Current and voltage bias stress effects in chemical vapor deposition made perovskite photovoltaic devices

Perovskites have been receiving a lot of attention as they can be tailored to specific semiconducting properties and lead to various functionality and optoelectrical applications. At the same time, chemical vapor deposition (CVD) is a system used as a deposition process for making perovskites due to the widespread control of its internal environment. The purpose of this study is to investigate the process of making perovskites with CVD and how the morphology, crystallite orientation, and temperature changes in the perovskite structures affect the current behavior observed in devices while under a range of voltage bias stress in both light and dark conditions. Lead-iodide films were made by spin coating a lead-iodide solution (463 mg/mL) on glass and using a powdered methyl-ammonium iodide as a reactant deposited on the glass in the CVD to create perovskites. The experiment was conducted with temperatures from 120 to 140 degrees Celsius; nitrogen levels at flow rates ranging from 100 to 150 mL of nitrogen; reaction times were sampled from two to four hours. Films were collected from the CVD when the deposition was finalized. The films were then analyzed with X-ray diffraction (XRD) to understand the perovskite morphology and improve the parameters for creating the perovskite. The results depict specific temperatures and reaction times for the CVD processes to create a full widespread perovskite made from the reaction between methylammonium iodide and lead iodide. The study produced parameters that consistently resulted in fully reactant thin films, leading to future projects and findings that can make better and more efficient films. The findings suggest that other approaches still need to be iii taken into consideration before getting a complete understanding of the morphology, crystallite orientation, and the electrical behavior of the thin films. Further surface analysis can also lead to more detailed findings on other techniques to produce better made films of different reactants.

CURRENT AND VOLTAGE BIAS STRESS EFFECTS IN CHEMICAL VAPOR DEPOSITION MADE PEROVSKITE PHOTOVOLTAIC DEVICES by Jenny Ngo A Senior Honors Thesis Submitted to the Faculty of The University of Utah In Partial Fulfillment of the Requirements for the Honors Degree in Bachelor of Science In Chemistry Approved: ______________________________ Dr. Luisa Whittaker-Brooks Thesis Faculty Supervisor _____________________________ Dr. Matthew S. Sigman Chair, Department of Chemistry _______________________________ Dr. Thomas G. Richmond Honors Faculty Advisor _____________________________ Sylvia D. Torti, PhD Dean, Honors College May 2022 Copyright © 2022 All Rights Reserved ABSTRACT Perovskites have been receiving a lot of attention as they can be tailored to specific semiconducting properties and lead to various functionality and optoelectrical applications. At the same time, chemical vapor deposition (CVD) is a system used as a deposition process for making perovskites due to the widespread control of its internal environment. The purpose of this study is to investigate the process of making perovskites with CVD and how the morphology, crystallite orientation, and temperature changes in the perovskite structures affect the current behavior observed in devices while under a range of voltage bias stress in both light and dark conditions. Lead-iodide films were made by spin coating a lead-iodide solution (463 mg/mL) on glass and using a powdered methyl-ammonium iodide as a reactant deposited on the glass in the CVD to create perovskites. The experiment was conducted with temperatures from 120 to 140 degrees Celsius; nitrogen levels at flow rates ranging from 100 to 150 mL of nitrogen; reaction times were sampled from two to four hours. Films were collected from the CVD when the deposition was finalized. The films were then analyzed with X-ray diffraction (XRD) to understand the perovskite morphology and improve the parameters for creating the perovskite. The results depict specific temperatures and reaction times for the CVD processes to create a full widespread perovskite made from the reaction between methylammonium iodide and lead iodide. The study produced parameters that consistently resulted in fully reactant thin films, leading to future projects and findings that can make better and more efficient films. The findings suggest that other approaches still need to be ii taken into consideration before getting a complete understanding of the morphology, crystallite orientation, and the electrical behavior of the thin films. Further surface analysis can also lead to more detailed findings on other techniques to produce bettermade films of different reactants. iii TABLE OF CONTENTS ABSTRACT ii INTRODUCTION 1 METHODS 3 RESULTS AND DISCUSSION 4 CONCLUSION 9 REFERENCES 10 iv 1 INTRODUCTION Inorganic-organic halide perovskites (OIHPs) have received an impressive amount of attention across many fields of research due to their tailorable semiconducting properties that create versatile functionality for a variety of optoelectronic applications such as solar cells,1 light-emitting diodes,2 photodetectors,3 resistive memory devices,4 magnetics,5 transistors,6 and lasing devices.7 However, regardless of the application, a common concern among the research on OIHPs is their lack of stability.8, 9 The power conversion efficiencies of organic-metal halide perovskite solar cells have been improved over the last decade using a wide variety of methods, such as composition manipulation, dopant introduction, and interfacial buffers. These methods, however, have taken little regard for the electronic and interfacial effects such alterations may cause within devices under voltage and current bias stresses -which are conditions required for most device operation. Several approaches have been proven successful to improve their stability, such as cross-linking additives, encapsulation with water-proof fluorinated polymers, or more simple compositional engineering of the OIHP with cation cascade techniques or partial halide ion substitution.1014 The issue remains, however, that even with a resolution that prevents OIHP degradation to external factors, these materials are still prone to experience internal current and voltage instabilities while under operation. Therefore, an important aspect that needs to be more carefully analyzed is the secondary effects these instabilities may cause on device performance. As slight compositional manipulations are commonly made to OIHPs to improve their performances, it becomes difficult to track any trends observed in these 2 materials, making it easy to draw bombastic conclusions about their power conversion efficiencies (PCEs) when incorporated into solar cell devices. For example, when focusing on OIHP solar cell devices, it is often challenging to understand the effects that variations in current and voltage under operating conditions have on the internal mechanisms occurring in the active layer, let alone their interactions with the corresponding charge transport materials. This is supported by the significant discrepancies observed on the recorded PCEs of OIHP solar cell devices that only have slight differences in composition.9, 15, 16 These differences in PCEs are not mechanistically well understood and still require much investigation. This paper will introduce the production of thin films that are prepared through chemical vapor deposition (CVD). The parameters of the formation of thin films from lead-iodide and methyl-ammonium iodide solutions will explore the use of CVD for consistent and effective production. The paper will also attempt to determine how interfacial recombination, morphological defects, ion migration, and charge transport are influenced under bias stress conditions with CVD made thin films. 3 METHODS Synthesis of PbI2 thin films A PbI2 thin film was constructed with a PbI2 and N, N dimethylformamide (DMF) solution. 462 mg of PbI2 was transferred to a scintillation vial, and 1 mL of DMF was mixed until PbI2 was fully dissolved. The mixture was done in a glove box and brought outside after the vial was removed of any air through the pump-purge process to be spincoated onto cleaned FTO glass slides. Glass FTO slides were plasma cleaned after being submerged in a soap solution, water solution, acetone, then finally cleaned in isopropanol. The glass slide was then dried and brought to be spin-coated with the PbI2 solution at various speeds as spin coat parameters were adjusted to determine the impact of the rate and time of the spin coater on the distribution and production of the thin film. The second coating of methylammonium iodide (MAI) was deposited onto the thin film through CVD. The CVD machine has various controllable factors of temperature, time, and nitrogen levels. With each of the controlled elements in the CVD, films were taken out of the machine once the second coat of MAI was deposited. The glass slides are taken out of the nitrogen-filled chamber of the CVD for structural analysis with the X-ray Diffractometer (XRD). The thin films were also taken to analyze voltage bias with a bias stress experiment conducted at pulsed voltages of 0.8V. 4 RESULTS AND DISCUSSION In this study, the various parameters and techniques are compared to distinguish the optimal parameters that will continuously produce thin films that can create a large amount of electrical activity and are distributed evenly on the entire glass slide. Some of the first tested parameters were made on changes in the duration of time that the reaction was left running and the varying times and speeds of the spin coater when coating the first layer of PbI2. Figure 1A is the parameter result, which led to no production of either the PbI2 or the second coat of MAI done in the CVD. The degradation of the PbI2 films led to a single rounded peak, and the color observed was a very light dark brown color with soft brown spots on the glass slide. A B Figure 1. A) XRD spectrum for the FTO glass slide without any coating present on the glass. With the parameters of no nitrogen gas, reaction temperature of 80°C, and reaction time of 40 minutes. B) XRD spectrum of one of the earlier experimental parameters of no nitrogen gas, reaction temperature of 110°C and reaction time of 80 min revealed that the first coating of PbI2 that was spin-coated onto the FTO glass slide. 5 Figure 1B contained a yellow coat film with brown spots, and the reaction parameter produced an uneven layer of PbI2. The resulting CVD parameters are likely from little to no amounts of MAI that reacted with PbI2 as the final product had close similarities to the yellow PbI2 film that was first placed into the CVD tube. Since the earlier parameters did not produce a perovskite from the previous experiments, the XRD was likely from the PbI2 coating. A B C D Figure 2: A) XRD parameters with nitrogen gas, reaction temperature of 120°C, and reaction time of 60 minutes, creating a production of a PbI2 x MAI perovskite film but likely contains other substances. B) XRD parameters with nitrogen gas, the temperature of 120°C and time of 80 minutes had a production of a PbI2 x MAI perovskite film but contained many other substances. C) XRD with the parameters of nitrogen gas, 110°C 6 and 60 min had no perovskite and only detecting PbI2 coat of the thin film. D) XRD with the parameters of nitrogen gas, 110°C and 60 min had no perovskite and only detecting PbI2 coat of the thin film. From the results seen in Figure 1, an adjustment was made by trying to increase the reaction time, so the time parameter was extended to 60 minutes. The adjustment began to produce a small amount of PbI2 x MAI perovskite, as shown in Figure 2A. The final product measured in the XRD had a dark brown coating that is the formation of the perovskite and can be seen by the peak at 27.65 and some smaller peaks at 60 and 77.65. However, when comparing Figure 1A to Figure 2A, the rounded shape of the FTO glass slide is seen. The figure shows that the reaction did not fully cover the entire glass, and this might have been from the distribution of the PbI2 coat, so another adjustment was made to the spin coater for a more even distribution. When the spin coating was changed to 4000 rpm for 30 sec the thin film was run under the conditions for the CVD was slightly modified to 110°C, but the XRD presents that the PbI2 reaction with MAI did not occur, leaving only a coat of PbI2 being detected in Figure 2C. The distribution was run with the speed of 2000 rpm for 30 seconds on the spin coater, and the results in Figure 2D produced similar results to Figure 2C, where only the production of PbI2 is present. The parameters for Figure 2A were maintained, but the reaction time was increased to 80 minutes resulting in a problem of uneven distribution. The change showed that a difference in the spin coat parameters might be able to adjust the amount of perovskite made. 7 Figure 3: XRD spectrum of a pure PbI2 x MAI perovskite thin film made under the parameters of nitrogen gas, 140°C, and for 100 min. After a few more data collection on the production of thin films from the changes in temperature, time, and volume of gas with various temperature parameters of 100°C, 110°C, 120°C, 140°C, and time parameters of 60, 80, 100, and 200 minutes. Several corrections in the parameters were made, finally, with the parameter of being under nitrogen gas, at 140°C and 100 min displayed in Figure 3, a clear distinction of a perovskite is being produced without the rounded curve of the FTO glass as seen in Figure 2A. The two peaks are very distinct for the perovskite, resulting in a dark grey perovskite evenly distributed on the glass slide. The experiment was rerun with the same 8 parameters, and the film results maintained the same XRD results depicting that the parameters are reproducible in forming a distinct perovskite. Figure 4: Voltage Bias Stress Experiments are conducted by measuring the current as a voltage of 0.8 V is pulsed on and off the cell. The stress experiments were done for 300 seconds, and the voltage was applied three times across the cell. The perovskites made with the parameters were seen in Figure 3 were placed under a voltage bias stress experiment. As 0.8 V was applied on and off, the solar cell showed that the current would gradually decrease from the peak of 1 mA after several seconds. This indicates that as a voltage is being constantly applied, there is likely a stress factor occurring, which leads to a degradation in the electrical properties of the cell. The pattern holds as in Figure 4 the voltage was applied on and off three times which is displayed with the same general structure of the decrease in electrical properties. 9 CONCLUSION Overall, this project produced a finalized parameter to create a perovskite with a CVD. However, some questions remain unanswered about the stability and electrical properties of these CVD made perovskite. While there are still some preliminary data on the bias stress of the perovskite, some data suggests electrical stress. Research on this topic of the production of perovskites through CVD production can continue to enhance the knowledge on the fabrication of efficient and long-lasting solar cells. As our reliance on energy increases, the demand will likely continue to grow. Solar energy, such as solar panels, has become of significant interest to the public, and there have been incentives to push for the use of solar panels. 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