Gary
Conboy
a,
Rupert G. D.
Taylor
a,
Neil J.
Findlay
a,
Alexander L.
Kanibolotsky
ab,
Anto R.
Inigo
a,
Sanjay S.
Ghosh
c,
Bernd
Ebenhoch
c,
Lethy
Krishnan Jagadamma
c,
Gopala Krishna V. V.
Thalluri
c,
Muhammad T.
Sajjad
c,
Ifor D. W.
Samuel
c and
Peter J.
Skabara
*a
aWestCHEM, Department of Pure and Applied Chemistry, Thomas Graham Building, University of Strathclyde, Glasgow, G1 1XL, UK. E-mail: peter.skabara@glasgow.ac.uk
bInstitute of Physical-Organic Chemistry and Coal Chemistry, 02160 Kyiv, Ukraine
cOrganic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, KY16 9SS, UK
First published on 13th November 2017
A series of copolymers containing the benzo[1,2-d:4,5-d′]bis(thiazole) (BBT) unit has been designed and synthesised with bisthienyl-diketopyrrolopyrrole (DPP), dithienopyrrole (DTP), benzothiadiazole (BT), benzodithiophene (BDT) or 4,4′-dialkoxybithiazole (BTz) comonomers. The resulting polymers possess a conjugation pathway that is orthogonal to the more usual substitution pathway through the 2,6-positions of the BBT unit, facilitating intramolecular non-covalent interactions between strategically placed heteroatoms of neighbouring monomer units. Such interactions enable a control over the degree of planarity through altering their number and strength, in turn allowing for tuning of the band gap. The resulting 4,8-BBT materials gave enhanced mobility in p-type organic field-effect transistors of up to 2.16 × 10−2 cm2 V−1 s−1 for pDPP2ThBBT and good solar cell performance of up to 4.45% power conversion efficiency for pBT2ThBBT.
Organic semiconductors are also often studied for use as functional active layer materials in organic photovoltaic (OPV) devices, as they can be engineered to have a broad absorption across the solar spectrum. When combined with a suitable acceptor species, resultant organic solar cells can have power conversion efficiencies (PCEs) of over 12%.18,19 In order to minimise the band gap, as well as to increase the carrier mobility and self-assembly within the bulk phase, it is beneficial to place alternating conjugated donor and acceptor units into the backbone of the conductive material to facilitate a push–pull effect.20,21 Such a structure is readily achievable in polymeric form through the copolymerisation of suitably functionalised donor and acceptor units. Many of these units contain multiple heteroatoms that offer further advantages, such as planarisation and strong interchain packing due to a combination of π–π stacking and non-covalent heteroatom/weak hydrogen bonding interactions in the bulk material.22–25 For example, the use of thiazole (rather than thiophene) facilitates these advantages in combination with deeper highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels26 to give increased device performance in both OFETs27,28 and OPVs.29,30 Additionally, intermolecular non-covalent interactions in the bulk are evident, further contributing to improved charge carrier properties.26,31–33
Recently we highlighted the use of a thiazole-containing benzobisthiazole (BBT) unit with an orthogonal (4,8- vs. traditional 2,6-) conjugation pathway (Fig. 1).25 Whilst BBT units possessing the 2,6-conjugation pathway are common and well-studied, those with 4,8-substitution have been underexplored in comparison,25,34–38 despite offering a better template with which to facilitate planarising intramolecular non-covalent interactions with neighbouring heterocycles.25 By carefully selecting the type and location of heteroatoms in the flanking heterocycles (Fig. 2), the planarity of the resultant molecule or polymer can be tuned and, in turn, their solubility and energy gap can be modified.
Fig. 1 Typical 2,6-BBT conjugation pathway (left) and alternate 4,8-BBT conjugation pathway (right). |
Fig. 2 Non-covalent intramolecular interactions between S and N atoms of a BBT core and flanking heterocycles. |
Non-covalent interactions are defined as contact distances between two atoms (often heteroatoms) which are shorter than the sum of the van der Waals radii of the two corresponding atoms. Through a combination of X-ray crystallography and computational simulations we have previously demonstrated that the inclusion of thiophene units either side of the BBT heterocycle (Fig. 2: X = CH, Y = S) gives a twisted, non-rigid structure. In the orientation depicted in Fig. 2, C–H⋯N hydrogen bonding interactions are offset by the repulsive S⋯S interactions, whilst S⋯N interactions (when the flanking heterocycles are flipped 180° relative to Fig. 2) are deterred by steric hindrance between the C–H and S groups.25 Conversely, utilising thiazole moieties in place of thiophene (Fig. 2: X = S, Y = N) results in four intramolecular S⋯N non-covalent interactions and a highly planarised structure (maximum torsion angle of 5.1° across the C–C bond connecting the BBT unit and heterocycle), and the use of furan (Fig. 2: X = CH, Y = O) leads to a similarly highly planarised structure (maximum torsion angle of 4.1°) through non-covalent S⋯O interactions.25 These, and other results, have shown that non-covalent S⋯N and S⋯O contacts offer favourable interactions of comparable strength, whilst S⋯S interactions are repulsive and N⋯O interactions are weak/negligible. It is important to note that additional intramolecular hydrogen bonding interactions with the BBT nitrogen atoms may be playing a role when X = CH (Fig. 2), however it has been unequivocally shown that heteroatom-heteroatom interactions are influential on the structure of such molecules.25
In this work we describe the synthesis of a series of BBT copolymers conjugated along the 4,8-substitution pathway (Fig. 1) and report their semiconducting properties in OFET and OPV devices. Bisthienyl-diketopyrrolopyrrole (DPP), dithienopyrrole (DTP), benzothiadiazole (BT), benzodithiophene (BDT) and 4,4′-dialkoxybithiazole (BTz) were selected as comonomers due to their reported behaviour in high performance OFET32,39 and OPV40 devices, but also to allow for band gap variation through HOMO and LUMO energy level tuning and to provide varying degrees of planarity through non-covalent heteroatom interactions (as discussed above).
Molecular weights (Table 1) of the resultant polymers were determined by gel permeation chromatography (GPC) in chloroform or o-dichlorobenzene solution and show a range of different values (14.4–96.0 kg mol−1). The limiting factor in molecular weight for each polymer is solubility – all polymers precipitated from solution during their respective polymerisations. Despite their very different molecular weights, pDPP2ThBBT and pDPPThFBBT have very similar solubility, suggesting that there is reduced rotational freedom in pDPPThFBBT due to intramolecular S⋯O and C–H⋯N interactions,25 facilitated by the furan rings flanking the BBT unit. In contrast, the thiophene moieties flanking the BBT unit in pDPP2ThBBT result in unfavourable S⋯S interactions and hence a more twisted structure, allowing the growing polymer chain to remain in solution longer before precipitating. Molecular weight variation between pDPPThBBT, pBT2ThBBT and pBDTBBT (which all feature thiophene units flanking the BBT units and hence contain unfavourable S⋯S interactions) is likely due to a combination of different alkyl chain lengths (leading to different solubility limits of the growing polymer chains during polymerisation) and the variety of polymerisation conditions used across the series.
Polymer | M w (kg mol−1) | PDI | E optgc (eV) | E electgd (eV) | Solutione | Film | HOMOd (eV) | LUMOd (eV) | T d (°C) | ||
---|---|---|---|---|---|---|---|---|---|---|---|
λ max (nm) | λ onset (nm) | λ max (nm) | λ onset (nm) | ||||||||
a Calculated from GPC using 0.5 mg ml−1 solutions in chlorobenzene at 80 °C. b Calculated from GPC using 1 mg ml−1 solutions in chloroform at 22 °C. c Calculated from the onset of the longest solid state wavelength absorption peak. d Found from CV, using the onset of redox activity and referenced to Fc/Fc+ (−4.8 eV). e Absorption spectra obtained from o-dichlorobenzene solutions. f Shoulder. | |||||||||||
pBTzBBT | — | — | 1.53 | 1.69 | 685, 740f | 795 | 657, 731f | 808 | −4.69 | −3.00 | 360 |
pDPPThBBT | 14.4a | 1.70 | 1.36 | 1.39 | 790 | 875 | 728 | 910 | −4.90 | −3.51 | 395 |
pDPP2ThBBT | 96.0a | 2.75 | 1.39 | 1.43 | 765 | 865 | 748 | 895 | −5.10 | −3.67 | 411 |
pDPPThFBBT | 18.0a | 2.04 | 1.43 | 1.26 | 716f, 751 | 855 | 705, 756 | 870 | −4.86 | −3.60 | 365 |
pBT2ThBBT | 17a | 1.80 | 1.58 | — | 548 | 690 | 620 | 785 | −5.20 | — | 451 |
pBDTBBT | 61b | 1.90 | 2.00 | 2.24 | 493 | 555 | 567 | 620 | −5.29 | −3.05 | 332 |
pDTPBBT | — | — | 1.84 | 2.11 | 530 | 620 | 536 | 675 | −4.80 | −2.69 | 401 |
Comparison of the optical band gaps reveals that the DPP containing polymers (pDDPThBBT, pDDP2ThBBT and pDPPThFBBT) have very similar optical properties, and the smallest optical band gaps (1.36–1.43 eV) of all the 4,8-BBT copolymers. This suggests that the strong electron-accepting nature of the DPP unit causes it to dominate the optical properties of these copolymers, resulting in red-shifted absorption. Copolymers featuring weaker acceptor units (BTz and BT) show slightly wider optical band gaps (1.53 and 1.58 eV for pBTzBBT and pBT2ThBBT, respectively), whilst those containing electron donating units have the widest optical band gaps (1.84 and 2.00 eV for pDTPBBT and pBDTBBT, respectively).
The optical properties of pBT2ThBBT, pBDTBBT and pDTPBBT reveal comparable absorption profiles to their equivalent 2,6-BBT copolymers (PBBTzBT-HD,46PBTHDDT47 and PBTDTP,47 respectively, Fig. 4) when measured in solution, with either similar or slightly hypsochromically shifted absorption onsets. As thin films, the absorption properties of pDTPBBT and its 2,6-substituted counterpart (PBTDTP)47 are also similar, and result in essentially identical optical band gaps (1.84 and 1.85 eV, respectively). However, the solid state absorption profiles of pBT2ThBBT and pBDTBBT are bathochromically shifted compared to their 2,6-analogues, resulting in lower optical band gaps (1.58 vs. 1.7 eV46 for the BT-containing polymers and 2.00 vs. 2.13 eV47 for the BDT-containing polymers). Moreover, the absorption window of pBT2ThBBT is significantly broader than that of its 2,6-analogue (PBBTzBT-HD),46 allowing for increased photon absorption.
Fig. 4 Previously reported 2,6-BBT copolymers featuring DTP (top left),47 BDT (top right)47 and BT (bottom) units.46acalculated from photoelectron yield spectroscopy, bcalculated using (Eg (opt) + EHOMO). |
pDPPThBBT and pDPP2ThBBT exhibit relatively similar HOMO and LUMO energy levels, and by extension closely matching electrochemical band gaps (1.39 and 1.43 eV, respectively). pDPPThFBBT shows a slightly lower electrochemical band gap of 1.26 eV, which is likely due to a combination of the lower resonance stabilisation energy of the furan groups flanking the BBT unit and increased conjugation through planarising intramolecular S–O interactions. The electrochemical data of the DPP-containing 4,8-BBT copolymers is consistent with the optical data in demonstrating that the strongly electron-accepting nature of the DPP unit results in it dominating the materials electronic behaviour, although thiophene or furan flanking units can fine tune this behaviour further.
In agreement with the optical data and comparing to those copolymers containing DPP, copolymers featuring weaker electron acceptor units have slightly wider electrochemical band gaps (1.69 eV for pBTzBBT), whilst those featuring electron-donating units (pDTPBBT and pBDTBBT) have much wider electrochemical band gaps (2.11 and 2.24 eV, respectively). In comparison to its 2,6-analogue, pBDTBBT has a smaller electrochemical band gap (2.24 eV vs. 2.37 eV for PBTHDDT),47 which is in agreement to the bathochromic shift seen in the absorption measurements.
Device optimisation studies were carried out for pDPP2ThBBT (selected for its solubility and relatively high molecular weight, which has been shown to result in improved charge carrier mobility)48,49 through variation of the annealing temperature (as cast, 60, 100, 150 and 200 °C) and solvent (chloroform, chlorobenzene or o-dichlorobenzene). Solutions of 10 mg ml−1 concentration were prepared and deposited onto the prefabricated substrates in accordance with the previously stated procedure, with the same device tested at each annealing temperature increment. To facilitate accurate comparison of the materials and limit further processing steps, the use of self-assembled monolayers (e.g. pentafluorobenzenethiol)50 or processing additives were avoided. Summarised data for devices based on pDPP2ThBBT are shown in Table 2 and Fig. 5, with full device data in Fig. S21–S33 (ESI†).
Solvent | Annealing temperature (°C) | μ h (cm2 V−1 s−1) | I on/Ioff | V th (V) |
---|---|---|---|---|
a Devices gave no response due to poor film morphology. | ||||
o-Dichlorobenzene | As cast | 1.17 × 10−2 | 103 | 12 |
60 | 1.63 × 10−2 | 103 | 14 | |
100 | 2.16 × 10−2 | 103 | 5 | |
150 | 1.20 × 10−2 | 103 | 0 | |
200 | 8.56 × 10−3 | 102 | −6 | |
Chlorobenzene | As cast | 3.97 × 10−3 | 103 | 13 |
60 | 6.35 × 10−3 | 103 | 14 | |
100 | 7.89 × 10−3 | 103 | 6 | |
150 | 7.76 × 10−3 | 103 | 1 | |
200a | — | — | — | |
Chloroform | As cast | 8.98 × 10−3 | 102 | 12 |
60 | 1.18 × 10−2 | 103 | 11 | |
100 | 1.28 × 10−2 | 103 | 1 | |
150 | 8.76 × 10−3 | 103 | −1 | |
200a | — | — | — |
Fig. 5 shows that o-dichlorobenzene is the best solvent for preparing OFETs from pDPP2ThBBT, with the devices outperforming analogous devices prepared from chlorobenzene or chloroform solutions at all annealing temperatures. However, annealing was only beneficial up to 100 °C, after which point further annealing resulted in a drop-off in device performance due to visibly poor film morphology. This is likely a result of the polymer exhibiting significant rigidity, meaning that modest annealing temperatures are enough to force the as cast film towards the thermodynamically most stable (crystalline) state resulting in grain boundaries. Accordingly, OFETs made from other 4,8-BBT copolymers were processed from o-dichlorobenzene solution and annealed at 100 °C; data obtained from these OFETs are shown in Table 3, with output and transfer characteristics in Fig. S34–S39 (ESI†).
Polymer | μ h (cm2 V−1 s−1) | I on/Ioff | V th (V) |
---|---|---|---|
pBTzBBT | 2.16 × 10−3 | 105 | −2 |
pDPPThBBT | 3.23 × 10−6 | 105 | −10 |
pDPP2ThBBT | 2.16 × 10−2 | 103 | 5 |
pDPPThFBBT | 2.03 × 10−3 | 105 | −2 |
pBT2ThBBT | 3.60 × 10−3 | 103 | −5 |
pBDTBBT | 8.69 × 10−5 | 103 | −2 |
pDTPBBT | 2.11 × 10−5 | 103 | 11 |
The hole mobility values obtained show a large variation, spanning four orders of magnitude. pDPP2ThBBT based OFETs gave the highest hole mobility (2.16 × 10−2 cm2 V−1 s−1) in combination with a moderate Ion/Ioff ratio (103) and low threshold voltage (5 V) for a p-type device, but suffer from increased hysteresis in both output and transfer characteristics (Fig. S21–S33, ESI†). This could be indicative of charge trapping in the bulk film or non-optimal device structure. pDPPThFBBT exhibits a slightly lower hole mobility (2.03 × 10−3 cm2 V−1 s−1), possibly due to its lower molecular weight48,49 and larger threshold voltage for p-type devices (−2 V). This is likely due to a non-ohmic contact between the gold electrode (work function −5.0 to −5.1 eV) and the shallow HOMO of pDPPThFBBT (−4.86 eV) resulting in poor charge injection. AFM images of the thin films of pDPP2ThBBT and pDPPThFBBT cast on OFET substrates (Fig. 6a and b) show very smooth uniform films with a root mean square (RMS) roughness of 0.55 and 0.34 nm, respectively, and grain boundaries on the order of 0.1 μm or less. Similarly acquired images of pDPPThBBT (Fig. 6c) show a much rougher film (RMS roughness of 2.82 nm) with domains extending up to 5 μm, and grain boundaries greater than 1 μm wide. This poor film morphology combined with the relatively low molecular weight of pDPPThBBT is likely the cause of the significantly inferior hole mobility (3.23 × 10−6 cm2 V−1 s−1).49
To overcome the poor solubility of pBTzBBT, all solutions used for OFET fabrication were pre-stirred at 100 °C for three hours then spin-coated whilst hot to prevent precipitation of the material. In spite of this, pBTzBBT produced the most ideal OFETs of those presented in this work, with modest hole mobility (2.16 × 10−3 cm2 V−1 s−1), but crucially a high Ion/Ioff ratio (105) and a low driving voltage (−2 V). AFM (Fig. 6d) revealed that the film morphology of pBTzBBT was very rough (RMS roughness = 15.63 nm), which is likely due to the poor solubility of the polymer (despite the extra pre-treatment), meaning that further optimisation of the device preparation, or increasing the solubility of the polymer through the use of longer alkyl chains, could lead to a higher-performing solution-processed OFET.
An additional example of the impact of the BBT unit on OFET performance is evident in pBT2ThBBT. OFETs featuring pBT2ThBBT showed modest mobility (3.60 × 10−3 cm2 V−1 s−1), but this is over two orders of magnitude higher than a literature example featuring only BT units,51 highlighting the enhanced crystallinity afforded by incorporation of BBT units into the polymer backbone through increased potential for non-covalent interactions and 2D conjugation.
Polymer |
PCE
avg. (%) |
PCE
best (%) |
V OC (V) | J SC (mA cm−2) | FF (%) |
---|---|---|---|---|---|
a Inverted architecture: ITO/Cs2CO3/BHJ/MoO3/Ag. The data are for the active layer blend ratios (donor/acceptor, w/w) giving the best and average (over 4 to 8 OPV devices) power conversion efficiency, namely pDPPThBBT:PC71BM, 1:3. pDPP2ThBBT:PC71BM, 1:2. pDPPThFBBT:PC71BM, 1:3. pBT2ThBBT:PC71BM, 1:1 (3% DIO). pBDTBBT:PC71BM, 1:1 (3% DIO). pDTPBBT:PC71BM, 1:1. Optimisation details of these devices are included in the ESI. The error bars (±) are standard deviations of the measured data. | |||||
pBTzBBT | — | — | — | — | |
pDPPThBBT | 0.5 ± 0.03a | 0.53a | 0.62 ± 0.03a | 1.60 ± 0.05a | 50.2 ± 1.0a |
pDPP2ThBBT | 0.9 ± 0.3 | 1.2 | 0.6 ± 0.02 | 2.40 ± 0.50 | 60.0 ± 5.0 |
pDPPThFBBT | 0.90 ± 0.02 | 0.92 | 0.64 ± 0.03 | 2.50 ± 0.12 | 55.0 ± 1.1 |
pBT2ThBBT | 4.33 ± 0.12 | 4.45 | 0.65 ± 0.02 | 14.3 ± 0.39 | 48.0 ± 1.3 |
pBDTBBT | 0.82 ± 0.02 | 0.84 | 0.81 ± 0.02 | 2.71 ± 0.05 | 39 ± 0.78 |
pDTPBBT | 0.57 ± 0.04 | 0.62 | 0.69 ± 0.19 | 2.80 ± 0.06 | 30.8 ± 7.7 |
pDPP2ThBBT and pDPPThFBBT exhibit very similar device performances across all measurements (PCEs of 0.89 and 0.87%, respectively) representing a small improvement over the simpler pDPPThBBT material (PCE 0.50%). The increased PCEs are attributed to the higher FF of 60% and 55% respectively, compared to 50% for pDPPThBBT. The FF obtained for each of the DPP-containing BBT copolymers increases proportionally with increasing hole mobility, which is in good agreement with previously published data.52
The highest PCE (4.45%) was obtained from pBT2ThBBT using a 1:1 blend with PC71BM and 3% diiodooctane (DIO) as an additive. The high PCE can be largely attributed to a very high JSC (14.32 mA cm−2) as well as moderate VOC and FF (0.65 V, 48%). The impressive JSC generated from this device, comparable to that of PTB7,53 indicates that this material is a promising candidate for high-efficiency OPV devices. The J–V characteristics, external quantum efficiency (EQE) and absorption spectra of the active layer blend components (pBT2ThBBT and PC71BM) corresponding to the best OPV device are shown in Fig. 7. The higher photocurrent of this polymer compared to others can be due to its broad absorption (as shown in Fig. 3) and the higher extinction coefficient (Fig. S44, ESI†) compared to other BBT polymers. Moreover, the exciton diffusion length of pBT2ThBBT determined by time resolved fluorescence studies is found to be ∼10 nm which is higher than many donor–acceptor polymers. For example, the reported value of for PTB7 of 4–5 nm54 would give of 6–7 nm. This enhanced exciton diffusion length of the pBT2ThBBT polymer can also contribute towards the increased photon harvesting of the pBT2ThBBT:PC71BM blend. The experimental details and calculations of exciton diffusion length are included in the ESI.†
Fig. 7 (a) J–V characteristics and (b) EQE spectra for the best performing pBT2ThBBT:PC71BM OPV devices. |
In comparison to a 2,6-BBT analogue (PBBTzBT-DT),46pBT2ThBBT gives a higher base PCE (4.45 vs. 2.37%) using the same conventional device structure. However, PCEs of PBBTzBT-DT were shown to improve to 3.84%46 and 6.53%55 by utilising an inverted device structure and a ZnO electron transport layer respectively, suggesting further improvements in pBT2ThBBT based devices are possible.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7tc03959j |
This journal is © The Royal Society of Chemistry 2017 |