Transcript 16.528 Alternative Energy Sources

16.528 Alternative Energy Sources
Organic Photovoltaic (OPV)
Timothy McLeod
Summer 2006
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
Workings – Inorganic vs. Organic Photovoltaic Cell (OPV)
(c) Modern p & n junction cell
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 (njunction)
» 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
For organic solar cells, overall power conversion efficiencies
are determined by 4 steps:
• Photon absorption and exciton generation
• Exciton diffusion to donor-acceptor interface
• Exciton split/Charge carrier generation at donoracceptor interface
• Carrier diffusion to respective electrodes
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
Single-crystal Si
Amorphous Si
*DSSC
Molecular OPV
Polymer OPV
Po
100
100
100
150
80
Voc
0.696
0.887
0.721
0.58
0.825
Isc
42
19.4
20.5
18.8
5.25
ff(%)
83.6
74.1
70.4
52.0
61.1
Efficiency(%)
24.4
12.7
10.4
3.6
3.3
*Dye-Sensitive Solar cell (hybrid OPV)
Note: A hybrid OPV based on C60 in OPV was reported to have efficiencies of >5%
Comments from Above Comparison Table
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•
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Silicon based solar cells have greater efficiencies than OPV
OPV are similar in Po and Voc
OPV have lower Isc, and fill factor