Experimental Investigation and Theoretical Calculation of Molecular

Article pubs.acs.org/JPCC

Experimental Investigation and Theoretical Calculation of Molecular Architectures on Carbazole for Photovoltaics Feiyan Wu,† Lie Chen,†,‡ Hongming Wang,† and Yiwang Chen*,†,‡ †

Institute of Polymers/Department of Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China Jiangxi Provincial Key Laboratory of New Energy Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China

‡

S Supporting Information *

ABSTRACT: A new conjugated polymer containing 6H-phenanthro[1,10,9,8cdefg]carbazole (PC) and 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT) units, so-called PPCDTBT, is synthesized based on the further modification of carbazole moieties for poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT). The resulting polymer exhibits a narrow band gap with 1.77 eV, resulting from the broader conjugation, while maintaining a low-lying HOMO energy level. The polymer geometry is severely transformed by the large fused block phenanthrocarbazole (PC). Through the density functional theory and time-dependent density functional theory calculations at the B3LYP/6-31G(d,p) level on the polymer dimer models, a big torsion angle is the main reason for breaking the backbone coplanarity and, consequently, the conjugation and organization. Moreover, a different transition from the HOMO-2 orbital is responsible for the absorption shoulder at a short wavelength. After ordinary optimization, the best power conversion efficiency of 2.3% is achieved with a preferable Voc of 0.80 V and Jsc of 7.9 mA/cm2. Additionally, for holding extended conjugation from the fused carbazolelike unit and suppressing the strong torsion, naphthocarbazole (NC) and the counterpart alternative polymer of NC and DTBT (PNCDTBT) are proposed and simulated, which would be more planar for better intra- and intermolecular interactions.

■

INTRODUCTION Over the past two decades, the vogue of bulk heterojunction polymer solar cells (PSCs) as renewable energy sources was originated from their unique compatibility with cost-effective large area fabrication and mechanical flexibility.1−3 Moreover, the power conversion efficiency (PCE) has increased quickly through a three-stage development, from less than 1% on poly(phenylenevinylene) (PPV)/PC61BM in 1995,4 to 4−5% by poly(3-hexylthiophene) (P3HT)/PC61BM in 2005,5,6 and over 9% very recently.7,8 Although the optimization of device structure9,10 or other morphology control11,12 advanced the development of PCEs, the essential driving force for the breakthrough consisted of the innovation of the active layer, which was composed of solution-processable electron-donating conjugated polymers and electron-accepting fullerene derivatives. In particularly, the band gap, energy levels (both HOMO and LUMO), and geometry of polymer materials were crucial factors to determine the PCEs in terms of short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF). Up to now, it was still an urgent task for polymers to pursue a 15% performance, which was regarded as being a prerequisite for large-scale commercialization.13 Fortunately, the synthetic versatility of organic chemistry allowed the polymer engineering to become possible: the electronic and optical properties of π-conjugated polymers relating to cell efficiency could be modulated by tailoring the molecular structure. Currently, the donor−acceptor (D-A) alternating conjugated polymers were the most effective and prevalent materials for this application. It © 2013 American Chemical Society

has been demonstrated to be powerful in both band-gap engineering and tuning of the energy levels by choosing apt donor and acceptor units as the result of intramolecular charge transfer (ICT).14 Many donor units, such as carbazole, fluorene, dithienosilole (DTS),15 and benzodithiophene (BDT),16 while other acceptor units, such as dithienbenzothiadiazole (DTBT)17 and diketopyrrolopyrrole (DPP),18 have impelled exciting advances in the synthesis of novel low-band-gap polymers, and PCEs over 7% have been widely reported from them.19−22 Among the numerous building blocks, carbazole is one of the most well-known blocks for its fully aromatic fusing ring, serving as an electron-rich structure and providing a better chemical and environmental stability.23 Different acceptor units have been copolymerized with 2,7-carbazole toward a rational design of low-band-gap materials, which was systematically investigated by Mario Leclerc.24 It has been found that the polymer poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) had the best potential for photovoltaics, and subsequently became the pioneered and the most important low-band-gap polymer to achieve a high PCE of 6.1% in 2009.25 Since then, the another structure, 3,6-carbazole, has also been introduced into alternating polymers or random copolymers for photovoltaics Received: February 12, 2013 Revised: April 18, 2013 Published: April 22, 2013 9581

dx.doi.org/10.1021/jp401552f | J. Phys. Chem. C 2013, 117, 9581−9589

The Journal of Physical Chemistry C

Article

in the literature,26,27 but their performances were incomparable with 2,7-carbazole derivatives. Additionally, some other modifications on carbazole have been accomplished by connecting aromatic groups on the “N” atom to increase conjugation.28 Furthermore, the atom “N” has been replaced with “S” in dibenzothiophene, which copolymerized with DTBT only achieved PCEs of 1.71%.29 It even has further been designed by replacing “N” with the “Si” atom.30 However, these attempts were not successful enough to surpass the representative PCDTBT till now. Considering that the band gap of polymer PCDTBT was about 1.9 eV through intramolecular charge transfer (ICT) from the electron-rich unit to the electron-deficient moiety, it was relatively large to matching the solar flux well. There was still space to rebuild the structure for better light harvesting and adjust the architecture for better hole mobility. At present, some unconventional modification on carbazole and its corresponding polymers has been carried out in our group by imposing an acceptor unit or donor unit on the carbazole aromatic ring. First, a new architecture, N-alkyl-carbazole[3,4c:5,6-c]bis[1,2,5]thiadiazole (CBT), was synthesized through fixing two electron-deficient units 1,2,5-thiadiazole on carbazole in 3-, 4- and 5-, 6- positions symmetrically.31,32 The LUMO energy level dropped drastically from −0.62 eV in carbazole to −2.12 eV in CBT, altering the unit to an apparent acceptor block. The resulting polymer PCBTT using thiophene as a π bridge showed a narrow band gap of 1.65 eV, but the PCE only received ∼0.5% due to the low carrier mobility coming from the too tight organization of the polymer chain. Herein, another donor unit perylene is integrated with carbazole to create a new building block, 6H-phenanthro[1,10,9,8-cdefg]carbazole (PC), subsequently coupling with the acceptor unit DTBT to form a new PCDTBT derivative polymer, PPCDTBT. Perylene is one of the most typical large fused rings and is notable for its perfect planar structure as well as strong electron-donating ability. Usually, a π-extended fused ring in the polymer backbone will be very helpful for enhancing the intermolecular π overlap and obtaining the close π−π stacking, which can contribute to bandgap lowering and carrier mobility improving.33 It has been widely employed in preparing molecular dyes,34 perylene diimide for a polymer acceptor in all-polymer PSCs,35 and few in D−A alternating polymers for donor materials.36 Therefore, a new structure PC is proposed with a combination of perylene and carbazole to offer a large coplanar fused ring, with expected better energy levels and carrier mobility simultaneously. The mentioned three types of monomer structures are depicted in Scheme 1. The polymer PPCDTBT

is synthesized for comparison with analogue polymers PCDTBT and PCBTT, all of which are almost based on the assemblance of thiadiazol and benzene rings by organic chemistry. Furthermore, on the basis of the experiments, we also discuss how modification of the carbazole structure influences the optical and electronic properties through theoretical calculation.

■

EXPERIMENT AND CALCULATION DETAILS Synthesis of Polymer PPCDTBT via Stille Coupling Polymerization. The polymer PPCDTBT was synthesized via Still cross-coupling polymerization using Pd(PPh3)4 catalyst, which was end-capped with bromobenzene and phenylboronic acid, as shown in Scheme 2. Monomer PC (331 mg, 0.5 mmol) Scheme 2. Synthesis of Polymer PPCDTBT

and monomer distannylated DTBT (313 mg, 0.5 mmol) were added in 13 mL of dry toluene in a flask under argon. The reaction mixture was purged with argon for 15 min. The catalyst Pd(PPh3)4 (19.3 mg, 3%) was added quickly under a stream of argon, and then the reaction mixture was purged with argon again for 15 min. Subsequently, the reaction mixture was heated to reflux for 48 h with stirring. Phenylboronic acid (0.6 mg, 0.5 mmol) was then added and stirred at reflux for 3 h. Bromobenzene (5 mL) was then added under nitrogen, and the mixture was stirred for another 3 h. Next, the reaction mixture was cooled to ambient temperature and dropped into methanol. The polymer was precipitated and then collected by filtration. The crude polymer was then extracted subsequently with methanol, ethyl acetate, hexane, and CHCl3 in a Soxhlet’s extractor. The residue after extracting with CHCl3 was collected and dried under reduced pressure to give the polymer PPCDTBT (330 mg, 80%) as a dark brown solid. Calculation. Geometry optimization of each unit and of each copolymer (dimer models) in the ground state was carried out with DFT/B3LYP/6-31G(d,p). All optimized geometries were confirmed to be the minimum-energy structures using a normal-mode analysis. The singlet ground states were calculated with the spin-restricted DFT to estimate the ionization potentials and the electron affinities. At the final geometries, the vertical singlet−singlet electronic transition energies were calculated at the same levels of theory B3LYP/631G(d,p) using TD-DFT implemented in the Gaussian 09 program.37 All of these calculations were carried out in the gas phase to neglect the solvent effect.

Scheme 1. Structure of Monomer Carbazole, PC, CBT, and Related Polymers

9582



■

Article

RESULTS AND DISCUSSION Detailed information of synthetic products is described in the Supporting Information (see Figures S1−S3). The counterpart polymer PCDTBT was obtained from Solarmar Materials Inc. with a molecular weight of 22 kg/mol, and the PCBTT was prepared according to our previous work with a molecular weight of 14 kg/mol.32 The bridging nitrogen atom in all molecules offers a center for lateral chain functionality to ensure the solubility of the polymers, which is critical for solution processing in fabrication. Branched C17-alkyl groups can be readily attached on carbazole and PC, while the ndodecyl group is introduced to CBT more difficultly. Because of the differences in backbone structures and side chains, the solubilities of PPCDTBT and PCDTBT are much better than that of PCBTT. The molecular weight of the present polymer PPCDTBT was determined by GPC in chloroform using polystyrene as standard, and the measured number-average molecular weight (Mn) is 19.5 kg/mol with polydispersity indices (PDIs) of 1.82. The thermal property was determined by TGA, and the corresponding data are shown in Table 1. Seen from the curve

monomer were modeled using density functional theory (DFT) in the Gaussian 09 software package at the B3LYP/631G(d,p) level of theory,38 which have been routinely used in organic molecular calculation widely.39,40 The optimized geometry and electron density distributions in the HOMO and LUMO, together with the associated energy levels for the different building blocks, are shown in Figure 2. Seen from the side view, all the blocks keep a perfect coplanar structure. It appears that the electron delocalized along the whole rings well on both the HOMO and LUMO of carbazole and PC, but the delocalized scope in the PC ring is much larger for the bigger fused ring. Differently, the LUMO of CBT is mainly localized on thiadiazole, distinguished from the HOMO level. The variation is more impressive on the energy levels; the LUMO energy level of CBT is −2.12 eV, showing a perceptible electron-withdrawing ability. The HOMO energy level of PC is elevated about 0.46 eV compared to that of carbazole, resulting from the electron delocalization in the large fused ring, and the energy gap (HOMO−LUMO) of PC is thus decreased as well. These results suggest that the structure modification on the carbazole ring alters the orbital electron distribution severely, consequently leading to the variation of the energy levels. Afterward, molecular simulation of polymers was performed on their dimers (n = 2) for representation in Figure 3. To simplify the calculations, the long appending chain anchoring on the nitrogen atom in each polymer was replaced by a methyl group. The HOMO levels of all polymers are well-delocalized along the conjugated backbones. On the other hand, the LUMO is largely localized on the BT segment, in both PCDTBT and PPCDTBT, but that is distributed on the whole chain in PCBTT, which further demonstrates the donor− acceptor alternating character of PCDTBT and PPCDTBT and the donor−acceptor integration feature in PCBTT with the thiophene as the π bridge.32 The HOMO energy level of PPCDTBT is the highest among the polymers owing to the strongest donor PC, whereas the LUMO levels of the three polymers are quite similar, which were determined by acceptor units. Consequently, the calculated band gap of the dimers shows the following trend: PPCDTBT (2.05 eV) < PCDTBT (2.24 eV) < PCBTT (2.40 eV). The energy-minimized dihedral angle between the planes of donor and acceptor segments in each repeating unit is one of the important characters to affect coplanarity and, therefore, the conjugation of the polymer backbone. The dihedral angle between the two thienyl groups with a central benzothiadiazole (BT) is about 6°, and we thus focused on the dihedral angle between the thienyl group and the carbazole-like unit to investigate the effects of twisting hindrance. The dihedral angle of PCDTBT is found to be 24−27°, in agreement with the value reported in the literature,41−43 which is small and indicative of conjugation through π orbitals. Surprisingly, through incorporating benzene rings on carbazole, the dihedral angle dramatically increases to ∼45° in PPCDTBT due to the large rigid structure of PC. However, the dihedral angle notably decreases to nearby 12° in PCBTT, revealing the good coplanarity in the backbone. This pronounced difference in dihedral angle will affect the molecular geometry and relevant properties. The optical properties of the polymers were investigated by UV−vis absorption analysis in both solution and as-cast thin films. As shown in Figure 4, steady-state absorption spectra of the polymers span most of the visible spectrum from 300 to 800 nm, showing two characteristic absorption bands: a

Table 1. Molecular Weight and Thermal Properties of Polymers polymer

Mna (kg/mol)

PDIa

Tdb (°C)

PPCDTBT

19.5

1.82

353

Determined by GPC in chloroform at 30 °C using polystyrene as standard. bThe decomposition temperature corresponding to a 5% weight loss determined by TGA at a heating rate of 10 °C min−1. a

Figure 1. TGA plots of PPCDTBT with a heating rate of 10 °C·min−1 under an inert atmosphere.

of thermal analysis in Figure 1, the decomposition temperature corresponding to 5% weight loss (Td) is 353 °C, displaying a high thermal stability as a result of the rigid fused ring structure. Differential scanning caloriometry (DSC) measurement on the copolymer reveals neither endo- nor exothermic processes in the range of 40−200 °C (Figure S4, Supporting Information). In comparison with the original carbazole unit, PC has a much larger π-conjugation system with a stronger electrondonating ability, whereas CBT possesses a penta-fused ring with electron-acceptor ability. A computational study of the three blocks was performed to deepen our understanding of the differences in electronic properties accompanying with the structure modification. The electronic structures of each 9583



Article

Figure 2. Electronic sturcture and energy levels of monomer carbazole, CBT, and PC, which were simulated by DFT/B3LYP/6-31G(d,p).

PPCDTBT to ∼354 nm in PCBTT and ∼397 nm in PCDTBT. Contrarily, all the three λmax of low-energy bands are mainly located in the range of 560−600 nm. Additionally, there is a distinct peak shoulder within PPCDTBT emerging at ∼472 nm. The absorption onset (λmax) of thin films are all red shifted with respect to the corresponding spectra in solutions, which can be ascribed to solid-state packing effects,44 where the biggest Δλonset of PPCDTBT achieved 55 nm, which came from the depression of big torsion angles in the film. On the other hand, the good coplanarity of PCBTT endows tight intermolecular interaction, confirmed by the prominent absorption shoulder of π−π stacking. In addition to the broadening of the spectra, the band gaps are also narrowed by tailoring structures of the polymers. The absorption maxima (λmax), the onset and optical band gaps for all the polymers are listed in Table 2. Time-dependent density functional theory (TD-DFT) at the B3LYP/6-31G(d,p) level has been used to gain insight into the vertical singlet electronic transitions. Figure 5 shows the absorption spectra of the three copolymers from the calculation with dimer models, emphasizing the transition energies (peak positions) and oscillator strengths (peak heights). The first two strong singlet−singlet electronic transitions are marked as a vertical bar, labeled “ex” in the figure. Detailed information, such as the transition energy, the oscillator strength ( f, the transition probability), and the major molecular orbital (MO), involved in the transition are given in Table 3. The strong transitions ( f > 0.5) of all copolymers in the UV/visible range correspond to the transitions from HOMO to LUMO or to a higher orbital. Exceptionally, the ex 6 of PPCDTBT is transited from HO-2 to LU+1, consistent with the absorption shoulder at 472 nm in the experimental spectra. Seen from the HO-2 of PPCDTBT in Figure S5 (Supporting Information), distinctly different from the HOMO orbital, the electron delocalization is distributed nonuniformly, which is mostly localized on the DTBT segment while little on the PC part. The lowest-energy (HOMO-to-LUMO) transition is associated with the intramolecular charge transfer from the donor unit to the BT unit, which can also be seen from the Figure 5. This transition occurs after 580 nm with high oscillator strengths (f > 1.2), arising in the sequence, 588 nm (PCBTT) < 641 nm (PCDTBT) < 707 nm (PPCDTBT), indicating the weakest ICT in the PCBTT and the strongest one in

Figure 3. Optimized geometry, HOMO/LUMO wave functions, and energy levels of PPCDTBT, PCDTBT, and PCBTT simulated by DFT/B3LYP/6-31G(d,p), which were carried out with a chain length of n = 2; the dihedral angle was between the neighboring blocks in the structure.

Figure 4. Absorption spectra of PPCDTBT, PCDTBT, and PCBTT in CHCl3 solution and as thin film spin-cast from chlorobenzene solution.

common high-energy band near 450 nm and a low-energy band between 450 and 800 nm associated with the intramolecular charger transfer (ICT). All the absorption spectra were normalized based on the low-energy band in order to compare the ICT conveniently. Seen from the spectra, the λmax of highenergy bands are widely distributed: from ∼300 nm in 9584



Article

Table 2. Summary of Optical Data for the Polymers PPCDTBT, PCDTBT, and PCBTT thin films

solution polymer

λmax [nm]

λonseta [nm]

λmax [nm]

λonseta [nm]

Δλonsetb [nm]

Egoptc [eV]

PPCDTBT PCDTBT PCBTT

301, 538 397, 562 354, 547

647 652 728

304, 566 397, 579 359, 602

702 671 753

55 19 25

1.77 1.85 1.65

λonset calculated from the intersection of the tangent drawn to the lowest-energy absorption edge with the baseline. bΔλonset = λonset (film) − λonset (solution). cEgopt = 1240/λonset (film). a

occupied molecular orbital to a different unoccupied molecular orbital. The TDDFT (B3LYP) calculation with the dimer models reproduces well the mostly experimental absorption spectra of the copolymers. However, the low-frequency absorption peaks calculated for PPCDTBT are off from the measured ones by >100 nm, most likely coming from the limited size of the dimer model to represent the hybridization in the HOMO/LUMO and the large torsion angle exhibited in PPCDTBT. In detail, the coplanarity of PCBTT makes the low-energy absorption peak red shifted from 588 nm by calculation to 602 nm in measured, whereas the large torsion angle in PPCDTBT makes that peak blue shifted from 707 nm by calculation to 566 nm in experiment. The electrochemical property of the PPCDTBT was investigated using cyclic voltammetry (CV), and the resulting voltammogram is shown in Figure S6 (Supporting Information). On the basis of the oxidation onset, the estimated HOMO energy is −5.38 eV. However, no clear reductive peak is observed; thus, the LUMO energy is derived by subtracting the optical band gap from the electrochemical oxidation data. The HOMO and LUMO energy levels are calculated as detailed in the Supporting Information, and the related data are summarized in Figure 6. Evidently, the LUMO levels of all copolymers locate at the similar values in both theoretical and experimental, because it is only determined by the BT unit regardless of its specific position in the polymer. In an agreement with the former discussion, the HOMO of PPCDTBT shows a little bit of elevation compared to that of PCDTBT, but is still lower than that of PCBTT in practical. The solid-state morphology of PPCDTBT was observed by X-ray diffraction analysis on drop-film to investigate the molecular organization, shown in Figure 7. The related data of them are shown in Table 4. Distinguished from the other two polymers, no conspicuous reflection peak appeared at 2θ = 4−5°, which often corresponds to a lamellar ordering (d1 in Table 4).45 The absence of a reflection peak at low angle manifests the poor crystallinity of the polymer PPCDTBT, which may be caused by the serious torsion of the polymer backbone. However, a weak diffraction peak exists at 2θ = 23.3°, showing a d-spacing of 0.38 nm (d2 in Table 4), between the values in PCBTT and PCDTBT. The peak reflects a π−π stacking originating from the large fused ring of PC. Moreover, there is no effect created on the curve under annealing. The photovoltaic performance of the present polymer PPCDTBT was explored by fabricating bulk heterojunction solar cells with a general device structure of ITO/ PEDOT:PSS/polymer:PC61BM/LiF/Al. Ordinary optimization processes, such as processing solvent, weight ratios of copolymer and PC61BM, spinning rates (film thickness), and annealing, were investigated. Figure 8 shows the optimal current−voltage curve (I−V curve) of the device under illumination (AM 1.5G 100 mW/cm2). A best PCE of 2.3%

Figure 5. Absorption spectra of PPCDTBT, PCDTBT, and PCBTT based on the TDDFT/B3LYP/6-31G(d,p) calculations of the transition energy and oscillator strength.

PPCDTBT. The next-lowest-energy transitions occur with the same order, ∼353 nm (PCBTT), ∼420 (PCDTBT) nm, and 510 nm (PPCDTBT), which transited from a different 9585



Article

Table 3. Electronic Transitions of the Copolymers (Dimer Model) polymer PPCDTBT PCDTBT PCBTT a

ex ex ex ex ex ex

1 6 1 11 1 12

transition energy (nm)a

osciliator strength f a

major MO involved in the transition

experimental UV−vis λmax (nm)b

707 510 641 420 588 353

1.23 0.56 1.58 1.66 1.44 0.67

HO → LU (61%) HO-2 → LU+1 (54%) HO → LU (61%) HO → LU+1 (65%) HO → LU (70%) HO → LU+4 (65%)

566 472 579 397 602 359

Seen from Figure 5. bFrom the absorption of corresponding copolymers as thin film.

Figure 8. Current−voltage curve of device structure of ITO/ PEDOT:PSS/PPCDTBT:PC61BM/LiF/Al under 100 mW/cm2 1.5 AM sunlight illumination.

Figure 6. Experimental and theoretical energy levels for the three polymers: PCDTBT, PCBTT, and PPCDTBT.

Table 5. Summary of Optimized Photovoltaic Properties of the Three Polymers

Table 4. d-Spacing Data from XRD of the Three Polymers polymers

d1 (nm)

d2 (nm)

2.09 1.73

0.38 0.44 0.32

Voc (V)

Jsc (mA/cm2)

FF

PCE (%)

PPCDTBT PCDTBT24 PCBTT32

0.80 0.86 0.67

7.9 6.8 2.7

0.37 0.56 0.27

2.3 3.6 0.48

The low FF, which is the main reason to discourage the overall PCE, is correlated with the unfavorable molecular conformation as well as the poor crystallinity. It is clear that the new materials PPCDTBT and PCBTT, based on two new carbazole-like blocks, successfully modulate the polymer’s energy levels, band gap, and other properties. The best absorption is obtained in PCBTT; however, the high HOMO energy level and disadvantageous molecular stacking frustrate the performance. On the other hand, the lower HOMO energy level and excellent conjugated absorption prompts the PCE of PPCDTBT to achieve 2.3%, although the too large twist angle damages the film morphology with low FF. Therefore, for the purpose of lowering the torsion angle and maintaining the benefits of the fused ring, the carbazole is further modified by just adding one or two conjugated benzene rings on the carbazole ring (namely, benzocarbazole (BC) and naphthocarbazole (NC), seen Figure 9) to decrease the block’s bulk and rigidity. Meanwhile, the “N” atom on carbazole is still maintained for attaching hydrocarbon chains. The MO energy diagrams of the newly designed blocks and the related dimer models of counterpart copolymers PBCDTBT and PNCDTBT are simulated from the same type of calculations as those carried out in the previous section. From the calculation, BC and NC show the HOMO at −5.16 and −5.23 eV, respectively, which are much lower than that of PC and slightly higher than that of carbazole. Distinctly, the

Figure 7. XRD diffraction pattern of the PPCDTBT film.

PPCDTBT PCDTBT24 PCBTT32

polymer

with a Jsc of 7.9 mA/cm2, a Voc of 0.80 V, and a low FF of 0.37 is achieved based on a 1:2 polymer/PC61BM weight ratio without thermal annealing (shown in Table 5). Compared to that of the counterpart polymer PCDTBT, a slightly low Voc is brought about by the slight elevation in HOMO energy level of PPCDTBT. The relatively high Jsc value can be understood from the narrower band gap than PCDTBT, and the higher mobility accompanying with a large fused ring of PC, which has been demonstrated in many perylene-containing polymers.33 9586



Article

organization is also affected by the introduction of a large fused ring PC, showing effective intermolecular π−π stacking in the absence of good crystallinity. Through investigation by the DFT and TDDFT calculations at the B3LYP/6-31G(d,p) level on the polymer dimer models, a big torsion angle is the main origin to destroy the backbone coplanarity, consequently influencing the chain conjugation and polymer organization. Meanwhile, a different transition from the HOMO-2 orbital is responsible for the absorption shoulder at a short wavelength. With an optimized blend ratio of PPCDTBT (1:2, w/w) with PC61BM, a moderated PCE of 2.3% is achieved with a preferable Voc of 0.80 V and Jsc of 7.9 mA/cm2, which were mainly controlled by the relatively low FF induced by the chain conformation. On the basis of these results, another two new blocks, BC and NC, are proposed and simulated to decrease the strong torsion. According to the calculation, the analogous polymer PNCDTBT is confirmed to be a more planar structure for favorable intramolecular charge transfer and intermolecular stacking.

Figure 9. Optimized geometry, HOMO/LUMO wave functions, and energy levels of the newly designed blocks and comparing to present units simulated by DFT/B3LYP/6-31G(d,p).

■

ASSOCIATED CONTENT

S Supporting Information *

two blocks are weaker donor units than PC, which is appreciated in constructing ideal D−A copolymers for photovoltaics.46 After coupling with DTBT, the torsion angle between the two segments in both polymers is reduced from 45° in PPCDTBT to ∼29° (Figure 10), very close to that of PCDTBT. Simultaneously, the HOMO and LUMO of their dimers keep the similar values with PCDTBT. It is inferred that the new blocks realize the depression of the big torsion as expected and maintain the band gap at the same time. Providing with almost the same electronic properties and geometry, the NC-containing polymer is preferential than the BC-containing one because its larger fused ring is better for carrier mobility. From this speculation, the newly designed copolymer, PNCDTBT, is expected to possess not only a suitable electronic structure (narrow band gap and low HOMO level) but also a better structural organization through a favorable interchain stacking, owing to its planar structure and large fused ring. We thus propose the PNCDTBT as another promising candidate from carbazole derivative to be used as polymer donor in the PSCs. The synthesis of this polymer is ongoing now, and its characteristics will be studied in future work.

Instruments and measurements, materials, 1H NMR spectrum of the monomer and polymers, DSC plot of PPCDTBT, electrochemical studies, map of the HOMO-2 orbital of PPCDTBT, and device fabrication. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel: +86 791 83969562. Fax: +86 791 83969561. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51273088, 51263016, and 51003045).

■

REFERENCES

(1) Thompson, B. C.; Fréchet, J. M. J. Polymer−Fullerene Composite Solar Cells. Angew. Chem., Int. Ed. 2008, 47, 58−77. (2) Chen, L.-M.; Hong, Z.; Li, G.; Yang, Y. Recent Progress in Polymer Solar Cells: Manipulation of Polymer:Fullerene Morphology and the Formation of Efficient Inverted Polymer Solar Cells. Adv. Mater. 2009, 21, 1434−1449. (3) Heeger, A. J. Semiconducting Polymers: The Third Generation. Chem. Soc. Rev. 2010, 39, 2354−2371. (4) Yu, G.; Gao, J.; Hummelen, J.; Wudl, F.; Heeger, A. J. Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions. Science 1995, 270, 1789−1791.

■

CONCLUSIONS A new conjugated polymer PPCDTBT was prepared by modification on carbazole to develop carbazole-like copolymers for photovoltaics. Through the optical and electronic analysis, the resulting polymer exhibits a narrower band gap with 1.77 eV from the larger conjugation, while maintaining the low-lying HOMO level with −5.38 eV. At the same time, the polymer

Figure 10. Optimized geometry, HOMO/LUMO wave functions, and energy levels of the two newly designed polymers simulated by DFT/B3LYP/ 6-31G(d,p), which were carried out with a chain length of n = 2; the dihedral angle was between the neighboring blocks in the structure. 9587



Article

(5) Li, G.; Shrotriya, V.; Huang, J. S.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. High-Efficiency Solution Processable Polymer Photovoltaic Cells by Self-Organization of Polymer Blends. Nat. Mater. 2005, 4, 864−868. (6) Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K. H.; Heeger, A. J. Thermally Stable, Efficient Polymer Solar Cells with Nanoscale Control of the Interpenetrating Network Morphology. Adv. Funct. Mater. 2005, 15, 1617−1622. (7) He, Z.; Zhong, C.; Huang, X.; Wong, W.; Wu, H.; Chen, L.; Su, S.; Cao, Y. Simultaneous Enhancement of Open-Circuit Voltage, Short-Circuit Current Density, and Fill Factor in Polymer Solar Cells. Adv. Mater. 2011, 23, 4636−4643. (8) He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Enhanced Power-Conversion Efficiency in Polymer Solar Cells Using an Inverted Device Structure. Nat. Photonics 2012, 6, 591−595. (9) Lin, H.-W.; Chiu, S.-W.; Lin, L.-Y.; Hung, Z.-Y.; Chen, Y.-H.; Lin, F.; Wong, K.-T. Device Engineering for Highly Efficient TopIlluminated Organic Solar Cells with Microcavity Structures. Adv. Mater. 2012, 24, 2269−2272. (10) Li, X.; Choy, W. C. H.; Huo, L.; Xie, F.; Sha, W. E. I.; Ding, B.; Guo, X.; Li, Y.; Hou, J.; You, J.; et al. Dual Plasmonic Nanostructures for High Performance Inverted Organic Solar Cells. Adv. Mater. 2012, 24, 3046−3052. (11) Etzold, F.; Howard, I. A.; Forler, N.; Cho, D. M.; Meister, M.; Mangold, H.; Shu, J.; Hansen, M. R.; Müllen, K.; Laquai, F. The Effect of Solvent Additives on Morphology and Excited-State Dynamics in PCPDTBT:PCBM Photovoltaic Blends. J. Am. Chem. Soc. 2012, 134, 10569−10583. (12) Verploegen, E.; Miller, C. E.; Schmidt, K.; Bao, Z.; Toney, M. F. Manipulating the Morphology of P3HT−PCBM Bulk Heterojunction Blends with Solvent Vapor Annealing. Chem. Mater. 2012, 24, 3923− 3931. (13) Wang, H.; Cheng, P.; Liu, Y.; Chen, J.; Zhan, X.; Hu, W.; Shuai, Z.; Li, Y.; Zhu, D. A Conjugated Polymer Based on 5,5′Bibenzo[c][1,2,5]thiadiazole for High-Performance Solar Cells. J. Mater. Chem. 2012, 22, 3432−3439. (14) Havinga, E.; ten Hoeve, W.; Wynberg, H. Alternate DonorAcceptor Small-Band-Gap Semiconducting Polymers; Polysquaraines and Polycroconaines. Synth. Met. 1993, 55, 299−306. (15) Guo, X.; Zhou, N.; Lou, S. J.; Hennek, J. W.; Ponce Ortiz, R.; Butler, M. R.; Boudreault, P. L.; Strzalka, J.; Morin, P. O.; Leclerc, M.; et al. Bithiopheneimide−Dithienosilole/Dithienogermole Copolymers for Efficient Solar Cells: Information from Structure−Property− Device Performance Correlations and Comparison to Thieno[3,4c]pyrrole-4,6-dione Analogues. J. Am. Chem. Soc. 2012, 134, 18427− 18439. (16) Liang, Y.; Feng, D.; Wu, Y.; Tsai, S.-T.; Li, G.; Ray, C.; Yu, L. Highly Efficient Solar Cell Polymers Developed via Fine-Tuning of Structural and Electronic Properties. J. Am. Chem. Soc. 2009, 131, 7792−7799. (17) Huo, L.; Hou, J.; Zhang, S.; Chen, H.-Y.; Yang, Y. A Polybenzo[1,2-b:4,5-b’]dithiophene Derivative with Deep HOMO Level and Its Application in High-Performance Polymer Solar Cells. Angew. Chem., Int. Ed. 2010, 49, 1500−1503. (18) Li, W.; Roelofs, W. S.; Wienk, M. M.; Janssen, R. A. Enhancing the Photocurrent in Diketopyrrolopyrrole-Based Polymer Solar Cells via Energy Level Control. J. Am. Chem. Soc. 2012, 134, 13787−13795. (19) Amb, C. M.; Chen, S.; Graham, K. R.; Subbiah, J.; Small, C. E.; So, F.; Reynolds, J. R. Dithienogermole as a Fused Electron Donor in Bulk Heterojunction Solar Cells. J. Am. Chem. Soc. 2011, 133, 10062− 10065. (20) Zhou, H.; Yang, L.; Stuart, A. C.; Price, S. C.; Liu, S.; You, W. Development of Fluorinated Benzothiadiazole as a Structural Unit for a Polymer Solar Cell of 7% Efficiency. Angew. Chem., Int. Ed. 2011, 123, 3051−3054. (21) Son, H. J.; Wang, W.; Xu, T.; Liang, Y.; Wu, Y.; Li, G.; Yu, L. Synthesis of Fluorinated Polythienothiophene-co-benzodithiophenes and Effect of Fluorination on the Photovoltaic Properties. J. Am. Chem. Soc. 2011, 133, 1885−1894.

(22) Chu, T.-Y.; Lu, J.; Beaupré, S.; Zhang, Y.; Pouliot, J.-R.; Wakim, S.; Zhou, J.; Leclerc, M.; Li, Z.; Ding, J.; et al. Bulk Heterojunction Solar Cells Using Thieno[3,4-c]pyrrole-4,6-dione and Dithieno[3,2b:2′,3′-d]silole Copolymer with a Power Conversion Efficiency of 7.3%. J. Am. Chem. Soc. 2011, 133, 4250−4253. (23) Blouin, N.; Michaud, A.; Leclerc, M. A Low-Bandgap Poly(2,7carbazole) Derivative for Use in High-Performance Solar Cells. Adv. Mater. 2007, 19, 2295−2300. (24) Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.; Neagu-Plesu, R.; Belletête, M.; Durocher, G.; Tao, Y.; Leclerc, M. Toward a Rational Design of Poly(2,7-carbazole) Derivatives for Solar Cells. J. Am. Chem. Soc. 2008, 130, 732−742. (25) Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Bulk Heterojunction Solar Cells with Internal Quantum Efficiency Approaching 100%. Nat. Photonics 2009, 3, 297−302. (26) Berton, N.; Ottone, C.; Labet, V.; de Bettignies, R.; Bailly, S.; Grand, A.; Morell, C.; Sadki, S.; Chandezon, F. New Alternating Copolymers of 3,6-Carbazoles and Dithienylbenzothiadiazoles: Synthesis, Characterization, and Application in Photovoltaics. Macromol. Chem. Phys. 2011, 212, 2127−2141. (27) Fu, Y.; Cha, H.; Lee, G.-Y.; Moon, B. J.; Park, C. E.; Park, T. 3,6Carbazole Incorporated into Poly[9,9-dioctylfluorene-alt-(bisthienyl)benzothiadiazole]s Improving the Power Conversion Efficiency. Macromolecules 2012, 45, 3004−3009. (28) Wang, L.; Fu, Y.; Zhu, L.; Cui, G.; Liang, F.; Guo, L.; Zhang, X.; Xie, Z.; Su, Z. Synthesis and Photovoltaic Properties of Low-Bandgap Polymers Based on N-Arylcarbazole. Polymer 2011, 52, 1748−1754. (29) Jin, E.; Du, C.; Wang, M.; Li, W.; Li, C.; Wei, H.; Bo, Z. Dibenzothiophene-Based Planar Conjugated Polymers for High Efficiency Polymer Solar Cells. Macromolecules 2012, 45, 7843−7854. (30) Beaupre, S.; Boudreault, P. L. T.; Leclerc, M. Solar-Energy Production and Energy-Efficient Lighting: Photovoltaic Devices and White-Light-Emitting Diodes Using Poly(2,7-fluorene), Poly(2,7carbazole), and Poly(2,7-dibenzosilole) Derivatives. Adv. Mater. 2010, 22, E6−E27. (31) Cai, S.; Chen, L.; Zha, D.; Chen, Y. A Novel Planar D-A Alternating Copolymer with D-A Integrated Structures Exhibiting Haggregate Behaviors for Polymer Solar Cells. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 624−634. (32) Zha, D.; Chen, L.; Wu, F.; Wang, H.; Chen, Y. Donor− Acceptor-Integrated Conjugated Polymers Based on Carbazole[3,4c:5,6-c]bis[1,2,5]thiadiazole with Tight π−π Stacking for Photovoltaics. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 565−574. (33) Chen, H.; He, C.; Yu, G.; Zhao, Y.; Huang, J.; Zhu, M.; Liu, H.; Guo, Y.; Li, Y.; Liu, Y. Phenanthro[1,10,9,8-cdefg]carbazole-Containing Copolymer for High Performance Thin-Film Transistors and Polymer Solar Cells. J. Mater. Chem. 2012, 22, 3696−3698. (34) Jiao, C.; Huang, K.-W.; Luo, J.; Zhang, K.; Chi, C.; Wu, J. Bis-Nannulated Quaterrylenebis(dicarboximide) as a New Soluble and Stable Near-Infrared Dye. Org. Lett. 2009, 11, 4508−4511. (35) Chen, X.-K.; Zou, L.-Y.; Guo, J.-F.; Ren, A.-M. An Efficient Strategy for Designing N-type Organic Semiconductor Materials Introducing a Six-Membered Imide Ring into Aromatic Diimides. J. Mater. Chem. 2012, 22, 6471−6484. (36) Kim, J.-H.; Kim, H. U.; Mi, D.; Jin, S.-H.; Shin, W. S.; Yoon, S. C.; Kang, I.-N.; Hwang, D.-H. Introduction of Perylene Units for Enhanced Interchain Interaction in Conjugated Polymers for Organic Photovoltaic Devices. Macromolecules 2012, 45, 2367−2376. (37) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, revision A.01; Gaussian, Inc.: Wallingford, CT, 2009. (38) Cohen, A. J.; Mori-Sánchez, P.; Yang, W. Challenges for Density Functional Theory. Chem. Rev. 2012, 112, 289−320. (39) Popere, B. C.; Della Pelle, A. M.; Poe, A.; Balaji, G.; Thayumanavan, S. Predictably Tuning the Frontier Molecular Orbital Energy Levels of Panchromatic Low Band Gap BODIPY-Based Conjugated Polymers. Chem. Sci. 2012, 3, 3093−3102. 9588



Article

(40) Risko, C.; McGehee, M. D.; Brédas, J.-L. A Quantum-Chemical Perspective into Low Optical-Gap Polymers for Highly-Efficient Organic Solar Cells. Chem. Sci. 2011, 2, 1200−1218. (41) Banerji, N.; Gagnon, E.; Morgantini, P.-Y.; Valouch, S.; Mohebbi, A. R.; Seo, J.-H.; Leclerc, M.; Heeger, A. J. Breaking Down the Problem: Optical Transitions, Electronic Structure, and Photoconductivity in Conjugated Polymer PCDTBT and in Its Separate Building Blocks. J. Phys. Chem. C 2012, 116, 11456−11469. (42) Zhang, S. S.; Qu, Z. X.; Tao, P.; Brooks, B.; Shao, Y. H.; Chen, X. Y.; Liu, C. G. Quantum Chemical Study of the Ground and Excited State Electronic Structures of Carbazole Oligomers with and without Triarylborane Substitutes. J. Phys. Chem. C 2012, 116, 12434−12442. (43) Kim, J.; Kwon, Y. S.; Shin, W. S.; Moon, S. J.; Park, T. Carbazole-Based Copolymers: Effects of Conjugation Breaks and Steric Hindrance. Macromolecules 2011, 44, 1909−1919. (44) Guo, X.; Puniredd, S. R.; Baumgarten, M.; Pisula, W.; Müllen, K. Benzotrithiophene-Based Donor−Acceptor Copolymers with Distinct Supramolecular Organizations. J. Am. Chem. Soc. 2012, 134, 8404− 8407. (45) Ahmed, E.; Subramaniyan, S.; Kim, F. S.; Xin, H.; Jenekhe, S. A. Benzobisthiazole-Based Donor−Acceptor Copolymer Semiconductors for Photovoltaic Cells and Highly Stable Field-Effect Transistors. Macromolecules 2011, 44, 7207−7219. (46) Zhou, H.; Yang, L.; You, W. Rational Design of High Performance Conjugated Polymers for Organic Solar Cells. Macromolecules 2012, 45, 607−632.

9589


Experimental Investigation and Theoretical Calculation of Molecular

Recommend Documents