Here is an example response curve with internal gain: http://www.sionyx.com/advantage.html. The QE for a black silicon solar cell can be found in figure 4 of this patent: http://www.freepatentsonline.com/20100224229.pdf. Solar cell performance, measured at 0 bias, is an experimental way to look at the response without internal gain.

Because thin layers of black silicon can absorb as much light as thick layers of ordinary silicon, it is possible that black silicon solar cells would be cheaper than ordinary silicon solar cells. The reason that the most progress has been made on detectors rather than solar cells is that it is easier to make a profit selling detectors at smaller scale. It is not so much the cost floor that has thus far prevented the appearance of black silicon solar cells, but rather just that it is currently not made at a scale that would lead to affordable solar cells. As technologies are developed for making larger quantities of black silicon, I would not be surprised if it or a related material started finding its way into solar cells. An example response spectrum can be found here: http://www.sionyx.com/advantage.html. Note that this plot shows internal gain, which some variants of the material possess at >2V reverse bias. The response function when running at small 1V reverse bias is comparable to that of ordinary silicon, but extended deeper into the IR (out to 1300 nm instead of silicon's 1100 nm).

I work on silicon-chalcogen alloys like the material SiOnyx uses but also including Si:Se and Si:Te. I encourage you to attend next year's black silicon symposium (http://www.army.mil/-news/2009/08/26/26478-bent-laboratories-line-of-sight-goes-beyond-cannon/). The article is a year old, but the most recent symposium did include discussions of several types of black silicon beyond what is used by SiOnyx.

Black Silicon is not made in a deposition process. Instead, ordinary silicon is shot with a femtosecond pulsed laser in the presence of a sulfur-containing gas. The laser causes sulfur to be incorporated while also structuring the surface of the silicon. Thus it changes both the chemical state and physical morphology of the material. I encourage you to check out the following freely available Ph.D thesis for more information: mazur-www.harvard.edu/publications.php?function=display&rowid=648.

The improvement realized by black silicon depends both on the kind of detector in which it is used and the wavelength it's trying to detect. An application for black silicon that the research community takes very seriously at the moment is detection of light at a wavelength of 1064 nm. This is the main emission line of Nd:YAG lasers, which are already used in a variety of applications (see: http://en.wikipedia.org/wiki/Nd-YAG_laser#Military_and_defense). At that wavelength, black silicon detectors are at least twice as good as traditional silicon devices and can be made 100-1000x thinner.

The S is typically either SF6 or H2S gas. The wavelength of the femtosecond laser isn't especially important; the key is that the laser fluence (energy per area) be above the ablation threshold of the silicon (between 0.1 and 1 J/cm^2 for the relevant pulse durations). The laser spot size is typically a fraction of a millimeter on a side, but it can be rastered over a silicon wafer to make a large-area black silicon film. There is a recent Ph.D thesis available for free at: mazur-www.harvard.edu/publications.php?function=display&rowid=648 that gives a complete recipe for making black silicon.

I'm getting my Ph.D researching black silicon. If you have science or engineering questions about it, post them in reply to this comment. I'll check back at around 3 PM EST and will do my best to answer the questions I find then.