Exciton Regeneration at Polymeric Semiconductor Heterojunctions

Control of the band-edge offsets at heterojunctions between organic semiconductors allows efficient operation of either photovoltaic or light-emitting diodes. We investigate systems where the exciton is marginally stable against charge separation, and show via E-field-dependent time-resolved photoluminescence spectroscopy that excitons that have undergone charge separation at a heterojunction can be efficiently regenerated. This is because the charge transfer produces a geminate electron-hole pair (separation 2.2-3.1nm) which may collapse into an exciplex and then endothermically (E=100-200meV) back-transfer towards the exciton.Comment: 10 pages, 4 figures. Manuscript in press in Phys. Rev. Let

Tuning the wavelength of lasing emission in organic semiconducting laser by the orientation of liquid crystalline conjugated polymer

We report the optical pumping of one-dimensional distributed feedback (DFB) conjugated polymer devices using a uniaxially aligned liquid crystalline polymer, poly(9,9-dioctylfluorene). We can independently select the alignment direction (via a rubbed polyimide layer) and the DFB structure (via nanoimprinting). In comparison with unaligned film, we show that lasing threshold is substantially reduced when absorption is parallel to the aligned direction (similar to 20.0 mu J cm(-2) pulse(-1)). This is mainly due to the higher absorption coefficient estimated in the table by calculating the exciton densities at each threshold value. We also report the control of lasing wavelength through independent selection of alignment direction and DFB orientation, which is achieved through the control of the effective refractive index of waveguide (n(eff)).open171

Morphology, structure and optical properties of low bandgap organic-inorganic halide perovskite based on CH₃NH₃SnxPb₁₋ₓI₃

Herein, we investigate morphology, structure and optical properties of low band gap organic-inorganic halide perovskite based on a mixture of lead and tin as the divalent cation in ABX3 structure. A significant change in morphology of CH3NH3SnxPb1−xI3 perovskite with x as well as an alteration in crystal structure from I4cm (β-phase) to pseudocubic P4mm (α-phase) space groups is observed when Sn is the dominant divalent cation (x ≥ 0.5). Photo thermal defection optical absorption spectroscopy (PDS) and photoluminescence (PL) of CH3NH3SnxPb1−xI3 show the non-linear change in the band edge of perovskite. The bandgap as low as 1.17 eV and the most red-shifted PL at 1035nm is achieved for perovskite with x=0.8. In addition, higher electronic disorder is measured for CH3NH3SnxPb1−xI3 compounds with higher x

Organic solar cells based on non-fullerene acceptors.

Organic solar cells (OSCs) have been dominated by donor:acceptor blends based on fullerene acceptors for over two decades. This situation has changed recently, with non-fullerene (NF) OSCs developing very quickly. The power conversion efficiencies of NF OSCs have now reached a value of over 13%, which is higher than the best fullerene-based OSCs. NF acceptors show great tunability in absorption spectra and electron energy levels, providing a wide range of new opportunities. The coexistence of low voltage losses and high current generation indicates that new regimes of device physics and photophysics are reached in these systems. This Review highlights these opportunities made possible by NF acceptors, and also discuss the challenges facing the development of NF OSCs for practical applications

The role of intermolecular coupling in the photophysics of disordered organic semiconductors: Aggregate emission in regioregular polythiophene

We address the role of excitonic coulping on the nature of photoexcitations in the conjugated polymer regioregular poly(3-hexylthiophene). By means of temperature-dependent absorption and photoluminescence spectroscopy, we show that optical emission is overwhelmingly dominated by weakly coupled H-aggregates. The relative absorbance of the 0-0 and 0-1 vibronic peaks provides a powerfully simple means to extract the magnitude of the intermolecular coupling energy, approximately 5 and 30 meV for films spun from isodurene and chloroform solutions respectively.Comment: 10 pages, 4 figures, published in Phys. Rev. Let

Singlet exciton fission in solution.

Singlet exciton fission, the spin-conserving process that produces two triplet excited states from one photoexcited singlet state, is a means to circumvent the Shockley-Queisser limit in single-junction solar cells. Although the process through which singlet fission occurs is not well characterized, some local order is thought to be necessary for intermolecular coupling. Here, we report a triplet yield of 200% and triplet formation rates approaching the diffusion limit in solutions of bis(triisopropylsilylethynyl (TIPS)) pentacene. We observe a transient bound excimer intermediate, formed by the collision of one photoexcited and one ground-state TIPS-pentacene molecule. The intermediate breaks up when the two triplets separate to each TIPS-pentacene molecule. This efficient system is a model for future singlet-fission materials and for disordered device components that produce cascades of excited states from sunlight.B.J.W. was supported by a Herchel Smith Research Fellowship. A.J.M. received funding from a Marie Curie Scholarship. D.B. is a FNRS Research Director. Both A.J.M and D.B. acknowledge support from the European Community’s Initial Training Network SUPERIOR (PITN-GA-2009-238177). Further funding for this project came from the Engineering and Physical Sciences Research Council (EPSRC) and a pump-prime grant from the Winton Programme for the Physics of Sustainability.This is the accepted version of an article originally published in Nature Chemistry 5, 1019–1024 and available online at http://www.nature.com/nchem/journal/v5/n12/full/nchem.1801.html. Nature Publishing Group's conditions for reuse are detailed at http://www.nature.com/authors/policies/license.html

Critical Assessment of the Use of Excess Lead Iodide in Lead Halide Perovskite Solar Cells.

It is common practice in the lead halide perovskite solar cell field to add a small molar excess of lead iodide (PbI2) to the precursor solution to increase the device performance. However, recent reports have shown that an excess of PbI2 can accelerate performance loss. In addition, PbI2 is photoactive (band gap ∼2.3 eV), which may lead to parasitic absorption losses in a solar cell. Here we show that devices using small quantities of excess PbI2 exhibit better device performance as compared with stoichiometric devices, both initially and for the duration of a stability test under operating conditions, primarily by enhancing the charge extraction. However, the photolysis of PbI2 negates the beneficial effect on charge extraction by leaving voids in the perovskite film and introduces trap states that are detrimental for device performance. We propose that although excess PbI2 provides a good template for enhanced performance, the community must continue to seek other additives or synthesis routes that fulfill the same beneficial role as excess PbI2, but without the photolysis that negates these beneficial effects under long-term device operation