Chapter V July 15, 2015 Junctions of Photovoltaics

Electron Affinity and Vacuum Level

Quasi Thermal Equilibrium under applied electric bias or exposure of light Quasi Fermi level distribution

Quasi Thermal Equilibrium under applied electric bias or exposure of light Quasi Fermi level In general, T n =T B =T

Quasi Thermal Equilibrium under applied electric bias or exposure of light

Current Density under Bias – Boltzmann Transport Equation and the Relaxation Time Approximation J(r) = J n (r) + J p (r)

Transport Equation in Crystal

Charge Separation of a Photovoltaic Device

Origin of Photovoltaic Action

Difference in work function Difference in electron affinity due to compositional gradient creating an effective field A gradient in the effective conduction band density of states

Contact of n-type semiconductor and metal with Φ m >Φ n Electron flow Hole flow

Electron flow Hole flow

Reverse biasForward bias

Contact of p-type semiconductor and metal with Φ m <Φ p

Ohmic Contacts: a low resistance contact for the majority carriers Φ m >Φ p Φ m <Φ n Upon illumination, no photovoltage across the junction can be established.

Limitations of the Schottky Barrier Junction

The junction region is depleted of both electrons and holes and always presents a barrier to majority carriers, and a low resistance path to minority carriers. It drives the collection of minority carriers which are photogenerated throughout the p and n layers, and reach the junction by diffusion.

Can lead to enhanced recombination in the junction region

Surface and Interface States

V bi

Effect of Interface States on a p-n Junction More of the potential difference is dropped on the n side, large amount of mobile holes will accumulate beside the interface

Interface States at a metal- semiconductor junction This is small for n-type- metal contact, but is large for p-type-metal contact.