Influence of Blend Composition on Ultrafast Charge Generation and

Article pubs.acs.org/JPCC

Influence of Blend Composition on Ultrafast Charge Generation and Recombination Dynamics in Low Band Gap Polymer-Based Organic Photovoltaics Giulia Grancini,†,‡ Nicola Martino,†,‡ Maria Rosa Antognazza,*,† Michele Celebrano,‡ Hans-Joachim Egelhaaf,§ and Guglielmo Lanzani†,‡ †

Center for Nano Science and Technology @ PoliMi, Istituto Italiano di Tecnologia, Via Pascoli 70/3, 20133 Milano, Italy Dipartimento di Fisica, Politecnico di Milano, Piazza L. da Vinci, 32, 20133 Milano, Italy § Konarka Technologies GmbH, Landgrabenstrasse 94, 90443 Nürnberg, Germany ‡

S Supporting Information *

ABSTRACT: A tremendous advancement in the performances of bulk heterojunction organic solar cells has been motivating a comprehensive investigation of many fundamental aspects regarding the structure−function relationship of these devices. The blend morphology has a crucial role in determining the device operation, affecting both charge separation and transport properties. Despite extensive spectroscopic investigations have been carried out to understand the fundamental photophysical processes, the charge separation mechanisms are still debated. Here we use ultrafast pump−probe spectroscopy to monitor directly photoexcited states dynamics in a promising polymer blend for photovoltaics, based on a low band gap polymer, poly[2,6(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT), integrated with a fullerene derivative, [6,6]-phenyl C61-butyric acid methyl ester (PCBM). We find that exciton quenching leads directly to two charge populations: charge-transfer states and free charges. Our experimental findings highlight how the different intrinsic charge generation pathways are strongly depending on the blend morphology and composition. The derived picture contributes to shed light on the fundamental photophysical processes that directly control the photovoltaic devices performances.

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INTRODUCTION Bulk heterojunction (BHJ) solar cells based on conjugated polymer blends have emerged as a promising technology in the development of low-cost photovoltaic power generation.1−5 In these composites, the donor/acceptor phases are dispersed to form an interpenetrating nanoscale network that provides both large interfacial area and percolation pathways, essential for efficient charge separation and transport.5−12 Charge generation occurs via a multistep process: after photoexcitation excitons diffuse toward an interfacial site, where they can separate into free charges (FCs) directly, via an ultrafast hot state dissociation, or through an intermediate state, usually referred to as charge transfer (CT) state.9,13−21 The latter consists of a pair of localized charges (polarons), Coulombically bound, with typical binding energy of about 0.1 to 0.5 eV.9,15−17 The role of the CT state has been largely investigated: evidence of the presence of this state has been reported by different techniques, such as time-resolved fluorescence,18−22 pump−probe spectroscopy, and photocurrent investigations.23−29 Primary events of relaxation following light absorption are of utmost importance in determining the photovoltaic solar cells performance. In general terms, several efforts have been done in multiple linked © 2012 American Chemical Society

directions to optimize the device performances, in particular toward: (i) elucidating the structure−function relationship,9,14,28 (ii) developing a comprehensive understanding of the charge generation process and the photophysics of the involved states at the BHJ interface,15,16,29−31 and (iii) pushing up the absorption spectral range with the development of a new class of copolymers with lower and lower band gap, thus covering the near-IR spectral region.32−37 Here we investigate the ultrafast photoexcitation scenario in a new class of low-bandgap-polymer/fullerene derivative systems that has recently boosted the efficiency of polymer cells to above 8%.1,34 In particular, we study poly[2,6-(4,4-bis-(2ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT). This is one of the first candidates of a new promising class of copolymers for organic photovoltaics adopting a cyclopentadithiophene unit as the donor block in the polymer chain.34 The success of PCPDTBT stems from several factors: (i) it shows improved chargetransport properties with mobility values as high as 2 × 10−2 Received: March 23, 2012 Revised: April 13, 2012 Published: April 16, 2012 9838

dx.doi.org/10.1021/jp302787u | J. Phys. Chem. C 2012, 116, 9838−9844

The Journal of Physical Chemistry C

Article

cm2 V−1 s−1,7,35 (ii) it has good processability, and (iii) it has an ideal energy gap of Eg = 1.46 eV.35 This ensures light harvesting in the near-infrared region. Moreover, when processed with peculiar additives (i.e., 1,8-diiodooctane, DIIO), the power conversion efficiency of the PCPDTBT:PCBM blend is enhanced by a factor of 2.27,30 The additives strongly affect phase separation and morphology, acting both on charge transport and FC generation.32,33 In this work, by means of ultrafast pump−probe spectroscopy, we unveil the charge generation dynamics in the PCPDTBT:PCBM blend, depending on the relative donor/ acceptor concentration. Moreover, we investigate the influence of processing additives on the photoexcitation scenario.

tapping mode and collecting the feedback signal, which allows stabilizing the tip−sample distance by a proper control over the oscillation induced on the tip. This approach has the potential to suppress the tip friction with the analyzed surface, hence dramatically reducing the degradation of soft samples, polymers in our specific case.

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EXPERIMENTAL RESULTS The normalized absorption spectra of the pristine PCPDTBT, the PCPDTBT:PCBM blends with different relative concentrations, and the blend processed with DIIO are shown in Figure 1. The red shift of the absorption onset observed in

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EXPERIMENTAL METHODS PCPDTBT polymer has been obtained by Konarka Technologies (molecular weight 27 700 g/mol; polydispersity index 1.9; purity >95%). PCBM and DIIO were purchased from American Dye Source and Sigma Aldrich, respectively, and used without any further purification. Solutions with different mixing ratios PCPDTBT:PCBM (1:0, 1:1, 1:3) were prepared with a concentration of 10 mg/mL of PCPDTBT in chlorobenzene. The additive was diluted with a concentration of 10 μL/mL. The solutions were deposited by spin coating on glass substrates; finally, the samples were annealed for 10 min at 140 °C. The absorption spectra have been measured in solid film by using a UV−vis-NIR spectrophotometer (Lambda 1050, Perkin Elmer). The transient changes of the excited-state absorption of the pristine polymer as well as of its blends were monitored by ultrafast pump−probe measurements with ∼150 fs time resolution. This technique provides a sensitive probe to depict a quantitative understanding of the excited-state population transitions and a clear picture of the time scale in which the cascade of photophysical events (photon absorption, exciton quenching, charge separation, and charge extraction) occurs. In a typical pump−probe experiment, the system under study is photoexcited by a short pump pulse, and its subsequent dynamical evolution is detected by measuring the transmission changes ΔT of a delayed probe pulse as a function of pump− probe delay τ and probe wavelength λ. The signal is given by the differential transmission ΔT/T = [(Tpump on − Tpump off)/ Tpump off]. The pump−probe setup is driven by 1 kHz repetition rate pulse train at λ = 780 nm center wavelength with 150 fs duration coming from a regeneratively amplified mode-locked Ti:sapphire laser (Clark-MXR model CPA-1). A fraction of this beam is used as the excitation pulse at 780 nm wavelength. Another small fraction of the Ti:sapphire amplified output is independently focused into a 2 mm thick sapphire plate to generate a stable single-filament white-light supercontinuum that serves as a probe pulse, spanning in the visible near-IR region (450−1000 nm). The pump (spot diameter ∼50 μm) and probe beams are spatially overlapped on the sample, and the time delay is controlled by a motorized translation stage. Details of the system can be found elsewhere.38,39 The pump beam energy used in this work was kept deliberately low, ∼30 nJ, thus avoiding bimolecular processes. All measurements were performed in open air at room temperature, and no degradation was observed. AFM measurements were performed prior to the optical characterization, using a commercial setup (α-SNOM, Witec). Topographical maps were collected by running the apparatus in

Figure 1. Optical absorption spectra of the polymer PCPDTBT, pristine (dashed black line), and in blend with PCBM, at various mixing ratios (in full dots the 1:1 blend, in open dots the 1:3 blend) and by addition of 1.8-diiooctane (DIIO) additive (red solid line). Spectra have been normalized to the main absorption peak in the NIR region. Chemical structures of the materials are also shown.

PCPDTBT:PCBM with DIIO sample can be attributed to an enhanced supramolecular order induced by aggregation of the polymer chains through the additive action.26,30,32,33 The molecular structures of the materials are reported in the inset. We note here that although there is an overlap in the absorption spectra of PCPDTBT and PCBM around 400 nm, photoexcitation at 780 nm can be used to excite selectively the polymer in the composite. First, we report (Figure 2) on the ultrafast dynamics in the neat PCPDTBT films by showing pump−probe spectra at several delay times from 200 fs up to 400 ps in the visible and in the near-IR (NIR) spectral regions (Figure 2a). Three main spectral features are present at 200 fs pump−probe delay: two broad positive bands spanning, respectively, from 550 to 800 nm and in the near-IR spectral region, and a negative band at shorter wavelengths, peaking around 480 nm. The first positive band is attributed to photobleaching (PB) due to the close resemblance to the optical absorption spectrum; the NIR band is assigned to stimulated emission (SE) from the excited singlet state to the ground state (S1→S0) according to the polymer fluorescence spectrum.20 The negative band is assigned to singlet−singlet (S1→Sn) photoinduced absorption (PA1). The PA1 band is almost completely decayed within the first 100 ps, in fair accordance with singlet lifetime values obtained from single photon counting fluorescence experiments.20 Note that the SE and PA1 dynamics (Figure 2b), at 950 and 480 nm, respectively, display the same temporal decay because they are 9839



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both related to transitions from the first excited singlet state, as depicted in Figure 2b, inset. In the BHJ blends (PCPDTBT:PCBM), optionally treated by DIIO addition, new channels for exciton decay are made available, giving rise to a more complex photoexcitation scenario. The presence of PCBM and DIIO strongly affects the nanomorphology and the segregation of the blend components. Figure 3 shows the atomic force microscopy (AFM) images comparing the PCPDTBT:PCBM blends (1:1 and 1:3) without (top) and with (bottom) the use of the additive. The images clearly resolve larger phase segregation in the presence of the additive, responsible for creating polymer nanoclustering with features ranging from ∼10 nm to about a few hundreds of nanometers. PCPDTBT:PCBM photophysical scenario is complicated by a zoo of possible spectral features related to singlets, triplets, charged pairs, and FCs. In the following, we report on the most significant photophysical pictures in three different study cases: (1) PCPDTBT:PCBM (1:1), presenting the same weight percentage of the PCPDTBT respect to the acceptor component; (2) PCPDTBT:PCBM (1:3), increasing the acceptor concentration; and (3) PCPDTBT:PCBM (1:3) processed with DIIO. (The latest was selected on the base of results, reported by Lee at al.,30 on optimized solar cells, with certified efficiency of more than 5%.) Additional blend compositions (PCPDTBT:PCBM (3:1) and PCPDTBT:PCBM (1:1) processed with DIIO) are reported in the Supporting Information section (Figures S1 and S2). We stress here that the goal of this work focuses on how the photophysical process and dynamics change by varying specific morphological parameters. Figure 4 presents selected transient absorption spectra (Figure 4a) and dynamics (Figure 4b) for the first study case (relative concentration 1:1 (w/w)) in the visible and in the NIR probe spectral regions.

Figure 2. Pristine PCPDTBT. Pump−probe spectra at selected probe delay times in the visible and in the NIR spectral regions (a) and photoexcitation dynamics at different probe wavelengths (b). Kinetics have been reproduced by a three-exponential decay fitting (solid lines).

Figure 3. AFM maps of PCPDTBT:PCBM blends (1:1 and 1:3, left and right, respectively) without (top) and with (bottom) the use of the DIIO additive. Polymer nanoclustering is negligible in the absence of the additive, whereas it is clearly visible and larger than the system lateral resolution in the presence of the additive for both compositions blends. Scale bar: 1 μm. 9840



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showing a long-lived charge PA band in the same spectral region (see Supporting Information, Figure S4). Note that usually pump−probe measurements report on the charge band monitored in the near-IR spectral region,15 which is easier to detect because it is free from spectral overlap with PB and SE bands. However, charged species are known to have several absorption bands, also at higher energies.42,43 Even if spectral congestion usually prevents discriminating this band, in our case, the spectral dip in the absorption spectrum around 500 nm of PCPDTBT enables us to monitor clearly the charge dynamics around 550 nm. Note that a third spectral feature at 520 nm can be observed in Figure 4a. We noticed (data not shown) that its temporal evolution fairly matches the 950 nm dynamics, possibly indicating a common origin; however, as it will be shown in the following, this spectral region is characterized by the presence of multiple signals, and their pronounced spectral overlap makes difficult to discriminate it clearly from the freecharge fingerprint as well as from the singlet−singlet absorption, observed upon the addition of the additive (see below). For this reason, we prefer to investigate the CT state dynamics only in the NIR region. After the formation in the first few picoseconds, the FC population at 550 nm shows an initial decay component that continues beyond our temporal window of 400 ps. Figure 5 reports the pump−probe spectra (Figure 5a) at fixed probe delay times and photoexcitation dynamics (Figure

Figure 4. PCPDTBT:PCBM (1:1 w/w). Pump−probe spectra at different probe delay times in the visible and in the NIR spectral regions (a) and photoexcitation dynamics at selected probe wavelengths (symbols) and fitting curves (solid lines) (b). Note the appearance of the PA2 and PA3 bands and the concomitant suppression of the SE and PA1 signals.

As in the case of pristine PCPDTBT, the PB band is still observed; on the contrary, the PA1 band peaking around 480 nm, fingerprint of S1→Sn transitions in the neat film, is almost entirely suppressed as well as the SE in the 880−980 nm spectral region (a residual sign of these spectral features can still be identified in the first few hundreds of fs; see Supporting Information, Figure S3).37,40,41 At the lower and higher wavelengths sides of the spectrum, new negative bands (PA2 and PA3, respectively) appear, which we relate to a fingerprint of charged populations. Figure 4b shows the temporal evolution of PA2 and PA3 bands, at selected wavelengths 550 and 950 nm, respectively. Their different temporal decay reflects that we are monitoring different species, both related to the charge-generation process. In particular, the PA3 band is assigned to CT state population, according to existing literature.41 The temporal trace at 950 nm (full red squares) shows an initial decay with time constant ∼2 ps, followed by longer-lived components. (See the Discussion below.) The dynamic at 550 nm (open green squares) displays an instantaneous formation, beyond our temporal resolution, and, interestingly, a rising component, complementary to the initial decay at 950 nm (i.e., characterized by the same time constant). This matching clearly indicates that the PA2 band is related to FC population, generated upon CT dissociation. This assignment is also supported by CW-PA measurements,

Figure 5. PCPDTBT:PCBM (1:3 w/w). Pump−probe spectra at different probe delay times in the visible and in the NIR (a) and photoexcitation dynamics at selected probe wavelengths (symbols) and fitting curves (solid lines) (b). As compared with the previous case, longer time constants are obtained from data fitting. The intensity of the 550 nm trace is multiplied by a factor of 2 for the sake of clarity. 9841



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5b) at selected probe wavelengths for PCPDTBT:PCBM (1:3) blend. The spectra show similar features with respect to the (1:1) sample: the initial SE component and singlet absorption (PA1) (see Supporting Information, Figure S5) are now totally quenched, whereas PA2 and PA3 bands are clearly visible and again show a complementary behavior at the earliest times. However, the time constant for the initial decay of the CT state and consequently the formation of FCs is now of ∼6 ps (in addition to the instantaneous component, as observed in the previous case), as reported in Table 1. Again, the FC population shows an initial decay component within the first 400 ps but with a longer time constant. Table 1. Time Constants for Charge Dynamicsa PCPDTBT:PCBM (1:1 w/w) PCPDTBT:PCBM (1:3 w/w) PCPDTBT:PCBM + DIIO (1:3 w/w)

550 nm

950 nm

τR ≈ 2 ps τD1 ≈ 150 ps τR ≈ 6 ps τD1 ≈ 320 ps τR ≈ 15 ps τD1 > 400 ps

τS ≈ 2.6 ps τD3 ≈ 35 ps τS ≈ 8 ps τD3 ≈ 130 ps τS ≈ 12 ps τD3 ≈ 220 ps

Time constants extracted from the fitting procedure of the photoexcitation dynamics at the selected wavelengths 550 and 950 nm for the different sample compositions. τR and τD1 are, respectively, the rising and the first decay time constant used for the FC dynamics, whereas τS and τD3 refer to the CT separation process and geminate recombination, respectively. Note the fair accordance between τR and τS in all considered samples and the increasing trend of τD1 going from (1:1) to (1:3) to (1:3) + DIIO sample. Two other decay time constants (τD2 and τD4) were required to obtain a good accordance of the fitting curves to the experimental data, which, however, resulted well beyond our temporal window (≫400 ps). a

Figure 6. PCPDTBT:PCBM (1:3 w/w) + DIIO. Pump−probe spectra at different probe delay times in the visible and in the NIR (a) and photoexcitation dynamics at selected probe wavelengths (symbols) and fitting curves (solid lines) (b). Note that all of the previously assigned PA and SE signals appear in this case. The intensity of the 550 nm trace is multiplied by a factor of 2 for the sake of clarity.

The effect of the additive DIIO on the photophysics of PCPDTBT:PCBM (1:3) film is finally investigated. Selected pump−probe results are reported in Figure 6. At variance with the blend without the additive, we clearly detect the presence of a PA1 band peaking at 480 nm. According to its spectral shape and temporal evolution, by analogy to the measurements in the pristine material (Figure 2), we assign it to S1→Sn excited-state transitions. The PA3 signal at 950 nm is accordingly overlapped with a residual SE band. Indeed, the apparent initial formation of the signal at 950 nm in a few picoseconds, due to SE quenching, matches the PA1 singlet decay at 480 nm. Note, however, that on longer time scales the dynamics at 480 nm reveals a long-lived component due to the spectral overlap with the tail of charges PA2 band. Again, the dynamics at 550 nm shows a rising component with a much longer time constant (τ ≈ 16 ps, see Table 1) with respect to the previous case, originated from the decay of the CT population at 950 nm; interestingly, within the first 400 ps, no sign of recombination is detected in this case for the FC population.

(Figure 7a). (ii) Exciton dissociation can occur from higher energy CT states, giving rise to spatially separated FCs before thermal relaxation occurs (a process still under debate9,17,21,34), as shown in Figure 7b. The probability of dissociation is strictly related to the energetics of the CT state, which is, in turn, strongly influenced by blend composition and morphology. Disorder in the blend induces a distribution of CT energies44 that results in a distribution of dissociating probability. Reported results are consistent with the occurrence of both charge generation processes. Mechanism (ii), taking place on ultrafast time scales ( 400 ps); conversely, in the other considered morphologies, an initial FC decay, ascribable to geminate recombination,41 is already present. However, because the complete recombination of both CT state and FC population cannot be observed within our temporal window of 400 ps, we avoid speculating on these processes, which should be studied on the nanosecond−microsecond time scale.

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CONCLUSIONS In this work, we have presented a thorough study of the relations existing between the sample morphology, its chemical composition, and the photoexcitations dynamics. We monitored different processes: exciton diffusion, CT dissociation and FC formation, FC initial decay, and CT geminate recombination. The 1:1 blend photophysics is explained by the fine mixing of the PCPDTBT and PCBM domains: here charges are generated upon exciton diffusion and recombine on few hundred picoseconds, possibly due to the lack of percolation pathways, thus explaining both the low fill factor and open circuit voltage of devices based on this active material.31 In the 1:3 blend, the increased amount of PCBM leads to a higher interfacial area, thus speeding up the charge separation step and favoring charge percolation, as evident from reduced FC recombination rate. Finally, the blend processed with the additive, characterized by larger domains, shows an increased fraction of longer-lived FC, free from recombination. Moreover, we infer that the geminate recombination of FCs is reduced, thus explaining the previously measured increased charge photogeneration efficiency under short-circuit conditions.30 In conclusion, our experimental findings are relevant both from a photophysical point of view and in terms of photovoltaic device performance, thus opening up the possibility to understand the crucial structure−function relationship by carefully monitoring the primary photophysical processes.

Figure 7. Sketch of the primary photophysical events occurring upon photoexcitation in the photovoltaic blend. Free charges can be generated both by an Onsager-like process mediated by an intermediate state (i.e., the CT state) (a) or directly by ultrafast exciton quenching (b). The relative ratio is strongly dependent on the sample composition and morphology.

separation process), τD3, and τD4 (geminate recombination), and the FC dynamics by an exponential increase (τR), followed by a biexponential decay (τD1 and τD2). Fitted curves are reported in Figures 4b, 5b, and 6b as solid lines, and extracted time constants are listed in Table 1. Note that longer-lived components with time constants τD2 and τD4 are required to get a fair accordance of the fitted curves to the experimental data, but in all considered cases, they are beyond our temporal window (τD2, τD4 ≫ 400 ps). By varying the ratio from 1:1 to 1:3 and processing the blend with DIIO, the competition between CT dissociation in FCs and its geminate recombination becomes more and more efficient, as evident from the increasing trend of the FC rising time (at 550 nm probe wavelength). In particular, in 1:1 blend, FCs are formed with a time constant growing from ∼2 ps in 1:1, to ∼6 ps in 1:3, and reaching ∼16 ps in 1:3 film processed with the additive. We argue that the different interfacial morphology has the effect of shifting CT density of states progressively toward higher energies, thus enhancing the probability of charge separation. Reported results highlight that the exciton diffusion process strongly depends on phase-separation length scale of PCPDTBT:PCBM domains. Immediately after photoexcitation in the 1:1 blend the exciton diffuses toward a PCBM phase, as confirmed by residual SE signal; in the 1:3 blend, the large amount of interfacial area, due to the increased PCBM content, leads to instantaneous exciton quenching, totally suppressing the SE signal within our temporal resolution; in the (1:3) film processed with DIIO, a clear signature of singlet exciton reappears (SE quenched in the first picoseconds). It has been demonstrated that the additive induces aggregation of polymer chains into more ordered supramolecular polymer structures during film deposition prior to complete drying of the

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ASSOCIATED CONTENT

S Supporting Information *

Extensive figures and data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. D. Fazzi for helpful discussions. REFERENCES

(1) Konarka, 2011. www.konarka.com (accessed Mar 10, 2012).

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Influence of Blend Composition on Ultrafast Charge Generation and

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