Dark current reduction and bandgap-voltage offset in solution-processed nanocrystal solar cells

In this work, we report dark current reduction in solution-processed PbS nanocrystal-metal Schottky junction solar cells via improved LiF passivation of the interface between PbS nanocrystal films and the metal electrode. For the optimized LiF interfacial layer, the dark saturation current density (J0) is decreased by at least one order of magnitude, resulting in very high open-circuit voltage (Voc) of 692±7 mV under one sun illumination for ~1.4 eV PbS nanocrystals. Using different size of PbS nanocrystals and therefore different bandgaps, we also demonstrate Voc (mV)=553Eg/q-59 as a function of the PbS nanocrystal bandgap (Eg). For different types of junctions employed for solution-processed PbS nanocrystal solar cells, we plot the bandgap-voltage offsets (Eg/q-Voc) under open-circuit conditions, showing strong dependence of the Voc on the Eg regardless of the types of junction used. Similar dependence is also found in solution-processed and sintered CdTe nanocrystal solar cells. These results suggest that suppressing the non-radiative recombination contributions to the dark current, such as improved passivation of nanocrystal surfaces, is more critical to improve the Voc in nanocrystal solar cells, rather than optimizing the device architecture with varying the n-type semiconducting materials.