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Françoise Provencher*, Maciej Sakowicz, Colin-Nadeau Brosseau, Richard Leonelli, Carlos Silva

Département de physique et Regroupement québécois sur les matériaux de pointe, Université deMontréal, C.P. 6128, Succursale centre-ville, Montréal (Québec), H3C 3J7, Canada

ABSTRACT

In organic photovoltaic diodes, singlet intrachain excitons dissociate into geminate polaron pairs (GPP) at theheterojonction, which further dissociate into photocarriers or relax into charge transfer exciton (CTX) states. Ourtemperature-dependent, time-resolved spectroscopic approach unravels the dynamics of those species in films ofpolycarbazole/fullerene derivative blend. We find that GPP act as a dark reservoir that feeds the CTX. At lowtemperature, the GPP are trapped and feed the CTX via tunneling, but not the free photocarriers. At room temperature,some of the GPP can overcome the Coulomb barrier and feed the CTX promptly, or be deeply trapped and feed the CTXon several timescales. We find that at room temperature, 16% of the geminate recombination is accounted for by trappedgeminate polaron pairs.

Keywords: Organic solar cells, semiconductor polymers, charge-transfer excitons, polaron-pair recombinationdynamics

1. INTRODUCTION Research on organic solar cells is steadily improving those devices performances, with record power-conversionefficiencies in excess of 7-8%1. While efforts in material synthesis and device engineering are paramount to advance thefield, understanding the underlying physics is also of utmost importance. The naive photophysical scenario in an organicdonor-acceptor photovoltaic blend is that the primary photoexcitation creates an intrachain exciton on the donormolecule, which then dissociates at the donor-acceptor interface to form free charge carriers that are then transported tothe electrodes. But there are also charge-transfer (CT) states, which are a variety of electronic states where the electronand the hole are localized on different molecules but are still bound by Coulomb forces. These CT states can act asprecursors to free charges, or act as traps at the donor/acceptor interface where the electron and hole are likely torecombine. Charge recombination is an important loss channel that hampers the efficiency of organic solar cells andefforts are deployed to understand its mechanisms.

Figure 1. Chemical structure of PCDTBT and PC70BM.

*francoise.provencher@umontreal.ca; phone 1 514 343-6111 #1603; http://www.phys.umontreal.ca/~silva/

Physical Chemistry of Interfaces and Nanomaterials X, edited by Carlos Silva, Proc. of SPIE Vol. 8098, 80980L · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.894025

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To address CT states recombination dynamics in an efficient organic photovoltaic device, we study a high-performancepolymer:fullerene photovoltaic system that demonstrated high power conversion efficiencies in both standard andinverted solar cell architectures, namely PCDTBT:PC70BM (see Figure 1 for the chemical structures). Efficiencies ashigh as 7.2%1 have been reported for optimized devices using this blend as an active layer. PCDTBT:PC70BM hasalready been scrutinized by photocurrent2 and time-resolved spectroscopic studies3,4, but we are the first to report slowrecombination dynamics of geminate charge pairs in this system.

In the present work, we implement time-resolved photoluminescence (PL) spectroscopy at 14 K and at room temperatureto probe charge recombination dynamics via CTX emission. We also perform photon-energy-dependent photocurrentmeasurements to probe the photocarriers. We find that at long-times, CTX are generated at the expense of geminate-polaron pairs. We conclude that in PCDTBT:PC70BM, a subset of the population is trapped and can only populate CTXwith a broad distribution of relaxation rates, even at room temperature.

2. EXPERIMENTAL SECTION2.1 Materials and sample preparation

PCDTBT (Mn = 13000; Mw = 30000; PDI = 2.3) was synthesized as described elsewhere5. PC70BM was purchased fromNano-C and used as received. Films were drop-cast from a 1:4 PCDTBT:PC70BM chloroform solution (7 mg/ml, Aldrichspectrophotometric grade chloroform) onto fuzed silica (Vitrosil) substrates in an inert atmosphere. Chemical structureof the molecules are shown on Figure 1.

2.2 Photovoltaic device fabrication and characterization

Photodiodes were made on a glass substrate with a ITO/PEDOT:PSS/PCDTBT:PC70BM/Al architecture. The deviceswere fabricated on pre-patterned indium tin oxide (ITO)-coated glass substrates cleaned by sonication in deionizedwater, ethanol, acetone, and isopropyl alcohol for 15 minutes each, followed by a 15 min oxygen plasma treatment.Then, a PEDOT:PSS layer was spin-coated at 5000 rpm, then annealed at 130 C for 15min. The activePCDTBT:PC70BM layer with 1:4 stoechiometry was spin-coated from a chloroform solution (spectrograde, Aldrich) at2000 rpm in a glovebox and left to dry without thermal annealing. The 100 nm thick aluminum cathode was evaporatedunder vacuum (2 × 10−5 Torr) at a rate of 2 Å/s. The devices were always handled and measured in oxygen- and water-free environments (N2 atmosphere or vacuum). The power conversion efficiency (PCE) of the devices was measuredwith a grade AAA solar simulator, under AM 1.5 conditions. The initial value of the PCE for all devices was between2.1-2.3%, and dropped to 1.3% three weeks later, with the VOC of all the devices in the range of 0.91 ± 0.03 V, inagreement with reported VOC in the literature.6,7

2.3 Electroluminescence

For the electroluminescence measurement, the biased cell was kept under vacuum. Lenses focused the emitted light ontoan optical fiber which was connected to the entrance slit of a spectrometer (Andor SR163) and was detected by a CCD(charge coupled device) Si array (Andor iDUS) cooled to -45 C.

2.4 Time-resolved PL spectroscopy

Time-resolved PL spectra were measured with an intensified CCD camera (Princeton-Instruments PIMAX 1024HB).The samples were excited with the output of an optical parametric amplifier (Light-conversion TOPAS) pumped by anamplified Ti:sapphire laser system (KMLabs Dragon), producing tunable, 40 fs pulses at 1 kHz. We generated 490 nm(2.53 eV) pulse trains, which is above the optical bandgap of both the PCDTBT and the PC70BM. The measurementswere carried out in a dynamic vacuum of < 10−6 mbar at 14 K and at 293 K in a cold finger continuous-flow cryostat.

2.5 Photocurrent spectrum

The photocurrent excitation spectrum was measured as follows. The photodiode was kept under vacuum while it wasilluminated with an ultrafast tunable laser output (Light-conversion TOPAS). The photocurrent was measured as the peakof the transient photocurrent read on an oscilloscope with a 50-Ω termination. The photocurrent intensity was correctedfor the laser intensity. The laser power varied for different wavelengths, but was kept in the range of 1 μW to 8 μW. Thephotocurrent was found to be linear with illumination power up to 10 μW.

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3. RESULTS AND DISCUSSION3.1 Absorption enhancement of PCDTBT

We present in Figure 2 steady-state absorption measurements of the neat PCDTBT and PCDTBT:PC70BM film. Steady-state absorption spectrum of the films were obtained on a Varian Cary 500 spectrophotometer. These spectra show thataddition of PC70BM to the neat PCDTBT polymer enhances the absorption in the ~2.7 eV region.

Figure 2. Normalized absorption spectra of neat PCDTBT and PCDTBT:PC70BM films in air at roomtemperature.

3.2 Charge transfer exciton spectral signature

Due to the lower energy level of the CTX, its emission appears as a red-shifted peak compared to the emission of theneat donor and acceptor. This behavior has been observed in the PL spectrum8-11 and the EL spectrum12 of many donor-acceptor blends. The identification of these spectral features in a wide variety of materials suggests the ubiquity of CTXas a recombination channel in donor-acceptor blends. PCDTBT:PC70BM is no exception, as can be seen from Figure 3.The red-shifted PL peaks at 1.44 eV, while the EL emission peaks at 1.35 eV. We thus propose to study the near-IR PLspectrum of PCDTBT:PC70BM films to unravel the dynamics of CTX.

Figure 3. Photoluminescence and electroluminescence emission spectra of neat PCDTBT and PCDTBT:PC70BMblend in 1:4 ratio.

3.3 Time resolved PL

We now turn our attention to the time evolution of the PL spectrum of the blend. Figure 4 shows that the decay of thesinglet exciton emission at 1.7 eV is fast, disappearing completely in less than 7 ns. Meanwhile, a broad feature emergesin the near-IR and lasts for hundreds of nanoseconds. This long-lived signal corresponds to the tail of the CTX emission.

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Figure 4. Normalized PL spectra of PCDTBT:PC70BM blend film at different times after the excitation pulse (490 nm).

Figure 5. Absorbance and delayed PL excitation spectra of PCDTBT:PC70BM film, and photocurrent spectrum of aITO/PEDOT:PSS/PCDTBT:PC70BM/Al device at 295K.

In order to track the dependence of the CTX on the excitation energy, we integrate spectrally the intensity of the time-resolved PL on a time window from 100 to 1000 ns, where the emission from the singlet exciton relaxation has totallyceased. We thus obtain a delayed PL excitation spectrum and find that it correlates with the absorption spectrum of theblend (see Figure 5). The delayed PL fluence dependence is sublinear (see Figure 6),

with a power-law exponent of 0.7. This sub-linear fluence dependence is due to early-time exciton-charge annihilation(see Ref. 13 and references within).

Figure 6. Fluence dependence of the delayed PL at 7 K. The fit is a power law with an exponent of 0.7.

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We further address the photogeneration of carriers by photocurrent excitation measurements on a PCDTBT:PC70BMsolar cell. Figure 5 shows that the photocurrent excitation spectrum is also correlated with the absorption spectrum. Wewould like to point out that similar photocurrent measurements in PCDTBT:PC70BM devices have been reported recentlyby Street et al.2

Since both the photocurrent and the delayed PL spectra follow the absorption spectrum, which probes the generation ofintrachain exciton, we thus conclude that there is a branching from the intrachain exciton to both the free carriers and theCTX. It is likely that this branching doesn’t occur directly, but is a multistep process via a dark intermediate charge-transfer state termed a geminate polaron pairs (GPP). The GPP is an electron-hole pair with a weaker Coulomb bindingenergy than the CTX and is a precursor to both the spatially separated free carriers, as well as to the tightly bound CTX.

To observe the dynamics of the CTX recombination, we integrate a narrow window of energy (1.50-1.52 eV) in the PLspectrum, as is highlighted on Figure 4, and track its intensity as a function of time on several orders of magnitude and attwo temperatures. The resulting PL decay curves are represented in Figure 7. Two main features are striking. First, thePL decay follows the same a power-law behavior at long times (>10 ns) at both temperature. A multipletrapping/detrapping theoretical model by Tachiya and Seki predicts a temperature-dependent power-law for the slowdecay kinetics of the density of charges for non-geminate bulk recombination.14 Since our results show a temperature-independent behavior, it means that the observed slow PL decay originates from geminate charge recombination. Thisconfirms that the near-IR PL signal is from the CTX. Secondly, the PL signal decays much faster at room temperaturethan at 14 K. Indeed the signal is one order of magnitude less after 10 ns at 293 K than at 14 K. This means that there is afast decay channel that is thermally activated. Note that we also studied the time-resolved PL decay of neat PCDTBTand neat PC70BM and they are different from the blend, asserting we are effectively probing the CTX state.

Figure 7. Time-resolved PL decay of PCDTBT:PC70BM at 293 K (open squares) and 14 K (open circles). Lines are fits tothe data by inverse Laplace solving of the CTX population rate equation.

3.4 Photophysical model

To explain the temperature-dependence of the PL decay behavior, we consider the following photophysical scenario. Aphoton is absorbed by the donor molecule and an intrachain exciton is created. The latter is separated at the interfacebetween the donor and the acceptor to form a bound geminate polaron pair. The distribution of geminate polaron pairsbinding energies is very broad, so the GPP above a certain temperature-dependent threshold are untrapped and can relaxpromptly, giving way to the fast decay at short times at room temperature. Meanwhile, a certain fraction of the GPPbelow this threshold is deeply trapped. Given enough time, the trapped GPP will relax via quantum tunneling, and thisstep is the slow recombination process bottleneck that produces a power law behavior. Once relaxed in the CTX state,the electron and hole will recombine with a lifetime of 1-2 ns. This scenario is represented graphically in Figure 8.

We can simplify this photophysical model by ignoring the fast processes of exciton generation and dissociation. We alsoconsider that the relaxation of the untrapped GPP to the CTX state is prompt. Thus, in this simplified model, the initialphotoexcitation populates the trapped GPP (level 2) and the CTX states (level 1) directly. The fraction of the total initialGPP population that is trapped is α and the fraction of untrapped GPP that populate promptly the CTX state is (1-α). The

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trapped GPP relax into the CTX state via quantum tunneling with a distribution of waiting times R(t). The CTX thenrecombine radiatively with a rate constant of Γ1. The simplified model is schematically represented on Figure 8.

Figure 8. Graphical representation of the photophysical models described in the text.

We translate this model into rate equations to express the population in the CTX state as a function of time in the Laplacedomain. The CTX population is proportional to the observed PL decay of Figure 7. We use a MatLab script to obtain theinverse Laplace transform of the rate equations numerically and fit it to the PL decay data. We find that at 14 K α=1,which means that at low temperature, the GPP lack the thermal energy to overcome the energetic barrier and they all aretrapped. On the other hand, at 293 K α=0.16, which means that at room temperature 16% of the geminate recombinationis accounted for by trapped GPP. These trapped GPP do not contribute to the photocurrent produced by the device, andthus hamper its efficiency. According to a recent paper by Etzold et al., the proportion of geminate vs non-geminaterecombination in PCDTBT:PC70BM films is 11% at room temperature4. Thus if 16% of these geminate recombinationscome from trapped GPP, that means that ~2% of all the recombination losses are due to trapped GPP at roomtemperature.

4. CONCLUSIONIn summary, we observe slow geminate charge recombination in an efficient organic photovoltaic blend. We investigatethe kinetics of the CTX via time-resolved PL and photocurrent measurements in a PCDTBT:PC70BM blend film. Ourphotophysical model reveals that deeply trapped GPP account for 16% of geminate charge recombination at roomtemperature. This infers that ~2% of the charge population is trapped Coulombically on microsecond timescales at roomtemperature. Even if these irremediably trapped charges do not account for a large amount of efficiency losses inPCDTBT:PC70BM, their study is a rich source of information about the photophysics of organic solar cells, hence theneed to further explore the molecular nature of slow recombination of trapped GPP.

ACKNOWLEDGEMENTS

CS acknowledges funding from NSERC and the Canada Research Chair in Organic Semiconductor Materials. CS andML acknowledge FQRNT for a Team Grant. CS and GL acknowledge mobility funding from the VIIe Sous-commissionmixte Québec-Italie. FP acknowledges a NSERC CGS-D Scholarship. MS acknowledges the MELS/FQRNT for a MeritScholarship. The authors thank Serge Beaupré and Mario Leclerc for synthesizing the PCDTBT.

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REFERENCES

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[2] Street, R., Song, K., Northrup, J. and Cowan, S. , “Photoconductivity measurements of the electronic structure oforganic solar cells,” Phys. Rev. B 83(16), 165207 (2011).

[3] Tong, M., Coates, N., Moses, D., Heeger, A., Beaupré, S. and Leclerc, M. , “Charge carrier photogeneration anddecay dynamics in the poly (2, 7-carbazole) copolymer PCDTBT and in bulk heterojunction composites withPC70BM,” Phys. Rev. B 81(12), 125210 (2010).

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[12] Tvingstedt, K., Vandewal, K., Gadisa, A., Zhang, F., Manca, J. and Inganas, O. , “Electroluminescence from chargetransfer states in polymer solar cells,” J. Am. Chem. Soc 131(33), 11819–11824 (2009).

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