High open-circuit voltage photovoltaic cells with a low bandgap copolymer of isothianaphthene, thiophene, and benzothiadiazole units

Jung Yong Kim, Yang Qin, Derek M. Stevens, Vivek Kalihari, Marc A. Hillmyer, C. Daniel Frisbie

Research output: Contribution to journalArticlepeer-review

21 Scopus citations

Abstract

A novel conjugated copolymer (PITN-co-ThBTD) composed of alternating isothianaphthene, thiophene, and benzothiadiazole units was synthesized and characterized. The polymer has a low bandgap of 1.55 eV as a result of the intrachain coupling between electron-donating/withdrawing units. Thermal analysis and wide-angle X-ray scattering (WAXS) reveal that the polymer has a largely amorphous structure. Blends of PITN-co-ThBTD with the electron acceptor methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) were studied as a function of increasing PCBM content by WAXS, atomic force microscopy, charge transport, and photovoltaic measurements. The PCBM solubility limit, i.e., the phase-separation point, was estimated to be 30 wt % PCBM, beyond which charge carrier transport switches from hole only to ambipolar (both electron and hole) in a field-effect transistor testbed. Bulk heterojunction solar cells were constructed from PITN-co-ThBTD films blended with varying weight fractions of PCBM. The best performance was observed at high PCBM compositions (∼70-80% PCBM) rather than at the phase separation point. The power conversion efficiency of 0.9% with short circuit current, Jsc = 3.4 mA/cm2, open circuit voltage, Voc = 0.83 V, and fill factor, FF = 32%, was measured under AM 1.5, 100 mW/cm2 illumination. The high Voc is a promising result for low bandgap polymer-based photovoltaics, while the low FF is a performance-limiting factor originating from the disordered structure of the polymer and the thickness of the film (100 nm).

Original languageEnglish (US)
Pages (from-to)21928-21936
Number of pages9
JournalJournal of Physical Chemistry C
Volume113
Issue number52
DOIs
StatePublished - 2009

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