We identify the existence of nonlinear optical (NLO) activity in a number of novel ABX3-type metal-free perovskites, where A is a highly tuneable organic cation, B is a NH4 cation and X a halide anion. Through systematic first-principles calculations, we identify important trends to chart the second-harmonic generation of this class of materials. We study three perovskites MDABCO- NH4 I3 , CNDABCO-NH4 I3 and ODABCO-NH4 I3 for use as deep-UV second-harmonic generation materials. We identify the role of the dipole moment imparted by the organic group on the A cation as an important parameter to tune the NLO properties of these materials. We apply this knowledge functionalising the organic group DABCO with the highly polar cyanide CN group, and we demonstrate a significant improvement of the NLO response in this family of materials. These findings can accelerate the application of metal free perovskites as inexpensive, non-toxic, earth-abundant materials for the next generation of optical communication applications.