Incompatible Magnetic Order in Multiferroic Hexagonal DyMnO3

Magnetic order of the manganese and rare-earth lattices according to different symmetry representations is observed in multiferroic hexagonal (h-) DyMnO$_3$ by optical second harmonic generation and neutron diffraction. The incompatibility reveals that the 3d-4f coupling in the h-$R$MnO$_3$ system ($R$ = Sc, Y, In, Dy - Lu) is substantially less developed than commonly expected. As a consequence, magnetoelectric coupling effects in this type of split-order parameter multiferroic that were previously assigned to a pronounced 3d-4f coupling have now to be scrutinized with respect to their origin

Continuous lasing for perovskites

Optically generated local phase changes in methylammonium lead iodide produce a transient quantum well structure with robust optical gain. The result is a perovskite laser that supports continuous-wave lasing under optical pumping.PostprintNon peer reviewe

Charge-carrier dynamics and mobilities in formamidinium lead mixed-halide perovskites

The mixed‐halide perovskite FAPb(BryI1–y)3 is attractive for color‐tunable and tandem solar cells. Bimolecular and Auger charge‐carrier recombination rate constants strongly correlate with the Br content, y, suggesting a link with electronic structure. FAPbBr3 and FAPbI3 exhibit charge‐carrier mobilities of 14 and 27 cm2 V−1 s−1 and diffusion lengths exceeding 1 μm, while mobilities across the mixed Br/I system depend on crystalline phase disorder

High-performance inverted planar heterojunction perovskite solar cells based on lead acetate precursor with efficiency exceeding 18%

Organic-inorganic lead halide perovskites are emerging materials for the next-generation photovoltaics. Lead halides are the most commonly used lead precursors for perovskite active layers. Recently, lead acetate (Pb(Ac)2) has shown its superiority as the potential replacement for traditional lead halides. Here, we demonstrate a strategy to improve the efficiency for the perovskite solar cell based on lead acetate precursor. We utilized methylammonium bromide as an additive in the Pb(Ac)2 and methylammonium iodide precursor solution, resulting in uniform, compact and pinhole-free perovskite films. We observed enhanced charge carrier extraction between the perovskite layer and charge collection layers and delivered a champion power conversion efficiency of 18.3% with a stabilized output efficiency of 17.6% at the maximum power point. The optimized devices also exhibited negligible current density-voltage (J-V) hysteresis under the scanning conditions

Quantifying Efficiency Loss of Perovskite Solar Cells by a Modified Detailed Balance Model

A modified detailed balance model is built to understand and quantify efficiency loss of perovskite solar cells. The modified model captures the light-absorption dependent short-circuit current, contact and transport-layer modified carrier transport, as well as recombination and photon-recycling influenced open-circuit voltage. Our theoretical and experimental results show that for experimentally optimized perovskite solar cells with the power conversion efficiency of 19%, optical loss of 25%, non-radiative recombination loss of 35%, and ohmic loss of 35% are the three dominant loss factors for approaching the 31% efficiency limit of perovskite solar cells. We also find that the optical loss will climb up to 40% for a thin-active-layer design. Moreover, a misconfigured transport layer will introduce above 15% of energy loss. Finally, the perovskite-interface induced surface recombination, ohmic loss, and current leakage should be further reduced to upgrade device efficiency and eliminate hysteresis effect. The work contributes to fundamental understanding of device physics of perovskite solar cells. The developed model offers a systematic design and analysis tool to photovoltaic science and technology.Comment: 21 pages, 9 figures, 3 table

Carriers trapping and recombination: the role of defect physics in enhancing the open circuit voltage of metal halide perovskite solar cells

One of the greatest attributes of metal halide perovskite solar cells is their surprisingly low loss in potential between bandgap and open-circuit voltage, despite the fact that they suffer from a non-negligible density of sub gap defect states. Here, we use a combination of transient and steady state photocurrent and absorption spectroscopy to show that CH3NH3PbI3 films exhibit a broad distribution of electron traps. We show that the trapped electrons recombine with free holes unexpectedly slowly, on microsecond time scales, relaxing the limit on obtainable Open-Circuit Voltage (Voc) under trap-mediated recombination conditions. We find that the observed VOCs in such perovskite solar cells can only be rationalized by considering the slow trap mediated recombination mechanism identified in this work. Our results suggest that existing processing routes may be good enough to enable open circuit voltages approaching 1.3 V in ideal devices with perfect contacts

Preparation of Single-Phase Films of CH3NH3Pb(I1-xBrx)3 with Sharp Optical Band Edges.

Organometallic lead-halide perovskite-based solar cells now approach 18% efficiency. Introducing a mixture of bromide and iodide in the halide composition allows tuning of the optical bandgap. We prepare mixed bromide-iodide lead perovskite films CH3NH3Pb(I1-xBrx)3 (0 ≤ x ≤ 1) by spin-coating from solution and obtain films with monotonically varying bandgaps across the full composition range. Photothermal deflection spectroscopy, photoluminescence, and X-ray diffraction show that following suitable fabrication protocols these mixed lead-halide perovskite films form a single phase. The optical absorption edge of the pure triiodide and tribromide perovskites is sharp with Urbach energies of 15 and 23 meV, respectively, and reaches a maximum of 90 meV for CH3NH3PbI1.2Br1.8. We demonstrate a bromide-iodide lead perovskite film (CH3NH3PbI1.2Br1.8) with an optical bandgap of 1.94 eV, which is optimal for tandem cells of these materials with crystalline silicon devices.We acknowledge funding from the Engineering and Physical Sciences Research Council (EPSRC) and the Winton Programme (Cambridge) for the Physics of Sustainability. THT acknowledges funding from Cambridge Australia Scholarships and the Cambridge Commonwealth Trust. D.C. acknowledges support from St. John's College Cambridge and the Winton Programme (Cambridge) for the Physics of Sustainability.This is the final published version. It's also available at: http://pubs.acs.org/doi/abs/10.1021/jz501332v

Direct measurement of the exciton binding energy and effective masses for charge carriers in organic–inorganic tri-halide perovskites

Solar cells based on the organic-inorganic tri-halide perovskite family of materials have shown remarkable progress recently, offering the prospect of low-cost solar energy from devices that are very simple to process. Fundamental to understanding the operation of these devices is the exciton binding energy, which has proved both difficult to measure directly and controversial. We demonstrate that by using very high magnetic fields it is possible to make an accurate and direct spectroscopic measurement of the exciton binding energy, which we find to be only 16 meV at low temperatures, over three times smaller than has been previously assumed. In the room temperature phase we show that the binding energy falls to even smaller values of only a few millielectronvolts, which explains their excellent device performance due to spontaneous free carrier generation following light absorption. Additionally, we determine the excitonic reduced effective mass to be 0.104me (where me is the electron mass), significantly smaller than previously estimated experimentally but in good agreement with recent calculations. Our work provides crucial information about the photophysics of these materials, which will in turn allow improved optoelectronic device operation and better understanding of their electronic properties