Researchers from China's Wuhan University and South China Normal University have developed a two-terminal (2T) monolithic all-perovskite tandem solar cell that uses a tin-lead (Sn-Pb) perovskite material for the top cell.
The team explained that mixed Sn-Pb perovskites have a narrow bandgap (NBG) of approximately 1.26 eV, which makes them ideal for efficient light harvesting and current-matching with wide bandgap (WBG) subcells in all-perovskite tandem cells.
However, the scientists also specified that NBG Sn-Pb perovskite solar cells show inferior efficiency and stability in comparison with their Pb-based counterparts because of the fast crystallization, which would lead to nonuniform nucleation. The rapid crystallization of Sn perovskites causes poor quality of Sn-Pb perovskite films, hindering the fabrication of efficient all-perovskite tandem cells and impeding their commercialization.
The research team set out to address this issue by gauging the mismatch in crystallization rates between Sn and Pb perovskites and modulating the grain growth and crystallization processes. It used the N-(carboxypheny)guanidine hydrochloride (CPGCl) molecule, which is also known as 4-guanidinobenzoic acid hydrochloride, to reduce defects and tin oxidation in the perovskite absorber of the bottom cell.
According to the scientists, CPGCl molecules were utilized as dopants at a mole ratio of 1% in Sn-Pb perovskite precursor solutions. These molecules had a greater impact on the delay of crystallization and grain growth in Sn perovskites, resulting in a delayed and balanced crystallization process in Sn-Pb mixed perovskites.
The team initially built the bottom cell with an indium tin oxide (ITO) substrate, a hole transport layer (HTL) based on PEDOT:PSS, the Sn-Pb perovskite absorber, an electron transport layer (ETL) based on a buckminsterfullerene (C60), a bathocuproine (BCP) buffer layer, and a copper (Cu) metal contact.
Tested under a reverse voltage scan, this cell achieved a power conversion efficiency of 23.15%, an open-circuit voltage of 0.88 V, a short-circuit current density of 32.77 mA cm–2, and a fill factor of 80.11%. The device was also able to retain 97.45% of its original efficiency after 3,500 h. The team attributed the improved device performance primarily to the reduction in carrier recombination. Further enhancements in device stability could involve the use of solid encapsulation and stable charge transport layers, the academics said.
The research team used then this cell to build the perovskite-perovskite tandem device. The latter was built with an ITO substrate, a hole transport layer (HTL) made of nickel(II) oxide (NiOx) and phosphonic acid called methyl-substituted carbazole (Me-4PACz), a wide bandgap (WBG) perovskite absorber, an ETL based on C60, a tin oxide (SnOx) buffer layer, an HTL based on PEDOT:PSS, the Sn-Pb perovskite absorber, an ETL based on C60, a BCP buffer layer, and Cu metal contact.
Tested under standard illumination conditions, the tandem call reached a maximum efficiency of 28.20 and a certified efficiency of 27.35%. It also achieved an open-circuit voltage of 2.13 V, a short-circuit current density of 16.27 mA cm–2, and a fill factor of 78.94%. This device was also able to retain 95.7% of the initial efficiency after being stored in a glovebox for 2,200 h.
The scientists said that despite achieving high efficiencies in individual subcells, the efficiency of all-perovskite tandem cells lags significantly behind their theoretical limits,and so to fully exploit the potential of high-efficiency subcells, research is required to develop stable interconnection layers that reduce both electrical and optical losses in tandem cells.
