Photoconductivity secondary

Another type of absorption is also possible, i.e., exciton absorption which enriches the crystal in free excitons if the latter annihilate then on the lattice defects, causing a change in the charged state of the defects and leading to the appearance of free carriers in the crystal. In this case photoconduction arises as a secondary effect. 

In the photoconductive sensor, the electrode is ohmic where carrier replenishment occurs. Electron photoconductivity is the dominant mode. In this case, the photocurrent is a secondary current. Photocurrent and photoconductive gain are given by (Bube, 1961)... 

The reverse bias p-i-n sensor is a primary photoconductor in which injection from the contacts is prevented by the junction. A secondary photoconductor allows charge to flow in from the contacts and offers the possibility of photoconductive gain. Gain occurs when the... 

On the contrary, in the PPVs, the onset of photoconductivity coincides with the onset of absorption. In the PPVs, mobile charge carriers are photogenerated (see Fig. VD-1). The data in Fig. IVD-1 are consistent with a weak exciton binding energy, less than approximately 0.1 eV. In the exciton model, however, this coincidental onset of photoconductivity and absorption must be explained as an accident excitons are photogenerated as a first step with subsequent dissociation as a result of secondary processes. 

Thus, the relevant question is whether the photoconductive response in PPV results principally from secondary processes following the photogeneration of neutral geminate pairs, in agreement with the exciton model, or from separated, mobile, positive and negative charged polarons. 

Steady state photoconductivity (PC) measurements provide additional insight. Not only is the onset of the steady state PC coincident with the onset of optical absorption but, using an established theoretical analysis which assumes that every absorbed photon creates a pair of charge carriers, the PC spectral response has been calculated from the measured absorption profile with results in excellent agreement with the PC data [146]. By contrast, in the case of strong exciton binding, the PC response from secondary processes is expected to be similar to the absorption spectrum this is not observed in the experimental results [146]. 

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