Publication database

Conventional photoluminescence (PL) yields at most one emitted photon for each absorption event. Downconversion (or quantum cutting) materials can yield more than one photon by virtue of energy transfer processes between luminescent centers. In this work, we introduce Gd₂O₂S:Tm³⁺ as a multi-photon quantum cutter. It can convert near-infrared, visible, or ultraviolet photons into two, three, or four infrared photons of ∼1800 nm, respectively. The cross-relaxation steps between Tm³⁺ ions that lead to quantum cutting are identified from (time-resolved) PL as a function of the Tm³⁺ concentration in the crystal. A model is presented that reproduces the way in which the Tm³⁺ concentration affects both the relative intensities of the various emission lines and the excited state dynamics and providing insight in the quantum cutting efficiency. Finally, we discuss the potential application of Gd₂O₂S:Tm³⁺ for spectral conversion to improve the efficiency of next-generation photovoltaics.

Organic photodetectors are interesting for low cost, large area optical sensing applications. Combining organic semiconductors with discrete absorption bands outside the visible wavelength range with transparent and conductive electrodes allows for the fabrication of visibly transparent photodetectors. Visibly transparent photodetectors can have far reaching impact in a number of areas including smart displays, window-integrated electronic circuits and sensors. Here, we demonstrate a near-infrared sensitive, visibly transparent organic photodetector with a very high average visible transmittance of 68.9%. The transmitted light of the photodetector under solar irradiation exhibits excellent transparency colour perception and rendering capabilities. At a wavelength of 850 nm and at −1 V bias, the photoconversion efficiency is 17% and the specific detectivity is 10¹² Jones. Large area photodetectors with an area of 1.6 cm² are demonstrated.

Perovskite solar cells with submicrometre-thick CH₃NH₃PbI₃ or CH₃NH₃PbI_3–xCl_x active layers show a power conversion efficiency as high as 15%. However, compared to the best-performing device, the average efficiency was as low as 12%, with a large standard deviation (s.d.). Here, we report perovskite solar cells with an average efficiency exceeding 16% and best efficiency of 17%. This was enabled by the growth of CH₃NH₃PbI₃ cuboids with a controlled size via a two-step spin-coating procedure. Spin-coating of a solution of CH₃NH₃I with different concentrations follows the spin-coating of PbI₂, and the cuboid size of CH₃NH₃PbI₃ is found to strongly depend on the concentration of CH₃NH₃I. Light-harvesting efficiency and charge-carrier extraction are significantly affected by the cuboid size. Under simulated one-sun illumination, average efficiencies of 16.4% (s.d. ± 0.35), 16.3% (s.d. ± 0.44) and 13.5% (s.d. ± 0.34) are obtained from solutions of CH₃NH₃I with concentrations of 0.038 M, 0.050 M and 0.063 M, respectively. By controlling the size of the cuboids of CH₃NH₃PbI₃ during their growth, we achieved the best efficiency of 17.01% with a photocurrent density of 21.64 mA cm^–2, open-circuit photovoltage of 1.056 V and fill factor of 0.741.

Luminescence upconversion nanocrystals capable of converting two low-energy photons into a single photon at a higher energy are sought-after for a variety of applications, including bioimaging and photovoltaic light harvesting. Currently available systems, based on rare-earth-doped dielectrics, are limited in both tunability and absorption cross-section. Here we present colloidal double quantum dots as an alternative nanocrystalline upconversion system, combining the stability of an inorganic crystalline structure with the spectral tunability afforded by quantum confinement. By tailoring its composition and morphology, we form a semiconducting nanostructure in which excited electrons are delocalized over the entire structure, but a double potential well is formed for holes. Upconversion occurs by excitation of an electron in the lower energy transition, followed by intraband absorption of the hole, allowing it to cross the barrier to a higher energy state. An overall conversion efficiency of 0.1% per double excitation event is achieved.

Dye-sensitized solar cells are a promising alternative to traditional inorganic semiconductor-based solar cells. Here we report an open-circuit voltage of over 1,000 mV in mesoscopic dye-sensitized solar cells incorporating a molecularly engineered cobalt complex as redox mediator. Cobalt complexes have negligible absorption in the visible region of the solar spectrum, and their redox properties can be tuned in a controlled fashion by selecting suitable donor/acceptor substituents on the ligand. This approach offers an attractive alternate to the traditional I₃⁻/I⁻ redox shuttle used in dye-sensitized solar cells. A cobalt complex using tridendate ligands [Co(bpy-pz)₂]^3+/2+(PF₆)_3/2 as redox mediator in combination with a cyclopentadithiophene-bridged donor-acceptor dye (Y123), adsorbed on TiO₂, yielded a power conversion efficiency of over 10% at 100 mW cm⁻². This result indicates that the molecularly engineered cobalt redox shuttle is a legitimate alternative to the commonly used I₃⁻/I⁻ redox shuttle.

Multi-photon ionization (MPI) spectroscopy of solid surfaces under ambient conditions and in nitrogen has been established and exemplified for a variety of materials. This was accomplished using a dedicated experimental setup that monitors the photoelectron yield as a function of the laser wavelength. The MPI spectra resemble the absorption characteristics, however, possess more peaks and are more detailed. This demonstrated the possibility to apply MPI spectroscopy for fast analysis of solids. The dependence of the signals upon the laser flux implies that the ionization mechanism depends on the examined molecule and in many cases it is a two-step process, via a long-living intermediate energy state. The method provides quantification in the pmole range and allows for surface imaging.

A new nondestructive analytical method for diagnosis of tooth caries is presented. The method is based on the Multiphoton Ionization (MPI) fast conductivity signals measured from tooth surfaces. The signals are acquired for a series of laser wavelengths, thus obtaining full MPI spectra. The results indicate a good correlation between the MPI results and the degree of severity of the caries, as diagnosed using traditional inspection. Moreover, the spectral information can be reduced (using least squares fitting) to a single parameter that provides an objective quantitative estimation of the caries severity. The MPI data can be obtained for tiny points on the dental surface and it is suggested that mapping is possible by scanning method.

Using metallorganic vapor-phase epitaxy, thin films of gallium nitride activated by Eu³⁺(GaN : Eu³⁺) have been deposited on sapphire substrates at atmospheric pressure. Luminescence from Eu³⁺ ions in GaN has been investigated using photoluminescence (PL) and PL excitation spectroscopy. Experimental results show that Eu³⁺ ions are excited via energy transfer from the host. Analyses of the observed emission and excitation spectra indicate occupancy of multiple sites in the nitride lattice. Using a pulsed laser source, variation of emission intensity with increasing excitation intensity has also been examined. The possibility of emission saturation at high excitation intensity is discussed from the perspective of application in light-emitting diode sources.