Sangam Chatterjee

Charge transfer at organic-inorganic interfaces—Indoline layers on semiconductor substrates

We studied the electron transfer from excitons in adsorbed indoline dye layers across the organic-inorganic interface. The hybrids consist of indoline derivatives on the one hand and different inorganic substrates (TiO2, ZnO, SiO2(0001), fused silica) on the other. We reveal the electron transfer times from excitons in dye layers to the organic-inorganic interface by analyzing the photoluminescence transients of the dye layers after femtosecond excitation and applying kinetic model calculations. A correlation between the transfer times and four parameters have been found: (i) the number of anchoring groups, (ii) the distance between the dye and the organic-inorganic interface, which was varied by the alkyl-chain lengths between the carboxylate anchoring group and the dye, (iii) the thickness of the adsorbed dye layer, and (iv) the level alignment between the excited dye ( π*-level) and the conduction band minimum of the inorganic semiconductor.

Ternary Mixed-Valence Organotin Copper Selenide Clusters

Reactions of the organotin selenide chloride clusters [(R1 SnIV )3 Se4 Cl] (A, R1 =CMe2 CH2 C(O)Me) or [(R1 SnIV )4 Se6 ] (B) with [Cu(PPh3 )3-x Clx ] yield cluster compounds with different inorganic, mixed-valence core structures: [Cu4 SnII SnIV6 Se12 ], [Cu2 SnII2 SnIV4 Se8 Cl2 ], [Cu2 SnII SnIV4 Se8 ], [Cu2 SnII2 SnIV2 Se4 Cl4 ], and [Cu2 SnIV2 Se4 ]. Five of the compounds, namely [(CuPPh3 )2 {(R1 SnIV )2 Se4 }] (1), [(CuPPh3 )2 SnII {(R2 SnIV )2 Se4 }2 ] (2), [(CuPPh3 )2 (SnII Cl)2 {(RSnIV )2 Se4 }2 ] (3) [(CuPPh3 )2 (SnII Cu2 ){(R1 SnIV )2 Se4 }3 ] (4), and [Cu(CuPPh3 )(SnII Cu2 ){(R1 SnIV )2 Se4 }3 ] (5) are structurally closely related. They are based on [(CuPPh3 )2 {(RSnIV )2 Se4 }n ] aggregates comprising [(RSnIV )2 Se4 ] and [CuPPh3 ] building units, which are linked by further metal atoms. A sixth compound, [(CuPPh3 )2 (SnII Cl)2 {(R1 SnIV Cl)Se2 }2 ] (6), differs from the others by containing [(RSnIV Cl)Se2 ] units instead, which affects the absorption properties. The compounds were analyzed by single-crystal X-ray diffraction, NMR and 119 Sn Mössbauer spectroscopy, DFT calculations as well as optical absorption experiments.

Photoelectrochemical response of GaN, InGaN, and GaNP nanowire ensembles

The photoelectrochemical responses of GaN, GaNP, and InGaN nanowire ensembles are investigated by the electrical bias dependent photoluminescence, photocurrent, and spin trapping experiments. The r ...

Charge carrier dynamics at the pentacene-C60 interface (Conference Presentation)

Organic molecular solids feature various properties considered advantageous for next-generation photovoltaic devices such as mechanical flexibility and ease of fabrication by, e.g., large-scale and large volume printing. Additionally, Singlet-Exciton Fission may allow surpassing the Shockley-Queisser limit. Here, one photoexcited singlet-type exciton decays into two triplet-type excitons, effectively doubling the number of excited charge carriers. Hence, above-unity quantum efficiencies may be achieved in photovoltaics and have been reported in for example, pentacene (PEN) –C60 heterojunctions. Here, we study the carrier dynamics at well-defined PEN-C60 interface model systems by time-resolved photoluminescence spectroscopy experiments for different excitation photon energies. Thereby, we disentangle charge transfer and excitation dynamics, i.e., injection, transport, dissociation, and extraction. The photoluminescence spectra reveal two distinct transition energies associated with charge-transfer (CT) states expected from photoelectron spectroscopy experiments. These long-lived transitions show a clear dependence on excitation energy, corroborating the proposed CT transitions and revealing the fact that carriers need to be created in both individual constituents for CT transitions to be observable. Additionally, the C60 photoluminescence efficiency strongly quenches for increasing PEN coverage while the lifetime is drastically enhanced yielding strong evidence for an electron transfer between the PEN ground state and C60 when only the latter is photoexcited.

Towards exploitation of singlet-exciton fission in organic crystals and potential integration with inorganic semiconductors

Molecular semiconductors offer intriguing electronic properties. In particular, singlet-exciton fission, the nonradiative decay of one singlet exciton into two triplet excitons effectively doubles the amount of carriers available for, e.g., photovoltaic current generation, thereby effectively surpassing the Shockley-Queisser-limit. An efficient use of singletexciton fission in actual devices, however, requires a detailed understanding of the decay dynamics in donor-acceptor heterostructures. We present a quantitative study on model single-crystalline perfluropentacene at cryogenic temperature and related heterostructures to reveal the intricate interplay between singlet-exciton fission and the nanoscopic molecular arrangement, the role of charge-transfer into and out of molecular systems and discuss the potential for functionalizing inorganic semiconductors. Finally, the potential implications in heterosystems and for functionalization of inorganic semiconductor devices are discussed.

Ultra-low threshold supercontinuum generation in SnS-based clusters (Conference Presentation)

Invited presentation by Prof. B. Witzingmann The development of supercontinuum sources is advancing fast in the last decades. As do all nonlinear effects, the supercontinuum generation strongly relies on the nonlinearity of the active material. This nonlinearity may be greatly enhanced in specially designed photonic-crystal fibers, making supercontinuum sources widely available. Nevertheless, pulsed lasers are required to supply high enough field strengths to overcome the threshold for supercontinuum generation. We study a new cluster based class of nonlinear media that exhibits ultra-low thresholds for supercontinuum generations, thus enabling the use of a low coast steady-state laser diode as the driving laser [1]. The clusters are composed of a tin sulfide based core that is surrounded by four organic ligands. The core adopts an adamantane-like architecture, [Sn4S6]. It has a tetrahedral shape and thus lacks inversion symmetry, enabling nonlinear processes. The four ligands (R = 4-(CH2=CH)-C6H4) are consolidating the structure of the core. Yet, as they are randomly oriented around the Sn-C bonds, they are also preventing any long-range order in the solid phase of the compound. As a result, the compound is obtained as a white powder with totally frustrated order. This powder has been studied in respect to its optical properties. When irradiated with a continuous-wave infrared laser of sufficient intensity it emits a warm white spectrum that is virtually independent from the pump-wavelength in a range of 725-1050 nm. Lowering the pump intensity, however, changes the spectral weight to the red, similar to dimming of thermal emitters. The input-output characteristics, however, exclude a thermal process as the source of the observed white-light. Additionally semi-classical calculations of the white-light generation process are performed, underlining this statement. [1] Rosemann N.W., et al.; Science, 2016, 352, 1301-1304

Carrier dynamics in pentacene-perfluropentacene heterocrystals (Conference Presentation)

Aromatic molecules are among the most promising materials in the field of organic optoelectronic due to the favorable properties of the delocalized -electron system present in those molecules. One of the most studied systems in this material class is the planar molecule of pentacene. An interesting application for pentacene is the incorporation into a donor-acceptor heterojunction in combination with its perfluorinated counterpart. Such samples may be deposited as intermixed blends (molecular alloys) or as alternating layered stacks. The out-of-plane delocalized -electron systems cause significant intermolecular coupling, even enabling the formation of charge-transfer excitons across heterointerfaces. Hence, studying this model system forms the optimal platform to investigate excitation transfer and charge separation in organic solar cells. We present a comprehensive study of the optical properties of pentacene -perfluoropentacene heterosystems. The samples are grown as crystalline thin films in different molecular configurations: either layered or as intermixed blends, both, in standing and lying molecular orientation. Time resolved luminescence and linear absorption spectroscopy are performed to obtain the carrier dynamics of the charge transfer states and response of the pure materials. The influence of different packing motifs on the optical properties is investigated, revealing a radiationless long-range energy transfer in addition to the local occupation of charge-transfer states.

Solar Cell Characterization with High Spatial Resolution

New techniques for a solar cell characterization with high spatial resolution are introduced and evaluated both by experiments on test structures and numerical simulations. The reliability is demonstrated and technical limits are assessed.