Hirotsugu Kida

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.

Steady-state photoconductivity in amorphous semiconductors containing correlated defects

Abstract The theory of steady-state photoconductivity is presented for amorphous semiconductors containing correlated defects and exponentially distributed trap states. It is demonstrated that the well-known excitation-intensity dependence G γ (0·5 < γ < 1) does not apply to amorphous semiconductors containing defects characterized by a negative correlation energy U (chalcogenide glasses), but is applicable when U is positive (a-Si: H) as long as the temperature is lower than a threshold temperature. For a-Si: H, the theoretically predicted variation of the temperature dependence with doping is compared with the experimental results to yield estimates of the magnitude of U and the position of the defect states in the gap. The theory presented here can be used to give a broad picture of the overall photoconductivity characteristics, and also to obtain information on the physical parameters related to recombination through the temperature and excitation-intensity dependences of the steady-state photoconduct...

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.

Below-gap primary photocurrent associated with correlated defects in hydrogenated amorphous silicon

Abstract The statistics for correlated defects under non-equilibrium conditions are derived and applied to formulate the d.c. and a.c. below-gap primary photocurrents (PPC) of undoped a-Si:H. Through a detailed theoretical examination, it has been found that the a.c. PPC originating from different optical transitions associated with the correlated defects exhibit their own specific phase shifts. The theoretically predicted characteristics of the d.c. and a.c. PPCs have been verified by experimental results on an a-Si:H p-i-n junction photocell, leading to the conclusion that the optical transition from the valence band to the doubly occupied dangling bond state makes a predominant contribution to the below-gap PPC in undoped a-Si:H. By means of a vector analysis of the a.c. PPC spectrum, the positions of the dangling bond states have been evaluated to be about 0–90 eV (singly occupied state) and 0–56 eV (doubly occupied state) below the conduction band, and the magnitude of the correlation energy U to be ...

Measurement of deep states in undoped amorphous silicon by current transient spectroscopy

Localized deep states in undoped a‐Si:H have been investigated by current transient spectroscopy (CTS). After a trap‐filling light pulse, current transients associated with thermal emission of trapped carriers are measured over a wide time range under the isothermal condition. Slow saturation of the CTS signal J×t, with increased filling‐pulse duration is well explained by taking account of spatially and energy dependent carrier capture and emission processes, yielding an estimate of attempt‐to‐escape frequency. The attempt‐to‐escape frequency has been found to be energy dependent and to range from 1012 to 1013 s−1. Detailed analysis of the CTS signal has revealed that deep‐states distribution displays a broad peak locating near the center of the gap and a less pronounced structure at 0.5–0.6 eV below the conduction band edge.

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 light and current induced effects in hydrogenated amorphous silicon

A systematic investigation has been made on the changes in mobility‐lifetime product and defect density induced by light and current injection in amorphous silicon  p‐i‐n junction. It has been found that these changes involve the creation and interconversion of two kinds of metastable defects, being induced merely by carrier trapping phenomenon.

A New Model for Simulating Photocarrier Collection in Amorphous Silicon Solar Cells

A new carrier transport model describing the characteristic features of the photovoltaic property in a-Si solar cells has been developed, where transport of both electrons and holes is taken into consideration. Based upon this model, collection efficiency spectra have been theoretically calculated and compared with experimental data to evaluate some basic parameters; mobility lifetime products and effective surface recombination velocities. Through the systematic examinations of thus obtained parameters in various a-Si solar cells, a correlationship between the photovoltaic performance and the cell fabrication conditions has been made clear.

Analysis of Weighting Function in Transient Spectroscopy for Precise Measurement of Deep States

The weighting function in transient deep-level spectroscopy has been analyzed for precise measurement of deep states. The relation among weighting functions of different measurement modes is discussed. It is shown that unless the deep energy level is a well-defined discrete one, reconstruction of the density-of-states from the observed quantity is not straightforward and the weighting function greatly depends upon the energy dependence of attempt-to-escape frequency or capture cross section of the states. Approximate and exact methods to deduce the density-of-states are presented emphasizing that the neglect of energy dependence of the attempt-to-escape frequency brings a large error in the density-of-states estimation.

Phase shift analysis of below-gap primary photocurrent in hydrogenated amorphous silicon

Abstract Below-gap primary photocurrent associated with dangling bond defects in hydrogenated amorphous silicon has been investigated by measuring the amplitude and phase shift spectra with respect to the chopped excitation light. Theoretical analysis has been made on the temperature and chopping frequency dependence of the phase shift, yielding a conclusion that the doubly occupied dangling bond states are located at 0.5-0.6 eV below the conduction band edge.