Micropower voltage regulator with discrete components
A simple shunt regulator would be best, since your photodiodes produce a bit more current than you need, and you can control their voltage by absorbing excess current. 

This is only approximate and won't provide perfect regulation, but it's simple. You will probably need to adjust R2 to hit exactly 1V under typical situations; even then I don't know if it'll meet 5% error across your desired range of lighting conditions. Some testing required.

4 diodes will (according to the datasheet) produce 1.4V open circuit and up to 70uA into a short circuit. 

Q1 is any low power NPN transistor with a reasonably high current gain (400) like a BC547C or BC847C (C grade is the high gain grade).

As the voltage builds up to close to 1, the transistor will start to turn on : at the microamp level, this happens well below the crude "0.6V" or "0.7V" approximate value, and 0.2 uA base current should be enough to absorb all the excess current produced by the photodiodes.

A MOSFET with a threshold voltage around 0.5V would also work, but the variability between devices is likely to be much greater than Vbe with bipolar transistors.







simulate this circuit &ndash; Schematic created using CircuitLab