Electron-Blocking Layer / Hole-Transport Layer for Organic Photovoltaics

INVENTION: An electron-blocking / hole transport layer has been developed for organic photovoltaics. The overall power conversion efficiency of MDMO-PPV:PCBM and P3HT:PCBM active layer devices exhibited significantly greater open circuit voltage with electron blocking layer incorporation versus controls.

ADVANTAGES: Increased organic photovoltaics efficiency promises performance suitable for special market commercial applications of these devices.

SUMMARY: Organic photovoltaics (OPV) offer several advantages over the more traditional inorganic photovoltaic cells, including fabrication with flexible substrates, lightweight, and production by inexpensive techniques such as spin-coating and printing. However, the power conversion efficiency (hp ≈ 5%.) of current bulk heterojunction (BHJ) solar cells is too low for commercial use. It is estimated that organic solar cell efficiencies of about 10%, would make OPVs commercially viable for certain markets. This invention focuses on improving one key parameter in solar cell performance namely, the open-circuit voltage (Voc), believed to originate from the difference in energy between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor.

Typical BHJ solar cells fabricated using either MDMO-PPV or P3HT as a donor polymer and PCBM, a fullerene derivative, acceptor material have hp values around 2.5% and 4.4%, respectively. One problem with BHJ solar cells is that both the donor polymer and the acceptor molecule are touching both electrodes. This means that electrons in the PCBM may be formed at an interface very close to the ITO anode, which typically collects the holes. Any electrons transferred to the ITO would essentially recombine with holes and reduce the working voltage of the device. Thus the Voc, would also be reduced. A similar effect should take place at the cathode if holes are produced in the HOMO of the polymer very near the aluminum.

This invention provides an electron blocking layer (EBL) between the active layer blend and the ITO anode that serves to prevent electrons in the PCBM from traveling to the ITO, thus forcing the electrons to flow in the correct direction for collection at the cathode. Comparison of a control MDMO-PPV:PCBM solar cell (Jsc = 6.1 mA/cm2, Voc = 0.67 V, and hp = 1.73%) and a similar device modified with the EBL ( Jsc = 5.9 mA/cm2, Voc = 1.08 V, and hp = 2.21%) exhibits a 28% increase in hp with the EBL modification. The 1.08 Voc is the highest recorded for this OPV materials system (Figure). EBL incorporation in P3HT:PCBM active layer solar cells showed similar Voc increase.

STATUS: A patent application has been filed and Northwestern University seeks to develop the invention.

Inventor(s): Tobin Marks, Alexander Hains, Michael Irwin, He Yan

Type of Offer: Licensing

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