Basic Research Needs for Solar Energy Utilization

Table 2 Are There Enough Materials for Energy-significant PV Production?

Technology Material

World Production

(MT/yr)a

Materials

Required

(MT)a,b

% of

Current

Production

Annual

Growth

Needed (%)

Crystalline silicon Purified silicon 25,000b 130,000 520 3.7c
Silver (grids, cell
pads)

20,000 6,000 30 0.53

Thin-film Cu (In,
Ga)Se 2 alloys

Indium 250

(by-product)

400 160 2.0d

Selenium 2,200 800 36 0.6e
Gallium 150 70 47 0.9f

Thin-film CdTe Tellurium 450
(2,000 unused by-
product)

933 38

(of total,

including

unused)

2.2

Cadmium 26,000
(by-product)

8 3 0.06

Thin-film silicon Germanium 270
(3,200 unused by-
product)

40 1

(of total,

including

unused)

0.7

a Necessary production for each type of PV technology to produce 20 GW/yr by 2050
b Metric tons
c Elemental silicon is not constrained by supply; current production is low because of low demand.
d Indium is a by-product of zinc, which has been growing at 3%/yr for 50 years. Indium growth will

probably exceed demand because of growth in zinc.
e Selenium is a by-product of copper; an increase of only 0.16% per year would keep pace with demand.
f Gallium is not constrained by supply; current production is low because of low demand.

Transparent Conducting Oxides. Transparent conducting oxides (TCOs) play an important
role in all thin-film solar cells. The key properties for high-quality TCOs are high optical
transmission (high band gap for window materials), low electrical resistivity and high carrier
mobility, low surface roughness (for most devices), good thermal and chemical stability, good
crystallinity (for most devices), adhesion and hardness, and low processing cost. Typical
processes used to fabricate TCOs include chemical and physical vapor deposition, sputtering,
laser ablation, spraying, and solution growth (e.g., chemical bath deposition). Some commonly
used n-type TCOs include indium tin oxide (ITO) and SnO 2 (both available commercially coated
on glass), ZnO, and cadmium stannate (Cd 2 SnO 4 ). Developing p-type TCOs is an important goal,
because it would open up more possibilities for thin-film device structures, particularly
multijunction devices. Materials being investigated include CuAlO 2 , CuInO 2 , CuSrO 2 , and (N,
Ga)-doped ZnO.