Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells
March 19th, 2017
Here, we present a protocol to adjust the properties of solution-processed CH3NH3PbI3 through the incorporation of monovalent cation additives in order to achieve highly efficient perovskite solar cells.
The overall goal of this procedure is to enhance the optoelectronic properties of lead halide perovskite through monovalent cation doping to fabric highly efficient perovskite solar cells. Incorporation of monovalent cations into lead halide perovskite significantly improves the semiconductor quality and photo effect behavior of this material. Adding a rational amount of low cost monovalent cation dopants to perovskite materials enhances charge mobility and reduces energetic disorder by an order of magnitude.
To begin comparing the substrate, use semi-transparent acrylic adhesive tape to cover 2/3 of the conductive side of an FTO coated glass slide. Then coat the uncovered areas with zinc powder. Pour a solution of 2 molar hydrochloric acid in distilled water over the slide to etch the substrate.
Use a cotton swab to wipe the residue from the exposed sections of the slide. Rinse the etched FTO substrate with distilled water and then remove the tape. Wash the etched surface in a 2%weight by volume solution of alkaline liquid detergent concentrate and water.
Sonicate the FTO substrate for 10 minutes each in acetone and isopropanol baths in sequence. Treat the FTO substrate in an oxygen plasma cleaner for 15 minutes to finish cleaning the etched substrate. To deposit the whole blocking compact titanium oxide layer, first place the etched and cleaned FTO substrate on hot plate at 450 degrees Celsius.
Immediately cover the contact area with a precut glass slide and allow the substrate to heat to 450 degrees Celsius. Mix 0.6 milliliters of TAA with 7 milliliters of isopropanol. With the substrate is at 450 degrees Celsius, apply the TAA solution to the substrate by spray pyrolysis using air as the carrier gas.
Allow the sample to remain at 450 degrees Celsius for 30 minutes. Then allow the sample to cool to room temperature. Remove the glass cover once the substrate has cooled.
To deposit the electron transport layer, first dilute 30 nanomolar titanium oxide paste with ethanol to a 2:7 weight ratio. Sonicate the suspension for 30 minutes. Then spin coat the sample with the suspension for 30 seconds at 5, 000 RPM with a ramp rate of 2, 000 RPM per second.
Annihile the titanium film at 500 degrees Celsius for 30 minutes to yield a mesoporous titanium oxide film. Then immerse the sample in a 40 millimolar solution of titanium chloride and distilled water. Treat the samples at 70 degrees Celsius for 20 minutes.
Annihile the titanium chloride treated sample at 450 degrees Celsius for another 30 minutes. Transfer the sample to a nitrogen-filled glove box with a humidity of less than 1%In a low-water, low-oxygen, inert atmosphere, add 1 milliliters of DMF to 553 milligrams of lead iodide. Heat the mixture to 80 degrees Celsius while stirring continuously until the lead iodide has dissolved.
Then prepare a 0.02 molar solution of the chosen monovalent cation halide in the lead iodide solution. Apply 80 microliters of the monovalent cation halide in lead iodide solution to the substrate. Spin coat the substrate for 30 seconds at 6, 500 RPM with a ramp rate of 4, 000 RPM per second.
Bake the resulting thin film at 80 degrees Celsius for 30 minutes. Next, dissolve 40 milligrams of methylammonium iodide in 5 milliliters of isopropanol. Place the lead iodide coated substrate in a spin coater and apply 120 microliters of the MAI solution.
Allow the solution to sit on the substrate for 45 seconds and then spin coat the substrate with MAI for 20 seconds at 4, 000 RPM. Annihile the perovskite film at 100 degrees Celsius for 45 minutes. To deposit the whole transport layer, first add 1 milliliter of chlorobenzene to 72.3 milligrams of spiro-methoxyTAD.
Shake the mixture until the solution appears transparent. Prepare a stock solution of 520 milligrams of lithium TFSI in 1 milliliter of acetonitrile. Then mix 17.5 microliters of the lithium TFSI solution in 28.8 microliters of 4-tert-butylpyridine with the spiro-methoxyTAD solution.
Spin coat the sample with this mixture for 30 seconds at 4, 000 RPM with a ramp rate of 2, 000 RPM per second. Cover the sample with a mask pattern for the top contacts of the solar cell. Deposit an 80 nanometer layer of gold onto the sample at a rate of 0.01 nanometers per second to finish fabricating the solar cell.
Thin films of pristine and additive-based perovskites were compared to the corresponding lead iodide films. When sodium iodide was used as an additive, large branch-shaped crystals of lead iodide were observed by FESEM along with large, asymmetric perovskite crystals. When cooper iodide and silver iodide were used as additives, uniform, pinhole-free perovskite layers were observed.
X-ray diffraction showed that the cation doping had no effect on the perovskite crystal structure. No peak corresponding to unconverted lead iodide was observed when sodium iodide or copper bromide was used as an additive. Kelvin probe force microscopy showed that the doped perovskite fermi levels shifted toward the valence band.
The doped samples showed increased conductivity in electron and whole mobilities, particularly for the sodium iodide and copper bromide samples. Increases were also observed for the short circuit current in the fill factor. The greater short circuit current increase in copper bromide and sodium iodide based cells was attributed to the full conversion of lead iodide and improved charge mobility.
Improvements in open circuit voltage were observed for copper iodide and silver iodide based solar cells, likely because of their uniform pinhole-free surfaces. The power conversion efficiencies of sodium iodide, copper bromide, and copper iodide based solar cells were all notably higher than that of a pristine perovskite cell. We have demonstrated a facile doping method that is compatible with the solution processing of methylammonium lead halide perovskite as an absorber layer in the mesoscopic perovskite solar cell structure.
The optoelectronic and structural properties of lead halide perovskite materials can be tuned by control amounts of monovalent cation dopants, which can lead to superior photovoltaic performance. Our method paved the way for researchers in the field of photovoltaics to explore this technique in other configurations of perovskite solar cells to further improve the electronic quality of perovskite thin films.
Overview
This article presents a protocol for enhancing the optoelectronic properties of lead halide perovskite solar cells through monovalent cation doping. The incorporation of these additives significantly improves semiconductor quality and charge mobility.
Key Study Components
Area of Science
- Materials Science
- Photovoltaics
- Optoelectronics
Background
- Lead halide perovskites are promising materials for solar cells.
- Monovalent cations can enhance the properties of these materials.
- Improving charge mobility is crucial for solar cell efficiency.
- Solution processing techniques are commonly used in fabricating perovskite layers.
Purpose of Study
- To develop a method for doping lead halide perovskite with monovalent cations.
- To assess the impact of doping on the optoelectronic properties of perovskite solar cells.
- To compare the performance of doped and undoped perovskite films.
Methods Used
- Preparation of FTO substrates through etching and cleaning.
- Application of titanium oxide layers via spray pyrolysis and spin coating.
- Doping of lead iodide with monovalent cation halides.
- Characterization of the resulting films using various microscopy and diffraction techniques.
Main Results
- Doped perovskite films exhibited improved charge mobility and conductivity.
- Enhanced short circuit current and fill factor were observed in doped cells.
- No adverse effects on the crystal structure were noted due to doping.
- Power conversion efficiencies of doped cells surpassed those of pristine cells.
Conclusions
- The doping method is effective and compatible with solution processing.
- Monovalent cation additives significantly enhance the performance of perovskite solar cells.
- This study provides a pathway for future improvements in solar cell technology.
Frequently Asked Questions
Executive Industry Relevance
Monovalent cation doping of perovskite materials addresses key challenges in optoelectronic material quality, including charge mobility and energetic disorder, which directly impact photovoltaic efficiency. This approach enables predictive confidence in material performance by reducing defect density and enhancing charge transport, supporting downstream device optimization. The method provides a scalable, solution-compatible strategy for tuning electronic properties in perovskite-based systems, relevant to early-stage material screening and lead identification in photovoltaic R&D pipelines.
Strategic Applications in Biopharma R&D
Early Discovery & Target Validation
- Scientific Value: Enables interrogation of dopant effects on charge carrier dynamics and defect passivation in semiconductor materials.
- Operational Value: Supports rapid screening of cation additives to identify those that improve optoelectronic functionality.
Screening & Assay Development
- Scientific Value: Facilitates preparation of doped perovskite films with standardized optoelectronic outputs for comparative analysis.
- Operational Value: Enhances reproducibility of charge mobility and conductivity measurements across additive variants.
Translational & Preclinical Research
- Scientific Value: Demonstrates continuity from material doping to functional device performance, supporting risk-adjusted advancement decisions.
- Operational Value: Provides quantitative benchmarks (e.g., open circuit voltage, short circuit current) for evaluating translational potential.
Pipeline & Workflow Integration
The doping method integrates into the discovery continuum by enabling early material optimization that informs lead identification and preclinical device validation in photovoltaic research.
- Discovery Biology: Supports hypothesis testing on how monovalent cations influence defect density and charge transport in perovskite lattices.
- Screening: Enables assay-ready preparation of doped films with improved hole mobility and reduced sub-bandgap absorption.
- Analytics: Generates quantitative outputs such as Fermi level shifts, conductivity enhancements, and photovoltaic parameter improvements for comparative evaluation.
- Translational Research: Links improved bulk properties to enhanced device metrics like open circuit voltage and photocurrent density, supporting preclinical continuity.
- Enterprise Reuse: Establishes a reusable doping protocol applicable across various perovskite compositions and device architectures.
Operational & Enterprise Impact
- Scientific Value: Predictive confidence in material quality through reduction of energetic disorder and enhancement of bulk hole mobility.
- Operational Value: Standardization of solution processing and additive incorporation for reproducible optoelectronic tuning.
- Strategic Value: Informs go/no-go decisions on material candidates based on measurable improvements in voltage and current output.
- Portfolio Impact: Enables risk-adjusted prioritization of doped perovskite formulations demonstrating superior power conversion efficiency.
Implementation Considerations
- Expertise in semiconductor materials, solution processing, and thin-film deposition techniques.
- Instrumentation for spin coating, thermal annealing, and optoelectronic characterization (e.g., FESEM, XRD, KPFM).
- Standardization of additive concentration and substrate preparation across experimental batches.
- Adaptation considerations for different monovalent cation halides and perovskite compositions.
- Practical limitations include ensuring uniform film formation and controlling crystallization kinetics during additive incorporation.
