Independent Control of Bulk and Interfacial Morphologies of Small Molecular Weight Organic Heterojunction Solar Cells

Abstract

We demonstrate that solvent vapor annealing of small molecular weight organic heterojunctions can be used to independently control the interface and bulk thin-film morphologies, thereby modifying charge transport and exciton dissociation in these structures. As an example, we anneal diphenyl-functionalized squaraine (DPSQ)/C(60) heterojunctions before or after the deposition of C(60). Solvent vapor annealing of DPSQ before C(60) deposition results in molecular order at the heterointerface. Organic photovoltaics based on this process have reduced open circuit voltages and power conversion efficiencies relative to as-cast devices. In contrast, annealing following C(60) deposition locks in interface disorder found in unannealed junctions while improving order in the thin-film bulk. This results in an increase in short circuit current by >30% while maintaining the open circuit voltage of the as-cast heterojunction device. These results are analyzed in terms of recombination dynamics at excitonic heterojunctions and demonstrate that the optimal organic photovoltaic morphology is characterized by interfacial disorder to minimize polaron-pair recombination, while improved crystallinity in the bulk increases exciton and charge transport efficiency in the active region.

Noise-induced quantum coherence drives photo-carrier generation dynamics at polymeric semiconductor heterojunctions

“…Clearly, increasing the interfacial bias shifts the fraction of states capable of producing current towards lower energies to the extent that they begin to overlap the absorption spectrum of the system. It should also be noted that the energetic onset for producing current is somewhat higher for the DOD cases suggesting that disorder at the interfacial region may facilitate efficient charge separation as suggested by Forrest 15,30 .…”

“…On one hand, energetic disorder at the interface would provide a free energy gradient for localized charge-transfer states to escape to the asymptotic regions. In essence, the localized polarons in the interfacial region could escape into a band of highly mobile polarons away from the heterojunction region 15 . On the other hand, energetic disorder in the regions away from the interface would provide an entropic driving force by increasing the density of localized polaron states away from the interfacial region, allowing the polarons to hop or diffuse away from the interface before recombination could take place 16 .…”

Growth and modelling of spherical crystalline morphologies of molecular materials

“…Most of the organic thin film growths by VTE result in amorphous or nanocrystalline morphologies, while the substrate is usually heated in order to enhance crystallinity. Substrate templating involves using thin crystalline films of small molecular organic materials 12,13 or thin membranes of graphene 14 as seed layers. The degree of vapour saturation of organic material at the substrate is an important parameter that determines film morphology.…”

Singlets lead to photogeneration inC60-based organic heterojunctions

“…A device area of 1 mm 2 was defined by a shadow mask during metal cathode deposition. The EQE was measured as previously reported [24]. Fits to the EQE were performed using the method of Peumans, et al [1] on C 60 in its absorption range of λ=380 nm to 520 nm.…”