Efficient Heterojunction Photovoltaic Cell Utilizing Nanocomposites of Lead Sulfide Nanocrystals and a Low‐Bandgap Polymer

Hybrid polymer–inorganic nanocrystal (NC) solar cells consisting of a hole-conducting conjugated polymer and inorganic semiconducting quantum dots, like cadmium selenide (CdSe), lead sulfi de (PbS) and lead selenide (PbSe), have attracted considerable research attention, since the advantages of two classes of materials can be effectively combined. Some of the potential advantages offered by this hybrid approach include: a) low fabrication costs due to easy solution-based processing; b) high optical absorbance and fl exible substrate manufacturing of the conjugated polymer matrix; and c) high electron conductance, tunable optical bandgap, and carrier multiplication due to the semiconducting NCs. [ 1–14 ] The performance of these hybrid devices depends on energy band matching, blend fi lm morphology, composition ratio, and NC shape, etc . [ 15–20 ] In addition, it is crucial to control interfacial contact at nanometer length scales in hybrid blend fi lms to ensure effi cient exciton generation and charge transfer. Generally, it is desirable to perform ligand exchange on the as-prepared NCs since they typically are shielded by long alkyl chain ligands such as trioctylphosphine oxide (TOPO) and oleic acid (OA) that act as insulating layers. For example, ligand exchange with shorter ligands like pyridine and butylamine have been shown to result in better charge photogeneration and effi cient charge transport. [ 16 , 19 , 21 , 22 ] Such pre-fabrication ligand exchange processing requires that good dispersibility of the NCs is maintained for fi lm fabrication. This method has been successfully applied to CdSe/poly(3-hexyl thiophene) (P3HT) hybrid devices to show signifi cantly improved performance. [ 15 ] Recently, the use of a low-bandgap polymer with CdSe NCs resulted in power conversion effi ciency of 3.2% with a broad absorption spectrum from UV to ∼ 800 nm. [ 23 ] To achieve better light harvesting including nearand mid-infrared (IR) wavelengths, PbS and PbSe NCs have been investigated in hybrid polymer solar cells. [ 24–28 ] While these materials will require appropriate encapsulation and proper disposal to avoid environmental contamination, they represent the best choice