Carbon is one of the most abundant elements in the earth's crust and is found in several structural forms (allotropes). Carbon atoms can be arranged in many ways to produce a wide variety of compounds that have unique and interesting physical, chemical, and electronic properties. Carbon allotropes include fullerenes (cage-like molecules of carbon), carbon nanotubes (high aspect-ratio cylindrical nanoparticles of carbon), and graphene (two-dimensional sheet of carbon). Using a combination of these materials, it is possible to fabricate devices composed entirely of carbon-based components.1 Due to its abundance and ease of processing in solution, carbon-based devices can potentially be made cheaply and in large quantities. Additionally, these materials exhibit exceptional electrical and optical properties, so they are highly tunable and can be used in many types of devices such as transistors, solar cells, displays, and supercapacitors. Carbon devices can also have high chemical, thermal, and physical endurance as compared to those made with other materials.
Recently, scientists have reported solar-cell devices with power conversion efficiencies (PCEs) of 0.1–1.3%. These solar cells consisted of an all-carbon photoactive layer using semiconducting single-walled carbon nanotubes (SWNTs) as the light-absorbing component and charge-donating (donor) material, with a fullerene layer as the charge-accepting (acceptor) material.2–4 These reports demonstrated the potential of carbon-based materials as the active elements in a solar cell. However, these devices use standard electrodes, such as indium tin oxide (ITO) as the bottom electrode (anode) and silver or aluminum as the top electrode (cathode). These materials are expensive, not solution processable, and not flexible—thus, they are not ideal choices.
Figure 1. Structure of the carbon-based solar cell, showing the components of each layer and the process of electron-hole pair (exciton) generation and separation when light is absorbed. Devices with both standard electrodes and carbon-based electrodes were fabricated. P3DDT and PEDOT are conductive polymers, while PDMS is a flexible silicone polymer. CNT: Carbon nanotube. ITO: Indium tin oxide. Sc-CNT: Semiconducting CNT. C60: A fullerene.
To address these issues, we fabricated the first all-carbon solar cell device (see Figure 1).5 We tested our device using both standard electrodes (ITO for the anode and silver for the cathode) or carbon-based electrodes (reduced graphene oxide for the anode and donor-doped SWNTs for the cathode). The active layer was composed of a bilayer structure with polymer-sorted semiconducting SWNTs as the donor layer, and fullerenes (C60) as the acceptor layer, which separates hole-electron pairs (excitons) produced when light is absorbed by the SWNTs. We sorted the SWNTs using a solution-based technique in which an organic polymer selectively dispersed the semiconducting SWNTs, a method previously developed by our group.6 This step was critical for good device reliability and performance, because unsorted, metallically-conductive SWNTs in this layer would cause shorts in the device and reduce efficiency.
After we optimized the active layer by determining the ideal solution concentration and deposition method of the SWNT film, as well as the thickness of the SWNT and C60 films, we fabricated solar-cell devices with ITO/silver electrodes with a maximum PCE of 0.46% under one-sun illumination. Next, our main challenge was to replace the electrodes with carbon-based ones. For the anode, we used reduced graphene oxide (rGO), which we fabricated on quartz substrates by thermally reducing spin-coated graphene oxide, a technique previously developed by our group.7 Using rGO can prevent device shorting because the smooth films produced allow for good morphology of the subsequent layers. The sorted SWNTs were spin-coated on top of the rGO films, followed by the thermal deposition of the C60 film. For the cathode, we used spray-coated SWNT films on elastomeric substrates. These films were n-type (donor) doped using a molecular dopant previously reported by our group.8 (N-type doping of this film is necessary because the cathode collects electrons, but carbon-based materials are typically hole carriers.) These SWNT cathodes were then laminated on top of the rGO/sorted-SWNT/C60 layer to complete the device, which had a PCE of ∼0.005%. We attributed the drop in performance when using the carbon electrodes to the contact resistance between the electrodes and the active layer, and to resistive losses and reduced device transparency from these electrodes.
We created a proof-of-concept device that demonstrated the feasibility of producing an all-carbon solar cell. However, there are many improvements necessary to increase the PCE of our device. For the photoactive layer, we are exploring different types of SWNTs to increase the absorption of light in the device, and we hope to improve the interface morphology between the fullerene and SWNT layers to enhance charge separation and extraction. We are also working to improve the contact between the active layer and the carbon-based electrodes by optimizing processing conditions and improving the conductivity of the electrodes. We believe that with these improvements, devices with PCEs of >1% can be achieved.
The authors would like to thank the Global Climate and Energy Project (GCEP) at Stanford University and the Air Force Office for Scientific Research (FA9550-12-1-01906) for funding.
Zhenan Bao, Michael Vosgueritchian
Zhenan Bao is a professor and a former distinguished technical staff member at Bell Labs from 1995 to 2004. She is a Fellow of SPIE, ACS, and AAAS. Her recent awards include the 2013 ACS Carl Marvel Creative Polymer Chemistry Award and the 2011 ACS Cope Scholar Award.
Michael Vosgueritchian received his BS in Chemical Engineering and Materials Science from the University of California at Berkeley in 2006. Subsequently, he worked for two years as a process engineer at Samsung Austin Semiconductors in Austin, Texas. In 2008, he joined Zhenan Bao's group as a doctoral student. His research is focused on using carbon nanotubes in stretchable electronic applications for transistors, electrodes, and solar cells.
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