Chapter 8 Thin Film Solar Cells July 12, 2015

Requirements for suitable materials Doping and charge transport are more difficult for a-Si.

Amorphous silicon

Absorption

Doping

Low doping efficiency means P is much less than N D. Femi level moves toward the defect level in the center of band gap and majority carrier activation energy is large. The built-in potential is less. In addition, recombination centers for minority carriers increases.

More explanations V bi = [E Fn (w n ) – E Fp (w p )]/q, V bi = φ n –φ p = -(E i – E F )/q – [-(E i – E F )]/q = kTln[N A N D /n i 2 ]/q P p0 = p n0 exp[qV bi /kT] V oc could not increase by increasing N D or N A.

Transport

The small diffusion length of a-Si is attributed to the small mobility of the carriers. Mobility of c-Si is 0.5 m 2 V -1 s -1 for electron with low doping.

Stability

C; absorption by defects of dangling bond. B: absorption by Urbach tail due to variation of bond length and angle.

the i-region thickness to around 0.5 μm. Amorphous Silicon Solar Cell Design

The p-i-n solar cell device physics

Fabrication of a-Si solar Cells

Light induced degradation: The Staebler Wronski effect is the most important barrier to widespread use of a-Si solar cells. Light-induced degradation is stronger Strategies to improve a-Si cell performance

where E F is above E C or below E V.