16.528 Alternative Energy Sources

Organic Photovoltaic (OPV)

Timothy McLeodSummer 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

Devices Po Voc Isc ff(%) 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