16.528 Alternative Energy Sources

1. 16.528 Alternative Energy Sources Organic Photovoltaic (OPV) Timothy McLeod Summer 2006

2. 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

3. Workings – Inorganic vs. Organic Photovoltaic Cell (OPV) (c) Modern p & n junction cell

4. Molecular (Energy) Level – Inorganic vs Organic Photovoltaic (OPV) • Frits and Modern cell - (inorganic elements) Based on metallic bond – “sea of free flowing electrons” (CB) • Tang cell - (organic compounds) Based on chemical bonding – “no sea”, need bridge

5. Similarities and Differencesbetween 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

6. 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:

7. 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

8. Comparison of OPV and Inorganic Cells Comparison of OPV Performance with Other Technologies DevicesPoVocIscff(%)Efficiency(%) Single-crystal Si 100 0.696 42 83.6 24.4 Amorphous Si 100 0.887 19.4 74.1 12.7 *DSSC 100 0.721 20.5 70.4 10.4 Molecular OPV 150 0.58 18.8 52.0 3.6 Polymer 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