Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES
Article
Effects of Fluorination and Side Chain Branching on Molecular Conformation and Photovoltaic Performance of Donor-Acceptor Copolymers Sebastian Wood, Ji-Hoon Kim, Do-Hoon Hwang, and Ji-Seon Kim Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b01503 • Publication Date (Web): 03 Jun 2015 Downloaded from http://pubs.acs.org on June 9, 2015
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 12
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
Effects of Fluorination and Side Chain Branching on Molecular Conformation and Photovoltaic Performance of Donor-Acceptor Copolymers Sebastian Wood,† Ji-Hoon Kim,§ Do-Hoon Hwang,§ Ji-Seon Kim†,* †
Department of Physics and Centre for Plastic Electronics, Imperial College London, London, SW7 2AZ, United Kingdom §
Department of Chemistry, and Chemistry Institute for Functional Materials, Pusan National University, Busan 609735, Republic of Korea
ABSTRACT: The performance of donor-acceptor type conjugated polymers in photovoltaic devices is strongly dependent on the morphology of the thin film, which can be controlled by the substitution of various side groups on to the conjugated backbone. We investigate the effects of fluorination of the acceptor unit, and differences in the symmetry of alkyl side chain branching on the resulting photovoltaic device efficiency. Chemical variants of the donor-acceptor copolymer poly{4,8-bis[(triisopropylsilyl)ethynyl)benzo[1,2-b:4,5-b′]-dithiophene-alt-2-(heptadecan-9-yl)-4,7-bis(thiophen-2-yl)-2Hbenzo[d][1,2,3]triazole} (PBDTBTz) are considered. Using Raman spectroscopy we find that the benzodithiophenethiophene (BDT-T) inter-unit bond dihedral torsion is minimized by using a symmetrically branched alkyl side chain on the benzotriazole (BTz) unit. This leads to an increased quality of molecular packing, resulting in the highest hole mobility (~3.8 × 10-3 cm2/Vs) and overall device efficiency (~5.5 %). By contrast, fluorination of the BTz unit is detrimental to device performance. In this case, whilst the BTz-T inter-unit bond dihedral angle is reduced, the neighboring BDT-T inter-unit bond dihedral is increased, and the π-π packing distance is increased (from 4.05 to 4.60 Å) in the neat film. We conclude that, whilst the fluorination tends to planarize the polymer backbone, it also introduces repulsive fluorinehydrocarbon interactions, which oppose close molecular packing, and hinder efficient charge generation in the photovoltaic blend.
(BTz) flanked with bridging thiophene (T) units is an attractive choice of acceptor unit since side chains can be substituted onto the triazole unit to give good solubility and polymer backbone planarity.10,11 For these reasons, donor-acceptor type copolymers comprising these units can be expected to perform well in solar cells. In addition to the chemical structure of the conjugated backbone, it has also been established that the optoelectronic properties of conjugated polymer materials are strongly affected by the substitution of different non-conjugated side groups. In particular, fluorine substitution onto the acceptor unit has been found to dramatically improve the efficiency of photovoltaic devices.10,12–15 Additionally, variations in the length and branching of alkyl side chains are found to have significant effects on the device perfor-
INTRODUCTION Donor-acceptor copolymers are an important class of materials for high-performance organic electronic devices. They typically offer good control over the electronic energy levels resulting in tunable light absorption and emission for organic photovoltaic (OPV) and organic lightemitting diode (OLED) applications.1–5 Recently reports have found benzodithiophene (BDT) to be an important donor unit in polymers for high efficiency photovoltaic devices.6,7 Furthermore, it has been found that the addition of triisopropylsilylethynyl (TIPS) side groups can improve charge transport and deepen the highest occupied molecular orbital (HOMO) energy level to increase the open-circuit voltage (Voc).8,9 Benzotriazole
1
ACS Paragon Plus Environment
Chemistry of Materials
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
mance as well as controlling the solubility of the polymer.16,17
Page 2 of 12
The side group variations considered in this study have strong effects on the molecular conformation and device performance of these polymers, which we correlate with changes in the conjugated backbone torsion. Raman spectroscopy enables us to distinguish the changes in dihedral angle of the two different inter-unit bonds present in these polymers i.e. BDT-T and BTz-T.20,24,25 We find that the asymmetrically branched alkyl side chain has little effect on the BTz-T bond planarity but induces an increased torsion of the BDT-T bond, leading to a disruption of the long range molecular packing. On the other hand, fluorination of the BTz unit results in a planarization of the BTz-T bond and a slightly increased torsion of the BDT-T bond, maintaining a high quality of molecular packing but with a slightly increased π-π packing distance. For this material series, both fluorination and asymmetric side-chains are found to be unfavorable for both the charge carrier mobility and overall photovoltaic performance.
The importance of the choice of side groups for tuning the performance of conjugated polymer thin film devices is widely appreciated, although the mechanisms are not clearly understood. The impact of side groups on the quality of molecular packing, intermolecular spacing, and phase segregation is well-documented but these are indirect consequences of the chemical structural changes.12,13,16–18 In this work, we consider the fundamental effects of side group substitution on the planarity of the conjugated polymer backbone. We make use of our indepth experience of Raman spectroscopy as an experimental probe for the molecular conformation of the polymers and compare this with other optoelectronic, morphological and charge transport properties in order to correlate the different side group substitutions with the photovoltaic device performance.19–23 A series of novel donor-acceptor copolymers was considered based on poly{4,8-bis[(triisopropylsilyl)ethynyl) benzo[1,2-b:4,5-b′]-dithiophene-alt-2-(heptadecan-9-yl)-4,7bis(thiophen-2-yl)-2H-benzo[d] [1,2,3]triazole} (SPBDTBTz). These polymers all comprise a BDT donor unit with bulky TIPS side groups and a BTz acceptor unit with bridging T rings. In order to understand the effects of changing the symmetry of the solubilizing alkyl side chain and the impact of fluorinating the BTz unit, three materials are compared, whose structures are shown in Figure 1: S-PBDTBTz has a symmetrically branched alkyl side chain; A-PBDTBTz has an asymmetrically branched alkyl side chain; and F-PBDTBTz has the symmetrically branched alkyl side chain but the two hydrogen atoms on the BTz unit have been substituted with fluorine.
EXPERIMENTAL METHODS Materials A series of donor-acceptor copolymers was prepared comprising (triisopropylsilyl)ethynyl (TIPS)-substituted 2,6-bis(trimethylstannyl)benzo[1,2-b:4.5-b′]dithiophene (BDT) as the donor unit and benzotriazole (BTz) as the acceptor unit with bridging thiophene rings. Synthesis of S-PBDTBTz was previously described by Kim et al. and the syntheses of A-PBDTBTz and F-PBDTBTz are described in the Supporting Information.8 Branched alkyl side chains on the BTz unit provide the solubility required for facile deposition techniques. See Figure 1 for chemical structures of the materials. Molecular weight (Mn) and Polydispersity (PDI) for the materials were: S-PBDTBTz (Mn = 23 kg/mol, PDI = 2.5); A-PBDTBTz (Mn = 15 kg/mol, PDI = 2.7); F-PBDTBTz (Mn = 20 kg/mol, PDI = 2.5). These values were measured by gel permeation chromatography (Waters high-pressure GPC assembly model M590) with chloroform eluent, and calibrated for the polymers using polystyrene standards. Phenyl-C71-butyric acid methyl ester (PC71BM) was received from EM-index for thin film blends. Sample Preparation Thin film samples were prepared from solutions of material dissolved in chlorobenzene at room temperature. In some cases, 1,8-diiodooctane (DIO) was used as a solution additive to form a mixed solvent with 3 % DIO by volume. Solution concentrations of 10 mg/ml were used for neat polymer, and 17 mg/ml (total) for polymer:PC71BM (1:1
Figure 1. Chemical structures of polymers: S-PBDTBTz (symmetrically branched alkyl side chain), A-PBDTBTz (asymmetrically branched alkyl side chain), and FPBDTBTz (symmetrically branched alkyl side chain with fluorinated benzotriazole unit).
2
ACS Paragon Plus Environment
Page 3 of 12
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials at 100 mW/cm2 intensity, calibrated by a standard Si photodiode from PV Measurements Inc., itself calibrated at the National Renewable Energy Laboratory (NREL).
weight ratio) blend films. Films were deposited by spin coating to a thickness of 80 - 100 nm. Samples for spectroscopic measurements were fabricated on fused silica (Spectrosil 2000) substrates. Photovoltaic devices were fabricated with the structure glass/ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al. Substrates were cleaned by ultrasonication in distilled water, methanol, and acetone. PEDOT:PSS (Clevios P from Heraeus) was deposited by spin coating to a thickness of 45 nm and baked to drive off residual water. Calcium (2 nm) and aluminium (120 nm) contacts were added by vacuum deposition at
