Novel acceptor molecules for bulk heterojunction organic solar cells [electronic resource]
Keywords: Materials Science and Engineering.

Solution-processable organic solar cells offer the promise of clean energy generation at lower cost than conventional technologies due to high-throughput roll-to-roll manufacturing, cheap and abundant materials and the lower installation costs associated with lightweight and flexible solar modules. Power conversion efficiencies of organic solar cells have surpassed 10% due in large part to the discovery and design of new materials for the donor half of the donor-acceptor heterojunction. However, the vast majority of organic photovoltaic devices contain fullerene derivatives as the electron acceptor material. Devices containing fullerenes as the electron acceptors have been shown to be energetically limited to open-circuit voltages of 1.0 V or less, thus limiting their maximum efficiency and potential for use as the high-voltage top cell in tandem solar cell architectures. This is in addition to other drawbacks of fullerenes such as their high synthetic cost and relatively poor light absorption. A phenyl imide-based electron acceptor molecule, HPI-BT, has been developed as an alternative to fullerene derivatives to address some of these drawbacks. Device efficiencies of up to 3.7% with the common electron donating polymer poly (3-hexyl thiophene) -- P3HT -- have been achieved through detailed optimization. While these devices have open-circuit voltages of 0.94 V (0.31 V higher than comparable devices with P3HT and PC61BM, a common fullerene derivative), the quantum efficiency is 20% lower than the equivalent fullerene-containing device. Through investigation of the dependence of quantum efficiency on applied electric field and light intensity in these devices and others using additional electron donating polymers, the primary cause of lower quantum efficiency in these devices is found to be recombination of geminate charge pairs before they are able to reach their fully charge-separated state. Recent research reports show that the microstructure of a typical bulk heterojunction organic solar cell consists of a relatively pure electron donor phase (P3HT), a relatively pure acceptor phase (PC61BM) and a two-component mixed phase at the interface of the two pure phases. This interfacial mixed phase is believed to provide an energetic driving force for charge separation from the mixed phase into the pure phases, thus providing high quantum efficiencies in fullerene-based devices. X-ray diffraction studies on blends of polythiophene and HPI-BT show no evidence of a strongly mixed third phase. The lower quantum efficiency of devices containing HPI-BT without this third mixed phase is explained by the favorable energetic offsets created in this three-phase morphology. Alternatively, the inability of fullerenes to effectively absorb light can be partially mitigated by the addition of a third molecule providing additional absorption bandwidth in a ternary blend organic solar cell. The addition of up to 20% (by weight) of a conjugated dye molecule, tetra-tert-butyl functionalized silicon naphthalocyanine (t-butyl SiNc), to a typical bulk heterojunction solar cell with P3HT and PC61BM results in the generation of additional photocurrent from dye absorption in the near-infrared region of the light spectrum. The effect of the tert-butyl functionalization on the incorporation of the dye molecule is discussed along with the potential for improved efficiency of ternary blend organic solar cells relative to their binary blend counterparts.