a spectroscopic and dft study of the electronic properties of carbazole-based d–a type copolymers

A Spectroscopic and DFT Study of the Electronic Properties ofCarbazole-Based D−A Type CopolymersMatthew E. Reish,† Sanghun Nam,‡ Wonho Lee,‡ Han Young Woo,‡ and Keith C. Gordon*,†

†MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Chemistry, University of Otago, P.O. Box 56,Dunedin 9054, New Zealand‡Department of Nanofusion Technology (BK21), Department of Cogno-Mechatronics Engineering (WCU), Pusan NationalUniversity, Miryang 627-706, Republic of Korea

ABSTRACT: The structural and electronic properties ofthree carbazole containing copolymers used in organicphotovoltaic applications, poly[N-1-octylnonyl-2,7-carbazole-alt-5,5-(4′ ,7′-di-2-thienyl-2′ ,1′ ,3′-benzothiadiazole)](PCDTBT), poly[N-1-octylnonyl-2,7-carbazole-alt-4,7-(2′,1′,3′-benzothiadiazole)] (PCBT), and poly[N-1-octylnon-yl-2,7-carbazole-alt-4,7-(2′ ,1′ ,3′-benzoselenadiazole)](PCBSe) have been studied using resonance Raman (RR) andtransient absorption (TA) spectroscopies and density func-tional theory (DFT) calculations. Enhancement of Ramanmodes centered on the acceptor unit when a Raman excitationwavelength is coincident with lowest energy electronicexcitation suggests that the excitation involves charge transferfrom the carbazole donor to the varying benzodiazole acceptors. The pattern of the enhancement when the excitation wavelengthis coincident with the higher energy transition indicates that this transition is π to π* in nature; this is consistent with TD-DFTcalculations. Nanosecond transient absorption studies show long-lived excited state signals for PCDTBT (126 ± 4 ns and 1.56 ±0.1 μs) and PCBSe (1.82 ± 0.1 μs), suggesting that population of the triplet state is appreciable. No transient signal could bedetected in PCBT. B3LYP TD-DFT calculations of the monomer through to the hexamer indicate a broadly delocalized excitedstate orbital for PCDTBT as indicated by the linear decrease in excitation energy with an increased number of repeat units, whilefor PCBSe and PCBT, the reduction in excitation is sublinear. The highest occupied (HOMO) and lowest unoccupied molecularorbitals (LUMO) of PCBSe and PCBT polymers compared to PCDTBT are similarly diffuse, but the population of higher orderorbitals is decreased when compared with PCDTBT. CAM-B3LYP calculations reduce the delocalization of the frontier orbitalsand show less reduction in excitation energy with additional repeat units for each polymer.

■ INTRODUCTION

Organic photovoltaics (OPVs) show promise as a technologythat could reduce the cost of solar energy.1−3 The low cost ofmaterials and the solution processability of the polymers mayfacilitate the creation of devices that cost less than traditionalsemiconductor cells.4,5 However, the overall power conversionefficiency (PCE) of these devices is currently limited by severalfactors. These include a lack of absorption of the complete solarspectrum and low charge carrier mobility, which limits short-circuit current density (Jsc), energy level mismatch of electronrich and electron deficient layers, which limits the open-circuitvoltage (Voc), and the recombination of various excited statesmeans many absorbed photons are wasted. The main approachthat has been utilized in order to optimize solar cell efficiencyhas been the design of polymer donors that meet the criteria ofa small band gap for increased absorption efficiency of the solarspectrum and orbital levels engineered for increased opencircuit voltage.6 Design of electron-donating polymers may beaccomplished using empirical knowledge but recently designstrategies have often been guided by theory.6−8 The use of

donor−acceptor copolymers has led to tunable highestoccupied (HOMO) and lowest unoccupied molecular orbital(LUMO) levels and allowed polymers to be designed to fit theenergy levels PCBM while maintaining small band gaps.9−15

Along with morphological optimization, guided design of theactive layer has quickly pushed the PCE from around 5% toover 9% which encourages optimism that devices could soonreach a practical threshold.13,16−18 Leclerc and co-workerspioneered the use of carbazole donors and achieved some ofthe earliest high efficiency OPV cells in the post P3HTparadigm.11,12,18−20 Each of the three polymers used in thisstudy uses a carbazole donor unit and a varying benzodiazoleunit as an electron acceptor, and the synthetic route to each isshown in Figure 1.Quantum chemical methods may be used to predict the

properties of new dyes, and this is leading to a change in the

Received: July 31, 2012Revised: September 17, 2012Published: September 18, 2012

Article

pubs.acs.org/JPCC

© 2012 American Chemical Society 21255 dx.doi.org/10.1021/jp307552z | J. Phys. Chem. C 2012, 116, 21255−21266

pubs.acs.org/JPCC

way polymers are designed as the properties of as yet to besynthesized compounds can be investigated without performingcostly and time-consuming synthesis on each new material.Several studies have been undertaken that indicate that DFTcalculations on oligomers representing conjugated OPVpolymers can be done with good relative accuracy such thatDFT can be predictive.7,21 DFT has also been used as a designguide to predict the properties of polymers before they aresynthesized.12,20 While computational chemistry has become animportant tool for design, correlation of experimental proper-ties with calculated properties has largely been limited toexcitation energies and HOMO/LUMO levels, while exper-imental validation of other properties, such as orbitaldistribution during electronic transitions, is largely absent.Resonance Raman (RR) spectroscopy can be used in order toinvestigate the nature of the electronic transitions as resonantenhancement of normal modes is directly related to thedistortion of bonds that are coincident with those normalmodes upon excitation into the Franck−Condon excitedstate.22,23 In charge transfer excitations, such as those expectedfor the lowest energy electronic excitation of each of thepolymers studied here, resonant enhancement of Raman modeswill be limited to those that are confined to the electronacceptor unit. This is a result of the greater distortion thatoccurs in the acceptor unit as opposed to the donor.24 Byanalyzing the enhancement pattern in the RR spectra usingRaman excitation wavelengths coincident with the varioustransitions in the donor−acceptor copolymers, it is possible todescribe excited state geometric distortion, which can be usedto define the nature of the electronic excitation. Using thismethod, we confirm experimentally the charge transfer natureof the lowest energy excitation and the π to π* nature of thenext higher energy excitation.One strategy that has been employed to lower the band gap

in conjugated polymers is to substitute heavier atoms of thesame group into the acceptor. When the sulfur of thebenzothiadiazole unit is exchanged for selenium, there is adecrease in the band gap of PCBT of 0.097 eV. However,devices utilizing the BSe acceptor unit consistently have lower

PCE when used in photovoltaic devices.25−27 This has beenexplained in terms of weaker absorption of the seleniumcontaining compounds or hole and electron transportproblems, which may be the result of poor morphology.6

Another phenomenon that may affect the conversion efficiencyof cells is the rate of intersystem crossing, which leads to tripletstates that can act as trap sites for the recombination ofelectron−hole pairs.16,28 The addition of heavy atoms increasesthe efficiency of intersystem crossing (ISC). Population of thetriplet state is observed using transient absorption spectroscopyand the efficiency of ISC observed in PCBSe is much greaterthan in PCBT.Studies on the transient absorption and photoluminescence

of PCDTBT are extensive in thin film and solution.29−32 Thesestudies have broadly differing results, but there is generalagreement that, in PCDTBT/PCBM blends, there is a very fastseparation (∼100 fs) of the bound exciton into chargedpairs.30,31 In this study, we have performed nanosecondtransient absorption in solution and find that there is significantpopulation of the triplet state but the time scale for intersystemcrossing in PCDTBT is in the nanoseconds and that tripletformation should have a negligible effect on PCE.Previous spectroscopic studies of PCDTBT have shown fast

charge separation and diffuse excitons have been discussed as ameans to facilitate this, but there is no consensus on excitondelocalization.30 TD-DFT calculations can help to clarifydelocalization of the exciton as the electron density distributionof orbitals populated in the S1 state can be used as anapproximation of the exciton.16 However, these results are alsoquestioned because of the underestimated band gap from DFTand the tendency of DFT to overestimate exciton delocalizationdepending on the degree of exchange correlation (XC).33 Thislimitation in DFT calculations has been addressed by thecreation of several different range separated functionals that,instead of maintaining constant HF exchange, vary the degreeof exchange with interion distance (r12

−1) in the followingway:34−36

Figure 1. Synthetic routes to carbazole-based copolymers.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp307552z | J. Phys. Chem. C 2012, 116, 21255−2126621256

μ μ=

−+

⎛⎝⎜

⎞⎠⎟

⎛⎝⎜

⎞⎠⎟r

rr

rr

1 1 erf( ) erf( )

12

12

12

12

12

where the first term describes shorter range coulomb potentialusing the DFT exchange, the second term describes longerrange interaction with HF exchange, and μ describes the rate offall off of DFT exchange with distance. This method has provensuccessful in rectifying incorrect orbital and excitation energiesin broadly delocalized π systems using the LC-BLYP func-tional.37,38 An extension of this method, termed CAM-B3LYP,adds two adjustable parameters to the electron repulsionoperator that allow for adjustable amounts of DFT and HFexchange at the distance extremes.39 In this approach, theerf(μr12) in both the short- and long-range terms becomes α +β erf(μr12), and in this way, the fraction of HF exchange that isutilized as r12 approaches 0 approaches α, and for larger valuesof r12, the fraction of DFT exchange is β.It has also been shown that the addition of a solvent model

can significantly improve excitation and fluorescence energiesfor conjugated systems.40,41 In accordance with this, we havealso included a PCM solvent field, the long-range correctedcalculations, in order to assess the effect of solvation onexcitation energies and electron delocalization in each system.Here, we have used the CAM-B3LYP and LC-BLYP

functionals to gauge the effect of long-range correction onthe orbital and excitation energy for each of the polymersstudied.39,42 Using the functionals described above, we haveinvestigated the delocalization of the excited state orbitals asfound by DFT and perform a comparative study betweenPCDTBT, PCBT, and PCBSe in order to investigate relativeexciton delocalization. Using the B3LYP functional, we findchanges in orbital delocalization between the polymers, withPCDTBT retaining greater reduction of excitation energy witheach additional unit than PCBT and PCBSe as the oligomerlength is increased. Using range corrected functionals, we findexcitation energies to be much higher than those foundexperimentally but still a large degree of delocalization of the S1state. For CAM-B3LYP, there is greater mixing with higherenergy orbitals in the excited state band, but these orbitals aremore localized than the B3LYP orbitals. While LC-BLYPcalculations overestimate the excitation energy even more thanCAM-B3LYP, it describes fully delocalized orbitals. B3LYPcalculations lend support to delocalization as an explanation offast exciton migration, with an exciton that can extend to 10 nmor more. This may help explain charge extraction in efficientsolar cells, which proceeds more rapidly than can be explainedby charge-hopping alone.30,31

■ EXPERIMENTAL SECTION

Synthesis. The monomers, 9-(1-octylnonyl)-2,7-bis-(4′,4′,5′,5′-tetramethyl-1′,3′,2′-dioxaborolan-2′-yl)carbazole(1), 4,7-bis(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (2), 4,7-dibromo-2,1,3-benzothiadiazole (3), and 4,7-dibromo-2,1,3-benzoselenadiazole (4) were prepared according to theprevious literature methods.43

Suzuki Polycondensation Polymerization. 9-(1-Octylnon-yl)-2,7-bis(4′,4′,5′,5′-tetramethyl-1′,3′,2′-dioxaborolan-2′-yl)-carbazole (200 mg, 0.304 mmol) (1), dibromo-monomer(0.304 mmol) (2, 3, or 4), and Pd(pph3)4 (3 mol %) weredissolved in a mixture of toluene (3.00 mL) and aq. 2 M K2CO3solution (1.40 mL). The mixture was reacted at 140 °C for 40min in a microwave reactor. After the reaction was completed,

the mixture was precipitated into methanol and stirred for 2 h.The crude polymer was purified by short-column chromatog-raphy (eluent, toluene) and Soxhlet extraction (solvent,acetone).

Poly[N-1-octylnonyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT). Yield: 80 mg(36%). 1H NMR (300 MHz, CDCl3): δ (ppm) 8.11 (br, 2H),8.02 (br, 2H), 7.91 (br, 2H), 7.71 (br, 2H), 7.50 (br, 2H), 7.45(br, 2H), 4.73 (br, 1H), 2.42 (br, 2H), 2.05 (m, 2H), 1.40−1.00 (br, 24H), 0.70 (br, 6H). Mn = 8600 g/mol (PDI = 2.1).

Poly[N-1-octylnonyl-2,7-carbazole-alt-4,7-(2′,1′,3′-benzo-thiadiazole)] (PCBT). Yield: 110 mg (64%). 1H NMR (300MHz, CDCl3): δ (ppm) 8.58 (br, 2H), 8.30 (s, 2H), 7.98 (br,2H), 7.70 (br, 2H), 4.78 (br, 1H), 2.52 (br, 2H), 2.05 (m, 2H),1.40−1.00 (br, 24H), 0.70 (br, 6H). Mn = 24 000 g/mol (PDI= 1.5).

Poly[N-1-octylnonyl-2,7-carbazole-alt-4,7-(2′,1′,3′-benzo-selenadiazole)] (PCBSe). Yield: 140 mg (75%). 1H NMR (300MHz, CDCl3): δ (ppm) 8.15 (br, 2H), 8.01 (br, 2H), 7.80 (br,2H), 7.35 (br, 2H), 4.73 (br, 1H), 2.52 (br, 2H), 2.05 (m, 2H),1.40−1.00 (br, 24H), 0.70 (br, 6H). Mn = 20 000 g/mol (PDI= 1.8).

Measurements. 1H and 13C NMR spectra were recordedon a JEOL (JNM-AL300) FT NMR system operating at 300and 75 MHz, respectively. The polymerization reaction wasdone by a microwave reactor (Biotage Initiator). The number-and weight-average molecular weights of the copolymers weredetermined by gel permeation chromatography (GPC) onAgilent GPC 1200 series, using THF as an eluent, relative to apolystyrene standard.FT-Raman spectra were obtained using a Bruker Equinox 55

interferometer coupled with a FRA-106 Raman module and aD418T liquid-nitrogen-cooled germanium detector, controlledby the Bruker OPUS v5.5 software package. A Nd:YAG laseroperating at 1064 nm and 120 mW of power was used. Thespectra were acquired with a resolution of 4 cm−1. Thepowdered samples were dispersed in KBr and pressed into adisk to minimize laser heating.Resonance Raman measurements were performed as

previously described.44,45 A continuous-wave Innova I-302krypton-ion laser (Coherent, Inc.) was used to generate RRscattering. A Pellin-Broca prism was used to separate Kr+

plasma lines. Typically, the laser output was adjusted to givearound 30 mW at the sample. The incident beam and thecollection lens were arranged in a 135° backscattering geometryto reduce Raman intensity loss by self-absorption. An aperture-matched lens was used to focus scattered light through anarrow band line-rejection (notch) filter (Kaiser OpticalSystems) and a quartz wedge (Spex) and onto the 100 μmentrance slit of a spectrograph (Acton Research SpectraPro500i). The collected light was dispersed in the horizontal planeby a 1200 grooves/mm ruled diffraction grating (blazewavelength 500 nm) and detected by a liquid-nitrogen-cooledback-illuminated Spec-10:100B CCD controlled by a ST-133controller and WinSpec/32 (version 2.5.8.1) software (RoperScientific, Princeton Instruments). Wavenumber calibrationwas performed using Raman bands from a 1:1 v/v acetonitrile/toluene sample. Peak positions were reproducible to within 1−2 cm−1. Spectra were obtained with a resolution of ∼5 cm−1.Absorption spectra were recorded on a Varian Cary 500 scan

UV−vis-NIR spectrophotometer utilizing Cary Win UV ScanApplication software. Spectra were recorded from 800 to 300



nm. Solutions were typically 10−6 mol L−1 and were containedin a 1 cm cell.Fluorescence spectra were recorded with a Perkin-Elmer

luminescence spectrometer LS50B with FL Winlab v. 4.00.02.To determine the fluorescence quantum yield (ΦF) solutions ofeach polymer and coumarin-6 as a standard were made upstarting at ∼0.1 absorbance and diluted down to ∼0.08, 0.06,0.04, and 0.02 absorbance in DCM. The ΦF of coumarin-6 inDCM is 0.96.46 The integrated fluorescence of each was thenrecorded with an excitation wavelength of 470 nm and plottedagainst absorbance. The following relationship was then used todetermine ΦF:

47

ηη

Φ = Φ⎛⎝⎜

⎞⎠⎟⎛⎝⎜

⎞⎠⎟⎛⎝⎜⎜

⎞⎠⎟⎟

AA

FFx

x

x

xs

s

s2

s2

where A is absorbance, F is integrated fluorescence intensity, ηis solvent refractive index, the subscript x denotes the unknown,and the subscript s denotes the standard.Excited state absorption spectra were acquired using a

LP920K system (Edinburgh Instruments) as previouslydescribed.48 Excitation was provided by a pulsed Brilliant(Quantel) Nd:YAG laser operating at 10 Hz. For all spectra,the second harmonic at 532 nm was utilized. Temporal pulsehalf width at half-maximum intensity estimated to be 4−6 ns,while probe radiation came from a Xe900 450 W xenon arclamp. Collected probe light was dispersed on a 1800 groove/mm grating in a TMS300-A Czerny−Turner monochromatorand recorded on a TDS3012C (Tektronix) digital oscilloscope.Transient data were collected at single wavelengths with aspectral bandwidth of 0.10−18 nm, which was adjusted by a slit,and mapped to create spectra. Calculation of the differentialoptical density, ΔOD, included a fluorescence and baselinecorrection.48

DFT calculations of the optimized geometry, vibrationalfrequencies, and electronic excitations using time-dependentDFT (TD-DFT) were performed using the Gaussian09software package.49 Long alkyl chains were substituted withmethyl groups to decrease computational cost. B3LYP, CAM-B3LYP, and LCBLYP functionals were used with the 6-31G(d)basis set to perform geometry optimization and TD-DFTcalculations. CAM-B3LYP calculations were performed usingthe default α, β, and μ coefficients in the Gaussian09 program(α = 0.19, β = 0.46, and μ = 0.33). LC-BLYP calculations usethe Gaussian09 default value for μ of 0.47. Additionally,optimizations and TD-DFT calculations including a solventfield using the CAM-B3LYP and LC-BLYP functionals wereperformed with the integral equation formalism polarizablecontinuum model (IEFPCM). The solvent field used was thedefault chloroform field in Gaussian09, which simulates adielectric constant of (εr) of 4.7133. This solvent was chosen asit is at the higher end of estimated dielectrics that can beexpected in polymer solar cells, generally reported to bebetween εr = 2−4.17For theoretical Raman vibrational energies, the three unit

oligomer was used as higher oligomers proved to be toocomputationally expensive. A scale factor of 0.97 wasdetermined by minimizing mean absolute deviation (MAD)between calculated and experimental modes and was found tobe similar to recent recommendations.50 Intensity correctionfor calculated spectra was applied to convert the Raman activitygiven by the Gaussian09 program to Raman scattering cross-sections.51−53 The differential Raman cross-section of the jth

mode, (∂σ)/(∂Ω), is related to the Raman activity, Sj, given bythe Gaussian09 frequency calculation (Gaussian keyword:Freq=Raman) as follows:

σ π ν ν

π ν

∂∂Ω

=−

−ν−

⎛⎝⎜

⎞⎠⎟⎛

⎝

⎜⎜⎜ ⎡⎣⎢

⎤⎦⎥

⎞

⎠

⎟⎟⎟⎛⎝⎜⎜

⎞⎠⎟⎟h

cS

245

( )

1 exp 8j j

hc

kTj

j

4 40

4

2j

where ν0 is the laser excitation frequency and νj is the frequencyof the jth mode.

■ RESULTS AND DISCUSSIONSynthesis. Three carbazole-based copolymers were synthe-

sized via Suzuki polycondensation between 9-(1-octylnonyl)-

2,7-bis(4′,4′,5′,5′-tetramethyl-1′,3′,2′-dioxaborolan-2′-yl)-carbazole and the dibromo monomers (2, 3, or 4) under themicrowave condition. All monomers were synthesized byfollowing the previous procedures reported elsewhere.43 Thestructures of synthesized monomers and copolymers wereconfirmed by 1H and 13C NMR spectroscopy. The number-average molecular weight (Mn) of the synthesized copolymerswas ranged from 8000 to 24 000 g/mol by gel permeationchromatography (eluent, THF).

Raman Studies. Raman spectra with a 1064 nm excitationsource give some indication of the changing structuralproperties in the series of compounds (Figure 2). For PCBTand PCBSe, the carbazole centered vibrational mode at 1627cm−1 is unshifted, while the equivalent mode in PCDTBT isshifted in relation to the other two compounds to 1623 cm−1.This is an indication that the bonding in the carbazole donor isunaffected by the change from sulfur to selenium but theelectronic structure is altered by changing the acceptor unitwith the addition of the two thiophenes. This is supported byDFT calculations, which indicate greater conjugation along thepolymer backbone chain in PCDTBT than in PCBT andPCBSe. This is evidenced by DFT calculations that show areduced torsional angle between monomeric units in PCDTBTas opposed to PCBT and PCBSe and a more linear decrease inabsorption energy with increasing oligomer length in PCDTBTthan in PCBT and PCBSe (Table 1).The strong band around 1540 cm−1 is identified by DFT

calculations as a stretch of the two rings making up thebenzothiadiazole moiety. The transition for this vibration is

Figure 2. Vibrational mode focused on the carbazole unit. For PCBTand PCBSe (solid line and dotted line), there is minimal change in theenergy of this mode indicating a similar ground state structure, but forPCDTBT (dashed line), the mode shifts by 4 cm−1. This is anindication that there is a change in the ground state structure ofcarbazole between PCDTBT and the other two polymers.



observed at 1546 cm−1 in PCBT and shifts down to 1534 cm−1

with substitution of the heavier selenium, as expected. Thebenzothiadiazole ring-stretch in PCDTBT is coincident withthe stretching frequency of the thiophene rings with one at1529 cm−1 and the other appearing at 1541 cm−1. To assignthese modes, it was necessary to refer to increased resonantenhancement of the mode at 1541 cm−1 and compare theorbital density of the resonant state, which shows the greatestconcentration on the benzothiadiazole portion of the DBTacceptor, and is therefore assumed to couple more effectivelywith the 1541 cm−1 mode. The most prominent vibrationpresent in the PCDTBT spectrum is observed at 1447 cm−1

and was assigned to a broad ring stretch focused on the -DTBTacceptor unit that is delocalized across the benzothiadiazole andthe two thiophenes.Raman spectroscopy shows that the ground state structure of

PCBT and PCBSe are similar and that the substitution of sulfurfor selenium has a minimal effect on bonding through thepolymer backbone. However, there is a significant shift in theequivalent modes in PCDTBT, which were interpreted, along

with other evidence, to be changes related to the effectiveconjugation path length.

Resonance Raman and TD-DFT. The electronic absorp-tion spectra and simulated excitations from TD-DFTcalculations are shown in Figure 3. In all cases, two transitionsare observed in the visible region. For PCBT and PCBSe, thesetransitions both occur at approximately 350 and 470 nm with aslight redshift for PCBSe. In the case of PCDTBT, thetransitions are red-shifted with the higher energy transitionoccurring at 382 nm and the lower energy transition at 535 nm.Calculations using B3LYP/6-31G(d) show good qualitativeagreement with the experimental data. The calculated excitationenergy for each transition is underestimated, which has oftenbeen observed in conjugated polymers and is covered in moredetail below.54,55 Resonance Raman was performed usingexcitation wavelengths coincident with the two lowestexcitations of each compound. Each of the polymers isfluorescent, but using Raman excitation wavelengths on theblue edge of the electronic absorptions of each compoundallowed reliable data on the two lowest energy absorptions ofeach compound to be collected (Figure 4).In PCDTBT, the Raman excitation wavelengths of 350.7 and

406.7 nm are coincident with the higher energy electronicabsorption with an absorption maximum at 382 nm (Figures 3and 4). The resonant enhancement observed at thesewavelengths is nearly even across all of the modes with someincreased enhancement of the band at 1623 cm−1, which isassociated with the carbazole based mode. This pattern ofenhancement is consistent with a π → π* transition as thedistortion of the excited state geometry will be along all theconjugated bonds that make up the backbone of the polymer.TD-DFT calculations are consistent with this assumption asthey indicate that the excitation with the maximum absorbanceat 382 nm is from the HOMO, a conventional polyenedelocalized π orbital, to the LUMO, which is the equivalent π*orbital. The carbazole mode shows increased resonanceenhancement relative to acceptor based modes, and this isalso supported by calculations, which show modest localizationof the LUMO orbital onto the carbazole unit, though in generalit is still broadly delocalized (Figure 5).Raman excitation wavelengths of 444 and 457.9 nm are

coincident with the lower energy excitation of PCDTBT andshow a different band enhancement pattern compared to the350.7 nm excitation (Figure 4). Enhancement of bands ofDTBT acceptor based modes, at 1374 and 1541 cm−1, isconsistent with increased distortion of the acceptor during thelower energy excitation. In contrast, the 1623 cm−1 bandassociated with a carbazole mode is almost completely absent,

Table 1. Calculated Structural and Electronic Excitation Data for the Six Repeat Unit Structure PCBT, PCBSe, and PCDTBT;Experimental Excitation Energy of Polymer Is Provided for Comparison

donor−acceptor torsional angle (deg)/bond length (Å)

polymer B3LYP CAM-B3LYP CAM-B3LYP (PCM) LC-BLYP LC-BLYP (PCM)

PCBT 36.3/1.48 37.9/1.48 39.1/1.48 38.3/1.48 39.4/1.48PCBSe 37.2/1.48 38.8/1.48 39.8/1.48 39.2/1.48 40.0/1.48PCDTBT 25.8/1.45 29.8/1.46 28.6/1.47 30.9/1.47 29.5/1.47

excitation energy (eV)

polymer B3LYP CAM-B3LYP CAM-B3LYP (PCM) LC-BLYP LC-BLYP (PCM) experimental

PCBT 2.24 3.05 3.08 3.64 3.67 2.72PCBSe 2.12 2.92 2.96 3.52 3.55 2.62PCDTBT 1.80 2.57 2.58 3.18 3.20 2.32

Figure 3. (A) PCDTBT, (B) PCBSe, and (C)PCBT. UV−vis spectraand TD-DFT calculated transitions of each compound with wave-lengths used for resonance Raman experiments overlaid (coloredarrows). The left scale is the experimental extinction coefficient, andthe right scale is the calculated oscillator strength from TD-DFT andshown using vertical red bars. TD-DFT calculations at the B3LYP 6-31G(d) level with 3 repeat units.



indicating that distortion and increased electron density is notseen on the carbazole unit during the lower energy excitation.This is consistent with DFT calculations that show that, in thelower energy excited state, the electron density changes areassociates with the acceptor unit. Another feature of the lowerenergy excitation RR spectrum is the enhancement of bands at844 and 874 cm−1. The modes for these bands are difficult toassign unambiguously because of their low intensity in thenormal Raman spectrum and in the calculation but aretentatively assigned as in-plane bending modes confined tothe acceptor and calculated at 835 and 854 cm−1 respectively.For PCBSe and PCBT, the higher energy excitation at 350.7

nm (Figure 4B,C) shows enhancement in the RR spectrum ofall of the bands observed in the normal Raman spectrum. Thisis expected for a π → π* excitation with change in electrondensity across the entire frame of the molecule. For bothpolymers, there are distinct enhancements of lower wave-number bands at 764 cm−1 for PCBSe and 855 cm−1 for PCBT.

This is in contrast to the enhancement pattern seen inPCDTBT where similar enhancements were observed with406.7 nm excitation. It appears that some resonance enhance-ments at 350.7 nm in PCBT and PCBSe are due to resonancewith the lower energy CT state. Furthermore, there is a greaterenhancement of other benzodiazole based modes, especially inPCBT, which has a relatively blue-shifted electronic absorption.These modes are therefore considered to be enhanced byresonance effects associated with the lower energy transition. Inorder to confirm this, shorter wavelength UV excitation wouldbe necessary but were not available during this study.The lower energy transitions of PCBSe and PCBT were

probed with Raman excitation wavelengths of 457.9 and 406.7nm, respectively. For PCBT, high fluorescence yield leads to alarge fluorescence baseline and a consequential reduction in S/N at 406.7 nm, but the modes with the strongest enhancementare still observed. The band calculated at 1534 cm−1 andobserved at 1546 cm−1 is enhanced and is associated with a

Figure 4. FT-Raman and resonance Raman spectra of (A) PCDTBT, (B) PCBSe, and (C) PCBT. For each panel, the FT-Raman spectrum recordedwith an excitation wavelength of 1064 nm is shown at the bottom in black and Raman excitation wavelengths of each spectrum are given on the right.Bands that show variable enhancement with excitation wavelength are indicated by a dashed line and described in the text. Solvent bands are notedby *.



benzothiadiazole mode (Figure 4C). The band calculated at1345 cm−1 and observed at 1357 cm−1 is also enhanced. Theassignment of this mode is less clear-cut as there are twocalculated transitions close to this energy; one is associated withthe benzothiadiazole, the other with carabazole. Assignment ofthis band to the acceptor based mode is not inconsistent withthe data for the 1534 cm−1 band.PCBSe shows a similar enhancement pattern to that of

PCBT when probing the lower energy CT transition. The bandcalculated at 1527 cm−1 and observed at 1534 cm−1, shown inFigure 4B, is enhanced. This mode closely resembles the

benzothiadiazole based mode for PCBT observed at 1546 cm−1,and the change in energy is attributed to the change in reducedmass by the exchange of the heavier selenium atom with sulfur.A reduction in the enhancement of the carbazole mode is alsoobserved but not to the extent that is seen for the PCDTBT.

Computational Chemistry. Structural optimization andelectronic excitation calculations using the TD-DFT methodwere performed on the monomer to hexamer of each polymer.Figure 6 shows the effect of increasing the number of repeatunits on excitation energy calculated by TD-DFT. Theexcitation energy is reduced approximately linearly with theinverse of the number of carbon atoms in the shortest paththrough the conjugated backbone (neff).One explanation for decreased electronic excitation energy

with increasing conjugation length is that there is increaseddelocalization of the HOMO and LUMO orbitals, which lowersthe bandgap as the excitation from the π to the π* state and islower in energy, analogous to an increase in path length of theparticle in a 1D box problem.56 However, delocalization of theHOMO and LUMO can only partially explain the effect ofincreasing conjugation length. It can be seen in Figure 7 thatthe HOMO and LUMO (NTO 1) in PCBSe are more diffusein relation to the total length of the oligomer than the sameorbitals in PCDTBT. This appears to be inconsistent with thereduction in excitation with increased oligomer length seen inFigure 6. In this figure, PCDTBT shows a nearly linearreduction in excitation energy with increasing conjugation path,while PCBSe and PCBT sublinear behavior. In order to morefully describe this effect, it is necessary to consider thepopulation of additional molecular orbitals in the ground andexcited states. This is because the electronic structure of thepolymers leads to a nearly degenerate band-type structure inwhich several electronic states lie in close proximity. Toinvestigate the nature of the additional states involved, we usemolecular orbital diagrams, and to simplify the complex natureof the excitation that involves varying contributions from manyorbitals, natural transition orbitals (NTOs) are used.57 NTOssimplify the description of the excitation by representing theelements of the transition density matrix as a set of orbitals withcorresponding eigenvalues (λ), which are the fractionalcontributions to the overall excitation with the first 4 NTOsfor PCDTBT and PCBSe shown in Figure 7.Analysis of the ground and excited state frontier molecular

orbitals of each polymer shows that the allowed states exist as acombination of close-lying, nearly degenerate levels. Thedensity of these can be approximated by a modified particle-in-a-box problem, where the lower energy orbitals are localizedand higher energy orbitals are delocalized. It is the higherenergy orbitals that help to explain the difference between theenergy reduction with additional units found for PCDTBT asopposed to PCBT and PCBSe. Figure 8A shows the calculatedenergy for the first frontier molecular orbitals of each polymer,and it is noted that, for PCDTBT, the gap in energy betweeneach orbital and its neighbors is reduced. The reduced energygap between neighboring energy levels allows greater access tothe higher energy, more delocalized orbitals. This is illustratedof Figure 8B with transition coefficients (λ) to each of theNTOs plotted against NTO number. The broader shape forPCDTBT indicates greater access to the higher energy NTOs.The increasing band-type structure and delocalized nature of

PCDTBT is an intuitive result that can be arrived at by manyother descriptions of the system including bond lengthalternation and planarity but the significance of the NTO

Figure 5. Electron isodensity surfaces of the orbitals involved in thetwo lowest energy transitions calculated by TD-DFT are shown nextto vibrational eigenvectors corresponding to modes that show thegreatest enhancement. (A) PCDTBT, (B) PCBSe, and (C) PCBT.Monomer MOs shown for clarity with extended oligomer MOs inFigure 7.

Figure 6. Dependence of the calculated excitation energy on theinverse of effective conjugation path (neff). A line through the first twopoints for each polymer is shown as a visual aid and highlightsdeviations from linearity. (Triangles, PCBT; circles, PCBSe; squares,PCDTBT. Blue, B3LYP; red, CAM-B3LYP; black, LC-BLYP; green,experimental).



approach is that it shows that the delocalization of the HOMOand LUMO do not fully describe the electron and hole created byphotoexcitation. This is important in the case of OPV polymersas the nature of the electron and hole determine excitonbinding energy as well as charge mobility, which are bothimportant when considering loss mechanisms in solar cells. The

increased access to higher energy orbitals that are on 10 nmlength scale seen for PCDTBT is consistent with recent resultsthat show that the exciton can be harvested at near unity in∼100 fs in thin-film cells.30,31 Such fast charge extraction isincompatible with a charge-hopping, Frenkel-type exciton.30,58

However, it must be noted that the B3LYP functional has atendency to estimate greater delocalization than is seenexperimentally, and therefore, to investigate this phenomenon,we have also used the CAM-B3LYP and LC-BLYP functionals.Using the B3LYP functional, the calculated energy is

underestimated by ∼0.5 eV for each polymer (Figure 6,Table 1). The underestimation of energy in calculations hasoften been attributed to the tendency of DFT functionals, withincorrect asymptotic behavior, to overestimate conjugation inextended systems.34−36 This is caused by an incorrect treatmentof Coulomb terms in the exchange partition as explained in theintroduction section. This approximation in DFT is known todecrease long-range forces and is especially important for longconjugated systems as the delocalization of orbitals becomesmore favorable. To investigate the effect that long-rangetreatment has on orbital delocalization and excitation on thesystems, the CAM-B3LYP and LC-BLYP functionals have beenalso been used. While TD-DFT excitation energy using theB3LYP functional is too low by around 0.5 eV, the use of the

Figure 7. Natural transition orbitals for the lowest energy transition of PCDTBT (left) and PCBSe (right). PCBT is nearly identical to PCBSe and isomitted here. Both calculations were performed using the hexamer.

Figure 8. (A) Energy of the frontier orbitals (lower = occupied; upper= unoccupied) that play a significant role in the lowest energytransition of each polymer. (B) Transition coefficient (λ) plottedagainst the NTO number for each polymer. (PCDTBT, blue triangles;PCBSe, black circles; PCBT, red squares.) PCBT and PCBSe overlapnearly completely.

Table 2. Transition Coefficient (λ) for the First 7 NTOs ofPCBT, PCBSe, and PCDTBT

PCBT PCBSe PCDTBT

NTOB3LYP(λ)

CAM-B3LYP(λ)

B3LYP(λ)

CAM-B3LYP (λ)

B3LYP(λ)

CAM-B3LYP(λ)

1 0.722 0.455 0.716 0.448 0.625 0.3342 0.195 0.248 0.197 0.246 0.226 0.2353 0.056 0.143 0.059 0.149 0.086 0.1904 0.017 0.076 0.019 0.081 0.037 0.1135 0.007 0.051 0.008 0.052 0.016 0.0656 0.001 0.009 0.001 0.007 0.004 0.0167 0.0004 0.005 0.001 0.0004 0.003 0.011



CAM-B3LYP functional causes an overestimation of the energyof approximately 0.3 eV for each polymer. The LC-BLYPexcitation energy is higher still with an overestimation of theexcitation energy of around 0.9 eV. Using CAM-B3LYP andLC-BLYP, there is also a significant change in the description ofthe excitation energy with number of repeat units. For PCBTand PCBSe, the departure from linearity in the plot of inverseoligomer length versus excitation is much faster for the CAM-B3LYP and LC-BLYP than for B3LYP, while for PCDTBT, therate of decrease in energy occurs at a lower rate (Figure 6). TheCAM-B3LYP and LC-BLYP optimized structures also showchanges related to loss of delocalization. This is reflected in theincrease of the torsional angle of ∼1.5° between the carbazole

donor and the acceptor for PCBT and PCBSe and ∼4° forPCDTBT. This is expected as the CAM-B3LYP and LC-BLYPfunctionals decrease delocalization of the electronic structure,and therefore, adding more units will not increase theconjugation path length (Table 1).It was also noted that, when using CAM-B3LYP, there is a

significant shift in the NTO description of the excitation. WhenCAM-B3LYP is used, the λ values for higher NTOs increase inrelation to the B3LYP calculation as seen in Table 2. Using thesame argument for the increase in band-type structure that wasused for B3LYP calculations, we should expect an even greaterreduction in excitation energy with increased length. However,the CAM-B3LYP higher order NTOs do not show the sameincreased delocalization that is seen for B3LYP calculations. InFigure 9, it can be seen that the higher NTOs are no moredelocalized than the lower NTOs. Several generic solutions to asloping quantum well problem are overlaid on the B3LYPportion of Figure 9 to highlight the band-structure seen inB3LYP calculations. The Coulomb potential acts as the slopingwalls of the quantum well and localizes the wave function.59

The Coulomb correction to the exchange partition in theCAM-B3LYP functional means that the Coulomb potential at adistance drops at a lower rate and that the wave function isfurther localized. This can be seen in the right side of Figure 9as the first 3 NTOs using the CAM-B3LYP method fit theparticle in a box description but are localized. Additionally, thefourth NTO is no longer delocalized, as the wall steepness ofthe quantum well in CAM-B3LYP means additional orbitalsnow find lower energy in localization rather than theproceeding to the next particle-in-a-box type orbital.Surprisingly, the LC-BLYP calculations show no localization

of the orbitals involved in the CT transition. It should be notedthat the LC-BLYP gives the poorest correlation withexperimental transition energies. Although it is possible toimprove the correlation of LC-BLYP by adjusting the μparameter, which has been shown to be necessary for largerconjugated systems, this is beyond the scope of thisstudy.37,40,41

The inclusion of solvent effects has also been shown to beimportant in the prediction of molecular properties. We haveincluded an IEFPCM field with a dielectric constant of 4.7133in the range separated calculations of optimized geometries andexcitation energies for comparison with in vacuo calculations.The changes in geometry and excitation energy for the 6 unitoligomer are presented in Table 1. There is an increase in thetorsional angle and excitation energy for the extendedoligomers of each polymer indicating some loss of conjugation.The effect of solvent field on the excitation energy is small. The

Figure 9. NTOs of PCDTBT using both the B3LYP (left) and CAM-B3LYP (right) functionals.

Figure 10. Effect of a solvent field on the TD-DFT excitation energyof range separated calculations. Stars, CAM-B3LYP; squares, LC-BLYP. Red, IEFPCM solvent field; blue, in vacuo.



solvent field decreases the predicted excitation energy of theshorter oligomers but increases the excitation of the longeroligomers (Figure 10).Excited State Dynamics. For PCBSe, a transient

absorption spectrum was collected from 350 to 500 nm(Figure 11). At short times (ns), there is a rapid ground statedepletion of the two bands centered at 342 and 476 nm and aconcurrent increase in absorbance that is narrow and well-defined relative to the PCDTBT early time absorption centeredaround 375 nm. The lifetime of this long-lived state is 1.83 ±0.1 μs. In contrast, PCBT showed no measurable excited statepopulation in the nanosecond regime as a result of its lack oftriplet population. This is supported by the high quantum yieldof fluorescence seen in PCBT. The quantum yield measured forPCBT is 52%, while the quantum yield for PCBSe is only 5%.This is expected to be a result of the increased spin orbitcoupling in PCBSe.The fluorescence quantum yield PCDTBT is 8%, much

lower than for PCBT even though there are no heavy atoms tofacilitate spin orbit coupling. One possible explanation for thisis the reduced binding energy of singlet excitons in PCDTBTgenerated by the more diffuse nature of the excited state (seeabove).60 For PCDTBT, the transient absorption spectrum wascollected from 300 to 530 nm. Initially, there is a ground statedepletion that is contained within the laser pulse. Following thisis a strongly absorbing state with a decay constant of 126 ± 12ns that dominates the visible spectrum. The decay of this statereveals a longer lived state with a lifetime of 1.56 ± 0.1 μs. Aprobable assignment for the two states are the triplet of thehigher energy excited state for the shorter lifetime state and thetriplet of the lower energy state for the longer lived state.31,32

The lower energy triplet has been widely observed, but the

triplet of the higher energy state receiving significant populationis not mentioned in recent studies of PCDTBT. The triplet isgenerally considered to be a trap state in OPV cells, andefficient population of the triplet state will have a negative effecton device efficiency. Furthermore, increased triplet populationcan lead to oxidative damage as the relatively high energy tripletis long-lived and more reactive than the ground state. However,the population of the triplet state may be reduced when thepolymer is used in the conventional manner in a heterojunctioncell as the singlet lifetime should be reduced by the rapid chargeseparation into an electron transport compound (PCBM), andthe cells are in the solid phase, as the structures will be morerigid, which often leads to reduced ISC compared to thesolution phase.

■ CONCLUSIONS

The electronic properties of three donor−acceptor copolymersbased on a carbazole donor and a benzodiazole acceptor arereported. Resonance Raman studies give experimental evidenceof the charge transfer nature of lowest energy electronictransitions in OPV copolymers where previously this had onlybeen accomplished theoretically. The lower energy transitionfor each polymer was shown to be strongly charge-transfer incharacter, with selective enhancement of Raman bands that areassociated with modes that involve motion of the acceptor unit.In PCDTBT, these enhancements are partitioned, withcarbazole modes not being enhanced with lower energyexcitation. For the higher energy transition, each compoundshows a broad enhancement of modes that include both thedonor and acceptor indicating a uniformly distributed excitedstate density. The description of stuctural distortion that is

Figure 11. Transient absorption data for PCDTBT (A,B) and PCBSe (C,D). The excitation wavelength for both is 532 nm. (A) Contour plot ofPCDTBT transient absorption. (B) Temporal evolution of the change in optical density (ΔOD) at 338 nm. (C) Contour plot of PCBSe transientabsorption. (D) Temporal evolution of ΔOD at 380 nm.



obtained from resonance Raman studies correlates well with thedescription of orbital density changes calculated by TD-DFT.TD-DFT calculations also give an indication that the effective

conjugation pathway in PCDTBT is larger than that in bothPCBT and PCBSe. This is noted in the greater linearity in thereduction of excitation energy upon increasing the oligomerlength. The origin of this difference is not readily apparentwhen simply examining the delocalization of the HOMO andLUMO, but when we consider the higher density of states forPCDTBT and higher population of diffuse orbitals in the lowerenergy excitation, we find evidence of a diffuse exciton forPCDTBT that extends as much as 11 nm. This is consistentwith experimental transient absorption studies by Etzold et al.31

and Banerji et al.30 that suggest that charge extraction fromPCDTBT proceeds faster than is possible through the charge-hopping mechanism and that charge extraction occurs at nearunity. Using the CAM-B3LYP functional decreases thecalculated delocalization of the exciton and indicates a highlylocalized excited state, but CAM-B3LYP also greatly over-estimates the excitation energy and therefore probably gives anoverly localized description of the exciton.Using nanosecond transient absorption, there is appreciable

population of the triplet state in dilute solution in bothPCDTBT and PCBSe, but triplet formation was not observedin PCBT. PCBSe and PCBT are electronically very similar, andthe increased population of the triplet of the state is thought tobe due to the heavy atom effect. As population of the tripletstate is usually considered a loss mechanism for an OPV andcan lead to oxidative damage of the cells, the use of heavieratoms to reduce the bandgap of the polymers may also carrysome negative effects for solar cell efficiency and longevity.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe would like to thank The Ministry for Science andInnovation (MSI), The International Science and Technologieslinkages fund (ISAT), The Royal Society of New Zealand, TheMacDiarmid Institute, and The University of Otago for supportduring this work. This work was also supported by NRF (2009-00605) and by the Korea Institute of Energy TechnologyEvaluation and Planning (KETEP) grant (2011T100200034)funded by the Korea government Ministry of KnowledgeEconomy.

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a spectroscopic and dft study of the electronic properties of carbazole-based d–a type copolymers

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