Perovskite solar cells (PSCs) have emerged as a promising technology with their low-cost fabrication and impressive power conversion efficiency (PCE). Despite their potential, long-term stability issues have hindered the commercialization of PSCs. To address this, researchers are actively exploring inverted PSCs with a p-i-n architecture, which combines good stability with decent efficiency. In recent years, there has been significant progress in achieving high efficiency inverted PSCs, and this blog explores these advancements, focusing on key elements such as perovskite compositions, fabrication methods, and counter electrode materials.

Optimizing Passivation in Inverted Perovskite Solar Cells

An international research team, led by Saudi Arabia’s King Abdullah University of Science and Technology (KAUST), has made substantial progress in optimizing passivation in inverted perovskite solar cells. They introduced thin layers of low-dimensional perovskite at the top and bottom interfaces of the solar cell, achieving remarkable results. The resulting cell exhibited an open-circuit voltage of 1.19 V, a short-circuit current density of 24.94 mA cm2, and a fill factor of 85.9%.

The novel approach involved a double-side heterojunction, incorporating 2D perovskite layers at both the top and bottom interfaces of a 3D perovskite film. In the traditional p-i-n architecture, the hole-selective contact is at the bottom of the intrinsic perovskite layer, with the electron transport layer at the top. This design allows the solar cell to be illuminated through the electron-transport layer (ETL) side. Achieving optimal passivation in such cells requires precise control over the thickness, purity, and dimensionality of the low-dimensional layers.

The researchers identified a specific ligand that exhibited the most effective interaction with the 3D perovskites for double-side passivation. This innovative design resulted in a power conversion efficiency of 25.6%, placing it among the top performers in the field. Crucially, the cell demonstrated a high level of stability, with only a 5% decrease in efficiency after 1,000 hours of exposure to real-world conditions. This stability is a crucial factor for the commercialization of perovskite solar cells.

Understanding the Cell Structure

The inverted perovskite solar cell fabricated by the research team followed a specific structure. The substrate consisted of glass and indium tin oxide (ITO), followed by layers of dimethoxy carbazole (Me-2PACz), a 2D perovskite layer, a 3D perovskite absorber, another 2D perovskite layer, a buckminsterfullerene (C60) electron transport layer, a bathocuproine (BCP) buffer layer, and finally, a silver (Ag) metal contact.

Tests conducted under standard illumination conditions showcased the impressive performance of the cell, with a power conversion efficiency of 25.63%, an open-circuit voltage of 1.19 V, a short-circuit current density of 24.94 mA cm2, and a fill factor of 85.9%. These results were further validated by an accredited testing center, certifying an efficiency of 25.0%, an open-circuit voltage of 1.17 V, a short-circuit current density of 25.0 mA cm2, and a fill factor of 85.7%.

The cell’s ability to retain around 95% of its initial efficiency after 1,000 hours, and 90% under maximum power point tracking (MPPT), indicates the enhanced energy barrier for ion migration, potentially improving perovskite crystal stability.

Research Context: Recent Progress in Inverted Perovskite Solar Cells

The groundbreaking research on inverted perovskite solar cells is part of the broader landscape of advancements in this field. A review published on May 5, 2023, delves into the recent progress in the development of high-efficiency inverted perovskite solar cells. This review acknowledges the attractiveness of PSCs due to their low-cost fabrication and high PCE, while also addressing the stability challenges that have impeded their commercialization.

The p-i-n architecture of inverted PSCs, where the hole-selective contact is at the bottom of the intrinsic perovskite layer with the electron transport layer at the top, is recognized for its concurrent good stability and decent efficiency. The PCE of inverted PSCs has seen significant improvement in recent years, nearly reaching that of n-i-p PSCs. The review highlights the evolution of perovskite compositions, fabrication methods, and counter electrode materials (CEMs) as key factors influencing the efficiency and stability of inverted PSCs.

Understanding the Landscape of Perovskite Solar Cell Development

Perovskite solar cells have undergone more than a decade of development, achieving a remarkable PCE of 25.7%. These cells, with certified efficiencies exceeding 25%, predominantly follow n-i-p structures, categorized into mesoporous and regular structures. The evolution from a dense TiO2 layer to the use of SnO2 as an electron transport material (ETM) has contributed to this efficiency milestone. However, challenges such as hysteresis and stability issues persist, particularly in regular PSCs with hygroscopic hole-transport layers.

In contrast, inverted p-i-n structures have gained attention for their potential to address stability concerns. The development of inverted PSCs has seen significant milestones, starting with the first reported inverted PSC in 2013 with an efficiency of 3.9%. Subsequent advancements led to a certified PCE exceeding 22% in 2020, and by 2022, inverted PSCs surpassed 25% in efficiency. Additionally, inverted perovskite mini-modules demonstrated certified PCEs of 19.2% and 21.07%, approaching the efficiency of regular mini-modules.

The Role of Perovskite Compositions

The composition of the perovskite layer plays a crucial role in the performance of inverted PSCs. Early inverted PSCs utilized methylammonium lead iodide (MAPbI3), but its thermal instability prompted a shift to formamidinium (FA)-based perovskites. FA, with its stronger hydrogen bonding with PbX6 octahedra, exhibited better thermal stability. The review emphasizes the importance of perovskite compositions, noting the transition from MAPbI3 to double-cation (FAMA) perovskite, triple-cation (CsFAMA) mixed-halide perovskite, and FA-Cs mixed-cation perovskites.

Researchers explored the impact of different compositions on device performance, revealing higher short-circuit current (JSC) in FA-based PSCs compared to MA-based PSCs. Introducing Cs+ as a partial replacement for MA+ enhanced crystallization, controlling the crystallization rate and improving perovskite film quality. The size mismatch between Cs+ and FA+ induced beneficial effects, reducing vibrational intensity and enhancing thermal stability.

Advancements in Perovskite Fabrication Methods

The quality of the perovskite film is pivotal for efficient charge generation and collection in inverted PSCs. Different fabrication methods have been explored to achieve compact, uniform, and high-quality perovskite films. Vacuum deposition stands out as a method offering precise control over film thickness and morphology, leading to conformal and dense perovskite films. Despite its advantages, vacuum deposition faces challenges such as high energy consumption and complexity.

Solution methods, including two-step and one-step processes, have gained popularity due to their simplicity and cost-effectiveness. However, achieving uniform and high-quality perovskite films through solution methods remains a challenge, especially for large-scale production. The review outlines strategies for improving film quality and device performance, emphasizing the importance of scalable fabrication methods for large-area perovskite solar modules.

Conclusion

The advancement of inverted perovskite solar cells represents a significant step towards addressing stability concerns and enhancing efficiency in the field of photovoltaics. Achieving a PCE of 25.6% demonstrates the potential of inverted PSCs as a viable alternative for commercial solar energy applications. Continued research into perovskite compositions, fabrication methods, and stability enhancement strategies will be pivotal for realizing the full potential of inverted PSCs and accelerating their adoption in the renewable energy sector.