Suppression Of Nonradiative Recombination Research Articles

Time-Resolved Microphotoluminescence Study of Cu(In,Ga)Se2

The carrier recombination processes in Cu(In1-x,Gax)Se2 (CIGS) thin films were investigated by time-resolved microscopic-photoluminescence (µ-PL) measurement at room temperature. For films with x = 0.45, the spatial distribution of the donor–acceptor pair luminescence is much larger than the grain size. The PL decay lifetime is directly correlated with the µ-PL intensity, but the spectral shape is identical regardless of the sample position. These results suggest that the nonuniform distribution of nonradiative recombination centers mainly affects the carrier recombination in CIGS thin films. At relatively high Ga concentrations (x ≥0.7), the spatial inhomogeneity in µ-PL intensity is enhanced and decay accelerated, suggesting an increase in the density of nonradiative recombination centers. Thus, suppression of nonradiative recombination is critical to enhancing the performance of CIGS-based solar cells.

Violet-to-red photoluminescence of spiro-TAD organic films doped with different organic dyes

Abstract Photoluminescence (PL) and photoluminescence excitation (PLE) spectra of undoped spiro-TAD films and spiro-TAD films doped either by the organic dyes coumarin 7 or DCM as well as by both of these dyes simultaneously were investigated at different dye concentrations. A widened PL spectrum caused by doping was encountered and violet-to-red emission was obtained. It was established that excitation of the dyes is realized most efficiently through spiro-TAD. The overall integral PL intensity of the coumarin 7 doped films increased with dye concentration due to the suppression of nonradiative recombination in the film caused by a transfer of spiro-TAD excitation energy to the dye molecules. Mainly radiative energy transfer from semiconductor to dye molecules occurs in the case of DCM doping. No mutual influence on the luminescence of both dyes in the spiro-TAD film was observed and as a consequence, the PL band intensity of each dye can be adjusted separately.

Suppression of nonradiative recombination in small size semiconductors

Enhanced optical efficiency of nanosized semiconductors is due to reduction in the nonradiative recombination rate, as the size of the grain is decreased. A theoretical basis for such a reduction is obtained by a classical approach by calculating surface recombination velocity in a confined configuration. This explains the reduction of nonradiative recombination in small size grains.

Control of dark currents in multi-quantum well solar cells fabricated by atomic H-assisted molecular beam epitaxy

We demonstrate that the dark current of a multi-quantum well (MQW) solar cell can be reduced significantly if grown by atomic H-assisted molecular beam epitaxy (H-MBE). Atomic H irradiated during MBE growth acts to passivate the defects and smooth heterointerfaces are formed. Further, it is shown that the dark current can be reduced in a modified MQW configuration consisting of two-step QWs as opposed to conventional square-shaped QWs. The suppression of non-radiative recombination at the MQW heterointerfaces, which is achieved by both schemes, is expected to realize improved performance of MQW-based solar cells and photovoltaic devices.

Dynamics of Zero-Dimensional Excitons in a Semimagnetic Environment

Time-resolved measurements on self-assembled CdSe/ZnMnSe quantum dots are presented. A rather long excitonic decay time of 580 ps is found which indicates a suppression of nonradiative recombination via the internal Mn2+ transition. The long excitonic lifetime allows for the formation of magnetic polarons, as evidenced by a transient redshift of the luminescence emission with a time constant of 125 ps. In a magnetic field, a decrease of the measured excitonic decay time is observed and a possible mechanism based on an interplay between bright and dark excitons is discussed.

Dynamics of Zero-Dimensional Excitons in a Semimagnetic Environment

Time-resolved measurements on self-assembled CdSe/ZnMnSe quantum dots are presented. A rather long excitonic decay time of 580 ps is found which indicates a suppression of nonradiative recombination via the internal Mn 2+ transition. The long excitonic lifetime allows for the formation of magnetic polarons, as evidenced by a transient redshift of the luminescence emission with a time constant of 125 ps. In a magnetic field, a decrease of the measured excitonic decay time is observed and a possible mechanism based on an interplay between bright and dark excitons is discussed.

Comparison Study of Structural and Optical Properties of InxGa1−xN/GaN Quantum Wells with Different In Compositions

The effect of In on the structural and optical properties of InxGa1−xN/GaN multiple quantum wells (MQWs) was investigated. These were five-period MQWs grown on sapphire by metalorganic chemical vapor deposition. Increasing the In composition caused broadening of the high-resolution x-ray diffraction superlattice satellite peak and the photoluminescence-excitation bandedge. This indicates that the higher In content degrades the interface quality because of nonuniform In incorporation into the GaN layer. However, the samples with higher In compositions have lower room temperature (RT) stimulated (SE) threshold densities and lower nonradiative recombination rates. The lower RT SE threshold densities of the higher In samples show that the suppression of nonradiative recombination by In overcomes the drawback of greater interface imperfection.

A Comparison of the Optical Characteristics of AlGaN, GaN, and InGaN Thin Films

We have investigated the optical properties of InGaN, GaN, and AlGaN epilayers using photoluminescence (PL), PL excitation, time-resolved PL, and optically pumped stimulated emission (SE) spectroscopy. The InGaN layers had a larger (i) Stokes shift, (ii) spectral broadening, (iii) decay time, and (iv) PL redshift with time than AlGaN layers of comparable alloy composition. The optically pumped SE behavior of InGaN layers is significantly different than that of GaN and AlGaN layers. This is attributed to the suppression of nonradiative recombination and the elimination of below-gap induced absorption in highly excited InGaN alloys, effects that accompany the incorporation of indium into GaN. In contrast, the SE behavior of AlGaN layers is similar to that of highly excited GaN.

Comparison Study of Structural and Optical Properties of InxGa1-xN/GaN Quantum Wells with Different in Compositions

AbstractThe effect of In on the structural and optical properties of InxGa1−xN/GaN multiple quantum wells (MQWs) was investigated. These were five-period MQWs grown on sapphire by metalorganic chemical vapor deposition. Increasing the In composition caused broadening of the high-resolution x-ray diffraction superlattice satellite peak and the photoluminescence-excitation bandedge. This indicates that the higher In content degrades the interface quality because of nonuniform In incorporation into the GaN layer. However, the samples with higher In compositions have lower room temperature (RT) stimulated (SE) threshold densities and lower nonradiative recombination rates. The lower RT SE threshold densities of the higher In samples show that the suppression of nonradiative recombination by In overcomes the drawback of greater interface imperfection.