Phototransistor and Photodiodes capacitances when some in an array sense light
You can get far more gain-bandwidth with a stable photo-diode, PD and OpAmp with feedback R than any phototransistor.  

Using tandem PT’s can double I and C so the dV/dt=I/C is the same for same R . But using a TIA can achieve 1e6 gain at 1Hz, which would be excessive so R feedback reduces the voltage gain using a TIA configured Op Amp.

Choice of narrow beam emitter and narrow beam PD increases gain by 2x for each reduction of beam angle /2 on the detector.  A black daylight blocking filter is integrated into most lenses for IR transmission is highly recommended.