Influence of Processing Additives on Charge-Transfer Time Scales

Letter pubs.acs.org/JPCL

Influence of Processing Additives on Charge-Transfer Time Scales and Sound Velocity in Organic Bulk Heterojunction Films Loren G. Kaake, Gregory C. Welch, Daniel Moses, Guillermo C. Bazan, and Alan J. Heeger* Center for Polymers and Organic Solids, University of California, Santa Barbara, California 93106, United States S Supporting Information *

ABSTRACT: The role of processing additives in organic bulk heterojunction thin films was investigated by means of transient absorption spectroscopy. The rate of ultrafast charge transfer was found to increase when a small amount of diiodooctane was used during film formation. In addition, coherent acoustic phonons were observed, and their velocity was determined. A strong correlation between the sound velocity and the charge-transfer time scale was observed, both of which could be explained by a subtle increase in thin film density.

SECTION: Spectroscopy, Photochemistry, and Excited States he need to find means of energy production to replace fossil fuel combustion has led to widespread interest in photovoltaic technologies. One example is the development of “plastic” bulk heterojunction (BHJ) solar cells, frequently based on blends of semiconducting polymers and fullerene derivatives. However, the challenges of reproducible polymer synthesis in terms of purity, molecular weight, and polydispersity have brought attention to the benefits of solution processable small molecules1−3 as an alternative approach to provide more readily scalable materials. The well-controlled nature of the small molecule system also makes it attractive for photophysical studies. Controlling the nanoscale morphology of the phaseseparated blend of electron-donating and electron-accepting molecules known as a BHJ material has emerged as a principal challenge in organic photovoltaic research. Several methods to control the nanoscale morphology have been devised, including thermal4 and solvent annealing.5,6 The most versatile and widely employed technique, however, is the use of solvent additives during film deposition7 which increase solar cell efficiencies by modifying the morphological length scales of the donor and acceptor phases.8−10The use of solvent additives has also been shown to increase the carrier mobility11,12 and thereby improve the competition between sweep out and recombination.13 However, questions remain concerning the mechanism by which solvent additives lead to improvements in cell performance. One of the most important basic scientific questions regarding the operation of organic solar cells is the mechanism underlying charge separation following photon absorption. The process is commonly described as occurring in four steps: exciton formation, exciton migration to a donor−acceptor

T

© 2012 American Chemical Society

interface, interfacial charge transfer (CT) into a bound state (CT exciton) at the donor−acceptor interface, followed by field-induced dissociation of the CT exciton at the donor− acceptor interface.14 However, this scenario is at variance with experimental observations. Charged species can be created on ultrafast time scales prior to the formation and diffusion of excitons;15−17 moreover, charges can be collected with unit internal quantum efficiency.18 Finally, estimates of the binding energy of the CT exciton are typically an order of magnitude greater than the thermal energy at room temperature,14 and temperature- and electric-field-independent charge-separation rates have been observed.19−22 As a result, a vigorous search into the nature of the CT and separation process is continuing.23 Far from being an isolated problem, a clear understanding of the CT mechanism is important in the development of better materials and processing conditions for organic photovoltaics. To address these questions, we employed femtosecond timeresolved transient absorption spectroscopy on spin-cast films composed of the small molecule, d-DTS(PTTh2)2,, and the fullerene acceptor PC70BM.24 BHJ solar cells based on a structurally similar molecule were recently shown to exhibit a power conversion efficiency of 6.7%. The cell performance was extremely sensitive to small concentrations of the solvent additive, diiodooctane (DIO), increasing to a maximum upon the addition of 0.25% DIO and decreasing when larger concentrations of the additive were utilized.1 Received: March 23, 2012 Accepted: April 27, 2012 Published: April 27, 2012 1253

dx.doi.org/10.1021/jz300365b | J. Phys. Chem. Lett. 2012, 3, 1253−1257

The Journal of Physical Chemistry Letters

Letter

Figure 1 shows the molecular structure of d-DTS(PTTh2)2 and PC70BM. Also shown are the steady-state absorption

Figure 2. Transient absorption dynamics. (a) Dynamics of 850 nm (positive) 720 nm (negative) for d-DTS(PTTh2)2/PC70BM blends cast from solutions with varying amounts of solvent additive. (b) Charge-transfer time scale extracted through curve fitting transient absorption dynamics, plotted versus the concentration of solvent additive.

Figure 1. System description and basic observations. (a) Molecular structures of compounds used. (b) Steady-state absorption spectra. (c) Transient absorption spectra of a d-DTS(PTTh2)2/PC70BM blend.

To illustrate more clearly how the CT rate changes with processing conditions, it was necessary to fit the dynamics of the PIA by convolving the temporal response of the instrument with a model that includes a rise time. A phenomenological model was chosen; it describes the population of charges by an exponential growth that decays via two independent channels as described by a biexponential term. This was the simplest model that adequately fit the data in the range of 0−5 ps. The instrument response function is essentially the convolution of pump and probe pulses and could be extracted from the ground-state photobleach dynamics. The photobleach dynamics were modeled as an instantaneous growth and two independent decay channels by multiplying a step function and a biexponential decay term. Therefore, the leading edge of the photobleach dynamics can be fit to extract the instrument response function, assumed to have a Gaussian shape. The response function (which had a width of 100 fs) was then held constant and convolved with the aforementioned CT model. Details of the fitting procedure are discussed in the Supporting Information. The results of the fitting are depicted in Figure 2b. They show that the major CT process occurs in the sub-100 fs time regime and that the rate gets faster with increasing [DIO] used during processing, becoming indistinguishable with an instantaneous process at 0.16 and 0.25% v/v. Because the time scales extracted are shorter than the temporal width of the instrument response function, the results of the fitting are to be taken as estimates, characterized by an error bar of about 20 fs. The general trend is identifiable and shows that a small increase in [DIO] increases the CT rate. Upon incorporating DIO with concentration higher than 0.25% v/v during film formation, the CT rate slows. The entire trend corresponds well with solar cell efficiencies in this class of materials, which increase with small amounts of solvent additive, reaching an optimal performance

spectra of d-DTS(PTTh2)2, PC70BM, and the d-DTS(PTTh2)2/PC70BM blend (70:30) (w/w) cast from chlorobenzene. The spectrum of d-DTS(PTTh2)2 shows the vibronic splitting typical of a crystalline molecular material. As is typical for BHJ materials, to a good approximation the absorption spectrum of d-DTS(PTTh2)2/PC70BM is a superposition of the spectra of the two components of the phaseseparated blend. Figure 1c shows the result of a transient absorption measurement carried out on a film of d-DTS(PTTh2)2/ PC70BM (70:30) (w/w) cast from chlorobenzene and processed with 0.25% DIO (v/v) solvent additive. The measurement consisted of a 400 nm pump pulse, followed by a white light probe. The spectrum contains two major features: a doublet photobleaching signal (peaked at roughly 650 and 720 nm) and a photoinduced absorption (PIA) (peaked at 900 nm). By comparison with the spectra in Figure 1b, the photobleaching signal arises from the ground-state absorption of d-DTS(PTTh2)2. The increased PIA at longer wavelengths is typical of that observed from photoexcited mobile carriers in BHJ materials.25−29 To study the rate of CT within the film, the time-dependent intensity of the PIA was monitored. Figure 2a shows the response at 850 nm (positive signal) and at 720 nm (negative signal) corresponding to the PIA and photobleaching signals, respectively. The rise time of the positive signal indicates ultrafast photoinduced CT. Note that the CT rate is sensitive to the [DIO]; the addition of DIO during film formation leads to faster charge separation. In contrast, the appearance of the photobleach is determined by the instantaneous photoexcitation of d-DTS(PTTh2)2 and does not depend upon the [DIO] used during processing. Therefore, the onset of the photobleach is expected to be limited solely by the time resolution of the measuring system. 1254

dx.doi.org/10.1021/jz300365b | J. Phys. Chem. Lett. 2012, 3, 1253−1257

The Journal of Physical Chemistry Letters

Letter

Figure 3. Extracting sound velocities. (a) Transient absorption spectra obtained in films of d-DTS(PTTh2)2. (b) Peak position of the feature near 720 nm as a function of time plotted for different film thicknesses and offset for clarity. (c) Period of the spectral oscillations plotted as a function of film thickness. (d) Sound velocity for individual blended films of d-DTS(PTTh2)2/PC70BM plotted as a function of solvent additive concentration. Thick lines on the y axis display the measured sound velocity in the individual components.

at concentration of ∼0.25% v/v, then decreasing with the use of higher [DIO].1 To explain the dependence of the CT rate on processing conditions, one needs information concerning the effect of processing with DIO on the morphology of the film. It would therefore be useful to find an independent phenomenon that is influenced by the structure of the film. In our case, dDTS(PTTh2)2 is particularly sensitive to the generation of coherent acoustic waves created by laser heating, as is displayed in Figure 3. Figure 3a shows the basic observation: the positions of the photobleach peaks oscillate with pump−probe delay time. The effect is well known and is caused by the propagation of pressure waves generated in the film as a result of laser heating;30−32 the spectral shift results from the transiently increased pressure.33,34 The magnitude of the peak shift can be extracted by fitting the transient absorption spectra with a series of peaks that are allowed to change in position and intensity. The oscillation period (T) is proportional to the sample thickness (l) and inversely proportional to the sound velocity (vS) of the material:

T=

4l vs

results are plotted in Figure 3d; experimental error was estimated by accounting for film roughness and the uncertainty in extracting the oscillation period. Films cast from solutions containing relatively small amounts of DIO show decreasing sound velocity with increasing [DIO]. At higher [DIO], the trend reverses, and sound velocity increases. In all cases, the sound velocity was between that of PC70BM and d-DTS(PTTh2)2. More importantly, the data indicate a clear correlation between the sound velocity and the CT time scale. Both decrease with increasing [DIO] before reversing direction at [DIO] higher than 0.25%. To understand the correlation between CT time and sound velocity, it is useful to describe the sound velocity in terms of the bulk elastic modulus (C) and the film density (ρ)36 vs =

C ρ

(2)

Thus, one can describe a decrease in sound velocity in terms of decreasing bulk modulus, increasing density, or both. In a composite composed of two materials that form an interpenetrating network, the modulus is relatively insensitive to changes in morphology.37 Thus, an increase in film density is a more likely explanation for the decrease in sound velocity with small amounts of additive. Increasing film density also provides an alternative explanation for recent X-ray scattering measurements, which show an increase in electron density through the use of solvent additives.38 In addition, increased film density is also consistent with an increase in local crystallinity and more efficient packing at the donor−acceptor interface, both factors which should contribute to faster CT. Finally, the reduction in film density at higher [DIO] can be understood by large scale coarsening of the grain structure, an interpretation supported by AFM measurements that show an increase in surface roughness of more than an order of magnitude. Grain coarsening also explains slower CT time scales because molecular contact at the donor−acceptor interface is made worse.

(1)

Note that the peak shift appears to be 180° out of phase with what one might expect from a rapid pressurization of the film (pressurization causes a red shift).33,34 However, features in the transient absorption spectrum that arise from a peak shift take a derivative shape, the negative node of which shifts in the opposite direction of the real peak shift. Figure 3c is a plot of the spectral oscillation period versus film thickness in a series of pure d-DTS(PTTh2)2 films. As described above in eq 1, the slope of this plot yields the sound velocity of the material. The deduced sound velocity in dDTS(PTTh2)2 is slightly higher than that in polystyrene (2.3 × 105 cm/s) and about half that of pyrex glass (5.6 × 105 cm/s).35 Similarly, the sound velocity was derived for films of PC70BM and d-DTS(PTTh2)2 and for each of the d-DTS(PTTh2)2/ PC70BM composite films prepared with varying [DIO]. The 1255

dx.doi.org/10.1021/jz300365b | J. Phys. Chem. Lett. 2012, 3, 1253−1257

The Journal of Physical Chemistry Letters

■

In conclusion, CT between the small molecule donor, dDTS(PTTh2)2, and the PC70BM acceptor is observed to occur in 12 h prior to measurement to remove any residual solvent and solvent additive. PC70BM films were cast from 15 mg/mL solutions at 1000 rpm. The d-DTS(PTTh2)2 films were cast from 40 mg/ mL solutions that had been heated to 60 °C at spin speeds of 6000, 3000, and 1500 rpm. Blended films of d-DTS(PTT2)2/ PC70BM were cast from 40 mg/mL (total weight/vol) solutions at 7/3 weight ratio of d-DTS(PTTh2)2 to PC70BM. Solutions were heated to 60 °C and filtered before spin-casting at 3000 rpm. 1,8-Diiodooctane (DIO) was included as a solvent additive as part of the d-DTS(PTTh2)2/PC70BM solutions. All films were characterized after transient absorption measurements according to their thickness and their steady state absorption characteristics. Absorption measurements were taken using a Beckman Coulter DU 800 UV−vis spectrometer in transmission mode. Thickness measurements were performed using an Asylum Research MFP-3D AFM. Samples were scratched using fine-tipped steel tweezers to image film thickness. Transient absorption measurements were conducted with a pulsed laser system at a repetition rate of 1 kHz. The laser consists of a titanium sapphire oscillator (Spectra Physics Tsunami) that is pumped with a Nd:YAG laser (Spectra Physics Millenia). The pulses were fed into a regenerative amplifier (Spectra Physics Spitfire) that was pumped with a high power Nd:YAG laser (Spectra Physics Empower). 800 nm pulses were generated with a pulse width of 100 fs. The pulses were split into pump and probe paths. The pump pulse was frequency doubled to 400 nm and focused onto the sample with a beam diameter of 1 mm and pulse energies of 30−120 μJ/cm2. The pump pulse was put through a delay stage to achieve time resolution. The probe pulse was focused into a 1 mm sapphire disk to generate the white light continuum used to measure visible and near-IR spectra. It was split before reaching the sample to provide a reference path to aid in the correction of intensity fluctuations. All spectra were manually corrected for the temporal chirp present in the white-light continuum. The polarization angle between pump and probe beams was 54 ± 1°. Samples were held under rough vacuum (p ≈ 10−3 Torr) during the measurements.

■

REFERENCES

(1) Sun, Y. M.; Welch, G. C.; Leong, W. L.; Takacs, C. J.; Bazan, G. C.; Heeger, A. J. Solution-Processed Small-Molecule Solar Cells with 6.7% Efficiency. Nat. Mater. 2012, 11 (1), 44−48. (2) Roncali, J. Molecular Bulk Heterojunctions: An Emerging Approach to Organic Solar Cells. Acc. Chem. Res. 2009, 42 (11), 1719−1730. (3) Li, Y. W.; Guo, Q.; Li, Z. F.; Pei, J. N.; Tian, W. J. Solution Processable D-a Small Molecules for Bulk-Heterojunction Solar Cells. Energy Environ. Sci. 2010, 3 (10), 1427−1436. (4) Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K.; Heeger, A. J. Thermally Stable, Efficient Polymer Solar Cells with Nanoscale Control of the Interpenetrating Network Morphology. Adv. Funct. Mater. 2005, 15 (10), 1617−1622. (5) Li, G.; Shrotriya, V.; Huang, J. S.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. High-Efficiency Solution Processable Polymer Photovoltaic Cells by Self-Organization of Polymer Blends. Nat. Mater. 2005, 4 (11), 864−868. (6) Wei, G.; Wang, S.; Sun, K.; Thompson, M. E.; Forrest, S. R. Solvent-Annealed Crystalline Squaraine: Pc70bm (1:6) Solar Cells. Adv. Energy Mater. 2011, 1 (2), 184−187. (7) Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.; Bazan, G. C. Efficiency Enhancement in Low-Bandgap Polymer Solar Cells by Processing with Alkane Dithiols. Nat. Mater. 2007, 6 (7), 497−500. (8) Hoven, C. V.; Dang, X. D.; Coffin, R. C.; Peet, J.; Nguyen, T. Q.; Bazan, G. C. Improved Performance of Polymer Bulk Heterojunction Solar Cells through the Reduction of Phase Separation Via Solvent Additives. Adv. Mater. 2010, 22 (8), E63. (9) Rogers, J. T.; Schmidt, K.; Toney, M. F.; Kramer, E. J.; Bazan, G. C. Structural Order in Bulk Heterojunction Films Prepared with Solvent Additives. Adv. Mater. 2011, 23 (20), 2284. (10) Salim, T.; Wong, L. H.; Brauer, B.; Kukreja, R.; Foo, Y. L.; Bao, Z. N.; Lam, Y. M. Solvent Additives and Their Effects on Blend Morphologies of Bulk Heterojunctions. J. Mater. Chem. 2011, 21 (1), 242−250. (11) Moon, J. S.; Takacs, C. J.; Cho, S.; Coffin, R. C.; Kim, H.; Bazan, G. C.; Heeger, A. J. Effect of Processing Additive on the Nanomorphology of a Bulk Heterojunction Material. Nano Lett. 2010, 10 (10), 4005−4008. (12) Chen, H. Y.; Yang, H. C.; Yang, G. W.; Sista, S.; Zadoyan, R. B.; Li, G.; Yang, Y. Fast-Grown Interpenetrating Network in Poly(3Hexylthiophene): Methanofullerenes Solar Cells Processed with Additive. J. Phys. Chem. C 2009, 113 (18), 7946−7953. 1256

dx.doi.org/10.1021/jz300365b | J. Phys. Chem. Lett. 2012, 3, 1253−1257

The Journal of Physical Chemistry Letters

Letter

(34) Schmidtke, J. P.; Kim, J. S.; Gierschner, J.; Silva, C.; Friend, R. H. Optical Spectroscopy of a Polyfluorene Copolymer at High Pressure: Intra- and Intermolecular Interactions. Phys. Rev. Lett. 2007, 99, 16. (35) Lide, D. R. CRC Handbook of Chemistry and Physics: A ReadyReference Book of Chemical and Physical Data; CRC Press: Boca Raton, FL, 2004. (36) Gersten, J. I.; Smith, F. W. The Physics and Chemistry of Materials; Wiley: New York, 2001. (37) Nielsen, L. E. Polymeric Composite Systems with 2 Continuous Phases. J. Appl. Polym. Sci. 1977, 21 (6), 1579−1584. (38) Lou, S. J.; Szarko, J. M.; Xu, T.; Yu, L.; Marks, T. J.; Chen, L. X. Effects of Additives on the Morphology of Solution Phase Aggregates Formed by Active Layer Components of High-Efficiency Organic Solar Cells. J. Am. Chem. Soc. 2011, 133, 20661−20663.

(13) Cowan, S. R.; Roy, A.; Heeger, A. J. Recombination in PolymerFullerene Bulk Heterojunction Solar Cells. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82 (24), 245207. (14) Clarke, T. M.; Durrant, J. R. Charge Photogeneration in Organic Solar Cells. Chem. Rev. 2010, 110 (11), 6736−6767. (15) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Photoinduced Electron-Transfer from a Conducting Polymer to Buckminsterfullerene. Science 1992, 258 (5087), 1474−1476. (16) Hwang, I. W.; Soci, C.; Moses, D.; Zhu, Z. G.; Waller, D.; Gaudiana, R.; Brabec, C. J.; Heeger, A. J. Ultrafast Electron Transfer and Decay Dynamics in a Small Band Gap Bulk Heterojunction Material. Adv. Mater. 2007, 19 (17), 2307−+. (17) Kaake, L. G.; Jailaubekov, A.; Williams, K. J.; Zhu, X. Y., Probing Ultrafast Charge Separation at Organic Donor/Acceptor Interfaces by a Femtosecond Electric Field Meter. Appl. Phys. Lett. 2011, 99 (8). (18) Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Bulk Heterojunction Solar Cells with Internal Quantum Efficiency Approaching 100%. Nat. Photonics 2009, 3 (5), 297−U5. (19) Pensack, R. D.; Asbury, J. B. Barrierless Free Carrier Formation in an Organic Photovoltaic Material Measured with Ultrafast Vibrational Spectroscopy. J. Am. Chem. Soc. 2009, 131 (44), 15986−+. (20) Jamieson, F. C.; Agostinelli, T.; Azimi, H.; Nelson, J.; Durrant, J. R. Field-Independent Charge Photogeneration in PCPDTBT/ PC70BM Solar Cells. J. Phys. Chem. Lett. 2010, 1 (23), 3306−3310. (21) Kniepert, J.; Schubert, M.; Blakesley, J. C.; Neher, D. Photogeneration and Recombination in P3HT/PCBM Solar Cells Probed by Time-Delayed Collection Field Experiments. J. Phys. Chem. Lett. 2011, 2 (7), 700−705. (22) Grzegorczyk, W. J.; Savenije, T. J.; Dykstra, T. E.; Piris, J.; Schins, J. M.; Siebbeles, L. D. A. Temperature-Independent Charge Carrier Photogeneration in P3HT-PCBM Blends with Different Morphology. J. Phys. Chem. C 2010, 114 (11), 5182−5186. (23) Bakulin, A. A.; Rao, A.; Pavelyev, V. G.; van Loosdrecht, P. H. M.; Pshenichnikov, M. S.; Niedzialek, D.; Cornil, J.; Beljonne, D.; Friend, R. H. The Role of Driving Energy and Delocalized States for Charge Separation in Organic Semiconductors. Science 2012, 335 (6074), 1340−1344. (24) Welch, G. C.; Perez, L. A.; Hoven, C. V.; Zhang, Y.; Dang, X. D.; Sharenko, A.; Toney, M. F.; Kramer, E. J.; Nguyen, T. Q.; Bazan, G. C. A Modular Molecular Framework for Utility in Small-Molecule Solution-Processed Organic Photovoltaic Devices. J. Mater. Chem. 2011, 21 (34), 12700−12709. (25) Fesser, K.; Bishop, A. R.; Campbell, D. K. Optical-Absorption from Polarons in a Model of Polyacetylene. Phys. Rev. B 1983, 27 (8), 4804−4825. (26) Osterbacka, R.; An, C. P.; Jiang, X. M.; Vardeny, Z. V. TwoDimensional Electronic Excitations in Self-Assembled Conjugated Polymer Nanocrystals. Science 2000, 287 (5454), 839−842. (27) Karsten, B. P.; Janssen, R. A. J. Small Band Gap Oligothieno 3,4B Pyrazines. Org. Lett. 2008, 10 (16), 3513−3516. (28) Karsten, B. P.; Viani, L.; Gierschner, J.; Cornil, J.; Janssen, R. A. J. An Oligomer Study on Small Band Gap Polymers. J. Phys. Chem. A 2008, 112 (43), 10764−10773. (29) Karsten, B. P.; Bijleveld, J. C.; Viani, L.; Cornil, J.; Gierschner, J.; Janssen, R. A. J. Electronic Structure of Small Band Gap Oligomers Based on Cyclopentadithiophenes and Acceptor Units. J. Mater. Chem. 2009, 19 (30), 5343−5350. (30) Thomsen, C.; Strait, J.; Vardeny, Z.; Maris, H. J.; Tauc, J.; Hauser, J. J. Coherent Phonon Generation and Detection by Picosecond Light Pulses. Phys. Rev. Lett. 1984, 53 (10), 989. (31) Kanner, G. S.; Vardeny, Z. V.; Hess, B. C. Picosecond Acoustics in Polythiophene Thin Films. Phys. Rev. B 1990, 42 (8), 5403−5406. (32) Kanner, G. S.; Frolov, S.; Vardeny, Z. V. Detection of Electronic Excited States in Conjugated Polymers by Picosecond Transient Strain Spectroscopy. Phys. Rev. Lett. 1995, 74 (9), 1685−1688. (33) Hanfland, M.; Brillante, A.; Syassen, K.; Stamm, M.; Fink, J. Polyparaphenylene under Pressure - Optical-Absorption and Vibrational-Modes. J. Chem. Phys. 1989, 90 (3), 1930−1934. 1257

dx.doi.org/10.1021/jz300365b | J. Phys. Chem. Lett. 2012, 3, 1253−1257