Hot exciton dissociation in polymer solar cells

The standard picture of photovoltaic conversion in all-organic bulk heterojunction solar cells predicts that the initial excitation dissociates at the donor/acceptor interface after thermalization. Accordingly, on above-gap excitation, the excess photon energy is quickly lost by internal dissipation. Here we directly target the interfacial physics of an efficient low-bandgap polymer/PC60BM system. Exciton splitting occurs within the first 50 fs, creating both interfacial charge transfer states (CTSs) and polaron species. On high-energy excitation, higher-lying singlet states convert into hot interfacial CTSs that effectively contribute to free-polaron generation. We rationalize these findings in terms of a higher degree of delocalization of the hot CTSs with respect to the relaxed ones, which enhances the probability of charge dissociation in the first 200 fs. Thus, the hot CTS dissociation produces an overall increase in the charge generation yield.

Doc.: http://www.nature.com/nmat/journal/v12/n1/pdf/nmat3502.pdf

Grancini, Maiuri, Fazzi, Petrozza, Egelhaaf, Brida, & Cerullo. (09 de 12 de 2012). Hot exciton dissociation in polymer solar cells. Nature Materials, 29-33.

Nanowire solar cells raise efficiency limit.

At the Nano Science Center at the Niels Bohr Insitute in Denmark, a group of scientists have demonstrated that a single nanowire can concentrate sunlight up to 15 times the normal intensity of sunlight. 

One the left an image of a SEM of the GaAs nanowire crsytal, next to it an image of a TEM of a single nanowire and on the right side a capture of a STEM of the nanowire's surface.

The nanowire naturally concentrates the sun's rays into a very small area in the crystal by up to a factor 15. This happens because the diameter of the nanowire is smaller than the wavelength of the light that comes form the sun and it causes resonances in the intensity of the ray of light in and around the Thus, the resonances can give a concentrated sunlight, where the energy is converted, which can be used to give a higher conversion effeciency of the sun's energy.

Fuente: Nanowerk, (2013). Nanowire solar cells raise efficiency limit. Recuperado el 11 de mayo de 2012, de http://www.nanowerk.com/news2/newsid=29679.php