Chetan R. Singh

Morphology, Crystal Structure and Charge Transport in Donor–Acceptor Block Copolymer Thin Films

We studied structure and charge transport properties of thin films of donor-acceptor block copolymers, poly(3-hexylthiophene-block-perylene bisimide acrylate), using a combination of X-ray scattering, AFM and vertical charge transport measurements in diode devices. Block copolymer self-assembly and crystallization of the individual components are interrelated and different structural states of the films could be prepared by varying preparation conditions and thermal history. Generally the well-defined microphase structures found previously in bulk could also be prepared in thin films, in addition alignment induced by interfacial interactions was observed. Microphase separated block copolymers sustain ambipolar charge transport, but the exact values of electron and hole mobilities depend strongly on orientation and connectivity of the microdomains as well as the molecular order within the domains.

Plasmonic nanomeshes : their ambivalent role as transparent electrodes in organic solar cells

In this contribution, the optical losses and gains attributed to periodic nanohole array electrodes in polymer solar cells are systematically studied. For this, thin gold nanomeshes with hexagonally ordered holes and periodicities (P) ranging from 202 nm to 2560 nm are prepared by colloidal lithography. In combination with two different active layer materials (P3HT:PC61BM and PTB7:PC71BM), the optical properties are correlated with the power conversion efficiency (PCE) of the solar cells. A cavity mode is identified at the absorption edge of the active layer material. The resonance wavelength of this cavity mode is hardly defined by the nanomesh periodicity but rather by the absorption of the photoactive layer. This constitutes a fundamental dilemma when using nanomeshes as ITO replacement. The highest plasmonic enhancement requires small periodicities. This is accompanied by an overall low transmittance and high parasitic absorption losses. Consequently, larger periodicities with a less efficient cavity mode, yet lower absorptive losses were found to yield the highest PCE. Nevertheless, ITO-free solar cells reaching ~77% PCE compared to ITO reference devices are fabricated. Concomitantly, the benefits and drawbacks of this transparent nanomesh electrode are identified, which is of high relevance for future ITO replacement strategies.

Nanoscale Morphology from Donor–Acceptor Block Copolymers: Formation and Functions

General design principles of donor–acceptor block copolymers are reviewed and specific results arising from block copolymers consisting of semicrystalline poly(3-hexylthiophene-2,5-diyl) (P3HT) blocks of appreciably high molecular weight and acceptor blocks carrying pendant perylene bisimides or fullerene derivatives are summarized. The chapter is structured according to the building blocks P3HT, poly(perylene bisimide acrylate), and a polystyrene copolymer grafted with phenyl-C61-butyric acid methyl ester used for the synthesis of the corresponding block copolymers, and in each part the synthetic challenges, structure formation, and consequences for charge transport, and in some cases photovoltaic properties, are addressed.

Consequence of thermal annealing on PCDTBT-based solar cells performance and composition profile (presentation video)

As reported earlier, the photovoltaic performance of PCDTBT:PCBM polymer solar cells drastically decreases upon thermal annealing. It was demonstrated in the literature, that thermal annealing leads to increased trap formation and as a consequence disturb solar cell performance, especially via a reduced fill factor. This has been demonstrated by space-charge-limited-current analysis and ellipsometry, as well as, structural changes analyse of PCDTBT upon annealing. However, we decided in addition to investigate morphological changes occurring within PCDTBT:PCBM photoactive blend layers upon thermal annealing, as these must have an impact on charge transport. By application of several characterizations techniques, and especially supported by results of Impedance Spectroscopy and Auger Electron Spectroscopy (AES), indeed the existence of an unfavourable compositional gradient within the photoactive layer could be revealed. This compositional gradient may be in part accounted for harming the transport of electrons and holes in either direction.