Photoconductive films for optoelectronics

Optoelectronic films are divided into: conductive films (metal conductive films, transparent conductive ITO films); photoconductive films (CdS and CdSe films, α-Si:H films); electroluminescent films (ZnS:Mn films, organic electroluminescent films); electrochromic films (WO3 films); liquid crystal films.

Photoconductive thin films are thin film devices in which the carrier concentration of certain films changes under external light, forming photogenerated carriers and thus changing the electrical conductivity of the film, or producing the photovoltaic effect. These thin films have important applications in photodetection, image sensing and solar energy utilization.

Light absorption in thin films

When light of a certain wavelength is shone on a semiconductor material, the electrons absorb the energy of the photons and jump from the valence band to the conduction band.

There are various mechanisms for light absorption by materials. Intrinsic absorption, impurity absorption - producing photoconductivity;

Exciton absorption, free carrier absorption, lattice absorption - no photoconductivity, with thermal effects.

Properties of photoconductive films

1, absorption coefficient α: light in the medium spread 1/α distance when the energy is reduced to 1/e of the original energy, that is, dI / dx = -αI. 

2, photoelectric conductivity effect: semiconductor or insulator in the light of its internal carrier concentration increases and leads to the phenomenon of conductivity increase;

3, gain: each absorption of a photon between the electrodes after the number of carriers, G = I/eF, I/e for the number of photoelectrons per second, F is absorbed per second to produce the number of carriers of photons;

4, light sensitivity: by the absorption of a photon per second and make the flow through the electrode increased by the number of carriers.

CdS/CdSe photoconductive films

Cadmium sulphide and cadmium selenide are group II-IV semiconductors with a visible optical harmonic response. The peak photo-resonance response is 0.5 μm for CdS and 0.7 μm for CdSe.

Preparation: Vacuum thermal evaporation technique, 200 °C - 300 °C, doping (Cl, Cu) shifted the peak spectral response to 560 nm and 770 nm.

Response time: The response time of doped CdS/CdSe photoconductor films can reach 6-8ms (rise time), while the response time of undoped films can reach 100ms.

α-Si:H photoconductive films

Preparation: Glow discharge plasma chemical vapour deposition of amorphous silicon films. 200-400°C, high purity silane gas reacted with hydrogen gas.

Properties: Absorption of all wavelengths below 500nm-600nm, gradual transmission of light from 620nm onwards; Applications: solar cells, thin film transistors (TFTs), electrostatic copying, memory devices and display devices.

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