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, 31