1. Chapter V July 15, 2015 Junctions of Photovoltaics

2. Electron Affinity and Vacuum Level

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

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

6. Quasi Thermal Equilibrium under applied electric bias or exposure of light

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

12. Transport Equation in Crystal

15. Charge Separation of a Photovoltaic Device

16. Origin of Photovoltaic Action

17. 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

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

23. Electron flow Hole flow

24. Reverse biasForward bias

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

27. 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.

29. Limitations of the Schottky Barrier Junction

30. 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.

32. Can lead to enhanced recombination in the junction region

33. Surface and Interface States

34. V bi

36. 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

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