Hiroshi Hashigami

Overview of light degradation research on crystalline silicon solar cells

Recent research on light degradation of crystalline Si materials and solar cells is reviewed. The first paper on the issue was published in 1973 when efficiency of solar cells using 1 Ω cm, B-doped CZ wafers degraded under illumination and recovered by annealing at a low temperature of around 200°C. In the 1990s, several studies have been performed to investigate the mechanism of the light degradation and also to provide practical solutions to suppress the degradation. Numerous experiments have been carried out regarding the effects of impurities including B, Ga, P, O and processing parameters such as oxidation temperature. To suppress the degradation, reducing the concentration of B and O or substituting boron by gallium as dopant was found to be effective. These findings are in agreement with a model attributing the lifetime degradation to oxygen and boron. Copyright

Effect of illumination conditions on Czochralski-grown silicon solar cell degradation

The substitutional B-interstitial O-related defects induced under an illumination, specific to B-doped Czochralski-grown silicon (Cz-Si), results in a solar cell efficiency loss of up to 3%. For a fundamental interpretation, the relationship between the solar cell performance and the minority-carrier lifetime degradation, and influence of the illumination conditions were investigated. A simulation of the cell performance degradation clearly represented that the degradation was explained as a result of the carrier diffusion length degradation. From a spectroscopic illumination investigation, the degradation was found to be independent of the stress-light wavelength. A blue light illumination on a solar cell resulted in the red response degradation, suggesting that the injected carriers activated the defect in the bulk. However, the correlation between carrier injection level and the cell performance decay time was nonlinear. The observed decay times were almost independent of the illumination intensity in ...

Suppression of light degradation of carrier lifetimes in low-resistivity CZ–Si solar cells

An international joint research has been conducted to investigate light degradation of low-resistivity Si CZ wafers and also to provide practical solutions to suppress the degradation. Ten kinds of CZ, MCZ and FZ Si wafers were evaluated under AM1.5 irradiation and processed to fabricate solar cells using low- and high-temperature processes. Lifetime degradation was suppressed using MCZ Si wafers with low oxygen content and Ga-doped CZ wafers with high oxygen content. In addition, high-temperature oxidation was also effective to reduce light degradation remarkably. No degradation of solar cells could be realized by using B-doped MCZ and Ga-doped CZ wafers combined with a high-temperature processing.

Characterization of the Initial Rapid Decay on Light-Induced Carrier Lifetime and Cell Performance Degradation of Czochralski-Grown Silicon

The rapid initial degradation of Czochralski-grown silicon (Cz-Si) solar cell performance has been investigated. The initial rapid degradation occurs at the beginning of the degradation process and is followed by a second slower degradation. The surface effect due to interface state degradation between the passivation layer and the Si substrate was investigated on several kinds of Cz- and float zone Si (FZ-Si) wafers. Carrier lifetime degradation induced by an illumination for 30 s was observed on every Cz-Si wafer, but no degradation was observed on FZ-Si wafers. Moreover, only the decrease of IR response of a cell was observed as a result of the initial degradation. The results suggest that the degradation occurs in the bulk. The magnitude and decay time of the degradation appear to depend on the wafer resistivity. Defects very similar to boron–oxygen complexes specific to Cz-Si were considered to be responsible. The distinctly small activation energy of 0.19 eV was obtained for the initial degradation, which suggests a fundamentally different defect reaction process. The activation energy of 1.6 eV for the defect annihilation process was obtained for both defects as well.

Performance Degradation of Czochralski-Grown Silicon Solar Cells by Means of Current Injection

Performance degradation of Czochralski-grown silicon (Cz–Si) solar cells caused by forward bias voltage application has been investigated. Because of the similarity to light-induced degradation, comparative experiments are carried out on the phenomena. Cell performance decay time evaluations reveal that current injection has a weaker effect on the degradation than illumination in the low-injection region. A difference in the energy of injected electrons is considered to be the reason. Degradation in the high injection region appears identical. Jsc–Voc curve analysis that evaluates bulk lifetime degradation identifies the phenomena.

Interpretation of light-induced cell performance degradation by means of spectroscopic light illumination

Light-induced cell performance and lifetime degradation are investigated using blue and red light illuminations in order to clarify the responsible factor for the metastable defect activation. The evaluation of the cell-performance decay curves with exponential decay fittings results in the same decay time constants for the degradation. Spectral response degradation is investigated under blue-light illumination. Only the decrease in red response indicates deep distribution of the activated metastable defect into the bulk. Injected carriers are responsible for the defect activation. Carrier diffusion lengths of more than 100 μm in a cell at a degraded state is considered to make blue light possible to degrade the bulk.

Light-Induced Lifetime Degradation of Multicrystalline Silicon Wafers and Solar Cells

Light-induced lifetime degradation of conventional cast, electromagne tic cast and heat exchanger method boron doped multicrystalline silicon wafers have bee n ext nsively studied. Results show that the carrier lifetimes of boron-doped conventional c ast , electromagnetic cast and heat exchanger method silicon wafers degrade as boron-doped Cz si licon wafers. However, boron-doped, electromagnetic cast and cast fabricated solar cells show a slight degradation of I sc, Voc and efficiencies comparing to wafer lifetime degradation. The degree of lifetime degradation depends on boron content and crystal quality suggesting the boron-oxygen compl ex models as a mechanism for lifetime degradation. Quality and stability of t he gallium-doped multicrystalline-Si wafers were intensively studied by means of quasi-steady-sta te photoconductance lifetime measurement. Results show that as-grown gallium-doped multicrystal line-Si wafers have high lifetimes and no significant degradation was observed under illuminat ion. The gallium-doped multicrystalline-Si wafers are a promising material for future photovolt aics.

Light-Induced Performance Degradation and Its Annihilation of Crystalline Silicon Solar Devices

Recent research on light-induced degradation of crystalline Si solar cells and modules is reviewed. The first paper on the issue was published in 1973 when efficiency of solar cells using 1 Ω·cm, B-doped CZ wafers degraded under illumination. Since mid 1990’s, several studies have been performed to investigate the mechanism of the light degradation and also to provide practical solutions to annihilate the degradation. Numerous experiments have been carried out regarding the effects of impurities including B, Ga, P, O and processing parameters such as annealing temperature. Recent analysis on weathered solar modules supported the LID is in agreement with a model attributing the lifetime degradation to oxygen and boron complex. The reduction of the concentration of B and O or substitution of boron by gallium are found to suppress the degradation and to realize cell efficiency further.Copyright