The back contact influence on characteristics of CdTe/CdS solar cells

Thin Film Cadmium Telluride Solar Cells

“…These defect states will introduce energy levels within the forbidden energy gap of CdTe that can affect minority carrier lifetimes in particular. 8,61-63 Self-compensation of CdTe limits the acceptor level that can be achieved; [2][3][4][5][6][7][8]64 if this effect can be reduced to increase p-type doping in CdTe, overall PV cell efficiencies can improved. 2,4,8…”

“…5,25 Finding a suitable metal for this purpose is a challenge due to the high electron affinity of CdTe and difficulty in obtaining high p-type conductivity to form a good ohmic contact. [5][6][7]25 This can lead to the formation of the Schottky barrier (f b ) which would restrict majority (hole) carrier conduction.…”

“…5,25 Finding a suitable metal for this purpose is a challenge due to the high electron affinity of CdTe and difficulty in obtaining high p-type conductivity to form a good ohmic contact. [5][6][7]25 This can lead to the formation of the Schottky barrier (f b ) which would restrict majority (hole) carrier conduction. [5][6][7] In these conditions, the device can be electrically described [75][76][77] as a two diode model, with the p-n junction representing one diode and the back contact acting as an opposed diode with associated shunt resistance.…”

“…75,76 The presence of a large barrier at the back contact results in 'roll-over' (kink) effects, observed in I-V curves, due to current saturation at high forward voltage. 5,75,76 Another approach for investigating barrier formation at the back contact is to consider the space charge regions at the p-n junction and back contact using the energy band diagrams. 75,78 When the two space charge regions overlap, depending on the barrier height and CdTe thickness, the effect of the barrier can be reduced due to closer CdS-CdTe band alignment.…”

“…[2][3][4][5][6][7][8] Unwanted defect levels acting as carrier traps are introduced into CdTe due to its self-compensation nature and require passivation using post-growth treatment with CdCl 2 deposition and annealing, as discussed further in this chapter. The conductivity of CdTe can be induced by depositing under certain conditions.…”