This is pretty basic.
Your DMM is 10 MΩ input which shunts the low current into a higher voltage instead of 10 kΩ. Since 10 MΩ is 1k bigger than 10 kΩ, so is the voltage yet the same power P = VI.
Depending on your experiment and desired dynamic range of voltage, change the value of R1 to obtain the desired conversion of PD from I to V.
(conversion factor is just Ohm's Law V=IR). Perhaps 100k to 1M is more suitable.
Panasonic makes an inexpensive 5mm radial "Light sensor" that compresses the range with a log scale so >4 decades of light input from near dark to bright sun gives an output to 5V output with a selected R-value to choose your optimum input light range.
The sensitivity, S for Silicon, is 0.6 A/W at λ=λp with a declining sensitivity from IR towards blue. This is the same as 0.6 uA/uW. The output current depends on the surface area of ~ 5 x 5 mm like a tiny PV power source.
Consider 1uA times R1 = 1Mohm * 1uA = 1V output. This simply uses the shunt resistor to convert the photon microamps * R = microvolts.
For overvoltage protection, the analog inputs are usually ESD protected with Diodes but limited to 5mA or so RMS so in bright sunlight at 100 klux, you might expect a short circuit current of 13 uA/100 lux * 100 klux = 13 mA.
To prevent this from flowing into the internal Analog port, they use an ESD-protection shunt diode to Vdd, but it is rated for only 5mA steady or so.
You can solve this in many ways to prevent Vin > 5V by PD cathode voltage drops (0.7V) from 5V or add an input series R to the Analog port from 1M shunt to 10k series.
"Shunt" means parallel or bypass.
Can you figure out my suggestions?