Photon upconversion is promising for applications such as biological imaging, data storage or solar cells. Here, we have investigated upconversion processes in a broad range of gadolinium-based nanoparticles of varying composition. We show that by rational design of a core-shell structure with a set of lanthanide ions incorporated into separated layers at precisely defined concentrations, efficient upconversion emission can be realized through gadolinium sublattice-mediated energy migration for a wide range of lanthanide activators without long-lived intermediary energy states. Furthermore, the use of the core-shell structure allows the elimination of deleterious cross-relaxation. This effect enables fine-tuning of upconversion emission through trapping of the migrating energy by the activators. Indeed, the findings described here suggest a general approach to constructing a new class of luminescent materials with tunable upconversion emissions by controlled manipulation of energy transfer within a nanoscopic region.

Tuning upconversion through energy migration in core–shell nanoparticles

Photovoltaic (PV) technologies for solar energy conversion represent promising routes to green and renewable energy generation. Despite relevant PV technologies being available for more than half a century, the production of solar energy remains costly, largely owing to low power conversion efficiencies of solar cells. The main difficulty in improving the efficiency of PV energy conversion lies in the spectral mismatch between the energy distribution of photons in the incident solar spectrum and the bandgap of a semiconductor material. In recent years, luminescent materials, which are capable of converting a broad spectrum of light into photons of a particular wavelength, have been synthesized and used to minimize the losses in the solar-cell-based energy conversion process. In this review, we will survey recent progress in the development of spectral converters, with a particular emphasis on lanthanide-based upconversion, quantum-cutting and down-shifting materials, for PV applications. In addition, we will also present technical challenges that arise in developing cost-effective high-performance solar cells based on these luminescent materials.

Enhancing solar cell efficiency: the search for luminescent materials as spectral converters

Lanthanide-doped upconversion nanocrystals enable anti-Stokes emission with pump intensities several orders of magnitude lower than required by conventional nonlinear optical techniques. Their exceptional properties, namely large anti-Stokes shifts, sharp emission spectra and long excited-state lifetimes, have led to a diversity of applications. Here, we review upconversion nanocrystals from the perspective of fundamental concepts and examine the technical challenges in relation to emission colour tuning and luminescence enhancement. In particular, we highlight the advances in functionalization strategies that enable the broad utility of upconversion nanocrystals for multimodal imaging, cancer therapy, volumetric displays and photonics.

Controlling upconversion nanocrystals for emerging applications

The design and characterization of metal–organic complexes for optoelectronic applications is an active area of research. The metal–organic complex offers unique optical and electronic properties arising from the interplay between the inorganic metal and the organic ligand. The ability to modify chemical structure through control over metal–ligand interaction on a molecular level could directly impact the properties of the complex. When deposited in thin film form, this class of materials enable the fabrication of a wide variety of low-cost electronic and optoelectronic devices. These include light emitting diodes, solar cells, photodetectors, field-effect transistors as well as chemical and biological sensors. Here we present an overview of recent development in metal–organic complexes with controlled molecular structures and tunable properties. Advances in extending the control of molecular structures to solid materials for energy conversion and information technology applications will be highlighted.

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Recent progress in metal–organic complexes for optoelectronic applications

Photon upconversion of near-infrared photons is a promising way to overcome the Shockley–Queisser efficiency limit of 32% of a single-junction solar cell. However, the practical applicability of the most efficient known upconversion materials at moderate light intensities is limited by their extremely weak and narrowband near-infrared absorption. Here, we introduce the concept of an upconversion material where an organic near-infrared dye is used as an antenna for the b-NaYF4:Yb,Er nanoparticles in which the upconversion occurs. The overall upconversion by the dye-sensitized nanoparticles is dramatically enhanced (by a factor of ∼3,300) as a result of increased absorptivity and overall broadening of the absorption spectrum of the upconverter. The proposed concept can be extended to cover any part of the solar spectrum by using a set of dye molecules with overlapping absorption spectra acting as an extremely broadband antenna system, connected to suitable upconverters.

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Broadband dye-sensitized upconversion of near-infrared light

The use of lanthanide ions to convert photons to different, more useful, wavelengths is well-known from a wide range of applications (e.g. fluorescent tubes, lasers, white light LEDs). Recently, a new potential application has emerged: the use of lanthanide ions for spectral conversion in solar cells. The main energy loss in the conversion of solar energy to electricity is related to the so-called spectral mismatch: low energy photons are not absorbed by a solar cell while high energy photons are not used efficiently. To reduce the spectral mismatch losses both upconversion and downconversion are viable options. In the case of upconversion two low energy infrared photons that cannot be absorbed by the solar cell, are added up to give one high energy photon that can be absorbed. In the case of downconversion one high energy photon is split into two lower energy photons that can both be absorbed by the solar cell. The rich and unique energy level structure arising from the 4fn inner shell configuration of the trivalent lanthanide ions gives a variety of options for efficient up- and downconversion. In this perspective an overview will be given of recent work on photon management for solar cells. Three topics can be distinguished: (1) modelling of the potential impact of spectral conversion on the efficiency of solar cells; (2) research on up- and downconversion materials based on lanthanides; and (3) proof-of-principle experiments. Finally, an outlook will be given, including issues that need to be resolved before wide scale application of up- and downconversion materials can be anticipated.

Lanthanide ions as spectral converters for solar cells

Efficient quantum cutting by the Pr3+-Yb3+ couple is demonstrated for the conversion of blue/green to near-infrared (NIR) radiation with quantum efficiencies close to 200%. The resonant two-step energy transfer process, which involves one higher-energy photon becoming two NIR photons, may lead to the reduction of energy loss in solar cells, revealing how spectral conversion is a promising avenue for boosting solar-cell efficiency.

Near‐Infrared Quantum Cutting for Photovoltaics

Downconversion is a promising avenue to boost the efficiency of solar cells by absorbing one higher energy visible photon and emitting two lower energy near-infrared (NIR) photons. Here the efficiency of downconversion for the (Er3+,Yb3+) couple is investigated in NaYF4, a well-known host lattice for efficient upconversion with (Er3+,Yb3+). Analysis of the excitation and emission spectra for NaYF4 doped with 1% Er3+ and codoped with 0%, 5%, 10%, or 30% Yb3+ show that visible to NIR downconversion is inefficient. Downconversion by the scheme based on the reverse of the upconversion process is hampered by fast multiphonon relaxation from the F47/2 level (the starting level for downconversion) to the S43/2 level. Energy transfer from the S43/2 level of Er3+ to Yb3+ is shown to be inefficient. Efficient downconversion from the G411/2 of Er3+ level is observed, resulting in emission of two photons (one around 980 nm and one around 650 nm) after absorption of a single 380 nm photon.

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Downconversion for solar cells in NaYF4:Er,Yb

Energy losses inherent to the conversion of sunlight to electricity in solar cells are mainly due to the so-called spectral mismatch: low energy photons are not absorbed while the energy of high energy photons is only partly used by the solar cell. The losses can be significantly reduced by adapting the solar spectrum. A promising avenue is the use of a downconversion material where one higher energy visible (blue-green) photon is “cut” into two lower-energy near-infrared photons that both can be used by the solar cell. Here the efficiency of downconversion for the (Nd3+,Yb3+) couple in YF3 is studied to investigate if efficient two-step energy transfer occurs from the 4G9/2 level of Nd3+ (situated around 21?000?cm?1 or 470 nm) exciting two neighboring Yb3+ ions to the 2F5/2 level (around 10?000?cm?1 or 1000 nm). Optical measurements of YF3 doped with Nd3+ and Yb3+ show that there is efficient energy transfer from Nd3+ to Yb3+, but downconversion from the 4G9/2 level does not occur due to fast multiphonon relaxation. Relaxation from this level to lower-energy levels populates the 4F3/2 level of Nd3+ from which efficient one-step energy transfer to Yb3+ occurs. Analysis of the luminescence decay curves for different Yb3+-concentrations using Monte Carlo simulations reveals a high nearest neighbor transfer rate (3.3×105?s?1) through a dipole-dipole interaction mechanism. Downconversion is observed from the 4D3/2 level (situated in the UV, around 28?000?cm?1 or 360 nm) with an estimated quantum efficiency up to 140%. For application in solar cells this UV to 2 NIR downconversion will only result in a marginal reduction of spectral mismatch losses.

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Downconversion for solar cells in YF3:Nd3+, Yb3+

Energy losses in solar cells caused by the spectral mismatch can be reduced by adapting the solar spectrum using a downconversion material where one higher energy visible photon is ‘cut' into two lower energy near-infrared photons that both can be absorbed by the solar cell. Downconversion with the (Pr3+, Yb3+) couple in YF3 is investigated. Based on analysis of luminescence and diffuse reflectance spectra it is evident that two-step energy transfer takes place from the 3P0 level of Pr3+ (around 490 nm) exciting two Yb3+ to the 2F5/2 level giving emission around 980 nm. The transfer efficiency increases with Yb3+ concentration and is 86% for YF3 doped with 0.5% Pr3+ and 30% Yb3+. Due to concentration quenching the intensity of emission from Yb3+ is strongly reduced and the 2F5/2 emission intensity reaches a maximum for the sample with 0.5% Pr3+ and 2–5% Yb3+ at 300 K. Temperature dependent measurements reveal the role of the Pr3+ 1G4 level in the energy transfer between Pr3+ and Yb3+. Back-transf...

Linda Aarts

Papers

Lanthanide ions as spectral converters for solar cells

Near‐Infrared Quantum Cutting for Photovoltaics

Downconversion for solar cells in NaYF4:Er,Yb

Downconversion for solar cells in YF3:Nd3+, Yb3+

Downconversion for Solar Cells in YF3:Pr3+, Yb3+