Substrates

Requirements on the substrate are: (i) low cost, (ii) high stability (thermal, me­chanical, chemical), (iii) good isolation (electrical, chemical), (iv) small thermal expansion mismatch to thin films, (v) low weight, and, perhaps, (vi) flexibility and (vii) transparency. No substrate fulfills all requirements. For the superstrate con­figuration transparency is important. Remarkably, glasses of very different thermal expansion coefficients were used for superstrate CdTe cells such as soda lime glass [540], borosilicate glass [541], and aluminosilicate glass (see Table 5.1). FeOx, being an additive to the standard soda lime glass, induces optical absorption which accounts for ~1 mA cm-2 Jsc loss compared to borosilicate glass [541]. The loss can be reduced by use of low iron soda lime glass (white glass). Also the low softening

Chalcogenide Photovoltaics: Physics, Technologies, and Thin Film Devices. Roland Scheer and Hans-Werner Schock

Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31459-1

Table 5.1 Properties of substrates for chalcogenide solar cells.

Substrate

Root mean square roughness (nm)

Thermal expansion coefficient (300 K; 10-6 K-1)

Softening temperature (0 C)

Comment

Float glass

0.5 [543]

9.0

550

Contains -15% Na2O

Borosilicate

0.5

3.0-4.0

820

Contains ~4% Na2O,

glass

Aluminosilicate

~5.0

900

-13% B2O3 Contains -36% Al2O3

glass Kapton E

16 (320-570 K)

Polyimide foil

Upilex S

12.0-24.0

Polyimide foil

Ti sheet

(290-670 K)

8.6-9.7 (290-770 K)

Steel sheet

20 [544]

13.0

Cr steel

20-40 [544]

11.0

Contains -16 at% Cr

Mo

4.8-5.9 (290-870 K)

temperature of soda lime glass is a problem for large substrates (see Table 5.1), especially in bottom-up thin film deposition. The current world record CdTe solar cell has been prepared on borosilicate glass (Corning 7059) of 1 mm thickness [8]. Also, polymer substrates allow us to realize the superstrate configuration. By the use of 10 pm Upilex foils, more than 11% efficiency has been realized [542].

For a substrate-type CdTe cell, metal and polymer foils can be employed. As metal substrate, molybdenum has been used. This metal does not form a too large electric barrier [545]. The barrier can be further reduced by applying a nitrogen-doped ZnTe interfacial layer (for back contacts see Section 5.1.8). A problem of metal substrates is the high surface roughness, which depends on the production process. Adhesion of the thin films may become poor at dimples and cavities. Particles, loosely attached to the metal surface, can cause shunts if they detach during the solar cell process. Polishing and brushing of the metal foil can reduce these problems.

5.1.2

Updated: July 1, 2015 — 12:14 am