Chapter 8 Thin Film Solar Cells

April 21, 2023

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 ND.

• 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

• Vbi = [EFn(wn) – EFp(wp)]/q,

• Vbi = φn –φp = -(Ei – EF)/q – [-(Ei – EF)]/q

= kTln[NAND/ni2]/q

Pp0 = pn0exp[qVbi/kT]

Voc could not increase by increasing ND or N

A. kTEEc

FceNn /)(

Transport

The small diffusion length of a-Si is attributed to the small mobility of the carriers. Mobility of c-Si is 0.5 m2V-1s-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 EF is above EC or below EV.