Impact of nanometer air gaps on photon recycling in mechanically stacked multi-junction solar cells.

We investigate photon recycling at the top subcell in mechanically stacked multi-junction solar cells with nanometer air gaps between the subcells. We determine the incident-angle-dependence of the reflectivity from the rear surface of the top subcell. The results show that more than 30% of the luminescence at the top subcell is reflected at the air gap even for an air gap thickness of 10 nm. In addition, we demonstrate enhanced luminescence extraction in GaAs//InGaAsP dual-junction devices with nanometer air gaps compared to a device with no gap between the subcells. Our findings indicate that an efficient photon recycling can be realized even for air gaps of a few tens of nanometers.

Photoluminescence investigation of strictly ordered Ge dots grown on pit-patterned Si substrates.

We investigate the optical properties of ordered Ge quantum dots (QDs) by means of micro-photoluminescence spectroscopy (PL). These were grown on pit-patterned Si(001) substrates with a wide range of pit-periods and thus inter QD-distances (425-3400 nm). By exploiting almost arbitrary inter-QD distances achievable in this way we are able to choose the number of QDs that contribute to the PL emission in a range between 70 and less than three QDs. This well-defined system allows us to clarify, by PL-investigation, several points which are important for the understanding of the formation and optical properties of ordered QDs. We directly trace and quantify the amount of Ge transferred from the surrounding wetting layer (WL) to the QDs in the pits. Moreover, by exploiting different pit-shapes, we reveal the role of strain-induced activation energy barriers that have to be overcome for charge carriers generated outside the dots. These need to diffuse between the energy minimum of the WL in and between the pits, and the one in the QDs. In addition, we demonstrate that the WL in the pits is already severely intermixed with Si before upright QDs nucleate, which further enhances intermixing of ordered QDs as compared to QDs grown on planar substrates. Furthermore, we quantitatively determine the amount of Ge transferred by surface diffusion through the border region between planar and patterned substrate. This is important for the growth of ordered islands on patterned fields of finite size. We highlight that the Ge WL-facets in the pits act as PL emission centres, similar to upright QDs.

Enhanced photocarrier generation in large-scale photonic nanostructures fabricated from vertically aligned quantum dots.

We demonstrate enhanced photocarrier generation using photonic nanostructures fabricated by a wet etching technique with vertically aligned quantum dots (QDs). Using photoluminescence excitation spectroscopy, we found that the photocarrier generation in Ge/Si QDs placed close to the surface is enhanced below the band gap energy of crystalline silicon. The enhancement is explained by light trapping owing to the photonic nanostructures. Electromagnetic wave simulations indicate that the photonic nanostructure with a subwavelength size will be available to light trapping for efficient photocarrier generation by increasing their dip depth.

Enhanced photocarrier generation in large-scale photonic nanostructures fabricated from vertically aligned quantum dots.

We demonstrate enhanced photocarrier generation using photonic nanostructures fabricated by a wet etching technique with vertically aligned quantum dots (QDs). Using photoluminescence excitation spectroscopy, we found that the photocarrier generation in Ge/Si QDs placed close to the surface is enhanced below the band gap energy of crystalline silicon. The enhancement is explained by light trapping owing to the photonic nanostructures. Electromagnetic wave simulations indicate that the photonic nanostructure with a subwavelength size will be available to light trapping for efficient photocarrier generation by increasing their dip depth.

Investigation of the open-circuit voltage in solar cells doped with quantum dots.

Quantum dots (QDs) have attracted much attention for use in photovoltaic applications because of their potential for overcoming the limits of conventional single-junction devices. One problem associated with solar cells using QDs is that the open-circuit voltage (V(oc)) always decreases with the addition of QDs with respect to the reference cell without QDs. Here, we report the investigation of current-voltage characteristics in Ge/Si QD solar cells in the temperature range from 100 to 300 K. We show that even though V(oc) decreases with increasing temperature, it depends on the nominal Ge thickness, indicating that V(oc) reduction is primarily caused by a decrease in the bandgap energy of the cell. From photoluminescence decay measurements, we found that rapid carrier extraction from QDs occurred in the solar cells; this process eliminates the quasi-Fermi energy splitting between the QDs and the host semiconductor and causes V(oc )reduction in QD solar cells.

Simultaneous enhanced photon capture and carrier generation in Si solar cells using Ge quantum dot photonic nanocrystals.

We propose a novel solar cell structure with photonic nanocrystals coupled to quantum dots (QDs) for advanced management of photons and carriers. The photonic nanocrystals at the surface create an extra interaction between the photons and the QDs, which promotes light trapping. Photo-generated carriers can be efficiently transported by preparing vertically aligned QDs with electronic coupling. Implementation of the proposed structure was realized in crystalline Si solar cells with Ge QDs by development of a simple and practical formation method based on a wet chemical process without any lithography techniques. The wet process utilizes a periodically modulated etching rate induced by self-organized Ge QDs. The effectiveness of the proposed solar cell was demonstrated by the marked increase of the absolute conversion efficiency when compared with the control crystalline Si solar cells. It is found that light trapping by the photonic nanocrystals has a larger contribution to the efficiency improvement than the contributions from the carrier transport of the vertically aligned QDs.

Temperature dependence of photoluminescence spectra of nondoped and electron-doped SrTiO3: crossover from auger recombination to single-carrier trapping.

We report unusual photoluminescence (PL) behaviors in highly photoexcited SrTiO(3) crystals at low temperatures. The PL spectrum and dynamics show abrupt changes below 150 K in both nondoped and electron-doped SrTiO(3) samples. We clarified that the PL dynamics in both nondoped and electron-doped SrTiO(3) is well described by the same simple model involving single-carrier trapping, radiative bimolecular recombination, and nonradiative Auger recombination. The unusual temperature dependence of PL dynamics is caused by the crossover from Auger recombination at high temperatures to single-carrier trapping at low temperatures. We discuss the temperature-dependent PL dynamics in conjunction with the high carrier mobility of SrTiO(3) at low temperatures.

Direct and stepwise energy transfer from excitons to plasmons in close-packed metal and semiconductor nanoparticle monolayer films.

We studied the dynamics of photoluminescence (PL) and energy transfer in close-packed monolayer films of CdSe and Au nanoparticles (NPs) assembled using the Langmuir-Blodgett technique. The PL intensity and dynamics depended on the ratio of CdSe to Au NPs in the mixed films. The PL quenching of CdSe NPs occurs through rapid energy transfer from excitons in CdSe NPs to plasmons in Au NPs. The PL decay curves of the mixed NPs monolayers are determined by three decay rates: the direct energy transfer between the nearest-neighbor CdSe and Au NPs (CdSe-->Au), the stepwise energy transfer from CdSe to CdSe to Au NPs (CdSe-->CdSe-->Au), and the radiative recombination in CdSe NPs.

Metal dilution effects on the spin-crossover properties of the three-dimensional coordination polymer Fe(pyrazine)[Pt(CN)4].

The 1A1 left arrow over right arrow 5T2 spin transition has been investigated in the solid solutions of Fe(x)M(1-x)(pyrazine)[Pt(CN)4] (M = Ni or Co, 0 < or = x < or = 1) having a three-dimensional polynuclear structure. Both Ni and Co dilutions tend to decrease the hysteresis width and smooth the transition curves. The enthalpy (entropy) change associated with the spin transition was found to decrease from 26 kJ mol(-1) (84 J K(-1) mol(-1)) for x = 1 to 12 kJ mol(-1) (47 J K(-1) mol(-1)) for 47% Co dilution and to 15 kJ mol(-1) (54 J K(-1) mol(-1)) for 59% Ni dilution. Raman spectroscopy revealed a mixed one- and two-mode behavior in the solid solutions. For the first time, a correlation between vibrational frequencies exhibiting one-mode behavior and the entropy change, which drives the spin crossover, is established.