Solar cell driven transistors - circuit explanation
The intent of that resistor would be as a 'pull down' resistor, to give a path to gnd fkr the PV current when it's prodicing less than the necessary ~1uA to cause an 0.7V voltage drop across the 1M resistor &amp; reach the 0.7V required to forward-bias the BE junction of Q3.

PV cells' output power varies more in the realm of current than of open-circuit voltage, so it's necessary to provide a resistor to 'set the threshold.' If you lower the value of R23 it will raise the amount of light that it takes hitting the PV to generate anough current to turn off the led.