High production cost and low efficiency are the key roadblocks to realizing advances in solar technology. At present, solar photovoltaics can only account for less than 2% of today’s global electricity. More so, a complex inhibition is encountered when attempts to increase the power conversion efficiency of a single-junction solar cell are made. As a result, inquest into the formation of multiple-junctions in a cell is receiving considerable attention. In recent advancements, organometal halide perovskite-based solar cells have emerged as promising top cell candidates due to their suitable electrical and optical properties. To achieve highly efficient tandem devices, the realization of a high efficiency semi-transparent perovskite solar cell is still an essential challenge. Moreover, the development of a suitable deposition process that does not deteriorate the low-cost advantage and maintains the high performance of the perovskite solar cells will be of great importance.
A team of researchers led by Doh-Kwon Lee at the Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology designed semi-transparent inverted planar perovskite solar cells for monolithic tandem construction with an electrodeposited copper-indium-selenide solar cell. Their aspirations were to attempt and realize a high-efficiency monolithic thin film tandem device. In order to achieve this, they hoped to eliminate the severe degradation of the perovskite/PCBM layers during the sputtering process of the transparent conducting oxide, using a thin zinc oxide-nanoparticle introduced onto the electron-extracting PCBM layer by a simple solution process without a post-annealing step. Their research work is now published in the journal, Journal of Materials Chemistry A.
The procedure undertaken involved selecting a sputter aluminum-doped zinc oxide layer as the top transparent conducting oxide while employing a thin zinc oxide-nanoparticles layer prepared by a simple solution process as a buffer layer to protect the underlying PCBM and/or perovskite layers from possible damage during the sputter deposition of the aluminum-doped zinc oxide electrode. To demonstrate the feasibility of all-solution processed, monolithic two-terminal tandems, an electrodeposited copper-indium-selenide thin-film solar cell with a power conversion efficiency of ca. 10% was used as a bottom cell. An intrinsic zinc oxide/ aluminum-doped zinc oxide double layer was applied as a transparent recombination layer at the junction between the sub-cells by tuning its thickness. As a result, monolithically integrated, two-terminal copper-indium-selenide/perovskite tandem solar cells with a higher power conversion efficiency than the constituent single-junction devices were realized.
The research team observed that the thin zinc oxide-nanoparticles layer with an optimal thickness facilitates the electron transfer from PCBM to the silver back contact in opaque devices while also helping to protect the underlying layers from plasma-induced damage in semi-transparent devices. As a result, semi-transparent perovskite solar cell devices with an inverted architecture having a sputtered aluminum-doped zinc oxide top electrode are realized with power conversion efficiencies over 10%.
Within a semi-transparent perovskite solar cells in the p–i–n architecture, in which a solution processed zinc oxide-nanoparticles layer was introduced as an effective charge selective layer between the PCBM and transparent conducting oxide layers, has been demonstrated. It has been seen that due to the sputter-buffering ability of the zinc oxide-nanoparticles layer, an aluminum-doped zinc oxide top transparent conducting oxide layer could be deposited without causing severe plasma damage to the underlying perovskite/PCBM layers. The technique presented in their paper is important in that it allows current matching between the sub-cells, optimization of the recombination layer for good transmission of low-energy photons and low interfacial resistance and the development of a fabrication strategy that is viable for industrial use. All in all, the results obtained shed light on the possibility of all-solution-processed, highly efficient tandem solar cells.
PCBM – phenyl-C61-butyric acid methyl ester
Yoon Hee Jang, Jang Mi Lee, Jung Woo Seo, Inho Kim, Doh-Kwon Lee. Monolithic tandem solar cells comprising electrodeposited CuInSe2 and perovskite solar cells with a nanoparticulate ZnO buffer layer. J. Mater. Chem. A, 2017, 5, 19439–19446