Nanoplasmonic Sensing of CH 3 NH 3 PbI 3 Perovskite Formation in Mimic of Solar Cell Photoelectrodes

Hybrid metal-halide perovskites have emerged as leading class of semiconductors for photovoltaic devices with remarkable light harvesting efficiencies. The formation of methylammonium lead iodide (CH3NH3PbI3) perovskite into mesoporous titania (TiO2) scaffold by a sequential deposition technique is known to offer better control over the perovskite morphology. The growth reactions at the mesoporous TiO2 film depend on reactants concentration in the host matrix and the reaction activation energy. Here, we are characterizing formation of CH3NH3PbI3 perovskite in mimic solar cell photoelectrodes utilizing the developed NanoPlasmonic Sensing (NPS) approach. Based on dielectric changes at the TiO2 mesoporous film interface, the technique provides time-resolved spectral shifts of the localized surface plasmon resonance that varies widely depending on the different operating temperatures and methylammonium iodide (CH3NH3I) concentrations. Analytical studies included Ellipsometry, Scanning Electron Microscopy, and X-ray diffraction. The results show that perovskite conversion can be obtained at lower CH3NH3I concentrations if reaction activation energy is lowered. A significant finding is that the NPS response at 350 nm mesoporous TiO2 can widely change from red shifts to blue shifts depending on extent of conversion and morphology of perovskite formed at given reaction conditions.

Methylammonium lead halides exhibit several advantages of unique optical characteristics with band gap tunability, high mobility and long carrier lifetime [8][9][10][11][12][13].Mixed-halide perovskites have shown significant improvement in tuning the bandgap by adjusting the stoichiometry [14].Formation dynamics of Methylammonium lead iodide has been found to be processing specific [10].Perovskite films have been deposited either via thermal evaporation [15,16], or solution processing [5,11,12].The two common pathways often applied during film formation are one-step method in which the reactants are thoroughly mixed prior to deposition [17,18] and two-step route where the precursors are sequentially deposited [1,8,13].Control over perovskite morphology was observed using the twodeposition route to either thick or thin mesoscopic metal oxides [11,12,19].The growth and fabrication conditions of these films significantly affect the performance of the fabricated photovoltaic devices especially for mixed-halide perovskites.While conversion of polycrystalline perovskite is preferred for solar cells, growth of single crystals into multidimensional shapes has gained much attention in recent years [18].
In prior work, Rajab et al [20] used nanoplasmonic sensing (NPS) to detect methylammonium lead iodide (CH3NH3PbI3) perovskite at the interface of compact/650 nm mesoporous TiO2 films with Au nanodisks, where complete conversion of perovskite formation was characterized by slow NPS red shifts compared with fast methylammonium iodide (CH3NH3I3) crystallization in incomplete reactions.When complete reaction activation energies are reduced, perovskite formation was characterized by relatively fast 4 NPS red shifts.Overall NPS red shifts were primarily due to sufficient perovskite conversion required for dielectric constant change at the interface of the gold nanosensors.
In this paper, we demonstrate the use of NPS to detect the faster formation kinetics of CH3NH3PbI3 perovskite at the interface of compact/350 nm mesoporous TiO2 films with Au nanodisks.We monitor time-resolved spectral shifts of the Localized Surface Plasmon Resonance (LSPR) peak induced by the embedded plasmonic Au sensors.The rate constants of the compact/350 nm mesoporous TiO2 films are orders of magnitude higher than in the previous studied compact/650 nm mesoporous TiO2 films [20].We assess the formed perovskite structures according to the analytical results obtained by the characterization techniques.Additionally, we characterize the growth of multi-dimensional objects by their NPS shifts and reaction rate constants.

2.2.Film formation
Standard films comprising of fused silica coated with Au nanodisks (100 nm diameter and 20 nm height) and 10 nm dense layer of compact TiO2 as a dielectric spacer layer were used to analyze the nanoplasmonic sensing of Au nanosensors, similar to the early studied NPS experimental system arrangements [21,22,23,24].These films were then spin-coated with mesoporous TiO2 films prepared by mixing a commercial TiO2 paste from dyesol and ethanol (2:7 wt %) at 5000 rpm for 30 s.The films are calcinated at 500 º C for 30 min.The thickness of mesoporous TiO2 films was confirmed using VASE Ellipsometer VB-400.Then, 1M (462 mg/mL) of lead iodide (PbI2) in dimethylformamide (DMF) was prepared under stirring at 70 º C. The mesoporous TiO2 films were consequently infiltrated with PbI2 by spin coating at 6000 rpm for 90 s and dried at 70 º C for 30 min.Different concentrations of MAI in 2-propanol ranging from 5 to 15 mg/mL at various temperatures from 25 to 53 º C were prepared for in situ monitoring of perovskite formation.

In situ nanoplasmonic sensing of MAPbI3 perovskite formation
Insplorion Xnano was used to monitor nanoplasmonic peaks during MAPbI3 perovskite formation as described elsewhere.[20] Initially, a blank measurement was taken for a fused silica substrate for subtraction.The coated Au sensor film was placed inside the system chamber and wavelength scan shows the nanoplasmonic peak position of a sensor is located at 800 nm-900 nm depending on the Au discs size and discs spacing.The Au nanodiscs, which act as optical antennas respond to events occurring at the interface between the sensor surface and sample material.A peak-fitting method proposed by Dahlin et al. [25] is applied to enable monitoring the spectral shifts on the order of 1 nm or less with a 0.01 nm resolution limit.A fresh pure solvent was injected via the system pump to flush the tubes and to take a baseline measurement as settings are set at continuous, optimum flow rate of 0.1 µL/min to avoid bulk concentration gradient.At this point, minor spectral shifts were observed in both nanoplasmonic MAI at 2θ = 19.74°and 29.79° along with a hexagonal PbI2 at 2θ = 12.8°.The XRD patterns on different substrate surfaces were acquired as control samples.As complementary characterizing technique, scanning electron microscopy (SEM) (JEOL-6300F, 5 kV), was used to obtain SEM images, which were analyzed using Image J software to generate histograms of particle size distributions.

Effect of MAI concentration on mesoporous TiO2 films
The effect of changing MAI concentrations from 5 to 15 mg/mL at a high temperature of 41 °C on NPS response using Au sensors/compact TiO2/350 nm mesoporous TiO2/PbI2 was evaluated.Tetragonal MAI at 2θ = 19.74°for MAI concentration of 15 mg/mL along with hexagonal PbI2 at 2θ = 12.8° for MAI concentration of 7.5 mg/mL are maximum.

Effect of Temperature on mesoporous TiO2 films
The effect of operating temperature (from 25 to 53 °C) at a fixed high MAI concentration of 12.5 mg/mL on NPS response for Au sensors/compact TiO2/350 nm mesoporous TiO2/PbI2 was evaluated.
peak position and extinction due to change in the dielectric constant of the medium near the sensors.Upon diffusion of CH3NH3I into mesoporous TiO2 film at the different concentrations and temperatures, the refractive index at Au nanodiscs changes with film color change (dark brown), known of perovskite reaction, as major spectral shifts of resonance wavelength and extinction of the previously located, sensor-specific peak, are simultaneously, in real time collected and monitored via Insplorer software.We have evaluated the injection time in the current study from the starting point of MAI injection until the nanoplasmonic peak has plateaued, corresponding to the transition stage in Figs.1(a), 4(a).Typical time scale ranging from 18 s to 57 s has been calculated for the thinner 350 nm TiO2 films, depending on MAI concentration used and operating temperature.Much prolonged injection time was observed in case of thicker 650 nm films with a range from 31 s up to 120 s[20].In the standard sequential deposition method, 20 s dipping time has been applied using the 10 mg/mL MAI concentration under stagnant conditions[1].The corresponding injection time in our study for the same MAI concentration (under continuous flow conditions) was 22 s.For comparison, fresh solvent can be pumped again for rinsing the films.The sensors were removed for characterization.X-ray diffraction (XRD), Bruker AXS D4 Endeavour X diffractometer using Cu Kα1/2, λα1=154.060pm, λα2=154.439pm radiation, was used to obtain XRD measurements on different substrates to probe perovskite phase formation as well as content of reactants and products.As indicated in, Fig.S1, the control samples show peaks of tetragonal Preprints (www.preprints.org)| NOT PEER-REVIEWED | Posted: 7 September 2018 doi:10.20944/preprints201809.0124.v1

Figure 1 .Figure 2 .
Figure 1.Characteristic MAI concentration curves measured for Au sensors/compact TiO2/350 nm mesoporous TiO2/PbI2 prepared by continuous spin coating program showing (a) the nanoplasmonic peak position shifts for MAI concentrations ranging from 5 to 15 mg/mL at 41 °C and (b) the rate constants derived from the NPS peak position shifts.Both red and blue shifts are presented as positive rate constants.

Figure 4 (
a, b) and Fig.S1 (b) show characteristic temperature curves on nanoplasmonic peak position shifts and peak extinction shifts using compact/350 nm mesoporous TiO2 films.The characteristic curves in Fig. 4 (a, b) show red (positive) shifts of low rate constants of 3.4-12 s -1 in the nanoplasmonic peak position for temperatures from 25-36 °C indicative of slow conversion.They also show blue (negative) shifts of higher rate constants of 34-87 s -1 for temperatures from 41-53 °C indicative of fast conversion.The effect of changing the temperature on perovskite formation at high MAI concentration shows a non-linear relationship between the rate constant and temperature.

Figure 4 .
Figure 4. Characteristic temperature curves measured for Au sensors/compact TiO2/350 nm mesoporous TiO2/PbI2 showing (a) the nanoplasmonic peak position shifts for temperatures ranging from 25 to 53 °C at MAI concentration of 12.5 mg/mL and (b) the rate constants derived from the NPS peak position shifts.Both red and blue shifts are presented as positive rate constants.The conversion of PbI2 with a dielectric constant (Ɛ=6) to MAPbI3 perovskite with