UV-filtering down-conversion layer for efficient and stable organic solar cell

Abstract

Solar cells made up of organic materials such as polymers and small molecules have been established as an affordable alternative to conventional silicon solar cells. Although the performance had significantly improved through the years, organic solar cells (OSCs) still exhibit relatively low efficiency and high device instability preventing their commercialization. The theoretical energy conversion efficiency limit of any single-junction solar cell is a function of the band-gap energy of the semiconductor, referred to as Shockley-Queisser (SQ) limit. Down-conversion (DC) is one of the suggested approaches to overcome the SQ limit wherein high-energy photons are converted into low-energy photons of which the energy is just enough to overcome the band-gap energy to minimize the thermalization of the charge carriers. In the case of organic emitters, DC is not only limited to minimizing the lattice thermalization loss. Since the bandwidths of organic materials are limited, they have a limited absorption spectrum, thus some wavelengths will be unabsorbed causing a transparency loss. In this work, we discuss the effect of an efficient organic light-emitting material as an ultra-violet (UV)-filtering down-conversion (UVDC) layer on the efficiency of a flexible organic solar cell. With the help of those UVDC layers, high-energy photons in the UV spectrum can be converted into photons within the 600 nm range, which falls on the absorption spectra of the active layer (PBDB-T-2F: BTP-4Cl). Absorption of these down-converted photons within the cell leads to an increase in the photocurrent in the device and eventually improves the power conversion efficiency. Adding the UVDC layer on the OSCs not just increases the efficiency but also provides an additional filter to prevent the UV light from harming the active layer. But although the device degradation is not fully prevented, the UVDC layer is able to successfully improve the device stability under UV and AM1.5G illumination.