Organic Salts as a Route to Energy Level Control in Low Bandgap, High Open-Circuit Voltage Organic and Transparent Solar Cells that Approach the Excitonic Voltage Limit

A new series of organic salts with selective near‐infrared (NIR) harvesting to 950 nm is reported, and anion selection and blending is demonstrated to allow for fine tuning of the open‐circuit voltage. Extending photoresponse deeper into the NIR is a significant challenge facing small molecule organic photovoltaics, and recent demonstrations have been limited by open‐circuit voltages much lower than the theoretical and practical limits. This work presents molecular design strategies that enable facile tuning of energy level alignment and open‐circuit voltages in organic salt‐based photovoltaics. Anions are also shown to have a strong influence on exciton diffusion length. These insights provide a clear route toward achieving high efficiency transparent and panchromatic photovoltaics, and open up design opportunities to rapidly tailor molecules for new donor–acceptor systems. Organic salt photovoltaics with open‐circuit voltages approaching the excitonic limit are demonstrated. Anion exchange is shown to enhance energy level alignment and nearly double the exciton diffusion length. Anion alloying enables a design strategy for fine energy level tuning to simultaneously optimize open‐circuit voltage and photocurrent in new organic salt systems for multijunction and transparent phovoltaics with deeper near‐infrared response.