The problem with your circuit design and why you are seeing the low output, is due to cumulative effects of adding more IR detectors and using them in a forward bias configuration (think paralleling several resistors.... the resistance cumulatively gets lower and lower) .... The key is to use the IR detector in a reverse bias mode where it acts like a capacitor and not a diode. In this mode the "capacitive" leakage is proportional to the amount of light falling on the IR detector. So the circuit "charges" the "capacitor" and any light falling on the IR detector discharges the "capacitor" proportionally.
For this circuit, personally I would do away with an Op-Amp, and use a Collector feedback bias configuration with an ordinary NPN (2n3904) transistor and use the IR detector in a reverse bias configuration. See "Collector feedback bias" at the following link ... **broken link removed**
The Cathode of the IR detector should go to the transistors Base, while the Anode of the IR should go to ground. Your 4.7M resistor would be fine across the Base-Collector (Rb) of the transistor. (Rc) should be anywhere from 10k to 47k. Your output is from the Collector of the NPN transistor. This is a self biasing configuration as well, so there shouldn't be any adjustments once you find the values that work for you.