Barrier-layer photovoltaic cells

Barrier-Layer Photovoltaic Cells The photovoltaic cell is a simple device that is used for delecting and measuring radiation in the visible region. The typical cell has a maximum sensitivity at about. . SO nm the response falls ofl to about 0% of... 

The detector in a spectrometer must produce a signal related to the intensity of the radiation falling on it. For instruments operating in the visible region a photovoltaic or barrier-layer cell is the simplest and cheapest available. Current produced when radiation falls on a layer of a semiconductor material, e.g. selenium, sandwiched between two metallic electrodes, is proportional to the power of the incident radiation and can be monitored by a galvanometer. Barrier layer cells are robust and are often used in portable instruments but they are not very sensitive and tend to be unstable during extended use. 

Photodetectors are devices that convert light into an electric signal that is proportional to the number of photons striking its photosensitive surface. The photomultipfier tube is a commonly used photodetector for measuring light intensity in the UV and visible regions of the spectrum. Photodiodes are solid-state devices that are also used in modern instruments. In older instruments, barrier layer cells (also known as photovoltaic cells) were used as photodetectors, because they were rugged and less expensive. ... 

Figure 7.13. Schematic diagram of three common detectors used in the ultraviolet-visible region. A The barrier-layer or photovoltaic cell. B A vacuum phototube. C The vacuum photomultiplier.

Krebs, F.C., Carl, J.E., Cruys-Bagger, N., Andersen, M., Lilliedal, M.R., Hammond, M.A., Hvidt, S., 2005. Lifetimes of organic photovoltaics photochemistry, atmosphere effects and barrier layers in ITO-MEHPPViPCBM-aluminium devices. Sol. Energ. Mat Sol. Cells 86,499-516. 

In a barrier layer cell, also called a photovoltaic cell, a current is generated at the interface of a metal and a semiconductor when radiation is absorbed. For example, silver is coated onto a semiconductor such as selenium (see Figure 5.17) that is joined to a strong metal base, such as iron. To manufacture these cells, the selenium is placed in a container and the air pressure reduced to a... 

Light sensors made from a-Si H are either p-i-n or Schottky barrier structures. Unlike crystalline silicon, a p-n jimction is ineffective without the undoped layer, because of the high defect density in doped a-Si H. Illumination creates photoexcited carriers which move to the junction by diffusion or drift in the built-in potential of the depletion layer and are collected by the junction. A photovoltaic sensor (solar cell) operates without an externally applied voltage and collection of the carriers results from the internal field of the junction. When the sensor is operated with a reverse bias, the charge collection generally increases and the main role of the doped layers is to suppress the dark current. A Schottky device replaces the p-type layer with a metal which provides the built-in potential. 

In practice it has been found that intimate contacts formed between metal films and crystalline semiconductors exhibit poor photovoltaic response. This is caused by the fact that the thermionic emission dark current at the Schottky barrier leads to significantly higher dark currents than is normally encountered in a homojunction or heterojunction structure. This problem, however, can be got round while still preserving the potential advantages of the Schottky barrier by allowing a very thin oxide or insulating layer to be formed between the semiconductor and the metal contact. The introduction of this layer leads to the ccxnmon form of Metal-Insulator-Semiconductor Schottky Barrier Solar Cell (MIS SBSC) with hich we are primarily concerned here. 

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