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C3.04 - Efficient Solution-Processed Small Molecule Solar Cell by Incorporating High Molecular Weight Polystyrene 
April 22, 2014   9:30am - 9:45am

Bulk-heterojuction (BHJ) solar cells with bicontinuous interpenetrating network of organic donor (D) and acceptor (A) offer promising merits such as low cost, lightweight and flexibility and have been considered as potential contender for novel energy market. The power conversion efficiencies (PCEs) of BHJ solar cells based on conjugated polymer donors have been exceeding to 10 %. Recently, molecular donors with well-defined dimensions are being studied intensively, leading to small molecule BHJ solar cells which exhibit comparable PCEs ~7 %. Although there are much attractive properties along with molecular donors, it is generally pretty difficult to obtain film with good wettability from molecular donors when compared to analogous polymer systems. Moreover, the molecular films are so thin that it is not possible to achieve sufficiently high optical densities upon solar spectrum. To circumvent these problems, a novel method of active layer control by incorporating a small addition of an inert high molecular weight polymer to molecular D/A system was investigated. It is found that one can obtain increase in film thickness without sacrificing desirable morphology of phase separation and structural order. Specifically, high molecular weight polystyrene (PS, Mn=20,000,000) is incorporated to a p-DTS(FBTTh2)2:PC70BM blend on an order of 1-5 % by weight relative to the BHJ components. It shows that the device performance is extremely beneficial with a PCE increasing from 7.0 % to 8.2 %. Furthermore, different techniques such as grazing incidence wide-angle X-ray scattering (GIWAXS), dynamic secondary ion mass spectroscopy (DSIMS), cross-sectional transmission electron microscopy (TEM) and bright-field TEM were employed to illustrate the film composition and morphology. It reveals that the critical accumulation of insulating PS within the interior of film and away from the charge-collecting electrodes.

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Keynote Address
Panel Discussion - Different Approaches to Commercializing Materials Research
Business Challenges to Starting a Materials-Based Company
Fred Kavli Distinguished Lectureship in Nanoscience
Application of In-situ X-ray Absorption, Emission and Powder Diffraction Studies in Nanomaterials Research - From the Design of an In-situ Experiment to Data Analysis