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Showing papers by "Michael Grätzel published in 2020"


Journal ArticleDOI
02 Oct 2020-Science
TL;DR: A deposition method using methylammonium thiocyanate vapor treatment to convert δ-FAPbI3 to the desired pure α-phase below the thermodynamic phase-transition temperature is shown.
Abstract: Mixtures of cations or halides with FAPbI3 (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-FAPbI3 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-FAPbI3 below the thermodynamic phase-transition temperature. We used these low-defect-density α-FAPbI3 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.

446 citations


Journal ArticleDOI
TL;DR: It is found that simple surface treatment with tBBAI significantly accelerates the charge extraction from the perovskite into the spiro-OMeTAD hole-transporter, while retarding the nonradiative charge carrier recombination, boosting the power conversion efficiency (PCE) of the PSC and reducing the hysteresis to barely detectable levels.
Abstract: Passivation of interfacial defects serves as an effective means to realize highly efficient and stable perovskite solar cells (PSCs). However, most molecular modulators currently used to mitigate such defects form poorly conductive aggregates at the perovskite interface with the charge collection layer, impeding the extraction of photogenerated charge carriers. Here, a judiciously engineered passivator, 4-tert-butyl-benzylammonium iodide (tBBAI), is introduced, whose bulky tert-butyl groups prevent the unwanted aggregation by steric repulsion. It is found that simple surface treatment with tBBAI significantly accelerates the charge extraction from the perovskite into the spiro-OMeTAD hole-transporter, while retarding the nonradiative charge carrier recombination. This boosts the power conversion efficiency (PCE) of the PSC from ≈20% to 23.5% reducing the hysteresis to barely detectable levels. Importantly, the tBBAI treatment raises the fill factor from 0.75 to the very high value of 0.82, which concurs with a decrease in the ideality factor from 1.72 to 1.34, confirming the suppression of radiation-less carrier recombination. The tert-butyl group also provides a hydrophobic umbrella protecting the perovskite film from attack by ambient moisture. As a result, the PSCs show excellent operational stability retaining over 95% of their initial PCE after 500 h full-sun illumination under maximum-power-point tracking under continuous simulated solar irradiation.

264 citations


Journal ArticleDOI
TL;DR: Stabilization of the desired α-FAPbI3 perovskite phase by protecting it with a two-dimensional (2D) IBA2F APb2I7 (IBA = iso-butylammonium overlayer, formed via stepwise annealing) is reported on.
Abstract: 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.

170 citations


Journal ArticleDOI
TL;DR: Crown ethers are employed to modulate perovskite films, affording passivation of undercoordinated surface defects, which reduces the concentration of surface electronic defects and suppresses nonradiative recombination, accompanied by enhanced stability under ambient and operational conditions.
Abstract: The use of molecular modulators to reduce the defect density at the surface and grain boundaries of perovskite materials has been demonstrated to be an effective approach to enhance the photovoltaic performance and device stability of perovskite solar cells. Herein, we employ crown ethers to modulate perovskite films, affording passivation of undercoordinated surface defects. This interaction has been elucidated by solid-state nuclear magnetic resonance and density functional theory calculations. The crown ether hosts induce the formation of host-guest complexes on the surface of the perovskite films, which reduces the concentration of surface electronic defects and suppresses nonradiative recombination by 40%, while minimizing moisture permeation. As a result, we achieved substantially improved photovoltaic performance with power conversion efficiencies exceeding 23%, accompanied by enhanced stability under ambient and operational conditions. This work opens a new avenue to improve the performance and stability of perovskite-based optoelectronic devices through supramolecular chemistry.

129 citations


Journal ArticleDOI
TL;DR: Copper thiocyanate is shown to assist hole transport in photoelectrodes and enable a 4.55% solar-to-hydrogen efficiency in tandem devices.
Abstract: Photoelectrochemical water splitting provides a promising solution for harvesting and storing solar energy. As the best-performing oxide photocathode, the Cu2O photocathode holds the performance rivaling that of many photovoltaic semiconductor-based photocathodes through continuous research and development. However, the state-of-the-art Cu2O photocathode employs gold as the back contact which can lead to considerable electron-hole recombination. Here, we present a Cu2O 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 Cu2O 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.

111 citations


Journal ArticleDOI
TL;DR: A ligand-assisted surface matrix strategy to engineer the surface and packing states of Pe-QD solids is demonstrated by a mild thermal annealing treatment after ligand exchange processing triggered by guanidinium thiocyanate, which exhibits remarkably enhanced charge mobility and carrier diffusion length compared to control ones.
Abstract: Metal halide perovskite quantum dots (Pe-QDs) are of great interest in new-generation photovoltaics (PVs). However, it remains challenging in the construction of conductive and intact Pe-QD films to maximize their functionality. Herein, a ligand-assisted surface matrix strategy to engineer the surface and packing states of Pe-QD solids is demonstrated by a mild thermal annealing treatment after ligand exchange processing (referred to as "LE-TA") triggered by guanidinium thiocyanate. The "LE-TA" method induces the formation of surface matrix on CsPbI3 QDs, which is dominated by the cationic guanidinium (GA+ ) rather than the SCN- , maintaining the intact cubic structure and facilitating interparticle electrical interaction of QD solids. Consequently, the GA-matrix-confined CsPbI3 QDs exhibit remarkably enhanced charge mobility and carrier diffusion length compared to control ones, leading to a champion power conversion efficiency of 15.21% when assembled in PVs, which is one of the highest among all Pe-QD solar cells. Additionally, the "LE-TA" method shows similar effects when applied to other Pe-QD PV systems like CsPbBr3 and FAPbI3 (FA = formamidinium), indicating its versatility in regulating the surfaces of various Pe-QDs. This work may afford new guidelines to construct electrically conductive and structurally intact Pe-QD solids for efficient optoelectronic devices.

101 citations


Journal ArticleDOI
TL;DR: A green, efficient and stable solar cell based only on water and safe and cheap elements of the periodic table is proposed in this work, finally consolidating (also from a sustainability viewpoint) the concept of “artificial photosynthesis” studied for decades by the scientific community.
Abstract: A green, efficient and stable solar cell based only on water and safe and cheap elements of the periodic table is proposed in this work, finally consolidating (also from a sustainability viewpoint) the concept of “artificial photosynthesis” studied for decades by the scientific community. The concept of dye-sensitized solar cells is re-proposed here with a metal-free organic dye, an iodine-based electrolyte in a 100% aqueous environment and a new cathode (cationic PEDOT) synthesized for the first time with the aim of inhibiting the repulsion between the anions of redox couples and the PEDOT:PSS matrix commonly used as the counter-electrode. This elegant setup leads to a record efficiency of 7.02%, the highest value ever obtained for a water-based solar cell and, in general, for a photovoltaic device free of both organic solvents and expensive/heavy metals.

88 citations


Journal ArticleDOI
TL;DR: In this paper, the lead-free perovskite solar cells hold the most promise among lead free PSCs, but they are plagued with inadequate environmental stability and power-conversion efficiency (PCE).
Abstract: Tin-based halide perovskite solar cells (PSCs) hold the most promise among lead-free PSCs, but they are plagued with inadequate environmental stability and power-conversion efficiency (PCE). Here w...

87 citations


Journal ArticleDOI
TL;DR: The use of quasi-3D azetidinium lead iodide (AzPbI3) as a secondary layer on top of the primary 3D perovskite film that results in significant improvements in the photovoltaic parameters.
Abstract: The employment of 2D perovskites is a promising approach to tackling the stability and voltage issues inherent in perovskite solar cells. It remains unclear, however, whether other perovskites with...

86 citations


Journal ArticleDOI
21 Aug 2020-iScience
TL;DR: A compact review study of the compositional and interface engineering is presented and promising strategies and directions of the PSCs are discussed.

85 citations


Journal ArticleDOI
TL;DR: It is revealed that the lone-pair electrons of BPQDs can induce strong binding between molecules of the CsPbI2Br precursor solution and phosphorus atoms stemming from the concomitant reduction in coulombic repulsion.
Abstract: Black phosphorus quantum dots (BPQDs) are proposed as effective seed-like sites to modulate the nucleation and growth of CsPbI2Br perovskite crystalline thin layers, allowing an enhanced crystallization and remarkable morphological improvement. We reveal that the lone-pair electrons of BPQDs can induce strong binding between molecules of the CsPbI2Br precursor solution and phosphorus atoms stemming from the concomitant reduction in coulombic repulsion. The four-phase transition during the annealing process yields an α-phase CsPbI2Br stabilized by BPQDs. The BPQDS/CsPbI2Br core-shell structure concomitantly reinforces a stable CsPbI2Br crystallite and suppresses the oxidation of BPQDs. Consequently, a power conversion efficiency of 15.47% can be achieved for 0.7 wt % BPQDs embedded in CsPbI2Br film-based devices, with an enhanced cell stability, under ambient conditions. Our finding is a decisive step in the exploration of crystallization and phase stability that can lead to the realization of efficient and stable inorganic perovskite solar cells.

Journal ArticleDOI
15 Jan 2020-Joule
TL;DR: In this article, an intermediate-phase engineering strategy was proposed to improve the inorganic perovskite/metal oxide interface by utilizing volatile salts, such as methylammonium and formamidinium.

Journal ArticleDOI
TL;DR: A bifunctional supramolecular modulator, 1,2,4,5-tetrafluoro-3,6-diiodobenzene, which interacts with the surface of the triple-cation double-halide perovskite material via halogen bonding is introduced.
Abstract: There has been an ongoing effort to overcome the limitations associated with the stability of hybrid organic-inorganic perovskite solar cells by using different organic agents as additives to the perovskite formulations. The functionality of organic additives has been predominantly limited to exploiting hydrogen-bonding interactions, while the relevant atomic-level binding modes remain elusive. Herein, we introduce a bifunctional supramolecular modulator, 1,2,4,5-tetrafluoro-3,6-diiodobenzene, which interacts with the surface of the triple-cation double-halide perovskite material via halogen bonding. We elucidate its binding mode using two-dimensional solid-state 19F NMR spectroscopy in conjunction with density functional theory calculations. As a result, we demonstrate a stability enhancement of the perovskite solar cells upon supramolecular modulation, without compromising the photovoltaic performances.

Journal ArticleDOI
TL;DR: Two new D-A-π-A featured sensitizers coded HY63 and HY64 are reported, which employ either benzothiadiazole or phenanthrene-fused-quinoxaline as the auxiliary electron-withdrawing acceptor moiety, and in depth studies of interfacial charge carrier dynamics show that PFQ is superior to BT in retarding charge recombination resulting in near quantitative collection of photogenerated charge carriers.
Abstract: Dye-sensitized solar cells (DSSCs) based on CuII/I bipyridyl or phenanthroline complexes as redox shuttles have achieved very high open-circuit voltages (VOC , more than 1 V). However, their short-circuit photocurrent density (JSC ) has remained modest. Increasing the JSC is expected to extend the spectral response of sensitizers to the red or NIR region while maintaining efficient electron injection in the mesoscopic TiO2 film and fast regeneration by the CuI complex. Herein, we report two new D-A-π-A-featured sensitizers termed HY63 and HY64, which employ benzothiadiazole (BT) or phenanthrene-fused-quinoxaline (PFQ), respectively, as the auxiliary electron-withdrawing acceptor moiety. Despite their very similar energy levels and absorption onsets, HY64-based DSSCs outperform their HY63 counterparts, achieving a power conversion efficiency (PCE) of 12.5 %. PFQ is superior to BT in reducing charge recombination resulting in the near-quantitative collection of photogenerated charge carriers.

Journal ArticleDOI
TL;DR: In this article, the morphology of perovskite material has been optimized to achieve high power conversion efficiencies for perovskiy solar cells, in the ongoing intense quest to increase the performance of solar cells.
Abstract: In the ongoing intense quest to increase the performance of perovskite solar cells, optimizing the morphology of perovskite material has become imperative to achieve high power conversion efficienc...

Journal ArticleDOI
TL;DR: Higher temperature thermal annealing was found to correlate with a shift in the photocurrent onset potential towards less positive potentials, assigned to a suppression of both back electron-hole recombination and of charge accumulation in intraband-gap states, indicating these intrusion states do not contribute directly to water oxidation.
Abstract: Operando spectroelectrochemical analysis is used to determine the water oxidation reaction kinetics for hematite photoanodes prepared using four different synthetic procedures. While these photoano...

Journal ArticleDOI
TL;DR: In this article, a scalable fabrication process for heteroepitaxial growth of mixed-cation hybrid perovskites (FA1-x-yMAxCsy)Pb(I 1-xBrx)3 in ambient atmospheric condition by using a Crystal Engineering (CE) approach is presented.

Journal ArticleDOI
TL;DR: Titanium dioxide (anatase, rutile) and quasi-amorphous tin dioxide are prepared on F-doped SnO2 in the form of dense thin films, which can serve as electron-selective layers in perovskite solar cel...
Abstract: Titanium dioxide (anatase, rutile) and quasi-amorphous tin dioxide are prepared on F-doped SnO2 in the form of dense thin films, which can serve as electron-selective layers in perovskite solar cel...


Journal ArticleDOI
TL;DR: The new semitransparent DSC with spring green to cyan colors have PCEs ranging from 6.7% to 8.1% and show stability for 1000 h under accelerated ageing test at 80 °C, superior to the zinc porphyrin DSCs.
Abstract: Semitransparent dye-sensitized solar cells (DSCs) are appealing as aesthetically pleasing and colorful see-through photovoltaics. Green semitransparent DSCs have been presented, but the best ones rely on green zinc porphyrin photosensitizers and high volatile electrolytes. For potential outdoor applications, the zinc porphyrin DSCs employing ionic liquid electrolytes merely reached a power conversion efficiency (PCE) of 6.3% even with opaque mesoporous TiO2 films. Herein, the new green DSC is realized by using a blue organic photosensitizer in conjunction with an orange ionic-liquid-based electrolyte, presenting a simple and an effective path for color tuning of photovoltaics. The new approach allows for broadly modulating the color from spring green to cyan by tuning the contributions of the light absorption by the dye-sensitized TiO2 film and the electrolyte layer. The new semitransparent DSCs with spring green to cyan colors have PCEs ranging from 6.7% to 8.1% and show stability for 1000 h under accelerated ageing test at 80 °C, superior to the zinc porphyrin DSCs. The findings pave a new way to achieve efficient and stable colorful solar cells.

Journal ArticleDOI
TL;DR: In this article, two spiro-MeTAD compounds were synthesized, characterized by experimental and quantum mechanical methods, and used as hole transporting materials (HTMs) in perovskite solar cells (PSCs).
Abstract: Two spiro-MeTAD compounds (1 and 2) were synthesized, characterized by experimental and quantum mechanical methods, and used as hole transporting materials (HTMs) in perovskite solar cells (PSCs). The new compounds differ from spiro-OMeTAD only by the presence of methyl substituents as compared to methoxy groups. This modification results in the absorption band blue shifting by ∼20 nm as compared to spiro-OMeTAD, increased glass transition temperature for 2, and reduced ionization potentials by 0.02–0.12 eV. Hole mobilities five times larger were obtained for spiro-MeTAD/spiro-MeTAD, which is maintained in the presence of additives. Despite this improvement, J–V measurements in PSCs resulted in a power conversion efficiency (PCE) of 17.2% and 17.05% for 1 and 2 HTMs, respectively, as compared to 19.24% for spiro-OMeTAD. Photoluminescence measurements of perovskite:HTM layers indicate much stronger quenching in the case of spiro-OMeTAD/spiro-MeTAD. These results point to the dominant importance of the perovskite:HTM interfacial properties as compared to the HTM hole-transport properties in the bulk. Given that improved hole-mobility and energy-level alignment are the main targets of the current research efforts in this domain, our results alert to the necessity to prioritize the improvement of perovskite–HTM interaction properties.

Journal ArticleDOI
24 Aug 2020
TL;DR: In this article, a C-2v-symmetric spiro-configured hole-transporting materials (HTM-1) for perovskite solar cells (PSCs) is presented.
Abstract: There is an urge to develop new hole-transporting materials (HTMs) for perovskite solar cells (PSCs), which can yield comparable power conversion efficiencies (PCEs) yet mitigate the issue of stability associated with the state-of-the-art HTM Spiro-MeOTAD Herein, we designed and prepared C-2v-symmetric spiro-configured HTM-1 comprising a central acridine-cyclopentadithiophene core unit flanked with triarylamine moieties PSCs containing a 40 nm thin HTM-1 layer for hole extraction yielded a stabilized PCE approaching 21% under standard illumination Owing to its higher hole mobility (mu(h)) at low electric field, an impressive short-circuit current density (J(SC)) of 247 mA cm(-2) and a high fill factor (FF) of 077 have been achieved More importantly, HTM-1-based PSCs presented an excellent long-term operational stability under continuous illumination for 400 h and thermal stability at 80 degrees C, which can be ascribed to its high glass transition temperature of 168 degrees C and superior moisture tolerance Arguably, the confluence of high performance and remarkable stability will lead to the development of technologically interesting new, stable, and efficient PSCs

Journal ArticleDOI
TL;DR: Copper redox mediators can be employed in dye sensitized solar cells (DSCs) both as liquid electrolytes or as solid state hole transport materials (HTMs) as mentioned in this paper.
Abstract: Copper redox mediators can be employed in dye sensitized solar cells (DSCs) both as liquid electrolytes or as solid state hole transport materials (HTMs). The solid state devices that employ copper...

Journal ArticleDOI
TL;DR: In this article, mixed cation perovskites, with x = 0.8, achieve high solar cell power conversion efficiencies while exhibiting long-term stability under ambient conditions, while exhibiting high energy efficiency.
Abstract: Mixed cation perovskites, [HC(NH2)2]xCs(1–x)PbI3, (FAxCs(1–x)PbI3) with x = 0.8, achieve high solar cell power conversion efficiencies while exhibiting long-term stability under ambient conditions....

Journal ArticleDOI
TL;DR: In this article, a two-dimensional hybrid coordination polymer (Pb(CH3NH2)I2]n with a well-defined layered structure is proposed for scalable perovskite ink and deposition.
Abstract: A two-dimensional hybrid coordination polymer [Pb(CH3NH2)I2]n featuring a well-defined layered structure is reported for scalable perovskite ink and deposition. This coordination polymer exhibits s...

Journal ArticleDOI
TL;DR: Transient-absorption spectroscopy demonstrated the role of nickel-coated (3D) TiO 2 core-shell nanorods in prolonging photogenerated charge in GaAs, resulting in a higher catalytic activity, which may open the potential of utilizing this low-cost nanostructured catalyst for decorating narrow band gap semiconductor photoanodes for PEC water splitting devices.
Abstract: Photoelectrochemical (PEC) water splitting is a promising clean route to hydrogen fuel. The best-performing materials (III/V semiconductors) require surface passivation, as they are liable to corrosion, and a surface co-catalyst to facilitate water splitting. At present, optimal design combining photoelectrodes with oxygen evolution catalysts remains a significant materials challenge. Here, we demonstrate that nickel-coated amorphous three-dimensional (3D) TiO2 core-shell nanorods on a TiO2 thin film function as an efficient hole-extraction layer and serve as a protection layer for the GaAs photoanode. Transient-absorption spectroscopy (TAS) demonstrated the role of nickel-coated (3D) TiO2 core-shell nanorods in prolonging photogenerated charge lifetimes in GaAs, resulting in a higher catalytic activity. This strategy may open the potential of utilizing this low-cost (3D) nanostructured catalyst for decorating narrow-band-gap semiconductor photoanodes for PEC water splitting devices.



Journal ArticleDOI
TL;DR: It is suggested that a new design principle for the preparation of stable perovskite structures at room temperature with cations that lie outside the Goldschmidt range through mixtures with size-complementary cations in such a way that the effective average cation radius of the mixture lies within the stability range.
Abstract: Lead halide perovskites with mixtures of monovalent cations have attracted wide attention due to the possibility of preferentially stabilizing the perovskite phase with respect to photovoltaically less suitable competing phases. Here, we present a theoretical analysis and interpretation of the phase stability of binary (CH6N3)x[HC(NH2)2](1−x)PbI3 = GUAxFA(1−x)PbI3 and ternary CsyGUAxFA(1−y−x)PbI3 mixtures. We first estimate if such mixtures are stable and if they lead to a stabilization of the perovskite phase based on static Density Functional Theory (DFT) calculations. In order to investigate the finite temperature stability of the phases, we also employ first-principles molecular dynamics (MD) simulations. It turns out that in contrast to the FA+-rich case of FA/Cs mixtures, although mixing of FA/GUA is possible, it is not sufficient to stabilize the perovskite phase at room temperature. In contrast, stable ternary mixtures that contain 17% of Cs+ can be formed that lead to a preferential stabilization of the perovskite phase. In such a way, the enthalpic destabilization due to the introduction of a too large/too small cation that lies outside the Goldschmidt tolerance range can be (partially) compensated through the introduction of a third cation with complementary size. This allows to suggest a new design principle for the preparation of stable perovskite structures at room temperature with cations that lie outside the Goldschmidt range through mixtures with size-complementary cations in such a way that the effective average cation radius of the mixture lies within the stability range.

Posted ContentDOI
14 Oct 2020-ChemRxiv
TL;DR: Using metadynamics, X-ray diffraction and Raman spectroscopy, the solid-solid phase transition occuring during two-step sequential deposition of methylammonium-lead iodide and formamidinium lead iodide was investigated in this article.
Abstract: It is well established that the lack of understanding the crystallization process in two-step sequential deposition has a direct impact on efficiency, stability and reproducibility of perovskite solar cells. Here, we try to understand the solid-solid phase transition occuring during two-step sequential deposition of methylammonium lead iodide and formamidinium lead iodide. Using metadynamics, X-ray diffraction and Raman spectroscopy, we reveal the microscopic details of this process. We find that the formation of perovskite proceeds through intermediate structures and report polymorphs found for methylammonium lead iodide and formamidinium lead iodide. From simulations, we discover a possible crystallization pathway for the highly efficient metastable α-phase of formamidinium lead iodide. Guided by these simulations, we perform experiments that results in the room temperature crystallization of α-formamidinium lead iodide.