Name of the Student  : Ms. Amritha Raj
Degree Registered    : Ph.D. Engineering 

 Advisor : Prof. Sushobhan Avasthi, CeNSE

 Date    : 23rd  September  2025  (Tuesday), 3:30 PM

 Venue   : Hybrid  : Seminar Hall  - https://shorturl.at/Sry81

Abstract:

This work presents a comprehensive study on establishing a reliable characterization methodology 
based on the Photoelectromagnetic (PEM) effect in semiconductors. When a semiconductor surface 
is incident by photons of energy higher than its bandgap, electron-hole pairs are generated on 
the surface. Due to the concentration gradient, these excess carriers diffuse into the bulk 
of the semiconductor. When an external magnetic field is applied perpendicular to the diffusion 
gradient, the photo-diffused carriers get deflected in the opposite direction. This generates an
 open circuit voltage and is called the PEM effect. When connected to an external circuit, a PEM
 short circuit current flows in the circuit, which is directly proportional to the mobility,
 magnetic field, photon flux, and effective diffusion length of the semiconductor. The PEM 
short circuit current is proportional to the product of mobility and effective diffusion 
length, whereas the conventional photocurrent is proportional to the mobility lifetime product.
 By combining the PEM effect with the photocurrent (PC) measurements, it is possible to extract
 key transport and recombination parameters such as carrier mobility, diffusion length, surface
 and bulk recombination velocity.

One of the most compelling aspects of the PEM-PC method is its ability to capture bulk properties
without being overshadowed by surface effects, which often complicate results in traditional measurements.
 The ratio of PC to PEM signal allows a direct measurement of the bulk carrier lifetime and bulk
 recombination velocity (BRV), which is defined as the D/Lb, where D is the diffusion coefficient 
and Lb is the bulk diffusion length of the semiconductor. This parameter D/Lb determines the 
open-circuit voltage in solar cells. BRV directly estimates the diode dark saturation current, J0, 
which measures the total recombination in a p−n junction solar cell. In thermal equilibrium, 
the bulk recombination current due to intrinsic defects (i.e., SRH recombination) 
is given as  where  is the minority carrier concentration, and  is the BRV. 
The J0 is directly related to the open circuit voltage, hence the BRV provides an estimate 
for the VOC measured in a practical solar cell. 

The combined PEM and PC data also allow the determination of the lower limit to bulk diffusion length,
 bulk excess carrier lifetime, and carrier mobility. While an accurate lifetime and SRV estimation 
is possible with the knowledge of mobility or diffusion coefficient, the lower bounds serve as a
 qualitative parameter for what values of lifetime and mobility to be expected.

In this study, we also investigate the possibility of characterising ultra-thin perovskite 
single crystals via the PEM-PC technique, specifically in the regime where the sample
 thickness is significantly smaller than the carrier diffusion length. As the focus 
shifts toward miniaturized and low-dimensional devices, the ultrathin limit poses new 
challenges for material characterization due to surface sensitivity and diffusion-limited 
carrier transport. By performing systematic PEM and photocurrent (PC) measurements, 
we established a practical and contact-efficient framework for extracting SRV and mobility 
in low-dimensional semiconductors.

In summary, this work highlights PEM–PC analysis as a powerful diagnostic tool that bridges the gap between material-level characterization and device-level performance, offering new opportunities for rational interface design and optimization of next-generation photovoltaic devices