Compact Layers of Hybrid Halide Perovskites Fabricated via the Aerosol Deposition Process : Uncoupling Material Synthesis and Layer Formation

We present the successful fabrication of CH3NH3PbI3 perovskite layers by the aerosol deposition method (ADM). The layers show high structural purity and compactness, thus making them suitable for application in perovskite-based optoelectronic devices. By using the aerosol deposition method we are able to decouple material synthesis from layer processing. Our results therefore allow for enhanced and easy control over the fabrication of perovskite-based devices, further paving the way for their commercialization

First of Their Kind : Solar Cells with a Dry-Processed Perovskite Absorber Layer via Powder Aerosol Deposition and Hot-Pressing

Preparing halide perovskite films by solvent-free, powder-based processing approaches currently attracts more and more attention. However, working solar cells employing dry, powder-based halide perovskite thin films, have not been demonstrated so far. Herein, perovskite solar cells are presented where the absorber layer is prepared by transferring readily synthesized perovskite powders into a compact thin film using a fully dry-powder-processing concept. Compact thin films are deposited via an optimized powder aerosol deposition (PAD) process. Pressing at 120 °C further improves the morphology and the optoelectronic film properties. Integrating the perovskite films in a solar cell configuration results in fully working devices, with champion power conversion efficiencies of >6%. While the (optoelectronic) properties of the PAD-processed films are found to be comparable with their solution-processed counterparts, investigations of the solar cell stack suggest deterioration of the electron-transport layer properties due to the PAD process, and the presence of hydrates at the perovskite surface to be important factors that contribute to the limited solar cell efficiency. Herein, perspectives to overcome the identified limitations are outlined, emphasizing the high potential and realizability of efficient perovskite solar cells based on dry-powder-processing approaches in the future

Organic Bidirectional Phototransistors Based on Diketopyrrolopyrrole and Fullerene

It is shown that simple bilayer devices consisting of the diketopyrrolopyrrole (DPP) monomer Ph-TDPP-Ph as donor and C-60 as acceptor feature J-V-characteristics of a bidirectional organic phototransistor where illumination intensity plays the role of the gate voltage as compared to a conventional field-effect transistor. The output current may therefore be controlled both electrically and optically. The underlying mechanism is based on the good charge transport in Ph-TDPP-Ph and C-60, the intrinsic dissociation properties of C-60, and the presence of an injection barrier for holes. In addition to this, it is demonstrated that the observed behavior of the DPP/C-60 system allows the realization of basic logic elements like NOT-, AND-, and OR-Gates, which may provide the basis for advanced analog and digital applications

What is the binding energy of a charge transfer state in an organic solar cell?

The high efficiencies reported for organic solar cells and an almost negligible thermal activation measured for the photogeneration of charge carriers have called into question whether photoinduced interfacial charge transfer states are bound by a significant coulomb attraction, and how this can be reconciled with very low activation energies. Here, this question is addressed in a combined experimental and theoretical approach. The interfacial binding energy of a charge-transfer state in a blend of MeLPPP:PCBM is determined by using energy resolved electrochemical impedance spectroscopy and is found to be about 0.5 eV. Temperature-dependent photocurrent measurements on the same films, however, give an activation energy that is about one order of magnitude lower. Using analytical calculations and Monte Carlo simulation the authors illustrate how i) interfacial energetics and ii) transport topology reduce the activation energy required to separate the interfacial electron–hole pair, with about equal contributions from both effects. The activation energy, however, is not reduced by entropy, although entropy increases the overall photodissociation yield. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimGerman Science Foundation DFG [GRK1640]; Bavarian State Ministry of Education, Science and the Arts through the Collaborative Research Network " Solar Technologies go Hybrid" (SolTech); EU-Marie-Sklodowska-Curie-ITN Network INFORM; Elite Network Bavaria (ENB); CONEX program; Universidad Carlos III de Madrid; European Union's Seventh Framework Programme for research, technological development and demonstration [600371]; Banco Santander; el Ministerio de Economia, Industria y Competitividad [COFUND2014-51509]; el Ministerio de Educacion, Cultura y Deporte [CEI-15-17

The Impact of Driving Force and Temperature on the Electron Transfer in Donor–Acceptor Blend Systems

We discuss whether electron transfer from a photoexcited polymer donor to a fullerene acceptor in an organic solar cell is tractable in terms of Marcus theory, and whether the driving force Δ<i>G</i>₀ is crucial in this process. Considering that Marcus rates are presumed to be thermally activated, we measured the appearance time of the polaron (i.e., the radical-cation) signal between 12 and 295 K for the representative donor polymers PTB7, PCPDTBT, and Me-LPPP in a blend with PCBM as acceptor. In all cases, the dissociation process was completed within the temporal resolution of our experimental setup (220–400 fs), suggesting that the charge transfer is independent of Δ<i>G</i>₀. We find that for the PCPDTBT:PCBM (Δ<i>G</i>₀ ≈ −0.2 eV) and PTB7:PCBM (Δ<i>G</i>₀ ≈ −0.3 eV) the data is mathematically consistent with Marcus theory, yet the condition of thermal equilibrium is not satisfied. For MeLPPP:PCBM, for which electron transfer occurs in the inverted regime (Δ<i>G</i>₀ ≈ −1.1 eV), the dissociation rate is inconsistent with Marcus theory but formally tractable using the Marcus–Levich–Jortner tunneling formalism which also requires thermal equilibrium. This is inconsistent with the short transfer times we observed and implies that coherent effects need to be considered. Our results imply that any dependence of the total yield of the photogeneration process must be ascribed to the secondary escape of the initially generated charge transfer state from its Coulomb potential