Single-junction a-Si solar cells with over 13% efficiency

Single junction hydrogenated amorphous silicon solar cells having a high conversion efficiency of 13.2% were developed by combining three approaches. First, a new type of p-layer, such as (a-Si/a-C)_n multilayers, was investigated. The high open-circuit voltage was obtained without lowering the short-circuit current and the fill factor. Second, alternately repeating deposition and hydrogen plasma treatment method was applied to the fabrication of an a-SiC or wide gap a-Si : H films for p/i interface layer. High photoconductive and wide bandgap materials were obtained applicable to the p/i buffer layers. Third, the relationships between defect density of films or fill factors of solar cells and hydrogen radical in plasma were investigated. It was suggested that the H^*/SiH^* ratio was an effective parameter to improve the defect and fill factor, and the excess hydrogen radical deteriorated quality of films and cells.     

p-i interface engineering and i-layer control of hot-wire a-Si:H based p-i-n solar cells using in-situ ellipsometry

In this paper we report on the effect of monitoring the i-layer region near the p-i interface with the help of in-situ kinetic and spectroscopic ellipsometry on the performance of hot-wire deposited hydrogenated amorphous silicon p-i-n solar cells. It is very clearly observed that the microstructure at the p-i interface region in terms of the Si---Si bond packing density and surface roughness significantly affects the cell performance. The filament temperature, T_Fil, was the main parameter varied to control the above mentioned two properties near the p-i interface as well as in the bulk i-layer. In order to achieve significant enhancement in the cell performance we extended the idea of the “soft start”, earlier employed for the glow discharge deposited solar cells, to the hot-wire deposited i-layer. We were able to control the i-layer properties at the p-i interface and in the bulk independently and correlate these to the cell performance. It is shown that a major increase in cell performance can be achieved by improving the microstructure of the growing film directly at the p-i interface. Most interestingly, no significant deterioration in cell efficiency has been observed if only the p-i interface was properly controlled but the i-layer was of lower quality. These results are also shown to be consistent with model calculations of a numerical simulation. Our results therefore provide a clue to prepare hot-wire a-Si:H based solar cells with high efficiency and in the whole at high growth rates, which is needed for a more economic a-Si:H solar cell production.     

Amorphous silicon/p-type crystalline silicon heterojunction solar cells with a microcrystalline silicon buffer layer

Undoped hydrogenated amorphous silicon (a-Si:H)/p-type crystalline silicon (c-Si) structures with and without a microcrystalline silicon (μc-Si) buffer layer have been investigated as a potential low-cost heterojunction (HJ) solar cell. Unlike the conventional HJ silicon solar cell with a highly doped window layer, the undoped a-Si:H emitter was photovoltaically active, and a thicker emitter layer was proven to be advantageous for more light absorption, as long as the carriers generated in the layer are effectively collected at the junction. In addition, without using heavy doping and transparent front contacts, the solar cell exhibited a fill factor comparable to the conventional HJ silicon solar cell. The optimized configuration consisted of an undoped a-Si:H emitter layer (700 Å), providing an excellent light absorption and defect passivation, and a thin μc-Si buffer layer (200 Å), providing an improved carrier collection by lowering barrier height at the interface, resulting in a maximum conversion efficiency of 10% without an anti-reflective coating.     

Hydrogenated amorphous silicon films with significantly improved stability

A new regime of plasma-enhanced chemical-vapor deposition (PECVD), referred to as “uninterrupted growth/annealing” method, has been proposed for preparation of high-quality hydrogenated amorphous silicon (a-Si:H) films. By using this regime, the deposition process no longer needs to be interrupted, as done in the chemical annealing or layer by layer deposition, while the growing surface is continuously subjected to an enhanced annealing treatment with atomic hydrogen created in the hydrogen-diluted reactant gas mixture at a relatively high plasma power. The intensity of the hydrogen plasma treatment is controlled at such a level that the deposition conditions of the resultant films approach the threshold for microcrystal formation. In addition, a low level of B-compensation is used to adjust the position of the Fermi level close to the midgap. Under these conditions, we find that the stability and optoelectronic properties of a-Si:H films have been significantly improved.     

Fabrication of microcrystalline silicon solar cells on a SnO2 coated substrate using seed layer insertion

We fabricated hydrogenated microcrystalline silicon (μc-Si:H) solar cells on SnO₂ coated glass using a seed layer insertion technique. Since rich hydrogen atoms from the μc-Si:H deposition process degrade the SnO₂ layer, we applied p-type hydrogenated amorphous silicon (p-a-Si:H) as a window layer. To grow the μc-Si:H layer on the p-a-Si:H window layer, we developed a seed layer insertion method. We inserted the seed layer between the p-a-Si:H layer and intrinsic bulk μc-Si:H. This seed layer consists of a thin hydrogen diluted silicon buffer layer and a naturally hydrogen profiled layer. We compared the characteristics of solar cells with and without the seed layer. When the seed layer was not applied, the fabricated cell showed the characteristics of a-Si:H solar cell whose spectral response was in a range of 400-800 nm. Using the seed layer, we achieved a μc-Si:H solar cell with performance of V_oc=0.535 V, J_sc=16.0 mA/cm², FF=0.667, and conversion efficiency=5.7% without any back reflector. The spectral response was in the range of 400-1100 nm. Also, the fabricated device has little substrate dependence, because a-Si:H has weaker substrate selectivity than μc-Si:H.     

Stability of a-Si:H solar cells deposited by Ar-treatment or by ECR techniques

Stability against light soaking was studied for amorphous silicon (a-Si:H) solar cells using three different i-layers; (a) device-quality a-Si:H (standard a-Si:H) with bandgap of 1.75 eV, (b) narrow bandgap (1.55 eV) a-Si:H fabricated by Ar* chemical annealing and (c) a-Si:H(Cl) fabricated from SiH₂Cl₂. Both the narrow bandgap a-Si:H and the a-Si:H(Cl) solar cells showed much improved stability than that of the standard a-Si:H solar cells: e.g., fill factor of the narrow bandgap a-Si:H cell only slightly decreased from 56% to 53%, while that of the standard a-Si:H cell degraded from 62% to 51%. In addition, mobility–lifetime products of the a-Si:H(Cl) cell also exhibited improved stability than that of the standard a-Si:H solar cell.     

Toward stabilized 10% efficiency of large-area (>5000 cm) a-Si/a-SiGe tandem solar cells using high-rate deposition

This paper reviews recent progress in large-area a-Si/a-SiGe tandem solar cells at Sanyo. Optimized hydrogen dilution conditions for high-rate deposition of hydrogenated amorphous silicon (a-Si:H) films and thinner i-layer structures have been systematically investigated for improving both the stabilized efficiency and the process throughput. As a result, a high photosensitivity of 10⁶ for a-Si:H films has been maintained up to the deposition rate of 15 Å/s. Furthermore, the world's highest initial conversion efficiency of 11.2% which corresponds to a stabilized efficiency of about 10% has been achieved for a 8252 cm² a-Si/a-SiGe tandem solar cell by combining the optimized hydrogen dilution and other successful technologies.     

Polycrystalline Si thin-film solar cell prepared by solid phase crystallization (SPC) method

The solid phase crystallization (SPC) method has been studied for fabricating polycrystalline (poly) Si thin films for solar cells. The approach was to optimize the “partial doping structure” (nondoped a-Si/phosphorus(P)-doped a-Si) which we proposed as a starting structure before SPC. A conversion efficiency of 6.3% was obtained by using nondoped a-Si with a large structural disorder. This cell showed a collection efficiency of 51% at a wavelength of 900 nm. In order to significantly reduce the incubation time which is the important factor for the enlargement of the grain size, P doping of more than 10²⁰ cm⁻³ was required for the P-doped layer.     

Wide optical band gap window layers for solar cells

In this paper, the preparation of amorphous silicon carbide with very wide optical band gap and high conductivity were reported. The films were fabricated under the “silane–plasma starving” and H₂ dilution condition in conventional capacitively coupled reactors. The silane–plasma starving condition and H₂ dilution play important roles in decreasing H content, modulating the material toward the ordered structure and enhancing the doping ratio. This is an easy way to prepare wide optical band gap and highly conductive p-type window layers for a-Si : H-based solar cells.     

Boron doped amorphous diamond window layer deposited by filtered arc for amorphous silicon alloy p-i-n solar cells

In order to improve the conversion efficiency of amorphous silicon (a-Si:H) alloy p-i-n solar cells, the original p-a-Si:H window layer is substituted by the boron-doped amorphous diamond (a-D:B) films deposited using filtered cathodic vacuum arc technology. The microstructural, optical and electrical properties as functions of the boron concentrations in the films were, respectively, evaluated by an X-ray photoemission spectroscopy, an ultraviolet-visible spectrometer and a semiconductor parameter analyzer. The photovoltaic parameters of the solar cell modules were also detected as functions of boron concentration. It has been shown that the conductive a-D:B films could be obtained and still remained a wide optical gap. The p-i-n structural amorphous silicon solar cell using the a-D:B window layer increased the conversion efficiency by a roughly 10% relative improvement compared to the conventional amorphous silicon solar cell because of the enhancement of short wavelength response.     

Development of high efficiency p-i-n amorphous silicon solar cells with the p-μc-Si:H/p-a-SiC:H double window layer

A structure is developed to help improve the TCO/p contact and efficiency of the solar cell. A p-i-n amorphous silicon (a-Si:H) solar cell with high-conversion efficiency is presented via use of a double p-type window layer composed of microcrystalline silicon and amorphous silicon carbide. The best efficiency is obtained for a glass/textured TCO/p-μc-Si:H/p-a-SiC:H/buffer/i-a-Si:H/n-μc-Si:H/GZO/Ag structure. Using a SnO₂/GZO bi-layer and a p-type hydrogenated microcrystalline silicon (p-μc-Si:H) layer between the TCO/p-a-SiC:H interface improves the photovoltaic performance due to reduction of the surface potential barrier. Layer thickness, B₂H₆/SiH₄ ratio and hydrogen dilution ratio of the p-μc-Si:H layer are studied experimentally. It is clearly shown that the double window layer can improve solar cell efficiency. An initial conversion efficiency of 10.63% is achieved for the a-Si:H solar cell.     

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.     

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.     

Widegap a-Si:H films prepared at low substrate temperature

Wide bandgap hydrogenated amorphous silicon (a-Si:H) films have been prepared by the PECVD method at a low substrate temperature (80°C) controlling the incorporation of hydrogen (bonded with silicon) into the film. Optimizing the deposition parameters viz. hydrogen dilution, rf power, a-Si:H film with E_g ∼ 1.90 eV and σ_ph ≥ 10⁻⁴ Scm⁻¹ has been developed. This film exhibited better optoelectronic properties compared to a-SiC:H of similar optical gap. The quantum efficiency measurement on the Schottky barrier solar cell structure showed a definite enhancement of blue response. Surface reaction as well as structural relaxation under suitable deposition condition have been claimed to be responsible for the development of such material.     

Development of thin film amorphous silicon oxide/microcrystalline silicon double-junction solar cells and their temperature dependence

We have developed thin film silicon double-junction solar cells by using micromorph structure. Wide bandgap hydrogenated amorphous silicon oxide (a-SiO:H) film was used as an absorber layer of top cell in order to obtain solar cells with high open circuit voltage (V_oc), which are attractive for the use in high temperature environment. All p, i and n layers were deposited on transparent conductive oxide (TCO) coated glass substrate by a 60 MHz-very-high-frequency plasma enhanced chemical vapor deposition (VHF-PECVD) technique. The p-i-n-p-i-n double-junction solar cells were fabricated by varying the CO₂ and H₂ flow rate of i top layer in order to obtain the wide bandgap with good quality material, which deposited near the phase boundary between a-SiO:H and hydrogenated microcrystalline silicon oxide (μc-SiO:H), where the high V_oc can be expected. The typical a-SiO:H/μc-Si:H solar cell showed the highest initial cell efficiency of 10.5%. The temperature coefficient (TC) of solar cells indicated that the values of TC for conversion efficiency (η) of the double-junction solar cells were inversely proportional to the initial V_oc, which corresponds to the bandgap of the top cells. The TC for η of typical a-SiO:H/μc-Si:H was −0.32%/ °C, lower than the value of conventional a-Si:H/μc-Si:H solar cell. Both the a-SiO:H/μc-Si:H solar cell and the conventional solar cell showed the same light induced degradation ratio of about 20%. We concluded that the solar cells using wide bandgap a-SiO:H film in the top cells are promising for the use in high temperature regions.     

Properties of amorphous silicon solar cells fabricated from SiH2Cl2

Chlorinated intrinsic amorphous silicon films [a-Si:H(Cl)] and solar cell i-layers were fabricated using electron cyclotron resonance-assisted chemical vapor deposition (ECR-CVD) and SiH₂Cl₂ source gas. n–i–p solar cells deposited on ZnO–coated SnO₂ substrates had poor photovoltaic performances despite the good electronic properties measured on the a-Si:H(Cl) films. Improved open–circuit voltage (V_oc) of 0.84 V and fill factor (FF) of 54% were observed in n–i–p solar cells by providing an n/i buffer layer and by using Ga-doped ZnO coated glass substrates. However, the FF improvement was still rather poor, which is thought to originate from high interface recombination in the ECR deposited solar cells. The V_oc and the FF showed much stable feature against light soaking.     

Optimization of ZnO for front and rear contacts in a-Si solar cells

The enhancement of the reflection from the rear contact of p-i-n a-Si solar cells using ZnO combined with metals (Ag/Al) as a back reflector was demonstrated theoretically and experimentally. Futhermore, the incorporation of unreacted H₂O as source gas in the ZnO films was clearly observed through the thermal evolution measurement, suggesting the need for employing the pre-annealing technique for ZnO films before using them as a front contact in p-i-n a-Si solar cells. By using these approaches, the a-Si solar cells with glass/annealed-ZnO/delta-doped p/buffer/i/n/ZnO/metals(Ag/Al) structure were successfully fabricated and a conversion efficiency of 12.1% (AM-1.5, area 3×3 mm²) was obtained. Moreover, the solar cells with a structure of AR coated glass/SnO₂/delta-doped/p/buffer/i/n/ZnO/metals(Ag/Al) were also fabricated and by optimizing the use of the ZnO layer at the rear contact, a conversion efficiency of 12.6% was obtained. To make the ZnO films more appropriate for solar cells application, the growth rate of the ZnO films was increased by increasing the flow rate of diethylzinc used as a source gas.     

Design and Optimization of Amorphous Based on Highly Efficient HIT Solar Cell

The objective of this paper is to improve the power conversion efficiency of HIT solar cell using amorphous materials. A high efficiency amorphous material based on two dimensional heterojunction solar cell with thin intrinsic layer is designed and simulated at the research level using Synopsys/RSOFT-Solar Cell utility. The HIT structure composed of TCO/a-Si:H(p)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(n⁺)/Ag is created by using of RSOFT CAD. Optical characterization of the cell is performed by Diffract MOD model based on RCWA (rigorous coupled wave analysis) algorithm. Electrical characterization of the cell is done by Solar cell utility using based on Ideal diode method. In addition, optimization of the different layer thickness in the HIT structure is executed to improve the absorption and thereby the photocurrent density. The proposed HIT solar cell structure resulted in an open circuit voltage of 0.751 V, a short circuit current density of 36.37 mA/cm² and fill factor of 85.37% contributing to the total power conversion efficiency of 25.91% under AM1.5G. Simulation results showed that the power conversion efficiency is improved by 1.21% as compared to the reference HIT solar cell. This improvement in high efficiency is due to reduction of resistive losses, recombination losses at the hetero junction interface between intrinsic a-Si and c-Si, and optimization of the thicknesses in a-Si and c-Si layers.     

Performance of spray-deposited ZnO:In layers as front electrodes in thin-film silicon solar cells

The surface morphology, optical and electrical properties of spray-deposited ZnO:In layers are characterized and compared to ASAHI U-type SnO₂:F. Both TCO layers were implemented as front electrodes in a-Si:H single-junction solar cells. The similar V_oc of the solar cells indicates a good electrical contact between the ZnO:In layer and the a-Si:H material. The difference in solar cell performance between cells on ASAHI U-type TCO (10%) and the spray-deposited ZnO:In (8.4%) is mainly due to a Jsc-loss, caused by the lower ZnO:In bandgap and insufficient surface texturing. Surface roughening experiments of ZnO:In layers have been carried out.     

Comparison of multicrystalline silicon surfaces after wet chemical etching and hydrogen plasma treatment: application to heterojunction solar cells

The modifications of the surface and subsurface properties of p-type multicrystalline silicon (mc-Si) after wet chemical etching and hydrogen plasma treatment were investigated. A simple heterojunction (HJ) solar cell structure consisting of front grids/ITO/(n)a-Si:H/(p)mc-Si/Al was used for investigating the conversion efficiency. It is found that the optimized wet chemical etching and cleaning processes as a last technological step before the deposition of the a-Si:H emitter are more favorable to HJ solar cells fabrication than the hydrogenation. Solar cells on p-type mc-Si were prepared without high-efficiency features (point contacts, back surface field). They exhibited efficiencies up to 13% for a cell area of 1 cm² and 12% for a cell area of 39 cm².