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Molecular Engineering of the Lead Iodide Perovskite Surface: Case Study on Molecules with Pyridyl Groups Lei Zhang, Lei Xu, Jing Su, Di Lu, and Jingfa Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07577 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 17, 2017

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The Journal of Physical Chemistry

Molecular Engineering of the Lead Iodide Perovskite Surface: Case Study on Molecules with Pyridyl Groups

Lei Zhangab*, Lei Xuab, Jing Sua, Di Lua and Jingfa Li ab

a

Jiangsu Key Laboratory for Optoelectronic Detection of Atmosphere and Ocean，Nanjing University of Information Science & Technology, Nanjing 210044, China. Email: [email protected] b

Department of Applied Physics, School of Physics and Optoelectronic engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China

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ABSTRACT

We computationally investigate the molecular engineering approach of the lead iodide perovskite surface, employing a pyridyl anchor-based molecular adsorbate as an example. The molecular adsorption approach on lead halide perovskite surfaces has been employed for passivation purposes in perovskite solar cells and demonstrated to successfully enhance the solar cell performance in previous experimental studies. It is an open question whether the structures and properties of the lead halide perovskite can be further modified via the molecular engineering approach, and this study serves to probe the molecular engineering approach in the lead halide perovskite surface. First principles calculations are employed to determine the nanoscopic structure of the lead halide perovskite surface with pyridyl anchor-based molecular adsorbates, and prove that the pyridyl anchor-based molecule resides stably on the perovskite surface and modifies the perovskite surface structure. In addition, the calculations demonstrate that the electronic and optical properties of the lead halide perovskites can be controlled by the molecular engineering method. Noteworthily, we find that the molecular engineering approach is especially effective to modify the optical properties of the lead halide perovskite layer investigated in this study. Such molecular engineering approach on the perovskite surface could be potentially applicable to further enhance the performance of perovskite solar cells and perovskitebased optoelectronic devices.

Introduction The lead halide perovskite solar cells have attracted many research attentions due to the high power conversion efficiency (PCE), low cost and facile fabrication processes.1–5 Recently perovskite solar cells have achieved PCE beyond 20% that is comparable to the silicon-based solar cells, while continuous efforts have been made to further improve the perovskite solar cell performance.6–12 The


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existence of perovskite surfaces is universal in perovskite solar cells due to the solution-based synthetic routes.13–15 The perovskite surfaces have been identified to be responsible for the water degradation in perovskite material and act as charge defects, and significantly affect the solar cell performance.16–20 The degradation of perovskite materials is considered to be one of the major issues that prevent the lead halide perovskite materials from extensive usage.21–23 Surface engineering methods have been developed to passivate the perovskite surface to retard the solar cell degradation process;24,25 for example, molecule-based approaches via the Lewis bases and pyridine molecules have been identified to effectively improve the stability and optoelectronic performance of the perovskite solar cell.21,26–28 Yet, there is a lack of theoretical understanding on the structures and properties of the lead halide perovskite surface with molecular adsorbates. It is an open question whether the structures and properties of the lead halide perovskite can be effectively modified via the molecular engineering approach. In this study, the nanoscopic structures of lead iodide perovskite (CH3NH3PbI3) surfaces adsorbed with pyridine molecules are revealed from first principles calculations. Based on the molecule/perovskite interfacial structure, a molecule-based surface/interface approach to enhance the optical properties of the lead halide perovskite is suggested via judicious molecular engineering the adsorbates on perovskite surface. Electronic and optical properties of the lead iodide perovskite surface such as band structures, projected density of states (PDOS) spectra and UV/vis light absorption spectra are discussed to reveal the impacts of the molecular adsorption approach on lead halide perovskite surfaces.

Computational Details A 8.8 Å x 8.8 Å perovskite surface along the (0 0 1) direction is prepared in a unit cell with a 20 Å vacuum layer along the c-axis. The (001) surface of the CH3NH3PbI3 perovskite is focused since it has been demonstrated to be one of the most stable surfaces by density functional theory (DFT) 3 ACS Paragon Plus Environment


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calculations and X-ray diffraction experiments.29 A pyridine molecule (1) and a conjugated azo molecule [(E)-N,N-diethyl-4-(pyridin-4-yldiazenyl)aniline, 2] (Figure 1) bearing a pyridyl moiety are placed in the unit cell. Three perovskite surface systems are investigated: a bare perovskite surface with no adsorbate (3), a perovskite surface with an adsorbate 1 (4), and a perovskite surface with an adsorbate 2 (5) (Figure 1). PBE functional and 340 eV cutoff energy are employed during the geometrical optimization (GO) step in CASTEP30 for all the systems, with pseudopotential treated at ultrasoft level. van der Waals interactions are considered using the Tkatchenko−Scheffler (TS) scheme.31 Spin-orbit coupling (SOC) effect is not included in the calculation, since the current PBE theory produces decent values due to error cancellations32–36 and enjoy low computational cost. Multiple terminations exist for the perovskite surface such as the PbI-termination and the cationtermination.29,34,37–39 In this study the perovskite surface with the PbI-termination is focused, since the PbI-termination exposes multiple Lewis-acid atoms which are more suitable for the adsorption of the Lewis-base pyridine molecule.25,26 A 6 x 6 x 1 Monkhorst-Pack grid size for the self-consistent field (SCF) calculation is employed for the electronic and optical properties calculations based on the optimized structures. A TiO2 layer (DSL 18NR-T, Dyesol) bearing a TiO2 particles size of ca. 20 nm is deposited onto a transparent glass via the doctor-blade method; this layer is sintered at 500 °C for 30 min to obtain the TiO2 underlayer. An equimolar mixture of HI and CH3NH2 is stirred for 2 h in an ice-water bath to obtain the CH3NH3I solution. The solution is then evaporated at 60 °C resulting in white CH3NH3I powders which are washed three times in diethyl ether and dried in vacuum. The CH3NH3I and PbI2 powders are dissolved in γ-butyrolactone at room temperature to get a 0.05 mol/L precursor solution. The resulting film is spin-coated on the TiO2 film glass at a rate of 2000 rpm using a benchtop machine for 30 seconds, and is then subjected to an annealing treatment in an oven at 90 °C. The prepared

TiO2/perovskite

film

is

sensitized

in

a

0.5

mM

solution

of

4(4-

Diethylaminophenylazo)pyridine (2) in anhydrous ethanol for 24 hours. The resulting sensitized film 4 ACS Paragon Plus Environment

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consisting of the TiO2/perovskite/2 trilayer is measured in a UV/vis spectrophotometer (TU-1810) to obtain its UV/vis light absorption spectra.

1

2

3

4

5

Figure 1 The chemical structures of the pyridine molecule (1), the azo molecule with a pyridyl moiety (2), the bare lead iodide perovskite surface (3), a lead iodide perovskite surface with a pyridine molecule (4), and a lead iodide perovskite surface with an azo molecule (5). Results and Discussion Interfacial Structures In the optimized structures of 3-5, the N…Pb distances between the nitrogen atom in the molecular adsorbate and the lead atom in the perovskite surface are 2.54 Å in 4 and 2.60 Å in 5, which are shorter than the S…Pb distance and the I…Pb distance in the case of thiophene-stabilized lead iodide perovskite surface, and halogen bond-stabilized lead iodide perovskite surface, respectively.40 This suggests a stronger interaction between the molecule and the perovskite surface in the presence of a pyridyl anchor. Compared with those in the bare perovskite surface 3, the average Pb-I bond length in the outermost layer in 4 and 5 increases by ca. 0.17 Å upon molecular adsorption (Table 1), and the average Pb-I-Pb angle is also modified upon adsorption. The adsorption energy in 4 and 5 are 1.26 eV and 1.06 eV respectively (Table 2), which are larger than those in non-pyridyl-based molecule/perovskite systems.40 The larger adsorption energies in the presence of the pyridyl anchor 5 ACS Paragon Plus Environment


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suggest that the pyridyl group is effective to enhance the coupling between molecular adsorbates and perovskite surfaces. Table 1 The Pb-I bond lengths and Pb-I-Pb angles for the outermost perovskite surface layer in 3-5. 3 is the bare lead iodide perovskite surface while 4-5 are molecularly modified lead iodide perovskite surfaces 3

4

5

Pb-I (Å)

3.33

3.50

3.49

Pb-I-Pb (º)

141

133

142

Table 2 Adsorption energies of the molecular adsorbate on the perovskite surface in 4 and 5 4

5

Ead (eV) 1.26 1.06 Optical Properties Figure 2 depicts the UV/vis light absorption spectra of 3-5. The UV/vis light absorption spectra of the bare PbI-terminated perovskite surface 3 exhibit efficient light absorption in the UV/vis region that is consistent with the literature.41,42 Upon molecular adsorption, the UV/vis light absorption spectra of 4 display negligible changes compared with those of the bare perovskite surface 3. However, when the conjugated azo adsorbate is adsorbed onto the perovskite surface in the case of 5, the optical performance revealed by the light absorption intensity is significantly enhanced in the visible region, revealed by the stronger absorption near 660 nm in 5. This improvement in the visible light absorption properties could be potentially important for the design of the halide perovskite light absorbers and their application in solar cells, considering the fact that the under-coordinated atoms serving as charge traps at the lead iodide perovskite surface are passivated by the pyridyl-based adsorbates.


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The UV/vis light absorption experiment is conducted to back up the theoretical calculations on the optical properties of the molecule/perovskite system. The experimental result agrees with our calculations and supports the effectiveness of the molecule-based approach on the perovskite optical performance: the light absorption performance in the visible region of the perovskite film is enhanced via the molecular adsorption approach revealed by the experimental UV/vis light absorption spectra (Figure S1). The light absorption enhancement in the visible region upon molecular adsorption via the pyridyl group is observed in both experiments and calculations, substantiating the calculations on the molecule-based perovskite surface/interface engineering approach. Nevertheless, slight differences exist between the calculation and the experimental results. For example, the calculation predicts a narrow band centered at 660 nm with light absorption intensity enhancement via the molecular approach, while the experimental result reveals a broad band from 450 nm to 700 nm that exhibits an enhanced UV/vis light absorption intensity. The dissimilarity might originate from the H-aggregation of molecular azo dyes on semiconducting substrates in the real scenario which leads to a blue-shifted light absorption spectra relative to its monomeric form.43,44 The coexistence of monomers and Haggregates of the azo dye results in a broadened band in the visible region of the fabricated perovskite film.

Figure 2 UV/vis light absorption spectra of 3-5



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PDOS Spectra To understand the UV/vis light absorption performance of the perovskite layer with pyridyl-based adsorbates, the PDOS spectra are plotted to reveal their electronic properties (Figure 3). The valence band between -3.5 eV and 0 eV is mainly contributed by the p orbital of iodine atoms and s orbital of lead atoms, while the conduction band between 1.5 eV and 3.3 eV is mainly contributed by the s orbital of lead atoms. These characteristics are the same as the bulk crystal properties.33,36,45 Negligible variations are observed in the PDOS of 4 bearing a small pyridine molecule compared with those of 3. However, when the small pyridine molecule adsorbate is molecularly engineered to the conjugated molecule bearing a pyridyl anchor and an azo moeity in 5, additional defect states near 0.62 eV are introduced in the band gap. These states are contributed by the adsorbate, and essentially “decrease” the band gap from ca. 1.51 eV to 0.62 eV; this could be the major cause that leads to the greater optical absorption intensity in the visible region in 5.

3

4

5

Figure 3 PDOS spectra of 3-5. Total: total density of states. Pb_s: s orbitals of lead. Pb_p: p orbitals of lead. I: iodine. C: carbon. H: hydrogen. N: nitrogen. Molecule: the organic adsorbate. The maximum of valence band is set to 0.

Band Structures


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The band structures plots of 3-5 along lines connecting high symmetry points G (0, 0, 0), X (0, 0.5, 0) and M (0.5, 0.5, 0) exhibit direct band gaps at Gamma points in the molecule/perovskite interfacial system (Figure 4). The band gaps are similar for 3 (1.51 eV) and 4 (1.56 eV), which suggests negligible impacts from the adsorption of the pyridine molecule on the perovskite surface band gap. In the case of 5, a straight band line at 0.62 eV is introduced when the conjugated azo molecule is adsorbed onto the perovskite surface. This band line is mainly contributed by the carbon and nitrogen atoms in the molecular adsorbate, and essentially leads to a “decreased” band gap by ca. 0.9 eV, resulting in stronger light absorption intensities in the visible region of the perovskite material.

3

4

5

Figure 4 The band structures of 3-5 along lines connecting high symmetry points G (0, 0, 0), X (0, 0.5, 0) and M (0.5, 0.5, 0). The maximum of valence band is set to 0.

Orbital Distributions The orbital distributions of 3-5 (Figure 5) reveal that the molecular adsorbates modify the spatial distributions of the valence band maximum (VBM) and the conduction band minimum (CBM). The CBM distribution in 5 is significantly different with that in 3 or 4. In the case of 5, the most prominent feature is that the CBM is distributed in the molecular adsorbate (Figure 5), which is not observed in 3



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or 4. The CBM of 5 corresponds to the band line at 0.62 eV (Figure 4), which has significant effects on the optical properties of the perovskite material.For the bare perovskite surface 3, the VBM-1 is located in the iodine and lead atoms in the uppermost and bottommost layer of the constructed perovskite surface, while the VBM is localized in the iodine and lead atoms in the uppermost layer. In contrast, the CBM and CBM+1 of 3 are delocalized in the lead atoms (p orbitals) throughout the layers. In the case of 4, the VBM-1 and VBM are distributed in the bottom layers, while the CBM+1 is distributed in the pyridine molecule. VBM-1

VBM

CBM

CBM+1

3

4

5

Figure 5 Orbital distribution of VBM-1, VBM, CBM and CBM+1 of 3-5. A 3 x 1 supercell is shown for clarity purpose.

Conclusions The pyridyl anchor leads to strong interactions between the molecular adsorbate and the perovskite surface, and such interactions offer effective passivation in the lead halide perovskite surface. The 10 ACS Paragon Plus Environment

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DFT calculations demonstrate that the molecular adsorbate modifies the perovskite surface structures and elongates the surface Pb-I bond lengths. A molecular engineering method is introduced to display that the adsorbates with pyridyl moiety can be molecularly engineered to further enhance the UV/vis light absorption properties of the perovskite material. Such prediction is successfully validated by the experimental UV/vis light absorption study. We propose that the molecular engineering method is not restricted to the pyridyl anchoring moiety, and could be an effective alternative surface/interface engineering approach to achieve high performance perovskite solar cells.

ACKNOWLEDGEMENTS

This work was supported by the Nanjing University of Information Science and Technology (NUIST) Startup Fund, the Natural Science Fund for Colleges and Universities in Jiangsu Province (Grant No. 16KJB150027 and 16KJB150026), the Natural Science Foundation of China (No. 51702165 and No. 51472123), the Six Talent Peaks Project of Jiangsu Province China (No. R2016L07), the Jiangsu province “Double Plan” R2016SCB02 and the Jiangsu Provincial Natural Science Foundation (Grant No. BK20160942 and BK20160941). The authors acknowledge computational support from NSCCSZ Shenzhen, China.

SUPPORTING INFORMATION

Figure of the experimental UV/vis light absorption spectra of the bare perovskite film

AUTHOR INFORMATION Corresponding Author *Email: [email protected]. 11 ACS Paragon Plus Environment


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Molecular Engineering of the Lead Iodide ... - ACS Publications

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