Organic solar cells or organic photovoltaics (OPV) based on conjugated polymers (e.g., poly-3-hexylthiophene, P3HT) and fullerene derivatives (e.g., [6,6]-phenyl C61 butyric acid methyl ester, PCBM) have attracted attention over the past decades because they may provide a cost-effective route to wide use of solar energy for electrical power generation1-3.
These organic semiconductors have the advantage of being chemically flexible for material modifications, as well as mechanically flexible for the prospective of low-cost, large scale processing such as screen-printing or spraying on flexible substrates. The world's next generation of microelectronics may be dominated by "plastic electronics" and organic solar cells are expected to play an important role in these future technologies. Figure 1 shows a conceptual drawing of flexible OPV array (upper panel) and the actual device on PET substrate (lower panel)4.
Figure 1. Conceptual drawing of flexible OPV array (upper panel) and the actual device on PET substrate (lower panel)
The photovoltaic process in organic solar cell devices consists of four successive processes: light absorption, exciton dissociation, charge transport, and charge collection: absorption of a photon creates an exciton (bounded electron-hole pair). The exciton diffuses to the interface of two different components, where exciton dissociation or charge separation occurs, followed by positive charges (holes) moving to the anodes and negative charges (electrons) to the cathode. Figure 2 demonstrates how electricity is generated in OPV device.
Figure 2. How electricity is generated in organic photovoltaics (OPV) device.
Several parameters determine the performance of a solar cell, namely, the open-circuit voltage (Voc), short-circuit current (Isc), and the so-called fill factor (FF). The overall power conversion efficiency η is defined as η = (FF)•(IscVoc)/Pm. Over the past decade, OPV efficiency has been significantly improved to over 5% in single cell3,5 and 1% in submodules5, owing to a better understanding of device physics, optimization of device engineering and developments of new materials2,3,6.
However, most of such OPV devices are developed in laboratories with fabrication process involving spin-coating for photoactive layer and the use of high vacuum to deposit the cathode. This conventional technique limits the real potential of OPV in commercial market: low-cost manufacturing, solution processable and high production rate7.
Recently, world-wide research efforts have been on developing transparent contact based on modified poly (3,4ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS) solution8. For large scale production, screen printing9, ink-jet printing10 and spraying11 have been demonstrated mostly in OPV single cells.
Organic Semitransparent Photovoltaic Energy Converter or abbreviated as OSPEC is a novelty since it combines three features of OPV: solution processable, vacuum free and large-scale compatible. OSPEC utilizes a special spray technique developed by researchers in Nanostructure Optoelectronics Lab in USF, partnering with New Energy Technologies, Inc and Florida High Tech Corridor, to fabricate OPV modules with transparent contacts12. Figure 3a shows a working OSPEC array, and 3b is its current-voltage characteristics. The overall power conversion efficiency is 0.42% under 1 sun irradiance.
Figure 3. (a) Working OSPEC array, (b) its current-voltage characteristics.
OSPEC is a 'green' solution for today's energy needs. It has various applications particularly in building integrated photovoltaic products. Most conventional solar cells are made up of silicon wafers, a brittle opaque substance that limits how they can be used. For instance, on window technology where transparency is a key issue, OSPEC can be made semitransparent; further more, OSPEC performs better than silicon solar cells under ambient light13, which offers new opportunities for indoor applications.
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7. F.C. Krebs "Fabrication and processing of polymer solar cells: A review of printing and coating techniques" Solar Energy Materials & Solar Cells 93 (2009) 394-412
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9. Shaheen et.al. APL 79, 2996, 2001
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11. Lim et.al. APL. 93, 193301, 2008
12. US provisional patent # 12/630,398.
13. X.Jiang et.al., unpublished work; Proc. Of IEEE, Vol 93, No.8, 1429(2005); A.Jäger-Waldau,'PV status report 2003' Institute for environment and sustainability, Eur. Commission, 2003
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