8 or 10 mm IR photo diode

I am develloping a simple IR sensor and now I got a problem..

I am using a 5 mm IR photodiode, but I got a verry small signal. If i change to a 8 or 10 mm IR diode I got more signal. Who are making 8 or 10 mm IR diodes?

Changing from 5 mm to 8 mm will gain the area with more than 2 times (

Regards

Benny