High Quality Inkjet Printed‐Emissive Nanocrystalline Perovskite CsPbBr3 Layers for Color Conversion Layer and LEDs Applications

Metal halide perovskites (MHPs) have shown outstanding optical emissive properties and can be employed in several optoelectronics devices. In contrast with materials of well-established technologies, which are prone to degradation or require expensive processes, MHPs can be obtained by solution processing methods and increase stability. Inkjet printing is proposed as an industrial friendly technique to deposit MHPs. The inks have been developed from colloidal CsPbBr3 nanocrystals and printing procedures that allow the deposition of thin layers with intense green emission. High emissive printed layers are assured by carrying out thermal annealing in vacuum oven, which is demonstrated to promote compact layers with low roughness, corroborated by SEM and AFM. XRD measurements show CsPbBr3 crystalline layers with cubic symmetry and XPS provides insight into the stoichiometric composition and local bonding. Optical properties of inkjet-printed CsPbBr3 films have been analyzed by UV-vis absorbance and photoluminescence (PL), to extract the bandgap energy and photoluminescence quantum yield (PLQY). CsPbBr3 printed layers emit at 524 nm with a narrow emission (FWHM ≈ 15 nm), exhibiting a PLQY up to 20%. These results enabled the large-scale fabrication by inkjet printing of CsPbBr3 color conversion layers (CCLs) and pave the way for flexible LEDs

Energy transfer versus charge separation in hybrid systems of semiconductor quantum dots and Ru-dyes as potential co-sensitizers of TiO2-based solar cells.

Hybrid structures of colloidal quantum dots (QDs) with Ru-dyes have been studied as candidates for panchromatic sensitizers for TiO2-based solar cells. Steady-state and time resolved photoluminescence spectroscopy and photocurrent measurements have been employed to identify the prevailing transfer mechanisms for photogenerated excitons between CdSe QDs capped with a traditional bulky organic ligand trioctylphosphine and Ru-dyes (N3 or Ru505) deposited onto inert glass or mesoporous TiO2 substrates. The type II energy level alignment between the QDs and both N3 and Ru505 offers a possibility for the directional charge separation, with electrons transferred to the QDs and holes to the dye. This scenario is indeed valid for the QD/Ru505 and TiO2/QD/Ru505 hybrid systems, with the negligible spectral overlap between the emission of the QDs and the absorption of the Ru505 dye. For the QD/N3 and TiO2/QD/N3 hybrid systems, the spectral overlap favors the longer range energy transfer from the QDs to N3, independently of the presence of the electron acceptor TiO2

Influence of the substrate on the bulk properties of hybrid lead halide perovskite films

In addition to the known effect of the substrate on the interfacial properties of perovskite films, here we show that the bulk properties of hybrid lead halide perovskite films depend on the type of substrate used for film growth. Despite the relative large film thickness, ∼600 nm, the roughness and nature of the substrate layer (glass, FTO, TiO and PEDOT:PSS) affect not just the degree of preferential orientation and crystal grain size but also the lattice parameters of CHNHPbI films synthesized from the PbCl precursor. The obtained changes in lattice parameters indicate that the Pb-Pb distance varies by around 0.7%. We suggest that the substrate roughness and chemical nature determine the concentration of defects mainly by varying the chlorine content and probably by the incorporation of oxygen and iodine vacancies during film nucleation and growth. These differences also have consequences in the observed light induced transformations. Upon laser illumination, the formation of additional defects, most probably related to oxygen, is revealed by 110 and 165 cm Raman peaks. With increasing laser power the chemical transformation into PbO is clearly identified by the 140 and 275 cm Raman peaks. The irreversible photoluminescence enhancement observed at low power with illumination time, also dependent on the substrate nature, is proposed to be due to the localization of the electron-hole excitons created in the vicinity of the light generated defects. These results shed light on the performance of the perovskite layer and help to understand how bulk processes, where ion migration is a conspicuous example, are severely affected by interfacial properties such as those imposed by the substrate.Funding by the Spanish Ministerio de Economía y Competitividad (MINECO) under Projects MAT2015-65356-C3-1-R and 2-R, MAT2014-54852-R and MAT2015-70611-ERC and Comunidad de Madrid Excellence Network under Project S2013/MIT-2740 (and Associated Lab LABCADIO belonging to CM net labs ref. 351) is acknowledged. We also acknowledge the MINECO for financial support and provision of synchrotron radiation facilities at ESRF, and thank María Vila for her assistance in using beamline BM25-SpLine. B. C. H. is grateful to the support of the National Council of Technological and Scientific Development (CNPq), Brazil, through the Science without Borders program

Theory of Impedance Spectroscopy of Ambipolar Solar Cells with Trap-Mediated Recombination

The analysis of recombination in solar cells suggests in many cases the presence of trap-mediated recombination in the absorber. We present a theory of the recombination of electrons and holes, the Shockley–Read–Hall model, using the impedance spectroscopy technique. We derive the impedance functions and the corresponding equivalent circuit model. After examining some cases of interest, we show that two semicircles can be obtained in the recombination circuit only if the chemical capacitance associated with traps is substantially larger than the chemical capacitances of free electrons and holes in the absorber bands, while in the other cases the normal behavior of one recombination arc will be obtained

Role of the Selective Contacts in the Performance of Lead Halide Perovskite Solar Cells

The effect of electron- and hole-selective contacts in the final cell performance of hybrid lead halide perovskite, CH3NH3PbI3, solar cells has been systematically analyzed by impedance spectroscopy. Complete cells with compact TiO2 and spiro-OMeTAD as electron- and hole-selective contacts have been compared with incomplete cells without one or both selective contacts to highlight the specific role of each contact. It has been described how selective contacts contribute to enhance the cell FF and how the hole-selective contact is mainly responsible for the high Voc in this kind of device. We have determined that the recombination rate is mainly governed by the selective contacts. This fact has important implication for the future optimization of perovskite solar cells. Finally, we have developed a method to analyze the results obtained, and it has been applied for three different electron-selecting materials: TiO2, ZnO, and CdS