SEMINAR: Addressing Fundamental Challenges in Organic Photovoltaics

Vikram Krishna Kuppa, PhD University of Dayton
When Jan 29, 2016
from 01:00 PM to 02:00 PM
Where Ernst Hall, Room 310
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Organic solar cells have attracted considerable attention in recent years due to their potential advantages of flexibility, low cost and ease of processing. The classic bulk heterojunction (BHJ) involves a conjugated polymer mixed with fullerene derivatives, to create the large interfacial area necessary for charge generation. However, the drawback of traditional BHJs is that at least half of the cell is comprised of optically inactive materials which are present merely to dissociate photogenerated excitons and to transport charge carriers. An alternative is to use a blend of semiconducting polymers, with the benefit of enhanced spectral coverage. Nevertheless, blend devices are much less efficient, due to complex phase behavior, and very poor charge mobility [11]. Our work has focused on designing cells that go beyond conventional BHJs, with the following features:

  1. a high fraction of photoactive polymers in the cell;
  2. efficiencies comparable or better than traditional devices;
  3. 100-fold increase in transport coefficients.

We have recently demonstrated how the efficiency of polymer-based solar cells increases three-fold by incorporating small amounts of pristine graphene into the active layer. The fundamental mechanisms underlying this enhancement are investigated, and are shown to arise from improvements in both charge transport and morphology. Three different types of devices employing different chromophores are studied, and a maximum of 200% enhancement in performance is indicated. We investigate the structure via small angle neutron scattering (SANS) studies of the systems with and without graphene. SANS reveals the existence of disk-like P3HT crystallites distributed in an amorphous matrix background. P3HT crystallinity was enhanced upon graphene addition, resulting in larger crystallites and a higher degree of ordering. These structural changes are accompanied by better charge transport, and an increase of the short-circuit current of the devices. Results on cell characteristics and recombination mechanisms are also reported, and indicate means of addressing fundamental problems in OPV systems