On the Limit of Excitonic Effect in Polymeric Photovoltaic Materials

Organic photovoltaic (OPV) has drawn a lot of attention during the last decade owningto its potential for low-cost and high-efficiency. The technology was triggered by theearlier breakthrough of the introduction of organic heterojunction that efficientlydissociate strongly bound excitons upon photo-excitation. The heterojunction structurehas become the design rule for material synthesis and device architecture. Recently,having the efforts from both fine tuning of the electronic properties of the organicphotoactive materials and device structures, over 10% power conversion efficiency (PCE)has been reported. However, in order to tackle the excitonic effect, the organicheterojunction requires an energy offset (>0.3eV) between the organic donor andacceptor materials, as a trade-off, the photovoltage and as a result of the PCE aresignificantly reduced by more than 30%.Recently the excitonic effect has been regarded as the major limitation on furtherimproving the performance in OPV. Despite of the encouraging theoretical results andpredictions on the ultimate PCE of 20% in OPV by reducing the excitonic effect,experimentally little success has been reported so far. There are two major obstacles intackling this issue: 1) There is lack of systemic investigation on the correlation betweenthe dielectric properties and excitonic effect in organic materials. It is still unclear howhigh the dielectric constant can completely diminish the exciton effect in the OPVprocesses. 2) There is lack of viable approach or demonstration to bring insight intoreducing the excitonic effect. It is unknown whether the dielectric constant can beeffectively increased by molecular structural modification or incorporating with otherhigh-dielectric (high-k) materials.There are two key objectives in this proposed research: In materials, we will increase thedielectric constant of existing OPV materials by structural modifications and developinghigh-k composites. We are aiming at reducing the exciton binding energy in existinghigh-efficiency conjugated low-bandgap polymers. Ultimately, we will explore thepossibility to realize free-charge generation without the heterojunction structure inOPV. In device physics, we will investigate the excitonic effect by developing techniquesbased on pump-probe and charge modulation spectroscopies to investigate the exciton tofree-charge evolution in different material systems. We are aiming at quantitativelyidentify the contribution of the material dielectric properties on the excitonic effect.Therefore, the success of this project would not only bring insight into developing moreefficient photovoltaic devices, but more fundamentally the excitonic effect in photo-carriergeneration process in photovoltaic materials.