First author: Havid Aqoma, Sang-Hak Lee.
Corresponding author: Sung-Yeon Jang
Correspondence unit: Ulsan National Institute of Science and Technology (UNIST), South Korea
Research highlights
1. Solve the problems of organic PQDs for solar cells, including inefficient ligand exchange and formation of photoactive delta phase;
2. Adopt a ligand exchange strategy of methylammonium iodide (MAI) in isopropyl alcohol (IPA) to replace the long-chain oil-based ligands with enhanced electronic coupling while maintaining a stable α phase;
3. The certified quasi-stable state (QSS) PCE of the solar cell is 18.1%, which exceeds many reported quantum dot-based solar cells and has higher stability.
1. Advantages and prospects of perovskite colloidal quantum dots
Colloidal quantum dots (CQDs) have attracted considerable research interest due to their unique optoelectronic properties. Recently, lead halide perovskites have emerged as core materials for CQDs and have shown more promising characteristics than traditional metal sulfides in optoelectronic applications. In perovskite-based CQDs (PQDs), environmentally stable photoactive α-phase perovskite crystals are achieved through nanoscale ligand-assisted surface straining. Furthermore, by changing their composition, size, and shape, the optoelectronic properties of PQDs can be manipulated while retaining their inherent defect-tolerant properties. In solar cell applications, compared with traditional CQDs, PQDs have lower trap density and significantly suppress non-radiative charge recombination, thereby achieving higher external quantum efficiency (EQE) and reducing energy loss. From a processing perspective, PQDs have advantages over perovskite films in industrial manufacturing by separating the crystallization process from the deposition process. This property also allows the use of more environmentally friendly solvents such as n-octane, methyl acetate, and ethyl acetate, rather than relying primarily on dimethylformamide for processing of perovskite films.
2. Introduction to results
Although lead halide perovskite-based colloidal quantum dots (PQDs) have emerged as promising photoactive materials in solar cells, research to date has mainly focused on inorganic cationic PQDs, although organic cationic PQDs have more favorable energy gaps. In this work, Sung-Yeon Jang et al. of Ulsan National Institute of Science and Technology (UNIST), South Korea, developed solar cells using narrow energy gap organic cationic PQDs and demonstrated significantly higher efficiency compared to their inorganic counterparts. We employ an alkylammonium iodide-based ligand exchange strategy and demonstrate that this strategy is more efficient than traditional methylacetate-based ligand exchange and can replace long-chain oleic acid ligands more effectively , while stabilizing the α phase of organic PQDs under ambient conditions. The vegetated solar cell whose organic cationic PQDs have a high certified quasi-steady-state efficiency of 18.1% under illumination under open circuit conditions and maintains stability for 300 hours at 80°C.
3. Results and Discussion
Point 1: Use traditional ligand exchange method to obtain FAPbI3 -PQD
FAPbI3 -PQDs capped with OAc and OAm were synthesized via a conventional hot injection method with an average size of 15.5 ± 3.3 nm. The absorption onset and photoluminescence peaks of FAPbI3 -PQD in n-octane are located at 782 and 783 nm, respectively, and the photoluminescence quantum yield is 72.40%, which is comparable to previous reports. Solar cell devices were prepared using FAPbI3 -PQD as the light absorbing material, and the PQD active layer was prepared using different ligand exchange methods. The device structure is shown in Figure 1a, and the preparation conditions are shown in the Methods section. The photoelectric conversion efficiency (PCE ) of the device was determined through the current density (J)-voltage (V) characteristics (Figure 1b). Using the reported traditional ligand exchange method (Pb(NO 3 ) in methyl acetic acid (MeAc), labeled PQD-PbNO 3 ), the PCE was 13.86%.
Figure 1 Photovoltaic performance and surface properties of PQD layer by different ligand exchange methods
During the ligand exchange process, chemical changes were analyzed using Fourier transform infrared (FTIR), X-ray photoelectron (XPS) and nuclear magnetic resonance (NMR) spectroscopy. The FTIR spectrum of the PQD-PbNO thin film (Fig. 1d) shows that the intensity of the peak at 2,750–3,000 cm −1 is reduced, corresponding to the vibration mode ν(C−Hx) of the oleic acid chain of OAc and OAm, compared to the synthesized PQD film (labeled PQD-pristine), which indicates that a large amount of oleic acid ligands remain in the PQD- PbNO film, which may hinder charge transport within the PQD layer.
Figure 1e shows the 1H-MAS-NMR spectrum of PQDs. The peaks at chemical shifts (δ) of 7.98 and 8.78 ppm are from protons in FA groups of PQDs (FANH and FACH, respectively), while the peak at 5.86 ppm is attributed to protons in OAmNH. The peaks at 0.70–5.00 ppm originate from protons in the oleic acid group of OAm and OAc. By integrating these peaks, it can be determined that the ratios of OAc/FA and OAm/FA in PQD-pristine are 0.088 and 0.196, respectively (Figure 1e, f). Considering the lattice parameter (0.635 nm) and size (approximately 15.5 nm) of PQD, the theoretical ligand density and ligand/FA ratio of the surface were calculated to be 2.48 ligands nm −2 and 0.293, respectively. The calculated ligand/FA ratio is consistent with the experimental value (0.284), indicating a high density of ligand coverage on PQD-pristine. For PQD-PbNO 3 , the OAc/FA ratio significantly decreased to 0.012, while the OAm/FA ratio remained at 0.106 (Fig. 1e, f). This result is consistent with previous literature, suggesting that the role of MeAc is to replace OAc, while Pb(NO 3 ) 2 only retains water molecules during the ligand exchange process. The abundant presence of OAm in PQD-PbNO may be the key reason for suboptimal charge extraction. Therefore, developing a more effective ligand exchange strategy to replace OAm may help improve charge transport in the active layer of FAPbI 3 -PQD.
Point 2: Targeted ligand exchange method to obtain FAPbI 3 -PQD and stability study
Shorter alkylammonium halide-based ligands were employed, which can effectively replace the long oleic acid ligands and enhance the electronic coupling of PQDs. In addition, short alkylammonium ions, such as FA + and MA + , can modify surface A-site defects that may occur during ligand exchange. These short alkyl ammonium halides are dissolved in the polar solvent IPA and can effectively remove OAm without damaging the PQD layer. FTIR spectra (Fig. 1d, c) and C 1s XPS spectra confirmed that FAPbI3 -PQDs exchanged with FAI/IPA (labeled PQD-FAI) contained less oleic acid ligands, especially OAm, compared to traditional PQD-PbNO 3 (Fig. 1f). The N 1s XPS spectrum of the PQD layer (Fig. 1g) shows that the peak intensity of OAm (about 401.6 eV) in PQD-FAI is significantly reduced compared with PQD- PbNO . Quantitative analysis of surface ligands showed that almost all the oleic acid ligands in PQD-pristine were effectively replaced in PQD-FAI, while a large amount of OAm was still retained in traditional PQD-PbNO (Figure 1f) .
Although the ligand exchange efficiency in PQD-FAI is much higher than that in PQD-PbNO ( Fig. 1f), the photoelectric conversion efficiency (PCE) of the solar cell using PQD-FAI (PQD-FAI device) is only slightly higher than that in PQD-PbNO 3 devices (15.05% vs. 13.86%) (Figure 1b and Table 1). PQD-FAI devices show higher short-circuit current density ( J SC ) but lower open-circuit voltage ( V OC ) than PQD- PbNO devices . Since the optical band gaps of PQD-PbNO and PQD-FAI are almost the same, the reduced V in PQD-FAI devices indicates a higher trap density. Figure 2a shows the time-resolved photoluminescence (TRPL) decay of PQD films measured using a time-correlated single photon counting (TCSPC) system. The PQD film exhibits biexponential decay characteristics, with an average relaxation time of 1.20 nanoseconds for PQD-FAI and 3.24 nanoseconds for PQD- PbNO . Compared with PQD-PbNO3 , the shorter carrier lifetime in PQD-FAI is attributed to increased trap-mediated nonradiative recombination, which may be responsible for the higher voltage loss in PQD-FAI devices.
Figure 2 Effects of different ligand exchange methods on PQD trap states
Figure 2b shows the density of trap states (t-DOS) with respect to voltage. At the maximum power point voltage (VMPP), the calculated t-DOS of the PQD-FAI device (4.84 × 10 16 cm −3 V −1 ) is larger than that of the PQD -PbNO 3 device (2.23 × 10 16 cm −3 V −1 ) is twice as high. There is a higher trap density in the PQD-FAI layer measured by space charge limited current (SCLC). Although the charge mobility of PQD-FAI is almost an order of magnitude higher (1.39 × 10 −3 cm −3 V −1 s −1 ) than that of PQD-PbNO3 (1.78 × 10 −4 cm −3 V −1 s −1 ), However, the calculated relative trap density of PQD-FAI (1.56 × 10 16 cm −3 ) is higher than that of PQD-PbNO3 (1.40 × 10 16 cm −3 ), which is consistent with their device performance trends.
The crystal structure of the PQD layer was examined using total reflection wide-angle X-ray diffraction spectroscopy (GIWAXS). The one-dimensional (1D) plot along the surface perpendicular to the surface (Fig. 2d) shows that PQD-FAI displays a significant δ-FAPbI3 perovskite peak at the position of 0.828 Å^ −1 , while PQD-pristine and PQD-PbNO 3 shows only a strong (100) cubic peak at the position of 0.978 Å −1 . The substitution of oleic acid ligands on PQD-FAI changes the surface strain and partially converts the α phase into the photoinactive δ phase, which can serve as a charge recombination center. These findings indicate that efficient ligand exchange of FAI/IPA for PQDs is beneficial to improve charge transport and J SC in the device ; however, it may also induce crystal phase transition (from α phase to δ phase), resulting in voltage loss in the device.
In order to suppress the δ phase formation in PQDs, MAI was chosen for ligand exchange because the incorporation of MA cations into the perovskite in FAPbI 3 can stabilize its α phase crystal due to the shrinkage of the cuboctahedral volume. The dipole moment of MA is higher than that of FA , resulting in a lower formation energy of α-phase perovskite, thus inhibiting the formation of δ-phase. Figure 2e gives a schematic diagram of the surface and crystal of organic PQDs according to different ligand exchange methods based on the characterization results. While the traditional ligand exchange method (PQD-PbNO 3 ) significantly retained OAm, PQD-FAI and PQD-MAI effectively replaced OAm. Although excessive replacement of ligand molecules leads to the deterioration of the crystal phase of PQD-FAI, MA cation exchange effectively prevents the formation of δ phase.
Point 3: Research on the performance and stability of organic PQD-based solar cells
After optimizing the MAI concentration, the optimal photoelectric conversion efficiency (PCE) of the PQD-MAI device in Figure 1b reaches 18.94%, which is much higher than that of the PQD-FAI device (15.05%). Statistical results regarding device performance (Fig. 3a and ) show that all parameters of PQD-MAI devices are significantly improved compared to PQD-FAI devices. The external quantum efficiency (EQE) of PQD-MAI devices (Fig. 1c) is higher than that of PQD-FAI devices at all wavelengths, indicating improved charge collection efficiency. The stable power output (SPO) of the PQD-MAI device measured at a bias voltage of 0.974 V produced a PCE of 18.61% (Figure 3b), consistent with the J–V scan results. The device has a certified QSS efficiency of 18.06% measured under QSS J–V scan, with a J of 20.89 mA cm , a V of 1.14 V , and a FF of 0.76, making it the best reported quantum dot-based device. One of the solar cells. The VOC (1.14 V) of the PQD-MAI device is significantly higher than that of the PQD-FAI device (Fig. 1b), which is attributed to the reduced trap density caused by the suppressed δ phase formation.
Figure 3 Solar cell performance of PQD-MAI devices
The charge recombination lifetime (τ rec ) of the device with respect to applied voltage, based on TPV analysis , is plotted in Figure 3d and summarized in Appendix Table 345. The PQD-MAI device exhibits a longer τ rec under VMPP conditions (122.84 μs) compared to 5.89 μs for the PQD-FAI device , which is attributed to reduced trap-assisted recombination. The changes in device V OC under different irradiation intensities were measured (Fig. 3e). The kT q − 1 of the PQD-MAI device is 1.24, which is lower than that of the PQD-FAI device (1.41), indicating reduced trap-assisted recombination, consistent with the results of TRPL, TPV and SCLC analysis (Fig. 2a, c) .
Figure 4 Long-term stability of P QD solar cells
The long-term stability of PQD-MAI devices was evaluated under various conditions. First, under N2 atmosphere, the photoelectric conversion efficiency (PCE) of the device after illumination under open circuit conditions was monitored. The PQD-MAI device still maintained more than 88% of the initial PCE after 1,152 hours under simulated 1 sun, while the PQD-FAI only maintained 40% after 1,224 hours (Figure 4a). The improved stability of PQD-MAI devices may be attributed to the lower trap density due to the absence of δ phase, as these phase impurities have been reported to act as degradation centers under illumination conditions. Furthermore, thermal stability evaluation was performed by placing the device on a hot plate inside an N2 atmosphere glove box at 60 °C and 80 °C ( Fig. 4b). To circumvent the degradation issues associated with hole transport materials (HTM), poly(triarylamine) (PTAA) was used instead of spiro-OMeTAD in the 80°C thermal aging test. As shown in Figure 4b, the conventional PQD-PbNO devices lost more than 60% of the initial PCE, while the devices using the ligand exchange method (PQD-MAI and PQD-FAI) only lost about 20% after 300 hours. Initial PCE, exhibits quite high thermal stability. In addition, the encapsulated PQD-MAI device still maintained more than 93% of the initial PCE after being stored in a dark environment for about 600 days (Figure 4c).
4. Summary
This work successfully prepared a quantum dot solar cell by developing a MAI-based ligand exchange strategy, with a certified photoelectric conversion efficiency of 18.1%. The resulting mixed organic (FA/MA) cation-based PQD layer enables efficient surface ligand replacement under ambient conditions without compromising its photoactive α phase. The solar cell device also exhibits impressive long-term photoelectric and thermal stability. The findings of this study open a path for further development of phase-stable organic PQDs for various optoelectronic devices.
5. References
Aqoma, H., Lee, SH. et al. Alkyl ammonium iodide-based ligand exchange strategy for high-efficiency organic-cation perovskite quantum dot solar cells. Nat Energy (2024).