Fabrication of selective-emitter silicon heterojunction solar cells using hot-wire chemical vapor deposition and laser doping

The Si heterojunction (HJ) solar cells were fabricated on the textured p-type mono-crystalline Si (c-Si) substrates using hot-wire chemical vapor deposition (HWCVD). In view of the potential for the bottom cell in a hybrid junction structure, the microcrystalline Si (μc-Si) film was used as the emitter with various PH₃ dilution ratios. Prior to the n-μc-Si emitter deposition, a 5 nm-thick intrinsic amorphous Si layer (i-a-Si) was grown to passivate the c-Si surface. In order to improve the indium-tin oxide (ITO)/emitter front contact without using the higher PH₃ doping concentration, a laser doping technique was employed to improve the ITO/n-μc-Si contact via the formation of the selective emitter structure. For a cell structure of Ag grid/ITO/n-μc-Si emitter/i-a-Si/textured p-c-Si/Al-electrode, the conversion efficiency (AM1.5) can be improved from 13.25% to 14.31% (cell area: 2 cm × 2 cm) via a suitable selective laser doping process.     

Tungsten filament effect on electronic properties of hot-wire CVD silicon films for heterojunction solar cell application

In this study, we describe the correlation between cell efficiency and wire aging during hot-wire chemical vapor deposition in detail. The new and aged tungsten (W) filaments were used to deposit the n-type microcrystalline silicon (μc-Si) films for heterojunction (HJ) Si solar cell applications. Tungsten silicide (WSi_x) was coated on the W catalyzer surface (center and end regions) after each deposition, and which was investigated and determined by scanning electron microscopy and electron probe microanalysis. The wire age has an effect on the resulting electronic properties of the grown film, thought to be related to differences in dark conductivity with aged versus new wires. It was found that the aging process is related to the formation of a silicide at the surface. A limited amount of silicon was observed in the bulk of catalyzer, suggesting that silicon diffusion into the wire has occurred. The original single-side HJ solar cell with efficiency of 15.3% has been fabricated using the new wires. The quality of n-type μc-Si films and efficiency of HJ solar cells were reduced when the aged W filament was employed. The quality of silicon films and the efficiency of HJ solar cell could be improved after regeneration process.     

Intrinsic microcrystalline silicon prepared by hot-wire chemical vapour deposition for thin film solar cells

Microcrystalline silicon (μc-Si:H) prepared by hot-wire chemical vapour deposition (HWCVD) at low substrate temperature T_S and low deposition pressure exhibits excellent material quality and performance in solar cells. Prepared at T_S below 250 °C, μc-Si:H has very low spin densities, low optical absorption below the band gap, high photosensitivities, high hydrogen content and a compact structure, as evidenced by the low oxygen content and the weak 2100 cm⁻¹ IR absorption mode. Similar to PECVD material, solar cells prepared with HWCVD i-layers show increasing open circuit voltages V_oc with increasing silane concentration. The best performance is achieved near the transition to amorphous growth, and such solar cells exhibit very high V_oc up to 600 mV. The structural analysis by Raman spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM) shows considerable amorphous volume fractions in the cells with high V_oc. Raman spectra show a continuously increasing amorphous peak with increasing V_oc. Crystalline fractions X_C ranging from 50% for the highest V_oc to 95% for the lowest V_oc were obtained by XRD. XRD-measurements with different incident beam angles, TEM images and electron diffraction patterns indicate a homogeneous distribution of the amorphous material across the i-layer. Nearly no light induced degradation was observed in the cell with the highest X_C, but solar cells with high amorphous volume fractions exhibit up to 10% degradation of the cell efficiency.     

Quantitative analysis of tungsten, oxygen and carbon concentrations in the microcrystalline silicon films deposited by hot-wire CVD

In order for hot-wire chemical vapor deposition to compete with the conventional plasma-enhanced chemical vapor deposition technique for the deposition of microcrystalline silicon, a number of key scientific problems should be cleared up. Among these points, the concentration of tungsten (nature of the filament), as well as the concentration of oxygen and carbon (elements issued when vacuum is broken between two runs), should not exceed threshold values, beyond which electronic properties of the films could be degraded, as in the case of monocrystalline silicon. Quantitative chemical analysis of these elements has been carried out using the secondary ion mass spectrometry technique through depth profiles. It has been shown that for a high effective filament surface area (S_f=27 cm²), the W content increases steadily from 5×10¹⁴ to 2×10¹⁸ atoms cm⁻³ when the filament temperature T_f increases from 1500 to 1800 °C. For a fixed T_f, the W content increases with the effective surface area S_f. Thus, considering our reactor geometry, the W content does not exceed the detection limit (5×10¹⁴ atoms cm⁻³) when T_f and S_f are limited to 1600 °C and 4 cm², respectively. For O and C elements, under deposition conditions of high dilution of silane in hydrogen (96%), O and C concentrations approaching 10²⁰ atoms cm⁻³ have been obtained. The introduction of an inner vessel inside the reactor, the addition of a load-lock chamber and a decrease in substrate temperature to 300 °C have led to a drastic decrease in these contents down to 3×10¹⁸ atoms cm⁻³, compatible with the realization of 6% efficiency HWCVD μc-Si:H solar cells.     

Microcrystalline silicon for solar cells deposited at high rates by hot-wire CVD

Using two tungsten (W) filaments and a filament–substrate spacing of 3.2 cm, we have explored the deposition of microcrystalline silicon (μc-Si) solar cells, with the i-layer deposited at high deposition rates (R_d), by the hot-wire CVD (HWCVD) technique. These cells were deposited in the n–i–p configuration on textured stainless steel (SS) substrates, and all layers were deposited by HWCVD. Thin, highly crystalline seed layers were used to facilitate crystallite formation at the n–i interface. Companion devices were also fabricated on flat SS substrates, enabling structural measurements (by XRD) to be performed on i-layers used in actual device structures. Using a filament temperature of 1750 °C, device performance was explored as a function of i-layer deposition conditions, including variations in i-layer substrate temperature (T_sub) using constant H₂ dilution, and also variations in H₂ dilution during i-layer deposition. The intent of the latter is to affect crystallinity at the top surface of the i-layer (i–p interface). We report device performance resulting from these studies, with all i-layers deposited at R_d>5 Å/s, and correlate them with i-layer structural studies. The highest device efficiency reported is 6.57%, which is a record efficiency for an all-hot-wire solar cell.     

Structural properties of microcrystalline Si films prepared by hot-wire/catalytic chemical vapor deposition under conditions close to the transition from amorphous to microcrystalline growth

The structural properties of microcrystalline Si films prepared by hot-wire/catalytic chemical vapor deposition, with various dilution ratios of silane in hydrogen, were investigated as regards to the role of hydrogen. A large surface roughness correlated with a low crystalline nuclei density was observed for microcrystalline Si films deposited near the transition from amorphous to microcrystalline growth. Investigations of hydrogen-related properties suggest the presence of molecular hydrogen in these films. We tentatively propose that the diffusion of atomic hydrogen into the subsurface layer of growing films, which leads to the relaxation of amorphous Si network and to the generation of molecular hydrogen, plays an important role for determining the film properties, besides top surface reactions.     

Incorporation of amorphous and microcrystalline silicon in n–i–p solar cells

We have investigated the material properties and n–i–p solar cell quality of hot-wire deposited amorphous and microcrystalline silicon. Although it is possible to make high quality amorphous silicon solar cells, occasionally many cells show shunting behavior. Therefore, better control over the variation in cell performance is needed. We prove that this behavior is correlated with the filament age and different methods for improving the reproducibility of the cell performance are presented. Furthermore, the influence of different deposition parameters of microcrystalline silicon layers on the material and solar cell properties was studied. Although some of these microcrystalline layers are porous and oxidize in air, an initial efficiency of 4.8% is obtained for an n–i–p cell on untextured stainless steel.     

Amorphous/crystalline silicon heterojunction solar cells with varying i-layer thickness

We study the effect on various properties of varying the intrinsic layer (i-layer) thickness of amorphous/crystalline silicon heterojunction (SHJ) solar cells. Double-side monocrystalline silicon (c-Si) heterojunction solar cells are made using hot-wire chemical vapor deposition on high-lifetime n-type Czochralski wafers. We fabricate a series of SHJ solar cells with the amorphous silicon (a-Si:H) i-layer thickness at the front emitter varying from 3.2 nm (0.8xi) to ~ 96 nm (24xi). Our optimized i-layer thickness is about 4 nm (1xi). Our reference cell (1xi) performance has an efficiency of 17.1% with open-circuit voltage (V_oc) of 684 mV, fill factor (FF) of 76%, and short-circuit current density (J_sc) of 33.1 mA/cm². With an increase of i-layer thickness, V_oc changes little, whereas the FF falls significantly after 12 nm (3xi) of i-layer. Transient capacitance measurements are used to probe the effect of the potential barrier at the n-type c-Si/a-Si interface on minority-carrier collection. We show that hole transport through the i-layer is field-driven transport rather than tunneling.     

Amorphous silicon carbide heterojunction solar cells on p-type substrates

The performance of silicon heterojunction (SHJ) solar cells is discussed in this paper in regard to their dependence on the applied amorphous silicon layers, their thicknesses and surface morphology. The emitter system investigated in this work consists of an n-doped, hydrogenized, amorphous silicon carbide a-SiC:H(n) layer with or without a pure, hydrogenized, intrinsic, amorphous silicon a-Si:H(i) intermediate layer. All solar cells were fabricated on p-type FZ-silicon and feature a high-efficiency backside consisting of a SiO₂ passivation layer and a diffused local boron back surface field, allowing us to focus only on the effects of the front side emitter system. The highest solar cell efficiency achieved within this work is 18.5%, which is one of the highest values for SHJ-solar cells using p-type substrates. A dependence of the passivation quality on the surface morphology was only observed for solar cells including an a-Si:H(i) layer. It could be shown that the fill factor suffers from a reduction due to a reduced pseudo fill factor for emitter thicknesses below 11 nm due to a lower passivation quality and/or a higher potential for shunting thorough the a-Si emitter to the crystalline wafer with the conductive indium tin oxide layer. Furthermore, the influence of a variation of the doping gas flow (PH₃) during the plasma enhanced chemical vapor deposition of the doped amorphous silicon carbide a-SiC:H(n) on the solar cell current-voltage characteristic-parameter has been investigated. We could demonstrate that a-SiC:H(n) shows in principle the same dependence on PH₃-flow as pure a-Si:H(n).     

Growth of c-GaN films on GaAs(100) using hot-wire CVD

Cubic gallium nitride (GaN) films were grown on nitrided layers of GaAs(100) by hot-wire chemical vapor deposition. The nitrided layer was also formed by NH_x radicals generated on a tungsten hot-wire surface. Nitridation conditions for the growth of GaN with a cubic-type structure were investigated. As a result, GaN film with a preponderant cubic phase was grown on the GaAs surface layer nitrided at a substrate temperature of 550 °C, a filament temperature of 1200 °C and an ammonia (NH₃) pressure of 1 Torr.     

Growth of GaN films on nitrided GaAs substrates using hot-wire CVD

Epitaxial growth of cubic-type gallium nitride (c-GaN) by hot-wire CVD on GaAs(100) substrates was investigated. Prior to the epitaxial growth, a nitridation layer was formed using ammonia plasma generated by electron cyclotron resonance (ECR). It was found that the crystal phase of the epitaxial layer was predominantly determined by that of the nitrided layer. The best nitridation condition using ECR plasma for the growth of the GaN films with preponderant cubic-type structure was obtained.     

Improvement of the efficiency of triple junction n-i-p solar cells with hot-wire CVD proto- and microcrystalline silicon absorber layers

Hot-wire chemical vapour deposition (HWCVD) was applied for the deposition of intrinsic protocrystalline (proto-Si:H) and microcrystalline silicon (μc-Si:H) absorber layers in thin film solar cells. For a single junction μc-Si:H n-i-p cell on a Ag/ZnO textured back reflector (TBR) with a 2.0 μm i-layer, an 8.5% efficiency was obtained, which showed to be stable after 750 h of light-soaking. The short-circuit current density (J_sc) of this cell was 23.4 mA/cm², with a high open-circuit voltage (V_oc) and fill factor (FF) of 0.545 V and 0.67.Triple junction n-i-p cells were deposited using proto-Si:H, plasma-deposited proto-SiGe:H and μc-Si:H as top, middle and bottom cell absorber layers. With Ag/ZnO TBR's from our lab and United Solar Ovonic LLC, respective initial efficiencies of 10.45% (2.030 V, 7.8 mA/cm², 0.66) and 10.50% (2.113 V, 7.4 mA/cm², 0.67) were achieved.     

Aluminum doped silicon carbide thin films prepared by hot-wire CVD: Investigation of defects with electron spin resonance

Al-doped p-type μc-SiC:H is prepared in a wide range of HWCVD preparation parameters like Al-doping ratio, deposition pressure, substrate and filament temperatures. We investigate the structural and electrical properties, and focus on identification of paramagnetic defect states by electron spin resonance (ESR). Nominally undoped μc-SiC:H is of a high n-type conductivity (σ_D = 10^− 6-10^− 1 S/cm) and shows a narrow central ESR line (g ≈ 2.003, peak-to-peak linewidth ?H_pp ≈ 4 G) with two pairs of satellites and a spin density N_S = 10¹⁹ cm^− 3. Al-doping results in the compensation of dark conductivity to as low as σ_D = 10^− 11 S/cm and at higher doping concentrations to effective p-type material. Increase of Al-doping results in reduction of crystallinity (I_C^IR), ESR line shifts to g ≈ 2.01 and becomes as broad as ?H_pp ≈ 30 G, not unlike to the resonance of singly occupied paramagnetic valence band tail states in a-Si:H. ESR spectrum of highly crystalline Al-doped μc-SiC:H however has a g-value very close to undoped μc-SiC:H. Electron spin density in compensated material decreases to 5 × 10¹⁷ cm^− 3 before it increases again for the highly doped material.     

High-rate deposition of amorphous silicon films using hot-wire CVD with a coil-shaped filament

To reduce the manufacturing cost of amorphous silicon (a-Si:H)-based photovoltaic devices, it is important to deposit high-quality a-Si:H and related materials at a high deposition rate. To this end, we designed and constructed a hot-wire deposition chamber with a coiled filament design and with multiple gas inlets. The process gas could be directed into the chamber through the filament coil and have maximum exposure to the high-temperature filament surface. Using such a chamber design, we deposited a-Si:H films at high deposition rates up to 800 Å s⁻¹ and dense, low-void a-Si:H at rates up to 240 Å s⁻¹.     

Preparation of n-type nanocrystalline 3C-SiC films by hot-wire CVD using N2 as doping gas

N-type nanocrystalline 3C-SiC films were prepared by hot-wire chemical vapor deposition from SiH₄/CH₄/H₂ and N₂ as a doping gas and the structural and electrical properties were investigated. The gas flow rates of SiH₄, CH₄ and H₂ were 1, 1 and 200 sccm, respectively. As the N₂ gas flow rate was increased from 0 to 10 sccm, the conductivity and the activation energy improved from 0.05 to 0.3 S/cm and from 45 to 28 meV, respectively. The Hall Effect measurement proved that the improvement of the electrical properties was caused by the increase in the carrier concentration. On the other hand, in the N₂ gas flow rate between 10 and 50 sccm, the conductivity and the activation energy remained unchanged. The crystallinity deteriorated with increasing N₂ gas flow rate. This gave rise to the unchanged electronic properties in spite of the increase in the intake of N atoms.     

Hydrogen passivation and junction formation on APIVT-deposited thin-layer silicon by hot-wire CVD

The hot-wire chemical vapor deposition (HWCVD) technique was employed to deposit μc-Si emitters and a-SiN_x:H passivation/antireflection films, and to hydrogenate silicon thin layers grown by atmospheric-pressure iodine vapor transport (APIVT). Photovoltaic devices with HWCVD μc-Si emitters on APIVT epitaxial silicon exhibit greater than 8% efficiency, similar to those made with diffused junctions. On polycrystalline APIVT-Si layers, a HWCVD-deposited μc-Si emitter reduces open-circuit voltage loss caused by grain boundaries. Hot-wire hydrogenation improves Hall mobility by approximately 50%. HWCVD a-SiN_x:H films improve minority-carrier lifetime significantly after thermal annealing at temperatures up to 500 °C.     

Microcrystalline silicon carbide thin films grown by HWCVD at different filament temperatures and their application in n-i-p microcrystalline silicon solar cells

To optimize the performance of microcrystalline silicon carbide (µc-SiC:H) window layers in n-i-p type microcrystalline silicon (µc-Si:H) solar cells, the influence of the rhenium filament temperature in the hot wire chemical vapor deposition process on the properties of µc-SiC:H films and corresponding solar cells were studied. The filament temperature T_F has a strong effect on the structure and optical properties of µc-SiC:H films. Using these µc-SiC:H films prepared in the range of T_F = 1800-2000 °C as window layers in n-side illuminated µc-Si:H solar cells, cell efficiencies of above 8.0% were achieved with 1 µm thick µc-Si:H absorber layer and Ag back reflector.     

Status report: solar cell related research and development using amorphous and microcrystalline silicon deposited by HW(Cat)CVD

This article reviews the research and development of a-Si:H and μc-Si:H based solar cells by using hot wire chemical vapor deposition (HWCVD). The groups involved and the present status of conversion efficiencies attained are listed and will be discussed for different cell structures realized entirely or partly using this method. There are three main advantages of HWCVD: a quite simple set up, higher useable deposition rates and higher stability of HW-a-Si:H. It will be discussed how these advantages can be exploited to make HWCVD an alternative to plasma enhanced chemical vapor deposition.     

Hot-wire deposition of amorphous and microcrystalline silicon using different gas excitations by a coiled filament

Microcrystalline silicon (μc-Si:H) and amorphous silicon (a-Si:H) films were deposited using a hot-wire CVD (HWCVD) system that employs a coiled filament. Process gasses, H₂ and Si₂H₆, could be directed into the deposition chamber via different gas inlets, either through a coiled filament for efficient dissociation or into the chamber away from the filament, but near the substrates. We found that at low deposition pressure (e.g. 20 mTorr) the structure of the films depends on the way gases are introduced into the hot-wire chamber. However, at higher pressure (e.g. 50 mTorr), Raman measurement shows similar results for films deposited with different gas inlets.     

Piezoresistive properties of nanocrystalline silicon thin films deposited on plastic substrates by hot-wire chemical vapor deposition

The piezoresistive property of n-type and p-type nanocrystalline silicon thin films deposited on plastic (PEN) at a substrate temperature of 150 °C by hot-wire chemical vapor deposition, is studied. The crystalline fraction decreased from 80% to 65% in p-type and from 84% to 62% in n-type films, as the dopant gas-to-silane flow rate ratio was increased from 0.18% to 3-3.5%. N-type films have negative gauge factor (− 11 to − 16) and p-type films have positive gauge factor (9 to 25). In n-type films the higher gauge factors (in absolute value) were obtained by increasing the doping level whereas in p-type films higher gauge factors were obtained by increasing the crystalline fraction.