High-efficiency μc-Si/c-Si heterojunction solar cells

P-type microcrystalline silicon (μc-Si (p)) on n-type crystalline silicon (c-Si(n)) heterojunction solar cells is investigated. Thin boron-doped μc-Si layers are deposited by plasma-enhanced chemical vapor deposition on CZ-Si and the V_oc of μc-Si/c-Si heterojunction solar cells is higher than that produced by a conventional thermal diffusion process. Under the appropriate conditions, the structure of thin μc-Si films on (1 0 0), (1 1 0), and (1 1 1) CZ-Si is ordered, so high V_oc of 0.579 V is achieved for 2×2 cm² μc-Si/multi-crystalline silicon (mc-Si) solar cells. The epitaxial-like growth is important in the fabrication of high-efficiency μc-Si/mc-Si heterojunction solar cells.     

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.     

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.     

a-Si:H(p)/c-Si(n)异质结太阳电池的模拟优化

通过AFORS-HET软件模拟了TCO/a-Si:H(p)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(n)/Ag结构的硅异质结电池中硅衬底电阻率、本征非晶硅薄膜厚度、发射极材料特性以及TCO功函数对电池性能的影响。结果表明:在其它参数不变的条件下,硅衬底电阻率越低,转换效率越高;发射极非晶硅薄膜厚度对短路电流有较大影响,发射极掺杂浓度低于7.0×10¹⁹cm^-3时,电池各项性能参数都极差;TCO薄膜功函数应大于5.2 eV,以保证载流子的输运收集。     

Low temperature μc-Si film growth using a CaF2 seed layer

This paper describes low temperature thin film Si growth by remote plasma chemical vapor deposition system for photovoltaic device applications. Using CaF₂/glass substrate, we were able to achieve an improved μc-Si film at a low process temperature of 300°C. The μc-Si film on CaF₂/glass substrate shows that a crystalline volume fraction of 65% and dark conductivity of 1.65×10⁻⁸ S/cm with the growth conditions of 50 W, 300°C, 88 mTorr, and SiH₄/H₂=1.2%. XRD analysis on μc-Si/CaF₂/glass showed crystalline film growth in (1 1 1) and (2 2 0) planes. Grain size was enlarged as large as 700 Å for a μc-Si/CaF₂/glass structure. Activation energy of μc-Si film was given as 0.49 eV. The μc-Si films exhibited dark- and photo-conductivity ratio of 124.     

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.     

Amorphous and microcrystalline silicon solar cells prepared at high deposition rates using RF (13.56 MHz) plasma excitation frequencies

This paper presents a-Si:H and μc-Si:H p–i–n solar cells prepared at high deposition rates using RF (13.56 MHz) excitation frequency. A high deposition pressure was found as the key parameter to achieve high efficiencies at high growth rates for both cell types. Initial efficiencies of 7.1% and 11.1% were achieved for a μc-Si:H cell and an a-Si:H/μc-Si:H tandem cell, respectively, at a deposition rate of 6 Å/s for the μc-Si i-layers. A μc-Si:H cell prepared at 9 Å/s exhibited an efficiency of 6.2%.     

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.     

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.     

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.     

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.     

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.     

Extension of the a-Si:H electronic transport model to μc-Si:H: use of the μτ product to correlate electronic transport properties and solar cell performances

The aim of this communication is to show that it is possible to extend the model of the electronic transport developed for amorphous silicon (a-Si:H) to microcrystalline silicon (μc-Si:H). By describing the electronic transport with the μ⁰τ^R products (mobility×recombination time) as a function of the Fermi level, we observed the same behaviour for both materials, indicating a similar type of recombination. Moreover, applying the normalised μ⁰τ⁰ product (mobility×life-time) obtained by combining the photoconductivity (σ_photo) and the ambipolar diffusion length (L_amb) measured in individual layers, we are able, as in the case of a-Si:H, to predict the quality of the solar cells incorporating these layers as the active i layer.     

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.     

Investigation of the M/a-Si : H/c-Si structure as a Schottky contact with a diffusion barrier layer

The electrical and photoelectrical characteristics of the a-Si : H/c-Si (p-type) structure are measured. The structure is analysed as a Schottky diode in which the a-Si : H is considered as a diffusion barrier layer. The conventional h.f. C–V theory is simplified and adapted to the analysis, which allows to estimate the initial band bending at the c-Si interface, the built-in electric field in the a-Si : H layer and the differential density of the a-Si : H/c-Si interface states.     

p-doped μc-Si:H window layers prepared by hot-wire CVD for amorphous solar cell application

We report on boron-doped μc-Si:H films prepared by hot-wire chemical vapor deposition (HWCVD) using silane as a source gas and trimethylboron (TMB) as a dopant gas and their incorporation into all-HW amorphous silicon solar cells. The dark conductivity of these films was in the range of 1–10 (Ω cm)⁻¹. The open circuit voltage V_oc of the solar cells was found to decrease from 840 mV at low hydrogen dilution H-dil=91% to 770 mV at high H-dil =97% during p-layer deposition which can be attributed to the increased crystallinity at higher H-dil and to subsequent band edge discontinuity between μc-Si:H p- and amorphous i-layer. The short circuit current density J_sc and the fill factor FF show an optimum at an intermediate H-dil and decrease for the highest H-dil. To improve the conversion efficiency and the reproducibility of the solar cells, an amorphous-like seed layer was incorporated between TCO and the bulk p-layer. The results obtained until now for amorphous solar cells with and without the seed layer are presented. The I–V parameters for the best p–i–n solar cell obtained so far are J_sc=13.95 mA/cm², V_oc=834 mV, FF=65% and η=7.6%, where the p-layers were prepared with 2% TMB. High open circuit voltages up to 847 mV could be achieved at higher TMB concentrations.     

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%.     

Cell structures with low-high heterojunction of c-Si and μc-Si: H under rear contact for improvement of efficiencies

For a remarkable improvement of conversion efficiencies of single-crystalline silicon (c-Si) solar cells, we have been investigating rear surface structures. The structure has a highly conductive boron (B) doped hydrogenated microcrystalline silicon (μc-Si:H) film with a wide optical bandgap between a p-type c-Si substrate and a rear contact instead of a heavily diffused layer. The conditions of depositing the μc-Si:H film were investigated. Both short-circuit current density (J_sc) and open-circuit voltage (V_oc) of the cell with the μc-Si:H film are much higher than those without the film. The V_oc obtained was higher than 650 mV and the efficiency was 19.6% for a 5 cm × 5 cm cell. It is confirmed that a low-high heterojunction of the c-Si substrate and the μc-Si:H film is very effective in preventing minority carriers near the rear contact from recombining.     

Numerical modelling of trap-assisted tunnelling mechanism in a-Si:H and μc-Si n/p structures and tandem solar cells

The trap-assisted tunnelling theory was developed to describe the tunnelling of charge carriers via bandgap energy levels in structures based on hydrogenated amorphous silicon and microcrystalline silicon. Its implementation into ASPIN numerical simulator is explained. Models that were verified on n/p single junctions were applied in the tunnel recombination junction area of a tandem solar cell. Thus, it is possible to study a multi-layer solar cell without separately simulating any of its components.     

Structural, optical and electrical characterizations of μc-Si:H films deposited by PECVD

The effects of the silane concentration f on the structural, optical and electrical properties of undoped hydrogenated silicon films prepared in a plasma-enhanced chemical vapour deposition system have been studied. The electrical conductivity and Hall mobility appear to be controlled by microstructures induced by silane concentration and a clear electrical transport transition from crystalline to amorphous phase has been found when 3%<f<4%. A two-phase model has been used to discuss the electrical properties.