Direct Optical Observation of Stimulated ... - ACS Publications

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The Journal of Physical Chemistry

Direct Optical Observation of Stimulated Emission from Hot Charge Transfer Excitons in Bulk Heterojunction Polymer Solar Cells Bhoj R. Gautam†, Andy Barrette†, Cong Mai†, Liang Yan‡, Qianqian Zhang‡, Evgeny Danilov§, Wei You‡, Harald Ade†, and Kenan Gundogdu†*

†

Department of Physics and Organic and Carbon Electronics Laboratory (ORaCEL), North

Carolina State University, Raleigh, North Carolina 27695, United States ‡

Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North

Carolina 27599, United States§ §

Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695,

United States *

E-mail:[email protected]

ABSTRACT: Charge transfer excitons (CTEs) play an important role in semiconducting polymer-based optoelectronic applications. In organic photovoltaics, they are an intermediate step between tightly bound excitons and free charges. Although CT state energies at the interface of bulk heterojunction organic solar cells have been reported using quantum chemical calculations and by sensitive external quantum efficiency (EQE) measurements, direct optical observation of CT states was limited to relaxed, low energy, CT levels. Here we used polarization anisotropy transient absorption experiments to measure emission from high-energy CT levels. These experimental methods provide means to study high energy CT state dynamics in BHJs with controlled molecular orientations and complement theoretical calculations of interfacial CT state energies.

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1. INTRODUCTION Charge transfer excitons (CTEs) are bound electron-hole pairs with electron and hole situated on different molecules or different segments of a molecule with donor acceptor character. In bulk heterojunction (BHJ) organic photovoltaic (OPV) cells, these excitons1-8 are formed at the donor acceptor interface by photoinduced charge transfer after optical excitation. CTEs are precursors of charge generation in OPVs and hence play a critically important role in device functionality.7,

9-10

Electroluminescence11-12, photoluminescence5,

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, and electro-

absorption spectroscopy measurements14 have been the main experimental tools for observation of CT excitons. These experiments measure the spectral response of either electrically or optically injected excitons. Since excitons relax to the bottom of the CT state manifold much faster than they radiatively recombine, luminescence measurements can only measure relaxed lower-energy CT states, which are in the range of 1.2 eV- 1.6 eV 2, 7, 13 for many polymer blends. Higher-energy levels in the CT state manifold can play a significant role in initiating of charge separation1 as the spectral overlap between the polymer states and the CT levels facilitates exciton diffusion to the interface via Förster transfer mechanism.15 However it is very challenging to experimentally observe emission from high-energy CT excitons. This is because spectral signatures of these states overlap with the polymer excitonic and CT photon-induced absorption (PIA) states. In addition, they are short lived because CT excitons charge separate or relax to the bottom of the CT band on sub-picosecond time scale. Time resolved photoemission spectroscopy has been used to measure the high energy CT levels with respect to the vacuum.16 But energy assignments of the high-energy CT levels with respect to the interfacial ground state mainly rely on theoretical calculations.4, 17


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Here we report unambiguous optical measurement of higher-level CT states in a blend of phenyl-C61-butyric acid methyl ester (PCBM) and electron-donating polymer poly [naphtha [2,1-b:3,4-b′] dithiophene–4, 7-di (thiophen-2-yl) benzothiadiazole] (PNDT–DTBT). The chemical structures of donor polymer (PNDT-DTffBT) and acceptor (PCBM) are shown in figure 1a. We used fluorinated (PNDT-DTffBT) polymer and processing conditions such that the polymer fibrils preferentially form a face-on interface with the PCBM molecules. This preferred molecular orientation of donor and acceptor is potential for increased wave function overlap between the polymer chains and PCBM and improves charge separation efficiency. 18-19 In order to observe CT excitons, we excite the blended sample using an ultrafast laser pulse (pump), and then we used a second pulse (probe) to stimulate the emission from the CT states, before they relax to the bottom of the CT band. In order to differentiate the CT emission from that of the relaxed Stokes shifted singlet emission of the polymer, we measured the polarization anisotropy of the stimulated emission. The schematic illustrations in Figure 1b and 1c display the polymer/fullerene interface and stimulated emission from the polymer and the CT states. The horizontal zigzag lines correspond to polymer chains, and the spheres represent PCBM. When a linearly polarized light with appropriate excitation energy is incident to thin film, it selectively excites those chromophores whose transition dipole moments (TDM) are oriented parallel to the polarization of the light. The TDM of a conjugated polymer is along its backbone; therefore, a pump pulse creates an exciton that is primarily delocalized in the backbone direction. On the other hand, when a CT exciton is formed, an electron transfers to the PCBM, leaving the hole on the polymer side of the interface. This photoinduced excitation transfer to the PCBM rotates the TDM and makes it perpendicular to that of polymer. Therefore, the probe pulse stimulates the emission of


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polymer excitons, when it has the same polarization with the excitation field, whereas it will stimulate the emission from the CT states, where it has the perpendicular polarization with respect to the excitation field.

Figure 1. Chemical structure of PCBM and PNDT-DTffBT (a), X in PNDT-DTffBT refers to fluorine. Schematic illustration of rotation of the transition dipole moment (TDM) due to transformation of polymer exciton (b) to CT exciton (c). Major axis of red oval indicating TDM direction has been rotated when polymer exciton is transformed to CT exciton. The blue dashed arrows are the polarization direction of the initial excitation. The oscillating red lines are the emission of polymer exciton (b) and CT exciton (c) with orthogonal polarizations.

2. EXPERIMENTAL SECTION Blend of PCBM and PNDT–DTffBT was prepared at weight ratio of 1:1 in dichlorobenzene (DCB) solution (10 mg/ml) and was heated at 120−140 °C for 6 h. Blend films were prepared by spin casting the hot solution on ultrasonically cleaned glass substrates at 500 rpm for 60 s. Films were dried at room temperature in a sealed petri dish for 12 hour. Neat PNDT-DTffBT films were prepared using same procedure from 15 mg/ml of solution. The thin film samples were encapsulated using UV curable glue before measurement.


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A Cary win 50 UV-spectrophotometer from Varian was used for the room temperature ground state absorption measurement. An Edinburgh Luminescence Spectrometer (Model: F900) equipped with a xenon lamp was used to measure the room temperature steady state photoluminescence (PL) spectra in the UV-NIR spectral region. The samples were excited with 2.02 eV excitation energy and the emitted PL was detected using a red sensitive PMT. In the transient absorption experiments the output of amplified Ti:Sapphire laser provides 100fs pulses centered at 800 nm at 1 kHz repetition rate. These pulses are then split into two different paths. The first one is used to pump an optical parametric amplifier (OPA) system to generate pump pulses to excite the sample. The second one is focused onto sapphire window to generate white light continuum pulses, which are used to probe the excited state dynamics in a broad spectral range from 1.0 eV to 2.4 eV. A CCD spectrometer combination measured the differential transmission of probe pulses at different time delays.

In order to resolve the

polarization anisotropy, experiments are repeated with parallel and perpendicular linearly polarized pump and probe pulses. 3. RESULTS AND DISCUSSION Figure 2 shows the absorption and photoluminescence (PL) spectra of neat PNDT– DTffBT and PNDT–DTffBT:PCBM blend films. Vibronic features above 1.7 eV are reflected in the absorption spectra of both films. The slight differences in relative intensities of the 0-0 and 01 vibronic transitions indicate slight variaton in aggregation of polymers when blended with PCBM. The PL spectra in both films exhibit an emission peak centered at 1.65 eV due to radiative recombination of polymer exciton. The PL emission in the blend also exhibits a weaker shoulder at 1.2 eV 13due to lowest energy relaxed CT exciton emission, which is absent in neat film.


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Figure 2. Absorption (a) and Photoluminescence (b) spectra (PL) of Neat PNDT-DTffBT and PNDT-DTffBT:PCBM blend films. The inset in Figure b shows the PL intensity comparison of neat PCBM and PNDT-DTffBT:PCBM blend films under same excitation energy and same intensity. The samples were excited with 2.02 eV excitation energy for PL measurement.

Figure 3 a and b show the transient absorption spectra (TAS) of PNDT–DTffBT:PCBM blend at three different delays in the spectral range from 1.55 eV to 2.2 eV for parallel and perpendicular pump and probe polarizations, respectively. There are three different features contributing to the transient spectra. Increased transmission above 1.77 eV, with similar vibronic structure in the absorption spectra, is due to bleaching of the ground state (GSB). A positive signal centered near 1.66 eV, evident at 0 fs, is due to stimulated emission (SE). The SE is overlapped with a photoinduced absorption (PIA) band, which is due to CT excitons17 and separated charges.20 The PIA dominates the transient absorption spectrum between 1.55 and 1.77 eV at later delay times as SE vanishes quickly. Interestingly, the SE is strong and lasts longer in the perpendicular pump-probe polarization configuration compared to the parallel configuration. Figure 3 c shows that, while the SE for parallel configuration has almost vanished,


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it still persists in the transient spectrum measured using perpendicular configuration, even at 360 fs. Importantly, even without further analysis, the stronger SE feature in the perpendicular configuration directly and unambiguously indicates that it is from an interface state, based on related arguments presented above and in Figure 1. It cannot be from singlet emission in the polymer phase. We will later show the pure polymer results supporting this assertion.

Figure 3 Room temperature transient absorption spectra of PNDT-DTffBT:PCBM blend at (a) parallel, (b) perpendicular probe polarization relative to pump polarization at three different delays. Transient absorption spectra at 360 fs delay with both polarization of probe relative to pump polarization (c). Excitation at 2.02 eV.

In order to measure the evolution of the CT exciton population, we calculated polarization anisotropy21 r (t ) =

I || − I ⊥ I || + 2 I ⊥

of the transient spectra. In this equation, I || and I ⊥ are

the intensity of the transient absorption signal obtained with polarization of the probe pulses parallel and perpendicular to the pump pulses, respectively. Figure 4a shows the anisotropy value of different excited state species at 0 fs. The initial anisotropy of GSB is positive and has a value in between 0.2-0.3. On the other hand, the anisotropy in PIA region shows both positive and negative values indicating the presence of different species in that probe range. The anisotropy


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value is ~0.2 for the PIA peak centered around 1.71 eV whereas SE peak anisotropy is ~ - 1 centered at 1.66 eV. For a single excitonic transition, polarization anisotropy takes positive values and decays to 0 as the orientation of the TDM randomizes.22-23 In an interface, however, as schematically shown in Figure 1, there are two excitonic transitions, which are perpendicular to each other. As the polymer exciton evolves into an interface exciton, the TDM rotates. As a result, the polarization anisotropy can take negative values.24-26 The absolute value of the polarization anisotropy depends on the number of TDMs, and their relative orientations. For a simple two level system, in which the TDMs are randomly oriented, the excited chromophores are photoselected with a cos2Θ projection of the polarization. This leads to an anisotropic response with the maximum value being 0.4.21 However if response from multiple transitions overlaps (PIA, SE and/or GSB), anisotropy could take a value between -∞ < r < ∞.27-29 The initial anisotropy for the spectral region above 1.8 eV is within the range of two-level system because it is GSB that contributes primarily to the transient spectra (Figure 4a). The value is less than theoretically expected value (0.4), due to ultrafast exciton hopping or fast electron or hole transfer processes that do occur on the time scale within the time resolution of our system.1, 23, 30

Below 1.8 eV, both PIA and SE contribute to the TAS. Since the PIA and SE are

fundamentally different processes, we deconvoluted their relative contributions, by using Gaussian functions to fit the spectra, and determined the anisotropy of the SE located at 1.66 eV. Figure 4b shows that initial SE anisotropy is -0.1 and decreases to -0.35 in the first 200 fs. Further negative growth of SE anisotropy suggests that at later delays, the SE from the polymer exciton decreases and the SE from the CT excitons persists (Figure 3), i.e. the exciton population continues to transfer from polymer states to interface states and the negative anisotropy grows.


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Figure 4 The anisotropy of the transient absorption spectra at 0 fs for the raw data PNDTDTffBT:PCBM blend (a) and the evolution of the anisotropy of SE derived after deconvoluting the SE and the PIA signatures (b). Two dashed horizontal lines in Figure (a) show the range of anisotropy for two level system. Vertical blue line separates GSB (right) and PIA/SE regions (left).

Although the larger SE signature in the perpendicular configuration is an unambiguous indication that its origin is from an interface state, we also performed, as a control, the polarization dependent transient absorption measurement on the neat PNDT-DTffBT. Figure 5a shows the transient absorption spectra for the parallel and perpendicular configuration at 0 fs. The TAS at this delay shows the purely stimulated emission at probe energy of 1.65 eV with little evidence of a PIA feature. This confirms that the PIA observed for the blends has to be associated with CT states. Both GSB and SE in TAS have positive initial anisotropy close to 0.4 as shown in Figure 5b, indicating that these excitations have not rotated at all and the relaxed excitons underlying the SE feature are likely still on the initially-excited chains. The difference in the magnitude and sign of initial anisotropy of SE between the neat and blended films is the


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key observation of this work and proves that the SE in the blend originates from a donoracceptor interface exciton.

Figure 5. Room temperature transient absorption spectra at 0 fs (a) and initial anisotropy (b) of neat PNDT-DTffBT film. Excitation energy was 2.02 eV.

As an additional control, we measured polarization dependent TAS of neat PCBM (Figure 6). Even at more than an order of magnitude higher excitation intensity, the signal amplitude in neat PCBM was an order of magnitude smaller compared to our measurements on the blended thin film. In Figure 6, the TAS of neat PCBM film does not show any GSB and SE feature for either parallel or perpendicular pump and probe configurations. The spectra exhibit PIA features at ~2.4 eV and 1.7 eV.31 At 1.7 eV spectrum exhibits negative anisotropy. But the fact that relatively high-power excitation of PCBM thin films resulted in relatively weak signal, as well the fact that it is not the SE but PIA feature that contributes to negative anisotropy in neat PCBM film, suggests that the observed negative anisotropy in the SE of blend is not an artifact of PCBM but is rather from CT emission in blended thin films.


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Figure 6. Room temperature transient absorption spectra at 0 fs (a) and initial anisotropy (b) of neat PCBM film. Excitation energy was 2.02 eV.

Having identified the SE features as a CT exciton, we address the question of whether it is hot or relaxed. The negative initial anisotropy value of SE indicates formation of high-energy CT excitons within the time resolution of our experiment. The singlet exciton in the polymer must be sufficiently delocalized and coupled to the high-energy CT states to have negative initial anisotropy. Also the spectral position of stimulated emission at 1.66 eV is in the energy range of CT states observed in sensitive EQE measurements, which represent CT states that are directly created by absorption and thus completely unrelaxed. At the same time, this CT state is significantly above the position of lowest energy of CT in polymer blend (Figure 2b) inferred from electroluminescence11-12 and photoluminescence.5, 13

4. CONCLUSIONS In conclusion, we directly and unambiguously observed with optical methods the hot CT state in a BHJ blend using polarization anisotropy methods. Our observation and simple methods open the door to assess the hot CT states in BHJ with different and controlled degree of


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molecular orientation and inter diffusion of donor and acceptor interfaces. This will greatly facilitate experimental and theoretical understanding of structure-property relations in these extensively studied systems by assessing CT state energies as well as creation and decay rates.

ASSOCIATED CONTENT Supporting Information. Additional experimental results “This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author * E-mail:[email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS This work was supported by Office of Naval Research (ONR) grant N000141310526 P00002 (B.R.G, A.B, C.M and K.G), ONR grant N000141410531 (H.A), ONR grant N000141410221 (Q.Z and W.Y) and NSF SNM grant ECCS-1344745 (L.Y and W.Y).

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