All entries for Jason Sussman

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Effect of Electron Spin on OPV Recombination

• Authors: J. M. Sussman, D. Ciudad, M. A. Baldo
• Sponsorship: U.S. Department of Energy

Organic photovoltaics (OPVs) are promising low-cost solar cells: they can be stacked in multi-junctions, and they are compatible with roll-to-roll processing. But as a solar cell’s installation costs are proportional to the area it covers, OPVs’ low efficiencies presently bar their widespread adoption. A significant source of loss in OPVs is the recombination of charges at the donor-acceptor interface: excited electrons combine with holes, returning the system to its ground state, rather than powering an external load. We therefore need to reduce the recombination rates in organic photovoltaics. We consider doing so by taking advantage of spin-disallowed transitions.

Excited states in OPVs come in two flavors of spin: singlets and triplets. Since the ground state is almost always a singlet, quantum mechanical rules prevent triplet excited states from relaxing, so triplets have longer lifetimes—i.e., lower recombination rates. In the absence of spin mixing processes, an OPV that produces electron-hole pairs in the triplet state should be more efficient than an OPV that produces singlets.

To prepare excited states in either the singlet or triplet state, we made a heterojunction solar cell with PTCBI (which produces singlets when excited) and pentacene (which produces triplets when excited ^[1] ). The spectral dependence of optical absorption in the two materials allows us to produce mostly triplets or singlets by exciting the device with 635-nm or 532-nm light, respectively. At room temperature the two show the same behavior; we are now examining the devices at much lower temperatures and under a magnetic field, where the mixing rate between the two states may be low enough to reveal the difference between the two.



Figure 1: Energy level schematic for the organic heterojunction photovoltaic device. When excited, pentacene produces triplets; PTCBI produces singlets. We expect to see a difference in device performance according to which of the two we excite. Energy levels from ^[2] .



Figure 2: Open-circuit voltage versus short-circuit current. Short-circuit current is a proxy for how many excitons are being created, while open-circuit voltage is a proxy for how many of those excitons recombine, so we would expect that the relationship between the two would be different for a 635-nm excitation (mainly triplets) versus a 532-nm excitation (mainly singlets). We see no difference at room temperature.




1. J. Lee, P. Jadhav, and M. A. Baldo, “High efficiency organic multilayer photodetectors based on singlet exciton fission,” Appl. Phys. Lett., vol. 95, p. 033301, July 2009. [↩]
2. J. Hwang, A. Wan, and A. Kahn, “Energetics of metal-organic interfaces: New experiments and assessment of the field,” Mater. Sci. Eng. R, vol. 64, pp. 1–31, Mar. 2009. [↩]

Reducing Recombination Losses in Planar Organic Photovoltaic Cells Using Multiple Step Charge Separation

• Authors: T. D. Heidel, D. Hochbaum, J. M. Sussman, V. Singh, M. E. Bahlke, I. Hiromi, J. Lee, M. A. Baldo
• Sponsorship: U.S. Department of Energy

Organic photovoltaics (OPVs) are promising low-cost solar cells: they can be stacked in multi-junctions, and they are compatible with roll-to-roll processing. But as a solar cell’s installation costs are proportional to the area it covers, OPVs’ low efficiencies presently bar their widespread adoption. A significant source of loss in OPVs is the recombination of charges at the donor-acceptor interface: excited electrons combine with holes, returning the system to its ground state, rather than powering an external load. We therefore need to reduce the recombination rates in organic photovoltaics. We do so through spatial separation of electrons from the holes they leave behind when excited.

We enhanced the efficiency of heterojunction solar cells by introducing a thin layer of material between the donor and acceptor layers. Normally an exciton (a bound electron and hole created by light exciting the PV) splits at the donor-acceptor interface, but the electron and hole are still attracted to each other and often recombine. By adding an interfacial layer that creates an energy gradient for charges crossing the interface, we spatially separate the electron from its hole, reducing recombination rates and thereby improving efficiency. The structure has proven successful ^[1] .



Figure 1: Energy level schematic for organic heterojunction photovoltaic device utilizing CuPC and C60. The ClAlPC layer provides a cascaded charge transfer from the CuPC donor layer to the C60 acceptor layer. From^[1].



Figure 2: Current density-voltage characteristics for CuPC/C60 photovoltaic devices with and without a ClAlPC interfacial layer. The control devices (black circle) have no interfacial layer. The insertion of a thin 1.5 nm interfacial layer (blue square) increases the short circuit current and the open circuit voltage. As the thickness of the interfacial layer is further increased to 3 nm (red diamond) and 4.5 nm (green triangle), the open circuit voltage increases but the short circuit current decreases. From^[1].






1. T. D. Heidel, D. Hochbaum, J. M. Sussman, V. Singh, M. E. Bahlke, I. Hiromi, J. Lee, and M. A. Baldo, “Reducing recombination losses in planar organic photovoltaic cells using multiple step charge separation,” J. Appl. Phys., vol. 109, 104502, May 2011 [↩]