Thickness Effect on Structural Defect-Related Density of States and

Article pubs.acs.org/JPCC

Cite This: J. Phys. Chem. C 2018, 122, 5881−5887

Thickness Effect on Structural Defect-Related Density of States and Crystallinity in P3HT Thin Films on ITO Substrates Vojtech Nádaždy,† Katarína Gmucová,†,* Peter Nádaždy,† Peter Siffalovic,† Karol Vegso,† Matej Jergel,† František Schauer,‡ and Eva Majkova† †

Institute of Physics, Slovak Academy of Sciences, Dúbravská cesta 9, 845 11 Bratislava, Slovak Republic Faculty of Applied Informatics, Tomas Bata University in Zlín, Nad Stráněmi 4511, 760 05 Zlín, Czech Republic

‡

S Supporting Information *

ABSTRACT: We report on a study of thickness effect on the formation of structural defect-related density of states (DOS) in the band gap of poly(3-hexylthiophene-2,5-diyl) (P3HT) thin films spincoated on ITO substrates. The energy-resolved electrochemical impedance spectroscopy and grazing-incidence wide-angle X-ray scattering were used to correlate the DOS with the degree of crystallinity in P3HT thin films. We found an exponential increase of the defect DOS in the band gap with increasing fraction of the amorphous phase when decreasing the film thickness. The exponent increases abruptly when reducing the thickness down to 30 nm, which indicates two thickness regions with different dynamics of the defect DOS formation driven by increasing the fraction of the amorphous phase. Moreover, we observed the co-existence of two P3HT polymorphic crystalline phases with different backbone spacings, which results in the appearance of a peculiar DOS satellite peak above the highest occupied molecular orbital. The volume of the minor, more dense, crystalline phase exhibits a thickness dependence with a maximum plateau around 40 nm. These results suggest an important effect of the substrate roughness on the crystallinity and polymorphism of P3HT thin films depending on the film thickness with general implications for polymer thin films.

■

INTRODUCTION The defect-related electronic states in the band gap have a profound influence on the efficiency of organic electronic devices such as solar cells,1 organic field-effect transistors,2 organic light-emitting diodes,3 and recently electrochemical transistors.4 Two classes of defects can be distinguished in organic semiconductors: extrinsic and intrinsic. The extrinsic defects are induced by chemical impurities in the semiconductor, whereas the intrinsic defects arise from the structural disorder in semiconductor bulk and/or surface. The origin of chemical defects is attributed to the influence of ambient atmosphere and synthesis residues. The structural defects perturb the surrounding energy levels and generate new electronic states in the band gap. As their effect is weak, no covalent bonds are altered.1 Recently, we have reported on the changes arising in the fine band gap electronic structure of the regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) as a consequence of the thin film exposure to the ambient air.5 The oxygen and water-related defects have been identified by comparing the energy levels of the observed defect states with the data from the literature that were obtained by a combination of optical and electrical spectroscopic methods or first-principles calculations. Preparation of the samples in an inert atmosphere prevents the formation of oxygen and water-related defects. However, © 2018 American Chemical Society

structural defects play an important role in the performance of organic electronic devices as well. The structure of solutionprocessed thin films of semiconducting conjugated polymers is characterized by ordered crystalline nanodomains embedded in an amorphous matrix.6 The density of states (DOS) related to the intrinsic structural defects may vary in these materials depending on the crystallinity7 that affects many microphysical properties.8 For example, charge transport is typically limited by the hopping process, resulting in low charge carrier mobilities (98%, the number average molecular weight Mn 45 000−65 000 g mol−1, and the polydispersity index (PDI) < 2 was purchased from SigmaAldrich. The ITO-coated glass substrates with sheet resistance of 7 Ω per sq and the rms surface roughness of 7 nm were delivered by Präzisions Glas & Optik, Germany. The ITO substrates were cleaned using ultrasonic bathing in acetone and isopropylalcohol. After drying out, the substrates were treated with UV−ozone for 10 min. Then, P3HT thin films were deposited by spin-coating at the room temperature in the nitrogen atmosphere in the glovebox. Before spin coating, all solutions were filtered using a 200 nm PTFE filter. The solvent annealing was performed in a Petri dish with the volume of 18 cm3 for 30 min straight after the spin-coating. The samples were then immediately thermally annealed at 110 °C for 5 min with 30 s rise time from the room temperature. The cooling down took place spontaneously on the tissue paper. The film thickness was measured with a Dektak 150 surface profiler (Veeco, USA). The film thicknesses with determined experimental errors, solution concentrations, and spinning rates are summarized in Table 1. Table 1. P3HT Thin Film Preparation Details film thickness (nm)

concentration (wt %)

spinning rate (rps)

7±2 10 ± 2 16 ± 2 30 ± 3 40 ± 3 50 ± 3 60 ± 3 90 ± 4 120 ± 5

0.3 0.3 0.5 1.0 1.0 1.0 1.0 1.5 1.5

30 28 30 60 45 30 28 35 30

Because the films were also electrochemically characterized in a liquid electrolyte, X-ray measurements of all samples were performed before and after wetting by the electrolyte to be sure that its penetration does not modify the polymer structure. This comparison experiment indicated a minor change of the diffraction intensity which did not affect the addressed correlation between the electronic structure and microstructure data (Supporting Information, section S3). Energy-Resolved Electrochemical Impedance Spectroscopy. The ER-EIS is a spectroscopic method capable to map DOS in organic semiconductor materials in the entire energy range from HOMO to the lowest unoccupied molecular orbital (LUMO). This method recently introduced by us28 was used for elucidation of the chemical defect-related DOS in P3HT5 and UV irradiation-induced defect states in polysilanes.29,30 The DOS function g(E) in the semiconductor at the Fermi energy EF,redox = eE can be expressed in terms of the 5882

DOI: 10.1021/acs.jpcc.7b11651 J. Phys. Chem. C 2018, 122, 5881−5887

Article

The Journal of Physical Chemistry C charge-transfer resistance Rct measured at the applied potential E as28 g (E F,redox = eE) = =

1 e 2ket[A]SR ct

GIWAXS pattern was calibrated by a silver behenate standard. The sample-to-detector distance was 250 mm. The exposure time for a single measurement was set to 1500 s.

■

dns d(jS) 1 = 2 d(eE) e ket[A]S dE

RESULTS AND DISCUSSION Structural Defect-related DOS. The DOS spectra measured with the ER-EIS technique are shown in Figure 1.

(1)

where S is the sample area. The charge-transfer resistance Rct = dU/d(jS) corresponds to the real part of complex impedance measured by harmonic voltage at a given frequency f and an amplitude dE superimposed on the applied potential E. The DOS function g(eE) may be directly derived from the chargetransfer resistance Rct(E) measured at an instantaneous position of the Fermi energy given by the applied voltage E using eq 1. The impedance/gain-phase analyzer Solartron analytical, model 1260, was used in the configuration described in our previous paper.28 The frequency, amplitude of ac voltage, and the sweep rate of the dc voltage ramp were set to 0.5 Hz, 100 mV, and 10 mV/s, respectively. Electrochemical microcells with a volume of about ≈200 μL were formed on ITO substrates with deposited regioregular P3HT thin films by gluing the plastic cone.5,28 In this way, the disc working electrode area of 12 mm2 was defined. A solution of 0.1 M TBAPF6 in acetonitrile was employed as the supporting electrolyte. The potential of the working electrode with respect to the reference Ag/AgCl electrode was controlled via a potentiostat. A Pt wire was used as the counter electrode. Considering that the standard potential of the reference Ag/ AgCl electrode is +0.22 V31 relative to the standard hydrogen electrode (+4.44 V), the energy scale of DOS is recalculated to the local vacuum level with an Ag/AgCl energy versus vacuum value of 4.66 eV. GIWAXS. GIWAXS is a reciprocal-space mapping technique widely used for structural characterization of organic thin films. In this technique, a monochromatic and collimated X-ray beam impinges on the sample surface at a small angle of incidence (usually αinc = 0.2°−1°) with respect to the sample surface. The scattered X-ray photons are collected by a position-sensitive Xray detector. The utilization of the two-dimensional (2D) X-ray detector allows simultaneous and fast acquisition of a large part of reciprocal space when compared to a point detector which scans the reciprocal space along a particular trajectory. The application of the GIWAXS technique to P3HT thin films was reviewed by Müller-Buschbaum.32 The GIWAXS measurements of P3HT thin films were performed on a customdesigned Nanostar device (Bruker AXS, Germany) with a liquid Ga metal-jet X-ray source (Excillum, Sweden) dedicated to the small-angle and wide-angle X-ray scattering. The metal-jet Xray source generates high-intense Ga Kα radiation with a photon energy of 9.25 keV (λ = 0.134 nm). The emitted X-ray photons were collected and collimated by a pair of multilayer mirrors arranged in Montel configuration. The photon flux behind Montel optics was 2 × 109 photons/s. The X-ray beam collimation was performed by two single-crystal Ge pinholes that were 550 cm in diameter and 50 cm apart each other. The photon flux behind the collimator was 3 × 108 photons/s. A vacuum compatible six-axis hexapod H-811 (Physik Instrumente, Germany) was employed to adjust the sample into the collimated X-ray beam. The angle of incidence was set to 0.2°. The GIWAXS pattern was recorded by a silicon-based 2D X-ray detector Pilatus 300 K (Dectris, Switzerland) working in the single-photon counting regime. The reciprocal space of the

Figure 1. The DOS of P3HT thin films of various thicknesses measured by the ER-EIS. The regions (I) and (II) are discussed in the text.

Considering the DOS maximum on the order of 1021 cm−3 eV−1,33 the defect-related DOS can be estimated to vary between 1015 and 1018 cm−3 eV−1, which is in good agreement with the sub-band gap DOS reported by Nogueira et al.34 and Boix et al.35 The quantification of g(E) depends on the area of the bulk electrolyte/polymer interface, S (see eq 1 and Supporting Information, section S4). The spectra are therefore corrected with respect to the effective area of the thickest P3HT layer supposing a linear increase of the effective area with thickness. Unlike a previously reported DOS of chemical defects in P3HT5, a wide continuum of states between the LUMO and approximately −0.25 V versus Ag/AgCl (−4.45 eV vs vacuum energy) is recorded for the P3HT films thinner than 30 nm which grows with decreasing thickness. On the other hand, a systematic DOS decrease in the band gap of the P3HT films thicker than 30 nm is measured. To explain this evolution, the presence of oxygen-related defects may be excluded because of the inert nitrogen atmosphere. Hence, the DOS increase in the band gap with the thickness decrease can be attributed to intrinsic structural defects in the P3HT thin films. Besides, there is an apparent satellite peak above the HOMO whose amplitude varies with the film thickness. Microstructure. The molecular stacking in semiconducting polymers forms crystalline domains. In particular, the regioregular P3HT forms lamellar crystalline domains by a π−π stacking of the thiophene rings of the neighboring backbone chains which are separated by amorphous regions. The spin-coated P3HT thin films typically exhibit the edge-on configuration of crystalline domains with the π−π stacking direction parallel to the substrate surface. The lattice constant of this stacking is controlled by the crystallization of alkane side chains.8,36 The organic molecules in thin films tend to form multiple crystalline structures known as polymorphs. In the case of P3HT, polymorphic phases can be stimulated by the solvent evaporation rate,37 surface morphology,38 or chemical additives39 used in the thin film processing. The most intense 5883


Article

The Journal of Physical Chemistry C

microstructure probed by GIWAXS, the structural defectrelated DOS in the band gap was compared with two structural parameters derived from GIWAXS. In particular, the relative amorphous volume fraction given as (1 − xcr) and the RB/A ratio quantifying the relative amount of the two P3HT polymorphs were employed. The ER-EIS method shows that the DOS drops to a minimum close to 1015 cm−3 eV−1 for the thickest film and increases with the reduction of film thickness in the whole band gap up to 1018 cm−3 eV−1 (Figure 1). In addition, a satellite peak above the HOMO is recognized. Let ΣI and ΣII be numerically integrated values of DOS in the regions I and II, respectively, as indicated in Figure 1. Region I refers to the transport gap determined for each film thickness from the band edges in linear scale following a procedure used by Deibel et al.42 Region II refers to the satellite peak above the HOMO fitted with a Gaussian and an exponential background. Hence, ΣI is the integral value of DOS in the band gap without the satellite peak contribution, whereas ΣII is the area under the Gaussian curve fit of the satellite peak. The ΣI dependence on the film thickness is shown in the semilogarithmic scale in Figure 3a. The thinner the film, the

100 diffraction in the GIWAXS pattern suggests the edge-on configuration of π−π stacking with a strong preferential texture along the surface normal (Supporting Information, Figure S1). The intensity distribution of this diffraction along qz axis at qy = 0 nm−1 exhibits an asymmetry (Figure 2 and Supporting

Figure 2. The intensity profile of 100 diffraction along qz axis at qy = 0 nm−1 for the 30 nm P3HT thin film measured by GIWAXS. A decomposition into two polymorphic phases denoted as A and B is indicated.

Information) which can be fully simulated by a contribution of two P3HT polymorphic phases labeled here as A and B. The dominant A phase has the backbone spacing dA100 ≅ 1.65 nm (qAz = 3.8 nm−1), which suggests a well-described bulk phase of P3HT8.40 The minor B phase has a smaller spacing dB100 ≅ 1.51 nm (qBz = 4.15 nm−1). Such a denser polymorphic phase was found in the P3HT sample crystallized under an extremely slow evaporation rate.37 It was also shown that the packing density and structural perfection of P3HT is affected by the size of P3HT crystals.38 The ratio of integral intensities RB/A of the overlapping 100 diffractions of the two P3HT crystalline phases exhibits a thickness dependence which can be correlated with the evolution of the satellite peak in DOS above the HOMO, as it will be shown later in the discussion. We also estimated a relative degree of crystallinity, xcr, in the P3HT thin films as41 xcr =

∫ qIcr(q) dq ∫ qI(q) dq

Figure 3. (a) The integral value of DOS in the band gap and (b) the relative amorphous volume fraction as a function of the film thickness.

higher the ΣI observed. The relative volume fraction of the amorphous phase (1 − xcr) exhibits a similar thickness dependence (Figure 3b). A relationship between these quantities is shown in Figure 4. The DOS in the band gap

(2)

Here, I(q) is the total scattered intensity including a contribution from the amorphous background and Icr(q) is the integral intensity of crystalline phase only (Supporting Information, Figure S2). Because of the restricted reciprocal space used for xcr evaluation (q = 1.4−4.7 nm−1), the calculated xcr values do not represent the absolute values of crystallinity degree but allow the quantification of relative changes in a set of samples with different film thicknesses. A more profound analysis of the crystallinity degree employing the pole figures was published by Jimison et al.26 However, because of a high degree of texture in the P3HT lamellar phase (see Figure S1 in Supporting Information), the approach used here is fully sufficient to describe the relative changes in P3HT crystallinity. These can be correlated with the DOS changes in the band gap as it is shown in the next section. Relation between the Electronic Structure and Microstructure. To relate the thickness evolution of the electronic structure probed by the ER-EIS method to that of the

Figure 4. The integral value of DOS in the band gap as a function of the relative amorphous volume fraction. The respective film thicknesses are those in Figure 3. 5884


Article

The Journal of Physical Chemistry C increases with the fraction of the amorphous phase. In particular, two distinct line fits imply two different exponential dependences, ΣI ∝ exp[k × (1 − xcr)], with remarkably different values of k coefficient, being 10 and 110 above and below the 30 nm film thickness, respectively. The amorphous volume fraction at this crossover thickness is 80%. In this regard, it is interesting to mention a paper by Jimison et al.26 who investigated the degree of crystallinity as a function of the P3HT film thickness by X-ray diffraction and found a steady increase with the film thickness before leveling off at around 30 nm. In our case, the existence of crossover thickness indicates different tendencies to the defect DOS formation induced by structural defects in the two thickness regions. Below 30 nm, the fraction of the amorphous phase is higher and the thinner the films are, the easier the various structural defects are formed and the faster the defect DOS are created. For very thin P3HT films, an amorphous-like phase with a highly distorted lattice of P3HT molecular chains formed at the interface with substrate and occupying a major part of the film volume may be expected. In addition, the interchain and intrachain disorder may be fostered by various structural defects inside the chains which also affect the DOS in the band gap. Theoretical calculations by Feng et al.17 showed that the isolated rotation of the arene rings or loss of the π−π stacking either through tilting or lateral displacement of adjacent polymer chains results in the formation of defect states above the HOMO. Additional defect states below the LUMO were found after increasing the geometrical distorsion of arene rings. Such structural defects are presumably responsible for the steep increase of the integral value of DOS in the band gap below 30 nm. Above 30 nm, higher content of the crystalline phase suggests less-defect molecular chains, adopting energetically more favorable configurations which decelerate the defect DOS formation in the band gap with increasing film thickness. The thickness dependences of ΣII (integral DOS of the satellite peak above the HOMO) and RB/A (intensity ratio of the two P3HT polymorphs) are shown in Figure 5. They exhibit similar shapes with a broad maximum centered around 40 nm. A linear dependence of ΣII on RB/A in the semilog scale (Figure 6) shows a correlation between the occurrence of B phase and the satellite peak above the HOMO. As mentioned

Figure 6. The integral DOS of the satellite peak above the HOMO as a function of the ratio of integral intensities of the 100 diffractions of the two P3HT polymorphs.

above, the dominant A phase with the spacing dA100 ≅ 1.65 nm is equivalent to the bulk phase of P3HT8.40 This phase is typically observed as the only phase in the P3HT thin films deposited on smooth polished substrates with rms roughness below 1 nm, such as silicon wafers, being manifested by a symmetric 100 diffraction peak at qz ≅ 3.8 nm−1. In our case, the substrate was a glass plate covered with the ITO layer (used e.g., as the front transparent electrode in organic solar cell) and has a much larger rms roughness of about 7 nm. Such a large roughness acts adversely on the mobility of P3HT molecular chains during the solvent and thermal annealing to adopt energetically the most favorable configurations and is responsible for the appearance of the minor B phase with dB100 ≅ 1.51 nm. It is noteworthy that a similar co-existence of two polymorphic phases with the backbone spacings of 1.64 and 1.55 nm was observed in the P3HT films deposited on mesoporous titania electrodes with a particular pore size of 40 nm.38 This compares well with the P3HT film thickness corresponding to the maximum content of B phase (Figure 5b). In contrast, only the regular 1.64 nm spacing was found in the electrodes with 12 nm pore size. In our case, the formation of B phase is suppressed at both small and large film thicknesses with respect to the 40 nm thick film. Small film thicknesses nearly approach the ITO substrate roughness. This may affect unfavorably the film morphology and continuity, impeding the formation of B phase over larger volumes during the solvent and thermal annealings. On the other hand, the effect of substrate roughness becomes weaker at larger film thicknesses, and the film structure is more akin to the bulk material.

■

SUMMARY The ER-EIS and GIWAXS techniques were applied to investigate the relationship between the electronic and molecular structures in P3HT thin films. An exponential increase of the DOS in the band gap with increasing fraction of the amorphous phase when decreasing the film thickness was found. The exponent increases abruptly by 1 order of magnitude when reducing the film thickness down to 30 nm (Figure 4), where the relative volume fraction of the amorphous phase reaches 80%. Hence, the films thinner than 30 nm exhibit a much faster formation of the defect DOS with increasing fraction of the amorphous phase than those thicker than 30 nm. The existence of crossover thickness is important for application of P3HT films on ITO substrate in electronic devices.

Figure 5. (a) The integral DOS of the satellite peak above the HOMO and (b) the ratio of integral intensities of the 100 diffractions of the two P3HT polymorphs as a function of the P3HT film thickness. 5885


Article


High-Performance Organic Electrochemical Transistors. J. Am. Chem. Soc. 2016, 138, 10252−10259. (5) Gmucová, K.; Nádaždy, V.; Schauer, F.; Kaiser, M.; Majkova, E. Electrochemical Spectroscopic Methods for the Fine Band Gap Electronic Structure Mapping in Organic Semiconductors. J. Phys. Chem. C 2015, 119, 15926−15934. (6) Nalwa, H. S. Handbook of Organic Conductive Molecules and Polymers; John Wiley and Sons Ltd: Chichester, U.K., 1997. (7) Spoltore, D.; Oosterbaan, W. D.; Khelifi, S.; Clifford, J. N.; Viterisi, A.; Palomares, E.; Burgelman, M.; Lutsen, L.; Vanderzande, D.; Manca, J. Effect of Polymer Crystallinity in P3HT: PCBM Solar Cells on Band Gap Trap States and Apparent Recombination Order. Adv. Energy Mater. 2013, 3, 466−471. (8) Prosa, T. J.; Winokur, M. J.; Moulton, J.; Smith, P.; Heeger, A. J. X-Ray Structural Studies of Poly(3-Alkylthiophenes): an Example of an Inverse Comb. Macromolecules 1992, 25, 4364−4372. (9) Kobashi, M.; Takeuchi, H. Inhomogeneity of Spin-Coated and Cast Non-Regioregular Poly(3-Hexylthiophene) Films. Structures and Electrical and Photophysical Properties. Macromolecules 1998, 31, 7273−7278. (10) Luo, C.; Kyaw, A. K. K.; Perez, L. A.; Patel, S.; Wang, M.; Grimm, B.; Bazan, G. C.; Kramer, E. J.; Heeger, A. J. General Strategy for Self-Assembly of Highly Oriented Nanocrystalline Semiconducting Polymers with High Mobility. Nano Lett. 2014, 14, 2764−2771. (11) Wang, G.; Swensen, J.; Moses, D.; Heeger, A. J. Increased Mobility from Regioregular Poly(3-Hexylthiophene) Field-Effect Transistors. J. Appl. Phys. 2003, 93, 6137−6141. (12) Skrypnychuk, V.; Wetzelaer, G.-J. A. H.; Gordiichuk, P. I.; Mannsfeld, S. C. B.; Herrmann, A.; Toney, M. F.; Barbero, D. R. Ultrahigh Mobility in an Organic Semiconductor by Vertical Chain Alignment. Adv. Mater. 2016, 28, 2359−2366. (13) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; et al. Two-Dimensional Charge Transport in Self-Organized, High-Mobility Conjugated Polymers. Nature 1999, 401, 685−688. (14) Rivnay, J.; Noriega, R.; Northrup, J. E.; Kline, R. J.; Toney, M. F.; Salleo, A. Structural Origin of Gap States in Semicrystalline Polymers and the Implications for Charge Transport. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, No. 121306(R). (15) Street, R. A.; Song, K. W.; Northrup, J. E.; Cowan, S. Photoconductivity Measurements of the Electronic Structure of Organic Solar Cells. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 165207. (16) Frost, J. M.; Kirkpatrick, J.; Kirchartz, T.; Nelson, J. Parameter Free Calculation of the Subgap Density of States in Poly(3Hexylthiophene). Faraday Discuss. 2014, 174, 255−266. (17) Feng, D.-Q.; Caruso, A. N.; Losovyj, Y. B.; Shulz, D. L.; Dowben, P. A. Identification of the Possible in Poly(3-Hexylthiophene) Thin Defect States Films. Polym. Eng. Sci. 2007, 47, 1359− 1364. (18) Poelking, C.; Andrienko, D. Effect of Polymorphism, Regioregularity and Paracrystallinity on Charge Transport in Poly(3Hexylthiophene) [P3HT] Nanofibers. Macromolecules 2013, 46, 8941−8956. (19) Maillard, A.; Rochefort, A. Structural and Electronic Properties of Poly(3-Hexylthiophene) π-Stacked Crystals. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 115207. (20) Lücke, A.; Schmidt, W. G.; Rauls, E.; Ortmann, F.; Gerstmann, U. Influence of Structural Defects and Oxidation onto Hole Conductivity in P3HT. J. Phys. Chem. B 2015, 119, 6481−6491. (21) DeLongchamp, D. M.; Kline, R. J.; Fischer, D. A.; Richter, L. J.; Toney, M. F. Molecular Characterization of Organic Electronic Films. Adv. Mater. 2011, 23, 319−337. (22) Botiz, I.; Stingelin, N. Influence of Molecular Conformations and Microstructure on the Optoelectronic Properties of Conjugated Polymers. Materials 2014, 7, 2273−2300.

The presence of two polymorphic crystalline A and B phases with different backbone spacings was found by GIWAXS. The minor B phase content has a maximum plateau around 40 nm and decays at both small and large film thicknesses. The integral DOS of the satellite peak located above the HOMO shows an exponential increase with the volume ratio RB/A, suggesting a principal role of the minor B phase in the occurrence of the satellite peak. Our results show that in the case of rough substrates such as ITO, the DOS in the band gap of P3HT thin films is strongly dependent on the film thickness as the substrate roughness induces various structural defects and leads to the formation of polymorphic crystalline phases. The implications for applications of polymers in high-performance organic electronics are straightforward.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b11651. Quantification of P3HT polymorphs, degree of crystallinity in P3HT thin films, influence of electrolyte on P3HT microstructure, and determination of the diffusion coefficient of electrolyte penetration into P3HT (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +421220910762. ORCID

Vojtech Nádaždy: 0000-0003-4127-5249 Katarína Gmucová: 0000-0003-4118-3336 Peter Siffalovic: 0000-0002-9807-0810 Matej Jergel: 0000-0002-4482-7881 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully thank to P. Kalinay (Institute of Physics SAS, Bratislava) for the model of electrolyte penetration into P3HT given in the Supporting Information. This research was supported by the Slovak Research and Development Agency, project no. APVV-0096-11, the Scientific Grant Agency VEGA, projects nos. 1/0501/15, 2/0163/17, 2/0092/18, and the project Applied Research of Advanced Photovoltaic Cells, ITMS code 26240220047, supported by the Research and Development Operational Programme funded by ERDF.

■

REFERENCES

(1) Carr, J. A.; Chaudhary, S. The Identification, Characterization and Mitigation of Defect States in Organic Photovoltaic Devices: A Review and Outlook. Energy Environ. Sci. 2013, 6, 3414−3438. (2) Diemer, P. J.; Hayes, J.; Welchman, E.; Hallani, R.; Pookpanratana, S. J.; Hacker, C. A.; Richter, C. A.; Anthony, J. E.; Thonhauser, T.; Jurchescu, O. D. The Influence of Isomer Purity on Trap States and Performance of Organic Thin-Film Transistors. Adv. Electron. Mater. 2017, 3, 1600294. (3) Ingram, G. L.; Zhao, Y.-B.; Lu, Z.-H. Exciton Dynamics of Luminescent Defects in Aging Organic Light-Emitting Diodes. J. Appl. Phys. 2017, 122, 245501. (4) Nielsen, C. B.; Giovannitti, A.; Sbircea, D.-T.; Bandiello, E.; Niazi, M. R.; Hanifi, D. A.; Sessolo, M.; Amassian, A.; Malliaras, G. G.; Rivnay, J.; et al. Molecular Design of Semiconducting Polymers for 5886


Article


Poly-3-Hexyl-Thiophene (P3HT). Angew. Chem., Int. Ed. 2012, 51, 11068−11072. (41) Roe, R. J. Methods of X-ray and Neutron Scattering in Polymer Science; Oxford University Press: New York, USA, 2000. (42) Deibel, C.; Mack, D.; Gorenflot, J.; Schöll, A.; Krause, S.; Reinert, F.; Rauh, D.; Dyakonov, V. Energetics of Excited States in the Conjugated Polymer Poly(3-Hexylthiophene). Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 085202.

(23) Tremel, K.; Ludwigs, S. Morphology of P3HT in Thin Films in Relation to Optical and Electrical Properties. Adv. Polym. Sci. 2014, 265, 39−82. (24) Porzio, W.; Scavia, G.; Arnautov, S.; Conzatti, L.; Charmet, J.; Laux, E.; Keppner, H.; Zerbi, G.; Brambilla, L.; Bolognesi, A. Nanoscale Structure and Morphology of Thin Films of Poly(2Chloroxylylene) Synthesized by the CVD Method on Different Liquids. Eur. Polym. J. 2011, 47, 1725−1735. (25) Scavia, G.; Barba, L.; Arrighetti, G.; Milita, S.; Porzio, W. Structure and Morphology Optimization of Poly(3-Hexylthiophene) Thin Films onto Silanized Silicon Oxide. Eur. Polym. J. 2012, 48, 1050−1061. (26) Jimison, L. H.; Himmelberger, S.; Duong, D. T.; Rivnay, J.; Toney, M. F.; Salleo, A. Vertical Confinement and Interface Effects on the Microstructure and Charge Transport of P3HT Thin Films. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 611−620. (27) Spano, F. C.; Silva, C. H- and J-Aggregate Behavior in Polymeric Semiconductors. Annu. Rev. Phys. Chem. 2014, 65, 477−500. (28) Nádaždy, V.; Schauer, F.; Gmucová, K. Energy Resolved Electrochemical Impedance Spectroscopy for Electronic Structure Mapping in Organic Semiconductors. Appl. Phys. Lett. 2014, 105, 142109. (29) Schauer, F.; Tkácǒ vá, M.; Nádaždy, V.; Gmucová, K.; Ožvoldová, M.; Tkác,̌ L.; Chlpík, J. Electronic Structure of Uv Degradation Defects in Polysilanes Studied by Energy Resolved Electrochemical Impedance Spectroscopy. Polym. Degrad. Stab. 2016, 126, 204−208. (30) Schauer, F.; Tkác,̌ L.; Ožvoldová, M.; Nádaždy, V.; Gmucová, K.; Jergel, M.; Šiffalovič, P. Effect of Crystallinity on UV Degradability of Poly[Methyl(Phenyl)Silane] by Energy-Resolved Electrochemical Impedance Spectroscopy. AIP Adv. 2017, 7, 055002. (31) Bates, R. G.; Macaskill, J. B. Standard Potential of the SilverSilver Chloride Electrode. Pure Appl. Chem. 1978, 50, 1701−1706. (32) Müller-Buschbaum, P. The Active Layer Morphology of Organic Solar Cells Probed with Grazing Incidence Scattering Techniques. Adv. Mater. 2014, 26, 7692−7709. (33) Vukmirović, N.; Wang, L.-W. Density of States and Wave Function Localization in Disordered Conjugated Polymers: A Large Scale Computational Study. J. Phys. Chem. B 2011, 115, 1792−1797. (34) Nogueira, A. F.; Montanari, I.; Nelson, J.; Durrant, J. R.; Winder, C.; Sariciftci, N. S. Charge Recombination in Conjugated Polymer/Fullerene Blended Films Studied by Transient Absorption Spectroscopy. J. Phys. Chem. B 2003, 107, 1567−1573. (35) Boix, P. P.; Garcia-Belmonte, G.; Muñecas, U.; Neophytou, M.; Waldauf, C.; Pacios, R. Determination of Gap Defect States in Organic Bulk Heterojunction Solar Cells from Capacitance Measurements. Appl. Phys. Lett. 2009, 95, 233302. (36) Brinkmann, M.; Rannou, P. Effect of Molecular Weight on the Structure and Morphology of Oriented Thin Films of Regioregular Poly(3-Hexylthiophene) Grown by Directional Epitaxial Solidification. Adv. Funct. Mater. 2007, 17, 101−108. (37) Yuan, Y.; Zhang, J.; Sun, J.; Hu, J.; Zhang, T.; Duan, Y. Polymorphism and Structural Transition around 54 Degrees C in Regioregular Poly(3-Hexylthiophene) with High Crystallinity as Revealed by Infrared Spectroscopy. Macromolecules 2011, 44, 9341− 9350. (38) Song, L.; Wang, W.; Pröller, S.; González, D. M.; Schlipf, J.; Schaffer, C. J.; Peters, K.; Herzig, E. M.; Bernstorff, S.; Bein, T.; et al. In Situ Study of Degradation in P3HT-Titania-Based Solid-State DyeSensitized Solar Cells. ACS Energy Lett. 2017, 2, 991−997. (39) Smith, B. H.; Clark, M. B., Jr.; Kuang, H.; Grieco, C.; Larsen, A. V.; Zhu, C.; Wang, C.; Hexemer, A.; Asbury, J. B.; Janik, M. J.; et al. Controlling Polymorphism in Poly(3-Hexylthiophene) through Addition of Ferrocene for Enhanced Charge Mobilities in Thin-Film Transistors. Adv. Funct. Mater. 2015, 25, 542−551. (40) Dudenko, D.; Kiersnowski, A.; Shu, J.; Pisula, W.; Sebastiani, D.; Spiess, H. W.; Hansen, M. R. A Strategy for Revealing the Packing in Semicrystalline π-Conjugated Polymers: Crystal Structure of Bulk 5887


Thickness Effect on Structural Defect-Related Density of States and

Recommend Documents