Normal p-n junctions (including solar panels and ordinary semiconductors) are light sensitive because one photon of light can strike and transfer its energy into creating one electron-hole pair (promoting one electron from the valence band to conduction band). This is called "optical generation", in contrast to "thermal generation". These electron-hole pairs are generated in the depletion region of the junction where they are quickly swept across by strong electric fields that arise from the junction itself. But in general, one photon = one electron-hole pair -- at most.

Avalanche photodiodes already exist and convert a single incident photon into multiple charge carriers. They do this using an externally-applied electric field. This externally-applied electric field enables "impact ionization" which means that one mobile charge carrier seems to be able to knock another one loose.

(As a thought experiment: imagine a bed of pebbles. Toss in another pebble, and it's likely to kick one of the existing pebbles free. Now, pretend the bed of pebbles is at a 45-degree angle to gravity. Toss a pebble in and it's more likely that the ejected pebble will itself kick another pebble loose when it lands. Make the angle steep enough and you get an "avalanche", hence the name.)

As far as I can tell, the novelty here is that the high electric field is created by the geometry and materials at the surface of the nanoscale needle structure, so some version of the avalanche effect happens without needing to apply an external electric field.

In the headline, efficiency refers to quantum efficiency (electrons per photon); conservation of energy still applies.