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

• Published in: Nano letters Vol. 12; no. 8; pp. 4366 - 4371
• Main Authors: Zimmerman, Jeramy D, Xiao, Xin, Renshaw, Christopher Kyle, Wang, Siyi, Diev, Vyacheslav V, Thompson, Mark E, Forrest, Stephen R
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 08-08-2012
• Subjects: Annealing; Applied sciences; Buckminsterfullerene; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Deposition; Electronic structure and electrical properties of surfaces, interfaces, thin films and low-dimensional structures; Electronic transport in multilayers, nanoscale materials and structures; Electronics; Energy; Exact sciences and technology; Excitation; Fullerenes; Heterojunctions; Low-dimensional structures (superlattices, quantum well structures, multilayers): structure, and nonelectronic properties; Molecular electronics, nanoelectronics; Morphology; Natural energy; Photovoltaic cells; Photovoltaic conversion; Physics; Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices; Solar cells; Solar cells. Photoelectrochemical cells; Solar energy; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); polaron-pair recombination; squaraine; annealing; Organic photovoltaic; fullerene; small-molecule organic semiconductor; Solar cells; Annealing; Charge transport; Crystallinity; Molecular weight; AND circuit; Photovoltaic cell; Thin films; Interfaces; Molecular electronics; Excitons; Morphology; Fullerenes; Heterojunctions; Heterointerface; Polarons
• DOI: 10.1021/nl302172w
• EISSN: 1530-6992
• ISSN: 1530-6984

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)/C60 heterojunctions before or after the deposition of C60. Solvent vapor annealing of DPSQ before C60 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 C60 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.