Thermal ideality factor of hydrogenated amorphous silicon p-i-n solar cells

The performance of hydrogenated amorphous silicon (a-Si:H) p-i-n solar cells is limited, as they contain a relatively high concentration of defects. The dark current voltage (JV) characteristics at low forward voltages of these devices are dominated by recombination processes. The recombination rate depends on the concentration of active recombination centers and the recombination efficacy of each of these centers. The first factor causes the ideality factor of the devices to be non-integer and to vary with voltage. The temperature dependence of the dark current can be expressed by its activation energy. For microcrystalline silicon solar cells the activation energy varies with voltage with a so-called thermal ideality factor of 2. This value was derived for devices with a spatially uniform defect distribution and reflects the recombination efficacy. Here we present results of a thickness series of a-Si:H p-i-n solar cells. We have matched the experimental curves with computer simulations, and show that the voltage-dependent ideality factor curve can be used to extract information on the cross sections for electron and hole capture. Also, the activation energy is used as a measure for the mobility gap, resulting in a mobility gap for a-Si:H of 1.69?eV. We find a thermal ideality factor close to 2 for all samples. This is explained with a theoretical derivation, followed by a comparison between the internal electric field strength and the spatial variation of the defect density in the intrinsic layer. The thermal ideality factor is shown to be insensitive to the defect distribution and the recombination profile in the device. It is, therefore, an appropriate parameter to characterize a-Si:H p-i-n devices, providing direct insight on the recombination efficacy.Electrical Sustainable EnergyElectrical Engineering, Mathematics and Computer Scienc

Exploring dark current voltage characteristics of micromorph silicon tandem cells with computer simulations

The transport mechanisms controlling the forward dark current-voltage characteristic of the silicon micromorph tandem solar cell were investigated with numerical modeling techniques. The dark current-voltage characteristics of the micromorph tandem structure at forward voltages show three regions: two with an exponential dependence and a third where the current grows more slowly with the applied voltage. In the first exponential region the current is entirely controlled by recombination through gap states of the top cell. In the second exponential region the current is controlled by the mixture of recombination through gap states of both the top and bottom cells and by free carrier diffusion along the a-Si:H intrinsic layer. In the third region the onset of the electron space charge limited current on the a-Si:H top cell can be observed along with some other mechanisms that are discussed in the paper. The high forward dark J-V curve of the tandem cell can be used as diagnosis tool to quickly inspect the efficiency of the recombination junction in recombining the electron-hole pairs generated under illumination in the top and bottom cells. The dark current at high forward voltages is highly influenced by the amount of electron-hole pairs thermally generated inside the recombination junction

Investigation of the oxygen-vacancy (A-center) defect complex profile in neutron irradiated high resistivity silicon junction particle detectors

Distributions of the A-center (oxygen-vacancy) in neutron silicon detectors have been studied using Deep Level Transient Spectroscopy. A-centers have been found to be nearly uniformly distributed in the silicon water depth for medium resistivity (0.1 {minus} 0.2 k{Omega}-cm) silicon detectors. A positive filling pulse was needed to detect the A-centers in high resistivity (>4 k{Omega}-cm) silicon detectors, and this effect was found to be dependent on the oxidation temperature. A discussion of this effect is presented. 16 refs