S. Nonomura

Variable minority carrier transport model for amorphous silicon solar cells

A new carrier transport model describing the photovoltaic characteristics of amorphous silicon (a-Si) p-i-n junction (where i denotes intrinsic material) solar cells is proposed. In the model, the operative i layer is divided into two regions at variable boundary; in each region, either electrons or holes are assumed to act just like the minority carriers dominating the carrier recombination rate. Based on this variable minority carrier transport model, comprehensive interpretations of the carrier collection efficiency spectra and dark and illuminated current density-voltage characteristics are given in terms of several basic parameters responsible for them, i.e. mobility-lifetime products, effective surface recombination velocities and conductivities at the p-i and i-n interfaces. The model presented here can be used not only for explaining and predicting the photovoltaic properties but also for evaluating these parameters in actual a-Si solar cells. Furthermore, an identification of the relevant properties in various types of a-Si solar cell enables us to make clear how to design and fabricate the cells in order to attain higher photovoltaic performances.

Mobility‐lifetime product and interface property in amorphous silicon solar cells

The mobility‐lifetime products ( μτ) and interface property have been examined through the photovoltaic studies in actual hydrogenated amorphous silicon (a–Si:H) p–i–n junction solar cells. A small amount of boron atoms included in a–Si:H enhances the μτ products of both electrons and holes up to the order of 10−7 cm2/V, which corresponds to the carrier diffusion length in excess of 5000 A. The doped window layer possessing inferior photoelectric property works as the recombination region for photocarriers generated in the active i layer, and practically dominates the interface property together with the surface recombination velocity S0 at the electrode/doped layer interface. The S0 at the SnO2/p a–Si:H interface is estimated to be about 3×102 cm/s with an assumption of the electron mobility at 0.1 cm2/Vs. Prolonged light exposure causes a reversible change of the μτ products in every layer composing the p–i–n junction. These experimental results are discussed in connection with photovoltaic performances.

A study of visible-light injection-electroluminescence in a-SiC/p-i-n diode

Abstract An injection mode electroluminescence has been first observed in a p a-SiC/i a-SiC/n a-SiC diode. White-green, yellowish-orange and red light emissions have been observed in these junctions at room temperature depending on the optical band gaps of the luminescent active i layers. From the relation between the injection current density and the total EL intensity, it has been shown that EL is dominated by the recombination of electron-hole pairs doubly injected into the i layer.

New types of high efficiency solar cells based on a‐Si

Three types of new structure a‐Si solar cells having more than 9% efficiency are presented. The first one has a high optical reflection back electrode metal alloyed with optically transparent n‐type μc‐Si deposited on the conventional glass substrate a‐SiC/a‐Si heterojunction solar cell. The second type structure is an inverted p‐i‐n solar cell having Ag/TiO2/a‐Si metal‐insulator‐semiconductor type back surface electrode which more efficiently collects longer wavelength photocarriers just above the band edge. The third structure demonstrated here has a‐Si/polycrystalline tandem junction to pick up the energy of longer wavelength photons passing through the front side of the a‐Si solar cell. All key technologies proposed here are practical and offer more promised real alternatives for the fabrication of high efficiency a‐Si solar cells.

A study of built-in potential ina-Si solar cells by means of back-surface reflected electroabsorption

The electroreflectance (ER) signal has been studied for the purpose of identifying the built-in field in practical amorphous silicon (a-Si∶H) solar cells. Through both theoretical and experimental considerations, it has been confirmed that the ER signal essentially comes from the light which is reflected at the back surface and hence experiences the internal electric field within thea-Si∶H layer. By analyzing the ER signal, which is really the back-surface reflected electroabsorption signal, the built-in potentialVbcan be evaluated. This method has been applied to various types ofp-i-n junctiona-Si solar cells.Vbof a usual homojunction solar cell was about 0.85 V. Increases ofVbby 50≈130mV have been found in heterojunction solar cells constructed withp-type amorphous silicon carbide (a-SiC∶H) and/orn-type microcrystalline silicon (μc-Si) as compared with homojunctionp-i-n solar cells. Moreover, a clear dependence ofVbon the substrate materials has been observed. These experimental results are described in connection with cell performances.

Detailed studies of optical edge and below gap absorption in a-Si1−xCx:H system

Abstract The optical absorption edge and below gap absorption of a-Si 1−x C x :H system are investigated by photoacoustic spectroscopy and electroabsorption method. Incorporation of carbon atoms introduces the broadening of Urbach tail and increasing of below gap absorption. The effect of substrate temperature is also demonstrated. A strong correlation between Urbach slope and shape of electroabsorption spectra are shown and discussed on the stand point of thermal and compositional disorder.

Analysis of photo-induced changes in the performance of amorphous silicon solar cells

Abstract Photo-induced changes in the performance of a-SiC:H/a-Si:H (where a-SiC:H and a-Si:H denote hydrogenated amorphous SiC and silicon) heterojunction solar cells were investigated under illuminations of 1 sun and 10 suns. The fill factor and the conversion efficiency decreased sharply with time under 10 suns illumination. In order to separate the origin of the photo-induced fill factor change from thermal effects on the fill factor, the temperature dependence and heat-treatment-induced recovery of the cell performance were also studied. With these systematic data, a series of experimental and theoretical analyses were carried out using the variable minority carrier transport model. The results show that the photo-induced changes are caused not only by the bulk Staebler-Wronski effect but also by changes in the surface recombination factor.

Back surface reflected electroabsorption as a new characterization method of internal potential in a-Si homo- and heterojunction

Abstract The electro-optical study in a-Si:H and a-SiC:H have been made utilizing newly developed technique; Back-Surface-Reflected-Electroabsorption (BASREA) method. Through the theoretical and experimental analyses on the electric field dependence of the BASREA spectrum, it has been shown that the BASREA method is useful tool for evaluating the definite band edge energy of the material and the built-in potential Vb of the a-Si solar cells. Utilizing this BASREA method, the built-in potential between a-Si:H, a-SiC:H and μc-Si have been identified for the basic study of solar cell technology.

Characterization of mobility-lifetime products and interface property in amorphous silicon p-i-n junctions

Abstract The mobility-lifetime products and interface property have been examined in actual a-SiC/a-Si/μc-Si heterojunction cells through the analysis of the drift type photovoltaic effect. It is made clear how the transport parameters responsible for the cell characteristics are influenced by the cell fabrication conditions and processes.

High quality a-Si film produced by horizontal plasma furnace

Abstract A series of investigation on the a-Si film quality in a p-i-n junction has been made by employing the separated three chamber system of horizontal plasma mode. A clear improvement of film quality has been experimentally verified in a-Si films prepared by this system as compared with those by the single chamber system. On the basis of these investigation, a-SiC/a-Si/μc-Si heterojunction solar cells having more than 9% conversion efficiency have been developed.