Real-space observation of unbalanced charge distribution inside a perovskite-sensitized solar cell; Victor W. Bergmann, Stefan A. L. Weber, F. Javier Ramos, Mohammad Khaja Nazeeruddin, Michael Grätzel, Dan Li, Anna L. Domanski, Ingo Lieberwirth, Shahzada Ahmad, Rüdiger Berger; Nat Commun, 2014, 5, 5001.
Perovskite-sensitized solar cells have reached power conversion efficiencies comparable to commercially available solar cells used for example in solar farms. In contrast to silicon solar cells, perovskite-sensitized solar cells can be made by solution processes from inexpensive materials. The power conversion efficiency of these cells depends substantially on the charge transfer at interfaces. Here we use Kelvin probe force microscopy to study the real-space cross-sectional distribution of the internal potential within high efficiency mesoscopic methylammonium lead tri-iodide solar cells. We show that the electric field is homogeneous through these devices, similar to that of a p-i-n type junction. On illumination under short-circuit conditions, holes accumulate in front of the hole-transport layer as a consequence of unbalanced charge transport in the device. After light illumination, we find that trapped charges remain inside the active device layers. Removing these traps and the unbalanced charge injection could enable further improvements in performance of perovskite-sensitized solar cells.
Yttrium-substituted nanocrystalline TiO2 photoanodes for perovskite based heterojunction solar cells; Peng Qin, Anna L. Domanski, Aravind Kumar Chandiran, Rüdiger Berger, Hans-Jürgen Butt, M. Ibrahim Dar, Thomas Moehl, Nicolas Tetreault, Peng Gao, Shahzada Ahmad, Mohammad K. Nazeeruddin and Michael Grätzel; Nanoscale; 2014, 6, 1508.
We have demonstrated an enhanced PCE with the use of 0.5% Y3+ substitution of TiO2 in the solid-state CH3NH3PbI3-based mesoscopic solar cells. With the one-step spin-coating process,the devices using 0.5%Y-TiO2 as the photoanode and spiro-OMeTAD as the hole transfer material yielded a PCE of 11.2% under 1 Sun illumination. Scanning probe microscopy allowed us to prove that the contact potential difference is homogeneously distributed at least on a 1 µm length scale. The localized properties complement the bulk properties, which clearly illustrates the efficient photo-induced charge separation process taking place in the cell.
Strategy for Good Dispersion of Well-Defined Tetrapods in Semiconducting Polymer Matrices; Jaehoon Lim, Lisa zur Borg, Stefan Dolezel, Friederike Schmid, Kookheon Char, Rudolf Zente; Macromol. Rapid. Commun., 2014, 35, 1685−1691.
The morphology or dispersion control in inorganic/organic hybrid systems is studied, which consist of monodisperse CdSe tetrapods (TPs) with grafted semiconducting block copolymers with excess polymers of the same type. Tetrapod arm-length and amount of polymer loading are varied in order to find the ideal morphology for hybrid solar cells. Additionally, polymers without anchor groups are mixed with the TPs to study the effect of such anchor groups on the hybrid morphology. A numerical model is developed and Monte Carlo simulations to study the basis of compatibility or dispersibility of TPs in polymer matrices are performed. The simulations show that bare TPs tend to form clusters in the matrix of excess polymers. The clustering is significantly reduced after grafting polymer chains to the TPs, which is confirmed experimentally. Transmission electron microscopy reveals that the block copolymer-TP mixtures (“hybrids”) show much better film qualities and TP distributions within the films when compared with the homopolymer-TP mixtures (“blends”), representing massive aggregations and cracks in the films. This grafting-to approach for the modification of TPs significantly improves the dispersion of the TPs in matrices of “excess” polymers up to the arm length of 100 nm.
Dual Functionalization of Nanostructures of Block Copolymers with Quantum Dots and Organic Fluorophores; Seungyong Chae, Jeong-Hee Kim, Patrick Theato, Rudolf Zentel, Byeong-Hyeok Sohn; Macromol. Chem. Phys., 2014, 215, 654−661.
Dual functionalization of nanostructures of block copolymers by simultaneous incorporation of two functionalities into the nanostructures is demonstrated. For this purpose, a blending strategy of two block copolymers is employed, which have the same basic chemical structure for each block but differ in small amounts of functional moieties in one of the blocks. Block copolymers containing thiol and fluorophore functionalities are synthesized, utilizing a block copolymer that features a reactive block of activated esters. With thiol-functionalized block copolymers, selective incorporation of quantum dots (QDs) to lamellar nanostructures is first validated by transmission electron microscopy. Fluorophore functionalization of lamellar nanostructures is also confirmed with fluorophore-anchored block copolymers by photoluminescence spectroscopy. Then, with a blending strategy of two functionalized block copolymers, dual functionalization of lamellar nanostructures of block copolymers is induced by simultaneous incorporation of QDs and organic fluorophores, which exhibit fluorescence resonance energy transfer due to nanoscale confinement.
Reduced efficiency roll-off in light-emitting diodes enabled by quantum dot–conducting polymer nanohybrids; Wan Ki Bae, Jaehoon Lim, Matthias Zorn, Jeonghun Kwak, Young-Shin Park, Donggu Lee, Seonghoon Lee, Kookheon Char, Rudolf Zentel, Changhee Lee; J. Mater. Chem.C, 2014, 2, 4974-4979.
We demonstrate QLEDs implementing wider active layers (50 nm) based on QD–conducting polymer nanohybrids, which exhibit a stable operational device performance across a wide range of current densities and brightness. A comparative study reveals that the significant suppression of efficiency roll-off in the high current density regime is primarily attributed to a sufficient charge carrier distribution over the wider active layer and improved charge carrier balance within QDs enabled by the hybridization of QDs with conducting polymers. Utilization of this finding in future studies should greatly facilitate the development of high performance, stable QLEDs at high current density or luminance regime toward displays or solid-state lighting applications.
Functionalization of TiO2 Nanoparticles with Semiconducting Polymers Containing a Photocleavable Anchor Group and Separation via Irradiation Afterward; Florian Mathias, Muhammad Nawaz Tahir, Wolfgang Tremel, Rudolf Zentel; Macromol. Chem. Phys., 2014, 215, 604–613.
The controlled radical polymerization (RAFT polymerization) of semiconducting polymers based on poly(4,4′-dimethyl-triphenylamine) is described. These polymers are afterward end-functionalized with a photocleavable group and an anchor unit (catechol) for oxidic nanoparticles (NPs). Serving as a reference, polystyrene oligomers with the same end groups are also synthesized. Using these polymers allows functionalization of the TiO2-NPs, leading to an improved solubility and miscibility in organic solvents or polymer matrices. Irradiation in the UV region is used to split the photocleavable group and remove the polymer chains from the NPs, which leads to their aggregation.