Interest in organic small molecules that exhibit second-scale phosphorescence at room temperature has grown immensely in recent years due to their potential applications in sensing, anticounterfeiting, and bioimaging. However, such material systems are rare—requiring second-scale triplet lifetimes, efficient intersystem crossing, and slow rates of nonradiative recombination. This third requirement has been met by isolating phosphors in a rigid matrix, or by aggregating them into densely packed crystals or powders to suppress the molecular vibrations that lead to recombination. While these techniques work well for a small subset of molecules with specific properties, most isolated molecules in a rigid matrix do not phosphoresce, and most macroscopic aggregates experience significant triplet quenching. In this work, we find a middle ground between these extremes by forming microscopic [approximately submicron sized] phosphor aggregates in rigid polymer matrices using a simple drop casting and thermal annealing process. Using this technique, we activate second-scale phosphorescence at room temperature in 20 molecules that do not otherwise phosphoresce using conventional matrix-isolation or crystallization approaches. We find that increased chromophore loading increases aggregate sizes. Excitons are thus able to diffuse further and interact more, and triplet-triplet annihilation dominates. Furthermore, we determine that excimer formation in some aggregates leads to increased rates of triplet generation—complementing the effect of nonradiative recombination suppression and further enhancing phosphorescence. In sum, the simplicity and robustness of this blending approach significantly loosens the design constraints to access second-scale emission with organic phosphors, allowing researchers to choose from a broader catalog of organic materials to match the desired properties for a given application.

Organic small molecules that exhibit second-scale phosphorescence at room temperature are of interest for potential applications in sensing, anticounterfeiting, and bioimaging. However, such materials systems are uncommon-requiring millisecond to second-scale triplet lifetimes, efficient intersystem crossing, and slow rates of nonradiative recombination. Here, a simple and scalable approach is demonstrated to activate long-lived phosphorescence in a wide variety of molecules by suspending them in rigid polymer hosts and annealing them above the polymer's glass transition temperature. This process produces submicron aggregates of the chromophore, which suppresses intramolecular motion that leads to nonradiative recombination and minimizes triplet-triplet annihilation that quenches phosphorescence in larger aggregates. In some cases, evidence of excimer-mediated intersystem crossing that enhances triplet generation in aggregated chromophores is found. In short, this approach circumvents the current design rules for long-lived phosphors, which will streamline their discovery and development.

Voltage-dependent characterizations of organic solar cells with brightly-emitting charge-transfer excitons reveal excitation dynamics and trends as a function of donor molecule.

Transparent photovoltaics (TPVs) can be integrated into the surfaces of buildings and vehicles to provide point‐of‐use power without impacting aesthetics. Unlike TPVs that target the photon‐rich near‐infrared portion of the solar spectrum, TPVs that harvest ultraviolet (UV) photons can have significantly higher transparency and color neutrality, offering a superior solution for low‐power electronics with stringent aesthetic tolerance. In addition to being highly transparent and colorless, an ideal UV‐absorbing TPV should also be operationally stable and scalable over large areas while still outputting sufficient power for its specified application. None of today's TPVs meet all these criteria simultaneously. Here, the first UV‐absorbing TPV is demonstrated that satisfies all four criteria by using CsPbCl 2.5 Br 0.5 as the absorber. By precisely tuning the halide ratio during thermal co‐evaporation, high‐quality large‐area perovskite films can be accessed with an ideal absorption cutoff for aesthetic performance. The resulting TPVs exhibit a record average visible transmittance of 84.6% and a color rendering index of 96.5, while maintaining an output power density of 11 W m −2 under one‐sun illumination. Further, the large‐area prototypes up to 25 cm 2 are demonstrated, that are operationally stable with extrapolated lifetimes of >20 yrs under outdoor conditions.

As the stability of organic and perovskite solar cells improves, accelerated ageing methods become increasingly essential to elucidate their long-term degradation mechanisms and to predict their real-world operational lifetimes. By effectively applying these underutilized tests, emerging photovoltaic technologies can be de-risked and their time to market can be expedited.

Perovskite CsPbI3 is a promising photovoltaic absorber material, thanks to its ideal bandgap for Si-tandem solar cell applications and its excellent thermochemical stability compared with hybrid organic–inorganic perovskites. However, CsPbI3 has its own stability challenges as its photoactive β- and γ-polymorphs are thermodynamically unstable at room temperature compared with the yellow non-perovskite δ-phase. Stabilizing CsPbI3 has, thus, been the subject of considerable research in recent years. While some approaches, such as alloying with halides and reducing crystalline domain size, have proven effective in improving phase stability, these benefits have, thus far, come at the expense of photovoltaic efficiency compared with the state-of-the-art CsPbI3 solar cells. In this perspective, we discuss the progress and limitations of inorganic perovskite stabilization techniques and look forward at how to achieve inorganic perovskite solar cells with both commercially viable efficiencies and lifetimes.

Organic photovoltaic cells that employ Y-series non-fullerene acceptors (NFAs) have recently achieved impressive power-conversion efficiencies (>18%). To fulfill their commercial promise, it is important to quantify their operational lifetimes and understand their degradation mechanisms. In this work, the spectral-dependent photostability of films and solar cells comprising several Y-series acceptors and the donor polymer PM6 is investigated systematically. By applying longpass filters during aging, it is shown that UV/near-UV photons are responsible for the photochemical decomposition of Y-series acceptors; this degradation is the primary driver of early solar cell performance losses. Using mass spectrometry, the vinylene linkage between the core and electron-accepting moieties of Y-series acceptors is identified as the weak point susceptible to cleavage under UV-illumination. Employing a series of device characterization, along with numerical simulations, the efficiency losses in organic photovoltaic cells are attributed to the formation of traps, which reduces charge extraction efficiency and facilitates non-radiative recombination as the Y-series acceptors degrade. This study provides new insights for molecular degradation of organic photovoltaic absorber materials and highlights the importance of future molecular design and strategies for improved solar cell stability.

All-inorganic beta-CsPbI3 has superior chemical and thermal stability compared to its hybrid counterparts, but the stability of state-of-the-art beta-CsPbI3 perovskite solar cells (PSCs) under normal operating conditions (i.e., under illumination in an inert atmosphere) remains inferior to their hybrid counterparts. Here, we found that the lattice distortion in CsPbI3 near the perovskite/electron transport layer (ETL) interface can induce polymorphic transformation in encapsulated CsPbI3 films aged under illumination. To suppress this lattice distortion, we introduced alkyltrimethoxysilane strain-release layers (SRLs) at the perovskite/ETL interface. We found the SRL with the longest alkyl chain is the most effective at reducing interfacial lattice distortion, leading to enhanced charge transfer at the perovskite/ETL interface and improved phase/device stability. Its incorporation in beta-CsPbI3 solar cells resulted in a power-conversion efficiency of 20.1% and an operational lifetime with an extrapolated T(80 )of > 3000 h for encapsulated devices tested under continuous illumination under maximum power point tracking conditions.

Transparent photovoltaic cells are an emerging technology that can provide point-of-use electricity generation for building-integrated applications. While most transparent solar cells to date target absorption of the photon-rich near-infrared portion of the solar spectrum, these devices compromise color neutrality and transparency because of parasitic absorption of long-wavelength visible light. One solution to eliminate parasitic absorption is to employ materials that absorb near-ultraviolet light with sharper absorption cutoffs. Herein, we demonstrate organic donor materials based on N,N'-diaryl-diamines that incorporate a series of aryl linkers to systematically tune their absorption profiles. When paired with acceptor 4,6-bis(3,5-di-4-pyridinylphenyl)-2-methylpyrimidine in an inverted architecture with an indium tin oxide top electrode and an organic optical outcoupling layer, the three best-performing transparent solar cells exhibit average photopic-response-weighted transmittances of 80.3-82.0% and color-rendering indices of 95.0-97.1, both of which are records for organic photovoltaics, with power-conversion efficiencies of 0.43-0.70%.

Thermal evaporation is a promising technique for the fabrication of uniform perovskite films over large areas that forgo the use of hazardous solvents. However, evaporation equipment, particularly at the laboratory scale, is often shared between different materials systems and it is, thus, important to understand the potential impact that halide perovskite evaporation can have on other films and devices processed in the same chamber. Here, we observe that evaporation of perovskite precursors such as PbI2 and CsI results in significant iodine contamination that is not efficiently removed by conventional decontamination procedures such as solvent cleaning, chamber bakeout, and foil replacement. X-ray photoelectron spectra show that this iodine contamination can incorporate itself into organic and metal films grown in the same chamber, which degrades the performance of thermally evaporated organic photovoltaic cells by ∼90%. To remove the contamination and restore the performance of other optoelectronic devices grown in the same chamber, a total resurfacing/replacement of all interior evaporator surfaces was required.

To understand degradation routes and improve the stability of perovskite solar cells (PSCs), accelerated aging tests are needed. Here, we use elevated temperatures (up to 110°C) to quantify the accelerated degradation of encapsulated CsPbI 3 PSCs under constant illumination. Incorporating a two-dimensional (2D) Cs 2 PbI 2 Cl 2 capping layer between the perovskite active layer and hole-transport layer stabilizes the interface while increasing power conversion efficiency of the all-inorganic PSCs from 14.9 to 17.4%. Devices with this 2D capping layer did not degrade at 35°C and required >2100 hours at 110°C under constant illumination to degrade by 20% of their initial efficiency. Degradation acceleration factors based on the observed Arrhenius temperature dependence predict intrinsic lifetimes of 51,000 ± 7000 hours (>5 years) operating continuously at 35°C.

Organic solar cells are ideal for semi-transparent applications given their "peaky" absorption, which allows them to selectively absorb photons outside the visible range while transmitting visible light. Such devices embody a fundamental tradeoff between transparency and power generation that must be optimized to fit the requirements of each potential application-for example, powering electrically dimmable smart windows. To inform the design of organic ultraviolet-absorbers and solar cells that target such applications, we computer-generate sets of optical constants with a range of absorption coefficients and absorption cutoff wavelengths that mimic those of real organic semiconductors. We then perform optical transfer-matrix simulations to determine the absorption and transmission spectra of full-stack photovoltaic cells, inserting these computer-generated optical constants to describe the photoactive absorbing layers. We find that solar cells having absorbers with a cutoff wavelength of 420 nm produce the most power without degrading transparency or color neutrality, and that absorption coefficients up to 5 x 105 cm 1 are needed to fully absorb the targeted wavelengths within practical photoactive layer thicknesses <300 nm in the absence of a reflecting electrode.

Transparent photovoltaics that harvest ultraviolet photons are promising point‐of‐use power sources for lower power applications, such as electrochromic windows that regulate the flow of visible and infrared photons for lighting and temperature regulation. Organic photovoltaic cells employing contorted hexabenzocoronene (cHBC) and its derivatives as chromophores have shown promise for transparent solar cells due to their high open‐circuit voltages, large‐area scalability, and high photoactive layer transparency. Here, the operational stability of such devices is investigated and it is found that the solar cell active layers that include peripherally halogenated chromophores undergo rapid morphological degradation during operation, while control cells employing cHBC and other non‐halogenated derivatives as donors with archetype C 70 as an acceptor are highly stable. This study suggests halogenation of chromophores can play an outsized role in determining the operational stability of devices comprising them, which should be considered during the molecular design process.

The power conversion efficiency of organic solar cells has rapidly increased, yet significantly less attention has been paid to materials stability and device longevity. For organic solar cells to make an impact in the marketplace, researchers, funding agencies and journals should do more to address this crucial gap.

Improving the long-term stability of perovskite solar cells is critical to the deployment of this technology. Despite the great emphasis laid on stability-related investigations, publications lack consistency in experimental procedures and parameters reported. It is therefore challenging to reproduce and compare results and thereby develop a deep understanding of degradation mechanisms. Here, we report a consensus between researchers in the field on procedures for testing perovskite solar cell stability, which are based on the International Summit on Organic Photovoltaic Stability (ISOS) protocols. We propose additional procedures to account for properties specific to PSCs such as ion redistribution under electric fields, reversible degradation and to distinguish ambient-induced degradation from other stress factors. These protocols are not intended as a replacement of the existing qualification standards, but rather they aim to unify the stability assessment and to understand failure modes. Finally, we identify key procedural information which we suggest reporting in publications to improve reproducibility and enable large data set analysis.

Organic photovoltaic cells are now approaching commercially viable efficiencies, particularly for applications that make use of their unique potential for flexibility and semitransparency1–3. However, their reliability remains a major concern, as even the most stable devices reported so far degrade within only a few years4–8. This has led to the belief that short operational lifetimes are an intrinsic disadvantage of devices that are fabricated using weakly bonded organic materials—an idea that persists despite the rapid growth and acceptance of organic light-emitting devices, which can achieve lifetimes of several million hours9. Here we study an extremely stable class of thermally evaporated single-junction organic photovoltaic cells. We accelerated the ageing process by exposing the packaged cells to white-light illumination intensities of up to 37 Suns. The cells maintained more than 87 per cent of their starting efficiency after exposure for more than 68 days. The degradation rate increases superlinearly with intensity, leading to an extrapolated intrinsic lifetime, T80, of more than 4.9 × 107 hours, where T80 is the time taken for the power conversion efficiency to decrease to 80 per cent of its initial value. This is equivalent to 27,000 years outdoors. Additionally, we subjected a second group of organic photovoltaic cells to 20 Suns of ultraviolet illumination (centred at 365 nanometres) for 848 hours, a dose that would take 1.7 × 104 hours (9.3 years) to accumulate outdoors. No efficiency loss was observed over the duration of the test. Overall, we find that organic solar cells packaged in an inert atmosphere can be extremely stable, which is promising for their future use as a practical energy-generation technology. Organic photovoltaic cells containing an intrinsically stable organic absorber layer that are likely to be stable for many decades under illumination are demonstrated.

For a suitably designed organic multilayer structure, optically or electrically generated electrons confined to a thin fullerene channel can diffuse over surprisingly long distances of several centimetres.