A diketopyrrolopyrrole molecule end-capped with a furan-2-carboxylate moiety: The planarity of molecular geometry and photovoltaic properties

Article abstract of DOI:10.1039/c3ta13534a

We designed and synthesized a diketopyrrolopyrrole (DPP) molecule with a fully-planar molecular geometry, 3,6-bis{5-[(ethylfuran-2-carboxylate)-2-yl] thiophene-2-yl}-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP(CF)₂), for solution-processable organic solar cells (OSCs). It is theoretically calculated that the dihedral angles between the two furan-2-carboxylate end-groups and the DPP core are both only 0.56°. Due to this negligible steric distortion, the molecular conformation of DPP(CF) ₂ can be considered fully coplanar, leading to a higher crystallinity for the DPP(CF)₂ film. As a result, the hole mobility of DPP(CF)₂ is one order of magnitude higher than that of the DPP derivative with thiophene-2-carboxylate as the end-group (DPP(CT)₂). DPP(CF)₂ exhibits both a low optical band gap (E_g) of 1.60 eV and a low-lying highest occupied molecular orbital (HOMO) energy level of -5.33 eV, implying that DPP(CF)₂ is a promising electron donor for OSCs. OSCs with DPP(CF)₂ or DPP(CT)₂ as the electron donor and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as the electron acceptor were fabricated. It is found that the DPP(CF) ₂-based devices exhibit much better photovoltaic performance than the DPP(CT)₂-based devices, with the highest power conversion efficiency of 5.37% and a short-circuit current density of 11.4 mA cm^-2. This phenomenon can be ascribed to the superior charge-transporting ability of DPP(CF)₂ due to its fully-planar molecular geometry. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3ta13534a

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Page 1 of 19
Journal of Materials Chemistry A
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DOI: 10.1039/C3TA13534A
A diketopyrrolopyrrole molecule end-capped with furan-2-carboxylate
moiety: planarity of molecular geometry and photovoltaic property
Lei Fu,a Weifei Fu,a Pei Cheng,b Zhixin Xie,a Congcheng Fan,a Minmin Shi,*a Jun Ling,a Jianhui Hou,b
Xiao-wei Zhanc and Hongzheng Chen*a
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
We designed and synthesized a diketopyrrolopyrrole (DPP) molecule with fullyꢀplanar molecular
geometry, 3,6ꢀbis{5ꢀ[(ethylfuranꢀ2ꢀcarboxylate)ꢀ2ꢀyl]thiopheneꢀ2ꢀyl}ꢀ2,5ꢀbis(2ꢀethylhexyl)pyrrolo[3,4ꢀ
c]pyrroleꢀ1,4ꢀdione (DPP(CF)2), for solutionꢀprocessable organic solar cells (OSCs). It is theoretically
10 calculated that the dihedral angels between the two furanꢀ2ꢀcarboxylate endꢀgroups and DPP core are
both only 0.56o. Due to this negligible steric distortion, the molecular conformation of DPP(CF)2 can be
considered fully coplanar, leading to a higher crystallinity for DPP(CF)2 film. As a result, the hole
mobility of DPP(CF)2 is one order of magnitude higher than that of the DPP derivative with thiopheneꢀ2ꢀ
carboxylate as the endꢀgroup (DPP(CT)2). And DPP(CF)2 exhibits both a low optical band gap (Eg) of
15 1.60eV and a lowꢀlying highest occupied molecular orbital (HOMO) energy level of ꢀ5.33 eV, implying
that DPP(CF)2 is a promising electron donor for OSCs. OSCs with DPP(CF)2 or DPP(CT)2 as the
electron donor and [6,6]ꢀphenylꢀC71ꢀbutyric acid methyl ester (PC71BM) as the electron acceptor were
fabricated. It is found that the DPP(CF)2ꢀbased devices exhibit much better photovoltaic performance
than the DPP(CT)2ꢀbased devices, with the highest power conversion efficiency of 5.37% and a shortꢀ
20 circuit current density of 11.4mA cmꢀ2. This phenomenon can be ascribed to the superior chargeꢀ
transporting ability of DPP(CF)2 caused by its fullyꢀplanar molecular geometry.
Introduction
Organic solar cells (OSCs) have attracted tremendous scientific and industrial interests in recent years due to their promising advantages,
such as lowꢀcost, simple manufacturing process, flexibility, and light weight1ꢀ5. Bulk heterojunction (BHJ) structure which involves a
25 blend of organic electron donor (conjugated polymers or small molecules) and electron acceptor ([6,6]ꢀphenylꢀC71ꢀbutyric acid methyl
ester (PC71BM) or other fullerene derivatives) has emerged as a robust way to fabricate highꢀperformance OSCs. In the last decade, the
PCE of OSCs has rapidly improved6ꢀ14, which can be mainly attributed to the development of many high performance polymer donors
with near ideal optical and electron properties. Meanwhile, OSCs with small molecules as electron donors also received increasing
attention for the advantages of easy synthesis and purification, wellꢀdefined structures, absence of end group contaminants, high charge
30 carrier mobility, and better batchꢀtoꢀbatch reproducibility5,15,16. Through persistent investigations by scientists, power conversion
efficiency (PCE) of over 8% has been achieved for small molecule organic solar cells (SMOSCs)17ꢀ19. For instance, Chen and coꢀworkers
achieved a PCE of 8.12% by developing a series of novel small molecule donors, with a benzodithiophene core, a terthiophene spacer,
and a rhodamine endꢀgroup20. Very recently, Bazan et al. reported an OSC based on the small molecule donor of pꢀDTS(FBTTh2)2
21ꢀ23
and found that barium cathode layer can significantly enhance the fill factor (FF) of the device via increasing the charge collection at the
35 cathode. Ultimately the PCE is improved to 9.02% and is the record value for SMOSCs.
Nowadays, several strategies to design organic donors for highꢀperformance OSCs have been established: (1) to form a conjugated
donorꢀacceptor (DꢀA) molecular backbone to achieve broad and strong light absorptions in the visible and nearꢀinfrared regions, (2) to
incorporate planar molecular structures which are closely packed for crystallization to enhance the charge carrier mobility and yield large
shortꢀcircuit current (JSC), and (3) to realize a lowꢀlying the highest occupied molecular orbital (HOMO) energy level to ensure high
40 openꢀcircuit voltage (VOC) and a suitable the lowest unoccupied molecular orbital (LUMO) energy level to provide enough energy offset
for efficient exciton dissociation24.
Since firstly reported by Nguyen and coꢀworkers25ꢀ28, diketopyrrolopyrrole (DPP) unit, with two fused electronꢀdeficient lactams, has
been widely used to design DꢀA polymers and small molecules for applications in the optoelectronic field. The thienyl groups chemically
decorated at 3,6ꢀpositions of the lactam improve intramolecular πꢀelectron delocalization and πꢀπ intermolecular interaction, leading to
45 intense absorptions in the visible region29. In the last few years, significant progress has been made for the application of DPPꢀcontaining
materials in semiconductors. Liu’s group30 reported two donor–acceptor (D−A) copolymer based on diketopyrrolopyrole and (E)ꢀ2ꢀ(2ꢀ
(thiophenꢀ2ꢀyl)vinyl) thiophene (TVT) units for solutionꢀprocessed fieldꢀeffect transistors (FETs), with a highest hole mobility of 8.2
cm2 Vꢀ1 sꢀ1. A linear solutionꢀprocessable small molecule based on 5ꢀalkylthiopheneꢀ2ꢀylꢀsubstituted benzodithiophene and DPP was
reported by two different groups, and the resulting OSCs show PCEs of 5.79%31 and 5.29%32, respectively. Recently, our group designed
50 a DPP molecule endꢀcapped with two thiopheneꢀ2ꢀcarboxylate moieties (DPP(CT)2), which owns appropriate energy levels (both narrow
band gap and lowꢀlying HOMO) as the donor in SMOSCs. The DPP(CT)2ꢀbased OSCs gave a PCE of 4.02%33. We found that the
relatively low hole mobility of DPP(CT)2 is one of main reasons for the moderate device performance. Nguyen and coꢀworkers

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presented that changing heteroatom substitutions in the aromatic endꢀgroups may significantly affect their thermal properties, solubility,
crystallinity, film morphology, charge carrier mobility, and solar cell performance34. Therefore, to obtain high performance SMOSCs, in
this work, while remaining the ester functionalization to tune its solubility and energy level, we replace thiopheneꢀ2ꢀcarboxylate with
furanꢀ2ꢀcarboxylate unit to design a new DPP derivative, 3,6ꢀbis{5ꢀ[(ethyl furanꢀ2ꢀcarboxylate)ꢀ2ꢀyl]thiopheneꢀ2ꢀyl}ꢀ2,5ꢀbis(2ꢀ
ethylhexyl)pyrrolo[3,4ꢀc]pyrroleꢀ1,4ꢀdione (DPP(CF)2). It is expected that DPP(CF)2 would exhibit better planarity of the molecular
structure than DPP(CT)2 because the oxygen atom in furan ring is smaller than the sulfur atom in thiophene ring. We demonstrate that
this improved molecular structure decreases the steric hinderance35 and results in stronger πꢀπ stacking, and the electronegative oxygen
atom stabilizes the HOMO energy level36. As a result, improved photovoltaic performance can be expected. Herein, molecular
geometries, crystallinities, chargeꢀtransporting abilities, and photovoltaic properties of DPP(CF)2 and DPP(CT)2 are investigated and
5
10 compared, in order to verify the feasibility of the above molecular design strategy.
Scheme 1
Results and discussion
Synthesis and Thermal Property
DPP(CF)2 was synthesized via traditional Pd(0)ꢀcatalyzed Stille coupling reaction between ethyl 5ꢀbromoꢀfuranꢀ2ꢀcarboxylate and the
15 stannyl derivative of DPP, as shown in Scheme 1. The obtained DPP(CF)2, as well as DPP(CT)2, was soluble in either chloroform (more
than 20 mg/ml), chlorobenzene, or dichlorobenzene, due to the alkyl chain in the end heteroatom substitutions. Thermal properties of
DPP(CF)2 were investigated by thermogravimetric analysis (TGA) (Fig. 1a) and differential scanning calorimetry (DSC) (Fig. 1b). The
DSC heating trace for DPP(CF)2 shows a melting peak at 198 °C, suggesting that this molecule is crystalline. During the cooling process,
a broad exothermic peak with a shoulder is observed from 75 to 130 °C, due to its quite slow crystallization process. From TGA curves,
20 it is found thatthe decomposition temperature (Td) with 5% weight loss of DPP(CF)2 is 398 °C (39 °C higher than that of DPP(CT)2),
indicating a better thermal stability. This thermal stability is good enough for the application in optoelectronic devices37, 38
.
Optical Property
Optical measurements provide essential information on the electronic properties and the ongoing electronꢀtransfer process within
materials39. UVꢀVis absorption spectra of DPP(CF)2 and DPP(CT)2 in dilute chloroform (CF) solutions and corresponding thin films are
25 compared in Fig. 2. Two molecules display similar characteristic absorption peaks due to the similar molecular structures 40, 41. However,
the absorption of DPP(CF)2 redꢀshifts by ~ 12 nm than that of DPP(CT)2 when they exist as monomeric dyes in solutions, implying
stronger conjugation effect between furan rings and DPP core in DPP(CF)2. The redꢀshift of the absorption becomes more obvious in
their solid films. Compared to the solutions, both films exhibit an additional absorption peak located at longer wavelength region, which
originates from strong intermolecular πꢀπ overlapping in the condensed state36. Because of the improved molecular arrangement caused
30 by better intramolecular conjugation and more planar molecular geometry, the absorption peak (representing πꢀπ stacking) of DPP(CF)2
film is more redꢀshifted and intense. As a result, the bandꢀedge (λonset) of DPP(CF)2 film extends to 773 nm, whereas that of DPP(CT)2
is 752 nm The optical band gaps (Egopt) are calculated as 1.60 eV and 1.65 eV for DPP(CF)2 and DPP(CT)2, respectively.
Electrochemical Property
The electrochemical properties of DPP(CT)2 and DPP(CF)2 were investigated by cyclic voltammetry (CV). Fig. 3 shows the cyclic
35 voltammograms of the two molecules in dichloromethane solutions. The CV curve was recorded versus the potential of standard calomel
electrode (SCE), which was calibrated by the ferroceneꢀferroceium (Fc/Fc+) redox couple (4.8 eV below the vacuum level). In the
positive potential region, the E1/2values for the first oxidation/reduction waves are calculated as the oxidation potential. From the
empirical formula EHOMO= ꢀ(Eox +4.4) eV, the HOMO energy level of DPP(CF)2 can be calculated as ꢀ5.33 eV. Compared to DPP(CT)2,
DPP(CF)2 also presents slightly lower LUMO energy level of ꢀ3.58 eV, due to the increased electron affinity of the furan ring. The
40 increased electron affinity of the furan ring can reduce both the HOMO and LUMO levels of DPP(CF)2 (induction effect). Moreover, the
furan ring possesses conjugation effect with DPP core, leading to the increasing of HOMO level and the decreasing of LUMO level of
DPP(CF)2 (conjugation effect). Therefore, the combination of the above two effects gives the identical HOMO level and the lower
LUMO level of DPP(CF)2.
Fig. 1
45
Fig. 2
Fig. 3
Geometry and Front Orbitals Simulated by Theoretical Calculations
To demonstrate the improved πꢀconjugation and molecular planarity of DPP(CF)2 ensured by the endꢀgroups of furanꢀ2ꢀylꢀ
carboxylate, the molecular conformation and front orbitals of DPP(CF)2, in the free states, were simulated by theoretical calculations35, 40
.
50 The simulated front orbitals, energy levels, and dihedral angels between the endꢀgroups and DPP core are presented in Fig. 4 and Table 1.
It is found that the dihedral angle between DPP core and endꢀgroup is merely 0.56o for DPP(CF)2, which is much smaller than that of
DPP(CT)2 (5.27o)33. Due to this negligible steric distortion, the two furan moieties at endꢀgroups can be considered to show complete

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coplanarity and excellent conjugation with DPP core, giving a narrower Eg of 2.05 eV, whereas the Eg of DPP(CT)2 is 2.11 eV.
Moreover, it is disclosed that the replacement of thiopheneꢀ2ꢀcarboxylate with furanꢀ2ꢀcarboxylate induces minor change in the HOMO
energy levels of the two DPP molecules, from ꢀ5.35 to ꢀ5.33 eV, which is in accordance with the data from CV measurements.
Fig. 4
5
Table 1
Charge-Transporting Property
Organic fieldꢀeffect transistors (OFETs) were fabricated to reveal the enhanced chargeꢀtransporting property of DPP(CF)2 due to
molecular planarity. For an average of 15 devices, the transfer curves of the corresponding devices are shown in Fig. 5. The asꢀcast films
show an average hole mobility of 1.1±0.4×10ꢀ4 cm2 Vꢀ1 sꢀ1 for DPP(CT)2 and 2.0±0.3×10ꢀ6 cm2 Vꢀ1 sꢀ1 for DPP(CF)2, respectively.
10 Through thermal annealing for 10 min at 100 °C, the hole mobility of DPP(CF)2 increases significantly by three orders of magnitude to
5.1±2.3×10ꢀ3 cm2 Vꢀ1 sꢀ1. However, the hole mobility of DPP(CT)2 increases moderately to 3.1±1.1×10ꢀ4 cm2 Vꢀ1 sꢀ1 after thermal
annealing. The highest mobilities of the annealed DPP(CF)2 and DPP(CT)2 films are 1.3×10ꢀ2 and 6.3×10ꢀ4 cm2 Vꢀ1 sꢀ1, respectively.
Fig.5
15 Photovoltaic Property
Photovoltaic properties of DPP(CT)2 and DPP(CF)2 were investigated by fabricating OSCs with a conventional structure of
ITO/PEDOT:PSS/DPP molecule:PC71BM/LiF/Al. JꢀV characteristics of the devices under one sun (simulated AM 1.5G irradiation at 100
mW cmꢀ2) are shown in Fig. 6a, with the performance parameters summarized in Table 2. It is found that DPP(CF)2:PC71BM blends with
weight ratios of either 2:1, 1:1, or 1:1.5 induce very poor device performances (PCE<0.5%). The composition ratio of 1.5:1 results a
20 relatively better PCE of 0.52% with a shortꢀcircuit current density (JSC) of 1.47 mA cmꢀ2, fill factor (FF) of 0.37 and a high openꢀcircuit
voltage (VOC) of 0.96 V. This composition ratio is fixed for further device optimization by thermal annealing and incorporating additive.
Thermal annealing leads to a decrease in VOC by 0.07ꢀ0.10 V. However, an increase in FF by 0.05ꢀ0.15 and a remarkable improvement in
JSC by several times are obtained. The devices exhibit the best PCE of 2.86% with a JSC of 6.23 mA cmꢀ2, VOC of 0.89 V, and a FF of 0.52
after annealing at 100 oC for 10 min. Incorporating 0.2% v/v additive of 1,8ꢀdiiodooctane (DIO)23 can only slightly increase the JSC and
o
25 FF, leading to a PCE of 0.86%. Nevertheless, after combined with annealing at 100 C for 10 min, the PCE of the device improves
tremendously to 4.35%, with a JSC of 8.80 mA cmꢀ2, VOC of 0.89 V, and a FF of 0.56. To further improve the photovoltaic performance
of the DPP(CF)2ꢀbased OSCs, a Ca layer is used to replace LiF layer, so that the cathode can collect electrons more efficiently37. Thus,
the PCE is elevated to 5.37%, with a much larger JSC of 11.40 mA cmꢀ2, VOC of 0.89 V, and a FF of 0.53. This efficiency of 5.37%
achieved by DPP(CF)2 is higher than those from most reported DPPꢀbased small molecule donors (4.0ꢀ4.8%)44ꢀ46, and even higher than
30 that from the polymer alternating DPP and furan rings (5.0%)36. Under the same device conditions, the PCE of the DPP(CT)2ꢀbased OSC
is 3.89% due to a smaller JSC of 8.89 mA cmꢀ2.
Table 2
Fig. 6
35
The external quantum efficiency (EQE) curves of the DPP(CF)2ꢀbased OSCs are shown in Fig. 6b. All of the devices show broad
photovoltaic response ranging from 300 to 770 nm. The highest EQE value of the devices without any active layer treatment is only ~
10%. Thermal annealing improves EQE in the whole spectrum, and the maximum value reaches 35% at 680 nm. Incorporating additives
slightly improves EQEs all over the response range, especially in the absorption range of DPP(CF)2 (500ꢀ700 nm). After combining
40 these two treatments, remarkable increase of EQE values covering the whole absorption range is observed, with the maximum EQE value
of 58.9% at 629nm.
To understand the underlying mechanism for the above improvements of the device performances, the effects of thermal annealing and
additive on the morphology of the active layer were investigated. Fig. 7 shows the height and the corresponding phase images obtained
by atomic force microscopy (AFM) for DPP(CF)2:PC71BM blended films with different treatment. It is found that the film without
45 treatment shows large domains in a scale of ~ 100 nm and thermal annealing cannot alter the phase separation degree of the blended
films. After 0.2% v/v DIO is added, the film becomes smoother and large domains disappeared. Further annealing at 100 oC for 10 min,
phase separation with very clear boundaries is observed with the domain size of about 40 nm, and domains are much more
interconnected. These interpenetrating networks of donors and acceptors not only provide interfaces for exciton dissociations but also
percolate pathways for charge carrier transport to corresponding electrodes, responsible for the enhancement of the device performance.
50
Fig. 7
Fig. 8
Fig. 8 presents Xꢀray diffraction (XRD) patterns of the active layers spinꢀcoated on ITO substrates. As shown in Fig. 8a, the asꢀcast
3

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DPP(CF)2:PC71BM film exhibits a broad and weak peak at 2θ= 5.45o, indicating a low degree of crystallinity. This peak becomes much
stronger by incorporating 0.2% v/v additive, indicating that DIO can promote the crystallization of DPP(CF)2. The annealed
DPP(CF)2:PC71BM film shows a shifted sharp peak at 2θ= 7.25o, indicating higher crystallinity with different molecular arrangement
than that in the asꢀcast film41. While the blended film is treated by thermal annealing together with DIO, this sharp peak gets more
pronounced, giving the highest crystallinity and ordered molecular packing. In Fig. 8b, it is shown that, under the conditions for best
device performances, the obtained DPP(CT)2:PC71BM film exhibits a much weaker peak at 2θ= 6.45o, due to lower cystallinity induced
by less molecular coplanarity of DPP(CT)2.
5
We further investigated the effect of thermal annealing and additive on chargeꢀtransporting properties of the active layers by using the
spaceꢀchargeꢀlimited current (SCLC) method. As shown in Fig. 9 and Table 3, films without annealing exhibit very low hole mobilities
10 and relatively high electron mobilities, no matter they are incorporated additive or not. The low hole mobilities and the unbalanced
electron/hole transport are the main reasons for the poor performance of these two types of devices prepared without annealing. Thermal
annealing can significantly enhance the hole mobilities in both types of devices. In addition, the electron mobilities are also improved
slightly, and the more balanced electron/hole transport is obtained. Under the same conditions, both hole and electron mobilities of
DPP(CF)2:PC71BM film are much higher than those of DPP(CT)2:PC71BM. Therefore, based on the above analysis, we conclude: (1) the
15 excellent photovoltaic property of DPP(CF)2 is mainly ascribed to its superior chargeꢀtransporting ability. This is because the fullyꢀ
planar molecular geometry of DPP(CF)2 is in favor of ordered molecular packing in the condensed state, especially after the treatment of
thermal annealing; (2)additive improves photovoltaic performance basically through the reduction of domain size in the active layer.
Table 3
20
Fig. 9
Conclusions
In summary, a DPP molecule endꢀcapped with two furanꢀ2ꢀcarboxylate moieties, DPP(CF)2 is successfully synthesized and
demonstrated as a promising donor for solutionꢀprocessable OSCs. The endꢀgroups of furanꢀ2ꢀcarboxylate endow DPP(CF)2 with a
25 narrow Eg of 1.60 eV, a lowꢀlying HOMO energy level of ꢀ5.33 eV, and typically complete planar molecular geometry, which is in favor
of ordered molecular packing in the condensed state. Incorporating furanꢀ2ꢀcarboxylate can efficiently improve the molecular planarity
and crystallinity, resulting in higher chargeꢀtransporting property and photovoltaic performance. As a result, DPP(CF)2 exhibits excellent
chargeꢀtransporting property with the highest FET hole mobility of 0.013 cm2 Vꢀ1 sꢀ1, and a high PCE of 5.37% after the optimizations of
thermal annealing and additive.
30 Experimental Section
Materials and Instrument
All reagents and solvents were purchased from Aldrich and used without further purification. DPP(CT)2was synthesized according to
our previous published precedures30. 1HꢀNMR and 13CꢀNMR spectra were measured on a Bruker Advance DMX300 (300 MHz) and 600
(600 MHz) nuclear magnetic resonance spectroscope. UVꢀvisible absorption spectra were taken on a Shimadzu UVꢀ2450
35 spectrophotometer. Elemental analyses were conducted on a Flash EA 1112 elemental analyzer. Thermogravimetric analysis (TGA) was
o
carried out on a WCTꢀ2 thermal balance under protection of nitrogen at a heating rate of 20 C/min. Differential scanning calorimetry
(DSC) was recorded on a PerkinꢀElmer Pyris 1 differential scanning calorimeterunder protection of nitrogen at a heating rate of 5oC/min.
Cyclic voltammetry (CV) was done on a CHI 660C electrochemical workstation with Pt disk, Pt plate, and standard calomel electrode
(SCE) as working electrode, counter electrode, and reference electrode, respectively, in
a
0.1 mol/L
40 tetrabutylammoniumhexafluorophosphate (Bu4NPF6) CH2Cl2 solution. Topographic images of the films were obtained on a Veeco Multi
Mode atomic force microscopy (AFM) in the tapping mode using an etched silicon cantilever at a nominal load of ~2 nN, and the
scanning rate for a 5 ꢁm × 5 ꢁm image size was 1.0 Hz. Xꢀray diffraction (XRD) patterns were recorded on a Bede D1 system highꢀ
solution Xꢀray diffractometer.
Theoretical Calculations
45 Geometry optimizations were carried out by the density functional theory (DFT) method at the B3LYP/6ꢀ31G(d) level. Energies of front
orbitals (HOMOs and LUMOs) were evaluated with the basis set of 6ꢀ311+G(d,p). All the calculations were performed using the
Gaussian 03 program. Model compounds of the two DPP derivatives were simplified by replacing pendant alkyl side chains with methyl
groups to save computational time due to the fact that the geometries and energies negligibly depended on the pendant alkyl groups33, 48
.
Synthesis
50 5-bromofuran-2-carboxylic acid (1)
Furanꢀ2ꢀcarboxylic acid (2.50 g, 22.3 mmol) and 50 mL of glacial acetic acid were poured into a 100 mL threeꢀnecked flask. Then, 1.32
mL (22.0 mmol) of bromine was added dropwise to the flask at room temperature. The mixture was warmed and stirred at 60 °C for 1 h.
The mixture was poured into 50 mL of cold water and sodium sulfite was added to remove excess bromine. After that, the mixture was

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filtered, and a white solid was obtained. The crude product was recrystallized from 30 mL of ethanol/water (1:2) to give 3.14 g (yield
78%) of product 1 as white needles. 1H NMR (600 MHz, DMSO) δ 13.31 (s, 1H), 7.22 (d, J = 3.6 Hz, 1H), 6.77 (d, J = 3.6 Hz, 1H).13C
NMR (600 MHz, DMSO) δ167.35, 145.19, 138.93, 127.33, 119.83.
Ethyl 5-bromo-furan-2-carboxylate (2)
5
1.2 g (6.51 mmol) compound 1 was soluble in 30 ml ethanol, and 1.24 ml trimethylchlorosilane was added into the solution under
nitrogen. After being stirred at the ambient temperature for 1 h, the mixture was heated at 65 °C for 24 h. After cooling to room
temperature, the solvent was removed by reduced pressure distillation. The crude product was purified by column chromatography (silica
gel) with dichloromethane and petroleum mixture (1:1, v/v) to yield a light yellow liquid 2(1.48 g, yield 86%).1H NMR (300 MHz,
CDCl3) δ 7.08 (d, J = 3.5 Hz, 1H), 6.42 (d, J = 3.5 Hz, 1H), 4.36 (q, J = 7.1 Hz, 2H), 1.38 (t, J = 7.1 Hz, 3H).13C NMR (300 MHz,
10 CDCl3) δ 157.80, 146.70, 127.46, 120.04, 114.05, 61.39, 14.47.
3,6-Bis[5-(trimethylstannyl)thiophene-2-yl]-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (3)
10 ml dry tetrahydrofuran (THF) was cooled to ꢀ10 ˚C under argon followed by addition of 3.19 ml nꢀbutyllithium (1.6 M in hexane).
Then a solution of 0.72 ml diisopropylamine was added slowly and the reaction mixture was stirred for 30 min. Then, the freshly
prepared solution of lithium diisopropylamine (LDA) in THF was added slowly to a cooled solution (ꢀ78 °C) of 3,6ꢀbis(5ꢀthiopheneꢀ2ꢀ
15 yl)ꢀ2,5ꢀbis(2ꢀethylhexyl)pyrrolo[3,4ꢀc]pyrroleꢀ1,4ꢀdione (1.07 g, 2.04mmol) in dry THF (20 ml). The reaction mixture was stirred for 1 h
at ꢀ78 °C under argon followed by addition of trimethyltin chloride (7.64 ml, 1M in hexane). The reaction mixture was allowed to warm
to room temperature and stirred for 2 h. Water was added to the reaction mixture followed by extraction with ether. The combined
organic phase was washed with water, dried over magnesium sulfate, filtered and concentrated in vacuum. Compound 3 was obtained as
sticky dark purple oil (1.68 g) with a yield of 97.5% and used without any further purification.
20 3,6-Bis{5-[(ethylfuran-2-carboxylate)-2-yl]thiophene-2-yl}-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP(CF)2)
1.42 g (1.32 mmol) compound 3 and compound 2 (1.15 g, 3.31 mmol) were put into a twoꢀneck 100mL round flask and were purged
three times with successive vacuum and nitrogen fill cycles. Then 30 mL of dry degassed toluene was added under the protection of
nitrogen and the solution was flushed with nitrogen before the addition of 70 mg of tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4).
The reaction mixture was further degassed and subsequently sealed before being heated to 110 °C for 48h. Removal of the solvent on a
25 rotary evaporator afforded the crude product, which was purified using column chromatography (silica gel)with dichloromethane and
petroleum mixture (5:1, v/v) to give a dark blue powder(0.48g, yield 54%).1H NMR (600 MHz, CDCl3) δ 8.93 (d, J = 4.2 Hz, 2H), 7.54
(d, J = 4.2 Hz, 2H), 7.24 (d, J = 3.6 Hz, 2H), 6.75 (d, J = 3.6 Hz, 2H), 4.39 (q, J = 7.1 Hz, 4H), 4.12 – 3.97 (m, 4H), 1.88ꢀ1.97 (m, 2H),
1.40 (t, J = 7.1 Hz, 6H), 1.40 – 1.27 (m, 16H), 0.90(t, J = 7.1 Hz, 6H), 0.86 (t, J = 7.1 Hz, 6H).13C NMR (600 MHz, CDCl3) δ 161.41,
158.37, 151.45, 144.47, 139.56, 136.62, 136.28, 129.91, 124.59, 119.89, 117.42, 109.00, 61.19, 46.04, 39.23, 30.28, 28.44, 23.66, 23.05,
30 14.35, 14.03, 10.54.Anal.Calcd for C44H52N2O8S2: C, 65.97; H, 6.54; N, 3.50. Found: C, 65.94; H, 6.56; N, 3.51.
Fabrication and Characterization of Organic Field-effect Transistors (OFETs)
A heavily doped silicon wafer was used as the gate electrode and substrate, with a 300 nm thermal oxidation SiO2 layer as gate
insulator covered with divinyltetramethyldisiloxanebis(benzocyclobute) (BCB). The DPP(CF)2or DPP(CT)2film was casted from
chloroform (CF) solution and annealed for 10 min at 100°C.Organic fieldꢀeffect transistors (OFETs) were constructed in a bottomꢀgated
35 configuration by depositing topꢀcontact source and drain electrodes (70 nm Au), with channel length of 50 ꢁm and width of 1 mm.
Transfer characteristics of the devices were measured in air using a Agilent 4155C semiconductor parameter analyzer. The measured
capacitance of the BCBꢀcovered SiO2/Si substrates was 10 nF/cm2, and this value was used for mobility calculation.
Fabrication and Characterization of Organic Solar Cells (OSCs)
Organic solar cells (OSCs) were fabricated on glass substrates commercially preꢀcoated with a layer of indium tin oxide (ITO). Prior to
40 fabrication, the substrates were cleaned using detergent, deionized water, acetone, isopropanol consecutively for every 15 min, and then
treated in an ultraviolet ozone generator for 15 min before being spinꢀcoated with
a layer of 35 nm poly(3,4ꢀ
ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). After baking the PEDOT: PSS in air at 140 oC for 15 min, the substrates
were transferred to a glovebox. The BHJ layer was spinꢀcast at 2500 rpm from a solution of DPP(CF)2(or DPP(CT)2) and PC71BM in
chloroform with or without 0.2% (v/v) 1,8ꢀdiiodooctane (DIO) at a total solids concentration of 16 mg/mL. The thickness of the active
45 layers is 90ꢀ100 nm, which was calibrated by an Ambios XPꢀ2 profilometer. Then the samples were loaded into a vacuum deposition
chamber (background pressure ≈ 5x10ꢀ4 Pa) to deposit 0.6 nm thick LiF(or 20nm thick calcium) and 100 nm thick aluminum cathode
with a shadow mask (device active area of 9 mm2).
Current density–voltage (J–V) curve was measured with Keithley 2400 measurement source units at room temperature in air. The
photocurrent was measured under a calibrated solar simulator (Abet 300 W) at 100 mW cmꢀ2, and the light intensity was calibrated with a
50 standard silicon photovoltaic reference cell. External quantum efficiency (EQE) spectrum was measured with Stanford lockꢀin amplifier
8300 unit.
The hole mobility of the blends was measured using the spaceꢀchargeꢀlimited current (SCLC) method. Holeꢀonly devices were
fabricated in a structure of ITO/PEDOT:PSS/DPP(CF)2(or DPP(CT)2):PC71BM/MoO3(10 nm)/Al(100 nm), electronꢀonly devices were
fabricated in a structure of Al(120 nm)/DPP(CF)2(or DPP(CT)2):PC71BM/Al(100 nm). The device characteristics were extracted by
55 modeling the dark current under forward bias using the SCLC expression described by the MottꢀGurney law:
5

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9
V 2
L3
J = εrε0ꢀ
8
Here, εr ≈ 3 is the average dielectric constant of the blended film, ε0 is the permittivity of the free space, ꢁ is the carrier mobility, L ≈ 70
nm is the thickness of the film, and V is the applied voltage.
ACKNOWLEDGMENT
5
The authors would like to gratefully acknowledge the financial support from the National Natural Science Foundation of China (Nos.
50990063, 51073135, 51261130582, 91233114) and Zhejiang Province Natural Science Foundation (No. LR13E030001). The work was
also partly supported by National High Technology Research and Development Program of China (863 Program) (No. 2011AA050520)
and 973 program (No. 2014CB643503).
Notes and references
10 a MOE Key Laboratory of Macromolecular Synthesis and Functionalization, State Key Lab of Silicon Materials, & Department of Polymer Science and
Engineering, Zhejiang University, Hangzhou 310027, P. R. China . Fax:+86 571 87953733; Tel: +86 571 87952557; Eꢀmail: minminshi@zju.edu.cn;
hzchen@zju.edu.cn
bInstitute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China.
cDepartment of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, P. R. China.
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45
Table 1 Energy levels and dihedral angels of the two DPP derivatives obtained by CV experiment and theoretical simulations.
CV
CV
opt
DFT
DFT
DFT
EHOMO
ELUMO
Eg
EHOMO
ELUMO
Eg
Molecule
Dihedral angels between the two endꢀgroups and DPP core (o)
(eV)
(eV)
(eV)
(eV)
(eV)
(eV)
DPP(CT)2
DPP(CF)2
ꢀ5.33
ꢀ5.33
ꢀ3.55
ꢀ3.58
1.60a
1.65a
ꢀ5.35
ꢀ5.33
ꢀ3.24
ꢀ3.28
2.11
2.05
5.27; 5.27b
0.56; 0.56
a Egopt= 1240/λonset
b Ref 33.
50
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Table 2 Device parameters of OSCs with or without additive DIO and with or without thermal annealing. The blend weight ratio of donor:acceptor is
1.5:1.
Thermal
annealing
VOC
(V)
JSC
PCE
(%)
Active layer
Solvent
Cathode
FF
(mA cmꢀ2)
DPP(CF)2:PC71BM
DPP(CF)2:PC71BM
DPP(CF)2:PC71BM
DPP(CF)2:PC71BM
DPP(CF)2:PC71BM
DPP(CF)2:PC71BM
DPP(CF)2:PC71BM
DPP(CT)2:PC71BM
CF
CF
CF
CF
LiF/Al
LiF/Al
LiF/Al
LiF/Al
LiF/Al
LiF/Al
Ca/Al
w/o
0.96
0.89
0.89
0.86
0.95
0.89
0.89
0.94
1.47
4.18
6.23
6.00
2.24
8.80
11.4
8.89
0.37
0.42
0.52
0.51
0.40
0.56
0.53
0.46
0.52
1.56
2.86
2.64
0.86
4.35
5.37
3.89
90oC
10 min
100oC
10 min
110oC
10 min
CF/DIO
(0.2% v/v)
CF/DIO
(0.2% v/v)
CF/DIO
(0.2% v/v)
CF/DIO
(0.2% v/v)
w/o
100 oC
10 min
100 oC
10 min
100 oC
10 min
Ca/Al
5
10
Table 3 Summary of hole and electron mobilities of DPP(CF)2:PC71BM and DPP(CT)2:PC71BM films prepared by different procedures.
ꢁh
ꢁe
Active layer
Solvent
Thermal annealing
(×10ꢀ5
(×10ꢀ4
ꢁe/ꢁh
cm2Vꢀ1 sꢀ1)
cm2Vꢀ1 sꢀ1)
DPP(CF)2:PC71BM
DPP(CF)2:PC71BM
DPP(CF)2:PC71BM
DPP(CF)2:PC71BM
DPP(CT)2:PC71BM
CF
w/o
0.80±0.06
4.11±0.27
1.12±0.19
4.65±0.87
0.97±0.15
3.02±1.06
4.93±0.55
4.10±0.58
8.08±1.73
0.43±0.12
38
12
37
17
4.4
CF
100oC 10 min
w/o
CF/DIO (0.2% v/v)
CF/DIO (0.2% v/v)
CF/DIO (0.2% v/v)
100 oC 10 min
100 oC 10 min
15
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i
ii
HOOC
Br
Br
COOH
COOC2H5
Sn(CH3)3
O
O
O
1
2
R
R
N
S
N
S
iii
O
S
O
S
O
O
N
N
(H3C)3Sn
R
R
3
C2H5OOC
O
R
S
N
O
iv
O
2 + 3
N
S
R
O
COOC2H5
R= 2-ethylhexyl
DPP(CF)2
(i) Br2, acetic acid, reflux 24 h; (ii) ClSi(CH3)3/C2H5OH, 60 oC for 12 h;
(iii) LDA/ClSn(CH3)3, THF -78 oC; (iv) Pd(PPh3)4, toluene, reflux 24 h.
5
Scheme 1Synthesis route of DPP(CF)2
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Fig. 1(a) TGA traces of DPP(CF)2 and DPP(CT)2. The heating rate is 20 oC/min. (b) DSC curve of DPP(CF)2 at the heating rate of 5 oC/min.
10
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Fig. 2 Normalized UVꢀvis absorption spectra of DPP(CF)2 and DPP(CT)2 in dilute chloroform solutions and thin films.
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Fig. 3 CV traces of DPP(CT)2 and DPP(CF)2.
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10 Fig. 4 Front orbitals of DPP(CF)2 obtained by theoretical calculation: (a) HOMO, top view; (b) HOMO, side view; (c) LUMO, top view; and (d) LUMO,
side view.
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Fig.5 Transfer characteristics of the OFETs based on (a) DPP(CF)2 and (b) DPP(CT)2 with the highest mobilities.
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Fig. 6 Current densityꢀvoltage (JꢀV) characteristics (a) and external quantum efficiency (EQE) curves (b) of OSCs with DPP(CF)2:PC71BM (1.5:1, by wt.)
blend as the active layer with or without additive and with or without thermal annealing.
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Fig. 7 AFM height images (a, b, c, d) and phase images (e, f, g, h) of DPP(CF)2:PC71BM blended films obtained by different procedures.
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Fig. 8 XRD patterns of (a) DPP(CF)2:PC71BM blended films obtained by different procedures and (b) DPP molecule:PC71BM blended films prepared
under the optimized conditions (thermal annealing at 100 oC for 10 min and 0.2 % v/v DIO).
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Fig. 9 J0.5ꢀV curves of (a) holeꢀonly devices (ITO/PEDOT:PSS/DPP(CF)2:PC71BM/MoO3/Al) and (b) electronꢀonly devices (Al/DPP(CF)2:PC71BM/Al).
The symbols represent experimental data and the solid lines are fitted according to the MottꢀGurney Law.
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Table of Content
A diketopyrrolopyrrole molecule end-capped with furan-2-carboxylate
moiety: planarity of molecular geometry and photovoltaic property
5
Lei Fu,a Weifei Fu,a Pei Cheng,b Zhixin Xie,a Congcheng Fan,a Minmin Shi,*a Jun Ling,a Jianhui Hou,b
Xiao-wei Zhanc and Hongzheng Chen*a
DPP(CF)2 exhibits fully-planar molecular geometry, which comes with excellent hole mobility of 0.013 cm2 V-1 s-1 and a high power
conversion efficiency of 5.37%.
10