Solid films made of binary ionic semiconducting nanocrystals (NCs), such as CdS, CdSe, PbS, PbSe and InAs have recently became very attractive as solution processable active materials in different types of advanced optoelectronic devices. The interest on this type of materials is determined by the unique dependence of their band gap magnitude on their size when the diameter becomes comparable to (or smaller then) the effective excitonic Bohr radius. Other attractive points of such materials are the room temperature facile solution processing (which imply possibility to use printing techniques for large areas) and the potential for controllable doping by straightforward surface modification with different types of ligands. PbS colloidal quantum dots (CQDs) are especially promising as a material for active layer for 3^rd generation solar cells because of low-price, scalable synthesis, the Earth-abundant constituent elements, the possibility to absorb light in the near IR part of the solar spectrum and the good stability.

    Major success in the power conversion efficiency of PbS solar cells were obtained via improvements in synthetic conditions, surface passivation of the nanocrystals, obtaining of better packing within the solid made of QDs, improving of absorption and transport properties and device architecture.

    One of the highest efficiency PbS solar cell structure today use PbS passivated with Tetrabutylammonium iodide  (TBAI) as a main absorbing layer together with a thin layer of PbS treated with 1,2-Ethanedithiol (EDT) as an electron blocking/hole extracting layer. Halide anions have a high affinity towards surface Pb²⁺ cations, thus they are widely investigated as a «single atomic approach» to passivate part or all of the surface states and to decrease the distance between nanocrystals.

    Here we will present the fabrication of PbS devices where the layer treated with EDT (p-type layer) is substituted with a layer treated with TBACl solution. TBACl treated PbS show p-type dominated behavior. This allows us to obtain the device showing power conversion efficiency higher than 7,5% with a short current density higher then 28 mA/cm², confirmed by EQE measurement. These devices show high stability also after long (more then 1 week) exposure to ambient conditions. The use of TBACl is highly attractive as it allows to completely avoid the treatment of the CQD layer with thiols making the fabrication of PbS devices more environmental friendly.