Solar cells, devices that can convert sunlight into electricity, are now widely used in many countries and are contributing to the reduction of greenhouse gas emissions on Earth. While most of the solar cells on the market today are based on silicon, energy engineers have been exploring the potential of other photovoltaic materials, including a class of materials known as perovskites.
Perovskites are materials with a characteristic crystal structure; the same structure of the mineral calcium titanium oxide CaTiO3. A promising solar cell design introduced over the past decades entails the stacking of silicon and perovskite layers to produce so-called tandem cells, a type of photovoltaics that can capture a broader range of the solar spectrum than single-layer solar cells.
Perovskite/silicon solar cells have been found to exhibit remarkable power conversion efficiencies, which essentially means that they convert a higher percentage of sunlight into electricity. Nevertheless, under some conditions (e.g., when they are situated in partly shaded areas) their performance tends to rapidly degrade over time.
Researchers at City University of Hong Kong, University of Oxford and other institutes recently carried out a study aimed at better understanding a process underlying the degradation of perovskite/silicon solar cells, known as reverse bias. Their observations led to the development of a new materials engineering strategy that improved both the efficiency and durability of these solar cells. The study is published in the journal Nature Energy.
“We demonstrate that interfacial electric field discontinuity caused by the dielectric constant mismatch between the perovskite and C60 layer triggers abrupt voltage breakdown and accelerates instability,” wrote Lina Wang, Zexin Yu and their colleagues in their paper.
“This dielectric mismatch drives carrier tunneling and interface reactions under reverse bias. To mitigate these effects, we have introduced graded dielectric layers that smooth the electric field profile, correct abnormal band bending to reduce tunneling current and inhibit halide ion accumulation.”
Smoothening electric fields inside tandem solar cells
Wang, Yu and their colleagues carried out a series of tests on perovskite/silicon tandem solar cells. These tests were aimed at better understanding why tandem solar cells are prone to reverse bias, an electrical state associated with the flow of voltage in the direction opposite to the one it should flow in, which can cause damage and a decline in performance.
Their experiments showed that the perovskite layer and carbon-based transport layer in the tandem solar cells had very different dielectric constants (i.e., their ability to store electrical energy differed). This caused sudden discontinuities in the electric fields, which could in turn trigger undesirable chemical reactions and the accumulation of halide ions inside the cells.
To limit or prevent these processes, the researchers engineered new dielectric layers, in which the dielectric properties of materials changed gradually, instead of abruptly. They then integrated these layers into tandem solar cells and tested their power-conversion efficiencies and stability over 1,000 hours of continuous operation.
“The tandem devices achieve efficiencies of 34.18% (certified: 33.76%) and 34.03% for silicon heterojunction and tunnel oxide passivated contact architectures, respectively, and retain over 92% of their initial efficiency after 1,000 h at ?15 V,” wrote the authors. “A large-area multicell string reaches 31.00% efficiency and maintains over 90% of the initial value after 1,000 h of shading stress.”
Towards efficient and more durable perovskite/silicon tandem cells
In initial tests, perovskite/silicon tandem solar cells based on the team’s newly engineered dielectric layers were found to achieve remarkable power conversion efficiencies. In addition, after 1,000 hours of operation in unfavorable conditions, they retained more than 90% of their performance.
The design strategy introduced by Wang, Yu and their colleagues could soon be refined and used to carefully engineer other materials for tandem solar cells. These materials could then be deployed in real cells and tested in a wide range of real-world conditions.
In the future, this recent study could contribute to the development of more durable and reliable perovskite/silicon tandem solar cells. These cells could eventually be commercialized and used to power residential buildings, offices, industrial facilities or even portable technologies.
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Publication details
Lina Wang et al, Improving the stability of monolithic perovskite/silicon tandems against reverse-bias stress using graded dielectric layers, Nature Energy (2026). DOI: 10.1038/s41560-026-02067-w
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