Inhomogeneous Deactivation with UV Excitation in Submicron Grainsof Lead Iodide Perovskite-based Solar Cell as Revealed

by Femtosecond Transient Absorption Microscopy

Tetsuro Katayama,*1,2,3 Akira Jinno,2 Eisuke Takeuchi,2 Syoji Ito,2 Masaru Endo,4 Atsushi Wakamiya,3,4

Yasujiro Murata,4 Yuhei Ogomi,5,6 Shuji Hayase,5,6 and Hiroshi Miyasaka*21Institute for NanoScience Design, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531

2Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-85313PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012

4Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-00115Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology,

2-4 Hibikino, Wakamatsu-ku, Kitakyushu, Fukuoka 808-01966CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012

(E-mail: tetsuro@insd.osaka-u.ac.jp)

Femtosecond transient absorption microscopy was appliedto the elucidation of ultrafast dynamics in a lead iodideperovskite solar cell. Space- and time-resolved detectionrevealed that rapid deactivation with a time constant of 0.7 pstook place without long-living species in submicron grains withthe (CH3NH3)PbI3 capping layer, while the long-living compo-nent was detected in other areas.

Hybrid inorganic­organic solar cells with organometaltrihalide perovskite as light absorbers have been attracting muchattention because of their large power conversion efficiency(PCE).1­13 This large PCE is attributed to several factors: (i) highmolar absorption coefficient9 of around 105M¹1 cm¹1, (ii) lowinternal resistant owing to extremely low thickness,10­12 and (iii)high conversion efficiency of charge separation owing to theextended diffusion length of excitons.12,13 In addition, it hasbeen reported that the structural properties of the perovskitelayer are of crucial importance for the high performance of thedevice. The mesoporous titanium oxide (mp-TiO2) layer, used asa scaffold, also plays a part in improving the crystalline structureof the perovskite layers.2 Not only the structure but also thethickness of the perovskite/mp-TiO2 layer would affect thephotoelectric conversion.14­16 The PCE improved14 from 6.5%to 9.5% with a decrease in the thickness of the perovskite/mp-TiO2 layer from 1.5¯m to 600 nm; this result was ascribedto the decrease in the internal resistance and dark current.14,15

In addition, the procedure for the production of the perovskitelayer, solution cast or vapor deposition, strongly affects theperformance of the device. Inhomogeneous crystal grains wereformed in the solution-processed layer,14 while a rather smoothmorphology was confirmed in the vapor-deposited layer.17 Theseresults indicate that the structure and morphology of theperovskite layer in a wide range of space, from atomic levelto mesoscopic scale, strongly affect the total performance of thedevice. Accordingly, space-resolved detection of the dynamicbehaviors is important to directly acquire information on thecorrelation between the structure and the performance of thedevice. Accordingly, in the present work, we have appliedfemtosecond transient absorption microscopy to the directelucidation of the space-resolved dynamics in inorganic­organicsolar cells with solution-processed perovskite layers.

Details for the preparation of the device are described inprevious papers.18 Briefly, patterned transparent conductingoxide substrates (FTO, 25 © 25mm2) were covered with acompact TiO2 layer by spray pyrolysis.19 Subsequent spin-coating of an ethanol suspension of a TiO2 paste (PST-18NR,TiO2 paste:ethanol = 1:3.5wt ratio) resulted in the depositionof ca. 200-nm-thick mesoporous films of TiO2 nanoparticles(particle diameter: ca. 20 nm). PbI2 was then introduced intothe TiO2 nanopores by spin coating a solution of PbI2 in DMF(1.0­1.1M) at 70 °C. Afterward, the films were dipped for 20 sin a 0.06M solution of CH3NH3I in 2-propanol. The films werethen quickly rinsed with 2-propanol and dried at 70 °C for30min. The hole-transporting layer was deposited on theperovskite layer by spin coating a solution of 2,2¤,7,7¤-tetra-kis(N,N-di-p-methoxyphenylamine)-9,9¤-spirobifluorene (spiro-OMeTAD) in chlorobenzene (0.058M) containing 4-tert-butyl-pyridine (0.19M) and lithium bis(trifluoromethylsulfonyl)imide(0.031M) as well as tris[2-(1H-pyrazol-1-yl)-4-tert-butylpyri-dine]cobalt(III) tris[bis(trifluoromethylsulfonyl)imide] (5.6 ©10¹3M) as dopants. For the back contact, a gold layer (80 nm)was deposited thermally on top of the device. The PCE of thedevice was 10.6% (see Supporting Information (SI)).

Space-resolved dynamics were measured by means of atransient absorption spectroscope combined with a microscope(Olympus, IX71). The light source was an optical parametricamplifier (OPA-800, Spectra Physics) pumped by a Ti:sapphirefemtosecond laser (Solistice, Spectra Physics, 1 kHz, 3.5W,800 nm). The harmonics of the output of OPA (signal and idler)at 420 and 760 nm were used for the pump and probe pulses.A triggered CCD camera (Allied Vision Technologies, pike-F032B) was employed for detecting the probe light intensity.Temporal and spatial resolutions were estimated to be ca. 200 fsand ca. 600 nm, respectively. For measuring steady-stateabsorption spectra under the microscope, unpolarized whitecontinuum light generated by focusing a femtosecond 1.2-¯mlaser pulse on a CaF2 plate was used. The reference andmonitoring light intensities were detected by multichannelphotodetectors (PMA-20, Hamamatsu).

Figure 1a shows a schematic representation of the planarheterojunction perovskite solar cell. The laser beam wasirradiated from the glass substrate (from the bottom in the figure)using an objective lens (NA = 0.8, Olympus, LMPlanFLN).

CL-140551 Received: June 1, 2014 | Accepted: July 15, 2014 | Web Released: July 23, 2014

1656 | Chem. Lett. 2014, 43, 1656–1658 | doi:10.1246/cl.140551 © 2014 The Chemical Society of Japan

A cross-sectional scanning electron microscope (SEM, JSM-6060, JEOL, at an acceleration voltage of 10 kV) image of thesolar cell is exhibited in Figure 1b, showing inhomogeneousthickness and grains with sub-micrometer sizes. The inhomoge-neous boundary and grains were typical of the solution-processedperovskite layer.4,11,17

Figure 2a shows spatial distribution of the absorptionintensity at 760 nm of the perovskite-based solar cell. Theabsorption at 760 nm corresponds to the electronic transitionbetween conduction and valence bands13,16 of the perovskitelayer. The highest absorbance (ca. 0.30) in this region is almosttwice than that of the smallest value. This difference in theabsorbance may reflect the difference in the thickness of thelayer shown in Figure 1.

Figure 2b shows the steady-state absorption spectra atvarious positions in Figure 2a, indicating that the spectral shapeis also position dependent. That is, not only the thickness ofthe layer but also the electronic structure is dependent on theposition. It was reported that the “capping layer”4,11 without ascaffold has different crystal structures.19 The difference in thespectral band shapes is attributable to this structural difference.15

The position-dependent ratio of the absorbance at 760 nmversus that at 610 nm is shown in Figure 2c. From thecomparison of Figure 2c with Figure 2a, one can find that the

position with higher absorbance at 760 nm has a larger value ofthis ratio.

Figure 3 shows the transient absorption image of the samesample in Figure 2 at 0.5, 10, and 100 ps, following a femto-second 420-nm excitation (8¯J cm¹2). The monitoring wave-length is 760 nm. The monitoring positions are the same as thosein Figure 2, within 200-nm accuracy. At position (1), thepositive signal appearing in the early stage after the excitation isfollowed by the rapid decay in the initial several picosecondtime region. On the other hand, a negative absorption signal atposition (4) shows no remarkable evolution in the initial 10 pstime region.

Time profiles of the transient absorbance at 760 nm areshown in Figure 3d. As was shown in the images in Figures 3a­3c, the positive signal at position (1) (filled blue circles)decreases to the baseline very rapidly. The solid line (red line)for this time profile is a curve calculated with a time constant of700 fs, reproducing the experimental results fairly well. In thetime window at least up to 3 ns, the transient absorbance stayson the baseline. This result strongly suggests that almost all thetransient species produced by the photoexcitation deactivatedwithin a few ps.

On the other hand, the negative signal at position (4) showsno remarkable decay in the initial 10 ps and gradually decreasesin a few nanosecond; the solid line (black line) for the timeprofile at position (4) is a curve calculated with a decay timeconstant of 540 ps. This time constant was also obtained for thetime profile of the emission (see SI). Although the experimentalresults are rather scattered, the solid line reproduces theexperimental results, indicating that the recovery of the negativesignal is ascribable to the decay of the excited state (hole­electron) in the perovskite layer.13 In addition, the negativesignal is observed even at a few nanoseconds following theexcitation. This result implies that the recovery of the bleachsignal is accompanied with hole transfer to spiro-OMeTADlayers.13 It should be noted that because the extinctioncoefficient of spiro-OMeTAD2+ (1­2 © 104M¹1 cm¹1 around420 nm, see SI) is sufficiently smaller than that of (CH3NH3)PbI3

Figure 1. (a) Schematic cross-section view of the device.Objective lens for the optical detection is also indicated. (b) Animage of the cross-sectional scanning electron microscope of theperovskite-based solar cell.

Figure 2. (a) The image of the absorbance at 760 nm. (b)Absorption spectra at each numbered position (① x = ¹1.1¯m,y = 0.8¯m, ② x = ¹0.9¯m, y = ¹0.8¯m, ③ x = 0.5¯m,y = 0.6¯m, ④ x = 0.7¯m, y = ¹0.8¯m). (c) The image ofthe ratio of the absorbance at 760 and 610 nm in perovskite-based solar cell.

Figure 3. Femtosecond transient absorbance images of theperovskite-based solar cell at pump­probe delays of (a) 500 fs,(b) 10 ps, (c) 100 ps. Exciting wavelength was at 420 nm, andmonitoring wavelength is at 760 nm, (d) Time profiles oftransient absorbance at each numbered position and averagedover 25¯m2 region. Inset is time profiles up to 3000 ps (①x = ¹1.1¯m, y = 0.8¯m, ④ x = 0.7¯m, y = ¹0.8¯m).

Chem. Lett. 2014, 43, 1656–1658 | doi:10.1246/cl.140551 © 2014 The Chemical Society of Japan | 1657

(2­3 © 105M¹1 cm¹1 around 420 nm9), the excited-state dy-namics in the hole-transfer material can be ignored.

For the time-resolved studies on the dynamic behaviors ofthe perovskite system, several investigations12,13,16 reported thatthe negative signal was observed for the transient absorption at760 nm. These studies were performed for rather wide spatialregions of the sample. To compare the present results with thoseprevious studies, we averaged time profiles over the area ofca. 25¯m2, the result of which is shown as triangles in Figure 3d.The dynamic behaviors thus obtained show a negative signal andgradual decay within ca. 500 ps. This result is consistent withthe previous results, and in turn, the space-resolved study revealsthe specific dynamics hidden in the averaged detection.

To elucidate the spatial distribution of the positive signalsmore precisely, position dependence of the steady-state andtransient absorption signals are shown in Figure 4, where theabsorption intensities are plotted along the line at x = 0¯m inFigure 3c. The position with positive signals in the transientabsorbance at 760 nm has rather large absorbance in the steady-state absorption. As was shown in Figure 3d, the positive signalsdisappeared within a few picoseconds following the excitation.On the other hand, the position with negative signals tended tohave rather weak steady-state absorption. It should be notedthat the present spatial resolution was ca. 600 nm. Hence, thedynamics at several positions in Figure 4 were not perfectlydiscriminated; for example, at ¹1.1¯m in Figure 4, the positiveabsorbance in the early stage after the excitation is replaced withnegative signals in the time region >1 ps. These results suggestthat two kinds of perovskite systems coexist in this monitoringspatial region.

The energy diagrams of the two regions are summarized inFigure 5. For the position where the negative signal wasobserved immediately after the 420-nm excitation, the hole invalence band 2 (VB2) of (CH3NH3)PbI3 transfers to VB1 withinthe time resolution of our apparatus, and then the hole in VB1transfers to spiro-OMeTAD within ca. 540 ps, as shown inFigure 5a.13 On the other hand, the deactivation processcoexisted within a few picoseconds in thick perovskite regime,as shown in Figure 5b. Some morphologies might prohibit thegeneration of free carrier from the exciton in the perovskitecrystal grain. As mentioned in the introduction, the PCE of theperovskite solar cell is related to the inhomogeneity of the

perovskite layer.15,17 The present results suggest that thedecrease in the PCE with increasing thickness is due to theincrease in the inhomogeneous (CH3NH3)PbI3 capping layer.

We are now focusing on more detailed studies on therelation between the structure and dynamics using polarization-dependent transient absorption microscopy, the results of whichwill be published soon.

Supporting Information is available electronically on J-STAGE.

References1 A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem.

Soc. 2009, 131, 6050.2 M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J.

Snaith, Science 2012, 338, 643.3 P. V. Kamat, J. Phys. Chem. Lett. 2013, 4, 908.4 J. H. Heo, S. H. Im, J. H. Noh, T. N. Mandal, C.-S. Lim, J. A.

Chang, Y. H. Lee, H. Kim, A. Sarkar, M. K. Nazeeruddin, M.Grätzel, S. I. Seok, Nat. Photonics 2013, 7, 486.

5 A. Abrusci, S. D. Stranks, P. Docampo, H.-L. Yip, A. K.-Y. Jen,H. J. Snaith, Nano Lett. 2013, 13, 3124.

6 H.-S. Kim, J.-W. Lee, N. Yantara, P. P. Boix, S. A. Kulkarni, S.Mhaisalkar, M. Grätzel, N.-G. Park, Nano Lett. 2013, 13, 2412.

7 J. H. Noh, S. H. Im, J. H. Heo, T. N. Mandal, S. I. Seok, NanoLett. 2013, 13, 1764.

8 W. Zhang, M. Saliba, S. D. Stranks, Y. Sun, X. Shi, U. Wiesner,H. J. Snaith, Nano Lett. 2013, 13, 4505.

9 S. Sun, T. Salim, N. Mathews, M. Duchamp, C. Boothroyd, G.Xing, T. C. Sum, Y. M. Lam, Energy Environ. Sci. 2014, 7, 399.

10 N.-G. Park, J. Phys. Chem. Lett. 2013, 4, 2423.11 H. J. Snaith, J. Phys. Chem. Lett. 2013, 4, 3623.12 S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. P.

Alcocer, T. Leijtens, L. M. Herz, A. Petrozza, H. J. Snaith,Science 2013, 342, 341.

13 G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Grätzel,S. Mhaisalkar, T. C. Sum, Science 2013, 342, 344.

14 H. J. Snaith, R. Humphry-Baker, P. Chen, I. Cesar, S. M.Zakeeruddin, M. Grätzel, Nanotechnology 2008, 19, 424003.

15 B. Conings, L. Baeten, C. De Dobbelaere, J. D’Haen, J. Manca,H.-G. Boyen, Adv. Mater. 2014, 26, 2041.

16 H.-S. Kim, C.-R. Lee, J.-H. Im, K.-B. Lee, T. Moehl, A.Marchioro, S.-J. Moon, R. Humphry-Baker, J.-H. Yum, J. E.Moser, M. Gräzel, N.-G. Park, Sci. Rep. 2012, 2, 591.

17 M. Liu, M. B. Johnston, H. J. Snaith, Nature 2013, 501, 395.18 A. Wakamiya, M. Endo, T. Sasamori, N. Tokitoh, Y. Ogomi, S.

Hayase, Y. Murata, Chem. Lett. 2014, 43, 711.19 J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao,

M. K. Nazeeruddin, M. Grätzel, Nature 2013, 499, 316.

Figure 4. Spatial profiles of steady-state absorbance andfemtosecond transient absorbance at the position of x = 0¯m.Monitoring wavelength was 760 nm.

Figure 5. Schematic energy diagrams of the each grainin perovskite-based solar cell (a) without (b) with thick(CH3NH3)PbI3 capping layer.

1658 | Chem. Lett. 2014, 43, 1656–1658 | doi:10.1246/cl.140551 © 2014 The Chemical Society of Japan