Molecular Orientation and Ultrafast Charge Transfer Dynamics Studies

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Molecular Orientation and Ultrafast Charge Transfer Dynamics Studies on the P3HT:PCBM Blend Bruno G. A. L. Borges, Amanda Garcez Veiga, Lazaros Tzounis, Argiris Laskarakis, Stergios Logothetidis, and Maria Luiza Miranda Rocco J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08056 • Publication Date (Web): 12 Oct 2016 Downloaded from http://pubs.acs.org on October 14, 2016

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The Journal of Physical Chemistry

Molecular Orientation and Ultrafast Charge Transfer Dynamics Studies on the P3HT:PCBM Blend B.G.A.L. Borges1, A.G. Veiga1, L. Tzounis2, A. Laskarakis2, S. Logothetidis2, M.L.M. Rocco1,* 1

Institute of Chemistry, Federal University of Rio de Janeiro, 21941-909, Rio de

Janeiro, RJ, Brazil 2

Lab for Thin Films, Nanobiomaterials, Nanosystems & Nanometrology (LTFN),

Physics Department, Aristotle University of Thessaloniki, 54124, Thessaloniki, Greece *Corresponding author. Tel.: +55-21-2562-7786; Fax: +55-21-2562-7265. E-mail address: [email protected] (M.L.M. Rocco).

Abstract Ultrathin films of poly(3-hexylthiophene) (P3HT) blended with 6,6.-phenyl-C61-butyric acid methyl ester (PCBM) were investigated by X-ray Absorption Spectroscopy (XAS) and resonant Auger spectroscopy (RAS) in the context of the core-hole approach at the sulfur K absorption edge. P3HT:PCBM blend is a well-known system widely used as a bulk heterojunction (BHJ) photoactive layer in organic photovoltaics (OPV). Its morphology, phase separation and ordering of the constituent phases have been proven to significantly affect the performance of the resulting OPV devices. Herein, thermally annealed P3HT:PCBM films at optimum conditions, in terms of power conversion efficiency (PCE) of the resulting fully-printed OPV modules (2.22% for 8 cm2 active area modules), have been proven to be well-ordered films as revealed by the XAS spectra measured at different angles of the incoming photons. Moreover, electron 1

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delocalization times were calculated as a function of the excitation energy, resulting in a very low delocalization time in the femtosecond regime.

2


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1.

Introduction

Poly(3-hexylthiophene) (P3HT – Scheme 1a) and other thiophene-based polymers are among the most researched materials to be used as electron donors in photoactive blends for organic photovoltaics (OPVs), since they can be solution-processed and potentially produce low-cost, flexible and lightweight solar cells1,2. Although the commercial availability of these organic-based optoelectronics has increased, their efficiency and more importantly their lifetime still require optimization. There are several parameters, which may affect the final performance of these devices, such as the film preparation methods, the use of post-processing annealing as i.e. thermal and solvent annealing, the OPV device architecture, the ratio between donor and acceptor units in the photoactive blend, the molecular orientation, and the electronic structure of all individual components among others2-8. In this sense, understanding the occupied and non-occupied electronic structure, as well as the charge transfer dynamics is highly desirable for those materials9. For instance, previous studies have pointed that charge separation is more efficient when the difference between the lowest unoccupied molecular orbitals (LUMO) of donor and acceptor units is around 0.3 eV10,11. On the other side, the blend morphology of P3HT mixed with electron acceptors such as the phenyl-C61-butyric acid methyl ester (PCBM), and the molecular ordering have also a dramatic influence on the exciton diffusion, charge separation and charge carrier transport of such organic optoelectronic devices12-15. For oriented poly(3-hexylthiophene) films, it is considered that p-type molecular orbitals delocalization occurs mainly due to − interchain stacking, increasing the conjugation length and shifting the LUMO energy level of the polymer to a lower value16,17. 3



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Regioregular poly(3-hexylthiophene) (P3HT) is one of the good candidates for low band gap p-type polymers in bulk-heterojunction solar cells, mostly because of its high charge mobility and a band gap of 1.9 - 2.0 eV, compatible with the strongest sun light18. When blended to [6,6]-phenyl C61 butyric acid methyl ester (PCBM – Scheme 1b) and after annealing treatment, P3HT:PCBM bulk heterojunction solar cells have achieved efficiencies of more than 5% for spin coated devices3,19, encouraging further research on this system.

O

O S

n

P3HT

PCBM

(a)

(b)

Scheme 1. Chemical structures of (a) Poly(3-hexylthiophene) – P3HT and (b) [6,6]-Phenyl C61 butyric acid methyl ester (PCBM).

In this work, our goal was to investigate the electronic structure, molecular orientation and ultrafast charge transfer dynamics of the P3HT:PCBM blend that has been printed onto flexible PET substrate by gravure method in to realize fully printed OPV modules. The 45 cm2 OPV modules (active area of the module is 8 cm2) fabricated in our 4


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previous work4 were composed of 8 interconnected cells and exhibited a maximum power conversion efﬁciency (PCE) of 2.22% at the optimum thermal annealing conditions of the P3HT:PCBM blend, which were found to be 140 °C for 1 min. To obtain information about the charge transfer dynamics, we applied here the core-hole clock approach (CHC) using resonant Auger spectroscopy, which emerges as an alternative for pulse laser pump-probe spectroscopy, with some advantages20,21. We have been using the CHC method systematically on thiophene-based polymer films and their blends, probing ultrafast charge transfer dynamics in the femtosecond regime22-26. The electronic structure and molecular orientation were evaluated through angulardependent S-K edge X-ray Absorption Spectroscopy (XAS) and X-ray Photoelectron Spectroscopy (XPS), which are very well-known techniques that have shown great potential to characterize low band-gap polymer films14,16,27-29. The XAS and RAS results of this study fully corroborate the optimum conditions of annealing for the maximised PCE of the OPV modules fabricated in our previous work. 2.

Experimental Section

The P3HT:PCBM blend was gravure printed onto zinc oxide (ZnO) films, printed initially by gravure, onto commercially available substrates consisting of heat stabilized PolyEthylene Terephthalate (PET) (thickness: 175 µm) with Indium Tin Oxide (ITO) (PET-ITO sheet resistance: ~50 Ω/sq). All details for the PET-ITO patterning can be found elsewhere4. Zinc oxide (ZnO) ink consisted of nanosized ZnO particles was used for the electron transport layer (ETL) for the OPV devices. The ZnO nano-particle ink was printed by gravure at a speed of 18 m/min and dried on a hot plate at 140 °C for 1 min. The Poly(3-hexylthiophene) (P3HT, Advent) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM, technical grade, 99.5%, Solenne BV) were blended to form the 5



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active bulk heterojunction (BHJ) layer (1:0.8 in o-DCB with a concentration of 160 mg/ml). P3HT:PCBM (160 mg/ml) was prepared in o-DCB the previous day and kept under stirring overnight at 80°C in N2 environment to dissolve completely the two blend components. Once the ZnO layer was dried the P3HT:PCBM was printed by gravure using the same printing speed of 18.0 m/min and 140 °C for 1 min for the dryingannealing time. Figure 1 illustrates schematically the sheet-to-sheet (S2S) gravure printing technique used throughout the OPV module fabrication and specifically the P3HT:PCBM layer studied in this work. The engraved plate surface pattern consisted of cells with tone at 100%, a line density of 120 lines/cm, and the pyramidal cells had a nominal volume of 12.7 ml/m2. The arrows indicate the substrate which is fixed onto the rolling cylinder, the blade for ink removal, the printing direction and the engraved cells. The pattern on the printing plates consisted of stripes with a width of 7 mm and length of 46 mm.

Fig. 1. Schematic illustration of the sheet-to-sheet (S2S) gravure printing process.

The XAS and RAS experiments were carried out at the Brazilian Synchrotron Light Source (LNLS), using the Soft X-ray Spectroscopy (SXS) beamline. The SXS is equipped with a Si(111) double crystal monochromator, which provides a photon bandwidth of 0.38 eV around the sulphur K-edge. Beamline details and experimental 6


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set-up can be found elsewhere26. S-K edge XAS spectra were acquired using the Total Electron Yield (TEY) mode by measuring the electron current at the sample (drain current) simultaneously with a photon flux monitor (Au grid). The LIII transition of metallic Molibdenium (2p3/2 4d) was used for energy calibration of the XAS spectra. S-K edge angular dependent XAS spectra were obtained by changing the angle between the incoming X-ray beam and the sample surface. Auger decay spectra were measured using a hemispherical electron energy analyzer with a pass energy of 20 eV. No damage effects were observed for photoabsorption or Auger decay spectra. A linear combination of Gaussian (G) and Lorentzian (L) peak shape function was used for curve fitting analysis of the resonant Auger spectra, measured at various photon energies around the S-K edge, and a Shirley background function was selected for an electron kinetic energy ranging from 2103 to 2120 eV.

3.

Results and Discussion

Figure 2 presents the angle-dependent S-K edge XAS spectra for P3HT:PCBM blends deposited over a PET-ITO substrate covered with ZnO. The XAS spectrum shows the electronic transitions from Sulphur 1s electron to unoccupied states. The first and most intense peak is assigned to the overlapping of the S 1s π* and S 1s σ* (S-C) transitions, both lying close in energy (the energy difference is around 1 eV) and labeled as B and C, respectively. A less intense peak can be observed at 2473.2 eV photon energy (labeled as D) at normal incidence (90 degrees) and it is better attributed to another S 1s σ* (S-C) type transition, whose transition moment is perpendicular to the polymer backbone, as previously discussed by H. Ikeura-Sekiguchi et al.16 and R. Onoki et al.30. XAS spectra for P3HT:PCBM blend (Figure 2) show opposite behavior of their resonances as moving from normal (90 degrees) to grazing (20 degrees) 7



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incidence angles. An enhancement of the B and D transitions is clearly observed at grazing incidence while the C transition becomes appreciable at normal incidence. These data point out to well-ordered films, with thiophene rings oriented preferably along the substrate normal. It is well-established that the thin film morphology is strongly affected by the processing conditions such as deposition methods4,5,13,31-33. Upon deposition from solution, two molecular orientations are mainly reported for P3HT thin films: edge-on and face-on, where the first one is characterized by π-stacking interactions with thiophene rings perpendicularly oriented with respect to the substrate plane. In face-on orientation, the thiophene rings lie flat on the substrate while πstacking interactions are oriented along the substrate normal. For films prepared under slow casting methods, realized under conditions close to the equilibrium, the edge-on orientation is assumed to be the thermodynamically favorable one

13,34-37

. Furthermore,

previous works on S-K edge XAS spectra for regioregular P3HT have also pointed to edge-on orientation for the thiophene ring16,28. In addition, the dependence of the processing conditions of the P3HT:PCBM blends with their morphology is also confirmed by the investigation of the vertical distribution of the P3HT and PCBM constituents in blend by Spectroscopic Ellipsometry (SE) measurements and their analysis by the Bruggeman Effective Medium Approximation in combination with an exponential gradient model. As it has been reported, the printing process of P3HT:PCBM films and subsequent annealing (at 140 °C for 1 min) on ZnO surfaces results to a blend structure that includes an enrichment of the top regions of the blend with the electron donor (P3HT) whereas the acceptor is segregated to the bottom region of the blend4,8. This is a more thermodynamically favorable morphology, mainly attributed to the differences in the surface energy of the P3HT and PCBM and the induced dipole–dipole interactions between PCBM and the ZnO layer4,8. This 8


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segregation of P3HT closer to the BHJ film surface is also correlated to higher surface roughness values and specific π-stacking formations of P3HT in the blend.

C B A

D

E

F

θ = 90o

θ = 70o I

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


θ = 45o

θ = 30o

θ = 20o

2470 2475 2480 2485 2490 2495 Photon Energy (eV)

Fig. 2. S1s angle-dependent XAS spectra of P3HT:PCBM blend thin film deposited on PET/ITO/ZnO substrate. The selected photon energies used for Sulphur KL2,3L2,3 Resonant Auger Decay spectra (labeled as A, B, C, D and E) are also shown.

Although it was shown that PCBM may inhibit the self-organization of polymers, like thiophene and benzothiadiazole copolymers14, it seems here that the inclusion of PCBM

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didn’t disturb the self-organization of the polymer in the blend, prepared under these conditions. Sulfur KL2,3L2,3 Auger decay spectra were acquired by tuning the excitation energy. Figure 3 plots P3HT:PCBM blend film Auger decay spectra (RAS) at the photon energies labeled from A to F in the XAS spectra, after the deconvolution procedure. As observed previously for poly(thiophene)22, poly(bithiophene)26 and other thiophenebased polymeric systems23,24,25, those spectra are characterized by the presence of different Auger decay channels: The feature at around 2112 eV constant kinetic energy is assigned to a 1D2 Normal Auger Decay channel (2 holes state), whose intensity increases from A to F incident photon energies and is dominant after the ionization potential energy (E and F photon energies). At the resonance energy positions (A, B, C and D), other peaks are observed, with different kinetic energy values, characterized by very well-known spectator Auger decay channels (2 holes and 1 excited electron final state). As discussed in our previous works22-26, the presence of both spectator and normal Auger decays at the resonance energies is an evidence of electron delocalization during the Auger decay occurring in the femtosecond regime. For the deconvolution procedure, the normal Auger peak position was kept at around 2112 eV while the other peaks could shift, and the full width at half maximum (FWHM) values of the normal Auger signal was maintained higher than the spectator contributions since the electrons involved in the charge transfer process have a more delocalized character38.

10


spectators 2h-1e

Normal 2h 1 D2

hv = 2470.8 eV 2108

2110

2112

C

B

CPS

CPS

A

hv = 2471.2 eV 2114

2116

2118

2120

2108

2110

Kinetic Energy (eV)

2112

hv = 2472 eV 2114

2116

2118

D

2112

2116

Kinetic Energy (eV)

2118

2120

2114

2116

2118

2120

2118

2120

CPS

hv = 2479.2 eV 2114

2112

F

CPS 2110

2110

Kinetic Energy (eV)

E

hv = 2473.2 eV 2108

2120 2108

Kinetic Energy (eV)

CPS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CPS

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2108

2110

2112

hv = 2490 eV 2114

2116

Kinetic Energy (eV)

2118

2120 2108

2110

2112

2114

Fig. 3. S KL2,3L2,3 Resonant Auger decay spectra for P3HT:PCBM blends at selected photon energies of the XAS spectrum (labeled as A, B, C, D, E and F, respectively).

Charge transfer times ( ) for P3HT:PCBM blends were derived using the branching ratios of the spectators and normal Auger signals through the core-hole clock method21,22,39, using 1.27 fs as the value for sulphur core-hole lifetime40. Through that method, the core-hole lifetime is used as an internal clock to monitor the electron delocalization dynamics in the femtosecond down to hundred attosecond range. The results are presented for each photon energy (A to F) in Table 1, indicating that for this system charge transfer dynamics occurs in the low femtosecond regime. All derived values are below 9 fs, with a maximum value of 8.49 fs at the resonance photon energy labeled as B. When compared to other thiophene-based polymeric films already measured22-26, this P3HT:PCBM blend showed the lowest charge transfer time ever derived at the S-K absorption edge using the core-hole clock methodology, which is 11


2116

Kinetic Energy (eV)


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also consistent with its high efficiency in devices and one of the most successful material for BHJ solar cell. Table 1. Charge transfer times ( ) values determined for A to F labeled XAS photon energies in P3HT:PCBM blend films.

4.

Label

Photon Energy (eV)

(fs)

A

2470.8

7.19

B

2471.2

8.49

C

2472.0

5.76

D

2473.2

1.69

E

2479.2

0.62

F

2490.0

0.22

Conclusions

S-K edge XAS spectra of P3HT blended with a fullerene derivative molecule (PCBM) have been obtained varying the incidence angle of the incoming photon. Well-ordered films were measured with a preferred edge on geometry of the thiophene ring. Resonant Auger spectroscopy following S K-edge photoexcitation was employed taking advantage of the core-hole clock approach. As a consequence, electron delocalization times were derived which occurred in the femtosecond regime, which are much shorter than that measured for polythiophenes and related polymers blended with fullerene. These spectroscopic findings corroborate the results obtained so far concerning the potential application of this methodology to gain information on the dynamics of polymeric systems. Finally, the XAS and RAS results and the findings thereof well 12


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support the optimum conditions found for annealing the P3HT:PCBM for the maximised PCE of the OPV modules fabricated in our previous work.

Acknowledgments Research partially supported by LNLS – National Synchrotron Light Laboratory, Brazil. M.L.M.R. would like to thank CNPq for financial support. The authors would also like to acknowledge the technical assistance of the soft X-ray group from LNLS. The NMP.2012.1.4-1 Project SMARTONICS (under Grant Agreement number 310229) part of the European Union Seventh Framework Programm is gratefully acknowledged for partially funding this work.

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Donor/acceptor Copolymers and Fullerene: Experimental and Theoretical Approaches.

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Tables 2001, 77, 1-56.

19



P3HT:PCBM S 1s XAS θ = 90o

Intensity (a.u.)

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θ = 20o

2470 2475 2480 2485 2490 2495 Photon Energy (eV)

20


Molecular Orientation and Ultrafast Charge Transfer Dynamics Studies

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