Pn-Junction nanorods in a polymer matrix: A paradigm shift from conventional hybrid bulk-heterojunction solar cells

Abstract

We report introduction of pn-junction nanorods in a polymer matrix to form hybrid bulk-heterojunction (BHJ) solar cells. This is a paradigm shift from conventional hybrid solar cells, where quantum dots or nanorods of an inorganic semiconductor and a conjugated polymer form a BHJ. The pn-junctions were formed in n-type CdS nanorods through a controlled cationic exchange process; here p-type Cu2S formed from one end of CdS due to selective reactivity of crystalline planes of the nanorods. Due to an epitaxial attachment between Cu2S and CdS semiconductors in the nanorod, a depletion region hence formed in the pn-junction, which is a classical example of type-II band-alignment at the interface. The junction separated charge carriers under illumination through a drift of minority carriers across the depletion region. Hybrid BHJs based on Cu2S|CdS pn-junction nanorods in a conventional polymer matrix therefore acted as efficient solar cells as compared to similar BHJ devices with nanorods of individual materials, that is Cu2S or CdS. We varied the relative lengths of p- and n-sections of the pn-junctions that in turn controlled charge separation and carrier transport processes to optimize the solar cell performance.