Influence of hydration water on CH 3 NH 3 PbI 3 perovskite films prepared through one-step procedure

Organic-inorganic perovskites were fabricated through a one-step procedure with different levels of hydration water in precursor solutions. The optical properties of CH3NH3PbI3 films were investigated through spectroscopic ellipsometry and photoluminescence measurements. With the measured optical constants, the efficiency limit of perovskite solar cells is predicted with a detailed balance model. By comparing the optical measurement to that of planar heterojunction solar cells, we conclude that the radiative efficiency and porosity of the perovskite film significantly influence the performance of perovskite solar cells. An optimized hydration-water concentration is obtained for the 3CH3NH3I:1PbAc2•xH2O precursor solution. 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Introduction
During the past several years, solar cells based on methylammonium lead iodide (CH 3 NH 3 PbI 3 ) perovskite have attracted much attention because of their high energy conversion efficiency [1].The power-conversion efficiency (PCE) of perovskite-based solar cells has rapidly increased to exceed 20% [2].The most common methods to increase the PCE of solar cells are structure optimization [3][4][5][6] and film-quality improvement [7].The quality of perovskite films has been improved using various methods such as large grain growth [8], surface smoothening [9,10], defect passivation [11,12], and process optimization [13,14].It has been reported that ultra-smooth and nearly pinhole-free CH 3 NH 3 PbI 3 films can be achieved using lead acetate (PbAc 2 ) instead of lead halides [15].Further, controlling the content of hydration water in precursors can significantly improve the smoothness and uniformity of perovskite solar cells [16].In addition, the use of hydration water can greatly simplify the one-step process of fabricating perovskite films, which enables the future mass production of perovskite solar cells [15,16].
One of the critical features of perovskite solar cells is that a relatively high short-circuit photocurrent density (J sc ) can be easily achieved [17].In an ideal solar cell, J sc is mainly controlled by the optical absorption of the absorber layer [18].As the CH 3 NH 3 PbI 3 layer is the critical light absorber in a perovskite solar cell, its optical properties need to be thoroughly studied.There are several reports on the refractive index and extinction coefficient of CH 3 NH 3 PbI 3 [18][19][20][21][22][23][24].However, the previous results are inconsistent with each other.Since the properties of perovskite films are strongly influenced by the precursors and process utilized in fabrication, the perovskite films fabricated using different processes may demonstrate different optical and electronic properties, although the final composition remains constant.Therefore, it is difficult to perform a fair comparison of different perovskite films, and a systematic study of perovskite films fabricated from similar precursors and processes is needed.Interestingly, recently work has shown that the use of hydration-watercontrolled precursors can improve the performance of devices based on perovskite [16].Furthermore, previous studies have demonstrated that moisture can assist the growth of CH 3 NH 3 PbI 3 and lead to better device performance [25,26].Therefore, the investigation of the optical properties of hydration-water-improved perovskite films is of significant importance.
Spectroscopic ellipsometry (SE) is an indirect measurement technique routinely used for determining the optical properties of thin films [27].It features fast and non-destructive detection over a wide photon-energy range, making it suitable for the characterization of many materials including metals [28,29], semiconductors [30,31], insulators [32], and nanocrystals [33].In the SE measurement, two parameters ψ and Δ can be revealed from the expression of complex reflectance ratio ρ [27,34]: tan( ) , where p r and s r are the Fresnel reflection coefficients of p-and s-polarized light, respectively.Further, ψ and Δ are the amplitude ratio of reflected p-to s-polarized light and the phase shift difference, respectively.The ellipsometric spectra can be fitted based on an optical model, which is constructed from the physical structure of the measured sample.Then, the dielectric functions can be solved.

Device fabrication
The solar-cell devices are constructed with a structure of ITO/ PEDOT:PSS/ CH 3 NH 3 PbI 3 / [6, 6]-phenyl-C61-butyric acid-methyl-ester (PBCM)/ C 60 / 2,9-dimethyl-4,7-diphenyl-1,10phenanthroline (BCP)/ Ag.PEDOT:PSS was spin-coated on cleaned ITO substrates at 4,000 rpm.The films were then annealed at 130 °C.Perovskite films were spin-coated from the prepared precursor solutions at 2,000 rpm and then annealed on a hotplate at 100 °C for 5 min.PCBM (dissolved in CB, 20 mg/ml) was spin-coated on top of the perovskite layer at 1,800 rpm.The fabrication of the devices was completed through the thermal evaporation of C 60 , BCP, and Ag.Detailed information for the solar cells can be found in a previous report by Li et al. [16].We prepared five solar cells for each hydration-water content, and the solar cells with the highest filling factor were selected for comparison.

CH 3 NH 3 PbI 3 film characterization
The optical constants of CH 3 NH 3 PbI 3 films, a relatively unstable material, are easily affected by moisture in the environment.To avoid degradation, all samples were sealed in a nitrogen environment until SE measurement.SE measurements (J.A. Woollam, Inc., M2000X-FB-300XTF) were performed from 1.38 to 4.13 eV at room temperature and at an incident angle of 65°.All the SE measurements were completed within 30 s after the exposure of the samples to air.The photoluminescence (PL) intensity was measured immediately after the SE measurement.A semiconductor laser with a wavelength of 447 nm was used as the pump light.The wavelength of PL was selected using a monochromator, and a long-pass filter (>500 nm) was placed before the monochromator to eliminate the influence of stray light from the pump laser.The PL signal was detected using a Si photodetector combined with a lock-in amplifier.
The film morphologies were determined with a scanning electron microscope (FEI Siron200, SEM) and an atomic force microscope (Bruker Dimension Icon VT-1000 System, AFM) in the tapping mode.The perovskite phases were analyzed using an X-ray diffractometer (Bruker-AXS D8, XRD) with a Cu-Kα radiation source (λ = 1.5406Å).

Solar-cell characterization
J-V curves were measured using an electrochemical workstation (ZAHNER CIMPS) under AM1.5 sunlight at 100 mV/m 2 irradiance generated by a Class AAA sun simulator (SF300-A Sciencetech-Inc.).A Si diode (Hamamatsu S1133) was used to calibrate the light intensity and stability.The solar cells were masked with a 3D-printed aperture to define the active area.

Results and discussions
To investigate the influence of hydration water on the formation of CH 3 NH 3 PbI 3 through the one-step method from a 3:1 solution (by moles) of CH 3 NH 3 I:PbAc 2 •xH 2 O in DMF precursor solutions, five CH 3 NH 3 PbI 3 samples were fabricated on quartz/ PEDOT: PSS substrates.The content of hydration water was carefully controlled in 3CH 3 NH 3 I:1PbAc 2 •xH 2 O with x equals to 0, 0.75, 1.50, 2.25, and 3.00.Figure 1 illustrates the morphology of CH 3 NH 3 PbI 3 films measured by AFM and SEM.The values of root-mean-square roughness (RMS roughness, R q ) of the five samples were 14.1, 12.2, 10.7, 10.7, and 10.8 nm, respectively.The flat surfaces of CH 3 NH 3 PbI 3 films enable SE measurements [18,23], and the surface roughness decreased with the increasing content of hydration water in PbAc 2 until it reaches x = 1.50 from x = 0, which indicates that hydration water plays an important role on the film-surface smoothness.However, the film roughness is not significantly improved with a hydrationwater concentration greater than 1.50.The SEM images show that the porosity of CH 3 NH 3 PbI 3 films tend to increase with increasing hydration-water concentration, indicating that excess hydration water may have a negative effect on the film quality.The XRD patterns of CH 3 NH 3 PbI 3 films are shown in Fig. 2. They show that all samples have a good crystalline profile.Recent reports have established that CH 3 NH 3 PbI 3 can exhibit a cubic (Pm 3 ~m) phase at high temperature and a tetragonal (I4/mcm) phase at room temperature [35,36].Our results indicate that the CH 3 NH 3 PbI 3 films prepared using PbAc 2 are in the tetragonal phase [37,38].A majority of the (110) plane and a very small portion of the (002) plane for the tetragonal phase can be observed in Fig. 2(b).The samples also contain small quantities of PbI 2 (2 orders of magnitude lower than the CH 3 NH 3 PbI 3 peak), which may originate from the annealing process or the degradation of perovskite.The ellipsometric spectra collected at a fixed incident angle of 65° were fitted to an optical model consisting of semi-infinite quartz substrate/ PEDOT: PSS buffer layer/ CH 3 NH 3 PbI 3 layer/ roughness layer/ ambient air based on the structure of the fabricated CH 3 NH 3 PbI 3 films.The surface-roughness layer is modeled on a Bruggeman effective medium approximation (EMA) with a mixture of 50% CH 3 NH 3 PbI 3 and 50% voids [39].To obtain the accurate dielectric constants of each layer, the ellipsometric spectra were collected for the quartz substrate, quartz/ PEDOT: PSS, and the final CH 3 NH 3 PbI 3 films, respectively [18].The dielectric functions of CH 3 NH 3 PbI 3 are described by the Tauc-Lorentz model [40].The imaginary part (ε 2 ) is obtained from the Tauc joint density of states and the Lorentz oscillator as follows: where A is the amplitude parameter, E p is the peak transition energy, C is the broadening term, and E is the photon energy.The real part (ε 1 ) of the dielectric function can be obtained through Kramers-Kronig integration: 1 2 2 ( ) 2 ( ) , where P is the Cauchy principal part of the integral and ε ∞ is the constant contribution to ε 1 at high frequency.Four Tauc-Lorentz oscillators were used to construct the dielectric functions of CH 3 NH 3 PbI 3. The Tauc gap E g was kept constant for all oscillators.The ε ∞ value of all samples is set to 1.0.The fitting of all CH 3 NH 3 PbI 3 samples displays good agreement with collected data.Figure 3 illustrates the hydration-water dependence of the complex refractive index and dielectric constants of CH 3 NH 3 PbI 3 films.The measured dielectric constants and XRD patterns indicate that the final composition of perovskite films remains constant.It can be seen from Figs. 3(a) and 3(c) that CH 3 NH 3 PbI 3 films prepared with a low-hydration precursor have higher ε 1 and n in the low-energy range.In the transparent range, where ε 2 is 0, the static dielectric constant is given by [27] 0 f 1 , where q i and l i are the electric charge of the electric dipole and distance between the charge pair in the material.ε 0 is the free-space permittivity, and E f is the electric field.The XRD patterns in Fig. 2 indicate that the structure parameters of all samples are identical; thus, refractive-index variation induced by atomic and electric polarization can be neglected.As previously reported, CH 3 NH 3 PbI 3 is filled with different percentages of voids [20] because of the evaporation of moisture during the fabrication process of CH 3 NH 3 PbI 3 films.Therefore, a high content of hydration water will result in a CH 3 NH 3 PbI 3 sample with high porosity and low refractive index.The SEM images in Fig. 2 show a similar result.To further understand the measured dielectric constants in Fig. 3, we list the fitted parameters of CH 3 NH 3 PbI 3 samples in Table 1.It can be seen that the band gap of CH 3 NH 3 PbI 3 films is approximately 1.6 eV, which is consistent with previous reports [18,24].This band gap is the difference between the valence-band maximum and conduction minimum of CH 3 NH 3 PbI 3 [24,41].The calculated Urbach energy of five samples are in the range of 12-14 meV, which indicates the formation of suppressed tail states in all CH 3 NH 3 PbI 3 samples.Absorption peaks at 2.5 eV are also observed in our measurement, which can be attributed to another direct transition in CH 3 NH 3 PbI 3 [18].It has been reported that PbI 2 has a sharp transition peak at 2.5 eV, while the CH 3 NH 3 PbI 3 peak is broad [42].The deceasing broadening term C 2 indicates that a higher level of hydration water results in a higher content of PbI 2 in perovskite films.The transition peak at approximately 3.2 eV is not as sharp as that reported by M. Shirayama et al. [18].This is probably due to the different sample preparation method employed here; the deliberately introduced moisture reduced the ε 2 value of CH 3 NH 3 PbI 3 .The last oscillator located at approximately 4 eV varies among different samples.This is due to inaccurate ellipsometric data at the short wavelength range induced by light scattering and depolarization.The measured extinction coefficient (k) decreases with the increasing content of hydration water.We attribute this phenomenon to the light scattering caused by the high surface roughness and the non-void structure formed by the fully dehydrated precursor [18].It should be noted that, although a high roughness can benefit the absorption of the perovskite layer, it may also result in more defects to solar cells.
Based on the obtained complex refractive index, the efficiency limit of CH 3 NH 3 PbI 3 perovskite thin-film solar cells was calculated with a detailed balance model [17,43,44].In the detailed balance model, the short-circuit photocurrent density J sc of an illuminated solar cell is given by where q is the elementary charge, AM1.5G G is the AM1.5 solar flux, E is the corresponding photon energy, and a(λ, L) is the absorptivity, which can be simulated from the film thickness L and absorption coefficient α.The dark saturation current density J 0 is formulated as where 0 ( ) λ G is the thermal emission spectrum of the absorber layer, which can be calculated from the law of black-body radiation.The open-circuit voltage V oc is hence determined by where k B is the Boltzmann constant and T is the working temperature of solar cells.Here, we assume T = 300 K and absorber thickness L = 300 nm.The simulated values of J sc and V oc for all samples are approximately 28.5 mA/cm 2 and 1.32 V, respectively.Owing to the identical band gap and high absorption coefficient, the calculated J sc and V oc for different samples are essentially consistent.We also prepared several planar heterojunction solar cells to compare experimental results with the simulation results.The measured J-V curves of selected solar cells in Fig. 4(a) show that the performance of CH 3 NH 3 PbI 3 solar cells strongly depend on hydration water.As illustrated in Fig. 4(b), the V oc values of the prepared solar cells increase with increasing level of hydration water, and the short-circuit photocurrent density J sc reaches the maximum value at x = 1.50.The low V oc values of our solar cells can be attributed to surface defect levels and metallic contacts [45,46], which were not considered in the simulation with the detailed balance model.The deviation among five samples indicates that the performance of perovskite solar cells is still influenced by hydration water.As all the solar cells have the same structure, the difference must originate from the perovskite films and is related to the content of hydration water in the precursor.To understand the difference between experiment and simulation, we further measured the PL spectra of CH 3 NH 3 PbI 3 films.As shown in Fig. 4(d), the PL emission peaks are located in the band gap of CH 3 NH 3 PbI 3 and originate from the recombination of charge carriers.The hydration water can significantly enhance the PL intensity, indicating that the introduction of hydration water can suppress the formation of non-radiative pathways in CH 3 NH 3 PbI 3 films to result in longer carrier lifetimes [26].Considering Rau's reciprocity relation [47], the darkcurrent density in Eq. ( 8) can be rewritten as where EL η is the absolute quantum efficiency of electroluminescence (EL).As previously reported, the PL emission in CH 3 NH 3 PbI 3 solar cells originates from the same type of charge pairs as in EL, and their efficiencies are approximately proportional to the PL emission [48] As indicated in Table 2, the calculated ΔV oc between 0H 2 O and 3.00H 2 O CH 3 NH 3 PbI 3 solar cells is approximately 0.074 V, which is less than the value obtained in the experiment.This discrepancy probably results from the fact that the PL measurements are performed under the thin-film condition, while V oc is collected for solar-cell devices.Furthermore, the different values of film-surface roughness may also affect the contact resistance.The short-circuit photocurrent density J sc depends on various factors in the CH 3 NH 3 PbI 3 layer, such as roughness, non-radiative pathway, and porosity.For 0H 2 O and 0.75H 2 O samples, the high roughness and non-radiative pathway makes the J sc values less than that for the 1.50H 2 O sample.The 2.25H 2 O and 3.00H 2 O samples have higher radiative recombination efficiency, but the high porosity in the absorbing layer influenced the output current density by blocking the transport of electrons and holes.However, the 1.5H 2 O sample has a good balance between the non-radiative pathway and porosity, and it has the highest PCE among all samples.

Conclusion
In summary, the optical and structure properties of hydration-water-improved CH 3 NH 3 PbI 3 perovskite films were studied using SE and PL measurements.The measured optical constants show that the band gap of the prepared CH 3 NH 3 PbI 3 films is approximately 1.6 eV, which is insensitive to the content of hydration water in the precursor, while the content of PbI 2 may increase with increasing hydration-water concentration.The SE and PL results indicate that, although hydration water can reduce the non-radiative pathway in the perovskite film, it increases porosity, which leads to a complex variation of PCE.Although the efficiency limitation can be predicted with a detailed balance model, the performance of perovskite solar cells is limited by the film quality.Solar cells based on CH 3 NH 3 PbI 3 with different hydration-water concentrations were fabricated, and their measured J-V curves show that hydration water in perovskite precursors favors the formation of a radiative pathway in CH 3 NH 3 PbI 3 films and leads to a higher open-circuit voltage.Simulations with the Rau's reciprocity relation also agree with the experiments.Therefore, the hydration-water concentration is confirmed to play an important role in CH 3 NH 3 PbI 3 films by influencing the surface roughness, non-radiative pathway, and film porosity.We found that the best hydration-water concentration is x = 1.50 for the 3CH 3 NH 3 I:1PbAc 2 •xH 2 O precursor solution.The results reported in this work provide a guideline for further optimization of the performance of perovskite thin-film solar-cell devices by using hydration water.

Fig. 4 .
Fig. 4. (a) Measured J-V curves of the selected planar heterojunction configuration for CH3NH3PbI3 solar cells.The solid and dashed lines indicate the forward and backward scanned data, respectively.(b)&(c) The performance of all fabricated solar cells.The plot indicates the average value, and the error bar indicates the upper and lower bounds of measured data.(d) Measured PL spectra of CH3NH3PbI3 films.