Photovoltaic nano-devices have largely been relying on charge separation in conventional p-n
junctions. Junction formation via doping, however, imposes major challenges in process control.
Here, we report on a concept for photovoltaic energy conversion at the nano scale without the need
for intentional doping. Our approach relies on charge carrier separation in inhomogeneously
strained germanium nanowires (Ge NWs). This concept utilizes the strain-induced gradient in
bandgap along tapered NWs. Experimental data confirms the feasibility of strain-induced charge
separation in individual vapor-liquid-solid grown Ge NW devices with an internal quantum
efficiency of 5%. The charge separation mechanism, though, is not inherently limited to a distinct
material. Our work establishes a class of photovoltaic nano-devices with its opto-electronic
properties engineered by size, shape, and applied strain.