Integrating upconversion nanoparticles (UCNPs) such as NaYF4:Yb/Er into perovskite solar cells (PSCs) presents a promising strategy to enhance light harvesting by converting sub-bandgap photons into usable higher-energy photons. These nanoparticles absorb near-infrared (NIR) light and emit visible or ultraviolet light, which can then be absorbed by the perovskite layer, thereby increasing the overall photocurrent and efficiency of the solar cell. The success of this approach depends on several factors, including the energy transfer mechanisms, nanoparticle placement, synthesis methods, quantum yield optimization, and spectral matching, as well as overcoming challenges like aggregation and parasitic absorption.

Upconversion is a nonlinear optical process where low-energy photons are converted into higher-energy photons through sequential absorption and energy transfer steps. In NaYF4:Yb/Er nanoparticles, ytterbium (Yb) acts as a sensitizer, absorbing NIR photons at around 980 nm, while erbium (Er) serves as the activator, emitting photons in the visible range (540 nm and 660 nm). The energy transfer occurs via Yb-to-Er cross-relaxation, where excited Yb ions transfer energy to neighboring Er ions, promoting them to higher energy states. Subsequent nonradiative relaxation and photon emission result in upconverted light that matches the absorption spectrum of the perovskite material, typically methylammonium lead iodide (MAPbI3) or formamidinium lead iodide (FAPbI3).

The placement of UCNPs within the solar cell architecture is critical for maximizing photon utilization. Two primary configurations are explored: front-side and rear-side integration. Front-side placement involves embedding UCNPs within or atop the electron transport layer (ETL), allowing NIR light to be upconverted before reaching the perovskite layer. This approach minimizes losses from parasitic absorption in other layers but may suffer from reduced NIR light penetration due to scattering or reflection. Rear-side integration positions UCNPs behind the perovskite layer, often within the hole transport layer (HTL) or atop the metal electrode. Here, unabsorbed NIR light that passes through the perovskite is upconverted and reflected back into the active layer. This method avoids interference with the primary light absorption process but requires efficient back-reflectance to prevent photon loss.

Synthesis methods for NaYF4:Yb/Er UCNPs must ensure high crystallinity, uniform size distribution, and minimal surface defects to maximize quantum yield. Hydrothermal and solvothermal techniques are commonly employed, offering control over particle size and morphology. For instance, a typical hydrothermal synthesis involves dissolving yttrium, ytterbium, and erbium precursors in a solvent mixture, followed by heating at 180-200°C for several hours. Oleic acid or other surfactants are often used to stabilize the nanoparticles and prevent aggregation. Post-synthesis ligand exchange with shorter molecules like polyethylene glycol (PEG) can improve dispersibility in polar solvents, facilitating integration into solar cell layers.

Quantum yield (QY) optimization is essential for practical applications. The QY of UCNPs depends on factors such as dopant concentration, crystal phase (hexagonal vs. cubic), and surface passivation. For NaYF4:Yb/Er, the hexagonal phase exhibits higher QY due to reduced nonradiative decay pathways. Optimal dopant concentrations typically range from 18-20% for Yb and 2-5% for Er, balancing absorption and emission efficiency. Core-shell structures, where an inert shell (e.g., NaYF4) encapsulates the doped core, further enhance QY by suppressing surface-related quenching. Reported QY values for optimized NaYF4:Yb/Er UCNPs under 980 nm excitation range from 0.1% to 1.5%, depending on excitation power density and measurement conditions.

Spectral matching between UCNP emission and perovskite absorption is crucial for efficient energy transfer. The emission peaks of NaYF4:Yb/Er at 540 nm and 660 nm align well with the absorption onset of MAPbI3 (≈800 nm), ensuring that upconverted photons are readily absorbed. However, the relatively weak emission intensity of UCNPs necessitates high nanoparticle loading, which can lead to aggregation and light scattering. Strategies to mitigate aggregation include embedding UCNPs in a transparent matrix (e.g., polymethyl methacrylate) or depositing them as a thin, uniform layer via spin-coating or spray pyrolysis.

Parasitic absorption in UCNP-integrated PSCs arises from unintended light absorption by non-active components, such as the HTL or ETL. For example, spiro-OMeTAD, a common HTL, exhibits weak absorption in the NIR region, which can compete with UCNPs. To minimize this, alternative HTLs with lower NIR absorption or optimized UCNP placement can be employed. Additionally, plasmonic effects from metal electrodes may enhance or interfere with upconversion, depending on the distance between UCNPs and the metal surface.

Experimental studies have demonstrated efficiency gains in UCNP-integrated PSCs. For instance, a PSC with rear-side NaYF4:Yb/Er nanoparticles showed a 1-2% absolute increase in power conversion efficiency (PCE), attributed to enhanced NIR harvesting. The exact improvement depends on UCNP loading, solar cell architecture, and illumination conditions. Theoretical modeling suggests that the maximum achievable efficiency gain from upconversion is limited by the QY of UCNPs and the fraction of sub-bandgap photons in the solar spectrum. Under AM1.5G illumination, the additional photocurrent from upconversion is estimated to be 1-3 mA/cm², assuming ideal photon management and a QY of 1%.

Challenges remain in scaling up UCNP-integrated PSCs for commercial applications. Stability concerns include potential degradation of UCNPs under prolonged illumination or thermal stress, as well as ion migration at the perovskite-UCNP interface. Cost-effective synthesis of high-QY UCNPs and their uniform integration into large-area solar cells are also ongoing research areas. Despite these hurdles, the combination of upconversion nanotechnology with perovskite photovoltaics holds significant potential for pushing the efficiency limits of solar energy conversion. Future work may explore alternative UCNP compositions, such as those doped with Tm or Ho for different emission spectra, or hybrid systems combining upconversion with downshifting or plasmonic enhancement.