Investigation on Excited-State Photophysical Characteristics of Low Bandgap Polymer APFO3

Li-li Qu,Ying-hui Wng,?,Zhi-hui Kng,Yu-gung M,Hn-zhung Zhng?

a.Femtosecond laser laboratory,College of Physics,Jilin University,Changchun 130012,China

b.State Key Laboratory of Supramolecular Structure and Materials,Jilin University,Changchun 130012, China

Investigation on Excited-State Photophysical Characteristics of Low Bandgap Polymer APFO3

Li-li Qua,Ying-hui Wanga,b?,Zhi-hui Kanga,Yu-guang Mab,Han-zhuang Zhanga?

a.Femtosecond laser laboratory,College of Physics,Jilin University,Changchun 130012,China

b.State Key Laboratory of Supramolecular Structure and Materials,Jilin University,Changchun 130012, China

The excited state photophysics of low bandgap polymer APFO3 has been investigated in detail.The chemical calculations conf i rm that the intrachain charge transfer(ICT)may occur after photo-excitation and is mainly responsible for the f i rst absorption band.The transient absorption results conf i rm that ICT indeed exists and competes with the vibrational relaxation at the same time,when APFO3 is in a monodisperse system.This ICT process would disappear due to the influence of interchain interaction when APFO3 is in the condensed phase,where the exciton decay would be dominant in the relaxation process after photoexcitation.The photoexcitation dynamics of APFO3 film blending with PC61BM are presented,which shows that the exciton may be dissociated completely as the percentage of PC61BM reaches～50%.Meanwhile,the photovoltaic performance based on blend heterojunction shows that the increase of photocurrent is little if the percentage of PC61BM exceeds～50%.Overall,the present study has covered several fundamental processes taking place in the APFO3 polymer.

Conjugated polymer,Transient absorption,Intrachain charge transfer

I.INTRODUCTION

Semiconductor polymers have attracted much attention in the f i elds of commerce and science,because of their potential application in the optoelectronic f i elds, such as organic f i eld effect transistor[1],organic emitting light diode[2],and polymer solar cell[3].These organic materials own many advantages over conversional semiconductors,such as the good solution processability and the mechanical properties,and are able to allow access to generation of cheap and f l exible devices.Recently,the solar cells based on polymer have increased very much and reached～10%in 2012[4]. In order to further improve the performance of photovoltaic devices,it is necessary to synthesize the lowbandgap polymers,so as to broaden the harvesting region of photon and enhance the short circuit current of photovoltaic device[5].The alternating polyfluorene copolymer(poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-5,5-(40,70-di-2-thienyl-20,benzothiadiazole)])(APFO3), with structure shown in Fig.1,is one kind of πconjugated copolymers,which is composed of the electron-donor unit(fluorine group)and the electronacceptor units(benzothiadiazole and two thiophene units).Its absorption bandedge has reached～700 nm in the solution and its carrier mobility is also excellent, which makes it suitable for photovoltaic application[7]. In addition,APFO3 has also been used as a model polymer to investigate the charge transfer and geminate recombination processes occurring in the polymer blending heterojunction with PCB60M[8]and PCB70M[9]. Although the photophysics of heterojunction based on APFO3 have been investigated in detail,some questions still need to be interpreted,such as evolution of photophysical properties under different situations,the direct photo-generation of separated charges in the excited state,and the role and origin of possible interchain electronic species.In order to answer these questions, it is necessary to present a broad investigation on this conjugated polymer and compare the photoexcitation dynamics of conjugated polymer under different conditions.

FIG.1 Molecular structure of APFO3.

In this work,we performed a series of investigations on the photophysics of conjugated polymer APFO3 to understand its photoexcitation dynamics in a monodisperse system and a condensed phase.Through intro-ducing the PC61BM in the APFO3 film,we also detected the exciton dissociation dynamics and further prepared the photovoltaic devices.In addition,we could employ the density functional theory(DFT)to know the basic electronic transition mechanism after the photoexcitation.Finally,we provided the spectroscopic studies of APFO3 in detail.

II.EXPERIMENTS

A.Materials

The chemicals used were all purchased from Lumtec Technology without further purif i cation.The polymerdissolvedinthechlorobenzene(CB)solvent with concentration of 100μg/mL.The APFO3 and APFO3:PC61BM thin films were prepared through spin coating the sample solution in the CB solution onto 2 mm thick fused silica glass substrates.

B.Experimental details

Steady-state absorption measurements were carried out using a UV-Vis spectrophotometer(Purkinje, TU-1810PC).Photoluminescence(PL)spectra were recorded by a f i ber optic spectrometer(Ocean Optics, USB4000)with excitation pulse at 400 nm.We employed a mode-lock Ti:Sapphire femtosecond laser system(Coherent),which offered 2.0 mJ,130 fs pulses at 800 nm with a repetition rate of 1 kHz.The setup of transient absorption(TA)measurement was reported in Ref.[10].Brief l y,the output of femtosecond laser beam was split into two parts.The major one was frequency-doubled by a 1 mm thick BBO crystal to generate 400 nm pulses,which will be used as the pump beam,while the minor one was focused into a 5 mm quartz cell filled with pure water to generate a white light continuum as the probe beam.The probe beam was also focused onto the sample to overlap with the pump beam.The transmittance change of probe pulse was detected by a photomultiplier tube (Zolix,PMTH-S1-CR131)connected to the lock-in Amplif i er(SR830,DSP).Photovoltaic devices were comprised of thin film with indium tin oxide(ITO)anode, PEDOT:PSS hole transport layer,active layer of spincoated PDPP-F/PC61BM blend,and capped with Al cathode.The J-V(current density-voltage)characteristics of photovoltaic device were tested under sun-like illumination.The light source was a Xenon lamp with an AM1.5G filter of which intensity was calibrated to 100 mW/cm2.All measurements were carried out at room temperature.

C.Computational methods

FIG.2Normalizedabsorptionandemissionspectra of(a)APFO3 in solution with different polarity and (b)APFO3 film(λexe=400 nm).The concentration of solution is 100μg/mL.

The ground geometry of APFO3n(n=1-4 repeat unit)was optimized with DFT[11],B3LYP functional [12],and 6-31G(d)basis set.All calculations were performed by the Gaussian 09 program package[13].The influence of peripheral carbon chains was believed to be so small that such chains were replaced by H to avoid excessive computation demand.Electronic transition in the optical absorption of the APFO32oligomer with two repeat units was computed with time-dependent DFT (TD-DFT)[14],CAM-B3LYP functional[15],and 6-31G(d)basis.The corresponding electronic properties and geometries were calculated by assuming APFO3 oligomers to be isolated molecules in the vacuum.

III.RESULTS AND DISCUSSION

As is depicted in Fig.2(a),the absorption spectra of APFO3 in the solution with different polarity are all composed of two broad absorption bands,peaking at 389(P2peak)and 548 nm(P1peak),respectively,which exhibits that the absorption features of APFO3 are almost independent of the polarity of solvent.Their emission spectra have a broad unstructured emission band centered at 653 nm(CB),664 nm(THF), and 674 nm(CHCl3),and their corresponding bandwidth seems to be a little narrower in comparison with that of the P1band in the absorption spectra.The polarity-dependent emission spectra of APFO3 show an apparent solvatochromism behavior.The emission band red shifts as the polarity of solvent gradually enhances,implying the intrachain charge transfer(ICT) may occur in this π-conjugated polymer[16].Moreover, the emission spectra in different solvents show a good enantiomorphous feature.The bandgap could be estimated from the intersection point between the normal-ized absorption and emission spectra in Fig.2(b),which is～2.05 eV(CB),2.04 eV(THF)and 2.02 eV(CHCl3), respectively.The Stokes shift,given by the frequency di ff erence between the emission maximum and the fi rst absorption peak,is～2935 cm-1(CB),3188 cm-1(THF)and 3412 cm-1(CHCl3),respectively,and gradually increases with the increasing of solvent polarity. It is noted that the geometry of D-A type conjugated polymer in excited state could be sensitive to the solvent polarity.For the APFO3 in the condensed phase,the fi rst absorption band may broaden from～4083 cm-1(in the solution)to～5234 cm-1.Due to the interchain interaction,the P1and P2peaks in the absorption spectra red shift from～390 and～546.5 nm to～397 and～576 nm,respectively.Meanwhile,the emission peak also red shifts to 708 nm,the bandgap decreases to 1.89 eV,the Stokes shift changes to～3292 cm-1.

FIG.3(a)Optimized geometry,HOMO and LUMO of APFO3 with two repeat units.(b)Calculated absorption spectrum of APFO3 oligomer with two repeat units.(c)The repeat unit-dependent HOMO-LUMO.

The ground geometry of APFO32oligomer with two repeat units is optimized by B3LYP/6-31G(d).Herein, the donor unit(fluorine group)and the acceptor units (benzothiadiazole unit and two thiophene units)all own planarity(as seen in Fig.3(a)),but a twist appears between the donor and the acceptor units.The corresponding value is～23.9?,which may has a little influence on the ICT character.As is seen in Fig.3(a),the highest occupied molecular orbital(HOMO)is almost distributed over the whole oligomer backbone,while the lowest unoccupied molecular orbital(LUMO)is mainly localized in the acceptor units.After photoexcitation,the charge cloud in oligomers clearly redistributes through the ICT process,and eventually the charge almost distributes on the acceptor units.The simulated absorption spectrum of APFO32oligomer with DT-DFT/6-31G is presented in Fig.3(b),and the spectral shape with a“camel back”character is known to be a f i ngerprint of the donor-acceptor conjugated copolymer structure with an ICT state[17].Similarly to the experimental data,the simulated absorption spectrum of APFO3 is also composed of two absorption band, but both of them show a little blue shift in comparison with the experimental data.According to the repeat unit-dependent HOMO and LUMO shown in Fig.3(c),we f i nd that both of them gradually converge to an extremum and the bandgap gradually also decreases to a constant,indicating that the increase of repeat unit would lead to the red shift of the simulated absorption spectra and reduce the difference between the simulated absorption spectrum and experimental data.In addition,the calculated electronic transition is assumed to be in the vacuum,which may be also responsible for the difference between the theoretical and the experimental data.Using the calculated data for the oligomer with two repeating units,the contribution of the frontier molecular orbitals to the electronic transitions is analyzed.The low energy absorption(P1peak)band may be ascribed to S0→S1transition and is mainly composed of HOMO→LUMO transition,where the ICT transition is dominant in S0→S1transition.The high energy absorption(P2peak)band may be attributed to the π→π?transition and its component is so complex,and the detailed electronic transitions in APFO32oligomer are shown in Table I.

TABLEI Calculated transition energies ETand oscillator strengths f for APFO32oligomer with two repeat units.

FIG.4(a)Time-dependent TA spectra of APFO32in CB solution with concentration of 100μg/mL(λexe=400 nm). (b)Temporal evolution of TA signal at λprobe=440,560,620,and 860 nm,the solid lines are fitted results.(c)The photophysical process of APFO32in a monodisperse system.

TABLE II Best-fit parameters of TA traces of APFO3 with multi-exponential functions.

The time-dependent TA spectra of the APFO3 in the CB solution with concentration of 100μg/mL at different time are shown in Fig.4(a),and the aggregation between copolymer is expected to be insignif i cant in this concentration[18].The TA spectrum of APFO3 at 1.0 ps has four main spectral features.Two positive absorption bands at～390 and～550 nm are attributed to ground state photo-bleaching(GSPB)because of the close resemblance to the optical absorption spectrum,and the other two negative excited absorption bands at～450(EA1)and above 600 nm(EA2and EA3)may correspond to the excited state absorption.It is interestingly found the GSPB1gradually blue shifts from 570 nm to 550 nm with time,indicating that the simulated emission(SE)should be overlapped with the GSPB1in this spectral region.In order to understand the photophysical character of the excited APFO3,we exhibit the temporal evolution of EA1, GSPB1,EA2,and EA3bands at selected λprobe=440, 560,620,and 860 nm,and all of them are fitted by a multi-exponential function.The fitted results are shown in Fig.4(b)and summarized in Table II.Their different decay behaviors ref l ect that the transient species that we are monitoring come from different energy states. The temporal trace of EA3at 860 nm shows an initial decay with the lifetime of～2.0 ps,showing that some transient species directly relax from the high energy excited state.Simultaneously,the dynamics of EA1also exhibit a fast rising dynamic behavior with the lifetime of～2.5 ps and is a little longer than that of the fast dynamic component in EA3trace,suggesting that some transient species in high energy excited state may relax to the low energy excited state through vibrational thermal relaxation.In addition,the GSPB1trace offers an initial decay with time constant～1.9 ps,indicating that some transient species maybe directly come back from the high energy state to the ground state.The EA2signal at 620 nm is accordingly overlapped with a residual GSPB band and displays an instantaneous formation(＜0.5 ps),beyond our temporal resolution,and a slow rising component with a long lifetime of～11 ps, indicating that a intermediate state is generated after photoexcitation.Considering the polarity-dependent emission(Fig.2(a))and the quantum chemical calculation(Fig.3(a)),we expect that this rising process should correspond to the inter-conversion from the initial high energy excited state to the ICT state.All the photophysical processes of APFO3 in a monodisperse system are summarized in Fig.4(c).The vibrational thermal relaxation(～2 ps),the relaxation to the ground state,and the ICT process with lifetime of～11 ps may occur at the same time,and then the last transient species would all gradually come back to the ground state.Herein,our analysis provides a systemic evidence of complex relaxation processes of APFO3 polymer in the monodisperse solution system.

FIG.5(a)Transient absorption spectra of pristine APFO3 film(λexe=400 nm).(b)Wavelength-dependent TA curves of pristine APFO3 film at 590 and 860 nm.(c)The photophysical process of APFO3 in condensed system.

FIG.6 (a)TA curves of pristine APFO3 and its blend films with different concentrations of PC61BM(λexe=400 nm). (b)Current-voltage curves measured for photovoltaic devices based on blend heterojunction(wAPFO3:wPC61BM=1:1 and 1:3).(c)The photophysical process of APFO3 blend film with PC61BM.

And then,we offer the TA spectra of the pristine APFO3 film at different times(as seen in Fig.5(a))so as to understand the photophysical character of the excited APFO3 in the condensed phase.The TA spectrum at～1.0 ps has a little difference from that in the CB solution,and the component of SE disappears in the condensed phase.It contains f i ve main spectral features:three negative bands at～450,720,and 845 nm, and two positive bands at～410 and～575 nm.The positive absorption bands are also assigned to the GSPB, and three negative absorption bands are the EA part, which may correspond to the singlet-singlet(S1→Sn) photo-induced absorption.Due to the interchain interaction,the GSPB features both red shift,meanwhile all the spectral features in TA spectra decay with time, showing that no intermediate transient species appears after photoexcitation.Note that the GSPB2and EA3dynamics(as depicted in Fig.5(b)),at 590 and 860 nm, respectively,display the similar temporal decay behavior,because they are both related to transitions from the f i rst excited singlet state as depicted in Fig.5(c). At this time,the interchain exciton relaxation may be dominant in the relaxation process of excited APFO3 in the condensed phase.The dynamics of GSPB2and EA3are fitted by tri-exponential function and the fitted results are also summarized in Table II.There is a difference between the GSPB2dynamics(hτi≈486 ps)and EA3dynamics(hτi≈974 ps),which may be assigned to the superimposition of the excited state absorption and the ground state absorption.The complicated dynamic behavior of GSPB2and EA3indicates that there are rich dynamic processes occurring in the film at the same time,which maybe involves the excition-exction annihilation,the excition migration and the polaron relaxation.

When the APFO3 fi lm is blended with PC61BM,the phase segregation and the heterogeneity occur in the fi lm morphologies.After photoexcitation,the exciton would di ff use in the polymer phase and dissociate on the interface between APFO3 and PC61BM phase.Figure 6(a)offer the normalized TA curves at 780 nm of the pristine APFO3 film and the blend films with different percentage of PC61BM.After the introduction of a little PC61BM(5%),the component of fast exciton relaxation process obviously weakens,as presented in Fig.6(c),indicating that a part of photo-generated exciton may dissociate and form charge-transfer state(CT-S)with long lifetime.As the percentage of PC61BM reaches～50%,the interface area between APFO3 and PC61BM increases.As seen in Fig.6(a),the fast exciton relaxation completely disappears,indicating that the photogenerated exciton has been completely dissociated after photoexcitation and the exciton dissociation yield almost reaches～100%.In addition,the dynamic process with long lifetime should be assigned to the recombination of separated carriers.The photovoltaic devices based on blend heterojunction have been performed and the weight proportion between APFO3 and PC61BM is 1:1 and 1:3,so as to ensure the high effective exciton dissociation.Their current density-voltage(J-V)curves under the illumination of AM1.5G,100 mW/cm2are shown in Fig.6(b).The measurement results show that Vocobviously increases from 0.38 V to 0.58 V and the Jscalso improves from 5.84 mA/cm2to 5.96 mA/cm2as the percentage of PC61BM increase from 50%to 75%.Their fill factors(FF)are invariable(～0.28).Finally, the PCE could increase from 0.61%to 1.01%.The enhancement of voltage is～200 mV,which may be responsible for the improvement of photovoltaic performance.The increasing of photocurrent is only 0.12 mA/cm2after the percentage of PC61BM further increases.This indicates that the increasing of PC61BM almost couldn’t influence the generation process of free charge after the percentage of PC61BM has exceeded～50%.

IV.CONCLUSION

We systemically study the excited state photophysical character of APFO3 in the monodisperse system, the condensed phase and the mixing phase.The chemical calculations and polarity-dependent emission spectra conf i rm that the ICT really occurs after photoexcitation,and is mainly responsible for the f i rst absorption band in the red region of absorption.The TA measurements conf i rm that ICT,as an intermediate state,really exists in a monodisperse system and competes with the vibrational thermal relaxation at the same time.Due to the influence of interchain interaction,the ICT process would disappear in the condensed phase.In addition,the photo-generated exciton could dissociate in the blended film with PC61BM, whose yield would reach～100%as the percentage of PC61BM increases to～50%.The photovoltaic parameters still exhibit that the increase of photocurrent is small after the percentage of PC61BM exceeds～50%. Several fundamental situations have been covered in the study of APFO3 in excited state which is benef i cial to further understand the conjugated polymers.

V.ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China(No.21103161 and No.11274142)and the China Postdoctoral Science Foundation(No.2011M500927).

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ceived on June 25,2013;Accepted on August 7,2013)

?Authors to whom correspondence should be addressed.E-mail：yinghui-wang@jlu.edu.cn,zhanghz@jlu.edu.cn