Carbon Nanotube‐Silicon Solar Cells

Due to the high cost of silicon photovoltaics there is currently great interest in finding alternative semiconductor materials for light harvesting devices. Single‐walled carbon nanotubes are an allotrope of carbon with unique electrical and optical properties and are promising as future photovoltaic materials. It is thus important to investigate the methods of exploiting their properties in photovoltaic devices. In addition to already extensive research using carbon nanotubes in organic photovoltaics and photoelectrochemical cells, another way to do this is to combine them with a relatively well understood model semiconductor such as silicon. Nanotube‐silicon heterojunction solar cells are a recent photovoltaic architecture with demonstrated power conversion efficiencies of up to ～14% that may in part exploit the photoactivity of carbon nanotubes.     

The Role of Nanotubes in Carbon Nanotube–Silicon Solar Cells

The mechanism of action of nanotube‐silicon heterojunction solar cells is under discussion with literature reports suggesting either p‐n or Schottky junction characteristics. The crux of the issue is whether the nanotubes contribute to the observed photocurrent or not. In order to further understand the mechanism of action of these solar cells, devices were fabricated using nanotubes sorted by (n,m) species, so that the excitonic transition is well defined and is outside the range of absorption of silicon and such that any contribution to the photocurrent from the nanotubes should be easily resolved from that of the silicon by analysis of the photocurrent spectrum. The devices exhibited the photocurrent spectra of silicon only, indicating that the nanotubes do not contribute to the photocurrent. However, by changing the back contact electrode material, results were obtained that appear to show such a contribution.     

Breakthrough Carbon Nanotube–Silicon Heterojunction Solar Cells

The latest advances in carbon nanotube–silicon heterojunction solar cells are combined with a new doping protocol based on the outstanding electron withdrawing properties and excellent silicon surface passivation ability of sulfonated polytetrafluoroethylene (Nafion). Using this new dopant for carbon nanotube–silicon solar cells, advanced substrate design, and an optimized antireflective texture fast etch with organic base, breakthrough performance is obtained from research grade devices with active areas of 1 and 5 cm², which yield power conversion efficiencies of 17.2 and 15.5%, respectively.     

Advances in Carbon Nanotube–Silicon Heterojunction Solar Cells

Heterojunctions of carbon nanotubes interfaced with silicon respond to light illumination and can be operated in the power regime as solar cells. Very significant advances have been made in the last 5 years both in terms of headline performance values and in fundamental understanding of the underlying operating principles, as well as the sophistication of the devices and studies being reported. The body of literature is growing rapidly, and the latest power conversion efficiency and active area records have now reached over 17% and 2 cm², respectively. Thus, the authors believe that it is now a useful time for an evaluation of the current state‐of‐the‐art and challenges going forward, as well as for a comprehensively updated review of progress made in the field. In addition, the authors provide a summary of the various fabrication schemes that have been used, analysis of some of the major device structure–property relationships revealed by comparison of published works, and a thorough breakdown of the various factors involved in improving performance, as well as a critical assessment of the real opportunities that may exist for this technology in the context of the wider silicon photovoltaics industry.     

High‐Efficiency Large‐Area Carbon Nanotube‐Silicon Solar Cells

Currently studied carbon nanotube‐silicon (CNT‐Si) solar cells are based on relatively small active areas (typically <0.15 cm²); increasing the active area generally leads to reduced power conversion efficiencies. This study reports CNT‐Si solar cells with active areas of more than 2 cm² for single cells, yet still achieving cell efficiencies of about 10%, which is the first time for CNT‐Si solar cells with an active area more than 1 cm² to reach the level for real applications. In this work, a controlled number of flattened highly conductive CNT strips is added, in simple arrangement, to form a CNT‐Si solar cell with CNT strips in which the middle film makes heterojunctions with Si while the top strips act as self‐similar top electrodes, like conventional metal grids. The CNT strips, directly condensed from as‐grown CNT films, not only improve the CNT‐Si junctions, but also enhance the conductivity of top electrodes without introducing contact barrier when the CNT strips are added onto the film. This property may facilitate the development of large‐area high‐performance CNT or graphene‐Si solar cells.     

Multifunctional Effect of p‐Doping,Antireflection, and Encapsulation by Polymeric Acid for High Efficiency and Stable Carbon Nanotube‐Based Silicon Solar Cells

Silicon solar cells among different types of solar energy harvesters have entered the commercial market owing to their high power conversion efficiency and stability. By replacing the electrode and the p‐type layer by a single layer of carbon nanotubes, the device can be further simplified. This greatly augments the attractiveness of silicon solar cells in the light of raw material shortages and the solar payback period, as well as lowering the fabrication costs. However, carbon nanotube‐based silicon solar cells still lack device efficiency and stability. These can be improved by chemical doping, antireflection coating, and encapsulation. In this work, the multifunctional effects of p‐doping, antireflection, and encapsulation are observed simultaneously, by applying a polymeric acid. This method increases the power conversion efficiency of single‐walled carbon nanotube‐based silicon solar cells from 9.5% to 14.4% and leads to unprecedented device stability of more than 120 d under severe conditions. In addition, the polymeric acid‐applied carbon nanotube‐based silicon solar cells show excellent chemical and mechanical robustness. The obtained stable efficiency stands the highest among the reported carbon nanotube‐based silicon solar cells.