Studies of Pyrite FeS2 Nanocrystals and Plasmonic Nanostructures for Low Cost and High Efficiency Inverted Bulk Heterojunction Photovoltaic Cells

Abstract

Solar energy is our most abundant and clean natural energy source which has made photovoltaics (PV) of great interest as a renewable and sustainable energy technology. Organic photovoltaics (OPVs) offer great potential for lower fabrication and materials costs as they are solution processable, compatible with high throughput roll-to-roll printing processes, flexible, lightweight and have tunable material properties. The encompassing goal of this work is to explore new materials, nanostructures and device designs that may aid the development of OPVs towards large scale, low cost and more efficient PV technologies. In the search for new PV materials, pyrite iron disulfide (FeS2) is noticeable as an earth abundant and environmentally benign semiconductor with low procurement costs and advantageous optoelectronic properties. Here, pyrite FeS2 nanocrystals (NCs) were synthesized and deployed in ternary organic-inorganic hybrid bulk-heterojunction (BHJ) solar cells with an inverted architecture. Three device performance regimes are observed when the pyrite NC concentration is varied from 0 to 4 wt% that appear linked to microstructure transitions in the active layer. The addition of FeS2 NCs consistently increased photocurrent and exposure in air actually rectified current leakage resulting in an average 28% increase in power conversion efficiency (PCE) of hybrid devices compared to control devices. The photocurrent enhancement and air-stability demonstrated by this inverted design offer a promising architecture for future pyrite FeS2 NC-based devices. Furthermore, the NC characterizations presented here provide insight to advance pyrite's development as a PV material. Nanostructures of various types can be used to enhance OPV performance. The effects of nanostructured ZnO electron transport layers (ETLs) were studied in high efficiency inverted OPVs on ITO substrates. Nano-ridged and planar ZnO ETLs were formed and consistently high FFs exceeding 70% and PCEs reaching 8.32% were achieved on both types of nanostructures when maintaining optimal active layer thickness. ITO is the most common transparent electrode used in OPVs, yet limited indium reserves and poor mechanical properties of ITO on flexible substrates make it non-ideal for large-scale OPV production. To replace ITO, plasmonic nanostructured electrodes were designed, fabricated and deployed as electrodes in inverted OPV devices. ZnO thickness was found to significantly impact active layer absorption due to resonant cavity effects with the plasmonic nanostructured electrodes. Devices with thinner ZnO ETLs showed PCEs as high as 5.70% and higher JSC’s than devices on thicker ZnO. ITO-free, flexible devices on PET showed a PCE of 1.82% and those fabricated on ultrathin and conformable Parylene substrates yielded an initial PCE over 1%. To our knowledge, this is the first time nanopatterned plasmonic electrodes have been applied to highly flexible, ITO-free OPVs. With further fabrication development to improve nanopattern quality on flexible substrates, these designs show promise for highly functioning conformable devices that can be applied to numerous needs for lightweight, ubiquitous power generation.