Initially, in a multijunction solar cell, a perovskite solar cell was usually combined with a silicon semiconductor with different absorption band width. For a multi-junction solar cell, the perovskite solar cell must be semi-transparent to transmit sunlight to the underlying silicon sub-cell. For this reason, it was necessary to apply an additional intermediate layer to prevent damage to the plasma during spraying.
Molybdenum oxide (MoOx) was mostly used as material as the buffer layer by thermal evaporation on hole transfer layer. It is clear that the semi-transparent perovskite cell with MoOxbuffer layer showed higher efficiency than pristine device. And we found that semi-transparent perovskite cell with MoOxlayer also showed poor stability in thermal ambient condition.
We attempted to solve this stability problem by coating additional metal oxide nanoparticles as a buffer layer before the MoOx coating. With a mixed buffer layer of nanoparticles and MoOx, the semi-transparent perovskite solar cell showed improved stability under both ambient and thermal environmental conditions.
Introduction
Perovskite material tuning with additives
By Yixin Zhao and Kai Zhu, addition of MACl led to better crystallinity of perovskite with both mesoporous and planar structure51. And during the thermal annealing process of perovskite films, it was found that amounts of Cl were removed. The released halide from additives then reacted with Pb 2+ ion and resulted in crystallization of perovskite.
They used MABr as an additive and the crystallinity of the perovskite film was maximized with 1.5 mol% MABr. Not only the organic halide additives, but also metal halide additives were used to improve the crystallinity of the perovskite film. These metal halide additives changed the morphology of the PbI2 film and increased the crystallinity of perovskite.
Adding SnF2 could fill the Sn vacancies of perovskite and subsequently reduce the defect density. Also, the addition of solvent to IPA erased the PbI2 defects on the perovskite surface and increased the crystallinity of perovskite.
Device Structure
Adaptation for tandem
The device with MoOxinterlayer showed higher performance of semi-transparent perovskite solar cell than pristine device. The most used buffer material in n-i-p structure was MoOx, which induces much higher performance of semi-transparent perovskite solar cell than pristine state. To fabricate semi-transparent perovskite solar cell with a certain stability, we tried to focus on how to modulate transparent electrode and buffer layer.
For the metal oxide nanoparticle buffer layer, we compared the semi-transparent perovskite solar cell conversion efficiency with ZnO, AZO, and ZTO nanoparticles. We thought that the ZTO nanoparticles did not affect the transmission of perovskite solar cells. To check the conversion efficiency of different semi-transparent perovskite cells, we built perovskite solar cells under different buffer layer conditions.
These phenomena were also found in the small active area (0.094 cm2) of perovskite solar cells. The best performance of a semi-transparent solar cell with a ZTO/MoOxbuffer layer is shown in the figure. We tried to solve the issue of the stability of semi-transparent solar cells by combining metal oxide nanoparticles MoOx and ZTO.
Huang, Stability of perovskite solar cells: A perspective of exchange of cation A and anion X. Effect of electrode interfaces on the stability of perovskite solar cells: Reduced degradation using MoOx/Al for hole collection.
Transparent Conducting Oxides
Motivation of the research
Buffer Layer and metal oxide nanoparticles
In the semi-transparent n-i-p perovskite device structure with TCO sputtering progress, MoOx was mainly used as a buffer layer to eliminate magnetron plasma damage to the organic hole transport layer and perovskite during TCO deposition. The use of MoOxinterlayer brought some advantages in higher device performance with increasing fill factor, but it was slightly inferior to the use of metal electrode in organic hole conductor. Like the p-i-n structure, TCO magnetron sputtering caused plasma damage to the organic electron collection layer (PCBM).
To reduce the degradation of organic layer in p-i-n structure, metal oxide materials were used, such as aluminum doped zinc oxide (AZO), zinc oxide (ZnO) and others. However, different from MoOx, metal oxide materials required much higher temperature annealing processes for coating. They used dispersed metal oxide nanoparticles (AZO nanoparticles dispersed in IPA) as a buffer layer between PCBM and ITO.
Also, when the nanoparticle layer was removed in opaque structure, the thermal stability of the cell rapidly deteriorated, while semi-transparent structure with metal oxide nanoparticle layer maintained better thermal stability. This shows that metal oxide nanoparticle layer acted as an insulating layer to prevent perovskite device degradation.
Experimental Method
Buffer Layer preparation and deposition
Transparent Electrode Deposition
The type of magnetron sputtering was radio frequency for uniform coating of steel oxide layers.
Characterization
Results and Discussion
Device performance with combined buffer layer
In the figure, all the perovskite cells with ITO electrode showed similar efficiency, but the perovskite cell with ZTO nanoparticles showed higher current density instead of and other conditions. To measure the effects of nanoparticles on absorbance, we checked the transmittance spectra of MoOx. Both ITO conditions with MoOx buffer layer showed 75% average transmittance from 300 to 1300 nm wavelengths.
On the contrary, the MoOxlayer affected the transmittance because the ZTO/ITO layer showed 7% higher transmittance than the other two conditions. In the complete device, the average transmission in the range above 800 nm wavelength showed over 70%. He showed that the semi-transparent solar cell was sufficient to send the near-infrared spectrum to the rear subcell.
Without any buffer layer for the ITO sputtering process, the perovskite cell showed the S-shape of its curve with degraded fill factor and efficiency. The efficiency of the perovskite cell was further increased when ZTO nanoparticles were added before the MoOx. In Figure 23, the ZTO/MoOx buffer layer condition showed the highest efficiency than the others.
The external quantum efficiency (EQE) and steady state of semi-transparent device with ZTO/MoOxbuffer layer were measured. The stability of semitransparent with each buffer layer condition was also checked under different storage conditions. The MoOxcoated solar cell showed higher initial efficiency than the ZTO single coated solar cell, recording over 11%.
But with the combined interlayer, the stability of the solar cell did not decrease significantly, maintaining almost 70% of the initial efficiency after 900 hours.
Conclusion
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