This paper describes the use, within p–i–n- and n–i–p-type solar cells, of hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (μc-Si:H) thin films (layers), both deposited at low temperatures (200°C) by plasma-assisted chemical vapour deposition (PECVD), from a mixture of silane and hydrogen. Optical and electrical properties of the i-layers are described. These properties are linked to the microstructure and hence to the i-layer deposition rate, that in turn, affects throughput in production. The importance of contact and reflection layers in achieving low electrical and optical losses is explained, particularly for the superstrate case. Especially the required properties for the transparent conductive oxide (TCO) need to be well balanced in order to provide, at the same time, for high electrical conductivity (preferably by high electron mobility), low optical absorption and surface texture (for low optical losses and pronounced light trapping). Single-junction amorphous and microcrystalline p–i–n-type solar cells, as fabricated so far, are compared in their key parameters (Jsc, FF, Voc) with the [theoretical] limiting values. Tandem and multijunction cells are introduced; the μc-Si: H/a-Si: H or [micromorph] tandem solar cell concept is explained in detail, and recent results obtained here are listed and commented. Factors governing the mass-production of thin-film silicon modules are determined both by inherent technical reasons, described in detail, and by economic considerations. The cumulative effect of these factors results in distinct efficiency reductions from values of record laboratory cells to statistical averages of production modules. Finally, applications of thin-film silicon PV modules, especially in building-integrated PV (BIPV) are shown. In this context, the energy yields of thin-film silicon modules emerge as a valuable gauge for module performance, and compare very favourably with those of other PV technologies. Copyright © 2004 John Wiley & Sons, Ltd.

Thin‐film silicon solar cell technology

High energy conversion efficiency is one of the most important keys to the commercialization of dye-sensitized solar cells (DSCs) in the huge electricity generation market. According to our experience in the persistent efforts that helped to achieve high efficiency DSCs, we selectively review the major progress of improving the energy conversion efficiency of DSCs which may be useful for future applications. We start the discussion from modelling the device by macroscopic equivalent circuit and then highlight some approaches to improve the device performance, such as the molecular engineering of novel dye sensitizers and light trapping effect, tuning the potential of redox shuttles and surface passivation of photoelectrodes, and optimizing the resistance. Finally, we illustrate a roadmap of possible future directions of DSCs with the challenges of how to further improve the efficiency to accelerate the progress in the commercialization of DSCs.

Highly efficient dye-sensitized solar cells: progress and future challenges

The present paper discusses the principle of dye-sensitized solar cells (DSCs) in terms of a new physics-based equivalent circuit model. This model is proposed following analysis by electrochemical impedance spectroscopy of the voltage dependence of the internal resistance elements of DSCs. The influence of these elements upon cell performance in areas such as short circuit current density (JSC), open circuit voltage (VOC) and fill factor (FF) was examined based on the equivalent circuit. It was demonstrated that the haze factor of TiO2 electrodes is a useful index when fabricating light-confined TiO2 electrodes to improve JSC, and that blocking the TiO2 surface with molecules is an effective way of reducing interfacial charge recombination at the TiO2 surface and of improving shunt resistance and VOC. FF was also improved by reduction of the internal series resistance, which is composed of the following three elements: the redox reaction resistance at the platinum counter electrode, the resistance of carrier transport by ions in the electrolyte, and resistance due to the sheet resistance of the transparent conducting oxide. Finally, the highest efficiency scores of 10.4% and 10.8% (aperture illumination area 1.004 cm2 and 0.2227 cm2, respectively) were confirmed by a public test center.

Improvement of efficiency of dye-sensitized solar cells based on analysis of equivalent circuit

Simulation and study of the influence of the buffer intrinsic layer, back-surface field, densities of interface defects, resistivity of p-type silicon substrate and transparent conductive oxide on heterojunction with intrinsic thin-layer (HIT) solar cell

We used a multi-step process to make aluminum-doped ZnO (AZO) films with a wide range distribution of textures for light trapping in thin film silicon solar cells, which includes AZO deposition, HCl etching, AZO re-deposition, and HCl re-etching. The large features created by the first etching provide an effective light trapping for long wavelength light; the small features from the second etching enhances the short wavelength light trapping. Microcrystalline silicon solar cells deposited on the above-mentioned AZO show an improved photocurrent density without loss in the open-circuit voltage and fill factor, resulting in an overall increase in efficiency by 14.64%.

Effective light trapping in thin film silicon solar cells from textured Al doped ZnO substrates with broad surface feature distributions

Microcrystalline silicon (µc-Si) films and solar cells were prepared by mercury-sensitized photochemical vapor deposition (photo-CVD). The changes in the structural properties of a series of µc-Si films grown under various H2 dilution and deposition pressure conditions were discussed. The results indicated that the properties of µc-Si films depend strongly on the atomic hydrogen. The microstructures of µc-Si films on textured SnO2 with different haze factors (from 13% to 65%) were observed with a scanning electron microscope and a transmission electron microscope. The observations revealed that the µc-Si layers grew from the initial stage of deposition with columnar grains and that they were conformal to the surface of textured SnO2. The grain boundary density of µc-Si film on SnO2 with higher haze factors was lower than that on SnO2 with lower haze factors. The effect of textured SnO2 with different haze ratios on p–i–n µc-Si cell characteristics was discussed and it was found that the higher haze factors showed a higher degree of light trapping in our µc-Si cells. We deposited µc-Si cells with an intrinsic µc-Si layer thickness of 430 nm on a textured SnO2 sample with a haze factor of 42% and achieved a conversion efficiency of 6.55%.

Microcrystalline Silicon Films and Solar Cells Prepared by Photochemical Vapor Deposition on Textured SnO2 with High Haze Factors

Hot Wire Cell (HW-Cell) method has been developed in order to grow microcrystalline silicon (µc-Si:H) thin films. The influence of various deposition parameters on the structural and electrical properties of the films was investigated to improve film quality. It was found that the concentrations of O and C atoms in µc-Si:H films could be reduced from the order of 1021 cm-3 to the order of 1020 cm-3 by decreasing the partial pressure of SiH4 from 100 mTorr to 3 mTorr. Then, a novel 2-step growth method was proposed in order to reduce the incubation layer in the initial growth of µc-Si:H i-layer. By using this method, Jsc largely increased (10.11 → 18.32 mA/cm2), and as a result, the conversion efficiency of 3.9% could be achieved. The influence of the incubation layer on solar cell performances was also investigated by a numerical analysis. To date, a conversion efficiency of 5.3% (Voc: 0.48 V, Jsc: 20.56 mA/cm2, F.F.: 0.54, active area: 0.086 cm2, AM1.5) was obtained for µc-Si:H solar cells with an i-layer thickness of 1.0 µm. Furthermore, high-rate depositions were investigated and a maximum deposition rate of 11.5 nm/s could be achieved. µc-Si:H solar cells fabricated at a high deposition rate of 1.5 nm/s showed a conversion efficiency of 2.8% (Voc: 0.42 V, Jsc: 12.31 mA/cm2, F.F.: 0.54, active area: 0.086 cm2, AM1.5).

2-Step Growth Method and Microcrystalline Silicon Thin Film Solar Cells Prepared by Hot Wire Cell Method

Hot wire cell (HW-cell) method has been developed in order to grow microcrystalline silicon (/spl mu/c-Si) thin films. A novel 2-step growth method was proposed in order to reduce an incubation layer in the initial growth of /spl mu/c-Si i-layer. By using this method, Jsc largely increased (10.11? 18.32 mA/cm/sup 2/), as a result, the conversion efficiency of 3.9% could be achieved. Up to now, a conversion efficiency of 6.0% (Voc: 0.50 V, Jsc: 19.69 mA/cm/sup 2/, F.F.: 0.61, AM1.5) was obtained for /spl mu/c-Si solar cells with an i-layer thickness of 0.8 /spl mu/m. Furthermore, high rate depositions were investigated and a maximum deposition rate of 11.5 nm/s could be achieved. /spl mu/c-Si solar cells fabricated at a high deposition rate of 1.5 nm/s showed a conversion efficiency of 2.8% (Voc: 0.42V, Jsc: 12.31 mA/cm/sup 2/, F.F.: 0.54, AM1.5).

Microcrystalline silicon thin film solar cells prepared by hot wire cell method

Amorphous Si (a-Si:H) and microcrystalline Si (µc-Si:H) materials were successfully deposited by the Hot Wire Cell method. We have studied these materials from the viewpoint of improving their qualities and the solar cell performances. As a result, it was found that the peak at 2100 cm-1 which originates from Si–H2 bonding in the infrared absorption spectra could be dramatically reduced at a low deposition pressure of 10 mTorr. Furthermore, it was shown by secondary ion mass spectroscopy (SIMS) analysis that the concentrations of O and C atoms in a-Si:H films could be reduced to the order of 1018 cm-3 at low deposition pressures of 1–10 mTorr. The conversion efficiency of 7.5% was achieved for superstrate-type pin a-Si solar cells with a deposition rate of 0.4 nm/s under AM1.5 insolation.

https://iopscience.iop.org/article/10.1143/JJAP.42.1521/pdf

Amorphous Silicon Thin Films Prepared by Hot Wire Cell Method and Its Application to Solar Cells

Boron doped microcrystalline silicon (p-µc-Si) films were successfully deposited by the Hot Wire Cell method using a gas mixture of pure silane (SiH4) and diborane (0.5% B2H6 in H2). The influence of various deposition parameters on the structural and electrical properties of the films was investigated to obtain highly conductive p-µc-Si films. A high dark conductivity (σd) of 84.6 S/cm and a low activation energy of 0.02 eV were achieved for 550-nm-thick films. For thin films with a thickness of 15 nm, a σd of 4×10-2 S/cm was obtained. The thin p-µc-Si was incorporated into p–i–n amorphous silicon (a-Si) and microcrystalline silicon (µc-Si) solar cells, in which intrinsic layers were deposited by photo chemical vapor deposition (photo-CVD). The initial conversion efficiencies of 9.04% and 6.17% were obtained for a-Si and µc-Si solar cells, respectively.

Yoshinori Ide

Papers

Microcrystalline Silicon Films and Solar Cells Prepared by Photochemical Vapor Deposition on Textured SnO2 with High Haze Factors

2-Step Growth Method and Microcrystalline Silicon Thin Film Solar Cells Prepared by Hot Wire Cell Method

Microcrystalline silicon thin film solar cells prepared by hot wire cell method

Amorphous Silicon Thin Films Prepared by Hot Wire Cell Method and Its Application to Solar Cells

Highly Conductive Boron Doped Microcrystalline Si Films Deposited by Hot Wire Cell Method and its Application to Solar Cells