Session T18: Focus Session: Organic Photovoltaics and Other Photonic Devices

Harvesting Lost Photons: Minimizing Sub-Bandgap Losses in Organic Photovoltaic Devices by Up-conversion

We report on a novel approach to increase the efficiency of organic photovoltaic (OPV) cells in the near-infrared region of the solar spectrum by blending the organic semiconductors with rare-earth doped nanoparticles with up-converting photophysical properties. The approach consists in (i) synthesizing lanthanide-doped nanoparticles capable of efficient energy transfer of up-converted near-infrared (NIR) photon energy to conjugated polymers; (ii) assembling these nanoparticles, in blends with p-type polythiophenes and n-type fullerenes, in solution-processed OPV cells capable to harvest NIR photons.   

In-situ X-ray characterization of thermal and solvent based annealing of thin P3HT and P3HT/PCBM films

We have studied the annealing of thin films of P3HT (polyhexylthiophene) and mixed P3HT and PCBM thin films using in-situ Grazing Incidence Angle x-ray scattering techniques at the National Synchrotron Light Source. The films, 50-200 nm thick, were prepared using spin coating from a volatile solution. Both thermal and solvent annealing techniques are well known to improve electrical properties yet the precise mechanism is not well understood. In our measurements, we have monitored the dependence of the diffraction peak positions and widths under a variety of different in-situ thermal and solvent conditions. A detailed comparisons between these methods provides new insight into how to improve the crystallinity beyond what can be obtained by thermal methods alone. This may eventually lead to better electrical properties in thin film organic photovoltaic devices.   

Excited-State Dynamics at Organic Photovoltaic Heterojunctions by Pump-Probe Photoelectron Spectroscopy

The critical process of charge separation in organic photovoltaic (OPV) devices is determined directly at the organic heterojunction, but these interfaces have been less extensively studied than organic/metal interfaces. We prepare model photovoltaic heterojunctions by deposition of ultrathin films of organic semiconductors on single-crystal metal substrates. The electronic structure of the component materials and their interfaces is determined with ultraviolet photoelectron spectroscopy (UPS) and two-photon photoemission (2PPE). The systems studied in this work involve phthalocyanines and analogs as donors and C$_{60}$ fullerene as acceptor. Time-resolved pump-probe experiments are applied to directly measure the excited state dynamics at these OPV heterojunctions. An ultrafast visible pump pulse selectively generates excitons in one material, followed by a time-delayed UV probe to interrogate the population of the acceptor charge transport level. Analysis of cross-correlations reveals the timescales of charge separation and recombination at the interface. Additionally, comparison will be made to structural and local spectroscopic studies of similar phthalocyanine/fullerene systems made by STM/STS.   

The role of triplet excitons in enhancing polymer solar cell efficiency: a photo-induced absorption study

Inclusion of heavy metal atoms in a polymer backbone allows transitions between the singlet and triplet manifolds. Interfacial dissociation of triplet excitons constitutes a viable mechanism for enhancing photovoltaic (PV) efficiencies in polymer heterojunction-based solar cells, which are now becoming feasible options for solar panels. The PV efficiency from polymer solar cells utilizing a ladder-type poly para phenylene polymer (PhLPPP) with trace quantity of Pd atoms and a fullerene derivative (PCBM) is almost 10 times more than its counterpart (MeLPPP) with no Pd atom. Evidence is presented for the formation of a weak ground-state charge-transfer complex (CTC) in the blended films of PhLPPP and PCBM, using photo- induced absorption (PIA) spectroscopy. Such complexes are not seen in the PIA spectrum of MeLPPP: PCBM blends. Possible mechanisms for the CTC state formation as well as the significance of this to the understanding and optimization of polymer blended solar cells will be discussed.   

Photophysics of charge-transfer excitons in thin films of $\pi$-conjugated polymers

We develop a theory of the electronic structure and photophysics of interacting chains of $\pi$-conjugated polymers to understand the differences between solutions and films. While photoexcitation generates only the intrachain exciton in solutions, the optical exciton as well as weakly allowed charge-transfer excitons are generated in films. We show that a significant fraction of ultrafast photoinduced absorptions (PAs) in films originate from the lowest charge-transfer exciton. Using sophisticated many-body approaches that take into account high order configuration interaction, we have calculated the full wavelength-dependent PA spectra of pairs of interacting PPV oligomers. Good qualitative agreement is obtained with the experimental PA spectra of thin films of $\pi$-conjugated polymers. The origin of each individual PA is explained within our theory. Our work resolves long-standing controversies regarding the nature of the primary photoexcitations in films.   

Photo-crosslinkable Polythiophenes for Efficient Thermally Stable Organic Photovoltaics

We report a new series of bromine-functionalized poly(3-hexylthiophene) (P3HT-Br) copolymers for use in solution processed organic photovoltaics (OPVs). P3HT-Br copolymers were synthesized from two different monomers, where the ratio of the monomers was carefully controlled to achieve a UV photo-crosslinkable layer while leaving the $\pi -\pi $ stacking feature of conjugated polymers unchanged. Photo-crosslinkable P3HT-Br was demonstrated as effective electron donors in OPVs. The crosslinking stabilizes P3HT-Br/PCBM blend morphology preventing the macro phase separation between two components, which lead to OPVs with remarkably enhanced thermal stability. The drastic improvement in thermal stabilities is further characterized by microscopy as well as grazing incidence X-ray scattering (GIXS). The use of these copolymers for solution processed efficient bilayer PVs is also described. Benefited from the little disturbance in $\pi -\pi $ stacking by crosslinkable units as evidenced in GIXS, P3HT-Br/PCBM bilayer device shows high power conversion efficiency at over 2.2{\%} and excellent thermal stability.   

Temperature Dependence of Biexciton Decay and Intermolecular Hopping in Zinc Phthallocyanine Films.

The femtosecond exciton dynamics of melt-pressed zinc phthalocyanine (ZnPC) films are studied in the temperature range of 90-400 K. In this range ZnPC goes through a transition from a crystalline solid to a liquid crystalline phase. For the entire temperature range, the excitons are shown to decay on the time scale of 10's of picoseconds, and these dynamics are nonlinear with respect to pump fluence. Such a behavior is well described by a biexciton recombination model under one dimensional diffusion constraints. The single exciton lifetime and the biexciton recombination crossection are extracted at all temperatures. From the latter, the exciton hopping time is calculated. The exciton hopping time decreases with temperature in the crystalline phase, but increases in the liquid crystalline phase. The role of temperature and structural order in the exciton hopping time will be discussed.   

White tandem OLED with carbon nanotube interlayer

White organic light emitting diodes (OLEDs) have become well recognized as an important candidate for future lighting and display applications. An existing idea to generate white color places R, G, B pixels in a side-by-side geometry. Also, white tandem OLEDs have been developed by vertically stacking in series multiple electroluminescent layers. However, such structures require a complex interfacial layer which is usually fabricated by strong dopants to form a p+/n+ interface. We have shown earlier that transparent carbon nanotubes (CNT) can be used as effective three dimensional charge injectors in polymer light emitting diodes[1] and OLEDs[2]. Now, we show that CNT can be used as an interlayer in two cell OLEDs with complimentary colors. We show that tandem devices with CNT interlayers, together with selective barriers and PEDOT:PSS coating can control the device color. In addition, the emission intensity can be controlled by independently tuning the driving voltage and current. In the case of overdoped p+/n+ interlayers we do not have this opportunity which is a great advantage of CNT injectors. We also compare the performance of multiwall CNTs vs. that of single wall CNTs in the tandem OLEDS. [1]R.H.Baughman et al.Science, 297,787-792(2002).[2]C.D.Williams et al.Appl. Phys. Lett. 93,183506(2008).   

Tuning optical properties of blue-emitting polyfluorenes via hydrostatic pressure

Polyfluorenes (PFs) represent a unique class of poly para- phenylene based blue-emitting polymers with intriguing structure-property relationships. Slight variations in the choice of functionalizing side chains result in dramatic differences in the inter- and intra-chain structures in PFs. Highlighting these differences are two prototypical PFs, poly (9,9-(di n, n-octyl) fluorene) (PF8) and poly (9,9-(di ethyl- hexyl) fluorene) (PF2/6). In addition to the nematic liquid crystal (n-LC) mesophase, PF8 is characterized by at least five structural phases. We present photoluminescence (PL) and Raman scattering studies of powder samples and thin films of PF8 under hydrostatic pressure. The powder sample was thermally annealed at 2GPa. The PL vibronics of the as-is powder sample red-shift at an average rate of 30 meV/GPa whereas the thermally annealed sample red-shifts at a higher rate of 50 meV/GPa, indicating a different crystalline mesophase for the annealed sample. The Huang-Rhys factor is found to increase with increasing pressures signaling a higher geometric relaxation of the electronic states. The Raman peaks harden with increasing pressures; the intra-ring C-C stretch frequency at 1600 cm $^{-1}$ has a pressure coefficient of 5 cm$^{-1} $/GPa and exhibits asymmetric line shapes at higher pressures.   

SERS Characterization of Self-Assembled Monolayers Embedded on Plasmonic Nano-structure

We discuss Raman spectroscopic analysis of self-assembled monolayers embedded in two different types of nano-structures capable of sustaining localized surface plasmon-surface plasmon polariton coupling via nanoscale gaps and curved surface features. Both structures cosnsist of metal-molecule-metal (M$^3$) junctions which can also allow charge transport through the molecular bridges. Our results indicate different electromagnetic and charge transport characteristics as a function of the top metal-molecule chemical interaction. We also report direct correlations between charge transport states, SERS response and inelastic vibrational scattering in selected M$^3$ molecular electronic device junctions.   

Non-linear optics and local-field factors in liquid chloroform: A time-dependent density-functional theory study

Chloroform is often used as a solvent and reference when measuring non-linear optical properties of organic molecules. We calculate directly the non-linear susceptibilities of liquid chloroform at optical frequencies, using molecular dynamics and the Sternheimer equation in time-dependent density-functional theory [X. Andrade et al., J. Chem. Phys. 126, 184106 (2007)]. We compare the results to those of chloroform in the gas and solid phases, and experimental values, and make an ab initio calculation of the local-field factors which are needed to extract molecular properties from liquid calculations and experimental measurements.   

Time resolved photoluminescence studies of long lived emissive specie in F8BT:PFB blends

Type-II heterojunctions play a crucial role in organic optoelectronic devices. We use donor-acceptor polyfluorene blends as a model system to understand excited-state dynamics at heterojunctions. These interfacial excitations are intrachain singlet and triplet excitons, geminate polaron pairs, and exciplexes (interfacial charge-transfer excitons). Time-resolved photoluminescence (PL) spectra were taken at 10\,K and room temperature to investigate the interconversion dynamics of these species. We observe delayed PL with sub-linear excitation fluence dependence. This implies that delayed singlet exciton generation involves a bimolecular annihilation mechanism. By means of kinetic modeling, we propose triplet-triplet exciton annihilation as a regeneration route to singlet excitons, and subsequently to exciplexes. This points to a significant ($<15$\,\%) yield of triplet excitons after interfacial charge separation, and to the central role of these species on the interfacial dynamics.