Beating the Detailed Balance Limit in Ideal Carbon Nanotube <i>pn</i> diodes

The detailed balance (DB) limit developed by Shockley-Queisser has been the guiding principle for the design of modern solar cells. According to the DB limit, a single junction pn diode exhibits the maximum open-circuit voltage (V_OC) when it operates in the ideal diode limit. An ideal pn diode is characterized by an ideality factor n=1 which correlates to a specific generation and recombination (GR) process in a diode. Though V_OC is directly proportional to n, typically a higher n does not correspond to a larger V_OC. This is due to higher GR process for diodes with n〉1 that directly correspond to an increased diode leakage current, which reduces the diode’s open-circuit voltage. Here, we exploit a previously overlooked parameter, the diode ideality factor n, to increase the V_OC above the ideal diode limit. We use dynamic gate modulation in a single-walled carbon nanotube diode to engineer a digitally tunable effective ideality factor that is decoupled from the diode leakage current. We show that the open-circuit voltage can be tuned in direct proportion to n and achieve a V_OC that is 3 times higher than that given by the DB limit of an ideal pn diode.