Inorganic perovskite nanocrystals, particularly cesium lead halide (CsPbX3, where X = Cl, Br, I), have emerged as promising top-cell materials in tandem solar configurations due to their exceptional optoelectronic properties. These materials exhibit tunable bandgaps, high absorption coefficients, and superior charge-carrier mobilities, making them ideal candidates for pairing with established photovoltaic technologies such as silicon or copper indium gallium selenide (CIGS) bottom cells. Their compatibility with solution-processing techniques further enhances their potential for scalable manufacturing.
The bandgap of CsPbX3 nanocrystals can be precisely tuned across the visible spectrum by varying the halide composition. For instance, CsPbI3 exhibits a bandgap of approximately 1.73 eV, while CsPbBr3 and CsPbCl3 have bandgaps of 2.3 eV and 3.0 eV, respectively. Mixed halide compositions, such as CsPb(Br/I)3, enable intermediate bandgaps, allowing optimization for spectral matching in tandem devices. This tunability is critical for maximizing the utilization of the solar spectrum. In a tandem configuration, the top cell absorbs high-energy photons, while the bottom cell captures lower-energy photons, thereby reducing thermalization losses and improving overall efficiency.
Thermal stability is a key advantage of inorganic perovskite nanocrystals over their organic-inorganic hybrid counterparts. CsPbX3 materials demonstrate higher resistance to thermal degradation, maintaining structural integrity at temperatures exceeding 300°C. This property is particularly important for industrial processing and long-term operational stability. However, phase instability in CsPbI3 at room temperature remains a challenge, often requiring compositional engineering or surface passivation to stabilize the desired cubic phase.
Synthesis of CsPbX3 nanocrystals is typically achieved through hot-injection or ligand-assisted methods. The hot-injection technique involves rapid injection of precursor solutions into a high-temperature solvent, resulting in monodisperse nanocrystals with controlled sizes and morphologies. Ligand-assisted methods, on the other hand, rely on the use of long-chain organic ligands such as oleic acid and oleylamine to stabilize the nanocrystals during growth. Both approaches yield high-quality materials with narrow emission linewidths and high photoluminescence quantum yields, often exceeding 80%. Recent advances have also demonstrated room-temperature synthesis routes, which could further reduce production costs.
In tandem solar configurations, spectral matching between the top and bottom cells is critical for optimal performance. Silicon bottom cells, with a bandgap of 1.1 eV, are commonly paired with CsPbX3 top cells having bandgaps in the range of 1.7–1.9 eV. This combination theoretically enables a maximum efficiency exceeding 35%, significantly higher than single-junction silicon cells. Similarly, CIGS bottom cells, with bandgaps adjustable between 1.0–1.7 eV, can be matched with wider-bandgap CsPbX3 top cells to achieve comparable efficiencies. The use of perovskite nanocrystals also allows for the fabrication of semi-transparent top cells, which transmit unabsorbed light to the underlying bottom cell.
Current matching between subcells is a major challenge in tandem device design. The photocurrent generated by the top and bottom cells must be closely matched to avoid efficiency losses. For CsPbX3/Si tandems, this requires careful optimization of the perovskite layer thickness and composition to ensure balanced absorption. Interfacial losses at the junction between the subcells further complicate device performance. Non-radiative recombination at interfaces and parasitic absorption in charge-transport layers can significantly reduce the open-circuit voltage and fill factor. Advanced interfacial engineering, such as the incorporation of ultrathin buffer layers or graded heterojunctions, has shown promise in mitigating these losses.
Record efficiencies for perovskite-based tandem solar cells have progressed rapidly in recent years. All-perovskite tandems have achieved efficiencies above 24%, while perovskite/silicon tandems have surpassed 29%. These results highlight the potential of CsPbX3 nanocrystals to outperform conventional single-junction technologies. However, scalability remains a critical consideration for commercial viability. Solution-processing techniques, such as blade coating or slot-die coating, are being explored for large-area deposition of perovskite layers. The compatibility of these methods with roll-to-roll manufacturing could enable cost-effective production of tandem modules.
Despite these advances, several challenges must be addressed to realize the full potential of CsPbX3 nanocrystals in tandem solar cells. Long-term stability under operational conditions, including exposure to moisture, oxygen, and light, requires further improvement. Encapsulation strategies and the development of more robust charge-transport materials are active areas of research. Additionally, the toxicity of lead-based perovskites has prompted investigations into less hazardous alternatives, such as tin or germanium-based compositions, though these often compromise performance.
The prospects for multi-junction devices incorporating CsPbX3 nanocrystals are promising. With continued advancements in material synthesis, device architecture, and interfacial engineering, these materials could play a pivotal role in next-generation photovoltaics. Their ability to combine high efficiency with scalable fabrication positions them as a leading candidate for meeting the growing demand for low-cost, high-performance solar energy solutions. Future research will likely focus on further improving stability, reducing material costs, and integrating perovskite top cells with emerging bottom-cell technologies to push the boundaries of photovoltaic performance.