LePing Yu

Heterojunction Solar Cells Based on Silicon and Composite Films of Polyaniline and Carbon Nanotubes

Composite films of polyaniline (PANI) and single-walled carbon nanotubes have been produced by a simple vacuum filtration process from aqueous solution, and the films have been applied on n-type silicon to form heterojunction solar cells. The performance of devices with the composite electrodes of different constituent ratios and of various film thicknesses has been investigated. It was found that the incorporation of the conducting polymer reduces the sheet resistance of the films, while the overall thickness influences both the conductivity and the amount of light transmitted through to the underlying silicon. By optimizing the composition and thickness of the films, a power conversion efficiency of 7.4% was obtained, which is a 60% increase over that obtained from devices without the polymer. Analysis of the DC electrical to optical conductivity ratios of the various films and comparison of this with the solar cell performance data shows that the improved output is not due solely to better electrical and optical properties but depends strongly on the exact nature of the junction as well, which changes with PANI content.

Application of a hole transporting organic interlayer in graphene oxide/single walled carbon nanotube–silicon heterojunction solar cells

The solid-state hole transporting material 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) has been applied as an interlayer for graphene oxide/single walled carbon nanotube–silicon (GOCNT/Si) heterojunction solar cells, forming a GOCNT/spiro-OMeTAD/Si structure. An organic–aqueous transfer method was developed to deposit the GOCNT electrode onto the spiro-OMeTAD coated Si surface without dissolving the organic layer. The influence of the thickness of the organic layer and the thin film GOCNT transparent conducting electrodes as well as the doping of the films with gold chloride (AuCl3) on device performance is explored. With the optimized thickness of the spiro-OMeTAD interlayer and the GOCNT electrode with transmittance above 80% at 550 nm, devices with solar power conversion efficiency of 12.83 ± 0.22% have been fabricated. This study reveals that adding a hole-conducting organic interlayer is able to significantly minimize the recombination at the heterojunction interface. In addition to improving performance, the spiro-OMeTAD behaves as a physical protection layer to significantly enhance device stability.

Synthesis of ultra-long hierarchical ZnO whiskers in a hydrothermal system for dye-sensitised solar cells

One-dimensional (1-D) ZnO structures are of great interest for many applications but the direct hydrothermal synthesis of ultra-long ZnO whiskers (>100 μm) remains a great challenge. Herein, we demonstrate the first synthesis of three kinds of ultra-long hierarchical ZnO whiskers, which are defined as ZnO-2 (>100 μm in length), ZnO-3 (>200 μm in length with relatively smooth surface) and ZnO-4 (>200 μm in length with relatively rough surface), via a one-pot hydrothermal process. The maximum length of hierarchical ZnO-4 whiskers can reach up to about 270 μm. The formation of oval-shaped quasi-hollow structural precursors plays a key role for the correct attachment of Zn2+-terminated and O2−-terminated active surfaces, producing well-ordered Zn2+⋯O2⋯Zn2+ bonds, and finally promoting the formation of ultra-long ZnO whiskers with hierarchical structures. When the synthesized ultra-long hierarchical ZnO-4 whiskers are mixed with commercial TiO2 for dye-sensitised solar cells (DSCs), the current density increases significantly from 13.68 mA cm−2 (commercial TiO2) to 16.81 mA cm−2 (TiO2–ZnO hybrid materials). The hybrid materials show a conversion efficiency of 7.95% which is higher as compared to that of commercial TiO2 (5.87%). This interesting performance of a hybrid material sheds light on the possibility of preparing ultra-long hierarchical ZnO whiskers (>100 μm) with tunable lengths through hydrothermal approaches and their application in DSCs.

Insights into chemical doping to engineer the carbon nanotube/silicon photovoltaic heterojunction interface

Graphene oxide/single-wall carbon nanotube (GOCNT) hybrid films have been used to fabricate heterojunction solar cells with silicon (Si) due to their compatibility with both aqueous and organic processing. In these cells GOCNT films are required to be both highly transparent and conducting. Different approaches are used to improve these optoelectronic properties of the GOCNT films, including hybridization with silver nanowires (AgNWs) and p-type doping with CuCl2, AuCl3, SOCl2, HCl, H2SO4, HNO3 and HClO4. UV-vis-NIR absorbance, Raman spectroscopy, and the sheet resistance of the films were used to evaluate the properties of the treated films and quantify doping. The most effective way to improve the optoelectronic properties of the GOCNT films was the incorporation of AgNWs which improved the figure of merit (FOM, the ratio of transparency and conductivity) by over 600%. However, GOCNT/Si heterojunction photovoltaic devices with HNO3 doped GOCNT films showed the highest solar photocurrent conversion efficiency (11.38 ± 0.26%). In terms of stability, CuCl2 and HCl doped films have the best electrode FOM stability, and devices made with such films have the most stable efficiency as well. This report suggests that the electronegativity of the active elements in the dopants has a strong influence on the optoelectronic properties of the films as well as the solar cell performance.

Use of Carbon Nanotubes in Third-Generation Solar Cells

There is a clear need to make energy production cheap, readily accessible, and deployable in a vast array of locations and circumstances, while ensuring its production does not enhance the greenhouse effect on climate. Of all the options available, photovoltaics offers the highest probability of delivering a meaningful and sustainable change in the way society produces its energy. Third-generation approaches to photovoltaics offer real opportunities to deliver energy to broad sections of society, which will ultimately provide energy security. This technology offers the advantages of cheap production, flexibility (and hence a range of deployment opportunities), and tunability of light absorption. Significant efforts to improve these photovoltaic systems have involved the use of carbon nanotubes. This chapter will primarily focus on those efforts. Carbon nanotubes have been used in virtually every component of the devices to help charge conduction, improve electrode flexibility, and in some cases act as active light absorbing materials.

Application of Hole-Transporting Materials as the Interlayer in Graphene Oxide/Single-Wall Carbon Nanotube Silicon Heterojunction Solar Cells*

Solid-state hole-transporting materials, including the traditional poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), and recently developed 4,4′-(naphthalene-2,6-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (NAP) and (E)-4′,4‴-(ethene-1,2-diyl)bis(N,N-bis(4-methoxyphenyl)-[1″,1‴-biphenyl]-4-amine) (BPV), have been applied as a hole-transporting interlayer (HTL) for graphene oxide/single-walled carbon nanotube–silicon (GOCNT/Si) heterojunction solar cells, forming a GOCNT/HTL/Si architecture. The influence of the thickness of the HTL has been studied. A new AuCl3 doping process based on bath immersion has been developed and proved to improve the efficiency. With the AuCl3-doped GOCNT electrodes, the efficiency of GOCNT/PEDOT:PSS/Si, GOCNT/NAP/Si, and GOCNT/BPV/Si devices was improved to 12.05 ± 0.21, 10.57 ± 0.37, and 10.68 ± 0.27 % respectively. This study reveals that the addition of an HTL is able to dramatically minimise recombination at the heterojunction interface.

Direct‐Patterning SWCNTs Using Dip Pen Nanolithography for SWCNT/Silicon Solar Cells

Dip pen nanolithography (DPN) is used to pattern single-walled carbon nanotube (SWCNT) lines between the n-type Si and SWCNT film in SWCNT/Si solar cells. The SWCNT ink composition, loading, and DPN pretreatment are optimized to improve patterning. This improved DPN technique is then used to successfully pattern >1 mm long SWCNT lines consistently. This is a 20-fold increase in the previously reported direct-patterning of SWCNT lines using the DPN technique, and demonstrates the scalability of the technique to pattern larger areas. The degree of the uniformity of SWCNTs in these lines is further characterized by Raman spectroscopy and scanning electron microscopy. The patterned SWCNT lines are used as thin conductive pathways in SWCNT/Si solar cells, similar to front contact electrodes. The critical parameters of these solar cells are measured and compared to control cells without SWCNT lines. The addition of SWCNT lines increases power conversion efficiency by 40% (relative). Importantly, the SWCNT lines reduce average series resistance by 44%, and consequently increase average fill factor by 24%.

Measuring the Density of States of the Inner and Outer Wall of Double-Walled Carbon Nanotubes

The combination of ultraviolet photoelectron spectroscopy and metastable helium induced electron spectroscopy is used to determine the density of states of the inner and outer coaxial carbon nanotubes. Ultraviolet photoelectron spectroscopy typically measures the density of states across the entire carbon nanotube, while metastable helium induced electron spectroscopy measures the density of states of the outermost layer alone. The use of double-walled carbon nanotubes in electronic devices allows for the outer wall to be functionalised whilst the inner wall remains defect free and the density of states is kept intact for electron transport. Separating the information of the inner and outer walls enables development of double-walled carbon nanotubes to be independent, such that the charge transport of the inner wall is maintained and confirmed whilst the outer wall is modified for functional purposes.

Efficiency Improvement Using Molybdenum Disulphide Interlayers in Single-Wall Carbon Nanotube/Silicon Solar Cells

Molybdenum disulphide (MoS2) is one of the most studied and widely applied nanomaterials from the layered transition-metal dichalcogenides (TMDs) semiconductor family. MoS2 has a large carrier diffusion length and a high carrier mobility. Combining a layered structure of single-wall carbon nanotube (SWCNT) and MoS2 with n-type silicon (n-Si) provided novel SWCNT/n-Si photovoltaic devices. The solar cell has a layered structure with Si covered first by a thin layer of MoS2 flakes and then a SWCNT film. The films were examined using scanning electron microscopy, atomic force microscopy and Raman spectroscopy. The MoS2 flake thickness ranged from 5 to 90 nm while the nanosheet’s lateral dimensions size ranged up to 1 μm2. This insertion of MoS2 improved the photoconversion efficiency (PCE) of the SWCNT/n-Si solar cells by approximately a factor of 2.

Back cover: Solar RRL 3-4∕2017

Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission. This is the back cover of Solar RRL 3-4∕2017 with an image relating to the article Sulfur-Doped Graphene with Iron Pyrite (FeS 2 ) as an Efficient and Stable Electrocatalyst for the Iodine Reduction Reaction in Dye-Sensitized Solar Cells (https://dspace.lboro.ac.uk/2134/24841)