CH3NH3PbI3 Heterostructure for

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 3

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Interface Engineering of Graphene/CHNHPbI Heterostructure for Novel P-I-N Structural Perovskites Solar Cells Yong-Hua Cao, Zun-Yi Deng, Ming-Zi Wang, Jintao Bai, Su-Huai Wei, and Hong-Jian Feng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04042 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018

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The Journal of Physical Chemistry

Interface Engineering of Graphene/CH3NH3PbI3 Heterostructure for Novel p-i-n Structural Perovskites Solar Cells

Yong-Hua Cao,a,b,d Zun-Yi Deng,a Ming-Zi Wang,a Jin-Tao Bai,a,b Su-Huai Wei c, * and Hong-Jian Feng a

a,*

School of Physics, Northwest University, Xi’an 710069, People’s Republic of China

b

Institute of Photonics & Photon-Technology, Northwest University, Xi’an 710069, People’s Republic

of China c

Beijing Computational Science Research Center, Beijing 100193, People’s Republic of China

d

School of Mechanical and Electrical Engineering, Henan Institute of Science and Technology,

Xinxiang 453003, People’s Republic of China Abstract: Functionalized graphene is widely used in various functional devices. Here, we introduce a simple plane capacity model and the density functional theory (DFT) to investigate the origin of charge transfer in the graphene/CH3NH3PbI3 interface, where graphene can be p-type or n-type doped by combined with different exposed surfaces of CH3NH3PbI3. Our calculations indicate that at the equilibrium distance, the work function of isolated graphene layer should be corrected by adding a value for assessing the charge transfer. After integrating the perovskite film with the functionalized graphene layer, we obtain a van der Waals heterostructure solar cell with p-i-n configuration, which introduces a built-in electrical field to facilitate the separation and transport of the photo-generated carriers. The new p-i-n junction highlights the interface effect on graphene in solar cell, which offers an avenue to design new photovoltaic devices with high performance.

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1. Introduction Since first obtained in 2004, graphene has attracted a great deal of attentions for its outstanding physical and chemical properties, such as excellent transparency, high electrical conductivity, remarkable flexibility, ambipolar transport characteristic, superior thermal and chemical stability,1-3 which renders wide applications of graphene-based materials in different fields.4-7 By chemical doping or introducing functional groups, graphene can be functionalized, which offers wide opportunities to regulate the electronic and chemical properties of graphene. Unlike conventional semiconductor doping, the electric-field effect and physisorption can control the carrier density and type (electron-like or hole-like) of graphene due to its unique band structure. 8 When deposited on various metal substrates, graphene can also be doped and exhibit different doping characteristics.9 The functionalization of graphene is a viable route to modulate its properties for diverse technological applications, including optoelectronics and photovoltaics. Among

the

current

photovoltaic

technologies,

hybrid

organic-inorganic

perovskite-based solar cells (PSCs) are one of the most promising candidates for next generation solar cells due to their high power conversion efficiencies (PCEs). This can be attributed to their excellent material properties, such as long-range carrier diffusion length, tunable band gap, broad absorption range, low-cost solution process and modest charge mobility.10-15 As a prototype of hybrid perovskites, methylammonium lead halide (CH3NH3PbX3, X = Cl, Br and I) and its derivatives are most widely investigated. Since Miyasaka et al. first used CH3NH3PbI3 as the sensitizer in dye-sensitized solar cells (DSSCs) and gained an efficiency of 3.81% in 2009,16 intense efforts have been devoted


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to improving the performance and stability of PSCs, for instance, adjusting chemical composition, optimizing crystallization process, controlling surface morphology and designing feasible interface.17-23 Recently, the CH3NH3PbI3-based thin-film solar cell reached a world record PCE of 22.7%,24 which has overtaken the present thin-film technologies, like cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS), and is approaching the commercialized crystalline silicon solar cells. A PSC device should be composed of perovskite absorber, charge transport materials (CTMs) and electrodes in which the photo-generated carries are collected by CTMs and transferred to corresponding electrodes. Among these materials, titanium dioxide (TiO2) and

2,29,7,79-tetrakis(N,N-di-p-methoxyphenylamine)9,99-spirobifluorene

(spiro-MeOTAD) are well known as electron transport layers (ETLs) and hole transport layers (HTLs) , respectively, inserting between the electrodes and perovskite layer.25-27 The efficient separation and transport of photo-generated carriers are crucial for developing high efficiency thin-film solar cells. In order to improve the PCE and stability of PSCs, graphene and its derivatives, such as N-doped graphene and graphene oxide (GO), have been integrated into the PSCs as CTMs and transparent electrodes. Incorporating graphene as a transparent bottom anode in p-i-n planar PSCs, graphene-based PSC containing MoO3 layer delivers a 17.1% PCE,28 which may be comparable to the 18.8% PCE of indium tin oxide (ITO)-based PSC. The performances of PSCs are strongly affected by the number of stacked graphene layers. Yan et al. fabricated an inversed n-i-p planar PSC with multilayer graphene as the top electrode and found that the PCE (12.37%) with double-layer graphene is higher than that with single-layer (9.18 %) and three-layer (11.45%) graphene.29 Moreover, the thin



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film PSC with low-temperatures ( W0 , electrons will transfer from graphene layer to perovskite side, that is, holes are

injected into graphene layer, as illustrated in Figure 7b. The charge transfer make graphene p-type doped, along with the Fermi-level shift downward. It is another thing for graphene/CH3NH3I interface because of WP < W0 . Then electrons will transfer from perovskite side to graphene layer, which makes the graphene n-type doped and


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Fermi-level shift upward, as displayed in Figure 7c. In a word, the charge transfer causes the functionalized (n- or p-type doped) graphene.

Figure 7. Schematic illustration of the Fermi level shift in functionalized graphene. (a) Isolated pristine graphene layer and CH3NH3PbI3 slab, (b) grphene/PbI2 and (c) grphene/CH3NH3I interfaces. (d) Schematic diagram of solar cell with graphene/CH3NH3PbI3/graphene archistructure. The red (blue) arrows represent the transport direction of electrons (holes).

Based on the functionalized graphene, we construct a van der Waals heterostructure solar cell with graphene/perovskite/graphene (G/P/G) configuration, in which the perovskite film possesses two different exposed surfaces, that is, PbI2- and CH3NH3I-termination, as shown in Figure 7d. In the G/P/G heterostructure, the work mechanism including modulation of graphene follows five steps. I) At dark state, because of the pristine work function mismatch between graphene layer and perovskite surface, electrons (holes) are pushed into corresponding graphene layer (see Figure 8a), which



will change the carrier concentration of graphene. As the result of charge transfer, the depletions regions emerge in the perovskite side, along with the formation of the built-in electric fields in both interfaces. II) As expected, the carrier concentration variations cause the Fermi level of graphene to move away from conical points, indicating that graphene layers are doped with electrons (n-type) or holes (p-type). III) Resulting from the Fermi level shift, the work functions of graphene layers are modified to reduce the contact barriers between graphene layer and adjacent perovskite surface, which provides an efficient charge extraction from perovskite film. IV) After graphene layers are doped, the G/P/G heterostructure acts as a p-i-n junction structure, with the intermediate perovskite film as intrinsic semiconductor, as shown in Figure 8b. V) As illuminated, the perovskite film absorbs incident photons and converts them into free electron-hole pairs. Then the built-in electrical field at the interfaces helps to separate the photo-generated electron-hole pairs and transfer them to the corresponding electrodes. It is concluded that the p-type (n-type) doped graphene serves as hole- (electron-) collecting layer and blocks electron (hole) injection.


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Figure 8 Schematic illustration of band diagram of G/P/G heterostructure solar cell structure (a) at dark state, (b) under illumination. The red (blue) arrows indicate the transfer directions of electrons (holes) and the orange arrows near the interfaces label the directions of the built-in fields, respectively. The dashed lines represent the Fermi levels of heterostructure. EF , E F 1 and EF 2 denote the pristine Fermi level of graphene layer, PbI2 and CH3NH3I surface, respectively. EFp and E Fn indicate the quasi-Fermi level of the graphene/PbI2 and graphene/CH3NH3I interfaces, respectively. The diagrams are not to scale and show only relative positions of the energy bands.

This G/P/G heterostructure combining the superior properties of functionalized graphene and perovskite materials has many advantages in versatile applications, including photovoltaic and photo-electricity field. Due to the physisorption on different exposed surfaces of CH3NH3PbI3 substrate, graphene can possess a desired carrier concentration and work function, which are more beneficial to separate and transfer charge from perovskite film. The modulations of graphene without any external impact will not degrade the excellent properties of graphene, and instead to improve its



conductivity. Moreover, the G/P/G heterostructure can also be used in transparent and flexible electronic devices because of the high optimal transparency and mechanism flexibility. Last but not least, the perovskite film sandwiched by hydrophobic graphene layers may improve the environmental stability of PSCs. In a word, the interface engineering makes the G/P/G structure more active in applications of high-performance electronic devices and PSCs. It is feasible in experiments to obtain the CH3NH3PbI3 film with different exposed surfaces. In our previous work, we used the CsBr and PbBr2 as the precursors to fabricate CsPbBr3/TiO2 thin films with two different interfaces (CsBr/TiO2 and PbBr2/TiO2) by the thermal evaporation technique.50 The charge transfer of the CsBr/TiO2 interface is superior to that of the PbBr2/TiO2 interface, which is in agreement with our DFT calculations. With the PbI2 and CH3NH3I as precursor materials, the CH3NH3PbI3 films with different exposed surfaces can be obtained by the similar thermal evaporation method. By spin-coating the PbI2 or CH3NH3I precursor solutions on the as-prepared CH3NH3PbI3 films, the thin overlayers can also be regarded as different exposed surfaces of perovskite films.

4. Conclusions In this work, we have systematically investigated the electronic properties of graphene/CH3NH3PbI3 heterostructure by employing the DFT calculation and plane capacity model. The graphene layer is physically absorbed on perovskite substrate which preserves its unique electronic structure. A plane capacity model is used to study the


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charge transfer driven by the work-function difference between graphene layer and perovskite surface, along with the Fermi-level shift of graphene layer. At the equilibrium distance, the work function of isolated graphene layer should be corrected by adding a value before assessing the charge transfer between graphene layer and perovskite slab. Calculations show that there are different charge transfer directions in graphene/PbI2 and graphene/CH3NH3I interfaces. A G/P/G van der Waals heterostructure based functionalized graphene forms a p-i-n solar cell, where the carrier concentration and work function of graphene are modified by charge injection. The built-in electric field at the interfaces will facilitates the separation and transport of photo-generated carriers. This work highlights the interface effect on electronic properties of graphene/CH3NH3PbI3 heterostructure and provides a new approach to enhance the performance of PSCs by means of interface engineering.

Corresponding Author Email: [email protected] (Prof. Hong-Jian Feng); Email: [email protected] (Prof. Su-Huai Wei)

Supporting Information The following files are available free of charge on the ACS Publications website. Optimized lattice constants, shortest distance variation between surface Pb/I atoms and adjacent C atoms of graphene layer, work functions, and fitting parameters.



Acknowledgements H.-J. F. was financially supported by the National Natural Science Foundation of China (NSFC) under Grants No. 51672214, No. 11304248 and No. 11247230, the Natural Science Basic Research Plan in Shaanxi Province of China (Program No. 2014JM1014), the Scientific Research Program Funded by Shaanxi Provincial Education Department (Program No. 2013JK0624), the Fund Program for the Scientific Activities of Selected Returned Overseas Professionals in Shaanxi Province of China, and Youth Bai-Ren (100 Talents Plan) Project in Shaanxi Province of China. S.-H. W was supported by NSFC Program for Scientific Research Center under Grant No. U1530401 and National Key Research and Development Program of China under Grant No. 2016YFB0700700. Y.-H.C was supported by the Social Science Project Funded by Henan Provincial Education Department (Project No. 2017-ZZJH-178).

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CH3NH3PbI3 Heterostructure for

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