Alternating and direct current characterization and photoinduced absorption studies of modified conjugated polymer thin films

Abstract

Control of interchain separation enables the optical properties of polymer thin films to be altered. We present a thin film preparation technique that increases the free volume fraction in spin coated polymer thin films resulting in reduced interchain interaction. The polaron yield, measured using photoinduced absorption, was significantly reduced as a direct result of the increased interchain separation, leading to a higher value for the measured photoluminescence efficiency. Impedance spectroscopy showed an increase in permittivity, probably due to greater polarizability. Increases approaching one order of magnitude were observed for direct current hole conductivity and mobility values. Space charge limited conduction analysis suggests a narrowing of the highest occupied molecular orbital band tail on treatment resulting in reduced trapping. Single layer light emitting devices prepared using this technique were found to be significantly brighter and to have longer operating lifetimes.

Spectroscopic investigation of excitons, photocarriers, and bias-induced carriers in regioregular poly(3-alkylthiophene)

“…The photophysics is important not only for discussing the photoexcitation dynamics in the slow time regime but also for enhancing the performance of the RR P3AT/fullerene composite solar cells that are designed for use under cw photoirradiation. 16,17 We here adopt a cw photoinduced absorption (PIA) technique, which has been shown to be effective for investigating the slow-regime photoexcitation dynamics of conjugated polymers 4,[13][14][15][18][19][20][21][22][23] and must also be effective for directly accessing the photophysics under cw photoirradiation. In the slow time regime, clarifying the behaviors of photogenerated polarons is the key to revealing the dynamics of the RR P3AT film since triplet excitons were shown not to be generated in the RR P3AT film by magnetic resonance techniques.…”

Computational steering in Monte Carlo simulations of thin film polystyrene

“…The physics of charge transport in semiconducting polymeric materials is determined by the dynamics of polarons and excitons. It has been seen experimentally that the interchain separation of polymers strongly affects the polaron generation and so the photoluminescent efficiency (Lipson et al 2004), and that the charge carrier mobilities are strongly affected by the composition of the device (Pacias et al 2003). This work aims to improve the methodology by which relaxed conjugated polymer structures can be found and so provide an understanding of the local configurations that control inter-chain hopping of polarons.…”

Interchain effects in the ultrafast photophysics of a semiconducting polymer:THztime-domain spectroscopy of thin films and isolated chains in solution

“…In the thin-film phase interchain interactions are present. 2,6,8,11,18,27,28 On short ͑subpicosecond͒ times, only a small number ͑Ͻ1%͒ of free charges is generated in the solid film, while this number is even smaller ͑by approximately 2 orders of magnitude͒ in solution. On longer timescales ͑Ͼ5 ps͒ mobile charges are not observed and the photoinduced changes in the THz wave form are mainly due to the polarizability of excitons.…”

“…10 However, most polymer solid samples are generally amorphous and disordered, as the large polymer molecules are very difficult to crystallize. This leads to a structure with a large variance in chain-chain distances 11 ͓see Fig. 1͑a͔͒, with only a small fraction of the polymer chain length "close" to neighboring chains.…”