16.528 Alternative Energy Sources Organic Photovoltaic (OPV) Timothy McLeod Summer 2006
Feb 13, 2016
Potential Benefits Using OPV
• Lower Material Cost• Organic compounds (cpd) can be mass produced decreasing material cost
compared to metal/semi-metal metals (inorganics)
• Flexibility• Organic cpd can be designed to be flexible whereas inorganics are not
• Lightweight/Large Area • Organic cpd are more lightweight than inorganics.• Could have extremely large areas of solar cells with little structural support
needed (think of a “plastic balloon” as solar cell)
• Disadvantages• Low power conversion efficiencies (up to 5% vs. up to 24% inorganic)• Technology not there yet, more research needed
Molecular (Energy) Level – Inorganic vs Organic Photovoltaic (OPV)
(a) Frits and Modern cell - (inorganic elements) Based on metallic bond – “sea of free flowing electrons” (CB)
(b) Tang cell - (organic compounds)Based on chemical bonding – “no sea”, need bridge
Similarities and Differences between Inorganic and Organic Cells• Similarities
• Both work on specific wavelength absorption to excite the electron (called Ionization Energy, IE)
• Both work on Redox potential or chemical potential between electrodes (causes electron to move).
• Differences• Inorganic cells
» use only elements: silicon (matrix), gallium (p-junction) and arsenide (n-junction)
» Absorbed light creates electron-hole pair and room temperature thermal energy allows electron to “break free of pair to “flow away” in the conductive band
» There is a continuous conductive energy band throughout the cell (“sea of free flowing electrons”)
• Organic cells» Use compounds, therefore need to look at Molecular Orbital Theory (MO)» No “sea of electrons”» Absorbed light does not completely frees electrons; creates excitons» Excitons are complexes where the electron-hole pair can not be separated by
room temperature thermal energy. » To separate, need to exceed exciton binding energy, Eb. » This occurs at Donor/Acceptor Interface where the Donor has smaller IE
(higher LUMO) and Acceptor has higher Electron Affinity (lower LUMO).» This difference is enough energy to “break-up” excitron to free the electron
Organic Photovoltaic - Overall Power Conversion Efficiencies
• Photon absorption and exciton generation• Exciton diffusion to donor-acceptor interface • Exciton split/Charge carrier generation at donor-
acceptor interface• Carrier diffusion to respective electrodes
For organic solar cells, overall power conversion efficiencies are determined by 4 steps:
Major Types of Organic Photovoltaic Cells
• Molecular Organic Photovoltaic Cells (OPV)• Working on removing single D/A interface and have
“interface” throughout matrix (called bulk heterojunction)• This would be accomplished by building block layers
• Polymer OPV• Working on removing single D/A interface by using
polymer blends that can be mixed together (combined donor/acceptor matrix)
• Hybrid OPV• Dye-Sensitive Solar cells (DSSC) – add dye to OPV• Most promising is C60 (nanotube) added to OPV
Comparison of OPV and Inorganic Cells
Comparison of OPV Performance with Other Technologies
Devices Po Voc Isc ff(%) Efficiency(%) Single-crystal Si 100 0.696 42 83.6 24.4Amorphous Si 100 0.887 19.4 74.1 12.7*DSSC 100 0.721 20.5 70.4 10.4Molecular OPV 150 0.58 18.8 52.0 3.6Polymer OPV 80 0.825 5.25 61.1 3.3
*Dye-Sensitive Solar cell (hybrid OPV)Note: A hybrid OPV based on C60 in OPV was reported to have efficiencies of >5%
• Silicon based solar cells have greater efficiencies than OPV• OPV are similar in Po and Voc • OPV have lower Isc, and fill factor
Comments from Above Comparison Table