This presentation will describe our research in designing molecules for solar energy conversion. One series of molecules will be ones that can self-assemble to create a molecularly defined pn-junction. The well-defined nature of this interface has important consequences on the properties in solar cells. A second series of molecules are a new class of chromophores for singlet fission. The design and characterization will be discussed along with plans to implement them into solar cells.

A recent major development in organic photovoltaic cells (OPCs) and organic thin-film transistors (OTFTs) involves the use of donor-acceptor (DA)-type polymers. The unique nature of DA-type polymers is manifested in the emergence of two distinct photoabsorption bands which correspond to charge-transfer (CT) and main-chain (MC) excitations, respectively. It is believed that the extended range of active photon energies arising from CT excitation might be crucial for high efficiencies in DA-type polymer OPCs. It has recently been pointed out that the efficiency of photocarrier generation is lower for CT excitation than for MC excitation.¹⁾ However, experimental evidences have not been established yet to discriminate between the individual photocarrier generation mechanisms associated with the CT and the MC excitations. In this study, we investigated photocarrier generation and recombination in response to the CT and the MC excitations in terms of the intrinsic photocurrent yields and photoluminescence lifetimes for a series of DA-type polymers, poly{[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl} (PCDTBT), poly{[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)oxycarbonyl]thieno[3,4-b]thiophenediyl]} (PTB7), and poly{2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene-2,6-diyl]} (PCPDTBT), as well as for the homopolymer P3HT as a reference.
Photoabsorption, photocurrent action spectra, and activation energy of photocurrent were measured for the pristine polymer films and the fullerene (PC₆₁BM)-blended films. The two unique photoabsorption bands corresponding to the CT and the MC excitations, respectively, can be clearly seen in the spectra of DA polymers. In the action spectra of the pristine films, the high-energy MC excitation exhibits a higher photocurrent yield than the low-energy CT excitation. Furthermore, activation energy required to generate a photocurrent through the low-energy CT excitation is larger than that through the high-energy MC excitation. These results provide evidence that the intrinsic photocarrier-generation mechanism for CT excitation differs from that for MC excitation. In addition, the activation energy for MC excitation in the pristine film is almost identical to that in the blend film, indicating that the photocarriers are generated directly by the MC excitation, even in the absence of a heterojunction. Based on the results, we discuss the role of CT excitation on efficient photocarrier generation in DA-type polymer OPCs.^2,3)
References: 1) N. Banerji et al., J. Phys. Chem. C 116 (2012) 11456. 2) J. Tsutsumi, et. al., Phys. Rev. Lett. 105 (2010) 226601. 3) J. Tsutsumi, et. al., J. Phys. Chem. C 116 (2012) 23957.

Poly (3-hexylthiophene) is one of the most studied polymers for OPV applications, owing to its relatively high hole mobility (~ 10-4 -10-3 cm2 V-1 s-1) and its tendency to self-assemble into crystalline aggregates. In particular, crystalline nanowires (or nanofibers) have attracted a great deal of interest for OPV applications because of the potential for efficient exciton or charge migration along the nanofiber axis, processes which strongly depend on structural order and molecular packing within the aggregate. Although chain folding has been suspected to be an important factor in determining electronic and structural properties of P3HT aggregates, the photophysical consequences of such conformational changes have not been specifically identified.
Here, we examine the photoluminescence spectra and polarization dynamics of isolated crystalline highly regio-regular P3HT nanofibers as a function of polymer molecular weight (Mn 10 - 65 kDa, with nominal PDI of 1.2). Evidence for polymer chain folding is seen in the TEM images of different nanofiber families, where the nanofiber widths appear to be constant for Mn > 20 kDa. The effect of increasing molecular weight is manifested in distinctly different dominant coupling types from H-type in the low Mn regime, to exclusively J-type in the high- Mn regime. We show further, with picosecond resolved polarization dynamics, evidence for a transition from two-dimensional coupling (both intra- and inter-chain) to almost exclusively intra-chain coupling as Mn is increased. These effects can be explained within a structural picture in which high- Mn P3HT chains fold on themselves with loss of thiophene ring registration in the subsequent lamellar packing.

Solar cells based on polymer semiconductors have composite active layers formed by phase separation of blended donor and acceptor materials. Achieving optimal device performance requires a delicate balance of trapping the blended material in a non-equilibrium configuration. We have been focused on understanding the relationship between the molecular structure of films of polymer semiconductors and the efficiency with which photogenerated charge is collected from the material. I will discuss experiments controlling the polymer semiconductor molecular orientation by confining it within nanometer-scale volumes using both imprint lithography as well as templating by means of lithography and self assembly. Synchrotron-based grazing incidence x-ray diffraction measurements allow us to correlate the material&’s molecular structure and its electronic and photovoltaic properties.

Hybrid solar cells based on a heterojunction between zinc oxide (ZnO) and a donor polymer potentially show high open-circuit voltage, have the opportunity to control the donor/acceptor interface through engineering the nanostructured zinc oxide film, and allow large-area processing at low cost. Yet, polymer:ZnO solar cells have not reached efficiencies comparable to organic bulk heterojunctions.
In this study, we show how charge generation and -transport in zinc oxide:P3HT (poly(3-hexylthiophene) devices is affected by the properties of the ZnO:P3HT interface, which depends strongly on the production and deposition method of ZnO.
We use a technique called time-delayed collection field (TDCF), which allows studying charge generation via fast charge extraction from the device after applying a short laser pulse. Recombination is studied by varying the delay time between laser pulse and voltage pulse for charge extraction.
Zinc oxide from various methods, such as sol-gel processing, sputtering and electrochemical growth of nanostructures is used to make solar cells and to compare charge generation and -recombination.
Furthermore, previous reports have shown that a monolayer of PCBA ([6,6]-Phenyl C61 butyric acid) can significantly enhance photocurrent generation at the interface between ZnO and P3HT. From spectroscopic measurements, both reduced recombination across the hybrid interface and enhanced charge carrier generation have been proposed to be responsible for the current enhancement. We quantify the effect of PCBA for differently produced zinc oxide layers and provide evidence for improved charge generation upon the introduction of PCBA. Our findings highlight the importance of understanding the zinc oxide:polymer heterojunction for optimizing charge generation and to overcome the limitations of polymer:metal oxide solar cells.

As a consequence of recent investigations that have used simple methods of lithography and material structuring, challenging the established Si nano- and micro-machining technologies, it has become apparent that hybrid organic-inorganic nanoarchitectonics can provide a unified strategy for the next generation of power sources [1]. Noticeably, electron-beam lithography merged with an ingenious resist composition and exposure modulation profile has enabled the direct writing of polymer three-dimensional (3D) lattices with periodicities and feature-sizes suitable for visible-spectrum photonic crystals. These superstructures can be easily co-integrated via transfer printing techniques with Si substrata accommodating classical photonic lattices obtained by novel deep-reactive ion etching protocols. Amongst the photonic crystals architectures, the 3D design remains the most challenging in fabrication and implementation. This originates from the stringent constitutional, quality and functional requirements. In this talk, a large area 3D structuring strategy for advanced photonic materials by adding the third dimension to two-dimensional (2D) colloidal etch masks is presented. Surface structuring by nanosphere lithography is merged with a novel silicon etching method to fabricate ordered 3D architectures. The SPRIE method, Sequential Passivation Reactive Ion Etching, is a one-step processing protocol relying on sequential passivation and reactive ion etching reactions using C₄F₈ and SF₆ plasma chemistries. Careful adjustments of both mask design and lateral etch extent balance allow the implementation of even more complex functionalities including photonic crystal slabs and precise defect engineering. The 3D photonic crystal lattices exhibit optical stop-bands in the infrared spectral region proving the potential of SPRIE for fast, simple and large-scale fabrication of photonic structures. Structural characterization and numerical modeling corroborate the optical response of the obtained photonic structures. The SPRIE protocol is presently investigated for the realization of tapered structures with axial diameter modulation designed to enhance the light absorption in Si solar cells or in Schottky junction solar cells with conducting polymers. Several classes of novel materials can be used in conjunction with these 3D Si platforms to design organic-group IV semiconductor hybrid solar cells [2].
[1] A. Vlad et al., Nano Letters 2009, 9, 2838; A. Vlad et al., Small 2010, 6, 1974; A. Vlad et al., Adv. Funct. Mater. 2013, 23, 1164; A. Vlad et al., in preparation.
[2] C. Aurisicchio et al., Adv. Funct. Mater. 2012, 22, 3209.

For photovoltaics to compete with grid electricity prices very high conversion efficiencies and low production costs are required. Multi-junction solar cells offer a proven pathway to very high efficiency, but due to the epitaxial growth process and current matching requirements they are too expensive for large scale applications. In this work we present a novel method for spectrum splitting that does not rely on sequential filtering but instead utilizes the unique optical properties of nanostructured materials.
We theoretically demonstrate subwavelength spectrum splitting in two closely spaced coupled core-shell nanowires comprising a silver core and a Cu₂O (1.95 eV band gap) and Cu₂S (1.2 eV band gap) shell respectively. In previous work we have shown that such metal-semiconductor core-shell nanowires can exhibit absorbtion cross sections near the semiconductor band gap that significantly exceed the geometrical cross section [1]. As a consequence the flow of light can be directed towards the favorable semiconductor when two of such nanowires are coupled: over 50% of photons in the high energy band are absorbed in Cu₂O (high band gap), while all other photons are absorbed in Cu₂S.
There are two big advantages to this concept: (1) as it is not constrained by current matching or epitaxial growth there is much more flexibility in the material choice, and (2) overall efficiencies can be higher. For example, with Cu₂O and Cu₂S the current matched efficiency is 28% while without current matching the efficiency is 33% (assuming a V_oc loss of 0.3 V below the band gap, a fill factor of 0.8 and an average EQE of 0.9). Even with 50% of high energy band photons absorbed in the low band gap material the efficiency is 29% and exceeds the ideal current matched case. Spectrum splitting based on the coupling properties of resonant nanostructures is therefore a very promising route for low-cost highly efficient next generation photovoltaics.
Reference:
[1] S.A. Mann, E.C. Garnett, Extreme light absorption in thin semiconductor layers wrapped around metal nanowires. Nano Letters 2013, DOI: 10.1021/nl401179h

With solution-process-ability, scale-able fabrication and purification, and cheap input materials, semiconducting single-walled carbon nanotube (SWNT) networks represent promising materials for solar cell (SC) applications. This promise has motivated a body of work not only developing solar cells[1-4] but also exploring alignment/deposition methods and SWNT photovoltaic physics. Despite this interest, there is to date no quantitative model of SWNT solar cell operation analogous to bulk semiconductor p-n junction photovoltaics, allowing a rigorous understanding of the physical tradeoffs driving experimental observations and informing what research will enable technological progress. We present a framework and model, the first, for describing the steady state operation of an active SWNT layer in a solar cell, capable of accounting for arbitrary distributions of nanotube chiralities, lengths, orientations, defect types and levels, bundle fraction and size, density, dielectric environment, and electrode combination. We achieve this by treating individual SWNT properties as random variables, and describing the network by the dependent distributions of those properties, yielding coupled stochastic differential equations for light absorption, exciton transport, and free carrier transport. We have applied the model to arbitrarily aligned and isotropic networks of monochiral (6,5) SWNT. The resulting predictions of optimal SC geometry are both qualitatively and quantitatively consistent with to-date experimental devices,[1-4] and motivate experimental investigations to improve them. In particular, it suggests prioritizing higher-density films, sensitivity to which explains the variation in observed EQE for existant devices, as well as vertical-alignment methods taking advantage of the high longitudinal diffusivity in SWNT. We also show that there is a strongly optimal film thickness that shifts with film density (and orientation of aligned films), reflecting an inherent tradeoff between light absorption (i.e. exciton generation) and diffusion to the electrodes.
[1] D. J. Bindl, M. J. Shea, and M. S. Arnold, Chem Phys 413, 29 (2013).
[2] R. M. Jain et al., Adv Mater 24, 4436 (2012).
[3] M. J. Shea, and M. S. Arnold, Applied Physics Letters 102, 243101 (2013).
[4] M. Bernardi et al., Acs Nano 6, 8896 (2012).

In this study, an approach to enhance the light-matter interactions in radial p-n junction GaAs/AlxGa1-xAs core-shell nanowires for very high solar cell efficiencies will be presented. Specifically, this work investigates the effects of the doping densities of the core and shell, material composition of the shell, diameter of the core, thickness of the shell, and length of the nanowires on light absorption and carrier transport. Consequently, this study reveals that optimized radial p-n junction GaAs/AlxGa1-xAs core-shell nanowires on Si substrates could exhibit solar cell efficiencies of approximately 35% and higher. In order to achieve that, this analysis shows that the doping density for the p-type GaAs core should be in the mid 10^19 cm^-3 whereas the doping density for the AlxGa1-xAs shell should be in the mid 10^16 cm^-3. With that combination of the core and shell doping densities, a considerably large separation of the quasi-Fermi energy levels can be achieved and that increases the open-circuit voltage. In addition, doping the p-type GaAs core heavily widens the absorption spectrum of GaAs at the longer wavelength end and increases the optical power absorbed by the nanowires. Besides, the amount of optical power absorbed by the nanowires is also closely related to the diameter of the core which plays an important role in the overall spectral overlap between the propagation modes of the nanowire and the AM1.5G solar spectrum. In this regard, this study shows that the above spectral overlap is optimal when the diameter of the core is around 400 nm. Another crucial factor which determines the amount of optical power absorbed by the nanowire is the length of the nanowire since light at shorter wavelengths does not penetrate GaAs as deeply as light at longer wavelengths. Concerning this, the analysis in this study indicates that the practical length of the nanowire should not be longer than 6 microns in order to avoid the detrimental effect of hole pile-up on the solar cell efficiency. As for the shell, it should be as thin as possible in order to minimize carrier recombination and the Al composition should not exceeds 0.2 otherwise the increasing height of the conduction-band energy barrier at the core-shell interface would degrade carrier collection. Taking all the above factors into considerations, this study shows that an optimized 6 micron-long radial p-n junction GaAs/Al0.2Ga0.8As core-shell nanowire could exhibit an effective short-circuit current density of around 45.1 mA/cm^2, an open-circuit voltage of approximately 1.03 V, and a solar cell efficiency of about 37.7 % under 1-sun AM1.5G solar spectrum illumination which is above the theoretical Shockley-Queisser limit (~34 %) for a planar single junction solar cell.

GaAs and other III-V compound nanowire (NW) arrays represent an approach to enable high-quality, lattice-mismatched tandem device growth via radial strain relaxation, and also reduce material usage and cost of photovoltaic and photoelectrochemical devices without compromising external quantum yield. Even at <5% fill fractions, NW arrays exhibit strong absorption due to light trapping. We report here on (i) strong light absorption and current collection properties of sparse GaAs NW arrays, supported by experiments, simulations and analytical theory, and (ii) optical design methods for broadband light absorption via geometric optimization of nanowire morphology, using simulation and analytical theory.
Experimental results reveal that 4% fill fraction GaAs NW arrays absorb 60-100% of incident light, and that absorption depends strongly on wavelength and incidence angle. These absorption characteristics indicate a minimum of a 25x enhancement in the effective cross section of NWs at resonant wavelengths. We assess the mechanism for the observed strong absorption enhancement and find that coupling to radial waveguide modes makes the dominant contribution.
Optical absorption for MOCVD-grown NW arrays (wire length=3um, wire diameter=150nm, pitch=600nm) as a function of wavelength (350-900nm) and illumination angle (0-60) was measured experimentally and modeled via 3D full-field electromagnetic simulations. Experiment, simulation and analytical modal analysis showed excellent qualitative and quantitative agreement across the spectrum and incident angles. The strong absorption of the wire arrays is explained by coupling into resonant leaky and guided optical waveguide modes, enabled by scattering of incident light from neighboring wires. The identification of specific TE and TM wire modes responsible for absorption peaks was carried out via a comparison of analytical solutions using fundamental optical waveguide theory and spatially-resolved electric field profiles of NW cross sections obtained from electromagnetic simulations.
To achieve near unity broadband absorption in sparse NW arrays, we introduce new resonant modes to exploit the large absorption cross sections of the NWs at resonant modes. The general approach is geometric modification to capitalize on array resonances, including exploration of different shape motifs and directed variation of NW array dimensions (radius, height, pitch). For instance, an array of truncated GaAs nanocones with tip and base radii of 40 and 100nm achieves a 25% improvement in absorbed current over the uniform NW array. This design among others will be discussed in detail, clearly illustrating the speed and cost advantages of simulation over experiment for rapid exploration of design space.
Ultimately, the elucidation of the mechanism responsible for high absorption in sparse GaAs NW arrays facilitated directed optimization of light absorption and thus enabled improved III-V NW array optoelectronic performance.

Chemically synthesized semiconductor nanocrystals have been extensively studied as both a test bed for exploring the physics of strong quantum confinement as well as a highly flexible materials platform for the realization of a new generation of solution-processed optical, electronic and optoelectronic devices. Due to readily tunable, size-dependent emission and absorption spectra, colloidal nanocrystals are especially attractive for applications in light-emitting diodes (LEDs), solid-state lighting, lasing and solar cells. It is universally recognized that the realization of these and other prospective applications of nanocrystals requires a detailed understanding of carrier-carrier interactions in these structures, as they have a strong effect on both recombination and photogeneration dynamics of charge carriers. For example, nonradiative Auger recombination [1] is one of the key factors limiting the performance of nanocrystal-based lasers [2] and LEDs [3]. On the other hand, the inverse of this process, carrier multiplication, plays a beneficial role in light harvesting and can be used to boost the efficiency of photovoltaics through increased photocurrent [4].
This presentation will overview recent progress in the understanding of multi-carrier processes in nanocrystals of various complexities including zero-dimensional spherical quantum dots, quasi-one-dimensional nanorods, and various types of core-shell heterostructures. Key insights into multi-carrier behaviors have been gained through comprehensive spectroscopic studies that emphasize femtosecond time-resolved techniques and advanced single-nanostructure characterization [5]. The specific focus of the presentation will be on recent efforts toward controlling multi-carrier interactions using traditional approaches such as size and shape control, as well as newly developed methods involving “interface engineering” [6] for suppression of Auger decay and engineering of intra-band cooling rates for enhancement of carrier multiplication.
1. V. I. Klimov, A. A. Mikhailovsky, D. W. McBranch, C. A. Leatherdale, and M. G. Bawendi, Science 287, 1011 (2000)
2. V. I. Klimov, A. A. Mikhailovsky, S. Xu, A. Malko, J. A. Hollingsworth, C. A. Leatherdale, H. J. Eisler, and M. G. Bawendi, Science 290, 314 (2000)
3. W. K. Bae, S. Brovelli, and V. I. Klimov, MRS Bulletin 38, 721 (2013)
4. R. D. Schaller and V. I. Klimov, Phys. Rev. Lett. 92, 186601 (2004)
5. C. Galland, Y. Ghosh, A. Steinbrück, M. Sykora, J. A. Hollingsworth, V. I. Klimov, and H. Htoon, Nature 479 203 (2011)
6. W. K. Bae, L. A. Padilha, Y.-S. Park, H. McDaniel, I. Robel, J. M. Pietryga, and V.I. Klimov, ACS Nano 7, 3411 (2013)

In a conventional solar cell the energy of an absorbed photon in excess of the bandgap is wasted as heat. Multiple exciton generation (MEG) in colloidal quantum dots (QDs) uses this energy to instead produce additional free charges, increasing the photocurrent and cell efficiency[1]. Theoretical predictions indicate that MEG has the potential to enhance the efficiency of a single gap cell from 33% to 42% [2], by minimisation of the energy threshold for MEG. An attractive interaction between excitons reduces the threshold by the biexciton binding energy, Bxx. This has been found to be small (-10meV) for type I QDs [3]. Previous calculations of Bxx in type II CdSe/CdTe QDs have found a large repulsion between excitons [4]. Here, we show that that a CdSe/CdTe core/shell QD exhibit large values of Bxx<0. Our theoretical methodology is based on an 14-band k.p Hamiltonian, with correct atomistic symmetry, C2v, of the zinc-blend material, which incorporates the effects of band mixing between the p-bonding, s-anti-bonding and p-anti-bonding states, SO interaction, crystal-field splitting, strain between core/shells and piezoelectric potentials [5]. Excitonic states were found using the full CI method, that incudes explicitly the effects of Coulomb interaction, exact exchange and correlations between many-electron configurations. Particular attention was paid to accurate modeling of the dielectric constant variation through the structure and surface polarization effects on core/shell and shell/solvent interfaces. Relevant material parameters are predicted using ab initio time-dependent density functional theory [6]. We conclude that: (i) it is not possible to predict biexciton binding using the Hartree approximation alone; it can only be predicted with a full CI Hamiltonian; (ii) CI predicts Bxx=-70 meV for QDs with 0.5 nm thick shell; (iii) by ignoring the dielectric confinement, it is not possible to predict biexciton binding for structures with shell thickness >0.75 nm; (iv) by changing the solvent dielectric constant from 1 to 2 the variation in the Bxx binding energy is as big as 100 meV; (v) a proper calculation of Bxx requires the inclusion of correlations and surface polarization effects but the effect of self-polarization is negligible. The strong biexciton binding found is explained by a stronger reduction in the Columbic repulsion between holes than reduction in the attraction between electrons and holes on the addition of the CdTe shell layer, which is a consequence of 4 fold degeneracy of the h-ground state imposed by symmetry of the structure.
[1] D.J. Binks, Phys Chem Chem Phys 13, 12693 (2011)
[2] V.I. Klimov, Appl Phys Lett 89, 123118 (2006)
[3]R. D. Schaller, et al, Nano Lett. 7, 3469 (2007)
[4] A. Piryatinski at al, Nano Lett 7, 108 (2007)
[5] S Tomic at al, J. Appl Phys 110, 053710 (2011)
[6] L. Bernasconi, S. Tomic et al, Phys Rev B 83, 195325 (2011)

Multiple-exciton generation (MEG) for single high-energy photon absorption is one of the most remarkable features of semiconductor quantum dots (QDs) and other nanoscale materials.1-5 It represents a possible new path toward increasing the conversion efficiency of solar cell devices. Here, we examine ultrafast multiple-exciton generation (MEG) in PbS, CdTe and Ag2S quantum dots (QDs) using femtosecond broadband transient absorption spectroscopy. For PbS QDs, we assess MEG in different sizes of QDs, and observe a strong size dependence of multiplication yields, providing significant new insights into the effects of quantum confinement on the process of multiple-exciton generation in semiconductor quantum dots. For instance, we achieved quantum efficiencies of 159, 129 and 106% per single-absorbed photon at pump photoexcitation of three times the band gap for QDs with band gaps 1.41 eV, 1.24 eV and 1.0 eV, respectively.
For Ag2S and CdTe QDs, we explored the carrier multiplications generated by single high-energy and multiple photon absorption. We found that, irrespective of the size of the QDs and how the multiple excitons are generated in the Ag2S and CdTe QDs, two distinct characteristic time constants of tens and hundreds of picoseconds are obtained for the non-radiative Auger recombination of the multiple excitons, indicating the existence of two binding excitons, namely tightly bound and weakly bound excitons. Furthermore, the results demonstrated that the dynamics of the formation and recombination of the multiple excitons are significantly different in single-photon excitation compared with the dynamics generated by multiple photon absorption. More importantly, the lifetimes of multiple-excitons in Ag2S QDs were about one and two orders of magnitude longer than those of comparable size PbS QDs and single-walled carbon nanotubes, respectively. This result is significant because it suggests that, by utilizing an appropriate electron acceptor, there is a higher possibility to extract multiple electron-hole pairs in Ag2S QDs, which should improve the performance of QD-based solar cell devices.
References
(1) Schaller, R. D.; Klimov, V. I. Phys. Rev. Lett. 2004, 92, 186601.
(2) McGuire, J. A.; Joo, J.; Pietryga, J. M.; Schaller, R. D.; Klimov, V. I. Acc. Chem. Res. 2008, 41, 1810-1819.
(3) Ji, M.; Park, S.; Connor, S. T.; Mokari, T.; Cui, Y.; Gaffney, K. J. Nano Lett. 2009, 9, 12171-1222.
(4) Wang, S.; Khafizov, M.; Tu, X.; Zheng, M.; Krauss, T. D. Nano Lett. 2010, 10, 2381-2386.
(5) Baer, R.; Rabani, E. Nano Lett. 2010, 10, 3277-3282.

In order to develop a working chemical intuition about electronically active organic materials, and particularly with the goal of developing design principles for organic photovoltaic materials, it is imperative to understand the relationship between molecular-level structure and the electronic excited state phenomena of exciton migration and charge separation dynamics both within conjugated polymers and at organic donor/acceptor interfaces. In this presentation, recent progress in simulating these processes, using a mixed quantum/classical molecular dynamics approach that employs an all-atom description of the intermolecular interactions coupled with semi-empirical electronic structure will be described. The results of exploring several systems at ambient temperature will be discussed, including phenylene-vinylene and thiophene oligomers, as well as cyanine and fullerene components. The roles of molecular structural fluctuations and intermolecular electronic couplings, as well as the roles of donor and acceptor excited state energy alignments will be discussed in the context of exciton transport and dissociation. The development of a molecular-level interpretation of experimental ultrafast time-resolved spectroscopic probes of these processes in a layered phtalocyanine-C60 system will illustrate the mechanistic insight accessible via such simulations.
The results reported here are based on work supported as part of the Energy Frontier Research Center “Understanding Charge Separation and Transfer at Interfaces in Energy Materials” (EFRC:CST), funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001091.

We show that measurements of P3HT:PCBM bulk heterojunction (BHJ) OPV device efficiency as a function of light intensity from 0.01 to > 100 mW/cm2 reveal and separate the internal processes of exciton dissociation, charge generation, charge recombination, and exciton-polaron annihilation. By measuring devices prepared through different fabrication procedures, using P3HT with different molecular weights, and replacing PCBM with ICBA we find that:
- There is a correlation between the density of the recombination centers and the dissociation efficiency; as the density of the recombination centers increases so does the dissociation efficiency.
- The recombination centers are located in the bulk of the active layer.
- The recombination centers&’ energy level and the LUMO level of the Fullerene are correlated.
Examining these observations we conclude that the recombination centers are the charge transfer (CT) states at the interface between the donor and acceptor phases. Hence, our results reflect the effect of the bulk morphology, i.e. how fine the phase separation is in the bulk heterojunction, on electrical properties of the OPV devices.
For all cells, the main loss source under low light intensities are the recombination centers, described as Shockley-Read-Hall "trap" assisted recombination, and under high intensities, 1 Sun and above, the exciton-polaron recombination becomes dominant. As we will show, all the losses parameters and their physical origin are quantifiable and provide crucial information for the device design.

Advances in synthetic methods allow a wide range of semiconductor nanocrystals (NCs) to be tailored in size and shape and to be used as building blocks in the design of NC solids. However, the long, insulating ligands commonly employed in the synthesis of colloidal NCs inhibit strong interparticle coupling and charge transport once NCs are assembled into the solid state as NC arrays. We employ short, compact ligands to exchange the long, insulating ligands used in synthesis and increase interparticle coupling. Synergistically, we use 1) thermal evaporation and diffusion and 2) wet-chemical methods to introduce extrinsic impurities and non-stoichiometry to passivate surface traps and dope NC solids, providing control over the density of states, the carrier statistics and the Fermi energy. Examples of II-VI and IV-VI semiconductor NC solid design will be given to realize high mobility n- and p-type materials for large-area, flexible, field-effect transistors and integrated circuits and for solar photovoltaics.

The combination of cadmium- and copper-based chalcogenides as semiconductor sub-units in nano-heterostructures has been receiving a strong attention because of their potential for photoenergy conversion.^{1, 2} Type II nano-heterostructures resulting from the combination of both materials are envisaged to enable spatial charge separation upon photoexcitation, with the low bandgap allowing for the absorption of a high portion of the sunlight. The deployment of such nano-heterostructures for photoenergy conversion will be ultimately determined by their optoelectronic properties, namely strong excitonic features in the cadmium chalcogenide sub-unit, and plasmonic properties in the copper-based, vacancy-doped one.^3-5
In this communication we present a thorough investigation of the optoelectronic properties of CdTe-Cu_2-xTe nano-heterostructures, which may serve as model systems for other cadmium- and copper-based chalcogenide nano-heterostructures. The samples were prepared via cation exchange which enables the size-tunability of both sub-units, while keeping the overall length constant. Our investigation is based on steady state and transient absorption spectroscopy as well as on theoretical calculations (discrete dipole approximation and bandstructure calculations) and takes into account the main characteristics of the disparate components. In short, our results suggest a negligible groundstate interaction between the excitonic and plasmonic resonances and an enhanced probability for an Auger-mediated recombination mechanism due to the presence of an increased carrier density in the Cu_2-xTe subunit. Taken altogether our results demonstrate that a shorter copper-based sub-unit and the suppression of a high carrier density within this sub-unit is advantageous for the application of type II chalcogenide-chalcogenide heterostructures based on copper and cadmium for photoenergy conversion.
1. J. B. Rivest, S. L. Swisher, L.-K. Fong, H. Zheng and A. P. Alivisatos, ACS Nano, 2011, 5, 3811-3816.
2. T. Teranishi and M. Sakamoto, The Journal of Physical Chemistry Letters, 2013, 4, 2867-2873.
3. J. M. Luther, P. K. Jain, T. Ewers and A. P. Alivisatos, Nature Mater., 2011, 10, 361-366.
4. I. Kriegel, C. Jiang, J. Rodríguez-Fernández, R. D. Schaller, D. V. Talapin, E. da Como and J. Feldmann, J. Am. Chem. Soc., 2012, 134, 1583-1590.
5. I. Kriegel, J. Rodríguez-Fernández, A. Wisnet, H. Zhang, C. Waurisch, A. Eychmüller, A. Dubavik, A. O. Govorov and J. Feldmann, ACS Nano, 2013, 7, 4367-4377.

Semiconductor nanostructures are increasingly being explored as key components for novel optoelectronic devices like photovoltaics. Though a strong effort has been made to understand their optical properties, such as absorption and photoluminescence, it still remains difficult to quantify the electrical properties of these small structures, especially at ultrafast time scales. Here we present results on the mobilities and carrier dynamics of silicon nanowires and silicon nanocrystals by utilizing a contactless electrical probe that has sub-picosecond time resolution. This technique relies on the absorption of THz radiation by free carriers in the nanostructures produced by an ultrafast laser pulse. We will present our observations of a drastic change in the free carrier dynamics as the Si geometry is decreased. The mobilities for the different Si structures for varying absorbed pump fluences were also studied. As the volume of Si is reduced, the mobility decreases, which we will show is due to the enhanced carrier-carrier and surface interactions. These experiments demonstrate the benefit of using a nanowire geometry as they can effectively, and directionally, transport charge. For SiNCs, however, charge transport would need to be enhanced through improved electrical coupling between dots, or transport to another conductive material, in order to effectively move carriers in a photovoltaic device.

Charged or multiply-excited colloidal quantum dots (QDs) have typically low emission because of fast nonradiative Auger process and this impedes the development of QD based lasers, light emitting diodes, and photovoltaics. However, in the last few years, strongly reduced Auger process has been observed in ultra-thick-shell CdSe/CdS nanocrystals and dot-in-rod structures with large particle sizes (~20 nm) where the electron confinement was minimal. This has been qualitatively attributed to various mechanisms, surface effects or interfacial alloying, but it remained very unclear whether Auger suppression could be achieved in small colloidal QDs (~5 nm) where the quantum confinement effect is preserved.
We therefore investigated the photoluminescence of charged colloidal dots with varying core/shell structures in order to design and tune the wavefunctions. With two electrons and one hole, the negative trion is arguably the simplest system to study the Auger process in colloidal QDs and, using confocal microscopy and controlled charging, we could investigate single dots in a defined -1 charge state at room temperature. In summary, type I CdSe/ZnS exhibit very short-lived trions, while with the weakly type II CdSe/CdS, the trion lifetime lengthens slowly with increasing shell thickness in accord with other results. However, the first study of the strongly type-II CdTe/CdSe QDs revealed that a long trion lifetime is obtained for very small particles and, uncharacteristically, that the trion lifetime is maximum (~4.5 ns) for an intermediate shell thickness. This leads to the smallest particles (~4.5 nm) with the brightest negative trion to date. At the single dot level, the negative trion exhibit non-blinking behavior and can reach almost the same brightness as the single exciton.
The unprecedented Auger suppression in such small dots is a significant departure from prior results with much larger nanoparticles. This leads us to a new proposal, whereas the optimum shell thickness in the type II structure is when the electron energy is exactly at the bottom of the conduction band of the core. Therefore, the Bloch functions for the electron and hole would have different central symmetries, leading to the vanishment of the transition matrix in the Auger process. We propose that this is also the main mechanism for the Auger suppression in the weakly type II CdSe/CdS, those needing a much larger shell to achieve the same effect.
This study is a significant improvement in the understanding of multiexciton recombination in colloidal quantum dots, and it may impact the design of widely tunable small bright charged QDs and help develop efficient light-emitting devices.

Understanding charge separation and recombination processes and developing materials that can efficiently direct charge carriers with nanoscale precision are of fundamental importance in advancing next-generation electronics, optoelectronics and energy technologies. As semiconductor heterostructures have enabled today&’s electronics and optoelectronics, the introduction of active heterojunctions can impart new and improved capabilities that will facilitate integration of colloidal quantum dots into high performance devices. With shape anisotropy that can be exploited for assembly and manipulating charge carriers, incorporating heterojunctions in colloidal semiconductor nanorods presents a promising direction. Challenges in materials synthesis/fabrication, unusual properties and prospects arising from the formation of heterointerfaces, and efforts to integrate these materials into photovoltaics and light-emitting diodes will be presented.

Very low emissivity of multicarrier states (e.g., charged excitons and multi-excitons) in nanocrystal quantum dots (QDs) is usually attributed to nonradiative losses due to Auger recombination whereby the recombination energy of an electron-hole pair is not converted into a photon but is instead captured by a third carrier. Nonradiative quenching by Auger recombination complicates many applications of QDs including light-emitting diodes, single-photon and photon-pair sources, and especially lasers, as all of these applications directly rely on efficient radiation from multicarrier states. Therefore, the development of “Auger-recombination-free” QDs is an important current challenge in the field of colloidal nanostructures. Our previous studies of so-called “giant”-QDs (g-QDs, a CdSe core surrounded with an ultra-thick CdS shell up to 19 monolayers), have shown that biexciton photoluminescence (PL) emission efficiencies (Q_BX) of individual g-QDs can be near-unity. However, the efficiency of biexciton emission was highly nonuniform across an ensemble of nominally identical QDs. It was speculated that in addition to dot-to-dot size variation, this spread originated from variations in the structure of the core/shell interface in QDs, which determined the shape of the confinement potential.
Here we present a direct side-by-side comparison of the effect of the core/shell interface on nonradiative Auger decay rates of individual core/shell CdSe/CdS nanocrystals that have either a sharp or a graded interface. The latter type of QDs (referred to as “alloyed QDs”) comprises a CdSe_xS_1-x alloyed layer of controlled composition and thickness between the CdSe core and the CdS shell. We observed that, while having essentially no effect on single-exciton dynamics, the interfacial layer leads to a systematic increase in the biexciton PL quantum yield (QY), as inferred from second-order intensity correlation (g⁽²⁾) measurements. Q_BX in alloyed QDs was found to be up to ~10 times higher than that in the reference QDs with a sharp interface. These results are corroborated by independent measurements of biexciton dynamics that show a considerable increase in biexciton lifetimes upon interfacial alloying, leading to a remarkable quantitative agreement between the Q_BX values derived by the g⁽²⁾-method and those inferred from the measured lifetimes of single- and bi-exciton. Finally, a statistical investigation of over 100 individual QDs with either sharp or graded interfaces shows that the CdS shell thickness has only a minor effect on biexciton emission QY. All of these findings point towards a significant role of the shape of the confinement potential in Auger recombination and indicate the possibility of controlling this effect via appropriate engineering of QD interfaces.

The narrow and precisely tunable emission of colloidal quantum dots enable the fabrication of light emitting diodes (QD-LEDs) with high luminescence efficiency and color purity. However, most quantum dots patterning methods, such as spin coating, mist deposition, inkjet printing are restricted to flat, rigid two dimensional substrates. In addition, the charge transport layers and the electrodes often require high vacuums deposition. The versatility of 3D printing offers a freeform fabrication alternative, enabling new geometrical form factors via direct writing of the quantum dots, charge transport layers, and electrodes into any desired shape. Here, for the first time we have demonstrated the direct writing of QD-LEDs with a custom-built printer exhibiting micron-scale resolution and multiple print-head functionalities in an ambient environment. Specifically, we have printed QD-LEDs containing CdSe/ZnS quantum dots as the emission layer, poly-TPD as the hole transport layer, ZnO nanoparticles as the electron transport layer, transparent silver grids as the cathode and highly conductive PEDOT:PSS as the anode. This abilities to 3D print this comprehensive variety of nanomaterials and polymers are enabled by formulation of the nanoparticle inks to produce thin and uniform layers after dispensing and heating processes. Most significantly, we demonstrate the direct writing of a three dimensional 3 × 3 × 3 LED array and its implementation as a simple holographic display system.

Monodispersed polystyrene-coated, gold core-shell nanoparticles (Au@PS NPs) with the core diameter of 30 nm and the total diameter of 90 nm were successfully synthesized for their applications in highly-efficient, air-stable, organic photovoltaics (OPVs) and organic light emitting diodes (OLEDs). The monodispersed Au@PS NPs were incorporated into poly(3,4-ethylenedioxy-thiophene):poly(styrene sulfonate) (PEDOT:PSS) film that was located between the ITO substrate and the active layer (or emitting layer) in organic electronic devices. The use of Au@PS NPs induced a simultaneous and remarkable enhancement in the device efficiency and ambient stability for OPVs and OLEDs. The enhancement of the device performance was attributed to the localized surface plasmon resonance of the Au@PS NPs, which was visualized by near-field scanning optical microscopy and other optical measurements including UV-Vis absorption spectra, spectral reflectance, photoluminescence and electroluminescence. While Au core of Au@PS NPs has been beneficial for device efficiency, the hydrophobic PS shell on Au@PS NPs could reduce hygroscopic and acidic nature of PEDOT:PSS film, thus resulting in a significant improvement in the device stability. The incorporation of Au@PS NPs into both OPVs and OLEDs implies the universal use of the core-shell type NPs to organic electronic devices.

Although a large number of threading dislocations (> 10^8cm^-2) exist in gallium nitride based light emitting diodes (LEDs), the operation of the efficient LED device the operation of the efficient LED device emitting blue wavelengths is ensured. The origin is usually attributed to carrier localizations, numerous possible mechanisms of which have been suggested. Some of them are related to the random distribution of indium atoms, the indium clustering, the hole localization in N-In-N chain, the well width fluctuation, and the V-defect.

In this talk, we particularly pay attention to the inverted-hexagonal-pyramid-shaped structural V-defect for the localized carriers. The structure looks graved in the basal InGaN/GaN quantum well (QW) active layers grown along [0001] direction. The vertex of V-defect is connected to the threading dislocation (TD) generated from the top of sapphire substrate.
Based on the photoluminescence spectra by the near-field scanning optical microscopy (NSOM) for InGaN/GaN quantum well structures, it is known that a local potential energy barrier is formed on a V-defect site around the TD due to the larger optical bandgap ascribed to the thinner quantum wells of relatively smaller indium contents in the V-defect sidewalls of (101-1) plane. Thus, the carriers to TD which acts as a non-radiative recombination center or a sink adversely working for efficient light emissions in the QW can be screened out by the V-defect. This plays a role of enhancing the quantum efficiency in blue GaN based LED devices with increased carriers which can participate in the radiative recombination.

The carrier distribution cannot be uniform especially in the in-plane QW with the presence of V-defect and TD, and the carrier recombination can be influenced by that. Even with this fundamental importance of carrier localization on optoelectronic properties, it has been rarely studied by theoretical work related to the V-defect and TD system in GaN based QWs.

In this presentation, we show a theoretical modeling which can describe inhomogeneous carrier distributions of in- and out-of-plane QW with taking into account a V-defect and TD. The internal quantum efficiency and non-radiative recombination are discussed as a function of current density, effective non-radiative domain size, local V-defect potential energy, and carrier density. The condition for more efficient screening of non-radiative recombination is also addressed.

In most single nanocrystal spectroscopy studies, little is known about the physical structure or composition of the individual nanocrystal; these properties are approximated by data gathered from the ensemble material. In reality, nanocrystals produced by colloidal methods exhibit a nontrivial variance in physical characteristics such as shape, size distribution, and elemental composition. Many of the observed fluorescence phenomena, including quantum yield and “on” / “off” dwell times, differ for each individual nanocrystal examined. Here we present a method of rectifying this disparity by performing sequential single particle fluorescence spectroscopy and scanning transmission electron microscopy on the same core / shell nanocrystals (NCs). Atomic number contrast scanning transmission electron microscopy (Z-STEM) of these NCs demonstrates that even seemingly monodisperse NCs cannot be described by the hexagonal or cubic structures that are commonly used to designate the ensemble properties of NCs. We show progress toward full correlation of a distribution of individual NCs in order to determine a structural description of optical phenomena such as fluorescence intermittency and “dark” particles. Instrument design, selection of a suitable substrate, and the use of fiducial markers to locate NCs on both an optical microscope and electron microscope will be presented. This innovative method promises to provide an unprecedented understanding of the interplay between structure and function on the single nanocrystal level, and lays the groundwork for future studies that will extensively study this relationship.