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Polymer photovoltaic materials have attracted considerable attention as a clean-energy harvesting technology for their highly tunable optoelectronic properties and ability to form flexible, large-area solar panels by low-cost solution processing methods. One of the critical issues in polymer-based solar cells is to improve their power conversion efficiency and long-term stability.
Solar energy is currently considered to have the highest economic impact among other renewable energy resources such as hydroelectric, biomass, and wind energy.
In the last ten years, organic photovoltaic (OPV) cells have drawn significant attention as a possible alternative to their inorganic counterparts.
Modern OPV technology is highly suitable for cost-effective roll-to-roll or printing high throughput production. The possibilities for lightweight OPV applications on flexible substrates provide an advantage over traditional inorganic solar cells.
Although power conversion efficiencies (PCE) of up to 17% have been recently reported for laboratory-scale OPV cells, significant work is still required to improve the PCE, spectrum absorbance range, and device stability to enable viable large-scale industrial applications.
Conductive Organic Molecules with Conjugated Atomic Bonds
Molecular semiconductors are relatively small organic molecules, typically with a non-repeating structure. On the other hand, polymer semiconductors are chains of linked repeating monomers or chains of two or more monomers (copolymers).
All electronic organic materials, whether monomer or polymer, contain a backbone of “conjugated” bonds, which is essential to achieving conductivity in the molecule.
Conjugation means that the polymer backbone consists of alternating single and double bonds between the carbon atoms. The robust single bonds between the carbon atoms are the so-called localized σ-bonds. In contrast, the double bonds provide weaker and less strongly localized π-bonds, resulting in high electronic polarizability.
Organic Materials as an Alternative to the Inorganic Semiconductors
However, the conductivity of these organic semiconductors is relatively low. Removing an electron from the valence band by oxidation (p-doping) or adding an electron to the conducting band by reduction (n-doping) results in an increased conductivity of the material. The doping process creates charge defects that can travel along the polymer’s backbone.
In such organic semiconducting materials, the energy levels of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) are analogous to the valence and conduction bands of conventional inorganic semiconductors, respectively.
The electrons photogenerated in the donor molecules are transferred from the LUMO of the donor to that of the acceptor. Conversely, the photogenerated holes in the acceptor molecule are transferred from the HOMO of the acceptor to that of the donor.
The significant limitations to PCE of the OPV devices arise from the short diffusion length of the charge carriers within the organic semiconductors and insufficient light absorption within the device's photoactive layers. Low charge mobility within the organic semiconductor materials requires the active layer thickness to be 100 nm, resulting in low light absorption efficiency.
Organic Semiconductors for Low-Cost Solar Cells
The development of the bulk heterojunction (BHJ) concept in the last decade of the 20th century, together with a range of organic semiconducting materials that can efficiently absorb light in the ultraviolet(UV)-visible part of the solar spectrum, paved the way to remarkable improvements in the OPV technology.
In the BHJ arrangement, conjugated donor polymers and acceptor materials, usually fullerene derivatives, are mixed in a well-controlled way to ensure a large donor/acceptor interfacial contact area and effective charge diffusion.
Placing the donor and acceptor molecules close together, typically less than 10 nm apart, reduces the required charge diffusion lengths below the electron-hole pair mean free-path lengths (the minimum charge diffusion distance before the electrons and the holes recombine).
OPV devices with BHJ structures benefit from improved electron/hole pair dissociation and charge transport, enhancing OPV devices' performance.
Three-Component Photoactive Layer with Improved Light Absorption
A research team from Hiroshima University in Japan, led by Prof. Itaru Osaka, recently developed an innovative OPV cell based on a three-component blend of high-crystallinity semiconducting polymers, fullerene derivatives, and photo-sensitizer material.
The three materials have different wavelength absorption ranges and compose a single photoactive layer with enhanced light absorption and significantly increased power conversion efficiency.
The blend consists of a high-crystallinity semiconducting polymer based on thiophene and thiazolothiazole (PTzBT) as a donor and a fullerene derivative as an acceptor. That combination allows the fabrication of a relatively thick photoactive layer, up to 360 nm, without adverse effects on the layer's charge carrier mobility.
The researchers also discovered that the light absorption within the photoactive layer does not increase linearly with the layer thickness. Instead, the maximum absorption occurred at some specific thickness due to the optical interference effect originating in the light reflection at the OPV cell's metal substrate (electrode). Therefore, careful control of the photoactive layer thickness and the ability to form thicker blended layers proved indispensable for the OPV cell's enhanced performance.
Novel Molecular Semiconductor Broadens Absorbance Range for OPV cells
The crucial point was adding a small amount of molecular semiconductor, called ITIC, to the photoactive layer as a non-fullerene acceptor.
Prof. Osaka's team considers ITIC a new generation of small electron-accepting organic molecules that can outperform fullerenes as electron acceptors for OPV applications. The unique molecular semiconductor exhibits strong light absorption from the visible to near-infrared parts of the spectrum with a peak at 700 nm, increasing the total absorption of an OPV device.
Optical Interference Helps to Boost Solar Cell Performance
To the scientists' surprise, adding only 6%wt of ITIC to the photoactive layer of the OPV cell vastly improved the solar cell’s efficiency, resulting in PCE of over 10%, compared to PCE of 7.4%, 1.5 times increase, for the binary photoactive layer (without ITIC).
The result indicates that the optical interference effect can intensify the sensitizer absorption and play an essential role in harvesting additional red-wavelength photons.
Although the device's overall PCE remained relatively low (around 10%) at present, sensitized ternary blends of organic semiconductor materials show an exceptional promise to improve OPV cell performance further.
References and Further Reading
M. Saito, et al. (2020) Significantly Sensitized Ternary Blend Polymer Solar Cells with a Very Small Content of the Narrow-Band Gap Third Component That Utilizes Optical Interference. Macromolecules, 53, 10623–10635. Available at: https://dx.doi.org/10.1021/acs.macromol.0c01787
M. Saito, et al., (2015) Highly Efficient and Stable Solar Cells Based on Thiazolothiazole and Naphthobisthiadiazole Copolymers. Scientific Reports, 5, 14202. Available at: https://doi.org/10.1038/srep14202
Hiroshima University (2020) New blended solar cells yield high power conversion efficiencies. [Online] https://www.hiroshima-u.ac.jp Available at: https://www.hiroshima-u.ac.jp/en/laboratory-updates/news/61990 (Accessed on 19 January 2021).
R. R. Rwenyagila, (2017) A Review of Organic Photovoltaic Energy Source and Its Technological Designs. International Journal of Photoenergy, 2017, 1656512. Available at: https://doi.org/10.1155/2017/1656512
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