Pradyumna Muralidharan

Hot hole transport in a-Si/c-Si heterojunction solar cells

The transport behavior of photogenerated minority carriers in an a-Si/c-Si heterojunction solar cell is dependent on the energy distribution function (EDF) of the carriers impinging on the hetero-interface. The high field region at the interface results in a strongly non-Maxwellian distribution of holes incident on the surface, which has implications for current collection and a significant impact on the overall efficiency of the device. This work studies the effect of the high field transport on photogenerated carriers at the hetero-interface through a combination of Monte Carlo simulations and analysis of defect assisted transport. A three band warped non-parabolic band model is implemented to describe the valence band in order to accurately represent high energy photocarriers. Also, percolation path theory is applied to study defect assisted transport in the intrinsic amorphous region by considering mechanisms such as defect capture through tunneling, emission through Poole - Frenkel effect, and emission through tunneling.

OPTODET - tool to model LWIR and MWIR region for HgCdTe photodetectors

The tool OPTODET has been developed to investigate and explain the device characteristics of p⁺-n HgCdTe Photodetector at low temperatures. In this paper the performance of narrow band gap Hg<inf>1−x</inf>Cd<inf>x</inf>Te (x=0.225) at 78 K and wide band gap Hg<inf>1−x</inf>Cd<inf>x</inf>Te (x=0.3) at 250 K have been analyzed. Our theoretical model considers complete Fermi-Dirac statistics and all the major recombination mechanisms. The performance of the device has been studied and simulated as a function of parameters such as doping and temperature. The dark current - voltage characteristics have been simulated and analyzed theoretically. Dark Current as low as I<inf>D</inf>=10⁻¹⁰ was obtained at 78 K. For x=0.225, a peak detectivity of equation was obtained.

A Multiscale Modeling Approach to Study Transport in Silicon Heterojunction Solar Cells

Single junction solar cells based on Silicon continue to be relevant and commercially successful in the market due to their high efficiencies and relatively low cost processing. Heterojunction solar cells based on crystalline (c-Si) and amorphous (a-Si) silicon (HIT Cells) have paved the way for devices with high VOCs (>700 mV) and high efficiencies (>20%) [1]. Panasonic currently holds the world record efficiency of 25.6% for its trademark a-Si/c-Si HIT cell [2]. The novel structure of the device precludes the usage of traditional methods (such as drift diffusion) to accurately understand the nature of transport. Theoretical models used by commercial simulators make a variety of assumptions that simplifies the transport problem (assumes a Maxwellian distribution of carriers) and thus lacks the sophistication to study defect transport. In this work we utilize a combination of Ensemble Monte Carlo (EMC) simulations, Kinetic Monte Carlo (KMC) simulations and traditional drift - diffusion (DD) simulations t...

Advanced tunneling models for solar cell applications

This paper studies the potential of wide bandgap tunnel junctions such as AlGaAs/GaAs and GaAs/GaAs configurations for multi - junction solar cells. Simulations were performed to study the dominant physical mechanisms in tunnel junctions such as band to band tunneling and trap assisted tunneling. 1-D Drift Diffusion simulations were performed to determine the different regions in the I-V characteristics, namely, the tunneling current, excess current and classical diffusion current. We outline the implementation of local and non-local tunneling models to understand the nature of peak current. The variation of peak voltage is examined with the addition of band gap narrowing and different effective masses.

Modeling of InAs/GaSb tunnel junction

We simulate a type III InAs/GaSb broken gap junction to evaluate its potential as a tunnel junction. The unique properties of broken gap will enhance current transport between the subcells of a multijunction solar cell and reduce the overall series resistance. A drift diffusion simulator has been developed to study the current characteristics of novel heterostructures. The effect all the major recombination mechanisms such as Shockley - Read - Hall, Radiative, and Auger on the current are included. Also, we investigate the effect of degeneracy on the device by considering the effect of band gap narrowing. In our model we also add effects of thermionic emission and band to band tunneling. As solar cells operate at low voltages we have added a model for low field mobility which might be a limiting factor.

Modeling of HgCdTe photodetectors in the LWIR region

We have developed a computer program that simulates the electrical characteristics of a p+ - n HgCdTe photodetector. Using solutions to the Poisson and Continuity equations we investigate low temperature behavior to determine optimum working conditions to enhance detectivity. Our model considers complete Fermi - Dirac statistics, major recombination mechanisms, band to band tunneling, trap assisted tunneling and impact ionization. Device performance was analyzed as a function of doping and temperature. Simulations show detectivity >; 1011 Jones at 77 K for Hg0.78Cd0.22Te.