Solution-processed lead halide perovskite thin-film solar cells have achieved power conversion efficiencies comparable to those obtained with several commercial photovoltaic technologies in a ...remarkably short period of time. This rapid rise in device efficiency is largely the result of the development of fabrication protocols capable of producing continuous, smooth perovskite films with micrometer-sized grains. Further developments in film fabrication and morphological control are necessary, however, in order for perovskite solar cells to reliably and reproducibly approach their thermodynamic efficiency limit. This Perspective discusses the fabrication of lead halide perovskite thin films, while highlighting the processing–property–performance relationships that have emerged from the literature, and from this knowledge, suggests future research directions.
Organic photovoltaics (OPV) have the advantage of possible fabrication by energy‐efficient and cost‐effective deposition methods, such as solution processing. Solvent additives can provide fine ...control of the active layer morphology of OPVs by influencing film formation during solution processing. As such, solvent additives form a versatile method of experimental control for improving organic solar cell device performance. This review provides a brief history of solution‐processed bulk heterojunction OPVs and the advent of solvent additives, putting them into context with other methods available for morphology control. It presents the current understanding of how solvent additives impact various mechanisms of phase separation, enabled by recent advances in in situ morphology characterization. Indeed, understanding solvent additives' effects on film formation has allowed them to be applied and combined effectively and synergistically to boost OPV performance. Their success as a morphology control strategy has also prompted the use of solvent additives in related organic semiconductor technologies. Finally, the role of solvent additives in the development of next‐generation OPV active layers is discussed. Despite concerns over their environmental toxicity and role in device instability, solvent additives remain relevant morphological directing agents as research interests evolve toward nonfullerene acceptors, ternary blends, and environmentally sustainable solvents.
Solvent additives are a versatile experimental handle for directing the bulk heterojunction morphology of solution‐processed organic photovoltaics, enabling enhanced device performance. The current understanding of solvent additives' impact on phase separation mechanisms, elucidated by in situ morphology characterization, is presented. Solvent additives will likely remain relevant as research interests evolve toward nonfullerene acceptors, ternary blends, and environmentally sustainable solvents.
The electronic devices that play a vital role in our daily life are primarily based on silicon and are thus rigid, opaque, and relatively heavy. However, new electronics relying on polymer ...semiconductors are opening up new application spaces like stretchable and self-healing sensors and devices, and these can facilitate the integration of such devices into our homes, our clothing, and even our bodies. While there has been tremendous interest in such technologies, the widespread adoption of these organic electronics requires low-cost manufacturing techniques. Fortunately, the realization of organic electronics can take inspiration from a technology developed since the beginning of the Common Era: printing. This review addresses the critical issues and considerations in the printing methods for organic electronics, outlines the fundamental fluid mechanics, polymer physics, and deposition parameters involved in the fabrication process, and provides future research directions for the next generation of printed polymer electronics.
Tin and lead iodide perovskite semiconductors of the composition AMX3, where M is a metal and X is a halide, are leading candidates for high efficiency low cost tandem photovoltaics, in part because ...they have band gaps that can be tuned over a wide range by compositional substitution. We experimentally identify two competing mechanisms through which the A-site cation influences the band gap of 3D metal halide perovskites. Using a smaller A-site cation can distort the perovskite lattice in two distinct ways: by tilting the MX6 octahedra or by simply contracting the lattice isotropically. The former effect tends to raise the band gap, while the latter tends to decrease it. Lead iodide perovskites show an increase in band gap upon partial substitution of the larger formamidinium with the smaller cesium, due to octahedral tilting. Perovskites based on tin, which is slightly smaller than lead, show the opposite trend: they show no octahedral tilting upon Cs-substitution but only a contraction of the lattice, leading to progressive reduction of the band gap. We outline a strategy to systematically tune the band gap and valence and conduction band positions of metal halide perovskites through control of the cation composition. Using this strategy, we demonstrate solar cells that harvest light in the infrared up to 1040 nm, reaching a stabilized power conversion efficiency of 17.8%, showing promise for improvements of the bottom cell of all-perovskite tandem solar cells. The mechanisms of cation-based band gap tuning we describe are broadly applicable to 3D metal halide perovskites and will be useful in further development of perovskite semiconductors for optoelectronic applications.
An overlooked factor affecting stability: the residual stresses in perovskite films, which are tensile and can exceed 50 MPa in magnitude, a value high enough to deform copper, is reported. These ...stresses provide a significant driving force for fracture. Films are shown to be more unstable under tensile stress—and conversely more stable under compressive stress—when exposed to heat or humidity. Increasing the formation temperature of perovskite films directly correlates with larger residual stresses, a result of the high thermal expansion coefficient of perovskites. Specifically, this tensile stress forms upon cooling to room temperature, as the substrate constrains the perovskite from shrinking. No evidence of stress relaxation is observed, with the purely elastic film stress attributed to the thermal expansion mismatch between the perovskite and substrate. Additionally, the authors demonstrate that using a bath conversion method to form the perovskite film at room temperature leads to low stress values that are unaffected by further annealing, indicating complete perovskite formation prior to annealing. It is concluded that reducing the film stress is a novel method for improving perovskite stability, which can be accomplished by lower formation temperatures, flexible substrates with high thermal expansion coefficients, and externally applied compressive stress after fabrication.
A previously uncharacterized metric affecting perovskite solar cells—film stress—is introduced and a direct connection is established to improved moisture and thermal stability with reduced tensile film stress. Three simple approaches for reducing stress and improving stability in perovskite films that involve the use of lower formation temperature, higher coefficients of thermal expansion substrates, and externally applied compression are developed.
As one of the most remarkable oxygen evolution reaction (OER) electrocatalysts, metal chalcogenides have been intensively reported during the past few decades because of their high OER activities. It ...has been reported that electron-chemical conversion of metal chalcogenides into oxides/hydroxides would take place after the OER. However, the transition mechanism of such unstable structures, as well as the real active sites and catalytic activity during the OER for these electrocatalysts, has not been understood yet; therefore a direct observation for the electrocatalytic water oxidation process, especially at nano or even angstrom scale, is urgently needed. In this research, by employing advanced Cs-corrected transmission electron microscopy (TEM), a step by step oxidational evolution of amorphous electrocatalyst CoS x into crystallized CoOOH in the OER has been in situ captured: irreversible conversion of CoS x to crystallized CoOOH is initiated on the surface of the electrocatalysts with a morphology change via Co(OH)2 intermediate during the OER measurement, where CoOOH is confirmed as the real active species. Besides, this transition process has also been confirmed by multiple applications of X-ray photoelectron spectroscopy (XPS), in situ Fourier-transform infrared spectroscopy (FTIR), and other ex situ technologies. Moreover, on the basis of this discovery, a high-efficiency electrocatalyst of a nitrogen-doped graphene foam (NGF) coated by CoS x has been explored through a thorough structure transformation of CoOOH. We believe this in situ and in-depth observation of structural evolution in the OER measurement can provide insights into the fundamental understanding of the mechanism for the OER catalysts, thus enabling the more rational design of low-cost and high-efficient electrocatalysts for water splitting.
Reduced-dimensional metal halide perovskites (RDPs) have attracted significant attention in recent years due to their promising light harvesting and emissive properties. We sought to increase the ...systematic understanding of how RDPs are formed. Here we report that layered intermediate complexes formed with the solvent provide a scaffold that facilitates the nucleation and growth of RDPs during annealing, as observed via in situ X-ray scattering. Transient absorption spectroscopy of RDP single crystals and films enables the identification of the distribution of quantum well thicknesses. These insights allow us to develop a kinetic model of RDP formation that accounts for the experimentally observed size distribution of wells. RDPs exhibit a thickness distribution (with sizes that extend above n = 5) determined largely by the stoichiometric proportion between the intercalating cation and solvent complexes. The results indicate a means to control the distribution, composition and orientation of RDPs via the selection of the intercalating cation, the solvent and the deposition technique.
Li2S is a high-capacity cathode material for lithium metal-free rechargeable batteries. It has a theoretical capacity of 1166 mAh/g, which is nearly 1 order of magnitude higher than traditional metal ...oxides/phosphates cathodes. However, Li2S is usually considered to be electrochemically inactive due to its high electronic resistivity and low lithium-ion diffusivity. In this paper, we discover that a large potential barrier (∼1 V) exists at the beginning of charging for Li2S. By applying a higher voltage cutoff, this barrier can be overcome and Li2S becomes active. Moreover, this barrier does not appear again in the following cycling. Subsequent cycling shows that the material behaves similar to common sulfur cathodes with high energy efficiency. The initial discharge capacity is greater than 800 mAh/g for even 10 μm Li2S particles. Moreover, after 10 cycles, the capacity is stabilized around 500–550 mAh/g with a capacity decay rate of only ∼0.25% per cycle. The origin of the initial barrier is found to be the phase nucleation of polysulfides, but the amplitude of barrier is mainly due to two factors: (a) charge transfer directly between Li2S and electrolyte without polysulfide and (b) lithium-ion diffusion in Li2S. These results demonstrate a simple and scalable approach to utilizing Li2S as the cathode material for rechargeable lithium-ion batteries with high specific energy.
Polymer aggregation and crystallization behavior play a crucial role in the performance of all-polymer solar cells (all-PSCs). Gaining control over polymer self-assembly via molecular design to ...influence bulk-heterojunction active-layer morphology, however, remains challenging. Herein, we show a simple yet effective way to modulate the self-aggregation of the commonly used naphthalene diimide (NDI)-based acceptor polymer (N2200), by systematically replacing a certain amount of alkyl side-chains with compact bulky side-chains (CBS). Specifically, we have synthesized a series of random copolymer (PNDI-CBS x ) with different molar fractions (x = 0–1) of the CBS units and have found that both solution-phase aggregation and solid-state crystallinity of these acceptor polymers are progressively suppressed with increasing x as evidenced by UV–vis absorption, photoluminescence (PL) spectroscopies, thermal analysis, and grazing incidence X-ray scattering (GIWAXS) techniques. Importantly, as compared to the highly self-aggregating N2200, photovoltaic results show that blending of more amorphous acceptor polymers with donor polymer (PBDB-T) can enable all-PSCs with significantly increased PCE (up to 8.5%). The higher short-circuit current density (J sc) results from the smaller polymer phase-separation domain sizes as evidenced by PL quenching and resonant soft X-ray scattering (R-SoXS) analyses. Additionally, we show that the lower crystallinity of the active layer is less sensitive to the film deposition methods. Thus, the transition from spin-coating to solution coating can be easily achieved with no performance losses. On the other hand, decreasing aggregation and crystallinity of the acceptor polymer too much reduces the photovoltaic performance as the donor phase-separation domain sizes increases. The highly amorphous acceptor polymers appear to induce formation of larger donor polymer crystallites. These results highlight the importance of a balanced aggregation strength between the donor and acceptor polymers to achieve high-performance all-PSCs with optimal active layer film morphology.
We introduce a novel siloxane-terminated solubilizing group and demonstrate its effectiveness as a side chain in an isoindigo-based conjugated polymer. An average hole mobility of 2.00 cm2 V–1 s–1 ...(with a maximum mobility of 2.48 cm2 V–1 s–1), was obtained from solution-processed thin-film transistors, one of the highest mobilities reported to date. In contrast, the reference polymer with a branched alkyl side chain gave an average hole mobility of 0.30 cm2 V–1 s–1 and a maximum mobility of 0.57 cm2 V–1 s–1. This is largely explained by the polymer packing: our new polymer exhibited a π–π stacking distance of 3.58 Å, while the reference polymer showed a distance of 3.76 Å.
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