NASA unveils its toughest challenges yet
17:15 09 February 2006
"The Station-Keeping Solar Sail Challenge
A solar sail pushed through space by the force of the Sun's photons to a target could earn $2.5 million with an equal amount available for keeping a solar sail for 90 days at a fixed point in space."
http://www.newscientistspace.com/channel/human-spaceflight/dn8701-nasa-unveils-its-toughest-challenges-yet.html

To be made into solar cells will require the sheets to made in both p-type and n-type semiconductors. The sheets appear to be formable into both types:

Strong, Transparent, Multifunctional, Carbon Nanotube Sheets.
SCIENCE,VOL 309, 19 AUGUST 2005, p. 1215-1219.
"However, black sheets of solution spun
MWNTs have been used as nontransmissive
hole-collecting electrodes in solar
cells (10), and transparent p-type SWNT
sheets have been used as hole-injection electrodes
in inorganic LEDs based on gallium
nitride (32)."
http://www.cnrs-imn.fr/GDRE_NanoE/Textes/transparentsheets.pdf

Transparent, Conductive Carbon Nanotube Films.
Science 27 August 2004:Vol. 305. no. 5688, pp. 1273 - 1276.
"For the nanotubes, the ease of chemical charge-transfer doping to obtain such transparency-versus-conductivity optimization (via exposure of the nanotubes to vapors of appropriate chemicals) provides an additional advantage for the t-SWNTs. Moreover, charge transport in these t-SWNTs is p-type, unlike the far more common transparent conducting oxides [e.g., indium tin oxide (ITO)], which are n-type. This should permit new complementary applications and alternative photonic coupling schemes (3)."
http://www.sciencemag.org/cgi/content/full/305/5688/1273

And there are various ways now available for making general nanotubes of either type:

CARBON NANOTUBE COMPUTER CIRCUITS
Novel processing and microfabrication lead to first single-molecule logic gate.
"The principal challenge in constructing NOT gates out of nanotubes, Avouris explained, is that invariably, without special processing, transistors fashioned from nanotubes are p-type--that is, they conduct positive charge carriers (holes). But NOT gates require n-type transistors, the type that conduct negative charge carriers (electrons), as well as p-type.
"Some researchers have demonstrated recently that doping nanotubes with electropositive elements such as potassium is a viable method for preparing n-type nanotube transistors. Now the IBM team has discovered another way to do it. Simply heating (annealing) p-type nanotube transistors in vacuum converts p-type into n-type, they reported."
http://pubs.acs.org/cen/topstory/7936/7936notw1.html

Nanotubes: Surprising Sensitivity To Oxygen Creates New Possibilities.
March 29, 2000
"We've demonstrated that carbon nanotubes can behave as both n-type and p-type semiconductors. Until now, all nanotube measurements had suggested p-type conducting behavior only."
"In their paper, the Berkeley researchers found that the degree of oxygen exposure is the determining factor as to whether a carbon nanotube functions as an n-type or p-type semiconductor. The ability to function as either type is critical if nanotubes are to ever replace conventional silicon devices."
http://www.lbl.gov/Science-Articles/Archive/zettl-nanotubes.html

And another research team may have already created thin-flim nanotube solar cells of comparable lightness to the tranparent nanotube films:

Sensational Materials: Cheap solar cells based on nanotubes.
Future Materials News - February, 2006
"Professor Nunzio Motta of QUT's School of Engineering Systems and Dr Eric Waclawik from the School of Physical and Chemical Sciences are developing the new renewable energy source using nanotechnology. Weighing only 10 micrograms per square centimetre, it's possible to generate additional power by linking up the polymer tiles in a patchwork that increases the size of the device to suit the user's needs."
http://www.future.org.au/news_2006/feb/cheap.html

Bob Clark