Fig 1 Schematic illustration of simplified PSC structure by replacing conventional PCBM/BCP with a new polyfullerene ETL named PFBCP.
Published:01 August 2024,
Published Online:26 June 2024,
Received:12 March 2024,
Revised:24 April 2024,
Accepted:04 May 2024
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In prevailing p-i-n perovskite solar cells (PSCs), solution-processible fullerene molecules are widely used as electron-transporting layers (ETLs) but they typically suffer from poor uniformity and undesirable stability issues. Additionally, a separate bathocuproine (BCP) layer is needed to block hole transfer, increasing fabrication complexity and cost. Here, we address these limitations by developing a novel polymeric ETL (named PFBCP) synthesized by polymerizing C60 with BCP. This innovative material achieves both efficient electron transport and hole blocking, while its excellent uniformity minimizes interface recombination and enhances stability. Consequently, our blade-coated PSCs utilizing PFBCP achieve a high power conversion efficiency exceeding 22% and retain 91% of initial efficiency after 1200 h of light exposure. This development not only paves the way for commercially viable PSCs but also opens avenues for future ETL design to realize even more efficient and stable PSCs.
Polyfullerene;
Perovskite solar cells;
Electron transporter;
Stability
Inverted or p-i-n perovskite solar cells (PSCs) are more promising for commercialization due to their low cost, low-temperature processibility, better stability, and compatibility with silicon photovoltaics for tandem cells compared with their n-i-p counterparts.[
In 2022, our group reported the first polyfullerene-type ETL for p-i-n PSCs that exhibits not only excellent electron-transporting capacities but also good stability, and realizes high-performance p-i-n PSCs and perovskite mini-modules.[
Fig 1 Schematic illustration of simplified PSC structure by replacing conventional PCBM/BCP with a new polyfullerene ETL named PFBCP.
PTAA (average Mn 7×103−1×104), BCP, lead iodide (PbI2, 99.999% trace metals), dimethyl sulfoxide (DMSO), L-α-phosphatidylcholine (LP), benzylhydrazine hydrochloride, 2-methoxyethanol (2-ME), and toluene were purchased from Sigma-Aldrich and used without further purification. C60 and PCBM were purchased from Nanjing Zhiyan Inc. Methylammonium iodide (MAI), formamidinium iodide (FAI), 4-fluoro-phenyethylammonium iodide (p-F-PEAI) and n-dodecylammonium iodide were purchased from GreatCell Solar. All chemicals without any notes were commercially available and used without further purification.
Fig 1 Synthetic route to PFBCP.
Compound 3
In a 100 mL round bottom flask, compound 1 (2.87 g, 3 equiv.), compound 2 (0.5 g, 1 equiv.), K2CO3 (1.25 g, 5 equiv.), and tetrakis(triphenylphosphine)palladium(0) (208 mg, 10 mol %) were added into THF:H2O (10 mL:2 mL, 5:1), and the mixture was heated at 80 °C for 24 h under nitrogen. After completion of the reaction, the resulting suspension was cooled to room temperature, and extracted in brine and DCM. The organic fraction was evaporated under reduced pressure. The residue was directly purified by silica gel chromatography (DCM:MeOH=50:1, V:V), and the product is a colorless viscous liquid, giving the final compound 3 with 60% (1.0 g) yield. 1H-NMR (400 MHz, CDCl3, δ, ppm): 7.77 (s, 1H), 7.49–7.41 (m, 2H), 7.08 (ddd, J=7.5, 3.5, 1.5 Hz, 1H), 7.00 (d, J=1.6 Hz, 1H), 4.80 (d, J=5.5 Hz, 2H), 3.94 (dd, J=8.7, 5.3 Hz, 2H), 2.99 (d, J=7.4 Hz, 3H), 1.86–1.78 (m, 1H), 1.49–1.21 (m, 35H), 0.91–0.83 (m, 6H). 13C-NMR (101 MHz, CDCl3, δ, ppm): 159.31, 158.78, 157.11, 148.48, 145.96, 138.97, 129.34, 128.49, 124.70, 124.31, 123.87, 122.99, 121.81, 120.86, 112.25, 70.97, 61.98, 53.42, 50.87, 38.08, 31.90, 31.87, 31.58, 29.99, 29.67, 29.61, 29.55, 29.33, 26.92, 25.97, 25.78, 22.68, 14.10. MALDI-TOF-MS (SA matrix) m/z: [M]+ calcd. for C64H105N2O4 1013.80; found 1013.80.
Compound 4
In a 100 mL round bottom flask, compound 3 (500 mg, 1 equiv.) and DCM (3 mL) were added, then 33% HBr·AcOH (495 mg, 2.5 equiv.) was added dropwise at 0 °C. After 30-min stirring, the color changed to a pale yellow. Monitoring with TLC (2% MeOH/DCM) showed a complete disappearance of the starting materials. The reaction mixture was diluted with DCM (50 mL), and the organic layer was washed with H2O (3 × 15 mL) and saturated brine (15 mL). The organic layer was dried over anhydrous MgSO4, and the solvent was removed under reduced pressure to give compound 4 as a yellow oil (225 mg, 40%). 1H-NMR (400 MHz, CDCl3, δ, ppm): 7.78 (s, 1H), 7.51–7.43 (m, 2H), 7.09–7.03 (m, 1H), 6.99 (dd, J=4.3, 1.6 Hz, 1H), 4.74 (s, 0H), 4.64 (s, 1H), 3.94 (d, J=5.2 Hz, 1H), 3.00 (s, 2H), 1.84 (h, J=5.9 Hz, 1H), 1.57–1.20 (m, 32H), 0.86 (td, J = 6.9, 2.1 Hz, 5H). 13C-NMR (101 MHz, CDCl3, δ, ppm): 158.86, 157.20, 157.10, 148.22, 145.95, 140.20, 130.77, 130.42, 126.36, 124.58, 123.82, 123.01, 121.69, 112.76, 112.67, 70.99, 64.88, 53.42, 41.36, 38.14, 38.11, 31.91, 31.89, 31.48, 30.01, 29.69, 29.63, 29.58, 29.34, 28.50, 26.95, 26.93, 26.02, 22.68, 14.11. MALDI-TOF-MS (SA matrix) m/z: [M]+ calcd. for C68H103Br2N2O4 1139.64; found 1139.64.
PFBCP
Extra dry 1,2-dichlorobenzene (15 mL), C60 (50 mg, 1 equiv.), CuBr (20 mg, 2 equiv.) and 2,2’-bipyridine (43 mg, 4 equiv.) were added into a flame-dried, N2-flushed flask and stirred at room temperature for 2 h. On confirmation of the solvation of C60, compound 4 (79 mg, 1 equiv.) was added and the mixture was stirred at 120 °C for 24 h. After cooling to room temperature, the 1,2-dichlorobenzene solvent was removed under vacuum distillation. The solid deposit was dissolved in a minimum of 1,2-dichlorobenzene, precipitated by addition to methanol, and recovered by filtration in a cellulose tube ready for Soxhlet extraction with acetone (2 days) and n-hexane (2 days). The polymer was dried under reduced pressure to a brown powder (47 mg, yield 40%). The molecular weight of PFBCP was determined with an Agilent GPC instrument with THF as the eluent (Mn: 2.8×104).
Patterned ITO glass substrates (1.5 cm × 1.5 cm) were first cleaned by ultrasonication with soap, deionized water and isopropyl alcohol, and then UV-ozone treated for 15 min before use. All perovskite solar devices were prepared by blade-coating at room temperature inside a fume hood with a relative humidity of 40%±10%. The PTAA solution at a concentration of 3.3 mg·mL−1 dissolved in toluene was blade-coated onto ITO glass substrates at a speed of 20 mm·s−1. The gap between blade-coater and ITO substrates was 150 μm. 1.35 mol/L MA0.7FA0.3PbI3 precursor solution was prepared by dissolving corresponding organic halide salts and lead iodide into 2-ME in a N2-filled glovebox with an O2 level <10 ppm. 0.83 mg·mL−1 n-dodecylammonium iodide, 0.27 mg·mL−1 LP, 0.14 vol% MAH2PO2, 1.40 mg ml−1 p-F-PEAI, 0.15 mg·mL−1 benzylhydrazine hydrochloride, 2.8 vol% DMSO were added as additives before blade-coating. Subsequently, the precursor solution was blade-coated onto the PTAA-covered ITO glass substrates with a gap of 250 μm at a movement speed of 20 mm·s−1. The air knife worked at 20 psi during blade-coating. After that, the perovskite films were annealed at 120 °C for 10 min in air. The filtered PCBM or PFBCP ETL solutions (20 mg·mL−1 in 1,2-dichlorobenzene) were coated onto perovskite layers at a speed of 20 mm·s−1. The optimal thickness of ETLs is around 30 nm. For PCBM-based PSCs, a thin layer of BCP (5 nm) was evaporated on the top of PCBM (0.1 Å·s−1). The solar cells were completed by thermally evaporating 100 nm copper (1 Å·s−1).
1H-NMR and 13C-NMR spectra were obtained on a 400 MHz Bruker AVANCE III-400 spectrometer. High-resolution mass spectrometry (HRMS) was done on a Bruker Daltonics MALDI TOF UltrafleXtreme. Thermogravimetric analysis (TGA) was conducted using a Netzsch STA449F3 thermogravimetric-differential scanning calorimetric thermal synchronous analyzer. The sample was heated from 30 °C to 600 °C at a heating rate of 10 °C·min−1 under a nitrogen atmosphere. UV-Vis absorption spectra were obtained with a Shanghai Metash UV-8000 spectrometer. Cyclic voltammetry was carried out on a ZAHNER IM6EX electrochemical workstation with a three-electrode configuration, using Ag/AgCl as the reference electrode, a Pt plate as the counter electrode, and a glassy carbon as the working electrode. The ETL solutions were drop-cast onto the electrodes to form thin films. 0.1 mol·L−1 tetrabutylammonium hexafluorophosphate in anhydrous acetonitrile was used as the supporting electrolyte. Potentials were referenced to the ferrocenium/ferrocene couple by using ferrocene as external standards in acetonitrile solutions. The scan rate is 0.1 V·s−1. Both steady-state PL and TRPL spectra were acquired on a HORIBA FL-3 fluorescence spectrophotometer at room temperature. The excitation wavelengths were 480 nm and 405 nm for PL and TRPL measurements, respectively. The TRPL curves were fitted with a bi-exponential formula:
1
where the t1 values of the two systems were found to be comparable (Table S1 in the electronic supplementary information, ESI), and here we used t2 to represent the lifetime of the films.
Atomic force microscopy (AFM) images were scanned from a Bruker Icon Atomic Force Microscope. Scanning electron microscopy (SEM) images were taken on TESCAN MIRA3 LMH operating at 5.0 kV. The J-V characteristics of solar cells were performed using an LED solar simulator (BG-LED3A-100S, Class AAA Solar Simulator) and the power of the simulated light was calibrated to 100 mW·cm−2 by a silicon reference cell (Newport 91150V). A metal mask with an aperture (6.8 mm2) aligned with the device area was used for measurements. All devices were measured using a Keithley 2400 source meter with a backward scan rate of 0.1 V·s−1 in air at room temperature, and the delay time was 10 ms. There was no preconditioning before measurement. The PSCs were encapsulated with cover glass sealed by an epoxy encapsulant on the back. After curing, the stability of encapsulated cells was monitored with a 91 PVKSOLAR MSCLT-1 automatic maximum power point (MPP) tracker. After connecting the PSCs to the MPP tracker, the cells kept working at MPP conditions under simulated AM 1.5G one sun illumination (100 mW·cm−2) in air (relative humidity: 40%±10%). The stabilized PCEs were recorded every 5 s. No fan or cooler was used to control the cell temperature, and the temperature of the cells was measured to be about 40 °C.
The electron mobilities were measured using the space-charge-limited-current (SCLC) method, employing a device architecture of glass/ITO/ZnO/ETL/PDINN/Ag. The mobilities were obtained by taking current-voltage curves and fitting the results to a space charge limited form, where the SCLC is described by:
2
where ε0 is the permittivity of free space, εr is the relative permittivity of the material (assumed to be 3), μ is the electron mobility, Vappl is the applied voltage, Vbi is the built-in voltage (0.7 V), Vs is the voltage drop from the substrate’s series resistance (Vs = IR, R is measured to be 10.8 Ω) and L is the thickness of the film. By linearly fitting J1/2 with Vappl-Vbi − Vs, the mobilities were extracted from the slope and L:
3
tDOS spectra were obtained with an Agilent E4980A precision LC meter. The energetic profile of tDOS was derived from the angular frequency dependent capacitance using the equation:
4
where C is capacitance, ω is the angular frequency, q is the elementary charge, kB is the Boltzmann’s constant and T is the temperature. Vbi and W are the built-in potential and depletion width, respectively, which were extracted from the Mott-Schottky analysis. The applied angular frequency ω defines an energetic demarcation,
5
where ω0 is the attempt-to-escape angular frequency (ω0 = 2πν0, ν0=1×109 Hz). The trap states below the energy demarcation can capture or emit charges with the give ω and contribute to the capacitance.
PFBCP exhibits good solubility in common organic solvents like chlorobenzene and 1,2-dichlorobenzene, making it suitable for solution processing as the ETL of p-i-n PSCs. PFBCP also shows good thermal stability with a decomposition temperature (5% mass loss) of over 360 oC (Fig. S1 in ESI). The UV-Vis absorption spectra of PFBCP and PCBM thin films are depicted in
Fig 2 (a) UV-Vis absorption spectra of PCBM and PFBCP thin films; (b) CV curve of PFBCP film; (c) Steady-state PL and (d) TRPL curves of the bare, PCBM- and PFBCP-coated perovskite films.
Perovskite film surface morphology was investigated using AFM and SEM before and after the deposition of ETLs. AFM height image (
Fig 3 AFM height (first row) and top-view SEM (second row) images of (a, d) bare, (b, e) PCBM-, and (c, f) PFBCP-coated perovskite films.
We evaluated the photovoltaic performance of PFBCP as an ETL in inverted planar PSCs. The PSCs employed a glass/indium tin oxide (ITO)/PTAA/MA0.7FA0.3PbI3/ETL/copper (Cu) architecture, with both perovskite layers and the HTL/ETL deposited via blade-coating under ambient conditions.[
Fig 4 (a) J-V characteristic curves, (b) EQE plots, (c) tDOS spectra, and (d) long-term stability testing results (MPP condition under simulated AM 1.5G illumination of 100 mW·cm−2) of the PSCs based on PCBM/BCP and PFBCP.
Thermal admittance spectroscopy (TAS) technique was then leveraged to investigate the trap density of states (tDOS) of the p-i-n PSCs.[
We report a new multifunctional ETL for p-i-n PSCs that simplifies device structure and enhances device performance. This ETL, named PFBCP, is solution-processible and combines efficient electron extraction with effective hole blocking in a single layer. Compared to conventional PCBM ETL, PFBCP offers superior film uniformity and coverage, leading to reduced recombination and lower trap density. Consequently, PSCs fabricated using blade-coating with PFBCP achieved a remarkable PCE of 22.3%, exceeding PCBM-based devices. Moreover, these PSCs retained an impressive 91% of their initial PCE after 1200 h of light soaking. This work presents a novel strategy for achieving high-performance PSCs with a simplified structure, paving the way for lower-cost manufacturing of perovskite solar technology.
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