Liu Yongsheng's JACS: New Selenophene-Based 2D Ruddlesden-Popper Perovskite Solar Cells
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Research highlights
- Selenophenylammonium iodide (SeMA) was used to control the crystallization of 2D RP perovskite films, obtaining films with large grain size, high crystallinity and preferred vertical orientation.
- Pre-deposited PCBM transport layer (PDTL) was used to passivate defects on the perovskite surface and densify the PCBM electron transport layer.
- Devices based on SeMA showed significantly enhanced humidity, thermal and light stability.
This study used Enlitech products for measurements.
Challenges in improving the performance of 2D RP perovskite solar cells
2D RP perovskites can enhance the moisture resistance of perovskite films and hinder ion migration. However, the presence of organic barriers disrupts the 3D perovskite structure, resulting in lower dielectric constant regions around the inorganic [MX6]4- sheets. This leads to quantum and dielectric confinement, increasing the exciton binding energy (Eb). To address this, organic ligands with high dielectric constants are usually introduced to lower Eb. The high Eb is also detrimental to charge transport, so another focus is enhancing charge transport in the perovskite films. Various techniques like hot-casting, additives, anti-solvent treatment, solvent vapor annealing and suitable organic ligands can produce high quality films with vertical orientation on the substrate. Additionally, interface engineering is considered an effective approach to solving this problem, especially adding an ultra-thin passivation layer between the perovskite and charge transport layer. However, research on reducing surface trap densities through interface engineering in 2D RP perovskite solar cells has been very limited. PCBM is an efficient electron transport material and has been shown to be a Lewis acid passivator for halide-induced traps. But when spin-coated on smooth perovskite films, the passivation effect may be restricted by the interface contact area. Hence, achieving effective contact between them presents a challenge.
Summary of results
Lowering the exciton binding energy of 2D RP perovskites is crucial as their high binding energy is detrimental to charge transport. To this end, Liu Yongsheng’s team at Nankai University successfully developed a selenophene-based spacer, selenophenylammonium iodide (SeMA), for application in 2D RP perovskite solar cells. The 2D perovskite films based on (SeMA)2MAn-1PbnI3n+1 (n=5) exhibited larger grain size, excellent crystallinity and preferred vertical orientation. Additionally, a pre-deposited PCBM transport layer (PDTL) strategy was adopted to effectively passivate defects on the perovskite surface and densify the PCBM electron transport layer, enabling faster electron extraction and transport. The champion PCEs of the optimized 2D RP (n=5) perovskite solar cells based on MA and FA cations reached 17.25% and 19.03% respectively. Moreover, the 2D RP perovskites showed superior film and device stability compared to 3D perovskites. For example, unpackaged 2D RP perovskite solar cells retained their original PCE after storage for 1008 hours under ambient conditions (30±5% RH), while MAPbI3 devices retained only 35% of their original efficiency after 336 h. The SeMA-based 2D RP perovskite solar cells also exhibited markedly improved thermal and light stability.
Results and discussion
Point 1: SeMA controls crystallization of 2D RP perovskite films
The authors used SeMA to control the crystallization process of the 2D RP perovskite films. The top view SEM image in Fig. 1e shows the SeMA-MA-Pb film exhibited complete surface coverage with a dense morphology and large grain size. XRD was used to study the crystallinity and orientation of the SeMA-MA-Pb films. As shown in Fig. 1f, two prominent diffraction peaks were observed for the SeMA-MA-Pb films at ~14.4° and 28.7°, corresponding to the (111) and (202) planes of the 2D RP perovskite crystal. No diffraction peaks were observed below 10°, indicating the SeMA-MA-Pb films preferred vertical orientation with respect to the substrate. This aligns with the GIWAXS data in Fig. 1g, which shows distinct and discrete Bragg spots without observation of low-n phases. These results demonstrate that the 2D RP perovskite grew vertically oriented on the substrate, ensuring efficient charge transfer between the front and back electrodes.
Point 2: Interface passivation improves charge transport and enhances device PV performance
Interface passivation provides a simple yet effective approach to improve charge transport, reduce non-radiative recombination and enhance PCE. To realize more efficient and stable 2D RP perovskite solar cells, the authors proposed a pre-deposited transport layer (PDTL) strategy, which aimed to passivate surface defects and enhance ETL performance by establishing enhanced PCBM coverage on the perovskite layer. The PDTL process flowchart is illustrated in Fig. 2a. To investigate the PV performance of SeMA-MA-Pb devices, the authors fabricated inverted structures of ITO/PEDOT:PSS/perovskite/PDTL/PCBM/BCP/Ag (Fig. 2b). The J-V curves in Fig. 2c show a PCE of 13.59% for SeMA-MA-Pb devices with JSC of 18.86 mA cm-2, VOC of 1.04 V and FF of 69.26%. With a thin pre-deposited PCBM layer, the PCE of the PDTL-based SeMA-MA-Pb PSCs increased to 17.25% with JSC of 21.85 mA cm-2, VOC of 1.09 V and FF of 72.45%. The SeMA-MA-Pb perovskite solar cells also showed good reproducibility, with an average PCE of 16.46% (Fig. 2d), much higher than devices without PDTL (PCEavg=13.08%). To verify the efficacy of the PDTL PCBM strategy, the authors fabricated and optimized 2D RP perovskites based on FA, (SeMA)2FAn-1PbnI3n+1-xClx (n=5, SeMA-FA-Pb). As shown in the J-V curves in Fig. 2e, the SeMA-FA-Pb devices had a PCE of 15.57% with JSC of 21.41 mA cm-2, VOC of 1.01 V and FF of 71.95%. With PDTL PCBM, the efficiency of SeMA-FA-Pb devices reached 19.03% with JSC of 23.00 mA cm-2, VOC of 1.08 V and FF of 76.52%. This further proves the effectiveness of pre-deposited PCBM in enhancing 2D RP device performance.
To elucidate the intrinsic mechanism of how the PDTL strategy impacts the perovskite solar cell performance, the authors performed DFT calculations to probe the interactions between PCBM and the perovskite surface. As shown in Fig. 3a, the first layer of PCBM bonded tightly to the perovskite surface with a binding energy of -12 kcal/mol. When the second layer of PCBM was placed on top of the first layer (Fig. 3b), the calculated binding energy between the two layers was -82 kcal/mol, significantly higher than between the first PCBM layer and perovskite. The larger binding energy indicates stronger stacking capability between the second and first PCBM layers, resulting in a more compact film morphology for the top PCBM layer. To further explore the critical role of the PDTL strategy, the authors used UPS to determine the energy levels of SeMA-MA-Pb and SeMA-MA-Pb/PDTL films. As shown in Fig. 3c, compared to the Fermi level (Ef) of 4.21 eV for the SeMA-MA-Pb film, the Ef of the PDTL-based SeMA-MA-Pb film increased to 3.81 eV, closer to the ECB. This indicates the SeMA-MA-Pb/PDTL film had more n-type characteristics, which can reduce trap-assisted recombination losses and facilitate efficient charge transfer between the perovskite and ETL, thus improving VOC. This provides further evidence that pre-deposited PCBM can enhance charge carrier transport.
Figure 2 Photovoltaic performance of the device under different conditions
Figure 3 Theoretical explanation of how pre-deposited PCBM improves carrier transport capability
Point 3: Crystallization control and interface engineering significantly enhance device stability
Since the stability of perovskite films significantly impacts the stability of the corresponding devices, the authors investigated the effects of incorporating the selenophene organic spacer and PCBM pre-deposition layer on the humidity, thermal and light stability of perovskite devices. By tracking the XRD data of perovskite films under different aging conditions, they monitored the moisture, thermal and light stability of the films. As shown in Fig. 5a-c, the films were aged by exposure to ambient air (RH, 45±5%), thermal stress (85°C, N2) or illumination (white LED, 100 mW cm-2). For the MAPbI3 film, an obvious diffraction peak (PbI2 peak) was observed at 12.8° after 288 h, indicating partial decomposition of MAPbI3. Notably, as shown in Fig. 5d-f, no obvious PbI2 peak was observed for the SeMA-MA-Pb films after 288 h of aging under the various conditions, demonstrating the superior moisture, thermal and light stability of the 2D RP perovskite. Further investigation of device stability under different aging conditions was carried out. As shown in Fig. 5g, even after storage for 1008 h under ambient conditions (30 ± 5% RH), the unpackaged SeMA-MA-Pb devices retained their initial efficiency, while MAPbI3 devices retained only 35% of their original efficiency after 336 h. The authors also studied the thermal stability of perovskite solar cells in a N2-filled glove box at 60°C (Fig. 5h). After 1008 h, the SeMA-MA-Pb perovskite solar cells retained 86% of their initial PCE. In contrast, the MAPbI3 devices showed rapid degradation, with only 41% of initial PCE remaining after 336 h. Additionally, under continuous illumination (white LED, 100 mW cm-2) in N2, the SeMA-MA-Pb devices maintained 97% of their initial efficiency after 1008 h, while the MAPbI3 devices retained 68% after 336 h (Fig. 5i). These results align with the superior stability of the films. The impressive stability can be ascribed to the inherent structural robustness of the 2D RP perovskite with SeMA spacer layers. Overall, the increased grain size, preferential vertical crystal orientation, prolonged carrier lifetimes and exceptional moisture resistance contributed to the enhanced PV performance and stability of the SeMA-MA-Pb films and devices.
Figure 4 Humidity, thermal and light stability of the device before and after optimization
Conclusion
In this work, the authors first developed a selenophene-based organic spacer SeMA. The perovskite films based on (SeMA)2MAn-1PbnI3n+1 (n=5) exhibited larger grain size, excellent film quality and preferred vertical crystal orientation. Additionally, combining ligand engineering and interface passivation yielded stable and efficient 2D RP perovskite solar cells. The authors further developed a PDTL method to passivate defects on the perovskite surface and densify the PCBM electron transport layer in 2D RP perovskites, thereby accelerating electron extraction and transport. The results show champion PCEs of 17.25% and 19.03% for the optimized 2D RP perovskite solar cells based on MA and FA cations, respectively. More importantly, the SeMA-based 2D RP perovskite solar cells displayed superior humidity, thermal and light stability. This demonstrates that selenophene organic spacers combined with pre-deposited transport layers provide a viable approach toward high-performance 2D RP perovskite solar cells.
References
Fu, Q., Chen, M., Li, Q. et al. Selenophene-Based 2D Ruddlesden-Popper Perovskite Solar Cells with an Efficiency Exceeding 19%. JACS (2023).
Doi: 10.1021/jacs.3c08604
