The present invention relates generally hybrid perovskites and, more particularly to humidity-driven crystallization of hybrid perovskite films such as may, for example, find use in solar cell and optoelectronic applications.
Hybrid perovskites are a mixture or combination of organic and inorganic ions with the same or similar crystal structure as calcium titanium oxide (CaTiO3). Halide perovskites are a subset of these materials containing halide ions such as fluoride, chloride or iodide, for example. An example of one such material is the iodide perovskite known as methylammonium lead iodide (CH3NH3PbI3).
Low cost organometal halide perovskite solar cells have emerged as prime candidates to meet the future energy generation demands at the gigawatt-scale, due to the certified-power conversion efficiency (PCE) of around 20% with simple solution-based processing approaches. Formation of such hybrid perovskites typically involves mixing precursors, both organic (e.g., CH3NH3I, CH3NH3Br, CH3NH3Cl, CH(NH2)2I, CH(NH2)2Br, CH(NH2)2Cl) and inorganic (e.g., PbI2, PbBr2, PbCl2), to form the organometal perovskite crystals.
One-step solution-based approaches to the production or growth of hybrid perovskite thin films have typically produced or resulted in the generation of a large density of pinholes or voids within the perovskite films, which can adversely impact photovoltaic performance, especially in planar heterojunction configuration devices. In addition, large variations in film morphology significantly limit device performance reliability, impacting yield.
As a result, two step solution-based processing techniques such as methylammonium iodide (CH3NH3I) vapor-assistance and the lead iodide (PbI2)/CH3NH3I bilayer interdiffusion have been developed in an effort directed to the growth of void-free perovskite thin films. These approaches typically employ high temperature (e.g., ≧100° C.) thermal-annealing to drive the diffusion of methylammonium (CH3NH3I) molecules into the dense lead iodide (PbI2) precursor layer to form compact, void-free methylammonium lead triiodide (CH3NH3PbI3) perovskite thin films. Thermal annealing for extended periods, however, has been found and is known to cause the decomposition of such perovskites into PbI2 which degrades the device performance by acting as defects in the perovskite thin films.
In addition, defects and grain boundaries in perovskite films detrimentally cause significant energy loss and decrease the performance of solar cells. Therefore, suppressing the effect of defects and grain boundaries is important for the pursuit toward the theoretical maximum efficiency with industrially realistic processing techniques.
There is a need and a demand for a simple and reliable solution processing approach for forming, producing, or otherwise making highly-crystalline perovskite films, that are desirably void-free, and which processing desirably does not require thermal annealing, as such processing can find desirable application in or for practical solar cell manufacturing technology, for example.
In accordance with one aspect of the subject development, methods for making hybrid perovskite films as well as the hybrid perovskite films themselves are provided.
In particular embodiments, methods for preferably making such a void-free hybrid perovskite film as well as the hybrid perovskite films themselves such as well-suited for low-cost high-performance planar heterojunction photovoltaic devices are provided.
In particular embodiments, methods for making vertical grain-boundary bulk-heterojunction hybrid perovskite films for solar cells and optoelectronic applications as well the hybrid perovskite films themselves are provided.
In accordance with one embodiment, a method for making a hybrid perovskite film involves sequentially depositing, e.g., spin coating, respective layers of inorganic and organic hybrid perovskite precursors onto a substrate to form a hybrid perovskite film precursor and subsequently exposing the hybrid perovskite film precursor to a humidified atmosphere to convert the inorganic and organic hybrid perovskite precursors to the hybrid perovskite. In one embodiment, such exposure can desirably be conducted at or near room temperature. Further, such exposure can desirably result in or produce well-oriented, highly-crystalline perovskite films without thermal annealing processing.
In accordance with one embodiment, methods for making the hybrid perovskite methylammonium lead iodide (CH3NH3PbI3) is provided. One such method involves sequentially spin coating layers of an inorganic hybrid perovskite precursor comprising PbI2 and layers of an organic hybrid perovskite precursor comprising CH3NH3I onto a substrate to form a hybrid perovskite film precursor. The hybrid perovskite film precursor is subsequently exposed to a humidified atmosphere corresponding to ambient air at room temperature with a humidity of at least 25% to convert the inorganic and organic hybrid perovskite precursors to form the hybrid perovskite.
In accordance with another aspect, there is provided a composition of matter comprising a hybrid perovskite film having crystalline columnar grains within a polycrystalline film.
In one embodiment, there is provided a composition of matter comprising a highly-crystalline void-free hybrid perovskite film formed by sequentially spin coating respective layers of inorganic and organic hybrid perovskite precursors onto a substrate to form a hybrid perovskite film precursor. The hybrid perovskite film precursor is exposed to a humidified atmosphere to convert the inorganic and organic hybrid perovskite precursors to the hybrid perovskite.
As used herein references to crystalline columnar grains within a polycrystalline film in general refer to the boundaries between the grains, including GBs and cracks, such as in accordance with aspects of the subject development may be infiltrated with, for example, a hole or electron transport material to create bulk heterojunctions between the columnar grains that enhance the electron and hole separation between grains. The infiltrated hole or electron transport material can act or serve as conduction pathways for holes or electrons to traverse to the p or n-type contact, respectively.
As used herein references to crystalline or, in some embodiments, references to highly crystalline films or materials are to be understood as generally referring to such films or materials that are 100% crystalline within the detection limits of TEM-SAD and X-ray diffraction.
As used herein, references to a vertical grain boundary (Vertical GB) generally refers to a grain boundary that has an origination or termination point at the hole transport layer/perovskite (top of the film shown in
As used herein reference to a “void-free” material, such as a perovskite film, are to be understood as generally referring to such a material wherein there are no voids detected within a 100 micron×100 micron SEM image. For example, in accordance with one aspect of the development, a void would have to traverse from the top of the perovskite film to the TiO2/perovskite interface to be detected. This technique would be able to detect voids with diameters larger than 10 nm, which would be exposed at the surface. Thus, there were be no voids detected within a 50 nm thick (thickness is defined as the direction into the page in image shown in
Other objects and advantages will be apparent to those skilled in the art from the following detailed description taken in conjunction with the appended claims and drawings.
The above-mentioned as well as other features and objects of the invention will be better understood from the following detailed description taken in conjunction with the drawings wherein:
Selected aspects of the present development concern methods for making hybrid perovskite films as well as such or resulting hybrid perovskite films themselves.
While the subject development will be described further below making specific reference to the hybrid perovskite methylammonium lead iodide (CH3NH3PbI3), the inorganic hybrid perovskite precursor PbI2 and the organic hybrid perovskite precursor CH3NH3I, those skilled in the art and guided by the teachings herein provided will understand and appreciate that the broader practice of the subject development is not so limited. That is, other suitable hybrid perovskites, such as other suitable halide perovskites, including, for example, methylammonium metal trihalide (CH3NH3BX3), formamidinum metal trihalide (H2NCHNH2BX3), methylammonium metal mixed halide (CH3NH3BX3-aYa), formamidinum metal mixed halide (H2NCHNH2BX3-aYa), where B is lead (Pb), or tin (Sn), both X and Y arc halogen atoms including iodine (I), bromine (Br) or chlorine (Cl), and a can be any number less than or equal to 3, and other corresponding inorganic and organic hybrid perovskite precursors, such as including methylammonium halide, formamidinum halide, metal halide, can be suitably employed.
As described in greater detail below, in accordance with one embodiment, a layer-by-layer sequential depositing method or technique, for example, by spin-coating, is desirably employed to grow “bilayer” CH3NH3I/PbI2 films. As will be appreciated by those skilled in the art and guided by the teachings herein provided, such a layer-by-layer technique can desirably provide or result in a high level or degree of controllability. Subsequently, interdiffusion between PbI2 and CH3NH3I layers can desirably be progressed or driven by a simple humidified air exposure, such as at room temperature, for making well-oriented, highly-crystalline perovskite films without necessitating or requiring thermal annealing processing. More particularly, the resulting high degree of crystallinity desirably can produce or result in a carrier diffusion length of ˜800 nm and a high device efficiency, such as device efficiencies in excess of 10%, in excess of 12%, in excess of 13%, in excess of 14%, or in excess of 15%, e.g., efficiency values which are comparable to values reported for thermally-annealed perovskite films. As will be appreciated, the simplicity and high device performance of such processing approach is or can be highly promising for direct integration into industrial-scale device manufacture.
It has been found that “air exposure”, such as herein described, can have an effect on perovskite crystal size. Further, it has been found that hybrid organometal perovskite materials are very sensitive to humidity, and degrade in humid air. However, the presence of water during film formation appears to solubilize some of the precursors promoting better mixing.
As detailed herein, the effect of humid air exposure during film formation can be utilized to reduce, minimize, avoid or preferably total eliminate the need for or conducting of a thermal annealing step.
Key findings or discoveries of the subject development include:
In view of the above, humidity exposure processing (HEP) methods are contemplated as an important or critical step in the formation in a variety of unique vertical grain boundary perovskite heterojunction (VGBPH) structures that can form through the infiltration of different media, after the formation of the crystals, or during the formation of the crystals.
It is further contemplated that the reduction, minimization, avoidance or total elimination of thermal processing of hybrid perovskite solar cells using HEP methods can or may produce or result in one more, alone or in combination, of the following beneficial features:
Further, with humidity exposure being found as a key to the growth of large single-crystals of perovskites which is important to form vertical grain boundaries from top to bottom through the entire active perovskite layer, enabling the VGBPH structures, then single-crystal perovskite films with vertical grain boundaries such as resulted from the HEP methods may result in or constitute compositions of matter wherein:
Experimental Section
This details a simple room-temperature, air-exposure process that removes, avoids or eliminates the thermal annealing step and drives the interdiffusion of PbI2 and CH3NH3I layers to synthesize highly-crystalline and pinhole-free CH3NH3PbI3 perovskite films. The interdiffusion process and the resulting formation of CH3NH3PbI3 films were characterized by in-situ X-ray diffraction (XRD), which revealed that ambient water molecules are a driving force for the interdiffusion. Perovskite thin film photovoltaic devices resulting from the new air-exposure process displayed a PCE of 15.6%, which is comparable to that of most thermally-annealed perovskite devices.
Device Fabrication and Characterization
A TiO2 precursor solution was spin-coated onto UV-Ozone treated ITO glass substrates at 2000 rpm for 60 seconds in air. After sintering the TiO2 precursor film in a furnace at 500° C. for 30 minutes to obtain the anatase phase of TiO2, a PbI2 solution (550 mg/mL) and CH3NH3I solution (70 mg/mL) were sequentially spin-coated onto the TiO2 layer at 6000 rpm for 30 seconds in a N2-filled glovebox. The as-spin-cast “PbI2/CH3NH3I” bilayer films were then maintained in ambient air (humidity of ˜30%) at room temperature to form the perovskite layer. A Spiro-OMeTAD solution (90 mg/mL in chlorobenzene) was doped by adding 10 μL 4-tert-butylpyridine (tBP) and 45 μl lithium bis(trifluoromethane sulfonyl) imide (Li-TFSI, 170 mg/mL in anhydrous acetonitrile). Then, the Spiro-OMeTAD solution was spin coated at 2000 rpm for 40 seconds, and then left in a desiccator overnight. Finally, silver was thermally-evaporated at deposition rate of 1 Å/s at a vacuum level of 10−6 mbar to form a 100 nm-thick film. For the thermally annealed device fabrication, the as-grown bilayer films were then placed in ambient air (humidity of ˜30%) for 60 minutes until the color of the films was dark brown. Subsequently, the films were thermally annealed at 100° C. for 2 hours covered by a glass petri dish in an N2-filled glovebox.
Current density-voltage (J-V) curves were measured using a Keithley 2400 source meter in a N2-filled glovebox. The devices were illuminated at 100 mW/cm2 (AM 1.5 G solar spectrum) from a solar simulator (Radiant Source Technology, 300 W, Class A). Prior to each measurement, the light intensity was carefully calibrated using a NIST-certified Si-reference cell. The J-V curves were obtained by scanning from reverse bias to forward bias, and forward bias to reverse bias, with a 50 ms sweep delay time. The active area was ˜6.5 mm2, which was measured and calculated by optical microscopy. For incident photon-to-current efficiency (IPCE) measurements, the light source was chopped at 35 Hz and the output electrical signal was collected by a lock-in amplifier (Merlin Radiometric Lock-In Amplifier, Newport) under short-circuit conditions: A calibrated silicon diode (70356-70316NS, Newport) with a known spectral response was used as a reference.
Film Characterization
An X-ray difflactometer (Panalytical X'Pert MPD Pro) with Cu-Kα radiation (λ=1.54050 Å) was used to measure the x-ray diffraction (XRD) patterns (0/20 scans). The step size and scan rate was 0.0167113° and 0.107815°/s, respectively. The X-ray pole figure was acquired using an X-ray diffractometer (Panalytical X'Pert MRD Pro) with Cu-Kα radiation (λ1.54050 Å). A two-theta-angle of 14.05° was employed to find the orientation of the (110) plane of the CH3NH3PbI3 perovskites.
Surface morphological images were measured with a scanning electron microscope (SEM, Zeiss Merlin) with an accelerating voltage of 5 kV. A Zeiss Libra 200MC was used to perform the transmission electron microscope (TEM) measurements with an operating voltage of 200 kV. Cross-sectional TEM specimens (thickness of ˜50 nm) were prepared by focused ion beam (FIB) milling with a final polish. An accelerating voltage of 10 kV and beam current of 20 pA were used to minimize beam induced surface damage. For the selected area diffraction pattern measurements, an aperture diameter of 325 nm, which exceeds the thickness of our perovskite layer in the device, was used.
Electron beam induced current mapping (EBIC) and associated scanning electron microscopy (SEM) measurements were conducted in a Hitachi 4800-CFEG, which was equipped with a Gatan EBIC system. A Stanford Research Systems model SR570 low-noise current preamplifier was applied to amplify the EBIC currents. In order to ensure a good signal-to-noise ratio and minimal electron beam induced damage, an accelerating voltage of 1.5 kV with beam currents of 30-50 pA was used. 5 kV Argon ion milling with a Gatan Illion+ was employed at liquid nitrogen temperatures to create a smooth cross-sectional surface to reduce artifacts due to topography. It should be emphasized that these samples can be damaged during argon ion milling and electron beam exposure, which can reduce the EBIC signal intensity, and only the first EBIC scan are displayed.
Photoluminescence (PL) spectra were measured using a spectrometer (Acton SP2300) equipped with a CCD (Princeton Instruments, Pixis 256), which was coupled to a microscope. The time-resolved PL was measured by using a time correlated single photon counting (TCSPC) (Horiba Scientific with Picosecond Photon Detection Module, PPD-850 and Fluorohub model: Horiba JY IBH). The PPD-850 was mounted to a second port of the same spectrometer. The perovskite films were excited using a second harmonic (400 nm) of a Ti:sapphire laser (Coherent, Mira 900) (800 nm, 5 ps pulses, 76 MHz repetition rate). To match TCSPC requirements, the laser repetition rate was reduced to ˜5 kHz using a pulse picker (Coherent). The output from the pulse picker was frequency doubled using an ultrafast harmonic generator (Coherent 5-050) and directed into a microscope to illuminate the perovskite films through a 10× microscope objective (beam spot size ˜21×39 μm) with an average power of ˜0.7 mW. In order to stabilize the specimens under illumination, they were exposed to a 400 nm laser light for ˜20-30 minutes. The quartz side of the samples was irradiated. The absorbance measurements were conducted with a Varian Cary 5000 spectrophotometer.
A HMS-3000 Hall Measurement System was employed to conduct the characterization of the carrier mobility of the air-exposed perovskite films, which were prepared on silicon substrate with dimension of 1 cm×1 cm and thickness of 350 nm. When combined with carrier lifetime measured previously, the carrier diffusion length was estimated according to equation
where, k is the Boltzmann constant, T is temperature, q is elementary charge, μ is carrier mobility, and τ is the carrier lifetime.
The surface morphological images were taken with a scanning electron microscope (SEM; Zeiss Merlin SEM). The SEM images were acquired by the Inlens model, line average scanning with a gun voltage of 5 kV. The transmission electron microscope (TEM) used in these studies was a Zeiss Libra 200MC operated at 200 kV. The electron energy loss spectroscopy (EELS) images were acquired with a beam current of 158 pA, as measured on the calibrated spectrometer CCD in vacuum, with an exposure time of 0.1 seconds per pixel and a pixel size of 5.3×5.3 nm2. Spectra were background subtracted and fit with a hydrogenic model scattering cross-section. With a known beam current and calibrated spectrometer, the fitting parameter could be interpreted as atomic areal density, and a quantitative value is obtained within ˜10%. Selected area diffraction was taken with an illuminated area of 325 nm for Pattern 1-3 and an illuminated area of 180 um for Pattern 4-6 as shown in
Discussion
The PbI2 layer and CH3NH3I layer were spin-coated sequentially onto TiO2/ITO glass substrates using dimethylformamide (DMF) and 2-propanol, respectively. As shown in
To investigate the evolution of the perovskite phase in the as-cast thin films during ambient exposure, in-situ X-ray diffraction was employed to monitor the PbI2 (001) and CH3NH3PbI3 (110) Bragg peaks as a function of time. As shown in
To assess the applicability of the films for solar cells, prototype devices were fabricated. The photovoltaic performance of electron transporting layer (ETL)-free devices (ITO/CH3NH3PbI3/Spiro-OMeTAD/Ag) based on our air-exposed perovskite films were first examined. In order to grow compact perovskite layers, a UV-ozone treatment for 10 minutes was used to generate a hydrophilic ITO glass surface, as shown by water contact angle measurements. Surprisingly, when swept from forward bias to reverse bias, the device without TiO2 yielded a JSC of 18.7 mA/cm2, a VOC of 1.02 V, a FF of 72%, and a PCE of 13.8% (
It should be noted that both types of devices, with and without TiO2, exhibited hysteresis characteristics. To reveal how the J-V hysteresis affects the maximum power output from the devices, a device was measured in both forward and reverse scan mode, which showed a PCE of 15.2% and 8%, respectively. The photocurrent as a function of time measured at the maximum power output point (0.792 V) under an illumination of 100 mW/cm2 was then acquired. The photocurrent density saturated at ˜16 mA/cm2, and the device showed a stable PCE of 12.7% despite of the strong J-V hysteresis.
To further understand the origin of their high photovoltaic performance, the time-resolved photoluminescence (PL) at ˜760 nm (
To understand the cross-sectional morphology and crystallinity of the devices, cross-sectional TEM images and SAED studies were performed. In
Electron beam induced current (EBIC) measurements were further employed to investigate the device operation mechanism. The cross sectional SEM and corresponding EBIC images are shown in
In summary, a simple room temperature air-exposure approach was developed to grow high crystalline, well-orientated perovskite films, resulting in a long carrier diffusion length of ˜800 nm. The devices with and without TiO2 ETL yielded PCE of 15.6% and 13.8%, respectively, which are comparable to that of most thermally-annealed perovskite devices. The non-thermal-annealing, room-temperature-air-exposure approach is compatible with roll-to-roll processing, which enables very low-cost flexible photovoltaics.
In one aspect of the subject development, perovskite thin films with large single crystalline grains and vertically oriented grain boundaries were grown through a simple solution-based layer-by-layer spin-coating method followed by air exposure and thermal annealing treatments. The grain boundaries (GBs) were shown to be infiltrated with p-type doped 2,2′,7,7′-tetrakis[N,N-di(p-methoxyphenylamine)]-9,9-spirobifluorene (Spiro-OMeTAD) to form a new type of vertical bulk heterojunction (BHJ) structure within the perovskite films as demonstrated by electron energy loss spectroscopy (EELS) study and electron beam induced current (EBIC) mapping. These measurements also showed that the BHJs at GBs can significantly reduce non-radiative recombination losses as well as enhance carrier collection in the devices. This approach leads to high performance devices with near 100% internal quantum efficiencies (IQEs) in the visible light range and high average PCE of 16.3±0.9%.
A sequential spin-coating method was applied to fabricate the PbI2/CH3NH3I bilayer films in a N2 filled glovebox. The as-casted bilayer films were initially exposed in ambient air (humidity of ˜30%) during the chemical formation of the perovskite layers. X-ray diffraction (XRD) was employed to examine the chemical reaction between the two precursor layers (PbI2/CH3NH3I) during room-temperature air exposure. The spin-coated PbI2/CH3NH3I bilayer film was found to have completely converted into a polycrystalline perovskite film after air exposure for 60 min, as shown by the disappearance of the PbI, Bragg peaks (in the top most XRD pattern shown in
The as-cast PbI2/CH3NH3PbI3/CH3NH3I “trilayer” films can convert to single phase CH3NH3PbI3 films even in a water-free environment (
The large perovskite grain films, as shown in.
External quantum efficiencies (EQEs) of these devices were also measured. A typical EQE curve is shown in
IQE near 100% evidences remarkable carrier generation and collection efficiencies in the subject devices. In order to reveal the origin of the exceptional cell performance, the vertical microstructure of the perovskite film in the devices was investigated. Transmission electron microscopy (TEM) was applied to image the cross-sectional morphology of the entire device with architecture (ITO/TiO2/CH3NH3PbI3/Spiro-OMeTAD/Ag). Large oriented-crystalline grains and vertically-aligned GBs were clearly identified (
EBIC measurements were further applied to reveal local electrical properties in single crystal grains as well as vertical GBs within the active layer. In EBIC, highly-energetic electrons were used to generate several electron-hole pairs in targeted areas of a semiconductor, which were separated due to internal electric fields and collected at electrodes. By scanning the electron beam, EBIC measurements revealed several important electrical characteristics, such as presence of internal electric fields, the location of p-n junctions at the mesoscopic scale, recombination centers (e.g., grain boundaries), and estimation of minority carrier diffusion lengths.
In order to understand the underlying causes of superior carrier collections in GBs, the presence of materials in the GBs had to be unveiled. Thus, the GBs between these vertically-aligned single-crystalline grains were examined with Z-contrast scanning transmission electron microscopy (Z-STEM) and EELS mapping.
In brief, as high PCE of 16.3±0.9% based on perovskite thin films containing large single-crystal grains and vertical GBs was thus obtained. Further, the hole transport medium Spiro-OMeTAD has been demonstrated to be infiltrated into the GBs to form vertical BHJs within a perovskite layer to suppress the recombination loss and enhance carrier collection in the vicinity of GBs, leading to IQEs approaching 100%. Thus, such an approach can present a facile and important alternative pathway to achieve the theoretical maximum PCE in hybrid perovskite solar cells.
In view of the above, it is to be appreciated that one aspect of the subject development involves a composition of matter such as composed of a vertical BHJ structure with large crystals and active grain boundaries. As described, such a composition can involve annealing and, if desired, infiltration of another material or media. Further, such a composition may rely on humidity processing such as follows:
Following annealing, structures such described or provided herein can be easily infiltrated such as, for example, herein shown to activate grain boundaries as active charge separation/collection channels in the solar cell in the BHJ geometry. Those skilled in the art and guided by the teachings herein provided will understand and appreciate that general BHJ structures such as herein described and provided are not necessarily limited to or require humidity processing.
The invention illustratively disclosed herein suitably may be practiced in the absence of any element, part, step, component, or ingredient which is not specifically disclosed herein.
While in the foregoing detailed description this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purposes of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.
This application claims the benefit of and priority to U.S. Provisional Application No. 62/237,168, filed on 5 Oct. 2015, the entirety of which application is incorporated herein by reference.
This invention was made with government support under Contract No. DE-AC05-00OR22725 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
Number | Date | Country | |
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62237168 | Oct 2015 | US |