Photocrosslinkable liquid-crystalline polythiophenes with oriented nanostructure and stabilization for photovoltaics

A novel stable and photocrosslinkable electron-donor material, liquid-crystalline polythiophene containing cyano-biphenyl mesogenic pendant, namely, poly{3-[6-(4′-cyano-biphenyloxy)hexyl]thiopheneylenethiophene-alt-3-(6-bromohexyl)thiophene} (PTcbpTT), was designed and synthesized. The structural anisotropy originating from cyano-biphenyl mesogens can induce the PTcbpTT to assemble into a well ordered morphology and consequently lead to the red-shift absorption, enhanced photoluminescence. The thermal treatment drives further development of the morphology of the copolymer and its blend films mixed with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), towards a state of microphase separation in the nanometer scale. Furthermore, the bulk heterojunction devices based on the PTcbpTT:PCBM (1:1 wt.%) active layer have been constructed. Without extensive optimization, the LC annealing device yields an enhancement of power conversion efficiency from 0.5% to 1.2%, showing a significantly increased Jsc and FF with respect to its untreated counterpart, thanks to the ordered microphase separation channels for charge transportation. The high Voc value of 0.731 V is due to the low HOMO level of PTcbpTT. Unlike devices prepared from PTcbpTT:PCBM blend without UV treatment, photocrosslinked PTcbpTT:PCBM devices are stable even when annealed for two days at the elevated temperature of 150 °C, implying that the photocrosslinked structure dramatically suppresses largescale phase segregation.

Bromine-functionalized liquid crystalline copolymer with cyano-bihenyl mesogenic pendant is developed to induce the well ordered morphology by the mesogens and improved the stability via photocrosslinking. After radiation by UV, well ordered morphology can be maintained by fixing both donor and acceptor in the photocrosslinked net work. Meanwhile, the photocrosslinked structure can freeze this optimum morphology at high temperature and preserve long term performance of PSC.Figure optionsDownload as PowerPoint slideHighlights
► Bromine-functionalized liquid crystalline copolymer with mesogenic pendant is developed to induce the well ordered morphology.
► Well ordered morphology can be maintained by fixing both donor and acceptor in the photocrosslinked net work.
► The photocrosslinked structure can freeze this optimum morphology at high temperature for preserving long term performance of PSC.