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