Photovoltaic cells are able to convert sunlight into electricity, providing enough of the most abundant and cleanest energy to cover our energy needs. However, the efficiency of current photovoltaics is significantly impeded by the transmission loss of sub-band-gap photons. Photon upconversion is a promising route to circumvent this problem by converting these transmitted sub-band-gap photons into above-band-gap light, where solar cells typically have high quantum efficiency. Here, we summarize recent progress on varying types of efficient upconversion materials as well as their outstanding uses in a series of solar cells, including silicon solar cells (crystalline and amorphous), gallium arsenide (GaAs) solar cells, dye-sensitized solar cells, and other types of solar cells. The challenge and prospect of upconversion materials for photovoltaic applications are also discussed

The inability to utilize near infrared (NIR) light has posed a stringent limitation for the efficiencies of most single-junction photovoltaic cells such as dye-sensitized solar cells (DSSCs). Here, we describe a strategy to alleviate the NIR light harvesting problem by upconverting non-responsive NIR light in a broad spectral range (over 190 nm, 670–860 nm) to narrow solar-cell-responsive visible emissions through incorporated dye-sensitized upconversion nanoparticles (DSUCNPs). Unlike typically reported UCNPs with narrow and low NIR absorption, the organic dyes (IR783) anchored on the DSUCNP surface were able to harvest NIR photons broadly and efficiently, and then transfer the harvested energy to the inorganic UCNPs (typically reported), entailing an efficient visible upconversion. We show that the incorporation of DSUCNPs into the TiO₂ photoanode of a DSSC is able to elevate its efficiency from 7.573% to 8.568%, enhancing the power conversion efficiency by about 13.1%. We quantified that among the relative efficiency increase, 7.1% arose from the contribution of broad-band upconversion in DSUCNPs (about ∼3.4 times higher than the highest previously reported value of ∼2.1%), and 6.0% mainly from the scattering effect of DSUCNPs. Our strategy has immediate implications for the use of DSUCNPs to improve the performance of other types of photovoltaic devices.

Cross-relaxation among neighboring emitters normally causes self-quenching and limits the brightness of luminescence. However, in nanomaterials, cross-relaxation could be well-controlled and employed for increasing the luminescence efficiency at specific wavelengths. Here we report that cross-relaxation can modulate both the brightness of single upconversion nanoparticles and the threshold to reach population inversion, and both are critical factors in producing the ultra-low threshold lasing emissions in a micro cavity laser. By homogenously coating a 5-μm cavity with a single layer of nanoparticles, we demonstrate that doping Tm³⁺ ions at 2% can facilitate the electron accumulation at the intermediate state of ³H₄ level and efficiently decrease the lasing threshold by more than one order of magnitude. As a result, we demonstrate up-converted lasing emissions with an ultralow threshold of continuous-wave excitation of ~150 W/cm² achieved at room temperature. A single nanoparticle can lase with a full width at half-maximum as narrow as ~0.45 nm.

Various energy trapping centers are employed to simultaneously suppress concentration quenching and tune luminescence output through efficiently confining the excitation energy in Er³⁺-sensitized upconversion nanoparticles.

Hexagonal NaYF₄:Nd³⁺/Yb³⁺/Er³⁺ microcrystals and nanocrystals with well-defined morphologies and sizes have been synthesized via a hydrothermal route. The rational control of initial reaction conditions can not only result in upconversion (UC) micro and nanocrystals with varying morphologies, but also can produce enhanced and tailored upconversion emissions from the Yb³⁺/Er³⁺ ion pairs sensitized by the Nd³⁺ ions. The increase of reaction time converts the phase of NaYF₄:Nd³⁺/Yb³⁺/Er³⁺ particles from the cubic to the hexagonal structure. The added amount of oleic acid plays a critical role in the shape evolution of the final products due to their preferential attachment to some crystal planes. The adjustment of the molar ratio of F⁻/Ln³⁺ can range the morphologies of the β-NaYF₄:Nd³⁺/Yb³⁺/Er³⁺ microcrystals from spheres to nanorods. When excited by 808 nm infrared laser, β-NaYF₄:Nd³⁺/Yb³⁺/Er³⁺ microplates exhibit a much stronger UC emission intensity than particles with other morphologies. This phase- and morphology-dependent UC emission holds promise for applications in photonic devices and biological studies.