|
|
| Fig. 1: Schematic of a layered organic photovoltaic cell, in which two electrodes sandwich two closely spaced layers of organic compounds: an electron donor and an acceptor. (Source: Wikimedia Commons) |
Turn on any news channel for an hour and barring the recent occurrence of a natural or civil disaster, one is bound to hear a discussion of energy in the broad sense of the energy production and budget of a region or nation. In our day and age, the discussion often broaches renewability and accessibility, two highly regarded traits in energy technology. A research group in Japan has demonstrated the ability to construct organic photovoltaic films, a new type of solar cell, using a felt-tip pen controlled by a human hand. This technique boasts both of those necessary traits. [1]
Cheaply and efficiently harnessing the sun's energy would present a constant and nearly limitless supply of energy, at least on the timescales of humankind. Currently, photovoltaic cells form the basis of renewable solar resources and technologies. Originally, the photovoltaic cell was a semiconductor device fabricated on silicon wafers, much like microchips and other integrated electronic devices.
The basis of the energy transfer is called the photovoltaic effect and it's discovery is credited to A.E. Becquerel (father of H. Becquerel!) in 1839. [2] He observed that illuminating a silver-chloride (AgCl) electrode immersed in an acidic solution subsequently generated a voltage between the AgCl electrode and another metal electrode in the same solution. At the time, electrons had yet to be discovered so the phenomenon was just that, a phenomenon. The modern conclusion was that electrons in the AgCl absorbed photons from the incident light and were subsequently excited to a higher energy state from which they could flow to generate current.
Contemporary photovoltaic solar cells were well understood in the 1950s, and the limits of their efficiency thoroughly explored by W. Shockley, the man also credited for leading the team that discovered the transistor effect. [3] Semiconductor photovoltaics often have a structure resembling that of a diode junction, which is a region with positive and negative charge carrier doping, usually of a silicon substrate. During operation, an incident photon excites an electron from the valence band of it's parent nucleus to the semiconductor conduction band of the material, from where it can flow to create current. [3,4] This process leaves behind a positively charged "hole", a quasiparticle that can flow in a very similar manner to an electron, yet with a positive charge. The charge-carrier doping discussed previously generates an electric field which forces the promoted electron-hole pair to flow each in a particular direction. If an external load is connected between the opposite sides of the junction, the flow of electron-hole pairs generates a current through the load, thus producing power from sunlight.
As always, there is a significant amount of complicated physics going on, such as the diffusion of charge carriers between the differently doped regions, but the description above captures the essence of a photovoltaic cell's behavior: light absorption and charge pair generation, subsequent separation of the charge pair to avoid recombination, and finally, charge transport to a load.
Although silicon and other semiconductor-based photovoltaics are still widely used, newer technologies have promising efficiency increases, such as those obtained with thin-film and organic photovoltaics. Here we choose to focus on the latter, which often makes uses of conductive polymers instead of semiconductor diode junctions as the means of converting photons to electrical energy. Incident photons are similarly absorbed by electrons in the material, but don't immediately generate a free electron or one constrained to a conduction band. Rather, the electron is promoted to higher molecular orbital and exists in a bound state with its hole, called an exciton, which must be promptly disassociated to generate electricity. [5]
Disassociation is accomplished simply by having a two closely spaced films of electron donor and acceptor molecules. The original incident photon generates an exiton in the donor layer. A neighboring acceptor molecule, with a high affinity for electrons, overcomes the exiton binding energy and captures the excited electron from the donor, leaving behind a positively charged hole. Metallic electrodes then transport the charge to a load, generating current just as in a semiconductor photovoltaic cell.
One of the beauties of organic photovoltaics is that the light-sensitive device which does all the heavy-lifting is a film! It can be deposited on a variety of surfaces and can even be flexible if its substrate is appropriately so. However, most efforts have focused on nano-fabrication type techniques, depositing the organic compounds necessary to build the photovoltaic cell on a transparent substrate wafer mad of such things like fused silica.
One of the biggest challenges in energy is the need to make a technology or resource accessible and economically viable for a global market. Semiconductor based solar cells have gotten much cheaper in recent years and are likely a viable energy solution for more and more consumers yet organic photovoltaics require expensive and complex instruments to fabricate. If the potential advantages of organic photovoltaics are ever to be utilized to their full extent, this must be rectified.
To address the increasingly important question of viable and renewable sources of energy, a group based in Japan has demonstrated that with the right solution of organic compounds, one can simply "paint" a solar cell onto any surface, assuming proper adhesion is achieved. [1] No longer is one required to spend hours belaboring various depositions performed in a cleanroom. Instead, you need only ask a chemist to brew up the proper solution, fill a felt-tip pen with said solution, and brush away, just as the authors of [1] have demonstrated.
One might ask how well such devices perform against those made with fancy machines and well-trained operators. In fact, the electrical current as a function of applied voltage, I-V curve, under illumination is almost identical between the hand-painted and more traditional organic photovoltaic cells. The external quantum efficiency, or how well the film converts photons of varying wavelength to electrons, is actually a small fraction better in the hand-painted version compared to its predecessor. Finally, the power conversion efficiency, a measure of how well a solar cell performs, does not decrease with increasing thickness of the hand-painted photovoltaic cell, which was a limitation of certain other traditionally fabricated organic photovoltaics, as the group demonstrates. [1]
However, this comparison must be understood to be a comparison solely between organic photovoltaics fabricated with clean-room technologies and those hand-painted with a special solution. [1] The efficiency and I-V curves of all varieties of organic photovoltaics are still lagging behind that of the well-developed semiconductor technology, and may do so forever. [3]
There are a number of issues with organic photovoltaic technology that are worth discussing. First and foremost, they have a significantly limited lifetimes compared to their silicon counterparts. [5-8] The organic molecules that make up organic solar cells suffer from oxidation, which changes the structure of the light-sensitive molecule to the extent that solar photons can no longer excite the flow of electricity. Indeed, it has been shown that encapsulated organic photovoltaics have a significantly extended lifetime compared to their brethren left open to the air, although both still pale in comparison to the silicon photovoltaic. [6] This makes them impractical for widespread use in their current form, with some only lasting 1 week. [6]
The efficiency of organic photovoltaics is also a very limiting factor. Semiconductor photovoltaics are more efficient owing to their inherent crystalline structure. [3,4] Organic photovoltaics, espeically those of the hand-painted variety, have essentially no crystalline structure and produce significantly less electrical current for a given photo-excited voltage within the cell. In 1961, Shockley reported an estimated conversion efficiency of nearly 14% (a number that has assuredly increased), whereas the hand-painted organic photovoltaic from 2017 was found to have a maximum efficiency of just 7.5%. [1,3]
As presented in Fig. 1, there is also a problem with the choice of electrodes. Not only do the organic constituents of the solar cell itself oxidize and cease to function, but the metal contacts through which electricity is extracted are subject to environmental strain. In particular, calcium oxidizes extraordinarily quickly and would be a poor choice of electrode. For semiconductor photovoltaics, one can produce a region of degenerate doping directly on a silicon surface, which serves as a conducting contact to extract electrons/holes photovoltaic-ly excited. [9]
With the ability to coat nearly any surface in an active photovoltaic cell, the possible reach and effectiveness of organic photovoltaics knows nearly no bounds. A new homeowner need only pick up a few specialized paintbrushes, a bucket of photovoltaic solution, some wires and a few batteries to store energy and they could be well on their way to much smaller electrical bill. Outside of cozy suburbia, the work with hand-painted photovoltaic cells allows nearly anyone who can afford it the opportunity to capitalize on the ever-present sun, regardless of building layout, composition or prior planning.
© Chas Blakemore. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] K. Suzuki et al., "Drawing Organic Photovoltaics Using Paint Marker Pens," AIP Adv. 7, 115002 (2017).
[2] A. E. Becquerel, "Mémoire sur les Effets Électriques Produits sous l'Influence des Rayons Solaires" Compt. Rend. 9, 561 (1839).
[3] W. Shockley et al., "Detailed Balance Limit of Efficiency of p-n Junction Solar Cells," J. Appl. Phys. 32, 510 (1961).
[4] R. N. Hall, "Silicon Photovoltaic Cells," Solid State Electron. 24, 595 (1981).
[5] O. Abdulrazzaq et al., "Organic Solar Cells: A Review of Materials, Limitations,and Possibilities for Improvement," Particul. Sci. Technol. 31, 427 (2013).
[6] M. Corazza et al., "Lifetime of Organic Photovoltaics: Linking Outdoor and Indoor Tests," Sol. Energy Mater Sol. Cells 143, 467-472 (2015).
[7] P. Ditiangkin, "Organic Light-Emitting Diodes and Organic Photovoltaics," Physics 240, Stanford Uniwersity, Fall 2013.
[8] A. Chew, "Organic Photovoltaics for a Greener Future," Physics 240, Stanford University, Fall 2015.
[9] A. P. Roth et al., "Band-Gap Narrowing in Heavily Defect-Doped ZnO," Phys. Rev. B 25, 7836 (1982).
