The efficiency of perovskite solar cells has increased to around 26%. However, large-scale production is still a challenge because of traditional methods like spin coating. To overcome this, researchers from the University of Rome Tor Vergata have refined the blade coating process to create 12.6% efficient large-area PV solar modules. For this, they have used a nickel oxide hole transport layer in ambient air along with a non-toxic solvent.
Aim of the Study – To demonstrate the progress in large-scale PV cell production offering long-term stability and efficiency.
12.6% Efficient Large-Area PV Solar Modules
To enhance the uniformity of the perovskite film, researchers introduced self-assembled monolayers between the layers. As a result, modules with 110 cm2 active area achieve 12.6% efficiency. Moreover, they maintained 84% of their initial efficiency after 1,000 hours at 85° C in the air.
2 main architectures based on the order of charge transport materials are followed to fabricate perovskite solar cells (PSCs):
- Normal (n-i-p)
- Inverted (p-i-n)
Here, inverted PSCs show improved stability and reduced hysteresis behavior. This makes them suitable for potential commercialization. For producing compact nickel oxide in large-area PV solar cells, different deposition methods are used, which are classified into printable and non-printable methods.
Despite that printable deposition techniques offer various benefits, there is a substantial gap between small-scale and large-scale printable NiOx-based PSCs. For example, moving to ambient air-deposited PSCs’ efficiency dropped to 20.7% in small-scale and 10.34% for large modules with 3.7 cm2 active area.
Procedures Adopted in the Study
Researchers established a procedure to print NiOx over 15 cm by 15 cm substrates without spin coating step. Modules with 110 cm2 active area were made with doctor-blading NiOx/MeO-2PACz/perovskite and thermal evaporation. Further by optimizing the NiOx ink and adding a self-assembled monolayer, the best module achieved around 12.6% PCE.
As a result, researchers observed that these modules outperformed previous large-area PV modules in terms of stability, efficiency, and performance.
Results and Discussion: NiOx Film Thickness And Uniformity Optimization
After depositing the NiOx film by doctor-blading at NiCl2·6H2O solution onto ITO substrates under ambient conditions. Then these films were annexed at 300° C to help decomposition and oxidation. Then, atmospheric oxygen was used to create the NiOx film.
Researchers deposited 4 concentrations on glass/ITO substrates, which is a reference solution at 0.15 M, along with 0.075 M (1:1 dilution), 0.050 M (1:2 dilution), and 0.037 M (1:3 dilution). The film thickness measured using ellipsometry was more than 140 by 140 mm.
The variation in film thickness is due to non-uniform doctor blading, which improved with higher solvent ratios. However, film thickness and viscosity decreased with dilution. Thus, the thickness decreased as precursor concentration diluted, resulting in: 42.2 nm (0.075 M), 40.0 nm (0.05 M), and 36.2 nm (0.037 M).
The following figure shows the X-ray photoelectron spectroscopy (XPS) and X-ray reflectometry (XRR) results. Both tests were used to assess the NiOx oxidation.
- XPS spectra show that in the 850-860 eV range, 4 peaks corresponded to Ni, NiO (Ni²⁺), NiOH (Ni²⁺), and Ni2O3 (Ni³⁺).
- Indicated binding energies of around 852.0, 853.5, 855, and 856 eV were bound by Gaussian fits. This aligns with the literature values.
- A dominant NiO peak indicates a highly oxidized film. They are considered suitable for applications like hole transport layers in PSCs.
- With 1:1 concentrations, it remains prominent but an increase in Ni and Ni203 contributions was also noticed. This further suggests that there was a reduction in oxidation efficiency.
- With 1:2 concentrations and better oxidation from thinner films, higher Ni203 shows more Ni³⁺.
- Further dilution to 1:3 concentrations shows an increase in Ni203 relative to NiO. This suggests non-uniformity in film thickness and oxidation.
Overall, researchers found that decreased precursor concentration correlates with increased nickel oxide complexity and higher oxidation states. However, lower NiO contribution reduces with less precursor.
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Result of XRR Measurement
No Kiessig fringes were noticed when XRR measurements examined electron density on NiOx deposited substrates. Critical angles correlate with scattering volume which shows the following:
- Uniformity due to low dispersion
- Heterogeneity due to high dispersion
There was an upward shift in electron density with decreased precursor concentration. This clearly indicates denser films with higher dilution are possible due to better oxidation or compaction during annealing. Thinner films show improvement in PV solar cell performance as they still exhibit enhanced properties.
Observations
- The most homogeneous is the reference sample and 1:2 concentrated NiOx deposition.
- Less uniform film was noticed in 1:3 concentration.
- Interface or surface roughness was estimated with the Fresnel Reflectivity. For reference sample and 1:2 concentration, it was around 4.5 (5) nm. This value remains constant across all patterns.
- Higher roughness dispersion was noted in samples obtained from 1:1 and 1:3 concentration plates. The values for the 1:1 XRR profile range from 2.5 (5) nm to about 4.5 (5) nm. For the 1:3 XRR profile, it is about 4.5 (5) nm to about 7.0 (5) nm.
Interface Engineering and Perovskite Film Morphology
This exploration of perovskite solar modules focused on the perovskite layer deposition, building on prior work with a two-step blade coating method using non-toxic solvents. We developed a double-cation perovskite (Cs0.15FA0.85PbI3−xBrx) through optimized parameters and additives to improve film quality. The two-step deposition involves using PbI2-(FAI)0.3-(CsI)0.15 in DMSO followed by FAI/FABr in isopropyl alcohol, with four drying methods proposed. This study tested these techniques on rigid substrates, producing high-quality films on 15 cm × 15 cm substrates, paving the way for a universal green perovskite formulation for varied devices and substrates.
Through SEM images it became clear that some defects were caused on the PV film by the non-optimized NiOx layer (0.15 M). Defects include pinholes and visible bars which were mainly due to uneven deposition. On the other hand, fewer defects were observed in the optimized NiOx layer (0.05 M), like smaller particles and fewer pinholes.
Other studies indicate the following:
- However, the results in the optimized layer were better than before but the occurrence of pinholes persisted. This indicates a major challenge related to the adhesion issues between the NiOx film and PV precursor inks.
- The UV-ozone and plasma conventional surface treatments have a negative effect on the NiOx film. This worsens the interface issues, like the formation of excess Pbl2. This can behave as a hole extraction barrier, reducing open circuit voltage of the device.
- Moreover, the low conductivity of the NiO can be harmful to the performance of the perovskite solar cells.
To avoid all these issues and improve the above situations, researchers used a self-assembled monolayer (SAM) of MeO-2PACz at the HTL/perovskite interface.
In another attempt, researchers discovered Ultrastable 2D Dion-Jacobson perovskites achieves 19.11% efficiency.
Observations
- The SEM image of the perovskite film confirms the effectiveness of the methods used: SAM layer.
- The perovskite films were highly uniform and there were no pinholes.
- The ellipsometry thickness map shows that the average film thickness achieved was 570 mm, which also supported the uniformity.
- Due to doctor blading process, there is a gradient with a starting thickness of 700 nm. Also, towards the end of the coating, there is a slight decrease in that.
- However, uniformity was achieved by using the SAM layer and adhesion issues were addressed with the same. The result was stable and industrial-ready perovskite solar modules.
Modules And Long-Term Stability of 12.6% Efficient Large-Area PV Solar Modules
Finally, the successful assembly of the perovskite solar module was done through evaporation of C60/BCP as the electron transport layer (ETL). Then the P2 laser scribing and subsequent evaporation of the copper electrode was done. Then the process was concluded with P3 scribing. The characteristics of the PV module with 22 series-connected cells are as follows:
- Efficiency – 12.6%
- Short-circuit current (ISC) – 98.13 mA
- Fill factor – 63.49%
- Open-circuit voltage (VOC) – 22.3 V
- Near-unity hysteresis index – 1.02
However, a remarkable consistency in performance between forward and reverse measurement scans is demonstrated by the near-unity hysteresis index. This highlights the reliable operation of the perovskite module.
Conclusion
In conclusion, this research advances the scalability of the 12.6% efficient large-area PV solar modules for commercial use. By using the doctor blading, creating a large-area PSC module with NiOx HTL became possible. The final result was a non-toxic perovskite formulation. And finally, the PSCs with improving stability and performance show their potential for future optimization and commercial applications.
