flash-infrared-annealing-for-perovskite-solar-cell-processing

Flash Infrared Annealing for Perovskite Solar Cell Processing

Organic&#8211;inorganic perovskites have an impressive potential for the design of next generation solar cells and are currently considered for upscaling ...

École Polytechnique Fédérale de Lausanne (EPFL)

The photopolymerization of Co(II)/Co(III) complexes for dye-sensitized solar cells (DSSCs) by means of a fast, inexpensive, in situ and inhibition-free process has been examined. We have succeeded in fabricating high-performance DSSCs able to retain a light-to-electricity power conversion efficiency exceeding 6.5% (8.5% at low intensity) after 1800 h of mixed (light on/off, temperature high/low) accelerated aging tests, thus revealing a possible way for the stabilization of these record-holding redox pairs.

Direct light-induced polymerization of cobalt-based redox shuttles: an ultrafast way towards stable dye-sensitized solar cells.

Solid-state dye-sensitized solar cells (sDSCs) in which a solid organic charge-transfer medium, or hole conductor (HC), is interposed between a dye-coated mesoporous oxide electrode and a conductive counter electrode, have attracted considerable interest as viable alternatives to the more ubiquitous mediator-electrolyte DSC. Of particular importance to efficient operation are, in addition to the useful processes contributing to current generation (light harvesting, electron injection and current collection), the recombinative deleterious processes. The organic HCs are highly reactive toward electrons in the oxide or the conducting glass support, therefore necessitating the inclusion of a carefully prepared thin blocking oxide underlayer support as well as the molecular design of special dark current-suppressing dyes. Initially (mid-1990s) sDSCs with organic small molecular weight hole conductors have undergone systematic investigation. At the same time the first tests of sDSCs with conducting polymer hole conductors were published, with subsequent emphasis on the in situ generation of the HC inside the pores. For both types of devices a light-to-electricity conversion efficiency, in the 5-10% range for several dye-HC combinations, approaches that of the most efficient DSCs with non-volatile liquid electrolytes, thereby encouraging further efforts for obtaining stable, efficient and inexpensive sDSCs. 

Dye-sensitized Solar Cells: New Approaches with Organic Solid-state Hole Conductors.

We developed a new donor-π-acceptor-type hole-transport material (HTMs) incorporating S,N-heteropentacene as π-spacer, triarylamine as donor, and dicyanovinylene as acceptor. In addition to appropriate frontier molecular orbital energies, the new HTM showed high photo absorptivity in the visible region. Without the use of p-dopants, solution-processed mixed perovskite devices using the HTM achieved power conversion efficiencies of up to 16.9% and high photocurrents of up to 22.2 mA cm(-2). These results demonstrate that heteroacene can be an excellent building block to prepare alternative HTMs for perovskite solar cells and hold promise for further advancement through fine-tuning the molecular structure.

High-Efficiency Perovskite Solar Cells Employing a S,N-Heteropentacene-based D-A Hole-Transport Material.

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.

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

Organometal halide perovskite solar cells have demonstrated high conversion efficiency but poor long-term stability against ultraviolet irradiation and water. We show that rapid light-induced free-radical polymerization at ambient temperature produces multifunctional fluorinated photopolymer coatings that confer luminescent and easy-cleaning features on the front side of the devices, while concurrently forming a strongly hydrophobic barrier toward environmental moisture on the back contact side. The luminescent photopolymers re-emit ultraviolet light in the visible range, boosting perovskite solar cells efficiency to nearly 19% under standard illumination. Coated devices reproducibly retain their full functional performance during prolonged operation, even after a series of severe aging tests carried out for more than 6 months.

Improving efficiency and stability of perovskite solar cells with photocurable fluoropolymers.

Solid-state dye-sensitized solar cells (sDSCs) in which a solid organic charge-transfer medium, or hole conductor (HC), is interposed between a dye-coated mesoporous oxide electrode and a conductive counter electrode, have attracted considerable interest as viable alternatives to the more ubiquitous mediator-electrolyte DSC. Of particular importance to efficient operation are, in addition to the useful processes contributing to current generation (light harvesting, electron injection and current collection), the recombinative deleterious processes. The organic HCs are highly reactive toward electrons in the oxide or the conducting glass support, therefore necessitating the inclusion of a carefully prepared thin blocking oxide underlayer support as well as the molecular design of special dark current-suppressing dyes. Initially (mid-1990s) sDSCs with organic small molecular weight hole conductors have undergone systematic investigation. At the same time the first tests of sDSCs with conducting polymer hole conductors were published, with subsequent emphasis on the in situ generation of the HC inside the pores. For both types of devices a light-to-electricity conversion efficiency, in the 5-10% range for several dye-HC combinations, approaches that of the most efficient DSCs with non-volatile liquid electrolytes, thereby encouraging further efforts for obtaining stable, efficient and inexpensive sDSCs.

The efficiencies of perovskite solar cells have gone from single digits to a certified 22.1% in a few years' time. At this stage of their development, the key issues concern how to achieve further improvements in efficiency and long-term stability. We review recent developments in the quest to improve the current state of the art. Because photocurrents are near the theoretical maximum, our focus is on efforts to increase open-circuit voltage by means of improving charge-selective contacts and charge carrier lifetimes in perovskites via processes such as ion tailoring. The challenges associated with long-term perovskite solar cell device stability include the role of testing protocols, ionic movement affecting performance metrics over extended periods of time, and determination of the best ways to counteract degradation mechanisms.

Promises and challenges of perovskite solar cells.

Currently, perovskite solar cells (PSCs) with high performances greater than 20% contain bromine (Br), causing a suboptimal bandgap, and the thermally unstable methylammonium (MA) molecule. Avoiding Br and especially MA can therefore result in more optimal bandgaps and stable perovskites. We show that inorganic cation tuning, using rubidium and cesium, enables highly crystalline formamidinium-based perovskites without Br or MA. On a conventional, planar device architecture, using polymeric interlayers at the electron- and hole-transporting interface, we demonstrate an efficiency of 20.35% (stabilized), one of the highest for MA-free perovskites, with a drastically improved stability reached without the stabilizing influence of mesoporous interlayers. The perovskite is not heated beyond 100°C. Going MA-free is a new direction for perovskites that are inherently stable and compatible with tandems or flexible substrates, which are the main routes commercializing PSCs.

Methylammonium-free, high-performance, and stable perovskite solar cells on a planar architecture.

Perovskite solar cells (PSCs) reached a certified 23.7% efficiency in 2018 by boosting their surprisingly high open-circuit voltage (VOC) and photocurrent. The suppressed recombination in PSCs significantly cut down the voltage loss between the bandgap energy and VOC, which encouraged the VOC to reach closer to the bandgap. In addition, the photocurrent is considerably closer to the theoretical value at a given bandgap, leaving almost no room for further improvement. This remarkable development in the performance of PSCs is mainly ascribed to high-quality perovskite material being consistently tailored in the progress of technology. At the beginning of the progress, the morphology of the perovskite was a major target for improvement to enhance the crystal quality. The need for compositional engineering of the perovskite was raised in later stages of the progress by considering the benefits from different compositions of perovskites and their structural stability. Here we review the overall progress in perovskite materials from two perspectives: morphological progress and compositional progress.

Morphological and compositional progress in halide perovskite solar cells.

All-inorganic metal halide perovskites are showing promising development towards efficient long-term stable materials and solar cells. Element doping, especially on the lead site, has been proved to be a useful strategy to obtain the desired film quality and material phase for high efficient and stable inorganic perovskite solar cells. Here we demonstrate a function by adding barium in CsPbIBr. We find that barium is not incorporated into the perovskite lattice but induces phase segregation, resulting in a change in the iodide/bromide ratio compared with the precursor stoichiometry and consequently a reduction in the band gap energy of the perovskite phase. The device with 20 mol% barium shows a high power conversion efficiency of 14.0% and a great suppression of non-radiative recombination within the inorganic perovskite, yielding a high open-circuit voltage of 1.33 V and an external quantum efficiency of electroluminescence of 10.

Ba-induced phase segregation and band gap reduction in mixed-halide inorganic perovskite solar cells.

Photoelectrochemical water splitting provides a promising solution for harvesting and storing solar energy. As the best-performing oxide photocathode, the CuO photocathode holds the performance rivaling that of many photovoltaic semiconductor-based photocathodes through continuous research and development. However, the state-of-the-art CuO photocathode employs gold as the back contact which can lead to considerable electron-hole recombination. Here, we present a CuO photocathode with overall improved performance, enabled by using solution-processed CuSCN as hole transport material. Two types of CuSCN with different structures are synthesized and carefully compared. Furthermore, detailed characterizations reveal that hole transport between CuO and CuSCN is assisted by band-tail states. Owing to the multiple advantages of applying CuSCN as the hole transport layer, a standalone solar water splitting tandem cell is built, delivering a solar-to-hydrogen efficiency of 4.55%. Finally, approaches towards more efficient dual-absorber tandems are discussed.

CuO photocathodes with band-tail states assisted hole transport for standalone solar water splitting.

Power conversion efficiency (PCE) of the perovskite solar cells (PSCs) has remarkably been increased from 3.1% to 25.2%. The fast expansion of the PSCs has been along with the development of compositional and interface engineering, which has been playing a critical role. For the PSCs with record high-efficiency and stability, the perovskite absorber layer has been changed from the initial MAPbI- to FAPbI-based compositions. Owing to the enormous engineering works, perovskite absorber layers with monolithic grains could be achieved, in which the interior defects are negligible compared with the surface defects. Therefore, interface engineering, which can passivate the surface defects and/or isolate the perovskite from the environmental moistures, has been playing a more and more important role to further boost the PCE and stability of the PSCs. Herein, a compact review study of the compositional and interface engineering is presented and promising strategies and directions of the PSCs are discussed.

Compositional and Interface Engineering of Organic-Inorganic Lead Halide Perovskite Solar Cells.

Mixtures of cations or halides with FAPbI (where FA is formamidinium) lead to high efficiency in perovskite solar cells (PSCs) but also to blue-shifted absorption and long-term stability issues caused by loss of volatile methylammonium (MA) and phase segregation. We report a deposition method using MA thiocyanate (MASCN) or FASCN vapor treatment to convert yellow δ-FAPbI perovskite films to the desired pure α-phase. NMR quantifies MA incorporation into the framework. Molecular dynamics simulations show that SCN anions promote the formation and stabilization of α-FAPbI below the thermodynamic phase-transition temperature. We used these low-defect-density α-FAPbI films to make PSCs with >23% power-conversion efficiency and long-term operational and thermal stability, as well as a low (330 millivolts) open-circuit voltage loss and a low (0.75 volt) turn-on voltage of electroluminescence.

Article	Year
Direct light-induced polymerization of cobalt-based redox shuttles: an ultrafast way towards stable dye-sensitized solar cells. Chemical communications (Cambridge, England)\| PubMed ID: 26364577	2015
Dye-sensitized Solar Cells: New Approaches with Organic Solid-state Hole Conductors. Chimia\| PubMed ID: 26507087	2015
High-Efficiency Perovskite Solar Cells Employing a S,N-Heteropentacene-based D-A Hole-Transport Material. ChemSusChem\| PubMed ID: 26813331	2016
Enhancing Efficiency of Perovskite Solar Cells via N-doped Graphene: Crystal Modification and Surface Passivation. Advanced materials (Deerfield Beach, Fla.)\| PubMed ID: 27515231	2016
Improving efficiency and stability of perovskite solar cells with photocurable fluoropolymers. Science (New York, N.Y.)\| PubMed ID: 27708051	2016
Dye-sensitized Solar Cells: New Approaches with Organic Solid-state Hole Conductors. Chimia\| PubMed ID: 28982463	2015
Promises and challenges of perovskite solar cells. Science (New York, N.Y.)\| PubMed ID: 29123060	2017
Methylammonium-free, high-performance, and stable perovskite solar cells on a planar architecture. Science (New York, N.Y.)\| PubMed ID: 30309904	2018
Morphological and compositional progress in halide perovskite solar cells. Chemical communications (Cambridge, England)\| PubMed ID: 30604789	2019
Ba-induced phase segregation and band gap reduction in mixed-halide inorganic perovskite solar cells. Nature communications\| PubMed ID: 31615978	2019
CuO photocathodes with band-tail states assisted hole transport for standalone solar water splitting. Nature communications\| PubMed ID: 31949135	2020
Compositional and Interface Engineering of Organic-Inorganic Lead Halide Perovskite Solar Cells. iScience\| PubMed ID: 32712463	2020
Vapor-assisted deposition of highly efficient, stable black-phase FAPbI perovskite solar cells. Science (New York, N.Y.)\| PubMed ID: 33004488	2020

Anders Hagfeldt

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