A self-assembled bilayer could enhance the thermal stability of perovskite solar cells

Over the past few years, photovoltaic (PV) technologies have become increasingly widespread, contributing to the ongoing quest to reduce greenhouse gas emissions. While most solar cells on the market today are made of silicon, other materials are emerging as promising alternatives for developing PV solutions.

Perovskites are among these materials, as they could be used to create more affordable solar cells that exhibit high power conversion efficiencies. Despite their potential, perovskite solar cells (PSCs) are not as stable as their silicon counterparts and their performance tends to decline at high temperatures and under fluctuating environmental conditions.

A feature of these solar cells that can contribute to their degradation over time is their reliance on hole-selective self-assembled monolayers (SAMs), molecular films that help to attract positive charge carriers. These films often do not adhere well to the cells’ surface and contribute to the thermal instability of PSCs.

Researchers at Xi’an Jiaotong University, Uppsala University and other institutes recently designed a self-assembled bilayer film that could overcome the limitations of conventional SAMs. Their paper, published in Nature Energy, shows that this structured bi-layer molecular film could adhere better to PSCs, enhancing their thermal stability and overall performance.

“The adoption of PSCs requires improved resistance to high temperatures and temperature variations,” wrote Bitao Dong, Mingyang Wei and their colleagues in their paper.

“Hole-selective SAMs have enabled progress in the performance of inverted PSCs, yet they may compromise temperature stability owing to desorption and weak interfacial contact. We developed a self-assembled bilayer by covalently interconnecting a phosphonic acid SAM with a triphenylamine upper layer.”

The new self-assembled bilayer designed by the researchers adds an upper layer to a conventional SAM based on phosphonic acid. This upper layer, made of the organic compound triphenylamine, forms covalent bonds with the SAM, producing a polymerized network.

“This polymerized network, formed through Friedel–Crafts alkylation, resisted thermal degradation up to 100 °C for 200 h,” wrote Dong, Wei and their colleagues. “The face-on-oriented upper layer exhibited adhesive contact with perovskites, leading to a 1.7-fold improvement in adhesion energy compared with the SAM–perovskite interface.”

Dong, Wei and their colleagues tested their self-assembled bilayer in a series of tests and found that it adheres better to perovskite surfaces than commonly employed mono-layer SAMs. Notably, the approach used to produce the bi-layer film is also versatile and can be applied to various SAM-forming molecules and alkylating agents.

The researchers also applied their self-assembled bilayer to inverted PSCs to explore how it affected their performance. Their findings were very promising, as introducing these films resulted in good power-conversion efficiencies while also limiting the cells’ loss of efficiency over time and boosting their stability at high temperatures.

“We reported power conversion efficiencies exceeding 26% for inverted PSCs,” wrote Dong, Wei and their colleagues. “The champion devices demonstrated less than 4% and 3% efficiency loss after 2,000 h damp heat exposure (85 °C and 85% relative humidity) and over 1,200 thermal cycles between −40 °C and 85 °C, respectively, meeting the temperature stability criteria outlined in the International Electrotechnical Commission 61215:2021 standards.”

In the future, the approach devised by this research group could be used to create other self-assembled bilayer films to improve the stability of PSCs. Collectively, these efforts could help to advance perovskite-based photovoltaics, which could contribute to their widespread adoption.

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