Silicon quantum dots (SiQDs) have emerged as a promising material for next-generation photovoltaic devices due to their unique optoelectronic properties. Their tunable bandgap, high absorption coefficient, and potential for carrier multiplication make them suitable for enhancing solar energy conversion. This article explores the application of SiQDs in three key photovoltaic systems: tandem solar cells, luminescent solar concentrators, and hot-carrier solar cells. The discussion also covers challenges and comparisons with perovskite and organic alternatives.

In tandem solar cells, SiQDs serve as an efficient top-layer material due to their adjustable bandgap, which can be tuned by controlling their size. The quantum confinement effect allows SiQDs to absorb higher-energy photons more effectively than bulk silicon, reducing thermalization losses. When integrated with a lower-bandgap bottom cell, such as crystalline silicon or perovskite, the tandem structure achieves a broader absorption spectrum. For example, a SiQD top layer with a bandgap of 1.7 eV can complement a 1.1 eV silicon bottom cell, enabling theoretical efficiency limits exceeding 30%. The SiQD layer also benefits from solution processability, allowing low-cost deposition techniques like spin-coating or inkjet printing. However, challenges remain in optimizing the interfacial layers to minimize recombination losses and ensuring efficient charge transfer between the SiQD layer and the bottom cell.

Luminescent solar concentrators (LSCs) leverage SiQDs as wavelength-shifting materials to enhance light harvesting. SiQDs absorb high-energy photons and re-emit them at longer wavelengths, which are then guided to the edges of the concentrator where photovoltaic cells are placed. The high photoluminescence quantum yield of SiQDs, often exceeding 50%, ensures minimal energy loss during the down-conversion process. Additionally, their narrow emission spectra reduce reabsorption losses, a common issue in traditional LSCs using organic dyes. The stability of SiQDs under prolonged illumination further makes them superior to organic alternatives, which often suffer from photodegradation. Despite these advantages, scattering losses and incomplete light trapping within the LSC structure remain hurdles for achieving high optical efficiency.

Hot-carrier solar cells represent another promising application for SiQDs, capitalizing on their ability to slow carrier cooling rates. In conventional solar cells, hot carriers lose energy as heat within picoseconds, limiting the maximum achievable efficiency. SiQDs, however, exhibit prolonged hot-carrier lifetimes due to quantum confinement and discrete energy levels. This property allows for the extraction of carriers before they thermalize, potentially increasing efficiency beyond the Shockley-Queisser limit. Experimental studies have demonstrated hot-carrier extraction from SiQDs on timescales of several hundred picoseconds, though practical implementation requires further development of energy-selective contacts and reduced interfacial losses.

A critical advantage of SiQDs in photovoltaics is their potential for carrier multiplication, where a single high-energy photon generates multiple electron-hole pairs. This phenomenon, observed in SiQDs with sizes below 5 nm, can significantly enhance photocurrent in solar cells. For instance, carrier multiplication efficiencies of up to 30% have been reported for SiQDs under specific excitation conditions. However, harnessing this effect in practical devices requires precise control over dot size distribution and surface chemistry to minimize non-radiative recombination.

Despite these benefits, SiQD-based photovoltaics face several challenges. Charge extraction losses arise from the insulating ligands often used to stabilize SiQDs in solution, which hinder inter-dot charge transport. Strategies such as ligand exchange with shorter molecules or in-situ doping have shown promise in improving conductivity. Stability under illumination is another concern, as surface oxidation can degrade SiQD performance over time. Encapsulation techniques and passivation layers are being developed to mitigate this issue. Additionally, the reliance on toxic solvents in some synthesis methods raises environmental concerns, driving research toward greener fabrication routes.

When compared to perovskite and organic photovoltaics, SiQDs offer distinct trade-offs. Perovskite solar cells boast higher efficiencies, with record values exceeding 25%, but suffer from instability under moisture and heat. SiQDs, in contrast, exhibit superior thermal and photostability, making them more suitable for long-term outdoor applications. Organic photovoltaics are lightweight and flexible but generally achieve lower efficiencies and degrade faster under UV exposure. SiQDs bridge the gap between stability and performance, though their current efficiencies lag behind perovskites. For instance, SiQD solar cells have demonstrated power conversion efficiencies of around 10%, with room for improvement through better material engineering and device architecture.

In summary, silicon quantum dots hold significant potential for advancing photovoltaic technology across multiple architectures. Their tunable optoelectronic properties enable efficient light harvesting, carrier multiplication, and hot-carrier extraction, addressing key limitations of conventional solar cells. While challenges in charge transport and stability persist, ongoing research is steadily overcoming these barriers. Compared to perovskite and organic alternatives, SiQDs offer a balance of stability and performance, positioning them as a viable candidate for future high-efficiency, durable solar energy solutions. The continued refinement of synthesis methods and device integration will be crucial for unlocking their full potential in real-world applications.