Nanooptics and Photonics 

Parametric study of p-Cd1-yMnyTe/n-CdTe graded bandgap solar cell
Danylov A.B.1, Ilchuk H.A.1, Petrus R.Yu.1, Sokolovskii B.S.2
1 Lviv Polytechnic National University, S. Bandera Str., 12, Lviv-79013, Ukraine.
E-mail: andriy.b.danylov@lpnu.ua
2 Ivan Franko Lviv National University, Universytetska str., 1, Lviv-79000, Ukraine.

Graded bandgap A2B6- and A3B5-semiconductors are perspective materials for photovoltaic devices. Such semiconductors are materials with variable coordinate dependent chemical composition, and band structure directly depends on the composition. Changing the composition of a solid solution leads to the change of bandgap width and inclination of the conduction band bottom and/or top of the valence band. Changing the position of the conduction band bottom creates a driving force for electrons  and for holes .  This force tries to move electrons to the region of lower field and holes to the region of higher field. And hence, in the region of the variable concentration appears a quasi-electric field  for electrons and   – for holes. This supplies effective separation of minority carriers in solar cell and as a result leads to decrease of carrier recombination and to increase of solar cell efficiency.
We consider the graded bandgap structure of p-Cd1-yMnyTe / n-CdTe, in which the ternary compound of p-type conductivity is characterized by a change in the concentration of manganese from y = 0.5 at the interface with air to y = 0 on the metallurgical boundary. In this case the ternary compound is doped with acceptor, and cadmium telluride – with donor impurities. The structure is irradiated from the p-region side. 
The numerical simulation of coordinate profile of carrier concentration based on continuity equations with boundary conditions was carried out. Current densities were determined considering generation and recombination of carriers for solar spectrum AM1,5. Volt-ampere characteristics for different parameter variation were built and fill factors and efficiencies of solar cells for different profiles of graded bandgap were calculated.