&Organic Electronics

Dithienoindophenines: p-Type Semiconductors Designed byQuinoid Stabilization for Solar-Cell Applications

Longbin Ren+,[a, b] Haijun Fan+,[a] Dazhen Huang,[a, b] Dafei Yuan,[a, b] Chong-an Di,[a] andXiaozhang Zhu*[a, b]

Abstract: Compared with the dominant aromatic conju-gated materials, photovoltaic applications of their quinoi-

dal counterparts featuring rigid and planar molecularstructures have long been unexplored despite their

narrow optical bandgaps, large absorption coefficients,and excellent charge-transport properties. The design and

synthesis of dithienoindophenine derivatives (DTIPs) bystabilizing the quinoidal resonance of the parent indophe-nine framework is reported here. Compared with the am-

bipolar indophenine derivatives, DTIPs with the fixed mo-lecular configuration are found to be p-type semiconduc-

tors exhibiting excellent unipolar hole mobilities up to0.22 cm2 V@1 s@1, which is one order of magnitude higherthan that of the parent IP-O and is even comparable to

that of QQT(CN)4-based single-crystal field-effect transis-tors (FET). DTIPs exhibit better photovoltaic performance

than their aromatic bithieno[3,4-b]thiophene (BTT) coun-terparts with an optimal power-conversion efficiency (PCE)of 4.07 %.

Organic photovoltaics (OPVs) has been developed into a prom-ising photo-electric conversion technique in terms of its poten-tial advantages of low cost, large area manufacture, and flexi-bility.[1] To maximize sunlight utilization, a variety of donor ma-terials with low optical bandgaps have been designed by the

“donor–acceptor” (D-A) approach,[2] most of which are basedon the aromatic frameworks. Surprisingly, donor materials with

purely quinoidal frameworks have rarely been reported todate, with low PCEs of ,1.1 %.[3] Generally, quinoidal oligothio-phenes (QOT)[4] with rigid and planar Kekul8 structures showunique optoelectronic properties such as narrow opticalbandgaps with large absorption coefficients,[5] singlet-fission

property,[3, 6] near-infrared fluorescence,[7] and excellent charge-transport properties,[8] which are highly desirable for OPV appli-

cations. However, the most well-known dicyanomethylene-ter-minated QOTs such as quinoidal bithiophene (QBT)[9] are typi-

cally n-type semiconductors because of their high electron af-finities. Indophenine derivatives (QBT with 1-alkylindolin-2-one

terminals) known for over one century[10] have recently been

found to show promising ambipolar properties (Figure 1).[11] Todemonstrate the potential of QOTs for OPV applications, p-type

QOTs with suitable HOMO and LUMO energy levels should bedeveloped preferentially to match [6,6]-phenyl-C71-butyric acid

methyl ester (PC71BM) acceptors.

Thieno[3,4-b]thiophene (TbT) can efficiently reduce opticalbandgaps by enhancing quinoidal resonance of aromatic con-

jugated systems, that is, destabilization of aromatic reso-nance.[11] Thus, we envisioned that TbT may also elevate the

LUMO energy level of indophene derivatives by stabilizing thequinoidal resonance. With the introduction of TbT into the in-dophenine framework, we designed and synthesized dithi-

enoindophenine derivatives, DTIP-i-O and DTIP-o-O, with proxi-mal and distal sulfur orientations (Figure 1). DTIPs inherit the

advantages of QOTs, and meanwhile are endowed with newcharacteristics such as stable E-configuration, large p-surface,and intermolecular S–S interaction that especially benefits hole

transport. Compared with the parent N-octylindophene (IP-O),the frontier orbital energies of DTIPs increased slightly for

HOMO and considerably for LUMO, indicating that the aroma-tization tendency of the QBT framework are restrained. Accord-ing to OTFT (organic thin-film transistor) measurements, wefound that DTIPs are p-type semiconductors with hole mobility

Figure 1. n-Type, ambipolar, and p-type quinoidal bithiophenes: QBT (n-type), IP-O (ambipolar), DTIP-i-O (p-type; R1: n-dodecyl ; R2 : n-octyl).

[a] L. Ren,+ Dr. H. Fan,+ D. Huang, D. Yuan, Prof. Dr. C.-a. Di, Prof. Dr. X. ZhuCAS Key Laboratory of Organic SolidsBeijing National Laboratory for Molecular SciencesChinese Academy of Sciences, Beijing, 100190 (P.R. China)E-mail : xzzhu@iccas.ac.cn

[b] L. Ren,+ D. Huang, D. Yuan, Prof. Dr. X. ZhuUniversity of Chinese Academy of Sciences, Beijing 100049 (P.R. China)

[++] These authors contributed equally to this work.

Supporting information and ORCID from the author for this article areavailable on the WWW under http ://dx.doi.org/10.1002/chem.201603112.

Chem. Eur. J. 2016, 22, 17136 – 17140 T 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim17136

CommunicationDOI: 10.1002/chem.201603112

of 0.22 cm2 V@1 s@1, which is higher than that of parent IP-O byone order of magnitude. Furthermore, DTIPs were successfully

employed as donor materials in bulk heterojunction (BHJ) solarcells and delivered decent power conversion efficiency (PCE) of

4.07 %, which is much higher than their aromatic counterparts,BTTs, and should not be underestimated if we consider the

low PCEs obtained at the similarly early stage of small-mole-cule solar cells utilizing aromatic donor materials.[13]

Instead of the traditional “indophenine reaction”,[10] the new

synthetic route was utilized for the synthesis of isomeric DTIPsbecause of the unsymmetrical TbT (Scheme 1). Bithieno[3,4-

b]thiophenes, compound 1 and compound 1’, were synthe-sized from 6-iodo-2-octylthieno[3,4-b]thiophene/4-iodo-2-oc-

tylthieno [3,4-b]thiophene and tributyl(2-octylthieno[3,4-b]thio-phen-6-yl)stannane/tributyl(2-octylthieno[3,4-b]thiophen-4-yl)-

stannane in 60 and 85 % yields, respectively.[8] Diols, compound2 and compound 2’, were then synthesized from bithieno[3,4-b]thiophenes by deprotonation and selective nucleophilic reac-

tion with N-octylisatin in 70 and 80 % yields, respectively. Final-ly, diols were reduced using SnCl2 to give DTIP-i-O and DTIP-o-

O in 40 and 80 % yields as blue solids. For comparison, IP-Owas synthesized using a similar procedure. The stereochemistry

of dialkylated indophenine was clarified by Tormos et al. in

1996,[14] which revealed the existence of up to six stereoiso-mers. In contrast to IP-O, dithienoindophenines show well-de-

fined 1H NMR spectra, revealing the absence of any isomer. InFigure 2, all hydrogens in the aromatic region of IP and DTIPs

are assigned. The hydrogens at 3-position of DTIP-i-O were sig-nificantly more downfield-shifted than those of DTIP-o-O, (d :

8.76 vs. 7.39 ppm), which suggests the intramolecular O@H1 in-teractions and confirms the explicit configurations, as shown in

Scheme 1. It has been previously assumed that the isomeric

phenomenon might not affect the device performance; how-ever, we believe that the configurationally fixed DTIPs may

avoid the energetic and morphology disorders, which is favor-able for enhancing device performance. In addition, the aroma-

tic counterparts, BTT-i-O and BTT-o-O, were synthesized by theKnoevenagel condensation between dialdehyde intermediates

and 1-octylindolin-2-one as blue solids.

The photophysical and electrochemical properties of DTIPs,IP-O, and BTTs were examined in dichloromethane and are

summarized in Table 1. DTIPs show structured absorption spec-tra with maximum absorptions at 665 nm (DTIP-i-O) and

639 nm (DTIP-o-O) that are similar to that of IP-O (634 nm; seeFigure 3 a) with comparable absorption coefficients and are

consistent with only the E-configuration. In thin films, the max-

imum absorptions of DTIPs are hypsochromically shifted by 49and 39 nm, respectively. Meanwhile, a new shoulder peak (707

and 696 nm) was observed at the long-wavelength side, whichsuggests the possibility of coexistence of H- and J- packingpatterns. Based on the absorption onsets in thin films, the op-

Scheme 1. Synthesis of dithienoindophenines and their aromatic counter-parts. Reagents and conditions: (a) (i) nBuLi, THF, @35 8C; (ii) N-octylisatin,THF; (b) SnCl2, toluene, room temperature (RT).

Figure 2. 1H NMR spectra of IP-O including six isomers, DTIP-i-O, and DTIP-o-O in CDCl3.

Table 1. Photophysical and electrochemical properties of DTIPs, IP-O, andBTTs.

Cpd. lmaxabs[a]

[nm]e[b]

[L@1 m@1]lmax

abs

(film [nm])EHOMO

[c]

[eV]ELUMO

[c]

[eV]Eg

opt[d]

(film [eV])

IP-O 634 48100 518 @5.16 @3.70 1.50DTIP-i-O 665 49700 618, 707(sh.)[e] @5.10 @3.58 1.56DTIP-o-O 639 48100 601, 696(sh.)[e] @5.13 @3.49 1.65BTT-i-O 659 49700 689 @4.97 @3.38 1.69BTT-o-O 661 47900 627 @5.01 @3.39 1.57

[a] Measured in dichloromethane. [b] Absorption coefficients[L mol@1 cm@1] . [c] The LUMO/HOMO energy levels were determined fromELUMO/HOMO =@(4.80 + Ered/ox

onset). [d] The optical bandgaps were determinedfrom Eg

opt = 1240/lonsetabs. [e] Shoulder peaks.

Chem. Eur. J. 2016, 22, 17136 – 17140 www.chemeurj.org T 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim17137

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tical bandgaps of DTIPs were estimated to be 1.56 and 1.65 eV,

respectively. The electrochemical properties of DTIPs were ex-amined by cyclic voltammogram (Figure S2 in the Supporting

Information). DTIPs show reversible or quasi-reversible oxida-tive and reductive processes, according to which the HOMO

and LUMO energy levels were determined to be @5.10/

@3.58 eV for DTIP-i-O and @5.13/@3.49 eV for DTIP-o-O. Com-pared with IP-O (@5.16/@3.70 eV), DTIPs show slightly elevatedHOMOs and considerably elevated LUMOs. The LUMOs ofDTIPs match the PCBM acceptor better than that of IP-O in

BHJ solar cells. BTTs show structured absorption spectra withmaximum absorptions at 659 nm (BTT-i-O) and 661 nm (BTT-o-

O) with comparable absorption coefficients. In thin films (Fig-ure 3 b), the maximum absorption of BTT-i-O is bathochromi-cally shifted by 30 nm, whereas that of BTT-o-O is hypsochrom-

ically shifted by 34 nm, which implies J-aggregation for BTT-i-Ofilm and main H-aggregation in BTT-o-O film. Accordingly, the

optical bandgaps of BTTs were estimated to be 1.69 (BTT-i-O)and 1.57 eV (BTT-o-O), respectively, which shows the opposite

tendency to DTIPs based on sulfur orientations. In addition,

BTTs show obviously higher HOMO and LUMO energy levels,@4.97/@3.38 eV for BTT-i-O and @5.01/@3.39 eV for BTT-o-O

than those of DTIPs, implying the effectiveness of loweringHOMO and LUMO energy levels with quinoidal cores.

The potential of DTIPs as p-type semiconductors was investi-gated by bottom-gate bottom-contact (BGBC) OTFT (organic

thin-film transistor) devices that were fabricated on octadecyl-trichlorosilane (OTS)-modified SiO2 (300 nm)/Si substrates. Thin

films were spin-coated as the organic semiconducting layerand 30 nm gold served as the source-drain electrodes. The op-

timized thin films were obtained by annealing at 120 8C.Figure 4 shows the I–V characteristics of the best DTIP-based

OTFTs. In accordance with the alignment of frontier orbital

energy levels, DTIP-based OTFT devices showed p-type trans-port property with negligible n-type behavior. Devices based

on DTIP-i-O showed higher hole mobilities (mhavg =

0.15 cm2 V@1 s@1, Vth = 3.1 V, Ion/Ioff = 105) than those of DTIP-o-O(mh

avg = 0.055 cm2 V@1 s@1, Vth = 6.3 V, Ion/Ioff = 104 ; see Table S2 in

the Supporting Information). A maximum mobility of0.22 cm2 V@1 s@1 was obtained at 120 8C for DTIP-i-O, which isnearly one order of magnitude higher than that of ambipolarIP (mh

max = 0.031 cm2 V@1 s@1)[9] and is comparable to that of

single-crystal FETs based on QQT(CN)4.[15] From AFM (atomicforce microscopy) investigations (Figure S3 in the SupportingInformation), we found that DTIP-i-O can form better continu-

ous thin films with less defects than DTIP-o-O. In DTIP-i-O,there are intramolecular O@H hydrogen bonds, suggesting the

enhanced rigidity of DTIP-i-O compared with DTIP-o-O. Accord-ingly, we observed that DTIP-i-O possessed better crystallinity

than DTIP-o-O according to the thin-film X-ray diffraction

images (Figure S4 in the Supporting Information), which mayalso have contributed to its higher hole mobility. In addition,

the hole-transport properties of BTTs have also been examined,0.014 and 0.036 cm2 V@1 s@1 for BTT-i-O and BTT-o-O, respective-

ly, which is significantly lower than those of the correspondingquinoidal counterparts.

Figure 3. UV/Vis absorption spectra of DTIPs, IP-O (a), and BTTs (b) in di-chloromethane and thin films.

Figure 4. Output (a, c) and transfer (b, d) characteristics of BGBC OTFTs basedon DTIP-i-O (a, b) and DTIP-o-O (c, d) with annealing temperature at 120 8C.

Chem. Eur. J. 2016, 22, 17136 – 17140 www.chemeurj.org T 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim17138

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To investigate the potential of DTIPs for photovoltaic appli-cations, solution-processed BHJ solar cell devices were fabricat-

ed with a conventional configuration of ITO/PEDOT:PSS/activelayer/PFN[16]/Al. Table 2 and Figure 5 summarize the optimal

photovoltaic parameters for such devices, all of which weretested under AM 1.5 G, 100 mW cm@2 simulated illumination.

Comparatively, the DTIP-o-O device shows a higher Voc

(0.92 eV) than that of the DTIP-i-O device (0.86 eV), which re-lates with the former donor’s relatively deeper HOMO. Mean-

while, the DTIP-o-O device also obtained a larger Jsc than thatof the DTIP-i-O, which can be attributed to its better phaseseparation. In the DTIP-o-O:PC71BM blend, smaller aggregateswere observed using atomic force microscopy (AFM), as shown

in Figure S5 (see the Supporting Information), which favors the

charge separation and transport. DTIP-o-O device displays a su-perior PCE (4.07 %) to that of DTIP-i-O device (3.41 %), whereas

devices based on BTTs both show low optimal PCEs, 2.47 %(BTT-i-O) and 1.00 % (BTT-o-O).

In summary, dithienoindophenine derivatives DTIPs withfixed molecular configuration were designed and synthesized

together with the reference aromatic BTTs. DTIPs exhibited ex-cellent unipolar hole mobility up to 0.22 cm2 V@1 s@1, which isone order of magnitude higher than that of the parent IP-O

and is even comparable to that of single-crystal FETs based onQQT(CN)4. DTIP-based solar cells delivered a decent PCE of4.07 %, which is significantly higher than those of aromatic BTTcounterparts. From this study on DTIPs, we believe that OPVapplications of quinoidal semiconductors should be investigat-ed further.

Experimental Section

DTIP-o-O : 2,2’-Dioctyl-4,4’-bithieno[3,4-b]thiophene (0.27 g,0.53 mmol) was dissolved in anhydrous THF (20 mL) under a nitro-gen atmosphere and cooled to @35 8C. After N,N,N’,N’-tetramethyl-ethane-1,2-diamine (TMEDA; 0.129 g, 1.11 mmol) was added in thefirst solution, n-butyllithium (0.76 mL, 1.21 mmol, 1.60 m in hexane)was then added using a syringe. After stirring at @35 8C for 0.5 h,the solution was added to an N-octylisatin (0.30 g, 1.16 mmol) solu-tion (THF, 13 mL) using a syringe at @35 8C and was stirred for an-other 1 h before the solution was warmed to room temperatureover 2 h. The reaction mixture was quenched with a few drops ofsaturated NH4Cl solution, and extracted with CH2Cl2. The organiclayer was combined, dried over MgSO4, and concentrated under re-duced pressure. The crude product was purified using silica-gelcolumn chromatography (CH2Cl2 and ethyl acetate) to give 0.460 gof compound 2’ in 80 % yield as a light-yellow solid without furtherpurification. Then, the product (0.400 g, 0.39 mmol) was dissolvedin dry degassed toluene (10 mL), and SnCl2 (0.089 g, 0.47 mmol)was added. The mixture was stirred at room temperature for about2 h and methanol (10 mL) was then added. The precipitation waswashed with hexane and purified on a silica-gel column chroma-tography together with recrystallization to give 0.279 g of DTIP-o-O in 80 % yield as a deep-blue solid. 1H NMR (400 MHz, CDCl3): d=7.76–7.75 (d, 3J = 8.4 Hz, 2 H), 7.39 (s, 2 H), 7.30–7.26 (t, 3J = 7.2 Hz,2 H), 7.20–7.18 (t, 3J = 7.6 Hz, 2 H), 6.91–6.89 (d, 3J = 8.0 Hz, 2 H),3.85–3.81 (t, 3J = 7.6 Hz, 4 H), 3.02–2.99 (t, 3J = 7.6 Hz, 4 H), 1.91–1.83(m, 4 H), 1.76–1.72 (m, 4 H), 1.41–1.27 (m, 40 H), 0.89–0.86 (t, 3J =6.6 Hz, 12 H); 13C NMR (75 MHz, CDCl3): d= 165.4, 159.6, 148.5,142.5, 141.1, 139.3, 127.0, 124.6, 123.1, 123.0, 121.3, 117.9, 110.9,107.6, 39.8, 32.0, 31.8, 31.6, 31.3, 29.6, 29.5, 29.37, 29.34, 29.29,28.0, 27.2, 22.7, 22.7, 14.2, 14.1 ppm; HRMS (MALDI-TOF) m/z calcdfor C60H79N2O2S4 [M + H]+ : 987.501890; found: 987.502244; elemen-tal analysis calcd (%) for C60H78N2O2S4 : C 72.97, H 7.96, N 2.84;found: C 72.96, H 7.95, N 2.85.

DTIP-i-O : 40 % yield; 1H NMR (400 MHz, CDCl3): d= 8.76 (s, 2 H),7.86–7.84 (d, 3J = 7.6 Hz, 2 H), 7.30–7.24 (t, 3J = 8.0 Hz, 2 H), 7.23–7.17 (t, 3J = 8.0 Hz, 2 H), 6.88–6.86 (d, 3J = 7.6 Hz, 2 H), 3.82–3.78 (t,3J = 8.0 Hz, 4 H), 3.00–2.96 (t, 3J = 8.0 Hz, 4 H), 1.87–1.80 (m, 4 H),1.76–1.68 (m, 4 H), 1.25 (br, 40 H), 0.89–0.86 (t, 3J = 6.6 Hz, 12 H);13C NMR (100 MHz, CDCl3): d= 164.9, 154.2, 145.8, 144.6, 144.0,141.5, 127.2, 124.9, 123.7, 123.6, 123.3, 121.4, 114.2, 107.4, 39.8,31.9, 31.8, 31.8, 29.5, 29.4, 29.4, 29.3, 27.9, 27.2, 22.7, 22.7, 14.14,14.10 ppm; HRMS (MALDI-TOF) m/z calcd for C60H78N2O2S4 [M]+ :986.496286; found: 986.495436; elemental analysis calcd (%) for

Table 2. Photovoltaic performance of DTIP- and BTT-based devices underoptimized conditions.

Cpd.[a] Voc [V] Jsc [mA cm@2] FF [%] PCE (average) [%]

DTIP-i-O 0.86 6.99 56.70 3.41 (3.35)DTIP-o-O 0.92 7.97 55.66 4.07 (4.05)BTT-i-O 0.73 6.82 49.52 2.47 (2.23)BTT-o-O 0.54 3.76 48.69 1.00 (0.95)

[a] Structure of ITO/PEDOT:PSS/donors:PC71BM (weight ratio: 1:0.5)/PFN/Al.

Figure 5. Characteristic J–V curves (a) and EQE curves (b) of the optimizedsolar cell devices based on DTIPs and BTTs.

Chem. Eur. J. 2016, 22, 17136 – 17140 www.chemeurj.org T 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim17139

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C60H78N2O2S4 : C 72.97, H 7.96, N 2.84; found: C 72.00, H 7.99, N2.84.

Acknowledgements

We thank the National Basic Research Program of China (973Program; No. 2014CB643502), the Strategic Priority Research

Program of the Chinese Academy of Sciences (XDB12010200),and the National Natural Science Foundation of China

(91333113, 21572234) for financial support.

Keywords: organic solar cell · organic thin-film transistors · p-type semiconductors · quinones · thiophene

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Received: June 29, 2016

Published online on October 19, 2016

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