How much slower does a photodiode become in photovoltaic mode?
The open cct voltage Voc with an infinite load R but except for extremely low light levels, Voc is nearly constant.  PV mode is used for extremely low steady light level measurements.

Since PV mode is high impedance and diode has maximum capacitance at 0V (Terminal capacitance in pF) this leads to a relatively slow RC time constant. It also reduces  with reverse voltage bias, \$V_R\$.

e.g. V/I diode output in PV mode means very high Z and the relatively high RC time constant.

e.g. VI curves for  S2387-66R = 1pA @10mV and ΔV/ΔI  ~ 1e-2/1e-12= 1e10 Ω=10GΩ   

Specs indicate 5 GΩ typ. for each sensor @ Vr=10mV. which agrees with curves.

from we expect with 1k load \$BW_{-3dB}= 0.35/t_R= 35kHz\$  

So TIA is needed with Vr max to obtain max BW with Feedback cap conjugate matching required to flatten response. 



PC mode is in between speed with Rs as a series-shunt load between Vcc &amp; 0V return.

Photovoltaic mode               Photoconductive mode
=================               ========================================
Zero bias                       Reverse bias to reduce diode capacitance
No "dark" current               Has "dark" current
Linear                          Nonlinear
Low noise (i.e. Johnson)        Higher noise (i.e. Johnson, Shot)
Used in precision applications  Used in high speed applications








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