Synthesis and Characterization of Low Bandgap Conjugated Polymers Containing Thiophene Units and Their Applications in Polymer Solar Cells

Abstract

First we have synthesized a new conjugated polymer PBDTDT containing 4 8-dioctyloxybenzo[1 2-b;3 4-b']di-thiophene and 2 2'-dithiophene via a Stille coupling reaction The optical bandgap of PBDTDT is equal to the electrochemical bandgap (2 01 eV) In order to investigate its photovoltaic properties polymer solar cell (PSC) devices based on PBDTDT were fabricated with a conventional device configuration of ITO/PEDOT:PSS/copolymer:PC61BM/LiF/Al under AM 1 5G illumination 100 mW/cm2 The bulk heterojunction (BHJ) polymer solar cells were fabricated with the conjugated polymer as the electron donor and 6 6-phenyl-C61-butyric acid methyl ester (PC61BM) as the electron acceptor The power conversion efficiency (PCE) of the solar cells based on PBDTDT/PC61BM (1:3) annealing at 110 oC for 20 min was 1 19 % with a short-circuit current density (JSC) of 4 72 mA/cm2 an open-circuit voltage (VOC) of 0 6 V and a fill factor (FF) of 53 3 % Second sensible design and synthesis of conjugating polymers is important to the development of polymer solar cells (PSCs) In this work we synthesized two dioctylfluorene-thiophene based conjugated copolymers via Stille and Suzuki polymerization reactions PFTDPP and PFTpBT having different acceptor groups on the backbone The optical bandgaps of PFTDPP and PFTpBT are 1 39 eV and 1 80 eV respectively As a result the photovoltaic properties of the copolymers blended with 6 6-phenyl-C61-butyric acid methyl ester (PC61BM) as an electron acceptor were obtained The polymer solar cell (PSC) based on a conventional device configuration ITO/PEDOT:PSS/copolymers:PC61BM/LiF/Al showed a power conversion efficiency (PCE) of 2 42 % and 3 02 % for PFTDPP and PFTpBT respectively A simple method methanol treatment was introduced to further optimize device performance The best PCEs could reach 4 25 % and 4 20 % after methanol treatment under the AM 1 5G illumination with an intensity of 100 mW/cm2 from a solar simulator Third we have enhanced the power conversion efficiency of PSCs by physically doping an organic dye into the active layer of the cells The organic dye has a complementary absorption wavelength with regard to the polymer active layer and energy transfer effect thus increasing the photovoltaic current of PSCs We chose 2 3 9 10 16 17 23 24-octakis(octyloxy)-29H 31H-phthalocyanine (OPc) as an organic dye and analyzed OPc’s characteristics with cyclic voltammetry (CV) UV-Vis spectrometry and thermogravimetry analysis (TGA) The PSCs were fabricated by blending the organic dye into the active layer (P3HT/PC61BM) at different concentrations (1 63 wt% 3 22 wt% 4 76 wt% and 6 30 wt%) The doped devices showed a 19% increase in the PCE It was proved that this increase in efficiency was due to the increase in photovoltaic current based on the measurement of external quantum efficiency (EQE)

Date of Award	2016 Jul 20
Original language	English
Supervisor	Lien-Chung Hsu (Supervisor)