Organic–inorganic lead halide perovskite solar cells are promising alternatives to silicon‐based cells due to their low material costs and high photovoltaic performance. In this work, thin continuous perovskite films are combined with copper(I) iodide (CuI) as inorganic hole‐conducting material to form a planar device architecture. A maximum conversion efficiency of 7.5% with an average efficiency of 5.8 ± 0.8% is achieved which, to our knowledge, is the highest reported efficiency for CuI‐based devices with a planar structure. In contrast to related planar 2,2′,7,7′‐tetrakis‐(N,N ‐di‐4‐methoxyphenylamino)‐9,9′‐spirobifluorene (spiro‐OMeTAD)‐based devices, the CuI‐based devices do not show a pronounced hysteresis when tested by scanning the potential in a forward and backward direction. The strong quenching of photoluminescence (PL) signal and comparatively fast decay of open‐circuit voltage demonstrates a more rapid removal of positive charge carriers from the perovskite layer when in contact with CuI compared to spiro‐OMeTAD. A slow response on a timescale of 10–100 s is observed for the spiro‐OMeTAD‐based devices. In comparison, the CuI‐based device displays a significantly faster response as determined through electrochemical impedance spectroscopy (EIS) and open‐circuit voltage decays (OCVDs). The characteristically slow kinetics measured through EIS and OCVD are linked directly to the current–voltage hysteresis.

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We report the discovery of a tandem catalytic process to reduce energy demanding substrates, using the [Ir(ppy) 2 (dtb-bpy)] + (1 + ) photocatalyst. The immediate products of photoinitiated electron transfer (PET) between 1 + and triethylamine (TEA) undergo subsequent reactions to generate a previously unknown, highly reducing species (2). Formation of 2 occurs via reduction and semisaturation of the ancillary dtb-bpy ligand, where the TEA radical cation serves as an effective hydrogen atom donor, confirmed by nuclear magnetic resonance, mass spectrometry, and deuterium labeling experiments. Steady-state and time-resolved luminescence and absorption studies reveal that upon irradiation, 2 undergoes electron transfer or proton-coupled electron transfer (PCET) with a representative acceptor (N-(diphenylmethylene)-1-phenylmethanamine; S). Turnover of this new photocatalytic cycle occurs along with the reformation of 1 + . We rationalize our observations by proposing the first example of a mechanistic pathway where two distinct yet interconnected photoredox cycles provide access to an extended reduction potential window capable of engaging a wide range of energy demanding and synthetically relevant organic substrates including aryl halides.

Impedance spectroscopy (IS) is emerging as a valuable tool for the characterization of perovskite-based solar cells (PSCs). However, earlier reports of the IS response of mesoscopic PSCs have revealed notable discrepancies, with the interpretation of their spectra having been generalized to planar PSC devices despite fundamental differences in the device operation. The present study analyzes the impedance response of planar PSC devices through the characterization of cells employing a variety of constituent layers. Distinctive high-frequency and low-frequency features are observed in IS measurements and are attributed to the charge recombination across the perovskite/contact interfaces and the dielectric relaxation in the interfacial regions of the perovskite layer, respectively. Comparison of the characteristic IS time constants with time-resolved photoluminescence (TRPL) and open-circuit voltage decay (OCVD) measurements further substantiates the proposed impedance model. This work provides an empirical foundation for the interpretation of impedance spectra in planar PSCs, and develops the prospects of IS as a valuable diagnostic tool for future characterization of planar PSC devices.

A facile method to produce metal halide perovskite solar cells with improved stability and efficiency.

Remarkable power conversion efficiencies (PCE) of metal halide perovskite solar cells (PSCs) are overshadowed by concerns about their ultimate stability, which is arguably the prime obstacle to commercialisation of this promising technology. Herein, the problem is addressed by introducing ethane-1,2-diammonium ( + NH 3 (CH 2 ) 2 NH 3 + , EDA 2+ ) cations into the methyl ammonium (CH 3 NH 3 + , MA + ) lead iodide perovskite, which enables, inter alia, systematic tuning of the morphology, electronic structure, light absorption and photoluminescence properties of the perovskite films. Incorporation of <5 mol% EDA 2+ induces strain in the perovskite crystal structure with no new phase formed. With 0.8 mol% EDA 2+ , PCE of the MAPbI 3 -based PSCs (aperture of 0.16 cm 2 ) improves from 16.7 ±0.6 % to 17.9 ± 0.4% under 1 sun irradiation, and fabrication of larger area devices (aperture 1.04 cm 2 ) with a certified PCE of 15.2 ± 0.5% is demonstrated. Most importantly, EDA 2+ /MA + -based solar cells retain 75% of the initial performance after 72 hours of continuous operation at 50% relative humidity and 50 ºC under 1 sun illumination, whereas the MAPbI 3 devices degrade by approximately 90% within only 15 hours. This substantial improvement in stability is attributed to the steric and coulombic interactions of embedded EDA 2+ in the perovskite structure.

repeating in three dimensions and typically comprising organic cations (A) such as methylammonium (MA) or formamidinium (FA), metallic cations (M) (usually Pb 2+ or Sn 2+ ), and halide ions (X) (I − , Br − , Cl − ) according to the formula AMX 3 . [8][9][10] These materials possess many attractive properties for photovoltaic applications including a high light-absorption coefficient, high charge-carrier mobility, compatibility with low-cost solution processing, and potential ease of high-volume fabrication on flexible substrates using conventional roll-to-roll (R2R) printing and coating technologies. [8][9][10][11][12][13] However, these so-called "3D-perovskites" suffer from a low environmental stability, caused by weak lightinduced interactions between the organic cations and surrounding halide anions, and a susceptibility to hydrolytic reactions of the organic cations on exposure to moisture. [14] This has been one of the main limiting factors in the commercialization of perovskitebased solar cells and other optoelectronic devices. [15,16] More recently, Ruddlesden-Popper layered perovskites (R 2 A n−1 M n X 3n+1 ; n = 1 → ∞), particularly the subset with n ≤ 4 comprising 2D stacks of 3D perovskites, have garnered more attention due to their interesting optoelectronic properties [17][18][19] and considerably higher stability compared to 3D perovskites. [20][21][22][23] These 2D-perovskite materials are typically prepared by utilizing larger organic cations (R) such as phenylethylammonium or butylammonium (BA) in the formulation. The longer alkyl chains or aromatic moieties of these larger cations not only tune the material optoelectronic properties, 2D organic-inorganic hybrid Ruddlesden-Popper perovskites have emerged recently as candidates for the light-absorbing layer in solar cell technology due largely to their impressive operational stability compared with their 3D-perovskite counterparts. The methods reported to date for the preparation of efficient 2D perovksite layers for solar cells involve a nonscalable spincoating step. In this work, a facile, spin-coating-free, directly scalable dropcast method is reported for depositing precursor solutions that self-assemble into highly oriented, uniform 2D-perovskite films in air, yielding perovskite solar cells with power conversion efficiencies (PCE) of up to 14.9% (certified PCE of 14.33% ± 0.34 at 0.078 cm 2 ). This is the highest PCE to date for a solar cell with 2D-perovskite layers fabricated by nonspin-coating method. The PCEs of the cells display no evidence of degradation after storage in a nitrogen glovebox for more than 5 months. 2D-perovskite layer deposition using a slot-die process is also investigated for the first time. Perovskite solar cells fabricated using batch slot-die coating on a glass substrate or R2R slot-die coating on a flexible substrate produced PCEs of 12.5% and 8.0%, respectively.

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