Carbon Nanomaterials and Their Application for Emerging Solar Cells

The fact that only one–thousandth of the Sun’s energy incident on the Earth is equal to the entire world's current energy needs means direct conversion of this energy into electricity–photovoltaic (PV) energy–is now a mainstream renewable energy source. There are many types of emerging PV cells. Dye–sensitized solar cells (DSSCs) are an attractive potential source of renewable energy due to their eco–friendliness and ease of fabrication. However, in DSSCs, the rarity and high cost of some electrode materials (e.g. platinum) and the inefficient performance caused by slow electron transport, poor light–harvesting efficiency, and significant charge recombination present significant limitations. Over the past several years, carbon nanomaterials including carbon particles, carbon nanotubes and graphene have played important roles in addressing these issues. Although excellent progress has been made in the application of carbon materials in DSSCs, the exact role of nanocarbons in both the photoelectrode and counter electrode (CE) of DSSCs is still unclear. Organic–inorganic halides based perovskite solar cells (PSCs) have attracted a great deal of attention due to the extremely rapid increases in efficiencies observed over the past few years. Although the efficiencies of the PCS have exceeded 20%, they do have some disadvantages such as use of expensive electrode materials, the high temperature processing required during production and poor stability when in use. In this regard, it is no surprise that carbonaceous materials would have significant role in the development of PSCs as nanocarbons have been extensively studied in various energy related applications because of their fascinating properties, low cost and abundance. Research into the potential application of carbon nanomaterials in PSC is still at an early stage and a lot remains to be explored. This Ph.D. project focuses on the application and development of carbon nanomaterials for emerging PV devices such as DSSCs and PSCs. The following research has been included in this thesis: 1) A hybrid structure consisting of SnO₂ and reduced graphene oxide (SnO₂-RGO) was synthesized via a microwave-assisted method and has been employed as a photoanode in DSSCs, for the first time. It was found that the incorporation of RGO into the SnO₂ film not only enhances the electron transfer rate of the photoanode, but it also increases the adsorption of dye molecules into the film. Both these effects greatly enhance the DSSC performance. 2) As an alternative to platinum (Pt), a hybrid electrocatalyst based on sulfur-doped graphene with FeS₂ microspheres (SGN-FeS₂) was designed and used as a CE of DSSCs. Benefiting from the high conductivity of SGN and excellent electrocatalytic activity of FeS₂, the bifunctional hybrid electrocatalyst based device displays an efficiency of 8.1%, which was comparable to that (8.3%) of expensive Pt CE based DSSC and also exhibits excellent stability in ambient conditions. 3) Solution processed transparent conductive graphene films are utilized, for the first time, as an alternative to traditional transparent conducting oxide (TCO) electrodes at the electron collecting layer in perovskite solar cells (PSCs). By optimising the sheet resistance (Rs) and transparency of the films, maximum power conversion efficiency of 0.62% was obtained. The successful incorporation of graphene structures into both compact TiO₂ and mesoporous TiO₂ layers of the PSCs was also demonstrated. 4) The influence of CNTs on the PV performance of 1D titanium dioxide nanofiber (TiO₂ NF) photoelectrode perovskite solar cells (PSCs) was systematically explored. It was found that in addition to the significant enhancement in the efficiency of PSCs with SWCNTs, the incorporation of SWCNTs into TiO₂ NFs reduced the hysteresis effect and improved the stability of the PSC devices both under light and during storage in ambient conditions. 5) Significant enhancement in the power conversion efficiency (PCE) and stability (light- and long-term storage-stability) of perovskite solar cells (PSCs) by incorporating single-walled carbon nanotubes (SWCNTs) into the nanocrystalline TiO₂ photoelectrode was reported. The TiO2-SWCNTs photoelectrode based PSC device exhibited a PCE of up to 16.11%, while the cell fabricated without SWCNTs displayed an efficiency of 13.53%.Thesis (Ph.D.) -- University of Adelaide, School of Chemical Engineering, 201

Near-Infrared Active Lead Chalcogenide Quantum Dots: Preparation, Post-Synthesis Ligand Exchange, and lications in Solar Cells

Quantum dots (QDs) of lead chalcogenides (e.g. PbS, PbSe, and PbTe) are attractive near‐infrared (NIR) active materials that show great potential in a wide range of applications, such as, photovoltaics (PV), optoelectronics, sensors, and bio‐electronics. The surface ligand plays an essential role in the production of QDs, post‐synthesis modification, and their integration to practical applications. Therefore, it is critically important that the influence of surface ligands on the synthesis and properties of QDs is well understood for their applications in various devices. In this Review we elaborate the application of colloidal synthesis techniques for the preparation of lead chalcogenide based QDs. We specifically focus on the influence of surface ligands on the synthesis of QDs and their solution‐phase ligand exchange. Given the importance of lead chalcogenide QDs as potential light harvesters, we also pay particular attention to the current progress of these QDs in photovoltaic applications.This work was financially supported by the Australian Research Council Discovery Projects DP110102877 and DP140104062, DP150101939 and Discovery Early Career Award DE16010056

Solution processed graphene structures for perovskite solar cells

Organometallic trihalide perovskite light absorber based solar cells have drawn increasing attention because of their recent rapid increase in power conversion efficiency (PCE). These photovoltaic cells have relied significantly on transparent conducting oxide (TCO) electrodes which are costly and brittle. Herein, solution processed transparent conductive graphene films (TCGFs) are utilized, for the first time, as an alternative to traditional TCO electrodes at the electron collecting layer in perovskite solar cells (PSCs). By investigating and optimizing the trade-off between transparency and sheet resistance (Rs) of the graphene films, a PCE of 0.62% is achieved. This PCE is further improved to 0.81% by incorporating graphene structures into both compact and mesoporous TiO2 layers of the solar cell. We anticipate that the present study will lead to further work to develop graphene-based transparent conductive electrodes for future solar cell devices

Plasmonic Gold Nanostars Incorporated into High-Efficiency Perovskite Solar Cells

Incorporating appropriate plasmonic nanostructures into photovoltaic (PV) systems is of great utility for enhancing photon absorption and thus improving device performance. Herein, the successful integration of plasmonic gold nanostars (AuNSs) into mesoporous TiO2 photoelectrodes for perovskite solar cells (PSCs) is reported. The PSCs fabricated with TiO2-AuNSs photoelectrodes exhibited a device efficiency of up to 17.72 %, whereas the control cells without AuNSs showed a maximum efficiency of 15.19 %. We attribute the origin of increased device performance to enhanced light absorption and suppressed charge recombination

Sulfur-doped graphene with iron pyrite (FeS 2 ) as an efficient and stable electrocatalyst for the iodine reduction reaction in dye-sensitized solar cells

As an alternative to platinum (Pt), hybrid electrocatalysts based on sulfur-doped graphene with FeS2 microspheres (SGN-FeS2) were used as a counter electrode (CE) in dye-sensitized solar cells (DSSCs). Benefiting from the high conductivity of SGN and excellent electrocatalytic activity of the FeS2, the bifunctional hybrid electrocatalyst-based device displays a power conversion efficiency (PCE) of 8.1%, which is comparable to that (8.3%) of traditional Pt CE-based DSSC, while also exhibiting excellent stability in ambient conditions. These characteristics, in addition to its low-cost and facile preparation, make the SGN–FeS2 hybrid an ideal CE material for DSSCs

Development and access to finance of small and medium-sized enterprises in Mongolia

This paper presents a recent survey on development and access to finance of small and medium-size enterprises (SMEs) in Mongolia. The survey covers a sample of 1922 SMEs from Ulaanbaatar (capital city) and 21 provinces. We find that banks and local government administration are the most supportive institution for SME development. Political instability, corruption and labour supply, high lending rate, short maturity loans, service fees are perceived as the major obstacles that SMEs face in their business environments. Our results also suggest that SMEs in Mongolia are less likely to have access to external finance because of tight credit condition, potentially explaining the lack of SMEs’ growth. Implementing country-specific reform strategy for SME development covering key building blocks is needed to promote SME financial inclusion and facilitate SMEs to contribute the economic growth

Carbon nanotubes in TiO₂ nanofiber photoelectrodes for high-performance perovskite solar cells

1D semiconducting oxides are unique structures that have been widely used for photovoltaic (PV) devices due to their capability to provide a direct pathway for charge transport. In addition, carbon nanotubes (CNTs) have played multifunctional roles in a range of PV cells because of their fascinating properties. Herein, the influence of CNTs on the PV performance of 1D titanium dioxide nanofiber (TiO2 NF) photoelectrode perovskite solar cells (PSCs) is systematically explored. Among the different types of CNTs, single-walled CNTs (SWCNTs) incorporated in the TiO2 NF photoelectrode PSCs show a significant enhancement (≈40%) in the power conversion efficiency (PCE) as compared to control cells. SWCNTs incorporated in TiO2 NFs provide a fast electron transfer within the photoelectrode, resulting in an increase in the short-circuit current (J sc) value. On the basis of our theoretical calculations, the improved open-circuit voltage (V oc) of the cells can be attributed to a shift in energy level of the photoelectrodes after the introduction of SWCNTs. Furthermore, it is found that the incorporation of SWCNTs into TiO2 NFs reduces the hysteresis effect and improves the stability of the PSC devices. In this study, the best performing PSC device constructed with SWCNT structures achieves a PCE of 14.03%

Electrically Sorted Single-Walled Carbon Nanotubes-Based Electron Transporting Layers for Perovskite Solar Cells

© 2019 The Author(s). This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Incorporation of as prepared single-walled carbon nanotubes (SWCNTs) into the electron transporting layer (ETL) is an effective strategy to enhance the photovoltaic performance of perovskite solar cells (PSCs). However, the fundamental role of the SWCNT electrical types in the PSCs is not well understood. Herein, we prepared semiconducting (s-) and metallic (m-) SWCNT families and integrated them into TiO2 photoelectrodes of the PSCs. Based on experimental and theoretical studies, we found that the electrical type of the nanotubes plays an important role in the devices. In particular, the mixture of s-SWCNTs and m-SWCNTs (2:1 w/w)-based PSCs exhibited a remarkable efficiency of up to 19.35%, which was significantly higher than that of the best control cell (17.04%). In this class of PSCs, semiconducting properties of s-SWCNTs play a critical role in extracting and transporting electrons, whereas m-SWCNTs provide high conductance throughout the electrode

Advances in emerging solar cells

There has been a continuous increase in the world's electricity generation and consumption over the years [...]

Recent advances in Perovskite‐based building‐integrated photovoltaics

Perovskite-based solar cells have attracted great attention due to their low cost and high photovoltaic (PV) performance. In addition to their success in the PV sector, there has been growing interest in employing perovskites in energy-efficient smart windows and other building technologies owing to their large absorption coefficient and color tunability. The major challenge lies in integrating perovskite materials into windows and building facades and combining them with added functionalities while maintaining their remarkable power conversion efficiencies. Herein, advances that have been made in the application of perovskites to building-integrated photovoltaics (BIPVs) in four areas are highlighted: semitransparent windows, colorful wall facades, electrochromic windows, and thermochromic windows. In addition, the opportunities and challenges of this cutting-edge research area and important roadmaps for the future use of perovskites in BIPVs are discussed