Showing papers by "Michael Grätzel published in 2015"
TL;DR: Heavy doped inorganic charge extraction layers in planar PSCs were used to achieve very rapid carrier extraction, even with 10- to 20-nanometer-thick layers, avoiding pinholes and eliminating local structural defects over large areas.
Abstract: The recent dramatic rise in power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) has triggered intense research worldwide. However, high PCE values have often been reached with poor stability at an illuminated area of typically less than 0.1 square centimeter. We used heavily doped inorganic charge extraction layers in planar PSCs to achieve very rapid carrier extraction, even with 10- to 20-nanometer-thick layers, avoiding pinholes and eliminating local structural defects over large areas. The robust inorganic nature of the layers allowed for the fabrication of PSCs with an aperture area >1 square centimeter that have a PCE >15%, as certified by an accredited photovoltaic calibration laboratory. Hysteresis in the current-voltage characteristics was eliminated; the PSCs were stable, with >90% of the initial PCE remaining after 1000 hours of light soaking.
1,936 citations
TL;DR: In this paper, the rate-dependent hysteresis seen in current-voltage scans of CH3NH3PbI3 perovskite solar cells is related to a slow field-induced process that tends to cancel the electric field in the device at each applied bias voltage.
Abstract: In this work we show that the rate-dependent hysteresis seen in current–voltage scans of CH3NH3PbI3 perovskite solar cells is related to a slow field-induced process that tends to cancel the electric field in the device at each applied bias voltage. It is attributed to the build-up of space charge close to the contacts, independent of illumination and most likely due to ionic displacement, which is enhanced when the device undergoes aging. This process can also lead to a reduction of the open-circuit voltage or the steady-state photocurrent and does not directly correlate with the development of the hysteresis if it is measured at a fixed voltage sweep rate.
1,150 citations
TL;DR: In this article, the authors show that planar perovskite solar cells using TiO2 are inherently limited due to conduction band misalignment and demonstrate, with a variety of characterization techniques, for the first time that SnO2 achieves a barrier-free energetic configuration, obtaining almost hysteresis-free power conversion efficiencies (PCEs).
Abstract: The simplification of perovskite solar cells (PSCs), by replacing the mesoporous electron selective layer (ESL) with a planar one, is advantageous for large-scale manufacturing. PSCs with a planar TiO2 ESL have been demonstrated, but these exhibit unstabilized power conversion efficiencies (PCEs). Herein we show that planar PSCs using TiO2 are inherently limited due to conduction band misalignment and demonstrate, with a variety of characterization techniques, for the first time that SnO2 achieves a barrier-free energetic configuration, obtaining almost hysteresis-free PCEs of over 18% with record high voltages of up to 1.19 V.
1,049 citations
TL;DR: A one-step solution-processing strategy using phosphonic acid ammonium additives that results in efficient perovskite solar cells with enhanced stability, enhancing the material's photovoltaic performance from 8.8 to 16.7% as well as its resistance to moisture.
Abstract: In the past few years, organic-inorganic halide perovskites have rapidly emerged as promising materials for photovoltaic applications, but simultaneously achieving high performance and long-term stability has proved challenging. Here, we show a one-step solution-processing strategy using phosphonic acid ammonium additives that results in efficient perovskite solar cells with enhanced stability. We modify the surface of methylammonium lead triiodide (CH3NH3PbI3) perovskite by spin-coating its precursor solution in the presence of butylphosphonic acid 4-ammonium chloride. Morphological, structural and elemental analyses show that the phosphonic acid ammonium additive acts as a crosslink between neighbouring grains in the perovskite structure, through strong hydrogen bonding of the -PO(OH)2 and -NH3(+) terminal groups to the perovskite surface. The additives facilitate the incorporation of the perovskite within a mesoporous TiO2 scaffold, as well as the growth of a uniform perovskite layer at the surface, enhancing the material's photovoltaic performance from 8.8 to 16.7% as well as its resistance to moisture.
1,000 citations
TL;DR: In this paper, a transparent silver nanowire electrode was used on perovskite solar cells to achieve a semi-transparent device, which was placed in a mechanically-stacked tandem configuration onto copper indium gallium diselenide (CIGS) and low-quality multicrystalline silicon (Si) to achieve solid-state polycrystalline tandem solar cells with a net improvement in efficiency over the bottom cell alone.
Abstract: A promising approach for upgrading the performance of an established low-bandgap solar technology without adding much cost is to deposit a high bandgap polycrystalline semiconductor on top to make a tandem solar cell. We use a transparent silver nanowire electrode on perovskite solar cells to achieve a semi-transparent device. We place the semi-transparent cell in a mechanically-stacked tandem configuration onto copper indium gallium diselenide (CIGS) and low-quality multicrystalline silicon (Si) to achieve solid-state polycrystalline tandem solar cells with a net improvement in efficiency over the bottom cell alone. This work paves the way for integrating perovskites into a low-cost and high-efficiency (>25%) tandem cell.
656 citations
TL;DR: In this article, the inverted perovskite solar cell fabricated using a two-step method exhibited the highest fill factor (FF) of 0.85 and good efficiency of 18% based on CH3NH3PbI3.
Abstract: The inverted perovskite solar cell fabricated using a two-step method exhibited the highest FF of 0.85 and good efficiency of 18% based on CH3NH3PbI3. A small amount of H2O was added into PbI2/DMF to make a homogenous precursor solution. A high quality PbI2 film with full coverage was formed on a PEDOT:PSS surface by spin coating of the homogeneous PbI2 precursor solution. The perovskite film fabricated from the high quality PbI2 film is highly pure, smooth and very dense even without any pinhole. The champion cell achieves a remarkable fill factor (FF) of 0.85, which is the highest value reported in perovskite solar cells. The FF value is also very reproducible with less than 10% deviation for 50 cells. The cell exhibits no current hysteresis and is stable under both dark and illumination conditions in dry and inert atmospheres. The results not only provide a strategy to fabricate high efficiency inverted perovskite solar cells but also reveal how the water additive in the PbI2/DMF solution may affect the properties of PbI2 and therefore the perovskite film prepared using the two-step method and the overall photovoltaic performance of the corresponding inverted solar cell.
518 citations
TL;DR: It is shown that methylammonium (but also formamidinium) iodoplumbates are mixed conductors with a large fraction of ion conduction because of iodine ions, providing insight into electrical charge transport in the hybrid organic-inorganic lead halide solar cells as well as into new possibilities of improving the photovoltaic performance by controlling the ionic disorder.
Abstract: The success of perovskite solar cells has sparked enormous excitement in the photovoltaic community not only because of unexpectedly high efficiencies but also because of the future potential ascribed to such crystalline absorber materials. Far from being exhaustively studied in terms of solid-state properties, these materials surprised by anomalies such as a huge apparent low-frequency dielectric constant and pronounced hysteretic current-voltage behavior. Here we show that methylammonium (but also formamidinium) iodoplumbates are mixed conductors with a large fraction of ion conduction because of iodine ions. In particular, we measure and model the stoichiometric polarization caused by the mixed conduction and demonstrate that the above anomalies can be explained by the build-up of stoichiometric gradients as a consequence of ion blocking interfaces. These findings provide insight into electrical charge transport in the hybrid organic-inorganic lead halide solar cells as well as into new possibilities of improving the photovoltaic performance by controlling the ionic disorder.
494 citations
TL;DR: The present review tries to address the highly topical role of water in DSSCs, trying to figure out if it is a poisoner or the keyword to success, by means of a thoroughly detailed analysis of all the established phenomena in an aqueous environment.
Abstract: Nowadays, dye-sensitized solar cells (DSSCs) are the most extensively investigated systems for the conversion of solar energy into electricity, particularly for implementation in devices where low cost and good performance are required. Nevertheless, a key aspect is still to be addressed, being considered strongly harmful for a long time, which is the presence of water in the cell, either in the electrolyte or at the electrode/electrolyte interface. Here comes the present review, in the course of which we try our best to address the highly topical role of water in DSSCs, trying to figure out if it is a poisoner or the keyword to success, by means of a thoroughly detailed analysis of all the established phenomena in an aqueous environment. Actually, in the last few years the scientific community has suddenly turned its efforts in the direction of using water as a solvent, as demonstrated by the amount of research articles being published in the literature. Indeed, by means of DSSCs fabricated with water-based electrolytes, reduced costs, non-flammability, reduced volatility and improved environmental compatibility could be easily achieved. As a result, an increasing number of novel electrodes, dyes and electrolyte components are continuously proposed, being highly challenging from the materials science viewpoint and with the golden thread of producing truly water-based DSSCs. If the initial purpose of DSSCs was the construction of an artificial photosynthetic system able to convert solar light into electricity, the use of water as the key component may represent a great step forward towards their widespread diffusion in the market.
373 citations
TL;DR: In this paper, the authors performed extensive stability tests to prove the durability of hole-conductor-free PSCs based on a triple-layer architecture employing carbon as a back contact, including outdoor tests in the hot desert climate and indoor long-term light soaking as well as heat exposure during 3months at 80-85 degrees C.
Abstract: Lack of proven stability has become a major obstacle on the path of metal halide perovskite solar cells (PSCs), in particular methylammonium lead triiodide (MAPbI(3)), towards commercial viability. This correlates with the intrinsic affinity of MAPbI(3) towards moisture and ambient air in particular, leading to its degradation in ambient conditions. We performed extensive stability tests to prove the durability of hole-conductor-free PSCs based on a triple-layer architecture employing carbon as a back contact, including outdoor tests in the hot desert climate and indoor long-term light soaking as well as heat exposure during 3months at 80-85 degrees C. These results show no evidence for device degradation under the test conditions, confirming that the triple-layer device architecture provides a promising path towards realizing efficient and stable perovskite photovoltaics.
331 citations
TL;DR: One-dimensional nanowire perovskite with the mean diameter of 100 nm showed faster carrier separation in the presence of hole transporting layer and higher lateral conductivity than the three- dimensional nanocuboid crystal, indicative of more localized exciton states in nanowires.
Abstract: Organolead iodide perovskite, CH3NH3PbI3, was prepared in the form of nanowire by means of a small quantity of aprotic solvent in two-step spin-coating procedure. One-dimensional nanowire perovskite with the mean diameter of 100 nm showed faster carrier separation in the presence of hole transporting layer and higher lateral conductivity than the three-dimensional nanocuboid crystal. Reduction in dimensionality resulted in the hypsochromic shift of both absorption and fluorescence spectra, indicative of more localized exciton states in nanowires. The best performing device employing nanowire CH3NH3PbI3 delivered photocurrent density of 19.12 mA/cm2, voltage of 1.052 V, and fill factor of 0.721, leading to a power conversion efficiency (PCE) of 14.71% at standard AM 1.5G solar illumination. A small I–V hysteresis was observed, where a PCE at forward scan was measured to be 85% of the PCE at reverse scan.
313 citations
TL;DR: Vacuum-assisted thermal annealing is demonstrated as an effective means for controlling the composition and morphology of the CH(3)NH( 3)PbI(3), which significantly improves the device stability and reproducibility with a standard deviation of only 0.92% in PCE as well as strongly reducing the photocurrent hysteresis.
Abstract: Solar cells incorporating lead halide-based perovskite absorbers can exhibit impressive power conversion efficiencies (PCEs), recently surpassing 15%. Despite rapid developments, achieving precise control over the morphologies of the perovskite films (minimizing pore formation) and enhanced stability and reproducibility of the devices remain challenging, both of which are necessary for further advancements. Here we demonstrate vacuum-assisted thermal annealing as an effective means for controlling the composition and morphology of the CH3NH3PbI3 films formed from the precursors of PbCl2 and CH3NH3I. We identify the critical role played by the byproduct of CH3NH3Cl on the formation and the photovoltaic performance of the perovskite film. By completely removing the byproduct through our vacuum-assisted thermal annealing approach, we are able to produce pure, pore-free planar CH3NH3PbI3 films with high PCE reaching 14.5% in solar cell device. Importantly, the removal of CH3NH3Cl significantly improves the de...
TL;DR: This study represents one of the first demonstrations of extended, stable operation of perovskite photovoltaics, whose large open-circuit voltage is shown to be particularly suited for this process.
Abstract: Artificial photosynthesis, mimicking nature in its efforts to store solar energy, has received considerable attention from the research community. Most of these attempts target the production of H2 as a fuel and our group recently demonstrated solar-to-hydrogen conversion at 12.3% efficiency. Here, in an effort to take this approach closer to real photosynthesis, which is based on the conversion of CO2, we demonstrate the efficient reduction of CO2 to carbon monoxide driven solely by simulated sunlight using water as the electron source. Employing series-connected perovskite photovoltaics and high-performance catalyst electrodes, we reach a solar-to-CO efficiency exceeding 6.5%, which represents a new benchmark in sunlight-driven CO2 conversion. Considering hydrogen as a secondary product, an efficiency exceeding 7% is observed. Furthermore, this study represents one of the first demonstrations of extended, stable operation of perovskite photovoltaics, whose large open-circuit voltage is shown to be particularly suited for this process.
TL;DR: Triazatruxene-based compounds are demonstrated as a new class of HTM for the fabrication of highly efficient perovskite solar cells and remarkable power conversion efficiency was achieved using 5,10,15-trihexyl-3,8,13-tris(4-methoxyphenyl)-10, 15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazole with compos
Abstract: Four center symmetrical star-shaped hole transporting materials (HTMs) comprising planar triazatruxene core and electron-rich methoxy-engineered side arms have been synthesized and successfully employed in (FAPbI3)0.85(MAPbBr3)0.15 perovskite solar cells. These HTMs are obtained from relatively cheap starting materials by adopting facile preparation procedure, without using expensive and complicated purification techniques. Developed compounds have suitable highest occupied molecular orbitals (HOMO) with respect to the valence band level of the perovskite, and time-resolved photoluminescence indicates that hole injection from the valence band of perovskite into the HOMO of triazatruxene-based HTMs is relatively more efficient as compared to that of well-studied spiro-OMeTAD. Remarkable power conversion efficiency over 18% was achieved using 5,10,15-trihexyl-3,8,13-tris(4-methoxyphenyl)-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole (KR131) with compositive perovskite absorber. This result demonstrates ...
TL;DR: The basic principles are involved in molecular engineering of efficient D-A-π-A sensitizers, providing a clear road map showing how to modulate the energy bands, rationally extending the response wavelength, and optimizing photovoltaic efficiency step by step.
Abstract: The dye-sensitized solar cell (DSSC) is one of the most promising photovoltaic technologies with potential of low cost, light weight, and good flexibility. The practical application of DSSCs requires further improvement in power conversion efficiency and long-term stability. Recently, significant progress has been witnessed in DSSC research owing to the novel concept of the D–A−π–A motif for the molecular engineering of organic photosensitizers. New organic and porphyrin dyes based on the D–A−π–A motif can not only enhance photovoltaic performance, but also improve durability in DSSC applications. This Spotlight on Applications highlights recent advances in the D–A−π–A-based photosensitizers, specifically focusing on the mechanism of efficiency and stability enhancements. Also, we find insight into the additional acceptor as well as the trade-off of long wavelength response. The basic principles are involved in molecular engineering of efficient D–A−π–A sensitizers, providing a clear road map showing how ...
TL;DR: This study provides direct evidence for the multihole catalysis of water oxidation by hematite, and demonstrates the hole accumulation level required to achieve this, leading to key insights both for reaction mechanism and strategies to enhance function.
Abstract: Water oxidation is a key chemical reaction, central to both biological photosynthesis and artificial solar fuel synthesis strategies. Despite recent progress on the structure of the natural catalytic site, and on inorganic catalyst function, determining the mechanistic details of this multiredox reaction remains a significant challenge. We report herein a rate law analysis of the order of water oxidation as a function of surface hole density on a hematite photoanode employing photoinduced absorption spectroscopy. Our study reveals a transition from a slow, first order reaction at low accumulated hole density to a faster, third order mechanism once the surface hole density is sufficient to enable the oxidation of nearest neighbor metal atoms. This study thus provides direct evidence for the multihole catalysis of water oxidation by hematite, and demonstrates the hole accumulation level required to achieve this, leading to key insights both for reaction mechanism and strategies to enhance function.
TL;DR: In this article, a mesoscopic TiO2/Al2O3/NiO/carbon structure was used as a framework for perovskite solar cells with a quadruple-layer architecture.
TL;DR: It is shown that a single solution processed organic-inorganic halide perovskite solar cell in tandem with a Fe2O3 photoanode can achieve overall unassisted water splitting with a solar-to-hydrogen conversion efficiency of 2.4%.
Abstract: Photoelectrochemical water splitting half reactions on semiconducting photoelectrodes have received much attention but efficient overall water splitting driven by a single photoelectrode has remained elusive due to stringent electronic and thermodynamic property requirements. Utilizing a tandem configuration wherein the total photovoltage is generated by complementary optical absorption across different semiconducting electrodes is a possible pathway to unassisted overall light-induced water splitting. Because of the low photovoltages generated by conventional photovoltaic materials (e.g., Si, CIGS), such systems typically consist of triple junction design that increases the complexity due to optoelectrical trade-offs and are also not cost-effective. Here, we show that a single solution processed organic–inorganic halide perovskite (CH3NH3PbI3) solar cell in tandem with a Fe2O3 photoanode can achieve overall unassisted water splitting with a solar-to-hydrogen conversion efficiency of 2.4%. Systematic elec...
TL;DR: In this paper, a new approach is demonstrated for forming the PbI2 nanostructure and the use of high CH3NH3I concentration which are adopted to form high-quality (smooth and pbII2 residue-free) perovskite film with better photovoltaic performances.
Abstract: The photovoltaic performance of perovskite solar cells (PVSCs) is extremely dependent on the morphology and crystallization of the perovskite film, which is affected by the deposition method. In this work, a new approach is demonstrated for forming the PbI2 nanostructure and the use of high CH3NH3I concentration which are adopted to form high-quality (smooth and PbI2 residue-free) perovskite film with better photovoltaic performances. On the one hand, self-assembled porous PbI2 is formed by incorporating small amount of rationally chosen additives into the PbI2 precursor solutions, which significantly facilitate the conversion of perovskite without any PbI2 residue. On the other hand, by employing a relatively high CH3NH3I concentration, a firmly crystallized and uniform CH3NH3PbI3 film is formed. As a result, a promising power conversion efficiency of 16.21% is achieved in planar-heterojunction PVSCs. Furthermore, it is experimentally demonstrated that the PbI2 residue in perovskite film has a negative effect on the long-term stability of devices.
TL;DR: A novel method for the deposition of an optically transparent amorphous iron nickel oxide oxygen evolution electrocatalyst that enables the preparation of a stable hematite/perovskite solar cell tandem device, which performs unassisted water splitting.
Abstract: Sunlight-driven water splitting to produce hydrogen fuel is an attractive method for renewable energy conversion. Tandem photoelectrochemical water splitting devices utilize two photoabsorbers to harvest the sunlight and drive the water splitting reaction. The absorption of sunlight by electrocatalysts is a severe problem for tandem water splitting devices where light needs to be transmitted through the larger bandgap component to illuminate the smaller bandgap component. Herein, we describe a novel method for the deposition of an optically transparent amorphous iron nickel oxide oxygen evolution electrocatalyst. The catalyst was deposited on both thin film and high-aspect ratio nanostructured hematite photoanodes. The low catalyst loading combined with its high activity at low overpotential results in significant improvement on the onset potential for photoelectrochemical water oxidation. This transparent catalyst further enables the preparation of a stable hematite/perovskite solar cell tandem device, which performs unassisted water splitting.
TL;DR: The small-molecule-based hole-transporting material methoxydiphenylamine-substituted carbazole was synthesized and incorporated into a CH3NH3PbI3 perovskite solar cell, which displayed a power conversion efficiency of 16.91%, the second highest conversion efficiency after that of Spiro-OMeTAD.
Abstract: The small-molecule-based hole-transporting material methoxydiphenylamine-substituted carbazole was synthesized and incorporated into a CH3NH3PbI3 perovskite solar cell, which displayed a power conversion efficiency of 16.91%, the second highest conversion efficiency after that of Spiro-OMeTAD. The investigated hole-transporting material was synthesized in two steps from commercially available and relatively inexpensive starting reagents. Various electro-optical measurements (UV/Vis, IV, thin-film conductivity, hole mobility, DSC, TGA, ionization potential) have been carried out to characterize the new hole-transporting material.
TL;DR: This work studies the role of the hole transport layer (HTL) spiro-MeOTAD and its thickness in a mesoscopic TiO2-based solar cell architecture and finds that a sufficiently thick HTL not only increases the charge carrier collection efficiency but also the light harvesting efficiency.
Abstract: A tailored optimization of perovskite solar cells requires a detailed understanding of the processes limiting the device efficiency. Here, we study the role of the hole transport layer (HTL) spiro-MeOTAD and its thickness in a mesoscopic TiO2-based solar cell architecture. We find that a sufficiently thick (200 nm) HTL not only increases the charge carrier collection efficiency but also the light harvesting efficiency. This is due to an enhanced reflection of a smooth HTL/Au–electrode interface. The rough CH3NH3PbI3 perovskite surface requires an HTL thickness of >400 nm to avoid surface recombination and guarantee a high open-circuit voltage. Analyses of the electroluminescence efficiency and the diode ideality factor show that the open-circuit voltage becomes completely limited by trap-assisted recombination in the perovskite for a thick HTL. Thus, spiro-MeOTAD is a very good HTL choice from the device physics’ point of view. The fill factor analyzed by the Suns-Voc method is not transport limited, but ...
TL;DR: In this article, a cost-effective spiro-type hole transporting material (PST1) was developed for perovskite solar cells (PSCs) that works efficiently even without a cobalt dopant.
Abstract: We developed a cost-effective spiro-type 4,4′,4′′,4′′′\-(2H,2′H,4H,4′H-3,3′-spiro-bi[thieno[3,4-b][1,4]dioxepine]-6,6′,8,8′-tetrayl)tetrakis(N,N-bis(4-methoxyphenyl)aniline) hole transporting material (PST1) for perovskite solar cells (PSCs) that works efficiently even without a cobalt dopant. The PST1 is obtained by employing facile synthetic routes and tends to crystallize in the solid state. An X-ray diffraction study of PST1 revealed a unique quasi-spiro molecular configuration and found multiple CH/π and π–π intermolecular contacts. For the first time, the crystal structure of 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) is also studied for comparison. The device based on PST1 exhibited a PCE of 13.44%, and a comparable 12.74% PCE was achieved for its undoped form, which paves the way for developing new low cost hole transporting materials and final industrialization of perovskite solar cells.
TL;DR: In this article, a solution processable, molecular organic semiconductor, 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene), was employed as hole transport material (HTM) in mesoscopic methylammonium lead iodide perovskite solar cells.
Abstract: A solution processable, molecular organic semiconductor, 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene), was employed as hole transport material (HTM) in mesoscopic methylammonium lead iodide perovskite solar cells. TIPS-pentacene is potentially cost effective, exhibits a relatively high hole mobility and has a favourable HOMO level with respect to the valence band of perovskite. The photovoltaic performance of perovskite solar cells with TIPS-pentacene as HTM in its pristine form and with a dopant/additive was investigated and compared with classical spiro-OMeTAD based devices. Through solvoneering (solvent engineering) and concentration optimization TIPS-pentacene in its pristine form gave a very competitive power conversion efficiency (PCE) of 11.8% under 1 sun conditions. The open circuit voltage of 0.92 V and a short circuit current density of 20.86 mA cm−2 for the devices with pristine TIPS-pentacene were higher compared to doped spiro-OMeTAD based devices under similar conditions, thus paving the use of TIPS-pentacene as an alternative to an expensive spiro-OMeTAD for large area integration in perovskite based solar cells.
TL;DR: In this article, the perovskite is infiltrated in a mesoscopic scaffold and halide substitution is completed within seconds or minutes after contact with the halide solution, and the time course of the substitution reactions is monitored by in situ photoluminescence, absorption spectroscopy, and X-ray diffraction measurements.
Abstract: We report on rapid halide exchange in metal halide perovskite of the general formula MAPbX3 (X = Cl, Br, or I). We find that when the perovskite is infiltrated in a mesoscopic scaffold, halide substitution on the perovskite lattice is strikingly fast, being completed within seconds or minutes after contact with the halide solution. An exception is the exchange of bromide by iodide, which is slower and incomplete. Halide substitution occurs rapidly even for planar perovskite films which are several tens of nanometers thick. However, with thicker films the reaction requires hours, showing that the mesoscale greatly accelerates the halide exchange process. The time course of the substitution reactions has been monitored by in situ photoluminescence, absorption spectroscopy, and X-ray diffraction measurements. We show that the halide exchange can be a powerful tool to effect perovskite transformations.
TL;DR: This work highlights the potential of CH3NH3PbI3 as a new candidate for ultrafast spin switches in spintronics applications and elucidates the electron spin relaxation lifetime to be ∼7 ps and that of the hole is ∼1 ps through a simple two-level model.
Abstract: Low-temperature solution-processed organic-inorganic halide perovskite CH3NH3PbI3 has demonstrated great potential for photovoltaics and light-emitting devices. Recent discoveries of long ambipolar carrier diffusion lengths and the prediction of the Rashba effect in CH3NH3PbI3, that possesses large spin-orbit coupling, also point to a novel semiconductor system with highly promising properties for spin-based applications. Through circular pump-probe measurements, we demonstrate that highly polarized electrons of total angular momentum (J) with an initial degree of polarization Pini ∼90% (i.e., -30% degree of electron spin polarization) can be photogenerated in perovskites. Time-resolved Faraday rotation measurements reveal photoinduced Faraday rotation as large as 10°/μm at 200 K (at wavelength λ = 750 nm) from an ultrathin 70 nm film. These spin polarized carrier populations generated within the polycrystalline perovskite films, relax via intraband carrier spin-flip through the Elliot-Yafet mechanism. Through a simple two-level model, we elucidate the electron spin relaxation lifetime to be ∼7 ps and that of the hole is ∼1 ps. Our work highlights the potential of CH3NH3PbI3 as a new candidate for ultrafast spin switches in spintronics applications.
TL;DR: A process for the fabrication of NIR-transparent perovskite solar cells is presented, which enables power conversion efficiencies up to 12.1% combined with an average sub-band gap transmission of 71% for photons with wavelength between 800 and 1000 nm.
Abstract: A promising way to enhance the efficiency of CIGS solar cells is by combining them with perovskite solar cells in tandem devices. However, so far, such tandem devices had limited efficiency due to challenges in developing NIR-transparent perovskite top cells, which allow photons with energy below the perovskite band gap to be transmitted to the bottom cell. Here, a process for the fabrication of NIR-transparent perovskite solar cells is presented, which enables power conversion efficiencies up to 12.1% combined with an average sub-band gap transmission of 71% for photons with wavelength between 800 and 1000 nm. The combination of a NIR-transparent perovskite top cell with a CIGS bottom cell enabled a tandem device with 19.5% efficiency, which is the highest reported efficiency for a polycrystalline thin film tandem solar cell. Future developments of perovskite/CIGS tandem devices are discussed and prospects for devices with efficiency toward and above 27% are given.
TL;DR: In this paper, hole transporting materials (HTMs) were used to improve the thermal stability of perovskite-based solar cells (PSCs) and the steady-state maximum power output of devices in working condition was monitored to assess the stability and predict the lifetime of PSCs prepared using different HTMs.
Abstract: In this work, we synthesized novel hole transporting materials (HTMs) and studied their impact on the stability of perovskite-based solar cells (PSCs). The steady-state maximum power output of devices in working condition was monitored to assess the stability and predict the lifetime of PSCs prepared using different HTMs. We showed that the HTM has a significant impact on the device lifetime and found that novel silolothiophene linked methoxy triphenylamines (Si-OMeTPAs) enable more stable PSCs. We reported Si-OMeTPA based devices with a half-life of 6 K h, compared to 1 K h collected for the state-of-the-art PSCs using spirofluorene linked methoxy triphenylamines (spiro-OMeTADs) as HTMs. We demonstrated that such a clear improvement is correlated to the superior thermal stability of silolothiophene compared to the spirofluorene linked triphenylamine HTMs.
TL;DR: The contribution of the former in the recombination is small, thus increasing the survival probability of the charges in the excited perovskite, and the power-dependent femtosecond transient absorption measurements support the ultrafast charge transfer and show strong Auger-type multiparticle interactions at early times.
Abstract: Organic-inorganic hybrid perovskite solar cells have emerged as cost effective efficient light-to-electricity conversion devices. Unravelling the time scale and the mechanisms that govern the charge carrier dynamics is of paramount importance for a clear understanding and further optimization of the perovskite based devices. For the classical FTO/bulk titania blocking layer/mesoporous titania/perovskite/Spiro-OMeTAD (FTO/TPS) cell, further detailed and systematic studies of the ultrafast events related to exciton generation, electron and hole transfer, ultrafast relaxation are still needed. We characterize the initial ultrafast processes attributed to the exciton-perovskite lattice interactions influenced by charge transfer to the electron and hole transporters that precede the exciton diffusion into free charge carriers occurring in the sensitizer. Time-resolved transient absorption studies of the FTO/perovskite and FTO/TPS samples under excitation at different wavelengths and at low fluence 2 (μJ cm(-2)) indicate the sub-picosecond electron and hole injection into titania and Spiro-OMeTAD, respectively. Furthermore, the power-dependent femtosecond transient absorption measurements support the ultrafast charge transfer and show strong Auger-type multiparticle interactions at early times. We reveal that the decays of the internal trap states are the same for both films, while those at surfaces differ. The contribution of the former in the recombination is small, thus increasing the survival probability of the charges in the excited perovskite.
TL;DR: In this article, a facile mechanochemical route for the preparation of hybrid CH3NH3PbI3 perovskite particles with the size of several hundred nanometers for high-efficiency thin-film photovoltaic devices was presented.
Abstract: We present a facile mechanochemical route for the preparation of hybrid CH3NH3PbI3 (MAPbI3) perovskite particles with the size of several hundred nanometers for high-efficiency thin-film photovoltaic devices. Powder X-ray diffraction measurements demonstrate that mechanosynthesis is a suitable strategy to produce a highly crystalline CH3NH3PbI3 material showing no detectable amounts of the starting CH3NH3I and PbI2 reagents. Thermal stability measurements based on the thermogravimetric analysis data of mechanosynthesized perovskite particles indicated that the as-ground MAPbI3 is stable up to 300 °C with no detectable material loss at lower temperatures. The optical properties of newly synthesized perovskite particles were characterized by applying steady state absorption and fluorescence spectroscopy, which confirmed a direct band-gap of 1.48 eV. Time resolved single photon counting measurements revealed that 70% of charges undergo recombination with a 61 ns lifetime. The solar cell devices made from mechanosynthesized perovskite particles achieved a power conversion efficiency of 9.1% when applying a one step deposition method.
TL;DR: In this article, the photovoltaic metrics of CH3NH3PbI3 perovskite solar cells over a wide temperature range from 80 to 360 K were investigated.
Abstract: We have tested the photovoltaic metrics of CH3NH3PbI3 perovskite solar cells over a wide temperature range from 80–360 K. Our investigation reveals that the open-circuit voltage reaches a maximum value at about 200 K close to the phase transition from tetragonal to the orthorhombic phase. The photocurrent is remarkably stable down to 240 K but drops precipitously upon approaching and below the phase transition temperature, implying inefficient charge carrier generation from the orthorhombic perovskite structure. The impedance spectroscopy measurement suggests ionic motion within the bulk of CH3NH3PbI3 after a phase transition. We propose a plausible mechanism for these phenomena and discuss implications for photovoltage generation and charge carrier transport in CH3NH3PbI3 perovskite solar cells.