Other than efficiency, stability has been a long-standing concern for perovskite PVs and their practical applications. To date, such stable materials for photovoltaics and data about them is insufficient and researchers are trying different materials to gather better insight. In this research study, the ultrastable and efficient Dion-Jacobson perovskite solar cells for photovoltaic applications are being tested.
The team has successfully designed blade-coated solar cells with scalable technology for Dion-Jacobson (DJ) perovskites which demonstrated around 19.11% power conversion efficiency in different environmental conditions.
Aim of the study – To perform systematic research on key factors of stability in DJ materials. To find the guidelines for designing stable DJ perovskites for stabilized and efficient scalable solar cells.
Performances of Ultrastable and Efficient Dion-Jacobson Perovskite Solar Cells
2D perovskites have been preferred over 3D ones due to their desirable photophysical and inherent structural properties. DJ-type perovskite with ditopic diammonium cations strengthens the connection between inorganic layers and increases the rigidity of the structure. This further enhances its stability. Various studies on Ruddlesden−Popper (RP) and Dion-Jacobson (DJ) perovskites show that the latter is less stable in the atmospheric environment.
For example, poor moisture stability was witnessed within a few hours in the 1,4-phenylenedimethanammonium- and m-phenylenediammonium-based DJ perovskites.
As per the structural analysis, these materials have a unique interlayer-displacement quantum and well configuration, setting them apart from other DJ perovskites. This leads to less interlayer spacing and optimized charge transport and structural integrity in the interlayer.
For interlayer cations, using cycloalkyl organic cations preserves the electronegativity and flexibility of the molecules. It is also advantageous over alkyl-chain and aryl organic cations. Lattice stress effectively minimizes in this approach, ultimately leading to overall structural stability.
Highlights
- Introduction of a new series of DJ (CDMA 1, 4-cyclohexanedimethanammonium, n≥1) (MA)n-1Pbn3n+1 for stable and efficient solar cells.
- These un-encapsulated cells retained 92% of their efficiencies for more than 4000 hours under ~90% relative humidity (RH) aging conditions.
- These cells show operational and 85° C thermal stability.
- No efficiency lost after 5000-hour treatment.
- No efficiency loss after operating at 45° C maximum power point (MPPT) for 6000+ hours under continuous light (100 mW cm-2).
The Process of the Research
Scalable blade coating deposition technique was used to demonstrate the potential of DJ perovskites for large-scale solar cells. The reference solar cells used the nominal n=5 PDMA DJ perovskite. These are known for enhancing device stability and have similar organic molecular configurations.
| Properties of the cells | Reference device | Nominal Device n = 5 CDMA-based perovskite solar cells |
| PCE | 14.87% | 19.11% |
| Open-circuit voltage (VOC) | 1.06 V | 1.16 V |
| Short-circuit current density (JSC) | 18.32 mA cm−2 | 20.41 mA cm−2 |
| Fill factor | 76.46% | 80.56% |
The highest efficiency for blade coating 2D perovskites is the PCE of CDMA, as per the researchers. The following figure shows the current density-voltage (J-V) curves of the DJ solar cells.
External Quantum Efficiency (EQE)
Researchers measured the short circuit current density (JSC) of the device with EQE. The PDMA device has lower EQE than CDMA. Over the visible light absorption region, the JSC of PDMA was 17.57 and CDMA was 19.58 mA cm-2. These values matched the measured JSC under the solar stimulator.
Stability of the Solar Cells Under Different Conditions
Further, the researchers studied the stability of these cells under heat, light stressing, and moisture.
Light stressing
Its operational stability was checked under a nitrogen-atmosphere glovebox using an MPPT tracking under a simulated 1-sun white LED (light-emitting diode). The test at around 45° C for 6000+ hours aging without a downward trend.
- PDMA – PCE reduced by about 30%.
- CDMA – Slight fluctuation around their initial efficiency.
Heat
The thermal stability of the solar cells was checked on a hotplate at 85° C.
- PDMA – 50% decline in initial efficiency.
- CDMA – Negligible efficiency even after 5000 hours.
This verifies the superior thermal tolerance of CDMA perovskite cells.
Moisture
The un-encapsulated solar cells were stored in a chamber with constant-temperature humidity, around ~90% RH at ~22° C. This causes the following effects on both devices.
- PDMA – Rapid decrease in PCE to zero after 500 hours of aging.
- CDMA – Shows around 92% initial efficiency after 4394 hours of aging.
By comparing the aged devices, we find that the active layer of the PDMA-based devices has become colorless, while the CDMA-based device shows no visible discoloration. This confirms that CDMA DJ perovskites have high humidity tolerance. As per the consolidated image shown below, the CDMA series is the best between 2D and 3D perovskite solar cells, as per the researchers.
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Device Characteristics
Characterization of Device
The following figure shows a statistical distribution of the calculated hysteresis factor. It was measured from the PCE of PDMA and CDMA solar cells. This indicates relatively lower and stable distributed hysteresis in CDMA which could be conducive in producing stable and scalable solar cells for practical applications.
Characterization of the Perovskite Films
The researchers used Top-view scanning electron microscopy (SEM) to study the morphology of perovskite films.
As per the observation, there was a uniform grain distribution in PDMA perovskite films. Whereas, CDMA has a distinct enlargement of grain sizes along with some small 2D wafers covering the corresponding grain boundaries. This possibly involves self-passivation of the interface to ensure stability. Additionally, it helps to enable ultraviolet-visible (UV-Vis) absorption spectra and XRD measurements of the blade coating.
CDMA films have a relatively narrower full width at half maximum (FWHM) along with the stronger intensity of diffraction peaks. This suggests improved crystallinity and enlarged grain sizes as per Scherrer’s equation.
As per the UV-Vis absorption spectra, both DJ perovskite films have 2D phase distributions. However, CDMA has high intensity suggesting abundant 2D phase distribution which is beneficial for its short-wave absorption and film stability.
Grazing incidence wide-angle X-ray scattering (GIWAXS) observation shows that CDMA has relatively strong and discrete Bragg spots. This indicates its better crystallization that benefits the charge transport.
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Characterization of Strain and Stability of DJ Perovskite Films
Researchers selected 2 depths to investigate the residual strain in both devices according to the phase distribution shown by the TA characterization. A superficial area characterized by 3D-like phases and a high concentration of 2D perovskites in a deeper region were observed in the material. According to the image below, the grazing incident X-ray diffraction (GIXRD) of PDMA perovskite films was relative to 310 crystal plane.
There was a rightward shift in diffraction peaks with the increase of Ψ (0° to 45°). This further shows a gradual decrease in crystal plane distance and that there is a compressive strain on the film.
In 2D perovskite-rich regions, PDMA films show a leftward shift followed by an increase in Ψ values. There was a slight deviation from the linear fitting direction in the trend of variation. This suggests an uneven distribution of 2D perovskite phases which negatively impacts the stability and performance of the device. On the contrary, in CDMA films, negligible displacement with an increase of Ψ in both regions was evident.
Results were consistent after calculating residual strain via Williamson-Hall plots. This suggests that DJ perovskite is almost residual strain free and this quality contributes towards its stability.
After stored for a month, the black phase of the film completely converted and decomposed into a broad bandgap phase. Whereas in CDMA films, after 100 days of storage, remarkable moisture stability without impurity diffraction peaks was noticed.
Additionally, the water resistance of the films was examined by measuring water contact angles on the film surface. CDMA exhibits a large water contact angle (51°) and PDMA has smaller angles (41°). This indicates high water resistance of CDMA perovskite films. The dynamic contact-angle curves shown in the image below confirm the excellent waterlogging resistance of CDMA perovskite films.
Conclusion
As a result, researchers discovered that incorporating flexible organic cations in the interlayer displacement can improve the structural stability of DJ perovskite configuration. Based on this, researchers designed a series of interlayer-displacement DJ perovskites. Further, the materials were applied to the blade-coating process. Higher efficiency and stability were achieved after this. Moreover, in aging tests, there was also minimal degradation in the device.
All in all, researchers conclude that the newly developed series has the potential for constructing stable 2D perovskites which can be used for various applications.
Source: Ultrastable and efficient slight-interlayer-displacement 2D Dion-Jacobson perovskite solar cells
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