Semiconducting single-walled carbon nanotubes (s-SWCNTs) have attracted significant attention as a photoactive component in thin film photovoltaics due to their strong absorptivity and high charge mobility. However, the external quantum efficiency (EQE) of s-SWCNT/C₆₀ heterojunction solar cells is limited by poor exciton diffusion to the heterointerface. Exciton diffusion is believed to be strongly dependent on the length between defects, which quench excitons. Here, we systematically study the influence of defects on s-SWCNT/C₆₀ planar heterojunction photovoltaic devices. We use covalent sidewall functionalization via diazonium chemistry to introduce sp³ defects to the s-SWCNTs at varying concentrations. We generate a defect-adding model to describe the position of defects on the surface of the nanotube, and use it to successfully reproduce the time-resolved exciton decay rate measured with transient absorption spectroscopy. We fabricate s-SWCNT/C₆₀ heterojunction photovoltaic cells and characterize the EQE as a function of defect concentration. We develop a diffusion limited contact quenching Monte Carlo model to reproduce the EQE trends we observe. The model allows us to predict the behavior of theoretical s-SWCNT devices with varying length distribution and defect concentration, toward the limit of long length and zero defects. This work guides the field of nanotube photovoltaics and illuminates the promise and limitations of s-SWCNT solar cell devices.