Solution-processed Sb2Se3 photocathodes under Se-rich conditions and their photoelectrochemical properties.

In this study, selenium (Se)-rich antimony selenide (Sb2Se3) films were fabricated by applying a solution process with the solvents ethylenediamine and 2-mercaptoethanol to optimize the photoelectrochemical (PEC) performance of the Sb2Se3 photocathode. Various antimony (Sb)-Se precursor solutions with different molar ratios of Sb and Se (Sb : Se = 1 : 1.5, 1 : 3, 1 : 4.5, 1 : 7.5, and 1 : 9) were prepared to attain Se-rich fabrication conditions. As a result, the Se-rich Sb2Se3 films fabricated using the Sb-Se precursor solution with a molar ratio of Sb : Se = 1 : 7.5 exhibited an improved PEC performance, compared to the stoichiometric Sb2Se3 film. The charge transport was improved by the abundant Se element and thin selenium oxide (Se2O3) layer in the Se-rich Sb2Se3 film, resulting in a decrease in Se vacancies and substitutional defects. Moreover, the light utilization in the long wavelength region above 800 nm was enhanced by the light-trapping effect because of the nanowire structure in the Se-rich Sb2Se3 film. Hence, the optimal Se-rich Sb2Se3 photocathodes showed an improved photocurrent density of -0.24 mA cm-2 at the hydrogen evolution reaction potential that was three times higher than that of the stoichiometric Sb2Se3 photocathodes (-0.08 mA cm-2).

Oriented Crystal Growth during Perovskite Surface Reconstruction.

Surface passivation has become a key strategy for an improvement in power conversion efficiency (PCE) of perovskite solar cells (PSCs) since PSCs experienced a steep increase in PCE and reached a comparably matured point. Recently, surface passivation using a mixed salt of fluorinated alkyl ammonium iodide and formamidinium bromide demonstrated a remarkable improvement in both performance and stability, which can be tuned by the length of the alkyl chain. Nevertheless, the role of the alkyl chain in manipulating surface-limited crystal growth was not fully understood, preventing a further progress in interface control. In this study, we found that the length of the fluorine-substituted alkyl chain governed the crystal formation dynamics by manipulating surface tensions of different crystal orientations. The overall enhancement of the (001) plane, being the most favored, commonly resulted from the surface reformation of the perovskite film regardless of the chain length, while the highly oriented (001) over (111) was monitored with a particular chain length. The enhanced crystal orientation during surface recrystallization was responsible for the low trap density and thus effectively suppressed charge recombination at the interface, resulting in a considerable increase in open-circuit voltage and fill factor.

Printable Free-Standing Hybrid Graphene/Dry-Spun Carbon Nanotube Films as Multifunctional Electrodes for Highly Stable Perovskite Solar Cells.

Perovskite solar cells (PSCs) have attracted immense attention owing to their outstanding power conversion efficiency (PCE). However, their counter electrodes are commonly produced by evaporating metals, such as Ag and Au, under high vacuum conditions, which make the PSCs costly, thereby limiting their large-scale production. In this study, a free-standing hybrid graphene/carbon nanotube film was carefully designed to replace noble metal PSC counter electrodes to reduce the cost and increase the stability of PSCs. A highly conductive and stable hybrid carbon thin film can be easily transferred to the various desired substrates by a simple rolling process. The PSCs with hybrid graphene/carbon nanotube films showed a high PCE of 15.36%. Moreover, the devices exhibited excellent stability and could retain 86% of their initial PCE after storage for 500 h in a high-moisture atmosphere (RH 50%). The outstanding stability of PCEs can be attributed to the efficient moisture blocking by the multilayered graphene/carbon nanotube present in the hybrid film. The thin, flexible, and easy-to-synthesize free-standing hybrid graphene/CNT film with high conductivity showed great potential for realizing the low-cost production of highly stable PSCs.

Stabilization of Highly Efficient and Stable Phase-Pure FAPbI3 Perovskite Solar Cells by Molecularly Tailored 2D-Overlayers.

As a result of their attractive optoelectronic properties, metal halide APbI3 perovskites employing formamidinium (FA+ ) as the A cation are the focus of research. The superior chemical and thermal stability of FA+ cations makes α-FAPbI3 more suitable for solar-cell applications than methylammonium lead iodide (MAPbI3 ). However, its spontaneous conversion into the yellow non-perovskite phase (δ-FAPbI3 ) under ambient conditions poses a serious challenge for practical applications. Herein, we report on the stabilization of the desired α-FAPbI3 perovskite phase by protecting it with a two-dimensional (2D) IBA2 FAPb2 I7 (IBA=iso-butylammonium overlayer, formed via stepwise annealing. The α-FAPbI3 /IBA2 FAPb2 I7 based perovskite solar cell (PSC) reached a high power conversion efficiency (PCE) of close to 23 %. In addition, it showed excellent operational stability, retaining around 85 % of its initial efficiency under severe combined heat and light stress, that is, simultaneous exposure with maximum power tracking to full simulated sunlight at 80 °C over 500 h.

Influence of Alkoxy Chain Length on the Properties of Two-Dimensionally Expanded Azulene-Core-Based Hole-Transporting Materials for Efficient Perovskite Solar Cells.

A series of two-dimensionally expanded azulene-core-based π systems have been synthesized with different alkyl chain lengths in the alkoxy moieties connected to the partially oxygen-bridged triarylamine skeletons. The thermal, photophysical, and electronic properties of each compound were evaluated to determine the influence of the alkyl chain length on their effectiveness as hole-transporting materials (HTMs) in perovskite solar cells (PSCs). All the synthesized molecules showed promising material properties, including high solubility, the formation of flat and amorphous films, and optimal alignment of energy levels with perovskites. In particular, the derivatives with methyl and n-butyl in the side chains retained amorphous stability up to 233 and 159 °C, respectively. Such short alkoxy chains also resulted in improved electrical device properties. The PSC device fabricated with the HTM with n-butyl side chains showed the best performance with a power conversion efficiency of 18.9 %, which compares favorably with that of spiro-OMeTAD-based PSCs (spiro-OMeTAD=2,2',7,7'-tetrakis[N,N-bis(p-methoxyphenyl)amino]-9,9'-spirobifluorene).

Site-selective Synthesis of ß-[70]PCBM-like Fullerenes: Efficient Application in Perovskite Solar Cells.

We report on the site-selective synthesis of PCBM-like [70]fullerene site-isomers, where the elusive ß-site-isomers are, for the first time, the major product in a (cyclo)addition chemical reaction involving [70]fullerene. The reaction involves an straightforward cyclopropanation of [70]fullerene from sulfonium salts, affording a mixture of α and ß site-isomers in good yields. Amazingly, the preference for the α- or ß-site-isomer can be efficiently controlled by means of the solvent polarity! DFT theoretical calculations (DMF and toluene) nicely predict that, although the formation of the α-adduct is, as expected, thermodynamically favored, the selectivity of the process is determined by the energy difference of the respective transition states. Furthermore, the employ of α or/and ß site-isomers, as pure materials or as a mixture of them, used as templating agent, has been evaluated in perovskite solar cells. The positive influence of the [70]fullerenes by passivating the voids/pin-holes and/or deep slits, is reflected in highly efficient and stable bulk heterojunction perovskite solar cells, whose performance (around 20 %) is slightly but consistently depending on the isomeric fullerene composition. These experimental findings pave the way to investigate a new reactivity on C70 and to explore the properties of the less-known ß-derivatives.

Bifunctional Organic Spacers for Formamidinium-Based Hybrid Dion-Jacobson Two-Dimensional Perovskite Solar Cells.

Three-dimensional (3D) perovskite materials display remarkable potential in photovoltaics owing to their superior solar-to-electric power conversion efficiency, with current efforts focused on improving stability. Two-dimensional (2D) perovskite analogues feature greater stability toward environmental factors, such as moisture, owing to a hydrophobic organic cation that acts as a spacer between the inorganic layers, which offers a significant advantage over their comparatively less stable 3D analogues. Here, we demonstrate the first example of a formamidinium (FA) containing Dion-Jacobson 2D perovskite material characterized by the BFA n-1Pb nI3 n+1 formulation through employing a novel bifunctional organic spacer (B), namely 1,4-phenylenedimethanammonium (PDMA). We achieve remarkable efficiencies exceeding 7% for (PDMA)FA2Pb3I10 based 2D perovskite solar cells resisting degradation when exposed to humid ambient air, which opens up new avenues in the design of stable perovskite materials.

Additives, Hole Transporting Materials and Spectroscopic Methods to Characterize the Properties of Perovskite Films.

The achievement of high efficiency and high stability in perovskite solar cells (PSCs) requires optimal selection and evaluation of the various components. After a brief introduction to the perovskite materials and their historical evolution, the first part is devoted to the hole transporting material (HTM), between photoelectrode and dark counter electrode. The basic requirements for an efficient HTM are stated. Subsequently, the most used HTM, spiro-OMeTAD, is compared to alternative HTMs, both small-molecule size species and electronically conducting polymers. The second part is devoted to additives related to the performance of the perovskite light-absorbing material itself. These are related either to the modification of the composition of the material itself or to the optimization of the morphology during the perovskite preparation stage, and their effect is in the enhancement of the power conversion efficiency, the long-term stability, or the reproducibility of the properties of the PSCs. Finally, a number of spectroscopic methods based on the UV-Vis part of the electromagnetic spectrum useful for characterizing the different perovskite material types are described in the last part of this review.

The effect of illumination on the formation of metal halide perovskite films.

Optimizing the morphology of metal halide perovskite films is an important way to improve the performance of solar cells when these materials are used as light harvesters, because film homogeneity is correlated with photovoltaic performance. Many device architectures and processing techniques have been explored with the aim of achieving high-performance devices, including single-step deposition, sequential deposition and anti-solvent methods. Earlier studies have looked at the influence of reaction conditions on film quality, such as the concentration of the reactants and the reaction temperature. However, the precise mechanism of the reaction and the main factors that govern it are poorly understood. The consequent lack of control is the main reason for the large variability observed in perovskite morphology and the related solar-cell performance. Here we show that light has a strong influence on the rate of perovskite formation and on film morphology in both of the main deposition methods currently used: sequential deposition and the anti-solvent method. We study the reaction of a metal halide (lead iodide) with an organic compound (methylammonium iodide) using confocal laser scanning fluorescence microscopy and scanning electron microscopy. The lead iodide crystallizes before the intercalation of methylammonium iodide commences, producing the methylammonium lead iodide perovskite. We find that the formation of perovskite via such a sequential deposition is much accelerated by light. The influence of light on morphology is reflected in a doubling of solar-cell efficiency. Conversely, using the anti-solvent method to form methyl ammonium lead iodide perovskite in a single step from the same starting materials, we find that the best photovoltaic performance is obtained when films are produced in the dark. The discovery of light-activated crystallization not only identifies a previously unknown source of variability in opto-electronic properties, but also opens up new ways of tuning morphology and structuring perovskites for various applications.

Room-Temperature Formation of Highly Crystalline Multication Perovskites for Efficient, Low-Cost Solar Cells.

A room-temperature perovskite material yielding a power conversion efficiency of 18.1% (stabilized at 17.7%) is demonstrated by judicious selection of cations. Both cesium and methylammonium are necessary for room-temperature formamidinium-based perovskite to obtain the photoactive crystalline perovskite phase and high-quality crystals. This room-temperature-made perovskite material shows great potential for low-cost, large-scale manufacturing such as roll-to-roll processing.

Effect of Cs-Incorporated NiO x on the Performance of Perovskite Solar Cells.

The effect of Cs-incorporated NiO x on perovskite solar cells with an inverted structure was investigated, where NiO x and PCBM were used as selective contacts for holes and electrons, respectively. It was found that the generation of an Ni phase in an NiO x layer was significantly suppressed by employing cesium. Furthermore, Cs-incorporated NiO x enabled holes to be efficiently separated at the interface, showing the improved photoluminescent quenching and thus generating higher short-circuit current. The effect of Cs incorporation was also prominent in the inhibition of recombination. The recombination resistance of Cs-incorporated NiO x was noticeably increased by more than three-fold near the maximum power point, leading to a higher fill factor of 0.78 and consequently a higher power conversion efficiency of 17.2% for the device employing Cs-incorporated NiO x .

Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance.

All of the cations currently used in perovskite solar cells abide by the tolerance factor for incorporation into the lattice. We show that the small and oxidation-stable rubidium cation (Rb+) can be embedded into a "cation cascade" to create perovskite materials with excellent material properties. We achieved stabilized efficiencies of up to 21.6% (average value, 20.2%) on small areas (and a stabilized 19.0% on a cell 0.5 square centimeters in area) as well as an electroluminescence of 3.8%. The open-circuit voltage of 1.24 volts at a band gap of 1.63 electron volts leads to a loss in potential of 0.39 volts, versus 0.4 volts for commercial silicon cells. Polymer-coated cells maintained 95% of their initial performance at 85°C for 500 hours under full illumination and maximum power point tracking.

Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency.

Today's best perovskite solar cells use a mixture of formamidinium and methylammonium as the monovalent cations. With the addition of inorganic cesium, the resulting triple cation perovskite compositions are thermally more stable, contain less phase impurities and are less sensitive to processing conditions. This enables more reproducible device performances to reach a stabilized power output of 21.1% and ∼18% after 250 hours under operational conditions. These properties are key for the industrialization of perovskite photovoltaics.

Enhancing Efficiency of Perovskite Solar Cells via N-doped Graphene: Crystal Modification and Surface Passivation.

Controlling the morphology and surface passivation in perovskite solar cells is paramount in obtaining optimal optoelectronic properties. This study incorporates N-doped graphene nanosheets in the perovskite layer, which simultaneously induces an improved morphology and surface passivation at the perovskite/spiro interface, resulting in enhancement in all photovoltaic parameters.