Effects of Spin States on Photovoltaic Actions in Organo-Metal Halide

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Effects of spin states on photovoltaic actions in organo-metal halide perovskite solar cells based on circularly polarized photoexcitation Wei Qin, Hengxing Xu, and Bin Hu ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00801 • Publication Date (Web): 04 Oct 2017 Downloaded from http://pubs.acs.org on October 6, 2017

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ACS Photonics

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Effects of spin states on photovoltaic actions in

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organo-metal halide perovskite solar cells based on

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circularly polarized photoexcitation

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Wei Qin, Hengxing Xu, and Bin Hu*

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Department of Materials Science and Engineering, University of Tennessee-Knoxville,

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Tennessee 37996, USA

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ABSTRACT

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Organo-metal halide perovskites have become very promising photovoltaic materials with

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triply non-degenerate spin states due to spin-orbital coupling effects. This paper reports the

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effects of optically operated spin states on photocurrent (Jsc) and photovoltage (Voc) in

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perovskite (MAPbI3) solar cells with the device architecture of ITO/TiO2 (compact)/TiO2

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(mesoporous)/MAPbI3/P3HT/Au. Specifically, we switch the photoexcitation from linear

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polarization to circular polarization to change the electron-hole pair population of spin singlet

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߮௠೗ୀ±ଵ߮↑௘↓௛ and spin triplet ߮௠೗ୀ଴ ߮↑௘↑௛ in the perovskite solar cells. Simultaneously, we

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investigate the photovoltaic actions upon optically shifting the spin population. We find that

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optically shifting the spin population by switching photoexcitation from linear to circular

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polarization can cause an increase on both Jsc and Voc in the perovskite solar cells under

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circular photoexcitation. Our studies present the first evidence that the perovskite solar cells

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are the only type of solar cells where spin states can be optically operated with the

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consequence of influencing the photovoltaic actions. Our results indicate that switching

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photoexcitation from linear to circular polarization can increase the population of spin triplet

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electron-hole pairs available for dissociation and consequently increases the Jsc. On the other

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hand, optically shifting the spin population can decrease the bulk polarization and

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consequently increases the Voc under circular photoexcitation. Therefore, our studies provide

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insightful understanding on the effects of optically operating spin states on photovoltaic

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processes in perovskite solar cells.

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Keywords: Organo-metal halide perovskites, Perovskite solar cells, Spin states, Spin-orbital

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coupling

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Organo-metal halide perovskites are very interesting multifunctional materials due to

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their high carrier mobility,1-4 semiconducting properties5-13 and extremely high performance

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on solar cells.14-22 On the other hand, theoretical studies have shown that the spin-orbital

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coupling can split the degenerate spin states to form non-degenerate states, J=3/2, J=1/2 and

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S=1/2.23-24 Obviously, operating spin states offer opportunities to generate different

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multifunctional properties.25-31 As a consequence, probing spin states becomes an important

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issue to understand multifunctional properties in organo-metal halide perovskites. In 1994, the

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magneto-absorption studies have provided the first experimental evidence that the spin states

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can be optically probed at low temperature (4.2 K) and high magnetic field (> 10 Tesla).32 In

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2015, magnetic field effects of photoluminescence (PL) and electro-luminescence (EL) have

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indicated that spin states can be formed with field-controllable spin mixing at room

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temperature and low field (< 200 mT) in organo-metal halide perovskites.9, 33 In this work, we

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use optical approach to probe spin states during the generation of photovoltaic actions in

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organo-metal halide perovskite solar cells by switching photoexcitation between linear and

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circular polarizations with identical intensities. The linearly and circularly polarized

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photoexcitations with the same intensities were perpendicularly incident onto the perovskite

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solar cells to avoid any difference in reflection between linear and circular polarizations. By

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switching photoexcitation between linear and circular polarizations, spin populations can be

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shifted leading to less and more spin triplet ߮௠೗ ୀ଴ ߮↑௘↓௛ electron-hole pairs during the

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development of photovoltaic actions34. We find that switching photoexcitation between linear

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and circular polarizations can change both photocurrent and photovoltage in perovskite solar

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cells. Therefore, switching photoexcitation between linear and circular polarizations present

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an approach to investigate the effects of different spin states on photovoltaic actions in

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perovskite solar cells.

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Optical properties of the organic/inorganic halide perovskite (MAPbI3) thin films are

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characterized through absorption and PL, as shown in Fig. 1a. The absorption onset is 790 nm, 4 Environment ACS Paragon Plus

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which is consistent with the bandgap (1.55 eV) reported in literature.19-20,

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temperature PL exhibits the typical emission peaked at 767 nm. The current-voltage

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characteristics show PCE=12.57% with JSC=20.61 mA/cm2, VOC=0.94 V, and FF=0.65 for the

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perovskite solar cell [ITO/TiO2 (compact)/TiO2 (mesoporous)/MAPbI3/P3HT/Au] (Fig.1b).

The room-

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In order to probe spin states during the generation of photovoltaic actions, we switch the

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photoexcitation between linear and circular polarizations with the identical intensity in

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photovoltaic measurements (Fig. 1c). Specifically, we combine a linearly polarized laser beam

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with a ¼ wave plate as a light source with switchable polarization to generate photovoltaic

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actions in perovskite solar cells. Rotating the ¼ wave plate with 0° and 45° can switch the

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photoexcitation between linear and circular polarizations but does not change the beam

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intensity. We can see in equation (1) that rotating the ¼ wave plate can only tune the phase

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difference to shift the photoexcitation between linear and circular polarizations, while the

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beam intensity is kept unchanged. r 1 T π r [ Ax sin(ωt )i + Ay sin(ωt + ) j ]2 dt = I ∫ T 0 2 . r r 2 1 T = ∫ [ Ax sin(ωt )i + Ay sin(ωt ) j ] dt = I T 0

I Circular = 14 I Linear

(1)

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Figure 2 shows Jsc and Voc values for the perovskite solar cells when the

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photoexcitation is switched between linear and circular polarizations. We can see that both JSC

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and VOC are increased by switching photoexcitation from linear to circular polarization: JSC

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increasing from 7.72 mA/cm2 to 7.88 mA/cm2 by 2 % and Voc increasing from 1.024 V to

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1.025 V by 0.1 %. Clearly, switching photoexcitation between linear and circular

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polarizations generates different Jsc and Voc values in perovskite solar cells. Here, we should

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consider whether switching the photoexcitation between linear and circular polarizations with

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the identical intensity can change the light-induced polarization and spin populations during

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the development of photovoltaic actions. If switching the photoexcitation between linear and

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circular polarizations changes the light-induced polarization in perovskite film, the


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photovoltaic actions can be changed through bulk polarization and built-in field in perovskite

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solar cells. This is named as polarization effects. On the other hand, when switching

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photoexcitation between linear and circular polarizations changes the spin states in perovskite

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film, the photovoltaic actions can be modified due to the fact that different spin states have

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different charge dissociation and recombination rates. This can be called spin effects.

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Practically, the domain structures are isotropic in perovskite film plane. Switching

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photoexcitation between linear and circular polarizations does not change light-induced

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polarization in perovskite film. Theoretically, we can see in equation (2) that both linearly and

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circularly polarized photoexcitations generate the same light-induced polarization in isotropic

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domains in perovskite film. The linear and circular light-induced polarizations (PLinear and

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PCircular) can be given by 1 ் ࡼ௅௜௡௘௔௥ = ɛ଴ ߯ ࡱ௅௜௡௘௔௥ = ɛ଴ ߯ න ൣ‫ܣ‬௫ sinሺ߱‫ݐ‬ሻ ࢏ +‫ܣ‬௬ sinሺ߱‫ݐ‬ሻ ࢐൧ ݀‫ݐ‬ ܶ ଴

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ଵ

்

= ɛ଴ ߯ ் ‫׬‬଴ ൣ‫ܣ‬௫ sinሺ߱‫ ݐ‬ሻ ࢏ +‫ܣ‬௬ sinሺ߱‫ ݐ‬+ ߨ/2ሻ ࢐൧ ݀‫ = ݐ‬ɛ଴ ߯ ࡱ஼௜௥௖௨௟௔௥ =ࡼ஼௜௥௖௨௟௔௥

(2)

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where ELinear (ECircular) is the electric field component of linearly (circularly) polarized light.

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Based on this analysis, we can see that the dependence of photoexcitation polarization on Jsc

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and Voc can be attributed to the effects of spin states on photovoltaic actions upon using

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linearly and circularly polarized lights.

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Essentially, the spin effects of photovoltaic action involve three critical processes: (i)

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formation of spin states, (ii) dissociation of spin states, and (iii) transport of dissociated

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carriers, as shown in Fig. 3. Here, the wavefunction of electron-hole pair can be given by:

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߮௠೗ ߮௠ೞ ௘ ௠ೞ ௛ , where ml=-1,0,+1 and ms=+1/2(↑) , −1/2(↓). By transferring incident circular

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polarization momentum to orbital momentum, left-hand (σ+) and right-hand (σ-) circular

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photoexcitations generate spin-polarized electron-hole pairs ߮௠೗ ୀାଵ ߮↑௘↓௛ and ߮௠೗ ୀିଵ ߮↑௘↓௛ ,

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respectively due to the total angular momentum conservation. While linear photoexcitation

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can be consider as combination left-hand (σ+) and right-hand (σ-) circular photoexcitations 6 Environment ACS Paragon Plus

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and generates electron-hole pairs ߮௠೗ୀାଵ ߮↑௘↓௛ and ߮௠೗ ୀିଵ ߮↑௘↓௛ simultaneously. It should be

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noted that SOC can cause a spin flipping in the spin wavefunction ߮↑௘↓௛ of electron-hole pairs

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and consequently changes the spin-singlet ߮௠೗ ୀ±ଵ ߮↑௘↓௛ to spin-triplet ߮௠೗ୀ଴ ߮↑௘↑௛ . The

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circular photoexcitation generated electron-hole pairs ߮௠೗ ୀାଵ ߮↑௘↓௛ possess an orbital

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momentum with the same direction, which can provide more total orbital momentum leading

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to enhanced spin flipping through SOC. Consequently, more spin-triplets are generated due to

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the enhanced spin flipping process, leading to an increase in Jsc under circular photoexcitation.

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On contrary, linear photoexcitation generated electron-hole pairs have the orbital momentum

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with opposite direction, which provides less total orbital momentum leading to reduced spin

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flipping process. Thus, less spin-triplets can be generated due to the weakened spin flipping

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process, leading to reduced Jsc under linear photoexcitation. If the charge dissociation time is

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comparable to spin relaxation time, shifting spin population towards spin triplet states can

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increase the density of dissociated carriers upon applying a circular photoexcitation since

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parallel pairs can more readily dissociate due to the forbidden recombination through Pauli

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Exclusion Principle. When the dissociated carriers are extracted to form a transport, shifting

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spin population towards parallel states can thus increase the photocurrent, leading to spin

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effects of photocurrent in perovskite solar cells. Here, it should be pointed out whether spin

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effects can be observed is determined by whether the charge dissociation time is comparable

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with the spin relaxation time regardless the charge extraction time in transport. It has been

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reported that the spin relaxation time is around 1 ps24 while the dissociation time is within 1~2

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ps36-37. Therefore, within the spin relaxation time, the dissociation can lead to different

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densities of photogenerated carriers when the populations of spin singlet ߮௠೗ୀ±ଵ ߮↑௘↓௛ and

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spin triplet ߮௠೗ୀ଴ ߮↑௘↑௛ are changed upon switching the photoexcitation between linear and

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circular polarizations. Consequently, spin effects of photocurrent can be observed in

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perovskite solar cells.


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To confirm the spin states operated by circularly polarized photoexcitation, we study the

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PL upon switching photoexcitation between linear and circular polarizations in the MAPbI3

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perovskite film. Fig. 4a shows the PL measured with linearly and circularly polarized

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photoexcitations with identical intensity. We should note that the absorption from the MAPbI3

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thin films does not show any detectable difference between linearly and circularly polarized

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lights with identical intensity (as shown in Fig. S3 in Supporting Information). However, the

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circularly polarized photoexcitation generates a lower PL as compared to linearly polarized

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photoexcitation. The PL intensity is decreased by 12.5 % when the photoexcitation is

9

switched from linear to circular polarization. It is known that switching the photoexcitation

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from linear to circular polarization can essentially change the populations of spin singlet

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߮௠೗ୀ±ଵ߮↑௘↓௛ and spin triplet ߮௠೗ୀ଴ ߮↑௘↑௛ electron hole pairs, changing the density of light-

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emitting excitons. Therefore, the PL dependence of photoexcitation polarization provides an

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evidence to indicate that spin states can be optically operated at room temperature in organo-

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metal halide perovskites. Furthermore, we investigate the bulk polarization in perovskite film

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when the spin states are optically operated by switching the photoexcitation between linear

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and circular polarizations. Fig. 4b shows the capacitance measured at the frequency of 100

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kHz from the ITO/TiO2(compact)/TiO2 (mesoporous)/MAPbI3/P3HT/Au device under

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linearly and circularly polarized photoexcitations. In general, the capacitance characteristics

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can

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electrode/medium/electrode devices. The low and high-frequency capacitance signals are

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related to the polarization processes occurring at electrode interface and within bulk medium.

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Usually, Strong polarization effect takes placed at interfaces in solar cells16, 22. In the previous

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studies we found that the high-frequency capacitance signals (> 4 kHz) can indicate bulk

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polarization while the low-frequency capacitance (< 4 kHz) is generated by charge

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accumulation at interface.38 Here, we monitor the high-frequency capacitance amplitude while

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switching the photoexcitation between linear and circular polarizations to understand the

be

divided

into

low

and

high

frequency


regimes

in

sandwiched

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effects of spin states on bulk polarization. We can see in Fig. 4b that linearly and circularly

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polarized photoexcitations generate higher and lower capacitance amplitudes, respectively.

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This implies that changing spin populations can change the bulk polarization in organo-metal

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halide perovskites. In fact, this result provides first evidence that spin triplet ߮௠೗ ୀ଴ ߮↑௘↑௛ and

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spin singlet ߮௠೗ୀ±ଵ ߮↑௘↓௛ correspond to lower and higher electrical polarizations in organo-

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metal halide perovskites, respectively.

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Now we discuss the underlying mechanisms accountable for effects of spin states on

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photovoltaic actions in perovskite solar cells. It is known that a circularly polarized

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photoexcitation can operate spin states through angular momentum conservation. Considering

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the spin angular momenta ±h of circularly polarized photons, the spin operator can be

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expressed as: 0 −i Sˆ photon =  . i 0 

12

(3)

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As a consequence, a circularly polarized photoexcitation can generate ߮௠೗ ୀ±ଵ ߮↑௘↓௛ electron-

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hole pairs through spin angular moment conservation

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polarized light can be considered as a superposition of right and left-handed circular

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polarizations, and an equal number for spin up/down states is created, as shown in Fig. 1d.

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Therefore, under circularly polarized photoexcitation, spin polarized ߮௠೗ ୀାଵ ߮↑௘↓௛ or

18

߮௠೗ୀିଵ ߮↑௘↓௛ electron-hole pairs is formed, while under linear excitation ߮௠೗ୀାଵ ߮↑௘↓௛ and

19

߮௠೗ୀିଵ ߮↑௘↓௛ are formed simultaneously. (Fig. 1d). We should note that the spin mixing can

20

occur between anti-parallel and parallel spin states through spin relaxation caused by spin-

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orbital coupling or spin scattering from charged traps24 and excited states.24, 41 Here we set

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electron and hole spin relaxation rates as te and th (0< te, th 1 AP te + th − 2te th

1

(4)

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Because spin populations quickly reach an equilibrium before losing spin coherence,9, 24 the

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PP/AP ratio can be essentially larger than 1 under circularly polarized photoexcitation. Clearly,

4

switching the photoexcitation from linear to circular polarization can increase the spin

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population on parallel states in organo-metal halide perovskites. Because only antiparallel

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spin pairs are allowed to recombine due to Pauli Exclusion Principle, increasing parallel spin

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pairs can decrease the PL, as illustrated in Fig. 4a. However, this increases the total spin pairs

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available for dissociation to generate photocurrent. This is why switching photoexcitation

9

from linear to circular polarization causes an increase on Jsc, as shown in Fig. 2. We should

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point out that changing spin populations by switching photoexcitation between linear and

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circular polarizations can also affect the Voc based on the following two experimental

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information. First, increasing parallel spin pairs can decrease the bulk polarization in the

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perovskite solar cells upon switching the photoexcitation from linear to circular polarization,

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according to Fig. 4b. Second, light soaking effect indicates that decreasing bulk polarization

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can increase the Voc by enhancing the built-in field in perovskite solar cells.38, 42 Therefore,

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shifting electron-hole pair population from spin singlet to spin triplet states can consequently

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increase the Voc when the photoexcitation is switched from linear to circular polarization.

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With increasing incident light intensity, the ∆JSC show a non-monotonic change: first

19

increase and then decrease, however, the ∆PL shows a monotonic change: increase and then

20

saturation, as shown in Fig. 6. Here, the ∆Jsc is defined as: ∆JSC (%) =

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the ∆PL is given by ∆PL(%) =

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intensity dependent ∆JSC and ∆PL, spin dependent dynamic equations are introduced for

23

electrons, holes, and electron-hole pairs,

JSC (C)- JSC (L) ×100% and JSC (L)

PL(C ) − PL( L) ×100% . To understand the phenomena of light PL( L)


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dN PP 1 = β L ne nh − γ PP N PP + γ AP N AP − k D N PP dt 2 dN AP 1 = β L ne nh − γ AP N AP + γ PP N PP − k D N AP dt 2 dne (nh ) = − β L ne nh + k D (E )N PP + k D N AP dt

1

(5)

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where N AP and N PP are the densities for anti-parallel and parallel electron-hole pairs, ne and

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nh are the electron and hole densities, γ AP and γ PP are the spin relaxation rates from anti-

4

parallel to parallel state and from parallel to anti-parallel states, βL is recombination

5

coefficient and k D is dissociation rate. To figure out spin relaxation rate between anti-

6

parallel and parallel electron-hole pairs, we consider (i) spin flipping effect induced by

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circular excitation, (ii) spin relaxation effects from spin-orbit coupling, and (iii) spin exchange

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interaction in electron-hole pairs, 2 rˆ rˆ rˆ rˆ rˆ rˆ Hˆ = Hˆ photon + Hˆ relax + Hˆ exchange = cS photon ⋅ S i + ∑ bS i ⋅ L + JS 1 ⋅ S 2 ,

9

(6)

i =1

10

where c is the constant determined to be 1 for circularly polarized light and 0 for linearly

11

polarized light, b is defined as the spin-orbit coupling constant, and J denotes the exchange

12

interaction strength in electron-hole pairs. By involving the time evolution operator,

13

Hˆ t Vˆ = ex p ( ) , spin relaxation rate between anti-parallel and parallel states can be obtained ih

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as:

γ PP

15 γ AP

1 1 (c 2 + J 2 + cJ ) 2 (c 2 + J 2 − cJ ) 2 1 2 2 = [8 − − ]γ 0 1 2 1 16 2 2 2 2 2 2(b + c + J + cJ ) 2(b + c + J 2 − cJ ) 2 2 2 1 2 1 2 2 2 2 (c + J + cJ ) (c + J − J ) 2 1 2 2 = [8 + + ]γ 0 1 2 1 2 16 2 2 2 2 2 2 2(b + c + J + cJ ) 2(b + c + J − cJ ) 2 2

(7)

γ 0 is a spin independent constant, normally assumed to be 106s-1,43 b and J are set as

16

where

17

0.5T and 0.01T, respectively, due to strong spin-orbit coupling23, 11 Environment ACS Paragon Plus

44

and low electron-hole

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binding energy associated with weak exchange interaction.8 The equation (7) indicates that

2

spin relaxation rates of anti-parallel and parallel spin states can be changed by switching

3

photoexcitation from linear to circular excitation.

4 5 6

By calculating the Npp based on Eqs. (5) and (7), the JSC under linear and circular polarizations can be obtained as

J SC ∝ (ne + nh )µ = (ne + nh ) µ0

1 , 1 + ξ N PP

(8)

7

where µ is carrier mobility, ξ is the parameter defined to be 1 for strong scattering and 0 for

8

negligible scattering by parallel spin states. We can see in equation (8) that, when circular

9

polarization generates parallel spin pairs (Npp), the forbidden recombination leads to more

10

electron-hole pairs available to generate photocurrent. This can cause an increase on Jsc by

11

increasing charge density. On the other hand, the parallel electron-hole pairs can act as a

12

neutral scattering centers,45-47 decreasing carrier mobility. Therefore, increasing parallel

13

electron-hole pairs generates two competing processes on Jsc upon circularly polarized

14

photoexcitation: (i) increasing the electron-hole pairs available for charge dissociation and (ii)

15

decreasing the charge mobility through scattering. The former and later become dominant

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processes at low and high light intensities (below and above 100 mW/cm2 at 532 nm),

17

according to the light intensity dependence of ∆Jsc shown in Fig. 6. We should point out that

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the carrier mobility through scattering between parallel electron-hole pairs has little effect on

19

PL. We can see in Fig. 6b that the ∆PL shows monotonic increase and saturation with

20

continuously increasing light intensity. This provides further evidence that operating spin

21

states by using circularly polarized photoexcitation generates two competing processes: (i)

22

increasing electron-hole pairs available for dissociation and (ii) decreasing carrier mobility

23

through scattering, dominated at low and high light intensities in perovskite solar cells.

24

In summary, by operating spin states upon switching photoexcitation between linear and

25

circular polarizations, our studies provide the first evidence that optically operating spin states 12 Environment ACS Paragon Plus

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can affect the photocurrent and photovoltage during the photovoltaic development in

2

perovskite solar cells. Specifically, we find that switching photoexcitation from linear to

3

circular polarization causes an increase on both Jsc and Voc in perovskite (MAPbI3) solar

4

cells. Our analysis indicates that changing photoexcitation from linear to circular polarization

5

can shift the population from spin singlet to spin triplet states in electron-hole pairs. We

6

observe that switching the photoexcitation from linear to circular polarization decreases the

7

PL intensity. This confirms that the spin population can be operated by switching

8

photoexcitation polarization at room temperature in organo-metal halide perovskites.

9

Essentially, switching photoexcitation from linear to circular polarization generates more spin

10

triplet electron-hole pairs available for dissociation, leading an increase on Jsc but a decrease

11

on PL. Furthermore, the impedance characteristics show that switching photoexcitation from

12

linear to circular polarization decreases the bulk electrical polarization by increasing the

13

population of spin triplet electron-hole pairs. This result provides direct experimental

14

information that spin singlet and spin triplet states correspond to stronger and weaker bulk

15

polarizations in organo-metal halide perovskites. As a consequence, shifting the electron-hole

16

pairs population from spin singlet to spin triplet can essentially increase the Voc by increasing

17

built-in field through bulk polarization. Clearly, our studies provide a deeper understanding on

18

the effects of optically operating spin states on photovoltaic actions in perovskite solar cells.

19 20 21 22

EXPERIMENTAL METHODS Device fabrication. The methylammonium iodide (CH3NH3I) with 40 mg/ml in IPA was

23

purchased from One-Material Chemscitech. The lead(II) iodide (PbI2) with 462 mg/ml in

24

DMF was obtained from Alfa Aesar. These solutions were heated at 100oC for 10 minutes

25

before use. The P3HT was dissolved in 1,2-Dichlorobenzene with 15 mg/ml concentration for

26

spin cast. Titanium diisopropoxide bis(acetylacetonate) was spin-coated on previously cleaned

27

ITO substrates at 6000 RPM for 30 s in N2 atmosphere, followed by annealing at 450 oC for


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30 minutes. The mesoporous TiO2 was spin-coated on compact TiO2 at 4000 RPM for 40

2

seconds, then annealed at 450 oC for 30 minutes in air. The PbI2 solution was spin-coated on

3

combined TiO2 layer at 6000 RMP for 35 seconds and dried at 70 oC in N2 atmosphere for 3

4

minutes then at 100 oC for 5 minutes. 200 µL MAI solution was spin-coated on the PbI2 layer

5

at 6000 RMP for 35 seconds. The ITO/TiO2/MAPbI3 substrates were dried at 100 oC for 2

6

hours in N2 atmosphere. Furthermore, the P3HT with 15 mg/ml was spin-coated on top of

7

MAPbI3 layer at 3000 RPM for 30 seconds. At last, the Au was evaporated as anode with the

8

thickness of 50 nm under the pressure of 10-6 torr. AFM characterizations indicate the

9

thickness of 290 nm for MAPbI3 layer and 90 nm for P3HT layer. The perovskite solar cells

10

have the active area of 0.1 cm2.

11

Experimental measurements. The PL measurements were performed by SPEX Fluorolog III

12

spectrometer. Solar simulator (Newport-Model 69911) was used for photovoltaic

13

characterizations. Before performing J-V measurements, the light intensity from solar

14

simulator was calibrated through Newport power meter and multi-crystalline Si cell. During

15

calibration, mismatch in Jsc between measured value and standard value was kept lower than

16

1%. The J-V characteristics were characterized through Keithley 2400. The scanning speed

17

was set at 0.01 NPLC (number of power line cycles) and 200 points were selected during

18

scanning measurements. The J-V curves were measured in both forward and reverse

19

directions between 2 V and -1 V in N2 atmosphere at room temperature and averaged based

20

on 10 devices. The J-V characteristics show very small hysteresis. The linearly polarized laser

21

beam with 2 mm dimeter was combined with a ¼ wave plate to generate switchable linearly

22

and circularly polarized photoexcitations with identical intensity when the ¼ wave plate was

23

rotated with 0° and 45° relative to the direction of linear polarization of laser beam. Before

24

experimental measurements, the devices were illuminated with simulated sunlight for 30

25

minutes to remove light soaking for developing stable JSC and VOC. The PL and absorption

26

detections are not sensitive to light polarization based on the combination of photodetector 14 Environment ACS Paragon Plus

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and long-pass filters. The control-experiment was done by using organic and Si solar cells, as

2

shown in Fig. S4 in Supporting Information.

3 4

ASSOCIATED CONTENT

5

Supporting Information

6

Supporting information contains: (1) Effects of circular polarization on photocurrent at

7

different wavelengths in perovskite solar cells. (2) Absorptions of MAPbI3 thin film under

8

linear and circular polarized lights. (3) Confirmation on linearly and circularly polarized

9

photoexcitation intensities. (4) Control experiment based on Si solar cell under linear and

10

circular polarizations.

11

The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

12

AUTHOR INFORMATION

13

Corresponding Author

14

* E-mail: [email protected]

15

Notes

16

The authors declare no competing ﬁnancial interest.

17 18

ACKNOWLEDGEMENT

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This research was supported by the Air Force Office of Scientific Research (AFOSR: FA

20

9550–15–1–0064), the National Science Foundation (CBET-1438181), and the Homeland

21

Security (DHS-16-DNDO-077-001). The authors also acknowledge the support from AOARD.

22

This research was partially conducted at the Center for Nanophase Materials Sciences based

23

on user project (CNMS2012-106, CNMS2012-107, CNMS-2012-108), which is sponsored at

24

Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. Department


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of Energy. The authors also acknowledge the supports from the National Significant Program

2

(2014CB643506, 2013CB922104) and NSFC Program (61475051) in China.

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a

b 2

J (mA/cm )

PL (a.u.)

0 Abs. (a.u.)

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JSC=20.61 mA/cm

2

VOC=0.94 V FF=0.65 PCE=12.57%

-20

1

400 500 600 700 800 900 Wavelength (nm)

0.0

0.2

0.4 0.6 0.8 Voltage (V)

1.0

2 3 4 5 6 7 8 9 10 11

Figure 1. Characterization for perovskite (MAPbI3) solar cells and experimental setup for switching photoexcitation between linear and circular polarizations. a, Absorption and PL for MAPbI3 thin film. b, J-V curves measured with forward and reverse scanning methods for MAPbI3 solar cells under simulated sunlight (100 mW/cm2). Power conversion efficiency (PCE) shows 12.57% with JSC=20.61 mA/cm2, VOC=0.94 V, and FF=0.65, averaged from 10 devices. c, Setup for probing spin effects on photovoltaic actions by rotating ¼ wave plate to switch photoexcitation polarization with identical light intensity. d, Spin-up and spin-down electrons operated by linear and (left-hand σ +) circular polarizations.

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-7.9 2

3 4 5

excitation

excitation

-7.8 Linear

-7.7

excitation

Linear excitation

b 1.0252

0

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Circular Circular

1.0248

excitation

excitation Linear

1.0244

Linear excitation

1.0240 -7.6

2

Circular

Circular

VOC (V)

a Jsc (mA/cm )

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40 60 80 100 Time (s)

excitation

0

20

40 60 80 100 Time (s)

Figure 2. JSC and VOC measured by switching linearly and circularly polarized photoexcitations with identical light intensity. a. JSC values. b. VOC values.

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2 3 4 5 6 7

Figure 3. Underlying processes involved in spin effects of photocurrent


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b

58 56

Linear

Linear

excitation

excitation

54 52 50

Circular

Circular

48

excitation

excitation

0 3 4 5 6 7 8

20 40 60 80 100 120 Time (s)

Linear

6.4304 C (nF)

a PL @770nm (a.u.)

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Linear

excitation

excitation Circular

6.4296

excitation

Circular excitation

6.4288

0

50 100 Time (s)

150

Figure 4. Photoluminescence (PL) and capacitance (C) measured under linearly and circularly polarized photoexcitations with identical light intensity. The capacitance signals were sampled at 100 kHZ in ITO/TiO2(compact)/TiO2 (mesoporous)/MAPbI3/P3HT/Au to show bulk polarization. a. PL signals. b. Capacitance signals.


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Figure 5. Schematic diagram to show effects of spin states on Jsc and PL under linearly and circularly polarized photoexcitations. Circularly and linearly polarized photoexcitations facilitate and hinder spin triplet states formation in electron-hole pairs, respectively.

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b 2.0 1.5

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Calculation Experiment

1.0 0.5

4 5 6 7 8

∆PL (%)

a

∆JSC (%)

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-4 0

0 200 400 600 800 1000 2 Laser intensity (mW/cm )

Experiment

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Calculation

0 200 400 600 800 1000 2 Laser intensity (mW/cm )

Figure 6. Experimental and theoretical data for ∆Jsc and ∆PL at different light intensities in ITO/TiO2 (compact)/TiO2 (mesoporous)/MAPbI3/P3HT/Au solar cell and MAPbI3 film. a, ∆Jsc data. b, ∆PL data.

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Effects of Spin States on Photovoltaic Actions in Organo-Metal Halide

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