Graded morphology in bulk-heterojunction solar cells based on colloidal semiconductor nanostructures: Directed charge-separation and facile carrier transport

Abstract

We introduce density gradient of p- and n-type compound semiconductor nanostructures in bulk-heterojunction (BHJ) solar cells. The graded BHJs (GBHJs) were formed with the p-type copper-zinc-tin-sulfide (Cu2ZnSnS4) nanoparticles and n-type Bi2S3 nanorods, both of which were based on nontoxic and earth-abundant elements and were grown at a moderate reaction temperature (180 °C). The concentration gradient of the nanostructures in the GBHJs provided a preferred directionality of nano-depletion regions for directed charge separation and also a controlled vertical segregation for ever-increasing carrier-transport pathways during the charge-extraction process. A comparison of solar cell characteristics having a bilayer, a BHJ, and a GBHJ structure is being presented. The performance of the thickness-optimized devices for highest efficiency shows that the GBHJ offered an improved short-circuit current as compared to the bilayer and the BHJ structures and a higher fill-factor as compared to the BHJ device. The overall energy conversion efficiency (η) of GBHJ exceeded that of the other two heterojunctions. The advantages of GBHJ structures in yielding an improved η have been explained through an increased exciton dissociation process along with a lower carrier recombination as compared to the bilayer and the BHJ structures, respectively. Series and shunt resistances, which were derived from current-voltage characteristics and impedance spectroscopy, supported such analyses.