Carrier Dynamics in Pentacene|C60 Bilayer Solar Cell Investigated

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Carrier Dynamics in Pentacene|C Bilayer Solar Cell Investigated through the Magnetoconductance Takuya Omori, Yusuke Wakikawa, Tomoaki Miura, Yuji Yamaguchi, Ken-ichi Nakayama, and Tadaaki Ikoma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp508799j • Publication Date (Web): 17 Nov 2014 Downloaded from http://pubs.acs.org on November 26, 2014

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The Journal of Physical Chemistry

Carrier Dynamics in Pentacene|C60 Bilayer Solar Cell Investigated through the Magnetoconductance Takuya Omori,† Yusuke Wakikawa,‡ Tomoaki Miura,† Yuji Yamaguchi,§ Ken-ichi Nakayama,§, ǁ and Tadaaki Ikoma,*, †, §, ¶ †

Graduate School of Science and Technology, Niigata University, 2-8050 Ikarashi, Nishi-ku,

Niigata 950-2181, Japan. ‡

Center for Fostering Innovative Leadership, Niigata University, 2-8050 Ikarashi, Nishi-ku,

Niigata 950-2181, Japan. §

Core Research for Evolutionary Science and Technology, Japan Science and Technology

Agency, 4-1-8 Honcho, Kawaguchi, 332-0012, Japan. ǁ

Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa

992-8510, Japan. ¶

Center for Instrumental Analysis, Niigata University, 2-8050 Ikarashi, Nishi-ku, Niigata 950-

2181, Japan.

ACS Paragon Plus Environment

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Abstract

We demonstrate the magnetoconductance (MC) effect originated from depressing the spin mixing in encounter pairs under the external magnetic field provides quantitative information about the singlet fission, the charge recombination and the trap-related dynamics with triplet exciton in a bilayer device of pentacene (Pen) and fullerene (C60). Three MC effects in low-, moderate- and high-fields were detected in the bilayer device at room temperature. Kinetic analysis of the low-field MC effect showed that the charge recombination yield at the Pen|C60 interface is ~1 %. Quantum mechanical simulations of spin-carrying pairs following the conservation rule of spin angular momentum in recombination showed that the moderate- and high-field MC effects are caused by, respectively, the trap-related dynamics with triplet exciton and the singlet fission with a maximum yield of 52 % in the layers. The quantitative information obtained by investigating the MC effect will contribute to the development of high-efficiency OSC devices.

Keywords Charge Recombination, Singlet Fission, Trap-related Dynamics, Spin Conservative Reaction, Magnetic-Field-Dependent Intersystem Crossing

1. INTRODUCTION Organic solar cells (OSCs) have great potential for use in light-weight, flexible solar energy conversion devices. In an effort to realize low-cost and high-efficiency OSCs, the


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application of several new concepts in electron spin engineering to device development have been investigated recently.1-5 The interactions of electron spins in organic semiconductors are much smaller than the thermal energy of ambient temperature, but the electron spin plays an essential role in photodynamics such as charge recombination, singlet fission, triplet fusion and so on, because of the strict conservation rule of spin angular momentum. Therefore even if the carrier spin dynamics has a small change it considerably affects the kinetics of exciton and carrier. In this paper, we will show that the magnetoconductance (MC) effect arisen from the magnetic field sensitive spin dynamics provides quantitative information about the exciton and carrier reactions in OSC devices. Although there are sophisticated experimental techniques to observe those dynamics,6-11 the measurement of the MC effect can be simply performed using a conventional electric current measurement apparatus equipped with a small additional electromagnet for applying a small magnetic field of only a few hundred milli-tesla and also is a convenient method requiring neither preprocessing nor decomposition of the device as illustrated in Figure 1.

Figure 1. Setup for measuring MC effect and structure of Pen|C60-bilayer solar cell. Details are given in Experimental Section.


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The MC effect is defined as the ratio of the variation of the current density J induced by applying an external magnetic field B to the current density in the absence of B: MC = [J(B)−J(0)]×100/J(0). It has been extensively investigated in organic solids,12-16 and it has recently been reported that OSC devices without magnetic electrodes also exhibit the MC effect in both dark and light conditions.17, 18 Numerous mechanistic studies on various organic semiconductors have shown that the carrier dynamics of spin-carrying pairs following the spin conservation rule during collisions are crucial for the intrinsic MC effect.19-28 Because an external magnetic field depresses the spin mixing in the spin-carrying pairs before the reaction, unless the magnetic field is higher than several tesla,24,29 the product fraction of the spinselective reaction changes with the field strength. The magnetic field dependence of the MC effect normally saturates in magnetic fields that are high in comparison with the internal magnetic field driving the spin mixing. The larger the internal magnetic interaction in the spincarrying pair, the higher the external magnetic field necessary to reach the saturation value of the MC effect (MCS). Hence the magnetic field at the half MCS (B1/2) can be a good measure for the internal magnetic interaction of the spin-carrying pair. Based on the simple concept for experiments on the MC effect in organic semiconductors, in this paper we clarify the charge recombination, the singlet fission and the trap-related dynamics with triplet exciton in a bilayer OSC consisting of pentacene (Pen) and fullerene (C60), which possesses a well-defined p/ninterface (Figure 1). Furthermore kinetic and quantum mechanical simulations allow us to discuss quantitatively on the exciton and carrier dynamics in the solar cell. The simple MC-based analysis methods are useful for the development of high-efficiency OSC, since non-destructive techniques to inspect the dynamics in devices with complicated internal structures is requisite.


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2. EXPERIMENTAL SECTION We fabricated devices having active layers consisting of two thin films of pentacene (Pen) and fullerene (C60). Using a spin-coat technique, a hole buffer layer of poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was coated on an anode of indium tin oxide (ITO) patterned on a glass substrate. After annealing the PEDOT: PSS for 20 min at 120 °C in air, the films in the active layers were stepwise formed on the buffer layer by a vacuum vapor deposition method. As a cathode, aluminum (Al) with a low work function was coated on the active layer by the vacuum vapor deposition. The cross-section structure of the device was |ITO|PEDOT:PSS|Pen (60 nm)|C60 (40 nm)|Al|, where the numbers in the parentheses are the thicknesses of each film. The size of the active area in the devices was 2 × 2 mm. Finally, the devices were encapsulated in a glove box to prevent degradation by air exposure. The device characteristics of current density (J) versus bias voltage (V) were observed using both a standard apparatus (CEP-2000, Bunko-keiki) and a lab-made measurement system combined with a source meter (2611, Keithley) and a solar simulator (HAL-C100, Asahi Spectra). LED lamps emitting visible light with wavelengths (λ) of 455 or 627 nm (LLS, Ocean Optics) were used for selective excitation of C60 or Pen, respectively. The light to generate the photocarriers was guided to the device from the side of the ITO electrode using optical fibers. To detect the MC effect, we applied a magnetic field (B), the strength of which was monitored by a gaussmeter (421, LakeShore) placed close to the device, orientated parallel to the electric field (E) of the bias voltage applied to device, using an electromagnet (TM-YSV6609J-021.5, Tamagawa Factory). Unwanted influences on the MC effect, due to drifting in the device characteristics induced by long term illumination under an electric field, were removed by


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subtracting the current at zero magnetic field J(0) just before each measurement at nonzero magnetic field J(B). All the measurements were performed at room temperature.

3. RESULTS MC Effects in Dark and Light. The Pen|C60-bilayer device exhibits a typical rectification effect in the dark and a reasonable photovoltaic effect in the light of a solar simulator, as shown in Figure S1. Figure 2 shows the magnetic field dependence of the MC effect of a Pen|C60-bilayer solar cell in dark and light. The dark and photo states exhibit negative and positive MC effects respectively. The opposite polarity of the MC effect for these two states stems from the differences in the method of carrier injection and the nature of the current. Electrode injection, which gives rise to a current from the anode to the cathode and no spin correlation between electrons and holes at the interface – a so called nongeminate pair – causes a negative MC. Photo-injection, which generates spin correlated electron–hole (e-h) pairs from excitons – so called geminate pairs – at the interface and a current from the cathode to the anode, causes a positive MC. The magnetic field dependence of the MC effect can be fitted by the following empirical Gaussian functions.30    1 − exp  − ln 2  B MC ∑ S,i  B1  i =1   2 

   

2

   .  

(1)

The MC curve of the dark state in Figure 2 was fitted using double Gaussians with linewidths of 2.1 (MC1) and 13 mT (MC2), corresponding to the B1/2. The MC curve of the photo state was reproduced by triple Gaussians with B1/2 = 4.3 (MC1), 20 (MC2) and 190 mT (MC3). The additional MC3 term covers the broadest shape appearing in the high fields outside ±100 mT.


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Figure 2. MC effects of Pen|C60-bilayer solar cell depending on the method of carrier injection. The MC effect was measured at 0.20 V in the dark (blue) and at 0.00 V under illumination by a 1.5AM-solar simulator (100 mW/cm2) (red). The black curves are fits using multiple Gaussians. The shapes for the individual Gaussians involved in the fitting are shown by dotted curves. Bias Dependence. For the dark state, the shape of the MC curve drastically varies with the applied bias voltage (V) although it keeps a negative phase. (Figure S2) All the MC curves were analyzed with double Gaussians as mentioned above. Figure 3A shows the voltage dependence of MCS for the narrow MC1 and middle MC2 components, which are prefactors for individual Gaussians. The MCS value of the narrow MC1 component has a maximum amplitude around 0.16 V and decreases with increasing bias voltage, while that of the middle MC2 component decreases monotonically. The J/V-characteristic in Figure 3B clearly has two critical points around 0.16 and 0.50 V. Based on their slopes, it can be said that the nature of the current switches from diffusion to drift around 0.16 V and exhibits a space-charge-limit above 0.50 V. The MCS peak position of the MC1 component agrees with the turning point of diffusion-to-drift in the current. This fact indicates that the narrow component is strongly related to the diffusion


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motion of the carriers.31 The appearance of the MC2 component in all current regimes suggests that the carrier dynamics for the middle component are constantly active.

Figure 3. Correlation between the current density and the MC effect in the dark for a Pen|C60bilayer solar cell. (A) Bias dependence of the saturation values for the narrow MC1 and middle MC2 components. The solid curves are Gaussian (red) and exponential (blue) functions fitting the MC1 and MC2 components, respectively. (B) Logarithmic plot of the bias dependence of the current density. The broken straight lines are eye guides. (C) Dissociation rate dependence of the magnetic field effect on the recombination yield φr calculated for a ratio between the singlet and triplet recombination rate constants (kS : kT) of 0.65 : 1.00 in a kinetic model for e-h pairs described in S-III of Supporting Information.


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Excitation Dependence. Figure 4A and A’ show MC curves observed for selective excitations of Pen and C60 by illumination with monochromatic light of 627 and 455 nm, respectively. Three types of positive MC effect with different linewidths were detected in both cases and could be well fitted by triple Gaussians in a similar manner to the case of a solar simulator (Figure 2). Three B1/2 values were estimated from fitting the MC curves in Pen excitations, and were similar to those for non-selective excitation using the solar simulator. The B1/2 value of the narrow MC1 in the C60 excitation was also the same as that of the MC1 component in the non-selective excitation, while the other two values of 180 ± 15 mT and more than 400 mT were much broader than those of the solar simulator. There was considerable individual device variation in the B1/2 value of the C60 excitation. The similarity of the MC and J/V curves (Figure S1A) between the Pen-selective and non-selective excitations indicates that it is mainly exciton of pentacene that initiates the photocarrier dynamics in a Pen|C60-bilayer device. Although the contribution of the C60 excitation to the photovoltaic effect of this device is not significant, the results obtained for C60 excitation imply that the high-energy excitation at short wavelengths and the inhomogeneity of the morphology in the C60 layer cause relatively complicated carrier dynamics. The MCS of the narrow MC1 component was almost independent of the incident light power, as shown in Figure 4B. On the other hand, the MC2 and MC3 components increased as the light power increased (Figure 4C and D).


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Figure 4. Excitation wavelength and light power dependences of the MC effect in light for a Pen|C60-bilayer solar cell detected at zero bias. Selective excitations of the Pen and C60 layers performed by illumination at λ = 627 (A) and 455 nm (A’), respecitvely, with a power density of 31.8 mW/cm2. The black solid and broken curves in A and A’ are fits using, respectively, multiple Gaussians and simulations calculated by a density matrix formalism (see S-IV of Supporting Information). The saturation values for the MC components with B1/2 values of (B) ~ 5.5 mT, (C) ~ 23 mT and (D) ~144 mT were obtained by fitting with triple Gaussians. The red and black data points indicate values for selective excitation of Pen layer and non-selective excitation by a solar simulator, respectively.

4. DISCUSSION The observed MC effects show that a solar cell with a Pen|C60 bilayer exhibits three types of MC components that have different B1/2 values. The narrow MC1 and middle MC2 components are observable in both dark and light, but the broad MC3 appears only in light. The polarity of the MC effects depends on the method of carrier injection; it is negative for electrode


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injection and positive for photoinjection. The mechanisms for these observed MC effects can be discussed based on spin conservative carrier reactions through encounter pairs with multiplet spin sublevels depending on the spin angular momentum. The Zeeman interaction on an encounter pair with an external B-field depresses the intersystem crossing (isc) among the spin sublevels.12, 14, 29 Recombination. Because the B1/2 value reflects the internal magnetic interactions driving the isc, the MC1 component with a small B1/2 value near 2–5 mT can be assigned to an effect due to the doublet–doublet (DD) pair mechanism that exhibits the hyperfine interactions with nuclear spins. The strong correlation of the MC effect due to the DD pair mechanism with the diffusion motion of charge carriers leads to recombination dynamics of electron–hole (e-h) pairs at the interface between the Pen and C60 layers as illustrated in Scheme 1. Scheme 1. General Scheme of DD Pair Mechanism (Upper Part) Corresponding to Charge Recombination Responsible for the Narrow MC Effect (Lower Part). S, D and T Mean Singlet, Doublet and Quintet States Respectively. In the Context of the Pen|C60 Solar Cell, Ineffective Singlet Channel (kS) Compared with Triplet Channel (kT) is Expressed by Shading.

In the diffusion regime the recombination rate would be comparable with the formative and dissociative diffusion rates expressed by the constants k1 and k-1. Such a rate-competing condition is necessary for the external magnetic field to change the recombination efficiency,


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which results in the MC effect. However, once the drift motion becomes dominant due to a reversal of the potential slope of the semiconductors under bias voltages higher than a built-in potential (Vbi), the e-h pairs generated at the interface no longer have an effective k-1 corresponding to a dissociation process for the e-h pair. In other words, k-1 implying a carrier moving away from the interface decreases with the bias voltage. Therefore, at high bias voltages most of the carriers at the interface are recombined in the bilayer device because of the difficulty for the carriers to enter into their counter layers due to a large band mismatch. The almost oneway process is why the MC effect of the DD pair mechanism drops off in the drift regime. The polarity of the MC effect depends on both the population of the spin sublevels and the spinselectivity for recombination of DD pairs. In the case of photoinjection, e-h pairs are born from excitons (ex) created by the absorption of light. A single exciton with an antiparallel configuration of spins (1ex) in the Pen layer has sufficient electronic energy to generate a geminate e-h pair at the interface. This one-photon process for creating an e-h pair is why the DD-pair MC component in light is independent of the light power and the spin angular momentum of the e-h pair under illumination is singlet. Because the short-circuit current is a drift current of carriers under Vbi, which is the dissociation of an e-h pair born at the interface toward the electrodes, the photoconductance is proportional to the dissociation yield of an e-h pair. The observed positive MC effect in light and the initial population of 1e-h pairs leads to a conclusion of a preferential recombination from 3e-h pairs rather than 1e-h pairs. The triplet recombination probably produces a triplet exciton (3ex) in the Pen layer, because an e-h pair has an energy between that of the triplet states of pentacene (3Pen, 0.86 eV) and fullerene (3C60, 1.57 eV).32 The observed negative phase of the dark MC effect due to the DD pair mechanism can be explained by a recombination current at the interface.33 The preferential triplet recombination to


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Pen and the ratio of the populations of 1e-h and 3e-h of 1:3, according to simple spin statistics,

results in a decrease of the recombination yield in the presence of a magnetic field. The magnetic field effect on the recombination yield corresponding to the MCS value due to the DD pair mechanism was calculated using a simple kinetic model for non-geminate e-h pairs,34, 35 described in S-III of Supporting Information. As shown in Figure 3C, the magnetic field effect is negative and becomes maximum when the dissociation rate is comparable to the recombination rate. Such a peak behavior is the same as the observed bias dependence of MCS for the MC1 component. Fitting the calculated peak height to the observed height with −0.23 % gave a rate constant ratio of kS : kT = 0.65 : 1.00. To obtain the relative dissociation rate constant at zero bias, fitting of the MC1 component in light, shown in Figure 2, which can be understood as being due to the magnetic field effect on the dissociation yield of a geminate 1e-h pair (see SIII of Supporting Information), was also performed by the same kinetic model. The calculated MC1 in light monotonically increases with decreasing dissociation rate, as shown in Figure S3B. The MC1 value with kS/kT = 0.65 in Figure 5 reaches the same level as the MCS for the MC1 component in light observed at zero bias (+0.14 %). The cross point of the calculated MC1 curve with the observed MCS line gives a relative dissociation rate constant of k-10/kT = 61 at zero bias. The estimated ratio of rate constants is kS : kT : k-10 = 0.65 : 1.00 : 61, from which we can calculate a recombination quantum yield of φr = 0.01 for an e-h pair formed at the interface under the short-circuit condition. This indicates that there is almost no loss due to the charge recombination in the Pen|C60 bilayer cell.


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Figure 5. Dissociation dependence of the magnetic field effect on the dissociation yield of a geminate 1e-h pair with a spin selectivity of kS : kT = 0.65 : 1.00 for recombination, calculated by the kinetic model. Details of the kinetic model are described in S-III of Supporting Information. MCS is the saturation value for the MC1 component observed by irradiation by a solar simulator, shown in Figure 2. Singlet Fission. The broad MC3 component, of which the B1/2 value reaches 150 mT, is observable only in light, indicating a substantial magnetic interaction in the encounter pair related with excitons. Pairs involving triplet exciton (3ex), such as triplet–doublet (TD) and triplet–triplet (TT) pairs, are possible for the broad MC effect since 3ex usually includes a magnetic dipolar interaction between two electron spins, the so-called zero-field interaction, which is larger than the hyperfine interaction. Also, the MC effect originating from the TD and TT pairs is consistent with the experimental increase in the broad MC3 component for high power incident light, because those pairs can be totally formed through a two-photon process. However, no appearance of the MC3 component in dark and clear observation in excitation of Pen strongly suggest the singlet fission in the Pen-layer shown in Scheme 2, which is the TT pair mechanism.


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Scheme 2. General Scheme of TT Pair Mechanism (Upper Part) Corresponding to Singlet Fission Responsible for the Broad MC Effect (Lower Part). Q Means Quintet State. Ineffective Quintet Channel (kQ) Compared with Other Channels (kS and kT) due to Energetic Reason Is Expressed by Shading.

A geminate 1TT pair born from a combination between singlet exciton (S1, 1ex) and ground state (S0, 1g) of Pen can separate into two triplet exciton (T, 3ex) and generate a higher triplet exciton (Tn) through the isc in the TT pair. Also, the 1TT pair would have sufficient energy to return to S1 at room temperature.36 The quintet channel (kQ) from the 5TT pair can be ignored because of the high electronic energy of a quintet product (Q1). Deceleration of the isc under a magnetic field reduces the efficiency of singlet fission, leading to a positive MC effect due to the suppression of the consumption of 1ex that is a source of photocarriers produced at the Pen-C60 interface. The density matrix formalism for a TT pair having a fission channel from the singlet spin state of the pair (see S-IV of Supporting Information) could simulate the MC3 component very well using the rate constants kS = k-1 = 2 × 1010 s-1 (Figure 4A). This simulation also provides a quantum yield of double 3ex from a TT pair (ϕTT) of 0.52, which can be regarded as an upper limit of total quantum yield of the singlet fission (ϕSF) because of the product relation ϕSF = ϕTT × ϕ-S, where ϕ-S is the generation yield of a 1TT pair from a 1ex.


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Trap-related Reactions. The B1/2 value of the middle MC component observed in dark and light can be expressed as 25 ± 10 mT, suggesting the encounter pair involving 3ex. The TD pair with quartet states and doublet states is consistent with the incident light power dependence in light and is the most probable mechanism because the TT pair mechanism has an even broader shape as described in the previous section. To interpret the difference between phases in dark and light, carrier detrapping and trapping by 3ex, which have a direct channel to produce the doublet product (D) as illustrated in Scheme 3, are proposed. Scheme 3. General Scheme of TD Pair Mechanism (Upper Part) Corresponding to Carrier Detrapping and Trapping By 3Ex Responsible for the Middle MC Effect (Lower Part). q Means Quartet State. Ineffective Quartet Channel (kq) Compared with Doublet Channel (kD) due to Energetic Reason is Expressed by Shading.

The channel to produce quartet product (q) from the 4TD pair above is negligible for the same reason the quintet channel in the TT pair mechanism can be ignored. The MC effect requires a change in the carrier number in a reaction through a TD pair. The trap-related reaction by a 3ex and a doublet carrier (2c) meets this requirement. In Scheme 3, Eq. A expresses a detrapping of a trapped carrier (2ct) by collision with a 3ex, and Eq. B expresses trapping of a carrier by collision with a trapped 3ex (3ext). Under the simple spin statistics, in the formation of the TD pair, which is an exciton–carrier (ex-c) pair, a substantial reduction of the isc in the presence of a magnetic field leads to a decrease in the carrier density in the detrapping and to an increase in the trapping.


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Therefore the detrapping and trapping reactions cause negative and positive MC effects, respectively.37-41 The actual polarity of the MC effect depends on whether the majority of trapped species is 2ct or 3ext. For electrode injection, a 3ex is produced only from charge recombination at the interface. The photoinjection allows an additional pathway of 3ex generation from a 1ex in the layer. Singlet fission in the Pen layer under illumination would be especially effective. The occupation of traps in whole layers with many 3ex in photoinjection contributes more positively to the MC effect, while the negative MC effect due to the detrapping becomes dominant for electrode injection because of the low density of 3ex. We calculated MC curves using a density matrix formalism for a TD pair having a trapping channel from the doublet spin state of an ex-c pair (see S-IV of Supporting Information). A simulation using the zero field splittings of D = +49.5 mT and E = -15.0 mT for a pentacene 3ex42 with reasonable rate constants for the trapping (kD = 1 × 108 s-1) and dissociation (k-1 = 1 × 108 s-1) of the TD pair43 reproduced the positive MC curve (Figure 4A).

5. CONCLUSION By applying only a small magnetic field, and without any decomposition of the OSC device, we observed the MC effects depending on the applied bias and the power and wavelength of incident light. In combination with kinetic and quantum mechanical simulations, we obtained quantitative information about the exciton and carrier dynamics, such as the singlet fission in the Pen-layer, the charge recombination at the interface and the carrier trapping/detrapping process by triplet exciton in both layers, which are quite difficult to explore by usual J/V measurements. The results of this research show that energy turning and/or harvest of triplet excitons are key issues for enhancing the performance of OSCs.


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AUTHOR INFORMATION Corresponding Author *Address: Graduate School of Science and Technology, Niigata University, 2-8050 Ikarashi, Nishi-ku, Niigata 980-2181, Japan. *Telephone: +85-25-262-7390 *E-mail: [email protected]

ACKNOWLEDGMENT This work was supported in part by a CREST grant from the JST, a Grant-in-Aid for Scientific Research (No. 2610088) from MEXT, a grant for the Promotion of Niigata University Research Projects, a grant from Nihon Kagaku Kenkyukai and a grant from the Network Joint Research Center for Materials and Devices.

Supporting Information Available Appendix S-I, Device characteristics; Appendix S-II, Bias dependence of dark MC curve; Appendix S-III, Kinetic model for the DD pair mechanism; Appendix S-IV, Quantum mechanical simulation for TD and TT pair mechanisms. This information is available free of charge via the Internet at http://pubs.acs.org.

REFERENCES


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(1)

Jadhav, P. J.; Mohanty, A.; Sussman, J.; Lee, J.; Baldo, M. A. Singlet Exciton Fission in

Nanostructured Organic Solar Cells. Nano Lett. 2011, 11, 1495-1498. (2)

Congreve, D. N.; Lee, J. Y.; Thompson, N. J.; Hontz, E.; Yost, S. R.; Reusswig, P. D.;

Bahlke, M. E.; Reineke, S.; Van Voorhis, T.; Baldo, M. A. External Quantum Efficiency Above 100% in a Singlet-Exciton-Fission-Based Organic Photovoltaic Cell. Science 2013, 340, 334-337. (3)

Jankus, V.; Snedden, E. W.; Bright, D. W.; Whittle, V. L.; Williams, J. A. G.; Monkman,

A. Energy Upconversion via Triplet Fusion in Super Yellow PPV Films Doped with Palladium Tetraphenyltetrabenzoporphyrin: a Comprehensive Investigation of Exciton Dynamics. Adv. Funct. Mater. 2013, 23, 384-393. (4)

Tritsch, J. R.; Chan, W. L.; Wu, X. X.; Monahan, N. R.; Zhu, X. Y. Harvesting Singlet

Fission for Solar Energy Conversion via Triplet Energy Transfer. Nature Commun. 2013, 4, 1-7. (5)

Rao, A.; Chow, P. C. Y.; Ge´linas, S.; Schlenker, C. W.; Li, C.-Z.; Yip, H.-L.; Jen, A. K.-

Y.; Ginger, D. S.; Friend, R. H. The Role of Spin in the Kinetic Control of Recombination in Organic Photovoltaics. Nature 2013, 500, 435-439. (6)

Clarke, T. M.; Durrant, J. R. Charge Photogeneration in Organic Solar Cells. Chem. Rev.

2010, 110, 6736-6767. (7)

Barsoukov, E.; MacDonald, J. R. (Eds.) Impedance Spectroscopy: Theory, Experiment,

and Applications; 2nd ed.; John Wiley & Sons; Hoboken, U.S.A., 2005. (8)

Watanabe, S.; Tanaka, H.; Ito, H.; Kuroda, S.; Mori, T.; Marumoto, K.; Shimoi, Y. Direct

Determination of Interfacial Molecular Orientations in Field-effect Devices of P3HT/PCBM Composites by Electron Spin Resonance. Org. Electron. 2011, 12, 716-723.


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(9)

Page 20 of 33

Lupton, J. M.; McCamey, D. R.; Boehme, C. Coherent Spin Manipulation in Molecular

Semiconductors: Getting a Handle on Organic Spintronics. ChemPhysChem 2010, 11, 30403058. (10) Brabec, C. J.; Dyakonov, V.; Parisi, J.; Sariciftci, N. S. (Eds.) Organic Photovoltaics: Concepts and Realization; Springer: Berlin, Germany, 2003. (11) Brütting, W.; Adachi, C. (Eds.) Physics of Organic Semiconductors; 2nd ed.; WileyVCH: Weinheim, Germany, 2012. (12) Birks, J. B. (Ed.) Organic Molecular Photophysics; John Wiley & Sons Ltd., 1973. (13) Schwoerer, M.; Wolf, H. C. Organic Molecular Solids; Wiley-VHC Verlag GmbH & Co. KGaA: Weinheim, Germany, 2007. (14) Steiner, U. E.; Ulrich, T. Magnetic-Field Effects in Chemical-Kinetics and Related Phenomena. Chem. Rev. 1989, 89, 51-147. (15) Shiraishi, M.; Ikoma, T. Molecular Spintronics. Phys. E 2011, 43, 1295-1317. (16) Dediu, V. A.; Hueso, L. E.; Bergenti, I.; Taliani, C. Spin Routes in Organic Semiconductors. Nature Mater. 2009, 8, 707-716. (17) Lee, T. H.; Guo, T. F.; Huang, J. C. A.; Wen, T. C. Modulations of Photoinduced Magnetoconductance for Polymer Diodes. Appl. Phys. Lett. 2008, 92, 153303. (18) Lei, Y. L.; Song, Q. L.; Zhang, Y.; Chen, P.; Liu, R.; Zhang, Q. M.; Xiong, Z. H. Magnetoconductance of Polymer-Fullerene Bulk Heterojunction Solar Cells. Org. Electron. 2009, 10, 1288-1292.


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Page 21 of 33

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(19) Ito, F.; Ikoma, T.; Akiyama, K.; Kobori, Y.; Tero-Kubota, S. Long-range Jump versus Stepwise Hops: Magnetic Field Effects on the Charge-transfer Fluorescence from Photoconductive Polymer Films. J. Am. Chem. Soc. 2003, 125, 4722-4723. (20) Ikoma, T.; Ogiwara, T.; Takahashi, Y.; Akiyama, K.; Tero-Kubota, S.; Suzuki, T.; Wakikawa, Y. Giant Magnetoresistance due to Electron-hole Pair Mechanism in Poly(Nvinylcarbazole). Synth. Met. 2010, 160, 285-290. (21) Kalinowski, J.; Szmytkowski, J.; Stampor, W. Magnetic Hyperfine Modulation of Charge Photogeneration in Solid Films of Alq3. Chem. Phys. Lett. 2003, 378, 380-387. (22) Hu, B.; Wu, Y. Tuning Magnetoresistance between Positive and Negative Values in Organic Semiconductors. Nature Mater. 2007, 6, 985-991. (23) Hu, B.; Yan, L. A.; Shao, M. Magnetic-Field Effects in Organic Semiconducting Materials and Devices. Adv. Mater. 2009, 21, 1500-1516. (24) Desai, P.; Shakya, P.; Kreouzis, T.; Gillin, W. P. Magnetoresistance in Organic Lightemitting Diode Structures under Illumination. Phys. Rev. B 2007, 76, 235202. (25) Gautam, B. R.; Nguyen, T. D.; Ehrenfreund, E.; Vardeny, Z. V. Magnetic Field Effect Spectroscopy of C60-based Films and Devices. J. Appl. Phys. 2010, 113, 143102. (26) Kersten, S. P.; Schellekens, A. J.; Koopmans, B.; Bobbert, P. A. Magnetic-Field Dependence of the Electroluminescence of Organic Light-Emitting Diodes: A Competition between Exciton Formation and Spin Mixing. Phys. Rev. Lett. 2011, 106, 197402.


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Page 22 of 33

(27) Schellekens, A. J.; Wagemans, W.; Kersten, S. P.; Bobbert, P. A.; Koopmans, B. Microscopic Modeling of Magnetic-field Effects on Charge Transport in Organic Semiconductors. Phys. Rev. B 2011, 84, 075204. (28) Janssen, P.; Cox, M.; Wouters, S. H. W.; Kemerink, M.; Wienk, M. M.; Koopmans, B. Tuning Organic Magnetoresistance in Polymer-fullerene Blends by Controlling Spin Reaction Pathways. Nature Commun. 2013, 4, 2286. (29) Hayashi, H. Introduction to Dynamics Spin Chemistry: Magnetic Field Effects on Chemical and Biochemical Reactions; World Scientific Pub. Co. Inc.: Singapore, 2004. (30) Mermer, Ö.; Veeraraghavan, G.; Francis, T. L.; Sheng, Y.; Nguyen, D. T.; Wohlgenannt, M.; Köhler, A.; Al-Suti, M. K.; Khan, M. S. Large Magnetoresistance in Nonmagnetic πConjugated Semiconductor Thin Film Devices. Phys. Rev. B 2005, 72, 205202. (31) Cox, M.; Janssen, P.; Zhu, F.; Koopmans, B. Traps and Trions as Origin of Magnetoresistance in Organic Semiconductors. Phys. Rev. B 2013, 88, 035202. (32) Yi, Y.; Coropceanu, V.; Brédas, J.-L. Exciton-Dissociation and Charge-Recombination Processes in Pentacene/C60 Solar Cells: Theoretical Insight into the Impact of Interface Geometry. J. Am. Chem. Soc. 2009, 131, 15777-15783. (33) Giebink, N. C.; Wiederrecht, G. P.; Wasielewski, M. R.; Forrest, S. R. Ideal Diode Equation for Organic Heterojunctions. I. Derivation and Application. Phys. Rev. B 2010, 82, 155305.


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Page 23 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(34) Wakikawa, Y.; Ikoma, T.; Yamamoto, Y.; Fukushima, T.; Aida, T. Magnetic Field Effect on the Photocarriers in Self-assembled Hexabenzocoronene Nanotubes. Synth. Met. 2010, 160, 275-279. (35) Wakikawa, Y.; Ikoma, T.; Yamamoto, Y.; Fukushima, T.; Aida, T.; Akiyama, K. Effect of Acceptor Lamination on Photocarrier Dynamics in Hole Transporting Hexabenzocoronene Nanotubular Self-assembly. J. Phys. Chem. C 2013, 117, 15295-15305. (36) Thorsmølle, V. K.; Averitt, R. D.; Demsar, J.; Smith, D. L.; Tretiak, S.; Martin, R. L.; Chi, X.; Crone, B. K.; Ramirez, A. P.; Taylor, A. J. Morphology Effectively Controls SingletTriplet Exciton Relaxation and Charge Transport in Organic Semiconductors. Phys. Rev. Lett. 2009, 102, 017401. (37) Mehl, W. Darkinjection of Electrons from Alkali-metals in Anthracene. Solid State Commun. 1968, 6, 549-551. (38) Levinson, J.; Weiz, S. Z.; Cobas, A.; Rolon, A. Determination of the Triplet ExcitonTrapped Electron Interaction Rate Constant in Anthracene Crystals. J. Chem. Phys. 1970, 52, 2794. (39) Geacintov, N. E.; Pople, M.; Fox, S. Magnetic Field Effects on Photo-enhanced Currents in Organic Crystals J. Phys. Chem. Solids 1970, 31, 1375-1379. (40) Frankevich, E. L.; Sokolik, I. A. On the Mechanism of the Magnetic Field Effect on Anthracene Photoconductivity. Solid State Commun. 1970, 8, 251-253. (41) Ern, V.; Bouchard, D.; Fourny, J.; Delocote, G. Triplet Exciton-Trapped Hole Interaction in Anthracene Crystals Solid State Commun. 1971, 9, 1201-1203.


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(42) Van Strien, A. J.; Schmidt, J. An EPR Study of the Triplet State of Pentacene by Electron Spin-echo Techniques and Laser Flash Excitation. Chem. Phys. Lett. 1980, 70, 513–517. (43) Barhoumi, T.; Monge, J. L.; Bouchriha, H. Application of the Short and Long Consecutive Pairs Model to the Triplet-Doublet Interaction in Molecular Crystals. Eur. Phys. J. B 2010, 77, 453-458.

The magnetoconductance effect measured by a non-destructive technique with high sensitivity is used to quantitatively explore charge recombination at the interface between two organic layers, and trap-related dynamics with triplet exciton and singlet fission in the layers.


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Figure 1. Setup for measuring MC effect and structure of Pen|C60-bilayer solar cell. Details are given in Experimental Section.



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Figure 2. MC effects of Pen|C60-bilayer solar cell depending on the method of carrier injection. The MC effect was measured at 0.20 V in the dark (blue) and at 0.00 V under illumination by a 1.5AM-solar simulator (100 mW/cm2) (red). The black curves are fits using multiple Gaussians. The shapes for the individual Gaussians involved in the fitting are shown by dotted curves. 64x63mm (300 x 300 DPI)


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Figure 3. Correlation between the current density and the MC effect in the dark for a Pen|C60-bilayer solar cell. (A) Bias dependence of the saturation values for the narrow MC1 and middle MC2 components. The solid curves are Gaussian (red) and exponential (blue) functions fitting the MC1 and MC2 components, respectively. (B) Logarithmic plot of the bias dependence of the current density. The broken straight lines are eye guides. (C) Dissociation rate dependence of the magnetic field effect on the recombination yield φr calculated for a ratio between the singlet and triplet recombination rate constants (kS : kT) of 0.65 : 1.00 in a kinetic model for e-h pairs described in S-III of Supporting Information. 120x218mm (300 x 300 DPI)



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Figure 4. Excitation wavelength and light power dependences of the MC effect in light for a Pen|C60-bilayer solar cell detected at zero bias. Selective excitations of the Pen and C60 layers performed by illumination at λ = 627 (A) and 455 nm (A’), respecitvely, with a power density of 31.8 mW/cm2. The black solid and broken curves in A and A’ are fits using, respectively, multiple Gaussians and simulations calculated by a density matrix formalism (see S-IV of Supporting Information). The saturation values for the MC components with B1/2 values of (B) ~ 5.5 mT, (C) ~ 23 mT and (D) ~144 mT were obtained by fitting with triple Gaussians. The red and black data points indicate values for selective excitation of Pen layer and non-selective excitation by a solar simulator, respectively. 59x22mm (300 x 300 DPI)


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Figure 5. Dissociation dependence of the magnetic field effect on the dissociation yield of a geminate 1e-h pair with a spin selectivity of kS : kT = 0.65 : 1.00 for recombination, calculated by the kinetic model. Details of the kinetic model are described in S-III of Supporting Information. MCS is the saturation value for the MC1 component observed by irradiation by a solar simulator, shown in Figure 2. 57x56mm (300 x 300 DPI)



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Scheme 1. General Scheme of DD Pair Mechanism (Upper Part) Corresponding to Charge Recombination Responsible for the Narrow MC Effect (Lower Part). S, D and T Mean Singlet, Doublet and Quintet States Respectively. In the Context of the Pen|C60 Solar Cell, Ineffective Singlet Channel (kS) Compared with Triplet Channel (kT) is Expressed by Shading. 34x17mm (300 x 300 DPI)


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Scheme 2. General Scheme of TT Pair Mechanism (Upper Part) Corresponding to Singlet Fission Responsible for the Broad MC Effect (Lower Part). Q Means Quintet State. Ineffective Quintet Channel (kQ) Compared with Other Channels ((kS and (kT) due to Energetic Reason Is Expressed by Shading. 45x23mm (300 x 300 DPI)



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Scheme 3. General Scheme of TD Pair Mechanism (Upper Part) Corresponding to Carrier Detrapping and Trapping By 3Ex Responsible for the Middle MC Effect (Lower Part). q Means Quartet State. Ineffective Quartet Channel (kq) Compared with Doublet Channel (kD) due to Energetic Reason is Expressed by Shading. 39x16mm (300 x 300 DPI)


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Synopsis: The magnetoconductance effect measured by a non-destructive technique with high sensitivity is used to quantitatively explore charge recombination at the interface between two organic layers, and traprelated dynamics with triplet exciton and singlet fission in the layers. 29x9mm (300 x 300 DPI)


Carrier Dynamics in Pentacene|C60 Bilayer Solar Cell Investigated

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