posted on 2017-10-31, 00:00authored byIeva Narkeviciute, Thomas F. Jaramillo
The nanostructuring
of light-absorbing materials in photoelectrochemical
applications can potentially improve the performance of charge transport
limited semiconductors by increasing incident light absorption as
well as the electrochemically active surface area. However, a drawback
associated with an increase in electrode surface area is the increased
effect of surface recombination on device performance. To understand
the interplay of the positive and negative impacts of nanostructuring,
we studied these effects by varying the nanowire length and thereby
surface area on the photoelectrochemical performance of tandem core–shell
Si/Ta3N5 photoanodes. Si/Ta3N5 nanowires of different lengths, 1.2–3.3 μm,
were fabricated by changing the reactive ion etch duration by which
the Si nanowires are formed and subsequently characterized by optical
UV–vis reflectance measurements, effective charge carrier lifetime
measurements, and photoelectrochemical ferrocyanide oxidation. Overall,
we show that as the nanowire length is increased, the photovoltage
decreases due to decreasing effective carrier lifetimes that arise
from higher surface recombination. On the other hand, the device photocurrent
increases as the nanowires become longer due to increasing electrochemically
active surface area and decreased light reflection, which in turn
increases absorption due to light trapping within the nanowires. Balancing
these effects is crucial toward developing high performance devices.