crystalline silicon (poly-Si) sheet for solar cells, namely the DC-RF hybrid plasma spray method. It has some advantages such as high deposition rate of more than 10 μm/s and large grain size of the obtained poly-Si films. Poly-Si films with a grain size of more than 20 μm and a defect density of 10⁶–10⁷ cm⁻² have been obtained at the initial stage trial. The solar cell conversion efficiency of 4.3% has been obtained using the plasma sprayed poly-Si. It is considered that the reasons for the low conversion efficiency are metallic impurity contamination, regions of micro grain due to rapid nucleation, and many defects in the films due to thermal stress.     

Ultra-thin bifacial CdTe solar cell

Developing a high-quality transparent back contact, while maintaining efficient light transmission through the top absorber layer, are key components for achieving high-efficiency II–VI polycrystalline thin-film tandem solar cells. Combining these two elements, we fabricated ultra-thin bifacial CdTe solar cells (0.68 μm) with ZnTe:N/ITO transparent back contact and achieved efficiencies of 5.7% and 5.0% with illumination from the glass and the contact side, respectively. Device analysis, using (J–V) and QE measurements, show that the loss in efficiency is due to higher R_S and J₀ as well as lower, side-dependent, photons absorption.     

Heterogeneous growth of microcrystalline silicon germanium

Microcrystalline silicon germanium films showing excellent opto-electronic properties have been prepared at a substrate temperature of 195°C by radio frequency plasma enhanced chemical vapor deposition at 13.56 MHz. A white light (AM 1.5) photoconductivity of 5×10⁻⁵/Ω cm and ambipolar diffusion length of 114 nm (from SSPG) established the device quality. Films are intrinsic (Fermi level near midgap; activation energy E_a (0.49 eV) is approximately half the band gap (1.01 eV)). Performance of preliminary n–i–p solar cells (with μc-SiGe:H i-layer) on stainless steel and molybdenum substrates justify their photosensitivities. A current density of 9.44 mA/cm² has been generated in an i-layer of only 150 nm thick without any back-reflector. A deposition rate of 0.75 Å/s for such a thin layer gives this material much advantage than a μc-Si cell, where a thickness of >2 μm is needed. A high V_oc of 0.43 eV has been achieved for such a low mobility gap cell (Ge fraction 60%).     

Effects of grain boundaries on cell performance of poly-silicon thin film solar cells by 2-D simulation

The effect of grain boundaries on the performance of poly-Si thin film solar cells was studied theoretically using a 2-D simulation assuming the presence of either rectangular-shaped or graded width grain boundaries in the i-layer of p/i/n structure of solar cells. The grain boundary had an adverse effect mainly on V_oc. J_sc gradually increased and saturated with increasing solar cell thickness in cells without grain boundaries, whereas it reached a maximum for an i-layer thickness of 5 μm in polycrystalline silicon cells. The calculation using the graded width model showed that the efficiency of the p⁺/p⁻/n⁺ structure was better than that of the p⁺/n⁻/n⁺ structure. A slight p-type doping of the i-layer was found to be effective in improving cell performance.     

Thin film poly-Si formation for solar cells by Flux method and Cat-CVD method

Two methods were examined for the formation of poly-Si films. One is flux method and the other is Cat-CVD method. Flux method was used for forming poly-Si seed films on glass substrates covered with rear electrode. Poly-Si films of a few μm grain size and of mainly (1 1 1) crystalline orientation were obtained at less than 600°C. To make the seed films function as BSF layer for solar cell, boron doping was applied and carrier concentration of 2×10¹⁹/cm³ was obtained which is suitable for highly efficient solar cells. Cat (catalytic)-CVD method was examined for forming poly-Si photo-active layers on the seed films. The films showed deposition gas pressure-dependent crystalline orientations and there was no amorphous incubation layer in (1 1 1) oriented films by Cat-CVD method when deposited on (1 1 1) oriented seed films prepared by Flux method. The electrical properties of the film are insufficient at present, may be due to high defect density and the film structure which allows impurity contaminations of oxygen and carbon after film deposition. Although the film quality needs to be improved, poly-Si films whose crystal fraction is more than 85% were obtained at deposition rate of up to around 40 Å/s. This result indicates high potential of Cat-CVD method for high throughput photo-active formation process necessary for low production cost thin film silicon solar cells.     

Back electrode formation for poly-Si thin film solar cells on glass having AIC-grown seeding layer

Various conductive materials (Al, Mo and TiN) were deposited onto glass substrates to evaluate whether poly-Si seed layers can be formed on such substrates by means of Al-induced crystallisation (AIC) of a-Si at low temperature around 450°C. The material located between the glass and the poly-Si film serves as the back electrode of a substrate-type thin-film solar cell configuration. The outcome of the investigation is that Mo is found to be not compatible with the AIC process. In contrast, Al and TiN showed moderate to good compatibility. TiN is the only viable choice for high-temperature applications (>540°C). Al has satisfactory back electrode properties whereas TiN has a medium high resistivity (120 μΩ cm) and an estimated low back reflectance at the near-infrared wavelengths critical for light trapping.     

High current, thin silicon-on-ceramic solar cell

Solar cells utilizing thin-film polycrystalline silicon can achieve photovoltaic power conversion efficiencies greater than 19%. These high efficiencies are a result of light trapping and back surface passivation. Optimum silicon thickness, for devices employing such technology, has been determined to be 20 μm (Blakers et al., Appl. Phys. Lett. 60 (1992) 2572) to 35 μm (Rand et al., Proceedings of the IEEE Photovoltaic Specialist Conference, Orlando, FL, May 1990, p. 263). Low cost is achieved by minimizing the required amount of silicon feedstock per watt of power output. The use of an electrically insulating supporting substrate allows for monolithic, series connected sub-modules. A solar cell with a 20 μm thick polycrystalline Si-layer on a ceramic substrate, utilizing both light trapping and back-surface passivation, was fabricated and characterized. A short-circuit current of 25.8 mA/cm² was measured and verified by the National Renewable Energy Laboratory (NREL).     

Photovoltaic properties of close-space sublimated CdTe solar cells

CdS/CdTe solar cells were fabricated by close-space sublimation with a screen-printed Te-rich CdTe source and their photovoltaic properties were investigated by varying the substrate temperature, cell area, and thicknesses of CdTe and ITO layers. The resistivity of CdTe layers employed in this study was 3×10⁴ Ω cm. The optimum substrate temperature and thickness for CdTe deposition were 600°C and 5 μm, respectively. The CdTe bulk resistance degraded the cell performance above 6 μm. As the cell area increased the V_oc remained almost constant, while the J_sc and FF were strongly degraded because of the increase of the lateral resistance of the ITO layer. The optimum thickness of the ITO layer in this study was 300–450 nm. In this experiment we obtained an efficiency of 9.4% in the 0.5 cm² cells. The series resistance of the cell should be further reduced to increase the fill factor and improve the efficiency.     

Polycrystalline silicon solar cells on mullite substrates

Polycrystalline silicon layers have been grown on various alumino-silicate substrates in a rapid thermal chemical vapor deposition (RTCVD) system at high temperatures (>1000°C). Structural analysis shows a columnar growth with grain sizes up to 15 μm and growth rates up to 5 μm/min. Solar cell devices on this fine-grained Si material result in a short-circuit current of about 13 mA/cm² but a poor open-circuit voltage (<0.4 V). Larger grains obtained by the zone melting recrystallization (ZMR) technique boosted the current up to 26.1 mA/cm², thanks to the light-trapping by the mullite substrate. Best efficiency is 8.2% on a 1 cm² cell made on a 20 μm thick poly-Si layer.     

Numerical analysis of bulk diffusion length in thin-film c-Si solar cells

We propose a novel technique of determining relationship between effective and bulk diffusion length of single-crystalline Si (c-Si) thin-film solar cells using two-dimensional device simulator. In addition, bulk diffusion length was obtained using the result of the simulation. Effective diffusion length was measured by LBIC method in order to presume bulk diffusion length of c-Si thin film. We obtained 6.7 μm for effective diffusion length of c-Si thin-film solar cell whose thickness was about 7 μm. We compared the result of measurement and simulation, bulk diffusion length of c-Si thin film prepared by CVD method was estimated more than 30 μm and recombination velocity was presumed <10⁴ cm/s for front surface and 10³ cm/s for rear surface of the cell.     

Crystalline silicon thin-film solar cells on ZrSiO4 ceramic substrates

The development of a low-cost substrate is one of the major technological challenges for crystalline Si thin-film solar cells. Zirconium silicate (ZrSiO₄) ceramics is a material which can meet the demanding physical requirements as well as the cost goals. Thin microcrystalline Si films were deposited by atmospheric pressure CVD on ZrSiO₄-based ceramic substrates coated with barrier layers. The Si film was transferred into a multicrystalline grain structure by zone-melting recrystallization (ZMR). Film growth was analyzed in situ and correlated with substrate and barrier layer properties. Thin-film solar cells were fabricated from selected coarse-grained films. The best solar cell achieved an efficiency of 8.3% with a short circuit current density of 26.7 mA/cm². The effective diffusion length obtained from internal quantum efficiency measurements was about 25 μm.     

Aluminium-induced crystallisation of silicon on glass for thin-film solar cells

Aluminium-induced crystallisation of amorphous silicon is studied for the formation of continuous polycrystalline silicon thin-films on low-temperature glass substrates. It is shown to be a promising alternative to laser crystallisation and solid-phase crystallisation. Silicon grain sizes of larger than 10 μm are achieved at temperatures of around 475°C within annealing times as short as 1 h. The Al doping concentration of the poly-Si films depends on the annealing temperature, as revealed by Hall effect measurements. A poly-Si/Al/glass structure presented here can serve as a seeding layer for the epitaxial growth of polycrystalline silicon thin-film solar cells, or possibly as the base material with the back contact incorporated.     

Quantum dot integration in heterostructure solar cells

InAs self-assembled quantum dots (SA-QDs) were incorporated into GaAlAs/GaAs heterostructure for solar cell applications. The structure was fabricated by molecular beam epitaxy on p-GaAs substrate. After the growth of GaAs buffer layer, multi-stacked InAs QDs were grown by self-assembly with a slow growth rate of 0.01 ML/s, which provides high dot quality and large dot size. Then, the structure was capped with n-GaAs and wide band gap n-GaAlAs was introduced. One, two or three stacks of QDs were sandwiched in the p–n heterojunction. The contribution of QDs in solar cell hetero-structure is the quantized nature and a high density of quantized states. I–V characterization was conducted in the dark and under AM1 illumination with 100 mW/cm² light power density to confirm the solar cell performance. Photocurrent from the QDs was confirmed by spectral response measurement using a filtered light source (1.1-μm wavelength) and a tungsten halogen lamp with monochromator with standard lock-in technique. These experimental results indicate that QDs could be an effective part of solar cell heterostructure. A typical I–V characteristic of this yet-to-be-optimized solar cell, with an active area of 7.25 mm², shows an open circuit voltage V_oc of 0.7 V, a short circuit current I_sc of 3.7 mA, and a fill factor FF of 0.69, leading to an efficiency η of 24.6% (active area).     

Manufacturing method for transparent electric windows using dye-sensitized TiO2 solar cells

The transparent electric windows based on dye-sensitized nanocrystalline TiO₂ solar cells have been prepared. The solar cell consists of dye-sensitized TiO₂ electrode with a TiO₂ layer of an about 8 μm thickness and of a 80×80 mm² active area, Pt counter electrode and redox electrolyte. The solar cell shows a transmittance of approximately 60% in the visible range and an open-circuit voltage (V_oc) of 0.64 V and a short-circuit photocurrent (J_sc) of 250 mA. A moderately transparent electric window composed of nine unit solar cells in series generates V_oc of 5.7 V and J_sc of 220 mA at one sun light intensity.     

Enhanced quantum yield in porphyrin/electron-donor double-layer solar cells

When the layer of 3-carboxymethyl-5-[(3-ethyl-2(3H)-benzothiazolylidine)ethylidene (MC(COOH)) is inserted into the Au/Zntpyp interface in Al/Zntpyp/Au sandwich-type solar cell (Zntpyp: 5,10,15,20-tetra(3-pyridyl)porphyrinatozinc), the photovoltaic properties are remarkably improved. For the Al/Zntpyp(thickness 10 nm)/MC(COOH)(20 nm)/Au cell, a short-circuit photocurrent (J_sc) of 0.93 μ Acm⁻², open-circuit photovoltage (V_oc) of 0.71 V, fill factor (ff) of 0.41, and energy conversion yield (η) of 3.6% are obtained when illuminated at the Al/Zntpyp interface with 455 nm monochromatic light of 7.5 μ Wcm⁻² intensity. A rapid electron-transfer from the donor MC(COOH) to photogenerated holes in Zntpyp suppresses the charge recombination of the photogenerated carriers. The energetically well-arranged valence band levels eventually enhance the η value about 9 times compared with the Al/Zntpyp/Au cell. Further the Al/HD(9 nm)/MC(COOH)(20 nm)/Au cell using a longer-lived sensitizer (HD) instead of Zntpyp gives a J_sc value of 2.36 μ Acm⁻², V_oc value of 0.69 V, ff value of 0.34, and η value of 4.8% when illuminated with 445 nm monochromatic light of 11.7 μ Wcm⁻² intensity at the Al/HD interface, where HD represents a heterodimer consisting of 5,10,15-tri(4-chlorophenyl)-20-(3-pyridyl)porphyrin(H₂pyp₃p(Cl)) and 5,10,15,20-tetra(2,5-dimethoxyphenyl)porphyrinatozinc(Zntpp(OMe)₂).     

Large-area high efficiency single crystalline silicon solar cells

This paper reports on a 100 cm² single crystalline silicon solar cell with a conversion efficiency of 19.44% (J_sc = 37.65 mA/cm², V_oc = 638 mV, FF = 0.809). The cell structure is as simple as only applying the textured surface, oxide passivation, and back surface field by the screen printing method. The comparison between cell performances of the CZ (Czochralski) and FZ (Floating zone) silicon substrates was investigated. The higher efficiency cells were obtained for the FZ substrate rather than the CZ substrate. The influence of the phosphorus concentration of the emitter on the cell efficiency has also been investigated. A good result was obtained when the surface concentration of phosphorus was 3 × 10²⁰ cm⁻³ and the junction depth was about 0.6 μm.