Role of nanowire length on the performance of a self-driven NIR photodetector based on mono/bi-layer graphene (camphor)/Si-nanowire Schottky junction.

In this article, we have demonstrated a solid carbon source such as camphor as a natural precursor to synthesize a large area mono/bi-layer graphene (MLG) sheet to fabricate a nanowire junction-based near infrared photodetectors (NIRPDs). In order to increase the surface-to-volume ratio, we have developed Si-nanowire arrays (SiNWAs) of varying lengths by etching planar Si. Then, the camphor-based MLG/Si and MLG/SiNWAs Schottky junction photodetectors have been fabricated to achieve an efficient response with self-driven properties in the near infrared (NIR) regime. Due to a balance between light absorption capability and surface recombination centers, devices having SiNWAs obtained by etching for 30 min shows a better photoresponse, sensitivity and detectivity. Fabricated NIRPDs can also be functioned as self-driven devices which are highly responsive and very stable at low optical power signals up to 2 V with a fast rise and decay time of 34/13 ms. A tremendous enhancement has been witnessed from 36 µA W-1 to 22 mA W-1 in the responsivity at 0 V for MLG/30 min SiNWAs than planar MLG/Si PDs indicating an important development of self-driven NIRPDs based on camphor-based MLG for future optoelectronic devices.

Effective light polarization insensitive and omnidirectional properties of Si nanowire arrays developed on different crystallographic planes.

In this paper, fabrication of vertical Si nanowire arrays (SiNWAs) by a facile metal assisted chemical etching approach on different crystallographic planes of Si has been reported. A very low specular reflectance (R spec ) of 0.04% and 0.03% has been achieved in the whole visible range for SiNWAs grown on Si(100) and Si(111) oriented substrates, respectively. High broadband enhancement has been detected for vertical SiNWAs due to multiple scattering paths inside the nanowire arrays. On the other hand, inclined nanowires showed a fascinating behavior at the longer wavelength regime, where light gets the longer path to reflect back-forth and ease to reflect back outward at normal incidence. Moreover, for [100] SiNWAs, transverse electric field component demonstrates the strong polarization insensitive properties at the expense of transverse magnetic field component with a minimum reflectance of <2% up to 1200 nm. The [100] SiNWAs demonstrates extraordinary omnidirectional properties at θ B ≥ 58°. Theoretical validation of COMSOL with an effective medium approach reveal the effective dipole coupling and the presence of strong absorption modes for vertical SiNWs at a typical wavelength regime. The highly bound states of the particle tunneling through classical forbidden region shows a strong dependence on the gradient in the refractive index (mi ) from 1 to 3.4. The high order scattering effect is observed at ∼520 cm-1 in a disordered optical medium. This novel finding of light localization properties for SiNWAs with different orientation gives a new route to support various photonic applications.

Electro-analytical investigation of potential induced degradation in mc-silicon solar cells: case of sodium ion induced inductive loop.

Potential induced degradation of the shunt type (PID-s) in multi-crystalline silicon (mc-Si) solar cells is becoming critical for performance reduction of solar panels in large scale photovoltaic (PV) power plants. In this article PID-s has been investigated by applying high voltage stress on mc-Si solar cells for their degradation and recovery and results have been explained on the basis of DC and AC characterization. The efficiency decreases drastically from 15.7% to 2.9% due to a high voltage stress of -800 V at 85 °C for 48 hours, which is attributed to a reduction in shunt resistance and an increase in depletion and diffusion capacitances. The reduction in electrical performance due to PID-s has been further explained by morphological, structural and elemental analysis. Observed negative capacitance behaviour in impedance spectra of mc-Si solar cells after PID-s has been attributed to structural deformation caused by potential induced migration of sodium ions (Na+) into mc-Si. The structural deformation induced by potential induced migration of Na+ ions has been confirmed by using non-destructive and lattice strain sensitive micro-Raman spectroscopy. The obtained experimental results have been correlated with existing theoretical understanding of p-n junction solar cells to explain the consequences of PID-s.