Materials Letters 76 (2012) 187–189

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Materials Letters

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Tuning photoconductive properties of organic–inorganic hybrid perovskitenanocomposite device via organic layer's thickness

Chaorong Li ⁎, Jing Wan, Yingying Zheng ⁎, Wenjun DongDepartment of Physics and Key Laboratory of ATMT Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China

⁎ Corresponding authors. Tel./fax: +86 571 8684357E-mail addresses: crli@zstu.edu.cn (C. Li), zhengyy17

0167-577X/$ – see front matter © 2012 Elsevier B.V. Aldoi:10.1016/j.matlet.2012.02.060

a b s t r a c t

a r t i c l e i n f o

Article history:Received 15 November 2011Accepted 14 February 2012Available online 19 February 2012

Keywords:NanocompositesInterfacesFunctionalMultilayer structureThin films

Two-dimensional (2D) layered organic–inorganic hybrid perovskite (CnH2n+1–NH3)2(CH3NH3)m−1PbmI3m+1

(abbreviated as CnPbmI3m+1; n=4, 8, 12; m=1, 2, 3), with its unique self-assembly and photoelectric proper-ties, has been proposed to functionalize for applications in photoconductive integrated devices. Special deviceswith the heterostructure of ITO/TiO2/TiO2:CnPbmI3m+1/Pt were fabricated through a facile physical-chemicalprocess. The relationship of the photoconductive performance of the devices and device structures was system-atically investigated. The photoconductivity can be optimized by adjusting the alkyl chain length (n) or inorganicsheet thickness (m) of the hybrid perovskite. The photocurrent of the device shows a negative and positive de-pendence with n and m, respectively. Particularly, a high ratio value of 3.96×104 of the photocurrent and darkcurrent (Ji/Jd) is achieved for the ITO/TiO2/TiO2:(C4H9NH3)2Pb3I10/Pt device at the bias voltage of 1.0 V. That isdue to the fact that both the shortened alkyl chain and thickened inorganic sheet can facilitate the exciton disso-ciation at the donor–acceptor interface and enhanced the carrier transport.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Organic–inorganic hybrid perovskite materials, which combinestructural flexibility and tunable electronic properties, are extraordi-narily promising for applications in many fields such as field-effecttransistors and photoconductive integrated devices [1]. The FTO/TiO2/(C6H13NH3)2(CH3NH3)m−1PbmI3m+1(m=1, 2):TiO2/Pt systemshows excellent photoconductive performance [2]. Moreover, a cellbased on p-type CH3NH3PbBr3 and CH3NH3PbI3 as visible-light sensi-tizer exhibits a high photovoltage about 0.96 V, and solar energy con-vert about 3.8% [3]. Compared with inorganic semiconductivematerials, organic–inorganic hybrid perovskite materials can bepromised to application in foldable photoconductive integrateddevices.

The 2D layered hybrid perovskites, (RNH3)2(CH3NH3)m−1PbmI3m+1,form the natural multiple quantum well structure (MQWs) which canbe tuned at the molecular level, such as the inorganic sheet thicknessand the physical–chemical properties of the organic cations [4–6].Complicated organics may enhance interactions with carriers in pe-rovskite plants for excitonic transfer [7]. However, details abouthow the hybrid perovskite with long-chain organic components af-fects the energy band structure and carrier mobilities are lacking.Moreover, the photoconductive activity of the heterostructure of thedevice, in which CnPbmI3m+1 with the different long-chain length

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serves as the p-type component and anatase TiO2 nanoparticles servesas the n-type component, has been rarely reported.

In this article, a facile technique for producing air-stable, tunablephotoconductivity devices, ITO/TiO2/TiO2:CnPbmI3m+1/Pt (n=4, 8,12; m=1, 2, 3), has been demonstrated. The photocurrent–voltage(J–V) performance of the photoconductive devices with different nvalue was systematically investigated to evaluate the effect of organicamine layer. On the basis of extensive characterization of the struc-tural and photoelectric properties of the photoconductive devices,it's concluded that the photocurrent reduces with the increasing n,and shortening of alkyl chains facilitates the exciton dissociationand transport.

2. Experiments

ITO glass (5 Ω/sq; 20 mm×20 mm) was soaking in 60 mM TiCl4aqueous solution at 70 °C for 30 min to form a relative dense TiO2

buffer layer, and then annealed at 450 °C for 1 h. Then, a mesoporousTiO2 thin film was prepared by casting the starchy mixture of P25TiO2 and ethanol through the doctor blade technique.

Stoichiometric amounts of CnH2n+1NH3I (n=4, 8, 12), CH3NH3Iand PbI2 were dissolved in N, N-dimethylformamide (DMF). Next,CnPbmI3m+1 (n=4, 8, 12; m=1, 2, 3) thin films were formed onITO/TiO2/TiO2 simply by spin-coating and then vacuum dried at80 °C for 30 min. Finally, the Pt electrode was prepared by ionsputtering on ITO/TiO2/TiO2:CnPbmI3m+1. The schematic diagramof the photoconductive device is shown in Fig. 1.

Fig. 1. The schematic diagram of the photoconductive device.

188 C. Li et al. / Materials Letters 76 (2012) 187–189

Structure of the perovskite thin film was identified by XRD (D8Discover, German) with Cu Kα-radiation (λ=0.15405 nm; 40 mA,40 kV) at a scanning speeding of 0.02°/s and 2θ=2.5°–40°. UV–visspectroscopy was recorded on a Hitachi U-3900 spectrophotometer.Photocurrent-voltage (J–V) characteristics of the devices were carriedout on Keithley 4200. A solar simulator as the excitation source (CHF-XM35-500W) was used to illuminate the sample at a distance of10 cm.

3. Results and discussion

The XRD patterns are shown in Fig. 2A. Correspondingly, Fig. 2A(a), (b) and (c) reveals the structure of the thin film Cn–PbI4 (n=4,8, 12) on the ITO/TiO2/TiO2 substrate, respectively. 2θ scans clearlyshow order (00 l, l=2, 4, 6, et al.) peaks. These remarkable diffractionpeaks are attributable to c-axis Bragg reflections intensity over-whelmingly and indicate the formation of well-crystallized and high-ly oriented hybrid perovskite films. Moreover, the highest diffractionpeaks (002) are observed at 2θ=6.70° (n=4), 4.93° (n=8) and3.75° (n=12), respectively. Calculated by the Bragg equation,nλ=2dsinθ, the interlayer d-spacing value (d) is 11.5 Å, 17.8 Å and23.4 Å, respectively. The results indicate that d is increased with n.

B

400 500 600 700

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a002

A

Fig. 2. (A) XRDpatterns of ITO/TiO2/TiO2:CnPbmI3m+1 films; (B) A schematic depiction of the strspectra of ITO/TiO2/TiO2: CnPbmI3m+1 films. (a–c: m=1 and n=4, 8, 12, respectively; d–f: m=

As shown in Fig. 2B, Cn–PbI4 (n=4, 8, 12) consists of layers of PbI64−

octahedra sandwiched between the organic alkylamine chainswhich adopt a tilted bilayer arrangement [8]. Increasing alkyl chains(from 4 to 12) would enhance organic layer thickness and reducethe interactions between the adjacent inorganic layers. Similarly, forthe ‘bi’ (Cn–Pb2I7) and ‘tri’ (Cn–Pb3I10) layered perovskite, the c-axisdiffraction peaks (00 l, l=2, 4, 6, et al.) would also dominate the dif-fraction patterns (as shown in Fig. 2A (d–f) and Fig. 2A (g–i), respec-tively) and d value would also increase along with the increasingcarbon chain length n. However, the crystalline property and self-assembly performance of the multilayer hybrid perovskites (m≥2)would lower with the increase of the inorganic sheet thickness.

Fig. 2C (a), (b) and (c), shows the UV–vis spectroscopy of ITO/TiO2/TiO2:Cn–PbI4 (n=4, 8, 12) thin films, respectively. Correspond-ingly, the significant peak appears at 512 nm, 504 nm and 490 nm.And it appears blue-shifted with n increased due to the increase ofthe band gap Eg. The Eg can be calculated from the following equation:

α ¼K hυ−Eg� �n

hυ

Where α is the absorption coefficient, K is a constant, and n is a con-stant equal to 1/2 for direct (2 for indirect) band gap semiconductor ma-terials [2,9]. The calculated Eg value is 2.38 eV, 2.48 eV and 2.54 eV,respectively. It is indicated that Eg is increased as the n increased in thefamily of Cn–PbI4. It is also appropriate for Cn–Pb2I7 and Cn–Pb3I10, andthe blue-shift appears in ITO/TiO2/TiO2:Cn–Pb2I7 (567→560→510 nm)and ITO/TiO2/TiO2:Cn–Pb3I10 (606→602→570 nm) films, as shown inFig. 2C (d–f) and Fig. 2C (g–i), respectively. In contrast, Eg is decreasedas the inorganic sheet thickness (m) increased which has been found inour previous work [2].

a b c

ucture and orientation of the alkyl chains in the layered CnPbI4 compounds; (C) Absorption2 and n=4, 8, 12, respectively; g–i: m=3 and n=4, 8, 12, respectively).

-1.0 -0.5 0.0 0.5 1.0

-0.008

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J sc(

mA

/cm

2 )

Voltage (V)

ab

c

Fig. 3. Typical J–V characteristics of the ITO/TiO2/TiO2:CnPbmI3m+1/Pt devices (n=4, 8,12; m=1).

Table 1The J–V characteristics data of ITO/TiO2/TiO2:CnPbmI3m+1/Pt devices.

CnPbmI3m+1 m 1 2 3

n

Jsc (mA/cm2) 4 0.012 0.148 2.4678 0.008 0.040 0.10512 0.006 0.032 0.056

Jd (mA/cm2) 4 3.036×10−6 7.404×10−6 6.242×10−5

8 1.407×10−5 1.720×10−5 3.713×10−5

12 1.428×10−4 6.302×10−4 7.309×10−4

Ji/Jd ratio 4 3.953×103 1.999×104 3.952×104

8 5.686×102 2.326×103 2.828×103

12 4.202×101 5.078×101 7.662×101

189C. Li et al. / Materials Letters 76 (2012) 187–189

Fig. 3 shows J–V characteristics of the ITO/TiO2/TiO2:Cn–PbI4/Pt(n=4, 8, 12; m=1), and the J–V characteristics data of ITO/TiO2/TiO2:CnPbmI3m+1 is shown in Table 1. For a bias voltage of 1.0 V, thephotocurrent density (Jsc) is decreased with n increased(Jsc=0.012→0.008→0.006 mA/cm2, corresponding to n=4→8→12). Similarly, the Jsc of the device which contains Cn–Pb2I7 (m=2)or Cn–Pb3I10 (m=3) is also decreased with n increased, and the Jscis increased as m increased [2]. The ratio of photocurrent and darkcurrent (Ji/Jd) is also decreased with n increased, but increased withm increased. The Jsc of the ITO/TiO2/TiO2:C4Pb3I10/Pt device withthickest inorganic sheet (m=3) and shortest organic chain length(n=4) reaches a maximum of 2.467 mA/cm2, which is much higherthan the dark current density of 6.242×10−5 mA/cm2. The Ji/Jdreaches a largest value of about 4×104.

When the photoconductive devices are illuminated under the sim-ulated sunlight, the free excitons are generated, and then are separatedinto holes and electrons effectively at the interface of the CnPbmI3m+1

(electron donor) and the TiO2 nanoparticle (electron acceptor). Theholes and electrons migrate to the corresponding electrodes due toenergy levels matched among each components of the heterojunctiondevice [2].

The alkyl chain length of organic compounds would produce someinfluences. (i) The dielectric organic layer, which controls the com-munication of excitons from one inorganic layer to the next, wouldchange the exciton bands of hybrid perovskites. As mentionedabove, the Eg increases with n increasing, which may signify smallereffective mass of carriers, electrons and holes [1,10], and would resultin the decrease of Jsc. (ii) As the n increases, interlayer d-spacing valueincreases. It means that the channels for carrier transport would beelongated and the weakness of interactions between inorganic sheetswould appear. Jsc depends on the relative rates of the electron andhole transfer across the interface, and the relative rates may be decid-ed by the distance between the electron donor and electron acceptor[11], which is increased with n. So the thickened organic layers wouldreduce Jsc greatly. (iii) Disorder of conformation and orientationoccurs in long chain compounds (n≥8), and the structural distortionbecomes more seriously with n increased, which make the carriertransport more difficult [12].

It has been confirmed that increasing the inorganic layer's thick-ness leads to the decrease of bandgap (Eg) and exciton binding ener-gy (Eb), and would be conductive to increase the carrier mobility[2,13]. Thus, the photoconductivity of the devices can be optimizedvia shortening the alkyl chains or thickening the inorganic layers ofCnPbmI3m+1. The calculated Eg of C4Pb3I10 is 1.95 eV, which is theminimum among that of CnPbmI3m+1 (n=4, 8, 12; m=1, 2, 3).And C4Pb3I10 has the shortest alkyl chain. Consequently, the ITO/TiO2/TiO2:C4Pb3I10/Pt device has highest photoconductive properties,with Jsc of about 2.47 mA/cm2 and Ji/Jd ratio about 3.96×104.

4. Conclusion

In summary, a facile physical–chemical process was used for pro-ducing special organized ITO/TiO2/TiO2:(CnH2n+1NH3)2(CH3NH3)m−1

PbmI3m+1/Pt (n=4, 8, 12; m=1, 2, 3) photoconductive devicesfor visible-light. Results demonstrate that photoconductive propertyof devices can be tuned by varying the length of alkyl amine chains(n) or inorganic sheet thickness (m) of hybrid perovskites. Jsc and Ji/Jdratio increase with n decreased. Furthermore, a relatively high Ji/Jd(3.96×104) can be obtained for ITO/TiO2/TiO2:C4Pb3I10/Pt photocon-ductive device.

Acknowledgments

This work was supported by NSF of China ((Nos. 51172209,91122022, 10874153 and 50972130), and Science Foundation of Zhe-jiang Sci-Tech University).

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