Trap-assisted recombination

Trap-assisted recombination was first proposed by Shockley et al. and it is also known as Shockley-Read-Hall recombination. This recombination model was first used to study the charge recombination in inorganic semiconductors, and later it was introduced into the field of OSC. This type of recombination involves a trapped charge and a mobile charge with the... 

Rai V. K, Sriva stava R., Chauhan G., Saxena K, Chand S., Kamalsanan M.N., Jpn. J. Appl. Phys. (2008) May pp. 3408-3411, Trap Assisted Carrier Recombination in 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran Doped Bis(2-(2-hydroxyphenyl)bezoxazolate) Zinc. 

There are many ways of increasing tlie equilibrium carrier population of a semiconductor. Most often tliis is done by generating electron-hole pairs as, for instance, in tlie process of absorjition of a photon witli h E. Under reasonable levels of illumination and doping, tlie generation of electron-hole pairs affects primarily the minority carrier density. However, tlie excess population of minority carriers is not stable it gradually disappears tlirough a variety of recombination processes in which an electron in tlie CB fills a hole in a VB. The excess energy E is released as a photon or phonons. The foniier case corresponds to a radiative recombination process, tlie latter to a non-radiative one. The radiative processes only rarely involve direct recombination across tlie gap. Usually, tliis type of process is assisted by shallow defects (impurities). Non-radiative recombination involves a defect-related deep level at which a carrier is trapped first, and a second transition is needed to complete tlie process. 

A second important decay mechanism for the iridium-associated trapped electron center involves the thermally assisted excitation of the trapped carrier to the conduction band. It is retrapped many times at other Ir3 + sites, so that the time before the electron is permanently annihilated by silver formation at an intrinsic sensitivity center or recombination is increased. When this decay mechanism predominates, the addition of Ir3 + reduces the emulsion s high intensity reciprocity failure (see Section I.B). 

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