- Carbon nanotube (CNT) photodiodes are a promising system for high-efficiency photocurrent generation due to the strong Coulomb interactions that can drive carrier multiplication. If the Coulomb interactions are too strong, however, exciton formation can hamper photocurrent generation. Here, we explore, experimentally and theoretically, the effect of the environmental dielectric constant (ε_env) on the photocurrent generation process in CNTs. We study individual ultra-clean CNTs of known chiral index in vacuum or dry nitrogen gas (ε_env = 1) and oil (ε_env = 2.15). The efficiency of photocurrent generation improves by more than an order of magnitude in oil. Two mechanisms explain this improvement. First, the refractive index of the environment optimizes the interference between incident and reflected light. Second, exciton binding energies are reduced in oil, changing the relaxation pathways of photoexcited carriers. We varied the axial electric field in the pn junction from 4 to 14 V/μm. Our measurements at high field indicate that autoionization of second-subband excitons can coexist with carrier multiplication. Dielectric screening makes this coexistence regime more accessible and allows us to reach photocurrent quantum yields greater than 100%.