Making use of the colloidal character of quantum dots has allowed us to investigate two new photovoltaic-technology driven concepts: PbS CQDs based bulk nano-heterojunctions (BNH) and homojunctions. The first relies on blending the p-type PbS dots with n-type ZnO nanocrystals (NC) for creating compound photoactive layers [1]. The resulting BNH devices show significantly improved figures of merit (Jsc=16 mA/cm2, Voc=0.64 V, FF=50 % and PCE=5.2 % under 1 sun) compared to simple bilayer (BL) devices (Jsc=12.07 mA/cm2, Voc=0.51 V, FF=43 %, PCE=2.64 %). The PCE of the BNH is further increased to 7% under low lighting conditions (10% sun). The improved performance of the BNH device is attributed to: a) improved separation and extraction of photogenerated carriers in the BNH layer due the increased p-n interface and respective conduction channels within it, b) remote passivation of the PbS CQDs' mid-gap trap states by electrons from ZnO NCs located close to the PbS CQDs. The latter hypothesis is supported by a number of physical properties: i) the BNH devices when in dark have significantly higher shunt resistance compared to BLs, ii) temperature (T) depended Voc measurements indicate that for T approaching 0 K the extrapolated Voc of the BNH devices is close to the PbS CQDs´ band gap, iii) the ideality factor of the BNH device is 1. Photoluminescence (PL) measurements further confirm that ZnO NCs located within 30nm from the PbS dots enhance the emission of the later.

The second work presented describes an electronic doping approach for transforming p-type PbS CQDs to n-type [2]. It relies on elemental doping of the dots with a trivalent cation which when replaces Pb2+ in the PbS structure, it provides extra electrons for raising the Fermi level of the material. This concept was recently demonstrated for the case of Bi3+ as the dopant, and allowed for the creation of Bi:PbS/PbS bilayer homojunctions working as air-stable solar cells with PCE=2.7 % under 1 sun illumination. The optoelectronic properties of the doped material and homojunctions depend on the Bi/Pb molar ratio (studied in the 0.1-4 % range) and suggest that while doping raises electron concentration, this process is limited by the formation of doping-induced traps. Continuing on this work we also investigate Sb, In, Sn as dopants. Systematic characterization of the materials and devices with increasing doping density has revealed the following: a) only Sb resembles the case of Bi for yielding an electron accepting material suitable for the formation of a homojunction, yet Sb is a less efficient dopant, ii) the aforementioned functionality is accompanied by PL quenching and crystal structure distortion iii) the degree of physical incorporation of the dopant in the CQD product is highly element depended and highest for the cases of effective electronic doping.