Hot-wire CVD thin silicon films on crystalline silicon for solar cell applications
C. VozIsidro MartínA. OrpellaMichael VetterJoaquim PuigdollersR. AlcubillaD. SolerM. FonrodonaJ. BertomeuJ. Andreu
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In this work we study the electronic passivation of crystalline silicon surfaces with thin silicon films deposited by hot-wire chemical vapour deposition. Both intrinsic hydrogenated amorphous silicon and p-doped nanocrystalline silicon films were evaluated on p- and n-type float zone silicon wafers (1-10 /spl Omega//spl middot/cm). The effective surface recombination velocity was measured by the contactless quasi-steady-state photoconductance technique. Heterostructures consisting of a p-doped nanocrystalline silicon layer with a 10 nm thick intrinsic amorphous silicon buffer allowed effective surface recombination velocities of 120 and 170 cm/spl middot/s/sup -1/ on p- and n-type crystalline silicon. Current density-voltage characteristics of rectifying heterojunctions were also measured. These studies are of great interest to evaluate the possibility to obtain high efficiency heterojunction solar cells fully processed at low temperatures.Keywords:
Nanocrystalline silicon
Passivation
Nanocrystalline material
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Silicon solar cell
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Deposition
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Surface passivation of p-type silicon wafers was performed by amorphous silicon carbide films (SiC x ) deposited by plasma enhanced chemical vapor deposition (PECVD). Stacks of two different SiC x layers were applied. The inner layer was rich in silicon and offered good passivation properties. The outer layer was a carbon rich, antireflective coating. Anneals in forming gas were performed to improve surface passivation. Simulation of lifetime measurements indicated the nature of the passivating mechanism. Finally, optical constants were determined by ellipsometry measurements.
Passivation
Anti-reflective coating
Ellipsometry
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The n-type hydrogenated microcrystalline silicon-incorporated silicon oxide (n-μc-SiO:H) films are prepared by 13.56 MHz plasma enhanced chemical vapor deposition (rf-PECVD) using SiH 4 , H 2 and CO 2 gases for an inter-layer between the amorphous silicon (a-Si:H) top-cell and the microcrystalline silicon (μc-Si:H) bottom-cell in the thin-film silicon tandem solar cells. The optical and electrical properties of n-μc-SiO:H prepared by varying deposition conditions are investigated. The refractive indices of the films are 2.1˜2.7 at the wavelength of 600 nm and the electrical conductivity is ˜ 5 × 10 -3 S/cm. Compared to ZnO:Al inter-layer, the n-μc-SiO:H provides reduced current loss of μc-Si:H bottom-cells as well as enhanced current gain of a-Si:H top-cell which might be due to the reduced absorption of incident light in the n-μc-SiO:H inter-layer. An analysis on tunnel recombination junction using current-voltage measurements shows the non-ohmic behavior when crystalline volume fraction and conductivity of inter-layer are low, resulting in the reduction of the FF and V oc in solar cells. The initial conversion efficiency of 10.66% is achieved in thin film silicon tandem solar cells with n-μc-SiO:H inter-layer.
Microcrystalline
Ohmic contact
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Ultrathin Crystalline Silicon Heterojunction Solar Cell Integrated on Silicon-on-Insulator Substrate
To achieve power generation on IC chips, a hydrogenated amorphous silicon (a-Si:H)/crystalline silicon (c-Si) heterojunction solar cell was designed and fabricated on silicon-on-insulator substrate, where a 9- $\mu \text{m}$ epitaxial p-type c-Si layer served as light absorption layer and the buried SiO 2 as back surface passivation layer. It was found that a 1- $\mu \text{m}$ heavily doped thin p + layer was vital for improving the cell performances. Efficiency up to 12.7% with an open-circuit voltage of 679.7 mV was achieved on a 1.0-cm $^{2}$ square cell. The device performance was also investigated by annealing at different temperatures. The results suggested that a relatively large thickness of a-Si:H and transparent conductive oxide layers could improve thermal stability of the solar cells at temperature above 300 °C.
Passivation
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P-type-microcrystalline-silicon/n-type-crystalline-silicon hetero-junction solar cell has been prepared by means of hot-wire chemical vapor deposition (HW-CVD) technique. The solar cell structure was illuminated on the opposite side of the normally-formed heterojunction. With this inverted structure, the photovoltaic cell has the design potential to improve the light-incident surface-texturing with the possibility to avoid the use of transparent conducting oxide (TCO). Solar cells were fabricated on Czochralsky (CZ)-grown phosphorous-doped crystalline-silicon (c-Si) substrates within 0.5 to 1 ohm-cm. HW-CVD has employed for the deposition of a very thin intrinsic hydrogenated amorphous silicon (i-a-Si) as a buffer-layer as a heterojunction interface, and boron-doped hydrogenated microcrystalline silicon (p-μc-Si) on c-Si substrate. The tungsten catalyst temperature (T fil ) was settled to 1600°C and 1950°C for i-a-Si and p-μc-Si films, respectively. Silane (SiH 4 ), hydrogen (H 2 ) and diluted diborane (B 2 H 6 ) gases were used for p-μc-Si at the substrate temperatures (T sub ) of 200°C. The obtained I-V characteristics under simulated solar radiation at 100mW/cm 2 are: Jsc =26.1 mA/cm 2 ; Voc = 545 mV; Jm = 21.4 mA/cm 2 ; Vm = 410 mV; FF = 61.7%, with total area efficiency of η = 8.8%. The solar cell has great potential to improve its conversion efficiency with proper surface passivation and antireflection coat.
Microcrystalline
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In this paper, intrinsic hydrogenated amorphous silicon films are prepared by plasma enhanced chemical vapor deposition. The plasma spectra obtained by an optical emission spectroscopy at different deposition regions are analyzed. By comparing these spectra, the nonuniform deposition regions that could significantly limit the solar cell performance can be found. Tilting the substrate with a certain angle can adjust the electrode-to-substrate distance at the required regions so that the uniformity can be improved. In the case of our experiment, a substrate tilt angle of 1.5° results in a uniformity of within ±8% and a 156 × 156-mm 2 heterojunction solar cell with conversion efficiency of 18.55% that is improved by 11% compared with the cell without substrate tilting.
Deposition
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p-microcrystalline-silicon/n-crystalline-silicon hetero-junction solar cell has been prepared by means of hot-wire chemical vapor deposition (HW-CVD) technique. The solar cell structure was illuminated on the opposite side of the normally-formed heterojunction. With this inverted structure, the photovoltaic cell has the design potential increasing the light-incident surface texturing and it avoids the use of transparent conducting oxide (TCO). The HW-CVD has employed for the deposition of a very thin intrinsic hydrogenated amorphous silicon (i-a-Si) as a buffer-layer, and boron-doped hydrogenated microcrystalline silicon (p-μc-Si) on crystalline-silicon (c-Si) substrate. Solar cells were fabricated on Czochralsky (CZ)-grown phosphorous-doped c-Si within 0.5 to 1 ohm-cm. The tungsten catalyst temperature (T fil ) was settled to 1600 °C and 1950 °C for i-a-Si and p-μc-Si films, respectively. Silane (SiH 4 ) and hydrogen (H 2 ) gases were used and diluted diborane (B 2 H 6 ) for p-doping at the substrate temperatures (T sub ) of 200 °C. The obtained I-V characteristics under simulated solar radiation at 100mW/cm 2 are: Jsc =26.1 mA/cm 2 ; Voc = 545 mV; Jm = 21.4 mA/cm 2 ; Vm = 410 mV; FF = 61.7%, with total area efficiency of η = 8.8%.
Microcrystalline
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P-type amorphous silicon was deposited on crystalline n-type silicon substrates to obtain hetero-junction diodes. Additionally, a thin intrinsic amorphous silicon layer was inserted between both the p-type film and the n-type substrate to study its passivation effect on the c-Si surface. We studied the influence of the quality of the amorphous films upon the performance of the hetero-junction diodes. In particular, the diode ideality factor and the saturation current density were determined by measuring the current-voltage characteristics in dark conditions. The amorphous films were obtained by the hot wire chemical vapor deposition (HWCVD) technique, using a tungsten filament and SiH 4 , H 2 and B 2 H 6 , where the deposition parameters such as gas flow, substrate temperature and filament temperature were varied. It is be shown that the presence of the intrinsic layer is fundamental for making good diodes, since devices made without this film cause the diodes to have high saturation current density and ideality factor (3 × 10 -5 A/cm 2 , n > 8) as compared to diodes with a good intrinsic layer (5 × 10 -9 A/cm 2 , n = 1.4). The results obtained are encouraging, but the quality of the intrinsic films still should be improved for applying them to HIT (Hetero-junction with Intrinsic Thin layer) solar cells.
Saturation current
Passivation
Saturation (graph theory)
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