A Chinese-Swedish research team has boosted the performance of tin-lead perovskite solar cells by modifying additives and post-treatment processes. The device also demonstrated improved stability, retaining 60% of its initial efficiency after 550 hours at 85 °C under maximum power point conditions.

Researchers from East China Normal University and Sweden’s Linköping University have developed an alternative passivation method for tin-lead (Sn-Pb) perovskite solar cells that improves both efficiency and stability by avoiding the use of tin fluoride (SnF₂). The approach combines a lead fluoride (PbF₂) post-treatment with lead powder in the precursor.

“We identified and elucidated a previously unrecognized factor that drives the photo-thermal instability of Sn-Pb perovskite solar cells,” Wenxiao Zhang, co-first and co-corresponding author of the research told pv magazine, explaining that the study established that SnF₂ “parasitic reactions” trigger perovskite decomposition and degradation of functional device layers.

“Whereas the markedly lower stability of Sn-Pb perovskites relative to their Pb-only counterparts is usually ascribed solely to the oxidation of stannous ion (Sn²⁺), antioxidant strategies alone have failed to deliver a substantial improvement in photothermal durability. This work pinpoints the underlying cause and proposes an effective alternative,” said Zhang.

“To avoid the adverse effects of SnF₂ on stability and hole transport, we replace SnF₂ additive with lead powders, known for its antioxidant and crystallization-regulating effects as reported in our previous work, to remove Sn⁴⁺ from the precursor, combined with a PbF₂ post-treatment to passivate surface defects,” he went on to say.

The Sn-Pb test cells measured 0.09 cm2. The basic stack was as follows: indium tin oxide (ITO) substrate, P3CT-Cs layer, perovskite, lead fluoride, electron transport layer (ETL) based on buckminster fullerene (C60), bathocuproine (BCP) and insulating lithium fluoride (LiF), and copper (Cu) contacts.

The strategy enabled the efficiency of the SnF₂-free tin lead perovskite solar cell to reach 24.07% compared to 16.43% of the control device. As for photothermal stability, the cells without the SnF₂ additive retained 60% of their initial efficiency after continuous operation at 85 C under maximum power point (MPP) conditions for 550 h.

The negative effect of SnF₂e was evident in testing. For example, the researchers noted that both Cu and ITO electrodes had reactions “even at room temperature or without light soaking,” indicating the “corrodibility of migrated ion and reaction products.”

The fabrication requires precision but the process is straightforward, according to the scientists. “Tin-containing perovskites require a carefully controlled atmosphere with extremely low oxygen levels, and the film-forming temperature along with associated processing parameters must be finely tuned while using high-purity SnI₂. Even so, device fabrication remains straightforward,” said Zhang.

The scientists concluded that the work has implications for overcoming the stability bottlenecks of Sn-Pb single-junction and all-perovskite tandem solar cells. Their work is described in “A tin fluoride-free, efficient and durable tin-lead perovskite solar cell,” published by nature communications.

“We are working on the simultaneous efficiency and stability improvements of all-perovskite tandem solar cells and tin-lead perovskite solar cells,” said Zhang, referring to the future direction of the team's work.