K. L. Narasimhan

On light‐induced effect in amorphous hydrogenated silicon

Amorphous silicon films prepared by a discharge of 10% SiH4‐90% H2 mixture are shown to have properties comparable to those prepared from 100% SiH4. These films are found to be quite stable against prolonged light exposure.

Band Gap Tuning of CH3NH3Pb(Br1–xClx)3 Hybrid Perovskite for Blue Electroluminescence

We report on the structural, morphological and optical properties of AB(Br(1-x)Cl(x))3 (where, A = CH3NH3(+), B = Pb(2+) and x = 0 to 1) perovskite semiconductor and their successful demonstration in green and blue emissive perovskite light emitting diodes at room temperature. The bandgap of perovskite thin film is tuned from 2.42 to 3.16 eV. The onset of optical absorption is dominated by excitonic effects. The coulomb field of the exciton influences the absorption at the band edge. Hence, it is necessary to explicitly account for the enhancement of the absorption through the Sommerfield factor. This enables us to correctly extract the exciton binding energy and the electronic bandgap. We also show that the lattice constant varies linearly with the fractional chlorine content satisfying Vegards law.

High response organic visible-blind ultraviolet detector

The authors demonstrate a high efficiency visible-blind ultraviolet organic photodetector with a response of 30mA∕W. The active layer is a blend of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) and tris(8-hydroxyquinoline) aluminum (Alq3). The authors show that the spontaneous as well as the electric field induced carrier generation efficiencies in the blend are enhanced over its constituents. The spontaneous carrier generation efficiency measured from total carrier collection measurements is 30% in the blend. The photoluminescence of the blend shows an efficient energy transfer from the TPD to Alq3 molecule. The mobility-lifetime (μτ) product in the blend is 2.2×10−12cm2∕V. The μτ product is weakly dependent on temperature.

Separating Charges at Organic Interfaces: Effects of Disorder, Hot States, and Electric Field.

Charge separation at organic-organic (O-O) interfaces is crucial to how many organic-based optoelectronic devices function. However, the mechanism of formation of spatially separated charge carriers and the role of geminate recombination remain topics of discussion and research. We review critically the contributions of the various factors, including electric fields, long-range order, and excess energy (beyond the minimum needed for photoexcitation), to the probability that photogenerated charge carriers will be separated. Understanding the processes occurring at the O/O interface and their relative importance for effective charge separation is crucial to design efficient solar cells and photodetectors. We stress that electron and hole delocalization after photoinduced charge transfer at the interface is important for efficient free carrier generation. Fewer defects at the interface and long-range order in the materials also improve overall current efficiency in solar cells. In efficient organic cells, external electric fields play only a small role for charge separation.

Anomalous temperature dependence of photoluminescence in porous silicon

We have studied the temperature dependence of luminescence in porous silicon between 10 and 300 K. We find that above 60 K the luminescence has a temperature dependence opposite in sign from that of the band gap. Using a Gaussian decomposition procedure we have identified three different processes which dominate the luminescence at different temperatures. With a knowledge of the temperature dependence of the relative intensities of the three components we explain the blue shift of the luminescence peak with increasing temperature. We also present arguments to show that the luminescence in porous silicon is due to a complex.

Optical Absorption in AlQ

In this paper, we report on the optical absorption of tris(8-hydroxyquinolato) aluminium (Alq) sublimed thin films between 0.7 and 5 eV and compare it with absorption in solution between 1.5 and 5 eV. Specifically, we show that above 2.5 eV the absorption in solution and in the thin film are very similar to each other. The absorption falls off exponentially with energy from 3.18 eV in both thin film and in solution. Below 2.5 eV the absorption spectrum of the thin film changes qualitatively and is nearly independent of energy down to 0.7 eV. Exposure of the film to atmosphere significantly affects the subband gap absorption and causes an increase in the defect density.

Effect of Thermal and Structural Disorder on the Electronic Structure of Hybrid Perovskite Semiconductor CH3NH3PbI3

In this Letter, we investigate the temperature dependence of the optical properties of methylammonium lead iodide (MAPbI3 = CH3NH3PbI3) from room temperature to 6 K. In both the tetragonal (T > 163 K) and the orthorhombic (T < 163 K) phases of MAPbI3, the band gap (from both absorption and photoluminescence (PL) measurements) decreases with decrease in temperature, in contrast to what is normally seen for many inorganic semiconductors, such as Si, GaAs, GaN, etc. We show that in the perovskites reported here, the temperature coefficient of thermal expansion is large and accounts for the positive temperature coefficient of the band gap. A detailed analysis of the exciton line width allows us to distinguish between static and dynamic disorder. The low-energy tail of the exciton absorption is reminiscent of Urbach absorption. The Urbach energy is a measure of the disorder, which is modeled using thermal and static disorder for both the phases separately. The static disorder component, manifested in the exciton line width at low temperature, is small. Above 60 K, thermal disorder increases the line width. Both these features are a measure of the high crystal quality and low disorder of the perovskite films even though they are produced from solution.

Measurement of deep states in hole doped organic semiconductors

In this paper, we report on the electrical properties of hole doped N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine and tris(8-hydroxyquinoline) aluminum. Tetrafluorotetracyanoquinodimethane is used as the dopant. From the frequency dependence of the capacitance measurements, we show that deep levels dominate the capacitance. From the frequency and bias dependence, we estimate the density of deep levels above the Fermi level to be about 1017 cm−3. These states presumably arise due to polarization and Coulomb energy fluctuations.

Effect of nitrogen doping on glow-discharge amorphous silicon films

Abstract Amorphous silicon films prepared from a d.c. discharge of 10% SiH4–90% H2 mixture are found to have electrical and optical properties similar to those made from 100% SiH4. The effect of nitrogen on the properties of these films is investigated. It is found that instead of behaving as a classical donor, nitrogen introduces deep levels in the material.

Current transport in a-Si:Ge alloy Schottky barriers

In this paper, we have investigated the temperature dependence of forward and reverse currents of a‐Si:Ge:H‐Pd barriers. In contrast to a‐Si:H, we find that the diode quality factor is 2 and is independent of temperature. We have confirmed quantitatively that the forward current is recombination limited and the reverse current is generation limited. The barrier height ΦB=0.7 eV, which is half the measured optical gap. The frequency, bias, and temperature dependence of capacitance have also been investigated. From the capacitance measurements, we infer that the density of states near midgap is approximately 2×1017 cm−3 eV−1.