E ﬀ ect of Low-Concentration Rb + Mixing on Semiconductor Majority Charge Carriers Type of Perovskite Light-Absorption Layer by Using Two-Step Spin-Coating Method

: In recent years, perovskite materials have been the subject of great progress in optoelectronic devices. The perovskite layer is the light absorption layer of perovskite solar cells (PSCs), and the majority charge carriers type play a crucial role in the formation of a P–N junction. In this paper, the light absorption layer of PSCs was Rb-mixed at a low concentrations by using a two-step spin-coating method, which could adjust the majority charge carriers type in perovskite ﬁlms from N-type to P-type, and it has little inﬂuence on the crystal structure and light absorption capacity of perovskite. In addition, low concentration Rb-mixing is di ﬀ erent from high concentration Rb-mixing. With increasing Rb-mixing concentration, the perovskite grains does not change shape. Although the quality of perovskite ﬁlms deteriorated and the PL peaks exhibit a slight blue shift after mixing, the e ﬃ ciency only slightly decreased, indicating that the new P-N hetero-junction was still formed after mixing, which provided a new idea for the future research of homo-junction PSCs.


Introduction
Hetero-junction perovskite solar cells (PSCs) have been extensively studied in the past decade. There are many factors that affect the performance of PSCs, such as the morphology of perovskite films, carrier recombination, long diffusion length and so on [1][2][3][4][5]. However, most of the previous research has focused on hetero-junction PSCs, and a few papers have been published on homo-junction PSCs [5][6][7][8].

Cleaning of FTO Substrate
Firstly, the transparent FTO conductive glass was cleaned with detergent and deionized water in appropriate ratio with ultrasonic vibration cleaners (KQ-100E, Skymen Cleaning Technology Shenzhen Co., Ltd., Shenzhen, China) for 20 min. The impurities soluble in detergent on the substrate surface were washed away with large amounts of deionized water.
Secondly, the processed FTO conductive glass was cleaned in ethanol with ultrasonic vibration cleaners for 20 min to remove all kinds of impurities on the surface that were easily soluble in absolute ethanol. Then we used a large amount of deionized water to rinse the substrate surface, thereby ensuring the absence of residual ethanol.
Thirdly, the FTO conductive glass was cleaned with the mixed solution of deionized water, isopropyl alcohol and acetone in a volume ratio of 1:1:1 with ultrasonic vibration cleaners for 20 min to remove all kind of impurities on the surface that were easily soluble in isopropanol and acetone. After this step, a large amount of deionized water was used to clean the conductive substrate surface to ensure the absence of residual isopropyl alcohol and acetone.
Finally, the UV light cleaner (BZS250GF-TC, Shenzhen Huiwo Technology Co., Ltd., Shenzhen, China) was used to remove the residual organics on the cleaned conductive glass for 15 min.

Fabrication of Electron Transport Layer (ETL)
The acidic titanium dioxide solution was spin-coated at 2000 rpm for 60 s on a FTO substrate. Then the FTO/c-TiO 2 annealed at 100 • C for 10 min on a hot plate. Finally, it was sintered at 500 • C for 30 min in a ceramic fiber muffle furnace (MF-0910p, Hgtong company, Beijing, China) to obtain the TiO 2 compact layer. The mesoporous layer was made with spin-coating the 18NR-T TiO 2 at 2000 rpm for 60 s on a FTO substrate, and then annealed at 100 • C for 10 min on a hot plate. The last, it was sintered at 500 • C for 30 min in a ceramic fiber muffle furnace.

Fabrication of Perovskite Light-Absorption Layer
Firstly, the yellow crystalline PbI 2 powder (0.600 g) was dissolved to mixture solution of DMF and DMSO (volume ratio: 0.95:0.05). Secondly, the different weight of RbI powder (0.00852, 0.01704 and 0.02556 g) were separately dissolved in the 1 mL pure PbI 2 solution to obtain a precursor solution with the concentration of 0.04, 0.08 and 0.12 mol/L, respectively. Then, the MAI (0.073 g) was dissolved in 1mL IPA solution to obtain the MAI solution. Next, the PbI 2 precursor solution with mixing different concentrations of RbI were spin-coated on the m-TiO 2 substrate at 1500 rpm for 30 s. Finally, the MAI solution was spin-coated on the PbI 2 substrate at 1500 rpm for 30 s and then annealed at 150 • C for 15 min on a hot plate. A schematic diagram of the spin coating process is shown in Figure 1. Firstly, the transparent FTO conductive glass was cleaned with detergent and deionized water in appropriate ratio with ultrasonic vibration cleaners (KQ-100E, Skymen Cleaning Technology Shenzhen Co., Ltd., Shenzhen, China) for 20 min. The impurities soluble in detergent on the substrate surface were washed away with large amounts of deionized water.
Secondly, the processed FTO conductive glass was cleaned in ethanol with ultrasonic vibration cleaners for 20 min to remove all kinds of impurities on the surface that were easily soluble in absolute ethanol. Then we used a large amount of deionized water to rinse the substrate surface, thereby ensuring the absence of residual ethanol.
Thirdly, the FTO conductive glass was cleaned with the mixed solution of deionized water, isopropyl alcohol and acetone in a volume ratio of 1:1:1 with ultrasonic vibration cleaners for 20 min to remove all kind of impurities on the surface that were easily soluble in isopropanol and acetone. After this step, a large amount of deionized water was used to clean the conductive substrate surface to ensure the absence of residual isopropyl alcohol and acetone.
Finally, the UV light cleaner (BZS250GF-TC, Shenzhen Huiwo Technology Co., Ltd., Shenzhen, China) was used to remove the residual organics on the cleaned conductive glass for 15 min.

Fabrication of Electron Transport Layer (ETL)
The acidic titanium dioxide solution was spin-coated at 2000 rpm for 60 s on a FTO substrate. Then the FTO/c-TiO2 annealed at 100 °C for 10 min on a hot plate. Finally, it was sintered at 500 °C for 30 min in a ceramic fiber muffle furnace (MF-0910p, Hgtong company, Beijing, China) to obtain the TiO2 compact layer. The mesoporous layer was made with spin-coating the 18NR-T TiO2 at 2000 rpm for 60 s on a FTO substrate, and then annealed at 100 °C for 10 min on a hot plate. The last, it was sintered at 500 °C for 30 min in a ceramic fiber muffle furnace.

Fabrication of Perovskite Light-absorption Layer
Firstly, the yellow crystalline PbI2 powder (0.600 g) was dissolved to mixture solution of DMF and DMSO (volume ratio: 0.95:0.05). Secondly, the different weight of RbI powder (0.00852, 0.01704 and 0.02556 g) were separately dissolved in the 1 mL pure PbI2 solution to obtain a precursor solution with the concentration of 0.04, 0.08 and 0.12 mol/L, respectively. Then, the MAI (0.073 g) was dissolved in 1mL IPA solution to obtain the MAI solution. Next, the PbI2 precursor solution with mixing different concentrations of RbI were spin-coated on the m-TiO2 substrate at 1500 rpm for 30 s. Finally, the MAI solution was spin-coated on the PbI2 substrate at 1500 rpm for 30 s and then annealed at 150 °C for 15 min on a hot plate. A schematic diagram of the spin coating process is shown in Figure 1.   Firstly, the Li-TFST (260 mg) was dissolved in acetonitrile solution (5 mL). Then, the Spiro-OMeTAD powder (14.46 mg) and 4-tert-butylpyridine (TBP) (35 µL) were dissolved in chlorobenzene (2 mL). Finally, the two solutions were mixed together to prepare the hole transport material (Spiro-OMeTAD) precursor solution. The Spiro-OMeTAD precursor solution was spin-coated on perovskite substrate at 3000 rpm for 30 s to prepare HTL. Then, the Spiro-OMeTAD film was left in an ambient atmosphere for 40 min with no annealing procedure.

Fabrication of Counter Electrode (CE)
We reguard the carbon/FTO composite counter electrode as the photoanode of the PSCs. Finally, the FTO conductive glass was used to collect the burning candle soot to prepare CE. The carbon film was aligned on the top of the device, and the two sides were clamped with clips to complete the device preparation [31]. All experiments were performed under air conditions at room temperature. The schematic diagram of the device structure as shown Figure 2.

Fabrication of Hole Transport Layer (HTL)
Firstly, the Li-TFST (260 mg) was dissolved in acetonitrile solution (5 mL). Then, the Spiro-OMeTAD powder (14.46 mg) and 4-tert-butylpyridine (TBP) (35 μL) were dissolved in chlorobenzene (2 mL). Finally, the two solutions were mixed together to prepare the hole transport material (Spiro-OMeTAD) precursor solution. The Spiro-OMeTAD precursor solution was spin-coated on perovskite substrate at 3000 rpm for 30 s to prepare HTL. Then, the Spiro-OMeTAD film was left in an ambient atmosphere for 40 min with no annealing procedure.

Fabrication of Counter Electrode (CE)
We reguard the carbon/FTO composite counter electrode as the photoanode of the PSCs. Finally, the FTO conductive glass was used to collect the burning candle soot to prepare CE. The carbon film was aligned on the top of the device, and the two sides were clamped with clips to complete the device preparation [31]. All experiments were performed under air conditions at room temperature. The schematic diagram of the device structure as shown Figure 2.

Characterization
The scanning electron microscope (SEM) (Zeiss SIGMA, Oberkochen, Germany) was used to obtain morphological images of perovskite layers.
The apparatus can also examine energy dispersive X-ray spectroscopy (EDS) (Zeiss SIGMA, Oberkochen, Germany) for elements composition.
Absorption spectra of the perovskite films with different mixing concentrations of Rb + on FTO/c-TiO2/m-TiO2 substrates were shown by an ultraviolet visible (UV-vis) absorption spectrometer (Avantes, Apeldoom, The Netherlands).
A solar simulator (Sol 3A, Oriel, Newport, RI, USA) with standard simulated air-mass (AM) 1.5 sunlight was used to perform J-V measurement. The active area of the device was 0.2 cm 2 .
The majority charge carriers type was measured by the Hall effect measurement system HL5500PC (QUATEK, Shanghai, China) [32,33].
In the Hall effect test, the perovskite film was prepared on a glass substrate. The electrode was formed by dropping the silver paste on the four corners of the square sample and then heated by hot plate at 60 °C for 10 min for heat treatment. Finally, we placed the sample with the electrode on the instrument for testing (as shown in Supplementary Figure S1). The schematic diagram of basic test circuit of hall is shown in Supplementary Figure S2. In other tests, the perovskite film are prepared on carrier transport layer. All the tests contain substrates. All the characterizations were performed at room temperature in air conditions.

Characterization
The scanning electron microscope (SEM) (Zeiss SIGMA, Oberkochen, Germany) was used to obtain morphological images of perovskite layers.
The apparatus can also examine energy dispersive X-ray spectroscopy (EDS) (Zeiss SIGMA, Oberkochen, Germany) for elements composition.
Absorption spectra of the perovskite films with different mixing concentrations of Rb + on FTO/c-TiO 2 /m-TiO 2 substrates were shown by an ultraviolet visible (UV-vis) absorption spectrometer (Avantes, Apeldoom, The Netherlands).
A solar simulator (Sol 3A, Oriel, Newport, RI, USA) with standard simulated air-mass (AM) 1.5 sunlight was used to perform J-V measurement. The active area of the device was 0.2 cm 2 .
The majority charge carriers type was measured by the Hall effect measurement system HL5500PC (QUATEK, Shanghai, China) [32,33].
In the Hall effect test, the perovskite film was prepared on a glass substrate. The electrode was formed by dropping the silver paste on the four corners of the square sample and then heated by hot plate at 60 • C for 10 min for heat treatment. Finally, we placed the sample with the electrode on the instrument for testing (as shown in Supplementary Figure S1). The schematic diagram of basic test circuit of hall is shown in Supplementary Figure S2. In other tests, the perovskite film are prepared on carrier transport layer. All the tests contain substrates. All the characterizations were performed at room temperature in air conditions. Coatings 2020, 10, 627 5 of 11

Results and Discussion
To explore the quality of Rb-mixed and un-mixed perovskite films firstly, scanning electron microscope (SEM) measurements were measured, as shown in Figure 3. From Figure 3a, the unmixed MAPbI 3 surface is flat and the grain size is larger. From the Rb-mixed SEM top view (Figure 3b-d), it can be seen that smaller grain size start stacked along the vertical direction, and the pinholes began to appear on the surface. The number of pinhole in the perovskite film is more than other films, when the RbI concentrations is further raised to 0.12 mol/L. We considered that the mixing facilitates nucleation of perovskite crystals [34,35]. It can also be seen from the corresponding cross section that with the increase of mixing concentration, the grains gradually become smaller and begin to stack along the vertical direction.

Results and Discussion
To explore the quality of Rb-mixed and un-mixed perovskite films firstly, scanning electron microscope (SEM) measurements were measured, as shown in Figure 3. From Figure 3 a, the unmixed MAPbI3 surface is flat and the grain size is larger. From the Rb-mixed SEM top view (Figure 3b-d), it can be seen that smaller grain size start stacked along the vertical direction, and the pinholes began to appear on the surface. The number of pinhole in the perovskite film is more than other films, when the RbI concentrations is further raised to 0.12 mol/L. We considered that the mixing facilitates nucleation of perovskite crystals [34,35]. It can also be seen from the corresponding cross section that with the increase of mixing concentration, the grains gradually become smaller and begin to stack along the vertical direction.
It is shows that the quality of perovskite films are affected with the increase of mixing concentrations. Perovskite grains are stacked vertically, which may lead to lower carrier collection efficiency in PSCs with multi-grain boundaries, since carriers are transported vertically in PSCs. Moreover, with the increase of mixing concentrations, the pinhole of perovskite films gradually increases (as shown in the red circles in Figure 3), which also leads to the decrease of PSCs efficiency.  It is shows that the quality of perovskite films are affected with the increase of mixing concentrations. Perovskite grains are stacked vertically, which may lead to lower carrier collection efficiency in PSCs with multi-grain boundaries, since carriers are transported vertically in PSCs. Moreover, with the increase of mixing concentrations, the pinhole of perovskite films gradually increases (as shown in the red circles in Figure 3), which also leads to the decrease of PSCs efficiency.  Figure S3 presents the XRD patterns of Rb-mixed perovskite films. It can be seen from Supplementary Figure S3 that the peak positions of all XRD patterns of perovskite do not change significantly, that is, no new peaks appear or disappear after mixing, indicating that the crystal type of perovskite does not change significantly with Rb-mixing of low concentrations.
However, high concentration of Rb-mixing is different from low concentration of Rb-mixing, high concentration (1 mol/L ≤ Rb ≤ 1.5 mol/L) Rb-mixed perovskite is a cubic structure. When the Rb-mixing concentration is increased to 1.8 mol/L, the crystal structure of the perovskite is an orthorhombic structure [23]. The research indicating that low concentration (≤0.12 mol/L) of Rb-mixing does not change the crystal structure of perovskite.
Supplementary Figure S4 shows the EDS diagram of the perovskite films mixed with rubidium ions. It can be seen from the Supplementary Figure S4 that the perovskite films contain rubidium ions. Rb and Si positions have been marked with dotted lines. Rb peak position is 1.6 KeV and Si peak position is 1.7 KeV. Table 1 shows the majority charge carriers type of Rb-mixed perovskite layers obtained by Hall test. As can be seen from Table 1, the unmixed perovskite layer is N-type, and the perovskite layer is P-type after the mixing of Rb + , which indicates that the two-step rubidium ions mixing has changed the perovskite from N-type to P-type.  Figure 4 shows the UV-vis absorption spectra of perovskite layers with different Rb + concentrations. As can be seen from the Figure 4, the UV-vis absorption spectra intensity and range of the Rb-mixed perovskite layer have little change. Especially, the absorption band edge of the spectra almost coincide. It indicates that the low concentrations of Rb-mixed has no significant effect on the absorption intensity and band of perovskite optical absorption layer.
Coatings 2020, 10, x FOR PEER REVIEW 6 of 11 significantly, that is, no new peaks appear or disappear after mixing, indicating that the crystal type of perovskite does not change significantly with Rb-mixing of low concentrations. However, high concentration of Rb-mixing is different from low concentration of Rb-mixing, high concentration (1 mol/L ≤ Rb ≤ 1.5 mol/L) Rb-mixed perovskite is a cubic structure. When the Rbmixing concentration is increased to 1.8 mol/L, the crystal structure of the perovskite is an orthorhombic structure [23]. The research indicating that low concentration (≤0.12 mol/L) of Rbmixing does not change the crystal structure of perovskite.
Supplementary Figure S4 shows the EDS diagram of the perovskite films mixed with rubidium ions. It can be seen from the Supplementary Figure S4 that the perovskite films contain rubidium ions. Rb and Si positions have been marked with dotted lines. Rb peak position is 1.6 KeV and Si peak position is 1.7 KeV. Table 1 shows the majority charge carriers type of Rb-mixed perovskite layers obtained by Hall test. As can be seen from Table 1, the unmixed perovskite layer is N-type, and the perovskite layer is P-type after the mixing of Rb + , which indicates that the two-step rubidium ions mixing has changed the perovskite from N-type to P-type.  Figure 4 shows the UV-vis absorption spectra of perovskite layers with different Rb + concentrations. As can be seen from the Figure 4, the UV-vis absorption spectra intensity and range of the Rb-mixed perovskite layer have little change. Especially, the absorption band edge of the spectra almost coincide. It indicates that the low concentrations of Rb-mixed has no significant effect on the absorption intensity and band of perovskite optical absorption layer. The photoluminescence (PL) spectra of the perovskite layer (deposited on TiO2) with different Rb-mixing concentrations are shown in Figure 5. It is clearly observed from Figure 5a that the peak of PL is fairly strong when the concentration of RbI is further increased to 0.12 mol/mL, which could be explained by the existence of pinholes in perovskite films, preventing the photogenerated carriers injecting into the TiO2 layer from the light absorption layer. This is consistent with the results The photoluminescence (PL) spectra of the perovskite layer (deposited on TiO 2 ) with different Rb-mixing concentrations are shown in Figure 5. It is clearly observed from Figure 5a that the peak of PL is fairly strong when the concentration of RbI is further increased to 0.12 mol/mL, which could be explained by the existence of pinholes in perovskite films, preventing the photogenerated carriers injecting into the TiO 2 layer from the light absorption layer. This is consistent with the results in Figure 3, so we speculate that it may cause J SC to decrease. The PL peaks exhibit a slight blue shift (as shown in Figure 5b), implying a widening of the bandgap and decreasing of the J SC via mixure of Rb [36,37].
Coatings 2020, 10, x FOR PEER REVIEW 7 of 11 in Figure 3, so we speculate that it may cause JSC to decrease. The PL peaks exhibit a slight blue shift (as shown in Figure 5b), implying a widening of the bandgap and decreasing of the JSC via mixure of Rb [36,37].
(a) (b)  Figure 6 shows the J-V curves of PSCs with different Rb + concentrations. We also have extracted the performance parameters from reverse scanning J-V curves in Figure 6, as shown in Table 2. The short-circuit current density and open circuit voltage of PSCs gradually decrease, which is the main reason why the final efficiency gradually decreases with the increase of Rb-mixed concentrations. It is consistent with the results of film morphology (Figure 3). A number of pinholes in the perovskite film will lead to the decrease of short-circuit current density. However, the efficiency is only slightly decreased, indicating that the P-type perovskite layer and ETL can still form a new P-N heterojunction after the majority charge carriers type was changed.   Figure 6 shows the J-V curves of PSCs with different Rb + concentrations. We also have extracted the performance parameters from reverse scanning J-V curves in Figure 6, as shown in Table 2.
The short-circuit current density and open circuit voltage of PSCs gradually decrease, which is the main reason why the final efficiency gradually decreases with the increase of Rb-mixed concentrations. It is consistent with the results of film morphology (Figure 3). A number of pinholes in the perovskite film will lead to the decrease of short-circuit current density. However, the efficiency is only slightly decreased, indicating that the P-type perovskite layer and ETL can still form a new P-N hetero-junction after the majority charge carriers type was changed.
Coatings 2020, 10, x FOR PEER REVIEW 7 of 11 in Figure 3, so we speculate that it may cause JSC to decrease. The PL peaks exhibit a slight blue shift (as shown in Figure 5b), implying a widening of the bandgap and decreasing of the JSC via mixure of Rb [36,37].
(a) (b)  Figure 6 shows the J-V curves of PSCs with different Rb + concentrations. We also have extracted the performance parameters from reverse scanning J-V curves in Figure 6, as shown in Table 2. The short-circuit current density and open circuit voltage of PSCs gradually decrease, which is the main reason why the final efficiency gradually decreases with the increase of Rb-mixed concentrations. It is consistent with the results of film morphology (Figure 3). A number of pinholes in the perovskite film will lead to the decrease of short-circuit current density. However, the efficiency is only slightly decreased, indicating that the P-type perovskite layer and ETL can still form a new P-N heterojunction after the majority charge carriers type was changed.   The un-mixed and Rb-mixed hetero-junction perovskite device structures are shown in Figure 7. The majority carriers type of perovskite has changed by mixing RbI, indicating that the position of PN junction has changed. In Figure 7a, when the majority carriers type of the un-mixed perovskite layer is N-type, the perovskite layer forms a P-N junction with the Spiro-MeOTAD layer (HTL). In Figure 7b, when the majority carriers type of the Rb-mixed perovskite layer is P-type, the perovskite layer forms a new P-N junction with the TiO 2 layer (ETL). We prepared un-mixed and Rb-mixed perovskite solar cells and tested the efficiency of devices (as shown in Table 2). This shows that the newly formed P-N junction can still work.
Coatings 2020, 10, x FOR PEER REVIEW 8 of 11 The un-mixed and Rb-mixed hetero-junction perovskite device structures are shown in Figure 7. The majority carriers type of perovskite has changed by mixing RbI, indicating that the position of PN junction has changed. In Figure 7a, when the majority carriers type of the un-mixed perovskite layer is N-type, the perovskite layer forms a P-N junction with the Spiro-MeOTAD layer (HTL). In Figure 7b, when the majority carriers type of the Rb-mixed perovskite layer is P-type, the perovskite layer forms a new P-N junction with the TiO2 layer (ETL). We prepared un-mixed and Rb-mixed perovskite solar cells and tested the efficiency of devices (as shown in Table 2). This shows that the newly formed P-N junction can still work.

Conclusion
In this paper, the light absorption layer of PSCs was Rb-mixed at low concentrations by the twostep method, and the majority charge carriers type of the perovskite could be changed by mixing Rb + into the perovskite film. In addition, the Rb-mixed at low concentration has little effect on the crystal structure of perovskite. Moreover, it has little effect on the light absorbing capacity of the perovskite layer. The Rb + was added to un-mixed N-type perovskite. With the increase of the concentration of Rb + , the quality of perovskite film became worse, the amount of photo-generated carriers injected into the TiO2 layer from the perovskite layer decreases, and the PL peaks exhibit a slight blue shift, resulting in the low device efficiency. However, the efficiency slightly decreased, indicating that the P-type perovskite layer and ETL could still form a new P-N hetero-junction and the PSCs could still work after the majority charge carriers type was changed, which provided a possibility for the homojunction PSCs.

Conclusions
In this paper, the light absorption layer of PSCs was Rb-mixed at low concentrations by the two-step method, and the majority charge carriers type of the perovskite could be changed by mixing Rb + into the perovskite film. In addition, the Rb-mixed at low concentration has little effect on the crystal structure of perovskite. Moreover, it has little effect on the light absorbing capacity of the perovskite layer. The Rb + was added to un-mixed N-type perovskite. With the increase of the concentration of Rb + , the quality of perovskite film became worse, the amount of photo-generated carriers injected into the TiO 2 layer from the perovskite layer decreases, and the PL peaks exhibit a slight blue shift, resulting in the low device efficiency. However, the efficiency slightly decreased, indicating that the P-type perovskite layer and ETL could still form a new P-N hetero-junction and the PSCs could still work after the majority charge carriers type was changed, which provided a possibility for the homo-junction PSCs.