Effect of Ammonium Halide Additives on the Performance of Methyl Amine Based Perovskite Solar Cells

CH3NH3PbI3-xClx species were fabricated as light-absorbing layers for perovskite solar cells (PSCs), by employing NH4I, NH4Br, and NH4Cl as additives via annealing at 100 °C for different times. Solutions containing CH3NH3I, PbI2, and PbCl2 (4:1:1 molar ratio) in N,N-dimethylformamide were used to prepare perovskites with NH4I, NH4Br, and NH4Cl as additives, at concentrations of 0.1 M and 0.3 M. The additives helped increase the grain size and reduce pinholes in the perovskite films, as confirmed by field-emission scanning electron microscopy. The X-ray diffraction profiles of CH3NH3PbI3-xClx clearly showed peaks at 14° and 28° for the samples with additives, indicative of crystallinity. The best PSC performance with a power conversion efficiency of 9.13%, was achieved using 0.1 M NH4I by annealing for 5 min, whereas the power conversion efficiency of the perovskite solar cells without additives was 5.40%.


Introduction
In 2009, perovskite was first used as a sensitizer for light absorption in dye-sensitized solar cells (DSSCs), which gave a power conversion efficiency (PCE) of 3.8% [1]. Perovskite has an ABX 3 structure, where A, B, and X represent an organic cation, metal cation, and halide ion, respectively. Given perovskites have high absorption coefficients in the visible region, perovskite nanocrystals, especially CH 3 NH 3 PbX 3 (X = Br, I, Cl), are used as light absorbers. Park et al. achieved an improved PCE of 6.54% by using CH 3 NH 3 PbI 3 in DSSCs, in 2011 [2]. In 2012, CH 3 NH 3 PbI 3 and CH 3 NH 3 PbI 2 Cl were used as absorbers in solid-state hybrid solar cells to obtain PCEs of 9.7% and 10.9%, respectively [3,4], and research to increase the PCE of perovskite solar cells (PSCs) is ongoing . In 2017, Seok and coworkers achieved a PCE of PSCs, as high as 22.1% [28].
The quality of the perovskite layer is important for increasing the PCE of PSCs [28][29][30][31][32]. Research to improve the quality of perovskite films by using additives is currently being pursued. For example, the addition of CH 3 NH 3 Cl [33], CaCl 2 [34], InCl 3 [35], NH 4 Cl [36,37] etc., has been found to improve the morphology of perovskite films, thereby improving the performance of PSCs. PSCs with Cl-containing additives, showed high electrical conductivity and a long carrier diffusion length. Additives including, I − also affect the performance of PSCs. When NaI [38], CuI [38], HI [39], and NH 4 I [40] were used as additives, the PCEs of the resulting PSCs were 15.14%, 15.25%, 17.60%, and 18.31% respectively, which were higher compared to that of the PSCs without additives. Previous studies have shown that additives improve the morphology of perovskite films and increase the PCE of PSCs.
In this study, NH4I, NH4Br, and NH4Cl were used as additives in the perovskite solution for film formation via spin-coating. The CH3NH3PbI3-xClx perovskite solution for spin-coating was prepared in dimethylformamide (DMF). The concentration of the additives was varied, between 0.1 M and 0.3 M. After spin-coating, the perovskite films were annealed at 100 °C for 1, 5, 10, and 30 min. PSCs with an indium tin oxide (ITO)/poly (3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/CH3NH3PbI3-xClx perovskite/[6,6]-phenyl-C60 butyric acid methyl ester (PCBM)/bathocuproine (BCP)/LiF/Al device structure were fabricated. This study mainly shows device optimization through the change of various additives (NH4I, NH4Br, NH4Cl), amounts of additives (0.1 M and 0.3 M), and annealing time (1, 5, 10, and 30 min), respectively. The characteristics of the CH3NH3PbI3-xClx PSCs, were evaluated by controlling the annealing time and the concentration of additives, demonstrating that the PCE of the PSCs was improved by the additives. The details of the characteristics of the PSCs and the effects of the additives are discussed herein.

Fabrication of CH3NH3PbI3-xClx PSCs
ITO-coated glass (Woo Yang GMS, 15 Ω sq −1 ) was used as the substrate. The glass was cleaned by successive ultrasonication in acetone, isopropyl alcohol, and deionized water for 15 min each. The substrate was then treated with UV-ozone for 15 min. A hole transport layer of PEDOT:PSS (Clevious) was spin-coated onto the glass substrate at 4000 rpm for 30 s, followed by heating at 150 °C for 15 min in air. The substrates were loaded into a N2-filled glove box. As for synthesis of the CH3NH3PbI3-xClx precursor, CH3NH3I, lead(II) iodide (PbI2), and lead(II) chloride (PbCl2) in a 4:1:1 molar ratio, were dissolved in an anhydrous DMF. Thereafter, 0.1 M or 0.3 M NH4I, NH4Br, and NH4Cl were added to the solution. The precursor solution was then spin-coated onto the PEDOT:PSS layer at 4000 rpm for 30 s. The substrates were subsequently annealed at 100 °C for 1, 5, 10, and 30 min. A PCBM (Nanoholdings) solution (40 mg mL −1 ) in chlorobenzene was then spin-coated on top of the perovskite film at 750 rpm for 15 s and the substrate was heated at 60 °C for 5 min. Subsequently, films of BCP (3 nm, Taewon Scientific CO, Seoul, Korea), LiF (1 nm, Taewon Scientific CO, Seoul, Korea), and Al (100 nm) were deposited on top of the electron transport layer (PCBM layer) under vacuum (<10 −6 Torr) using a thermal evaporator, as shown in Figure 1. BCP and LiF, were used as hole blocking layers and Al was used as an electrode.

Characterizations
The X-ray diffraction (XRD, D8-Advance/Bruker-AXS (Billerica, MA, USA)) patterns were measured in an angle range of 2θ = 10 • to 50 • . The morphology of the films was shown through field emission scanning electron microscope (FE-SEM, Carl Zeiss, SIGMA (Kawasaki-shi, Japan)) images. The current-voltage (J-V) characteristics of PSCs were measured with a Keithley 2400 semiconductor parameter analyzer. The measurements were conducted under AM 1.5 G 100 mW/cm 2 illumination using an Oriel 150 W solar simulator. All devices were measured by masking the active area with a thin mask (0.04 cm 2 ). The J-V characteristics for all devices were measured at a voltage scan rate of 0.1 V s −1 .

Results and Discussion
We investigated the effect of the additives on crystallization of the CH 3 NH 3 PbI 3-x Cl x film through XRD, as shown in Figure 2. The perovskite films were prepared on PEDOT:PSS-coated ITO glass. The samples with and without additives were annealed at 100 • C for various times. NH 4 I, NH 4 Br, and NH 4 Cl were used as additives. The XRD peaks of the CH 3 4 Cl showed weak peaks. All layers except 0.1 M NH 4 I showed weaker peaks than the layer without additive, when annealed for 5 min. In the case of annealing for 1 min, all layers except 0.1 M and 0.3 M NH 4 Cl showed a peak similar to the layer without additive. The samples treated with NH 4 Cl showed a stronger peak than the sample without additive. Thus, it was found that the annealing time and the additives influenced crystallization of the CH 3 NH 3 PbI 3-x Cl x films.
Color changes in the perovskite layers were observed according to the annealing time and the additives. The CH 3 NH 3 PbI 3-x Cl x solution was combined with different concentrations of NH 4 I, NH 4 Br, or NH 4 Cl as additives for spin coating. The samples were annealed at 100 • C for 1, 5, 10, or 30 min. Figure 3 shows optical images of the CH 3 NH 3 PbI 3-x Cl x layers, before and after annealing. Before annealing, the CH 3 NH 3 PbI 3-x Cl x films were brown, as shown in Figure 3a-c. There was only a slight difference depending on the additives. The color of the CH 3 NH 3 PbI 3-x Cl x films changed after annealing. As shown in Figure 3d, the samples prepared with 0.1 M NH 4 I were nearly black regardless of the annealing time. However, those prepared with 0.3 M NH 4 I were red and were similar to the layers prepared with 0.3 M NH 4 Br (Figure 3e). After annealing the perovskite layer with 0.1 M NH 4 Br for 1 min, the layer became green. However, the layer with 0.1 M NH 4 Cl was a lighter green than that prepared with 0.1 M NH 4 Br and annealed for 1 min. With the use of the 0.1 M additives, the color was similar to that of the reference cells after heat treatment. Therefore, it was confirmed that a small amount of additive had no significant effect on color. However, the effect of the annealing time in the crystallization of the perovskite could be confirmed through the different color of perovskite layers,   The color of the CH 3 NH 3 PbI 3-x Cl x layers, is illustrated in Figure 3. Notably, the surface quality and morphology have more influence on the performance of CH 3 NH 3 PbI 3-x Cl x PSCs, than the color of the perovskite layer [42]. The CH 3 NH 3 PbI 3-x Cl x films with additives were prepared on a PEDOT:PSS film. Figure 4 shows FE-SEM images of the perovskite layers annealed for different times, with different additives. When 0.1 M NH 4 I was added to the CH 3 NH 3 PbI 3-x Cl x solution, the grain size increased and there were some pinholes compared to that of the sample annealed for 1 min (Figure 4a). Figure 4b shows that when 0.3 M NH 4 I was added, the grain size increased as the annealing time increased, Materials 2018, 11, 1417 5 of 10 and annealing for 30 min resulted in grains that were clustered with each other. As shown in Figure 4c,d, the FE-SEM images of the perovskite layer prepared with NH 4 Br as the additive, showed an uneven surface. Particularly, the image of the sample prepared with 0.3 M NH 4 Br showed a bright spot, which is estimated to be PbI 2 , as confirmed by XRD. When NH 4 Cl was used as the additive, there were many differences in the FE-SEM images depending on the concentration. Comparison of the samples prepared with 0.1 M NH 4 Cl (Figure 4e) and 0.3 M NH 4 Cl (Figure 4f), shows that there were large pinholes in the film prepared with 0.3 M NH 4 Cl, which is expected to have a negative effect on the perovskite layer as the light absorbing layer in solar cells. The color of the CH3NH3PbI3-xClx layers, is illustrated in Figure 3. Notably, the surface quality and morphology have more influence on the performance of CH3NH3PbI3-xClx PSCs, than the color of the perovskite layer [42]. The CH3NH3PbI3-xClx films with additives were prepared on a PEDOT:PSS film. Figure 4 shows FE-SEM images of the perovskite layers annealed for different times, with different additives. When 0.1 M NH4I was added to the CH3NH3PbI3-xClx solution, the grain size increased and there were some pinholes compared to that of the sample annealed for 1 min (Figure 4a). Figure 4b shows that when 0.3 M NH4I was added, the grain size increased as the annealing time increased, and annealing for 30 min resulted in grains that were clustered with each other. As shown in Figure 4c,d, the FE-SEM images of the perovskite layer prepared with NH4Br as the additive, showed an uneven surface. Particularly, the image of the sample prepared with 0.3 M NH4Br showed a bright spot, which is estimated to be PbI2, as confirmed by XRD. When NH4Cl was used as the additive, there were many differences in the FE-SEM images depending on the concentration.   Figure 5 shows the current density-voltage (J-V) curves of the CH 3 NH 3 PbI 3-x Cl x PSCs with NH 4 I, NH 4 Br, and NH 4 Cl as additives. As a reference, a CH 3 NH 3 PbI 3-x Cl x PSC without any additive was also fabricated, as shown in Figure 5a. The open-circuit voltage (V OC ), short-circuit current (J SC ), fill factor (FF), and power conversion efficiency (PCE) of the PSCs employing the films with additives, are summarized in Table 1. The perovskite layers were annealed at 100 • C for different times. The annealing times were 1, 5, 10, and 30 min. For the reference CH 3 NH 3 PbI 3-x Cl x PSCs, the highest PCE of 5.40%, was achieved with the film annealed for 5 min. The corresponding J SC , V OC , and FF were 0.84, 9.45, and 0.68, respectively. The same PCE (4.64%) was achieved with annealing times of 10 min and 30 min. On the other hand, when the CH 3 NH 3 PbI 3-x Cl x layer was annealed for 1 min, the J SC , V OC , FF, and PCE values declined significantly to 1.64 mA cm −2 , 0.81 V, 0.15, and 0.20%, respectively. Thus, it could be deduced that the cubic perovskite was not formed when the perovskite layer was annealed for 1 min. However, the PCE of the CH 3 NH 3 PbI 3-x Cl x PSCs employing the films with additives was higher than that of the PSCs employing the films without additives annealed for 1 min. The V OC , J SC , FF, and PCE of 0.86 V, 12.2 mA cm −2 , 0.67, and 7.31%, respectively achieved using 0.1 M NH 4 I were significantly higher values. When 0.1 M NH 4 I was used as the additive and the perovskite layer was annealed for 5 min, the highest efficiency was obtained. The corresponding V OC , J SC , FF, and PCE values were 0.78 V, 16.3 mA cm −2 , 0.72, and 9.13%, respectively. The PCE was 69% higher than that of the PSCs employing the perovskites without additives, under the same conditions. It was confirmed that the PCE decreased with longer heat treatment. When the concentration of NH 4 I was increased to 0.3 M, the PSC performance declined. Unlike the case with 0.1 M NH 4 I, where the highest PCE was achieved with the film annealed for 5 min, the highest PCE attained with 0.3 M NH 4 I was achieved with annealing for 10 min. The corresponding V OC , J SC , FF, and PCE were 0.79 V, 10.8 mA cm −2 , 6.70, and 6.06%, respectively. Thus, it was confirmed that the higher the concentration of NH 4 I as the additive, the longer the required annealing time. The CH 3 NH 3 PbI 3-x Cl x PSCs with the NH 4 Br-treated film showed remarkably low performance when 0.3 M NH 4 Br was added, compared to 0.1 M NH 4 Br. The V OC , J SC , FF, and PCE values of the PSCs employing the 0.1 M NH 4 Br-treated film were 0.85 V, 11.7 mA cm −2 , 0.66, and 6.57%, respectively, whereas that of the PSCs with the 0.3 M NH 4 Br-treated films were 0.87 V, 3.09 mA cm −2 , 0.23, and 0.61%, respectively, when annealing was performed for 5 min. When 0.1 M NH 4 Br was used as the additive, the Br − in NH 4 Br did not affect the CH 3 NH 3 PbI 3-x Cl x structure significantly. However, when 0.3 M NH 4 Br was added, it is considered that Br − bonded with CH 3 NH 3 PbI 3-x Cl x and disrupted the cubic structure. On the other hand, the performance parameters of the PSCs were higher when NH 4 I and NH 4 Cl were used as additives. It is considered that I − and Cl − helped to maintain the cubic structure of the perovskite. The V OC , J SC , and FF were 0.80 V, 15.1 mA cm −2 , and 0.67, respectively, for the highest PCE of 8.13%, achieved with 0.1 M NH 4 Cl. With the use of 0.3 M NH 4 Cl, the V OC , J SC , FF, and PCE values were 0.67 V, 14.4 mA cm −2 , 0.63, and 6.13%, respectively. The performance of the CH 3 NH 3 PbI 3-x Cl x PSCs employing the films treated with NH 4 I, NH 4 Br, and NH 4 Cl as additives was investigated. The additive is related to the grain size of perovskite. As a result, the diffusion length of the active layer became longer as the grain size increased, confirming that J SC and PCE were increased. It can also be seen that a small amount of additive was more effective than a large amount. In addition, it was confirmed that the annealing time affected the characteristics of the perovskite solar cells due to the decrease in efficiency, as the annealing time became longer.   Figure 5 shows the current density-voltage (J-V) curves of the CH3NH3PbI3-xClx PSCs with NH4I, NH4Br, and NH4Cl as additives. As a reference, a CH3NH3PbI3-xClx PSC without any additive was also fabricated, as shown in Figure 5a. The open-circuit voltage (VOC), short-circuit current (JSC), fill factor (FF), and power conversion efficiency (PCE) of the PSCs employing the films with additives, are summarized in Table 1. The perovskite layers were annealed at 100 °C for different times. The annealing times were 1, 5, 10, and 30 min. For the reference CH3NH3PbI3-xClx PSCs, the highest PCE of 5.40%, was achieved with the film annealed for 5 min. The corresponding JSC, VOC, and FF were 0.84, 9.45, and 0.68, respectively. The same PCE (4.64%) was achieved with annealing times of 10 min    The samples were annealed at 100 • C for 1, 5, 10, and 30 min, respectively. The optimal annealing time was 10 min with PCE 5.40%. Current density-voltage curves of CH 3 NH 3 PbI 3-x Cl x PSCs with additives of (b) 0.

Conclusions
In summary, an innovative approach by adding the methylammonium halide to enhance the performance of CH 3 NH 3 PbI 3-x Cl x PSCs was developed. CH 3 NH 3 PbI 3-x Cl x films were fabricated for one-step coating with perovskite solution, using NH 4 I, NH 4 Br, and NH 4 Cl as additives. The XRD patterns and FE-SEM images of the CH 3 NH 3 PbI 3-x Cl x films with additives generally, showed the presence of strong peaks at 14 • and 28 • , and an increase in the grain size of the perovskite with additives, but some pinholes were present. Investigation of the photovoltaic performance of the PSCs with additives, showed that the PCE of the CH 3 NH 3 PbI 3-x Cl x PSCs with the 0.3 M NH 4 Br-treated films was lower than that of the PSCs employing films without additives. It is proposed that, Br − from NH 4 Br substituted the I − of CH 3 NH 3 PbI 3-x Cl x , thereby disrupting its structure, and I − reacted with Pb + to form PbI 2 , leading to a decrease in the PCE. The highest PCE of the PSCs employing the films treated with 0.1 M NH 4 I was 9.13%, which was 69% higher than that of the PSCs employing films without any additive. It was confirmed that small amounts of NH 4 I, NH 4 Br, and NH 4 Cl maintain structure and improve the properties of CH 3 NH 3 PbI 3-x Cl x PSCs. These results indicated that the additives, especially 0.1 M NH 4 I, were effective in improving the characteristics of CH 3 NH 3 PbI 3-x Cl x PSCs.