Technical / research

Researchers at Chinese Academy of Sciences (CAS) have fabricated an intermediate recombination layer (IRL) featuring a heavily doped boron/phosphorus polysilicon tunneling junction, with tunnel oxide passivated contact (TOPCon) silicon cells serving as the bottom cell for perovskite/TOPCon tandem solar cells (TSCs). 

In perovskite/silicon TSCs, the IRL is an important structure electrically connecting the top-side perovskite and bottom-side silicon sub-cells, significantly influencing the overall device performance. The traditional IRL often uses ITO materials to ensure high transmittance and good electrical properties, which, however, usually leads to issues such as sputtering damage and low temperature process limitations.

The properties of 3D halide perovskites, such as mixed ionic–electronic conductivity and feasible ion migration, have enabled them to challenge traditional memristive materials. However, issues like poor moisture stability and difficulty in controlling ion transport due to their polycrystalline nature have hindered their use as a neuromorphic hardware. Recently, 2D halide perovskites have emerged as promising artificial synapses owing to their phase versatility, microstructural anisotropy in electrical and optoelectronic properties, and excellent moisture resistance. However, their asymmetrical and nonlinear conductance changes still limit the efficiency of training and accuracy of inference. 

Now, researchers from Seoul National University, Korea University, Sungkyunkwan University, Pohang University of Science and Technology and University of Southern California have achieved highly linear and symmetrical conductance changes in Dion–Jacobson 2D perovskites. 

While formamidinium lead triiodide (FAPbI₃) perovskite can be used to create highly efficient perovskite solar cells (PSCs), the thermodynamically unstable α-phase poses a challenge to device long-term stability. Thermal annealing is essential for producing high-quality polycrystalline films that stabilize the α-FAPbI₃ phase, but it also induces partial decomposition of FAPbI₃ into PbI₂, leading to extra phase instability of FAPbI₃ films.

Researchers from the University of Science and Technology of China, Chinese Academy of Sciences, NingboTech University and Nankai University have developed a zero dimensional (0D) perovskite-decorated strategy to enhance the intrinsic stability of FAPbI₃ film by stabilization of the initially formed α-FAPbI₃ phase. 

Researchers from City University of Hong Kong, National Renewable Energy Laboratory (NREL) and Imperial College London have improved the long-term stability of perovskite solar cells with an atomic-layer deposition (ALD) method that replaces the fullerene electron transport layer with tin oxide. 

Professor Zhu Zonglong (left) and Dr Gao Danpeng of City University of Hong Kong hold their innovative solar cells. Image credit: Eurekalert

The team started by depositing the perovskite and the hole-transporter layer in a single step. Then, they used ALD to create an oxygen-deficient tin oxide layer to reduce the band offset to a thicker, overgrown layer of normal tin oxide. Solar cells had a power conversion efficiency of more than 25%, and they retained more than 95% of efficiency after 2000 hours of maximum power point operations at 65°C. 

Researchers from Thailand's Mahidol University, Rajamangala University of Technology Thanyaburi and Synchrotron Light Research Institute have presented two novel air-stable hole transporting materials (HTMs) based on a spiro[fluorene-9,9′-xanthene] (SFX) core functionalized with N-methylcarbazole (XC2-M) and N-hexylcarbazole (XC2-H) rings. 

These HTMs were synthesized via a straightforward, three-step process with good overall yields (∼40%) and low production costs. To further reduce device cost, carbon back electrodes were employed. The resulting PSCs, with a structure of FTO/SnO₂/Cs_0.05FA_0.73MA_0.22Pb(I_0.77Br_0.23)₃/HTM/C, achieved power conversion efficiencies (PCEs) of 13.5% (XC2-M) and 10.2% (XC2-H), comparable to the reference spiro-OMeTAD device (12.2%). 

Researchers from Sweden's Chalmers University of Technology recently gained new insights into the dynamics of prototypical 2D halide perovskites (HPs) based on MAPbI₃ as a function of linker molecule and the number of perovskite layers using atomic-scale simulations.

The team showed that the layers closest to the linker undergo transitions that are distinct from those of the interior layers. These transitions can take place anywhere between a few tens of Kelvin degrees below and more than 100 K above the cubic–tetragonal transition of bulk MAPbI₃. 

Interface engineering plays a significant role in the constant improvement in the performance of perovskite photovoltaics, but such devices still suffer from several issues, including unavoidable open circuit voltage (VOC) losses. Now, an international team of researchers from Università Degli Studi Di Pavia, King Abdullah University of Science and Technology (KAUST), Chinese Academy of Sciences (CAS), University of Cambridge, Istituto Italiano di Tecnologia (IIT), Slovak Academy of Sciences and Imperial College London have proposed a different approach by creating a photo-ferroelectric perovskite interface. 

Graphical representation of the 2D/3D/2D perovskite heterostructure. Image from: Nature Communications

By engineering an ultrathin ferroelectric two-dimensional perovskite (2D) which sandwiches a perovskite bulk, the scientists exploited the electric field generated by external polarization in the 2D layer to enhance charge separation and minimize interfacial recombination. As a result, they observed a net gain in the device VOC reaching 1.21 V, the highest value reported to date for highly efficient perovskite PVs, leading to a champion efficiency of 24%. 

Two-dimensional Ruddlesden-Popper (2DRP) phase perovskites have excellent long-term environmental and structure stability. However, the efficiency of 2DRP perovskite solar cells (PSCs) still lags behind that of their 3D counterparts due to the large exciton binding energy between the large-volume organic spacer and the inorganic plate compared to their 3D analogs.

To address this issue, researchers from China's Northwestern Polytechnical University and Xijing University have used a thin layer of self-assembled monolayer material between the transporting layer and the perovskite film for efficient and stable 2DRP-based PSCs. 

Slot-die coating (SDC) technology is a potential approach to mass produce large-area, high-performance perovskite solar cells (PSCs) at low cost. However, when the interface in contact with the perovskite ink has low wettability, the SDC cannot form a uniform pinhole-free perovskite film, which reduces the performance of the PSC.

Optimizing Slot-Die Coating for Commercial Solar Cell Production. Image credit: InfinityPV

Researchers from Korea's Jeonbuk National University have examined the correlation between interfacial roughness, wettability, and the overall efficiency of perovskite solar cells produced using slot-die-coating. This work offers a comprehensive understanding of how modifying the roughness of the hole transport layer (HTL) can improve the quality of perovskite films, enhance charge transport, and ultimately lead to high-efficiency perovskite solar cells with long-term stability.

Researchers from NingboTech University, Hunan Institute of Engineering, Hangna Nanofabrication Equipment Co. and University Malaysia Sabah have developed an inverted perovskite solar cell with an interface passivator based on lead carbanion (Pb–C^–), that reportedly achieved the highest open-circuit voltage ever recorded for an inverted perovskite PV device. The lead carbanion layer was responsible for reducing defects at the interface between the perovskite layer and the electron transport layer.

Inverted perovskite cells, or “p-i-n” cells, have the hole-selective contact p at the bottom of intrinsic perovskite layer i with electron transport layer n at the top. Conventional halide perovskite cells have the same structure but reversed – a “n-i-p” layout. In a n-i-p architecture, the solar cell is illuminated through the electron-transport layer (ETL) side; in the p-i-n structure, it is illuminated through the hole‐transport layer (HTL) surface. Inverted perovskite solar cells are known for their impressive stability but have been held back by relatively low efficiencies. This issue mainly arises at the point where the perovskite layer meets the electron transport layer, causing energy loss instead of being converted into useful power, primarily caused by carrier recombination, especially at the interface between perovskite and the electron transport layer.