Luminscent Graphene Quantum Dots for Organic Photovoltaic Devices

Luminscent Graphene Quantum Dots for Organic Photovoltaic Devices

Vinay Gupta,*Neeraj Chaudhary,Ritu Srivastava,Gauri Datt Sharma,Ramil Bhardwaj,and Suresh Chand

Organic and Hybrid Solar Cell Group,National Physical Laboratory,Dr.K.S.Krishnan Marg,New Delhi 110012,India

b

Supporting Information ABSTRACT:Recent research in organic photovoltaic (OPV)is largely focused on developing low cost OPV materials such as graphene.However,graphene sheets (GSs)blended conjugated polymers are known to show inferior OPV characteristics as compared to fullerene adduct blended with conjugated polymer.Here,we demonstrate that graphene quantum dots blended with regioregular poly(3-hexylthiophene-2,5-diyl)or poly(2-methoxy-5-(2-ethylhexyloxy)-1,4phenylenevinylene)polymer results in a signi ?cant improvement in the OPV characteristics as compared to GSs blended conjugated polymers.This work has implications for inexpensive and e ?cient solar cells as well as organic light emitting diodes.

T

he widespread commercialization of conjugated polymer-based organic photovoltaic (OPV)devices remains limited due to the use of high-cost donor and acceptor materials.1Therefore,development of new cost-e ?ective materials 2is critical for the future of OPVs.Quantum dots (QDs)(e.g.,CdSe,CdTe,PbTe)are emerging as the leading acceptor materials in photovoltaics 3,4due to their size-tuned optical response,e ?cient multiple carrier generation,and low cost.5However,their toxicity and hazardous nature are serious impediments for large-scale device applications.Therefore,benign nanomaterials with similar properties are needed.Graphene has been shown to be a good electron acceptor,with mobility as high as 7?104cm 2V à1s à1.Higher carrier motilities means that charges are transported to respective electrodes more quickly,which reduces current losses via recombination and therefore improves the e ?ciency of a solar cell.4à10However,solar cells based on conjugated polymers/soluble graphene 11à13or graphene oxide (GO)13,14show much inferior characteristics as compared to fullerene acceptor-based solar cells.1Theoretical and experimental studies of graphene have indicated that its bandgap and hence optical properties can be manipulated by reducing its size to the nano level.15à18Here,we show that a conjugated polymer blended with graphene quantum dot (GQD)acceptors exhibits a signi ?cant enhance-ment in OPV characteristics as compared to the corresponding conjugated polymer-GS blends.

The GQDs were synthesized from GSs by a hydrothermal approach as described elsewhere.19In brief,GSs were obtained by thermal oxidation of GO sheets.The GSs were subjected to hydrothermal treatment at 200°C,followed by a dialysis process.The dialysis process allowed separation of GQDs from larger molecules through a semi-permeable membrane with retained

molecular weight of 3500Da,as shown in Figure S1of the Supporting Information (SI).The transmission electron micro-scopy (TEM)image of the synthesized GQDs is shown in Figure 1a.The fast Fourier transform (FFT)of GQDs is shown in the inset of Figure 1a,which con ?rms that most GQDs consist of 1à2layers.20The particle size distribution measurements of GQDs were performed using a laser scattering particle size analyzer (HORIBA,LB 500X).The sizes were mainly in the range of 5à15nm (9nm average),as shown in the Figure 1b.The X-ray di ?raction and Raman spectra of GQDs show characteristics of sp 2carbon (Figure S2in SI)with disorder due to scattering at the edges.21

For solar cell applications,we have functionalized GQDs with aniline (ANI),similar to phenyl isocyanate used in the case of GSs.11For organic light emitting diode (OLED)applications,?uorescent dye is often mixed with polymer to make the emitting layer.22Here,in order to see the direct e ?ect of GQDs on the performance of poly(2-methoxy-5-(2-ethylhexyloxy)-1,4pheny-lenevinylene)(MEH-PPV)polymer,methylene blue (MB)dye is used because it is non ?uorescent (Figure 2b).The method

Figure 1.(a)TEM image,(b)diameter distribution,and (c,d)FTIR spectra of the GQDs.

Received:

September 25,2010

of preparation of ANI-GQDs and MB-GQDs is detailed in the SI.In brief,GQDs were ?rst treated in HNO 3to attach carboxyl groups 23(carboxylation reaction,scheme 1,SI).The alkyl chloride synthesis method was then followed,in which the carboxylated GQDs were reacted with SOCl 2in the presence of dimethylfor-mamide (catalyst).This resulted in the formation of COCl-GQDs 22,23(scheme 2,SI).After that,the acylation reaction was followed,in which aniline reacted with acyl chlorides (COCl-GQDs in this case),and C 6H 5-NH-CO-GQD (ANI-GQDs)anilide was formed 23à25(scheme 3,SI).The same acylation reaction was also used to attach MB to carboxylated GQDs (scheme 4,SI).These surface exchanges were analyzed by Fourier trans-formed infrared (FTIR)spectroscopy.The FTIR spectrum of GQDs (Figure 1c(i)),after carboxylation (Figure 1c(ii)),con-?rms the functionalization of GQDs with carboxyl group 23(àCOOH,νC d O ≈1721cm à1)and hydroxyl O àH group (νO àH ≈3423cm à1).Further,acylation of the carboxyl group resulted in the splitting of νC d O into a doublet 26(àCOCl,νC d O =1731and 1772cm à1),as shown in Figure 1c(iii).The FTIR spectrum of the ANI-GQDs (Figure 1c(iv))shows a shift of the C d O band to lower frequency (νC d O =1611à1671cm à1),23,27which indicates the formation of an amide bond.In addition,the presence of new bands at 1492and 1308cm à1corresponding to N àH in-plane and C àN bands,27respectively,con ?rms the amide functionalization.The modi ?cation of the FTIR spectra after treatment with MB con ?rms the formation of MB-GQDs,as shown in Figure 1d.The functionalization of GQDs was corroborated by X-ray photoelectron spectroscopy (Figure S3,SI)Thermogravimetric analysis (Figure S4,SI)showed weight losses of 9.6,56.6,and 62.9%for COOH-GQDs,ANI-GQDs,and MB-GQDs,respectively,which also con ?rms the functiona-lization of GQDs.11,12

The UV àvis absorption spectra and the optical images of GQDs,ANI-GQDs,and MB-GQDs (in ethanol)are shown in Figure 2a.The spectrum of GQDs shows typical absorption peaks at 292and 325nm,which are very di ?erent from the GSs peaks at ca.230nm due to πf π*transition of aromatic sp 2domains.19The two electronics transitions of 325nm (3.81eV)

and 292nm (4.24eV)observed in the photoluminescence (PL)spectrum of GQDs can be regarded as transitions from σand πorbitals,viz.highest occupied molecular orbital (HOMO)to lowest unoccupied molecular orbital (LUMO).19,28The UV àvis absorption spectra of pure aniline and MB are shown in Figure S5(SI).The UV àvis spectrum of ANI-GQDs (Figure 2a)gives a broad absorption peak in the visible region centered at ～525nm,which extends to 800nm,and its color is pink.The UV àvis spectrum of MB-GQDs is narrow,and its color is green (Figure 2a).Both aniline and MB do not show any detectable PL in the visible region (Figure 2b).The PL emission of GQDs (excitation wavelength E ex =325nm)appears at 430nm,13giving blue luminescence (inset of Figure 2b).19The PL spectrum of the GQDs changes signi ?cantly after functionalization.The PL spectrum of ANI-GQDs (Figure 2b)is extended from 400to 700nm (centered at 510nm);hence,the luminescence is changed from blue to white (inset of Figure 2b).The PL spectrum of MB-GQDs (475à700nm)is narrow (Figure 2b)and extended from 475to 700nm (centered at 525nm),giving green luminescence (inset of Figure 2b).The origin of PL in graphene depends on the chemical nature of the graphene edges,which is explained in detail by Radovic et al.28They proposed that the zigzag edges sites of graphene are carbine-like,with the triplet ground state being most common.Not only is this proposal consistent with the key electronic properties and sur-face (re)activity behavior of carbons,28but it can also explain the enhanced PL (Figure 2)in GQDs,19PEG-ylated nano graphene oxide,29CNTs,30and carbon nanoparticles.31

The PL emission spectra (E ex =510nm)of the ?lms of poly(3-hexylthiophene-2,5-diyl)(P3HT)blended with ANI-GQDs in di ?erent wt %ratios (0,0.5,1,3,5wt %)are shown in Figure 2c.The PL shows quenching behavior with saturation at 1wt %ANI-GQDs.In comparison,1wt %of nonfunctionalized GQDs do not show any quenching e ?ect.Among the three mechanisms for PL quenching,namely energy transfer,exciplex formation,and charge transfer,only the excited-state charge transfer (CT)can account for the PL quenching of P3HT because of the absence of additional spectral features in the PL spectra of the blends as compared to P3HT.In the CT process,the photo-induced excitons generated in P3HT are dissociated at the heterojunction interface between P3HT and ANI-GQDs,which leads to a nonradiant relaxation of the excitons.The PL emission spectra (E ex =450nm)of the ?lms of MB-GQDs blended with MEH-PPV in di ?erent wt %ratios (0,0.5,1,3,5wt %)are shown in Figure 2d.The PL emission of MEH-PPV shows an intense peak at 590nm accompanied by a shoulder located at 640nm.Also there is no new peak with the addition of MB-GQDs.As the MB-GQDs is added,the PL intensity of MEH-PPV is increased,and after 1wt %of MB-GQDs the PL intensity is decreased.At low concentrations,the enhancement in the PL intensity is been attributed to an energy transfer from GQDs to the MEH-PPV matrix,and after an optimum concentra-tion the energy transfer is from MEH-PPV to GQDs,which results in quenching.As we increase the concentration of GQDs,their agglomeration takes place,and because of agglomeration,phase separation takes place.In comparison,1wt %of nonfunctionalized GQDs do not show any quenching e ?ect at all due to ine ?ective blending,as discussed later.It is seen that the photoemission of the blended ?lm exhibits much higher luminescence intensity for 0.5à3wt %of MB-GQDs than that of the pure MEH-PPV.Quantitative PL intensity measurements,however,depend crucially on the alignment and illumination geometry,and in the

Figure 2.(a)UV àvis absorption,(b)PL spectra of GQDs,ANI-GQDS,MB,Aniline,and MB-GQDs (c)PL spectra of

P3HT/ANI-GQDs,and (d)PL spectra of MEH-PPV/MB-GQDs.

case of ?lms it should also re ?ect the local morphology.Co-casting of P3HT and ANI-GQDs as well as MB-GQDs could very well in ?uence the ?lm morphology,leading to di ?erent and spatially varying P3HT concentrations.To ascertain quenching or radiative rate modi ?cations,the excited-state dynamics of P3HT/1wt %ANI-GQDs (Figure 3a)and P3HT/MB-GQDs (1wt %)(Figure 3b)were measured by time-resolved PL spectroscopy.The contrasting PL decay behavior of ANI-GQDs and MB-GQDs is quite evident as PL decay of the P3HT/MB-GQDs (1wt %)(τ≈10à10s)is an order of magnitude faster than that of the P3HT/ANI-GQDs (1wt %)(τ≈10à9s).P3HT/ANI-GQD-based hybrid solar cells were fabricated by spin-casting a solution of 15mg mL à1P3HT in dichlorobenzene (DCB)with ANI-GQDs contents of 0.5,1,3,and 5wt %(ratio to P3HT)onto indium tin oxide (ITO),coated with poly(ethylene dioxythiophene)àpolystyrene sulfonic acid (PEDOT:PSS)con-ductive polymer,which acted as the bottom electrode.LiF and Al were then vacuum deposited as the top contact (Figure S6in SI)to form an OPV device having the structure ITO/PEDOT:PSS/P3HT:ANI-GQDs/LiF/Al.11For comparison,the solar cells of P3HT/ANI-GSs with ANI-GSs contents of 0.5,1,3,5,10,and 15wt %(ratio to P3HT)were also fabricated.The solar cell characteristics were measured under 100mW AM 1.5G illumina-tion.The obtained power conversion e ?ciency (η),open-circuit voltage (V oc ),short-circuit current density (J sc ),and ?ll factor (FF)are summarized in Table 1.It can be seen that the FF of the P3HT/ANI-GQDs hetrojunction device is much higher (0.53)

as compared to ～0.33for GSs.11,12Current density versus voltage (J àV )curves of the ANI-GQD and ANI-GS (optimized)PV devices are plotted in Figure 4.The correspon-ding band diagram is given in the inset of Figure 4.Maximum values of η=1.14,V oc =0.61V,J sc =3.51mA cm à2,and FF =0.53were obtained for 1wt %ANI-GQD in P3HT.In comparison,maximum values of η=0.65,V oc =0.88V,J sc =2.65mA cm à2,and a low FF =0.28were obtained for 10wt %ANI-GSs in P3HT,similar to those reported earlier.11,12The corresponding band diagram is shown in the inset of Figure 4.The LUMO and HOMO of GQDs,determined by cyclic voltammetry (CV),32were à3.55and à5.38eV,respectively (Figure S7,SI).The position of the LUMO (or work function)of GQDs between those of P3HT 1and Al suggests their suitability for OPV applications.

Since the photocurrent is mostly limited by photoinduced charge carrier generation and transport,the nanoscale morphol-ogy of the P3HT/ANI-GQDs blend ?lm is an important factor for the determining the value of FF.The atomic force microscopy (AFM)image of the P3HT/ANI-GSs ?lm (Figure 5a)shows that there are large domains (about 100à200nm diameter),indicating large-scale phase separation and much larger than the di ?usion length of excitons (10nm),whereas the AFM image of P3HT/ANI-GQD shows uniform and ?ne features,suggesting nanoscale phase separation.This results in enhancement of the exciton migration to the donor/acceptor interface,resulting in a decrease in the resistance and a corresponding increases in the FF.So the improved morphology of the P3HT/ANI-GQD results in improved performance of the solar cell.

OLED devices were fabricated under the optimized condition from MEH-PPV,MEH-PPV/GQDs(1%),and MEH-PPV/MB-GQDs(1%)solution in DCB by a process similar to that used for a solar cell device,i.e.ITO/PEDOT:PSS/MEH-PPV:MB-GQDs/LiF/Al (Figure S8,SI).Figure 6shows the J àV curves of

Figure 3.Time resolved PL spectra of P3HT/ANI-GQDs and P3HT/MB-GQDs co-casting ?lms excited at 510nm.

Table 1.Performance Details (V oc ,J sc ,FF and η)of the P3HT/ANI-GQDs and P3HT/ANI-GSs under Simulated AM 1.5G 100mW Illumination

type of graphene graphene wt %V oc [V]J sc [mA cm à2]FF

η[%]à00.430.0370.210.003ANI-GQDs 0.50.62 2.650.470.77ANI-GSs 0.50.710.150.190.02ANI-GQDs 10.61 3.510.53 1.14ANI-GSs 10.720.190.220.03ANI-GQDs 30.58 1.320.510.39ANI-GSs 30.860.550.270.13ANI-GQDs 50.590.360.520.12ANI-GSs 50.94 1.50.330.46ANI-GSs 100.88 2.650.280.65ANI-GSs

15

0.95

0.31

0.250.07

Figure 4.J àV characteristics of the photovoltaic devices based on ANI-GQDs with di ?erent GQDs content and ANI-GS (under optimized condition)annealed at 160°C for 10min,in AM 1.5G 100mW illumination.

Figure 5.AFM images

of (a)

P3HT/ANI-GSs,(b)P3HT/ANI-GQDs,and (c)MEH-PPV/MB-GQDs.

the di ?erent OLEDs.The corresponding band diagram is given in the inset of Figure 6.The turn-on voltage for the pure MEH-PPV sample (Figure 6a)is ～6V and is decreased to ～4V for MEH-PPV/MB-GQDs(1%)(Figure 6c).At higher concentra-tion (3%MB-GQDs),charge trapping as well as a shortening e ?ect is observed,possibly due to agglomeration (Figure 6d).The enhancement of the maximum light-emission intensity after blending MEH-PPV with MB-GQDs directly re ?ects the en-hanced e ?ciency.MEH-PPV/MB-GQDs(1%)exhibits strong yellow emission,as compared to the bright orange emission of MEH-PPV.The inset of Figure 6shows the corresponding electroluminescence (EL)spectra.The EL spectrum of MEH-PPV/MB-GQDs(1%)starts from lower wavelength as compared to MEH-PPV.The MB-GQDs dispersed in the MEH-PPV provides more electrical transport paths,which results in an enhancement of charge injection and hence causes increase in the carrier density,thus requiring lower turn-on voltage and much higher e ?ciency.The AFM image of MEH-PPV/MB-GQDs (Figure 5c)also shows that the ?lm has uniform and ?ne features.In conclusion,we have demonstrated that the GQDs dis-persed in conjugated polymers show enhanced OPV and OLED characteristics as compared to GSs due to improved morpholo-gical and optical characteristics.The performance of GQD-based devices can be further improved by choosing other polymers or di ?erent types of functionalization.GQDs can be a cost-e ?ective,environmentally friendly,and more stable material for photo-voltaics than current organic materials.

’ASSOCIATED CONTENT

b

Supporting Information.

Detailed characterization of GQDs,ANI-GQDs,and MB-GQDs;UV àvis spectra of anilne and MB.This material is available free of charge via the Internet at https://www.sodocs.net/doc/744232431.html,).

’AUTHOR INFORMATION

Corresponding Author

drvinaygupta@https://www.sodocs.net/doc/744232431.html,

’ACKNOWLEDGMENT

This work was supported by Indo-UK project “Advancing the e ?ectiveness and production potential of excitonic solar cells (APEX)”.The authors to thank the director,NPL,for his support and Dr.B.K.Gupta for assistance in absorption and PL measurements.’REFERENCES

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Figure 6.Measured current density of MEH-PPV with MB-GQDs (a)0%,(b)0.5%,(c)1%,and (d)3%as a function of the applied voltage (V ).The inset plots the electroluminescence spectrum of the MEH-PPV (red)and MEH-PPV/MB-GQDs (1%)(black).The inset also shows the band diagram of the MEH-PPV/MB-GQDs and the recorded brightness of MEH-PPV LED and MEH-PPV/MB-GQDs (1%)LED.