The effect of electron-and hole-selective contacts in the final cell performance of hybrid lead halide perovskite, CH 3 NH 3 PbI 3 , solar cells has been systematically analyzed by impedance spectroscopy. Complete cells with compact TiO 2 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 V oc 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: TiO 2 , ZnO, and CdS. SECTION: Energy Conversion and Storage; Energy and Charge Transport

Recently developed organic-inorganic hybrid perovskite solar cells combine low-cost fabrication and high power conversion efficiency. Advances in perovskite film optimization have led to an outstanding power conversion efficiency of more than 20%. Looking forward, shifting the focus toward new device architectures holds great potential to induce the next leap in device performance. Here, we demonstrate a perovskite/perovskite heterojunction solar cell. We developed a facile solution-based cation infiltration process to deposit layered perovskite (LPK) structures onto methylammonium lead iodide (MAPI) films. Grazing-incidence wide-angle X-ray scattering experiments were performed to gain insights into the crystallite orientation and the formation process of the perovskite bilayer. Our results show that the self-assembly of the LPK layer on top of an intact MAPI layer is accompanied by a reorganization of the perovskite interface. This leads to an enhancement of the open-circuit voltage and power conversion efficiency due to reduced recombination losses, as well as improved moisture stability in the resulting photovoltaic devices.

The chemical state, electronic properties, and geometric structure of methyl-terminated Si͑111͒ surfaces prepared using a two-step chlorination/alkylation process were investigated using high-resolution synchrotron photoelectron spectroscopy and low-energy electron diffraction methods. The electron diffraction data indicated that the methylated Si surfaces maintained a ͑1 ϫ 1͒ structure, where the dangling bonds of the silicon surface atoms were terminated by methyl groups. The surfaces were stable to annealing at 720 K. The high degree of ordering was reflected in a well-resolved vibrational fine structure of the carbon 1s photoelectron emission, with the fine structure arising from the excitation of C-H stretching vibrations having h = 0.38± 0.01 eV. The carbon-bonded surface Si atoms exhibited a well-defined x-ray photoelectron signal having a core level shift of 0.30± 0.01 eV relative to bulk Si. Electronically, the Si surface was close to the flat-band condition. The methyl termination produced a surface dipole of −0.4 eV. Surface states related to CH 3 and Si-C bonding orbitals were identified at binding energies of 7.7 and 5.4 eV, respectively. Nearly ideal passivation of Si͑111͒ surfaces can thus be achieved by methyl termination using the two-step chlorination/ alkylation process.

The development of medium-bandgap solar cell absorber materials is of interest for the design of devices such as tandem solar cells and building-integrated photovoltaics. The recently developed perovskite solar cells can be suitable candidates for these applications. At present, wide bandgap alkylammonium lead bromide perovskite absorbers require a high-temperature sintered mesoporous TiO2 photoanode in order to function efficiently, which makes them unsuitable for some of the above applications. Here, we present for the first time highly efficient wide bandgap planar heterojunction solar cells based on the structurally related formamidinium lead bromide. We show that this material exhibits much longer diffusion lengths of the photoexcited species than its methylammonium counterpart. This results in planar heterojunction solar cells exhibiting power conversion efficiencies approaching 7%. Hence, formamidinium lead bromide is a strong candidate as a wide bandgap absorber in perovskite solar cells.

Available online xxx a b s t r a c tThis overview addresses the atomistic aspects of degradation of layered LiMO 2 oxide Li-ion cell cathode materials, aiming to shed light on the fundamental degradation mechanisms especially inside active cathode materials and at their interfaces. It includes recent results obtained by novel in situ/in operando diffraction methods, modelling, and quasi in situ surface science analysis. Degradation of the active cathode material occurs upon overcharge, resulting from a positive potential shift of the anode. Oxygen loss and eventual phase transformation resulting in dead regions are ascribed to changes in electronic structure and defect formation. The anode potential shift results from loss of free lithium due to side reactions occurring at electrode/electrolyte interfaces. Such side reactions are caused by electron transfer, and depend on the electron energy level alignment at the interface. Side reactions at electrode/electrolyte interfaces and capacity fade may be overcome by the use of suitable solid-state electrolytes and Licontaining anodes.