Charge recombination in organic solar cells

Recombination is a key source of efficiency loss in OSC devices. There are two types of recombination in OSCs the first is geminate recombination, which happens before mobile charges are formed, and the second is charge recombination, which is the recombination between mobile charges. Since geminate recombination has been covered in previous chapters, here we focus on the charge 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 

Bimolecular recombination is the dominant recombination process in OSC devices. The bimolecular recombination rate can be calculated by eqn (10.2). 

Equations (10.2) and (10.3) describe the process when a pair of opposite charges encounter each other in space and recombine with the help of Coulomb attraction. According to the classical bimolecular recombination model,when the mean free path of charges in a material is smaller than the Coulomb capture radius r, bimolecular recombination will take place. The typical mean free path of organic semiconductors is about 1-2 nm, which is much smaller than r (10-20 nm at room temperature ), therefore bimolecular recombination is universally present in OSC devices.  

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