Parameters influencing the deposition of methylammonium lead halide iodide in hole conductor free perovskite-based solar cells

Perovskite is a promising light harvester for use in photovoltaic solar cells. In recent years, the power conversion efficiency of perovskite solar cells has been dramatically increased, making them a competitive source of renewable energy. An important parameter when designing high efficiency perovskite-based solar cells is the perovskite deposition, which must be performed to create complete coverage and optimal film thickness. This paper describes an in-depth study on two-step deposition, separating the perovskite deposition into two precursors. The effects of spin velocity, annealing temperature, dipping time, and methylammonium iodide concentration on the photovoltaic performance are studied. Observations include that current density is affected by changing the spin velocity, while the fill factor changes mainly due to the dipping time and methylammonium iodide concentration. Interestingly, the open circuit voltage is almost unaffected by these parameters. Hole conductor free perovskite solar cells are used in this work, in order to minimize other possible effects. This study provides better understanding and control over the perovskite deposition through highly efficient, low-cost perovskite-based solar cells.

In recent years, organo-metal perovskite has been used as a light harvester in solar cells.The high efficiency perovskite-based solar cells are simple to fabricate and low cost, making them a competitive source of renewable energy. 1,2 Kojima et al. were the first to use solution-processed methylammonium (MA) lead halide iodide (CH 3 NH 3 PbI 3 ) perovskite as a sensitizer on mesostructured TiO 2 using a liquid electrolyte, achieving a power conversion efficiency (PCE) of 3.8%. 3owever, these cells suffered from major stability issues.The use of solid hole conductor material in CH 3 NH 3 PbI 3 perovskite solar cells demonstrated a PCE of 9.7% 4 with much better stability.Recently PCEs of more than 16% were demonstrated for perovskite-based solar cells. 5,6 bstitution of the iodide with chloride achieved MAPbI (3−x) Cl x perovskites, which were deposited on mesoscopic TiO 2 and Al 2 O 3 achieving PCEs of 7.6% and 10.9%, respectively. 7One of the attractive properties of the perovskites is their ability to act both as light harvester and hole conductor with a low exciton binding energy, making the fabrication of solar cells based on these perovskites much simpler compared to organic photovoltaic (OPVs) or dye-sensitized solar cells (DSSCs). 8,9 oreover, the structure of perovskite solar cells is not restricted to mesoporous structure, and efficient planar perovskite solar cells have been demonstrated as well. 10,11 [14]5 The effect of the perovskite deposition on the solar cells performance is critically important; it determines the film coverage, film thickness, film quality, and the transport properties.Graetzel and co-workers have demonstrated the use of the two-step deposition technique as a powerful technique for achieving highly efficient perovskite solar cells. 12The two-step deposition enables better control over the perovskite crystallization by separating the perovskite deposition into the two precursors.This paper describes an in-depth study of the parameters influencing the two-step deposition of perovskite in the solar cell.Hole conductor free perovskite solar cells were fabricated in this work, in order to minimize other possible effects.The influence of spin velocity, dipping time, annealing temperature, and methylammonium iodide (MAI) concentration on the photovoltaic (PV) performance are investigated.This study provides further understanding and control over the perovskite deposition through highly efficient perovskite-based solar cells.
This paper discusses the study of different parameters influencing the two-step deposition of CH 3 NH 3 PbI 3 perovskite in photovoltaic solar cells.The perovskite solar cell structure discussed is presented in Figure 1(a).6][17][18] This dual function simplifies the cell structure and minimizes the effect of other parameters that influence the PV performance, such as inconsistency in pore filling by the hole conductor.PbI 2 synthesis: The first step in the two-step deposition technique includes the PbI 2 precursor.Not much attention has been devoted to the synthesis of the PbI 2 ; usually this precursor is purchased from a supplier.In this study, the PbI 2 was synthesized in the lab according to reaction 1: The Pb(NO 3 ) 2 was reacted with potassium iodide (KI) at a ratio of 1:2 in water at 100 • C. The PbI 2 was precipitated immediately, filtered and dried in a vacuum oven.X-ray diffraction (XRD) analysis was performed on the synthesized PbI 2 and on PbI 2 purchased from Sigma-Aldrich. Figure 2 shows XRD of the lab-synthesized PbI 2 and of the purchased PbI 2 .There is complete agreement between both spectra, indicating that the PbI 2 synthesized in the lab is the same as the standard purchased PbI 2 .X-ray photoelectron spectroscopy (XPS) analysis also confirmed that there is no difference between the two versions of PbI 2 .(For XPS spectra, please see Table S1 and Figure S1 in the supplementary material.) 19   crystals' size is dependent on the spin velocity (film thickness); therefore, in the case of 6500 rpm (the thinnest PbI 2 film thickness), the perovskite crystals were the biggest.
The difference in the film thickness of the PbI 2 and CH 3 NH 3 PbI 3 is supported by the absorbance spectra in Figure 4(a): two different spin velocities are presented, 2000 rpm and 6500 rpm.In the case of the absorbance of PbI 2 , there is a big difference between the samples, related to thicker film of PbI 2 at 2000 rpm.In the case of the absorbance of CH 3 NH 3 PbI 3 , the difference between the spin velocities can be observed, although it is lower than the difference between the PbI 2 samples.The explanation is related to the fact that the film thickness of the complete CH 3 NH 3 PbI 3 is also dependent on the dipping process of the PbI 2 electrode, which affected the final film thickness of the perovskite.
Table I and Figure 4(b) summarize the PV parameters and the current voltage curves (JV curves) of the cells at different spin velocities.The best PV results are achieved for the 2000 rpm sample with PCE of 7.8%.The data show that by changing the spin velocity, the current density of the solar cell is the parameter with the most influence.
Dipping time: The effect of dipping time in the CH 3 NH 3 I solution (process 2 in Figure 1(b)) on morphology and on PV performance was studied.Figure 5 presents dipping times of 20 s, 1 min, 20 min, and 3.5 h.It is noted that increasing the dipping time increases the size of the CH 3 NH 3 PbI 3 perovskite crystals (spin velocity was the same for all samples in these experiments), resulting in perovskite crystals the size of several micrometers.
The influence of dipping on PV performance is observed in Table II and Figure 6.The best PV performance was observed for the shortest dipping time of 20 s.Longer dipping time results in bigger crystals which affect transport through the perovskite film and enhance the recombination at longer dipping times, as indicated by the dark current measurements in Figure 6(a).The open circuit voltage (V OC ) and fill factors (FF) were lower for dipping times of 3.5 h and for 1 h, compared to the other dipping times.
Annealing temperature: The annealing step is the final stage before evaporation of the contact.(As indicated previously, no hole transport material is used in these cells.)To investigate the effect of the annealing temperature, several annealing temperatures were studied.concentration results in more MAI to react with the already deposited PbI 2 , which could increase the crystallization of the CH 3 NH 3 PbI 3 perovskite.However, when an excess of MAI is present in the dipping solution, the free MAI (which did not react with the PbI 2 ) could have the opposite effect.
The excess MAI might remain on the CH 3 NH 3 PbI 3 perovskite surface, reducing the perovskite conductivity.Moreover, excess MAI could cause desorption of the perovskite from the surface.
Table IV shows the PV parameters at different MAI concentrations in the dipping solution.At high MAI concentration, the PV performance decreases due to a decrease in the current density.For low MAI concentration, the current density is high, although the FF is lower than the 0.09M MAI.
Table IV shows that there is an optimum concentration where the FF and the current density are the highest, resulting in PCE of 9.4%, the highest efficiency in this study.Above this optimum point, both fill factor and efficiency decreased due to an excess of MAI in the dipping solution (Figure 9).Current voltage curves and internal quantum efficiency (IQE) graphs are presented in Figures 10(a) and 10(b), respectively.IQE is the ratio of the number of charge carriers collected by the solar cell to the number of photons of a given energy that is absorbed by the cell.The IQE spectra of the 0.09M MAI concentration achieved around 85% in the range of 400-550 nm.
In this research, the two-step deposition technique was used for the deposition of the CH 3 NH 3 PbI 3 in hole conductor free perovskite solar cells.The elimination of hole conductor in this study enabled isolating the effect of different parameters in the two-step deposition process and to investigate their influence on the PV performance.PbI 2 was synthesized in the lab avoiding contamination and non-desirable additives.
The effect on the photovoltaic performance was investigated by changing several parameters in the two-step deposition, i.e., spin velocity, dipping time, annealing temperature, and various methylammonium iodide concentrations.It was concluded that the spin velocity has the most influence on the J SC , dipping time on the FF, annealing temperature on the J SC , and MAI concentration on the FF and J SC. Interestingly, the V OC was almost not affected by these parameters.
Understanding the effect of these critical parameters on perovskite deposition could lead to highly efficient, low-cost perovskite-based solar cells.
We would like to thank the Israel Alternative Energy Foundation (I-SAEF) that financed this research and to the Office of the Chief Scientist of the Ministry of Industry, Trade and Labor Kamin Project No.50303.We would like to thank Dr. Vladimir Uvarov from the Harvey M. Krueger Center for Nanoscience and Nanotechnology at the Hebrew University for the XRD measurements.

FIG. 1 .
FIG. 1.(a) Schematic structure of the solar cell without a hole conductor; (b) the different stages involved in the two-step deposition of the CH 3 NH 3 PbI 3 perovskite.Methylammonium iodide (MAI) corresponds to CH 3 NH 3 I.
Two-step deposition includes several stages: (a) dropping the PbI 2 solution onto the TiO 2 electrode; (b) spin coating the PbI 2 and heating the PbI 2 ; (c) dipping the PbI 2 electrode into CH 3 NH 3 I (CH 3 NH 3 I = MAI) solution; (d) annealing of the CH 3 NH 3 PbI 3 perovskite.Figure 1(b) shows the different stages in the deposition process.This paper describes changes in several important parameters in the deposition of the perovskite, showing the influence of these parameters on morphology, optical properties, and photovoltaic performance.
Figure 2(b) presents high resolution scanning electron microscopy (HR-SEM) of the synthesized PbI 2 deposited on mesoporous TiO 2 .Spin velocity: The influence of spin velocity on PV performance was examined by changing the spin velocity of the PbI 2 on the TiO 2 film (process 1 in Figure1(b)).
FIG. 5. Top view HR-SEM micrographs of CH 3 NH 3 PbI 3 crystals deposited on TiO 2 film at four different dipping times.

FIG. 6 .FIG. 8 .
FIG. 6.(a) Dark current for various dipping times: 1 min, 1 h, 3.5 h.The arrow indicates the increase in the dark current.(b) Current voltage curves of the hole conductor free solar cells at different dipping times.
Figure 8(a) summarizes the influence of the annealing temperature on the current density and the efficiency of the hole conductor free solar cells.Obviously, the annealing temperature mainly influences the current density.Methylammonium iodide concentration: The concentration of MAI in the dipping solution has an important effect on the cells performance (process 3 in Figure 1(b)).Increasing the MAI 081502-7 Cohen, Gamliel, and Etgar APL Mater.2, 081502 (2014)

FIG. 10 .
FIG. 10.(a) Current voltage curves of the different cells made at different MAI concentrations and the corresponding (b) IQE graphs.

TABLE I .
PV parameters of the cells at different spin velocities.

TABLE II .
PV performance of the cells at 5 different dipping times.

TABLE IV .
PV parameters of cells made at different MAI concentrations in the dipping solution.
FIG. 9. Fill factor and efficiency of the hole conductor free perovskite solar cells at different MAI concentrations.