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 (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 DevicesPoVocIscff(%) Efficiency(%) Single-crystal Si Amorphous Si *DSSC Molecular OPV Polymer OPV *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