Exciton Engineering

Excitons are electron-hole pairs that form when light interacts with certain types of matter.  Their energetics and transport are central to solar energy processes.  The separation of the exciton into a free electron and free hole is the source of usable electrical energy in a polymer heterojunction photovoltaic, for example, or a dye-sensitized solar cell. The diffusion of excitons to an engineered interface that can subsequently split them into electrons and holes remains a central challenge in many photovoltaics. The Strano laboratory applies the tools of transport and reaction engineering to these important species, and considers synthesized nanostructures as “exciton reactors”.

Since the exciton is a neutral particle it can be described using the same population balances, mass transfer and chemical kinetics approaches that chemical engineers know well and practice extensively. Conceptually an exciton is similar to a hydrogen atom (Fig. 1(a)): one electron orbiting one proton (i.e. a hole), bound together by Coulomb interactions. Fig. 1(b) shows a Frenkel exciton in comparison: one electron was promoted from the valence band to the conduction band, leaving behind a localized, positively-charged hole. Coulomb interactions are relatively strong in low-dielectric or low-dimensional materials. However the binding energy is smaller and the particle size is larger than that of a hydrogen atom due to dielectric screening. Figure 1(c) shows a first order decay reaction: radiative recombination of an exciton, giving rise to photoluminescence. Another first other decomposition reaction is the defect-mediated non-radiative decay giving rise to a phonon (Fig. 1(d)). An example of a second order reaction is exciton-exciton annihilation (EEA), where upon the collision of two excitons one is annihilated, whereas the other used the energy from the collision to be promoted to a higher energy level (Fig. 1(e)). Exciton-energy transfer (EET) can be thought of as a form of diffusion (Fig. 1(f)). More on this topic can be found here.

Figure 1. 'Reactions' involving excitons.



Project Area: SWNT-P3HT photovoltaics

There is significant interest in combining carbon nanotubes with semiconducting polymers for photovoltaic applications, due to potential advantages from smaller exciton transport distances and enhanced charge separation. Since exciton diffusion to an interface capable of dissociating it into electrons and holes, such as a p-n junction, is often the bottleneck in photovoltaic performance, one idea is to use an anisotropic material such as a nanotube or nanowire that is capable of dissociating the exciton at its surface and transporting the resultant free electron to the cathode. In reaction engineering terms, this is analogous to circumventing diffusion-controlled reaction by increasing catalytic surface area.   In the case of carbon nanotubes, however, bulk heterojunction (BHJ) devices have demonstrated extremely poor efficiencies for reasons that were not quite understood, since their fullerenic counterparts, such as C60 and PCBM, and their derivatives, are very efficient electron acceptors and used routinely in BHJ devices. 

Since little is understood about the nanotube/semiconducting polymer interface, Ham and Paulus et al. constructed a planar nano-heterojunction photovoltaic device comprised of well-isolated millimeter-long single-walled carbon nanotubes underneath a poly(3-hexylthiophene) (P3HT) layer (Fig. 2). In this simple configuration, the resulting junctions displayed photovoltaic efficiencies per nanotube ranging from 3% to 3.82%, which exceed those of polymer/nanotube BHJ by a factor of 50–100. The increase is attributed to the absence of aggregate formation in this planar device geometry.


Figure 2. SWNT-P3HT planar heterojunction


Interestingly, a maximum photocurrent and efficiency for a 60nm thick P3HT-layer was observed, in contradiction to an expected value equal to the diffusion length of excitons in P3HT (8.5nm). Paulus et al. combined an optical T-matrix model with a KMC simulation to investigate photocurrent generation. The result of the optical model (the generation rate of excitons as a function of position in the device) serves as the input for a First Reaction Model, a specific type of KMC. The model demonstrates how a bulk exciton sink can explain this shifted maximum in the P3HT/SWCNT case, whereas the maximum is mainly determined by PCBM interdiffusing in P3HT in the P3HT/PCBM case.

Based upon the results of this model it will be possible to more intelligently design polymer hybrid solar cells (both planar and bulk) and optimize them towards higher efficiencies. 


Project Area: Exciton Antennas

Developing new photonic materials for optical concentration and photon collection is crucial for applications such as higher efficiency photovoltaic cells and infrared photoemitters/photodetectors. One-dimensional materials such as single-walled carbon nanotubes are promising candidates due to their aligned axial transition dipoles, large absorption cross sections and high-quantum efficiencies. Photonic applications of SWCNTs, however, have always been hampered by their tendency to aggregate in bundles of inhomogeneous composition and our previous inability to isolate optically distinct species. Recent advances have enabled this separation on preparative scales. Han and Paulus et al. have dielectrophoretically assembled SWNTs of homogeneous composition into aligned filaments giving rise to strong photoluminescence (PL). By engineering these filaments in a unique way one can take advantage of the aforementioned described Förster resonance energy transfer (FRET), where excitons residing on SWNTs with a larger band gap are prone to transfer their energy to excitons located on SWNTs with a smaller band gap (Fig. 3(a-b)). These filaments consist of an annular shell of larger band gap (6,5) SWCNTs (Eg =1.21 eV) surrounding a core of a variety of smaller band gap SWNTs (Eg = 1,17 eV for (7,5) SWCNTs to 0,98 eV for (8,7) SWNTs). Despite broadband absorption in the ultraviolet-near-infrared wavelength regime experimental results indicated quasi-singular photoemission at the wavelength that corresponds to the E11 band gap of the (8,7) SWNT (the SWNT with the smallest band gap in the filament) (Fig. 3(c)). Since these lowest band gap SWNTs are located in the center of the filament, light has essentially been concentrated, both energetically and spatially. As better separation of different SWNT chiralities becomes possible, it will be possible to engineer fibers such that light is focused to a desired wavelength, which may vary depending on the application. The experimental data also reveal an unusually sharp, reversible decay in photoemission that occurs as such filaments are cycled from ambient temperature to only 357 K. We set up a deterministic model taking into account exciton generation, FRET from larger band gap to smaller band gap SWNTs, radiative and nonradiative decay of excitons in the SWNT filaments and fit it to their PL experimental data. The radiative rate constant krad and the FRET rate constant kFRET show little temperature dependence in the range considered. The defect-mediated nonradiative rate constant knrad follows classical Arrhenius-behavior and the exciton-exciton annihilation rate constant kEEA is modeled with collision theory, resulting in modified Arrhenius expression with a temperature-dependent prefactor. This prefactor indicates that as the temperature increases, two excitons residing on the same SWNT diffuse faster along the length of that SWNT, increasing chances of a collision. This strongly temperature-dependent second-order EEA process is responsible for the PL quenching at elevated temperatures. These results have conclusively demonstrated the potential of specifically designed collections of nanotubes to manipulate and concentrate excitons in unique ways.

Figure 3. Exciton antenna.


This work was featured in a documentary entitled ‘Here comes the sun’, which aired on Danish national television. An excerpt from the video can be found below.

Project Area: The All-Carbon NIR Photovoltaic

We study the incorporation of single-walled carbon nanotubes (SWNTs) into next generation solar cells as near infrared absorbers to efficiently harness energy in the 1000nm to 1400nm range. We are interested in both fundamental materials questions to obtain the maximum possible efficiency, as well as device design considerations. For the first time in Jul 2012, Jain and Howden et al developed a polymer-free carbon based photovoltaic which relies on exciton dissociation at a SWNT/C60 interface, demonstrating the ability to harness near-infrared energy from a pure pure SWNT phase (Fig. 4).

Figure 4. All-carbon photovoltaic


Project Area: Plant Nanobionics and Solar Energy

Naturally occurring photosynthetic systems use elaborate pathways of self-repair to limit the impact of photo-damage. We have demonstrated that complex consisting of photosynthetic reaction centers, phospholipids and carbon nanotubes mimics this process and exhibits photoelectrochemical activity. The components self-assemble into a configuration in which an array of lipid bilayers adsorb on the surface of the carbon nanotube, creating a platform for the attachment of light-harvesting proteins. The system can disassemble upon the addition of a surfactant and reassemble upon its removal over an indefinite number of cycles. Our current work focuses on developing self-repairing bio-photoelectrochemical systems with indefinite lifetimes by interfacing nanomaterials with natural, abundant, and economic photosynthetic entities.

photoelectrochemical complex

Figure 5. Photoelectrochemical complex for solar energy conversion

This work was also featured in 'Here comes the sun':