Forest MIMS used Green diodes as light sensors in may projects - all the way back to the 1970s. connect cathode to ground and anode to a large value resistor and the other end of that resistor back to ground. Then measure off of the node between LED and resistor when exposing the Green LED to visible light. Any power should flow from that resistor/LED junction as well. Use the old-school LEDs, not the HB LEDs that have green phosphors on them. You will need UV to penetrate the phosphors and excite the LED diode surface (not efficient).

Remember, the little chip in an LED doesn't have nearly the surface area to capture sunlight that a full panel has. Therefore, power will be greatly reduced. Look at the LED "chip" on a clear LED with a magnifying glass for scale.

 

Also, if you put solar cells in series, there are more losses. Four in series might not deliver four times that power.

I have used four in series and in indirect sunlight (shade, but exposed to the full blue sky) they will deliver tiny but useful power.

That is because they're in the shade, not because they're in series. If you connected them in parallel, you wouldn't fare any better.

 

both photo diodes and led's are self generating when exposed to light

That applies to any PN junction. Back in the 70's when I was curious about what was inside of integrated circuits, I opened the case of a defective power transistor and "discovered" that the BE junction was photovoltaic.

I was refreshing my LED knowledge and saw that silicon diodes will emit light at 1100nm.

 

thats why transistors have light blocking metal or epoxy covers., diodes too were found to introduce noise when exposed to light. the light affected specs are not listed on the spec sheets. PIN type photo diodes are some of the most sensative photo detectors around.

 

I discovered my RPi2 was camera-shy due to a missing light cover on a die.
http://forum.allaboutcircuits.com/threads/raspberry-pi-daq-system.75543/#post-830866

 

PIN type photo diodes are some of the most sensative photo detectors around.

Thanks to that intrinsic (undoped silicon) layer.

I first used them back in the 70's when I was working with laser diodes and fiber optics; which were relatively new back then. One of my coworkers worked on a motor controller to pull the fibers. Another of my coworkers worked on a life test rig for laser diodes. Back then they were fairly primitive, required high bias currents (> 100mA), and lifetimes were relatively short. HP had one that incorporated a beam splitter and some circuitry to compensate for fluctuations in light output. I worked in applications and was working on a 100MHz optical scope probe. Sadly, it never became a product, but it was an interesting project.