Excimer formation by steric twisting in carbazole and triphenylamine-based host materials

This paper presents a detailed spectroscopic investigation of luminescence properties of 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP) and N,N,N’,N’-tetraphenylbenzidine (TAD) in solutions and neat films. These compounds are compared to their derivatives CDBP and TDAD that contain methyl groups in the 2 and 2’ position of the biphenyl core. We find that whereas steric twisting in CDBP and TDAD leads to a high triplet energy of about 3.0 and 3.1 eV, respectively, these compounds also tend to form triplet excimers in a neat film, in contrast to CBP and TAD. By comparison with N-phenylcarbazole (NPC) and triphenylamine (TPA), on which these compounds are based, as well as with the rigid spiro analogs to CBP and TAD we show that the reduced excimer formation in CBP and TAD can be attributed to a localization of the excitation onto the central biphenyl part of the molecule.We acknowledge support from the Federal Ministry of Education and Research (BMBF) through the project ‘Trip-Q’, the German Science Foundation (DFG) through the Research and Training Group GRK 1640 and the UK Engineering and Physical Sciences Research Council (grant number EP/G060738/1).This is the final published version. It first appeared at http://pubs.acs.org/doi/abs/10.1021/jp512772j

An Efficient Approach to the Synthesis of Novel Pyrene-Fused Azaacenes

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Organic Letters (copyright © American Chemical Society) after peer review and technical editing by the publisher. To access the final edited and published work see: http://dx.doi.org/10.1021/ol401438

Crystallinity Effects in Sequentially Processed and Blend-Cast Bulk-Heterojunction Polymer/Fullerene Photovoltaics

Although most polymer/fullerene-based solar cells are cast from a blend of the components in solution, it is also possible to sequentially process the polymer and fullerene layers from quasi-orthogonal solvents. Sequential processing (SqP) not only produces photovoltaic devices with efficiencies comparable to the more traditional bulk heterojunction (BHJ) solar cells produced by blend casting (BC) but also offers the advantage that the polymer and fullerene layers can be optimized separately. In this paper, we explore the morphology produced when sequentially processing polymer/fullerene solar cells and compare it to the BC morphology. We find that increasing polymer regioregularity leads to the opposite effect in SqP and BC BHJ solar cells. We start by constructing a series of SqP and BC solar cells using different types of poly(3-hexylthiophene) (P3HT) that vary in regioregulary and polydispersity combined with [6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM). We use grazing incidence wide-angle X-ray scattering to demonstrate how strongly changes in the P3HT and PCBM crystallinity upon thermal annealing of SqP and BC BHJ films depend on polymer regioregularity. For SqP devices, low regioregularity P3HT films that possess more amorphous regions allow for more PCBM crystallite growth and thus show better photovoltaic device efficiency. On the other hand, highly regioregular P3HT leads to a more favorable morphology and better device efficiency for BC BHJ films. Comparing the photovoltaic performance and structural characterization indicates that the mechanisms controlling morphology in the active layers are fundamentally different for BHJs formed via SqP and BC. Most importantly, we find that nanoscale morphology in both SqP and BC BHJs can be systematically controlled by tuning the amorphous fraction of polymer in the active layer. © 2014 American Chemical Society

P3HT-Based Solar Cells: Structural Properties and Photovoltaic Performance

Each year we are bombarded with B.Sc. and Ph.D. applications from students that want to improve the world. They have learned that their future depends on changing the type of fuel we use and that solar energy is our future. The hope and energy of these young people will transform future energy technologies, but it will not happen quickly. Organic photovoltaic devices are easy to sketch, but the materials, processing steps, and ways of measuring the properties of the materials are very complicated. It is not trivial to make a systematic measurement that will change the way other research groups think or practice. In approaching this chapter, we thought about what a new researcher would need to know about organic photovoltaic devices and materials in order to have a good start in the subject. Then, we simplified that to focus on what a new researcher would need to know about poly-3-hexylthiophene:phenyl-C61-butyric acid methyl ester blends (P3HT: PCBM) to make research progress with these materials. This chapter is by no means authoritative or a compendium of all things on P3HT:PCBM. We have selected to explain how the sample fabrication techniques lead to control of morphology and structural features and how these morphological features have specific optical and electronic consequences for organic photovoltaic device applications