Mott-Schottky Analysis of the P3HT:ZnS_{cubic} and P3HT:ZnS_{hexa} Bulk Heterojunction Solar Cells

Abstract

Bulk heterojunction solar cells made of conducting polymers represent a new stage in the evolution of photovoltaic devices and this type of excitonic cells are presently the most studied solar cells [1–5]. In the case of “bulk heterojunction”, the interface between two materials of different electrical properties is all over the bulk, where the electron accepting nanoparticles are mixed with the electron donating polymer and the exciton created in either material diffuse to the interface to enable the charge separation. Poly(3-hexyl) thiophene (or P3HT):ZnS are very interesting nanocomposites due to their applicability as an active layer for bulk heterojunction solar cells of high open circuit voltage [6–8] and the charge transport in this system determines the performance of solar cells made of this nanocomposites. This paper discusses the Mott–Schottky characteristics of solar cells made of inorganic semiconducting ZnS nanoparticles with organic P3HT. Here, ZnS nanoparticles are preferred as an electron acceptor because it is environment friendly, stable indefinitely, and can be synthesized easily and inexpensively. It is supposed here that the lowest unoccupied molecular orbital (LUMO) level of the ZnS is below at least 0.3 eV than that of P3HT for an effective exciton splitting and charge dissociation [9], since quantum confinement effect of the semiconducting nanocrystals shifts their LUMO level to upward and the highest occupied molecular orbital (HOMO) to downward and that enlarges the band gap [10]. The wide band semiconductor ZnS has high electron mobility (600 cm V−1 s−1 [11]) and has an electron affinity of about 3.9 eV [12] that makes ZnS as an attractive material to use as an electron acceptor in hybrid photovoltaic devices. Further, conduction band shifts upward as decreasing the size of the particle [10] that helps to match well with the LUMO level of P3HT,