Semiconductor grain boundary recombination

Chemical Control of Surface and Grain Boundary Recombination in Semiconductors... 

In summary, a simple chemical picture of surface recombination is presented. The surface or grain boundary recombination velocity decreases when the appropriate surface species is reacted with a strongly chemisorbed species. It increases when a species is weakly chemisorbed. We shall now illustrate this concept for six extensively studied semiconductors, Ge, Si, GaP, GaAs, InP and InSb. 

Based on our observations, we have reason to believe that single crystal performance will be approached in future thin film, polycrystalline semiconductor based solar cells with grain boundary recombination velocities reduced by strongly chemisorbed species. 

In a porous film consisting of interconnected nanometer sized semiconductor particles the effective surface area can be enhanced 1000-fold [121]. Therefore, nanostructured electrodes can be good for unravel the surface phenomena. By scrutinizing the effect of iodine on the performance of the electrode it was concluded that the effect of surface states was small in the nanostructured hematite electrode. It was stated that the bulk and grain boundary recombination remained dominant. This is in consistency with the report from Cherepy et al [43]. 

Heller A. (1981), Chemical control of surface and grain boundary recombination in semiconductors , ACS Symp. Ser. 146, 57-77. 

Recombination of electrons and holes at grain boundaries in thin, polycrystalline films of semiconductors is a key problem that requires solution if efficient and inexpensive solar cells are to be developed. This problem is quite similar to the extensively studied phenomenon of recombination at semiconductor surfaces. Both involve the electronic states associated with the abrupt discontinuity in the chemical bonding at an interface. 

Electron-hole recombination velocities at semiconductor interfaces vary from 102 cm/sec for Ge3 to 106 cm/sec for GaAs.4 Our first purpose is to explain this variation in chemical terms. In physical terms, the velocities are determined by the surface (or grain boundary) density of trapped electrons and holes and by the cross section of their recombination reaction. The surface density of the carriers depends on the density of surface donor and acceptor states and the (potential dependent) population of these. If the states are outside the band gap of the semiconductor, or are not populated because of their location or because they are inaccessible by either thermal or tunneling processes, they do not contribute to the recombination process. Thus, chemical processes that substantially reduce the number of states within the band gap, or shift these, so that they are less populated or make these inaccessible, reduce recombination velocities. Processes which increase the surface state density or their population or make these states accessible, increase the recombination velocity. 

Considering there are no grain boundaries present in the material, the quite low photoresponse may be due to i) intrinsic recombinations centers located in mid bandgap states or ii) slow interfacial kinetics at the nanostructured semiconductor/electrolyte interface. 

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