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Characterisation of hydrogenated amorphous silicon A-SI:H solar cells using impedance spectroscopy (IS)

Baban, A. (1998) Characterisation of hydrogenated amorphous silicon A-SI:H solar cells using impedance spectroscopy (IS). Masters by Research thesis, Murdoch University.

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Practical applications of hydrogenated amorphous silicon (a-Si:H) solar cell are increasing significantly, because of their versatility, reduced cost and improved efficiency. The purpose of this work was to experimentally examine the performance and degradation of p-i-n junction a-Si:H solar cells. The samples were fabricated by glow discharge (GD) technique often called plasma enhanced chemical vapour decomposition (PECVD). The simple p-i-n photovoltaic cells were prepared at a pressure of 0.1 torr and a temperature of 225°C on untextured glass substrates. The cells used were just over 5% efficiency and were made under deposition conditions that yield cells in the range 5% to over 7% efficiencies.

To investigate the performance and degradation of the cells, we have measured the current-voltage characteristic curves for each case, dark and illumination. The cell’s internal parameters, short circuit current (Ish), open circuit voltage (Voc), fill factor (FF), and the efficiency (η), as well as the series and the shunt resistances were calculated from the measurements.

The microscopic mechanisms and generation of electron/hole pairs in a-Si:H photovoltaic cells were studied by employing the technique of impedance spectroscopy (IS). The complex impedance measurements as a function of frequency and bias voltage have been used to derive the parameters of the equivalent circuit. The results showed that over a limited range of frequencies and bias voltages, an equivalent circuit composed of a shunt resistor and a capacitor is adequate to describe the measurements. Otherwise, the simple equivalent circuit model requires modification and may be interpreted by assuming that the values of one or more of the equivalent circuit elements are a function of frequency. We have related this microscopic model to the microscopic mechanisms within the material; the density of states and the generation lifetime of both electrons and holes. The measurements based on the potential of impedance spectroscopy (IS) technique to distinguish between processes that proceed at different rates, indicated that the mechanisms leading to generation of electron/hole pairs may be identified.

To probe the stability of the cells, we have light soaked the samples for up to 60 hours by exposing them to simulated AM 1.5 global illumination. The resulting reduction in short circuit current and fill factor caused a significant decline in the efficiency while the open circuit voltage remained relatively constant. The efficiency decline was assessed in terms of current density and fill factor loss. The deterioration of the cell’s performance was attributed to creation of a large number of defect states in the bulk that reduce the minority carrier lifetimes in the i-layer. Annealing at 180°C for 60 minutes restored the initial efficiency.

The capacitance-voltage measurements were performed in dark and illumination to investigate the deep defect mechanisms and detect the transitions within the depletion region by varying the d.c bias voltage. Photocapacitance of the charged carriers were determined from the values of capacitance in the dark and illumination. The photocapacitance was used as a measure of internal field change and recombination process.

Sub-bandgap energy photons can produce electron/hole pairs that may contribute to the primary photocurrent and improve the efficiency of the cell. This was investigated through illuminating the sample with red or infra red light (650nm and 950nm respectively). The results from these measurements were interpreted as being due to one or both of the suggested processes; the transition caused by multiple photons, or/and as a result of illumination the band tail states extend into the forbidden gap to an extent that a single photon is sufficient to stimulate an electron from the upper level of valence band tails to the lower conduction band tails.

The results from this study have provided valuable information about decay/recombination, the minority carrier lifetimes in the i-layer which limit the performance of the material, and production of photogenerated carriers under photon energies less than the optical gap. This information could lead to the production of devices with improved efficiencies.

Item Type: Thesis (Masters by Research)
Murdoch Affiliation: Division of Science
Supervisor(s): Cornish, John
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