2, Stanford University, Stanford, California, United States
The efficiency of perovskite solar cells has skyrocketed from 3.8% to 22.1% in the past few years. For the best perovskites, the open-circuit voltage deficit, defined as the difference between bandgap and open-circuit voltage (Voc), is only 0.37 V, approaching other best technologies such as GaAs . In contrast to the remarkable achievement in Voc, the fill factors (FF) of perovskite solar cells are usually around 60%-70%. Even in the champion device, the FF is only 80%, which is considerably lower than that of GaAs (86.5%) , despite the fact that it has higher bandgap and comparable Voc deficit. Therefore, understanding the FF loss, is the key to further boost the efficiency of perovskite solar cells.
Suns-Voc technique is well established for silicon solar cells to acquire pseudo-current-voltage characteristics, from which one could obtain the recombination-limited FF of the device. Furthermore, by comparing the pseudo-current-voltage curve to the one-sun current-voltage curve (IV), the series resistance at maximum power point can be determined. To apply this technique on perovskite solar cells, we equipped our IV tester with neutral-density filters. Distinct from the Sinton flash Suns-Voc tool, our setup utilizes a continuous light source, therefore it can measure accurately on cells even with hysteresis. Moreover, the continuous lamp has class A spectrum, and it maintains the same at reduced illumination by applying neutral-density filters, which prevents errors induced by the change of spectrum.
We first verified our setup by measuring a silicon cell, and it shows excellent agreement with the Sinton tool. Performing IV and Suns-Voc measurements on a methylammonium lead iodide (MAPI) perovskite solar cell, we observed the series resistance decreases when the operating point moves from maximum power point to Voc. In contrast, similar analysis on the silicon cell shows the series resistance stays constant, in other words, no illumination dependency. Such illumination-dependent series resistance was observed on organic solar cells before, and is believed to be caused by the low carrier mobility of the bulk material . However, in perovskite solar cells, the carrier transport is limited by the contact layers instead of bulk . We are further investigating this by applying different contact layers to perovskite, and by the time we present, we would have more insight about the origin of this illumination-dependent series resistance.