Supriya Pillai

Microwave probing can be used to study the charge carrier dynamics in photo-conductive materials. Such studies are of paramount importance considering the significant efforts dedicated towards photo-voltaic energy generation now-a-days. Contactless methods to quantify the generation of charge carriers in film-based materials, under the influence of light of varying intensity, are much needed to enable screening of such materials to identify the most promising candidates [1]. We describe the use of time-resolved microwave conductivity (TRMC), which is an effective and contactless technique, to study the dynamics of charge carrier generation and recombination in thin film-based conductive media. In this work, we present numerical simulations carried out in CST Microwave Studio, and a practical microwave probe setup, to extend the TRMC technique for enabling new measurement capabilities. Such microwave-based probe setup can be used in-line with bulk-fabrication processes for evaluating quality of thin-film organic photo voltaic materials in real time.

Significant photocurrent enhancement has been demonstrated using plasmonic light-trapping structures comprising nanostructured metallic features at the rear of the cell. These structures have conversely been identified as suffering heightened parasitic absorption into the metal at certain resonant wavelengths severely mitigating benefits of light trapping. In this study, we undertook simulations exploring the relationship between enhanced absorption into the solar cell, and parasitic losses in the metal. These simulations reveal that resonant wavelengths associated with high parasitic losses in the metal could also be associated with high absorption enhancement in the solar cell. We identify mechanisms linking these parasitic losses and absorption enhancements, but found that by ensuring correct design, the light trapping structures will have a positive impact on the overall solar cell performance. Our results clearly show that the large angle scattering provided by the plasmonic nanostructures is the reason for the enhanced absorption observed in the solar cells.

A low-temperature solution-based process for depositing silver nanowire (AgNW) networks for use as transparent conductive top electrode is demonstrated. These AgNWs when applied to Cu₂ZnSnS₄ solar cells outperformed indium tin oxide as the top electrode. Thinner nanowires allow the use of lower temperatures during processing, while longer wires allow lowered sheet resistance for the same surface coverage of NWs, enhancing the transmittance/conductance trade-off. Conductive atomic force microscopy and percolation theory were used to study the quality of the NW network at the microscale. Our optimized network yielded a sheet resistance of 18 Ω/□ and ∼95% transmission across the entire wavelength range of interest for a deposition temperature as low as of 60 °C. Our results show that AgNWs can be used for low-temperature cell fabrication using cheap solution-based processes that could also be promising for other solar cells constrained to low processing temperatures such as organic and perovskite solar cells.

In this work, we have demonstrated the incorporation of metal (Ag NPs) and dielectric nanoparticles (SiO ₂ NPs) into the hole transporting layers of inverted PSCs using facile deposition methods.

Solution processed thin‐film solar technologies, such as organic photovoltaics, and more recently, perovskite solar cells, may provide low‐cost electricity generation. These technologies suffer from insufficient light absorption due to thin absorber layers. Plasmonic nanostructures have been incorporated in both technologies, initially with the aim of increasing light absorption, but reports have also shown significant enhancement in electrical characteristics in devices. Enhancement mechanisms that are facilitated by plasmonic nanostructures such as improved exciton dissociation and charge carrier transport, can occur concurrently with improved light absorption. This work surveys the myriad enhancement mechanisms and, importantly, discusses the extent of current understanding, as well as insights gained in plasmonics applications thus far. Given the substantial opportunities, the continuous focus on characterization and interpretation of enhancement mechanisms is imperative to unlock the full potential of plasmonic organic and perovskite solar cells. In particular, electrical or electronic effects from plasmonic nanostructure integration deserves further attention as a promising complement to improvements in device performance from optical effects.

Silver demonstrates the unique optical properties that make it suitable for various plasmonic applications. Determining the accurate optical constants for silver and investigating its dependence on grain structures are necessary for reliable theoretical predictions. This work investigates the effects of different deposition conditions and the resultant variable grain sizes, upon the optical properties of the silver films in conjunction with ellipsometry and optical characterization. Annealing conditions of substrate SiN_x films were also studied to ensure good quality of deposited polycrystalline silver films. The effects of grain sizes of silver on its electronic relaxation times were studied and shown to be consistent with the theoretical electrical resistivity model. This study shows that voids and surface layers can affect the optical constant values and that reproducible results can be obtained by avoiding such artefacts. The results will have implications on the analysis of different silver based nanostructures like nanowires and nanorods where grain boundary scattering can affect the optical and electrical properties like reflection and resistivity.

The use of hybrid transparent conductive (TC) layer has drawn much attention as a replacement for Indium Tin Oxide (ITO). In this work, Graphene and random Silver NWs have been selected as a hybrid TC to achieve a potential replacement. A simple solution-based process was applied for stacking NWs to Graphene. Understanding percolation theory embarked on the idea that graphene and AgNWs can mutually collaborate to carry the charge on the surface. The optimum high transmission (>90%) and the low sheet resistance (<30Ω/□) was obtained using a simple method of production. This promising hybrid TC layer would be a possible option for in many optoelectronic and semiconducting devices.