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fabrication and organic electronics. In this hybrid system,extremely thin absorber (ETA) layers, such as molecular dyes1,3

tion of photogenerated charges, which then diuse to thecorresponding electrodes. Such charge separation and trans-port occurring at the ETAETM and ETAHTM interfaces play acrucial role to the device performance.2,3,8 In this vein, it is veryimportant to tune the optical anETA layers by varying the banquantum dots through quanemploying organic dyes with dlowest-unoccupied molecular ooptimize the energy level alignmhybrid system. However, applyin

(CH3NH3PbI2Br) with higher absorption coecients and ahigher CB edge, which are auspicious for the 1D-based devices.The resulting hybrid photovoltaic cells, fabricated by combining

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18GDepartment of Chemistry, The Hong Kong U

Water Bay, Kowloon, Hong Kong, China. E

1594; Tel: +852-2358-7362

Electronic supplementary information (Echaracterization details, including addphotovoltaic characteristics of cells basethickness. See DOI: 10.1039/c3nr00218g

This journal is The Royal Society ofd electronic properties of thed gaps of the semiconductortum connement9,10 or byierent highest-occupied andrbitals (HOMOLOMO)11,12 toent and light absorption in theg these ETA materials to the

TiO2 NWAs as the ETM, CH3NH3PbI2Br as the ETA and 2,20,7,70-tetrakis-(N,N-di-p-methoxyphenylamine) 9,90-spirobiuorene(spiro-MeOTAD) as the HTM, have achieved a 4.87% PCEwith anopen circuit voltage (Voc) of 0.82 V, and both the PCE and Voc aresignicantly improved over those of CH3NH3PbI3.

The 1D TiO2 NWAs were grown on seeded uorine-doped tinoxide (FTO) substrates via a hydrothermal process reportedpreviously with some modications (see ESI for details).22 Theor semiconductor quantum dots,47 are typically introduced tothe interface between the inorganic electron transport materials(ETMs) and the organic hole transport materials (HTMs), toenhance the light harvesting eciency. Upon light excitation,the ETA injects electrons into the conduction band (CB) of thesemiconductor and holes into the HTMs, allowing the separa-All-solid-state hyborganometal halidone-dimensional T

Jianhang Qiu, Yongcai Qiu,and Shihe Yang*

A novel organometal halide perovskite (CH3NH3PbI2Br) is synthe-

sized and used as a visible light absorber to sensitize one-dimen-

sional (1D) TiO2 nanowire arrays (NWAs) for all-solid-state hybrid

solar cells. It achieved a power conversion eciency (PCE) of 4.87%

and an open circuit voltage (Voc) of 0.82 V, both higher than those of

its analogue CH3NH3PbI3.

All-solid-state hybrid photovoltaics based on mesoscopic inor-ganic semiconductor nanostructures and organic conductingmaterials have been considered as promising next generationsolar cells, as they integrate the desirable advantages of nano-

1,2

Cite this: Nanoscale, 2013, 5, 3245

Received 12th January 2013Accepted 1st March 2013

DOI: 10.1039/c3nr00218g

www.rsc.org/nanoscaleniversity of Science and Technology, Clear

-mail: chsyang@ust.hk; Fax: +852-2358-

SI) available: Experimental procedures,itional SEM, TEM, EDS, XRD andd on TiO2 NWAs with dierent lm

Chemistry 2013id solar cells based on a newperovskite sensitizer and

O2 nanowire arrays

eyou Yan, Min Zhong, Cheng Mu, He Yan

conventional mesoscopic particulate lms could encounterobstacles such as inecient electron transport in the nano-crystalline lms and incomplete lling of organic HTMs in themesopores of the lms.13,14 Our strategy in this work is todevelop a hybrid photovoltaic cell by fabricating orderednanostructures for ETMs, andmeanwhile, establishing a simpleprocess to tune the optical and electronic properties of ETAs, soas to tune the energy band alignment between ETAs and ETMs,eventually improving the charge transfer and power conversioneciency (PCE) of the device.

One-dimensional (1D) nanowire arrays (NWAs) provide adirect path for thephotogenerated electron transport.15,16Ofnoteis that the straight channels among the nanowires could benetthe lling of the HTMs, thus enhancing the hole transport e-ciency.17,18 Between the 1D structure and the HTM lies an ETAlayer of organometal halide perovskite sensitizers. As an ETA,perovskite sensitizers have demonstrated photovoltaic potentialin hybrid cells constructed with mesoporous lms,1921 but theextension to 1D-based cells is yet to be studied. By a singlehalogen substitution, we have synthesized a novel sensitizer

View Article OnlineView Journal | View Issuemorphologies of the TiO2 NWAs were examined by a eldemission scanning electronmicroscope (FE-SEM). Fig. 1a showsa typical cross-sectional SEM image of the as-synthesized TiO2NWA lm. The NWAs lm is about 1.5 mm thick and mostnanowires grow approximately vertical to the FTO substrate.Viewing from the top as in Fig. 1b, the diameters of mostnanowires are 50150 nm. According to the X-ray diraction

Nanoscale, 2013, 5, 32453248 | 3245

CH3NH3PbI2Br sensitized photoanode are 2.55%, 7.97% and6.33%, respectively, translating to the expected 1 : 2 : 1 atomicratio (Fig. S4a, ESI), while those of I and Pb with CH3NH3PbI3are 17.23% and 9.31%, respectively, resulting in a 3 : 1 atomicratio that correlates with the formula unit (Fig. S4b, ESI). Theformation of the perovskite ETA layers has been furtherconrmed by XRD analysis. The XRD patterns forCH3NH3PbI2Br and CH3NH3PbI3 coated on TiO2 NWAs (Fig. 2a)and FTO (Fig. 2b) indicate that both materials have a similarcrystalline structure assignable to the perovskite system.23,24 TheCH3NH3PbI3 sensitized lm gives diraction peaks at 14.05,28.45, 31.87, 40.45 and 43.13, which can be assigned to the(110), (220), (310), (224) and (314) planes of the tetragonalperovskite structure with a 8.872 A and c 12.637 A, whileCH3NH3PbI2Br shows peaks at 14.40, 29.06 and 32.60 corre-sponding to the (110), (220) and (310) planes, respectively, of a

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18GView Article OnlineFig. 1 Cross-sectional (a) and top (b) view SEM images of TiO2 NWAs synthe-sized on FTO substrates. SEM images of CH3NH3PbI2Br (c) and CH3NH3PbI3 (d)spin-coated on TiO2 NWAs. (e) Cross-sectional SEM image of the TiO2 NWAs/perovskite sensitizer/spiro-MeOTAD hybrid photovoltaic cell. (f) Schematic illus-tration of the hybrid solar cell.(XRD) pattern (Fig. S1, ESI), the nanowires take a tetragonalrutile structure (JCPDS: 00-021-1276). Advancedmicrostructuresof the nanowires were determined by a transmission electronmicroscope (TEM). The high resolution transmission electronmicroscope (HRTEM) image (Fig. S2a, ESI) reveals a (110)interplanar distance of 0.326 nm and a (001) interplanardistance of 0.297 nm, both of which agree with the relevantinterplanar distances of rutile TiO2, suggesting the [001] growthdirection of the nanowires. The corresponding selected areaelectron diraction (SAED) pattern (Fig. S2b, ESI) demon-strates that the nanowires have a single crystalline rutilestructure, in accordance with the XRD analysis above.

For solar cell studies, the as-synthesized TiO2 NWAs werecoated with the organometal halide perovskite sensitizers(CH3NH3PbI3 and CH3NH3PbI2Br) (see ESI for details). Fromthe SEM images shown in Fig. 1c and d, it can be seen clearlythat a perovskite sensitizer layer composed of 1020 nm nano-particles has been formed on the surface of the TiO2 NWAs.These perovskite sensitized TiO2 NWAs were spin coated with aHTM layer (spiro-MeOTAD), followed by thermal evaporation ofa Au layer as the counter electrode. The cross-sectional SEMimage of the photovoltaic cell, shown in Fig. 1e, portrays thewhole device structure, which conforms well to the layout of thehybrid inorganicorganic FTO/TiO2 NWAs/perovskite sensi-tizer/spiro-MeOTAD/Au unit in Fig. 1f. The distribution ofperovskite sensitizers within the photoanodes was estimated byenergy dispersive X-ray spectroscopy (EDS) mapping analysis(Fig. S3, ESI). The weight percentages of Br, I and Pb in the

3246 | Nanoscale, 2013, 5, 32453248tetragonal perovskite structure with a 8.681 A, b 8.692 A andc 12.268 A. Compared to CH3NH3PbI3, the diraction peaks ofCH3NH3PbI2Br are shied uniformly to high angles, which canbe rationalized by the shrunken crystalline lattice wholly causedby the single-atom replacement of I with Br. Those atomic tostructural changes in the ETA layer cast important variations inits optical and electronic properties, thereby oering a handleto modulate the photovoltaic operation of the hybrid devices.

Fig. 3a shows the diuse reectance spectra of the twoorganometal halide perovskites spin-coated on TiO2 NWAs andthe corresponding absorption spectra (inset) convertedaccording to the KubelkaMunk relation (optical absorptioncoecients f F(R) (1 R)2/2R). While CH3NH3PbI3 absorbslight in a relatively longer wavelength region up to800 nm, theappreciable absorption of CH3NH3PbI2Br is mainly at

hybrid photovoltaic cells. Apparently, the higher CB edge ofCH3NH3PbI2Br elicits a larger driving force for the photo-generated electrons to transfer from the sensitizer to the TiO2NWAs, which could lead to an enhanced photogenerated elec-tron injection eciency from the CH3NH3PbI2Br sensitizer tothe TiO2 NWAs,10,25 compared to the CH3NH3PbI3-based device.Indeed, the CH3NH3PbI2Br-based photovoltaic cell gives higherincident photon-to-current conversion eciency (IPCE) valuesat 400650 nm than those of CH3NH3PbI3 as can be seen fromFig. 5a, which, although mainly caused by the higher opticalabsorption coecients at 400650 nm (inset of Fig. 3a), couldalso benet from the enhanced electron injection from theCH3NH3PbI2Br sensitizer to the TiO2 NWAs. The resultingshort-circuit current density (Jsc) of 10.12 mA cm

2 (Fig. 5b andTable 1) is comparable to that of the CH3NH3PbI3-based devicein spite of the much narrower absorption of CH3NH3PbI2Br

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18GView Article Onlinevalue for CH3NH3PbI3 is 1.57 eV, which is in good agreementwith those reported elsewhere,20,23 while it is 1.78 eV forCH3NH3PbI2Br, which is >0.2 eV larger, mainly caused by thehybridization of the Br (4p) orbitals with the I (5p) and Pb (6s)orbitals. Furthermore, such introduction of the Br (4p) orbitals

Fig. 3 (a) Diuse reectance spectra of the organometal halide perovskite-coated TiO2 NWAs and the corresponding absorption spectra obtained by theKubelkaMunk method (inset). (b) Plots of [F(R)hn]2 vs. photon energy (hn). (c)(e)UPS spectra of the perovskite-coated TiO2 NWAs.could give rise to the variation of the valence band (VB) edge ofthe perovskite sensitizers. Ultraviolet photoelectron spectros-copy (UPS) was performed to investigate the VB edge positionsof these two sensitizers. Shown in Fig. 3ce are the corre-sponding UPS spectra with respect to the He I photon energy(21.22 eV). The VB edge energy (EVB) values, with reference to thevacuum level, are calculated to be 5.43 and 5.40 eV forCH3NH3PbI3 and CH3NH3PbI2Br, respectively. The CB edgeenergies (ECB) can then be determined from the Eg and EVBvalues, yielding3.62 eV for CH3NH3PbI2Br that is substantiallyhigher than that of CH3NH3PbI3 (3.86 eV). On the basis of theresults, Fig. 4 sketches the band alignment scheme for the

Fig. 4 Schematic energy level diagrams of the TiO2 NWAs, organometal halideperovskite sensitizers CH3NH3PbI3 (a) and CH3NH3PbI2Br (b), and spiro-MeOTAD.

This journal is The Royal Society of Chemistry 2013(Fig. 3a).Contrasting the high IPCEs at 400650 nm, there is a smaller

dark current of the photovoltaic cell based on CH3NH3PbI2Brcompared to that of the CH3NH3PbI3-based device (Fig. 5b),meaning that the solar cell featuring the TiO2 NWAs/CH3NH3PbI2Br sensitizer/spiro-MeOTAD/Au junction can betterrestrain the recombination of the photogenerated electronsinjected into the TiO2 NWAs and the holes extracted into spiro-MeOTAD. Since the Voc and ll factor (FF) are closely related tothe recombination rate of the photogenerated charges, thelarger Voc (0.82 V) and FF (0.59) of the CH3NH3PbI2Br-based cellare thus expected (Fig. 5b and Table 1). Altogether, it is becauseof the improvements in IPCE (at 400650 nm), FF and Voc thatthe CH3NH3PbI2Br cells could achieve an overall PCE of 4.87%,almost 20% higher than that based on the CH3NH3PbI3 sensi-tizer (4.29%) (Table 1).

Of note is that the 1D TiO2 NWAs/CH3NH3PbI2Br/spiro-MeOTAD device performs best when the length of the 1D NWAs,or the lm thickness, is 1.5 mm (Fig. S5 and Table S1, ESI).This is contrary to the hybrid perovskite photovoltaic cells basedon mesoporous lms, the thickness of which is restricted to thesubmicron scale due partly to the limitations of electrontransport in nanocrystalline lms and the pore lling of poly-mer HTMs.19,20 This result indicates that the 1D NWAs-baseddevice oers the promise of employing thicker lms with larger

Fig. 5 (a) IPCE spectra of the TiO2 NWAs/perovskite sensitizer/spiro-MeOTADhybrid solar cells. (b) The corresponding current densityvoltage (JV) character-istics under 100 mW cm2 AM 1.5 illumination and in the dark.Nanoscale, 2013, 5, 32453248 | 3247

down the surface area of the 1D NWAs lm, resulting in a

5 H. Lee, H. C. Leventis, S. J. Moon, P. Chen, S. Ito, S. A. Haque,T. Torres, F. Nuesch, T. Geiger, S. M. Zakeeruddin, M. Gratzeland M. K. Nazeeruddin, Adv. Funct. Mater., 2009, 19,2735.

6 S. J. Moon, Y. Itzhaik, J. H. Yum, S. M. Zakeeruddin,G. Hodes and M. Gratzel, J. Phys. Chem. Lett., 2010, 1, 1524.

7 J. A. Chang, J. H. Rhee, S. H. Im, Y. H. Lee, H. J. Kim,S. I. Seok, M. K. Nazeeruddin and M. Gratzel, Nano Lett.,

Table 1 Photovoltaic properties of the hybrid solar cells based on the organo-metal halide perovskite sensitizers and 1D TiO2 NWAs

a

Perovskite sensitizerJsc(mA cm2) Voc (V) FF h (%)

CH3NH3PbI2Br 10.12 0.82 0.59 4.87CH3NH3PbI3 10.67 0.74 0.54 4.29

a The estimated error is5%. Preparation conditions of the TiO2 NWAs,

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In conclusion, a solution processed all-solid-state hybridsolar cell based on a new organometal halide perovskite sensi-tizer and 1D TiO2 NWA with a PCE of 5% has been demon-strated. Our results show that higher Voc and PCE can beachieved by Br-substitution in the perovskite sensitizerCH3NH3PbI3. The signicantly improved Voc and eciencyachieved here highlight the superior electronic and opticalproperties of the 1D TiO2 NWAs and CH3NH3PbI2Br sensitizer,opening up new avenues for advancing the development ofhybrid photovoltaic cells.

This work was supported by the HK-RGC General ResearchFunds (GRF no. HKUST 606511 and 605710). We thank Prof.JiannongWang andMr Shiguang Liu for providing access to themetal evaporation facility.

Notes and references

1 U. Bach, D. Lupo, P. Comte, J. E. Moser, F. Weissortel,J. Salbeck, H. Spreitzer and M. Gratzel, Nature, 1998, 395,583.

2 G. Hodes and D. Cahen, Acc. Chem. Res., 2012, 45, 705.3 H. J. Snaith, A. Petrozza, S. Ito, H. Miura and M. Gratzel, Adv.surface areas without sacricing the charge transport eciency.Besides the lm thickness, other morphology parameters of the1D NWAs, such as the density and diameter, can also inuencethe device performance signicantly. As shown in Fig. 1a and b,although the diameter of most nanowires is below 100 nm,there are still lots of nanowires with a diameter larger than150 nm grown on the substrate. Such thick nanowires thin

growth temperature: T 180 C, growth time: t 8 h.Funct. Mater., 2009, 19, 1810.4 R. Plass, S. Pelet, J. Krueger, M. Gratzel and U. Bach, J. Phys.Chem. B, 2002, 106, 7578.

3248 | Nanoscale, 2013, 5, 324532482010, 10, 2609.8 H. J. Snaith, A. J. Moule, C. Klein, K. Meerholz, R. H. Friendand M. Gratzel, Nano Lett., 2007, 7, 3372.

9 C. F. Chi, P. Chen, Y. L. Lee, I. P. Liu, S. C. Chou, X. L. Zhangand U. Bach, J. Mater. Chem., 2011, 21, 17534.

10 K. Tvrdy, P. A. Frantsuzov and P. V. Kamat, Proc. Natl. Acad.Sci. U. S. A., 2011, 108, 29.

11 R. Zhu, C. Y. Jiang, B. Liu and S. Ramakrishna, Adv. Mater.,2009, 21, 994.

12 N. Cai, S. J. Moon, L. Cevey-Ha, T. Moehl, R. Humphry-Baker,P. Wang, S. M. Zakeeruddin and M. Gratzel, Nano Lett., 2011,11, 1452.

13 L. Schmidt-Mende and M. Gratzel, Thin Solid Films, 2006,500, 296.

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

15 M. Law, L. E. Greene, J. C. Johnson, R. Saykally andP. D. Yang, Nat. Mater., 2005, 4, 455459.

16 X. J. Feng, K. Zhu, A. J. Frank, C. A. Grimes and T. E. Mallouk,Angew. Chem., Int. Ed., 2012, 51, 2727.

17 C. K. Xu, J. M. Wu, U. V. Desai and D. Gao, Nano Lett., 2012,12, 2420.

18 J. C. Cardoso, C. A. Grimes, X. J. Feng, X. Y. Zhang,S. Komarneni, M. V. B. Zanoni and N. Z. Bao, Chem.Commun., 2012, 48, 2818.

19 M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami andH. J. Snaith, Science, 2012, 338, 643.

20 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. Gratzel and N. G. Park, Sci. Rep., 2012, 2, 591.

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129, 4136.This journal is The Royal Society of Chemistry 2013

All-solid-state hybrid solar cells based on a new organometal halide perovskite sensitizer and one-dimensional TiO2 nanowire arraysElectronic...All-solid-state hybrid solar cells based on a new organometal halide perovskite sensitizer and one-dimensional TiO2 nanowire arraysElectronic...