Impact of alkyl side chain on the photostability and optoelectronic properties of indacenodithieno[3,2‐b]thiophene‐alt‐naphtho[1,2‐c:5,6‐ c′]bis[1,2,5]thiadiazole medium bandgap copolymers

Abstract

Three donor‐π‐acceptor (D‐π‐A) type alternating conjugated polymers, namely PIDTT‐DTNT‐C16, PIDTT‐DTNT‐HD and PIDTT‐DTNT‐OD bearing the same backbone of indacenodithieno[3,2‐b]thiophene (IDTT) as the D unit and naphtho[1,2‐c:5,6‐c′]bis[1,2,5]thiadiazole (NT) as the A moiety but with different flexible side chain (n‐hexadecyl (C16), 2‐hexyldecyl (HD) and 2‐octyldodecyl (OD)) substituted thiophene employed as π‐bridges, were synthesized and characterized. The effects of the side chain on absorption, photostability, energy levels, aggregation, backbone conformation, morphology and photovoltaic properties were systematically investigated. Because moderate D and strong A units were selected to construct the polymer backbone, a medium optical bandgap (ca. 1.66 eV) and low‐lying highest occupied molecular orbital energy level (EHOMO ≈ −5.36 V), thus resulting in a relatively higher open‐circuit voltage (VOC) of 0.80–0.83 V, were achieved. It was found that the side chain gave rise to an insignificant impact on absorption, aggregation and photostability in chlorobenzene solution and energy levels but a non‐negligible influence on absorption, photostability and aggregation behavior in the film state. It was found that PIDTT‐DTNT‐C16 with the densest and most ordered packing structure exhibited the best photostability. Inverted bulk heterojunction polymer solar cells based on PIDTT‐DTNT‐HD:PC61BM ([6,6]‐phenyl‐C61‐butyric acid methyl ester) showed at least a 1.5‐fold increase in power conversion efficiency, chiefly originating from its slightly improved absorption, more balanced μh/μe ratio and favorable morphology of the active layer as a result of incorporating branched HD side chains into the IDTT‐alt‐DTNT backbone. © 2019 Society of Chemical Industry

Elevated Photovoltaic Performance in Medium Bandgap Copolymers Composed of Indacenodi-thieno[3,2-b]thiophene and Benzothiadiazole Subunits by Modulating the π-Bridge

“…The interlayer material 3,3 -(1,3,8,10-tetraoxoanthra [2,1,9-def :6,5,10-d e f ]diisoquinoline-2,9(1H,3H,8H,10H)diyl)bis(N,N-dimethylpropan-1-amine oxide) (PDINO) was bought from Derthon optoelectronic materials science technology Co LTD (Shenzhen, China). Bistin 2,8-bis(trimethyltin)-6,6,12,12-tetra(4hexylphenyl)-indacenodithieno [3,2-b]thiophene (IDTTSn) was prepared according to our reported method [34]. Symmetric dibromide 4,7-bis(5-bromo-4-octylthien-2-yl)benzo[c] [1,2,5]thiadiazole (DTBTBr 2 ) and unsymmetric 4-bromo-7-(5-bromo-4-octylthien-2-yl)benzo[c] [1,2,5]thiadiazole (TBTBr 2 ), as well as fluorinated 4-bromo-7-(5-bromo-4-octylthien-2-yl)-5,6-difluorobenzo[c] [1,2,5]thiadiazole (TFBTBr 2 ) were synthesized and described in Supporting Information.…”

“…1 H NMR spectra, melting points test, C, H, and N elemental analyses, TGA analyses, molecular weights of polymer, UV-Vis absorption, electrochemical properties, and atomic force microscopy (AFM) and transmission electron microscopy (TEM) images were obtained according to our reported methods [75]. The photostability of polymers and thin film X-ray diffraction (XRD) were measured according to our reported methods [34].…”

“…Key monomer bistin IDTTSn was synthesized according to our reported work [34]. Unsymmetrical dibromide TBTBr 2 and TFBTBr 2 are described in Scheme S1 in Supporting Information.…”

“…By comparison with the fully developed low band gap (LBG, E g < 1.6 eV) copolymers, medium band gap (MBG, 1.6 < E g < 1.8 eV) and/or wide band gap (WBG, E g > 1.8 eV) copolymers were relatively less of concern [30,31]. However, MBG and WBG copolymers were still of importance in the PSCs, because the relatively large band gap cannot only render the low-lying highest occupied molecular orbital energy level (E HOMO ) in order to achieve the higher open-circuit voltage (V OC ), but also couple with the electron-acceptor so as to broaden the absorption range and to garner the high short-circuit current density (J SC ) [32][33][34]. Consequently, it was of significant importance and highly essential to explore the new MBG and WBG copolymers and make a thorough inquiry of the structure-performance relationship.Recently, highly coplanar aromatic/heteroaromatic units have attracted tremendous interest, because they possessed the following characteristics: (i) the elongated effective conjugation and improved overlap of π-orbitals along the polymer backbone direction in order to facilitate electron delocalization, which was instrumental in gaining a red-shifted absorption spectra to capture sunlight; (ii) the better intermolecular charge carrier hopping and restrained the interannular rotation so as to reduce the Marcus reorganization energy that could affect the rate of charge hopping, leading to an improved intrinsic charge mobility [22,24,[35][36][37].…”

“…By comparison with the fully developed low band gap (LBG, E g < 1.6 eV) copolymers, medium band gap (MBG, 1.6 < E g < 1.8 eV) and/or wide band gap (WBG, E g > 1.8 eV) copolymers were relatively less of concern [30,31]. However, MBG and WBG copolymers were still of importance in the PSCs, because the relatively large band gap cannot only render the low-lying highest occupied molecular orbital energy level (E HOMO ) in order to achieve the higher open-circuit voltage (V OC ), but also couple with the electron-acceptor so as to broaden the absorption range and to garner the high short-circuit current density (J SC ) [32][33][34]. Consequently, it was of significant importance and highly essential to explore the new MBG and WBG copolymers and make a thorough inquiry of the structure-performance relationship.…”