Numerical simulation and experimental validation of inverted planar

Superlattices and Microstructures 112 (2017) 383e393

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Numerical simulation and experimental validation of inverted planar perovskite solar cells based on NiOx hole transport layer Xiaoqing Wei a, b, Xian Wang a, b, Hailong Jiang a, b, Yongliang Huang a, b, Anjun Han a, Qi Gao a, b, Jiantao Bian a, Zhengxin Liu a, b, * a

Research Center for New Energy Technology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 201800, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 September 2017 Received in revised form 26 September 2017 Accepted 27 September 2017 Available online 30 September 2017

Numerical simulation of inverted planar perovskite solar cells based on NiOx hole transport layer was performed with AMPS-1D program. The simulated device parameters were shown to agree well with our experimental work. The simulated results revealed that the device contained typical p-i-n junction configuration. The optimum thickness of the absorber, the effects of the absorber quality, the defect density of interfaces, the effects of VBO and CBO, the interface contact at front and back electrodes were analyzed. Opencircuit voltage mainly depended on the defect density in CH3NH3PbI3 layer, the recombination at HTL/CH3NH3PbI3 and ETL/CH3NH3PbI3 interface, the values of VBO and CBO, while short-circuit current mainly depended on the thickness of CH3NH3PbI3 layer. Fill factor was significantly influenced by the interface contact at front and back electrodes. Remarkably, a power conversion efficiency of 21.8% is obtained under optimised conditions. Real devices with PCE of up to 15% were obtained by initially optimizing the preparation of CH3NH3PbI3 absorber layer. Our work can provide some important guidance for device design and optimization from the considerations of both theory and experiment. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Perovskite solar cell Numerical simulation Experimental validation

1. Introduction In 2009, organic-inorganic lead halide perovskite compounds CH3NH3PbI3 and CH3NH3PbBr3 were firstly used as sensitizers instead of organic or inorganic sensitizers in DSSC cells, achieving power conversion efficiencies of 3.81% and 3.13%, respectively [1]. Since then, great attention has been paid to the utilization of organometal halide perovskite materials in solar cells. Apart from the initial DSSC architecture, solid-state mesoscopic [2,3] and planar [4,5] perovskite solar cells have also been developed, sequentially. As a new kind of photovoltaic semiconductor, organometal halide perovskite materials can be prepared by various methods [1,2,5e9], with appropriate direct band gap, high absorption coefficient [10], excellent carrier transport properties [11e13], good tolerance of defects [14], small exciton binding energy [15,16] as well as tunable

* Corresponding author. Research Center for New Energy Technology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 235 Chengbei Road, Jiading, Shanghai 201800, PR China. E-mail address: [email protected] (Z. Liu). https://doi.org/10.1016/j.spmi.2017.09.048 0749-6036/© 2017 Elsevier Ltd. All rights reserved.

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X. Wei et al. / Superlattices and Microstructures 112 (2017) 383e393

composition and structure [17]. Due to these excellent material characteristics and continuously optimised device fabrication process, 22.1%, a new record conversion efficiency of mesoscopic perovskite solar cell (PSC) has been achieved [18]. Compared with mesoscopic counterparts, planar perovskite solar cells contain no mesoscopic TiO2 or Al2O3 layer so the fabrication process is relatively simpler. Besides, planar perovskite solar cells can be conveniently combined with silicon or CIGS cells to create tandem solar cells, which is a basic strategy to improve the performance of photovoltaic devices [19,20]. Therefore, more and more attention had been attracted by planar structure since high-quality perovskite films were prepared with dualsource vapor deposition method [5]. Planar perovskite solar cells contain two types: normal structure and inverted ones. The normal perovskite solar cells are usually composed of TCO/TiO2/CH3NH3PbI3/Spiro-OMeTAD/Metal, while the inverted ones with TCO/PEDOT:PSS or NiOx/CH3NH3PbI3/[6,6]-phenyl C61-butyric acid methyl ester (PC61BM)/Metal configuration. Up to now, the certified record power conversion efficiency (PCE) of normal planar PSCs is 19.42% with the active area of 0.1134 cm2 or 19.9% (0.0737 cm2) while that of inverted planar PSCs is 18.21% with the active area of 1.022 cm2 [21e23]. For uncertified record efficiency, both planar structures have achieved a PCE of 20% [24,25]. So it is still hard to identify which planar structure is superior at present. From the perspective of reliability, the inverted planar PSCs using NiOx as hole transport layer show better stability and less hysteresis. Unfortunately, inverted planar PSCs based on NiOx have suffered from a relatively lower power conversion efficiency compared with the normal ones. Analysis of Microelectronic and Photonic Structures-1D (AMPS-1D) is a very general solar cell simulation program, which numerically solves the three governing semiconductor device equations. It has been widely used to model solar cells such as amorphous silicon, hetero-junction silicon, CIGS, CZTS and CdTe solar cells [26e30]. Recently, it is also used to simulate normal planar perovskite solar cells [31,32]. However, the numerical simulation combined with experimental validation for inverted planar PSCs is still rare. Fabrication process of NiOx-based inverted planar PSCs involves the continuous deposition of NiOx, CH3NH3PbI3, PC61BM and Metal on transparent conduction oxide glass. Numerical simulation with AMPS-1D is a useful way to evaluate the role of various material parameters in solar cells. Herein, AMPS-1D is used to simulate the inverted planar PSCs. Firstly, the electrostatics and J-V characteristic of the device are analyzed. Then the thickness of the absorber, the effects of the absorber quality, the defect density of interfaces, the effects of VBO and CBO, the interface contact at front and back electrodes are examined. Besides, real devices with PCE of up to 15% are obtained by optimizing the preparation of CH3NH3PbI3 absorber layer. Our work can provide some important guidance for device design and optimization from the considerations of both theory and experiment. 2. Simulation model A detailed analysis of perovskite solar cells requires the simultaneous solution of Poisson's equation and the continuity equations for electrons and holes:

h i d dj εðxÞ ¼ q pðxÞ nðxÞ þ NDþ ðxÞ NA ðxÞ þ pt ðxÞ nt ðxÞ dx dx 1 dJn ¼ RðxÞ GðxÞ q dx 1 dJp ¼ GðxÞ RðxÞ q dx

(1)

(2)

(3)

where

dn dx

(4)

dp dx

(5)

Jn ¼ qnmn E þ kT mn Jp ¼ qpmp E kT mp

Here ε is the dielectric constant, j the electrostatic potential, x the position in the device, p free hole density, n free electron density, Nþ D the ionized donor-like doping concentration, NA the ionized acceptor-like doping concentration, pt trapped hole density, nt trapped electron density, Jn the electron current density, Jp the hole current density, mn the electron mobility, mp the hole mobility, respectively. G is the optical generation rate and R is the net recombination rate resulting from band-to-band recombination and S-R-H recombination through gap states. The density-of-states (DOS) profile of CH3NH3PbI3 in our simulation is shown in Fig. 1. The subgap quantum efficiency measurements show that the tail state density decays exponentially from the optical band edges [33]. Here, these tail states are modeled using an Urbach tail of donor-like states coming out of the valence band by gd(E) ¼ GDO exp(E/ED) and an Urbach tail of acceptor-like states coming out of the conduction band by ga(E0 ) ¼ GAO exp(E0 /EA). E is measured positively up from the valence band edge EV and E0 is measured negatively down from the conduction band edge EC. ED and EA represent the characteristic energies that determine the slopes of their tails, which are measured to be 16 meV and 18 meV, respectively. GDO and GAO are taken to be equal to 1 1016 cm3 eV1. Midgap defect densities of CH3NH3PbI3 have been estimated by


385

Fig. 1. Typical gap state profile of CH3NH3PbI3 used in the simulation.

measuring capacitance vs. frequency and show two peaks, one at 0.66 eV below the conduction and one at 0.24 eV below the conduction, which show excellent Gaussian shape [33]. The Gaussian distributions provided by AMPS-1D are of the form

" GA ðEÞ ¼ NAG exp

1 ðE EACPG Þ2 2 2 WDSAG

"

#

1 ðE EDONG Þ2 GD ðEÞ ¼ NDG exp 2 2 WDSDG

(6) # (7)

where NAG and NDG are the effective density of states, EACPG and EDONG are the peak energy position and WDSAG and WDSDG are the standard energy deviation of the Gaussian acceptor and donor levels, respectively. EACPG and EDONG for the Gaussian acceptor- and donor-like states are measured from the valence and conduction bands, respectively. The inverted planar PSC structure designed in the simulation is TCO/NiOx/CH3NH3PbI3/PC61BM/Metal. It is a p-i-n solar cell with low p-type doped absorption layer CH3NH3PbI3 sandwiched between p-type hole transport layer NiOx and n-type electron transport layer PC61BM. For the boundary conditions, considering the practical reflection of TCO and Metal, reflection coefficient at front surface is set as 0.1, while that of back surface is 1. The surface recombination velocities of electrons and holes at the front and back electrodes are both set to be 1 107 cm/s. Material parameters of the model are summarized in Table 1. And the experimental part is shown in the Supporting Information. 3. Electrostatics and J-V characteristic of perovskite solar cell The electrostatics of perovskite solar cell is governed by Poisson's equation. The right-hand side of the equation is called the net charge density. Among them, it is assumed that the dopants in different materials are fully ionized. Since there is a concentration difference of free holes and free electrons among NiOx, CH3NH3PbI3 and PC61BM, free holes diffuse from NiOx region into CH3NH3PbI3 region and similarly, free electrons from PC61BM region into CH3NH3PbI3 region. As the charge carriers diffuse, a built-in electric field is formed to impede the diffusion process. As shown in Fig. 2(a), p(x) and n(x) decrease Table 1 Basic parameters for the simulation of inverted planar perovskite solar cells in this work. Parameters and units

NiOx

CH3NH3PbI3

PC61BM

Dielectric Constant Band Gap (eV) Electron Affinity (eV) Thickness (nm) Electron and Hole Mobility (cm2 V1 s1) Acceptor Concentration (cm3) Donor Concentration (cm3) Effective Conduction Band Density (cm3) Effective Valence Band Density (cm3)

12 3.55 1.7 50 0.01, 0.01 2.66 1017 0 9.15 1017 4.54 1018

23.3 1.5 3.93 330 45, 45 6.2 1013 0 1.66 1019 5.41 1018

4 2.1 4.1 80 0.01, 0.01 0 5 1017 2.5 1019 2.5 1019

Fig. 2. (a) Charge carriers distribution as a function of position in thermodynamic equilibrium, (b) distribution of charge carriers trapped by Urbach tail states and Gaussian defect states, (c) distribution of the net charge, (d) electric field in thermodynamic equilibrium, (e) Energy band diagram in thermodynamic equilibrium in the dark, (f) the calculated electron, hole and total current densities as a function of position at the short-circuit condition, (g) Energy band diagram at the open-circuit condition under illumination.

dramatically near the NiOx/CH3NH3PbI3 interface and PC61BM/CH3NH3PbI3 interface, respectively. And in CH3NH3PbI3 region, p(x) and n(x) are both negligible, which indicates that the CH3NH3PbI3 absorption layer is fully depleted. As for trapped holes


387

and electrons derived from Urbach tail states and Gaussian defect states, their distributions are plotted in Fig. 2(b). Due to the characteristic energies ED and EA are both small, holes and electrons trapped by Urbach tail states are negligible compared with those of Gaussian defect states. It should be noted that to simplify the analyses, we neglect the Urbach tail states and Gaussian defect states in NiOx and PC61BM region. Therefore, the net charge density is shown in Fig. 2(c) and the calculated electric field by Poisson's equation is displayed in Fig. 2(d) in thermodynamic equilibrium. And there is no net current flow and Fermi energy is independent of position as shown in Fig. 2(e). Therefore, the PSC calculated with the parameters in Table 1 contains typical p-i-n junction configuration. This is in accordance with the real devices in related literature [34,35]. Under illumination, photogenerated holes are pushed to NiOx layer and photogenerated electrons to PC61BM layer by the built-in electric field. According to the continuity equations for electrons and holes, Jn and Jp as a function of position are calculated and shown in Fig. 2(f) at the short-circuit condition. The results indicate that holes are carrying all the current in NiOx region and electrons emerge carrying all the current in PC61BM region. Between them, holes and electrons share the current. At x ¼ 135 nm, Jn is equal to Jp. The recombination in CH3NH3PbI3 absorption layer is mainly caused by Gaussian defect states. So when its thickness is fixed, JSC is mainly affected by Gaussian defect states in CH3NH3PbI3 layer. Quasi-Fermi levels of electron and hole are introduced here, which are expressed as:

n NC p ¼ EV kTln NV

EFn ¼ EC þ kTln

(8)

EFp

(9)

The output voltage of perovskite solar cell is the potential difference between the electron quasi-level at the left side of CH3NH3PbI3 layer and the hole quasi-level at the right side of CH3NH3PbI3 layer. At open circuit, the output voltage, namely open-circuit voltage (VOC ) can be written using the bandgap Eg and charge carrier density:

VOC ¼

Eg kB T NC NV ln q q np

(10)

In Fig. 2(g), the plots of the conduction band edge EC , the valence band edge EV , the quasi-fermi levels EFn and EFp are shown according to the simulation. It can be seen that the quasi-Fermi levels are constant in a broad region across the middle of the solar cell. The separation of EFn and EFp in this region is 1.08 eV, which is in accordance with the calculated VOC . According to Eq. (10), VOC of perovskite solar cell is mainly determined by Eg, which can be reached at absolute zero temperature. Besides, higher VOC can be obtained by increasing the free charge carrier density np. That's why reducing recombination and increasing the illumination intensity can increase VOC . 4. Results and discussion 4.1. Dependence of the device performance on perovskite layer thickness Performance of p-i-n junction solar cell depends mainly on optoelectronic parameters and thickness of the absorption layer. On the basis of parameters in Table 1, simulations are firstly carried out to examine the J-V characteristics of the devices with thickness of the CH3NH3PbI3 absorption layer ranging from 50 nm to 1000 nm. As shown in Fig. 3, the short circuit current density (JSC) increases sharply with the thickness varying from 50 nm to 500 nm. After this, increasement is very slow and JSC saturates to a plateau (~22.95 mA/cm2) when the thickness reaches 1000 nm. It can be seen that a high JSC (~20 mA/ cm2) is obtained when the thickness of CH3NH3PbI3 absorption layer is only 200 nm. This is attributed to the high absorption coefficient of CH3NH3PbI3. As a direct band gap semiconductor, its conduction band minimum is derived from Pb p orbitals, while the valence band maximum is a mixture of Pb s and I p orbitals [14]. So the edge transition originates from mixed-(Pb s, I p) to Pb p orbitals with a high intra-atomic Pb s to Pb p transition probability. Besides, Pb p orbital exhibits few dispersion, leading to a high joint density of states (JDOS). Therefore, CH3NH3PbI3 shows high absorption coefficient, which is crucial for realizing economic and efficient solar cells. The open circuit voltage (VOC) experiences a slight decrease with the increase of CH3NH3PbI3 layer thickness. This mainly results from lower free hole concentration p in thicker CH3NH3PbI3 layer at open circuit, as shown in Fig. S1. Lower p leads to higher EFp so that VOC decreases. A maximum PCE of 19.16% is achieved when the thickness is 500 nm by balancing all the parameters. 4.2. Dependence of the device performance on the defects in perovskite layer Electrons and holes generated by photon absorption in perovskite layer will be separated and transported to the ETM and HTM in the built-in electric field, respectively. So recombination through bulk defects in CH3NH3PbI3 film is the vital mechanism limiting the performance of perovskite solar cell. According to the above discussions, we know that holes and electrons trapped by Urbach tail states are negligible compared with those of Gaussian defect states in perovskite layer. The

388


Fig. 3. Device parameters as a function of CH3NH3PbI3 layer thickness.

recombination in CH3NH3PbI3 absorption layer is mainly caused by Gaussian defect states. So the density of Gaussian defect states in CH3NH3PbI3 films play an important role in determining the performance of the device. The device parameters as a function of defect densities NAG and NDG are plotted in Fig. 4. It can be seen that all photovoltaic parameters decrease with the increase of bulk defect density. This is in accordance with the report that high-quality perovskite films contain less trap density, reducing charge recombination [5]. It should be pointed out that JSC is almost a constant when the defect density is smaller than 1 1017 cm3 eV1. When it reaches 1 1018 cm3 eV1, JSC begins to reduce rapidly. This is due to the total recombination rate in CH3NH3PbI3 film is negligible compared with the generation rate at short circuit when the defect density is smaller than 1 1017 cm3 eV1. It becomes comparable with the generation rate when the defect density reaches 1 1018 cm3 eV1, as shown in Fig. S2. 4.3. Effects of interface recombination on the device performance Structural deficiencies between two different materials could introduce deep defect states within bandgap. Besides, it has been identified that uncoordinated iodine ions and surface Pb atoms are responsible for the formation of trap states on the perovskite surface [17]. These trap states could lead to charge recombination in the devices. We can artificially simulate the interface recombination by inserting a thin perovskite layer with a large number of defect states distributed evenly within the bandgap. This is the so called constant distribution in AMPS-1D. Fig. 5 shows the simulation results as a function of interface trap density. With the increase of interface trap density, all the parameters of the devices decrease. When the interface trap

Fig. 4. Device parameters as a function of bulk trap density in CH3NH3PbI3 films.


389

Fig. 5. Device parameters as a function of interface trap density.

density is larger than 1 1011 cm2, VOC and PCE of the device begin to reduce rapidly. According to our experimental experience, no high performance devices will be obtained when the interfaces of NiOx/CH3NH3PbI3 and PC61BM/CH3NH3PbI3 are not smooth and clean. So great attention should be paid to the interfaces for inverted planar perovskite solar cell.

4.4. Effects of the valence band offset of perovskite/HTM layer The simulated J-V curves of the device with different valence band offset (VBO) values of perovskite/HTM layer are shown in Fig. 6. Here, VBO ¼ cHTM þ Eg_HTMe(cperovskite þ Eg_perovskite), where c is the electron affinity. The defect density at perovskite/HTM interface is set as 1 1011 cm2, while the recombination at perovskite/ETM interface is neglected. It is obviously seen that VOC increases monotonically with increasing the VBO from 0.43 to 0 eV, while Jsc is not sensitive to VBO. The activation energy (Ea) for carrier recombination at perovskite/HTM interface is represented by Eg_perovskite e jVBOj. The main recombination mechanism in the device is the recombination at perovskite/HTM interface when Ea is lower than Eg_perovskite [36e39]. The interface recombination increases with the decrease of Ea as shown in Fig. S3. Thus, Voc reduces. The results are consistent with those reported in the literature [40e42]. At short circuit, the recombination rate in the device is negligible compared with the generation rate so that JSC is almost a constant. It should be noted that JSC becomes sensitive to VBO when the defect density at perovskite/HTM interface reaches 1 1013 cm2, as shown in Fig. S4.

Fig. 6. J-V curves of the device as a function of valence band offset (VBO) value of perovskite/HTM layer.

390


Here, VOC is still the most vital parameter to determine the PCE of the devices. Therefore, the optimised VBO value is 0 eV for the inverted planar perovskite solar cell. 4.5. Effects of the conduction band offset of perovskite/ETM layer Fig. 7 shows the J-V curves of the device with different conduction band offset (CBO) values of perovskite/ETM layer. Here, CBO ¼ cperovskiteecETM. The defect density at perovskite/ETM interface is set as 1 1011 cm2, while the recombination at perovskite/HTM interface is neglected. Voc increases monotonically with increasing the CBO from 0.5 to 0 eV, while JSC is also almost a constant. The activation energy (Ea) for carrier recombination at perovskite/ETM interface is represented by Eg_perovskite e jCBOj. The main recombination mechanism in the device is the recombination at perovskite/ETM interface when Ea is lower than Eg_perovskite [36e39]. The interface recombination increases with the decrease of Ea so that VOC reduces. When the defect density at perovskite/ETM interface reaches 1 1013 cm2, the output parameters of the device decrease but the degree is smaller than that of VBO case, as shown in Fig. S5. This is due to the recombination at the back perovskite/ETM interface is lower than that of the front perovskite/HTM interface for inverted planar perovskite solar cell. Thus, the optimised CBO is also 0 eV. 4.6. Effects of back and front contact on the device performance Fig. 8(a) shows the device parameters versus work function of the back contact metal. FF and PCE both decrease while JSC and VOC remain nearly constant with the increase of the back contact metal work function. An ohmic contact between PC61BM and metal electrode is necessary to transport electrons efficiently to the metal electrode (ME). Fig. S6 displays the electric field distribution in PC61BM layer as a function of metal work function. It can be seen that the direction of the electric field near PC61BM/ME interface becomes positive when the work function is larger than 4.3 eV. This means the electric field direction is directed from PC61BM to ME, which impedes the electron transport. So FF decreases with the increase of the positive electric field near PC61BM/ME interface. An ohmic contact is formed when the work function is less than 4.3 eV. A similar phenomenon is observed for the TCO/NiOx interface contact. And the device parameters versus work function of TCO are shown in Fig. 8(b). 4.7. Contrastive analysis of simulative and experimental results Inverted planar perovskite solar cell based on two different kinds of CH3NH3PbI3 films are prepared to examine the validity of the above simulation. The detailed preparation process and device parameters are shown in the Supporting Information. Fig. 9 displays the simulative and experimental J-V curves of the devices. For 1# device, the simulation result is very close to the experimental result, which verifies the validity of the model to a certain extent. For 2# device, the perovskite film contains many pin-holes, where charge carriers recombine. This phenomenon is simulated by increasing the interface defect density at perovskite/ETM and perovskite/HTM from 1 1011 cm2 to 5 1012 cm2. It can be seen that the variation tendency of the simulated device parameters is consistent with that of the real device. It should be noted that some hysteresis behaviors exist for perovskite devices based on both kinds of CH3NH3PbI3 films, as shown in Table S1eS4. The hysteresis phenomenon may result from the trap states at the grain boundaries of the perovskite nanocrystals and the interfaces between the perovskite

Fig. 7. J-V curves of the device as a function of conduction band offset (CBO) value of perovskite/ETM layer.


Fig. 8. Device parameters vs work function of (a) the back contact metal and (b) the front contact material TCO.

Fig. 9. The simulative and experimental J-V curves of the devices with different CH3NH3PbI3 film.

391

392


and the other layers. On the basis of the results discussed above, an efficiency of 21.8% (JSC ¼ 22.38 mA/cm2, FF ¼ 0.86, VOC ¼ 1.13 V) is obtained under moderate conditions. The result indicates that inverted planar perovskite solar cells based on NiOx hole transport layer contain the great potential in achieving high PCE. 5. Conclusions Device simulation of inverted planar perovskite solar cells based on NiOx hole transport material is carried out with AMPS1D program. Electrostatics and J-V characteristic of the device are analyzed deeply. The effects of key material parameters on the device performance are systematically investigated. The results indicate that: (i) The thickness of CH3NH3PbI3 absorption layer is pivotal for JSC and slightly impacts VOC. The optimum thickness is 500 nm. (ii) With the increase of bulk defect density in CH3NH3PbI3 layer, VOC and FF decrease sharply due to serious charge carriers recombination. JSC begins to reduce rapidly when it reaches 1 1018 cm3 eV1 (iii) When the interface trap density is larger than 1 1011 cm2, VOC and PCE of the device begin to reduce rapidly. (iv) When the valence band of HTM is lower than CH3NH3PbI3, interface recombination is the main recombination mechanism in the device, leading to low VOC. The case is similar for CBO of perovskite/ETM layer. The optimised VBO and CBO are both 0 eV. (v) Ohmic contact at TCO/HTM interface and Metal/ETL interface is vital to obtain high FF in the device. Under optimised conditions, a maximum PCE of 21.8% is obtained, highlighting the great potential of inverted planar perovskite solar cell. The work reveals the work principle of inverted planar perovskite solar cell and analyzes the effects of key material parameters on the device performance. Besides, real devices with PCE of up to 15% are obtained by optimizing the preparation of CH3NH3PbI3 absorber layer. Our work can provide some important guidance for device design and optimization from the considerations of both theory and experiment. Acknowledgments This work was supported by International S&T Cooperation Program of China (2015DFA60570) and Key project of Zhangjiang National Innovation Demonstration Zone Special Development Fund (ZJ2015-ZD-001). The authors would like to thank Professor S. Fonash of the Pennsylvania State University for providing the AMPS-1D program. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.spmi.2017.09.048. References [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050e6051. €tzel, N.G. Park, Lead iodide perovskite [2] H.S. Kim, C.R. Lee, J.H. Im, K.B. Lee, T. Moehl, A. Marchioro, S.J. Moon, R.H. Baker, J.H. Yum, J.E. Moser, M. 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