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.

In addition to the well-known size-dependent optical properties of quantum dots, heterostructured nanocrystals and nanorods have interfaces between two or more semiconductor materials. The hetero-interfaces can lead to improved and/or new optical and electronic properties that depend on the energy band alignment, allowing the benefits of manipulating charge carriers including injecting, extracting and confining electrons and holes. Due to the inherent shape anisotropy, the ability to introduce such heterojunctions in anisotropic nanocrystals offers additional benefits of directionality in band structure engineering and assembly. Here, we have synthesized and characterized colloidal nanorod heterostructures that consist of two or more heterojunctions and maintain the anisotropic shapes. The heterojunctions can enable independent control over the electron and hole injection/extraction processes while maintaining high photoluminescence yields. Additionally, diversity of the epitaxial heterojunctions formed on the surface of nanorods beyond the usual heterojunctions formed at the tips can provide new morphologies with potentially new ways of controlling optical properties.

Photonic crystals, nanowires, and other nanostructures have shown promise for future PV technologies by reducing material usage while maintaining efficiencies. However, recent studies have suggested that there may be additional efficiency improvements based on appropriate nanostructuring of devices. Here we describe the limiting (and practical) energy conversion efficiencies for these devices based on the method of detailed balance, which was first applied to photovoltaics by Shockley and Queisser. As a test case, we will present results for both nanowire and photonic crystal-based solar cells.
We find two dominant effects in these nanostructures that can lead to increased efficiency. First, nanosctructures can act as effective concentrators of light, which yield maximum efficiencies that are comparable to those of planar cells with macroscopic concentrating optics. The high current densities can, in these structures, lead to improved voltage characteristics, and hence improved efficiencies. Second, photonic crystal devices can also cause a modification of the spontaneous emission at the semiconductor bandgap, which can effect the reverse saturation current. For high quality materials like GaAs, modest suppression of spontaneous emission can lead to a several percent efficiency improvement. In addition, a photonic crystal above a planar cell can lead to an effective modification of the energy bandgap of the material through an elevation of the quasi-Fermi level, which can subsequently change its maximum theoretical efficiency. We will discuss how these concepts can lead to efficiencies in excess of 40% for a single junction device.

Doped semiconductor nanocrystals (NCs) can address key problems in applications such as LED&’s, photovoltaics, spintronics, and bioimaging, since doping enables further control over the properties of NCs. In this work, we report the successful doping of ZnSe NCs and ZnTe magic sized clusters (MSCs) with Mn2+ via cation exchange. The parent ZnTe MSCs consist of three cluster families with sizes 1.3, 1.5 and 1.8 nm. The parent ZnSe NCs have a diameter of 3 nm (<5% dispersion) and are highly luminescent. Mn2+ emission is observed for both doped materials, with a radiative decay time that is characteristic of Mn2+ in ZnTe and ZnSe, respectively. The absorption spectra of the NCs and MSCs remain virtually unchanged upon doping. Interestingly, the excitation spectra of the Mn2+ emission are characterized by the same absorption transitions observed for the undoped ZnSe NCs and ZnTe MSCs. This shows that the Mn2+ ions are excited through the exciton states of the host, providing further evidence that the emission originates from Mn2+ ions incorporated in the host, rather than simply bound to the surface. For some applications, like spintronics, very low doping concentrations are required. The approach developed here can be used to achieve low dopant concentrations, since doped MSCs can serve as nuclei for the growth of an undoped shell. Moreover, this strategy is very versatile, since the size and shape of the NCs is preserved after the cation exchange.

Colloidal semiconductor nanocrystals (NC's) are highly attractive as optical gain media due to their size-dependent wavelength of emission, ease of fabrication and flexible surface chemistry which facilitates their incorporation into a wide range of optical cavities. Tremendous progress has been made since the demonstration of optically pumped amplified spontaneous emission (ASE) in NC's at low temperature in 2000, and extremely low pump thresholds have since been achieved via improvements in the NC architecture and/or optical cavity design. While most studies have focused on investigating the effects of Auger recombination on NC gain, a deeper understanding on key parameters to achieving ASE or lasing such as the NC volume fraction or NC quantum yield (QY) has proven elusive since nearly all NC gain media reported to date have been based on solid state devices. For example, in a solid state waveguide, dynamically tuning the volume fraction of NC's without significantly modifying the film thickness or morphology is a virtually intractable task. In this work, we introduce core-shell NC's in which the core is of alloyed composition. Despite the shell possessing a larger bandgap, the NC&’s surprisingly exhibited Type II behavior with a biexciton lifetime on the order of 1 ns. This allowed for the attainment of room temperature ASE at ultralow pump thresholds in solution within a microfluidic channel. By systematically varying the concentration of NC's in solution, the dependence of NC volume fraction on the ASE pump threshold was obtained whereas fluorescence quenching studies were performed to elucidate the dependence of QY on pump threshold. A phenomenological model is proposed to explain these results and provide a qualitative description on how ASE is achieved, thereby yielding a conceptual framework by which to systematically optimize the performance of NC gain media in general.

Nanostructured GaSb/GaAs systems are of interest for the composition and strain dependence of their energy band alignment, which has been reported as type-II for GaSb quantum dots in GaAs matrices. Such systems would have a variety of possible optoelectronic applications, including in photovoltaics and memory devices. Contrary to experimental observations, simple model-solid theory predicts a type-I band alignment for GaSb in GaAs. In the present work, the effects of strain and defects in these materials are studied in order to explain the observed type-II band alignment. Cross-sectional STM images of GaSb/GaAs systems, including a variety of atomic structures ranging from clusters to rings to dots, are used to determine composition profiles for input into a continuum model. Based on a rule-of-mixtures assumption about material properties as a function of composition, the lattice mismatch strain field is calculated and used with deformation potential theory to determine the resulting conduction and valence band offsets. The band alignment of the system is shown to undergo a large type-I to type-II transition when strain is considered, but with conduction band offsets greater than those observed experimentally via STS. Strain relieving misfit dislocations are then considered as the source of this quantitative disagreement; both strain and charging effects of the misfit dislocations are investigated computationally. First, the relaxed strain field of a full edge-type misfit dislocation loop is calculated and included in the deformation potential analysis. Then a screened electrostatic potential associated with charging of the defect is considered. These effects are found to induce a large inhomogeneous shift in the band profile and may explain the observed effective band alignment. To further consider the effects of strain relaxation through dislocation formation, the conduction and valence band wave functions are calculated in the multiband kp approximation. The resulting spontaneous emission spectra are computed to compare with experimental photoluminescence data.

Crystalline silicon has a relatively low absorption coefficient (α), and therefore, in thin silicon solar cells surface texturization plays a vital role in enhancing light absorption. Texturization is needed to increase the path length of light through the active absorbing layer. The most popular choice for surface texturization of crystalline silicon is the anisotropic wet-etching that yields pyramid-like structures. These structures have shown to be both simple to fabricate and efficient in increasing the path length; they outperform most competing surface texture. Recent studies have also shown these pyramid-like structures are not truly square-based 54.7° pyramids but have variable base angles and shapes. In addition, their distribution is not regular—as is often assumed in optical models—but random. For accurate modeling of generation rate profiles, it is important to investigate the true nature of the surface texture that is achieved using anisotropic wet-etching, and its impact on the efficiency. We have used atomic force microscopy (AFM) to characterize the surface topology by obtaining actual height maps that would serve as input to ray tracing software. The height map also yields the base angle distribution, which will be compared to the angular reflectance distribution measured by spectrophotometery to validate the shape of the structures and to determine the efficacy of the techniques for characterization of solar cells. It is important to state that AFM has a shortcoming: when structure heights grow beyond a certain limit the AFM probe cannot reach the bottom of the surface and hence the resulting height map has blind spots. So, in parallel, we are using a focused ion beam (FIB) that is equipped with a scanning electron microscope (SEM) to obtain height maps employing a ‘slice and view&’ approach. The slices are later combined using image processing to obtain a height map. A final approach that we are using is to take stereoscopic images of a solar cell surface using SEM and obtain a height map by combining them using a stereo-vision technique. All of these different approaches are being employed to capture the true topology of the crystalline silicon solar cell surface yielded by anisotropic wet-etching, and to investigate the effect of base angle distribution , size of the structures, and their randomness on solar cell efficiency.

In thin-film photovoltaics, highly absorptive materials are conventionally used. However, these materials have achieved efficiencies that are not comparable to those of thick crystalline silicon (c-Si) photovoltaics and, in some cases, suffer from their toxicity and low supply. A viable solution to these problems would be to use c-Si for thin-film photovoltaics. However, thin c-Si films absorb sunlight weakly because of its indirect band gap, and highly efficient light-trapping should be provided to achieve high efficiency. For thin-film photovoltaics, nanoscale structures are typically involved for light trapping because the film thickness becomes comparable to the wavelength of sun light. While diverse nanostructures have been studied to break the light-trapping limit of geometric optics, known as the Lambertian limit, highly efficient nanostructures that can be easily manufactured have not been demonstrated. We have previously predicted that symmetry-breaking in light-trapping periodic nanostructures on thin films can approach the Lambertian limit very closely. Herein, we will demonstrate how such structures can be realized experimentally using simple wet etching methods, using KOH and HNO ₃/HF. We make use of positive photoresist and interferometric lithography to create symmetry-breaking nanoscale features on c-Si(100) wafers without any off-cut, tilt angle. The use of positive photoresist prevents the resist liftoff, often observed during wet etching. We will discuss how the symmetry of the light-trapping nanostructures is systematically lowered within our experimental framework by reactant transport control. Further, using group theory predictions, we will explore the implications of the symmetry-breaking for light-trapping in thin-film c-Si photovoltaics.

Thinning the absorber layer in CIGSe solar cells down to 0.5 mu;m (ultra-thin) is critical to reduce the material consumption (e.g Indium) and cost. However, it will inevitably lead to incomplete light absorption and thus considerable drop of efficiency. Therefore, light trapping is crucial to maintain high efficiencies of the ultra-thin CIGSe solar cells. Although light-trapping technologies have been extensively investigated for the thin-film silicon solar cells, little work has yet been reported in CIGSe solar cells. In this work, light trapping based on dielectric photonic structures, plasmonic metallic nanoparticles and micro-textures is investigated on ultra-thin CIGSe solar cells.
Dielectric nanostructures are highly favoured to be placed in front of solar cells due to the low parasitic absorption. The periodic arrayed TiO2 nanoparticles were prepared by nano-imprinting directly on the surface of CIGSe solar cells. A broadband photocurrent enhancement over the full 450-1000 nm wavelength range is observed. The enhancement is ascribed to be a combination of anti-reflection and light trapping.
Metallic nanoparticles are preferably located at the back to avoid parasitic absorption and achieve strong broad angle scattering. We investigated the Ag nanoparticles at the interface of ITO/glass substrate and Au at ITO/absorber, respectively. To passivate the diffusion of Ag into CIGSe during deposition, a 150-nm Al2O3 film was deposited between the Ag nanoparticles and ITO layer and a-10 nm thick silica shell coated on the Au nanoparticles. It is found that both Ag and Au nanoparticles can scatter light back into the solar cells and thus improve the short-circuit current density (Jsc) of the solar cells. The influence on EQE agrees well with the resonance peak of the Ag nanoparticles.
Geometric light trapping of using micro-textured glass substrates with high haze and the flat Ag back reflector were investigated individually as well as together for the ultra-thin CIGSe solar cells.

The optical path length of an optoelectronic device like a solar cell or a photodiode can be defined as the distance that an (unabsorbed) photon can successfully travel within the device before escaping out. This feature is usually described in term of device thickness.
Anyway light entering a device engineered with good light trapping features may bounces back and forth many times, presenting an optical path dozen of times higher then its physical thickness. This effect may enhance light absorption and, generally speaking, device performance. An attractive approach to improve the light trapping involves the introduction of plasmonic nanostructures and in recent years, metallic nanoparticles coupled to absorbing semiconductors have been utilized to enhance absorption in thin film solar cells.
This work reports a theoretical study aimed to identify the plasmonic resonance condition for a system formed by metallic nanoparticles embedded in an a-Si:H matrix. The study is based on a Tauc-Lorentz model for the electrical permittivity of a-Si:H and a Drude model for the metallic nanoparticles and the polarizability of an Aluminium sphere-shaped particle with radius of 10-20 nm.
We also performed FDTD simulations of light propagation inside this structure reporting a comparison among the effects caused by a single nanosphere of Aluminum, Silver and, as a comparison, an ideally perfectly conductor. Simulation of a distribution of a linear array of equally spaced nano spheres is also presented, presenting results about the influence of the average distance among the nano particles.
The simulation results shows that is possible to obtain a plasmonic resonance in the red part of the spectrum (600-650 nm) when 20 nm aluminum spheres are embedded into a-Si:H. Also, it is possible to enhance the light absorption in this part of the spectrum by introducing a distribution of a linear array of nanospheres having an average distance of 180 nm.

Organic light emitting diodes (OLEDs) are highly promising as efficient and inexpensive light sources. While organic emitters that facilitate an internal quantum efficiency of nearly 100% have been demonstrated, the extraction efficiency of internally generated photons still limits the overall OLED efficiency. The approximate free-space extraction efficiency in a conventional OLED is only 20%, since a large amount of the internally generated light is trapped in the high-index organic and anode layers. Incorporating nanostructured materials into the thin film stack is a promising approach for increasing OLED efficiency. Guided light is scattered on the nanostructure and coupled out. A single-period grating nanostructure, however, Bragg-scatters a waveguide mode to particular wavelength-dependent directions. Thus, the OLED emission over the angle is modified and becomes partly directional, i.e., leading to an angle-dependent color impression for the viewer. We investigate compound binary grating nanostructures designed by the spatial superposition of multiple binary gratings with different periods. We demonstrate that changing the duty cycle of a single grating component controls the intensity ratio between the emission directions.
Compound binary grating nanostructures with different compositions were written using electron beam lithography. Cr (7 nm) and Au (30 nm) layers were evaporated on top of the wafer acting as adhesive layer and conductive plating base for the subsequent galvanic deposition of nickel (550 µm). Nickel electroplating was carried out in a boric acid containing nickel sulphamate electrolyte. Afterwards wire-cut EDM was used to separate the stamps. The master stamps were replicated with a depth of 30 nm in photoresist on a glass substrate using nanoimprint lithography. Subsequently, a 70-nm-thick organic emission layer (PPV-derivative “Super Yellow”) was deposited on the nanostructured layer and protected by a silicon monoxide layer (70 nm). The emission characteristics of the different samples were characterized using photoluminescence measurements. With a goniophotometer setup, the spectrum was measured as a function of emission direction. Experimental results are compared to calculated guided mode Bragg-scattering at compound binary gratings employing rigorous coupled wave analysis and a customized analytical model. We show in theory and experiment that each out-coupling direction&’s intensity is determined by the waveguide cavity and the nanostructure&’s spatial Fourier coefficients. The deterministic control over the compound binary grating composition may be used to design a viewing-angle independent nanostructured OLED for general lighting applications or a specific emission profile for sensing applications.

The low absorption and high transmittance of graphene (G), make it an ideal over layer material for semiconductor devices. Here, the results of experimental and theoretical investigations of the antireflection properties of G/SiO2 stack deposited onto planar silicon solar cells without any textured surface are reported. Theoretical investigations have been done using Finite Difference Time Domain (FDTD) simulation. The effect of number of G layers and thickness of SiO2 layer on the antireflection properties has been investigated. The G layers have been prepared on copper foil by atmospheric pressure chemical vapour deposition (APCVD) method, which are then transferred to quartz, silicon and silicon solar cell substrates. Optical, AFM and Raman studies confirm the formation of monolayer G with a transparency of more than 97% in spectral range of 300-1100 nm. Prior to G transfer, required thickness of the SiO2 layer is deposited by magnetron sputtering method. With SiO2 thickness of 60 nm, G/SiO2 based antireflection structures display a broad band reflection suppression in 450 to 1000 nm wavelength and a minimum reflectance at a wavelength of 600 nm which is very important for solar cells, as the maximum power point in the sun spectrum is at about 600 nm. A comparison of the antireflection properties of G/SiO2 structure with that of standard Si3N4 /textured surfaces has been carried out by comparing the optical properties (reflectance and transmittance) and solar cell response (current-voltage and spectral response). Preliminary investigation shows that a planar Si solar cell with G/SiO2 overlayer as antireflection coating shows 29% enhancement in short circuit current as compared to standard Si cell without any antireflection coating. It is also observed that G helps in reducing the series resistance of Si solar cell due to its superior conductivity and texturing of the solar cell surface (commonly used in the solar cell technology) is not required. Our results show that graphene-SiO2-planar surface a good alternate to the standard Si3N4 antireflection-textured combination being used in present day Si sola cell technology.

Arrays of resonant dielectric nanostructures placed on top of c-Si solar cells can significantly enhance the cell efficiency due to both an antireflection and a light trapping effect. This is due to the preferential scattering of light towards the high-index solar cell by geometrical Mie resonances in the nanoparticle (NP). Despite several recent studies on light scattering by these nanostructures, the practical aspects of their integration into realistic c-Si solar cell devices have not been investigated yet. In this paper we study the effect of light-trapping NP coatings taking into account realistic doping profiles, surface morphology, cell thickness and surface passivation for c-Si solar cells. When compared to standard high-efficiency c-Si cells, the NP coating yields up to 10% relative improvement in conversion efficiency.
First, we use numerical simulations to study the incoupling of light in wafer-based c-Si solar cells coated with arrays of Si and TiO2 Mie NPs embedded in ethylene vinyl acetate (EVA). We find that an array of Si nanopillars with 210 nm particle diameter, 260 nm height, 345 nm array pitch, coated with a 65-nm-thick Si3N4 layer and embedded in EVA, reduces the AM1.5-averaged reflectivity of a bare Si wafer to 2.1%, a value well below that of a flat Si3N4 anti-reflection coating (9.0%). Similarly, an optimized array of TiO2 nanopillars in EVA yields a reflectivity as low as 4.2%.
Next, we study the effects of ordered and disordered arrays of dielectric NPs on light coupling and trapping into c-Si solar cells. When applied on a thick Si wafer, periodic and random arrays of Mie NPs yield similar antireflection effects, suggesting that the single-particle scattering dominates over the collective scattering from the grating. For thinner c-Si solar cells, we find that random arrays lead to best light trapping as they cover a broader range of spatial frequencies required for matching to the waveguide modes wavevectors. For a 5-mu;m-thick c-Si solar cell, a random array of Si NPs yields an AM1.5-weighted average absorption of 77%, compared to 72% of the periodic array and 66% of a standard Si3N4 coating.
Finally, we perform 1-d electrical simulations of realistic homo-junction c-Si solar cell on which optimized Si and TiO2 NP coatings are used to couple light in the cell. The results are compared to that of an n-type architecture c-Si solar cell with efficiency of 20%, employing a standard Si3N4 antireflection coating in combination with upright pyramidal surface texture. We find that both Si and TiO2 NP arrays enhance the electrical performance of the cell. Due to reduced carrier recombination in the heavily doped emitter region and reduced surface recombination we find a relative improvement in conversion efficiency of up to 10%. Overall, our study opens new perspectives for using nanostructures for realizing high-efficiency c-Si solar cells both on thin-films and wafer-based solar cells.

It is critical to achieve broad band harvesting of solar photons to enhance the efficiencies of thin film solar cells. Organic solar cells have shown remarkable progress recently, but still only absorb less that 50% of the solar spectrum. P3HT-PCBM cannot effectively absorb red photons (lambda;> 600 nm) and deep blue photons (lambda;<480nm), because of long photon absorption lengths in these wavelength ranges. Similarly, the absorption of photons is very weak in PTB7 beyond 700nm.
To increase the broad band absorption of light we design periodically textured organic solar cells that can be experimentally realized. These periodic structures strongly diffract light resulting in waveguide modes, and in addition demonstrate plasmonic concentration of light. We utilize rigorous scattering matrix simulations [1] where Maxwell&’s equations are vectorially solved. Our optimum nano-photonic solar architecture consists of multiple photonic and plasmonic crystals. The cathode is periodically textured with a periodic array of nanoparticles, that both strongly diffracts light and generates plasmonic concentration of light intensity. In addition there is a polymer lens on the glass side, that focusses light on the nanoparticles at the absorber layer cathode interface, and enhances further the plasmonic effects. This nano-photonic architecture with pitch 500-600 nm leads to very large absorption enhancements of 48% and current enhancements of 56% relative to the flat cell, with the usual 100-190 nm P3HT-PCBM thicknesses. The absorption and photo-current approach the Lambertian limit. This architecture is experimentally feasible since it does not require spin coating on corrugated surfaces. Moreover patterning the organic layer before cathode deposition has been demonstrated in recent experiments [2] and lens arrays on glass are routinely developed. We will show electric field intensities inside absorber layer, identifying the regions of enhanced absorption. The proposed configuration is more feasible than patterning the entire organic solar cell [1]. We will also compare its performance in thin film silicon cells. The exciton dissociation and charge transport should not be affected by this. This architecture is an unique way to control the interaction of light with nanostructures with the potential to achieve >12% efficient single junction organic solar cells.
[1] R. Biswas, E. Timmons, Opt. Exp. 21, A841-A846 (2013).
[2] J. You et al, Adv. En. Mat. 2, 1203-1207 (2012).

A Photonics device requires uniform periodic structural arrangement. Various techniques have been used to fabricate these types of structures, which employs several steps of fabrication. This work proposes single step hierarchical array of equal submicron size porous structure fabricated through tuning electrospinning processing parameters. The dictating parameters were high voltage, tip to collector distance and solvent used on the evolving structure. Morphological and optical investigations suggested the uniform periodic topography and enhancement in light absorption, which is assumed due to internal reflection of light. This structure was evaluated for better light harvesting as active layer in organic photovoltaic devices using poly (3 hexyl thiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) blend, and further studying enhancement in photoelectrical characteristics.

Cu(In,Ga)Se2, (CIGS) is a direct band gap semiconductor material that can be used as absorption layer in thin film solar cell with the highest efficiency recorded of 20.4% so far. In recent years, researchers make efforts on reducing the use of materials such In, Se, and Ga sources to reduce material cost. Many possible approaches have been adopted to light absorption management to maintain cell efficiency while reducing the thickness of these absorption layers, such as nanostructured surface. Plasmonic effect, which is one of approaches for light trapping, has been successfully applied on polymer tandem solar cell and Si solar cell. In this study, we apply plasmonic effect on the CIGS flexible thin film solar cell prepared by a non-vacuum process. From angle-dependent reflectance measurement, an enhanced light absorption in the range of 400~600 nm can be observed, which is consistent with the increasing external quantum efficiencies (EQE) of these devices. 15.91% enhancement of short circuit current(Jsc) and 51.63 % improvement of power conversion efficiency have been attained by the light trapping effect utilizing plasmonic effect. This research shows a great potential for possible application of plasmonic effect on ultra-thin film CIGS solar cells.

The relatively higher cost than that of grid electricity is one of the main reasons that strongly hinder large-scale and broader applications for electricity generated from current-generation solar cells. Conventional silicon solar cells are usually hundreds of microns in thickness due to its small absorption coefficient, and great efforts have been devoted to investigating thin-film solar cells with less photovoltaic materials to potentially reduce the cost for solar cells, for which effective light trapping approaches are crucial to achieve high light absorption and thus good performance. However, current thin-film solar cells are still around several or tens of microns in thickness. Solar cells with much thinner active layers are little explored due to the lack of efficient light trapping mechanisms.
Recently, perfect absorbers or selective thermal emitters have been studied to achieve spectrally selective absorption or emission due to the excitation of magnetic resonance. These structures, or called film-coupled metamaterials, are usually made of subwavelength metallic gratings on a dielectric spacer and a ground metallic film. Magnetic resonance could occur at particular wavelengths with strong magnetic field confined inside the dielectric spacer between the top and bottom metallic layers, resulting in enhanced light absorption. Such strong optical energy confinement is expected to occur when the dielectric layer is replaced with a semiconductor material, leading to effective light trapping inside the active photovoltaic layer.
In this work, we will numerically demonstrate such a novel physical mechanism to achieve efficient light trapping inside an ultra-thin amorphous silicon solar cell of 100 nm or so, sandwiched by a subwavelength metallic concave grating on top and an opaque metal film at the bottom, due to the excitation of magnetic resonance. The finite-difference time-domain method will be used to calculate the spectral absorptance of this film-coupled metamaterial solar cell structure in the visible and near-infrared region. Comparison will be made between the proposed structure and a free-standing amorphous Si film of the same thickness, to demonstrate the enhanced light absorption. Electromagnetic field distribution will be plotted to explain the physical mechanism of enhanced light trapping as the excitation of magnetic resonance inside the structure. Furthermore, energy absorption by the metals and by the photovoltaic material will be shown separately, which is important for minimizing the losses and enhancing the conversion efficiency. The top metallic grating and bottom film not only strongly localize optical energy inside the a-Si active layer, but also readily serve as front and rear electrical contacts. Insight gained from this work would potentially lead to the development of next-generation, ultra-thin, low-cost, and high-performance solar cells.

Silver nanowires are simple to fabricate and their large size allows them to be investigated by optical microscopy. We observed the fluorescence intensity of P3HT near such nanowires was increased by a factor of 2 over the background. By comparing two configurations, first with nanowires separated by a thin layer of PEDOT:PSS from P3HT and second with nanowires mixed into PEDOT:PSS, we could show that enhanced fluorescence arises from electric field enhancement rather than from scattering on the nanowires only. Fluorescence decays of P3HT on a nanowire and off a nanowire were similar, hence the process of fluorescence emission was unchanged by the plasmonic effect. We conclude that the increased emission arises from increased light absorption due to electric field enhancement near the nanowires. These results are promising to further increase the efficiency of organic solar cells.

Thin-film solar cells using a-Si:H offer the benefit of reducing material consumption and fabrication costs. Additional, benefit includes advantages of light-weight and possible flexible devices by roll-to-roll deposition processing. However, such thin absorbing layer reduces the photovoltaic efficiency, due to the decrease in a-Si:H layer optical path length and its poor light absorption at red and near-infrared (NIR) wavelengths.
Metal NP such as Au can exhibit strong localized surface plasmon resonances at UV, visible and NIR wavelengths. Once excited, surface plasmons decay, result in scattering and in light absorption as well. The optical properties of NP can be turned by changing their size, shape, or by altering the local dielectric environment. Metal NP have been shown to increase the absorption in the active material and then cell performances. The process involved is based on two approaches: i) the increase of the electromagnetic field in the vicinity of the metal NP of small size (<50nm) when irradiated with sunlight having a wavelength close to the resonance excitation wavelength; or ii) the diffusion of incident light from metal particles of bigger size (~100nm). However, the optimal parameters of NP in such cell are not actually well determined. Therefore, our work deals to understanding NP influence in such cells, to perform an optimal structure, by increasing the amount of light absorbed within the cell using NP scattering and luminescence(optical trapping).
Modeling based in Mie theory is first carried out with bhmie program using bulk Palik data. The extinction, scattering and backscattering efficiencies of Au sphere are calculated for various diameters and refractive medium indexes. The resonance wavelength is observed around 600nm for NP having diameter of 100nm. A red shift of the surface plasmon resonance is detected while NP size increases and/or refractive medium index. In addition, normalized angular scattering distribution at SPR shows the influence of NP size on the backscattering cross section of light. This distribution allows defining the optimal position of NP in photovoltaic cells.
Using these parameters, 10nm thickness gold layer has been deposited on glass and Transparent Oxide Conductor substrates respectively by thermal evaporation in vacuum and sputtering, followed by thermal annealing from 200°C to 500°C in order to promote the NP growth.
MEB pictures show quasi-spherical Au NP shape with a mean size of 100nm. This diameter range switches extinction of NP in scattering regime. Annealing temperature (T) strongly affects the morphology of NP. Surface coverage decreases and sphericity appears to increase with T. UV-Visible spectroscopy displays distinct localized surface plasmon resonances around 600nm after annealing with a red shift while T increases.
Acknowledgments: The research project leading to these results has received funding from the region Centre, France under the name ARPPCM.

In this work, in order to enhance the performance of ZnO nanorod (NR)/P3HT:PCBM solar cells, core/shell metal/insulator nanoparticles (NPs), including Au/SiO2, Au/PSMA and Ag/SiO2, were incorporated into the active layer of P3HT:PCBM. The addition of metal/insulator NPs to ZnONR/P3HT: PCBM hybrid solar cell resulted in the improvements of short circuit current density and fill factor of the hybrid solar cells. A 40% enhancement of the overall power conversion efficiency is achieved in the hybrid solar cell with Au/SiO2 NPs. The effects of incorporating metal/insulator NPs on the performances of the hybrid polymer solar cells were further investigated by time-resolved photoluminescence (TRPL), electrochemical impedance spectroscopy (EIS), and external quantum efficiency (EQE) spectra. The results will be discussed in the presentation.

The wrinkle formations in semiconductor nanomembranes can be engineered to enhance the electrical and optical properties for applications in logic and optoelectronic devices. Previously, the controlled formation of wrinkles have been used to improve the light absorption and scattering in organic and inorganic optoelectronic devices via increased active area and texturing effect of the surface wrinkles. To control the formation of wrinkles, the compressive/tensile stresses in soft and hard bilayer structures have been engineered using thermal expansion/contraction, solvent swelling/shrinkage, and mechanical stretching/releasing of bilayer thin films. Although these methods have been successfully applied for the wrinkling process of various materials, the multi-directional and deterministic location of patterned wrinkle arrays is a big challenge.
Here, we report the wrinkling process of the III-V compound semiconductor nanomembranes (InGaAs) by using the vacuum-induced stress control of nanomembranes on polydimethylsiloxane (PDMS) microwell arrays. In this method, the size, location, and direction of wrinkle arrays can be easily controlled by changing the shape and location of the microwell arrays and the modulus of soft substrates. We also demonstrate that the strain of wrinkled III-V nanomembranes can be locally engineered, presenting the facile route for the local band structure engineering of semiconducting nanomembranes. The wrinkling process introduced in this presentation is applicable to various semiconducting nanomembranes for applications in photodetectors, light emitting diodes, and solar cells.

Multijunction photovoltaic technologies have recently eclipsed 44% energy conversion efficiency. Since a majority of the approaches are based on the InGaP/GaAs/Ge or InP materials systems, a significant reduction in materials cost could be enabled by using Si bottom cells. However, direct integration of III-V materials on Si is challenging due to defects related to polar on nonpolar growth and a significant lattice mismatch with direct gap alloys. To this end, we are exploring the growth of InGaP on Si microwire arrays and other alternative geometries for photovoltaic applications. In particular, the high aspect wire arrays offer significant light trapping benefits and the possibility of fast outgrowth of defects when combined with selective epitaxy. Coupled optoelectronic simulations with realistic materials parameters guide the design of such structures in parallel with the development of cell geometries guided by our results in materials growth.
Our initial model of In0.51Ga0.49P/Si wire tandem cell designs suggest that efficiencies ge; 20% are achievable with a relatively low 100 ps bulk recombination lifetime in the top cell as well as minimal performance impact from surface recombination velocities up to 10,000 cm/s at interfaces in the top cell. To extend this model further, additional In1-xGaxP layers of intermediate composition will be added to create a graded lattice parameter buffer to allow for full accommodation of the misfit between the Si and InGaP top cell with a minimum population of threading dislocations. Refractive indicies of these materials are generated from ellipsometric measurements of thin films grown on GaP(001) substrates and interpolation with available literature data. All other materials parameters will be generated by appropriate interpolation with known alloy compositions. Further improvements to the model can be made by adding the effects of threading dislocations on mobility and minority carrier lifetime explicitly. Comparisons to literature values of threading dislocation density in planar and microstructured materials can be then used to predict device performance in these geometries in the best and worst case scenarios.

Organic photovoltaics (OPVs) are a promising technology for inexpensive solar applications. These devices, which use a donor and acceptor material to convert photons to electricity, are generally engineered to have nanostructured bulk heterojunction (BHJ) morphologies. Controlling the morphology is crucial to the performance of the OPV because it must be optimized for the transport of both excitons (bound excited states) and charges, which have conflicting criteria. Developing a way to quickly and efficiently funnel excitons to the interface between the donor and acceptor materials should enhance device performance. Given compatible energy levels, excitons can jump from one material to another by a long range Foerster resonance energy transfer (FRET) mechanism; therefore, by manipulating FRET, it is possible to control where excitons go. By placing a FRET energy acceptor at the interface of a donor and acceptor, the excitons can be preferentially directed to the interface to improve the generation of charge. By simulating nanostructured organic semiconductor films designed for FRET, we demonstrate a strategy for eliminating losses due to small exciton diffusion lengths while improving charge collection pathways, which would lead to significant improvements in efficiency. We will present data on bi-continuous nano-morphologies that are representative of standard bulk heterojunctions and show how FRET will allow us to relax criteria so that a wider range of morphologies will be suitable for OPVs.

The search for efficient photovoltaic devices based on cheap solution-processable conducting polymers has spurred a massive research effort geared toward the development of new materials and strategies for controlled nanoscale morphologies. Much of this new research has been focused on poly-alkyl thiophene (e.g. P3HT)-based systems, which have served as prototype materials for research into the relationships between structural elements of semiconducting polymeric materials that govern their light-matter interactions. While a wealth of new information on photophysical processes has emerged from a variety of thin-film photoluminescence measurements, one of the key limitations inhibiting identification of the direct structure-property relationships is the significant nanoscale structural heterogeneity present in these bulk materials. Spectroscopic examination of individual, isolated nanostructures that form the substructures of photovoltaic active layers has served to elucidate the connection between molecular-scale polymer chain interactions and the photophysical properties that govern important photovoltaic processes such as charge generation, transfer, transport, and trapping.
The P3HT nanofiber is a self-assembled collection of π-stacked lamellar sheets, in which the individual polymer chain backbones are oriented orthogonally to the crystal growth axis. For this reason, we have used P3HT nanofibers, being quasi 2-dimensional crystalline model systems with well-defined structure, along with time- and polarization-resolved photoluminescence (TPRPL) spectroscopy to correlate photophysical properties directly with interchain (along the π-stack axis) and intrachain (along the polymer conjugation axis) electronic communication. Using a model system (nanofibers), and a novel spectroscopic technique (TPRPL) which we have designed specifically to take advantage of the directionality and different characteristic time scales of excitonic and polaronic processes, the molecular-scale structural properties that limit photovoltaic efficiencies and their dependence on experimentally controlled polymer properties (regioregularity, molecular weight and polydispersity) and solvent processing conditions have been identified. Insights gained and techniques developed from studies of polythiophenes are now be applied to new semiconducting polymer materials, and continue to be of use for piecing together the complex phase diagrams that describe them, and for assessing the viability of these materials for OPV applications.

Nano- or microstructured semiconductors can enhance light absorption and decrease minority carrier recombination in photovoltaic applications while minimizing materials consumption.¹ Close spaced vapor transport (CSVT) is a promising method for reducing the cost of III-V materials like GaAs by enabling growth from a solid source at atmospheric pressure without toxic and pyrophoric gas phase precursors. CSVT should also provide a route to low-cost growth of high quality nano- or microstructures when a sufficient understanding of nucleation, growth mechanisms, and the thermodynamic driving forces in the system is obtained.
We present initial results of selected area epitaxial nano- and microstructure using CSVT on single-crystal GaAs substrates. Substrates were prepared by depositing SiO₂ on GaAs by electron beam evaporation, patterning the surface via contact photolithography or nanoimprint lithography, and subsequently etching with HF to reveal GaAs beneath the SiO₂ mask. Growth by CSVT on (100) and (111) GaAs patterns results in rectangular and hexagonal microstructures, respectively, as a result of the favored growth directions on each crystal facet. Both structure types provide pyramidal top morphologies which should enhance light coupling. Little to no crystal growth is observed on the mask, indicating that nucleation on the amorphous SiO₂ is unfavorable compared to the single crystal GaAs. Studies exploring the growth mechanism and temperature dependence of microstructure morphologies are ongoing.
These GaAs structures are then studied using non-aqueous photoelectrochemistry, spectral response, and optical reflectance measurements to understand the relationship between optical and electronic properties.² Further characterization of crystalline quality using transmission electron microscopy and photoluminescence will be presented. Nanoimprint lithography allows structure size reduction for tuning of materials utilization and light collection. These nanostructures may also be of utility for GaAs-on-Si heteroepitaxial growth for controlled misfit termination within the GaAs nanostructures.
(1) Ritenour, A. J.; Levinrad, S.; Bradley, C.; Cramer, R. C.; Boettcher, S. W. ACS Nano2013, 7, 6840-6849.
(2) Ritenour, A. J.; Cramer, R. C.; Levinrad, S.; Boettcher, S. W. Appl. Mater. Interfaces2012, 4, 69-73.

ZnO has been considered as suitable material for ultraviolet light-emitter and X-ray scintillator due to its wide bandgap of 3.3 eV at room temperature, ultrafast recombination process (recombination lifetime of sub-nanosecond), and large exciton binding energy of 60 meV. Moreover, ZnO nanowires have shown opportunity of high-quality material synthesis via facile methods and excellent waveguiding properties. Intensive studies on optical properties of ZnO nanowires have been done from the perspective of fundamental understanding of exciton recombination in ZnO nanostructures. However, optoelectronic device applications of ZnO nanowires require reliable method of controlling luminescent properties of ZnO. There have been several reports on change of luminescence intensity of near band edge emission of ZnO nanowires by doping and ion irradiation. Doping and ion treatment tends to induce formation of defects acting as nonradiative recombination centers. Meanwhile hydrogen, an ubiquitous element and usually amphoteric dopant in semiconductors, can be a candidate of dopant for controlling radiative recombination properties of ZnO because hydrogen acts as donor only in ZnO. Here we present time-resolved and time-integrated photoluminescence spectroscopic results of hydrogen plasma treated ZnO nanowires.
ZnO nanowires were synthesized by catalyst-free metalorganic chemical vapor deposition on silicon substrate. Subsequently, as-grown ZnO nanowires were treated by hydrogen plasma under different plasma power. The time-integrated photoluminescence spectra of as-grown and hydrogen plasma treated ZnO nanowires were obtained with Heminus;Cd continuous wave laser (325 nm). The time-resolved photoluminescence results were obtained by time-correlated single photon counting method with frequency tripled Ti:Sapphire laser (266 nm). All photoluminescence measurements were performed at various temperatures in the range from 10 to 200 K. Hydrogen plasma treatment results in remarkable increase in near band edge emission of ZnO nanowires without noticeable visible emission and change of morphology. Particularly, neutral donor bound excitonic emission was enhanced whereas free excitonic emission was suppressed. Time-resolved photoluminescence spectroscopy reveals correlation between radiative recombination lifetime of excitons in ZnO and plasma power. Additionally, temperature-dependent time-resolved photoluminescence study enables us to distinguish the effect of doping and that of surface modification on exciton recombination dynamics of ZnO nanowires.

GaN is one of the critical materials used in energy-related applications, including optoelectronic devices, high speed transistors, high power electronics and biocompatible devices. Free-standing GaN films have multiple unique advantages such as strain relieve, light extraction, defect reduction and flexibility. An ultrathin high quality GaN film can also be integrated to silicon for photonics coupling applications. Fabricating ultrathin GaN film is impossible through the traditional laser lift-off, chemical or mechanical methods, which usually yield no thinner than ~10um GaN film with certain surface damages. In this work, ultrathin GaN freestanding films ~100nm are demonstrated by engineering the nanostructure of the sacrificial layer for optimally controlled laser absorption, while retaining high crystal quality in the defect-filtered GaN film. Specifically, the average In composition and effective bandgap in the sacrificial layer is critical for effective laser absorption, which is examined by XRD, PL and ellipsometry. The bandgap of the sacrificial layer is engineered from 1.55eV to 2.3eV with digital alloy structures, which also serves as a heat flux trap to confine the shock wave. Nd-YAG laser with a frequency doubler is used for the laser lift-off process. The optimal combination of laser fluencies and irradiation counts are studied. An elastomeric protection or rigid film cover can be used for easy handling and transfer. This technique can be used to fabricate freestanding GaN films with low defects and ultrathin thickness, especially at

Beside Anti-Reflection-Coatings (ARC) the texturing of a solar cell&’s front surface is an effective measure to reduce its reflectivity. Another positive effect which can be achieved by texture is a light guidance sideways within the cell, resulting in increased optical path lengths. This light guidance is getting more and more important for decreasing cell thickness. For monocrystalline silicon pyramidacally textured surfaces can easily be fabricated by treating the substrate in an alkaline solution. However, this simple wet chemical alkaline process is not effective for getting the uniformity of pattern and high optical trapping.
In this study, the photo resist resin (PR) was nanoimprinted as an etching mask to investigate the uniformity of honeycomb pattern and light trapping. This technology allows high resolution and high-throughput replication of a given pattern in a polymer using a moulding process. PR spin coated silicon substrate was imprinted with PDMS mold and heated to make structured PR mask as an etchant protective layer. Acidic (HF:HNO3: CH3COOH) was used as wet chemical etching medium. After these process the surface reflectivity was lowered under 10% and this value was good one compared to the reflectivity of silicon monocrystalline wafer untextured surface

We performed time dynamics studies of excitation and emission processes for single silicon quantum dots with varying temperature and excitation power. Individual silicon quantum dots were fabricated in three steps. Using electron beam lithography and reactive ion etching, as the initial steps, matrix of undulating silicon nano-walls were created on a silicon wafer. Nano-size silicon crystals were then formed inside the upper part of the walls by applying self-limiting oxidation. The well-defined position of single dots fabricated by this method allows performing repeatable single dot studies on them.
Optical absorption cross section and exciton lifetime of single silicon quantum dots were measured, using an avalanche photo diode connected to a micro-photoluminescence (PL) setup. Based on a single photon counting technique, the PL intensity traces of single dots were recorded under pulsed excitation (lambda;ex= 405 nm), and at different power densities. The slope of the PL rise time rate versus excitation flux gives absorption cross section values in the range of 10-14 cm2. The PL decay of individual dots is of mono-exponential character with the lifetime in a microsecond range at room temperature. Temperature dependent measurements of the PL emission lifetime exhibit an increase of the PL decay time by decreasing the temperature to 70 K. This behavior is similar to that observed in ensemble measurements as for porous silicon and is usually ascribed to a decrease of non-radiative recombination. Finally, based on the measured absorption cross section and considering spatial distribution of the emitted light the PL quantum efficiency is estimated around 10 % for individual nanocrystals at room temperature.

We summarize recent advances in making solar cells based on solution-synthesized, solution-processed, inorganic nanoparticles that offer quantum-size-effect-tuned bandgaps for multi-junction cells.

Next generation photovoltaic technology has shifted toward solution-processable materials due to the inherent reduction in fabrication costs and freedom of deposition. Inorganic semiconductor nanomaterials can be synthesized and solution processed to form thin-film absorber layers for solar cells. Devices based on these materials are typically built on top of transparent conducting oxides (e.g. ITO, FTO) and then completed with an evaporated top metal contact. Under these pristine conditions, only the active layers are truly solution processed. Here, we report on a top-to-bottom solution processed solar cell by utilizing precursor solutions for each layer of a working solar cell device on non-conductive glass substrates. Cadmium chalcogenide nanocrystals are known for their solution processability and utility in device fabrication after a CdCl2 annealing treatment to promote both ligand removal and grain growth. Likewise, solution precursors for ITO and Au can be similarly annealed to produce high-quality films for use as electrodes in solar cells to replace conventional methods. Preliminary data show that under simulated one sun, all-solution processed devices produced an open-circuit voltage (Voc) of 580+24 mV, a short-circuit current (Jsc) of 1.61+0.4 mAcm-2, and an efficiency (eta;) of 0.53% while pristine contacts gave Voc=408+18 mV, Jsc=13.1+1.0 mAcm-2, and eta;= 2.1%. The solution processed contacts are characterized by SEM, UV/Vis, XRD, and four-point probe methods, and these results correlated with the overall PV device performance obtained from dark and light JV characterization.

Colloidal quantum dot solar cells (CQDSC) based on lead sulfide (PbS) have drawn much attention due to their ability to deliver a power efficiency of 8%. PbS quantum dots (QDs) exhibit a low and tunable band gap with a high absorption coefficient, thus establishing these QDs as a promising candidate for colloidal quantum dot (CQD) solar cells. CQDSCs consist of a compact/mesoporous TiO2 layer on a transparent conducting oxide (TCO), followed by a PbS QD layer, and finally a thermally evaporated metal which serves as a back contact. In these devices, it is important to control the band gap as well the band position of the QDs to efficiently inject electrons into TiO2 and holes into the metal electrode. However, in the QD layer, both the electron and hole mobility suffers due to the high band gap organic passivating layer around the QDs. The similar energy levels present within the PbS QDs also increase the probability of electron-hole recombination.
In this work, we have carried out experimental and theoretical studies of the effect of surface ligands on band positions in colloidal QDs. We have tuned the band alignment of the PbS QDs through the dipole moment of the passivating ligand. The dipole moment creates an induced electric field, which significantly alters the electronic properties of the individual QDs. The variation of the band gap and ionization potential of the same size PbS QDs treated with ligands consisting of differently functionalized thiophenol molecules were experimentally measured by absorption and photoelectron spectroscopy in air (PESA). Our results show that the ionization potential of PbS QDs, having a diameter of 3 nm, can be tuned from -5.5 eV to -4.8 eV by changing the ligand from nitrothiophenol to methylthiophenol, whereas the band gap varies from 1.25 eV to 1.37 eV, respectively. DFT calculations were performed to evaluate the dipole moment of the ligands, and a comparison of the calculated and experimental results shows that the ionization potential varies linearly with the dipole moment of the ligand. CQDSCs were fabricated and tested using the ligand-modified quantum dots. We have also considered another approach to tune the band gap as well as the band alignment by tuning the composition of ternary PbSe(x)S(1-x). The results show a correlation between band alignment and solar cell performance metrics. We will discuss how the charge mobility and total power conversion efficiency of the solar cell is affected by band alignment.

Quantum dot solar cells from colloidal, inorganic nanocrystals are an emerging PV technology with high potential and have reached a 7.0% independently-verified power conversion efficiency employing a heterojunction device architecture using a wide-band gap metal oxide (ZnO or TiO2), ~400 nm thick 1.3 eV PbS quantum dot layer, and a thin hole-transport layer, MoOx. Further improvement in device efficiency will require understanding of interfacial energetics and kinetics and how photogenerated carriers are collected, either through drift or diffusion currents. A collaborative research endeavor involving institutions of the Center for Advanced Solar Photophysics EFRC was conducted to measure spatially resolved potentials and electric fields across the device stack using scanning Kelvin probe microscopy (SKPM). SKPM uses a conductive AFM probe to directly map surface potentials of a sample (or in our case, a working device cross-section). As the tip scans over the surface, a feedback system nullifies the contact potential difference between tip and sample by applying a DC voltage to the tip, thus providing a spatially resolved work function map with 10 mV energy resolution. Scanning of cross-sectioned ZnO/PbS heterojunction solar cells as a function of applied bias and illumination was accomplished to obtain operating potential and electric field profiles to characterize interfacial energy band alignments and drift/diffusion regions across the device. Initial studies focused on mapping device cross-sections with varying thicknesses of the PbS layer to measure the width of a depletion region formed between ZnO and PbS. It was determined that the depletion width is independent of applied bias and PbS layer thickness, which are not well described by the traditional p-n model applied to the ZnO/PbS heterojunction. Furthermore, capacitance voltage measurements on ZnO/PbS cross-sections show a bias-independent capacitance, emphasizing the lack of formation of a depletion region. Our results are more readily explained by an n-i-n-like energy band alignment, analogous to a p-i-n amorphous silicon solar cell. Here, the intrinsic PbS layer is sandwiched between two n-type semiconductors, ZnO and MoOx, with MoOx possessing a deeper work function than ZnO, where the work function offset between the contacts produces an electric field across the light absorbing PbS QD layer. Replacement of the PbS layer with intrinsic amorphous Si shows similar potential profiles throughout the stack, thus providing strong evidence for the true operating mechanism for these solar cells. Trends in SKPM profiles and current-voltage characteristics were similar for both these structures using either PbS or amorphous silicon as the active layer. These results provide key insight into the optimization of future devices, whereby selection of contacts are crucial to create a strong electric field within the PbS layer for better charge extraction of photogenerated carriers.

Nanocrystal quantum dots (NQDs) have been extensively investigated lately with a hope for realizing next-generation (low-cost, high-efficiency) solar cells. A noticeable photoconversion efficiency of nanocrystal quantum dot solar cells has been achieved, more than 7 % (Nat. Nanotech. 2012) and carrier collection efficiencies over 100 % was demonstrated in working solar cell (Science 2011). Achieving higher efficiencies with enhanced stability in NQD solar cells require understanding and controlling over the surface of NQDs. In this presentation, our recent efforts on a fundamental aspects of efficient nanocrystal quantum dots-based photovoltaics focusing on the surface chemistry of NQDs will be discussed. Shape originated size-dependent stability, defect controlled fabrication processes, and chemical approaches of controlling the carriers will be presented.

The physical properties of a semiconductor are determined both by the properties of the intrinsic material and by the presence of impurities and defects. Control of these defects, which reside either inside or on the surface (abrupt termination of surface bonds) of the semiconductor, determines the performance of semiconductor devices. This is particularly important for quantum dot (QD) solar cells, where the presence of surface defects induces recombination of photogenerated carriers and degrade three important parameters that determine the power conversion efficiency (eta;) of the cell: short-circuit current (J_SC), open-circuit voltage (V_OC) and fill factor (FF).
Here we describe a method to improve the performance of lead sulfide (PbS) QD solar cells through exposure of the QD film to a solution containing metal salts. The halide ions, such as chloride ion, passivate surface lead sites whereas alkali metal ions passivate surface chalcogen sites (or mend Pb vacancies). The simultaneous introduction of both positive and negative ion maintains charge neutrality of QDs. The QD films are exposed to a metal salt solution prior to the ligand exchange procedure, where metal cations and halide anions with small ionic radius have high probability of reaching the QD surface to eliminate surface recombination sites. Compared to the control device fabricated using only the ligand exchange procedure, devices with metal salt treatment show an increased J_SC and FF, accompanied by a distinct reduction in a crossover between light and dark J-V characteristics.

Colloidal quantum dot solar cells are promising due to the rapid increase in efficiency over the past years. Most of those improvements can be attributed to enhanced control of the quantum dot surface and film deposition. Meanwhile, the metal oxide component has received considerably less attention. By doping zinc oxide we show that the metal oxide can be equally important. Nitrogen doping reduces the carrier concentration in ZnO by up to two orders of magnitude, and we show that this reduction can be used to supress interfacial recombination in ZnO/PbS solar cells. Furthermore we show that magnesium doping raises the ZnO acceptor levels. Using large-bandgap PbSe quantum dots as the active layer in a ZnO/PbSe solar cell, we could dramatically increase the open-circuit voltage by magnesium doping ZnO. We identify that the sub-bandgap states of ZnO pose a fundamental challenge in designing colloidal quantum dot solar cells and that metal oxides with a cleaner bandgap will lead to more efficient devices.

Tomohiro Nozaki*, Yi Ding, and Ryan Gresback
Department of Mechanical Sciences and Engineering, Tokyo Institute of Technology
2-12-1 Ookayama, Meguro-ku, Tokyo Japan 152-8550.
*Email: [email protected]
Silicon nanocrystals (SiNCs) have drawn keen attention as it has several novel properties mainly resulting from the size-dependent quantum confinement effect such as tunability of optical emission and absorption features. This makes it attractive for device applications including solar cells, transistors, light emitting diodes. Different forms of silicon nanocrystals have been developed to meet the requirements of the vast application fields. Among them, freestanding silicon nanocrystals can form inks allowing for solution processing, which is seen as a promising route to reducing the cost of the semiconductor device manufacturing.
In this paper, highly crystalline SiNCs with mean size of smaller than 6 nm were synthesized from chlorinated silicon precursor (SiCl4) using flow-type, non-thermal VHF (70 MHz) plasma reactor [1,2]. SiNCs, as an electron acceptor, were blended with p-type organic semiconductor materials (P3HT or PTB7) using appropriate solvent, producing stable SiNCs containing inks [3-4]. After spin casting on ITO substrate and evaporation of metal contact, it forms bulk heterojunction (BHJ) type organic/inorganic hybrid solar cells with isolated SiNCs embedded in polymer matrix [5]. Performance was evaluated under AM1.5 solar simulator, resulting short circuit current of 10 mA/cm2, open circuit voltage of 0.55 V, and power conversion efficiency of 2% by now. The electronic band structure and its tunability by size, surface termination; -Cl (as produced), -H (HF treatment), -O (controlled oxidation), and controlled phosphorus light doping is the key to maximizing the power conversion capability of the device. Examining the performance of those devices would offer valuable insights into the electronic band structure of SiNCs and their tunability. Successful fabrication of highly efficient and inexpensive solar cell with SiNCs is the important goal to meet the future energy demand.
[1] L. Mangolini, E. Thimsen, and U. Kortshagen: Nano Lett. 5 (2005) 655.
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[3] R. Gresback, Y. Murakami, Y. Ding, R. Yamada, K. Okazaki and T Nozaki: Langumuir, 29(6), 1802-1807, 2013.
[4] C. Y. Liu, Z. C. Holman, and U. R. Kortshagen: Nano Lett. 9 (2009) 449.
[5] Y Ding, R Gresback, R Yamada, K Okazaki, T Nozaki: Jpn Jounal of Applied Physics, 52, in press, 2013.

The use of inorganic nanoparticles as acceptor materials in photovoltaic devices has seen promising increases in efficiency over recent years, with current records using P3HT as a donor polymer rivaling traditional all organic systems. Using nanoparticles as a replacement for organic acceptors has a variety of advantages including increased mobility, chemical stability, cost of synthesis and the ability of the acceptor to also absorb light alongside the donor polymer. The ability to absorb light is key to increasing photocurrents generated by photovoltaics by covering more of the solar spectrum but also we have recently shown the hole transfer process from the inorganic component to polymer to be more efficient than the converse electron transfer process.(1)
One of the major challenges with these so called hybrid solar cells is the difficulty with processing both the inorganic and organic components from a common solvent. This problem is traditionally overcome by the use of solublising capping ligands bound to the surface of the nanoparticles. These ligands allow the materials to be solution processed together, but they also directly inhibit charge generation. We reported an in-situ method for the production of the inorganic component within the polymer after co-deposition by utilising the decomposition of metal xanthate complexes.(2) This allows for a one-pot ligand free generation of hybrid heterojunctions and has been shown to give superior charge generation and higher power conversion efficiency in photovoltaic devices when compared to an equivalent ex-situ capped nanoparticle system.(3)
As well as a ligand free heterojunction the use of xanthate precursors also opens up the possibility of synthesising a variety of sulfide materials that have not previously been fully explored, including several relatively non-toxic materials in comparison to the current materials of choice, mainly cadmium sulfide. We present the synthesis of non-toxic heterojunctions of bismuth sulfide and antimony sulfide for use in hybrid photovoltaics. The control of the morphology of these heterojunctions is studied by tuning of the xanthate precusors and through varying processing parameters. These heterojunctions are characterised using a variety of techniques including SEM, TEM and XRD and spectroscopic studies of charge photogeneration using transient absorption spectroscopy (TAS) are used along with the fabrication of photovoltaic devices.
(1) Dowland, S. A.; Reynolds, L. X.; MacLachlan, A.; Cappel, U. B.; Haque, S. A. Journal of Materials Chemistry A 2013, 1, 13896.
(2) Leventis, H. C.; King, S. P.; Sudlow, A.; Hill, M. S.; Molloy, K. C.; Haque, S. A. Nano Letters 2010, 10, 1253.
(3) Reynolds, L. X.; Lutz, T.; Dowland, S.; MacLachlan, A.; King, S.; Haque, S. A. Nanoscale 2012, 4, 1561.

Since the global oil crisis of 1973 that followed the Fujishima-Honda paper of 1972 showing that under certain conditions semiconductor photoelectrodes could be used to photolytically split H2O into H2 and O2, there have been large international research and development efforts to produce cost effective solar fuels, such as H2 from H2O splitting or renewable carbon-based fuels from the photoreduction of CO2 with H2O, as occurs in biological photosynthesis. Parallel efforts to develop cost effective solar electricity from photovoltaic cells were also accelerated The intensity of both of these R&D efforts have waxed and waned over the ensuing 4O years depending upon political leadership, societal/political perceptions of the severity of the energy security problem and global climate change (and their respective relative priority), and the state of the national economy of nations funding R& D in this area. Over this rather significant time period there have been many importance advances in the associated and highly interdisciplinary science and technology, new innovations and concepts, and some persistently difficult technological barriers to penetrate that have prevented commercialization, especially in producing cost effective solar fuels. The scientific and technological history, advances, new concepts, persistent barriers, and present status of viable solar fuels and solar electricity production will be discussed.

In this paper, we discuss results of a comprehensive study of PbS quantum dots (QDs) using photoelectron spectroscopy and other spectroscopic techniques, such as scanning tunneling spectroscopy (STS), absorption spectroscopy, and inverse photoelectron spectroscopy, to probe both the energy of the valence and conduction bands of QDs. The spectra are recorded as a function of the QD size, surface chemistry, doping, substrate, and density. Thus far, we have determined that the valence band energy is independent of the QD size (3 and 8 nm diameter) and substrate (Au, Pt, and MoO2) when probing the sample with photoelectron spectroscopy. Although consistent with other photoelectron studies, the conclusions are in contrast with STS literature results, which find that the valence band does shift with respect to the Fermi energy when the QD size is changed. Therefore, studies are underway to directly compare the photoelectron spectroscopy and STS techniques, which will be influential in understanding the similarities and differences between these two spectroscopic techniques. Furthermore, the results will provide insight into the energy level alignment that occurs at interfaces that are relevant for the use of PbS QDs in optoelectronic devices such as QD solar cells.

A synthetic approach has been developed which results in Cu_xIn_yS₂ quantum dots (QDs) possessing localized surface plasmon resonance (LSPR) modes in the near infrared (NIR) frequencies. Importantly, these LSPRs stem from native cation vacancies acting as acceptor sites as determined through Rutherford backscattering spectroscopy (RBS), rather than being born of gradual oxidative leeching effects post-crystal formation. This renders the LSPRs synthetically tunable, as well as stable over many weeks of storage. In order to investigate the hypothetical benefits of near-field plasmonic effects centered upon photovoltaic absorber material, non-plasmonic counterparts (“twins”) were developed based on a modified literature method as an experimental control. Scanning transmission electron microscopy (STEM) with energy-dispersive X-ray spectroscopy (EDS) mapping, absorption spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy were used to verify the nearly identical nature of the species of Cu-In-S-containing QDs, differing only in their stoichiometries. Simple QD-sensitized solar cells (QD-SSCs) were assembled which show an 11.5% average increase in incident photon conversion efficiency (IPCE) in the plasmon-enhanced devices with respect to the non-plasmonic controls. We attribute this bolstering of performance to augmented absorption stemming from near-field “antenna” effects in the plasmonic Cu_xIn_yS₂ QD-SSCs. This study represents the first of its kind: direct interrogation of the influence of plasmon-on-semiconductor architectures on IPCE in photovoltaic systems.

Solution-processed colloidal nanocrystals are considered promising materials for photocatalysis, as control over topology, shape and size of nanocrystals offers the ability to design self-standing nanometric reactors (1-3). Particular attention has been devoted to nanocrystals comprising epitaxial interfaces between semiconductors able to absorb solar light and noble metals able to catalyze chemical reactions. Nanocrystals based on metal chalcogenide semiconductors, like CdSe and CdS, decorated with noble metals like Au and Pt have been successfully demonstrated in recent experiments to reduce water into molecular hydrogen upon illumination with solar light (4-8). Achieving efficient photocatalytic water splitting will however require several further steps in materials development, including the simultaneous presence of an oxidizing electrode and the ability to store several photoexcited charges inside the same nanocrystals to activate multi-electron reactions.
With this aim, we tested as a novel platform for photocatalysis octapod shaped CdSe/CdS nanocrystals (9-13). Taking advantage of the different chemical reactivity of the various crystal planes, we were able to controllably decorate the nanocrystals with Pt nanoclusters at selected locations. We measured the optical properties of such heterostructured nanocrystals, both with cw and ultrafast spectroscopy techniques. Our results demonstrate charge separation and storage of multiple photoexcited electrons in the Pt tips. A rational base is provided for the development of branched nanocrystals for photocatalysis, where each branch could host a different metal tips to catalyze both reduction and oxidation within the same nanocrystal, to connect nanocrystals among themselves or even support them over a substrate.
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13. M. Zanella et al., Chem. Commun. (Camb.) 47, 203-205 (2011).

The treatment and desalination of water has become inextricably linked to energy resources; for example, nineteen percent of California&’s electricity goes to water-related uses. Therefore, it is vitally important to develop desalination approaches that reduce primary energy use. We have developed a nanofluidic device for in situ desalination of water that is integrated directly with a thin-film solar cell and driven by coupling to the solar cell rather than an external voltage source. This integrated device utilizes the electric fields generated in a solar cell to tune the thickness of the electric double layer (EDL) that arises in nanopores during fluid flow. The EDL can be used to selectively reject ions attempting to pass through the nanopores in order to achieve desalination. In our experiments, we control the electric double layer by light that impinges on the solar cells through which an array of nanopores has been fabricated. We have also performed multiphysics simulations to model the electric fields that exist within the pores, particularly at the P-N junction, by shining light on the device submerged in the ionic solutions. We will discuss both modeling and simulations of the EDL tuning and selective ion rejection in nanopores, as well as experimental demonstrations of this concept.

This talk will cover two related topics: (1) progress in using single quantum dot spectroscopic methods to interrogate excitonic and multiexcitonic processes of nano crystals in the visible and short wave infrared (SWIR), on substrates and directly in solution, and (2) progress in quantum dot structures and device architectures leading to increased efficiencies in photovoltaic and light emitting applications.

The current description of the nanoparticle surface often invokes a static population of trap states associated with dangling bonds or defects. These trap states strongly impact interfacial charge transfer and, as a result, nanocrystal performance in lighting, photovoltaic and photocatalysis applications. However, our recent atomic resolution scanning transmission electron microscopy movies show that the surface of semiconductor nanocrystals is in a state of constant fluctuation during imaging. We propose that this is also the case during UV-excitation.The current description of the nanoparticle surface often invokes a static population of trap states associated with dangling bonds or defects. These trap states strongly impact interfacial charge transfer and, as a result, nanocrystal performance in lighting, photovoltaic and photocatalysis applications. However, our recent atomic resolution scanning transmission electron microscopy movies show that the surface of semiconductor nanocrystals is in a state of constant fluctuation during imaging. We propose that this is also the case during UV-excitation.¹ Although atomic motion under an electron beam is not unexpected, the images were obtained at low acceleration energies that impart energy to the nanocrystal similar to that of a UV photon. A fluid surface is further made plausible in the context of the drastically lowered melting temperature of nanoparticles, where surface pre-melting has been predicted to occur. The origin of broad spectrum emission from ultrasmall nanocrystals was elucidated using dynamic STEM imaging in conjunction with density functional theory. We showed that the atomic fluctuations can result in white light emission from ultrasmall CdSe nanocrystals.¹ In this talk, dynamic STEM movies will be presented that show fluxionality is not limited to ultrasmall nanocrystals, but is present for all sizes of nanocrystals. As the diameter of the nanoparticle is increased, a stable crystalline core manifests while a nanometer thick region of fluctuation persist at the surface. The effect of chemical composition (CdS or CdTe) and surface composition (oleic acid, phosphonic acid ligands and ZnS) has on the fluxional surface will be presented as well as possible origins of surface fluxionality including: dynamic surface ligands, surface oxidation and surface pre-melting.²
1. Pennycook, T.J.; McBride, J.R.; Rosenthal, S.J.; Pennycook, S.J.; Pantelides, S.T. Dynamic Fluctuations in Ultrasmall Nanocrystals Induce White Light Emission Nano Lett. 2012, 12 (6), 3038-3042.
2. McBride, J.R.; Pennycook, T.J.; Pennycook, S.J.; Rosenthal, S.J. The Possibility and Implications of Dynamic Nanoparticle Surfaces ACS Nano 2013, 7 (10), 8358-8365.

The ability of metal nanoparticles to capture light through plasmon excitations offers a unique opportunity for enhancing the optical absorption of semiconductor materials via plasmon-to-exciton energy transfer. This process, however, requires that the semiconductor component is electrically insulated to prevent a “backward” charge flow into metal and interfacial states, which causes a premature dissociation of excitons. Here we demonstrate that such an energy exchange can be achieved on the nanoscale by using non-epitaxial Au/CdS core/shell nanocomposites. These materials are fabricated via a multistep cation exchange reaction, which decouples metal and semiconductor phases leading to fewer interfacial defects. Ultrafast transient absorption measurements confirm that the lifetime of excitons in the CdS shell (tau; asymp; 300 ps) is much longer than in conventional, reduction-grown Au/CdS heteronanostructures. As a result, the plasmon energy can be efficiently utilized by the semiconductor component without undergoing significant energy losses due to non-radiative carrier decay, an important property for catalytic or photovoltaic applications. The reduced rate of exciton dissociation in the CdS domain of Au/CdS nanocomposites was attributed to the non-epitaxial nature of Au/CdS interfaces associated with low defect density and a high potential barrier of the interstitial phase.

Photoresponsive materials that adapt their morphologies, growth directions, and growth rates dynamically in response to the local incident electromagnetic field would provide a remarkable route to the synthesis of complex 3D mesostructures via feedback between illumination and the structure that develops under optical excitation. Recently, we have reported a novel template-free photo-assisted electrodeposition technique in which ordered mesoscale Se-Te amorphous alloy structures with 50-500 nm scale features evolve spontaneously via dynamic feedback with a polarized monochromatic light source, requiring only mW/cm^2 power densities. These inorganic mesostructures exhibited phototropic growth in which lamellar stripes grew toward the incident light source, adopted an orientation parallel to the light polarization direction with a period controlled by the illumination wavelength, and showed an increased growth rate with increasing light intensity. The lamellar stripes were found to have a period of approximately lambda;/2, where lambda; is the wavelength of light in the solution, and demonstrated aspect ratios greater than 10. [1] In contrast to conventional lithography, this technique utilizes time-dependent variations in the illumination conditions to directly write complex 3D structures in a bottom-up fashion, with morphology features depending on illumination wavelength, angle, polarization and intensity. Here, we present a simple quantitative nanophotonic model to describe the dynamic feedback process and gain insight into the physical mechanisms governing the temporal evolution of the system. We find that film morphological evolution occurs via faster growth at locations on the film surface with high local optical intensity. Our results are well described by a model that postulates a locally modified electrochemical driving force for film growth proportional to the light-induced quasi-Fermi separation. This model leads to complex lamellar, spiral and branched structures that closely resemble those seen in experiments. Additionally, we demonstrate a Monte Carlo algorithm for interface motion with rate coupled to intensity calculations from full wave finite difference time domain electrodynamic simulations, which allows us to visualize the formation of the phototropic structures. We will discuss the use of this quantitative model to predict formation of complex three-dimensional structures and early stages of growth on patterned surfaces that support resonant modes.
[1] B Sadtler, S P Burgos, N A Batara, J A Beardslee, H A Atwater, N S Lewis, “Phototropic growth control of nanoscale pattern formation in photoelectrodeposited Se-Te films”, 2013, Proceedings of the National Academy of Sciences, in press.

Nanowires are filamentary crystals with a tailored diameter in the nanoscale range. Their especial morphology and dimensions render them especially interesting for the study of low dimensional phenomena and for opto-electronic and energy harvesting applications. In this presentation, I will explain how to synthetize high quality nanowire based heterostructures with molecular beam epitaxy for photonic applications. In particular, I will show two of our latest results obtained with self-catalyzed GaAs nanowires: 1) the formation of extremely high quality GaAs quantum dots in an AlGaAs shell to be used in quantum information technology and 2) the advantages of nanowires in next generation photovoltaics which constitute a potential way to overcome the Shockley-Queisser limit in efficiency.

Graphene has attracted considerable research interest for fabrication of various devices such as transparent conductive electrodes, photovoltaic cells, field-effect transistors, sensors, and supercapacitors because of its unique electrical and optical properties. In this work, we have synthesized graphene oxide (GO) for fabrication of photosensor. The synthesized GO was structurally characterized using X-ray diffraction, SEM, and FT-IR spectroscopy. The XRD and SEM studies indicate that graphite has been exfoliated during synthesis. The current-voltage characteristics of the GO/p-Si junction were studied under dark and different light intensities. The I-V characteristics were used to estimate various junction parameters such as ideality factor, barrier height. It was observed that photocurrent, photocapacitance, and photoconductivity of GO/p-Si junction increases with increase in light intensity. For example, the maximum photocurrent of ~ 4.5 × 10-6, 1.9 × 10-5, and 3.4 × 10-5 A was observed for light intensity of 20, 70, and 100 mW/cm2, respectively. The photocapacitance of the diode increases sharply from 2.15 × 10-11 to 2.55 × 10-11 F after switching on the light with intensity of 50 mW/cm2. After switching off the light, the photocapacitance decreases to almost its original values. The initial rise in the capacitance suggests generation of more free charge carriers at junction on illuminating the diode, whereas the decay of the photocapacitance after switching off the light is due to trapping of the charge carriers in the deep levels. The results suggest that GO has high potential for photosensor applications.

Photocatalysis has drawn great attention due to its significant role in solving energy and environmental issues related to wastewater treatment and purification. Over the past decades, intense research in photocatalysis has been explored on semiconductor materials as prime candidate of photocatalysis because of their favorable combination of electronic structures that involve charge transport characteristics, bandgap, and excited state lifetimes. When considering semiconductors, many have reported on semiconductor particles, taking the advantage of the quantum size effect. Despite so, photocatalysis degradation and water splitting efficiency remain low, mainly due to the fast electron-hole recombination process than that of the transportation process. Great efforts to improve the photocatalytic performance via the formation of wide range of semiconductor heterostructures, such as semiconductor/metal and semiconductor/semiconductor, have aimed to promote the electron-hole pair separation and transportation as well as enhancing the functional time. To understand the electronic properties, the influence of orientation cannot be neglected. In this study, noble Au was deposited on perovskite oxide (100)-, (110)-, and (111)-oriented BiFeO3 (BFO) thin film by using pulsed-laser deposition, followed by post-thermal annealing under oxygen atmosphere to produce Au nanoparticles. We chose BFO because it is a visible light active photocatalytic. High-resolution X-ray diffraction and transmission electron microscopy indicate that Au (111) nanoparticles were embedded on the (100)-, (110)-, and (111)-oriented BFO surface. The in-plane epitaxial relationship between Au and (100)-, (110)-, and (111)-oriented BFO is [10-1]*Au//[010]*BFO, [10-1]*Au//[1-10]*BFO, and [10-1]*Au//[1-21]*BFO, respectively. Importantly, the energy levels and band bending of BFO were found to be orientation-dependent, thus affecting the recombination and transfer efficiency of the electron-hole pairs. Our results show that the orientation difference led to significantly variation of photocatalytic performance. A comprehensive study of the intricate interplay between charge, orbital, and spin degrees of freedom in semiconductor/metal by means of ultrafast pump-probe and X-ray photoelectron spectroscopy (XPS) has been performed. This architecture offers us a better insight into the interfacial charge transfer process in semiconductor/metal for photocatalysis.

Thin films involving an oxide heterojunction is a proven strategy for the catalysis of water splitting reactions using solar light. Hematite (α-Fe2O3) and tungsten oxide have suitable properties to achieve such heterojunctions. A major limitation of this strategy is the short charge carrier diffusion length in hematite. Ultra-thin films were implemented to address this low conductivity issue. Nevertheless, such ultrathin films do not absorb light efficiently. The present study explores light trapping strategies to increase the optical path length of photons in hematite.
Micelle suspensions were developed to obtain thin films composed of microspheroids array with a tungsten oxide core and a hematite nanometric overlay. This bottom-up approach allows a fine control of the spheroids dimensions at the micrometric to submicrometric scale. By tuning the spheroids dimensions, different photonic regimes were observed experimentally.
Using the Finite Difference Time Domain method, light propagation inside the microstructures was quantitatively simulated. The simulation results were coupled to an analysis of the photoelectrochemical response of the films. Experiments and simulation showed good agreement and bring important insights in the relationship between the light interaction with the microstructure and the photoanode performances.

We will report on the development of nanoparticle solids comprising a combination of semiconductor and metal nanocrystals, which could be deployed in printable electronics applications. Such hybrid morphology enables a strong plasmonic effect on optical properties of semiconductor nanocrystals, which could be precisely controlled via the film architecture. In particular, plasmon-exciton interactions in the film can be tuned to provide either enhanced light-emitting or light-harvesting characteristics. Meanwhile, a stable and inert semiconductor matrix should facilitate the compatibility of fabricated films with existing device platforms. Alternatively, semiconductor nanocrystal films doped with a small fraction of metal nanoparticles could be utilized in field effect transistors. In this case, metal domains can selectively inject n- or p-type carriers in response to electric field of the gate electrode.

Raman spectroscopy is a powerful tool to detect and identify analyte on the surfaces; however Raman cross-section being very small it is often a daunting task using common photo detectors. Surface enhanced Raman spectroscopy (SERS) can however be used to enhance the Raman scattering intensity by several orders of magnitude due to interfacial surface plasmon resonance of a metal nanostructure of appropriate dimension. Fabrication of highly reproducible SERS substrates which can provide reliable signal enhancements have attracted a great deal of attention. Beyond their application in Raman spectroscopy nanostructured electrodes and interfaces can enhance light absorption in solar cells due to more efficient light harvesting as well as an increased light path. Ultrathin films of an active layer material (C60) deposited on nanostructured plasmonic Ag pillars show increased absorption as a result of increased light concentration inside the structure. In this study nanoimprint by melt processing is used to fabricate polyacrylonitrile (PAN) pillars. Plasmonic nanostructured electrodes with various geometries and dimensions have been fabricated by deposition of Ag on PAN pillars. SERS measurements show significant signal enhancement on nanostructured samples in comparison to planar Ag substrates due to local electromagnetic field enhancement. SERS enhancement factor of 106 have been observed with Ag nanopillars of 300 nm height, 75 nm radius and 200 nm pitch. In addition to SERS enhancement of analyte molecules the cycalization of PAN into amourphous carbon at the PAN/Ag interface can be observed. Under potential deposition (UPD) of thallium was used to determine the absolute surface area which agrees well with geometrical calculations based on SEM images. Finite Difference Time Domain (FDTD) method is used to simulate electric field distribution inside the samples and provides confirmation of the experimental results. Depending on the dimensions of the nanopillars and their spacing enhancement in the electric field will change. At low coverage Ag dewets the polymer pillars creating a non continuous film which consists of Ag islands. Raman intensity of the thin coverage Ag pillars is less than fully coated pillars and in addition to being far less reproducible. The aforementioned SERS studies show charge delocalization (partial Ag-to-C60) at the interface of C60/Ag. Absolute intensity of the Raman signal for thiophenol (Tp) monolayers on both Pt ( non-enhancing) and Ag (enhancing) coated nano-pillars is collected; using instrumental and collection factors, surface area (Tp coverage), and Tp scattering intensities SERS enhancement factor were determined. Complete SERS study on C60/Ag interface and discussion on the observed Raman peaks will be presented.

Boron subphthalocyanine (SubPc) is a promising donor material for organic photovoltaics, having one of the highest reported open circuit voltages among bilayer OPVs when coupled with C60. Recently, C70 has attracted more attention than C60, largely due to a broader optical absorption spectrum, which leads to a higher current at relatively high voltages. The structure and electronic properties of SubPc derivatives on C70-fullerene were explored using density functional theory (DFT) with added Van der Waals interactions. Total-energy calculations were used to elucidate the initial adsorption derivatives on low index surfaces of C70. The dependence of the electronic and optical excitations on the interface morphology is studied within the Green&’s-function GW and Bethe-Salpeter approaches. Insights gained from these calculations, and how they can be used to improve device efficiency are discussed.

We have measured resonance Raman excitation profiles of the longitudinal optical (LO) phonon fundamental and its first overtone for organic ligand capped, wurtzite form CdSe nanocrystals of three different sizes (2.6 to 3.2 nm diameter) dissolved in chloroform. The absolute differential Raman cross-section for the fundamental is much larger when excited near the first excitonic absorption maximum than for excitation at shorter wavelengths where the absorbance is higher. That is, the quantum yield for resonance Raman scattering is reduced for higher-energy excitation. In contrast, the photoluminescence quantum yield is relatively constant with wavelength, but consistently shows slightly more structure than the absorption. The optical absorption spectrum and the resonance Raman excitation profiles and depolarization dispersion curves are reproduced through quantitative simulations with a model for the energies, oscillator strengths, electron-phonon couplings, and dephasing rates of the multiple low-lying electronic excitations. The Huang-Rhys factor for LO phonon in the two lowest excitonic transitions (1S_3/21S_e and 2S_3/21S_e) is found to lie in the range S = 0.05 to 0.25. The strong, broad absorption feature about 0.5 eV above the lowest excitonic peak, typically labeled as the 1P_3/21P_e transition, is shown to consist of at least two significant components that vary greatly in the magnitude of their electron-phonon coupling, with one of them having much stronger coupling than the lower excitonic states.
We have also explored the origin of the low-frequency shoulder on the CdSe LO phonon band, usually assigned as a “surface optical” (SO) mode. In keeping with previous reports, we find that the frequency and relative intensity of the low-frequency shoulder change only slightly when the organic ligands are replaced by a ZnSe shell. Atomistic calculations of the resonance Raman spectra of near-spherical CdSe nanocrystals have been carried out by using an empirical force field to calculate the equilibrium geometries and phonon frequencies, and a simple effective mass envelope function model for the electron and hole wavefunctions to generate the Huang-Rhys factors for each phonon. Surface reconstruction leads to a greater frequency dispersion of optical-type phonons in energy-minimized structures compared to structures with the unrelaxed bulk geometry. However, in neither case do the low-frequency components of the LO-type phonon band show significant localization on surface atoms. We propose that the ubiquitous low-frequency shoulder of the LO phonon band is not a surface mode but rather arises from changes in both the selection rules and the form of the phonon modes in nanocrystals relative to the bulk material.

Hybrid bulk heterojunction (BHJ) solar cells have attracted considerable attention as a promising third generation solar cell technologies. It combines the solution processing feasibility of polymer semiconductors and environmentally stable inorganic parts with high electron mobilities and tunable band gaps and energy levels. Tremendous advancement has been made for hybrid BHJ solar cells involving CdSe:P3HT as the active layer and the PCE as high as 3.13% has been achieved by using low banddap semiconducting polymer PCPDTBT and CdSe tetrapods. However, most of the investigations were focused on the conventional device architecture. The inverted device configuration, using a high work function anode, offers the benefit of enhanced stability. Here, we report the study of inverted hybrid BHJ solar cells with CdSe quantum dots (QDs) (~7 nm) blended with P3HT as the active layer, ZnO nanoparticles (NPs) as the electron transport layer (ETL), and PEDOT:PSS or MoO₃ or both as the hole transport layer (HTL). By using Finite-difference time-domain method (FDTD) and Transfer-matrix method (TMM), we optimized the entire device configuration and the thickness of ETL using transfer-matrix method (TMM). We evaluated the device performance in terms of using different CdSe QD loadings in the active layer, making the ETL using sol gel method with static and dynamic annealing as well as spin coating of ZnO colloid NPs, and making the HTL with PEDOT:PSS or MoO₃ or both. . We found that under the same conditions, devices with the double HTLs consisting of 10 nm MoO₃ and 10 nm PEDOT:PSS show the best performance. Shunt resistance and series resistance were obtained by fitting Shockley equation and were further used to analyze the device. The relationship between Voc, FF, Jsc and light intensity indicates that carrier recombination mechanism is a bimolecular process. The strategy with double HTLs could be extended to other hybrid BHJ solar cells in order to achieve high solar to electronic energy transfer efficiency and long device life.

Morphological control of heterostructure nanocrystals (HNCs) has enabled the manipulation of photogenerated charge recombination dynamics. In solar harvesting applications such as photovoltaics and photocatalysis, effective separation of electron and hole allows for increased external quantum efficiency or overall turnover frequency. Energy gap of semiconductor nanomaterials in strong quantum confinement regime can ben engineered through design of the dimension of respective structure; for instance, in core/shell nanostructures, the core size and shell thickness both affect how electron and hole behave in the heterostructure. Using this nano-engineering motif, hetero-nanostructured quantum dots can prolong electron-hole recombination which results in enhanced photocatalytic activity.
In this presentation, we report our study that examined the photocatalytic activity of PbSe/CdSe/CdS core/shell/shell heterostucture nanocrystals (HNCs) with varying morphologies such as sphere, pyramid and tetrapod and varying size and composition of gold and silver tips. The photocatalytic dye reduction study revealed that the morphology change leads to significant difference in photocatalytic activity. For instance, tetrapod-shaped HNCS have shown higher photocatalytic activity than that of pyramidal and spherical HNCs. The difference can be explained by the subtle morphological changes. In addition, metal tips turned out to affect the photocatalytic activity significantly. The interesting activity difference can be attributed to work function and surface reactivity changes in different metal tips. For instance, Au, Ag, or Au/Ag core/shell tips were grown at the tips with systematically controlled size and shape, and the photocatalysis revealed that the high Fermi level in Au compared to Ag or Au/Ag likely lead to electron transfer into the HNCs, thereby enhancing the photocatalytic activity.

We use quasi-steady state photoinduced absorption (PIA) spectroscopy to study charge generation in blends of poly-3-hexylthiophene (P3HT) with PbS quantum dots as a function of excitation energy. We find that, per photon absorbed, the yield of photogenerated holes present on the conjugated polymer increases with pump energy, even at wavelengths when only the quantum dots absorb. We interpret this result as direct evidence for hot hole transfer in these conjugated polymer/quantum dot blends, and show that our results are consistent with the wavelength dependent internal quantum efficiencies measured for hybrid polymer/quantum dot solar cells.

For charge carriers in light-harvesting semiconductors to drive the water splitting reaction in photoelectrochemical cells, the charge transfer and transport must successfully compete with fast relaxation mechanisms. Here we have designed an ultrafast setup based on the optical Kerr effect to resolve subpicosecond high-resolution spectral dynamics of nanostructures during photocatalysis. The setup high UV-gating efficiency and wide detection bandwidth allows to study how growth and treatment conditions modify the various surface and mid-gap states and consequently photocatalyst performance. To verify this, we use a high-performing - external quantum efficiency of 80 % in the UV - ZnO nanowire photoanode conformally coated with 1 nm of TiO₂ by atomic layer deposition, and compare it to its bare ZnO NW counterpart sample that displays poor photocatalytic activity. To complement the ultrafast emission studies, a series of steady-state photoluminescence and mid-IR absorption spectroscopy under various electrode potential are carried out for both samples. Such systematic studies provide insights about the nature of the various competing mechanisms such as carrier-carrier scattering, trapping and radiative recombination. Our results reveal contrasting bulk behavior, together with modified trapping rates following the 1 nm deposition of TiO₂ on ZnO.

Exhibiting control over the charge carriers in semiconductor nanocrystals is essential for tailoring structures to specific applications. High surface to volume ratios coupled with the strong geometric confinement of the exciton in nanocrystals lead to significant interaction of excited charge carriers with the surface. These surface interactions typically occur within the first 10 picoseconds after excitation and dictate the ultimate fate of the excited state. The surface contains an ensemble of trap states that can localize excited charges and provide relaxation pathways for non-radiative recombination, thereby reducing the efficiencies with which nanocrystals fluoresce. Here ultrafast fluorescence upconversion spectroscopy is utilized to demonstrate that charge carrier trapping at the nanocrystal surface can be tamed by synthetically inducing a radial variation in the chemical composition of the nanocrystal. This graded alloy structure is designed to decouple the exciton from the nanocrystal surface in order to minimize surface effects on the excited state. The charge carrier dynamics for the structural evolution of graded alloy CdS_xSe_1-x nanocrystal heterostructures have been examined as a function of chemical composition. It was found that charge trapping is significantly reduced as sulfur is incorporated into a predominantly CdSe core to form a sulfur-rich type-I graded alloy shell. Our findings demonstrate the potential for highly efficient nanocrystal emitters that are independent of their surface, which would allow for their incorporation into a diverse range of applications without experiencing adverse effects arising from dissimilar environments.

Color tunability, narrow emission bandwidth, high brightness and simple solution processability make semiconductor nanocrystals excellent candidate materials for optoelectronics and related applications. While the high photoluminescence (PL) quantum yield of core/shell heterostructure shows much promise, the type I straddling band offset may be less than ideal in device applications such as light-emitting diodes (LEDs). The large band gap shell prohibits physical access to the core and poses significant barrier for carrier injection. We have designed, fabricated and characterized LEDs incorporating double heterojunction nanorods (DHNRs) as the electroluminescent layer. These DHNRs consist of both type II (staggered) band offset that allows efficient injection of electrons and holes and type I band offset that confines the electrons and holes for high light emission efficiency. The double heterojunction can provide independent control over the electron and hole injection processes while maintaining high PL yields with narrow linewidths. Effects of DHNR structure/morphology, hole/electron transport layers, and DHNR film assembly on device performance will be discussed.

We investigated scintillation properties of LaF3:Ce/CdSeS nanocomposites for radiation detection applications. The light output of scintillation of CdSeS were much enhanced in the present of LaF3:Ce, which can be ascribed to an efficient resonance energy transfer (RET) from LaF3:Ce to CdSeS quantum dots in the nano-composite scintillators. The luminescent intensity of LaF3:Ce/CdSeS nanocomposite scintillators was further enhanced by tuning the size of CdSeS quantum dots (QDs) to increase the spectrum overlap between the absorption of CdSeS quantum dots and excitation of LaF3:Ce nanoparticles. This RET-based approach could have implication for developing novel functional nanomaterials used in radiation detectors.

Colloidal semiconductor nanocrystals (NCs) are an important class of functional materials with potential applications in optoelectronics. In a variety of semiconductors, Si is environmentally benign compared to IIminus;VI and IVminus;VI semiconductors which contain toxic elements. For better performance of optoelectronic devices made from Si-NCs, precise control of the energy state structure is indispensable. In addition to the size, shape and surface chemistry, impurity doping is a principal parameter to determine the energy state structure. However, research on impurity-doped Si-NCs, especially impurity-doped colloidal Si-NCs, has been still very limited. In order to control the HOMO-LUMO gap of Si-NCs by impurity doping, we have developed colloidal Si NCs in which n- (P) and p-type (B) impurities are simultaneously doped.[1] The fabrication process we have been developing is as follows. First, we sputter-deposit Si and phosphosilicate and borosilicate glasses simultaneously, and anneal the mixture films. The annealing results in the formation of B and P codoped Si-NCs in silicate matrices. By removing silicate matrices by hydrofluoric acid etching, codoped Si-NCs are extracted. In codoped Si-NCs, only a small fraction of B and P atoms are doped in the core and electrically active, which form acceptor and donor states, respectively, in the bandgap. On the other hand, majority of doped B and P atoms exist on the surface of Si-NCs and not electrically active. The surface high B and P concentration layer control the surface chemistry of Si-NCs and, surprisingly, codoped Si-NCs are dispersed in polar solvents without any surface functionalization [2]. The purpose of this work is engineering the HOMO-LUMO gap of colloidal Si-NCs by the combination of quantum size effects and impurity doping. We control the diameter of codoped colloidal Si-NCs from 1 to 14 nm. These NCs are dispersed in methanol without agglomeration and exhibit efficient size-tunable PL covering a very wide range from 1.8 to 0.85 eV. The PL energies are always 0.3-0.4 eV lower than those of undoped Si-NCs with comparable sizes. Remarkably, when the size is larger than about 5 nm, the PL appears below the bandgap energy of bulk Si crystal (1.12eV). The significant low energy shift of the PL by codoping indicates that the PL arises from optical transitions between donor and acceptor states. The PL lifetime is also strongly affected by impurity doping. The PL lifetime of codoped Si-NCs is always shorter than that of undoped Si-NCs detected at the same energy. The shortening of the lifetime by doping suggests that carriers are further localized at the impurity sites in codoped Si-NCs, which results in the enhancement of non-phonon quasi-direct optical transitions . [1] H. Sugimoto, et al., J. Phys. Chem. C 116, 17969 (2012). [2] J. Phys. Chem. C 117, 6807 (2013).

Colloidal CuInSexS2-x quantum dots (QDs) represent good candidates for solar cell applications due to their direct and near-optimal, yet tunable, band-gap, high absorption coefficient, and relatively low toxicity. Electric transport in coupled arrays of CuInSexS2-x QDs has not yet been investigated even though this is critical to high performance photovoltaics. Herein, for the first time, we report functional CuInSexS2-x QD-FETs with top source-drain and bottom gate architecture. These FETs demonstrate p-type behavior initially but ambipolar transport after treatment with Cd2+ ions. Treating the QDs films with 1,2-ethanedithiol (EDT) increases the mobility up to four orders of magnitude, resulting in some of the highest ambipolar mobilities ever attained for QD films (specifically, hole mobility reached 0.28 cm2 V-1 s-1 with an electron mobility of 0.11 cm2 V-1 s-1). These high mobilities are related with decreasing the distance between QDs by film ligand exchange, as has been observed in EDT-treated films of other QD materials, but is also related to novel material properties of CuInSexS2-x QDs. When we switch the source and drain contacts from silver to indium, and then briefly anneal the films, the FETs show n-type behavior. The ability to finely tune the Fermi level across the band gap, and respectively change the transport behavior from p-type to ambipolar to n-type, makes CuInSexS2-x QDs attractive for thin film solar cells and the high values of mobility we have achieved are very promising.

Plasmonic nanostructures based on transparent conducting oxides (TCOs) hold promise as active materials in energy applications including tunable smart windows and photovoltaics. However, reliable control over free carrier concentration in these materials via doping is complicated by compensation effects due to defects. Batch-to-batch, doped TCO nanocrystals can show significant variation in dopant activation, and hence in the optical response that is dominated by localized surface plasmon resonances (LSPR). By combining Vis-NIR spectroscopy and Drude modeling of LSPRs with X-ray core-level spectroscopy, we were able to investigate the link between variations in the optical properties of our nanocrystals and their underlying doping and defect profiles. Our results indicate that changes in the LSPR due to differences in the activation of tin species correlate strongly with the relative degree of tin surface segregation. Nanocrystals with more tin near the surface are observed to have lower activation and thus, lower LSPR frequencies, for a given doping level. These results implicate near-surface defects, possibly oxygen interstitials or surface trap states, as compensating agents in ITO nanocrystals. A better undestanding of these defects should lead to greater synthetic control over the optical properties of TCO nanocrystals.

Vacancy-doped semiconductors, and in particular vacancy-doped copper chalcogenide nanocrystals (NCs), have undergone extensive investigation in the last few years.[1-3] Their high tendency to self-doping (namely vacancy-doping) gives rise to a large number of holes in their valence band.[4] As a result thereof, vacancy-doped copper chalcogenide NCs can display localized surface plasmon resonances (LSPRs) whose resonance wavelength can be fine-tuned as a function of the charge carrier density. [5-6] This represents an additional means of LSPR tunability with respect to plasmonic metallic nanoparticles and thus, makes them interesting materials for nanoplasmonics. Apart from LSPR modulation by means of charge carrier density control, NC morphology is expected to play a significant role in the LSPR frequency as well. From the family of vacancy-doped copper chalcogenides, copper telluride NCs have been the ones the least investigated thus far. The poor size, morphology and monodispersity control attained via one-pot syntheses, together with their low stability, have been the main reasons for that.
In this communication we will show that copper telluride NCs with well-defined morphologies can be obtained by performing a Cd2+/Cu+ cation exchange reaction on pre-formed CdTe NCs. This allows for a careful analysis of the NIR plasmonic properties of Cu2-xTe NCs of various shapes. Our results reveal that charge carrier localization in the valence band of Cu2-xTe NCs may be a non-negligible effect and that the effect of localization of holes can be a common property for the whole class of vacancy-doped copper chalcogenide NCs.[7]
References
[1] Zhao, Y.; Pan, H.; Lou, Y.; Qiu, X.; Zhu, J.; Burda, C. J. Am. Chem. Soc. 2009, 131, 4253-4261.
[2] Kriegel, I.; Rodriguez-Fernandez, J.; Como, E. D.; Lutich, A. A.; Szeifert, J. M.; Feldmann, J. Chem. Mater. 2011, 23, 1830-1834.
[3] Luther, J. M.; Jain, P. K.; Ewers, T.; Alivisatos, A. P., Nat. Mater. 2011, 10, 361-366.
[4] Scotognella, F.; Della Valle, G.; Srimath Kandada, A. R.; Zavelani-Rossi, M.; Longhi, S.; Lanzani, G.; Tassone, F., Eur. Phys. J. B 2013, 86: 154.
[5] Dorfs, D.; Härtling, T.; Miszta, K.; Bigall, N. C.; Kim, M. R.; Genovese, A.; Falqui, A.; Povia, M.; Manna, L., J. Am. Chem. Soc. 2011, 133, 11175-11180.
[6] Kriegel, I.; Jiang, C.; Rodriguez-Fernandez, J.; Schaller, R. D.; Talapin, D. V.; da Como, E.; Feldmann, J. J. Am. Chem. Soc. 2012, 134, 1583-1590.
[7] Kriegel, I.; Rodriguez-Fernandez, J.; Wisnet, A.; Zhang, H.; Waurisch, C.; Eychmüller, A.; Dubavik, A.; Govorov, A.O.; Feldmann, J., ACS Nano 2013, 7, 4367-4377.

PbSe quantum dots (QD) in the size range of 2-6 nm generate multiple excitons by the absorption of UV photons. However, exploiting multiple exciton generation (MEG) is challenging as the electron-hole pairs produced within the QDs have to be dissociated before Auger recombination. Dissociation is facilitated by the band offset that exists at a clean interface between the QD and the material the QDs are in contact with. In this research a vacuum spray process is employed to deposit coatings of surfactant-free QDs. The spray process is integrated with laser ablation to embed the QDs in inorganic films. Using these process composite films of PbSe QDs embedded PbTe have been fabricated. The high electron and hole mobility of PbTe facilitates the transport of both carrier types to corresponding electrodes. Transmission electron microscopy (TEM) and optical absorption spectroscopy confirmed single crystal nature of the QDs and quantum confinement. IV characteristics show orders of magnitude increase in charge transport properties in PbSe QD-PbTe composite films in comparison to PbSe QD films with butylamine surfactants. For the investigation of multiple exciton generation-dissociation in these surfactant-free QDs, the hybrid structures of ITO/PbSe QD-PbTe/Al were fabricated. Photocurrent generation has been investigated with a wavelength tunable source in the range of 532 nm-1060 nm. For QDs with average band gap of 0.7 eV this wavelength range corresponds to a transition from a single exciton generation to 3 exciton generation. Enhancement in device current during this transition indicates effective multiple exciton dissociation. Comparison of wavelength dependent current for devices with PbSe QD-PbTe composites and PbSe QDs with surfactants will be presented.

Lead chalcogenides (PbSe, PbS and PbTe) are rock-salt structured, IV-VI semiconductor compounds characterized by small effective carrier masses, large exciton diameters, and small band gaps. The gap can even be reduced to zero upon lattice compression. The lattice dynamics of these materials also manifest some intriguing features: A softening of the transverse optical (TO) phonon mode and an anomalous dip of the longitudinal optical (LO) mode at Gamma have been reported for all compounds. The former indicates a near ferroelectric behavior of lead chalcogenides whereas the origin of the latter was traced back to a pseudo Jahn-Teller effect or a near Kohn anomaly [1, 2]. Features described, among others, appoint PbX (X = Se, Te, S) as highly promising materials for optoelectronic and photovoltaic devices as well as templates for thermoelectric applications. Growth of nanostructures (in particular nanocrystals), where the effects of quantum confinement can be exploited, and accurate determination of their characteristics are very actively pursued at the moment.
We have performed ab-initio calculations of the phonon dispersions of <001> - oriented PbSe slabs, of 6 up to 15 layer thickness. Under the quantum confinement model, association of the obtained phonon modes of these low-dimensionality systems with the bulk phonon modes is feasible [3]. The LO mode of the investigated nanostructures was found to blueshift with decreasing layer thickness, in contrast to most nanocrystalline materials, where the Raman active phonon modes shift down in frequency. This blueshift is attributed to quantum
confinement of the LO mode and validates experimental results of Raman spectra for PbSe nanocrystals [3]. In the case of the TO modes, quantum confinement and strain effects were investigated, showing that negative frequencies may be obtained independent of the layer thickness. The latter underlines the near-ferroelectric character of PbSe nanostructures and reveals their instability towards structural phase transitions.
References
[1] O. B. Maksimenko et al., J. Phys. Cond. Mat. 9, 5561 (1997)
[2] O. Kilian et al., Phys. Rev. B 80, 245208 (2009)
[3] J. Habinshuti et al., Phys. Rev. B 88, 115313 (2013)

Lead chalcogenide (IV-VI) nanocrystal quantum dots (NQDs) have currently received special interest because their tunable optical properties have useful opto-electronic applications such as photovoltaic device. This study describes the synthesis and characterization of PbSe/PbS core/shell nanocrystal quantum dots (NQDs) by controlling the size of core and thickness of shell. Optical properties are obtained by absorption and photoluminescence (PL) spectroscopy and core/shell composition are analyzed by high-resolution transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). PbSe core and PbSe/PbS core/shell NQDs are fabricated in quantum dot solar cell to study photovoltaic properties.

A great many of structural and optical properties of thin films depend on the size of the grains. The grain size depends on deposition method, because grain growth occurs during deposition of the thin films. A lot of different deposition method can be used for film fabrication. In this the paper, we used the successive ion layer adsorption and reaction technique (SILAR) chemical deposition method to fabricate high quality PbS thin films and to derive grain size function for a variety of deposition cases. The films were deposited by alternately dipping of a substrate into the aqueous solutions containing ions of each component in this method. To produce the films, the lead acetate solution (0.05 M) and thioacetamide (0.06 M) were used as cationic and anionic precursors, respectively. Glass substrates were immersed into cationic precursor solution for 20 s, and then rinsed with deionized water for 40 s to remove the unattached ions. Then the substrate was immersed into anionic precursor for another 20 s and rinsed again in the deionized water for 40 s to complete one full cycle of operation. The dipping cycle was repeated 15, 20 and 30 times for every thin film. All of the films obtained in different immersion cycles show cubic rock-salt (NaCl) structure. The films had almost the same lattice parameter values. This lattice constant values were very similar to bulk PbS. But the preferred orientation changed from the (110) direction to the (200) direction with increasing dipping cycles. In the same time, grain size increased from 16 nm to 111 nm for the (200) direction. Both the increase in the average crystal size in a film and the change in the average crystal orientation can be accepted as the results of grain growth. This case was observed in SEM images also. Moreover, the composition of the thin films was determined by energy dispersive X-ray spectroscopy. The transmission of the thin films was characterized by a UV-Vis spectrometer from 400 nm to 1100 nm. The film fabricated in 15 cycles showed the highest percentage in transmission 15.12 % at the wavelength of 600 nm while the films fabricated in 20 and 30 cycles had lower transmission value at 3.8% and 0.77%, respectively. The evaluation results can be explained by a correlation between light transmission and the size and shape of the grains. It was determined that the energy band gaps of the thin films shifted from 2 eV to 1.5 eV as the film thickness increased. The decrease in band gap may be attributed to the increase in grain size. Moreover, the refractive index increased, extinction coefficient decreased, real and imaginary dielectric constants increased with increasing immersion cycles at the wavelength of 600 nm. These observations could be useful when considering the thin films for electronic and optical applications.

Metal-organic frameworks (MOFs) are composed of organic linkers and coordinating metals that are self-assembled to form a crystalline material with permanent and tunable nanoporosity. Their synthetic modularity along with the inherent structural order of MOFs allows for their utility as new functional electronic materials. Recently, we showed that an electrical insulating MOF can be made conducting through the introduction of guest molecules in the pores. However, there has been little attention paid toward identifying and creating semiconducting MOFs by virtue of modification of the organic linker. Herein, we propose a series of new conjugated organic linkers that are capable of forming the same one-dimensional infinite metal-oxide secondary building units (SBUs) as the well-known MOF-74. The SBU allows the linkers to form a continuous π-π stacking network that should enable charge transport in an analogous fashion to organic semiconducting materials while retaining the highly porous nature of the MOF. The structural and electronic properties of the proposed linkers and MOFs have been computationally modeled using a non-empirically tuned range-separated functional. The theoretically motivated tuning of the exchange functional leads to significantly improved results for excitation energies compared to conventional DFT methods. This work demonstrates that the electronic properties of the MOFs (i.e. optical gap) can be readily tuned by modification of the organic linker. Our findings reveal that the proposed systems are capable of harvesting light in the visible spectrum due to their relatively small optical gaps and appreciable oscillator strengths. In addition, we show that these systems have favorable orbital alignments with known electron acceptors to facilitate charge transfer. The predicted properties are in good agreement with experiment (i.e. UV-vis absorption spectra) demonstrating the power of this computational approach for MOF design.

Maximizing the enhancement of solar absorption promises to dramatically lower the cost of solar cells. However, the conventional upper limit of solar absorption enhancement is derived from idealized materials with weak intrinsic absorption and cannot apply to real materials. Here we elucidate the true maximal solar absorption enhancement in semiconductor materials by leveraging on an intuitive model, coupled leaky mode theory. We also point out that key to maximize the solar absorption is to engineer the optical modes to be leaky. By following the principle, we present a design that can enable a layer of 10 nm thick a-Si or 50 nm thick CdTe or 30 nm thick CIGS to absorb 90% sunlight above the bandgap, which is one order of magnitude less in volume than any of the existing most sophisticated designs. The design is close to our predicted superabsorption and may be better than the conventional Lambertian limit.

Semiconductor quantum dot (QD) arrays emerged recently as promising structures for the next generation of high efficiency intermediate band solar cell (IBSC) [1], due to their ability to facilitate the formation of mini-bands. The quantum coupling effect, which exists between states in QDs of an array, influences its electronic and optical properties. So far the great efforts have been devoted to study the vertically coupled QD arrays [2-5]. We present here a method based on multi-band kp Hamiltonian combined with periodic boundary conditions, applied to predict the electronic and optical properties of InAs/GaAs QDs based lateral QD arrays. Formation of the IB in all cases was achieved via delocalisation of the electron ground state (e0). Such model is used to examine the 1st Brillouin zone of QDs placed in the periodic in-plane lattice. Those information are then used to predict relevant parameters for the drift-diffusion transport model. We show that the IB in a laterally coupled QD-IBSC is more robust against external electric field of PN junction than in vertically coupled arrangement. The IB band width of sim;50 meV, formed under Ez=0 in vertically coupled QD array, is lost when the external electric field of Ez=200 KV/cm is applied. However, under the same electric field conditions across the PN junction, the IB in the laterally coupled QD-array arrangement remains largely intact, due to the lateral periodicity being unaffected by the external field in zminus;direction. Our theoretical results for the efficiency of InAs/GaAs QD arrays under unconcentrated light, X=1, are in the range 18.9minus;21.3% [6] and in very good agreement with experimental efficiency of 18.9% for the similar structures [4]. For light concentration changed from X=1 to X=1000, the efficiency increased from 22% to 33% for QD array made of smaller closely packed QDs (b = 10). We have shown that the efficiency of analysed QD arrays is almost independent of whether the IB is pre-filed or not. This suggests that the electron dynamics between various bands in InAs/GaAs QD arrays is such that do not support the formation of the IB related quasi-Fermi level under illumination [7], i.e., that the InAs/GaAs QD array works in the QD solar cell regime, mainly influenced by IB-VB gap, rather than benefiting from dynamics related to IB and contribution from IB to CB transitions (IBSC regime). Still, smaller QD array shows systematically a larger efficiency than array made of larger QDs (b = 20 nm) under all regimes considered, despite smaller IB-CB gap [46 meV in former vs. 58 meV in later.
[1] A.Luque, A.Marti, Phys.Rev.Lett. 78, 5014 (1997)
[2] R. Oshima, et al, Appl. Phys. Lett. 2008, 93, 083111 (2008).
[3] S.M. Hubbard, et al Applied Physics Letters 92, 123512 (2008).
[4] S. Blokhin, at al, 43, 514 (2009).
[5] S.Tomic, et al, Appl.Phys.Lett. 93, 263105 (2008)
[6]S.Tomic,T.Sogabe,Y.Okada, Progress in Photovoltaics, in press (2013)
[7] S.Tomic, Phys.Rev.B 82, 195321 (2010)

We have investigated the light absorption properties of gallium nitride (GaN) and silicon (Si) nanorod (NR) array structures using Finite-Difference Time-Domain (FDTD) method. FDTD is a numerical analysis tool for modeling the interaction of light with the material and it is based on the solution of time dependent Maxwell equations in the time domain with space discretised on a mesh. In our study, we conducted FDTD simulation dependent on the geometry of NR structures and wavelength of the light. We observed that the light absorption increases with increasing spacing between NRs for both GaN and Si even when the filling factor was considered, which must be attributed to the enhanced light trapping or multiple scattering effects. We also studied the tailoring of absorption of specific wavelengths of light when the dimension of the NR was carefully adjusted. In addition, the light absorption profile in the NR as a function of wavelength was explored. The details of FDTD simulation parameters and its results will be presented in the meeting.

Photovoltaic technologies have been on the rise since a few years now. Nevertheless, enhancing the efficiency of solar cells with respect to energy conversion and costs remains a challenge. Despite various concepts, silicon is still the leading material for solar energy applications based on its properties, established technologies, and its availability. In order to increase the conversion efficiency of silicon based materials, nanostructures such as nanowires were proposed, e.g. by Kempa et al. [1], Kelzenberg, et al. [2] and Garnett & Yang [3]. These studies show evidently, that light interaction can be tailored by rational control over nanowire composition, geometry, alignment and configuration.
Based on this, we focused our studies on the optical absorption characteristics of bottom-up synthesized silicon nanowires with a radial p-n junction and with diameters ranging from 100 to 200 nm. Utilizing the well established gold catalyzed vapor-liquid-solid (VLS) method, p-type silicon nanowires were grown in a hot-wall CVD-chamber at 475°C based on a SiH4/B2H6 gas composition. Subsequently, nanowires were overgrown with a n-type shell using SiH4 and PH3 as process gasses. For accompanying electrical studies, individual core/shell nanowires were horizontally arranged with selective metal contact to core and shell material similar to Ref. [1]. In dependence on the illumination intensity an alternating photo-current can be observed with the known I-V characteristics.
In order to study the spectral optical behavior more in detail, various nanowire configurations were realized. This includes epitaxial growth of vertically aligned arrays on <111> silicon wafers, non-epitaxial arrays e.g. on SiO2 or sapphire, and horizontal alignment with varying density. Furthermore, VLS bottom-up synthesis allows to alter parameters such as diameter distribution, interspacing, nanowire geometry, doping, and core/shell thickness ratio readily. Spectral absorption measurements were done in transmission and reflection mode utilizing several optical setups. Based on nanowire configurations and synthesis parameters the optical behavior will be discussed in detail with emphasis to solar cell applications.
References:
[1] T.J. Kempa, J.F. Cahoon, S.-K. Kim, R.W. Day, D.C. Bell, H.-G. Park, and C.M. Lieber, PNAS 109 (2012) 1407
[2] M.D. Kelzenberg, S.W. Boettcher, J.A. Petykiewicz, D.B. Turner-Evans, M.C. Putnam, E.L. Warren, J.M. Spurgeon, R.M. Briggs, N.S. Lewis & H.A. Atwater, Nat. Mater. 9 (2010) 239
[3] E. Garnett & P. Yang, Nano Lett. 10 (2010) 1082

Three-dimensional (3-D) nanostructures have demonstrated promising potential to boost performance of photovoltaic devices primarily owning to the improved photon capturing capability. Nonetheless, rational design guidelines for 3-D nanostructured solar cells with the optimal optical and electrical performance are of paramount importance and still need to be further developed. In this work, regular arrays of 3-D nanospikes (NSPs) were fabricated on flexible aluminum foil with a roll-to-roll compatible process. The NSPs have precisely controlled geometry including periodicity and height which allow systematic investigation on the geometry dependent optical and photovoltaic device performance with experiments and device modeling. Intriguingly, it has been discovered that the power conversion efficiency of an amorphous-Si (a-Si) photovoltaic device fabricated on optimal geometry of NSP can be improved by as much as 43%, as compared to its planar counterpart. Furthermore, large scale flexible NSP solar cell devices have been fabricated and demonstrated as well. These works not only have shed light on the design rules of high performance nanostructured solar cells, but also demonstrated a highly practical process to fabricate efficient solar panels with 3-D nanostructures.

Solar cells based on semiconductor nanowires are under intense research and development. Unique optical and electrical properties make nanowires a versatile candidate for next generation multi-junction solar cells. Here we report GaAs nanowires with axial p-i-n junctions grown by selective area growth. Simulation shows that the axial junction design has great advantages over radial junction counterpart due to smaller junction area and flexibility in control of junction depth and doping concentration. Since nanowires are prone to surface recombination, the diameter also has a significant influence on the device performance. In this study we compared solar cells made from 100nm and 300nm nanowires and revealed large diameter is essential to mitigate effect of surface states while space charge limited transportation was observed in fully depleted thin wires. Effect of junction depth was also investigated. Devices with 150nm deep junction achieved almost one order of magnitude higher Jsc than a 600nm deep junction did. The best device showed 21.21mA/cm2 Jsc, 0.511V Voc, 0.608 FF leading to efficiency of 6.56%. Highest Voc of 0.65V was obtained which is among the highest reported values so far to the best of our knowledge. Cathodoluminescence measurements indicated non-radiative recombination dominates in heavily doped p-type emitter region. Voc as high as 0.72V was obtained when illuminated by 10mW 850nm laser spot. These results demonstrate that GaAs nanowires are good candidate for high-efficiency low-cost thin film solar cells and also open up great opportunities for multi-junction solar cells comprising mismatched materials.

Semiconductor metal oxide nanowires composed of earth abundant metal oxides are technologically important materials for future energy conversion and storage devices, and for physical, chemical and biological sensors. Highly anisotropic materials such as nanowires often outperform the commonly used isotropic polycrystalline or particulate films in applications that place complex and multifunctional demands on the materials because nanowires contain two different length scales (small diameter, large length) that can be independently tailored to match the characteristic lengths of disparate physical processes. Despite these advantages, the applications of metal oxide nanowires are limited because conventional synthesis methods cannot achieve their scalable, rapid, controllable and economical synthesis. Here, we present versatile flame synthesis methods for the growth of various semiconductor metal oxide nanowires, such as W18O49, WO3, γ-Fe2O3, and ZnO nanowires, WO3 nanotubes, and MoO3 nanobelts. The controllability of the flame synthesis methods over the morphology and composition of the metal oxide nanowires are studied, which is of crucial importance for the practical use of flame synthesis in the production of nanowires for diverse applications. For example, the morphology of α-MoO3 can be controlled from single to branched to flower-like nanobelt arrays, by simply varying the flame equivalence ratio, the source temperature, the growth substrate temperature, and the material and morphology of the growth substrate. Moreover, flame synthesis methods offer great controllability over the composition of the metal oxide nanowires, and are capable of growing various ternary oxides, doped oxides, and heterostructures containing several oxides, such as Cu3Mo2O9 nanowires, Co-doped TiO2 nanowires, CuO/MoO3 core/shell nanowires, Co3O4 nanoparticle-decorated CuO nanowires. With the demonstrated good controllability, we have successfully synthesized WO3/W-doped BiVO4 core/shell nanowire arrays on fluorinated tin oxide (FTO) substrate and studied their photoelectrochemical water splitting performance as a photoanode. The WO3/W-doped BiVO4 core/shell NW photoanode generates a photocurrent of 3.1 mA/cm2 at a potential of 1.23 VRHE under simulated sunlight, which is the highest produced by any BiVO4-based photoanode without any oxygen evolution catalysts. The critical advance in this work is the use of a conductive WO3 NW array to overcome the intrinsically poor charge transport of BiVO4 without compromising light absorption. These results highlight the key role that engineering of nanowire morphology and composition will play in future energy conversion materials, and point towards a general strategy for the removal of performance-limiting inefficiencies in other promising materials.

One dimensional (1D) semiconductor nanowires (NW&’s) have garnered much interest due to their promising applications as building blocks for micro-electromechanical devices (MEMS) and miniaturized optoelectronics. Furthermore, they represent platforms for the study of size dependence of thermal conductivity, magnetism, and electronic structure. In particular, the high surface to volume ratio of NW devices is beneficial for solar to fuel conversion due to a higher reactive site density and increased light absorption relative to planar wafers. Semiconductor alloys, such as InxGa1-xP (0le;xle;1), can be compositionally tuned to control their electronic properties such as band gap and band alignment for photoelectrochemical (PEC) applications. However, these materials are typically only accessible through techniques such as metal organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) that are too low yield and high cost to be scalable and of industrial importance. Here, we present a facile method for the solution phase synthesis of In(x)Ga(1-x)P NW&’s that can be compositionally tuned across the entire In:Ga stoichiometric range. The In(x)Ga(1-x)P NW&’s are characterized through optical (photoluminescence, absorption), structural (powder X-ray diffraction, high resolution transmission electron microscopy) and spectroscopic (electron energy loss spectroscopy, elemental dispersive spectroscopy) methods. The NW&’s are in-situ doped p-type with zinc. Next, these NW&’s are processed into flexible photocathodes that can harness a range of incident solar photon energies for PEC solar to fuel conversion. The NW photoelectrode quantum efficiencies (QE) are comparable to that of planar wafers yet utilize three orders of magnitude less material.

Solar energy utilization is an attractive option for new energy technology and economic development. Our research is the formulation of catalyst materials for solar production of hydrogen from water. Titanium(IV) oxide has been explored for water splitting; however, a major challenge is that titanium(IV) oxide can only absorb UV light. Visible light absorption can be increased by metal ion or anion doping by creating interband states. Most dopant protocols lead to deposition of dopant ions throughout the solid, and interfacial deposition has received very little attention. We have developed a method to selectively attach transition metal ions to the surface of titanium(IV) oxide nanorods. The present study demonstrates that Cr(III), Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and bimetallic systems such as Cr(III)-Co(II), Fe(II)-Ni(II), Co(II)-Cu(II) etc. were coordinated to the surface of oleic acid capped TiO2 nanorods (NRs) by post-synthesis method without any phase or morphology transformation. Metal ion loading could be carefully controlled, and we show a titration curve for addition of single transition metal ions to the nanorod surface. The materials were characterized with UV-visible spectroscopy, transmission electron microscopy, elemental analysis and powder X-ray diffraction.

We have used femtosecond transient absorption microscopy to investigate the charge carrier dynamics in Si nanowires. The structures, which are 30-50 microns in length and 30-200 nm in diameter, are excited by a femtosecond pump pulse that is focused to a diffraction limited spot by a microscope objective, exciting a localized region of the structure. The charge carrier dynamics are then probed by the change in transmission of a second probe pulse that is spatially overlapped with the excitation pulse, but delayed in time. The pump-probe microscope has a lateral resolution of 400 nm and a time resolution better than 500 fs. Photoexcitation of the nanowires produces a free carrier population of electrons and holes that results in a transient photobleach of the wire. The decay of the photobleach has contributions from both population relaxation, as well as carrier migration away from the excitation spot. We have used this microscope to characterize the surface recombination, and how it is affected by strain and doping. By controlling the lateral position of the probe beam relative to the pump, we can excite a structure in one location and detect the arrival of carriers at new location, thus enabling a direct visualization of the charge carrier motion.

The use of nanostructured materials in photovoltaics (PV) has been pushed by the demand for 3rd generation i.e. highly efficient and low-cost solar cells. The bottom-up design of nanowire (NW) structures with rational control of key material parameters like chemical composition, morphology and size paved the way towards novel devices and concepts in photovoltaics. Nevertheless, most of these NW solar cell concepts still rely on charge separation in a classical p-n junction, deduced from conventional planar PV technology. Particularly for NW based solar cells, junction formation via doping requires detailed control of the process parameters and remains a major challenge in NW synthesis. Here we report on the application of a new charge separation mechanism beyond the classical junction approach in intrinsic semiconductor nanowires: band gap modulation via mechanical strain.
We focussed on scanning photocurrent microscopy (SPCM) characterization and simulation of the band structure in tapered NWs monolithically integrated into a silicon-on-insulator (SOI) based straining module. NWs are predestined for the exploration of strain-related effects due to the exceptionally high strain values that can be imposed before fracture compared to bulk materials. In particular Ge NWs attracted considerable attention due to their predicted direct band gap nature at high strain levels. Our experimental approach relies on an adjustable strain gradient along a tapered Ge NW, resulting in a gradient in the conduction and valence band edges and thus in effective photoexcited charge carrier separation with an efficiency of ~5%. The fact that no intentional doping is required, possibly enabling high-efficiency solar cells with low Auger recombination rates. The charge separation mechanism, though, is not inherently limited to a distinct material and scalable from single NWs to arrays. Our work establishes a novel class of photovoltaic nano-devices with its opto-electronic properties engineered by size, shape and applied strain.
The possibility of strain-tunable solar cells from cheap and abundant materials which have not been considered for photovoltaic applications so far is believed to foster further research in the field.

At the nanoscale, interfaces and surfaces of materials dominate the performance of electronic and optoelectronic devices. The optical and electrical properties of these interfaces and their states are strongly coupled to how materials are processed. Electron beam induced current (EBIC) analyses can be used to decouple electrical and optical properties, to measure depth profiling of junctions, and to explore how the carrier dynamics change under voltage bias. Separating the response of electrical and optical properties will allow both to be optimized in structures that are engineered for energy harvesting from light. The work presented will focus on the EBIC characterization of the interfaces and junctions of Si nanowire metal insulator semiconductor (MIS) diodes. The Si nanowires in the MIS diodes are etched using the metal assisted chemical etching (MACE) process from a wafer with known doping concentration making these devices an ideal test case for probing the properties of interfaces and junctions without the complexity of doping profiles in nanowires. Probing the separate components of heterostructures in core/shell AlGaAs/GaAs nanowires with EBIC will be also discussed. The presentation will show how this technique can be adapted to study a wide range of devices with energy applications.
Work supported by NSF (DMR 0907381) and the US Dept of Education under the GAANN-RETAIN program (P200A100117). Research carried out in part at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.

Vapor-liquid-solid grown Cu-catalyzed Si microwire arrays have shown great promise in photovoltaic and photoelectrochemical applications. Integration of GaP and other III-V materials on Si wires is a route to increased performance through the enabling of larger open circuit voltages and tandem or multijunction designs. State of the art work in the field, leveraging careful understanding of Si(001) surface preparation and an atomic layer deposition-like nucleation layer growth, has demonstrated the possibility of metallorganic chemical vapor deposition (MOCVD) of GaP on Si(001) substrates with nearly pristine interfaces free of stacking faults, microtwins and anti-phase domains. In this work, we transfer the atomic layer epitaxy (ALE) nucleation layer optimization to the 3-dimensional Si wire surfaces.
Before growth, planar Si(001), Si(011), Si(112) and Si microwire arrays are chemically cleaned using standard techniques to remove organic and metallic surface contaminants. After a high temperature anneal, ALE nucleation is performed 450°C with alternating pulses of triethylgallium (TEGa) and tertiarybutylphosphine (TBP). After heating the sample to 600°C under TBP overpressure, thicker GaP layers are grown with conventional simultaneous supply of both precursors. Undoped 50 nm thick films grown on planar Si are almost perfectly pseudomorphic (3.7% relaxation) as characterized by high resolution x-ray diffraction. Transmission electron microscopy of cross sections reveal that defects are still present in the GaP layers grown on Si microwires, including twins and antiphase domains.

Thin, n-type GaP:Si layers have been grown on p-type Si microwires as a demonstration of a single wire heterojunction solar cell. Realistic materials parameters are used in conjunction with a coupled optical and electrical device simulation implemented in the Synopsys Sentaurus TCAD to assist in the design and interpretation of experimental results.

Indium phosphide nanowires (NW) are a promising material for solar cell applications, due to their low recombination velocity [1], large light absorption [2], and excellent solar spectrum suitability. Recently, it has been shown that in a solar cell based on large diameter <111> InP NW (180nm), an efficiency of 13.8% has been achieved due to increased light absorption [3]. A second path to high efficiency lies in the NW side facets cleaning, e.g. by Piranha etching, which yields an efficiency of 11.1%, using <111> NWs with a volume 4 times smaller of those in Ref.3 [4].
In this work a combination of the two methods will be discussed, since in such manner it is expected that a new record efficiency could be achieved. <111> NWs, though, normally exhibit a mixed crystal phase of Zincblende (ZB) and Wurtzite (WZ), producing several planar stacking faults, which could limit the performance of solar cells. Here we discuss the development of growth of NW in the <100> crystal direction, which show a pure zincblende crystal phase [5], and hence allows fabrication of single crystalline solar cells. Nanowire geometry and impurity doping are investigated and optimized for solar cell devices.
[1] H. Joyce et al., Electronic properties of GaAs, InAs and InP nanowires studied by terahertz Spectroscopy, 2013 Nanotechnology 24 214006
[2] S. L. Diedenhofen et al., Strong Geometrical Dependence of the Absorption of Light in Arrays of Semiconductor Nanowires, ACS Nano 2011, 5 (3), 2316minus;2323.
[3] J. Wallentin et al., InP Nanowire Array Solar Cells Achieving 13.8% Efficiency by Exceeding the Ray Optics Limit, Science 1 March 2013: 339 (6123), 1057-1060
[4] Yingchao Cui et al., Efficiency Enhancement of InP Nanowire Solar Cells by Surface Cleaning, Nano Letters 2013 13 (9), 4113-4117
[5] U. Krishnamachari et al., Defect-free InP nanowires grown in [001] direction on InP (001), Appl. Phys. Lett. 85, 2077 (2004)

Metal catalyst-assisted vapor-liquid-solid mechanism can be used to grow large areas of nanowires with compositional and doping control in either axial or core-shell geometries [1]. Here, vertical arrays of Si nanowires (NWs) were grown at 900 °C from SiCl₄/H₂/N₂ mixture using 100 nm Au nanoparticles randomly dispersed on p+-Si(111) substrates. The density of NWs was ~0.1 mu;m^-2. The axial p-i-n junctions were formed by the sequential growth of 4 mu;m long B-doped, nominally undoped, and P-doped sections of NWs, for a total length L ~ 12 mu;m. Passivation of SiNW surface was achieved by rapid thermal oxidation at 1000 °C for 1 min to a SiO₂ thickness of ~12 nm.
The NW arrays were planarized using SU-8 photoresist, followed by reactive ion etching to expose the NW tips. Top contact electrodes to the n-doped section were realized by sputter deposition of a 200 nm, transparent indium-zinc-oxide layer. The p-contact was made by backside metallization of the Si substrate.
Under AIM 1.5 illumination, the unpassivated Si NW arrays exhibited an open circuit voltage V_OC = 170 mV, a short circuit current density J_SC > 3.7 mA/cm² (with uncertainty due to the unknown fraction of properly contacted nanowires), and a fill factor of ~30%. After the passivation, V_OC, J_SC and FF increased to 250 mV, > 9.2 mA/cm² and ~36%, respectively. The measured normal reflectance of the planarized NW array was around 6% over the 400-1000 nm spectral range, whereas the diffuse reflectance was ~20% over a broad wavelength range, indicating strong light scattering and absorption by the NWs.
The photovoltaic performance of passivated Si NW arrays and single NW devices was compared using a 532 nm laser with a power density of about 10 W/cm². Higher values of V_OC and FF obtained for the former are explained by reduced reflectance and light trapping in the NW arrays.
[1] Bozhi Tian, Thomas J. Kempa and Charles M. Lieber, "Single nanowire photo- voltaics", Chem. Soc. Rev. , 38, 16-24 (2009).

Macroscopically ordered structures such as nanowire arrays or inverse opal structures have been used as the electrode film for photoelectrochemical devices. 3D inverse opal structures fabricated using colloidal crystal templates have unique advantages; they provide full 3D connectivity to facilitate photo-generated electron transport, similar to 1D nanotube arrays and present a well-developed/fully-connected pore structure that facilitates electrolyte infiltration and transport. In this talk, we present recent results of novel inverse opal-based structures for dye-sensitized solar cell application. First, the preparation of hierarchical twin-scale inverse opal structures is introduced. The twin-scale inverse opals were obtained from a template fabricated via the assembly of mesoscale colloidal particles in the confined geometry of a macroporous inverse opal structure. The application to dye-sensitized solar cells revealed that due to the competing effects of recombination and dye adsorption, the maximum efficiency of solar cell was observed at the optimum film thickness. We also demonstrate the incorporation of graphene into inverse opal structures. During sacrificial templating process using polystyrene opals, the relative sizes of the graphene sheets and the cavities in the template induced a local infiltration of graphene to the top several layers of the inverse opal structures. Graphene incorporation enhanced charge separation and transport of the electrode film. This led the enhancement of photocurrent density.

We demonstrate multiple plasmonic effects on photocurrent properties in organic solar cells using Ag or Au nanoparticles on gold grating surfaces. The photocurrent properties are enhanced by both the localized surface plasmon and propagating surface plasmon excitations. In this study, we fabricate solid-state dye sensitized solar cell and organic thin film solar cell. In the solid-state dye-sensitized solar cells, spiro-OMeTAD grating pattern is fabricated on dye-Au-TiO2 films by nanoimprinting technique. P25-TiO2 is used to fabricate Au-TiO2 layer and N-719 dye is used as a photo-sensitizer. Spiro-OMeTAD is used as a solid electrolyte. After spin-coating spiro-OMeTAD on dye-Au-TiO2 films, PDMS grating mold is imprinted on the film. Grating pattern of PDMS mold is transferred from the template. Finally, silver is deposited on the solid electrolyte by vacuum evaporation technique. Surface plasmon resonance (SPR) property of the fabricated cell is studied by using white light irradiation. Short-circuit photocurrent is increased when the propagating grating-coupled surface plasmon (SP) is excited on the gold gratings as compared to that without SP excitation. The photocurrent further increases when the gold nanoparticles are placed within the grating-coupled surface plasmon field.

M. Garín1,2,3, R. Fenollosa1,2, L. Shi, R. Alcubilla3, L.F. Marsal4 & F. Meseguer1,2
1 Unidad Asociada ICMM/CSIC-UPV, Universidad Politécnica de Valencia, 46022, Valencia, Spain.
2 Instituto de Ciencia de Materiales de Madrid CSIC, Madrid, 28049, Spain.
3 Dept. d&’Enginyeria Electrograve;nica, Universitat Politècnica de Catalunya. 08034 Barcelona, Spain.
4 Dept. d&’Enginyeria Electrograve;nica, Electrica I Automatica. Universitat Rovira i Virgili. 43007 Tarragona, Spain.
Single-junction photovoltaic devices suffer from intrinsic obstacles limiting their efficiency to a top value dictated by the well-known Shockley-Queisser (SQ) limit [1]. The development of photodiode devices on micro and nanophotonic structures has opened new possibilities over the standard technology. The impinging light is strongly confined inside those photonic struc-tures, enhancing optical absorption and photocarrier generation, as it has been reported for planar optical cavities and, more recently, for nanowire resonators [2,3]. The most fundamen-tal limitation is given by the energy bandgap of the semiconductor, which determines the minimum energy of photons that can be converted into electron-hole pairs. In the case of sili-con a large percentage of infrared sunlight, with energy value below the fundamental absorp-tion edge of silicon, is still useless. Here we show the first example of a photodiode developed on a micrometer size silicon spherical cavity whose photocurrent shows the Mie modes of a classical spherical resonator. The long dwell time of resonating photons enhances the absorp-tion efficiency of photons. Also the photocurrent response shows very rich spectra with plenty of high-Q resonant peaks in a similar manner as the scattering spectra of high order whisper-ing gallery modes (WGMs) of spherical microcavities. Also, as a consequence of the en-hanced resonant absorption, the photocurrent response extends far below the bandgap of crystalline silicon [4]. It opens the door for developing a new generation of solar cells and pho-todetectors that may harvest infrared light more efficiently than silicon based photovoltaic de-vices.
REFERENCES
1. Schockley, W. & Queisser, H. J. J. Appl. Phys. 32, 510-519, (1961).
2. Wallentin, J. et al. Science 339, 1057, (2013).
3. Krogstrup, P. et al. Nature Photon. 7, 306—310, (2013).
4. Garin, M., Fenollosa, Shi L., Alcubilla R., Marsal Ll., and Meseguer F. Submitted for publication.

Here, we demonstrate how to utilize the plasmonic scattering effect in organic solar cells (OSCs) as an efficient approach to improve the PCE of the OSCs. We successfully manipulated the external quantum efficiency (EQE) and absorption enhancement in OSCs in entire visible wavelength range with controlling the size and shape of metal nanoparticles (MNPs). We found that the forward scattering efficiency depending on the size of AgNPs considerably influences the device performance.1 In addition, we carefully controlled the shape of MNPs and designed Au@Ag nanocube (NC) structure. A thin Ag shell acted as a strong plasmonic scattering enhancer in Au@Ag NCs and the Au@Ag NCs showed strong and broadband plasmonic scattering efficiency. Highly efficient plasmonic OSCs were fabricated by Au@Ag NCs, and the power conversion efficiency was enhanced to 9.2 % from 7.9 % in polythieno[3,4-b]thiophene/benzodithiophene (PTB7):[6,6]-phenyl C71-butyric acid methyl ester (PC70BM)-based OSCs. Moreover, the Au@Ag NCs plasmonic OSCs showed 2.2-fold higher EQE enhancement compared to AuNPs devices at a wavelength of 450 ~ 700 nm.
1 Baek, S. W. et al. Plasmonic Forward Scattering Effect in Organic Solar Cells: A Powerful Optical Engineering Method. Scientific Reports 3, 1726 (2013).

The large spectral tuning range and strong oscillator strength of the coherent electron oscillations in localized surface plasmon resonance (LSPR) make it highly attractive for the enhancement of solar energy conversion efficiency. While the large scattering cross section of LSPR has been used successfully to increase light trapping at energies above the band edge, large enhancements in spectral conversion below the band edge have proven more difficult. If the plasmon does not scatter incident radiation, the absorbed energy can be transferred to the semiconductor by direct transfer of hot plasmonic electrons, or, as shown recently by our group, through the nonradiative plasmon induced resonant energy transfer (PIRET). Whereas hot electron transfer can occur after the plasmon has decayed, PIRET is mediated by the strong local electromagnetic field during the coherent oscillations of the plasmon. Both mechanisms allow an enhancement in photoconversion at energies below the band edge of the semiconductor.
In this presentation we systematically investigate the PIRET mechanism by combining transient absorption spectroscopy and action spectrum photocatalysis/IPCE measurements. A varying SiO₂ barrier in Au@SiO₂@Cu₂O core shell nanoparticles is used to determine the distance dependence of the plasmon-semiconductor coupling. Next the effect of spectral overlap in the semiconductor-LSPR coupling is investigated in a Fe₂O₃/Au hole array composite. Additionally, the effect of the interface on the PIRET excited carriers is investigated in Au@Cu₂O and Ag@Cu₂O nanoparticles. The results are compared to theory in order to guide optimal design for enhancing below band edge solar energy conversion through LSPR and PIRET.

Light trapping in thin-film solar cells using periodic metallic backreflectors has been extensively studied. It is now well established that a properly designed periodic or random pattern of metallic scatterers, integrated with the metallic back contact of the solar cell, can serve to efficiently scatter light to in-plane waveguide modes of the solar cell. This light trapping strongly enhances the red response of the cell and at the same time enables the fabrication of thinner solar cells without compromising on efficiency.
A major drawback of the metallic structures is that they also absorb a significant fraction of the incident light due to Ohmic dissipation, which reduces the light trapping effect. Here, we demonstrate a novel backscattering geometry in a-Si:H solar cells that is entirely based on dielectric scatterers. The dielectric scattering patterns efficiently scatter the light and absorption losses in the particle are minor. The dielectric scatterers are integrated in the AZO buffer layer between the silver back contact and the active silicon layer of the solar cell. The scattering geometries are made with a rounded shape enabling growth of conformal high-quality silicon layers. .
We use three-dimensional finite-difference-time-domain simulation to study the scattering behaviour of single AZO particles at the a-Si:H/AZO interface and find that these particles exhibit geometrical resonances with scattering cross sections well above the geometrical cross section over the entire 350-800 nm spectral range. Interestingly, when the particle diameter becomes close to the array pitch, the Si structure in between the AZO particles also acts as a resonant scatterer, with a scattering cross section up to 5 times as large as its geometrical cross section.
We optimize the design of a periodic array of AZO hemispheres on the back of 350 nm a-Si:H cells and find that the periodic particle array very efficiently couples the incident light to waveguide modes in the a-Si:H layer, leading to a broadband absorption enhancement. The optimized array geometry (550 nm pitch, 225 nm particle radius) results in a 45% absorption enhancement in the intrinsic a-Si:H layer (i-layer) averaged over the AM1.5 spectrum in the 500 to 750 nm wavelength range.
We fabricate the optimized devices by using substrate conformal imprint lithography (SCIL) to print periodic arrays of dielectric particles on top of glass substrates that are sputter-coated with Ag and AZO. We conformally coat the particles with a second AZO layer via sputtering and then grow the a-Si:H layers (350 nm i-layer) on top of the scattering substrates in n-i-p configuration. ITO layers are sputtered on top, followed by Ag grids. We present photocurrent measurements on our novel cell geometry.

Nanophotonics is an exciting new field of science and technology that is directed towards making the smallest possible structures and devices that can manipulate light. In this presentation, I will start by showing how semiconductor nanostructures can mold the flow of light well below the wavelength of light. I will the proceed by showing of dense arrays of subwavelength-arrays forming metasurfaces enable broadband light absorption enhancements. The strong absorption in thin films opens up the opportunity to effectively trap light in ultrathin solar and photo-electrochemical cells. I will discuss how the nanostructure sizes and shapes as well as their non-periodic arrangement on a cell can be optimized to effectively harvest solar energy from such thin cells and across the broad solar spectrum.

Photovoltaic device performance depends on efficient conversion of absorbed photons to electronic charge. Any mismatch of refractive indices between air and the solar cell front surface results in reflection of incident sunlight and reduced device performance. We detail a new approach for texturing silicon surfaces over arbitrarily large areas, combining selfassembly of block copolymer thin films and plasma-based etching. This process creates densely packed arrays of sub-wavelength size cones, whose tapered profile grades the refractive index transition between air and bulk silicon. The gradual change in refractive index greatly reduces reflection at the air/silicon interface from more than 40 percent in a flat film to less than 1 percent over a broad wavelength range from 350 nm to 1000 nm. Reducing the nanostructure separation distance to 42 nm allows us to achieve less than1 percent broadband reflectance using nanocones as short as 155 nm, with the height necessary for 1 percent reflectance increasing linearly with pitch. These antireflective structures maintain less than 5 percent broadband reflectance for incident light angles as high as 60 degrees. Optical simulations of the nanostructured surfaces using the Transfer Matrix Method qualitatively reproduce the observed antireflection behavior. However, the experimentally measured reflectance values are smaller than the simulation. Local measurements of electron energy loss spectra are consistent with a change in dielectric constant along the nanocone axial direction. Using these values improves the match between optical reflectance simulations and the experimental measurements.

Ge nanostructures (NS) are receiving a great attention for their chance to exploit the quantum confinement effects (QCE) for energy applications. In fact, compared to Si, Ge NS show a much larger exciton Bohr radius (20 nm in Ge against 5 nm in Si) which, combined with their lower synthesis temperature and larger optical absorption, make them valuable candidate for future application in optoelectronics and new generation photovoltaics. In this work we report on how to efficiently tune the optical properties of Ge NS by a precise control of their structural properties. A large band gap tuning and great enhancement in the absorption efficiency are shown in single amorphous Ge quantum wells (QWs) or in ordered arrays of Ge quantum dots (QDs).
In particular, structural and optical characterizations are presented for single film of amorphous Ge grown at room temperature by magnetron sputtering deposition. We used Transmission Electron Microscopy (TEM) and Rutherford Backscattering Spectrometry (RBS) to measure the QWs thickness (2-30 nm) and to check the proper atomic density. Light absorption spectroscopy was performed on samples deposited onto quartz substrates, by measuring the transmittance and reflectance spectra in the 200 - 2000 nm wavelength range. The optical absorption of single amorphous Ge QWs (2 to 30 nm thick) evidences a marked size-dependent blue-shift and an enhanced oscillator strength with reducing the QW thickness. Still, the experimental optical bandgap and the oscillator strength values have been modelled with a fully agreement within the EMA theory [1]. Such a behavior is a clear indication of the strong quantum confinement of excitons occurring at room temperature and for a disordered material as amorphous Ge QW.
An open question to clarify is whether the size of NS is the only parameter determining their light absorption properties. Recent experimental and theoretical studies gave indication of a strong role of QD ordering and strain on the efficiency of light absorption [2, 3]. To shed light on the role of QD-QD, the absorption mechanism of multilayered samples containing Ge QDs have been studied in this work. In particular, Multilayers of Ge quantum dots (QDs, 3 nm in diameter) embedded in SiO2, separated by SiO2 barrier layer (3, 9, or 20 nm thick), have been synthesized by sputter deposition and characterized by TEM and light absorption spectroscopy. Quantum confinement affects the optical bandgap energy (1.9 eV for QDs, 0.8 eV for bulk Ge). Moreover, the absorption probability greatly depends on the QD-QD distance. A strong electronic coupling among Ge QDs is evidenced, with a significant increase of the light absorption efficiency when the QD-QD distance is reduced. These data unveil promising aspects for light harvesting with nanostructures [4].
[1] Cosentino et al., NRL 8, 128 (2013)
[2] Uhrendeldt et al., JAP 109, 094314 (2011)
[3] Guerra et al., PRB 87, 165441 (2013)
[4] Mirabella et al., APL 102, 193105 (2013)

Conventional planar hydrogenated amorphous silicon photovoltaic devices have been widely relegated to low-efficiency, low-cost applications. This poor efficiency is mainly attributed to the low hole mobility of the bulk amorphous material, limiting the device current collection. Nanostructuring of the material, however, holds promise of significantly improving carrier extraction without limiting the advantageous light absorption properties, deposition flexibility, and low cost of the material. In particular, we investigate the application of ordered nanohole arrays with sub-wavelength hole diameters and pitches. Fabrication methods, as well as characterization of critical device performance properties, such as hole transport (via time-of-flight transient photocurrent measurements), structural properties (through FTIR and Raman spectroscopy), and relative light absorption will be presented for structured nanohole hydrogenated amorphous silicon solar cells.

Among the diverse applications of the photocatalytic materials, photoelectrochemical water splitting has recently undergone special awareness by virtue of the actual and future energetic juncture. Titanium dioxide is considered the material par excellence in water splitting applications despite its light absorption limitation caused by its large band gap (3.0 eV). Many attempts have been tried in order to circumvent such limitation and improve its overall photocatalytic efficiency. Among them, tailoring the interface through the control of the electrode nanostructure has shown promising results. In this regard, single crystalline TiO2 nanorods have been successfully grown on FTO substrates through a simple hydrothermal route. Since then, the number of publications based on this material that attempt to improve its photocurrent has increased, reaching values very next to the theoretical efficiency values for TiO2. However, the grounds of the photoactivity improvement of TiO2 nanorods have not been fully elucidated nor profoundly treated.
Other approximations such as doping with transition metals have been explored with relative success. On the other hand, doping with non-metals, particularly nitrogen, has been shown to be the best option for the improvement of titanium dioxide efficiency. Notwithstanding the large efforts that have been spent in studying this topic, the origin of such enhancement is still in question. While some authors argued that this improvement lies on the bandgap narrowing caused by substitution of nitrogen on oxygen sites, others consider that vacancy formation inherent to nitrogen doping is responsible for such activity.
In this work, we present and ammonia-induced reduction treatment of titanium dioxide rutile nanorods that triggered a synergistic surface modification of titania electrodes. The surface treatment enhanced its overall photoelectrochemical performance, besides introducing a new absorption band in the 420minus;480 nm range. A physical model has been proposed to reveal the role of each fundamental interfacial property on the observed behavior. On the one hand, by tuning the Fermi level position, charge separation was optimized by adjusting the depletion region width to maximize the potential drop inside titanium dioxide and also filling the surface states, which in turn decreased electronminus;hole recombination. On the other hand, by increasing the density of surface holes traps (identified as surface hydroxyl groups), the average hole lifetime was extended, depicting a more efficient hole transfer to electrolyte species. The proposed model could serve as a rationale for controlled interfacial adjustment of nanostructured photoelectrodes tailoring them for the required application, knowledge that can also extended to other oxides.

A mesoporous film of colloidal Nb-doped TiO2 nanocrystals has been developed that can independently tune near-IR and visible light absorption depending on applied electrochemical potential within a Li-ion cell. This behavior is promising for energy-saving “smart” windows, and can be exploited to dynamically control radiation transfer in interior spaces. Considering that roughly 20% of total U.S. energy demand is due to indoor lighting and thermal management (R. Judkoff, MRS Bulletin 2008), the development of smart windows has the potential to dramatically reduce energy consumption nationwide.
The doping of anatase TiO2 nanocrystals with substitutional donors such as niobium has previously been shown to increase carrier concentrations, leading to a promising new transparent conductive oxide and plasmonic nanomaterial. Conduction band electron donation from niobium ions, inserted at stoichiometries approaching 20% Nb per Ti atom, enables a localized surface plasmon resonance tunable from near to mid-IR frequencies (L. De Trizio, Chem. Mater. 2013). This LSPR can be reversibly blue-shifted by pseudocapacitive charging from surface lithium ions upon application of an electrochemical reducing bias, similar to the near-IR spectral modulation in semiconductor nanomaterials such as indium tin oxide nanoparticles demonstrated previously (A. Llordes, Nature, 2013). Furthermore, niobium doped TiO2 nanocrystals demonstrate a visible-range absorption peak, which can be reversibly colored and bleached through intercalation of lithium into the anatase TiO2 lattice. Lithium atoms are incorporated into the doped anatase TiO2 lattice through a phase transition to the lithium titanate LixTiO2 structure (x < 0.5), creating a tunable visible-range absorption feature with enhanced intensity, rapid electrochromic switching and unique optoelectronic properties. Intercalated lithium ions are in effect a secondary dopant within the nanocrystal, and the interaction between the inserted lithium and donor defect sites in the lattice generate novel optical functionality. By studying the optical response of this material upon synthetic variation and electrochemical cycling, insight into the role of doping in TiO2 nanocrystals on their functional optoelectronic properties can be probed.