In this work, the authors realize stable and highly efficient wide‐bandgap perovskite solar cells that promise high power conversion efficiencies (PCE) and are likely to play a key role in next generation multi‐junction photovoltaics (PV). This work reports on wide‐bandgap (≈1.72 eV) perovskite solar cells exhibiting stable PCEs of up to 19.4% and a remarkably high open‐circuit voltage (VOC) of 1.31 V. The VOC‐to‐bandgap ratio is the highest reported for wide‐bandgap organic−inorganic hybrid perovskite solar cells and the VOC also exceeds 90% of the theoretical maximum, defined by the Shockley–Queisser limit. This advance is based on creating a hybrid 2D/3D perovskite heterostructure. By spin coating n‐butylammonium bromide on the double‐cation perovskite absorber layer, a thin 2D Ruddlesden–Popper perovskite layer of intermediate phases is formed, which mitigates nonradiative recombination in the perovskite absorber layer. As a result, VOC is enhanced by 80 mV.

The fact that organic solar cells perform efficiently despite the low dielectric constant of most photoactive blends initiated a long-standing debate regarding the dominant pathways of free charge formation. Here, we address this issue through the accurate measurement of the activation energy for free charge photogeneration over a wide range of photon energy, using the method of time-delayed collection field. For our prototypical low bandgap polymer:fullerene blends, we find that neither the temperature nor the field dependence of free charge generation depend on the excitation energy, ruling out an appreciable contribution to free charge generation though hot carrier pathways. On the other hand, activation energies are on the order of the room temperature thermal energy for all studied blends. We conclude that charge generation in such devices proceeds through thermalized charge transfer states, and that thermal energy is sufficient to separate most of these states into free charges.

Sustained stimulated emission under continuous-wave (CW) excitation is a prerequisite for new semiconductor materials being developed for laser gain media. Although hybrid organic-inorganic lead-halide perovskites have attracted much attention as optical gain media, the demonstration of room-temperature CW lasing has still not been realized. Here, we present a critical step towards this goal by demonstrating CW amplified spontaneous emission (ASE) in a phase-stable perovskite at temperatures up to 120 K. The phase-stable perovskite maintains its room-temperature phase while undergoing cryogenic cooling and can potentially support CW lasing also at higher temperatures. We find the threshold level for CW ASE to be 387 W cm -2 at 80 K. These results indicate that easily-fabricated single-phase perovskite thin films can sustain CW stimulated emission, potential at higher temperatures as well, by further optimization of the material quality in order to extend the carrier lifetimes.

In molecular solids, the intense photoluminescence (PL) observed for solvated dye molecules is often suppressed by nonradiative decay processes introduced by excitonic coupling to adjacent chromophores. We have developed a strategy to avoid this undesirable PL quenching by optimizing the chromophore packing. We integrated the photoactive compounds into metal-organic frameworks (MOFs) and tuned the molecular alignment by introducing adjustable “steric control units” (SCUs). We determined the optimal alignment of core-substituted naphthalenediimides (cNDIs) to yield highly emissive J-aggregates by a computational analysis. Then, we created a large library of handle-equipped MOF chromophoric linkers and computationally screened for the best SCUs. A thorough photophysical characterization confirmed the formation of J-aggregates with bright green emission, with unprecedented photoluminescent quantum yields for crystalline NDI-based materials. This data demonstrates the viability of MOF-based crystal engineering approaches that can be universally applied to tailor the photophysical properties of organic semiconductor materials.

An ideal material for photon harvesting must allow control of the exciton diffusion length and directionality. This is necessary in order to guide excitons to a reaction center, where their energy can drive a desired process. To reach this goal both of the following are required; short- and long-range structural order in the material and a detailed understanding of the excitonic transport. Here we present a strategy to realize crystalline chromophore assemblies with bespoke architecture. We demonstrate this approach by assembling anthracene dibenzoic acid chromophore into a highly anisotropic, crystalline structure using a layer-by-layer process. We observe two different types of photoexcited states; one monomer-related, the other excimer-related. By incorporating energy-accepting chromophores in this crystalline assembly at different positions, we demonstrate the highly anisotropic motion of the excimer-related state along the [010] direction of the chromophore assembly. In contrast, this anisotropic effect is inefficient for the monomer-related excited state.

This work studies spontaneous enhancement in power conversion efficiency of multi-cation-halide perovskite solar cells which occurs days after device fabrication.

enormous interest in perovskite-based multi-junction photovoltaics (PV). [1] To go beyond Shockley-Queisser radiative efficiency limit for single-junction solar cells, wide-bandgap (WBG) perovskite top solar cells (E G > 1.6 eV) [5] are combined with high-efficiency low-bandgap (LBG) bottom solar cells made from Si, [6] CIGS [7] or LBG (E G < 1.3 eV) perovskite devices. [8][9][10] While tandem PV technologies based on market-dominant crystalline Si and CIGS bottom solar cells have recently demonstrated PCEs exceeding 28%, [6,11] all-perovskite tandem solar cells are still less advanced. In comparison to single junction PSCs, all-perovskite tandem solar cells still lack behind with record PCEs of 23.1% [12] and 25% [12] for of all-perovskite two-terminal (2T) and four-terminal (4T) tandem solar cells, respectively.The key challenges hindering the progress of all-perovskite tandem solar cells are the low performance and stability of the LBG perovskite bottom solar cells. To resolve these challenges, previous studies on LBG perovskite thin films addressed compositional engineering of the perovskite, strategies to improve the thin-film morphology, and routes to enhance the optical and electrical properties. [8,10,[12][13][14][15] LBG All-perovskite multijunction photovoltaics, combining a wide-bandgap (WBG) perovskite top solar cell (E G ≈1.6-1.8 eV) with a low-bandgap (LBG) perovskite bottom solar cell (E G < 1.3 eV), promise power conversion efficiencies (PCEs) >33%. While the research on WBG perovskite solar cells has advanced rapidly over the past decade, LBG perovskite solar cells lack PCE as well as stability. In this work, vacuum-assisted growth control (VAGC) of solution-processed LBG perovskite thin films based on mixed Sn-Pb perovskite compositions is reported. The reported perovskite thin films processed by VAGC exhibit large columnar crystals. Compared to the well-established processing of LBG perovskites via antisolvent deposition, the VAGC approach results in a significantly enhanced charge-carrier lifetime. The improved optoelectronic characteristics enable high-performance LBG perovskite solar cells (1.27 eV) with PCEs up to 18.2% as well as very efficient four-terminal all-perovskite tandem solar cells with PCEs up to 23%. Moreover, VAGC leads to promising reproducibility and potential in the fabrication of larger active-area solar cells up to 1 cm 2 .In recent years, hybrid organic-inorganic perovskite materials attracted tremendous attention due to their outstanding optoelectronic and piezoelectric properties. [1][2][3] The optoelectronic properties of the perovskite materials enables power conversion efficiencies (PCEs) as high as 25.2% in singlejunction perovskite thin-film solar cells. [4] Moreover, the wide range of bandgaps (E G ) of this class of materials generates Adv. Energy perovskite thin films are realized by careful compositional engineering, incorporating Sn at the site of Pb in multication perovskite crystal structures. [8,10,13] In this regard, the exact ratio of Sn to Pb is criti...

and with low trap-state densities. [ 3 ] This enabled the fabrication of perovskites solar cells that convinced the solar cell community with high performances such as power conversion effi ciencies of over 20%, [ 4,5 ] while offering the possibility for low cost production, e.g., by solution-processing. [ 6 ] Meanwhile, further device applications for organic-inorganic mixed halide perovskites have been discovered. For example, in 2014, low threshold levels for amplifi ed spontaneous emission showed that mixed halide perovskites can also be used for the facile fabrication of lasers with high quality factors. [7][8][9][10] Furthermore, it is known that most halide perovskite materials can exist in different crystal structures, depending on environmental conditions such as temperature. [11][12][13] Here, we show that a coexistence of tetragonal and orthorhombic phases within apparently the same crystalline grain can be optically induced into the halide perovskite CH 3 NH 3 PbI 3 at low temperatures, leading to amplifi ed spontaneous emission (ASE) simultaneously at two distinct wavelengths. The ASE feature associated with the (high temperature) tetragonal phase can be reproducibly written, read-out, and erased at 5 K by choosing appropriateThe photoluminescence in a lead halide perovskite is measured for different temperatures (5-300 K) and excitation fl uences (21-1615 µJ cm −2 ). It is found that amplifi ed spontaneous emission (ASE) is observed for an excitation density larger than about 1 × 10 18 cm −3 for both the tetragonal phase above 163 K and the orthorhombic phase below about 163 K. The fl uence that is required to obtain this excitation density depends on temperature and phase since the nonradiative decay of excitations is temperature activated with different activation energies of 85 20 ± and 24 5 meV ± for the tetragonal and orthorhombic phase, respectively. The ASE from the tetragonal phase-usually prevailing at temperatures above about 163 K-can also be observed at 5 K, in addition to the ASE from the orthorhombic phase, when the sample is previously exposed to a fl uence exceeding 630 µJ cm −2 at a photon energy of 3.68 eV. This additional ASE can be removed by mild heating to 35 K or optically, by exposing the sample by typically a few seconds with a fl uence around 630 µJ cm −2 . The physical mechanism underlying this optically induced phase transition process is discussed. It is demonstrated that this phase change can, in principle, be used for an all-optical "write-read-erase" memory device.