Nano-Micro Letters Qi Chen & Yu Chen improve the performance and lifetime PCE of perovskite solar cells by improving the gel performance of hole transport (22.52%)
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In a recent publication in the Nano-Micro Letters journal on July 10, 2023, researchers from the School of Materials Science and Engineering at Beijing Institute of Technology, led by Professor Qi Chen and Professor Yu Chen, presented a study on improving the stability of perovskite solar cells. The research focused on the gelation of the hole transport layer to enhance the performance and longevity of these solar cells.
This study presents a novel method for enhancing the performance and stability of perovskite solar cells (PSCs) by modifying the spiro-OMeTAD hole transport layer (HTL) with terephthalic acid (TA) to form a gel-like structure. The addition of TA to spiro-OMeTAD results in the formation of a yellow transparent gel-like polymer network known as poly(TA). The gelation of HTL effectively improves the compactness of the resultant HTL and prevents moisture and oxygen infiltration. In addition, TA passivates the perovskite defects and facilitates the charge transfer from the perovskite layer to HTL. The optimized PSCs based on the gelated HTL, fabricated by the research team, exhibit an improved power conversion efficiency (PCE) of over 22.52% with excellent device stability. The gelated HTL also prevents the aggregation of LiTFSI salt and maintains a high conductivity under humid conditions. The PSCs with gelated HTL, developed by the research team, maintain 85% of their initial PCE after 1000 hours of continuous illumination at 25°C and 92% of their initial PCE after 2500 hours in ambient air at 25°C. The gelated HTL strategy was also applied to PTAA, and similar improvements in humidity stability were observed. The findings, obtained by the research team, offer a simple and promising strategy to improve the spiro-OMeTAD-based HTL for highly efficient and stable PSCs.
the hole-transporting layer (HTL).
The HTL is a thin film that facilitates the extraction of positive charges (holes) from the perovskite layer to the electrode. The most commonly used HTL material is spiro-OMeTAD, which has excellent hole mobility and compatibility with perovskite materials. However, spiro-OMeTAD also has some drawbacks, such as its poor conductivity in its pristine state and its sensitivity to moisture. To overcome these issues, spiro-OMeTAD is usually doped with lithium salts, such as LiTFSI, which can increase its conductivity and lower its energy level.
However, doping with lithium salts also introduces new problems, such as the degradation of the HTL and the perovskite layer due to the hygroscopic nature of LiTFSI, and the formation of J-V hysteresis due to the migration of Li+ ions. Therefore, the research team has been exploring various strategies to improve the performance and stability of the HTL, such as developing new HTL materials, using alternative dopants, and optimizing the doping methods. In this post, the research team will review some of the recent advances in this field and discuss their advantages and limitations.
The experiments in this article employed materials that were obtained commercially and used as received, such as cesium iodide (CsI, 99.9%, Sigma-Aldrich), lead iodide (PbI2, Xi’an Polymer Light Technology), methylammonium chloride (MACl, Xi’an Polymer Light Technology), and materials for charge transporting layers (SnO2 (15 wt% colloidal dispersion, Alfa), 2,2’,7,7’-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9’-spirobifluorene (spiro-OMeTAD, Xi’an Polymer Light Technology), bis(trifluoromethane)sulfonimide lithium salt (LiTFSI, 99.95%, Sigma-Aldrich), thioctic acid (TA, 99%, Sigma-Aldrich)). The solvents used were Chlorobenzene (CB, Sigma-Aldrich, 99.9%), N,N-dimethylformamide (DMF, 99.99%, Sigma-Aldrich), dimethylsulfoxide (DMSO, 99.5%, Sigma-Aldrich), isopropanol (99.99%, Sigma-Aldrich), acetonitrile (ACN, 99.95%, Sigma-Aldrich), and tBP (99.9%, Sigma-Aldrich)). Additionally, formamidinium iodide (FAI, Dyesol) was further purified after purchasing.
The research team cleaned the ITO substrate with ultrapure water, acetone, and ethanol in an ultrasonic system for 30 minutes each. Then, it was dried with N2 gas and treated with UV-O3 for 30 minutes to improve its wettability. A compact SnO2 layer was spin-coated on the substrate at 4000 rpm for 30 seconds and annealed at 150 °C for 30 minutes. Prior to perovskite film deposition, the substrates were exposed to UV light for 10 minutes.
For the PbI2 precursor, the research team dissolved PbI2 and CsI in a mixture solvent of DMF:DMSO and stirred at 70 °C for 5 hours. The organic cation precursors were prepared by dissolving FAI and MACl in isopropanol. Both solutions were filtered with a 0.22 μm PTFE filter. The perovskite films were fabricated using a two-step process: spin-coating the PbI2 precursor followed by the organic cation precursor. After annealing at 150 °C for 10 minutes, a hole transport layer (HTL) was spin-coated on the perovskite film.
Two types of HTL precursors were used. For the Reference HTL, a solution of spiro-OMeTAD, TBP, and LiTFSI in CB was used. For the Target HTL, TA was added to the Reference HTL solution. After overnight oxidation, a 100-nm-thick Au film was deposited as the back contact. The device area was defined as 0.0805 cm2 using a metal shadow mask.
The research team conducted rheological measurements of poly(TA) using an Anton Paar instrument (Physica MCR 301, Germany) with a parallel-plate geometry. Strain sweep measurements were conducted at 25 °C with an angular strain range from 0.1 to 2500% and a frequency of 0.5 Hz. Temperature sweep measurements were performed from 25 to 100 °C at a strain of 1% and frequency of 0.5 Hz. Fourier-transform infrared spectroscopy (FTIR) was conducted using a Magna-IR 750 (Nicolet, USA). 1H NMR spectra were obtained using a Bruker AVANCE III 300 MHz NMR Spectrometer. XPS data were collected using an Axis Ultra XPS spectrometer (Kratos, U.K.) with Al Kα radiation. SEM imaging was done with a Hitachi Regulus 8230. Nano-FTIR experiments were carried out using a Bruker Dimension Icon IR with a PRUM-TNIR-D-10 probe. ToF–SIMS measurement utilized a PHI NanoTOF II instrument (ULVAC-PHI, Inc.) with a 30 keV Bi+ pulsed primary ion beam. UV–vis absorption spectra were acquired using a UV–visible diffuse reflectance spectrophotometer (UV–vis DRS, Japan Hitachi UH4150). Laser scanning confocal microscopy (Enlitech, SPCM-1000) with a 470 nm pulse laser and galvo-based scanner was used for 2D PL mapping. XRD data were obtained using a Bruker D8 Advanced with Cu Kα radiation. Steady-state PL and TRPL were measured using FLS1000 (Edinburgh Instruments Ltd) with a 450 W Xe lamp. The photovoltaic performance of the PSCs was evaluated with a source meter (Keithley 2400) under AM1.5G illumination from a 1000 W m−2 solar simulator (SS-F5-3A, Enlitech). J-V scans were conducted in both reverse and forward directions at a scanning speed of 50 mV s−1. EQE curves were recorded using an Enli Technology (Taiwan) EQE measurement system. A calibrated silicon diode served as a reference for the EQE measurement.
Results and Discussion
Gelation of Hole Transport Layer
TA, a naturally occurring small molecule, possesses hydrophobic 1,2-dithiolane and alkyl chain groups, as well as hydrophilic carboxylic acid groups. The unique structure of TA, which includes dynamic covalent disulfide bonds and noncovalent hydrogen bonds, makes it a potential crosslinker for forming a robust continuous network. When TA is dissolved in chlorobenzene and LiTFSI is added, it undergoes gelation, forming a yellow transparent gel-like polymer network known as poly(TA).
Rheological measurements were conducted by the research team to study the gelation behavior. A strain sweep test revealed a gel to sol transition at an oscillatory strain amplitude of approximately 340%. The gel network remained stable below this critical strain, but failure occurred at the crossover point of storage modulus (G′) and loss modulus (G′′) around 340%. The gel exhibited a reversible solid-to-liquid transition above 50 °C, as observed by rheological analysis. This supramolecular polymer showed dynamic reversibility, transforming into a viscous polymer solution when subjected to temperature increase or dilution by water. The gel’s transition temperature could be enhanced by increasing the concentration of the monomer solution or incorporating metal ions, such as Fe3+, Pb2+, Zn2+, and Ca2+.
FTIR analysis confirmed strong interaction between TA and LiTFSI, resulting in the formation of a crosslinked structure. The addition of TA facilitated the gel formation in the hole transport layer (HTL) precursor solution. Solvents like formic acid or ethanol could dissolve the gel, enabling the research team to prepare HTL films on perovskite by spin coating. The gelated HTL with TA exhibited improved film morphology compared to the reference HTL. SEM and AFM analysis showed a uniform and dense surface in the gelated HTL film, indicating the role of TA in enhancing film quality. The spatial distribution of TA in the gelated HTL film was confirmed by AFM-IR.
a Schematic representation of crosslinking polymerization of TA. b The pictures of the polymerization of the TA. c Storage modulus (G′) and loss modulus (G′′) for poly(TA) gels on strain sweep. d FTIR spectra of TA (red), mixture of LiTFSI and TA (blue), LiTFSI (yellow). e Scanning electron microscopy (SEM) images of spiro-OMeTAD and spiro-OMeTAD doped with TA films. f AFM images of Target film and g corresponding Nano-FTIR images. Nano-FTIR at an IR frequency of 1693 cm–1 (which is resonant with the C = O stretching absorption of TA)
Improved Humidity Stability
ToF-SIMS mapping was used by the research team to assess the compositional distribution in the gelated HTL film by adding TA. It was observed that after exposure to high humidity conditions, the Reference film exhibited obvious LiTFSI aggregation on its surface, while the Target film with gelated HTL showed mitigated LiTFSI aggregation. This indicates that the gelated HTL is more robust under high humidity conditions. The interactions between TA and LiTFSI were found to retard Li aggregation. AFM-IR and depth profile ToF-SIMS measurements further confirmed the effectiveness of gelation in preventing LiTFSI aggregation and migration.
The effect of the gelated HTL strategy on the humidity stability of the perovskite film was also investigated. Perovskite films coated with HTL were aged in humid air, and UV-vis absorption spectra were monitored. The Reference film showed a sharp decrease in absorbance after exposure to humid air, while the Target film exhibited negligible change. XRD measurements confirmed that the Reference film degraded into PbI2 and the photoinactive δ-phase, while the Target film displayed a retarded α-to-δ phase transition. PL mapping of the aged films revealed that the Target films had a narrower wavelength range, indicating better stability compared to the Reference films.
The gelated HTL strategy was also applied to PTAA, and similar improvements in humidity stability were observed. Contact-angle measurements demonstrated that the gelated HTL film exhibited reduced hygroscopicity compared to the reference film. These findings indicate that the humidity stability of perovskite films covered by the gelated HTL is significantly improved.
a 2D ToF–SIMS elemental mapping of Li+ of the a Reference film and b Target film before and after aging under high RH of 85–90% at 25 °C for 200 h. c UV–vis absorption spectra for Reference and Target perovskite films at 700–850 nm over time. d The normalized absorption at 750 nm of Reference film and Target film. e PL peak position mapping and statistical diagram of Reference. f Target films before and after aging under high RH of 85–90% at 25 °C for 500 h
Improved Performance and Stability of Device:
The research team investigated the effect of the gelated hole transport layer (HTL) on the photoelectric performance and stability of the device. N-i-p type planar solar cells with the architecture of ITO/SnO2/perovskite/spiro-OMeTAD(TA)/Au were fabricated to evaluate the photovoltaic performance. The Target devices, utilizing the gelated HTL developed by the research team, exhibited higher average power conversion efficiency (PCE) of 20.22% compared to the Reference devices with 18.11%. They also showed improved reproducibility and compactness of the HTL film. The best Target device achieved a PCE of 22.52% with higher values of VOC, JSC, and FF compared to the Reference devices. The stability of the Target devices, developed by the research team, was significantly enhanced, with 92% retention of initial PCE after 2500 hours of exposure to ambient atmosphere (RH of ≈30–60%). In contrast, the Reference devices retained only 60% after 1000 hours. The unencapsulated Target devices also exhibited promising stability under high RH (85-90%), with 85% retention after 1000 hours compared to 75% for the Reference devices after 530 hours. Additionally, the Target devices showed over 85% retention of initial PCE after 1000 hours of continuous LED illumination, while the Reference devices only retained about 40%. These results confirm that the gelated HTL strategy significantly improves the long-term stability of the solar cells.
Performance of OAI-modified PSCs and mini-module. a J–V curves. b Stabilized power output measured at the maximum power point (MPP). c Long-term stability measurements of OAI-modified device without encapsulation under ambient conditions with about 30% relative humidity. d J–V curves of 1.03 cm2 PSCs and 10.93 cm2 mini-module. Insets are the pictures of 1.03 cm2 PSCs and 10.93 cm2 mini-module
a Structure of PSC and the interface between perovskite and gelated HTL. b The statistical distributions of PCE of the Reference and Target devices. c J-V curves for the best performing target device with aperture areas of 0.0805 cm2. d EQE curve and its integrated JSC curve of Reference and Target device. e The corresponding stabilized power output data at bias voltages (1.00 V) near the maximum power point. Normalized PCE evolution of the Reference and Target devices under f ≈30–50% RH, g 85–90% RH and h continuous illumination at MPP condition
Improved Photovoltaic Performance:
To understand the reasons behind the enhanced efficiency and stability in the gelated hole transport layer (HTL) devices, the research team investigated the electrical conductivity of spiro-OMeTAD and gelated HTL films. The presence of TA in the gelated HTL significantly increased the electrical conductivity compared to pristine spiro-OMeTAD. This enhancement is attributed to the strong electronegativity of the S atom in TA, which promotes oxidation of spiro-OMeTAD. Steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectroscopy revealed that the gelated HTL facilitated the transfer and extraction of photogenerated holes at the perovskite/spiro-OMeTAD interface. Voc dependence on light intensity measurement showed improved ideality factors for the Target devices. Space charge limited current (SCLC) measurement demonstrated enhanced carrier mobility in the gelated HTL. X-ray photoelectron spectroscopy and FTIR measurements indicated a strong interaction between TA and the perovskite, leading to the passivation of under-coordinated Pb2+ defects. The gelated HTL strategy also reduced surface defects in the perovskite films, as evidenced by longer PL lifetime and suppressed nonradiative recombination. Overall, the gelated HTL improved carrier transport and reduced defects, resulting in enhanced photovoltaic performance.
a I–V curves of ITO/spiro-OMeTAD/Au and ITO/spiro-OMeTAD doped with TA/Au resistance devices. b PL curves of Reference and Target perovskite films with HTL. c TRPL decay curves of Reference and Target perovskite films with HTL. Note that the TRPL and PL for samples with HTL were measured at a short circuit. d XPS spectra of Pb 4f of the perovskite and perovskite/TA films. e FTIR spectra of TA and TA with PbI2 powders. f TRPL decay curves of Reference and Target perovskite films with HTL. Note that the TRPL for samples with HTL was measured at an open circuit
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