Figure 1: Triplet exciton transport in tetracene crystals with varying time delays. a, The prompt fluorescence profile. The image was taken with camera integration with a time delay of 0–0.5 μs upon laser excitation. b, c, d, e, Delayed fluorescence profiles with time delays of 0.5–1 μs, 1–2 μs, 2–3 μs, and 3 μs– upon laser excitations, respectively. f, Cross-section of images in a, b, c, d, e. The area of cross-section is denoted in image a.

Exciton transport is universal in every kind of organic optoelectronic devices – including organic light-emitting diodes (OLEDs) and organic solar cells ^[1] . In organic bilayer photovoltaic devices, exciton diffusion limits donor/acceptor thicknesses, restricting sufficient absorptions of photovoltaic materials. Exciton diffusion of singlet excitons (total spin of 0) is usually limited to tens of nanometers, significantly smaller than the absorption length at the visible spectrum (a ~ 1mm) ^[2] . However, triplet excitons (total spin of 1), having disallowed-transition to ground states, are capable of moving much longer distances, up to several mm ^[3] . The long-range triplet exciton transport can allow us to build more efficient solar cells.

Despite their long intrinsic lifetime, triplet diffusion is disorder-limited in amorphous or polycrystalline organic semiconductors ^[2] . Organic single crystals, however, provide defect-free environment where triplet excitons can diffuse over long distances without being quenched by defects ^[3] .

Long-range triplet exciton transport has been reported before in organic acene crystals. However, in previous studies, triplet exciton diffusion was measured using indirect methods, such as probing polarization- and wavelength-dependent photoconductivity ^[4] . In this work, we perform direct imaging of triplet excitons by monitoring delayed fluorescence in two archetypical organic single crystals: tetracene and rubrene crystals. The comparison between tetracene and rubrene crystals is interesting since they exhibit similar molecular structures but differ in crystal structures. Our study will contribute to a better understanding of long-range exciton transport and benefit the power conversion capability of organic solar cells by overcoming exciton diffusion bottlenecks.

1. P. Peumans, A. Yakimov, and S. R. Forrest, “Small molecular weight organic thin-film photodetectors and solar cells,” Journal of Applied Physics, vol. 93, pp. 3693, 2003.
2. R. R. Lunt, N. C. Giebink, A. A. Belak, J. B. Benziger, and S. R. Forrest, “Exciton diffusion lengths of organic semiconductor thin films measured by spectrally resolved photoluminescence quenching,” Journal of Applied Physics, vol. 105, pp. 053711, 2009.
3. M. Pope and C. E. Swenberg, Electronic Processes in Organic Crystals and Polymers. Oxford University Press, New York, 1999.
4. H. Najafov, B. Lee, Q. Zhou, L. C. Feldman, and V. Podzorov, “Observation of long-range exciton diffusion in highly ordered organic semiconductors,” Nature Materials, vol. 9, pp. 938, 2010.

Photolithography’s accuracy and scalability have made it the method for sub-micron-scale definition of single-crystal semiconductor devices for over half a century. Unfortunately, organic semiconductor devices are chemically incompatible with the types of resists, solvents, and etchants traditionally used. This work investigates the use of an uncommonly used chemically inert resist method ^[1] ((A. Han, D. Vlassarev, J. Wang, J. A. Golovchenko, and D. Branton, “Ice lithography for nanodevices,” Nano Letters, vol. 10, no. 12, pp. 5056-5059, Dec. 2010.))((D. Branton, J. A. Golovchenko, G. M. King, W. J. MoberlyChan, and G. M. Schürmann, “Lift-off patterning processing employing energetically-stimulated local removal of solid-condensed-gas layers” U.S. Patent 752443 B1, April 28, 2009.))((G. M. King, G. Schürmann, D. Branton, and J. A. Golovchenko, “Nanometer patterning with ice,” Nano Letters, vol. 5, no. 6, pp. 1157-1160, June 2005.))((J. Cuomo, C. Guarnieri, K. Saenger, and D. Yee, “Selective deposition with ‘dry’ vaporizable lift-off mask,” IBM Technical Disclosure Bulletin, vol. 35, no. 1, June 1992.)) that relies on physical phase changes for lift-off patterning of thin films of organic semiconductors and metals.

The resist gas is flowed over a cryogenically cooled substrate, where it freezes solid. This layer can be patterned by thermal excitation in a number of ways to define the areas where the desired thin film is to remain.  After the desired thin film or films are deposited, the substrate is brought up above the resist material’s sublimation point, leaving behind only the intended pattern. All the unwanted regions are lifted-off by the subliming resist.

Creating and defining the shadow mask on the surface of the substrate in this manner allow for patterning it with a stamp or roller with micron-scale features without changing the process conditions.  In this work, carbon dioxide is used as the sublimable mask material, and prototype stamps have been fabricated using SU-8 photoresist. A mask and the subsequent organic thin film are shown in Figure 2. This process may provide an alternative to shadow masks and provide a manufacturing solution for large area organic electronics.

1. W. Johnson, R. Laibowitz, and C. Tsuei, “Condensed gas, in situ lithography,” IBM Technical Disclosure Bulletin, vol. 20, no. 9, Feb. 1978.

Luminescent Solar Concentrators (LSCs) aim to reduce the cost of solar electricity by using an inexpensive collector to concentrate solar radiation without mechanical tracking (Figure 1a) ^[1] . Ideally, the dyes re-emit the absorbed light into waveguide modes that are coupled to solar cells attached to the edges of the collector (black arrows). However, some photons are always lost, re-emitted through the face of the LSC, and coupled out of the waveguide (grey arrows). The trapping efficiency, η_trap, is defined as the fraction of photons emitted from the edge versus photons emitted from the face and edge combined. Assuming the dyes emit their photons isotropically, η_trap is given by η_trap = . If the refractive index of the waveguide n_s is ~ 1.7, then 20% of the re-emitted photons are lost.

Such surface losses become compounded if photons trapped in the waveguide and travelling towards the edges are re-absorbed by other dye molecules in the waveguide; see Figure 1b. When such re-absorbed photons are re-emitted, the LSC will suffer more confinement losses, and the cycle repeats. As the number of re-absorption events increases, the overall efficiency of LSCs drops exponentially.

In this work, we aim to quantify the contribution of self-absorption to the surface losses of an LSC. We vary the amount of self-absorption by tuning the dye concentration of ALq₃ and DCJTB in the waveguide (n_s = 1.7). Figure 2 shows preliminary results for η_trap of an LSC with high and low self-absorption. The LSC with low self-absorption has η_trap of 78%, which approaches the theoretical η_trap of 81%. The slightly lower measured value of η_trap is likely to be explained by scattering losses from the LSC surface.

1. J. S. Batchelder, A.H. Zewail, and T. Cole, “Luminescent solar concentrators. 2: Experimental and theoretical analysis of their possible efficiencies,”Applied Optics, vol. 20, pp. 3733-3754, Nov. 1981.

Figure 1: Illustration of ray tracing under (a) normal (spontaneous) and (b) lasing (stimulated) conditions within an LSC. (c) Typical laser transfer characteristic, showing the dramatic change in efficiency above the threshold.

Many on-chip optical applications, including spectroscopy ^[1] ; sensing ^{[2] [3]} ; nonlinear optics ^{[4] [5] [6]} ; and optical communications require high-finesse ^{[7] [8]} , high-quality factor (high-Q) micro-lasers. Such lasers must be exceptionally transparent at their emission wavelength. But if high-Q micro-lasers exhibit correspondingly weak absorption at the pump wavelengths, they are challenging to excite. Here we demonstrate micro-ring lasers that exhibit Q > 5.2 × 10⁶ and a finesse of > 1.8 × 10⁴ with a direct-illumination, non-resonant pump.  The micro-rings are coated with a combination of three organic dyes. This cascaded combination of near and ultimately far field energy transfer reduces material-losses by a factor of more than 10⁴, transforming incoherent light to coherent light with high quantum-efficiency. The operating principle established here is general and can enable fully integrated on-chip, high-finesse micro-lasers without the complications of coupled pump and emitter resonators ^[9] .

We are now working on lasing luminance solar concentrators (LSC) or solar powered lasers based on the cascaded energy concept. Above threshold, all the fundamental properties of an LSC improve. Specifically, (i) the brightness of the lasing LSC can be orders of magnitude larger than conventional solar concentrators; (ii) stimulated emission enhances the photoluminescent efficiency; (iii) it increases the trapping efficiency of the LSC; (iv) and stimulated emission decreases self-absorption. A solar powered laser also, in essence, converts a portion of the incoherent solar spectrum into a coherent source. This conversion enables the solar powered laser light to be frequency converted in nonlinear crystals, allowing harvesting of more of the solar spectrum via efficient upconversion and downconversion for high efficiency photovoltaics. Figure 1 shows ray tracing of a below- and an above-threshold LSC.

1. C. Y. Chao, W. Fung, and L. J. Guo, “Polymer microring resonators for biochemical sensing applications,” IEEE J. of Sel. Top. Quantum Electron, vol. 12, no. 1, pp. 134-142, Jan. 2006.

2. [1]    F. Vollmer, D. Braun, A. Libchaber, M. Khoshsima, I. Teraoka, and S. Arnold, “Protein detection by optical shift of a resonant microcavity,” Applied Physics Letters, vol. 80, no. 21,  pp. 4057-4059, April 2002.

3. A. Serpenguzel, S. Arnold, and G. Griffel, “Excitation of resonances of microspheres on an optical fiber,” Opt. Lett., vol. 20, no. 7, pp. 654-656, Apr. 1995
4. R. K. Chang, and A. J. Campillo, Optical Processes in Microcavities. Singapore: World Scientific, 1996.
5. F. Treussart, V. S. Ilchenko, J. F. Roch, J. Hare, V. Lefevre-Seguin, J. M. Raimond, and S. Haroche, “Evidence of intrinsic Kerr bistability of high-Q microsphere resonators in superfluid helium,”  Eur. Phys. J. D., vol. 1, pp. 235-238, Jan. 1998.
6. S. M. Spillane, T. J. Kippenberg, and K. J. Vahala, “Ultra-low threshold Raman laser using a spherical dielectric microcavity,”  Nature, vol. 415, pp. 621-623, Feb. 2002.
7. B. E. Little, S. T. Chu, P. P. Absil, J. V. Hryniewicz, F. G. Johnson, F. Seiferth, D. Gill, V. Van, O. King, and M. Trakalo, “Very high-order microring resonator filters for WDM applications,”  IEEE Photon. Technol. Lett., vol. 16, no. 10, pp. 2263-2265, Oct. 2004.
8. M. Ferrera, L. Razzari, D. Duchesne, R. Morandotti, Z. Yang, M. Liscidini, J. E. Sipe, S. Chu, B. Little, and D. J. Moss, “Low power continuous-wave nonlinear optics in doped silica glass integrated waveguide structures,” Nature Photonics, vol. 2, pp. 737-740, Nov. 2008.
9. Rotschild, C., Tomes, M., Mendoza, H., Andrew, T. L., Swager, T. M., Carmon, T. and Baldo, M. A., “Cascaded Energy Transfer for Efficient Broad-Band Pumping of High-Quality, Micro-Lasers,” Advanced Materials, vol. 23, 2011

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.

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.

Photovoltaic solar concentrators aim to increase the electrical power obtained from solar cells.  Conventional solar concentrators track the sun to generate high optical intensities, often by using large mobile mirrors that are expensive to deploy and maintain.  High optical concentrations without excess heating in a stationary system can be achieved with a luminescent solar concentrator (LSC) ^[1] . The LSC consists of a dye dispersed in a transparent waveguide.  Incident light is absorbed by the dye and then reemitted into a waveguide mode. The energy difference between absorption and emission prevents reabsorption of light by the dye, isolating the photon population in the waveguide. The performance of LSCs has been limited by two factors: self-absorption losses and a scarcity of dyes that emit efficiently in the infrared region. We have previously made significant progress on the problem of self-absorption losses ^[2] . Now we address operation in the infrared region.

Neodymium (Nd³⁺) and ytterbium (Yb³⁺) are nearly the optimal infrared LSC materials: inexpensive, abundant, efficient, and spectrally well-matched to high-performance silicon solar cells. These rare earth ions are natural three or four-level systems, therefore reasonably transparent to their own radiation and capable of generating high optical concentrations. Neodymium’s and ytterbium’s disadvantage is their relatively poor absorption overlap with the visible spectrum, meaning that the rare earth ions will require sensitization in the visible spectrum. Transition metals (TMs) can efficiently transfer energy to Nd and Yb ions, a process depicted in Figure 1. With a selection of TMs, broad sensitization of the both the visible and near infrared regions of the solar spectrum is possible, as shown in Figure 2. Finally, these ions can be combined with a high-throughput and chemically robust glass-making process for low cost and stable LSCs. Sensitized neodymium and ytterbium should enable LSCs matched to silicon.

1. W. H. Weber and J. Lambe, “Luminescent greenhouse collector for solar radiation,” Applied Optics, vol. 15, no. 10, pp. 2299-2300, Oct. 1976.
2. M. J. Currie, J. K. Mapel, T. D. Heidel, S. Goffri, and M. A. Baldo, “High-efficiency organic solar concentrators for photovoltaics,” Science, vol. 321, no. 5886, pp. 226 -228, July 2008.

Collaborators

• T. Van Voorhis, MIT Dept of Chemistry
• T. Swager, MIT Dept of Chemistry

Graduate Students

• J. Currivan, Research Assistant, Harvard Physics
• T. Heidel, Research Assistant, EECS
• P. Reusswig, Research Assistant, EECS
• P. Jadhav, Research Assistant, EECS
• J. Lee, Research Assistant, EECS
• C.L. Mulder, Research Assistant, EECS
• N. Thompson, Research Assistant, DMSE
• M. Bahlke, Research Assistant, EECS
• J. Sussman, Research Assistant, DMSE

Postdoctoral Associate

• C. Rotschild

Support Staff

• C.M. Bourgeois

Publications

Lee J., K. Vandewal, S. Yost, M. Bahlke, L. Goris, M.A. Baldo, J.V. Manca, and T. Van Voorhis, “Charge Transfer State versus Hot Exciton Dissociation in Polymer-Fullerene Blended Solar Cells,” Journal of the American Chemical Society, 132 (34), 11878-11880 (2010).

Orf, N.D., O. Shapira, F. Sorin, S. Danto, M.A. Baldo, J.D. Joannopoulos, and Y. Fink, “Fiber draw synthesis,” Proceedings of the National Academy of Sciences, 108, 4743-4747 (2011).

Jadhav, P.J., A. Mohanty, J. Sussman, and M.A. Baldo, “Singlet-fission-based alternative to multijunction organic solar cells,” Nano Letters 11, 1495–149 (2011).

Heidel, T.D., D. Hochbaum, M. Bahlke, I. Hiromi, J. Lee, and M.A. Baldo, “Reducing recombination losses in planar organic photovoltaic cells using multiple step charge separation,” in press, Journal of Applied Physics (2011).

Rotschild, C., M. Tomes, H. Mendoza, T. L. Andrew, T. M. Swager, T. Carmon, and M.A. Baldo, “Efficient broad-band pumping of high-finesse, high quality-factor lasers,” in press, Advanced Materials (2011).

We are interested in using ferromagnetic materials to engineer more energy-efficient transistors and logic gates.  Transistors today are limited by the energy dissipated per switching operation, and this heat dissipation can potentially be greatly reduced by using a collective effect, such as the collective switching of magnetic moments.  In our research we are designing and fabricating a promising instantiation of magnetic logic, using the switching of a magnetic domain wall in a soft ferromagnet.  The state of the logic gate is read out using a magnetic tunnel junction.  This gate is nonvolatile and can have a fanout greater than one; additionally, each device is a universal NAND gate.  Figure 1 shows a cartoon of the device, for a soft ferromagnet with magnetic moments parallel to the plane of the wire.

The logic gate uses current-induced domain wall motion in a soft ferromagnetic wire such as NiFe or CoFeB to write the state of the device.  We ensure that there is only one 180° transverse domain wall by depositing an antiferromagnet such as IrMn on each end of the wire, creating an exchange bias that fixes the net magnetic moment of the ends.  The output current of the device will depend on the tunnel magnetoresistance (TMR) of the tunnel junction, using an insulating tunnel barrier such as MgO.  TMR values from 300% to 600% have been observed at room temperature ^[1] , allowing a possible fanout up to 6.

The device is fabricated using electron-beam lithography and UHV sputter deposition.  We are using micromagnetic simulations to understand the scaling of the device with size, and we are implementing the device in circuit designs.

1. S. Ikeda, J. Hayakawa, Y. Ashizawa, Y. M. Lee, K. Miura, H. Hasegawa, M. Tsunoda, F. Matsukura, and H. Ohno. “Tunnel magnetoresistance of 604% at 300 K by suppression of Ta diffusion in CoFeB/ MgO/ CoFeB pseudo-spin-valves annealed at high temperature.” Applied Physics Letters, vol. 93, p. 082508, 2008.

Singlet exciton fission may find application in more efficient solar cells. Fission can reduce thermalization losses because by splitting the exciton, a high energy photon can produce two charge carrier pairs instead of one. The two device implementations to date that exploit singlet exciton fission, a pentacene photodetector and a tetracene solar cell, produce more current by multiplying the number of excitons in the visible part of the spectrum ^{[1] [2]} . In this work we show that diphenyltetracene (DPT) also exhibits singlet exciton fission; in Figure 1 we present a DPT-C₆₀ device in which singlet exciton fission contributes negatively to the current, which is opposite behavior to the previous two implementations.

DPT differs from previous implementations of singlet exciton fission in that its triplet E_t = 1.2eV ^[3] is less than the energy of the DPT-C₆₀ charge transfer state (CT) E_CT = 1.25eV. Hence the triplets cannot break up to form charge carriers. This effect is seen in the positive magnetic field effect shown in Figure 2. The application of a magnetic field results in reduced singlet fission and hence an increase in the number of singlet excitons and thereby increased current. In contrast, a similar measurement with a lower CT energy yields the usual negative magnetic field dependence. For example, below we demonstrate a negative magnetic field effect in DPT-F₁₆CuPC, where F₁₆CuPC is fluorinated copper phthalocyanine.

The amorphous nature of the DPT film in the device results in an isotropic magnetic field effect. Consequently, the large magnetic field effect shown by the device, 5% at .45T, may be used in an isotropic magnetic field detector.

1. J. Lee, P. Jadhav, and M. A. Baldo, “High efficiency organic multilayer photodetectors based on singlet exciton fission,” Applied Physics Letters, vol. 95, pp. 033301-033303, July 2009
2. P. J. Jadhav, A. Mohanty, J. Sussman, and M. A. Baldo, “Singlet exciton fission in nanostructured organic solar cells,” Nano Letters, DOI: 10.1021/nl104202j.
3. C. Burgdorff, T. Kircher, and H. G. Löhmannsröben, “Photophysical properties of tetracene derivatives in solution,” Spectrochimica Acta Part A: Molecular Spectroscopy vol. 44, pp. 1137-1141, Apr. 1988.

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] .

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