Customized Electron Transport Layer Boosts Flexible Solar Cell Efficiency and Stability

Perovskite quantum dot (PQD) solar cells, specifically CsPbI3PQD, have shown significant potential for use in optoelectronic devices due to their extraordinary photo-electrical properties. The power conversion efficiency (PCE) of these solar cells can exceed 16% through optimization strategies. A key challenge in fabricating high-performance flexible CsPbI3PQD solar cells is the preparation of an electron transport layer (ETL) capable of forming high-quality thin films at low temperatures. A recent study designed a customized ETL using a UV sintering method, achieving a PCE of 12.70%, the highest among reported flexible quantum dot solar cells. This research could pave the way for wider adoption of flexible CsPbI3PQD solar cells.

What are Perovskite Quantum Dot Solar Cells and Why are They Important?

Perovskite quantum dot (PQD) solar cells are a type of solar cell that have shown significant potential for use in optoelectronic devices such as photodetectors, light-emitting diodes, and solar cells due to their extraordinary photo-electrical properties. Among the PQDs, CsPbI3PQD has emerged as a promising material for next-generation solar cells because of its appropriate bandgap energy, making it suitable for single and multi-junction solar cells.

The power conversion efficiency (PCE) of these solar cells has been shown to exceed 16% through optimization strategies such as ligand engineering and additive engineering. A distinct advantage of CsPbI3PQD is its ability to be easily deposited on any substrate at desired nanoscale thicknesses using a layer-by-layer (LBL) method at room temperature. This facilitates the fabrication of devices even on lightweight and flexible polymer-based substrates.

Moreover, CsPbI3PQD films are expected to provide better mechanical stability than bulk perovskite films due to their nanoscale grain boundaries and soft surface ligands, underscoring their potential as flexible device components. A flexible solar cell employing CsPbI3PQD as a light absorber can achieve the highest PCE among flexible colloidal quantum dot solar cells.

What is the Role of the Electron Transport Layer (ETL) in PQD Solar Cells?

The electron transport layer (ETL) is a critical component in PQD solar cells, significantly impacting their photovoltaic performance and stability. Low-temperature ETL deposition methods are especially desirable for fabricating flexible solar cells on polymer substrates.

The fabrication of CsPbI3PQD solar cells necessitates the sequential deposition of layers to construct the device architecture comprising transparent conducting oxide (TCO), an n-type ETL, CsPbI3PQD, a p-type hole transport layer (HTL), and a metal electrode. In order to realize flexible photovoltaic devices on plastic substrates, all procedures must employ a low-temperature process below 150°C compatible with polymer substrates.

Among these, n-type metal oxides traditionally used for ETLs necessitate high-temperature thermal annealing to enhance crystallinity and interparticle connectivity. Specifically, to prepare high-performance TiO2 and SnO2 ETL, the fabrication process typically necessitates temperatures of 450-550°C and 180-200°C respectively.

What are the Challenges in Fabricating High-Performance Flexible CsPbI3PQD Solar Cells?

The primary challenge in fabricating high-performance flexible CsPbI3PQD solar cells lies in the preparation of an ETL capable of forming high-quality thin films even at low temperatures (<150°C). A potential solution involves the use of low-temperature-processable colloidal tin oxide (SnO2) nanocrystals (SnO2NP) ETL, which are commonly used in flexible perovskite solar cells.

However, SnO2NP may not be the ideal ETL because low-temperature processing is known to induce surface defects and low crystallinity. Additionally, the compatibility of CsPbI3PQDs with ETLs initially designed for bulk perovskite solar cells may result in energy-level mismatches. Thus, it becomes imperative to develop a customized ETL with suitable energy levels that can be prepared at room temperature to attain high-performance flexible CsPbI3PQD solar cells.

How Can a Customized ETL Improve the Performance of Flexible CsPbI3PQD Solar Cells?

The design of an ETL specifically for flexible CsPbI3PQD solar cells can offer several benefits including improved photovoltaic performance through energy level control, enhanced device stability by providing an appropriate interface with the adjacent ETL, and minimized damage to the polymer substrate through the consistent use of a low-temperature process during the entire fabrication process.

In a recent study, a SnO2-based ETL using a UV sintering method was designed to fabricate high-performance flexible CsPbI3PQD solar cells. Initially, colloidal SnO2 curved nanorods (SnO2CNRs) were synthesized, which were capped with oleic acid (OA) and oleylamine (OAm) to ensure quality dispersibility and stability.

For the application of SnO2CNRs as an ETL, they were dispersed in hexane, after which the organic ligands were removed at a low temperature (50°C) via UV irradiation following a spin-coating process. The energy level of SnO2CNRs was further controlled by doping them with gallium.

What are the Results of Using a Customized ETL in Flexible CsPbI3PQD Solar Cells?

The proposed ETL-based CsPbI3PQD solar cell achieves a power conversion efficiency (PCE) of 12.70%, the highest PCE among reported flexible quantum dot solar cells, maintaining 94% of the initial PCE after 500 bending tests. Consequently, it was demonstrated that a systematically designed ETL enhances the photovoltaic performance and mechanical stability of flexible optoelectronic devices.

This research represents a significant step forward in the development of high-performance flexible CsPbI3PQD solar cells. By addressing the challenges associated with the fabrication of the ETL, the researchers were able to significantly improve the performance and stability of these solar cells. This work could pave the way for the wider adoption of flexible CsPbI3PQD solar cells in the future.

What is the Future of Flexible CsPbI3PQD Solar Cells?

The future of flexible CsPbI3PQD solar cells looks promising. The development of a customized ETL that can be prepared at room temperature could significantly improve the performance and stability of these solar cells, making them more suitable for use in a variety of applications.

However, further research is needed to continue improving the performance and stability of these solar cells. This includes further optimization of the ETL, as well as the development of new materials and fabrication techniques. With continued research and development, flexible CsPbI3PQD solar cells could play a significant role in the future of solar energy.