A p-doped wafer (boron concentration of 5 E17/cm^3) that is 200 m thick completely and homogenously absorbs a beam of photons, generating 2E19 excess free

A p-doped wafer (boron concentration of 5 E17/cm^3) that is 200 µm thick completely and homogenously absorbs a beam of photons, generating 2E19 excess free electrons (Δn) per cubic centimetre and per second. 

If the implied open-circuit voltage is 0.650V, what is the bulk carrier lifetime (in s) in the wafer? (Please write the anwser in a numerical format and without the units) 

You can assume the following values: 

Bandgap of silicon at room temperature : EG = 1.1 eV

 For silicon at RT, ni = 1.5E10 /cm^3

 Use the following values of band-edge effective densities of states for silicon: NC = 3.2E19/cm^3 NV = 1.8E19/cm^3