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C: Physical Processes in Nanomaterials and Nanostructures

Optical-Helicity Manipulated Photocurrents and Photovoltages in Organic Solar Cells Mengmeng Wei, Xiao-Tao Hao, Avadh B. Saxena, Wei Qin, and Shijie Xie J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03537 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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The Journal of Physical Chemistry

Optical-helicity Manipulated Photocurrents and Photovoltages in Organic Solar Cells Mengmeng Wei1, Xiaotao Hao1,2, Avadh Behari Saxena3, Wei Qin1* and Shijie Xie1 Affiliations: 1

School of Physics, State Key Laboratory of Crystal Materials, Shandong University,

Jinan 250100, China 2

ARC Centre of Excellence in Exciton Science, School of Chemistry, The University

of Melbourne, Parkville, Victoria 3010, Australia 3

Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545,

USA

1

ACS Paragon Plus Environment

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Abstract Performance of an organic functional device can be effectively improved through external field manipulation. Here, we experimentally demonstrate the optical polarization manipulation of the photocurrent or photovoltage in organic solar cells. Through switching the incident light from linearly polarized light to circularly polarized one, we find a pronounced change in the photocurrent, which is not observable in normal inorganic cells. There are two competing hypotheses for the primary process underlying the circular polarization dependent phenomena in organic materials, one involving the inverse Faraday effect (IFE), the other a direct photon spin-electron spin interaction. By way of ingenious device design and external magnetic field induced stimuli, it is expected that the organic IFE can be a powerful experimental tool in revealing and elucidating excited state processes occurring in organic spintronic and optoelectronic devices. Therefore, we believe that our results will potentially lead to the development of new multifunctional organic devices with integrated electronic, optical, and magnetic properties for energy conversion, optical communication and sensing technologies.

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INTRODUCTION In recent years, many breakthroughs have been realized in organic semiconductor materials based devices such as organic solar cells (OSC)1-3 and organic light emitting diodes (OLED)4-5. Based on new materials synthesis and novel device architecture fabrication, 14% power conversion efficiency6-7 and 29% luminescence efficiency have been achieved8. Additionally, external field manipulation is an important method to improve the performance, such as the strong response of electrical and optical properties of nonmagnetic organic semiconductors to a weak external magnetic field, known as organic magnetic-field effect (OMFE)9-11. Recently it has also been noted that, in some inorganic materials circularly polarized light can switch spin without the use of a magnetic field12-17. In the topological insulators Bi2Se3 and Bi2Te3, surface spin current can be controlled by applying circularly polarized light18-21, and helicity-dependent surface photocurrent can be induced by the angular momentum transfer from photons to carriers. Furthermore, illumination of semiconductor heterojunction with circularly polarized light could induce non-uniformly distributing photoexcited carriers, which results in a spin dependent photocurrent22. It is considered that circularly polarized light tunes spin states through two competing mechanisms23-24: (i) directly transferring photon spin angular momentum of circular polarization to electron angular momentum to induce exciton polarization25-28; (ii) generating effective magnetic field by electric field component to change spin states (inverse Faraday effect (IFE))29. In the topological insulators and

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2D materials, circular polarization induced spin polarized photocurrent originates only from the surface, not from the bulk. For organic solar cells, electron-hole pairs or excitons can be excited in the bulk of the organic active layers under light illumination. However, circularly polarized photon dependent spin states and their effects on photovoltaic process are still lack of study in organic solar cells.

METHODS Materials. PTB7, P3HT and PCBM were purchased from 1-Matericals, Sigma-Aldrich and Nano-C, respectively, and used as received without further purification. Both PTB7:PC61BM and PTB7:PC71BM composites were blend in a weight ratio of 1:1.5 and dissolved in chlorobenzene at a concentration of 25mg/mL for device fabrication. The solutions were stirred at 70 °C for 24 h. The P3HT and PC61BM composites were prepared with the weight ratio of 1:1 in dichlorobenzene. The solution concentration of it was set at 20 mg/mL. For the ZnO, one gram of zinc acetate dehydrate and 0.28 gram of ethanolamine in 2-methoxyethanol (10mL) were dissolved under vigorous stirring for 24 h in air and the precursor was aged at room temperature for 1 day. Device Fabrication. The organic solar cells with an inverted structure of ITO/ZnO/active layer/MoO3/Ag were fabricated by using the following procedures. The ZnO precursor was spun on the ITO substrates at 3000 rpm for 50 s. After this process, the samples were baked in air at 200°C for 50 min. The PTB7 based solar cells were fabricated by spin-casting PTB7:PC61BM or PTB7:PC71BM blend

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solutions on the ZnO coated substrates at 1500 rpm for 60 s, and the P3HT based solar cells were at 800 rpm for 10 s and then1500 rpm for 50 s. These processes were in a N2 purged glove box. Finally, a 8 nm thick MoO3 hole transporting layer and a 100 nm thick silver top contact were deposited by thermal evaporation in a vacuum system. Experimental measurements. The current density-voltage (J-V) characteristics of the OSCs were measured under AM 1.5G illumination of 100 mW/cm2 using a programmable voltage-current source meter (Keithley 2400). The external quantum efficiency (EQE) of the OSCs was measured under nitrogen environment. The switchable linearly and circularly polarized photoexcitations were realized through combining a linearly polarized laser with a quarter wave plate. The temperature dependent photovoltaic measurements in solar cells were carried out on a cryostat with optical windows.

RESULTS AND DISCUSSION In this work, we have developed a diagnostic tool to exploit the notion of circularly polarized light induced photovoltaic effects. By way of fabricating organic photovoltaic devices, circularly polarized light dependent excited spin states are studied. Based on the analysis of circularly polarized light dependent photocurrent and photovoltage, we have uncovered the dominant mechanism for the tunability of spin states by circular polarization. Moreover, circularly polarized light dependent photoluminescence (PL) is investigated to further our understanding of the

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spin-photon interaction mechanism. The performance of PTB7 (Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b'] dithiophene-2,6-diyl] 3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]) based solar cell, ITO/ZnO/PTB7:PC71BM/MoO3/Ag, is presented in Figure 1a. The power conversion efficiency is 8.52% with Jsc=15.9 mA/cm2, Voc=0.79 V and FF=0.68. External quantum efficiency (EQE) is studied to further confirm the value of Jsc (the inset of Figure 1a). If we use PC61BM instead of PC71BM to fabricate the devices, PCE decreases to 6.3%, as shown in Figure S1. In addition to neutral white light irradiation, the setup for polarized light excitation of organic solar cells is shown in Figure 1b. Rotating the quarter wave plate with 0° and 45° can switch the polarization between linear and circular polarization. Theoretically, rotating the quarter wave plate only tunes the phase difference; it cannot change the light intensity. It means that circularly and linearly polarized light are of identical intensity to excite organic solar cells. Experimentally, we also have performed a control measurement to confirm that through rotating the quarter wave plate by 45o, light intensities are the same for the circularly polarized light and the linearly polarized light, as shown in Figure S2. Following the switching of the incident light from linearly polarized to circularly polarized, the photocurrent of PTB7 based solar cells provides a response accordingly, as shown in Figure 1c. Circularly polarized light can effectively increase the photocurrent. However, this phenomenon cannot be observed in Si solar cells, as shown in Figure S3. To compare with the case of linearly polarized light, we define

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the circular polarization induced photocurrent (PPC), photovoltage (PPV) and photoluminescence (PPL) as, PPS= S (C )  S ( L ) ,

(1)

where PPS denotes (collectively the three measurables, namely) cell photocurrent, photovoltage and photoluminescence signal difference, S(C) (S(L)) are collectively the values of photocurrent, photovoltage and photoluminescence under circularly (linearly) polarized light irradiation, respectively. For the present measurement, we obtain PPC=0.7 mA/cm2.

Figure 1. (a) J-V curve of the device ITO/ZnO/PTB7:PC71BM/MoO3/Ag under 100 mW/cm2 solar light illumination, the inset is the EQE curve. (b) Schematic of the circularly polarized light and the linearly polarized light excitation on PTB7 based organic solar cells, ITO/ZnO/PTB7:PCBM/MoO3/Ag. (c) The circularly and linearly polarized

light

dependent

photocurrent

of

the

device

ITO/ZnO/PTB7:PCBM/MoO3/Ag with 635 nm incident laser beam.

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In addition to the tunability of photocurrent by circularly polarized light, photovoltage can also be tuned by switching light from linearly polarized to circularly polarized, as shown in Figure 2a. Similarly, we obtain the circularly polarized light induced photovoltage PPV=2 mV. For the PL, the result is opposite. As shown in Figure 2e, it is found that the value of PL decreases when switching the light from linearly polarized to circularly polarized. The PTB7 PL spectrum is presented in Supporting Information as Figure S4. The circularly polarized light dependent photocurrent (Figure S5), photovoltage (Figure S6) and photoluminescence are sensitive to the intensity of light. As shown in Figs. 2b-f, these values tend to vanish with the decrease of the incident light intensity. In addition, some details of circularly polarized light dependent photocurrent and photovoltage effect of this device under different light intensity are shown in S5 and S6.

Figure 2. Photovoltage response by switching incident light from the circular polarization to the linear polarization, 200 mW/cm2 (a) and 10 mW/cm2 (b). (c) Incident

light

intensity

dependent

PPV.

Solid

line

is

fitting

curve,

8


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PPV=0.12×I0.5+0.002×I, I is light intensity. (d) Light intensity dependent PPC. Solid line is fitting curve, PPC=0.012×I0.5+0.0002×I. (e) Circular polarization and linear polarization dependent photoluminescence of PTB7. (f) PTB7 PL difference between circular polarization and linear polarization excitation with different incident light intensity. The solid line is a fitting curve, PPL=8.5×I0.5+0.36×I.

It is known that exciton dissociation and recombination are closely related to the spin states in organic materials. If the circularly polarized light could tune the spin of excited states, carrier density should be changed, which will result in a circular polarization dependent photocurrent and photovoltage in organic solar cells. Based on the interaction between circularly polarized photons and carrier spins, circularly polarized light can tune spin states in organic materials through two different mechanisms: (i) direct electron spin-photon interaction

23-28

; (ii) indirectly, IFE from

the spin-orbit interaction, as shown in Figure S7. For the mechanism (i), spin of circularly polarized light photon could have a direct interaction with electron spin, which can be realized through25-28 H

q 2   ( E ( x, t )  A( x, t )) , where q is the charge of electron, m the mass of electron, 4m 2 c 2

c the speed of light,  the Planck constant,  the relevant Pauli matrix, E ( x, t ) the electric field component of circularly polarized light, and A( x , t ) the magnetic vector potential. For the linear polarization, the Hamiltonian is identically zero25-28. This coupling, arising from the relativistic effect inherent in the Dirac equation, can lead to spin precession to further result in the creation of photo-generated polarized excitons

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(triplet excitons). Due to the large dissociation rate of triplet states in organic materials30-31, the coupling of the spin of circular polarization with electron spin will enhance the performance of photovoltage and photocurrent in organic photovoltaic devices.

Figure 3. Circularly polarized light dependent photovoltage (a) and photocurrent (b). Light

intensity

dependent

PPV

(c)

and

PPC

(d)

in

the

ITO/ZnO/P3HT:PCBM/MoO3/Ag devices. Solid lines in (c) and (d) are fitting curves, and the functions of fitting curves are PPV=-0.13×(I-6)0.5+0.0046×I and PPC=-0.006×(I-20)0.5+10-5×I, respectively.

To further test whether mechanism (i) is the dominant one to induce PPC and PPV effects in organic photovoltaic devices, a P3HT based solar cell, ITO/ZnO/P3HT:PCBM/MoO3/Ag, is fabricated. As shown in Figure 3, it is noted that circularly polarized light decreases the value of both photocurrent and photovoltage. Some details of circularly polarized light dependent photocurrent and photovoltage 10


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effect of this device under different light intensity are shown in Figure S8. Negative PPC and PPV effects are observed, which are completely opposite phenomena as compared to those observed in PTB7 based solar cells. As discussed above, if mechanism (i) is the dominant one, circular polarization induced higher triplet ratio should increase the values of photocurrent and photovoltage as compared to the linear polarization case. However, PPC and PPV are observed to be positive effects in PTB7 photovoltaic devices, while PPC and PPV are observed to be negative effects in P3HT photovoltaic devices. Thus, it is untenable that mechanism (i) is the dominant one that leads to circularly polarized light effects on photocurrent and photovoltage in organic devices. The other mechanism for optical helicity-driven spin dynamics is effect (ii), the IFE. The circularly polarized light induces an effective magnetic field whose strength is proportional to the Verdet constant Vver and the intensity difference between the right and left circularly polarized light32-34 Beff  Vver  I R  I L  ,

(2)

where IR (IL) is the intensity of right (left) circularly polarized light. Usually the strength of the effective magnetic field is very weak. However, with an intense laser focusing on symmetry-breaking materials, values in the range 1-10 Gauss or higher are considered reasonable. An experimental study of the IFE in plasmas reported that the circularly polarized light is able to create an axial magnetic field of tens of kilogauss35. Considering the non-symmetric structure of organic donor-acceptor composites and the localization of carriers in organic materials, it is expected that the

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effective magnetic field may be even larger.

Figure 4. (a) External magnetic field dependent photovoltages and photocurrents in the ITO/ZnO/PTB7:PCBM/MoO3/Ag device. The inset depicts the external magnetic field dependent photovoltage of the ITO/ZnO/P3HT:PCBM/MoO3/Ag device. (b) External magnetic field dependent photoluminescence of PTB7. The direction of magnetic field is parallel to the surface of organic thin film devices.

For the organic magnetic field effects, it has been well studied that an external magnetic field could tune spin states due to spin-dependent pairing and spin mixing in organic semiconducting materials, wherein the photocurrent and the photovoltage can be tuned by a magnetic field36-37. Following this reasoning, an external magnetic field is applied on PTB7 based devices to simulate the effective field that is induced by the electric component of circularly polarized light. As shown in Figure 4a, under the stimulus of the external magnetic field, both the photocurrent and the photovoltage are obtained, as depicted in Figure S9. It shows a tendency similar to the effect of circularly polarized light effects. With the effect of an external magnetic field, both

Jsc  Jsc ( B )  Jsc (0)

and

Voc  Voc( B)  Voc(0) increase with external

magnetic field B in PTB7 based solar cells, as shown in Figure 4a. Just like the 12


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circularly polarized light case, with the increase of the incident intensity, the effective magnetic field is enhanced. Thus, both PPC and PPV become more pronounced (Figure 2c and 2d) with incident light intensity. Furthermore, as shown in Figure 4b, increasing external magnetic field will also tune the spin-dependent pairing, which results in PL quenching. The PL quenching tendency with magnetic field is consistent with the observed circularly polarized light effect on PL (Figure 2f). Moreover, external magnetic field decreases the value of photovoltage in P3HT solar cells (Figure 4a). Based on these analyses, it is deduced that a circularly polarized light has nearly the same effect as an external magnetic field on the tunability of photocurrent and photovoltage in both PTB7 and P3HT based solar cells. Therefore, it is concluded that the IFE is the dominant mechanism that leads to the circularly polarized light effect.

Figure 5. Angle (rotating wave plate) dependent photovoltage and photocurrent in the ITO/ZnO/PTB7:PCBM/MoO3/Ag solar cell, (a) Photovoltage; (b) Photocurrent. 0O corresponds to the linear light polarization. Purple squares are the measured results, while the green line is the fitting of PPC or PPV to Beff  B0 sin 2 2 .

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Next, we study the dependence of the photocurrent and the photovoltage on the polarizing angel 

(the definition of 

is shown in Figure 1b).   0,  / 2

corresponds to the linearly polarized light.    / 4 corresponds to the right circularly polarized and   3 / 4 to the left circularly polarized light, as depicted in Figure 1c. External magnetic fields with positive and negative directions have the same tunability on photovoltage, as shown in Figure S10. From the IFE in Eq. (2), we obtain the effective magnetic field strength that induced by circularly polarized light as Beff  B0 sin 2 2 . The results are shown in Figure 5a and Figure 5b for PTB7 based solar cells, ITO/ZnO/PTB7:PCBM/MoO3/Ag. Apparently, the photocurrent and the photovoltage oscillate with the polarizing angle, which is essentially consistent with the behavior of the effective magnetic field (Figure 5). Furthermore, thermal effects are studied in organic devices. Although the above measurements are carried out at room temperature, it is found that the thermal effects on the photocurrent and the photovoltage are still apparent. As shown in Figure S11, PPV decreases with temperature due to the temperature induced fast spin relaxation. CONCLUSIONS

In summary, circularly polarized light dependent photovoltage and photocurrent are

studied

in

organic

solar

cells

ITO/ZnO/PTB7:PCBM/MoO3/Ag

and

ITO/ZnO/P3HT:PCBM/MoO3/Ag. Circularly polarized light could increase the value of photocurrent and photovoltage in PTB7 based solar cells through the inverse Faraday effect (IFE). Due to the effect of circularly polarized light induced effective magnetic field, more triplet electron-hole pairs are generated to increase charge carrier

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dissociation which results in the increase of photovoltage and photocurrent. Although the spin of circularly polarized light photon can directly interact with the electron spin to tune spin states of carriers in organic materials, the effect is much weaker than IFE. Overall, this work provides a unique perspective to understand the underlying photovoltaic mechanism and paves the way for observing new physical phenomena and future applications of organic opto-spintronic devices.

ASSOCIATED CONTENT Supporting Information

The Supporting Information 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 interest.

ACKNOWLEDGEMENTS

This work was supported by National Natural Science Foundation of China (Grant No. 11504257, 11774203, 11574180, 11574181), 111 Project B13029, Qilu Young Scholar Award of Shandong University and the US Department of Energy.

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