Department of Materials Science and Engineering and California NanoSystems Institute, University of California Los Angeles, Los Angeles, CA 90095, USAInstitute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123, China
Department of Materials Science and Engineering and California NanoSystems Institute, University of California Los Angeles, Los Angeles, CA 90095, USAState Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, China
5 These authors contributed equally 6 Present address: College of Chemistry, University of California, Berkeley, Berkeley, CA 94720, USA 7 Lead Contact
Caffeine enhances both thermal stability and efficiency
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The stabilized power conversion efficiency of 19.8% was realized
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Thermally stable solar cells at 85°C for over 1,300 h were achieved
Context & Scale
To overcome the barrier of the commercialization of metal halide perovskite solar cells, a simple, cost-effective, and generalized strategy that mitigates the intrinsic thermal instability is strongly needed. Here, caffeine is introduced to simultaneously enhance the efficiency and thermal stability of the solar cells based on various kinds of perovskite materials. The strong interaction between caffeine and Pb2+ ions serves as a “molecular lock” that increases the activation energy during film crystallization, delivering a perovskite film with preferred orientation, improved electronic properties, reduced ion migration, and greatly enhanced thermal stability. Ultimately, a champion-stabilized efficiency of 19.8% with 1,300 h thermal stability at 85°C in nitrogen was achieved.
Summary
To increase the commercial prospects of metal halide perovskite solar cells, there is a need for simple, cost-effective, and generalized approaches that mitigate their intrinsic thermal instability. Here we show that 1,3,7-trimethylxanthine, a commodity chemical with two conjugated carboxyl groups better known by its common name caffeine, improves the performance and thermal stability of perovskite solar cells based on both MAPbI3 and CsFAMAPbI3 active layers. The strong interaction between caffeine and Pb2+ ions serves as a “molecular lock” that increases the activation energy during film crystallization, delivering a perovskite film with preferred orientation, improved electronic properties, reduced ion migration, and greatly enhanced thermal stability. Planar n-i-p solar cells based on caffeine-incorporated pure MAPbI3 perovskites, which are notoriously unstable, exhibit a champion-stabilized efficiency of 19.8% and retain over 85% of their efficiency under continuous annealing at 85°C in nitrogen.
Organic-inorganic hybrid halide perovskite (PVSK) materials have attracted considerable attention because of their unique photoelectric properties, which can be applied to an extensive variety of applications.
Efficient planar heterojunction perovskite solar cells by vapour deposition many different photovoltaic technologies are being developed for large-scale solar energy conversion.
Despite the tremendous achievements that have been made toward pushing the efficiency, long-term stability is still the challenge for the commercialization of PVSK solar cells.
such that methylammonium (MA)-based PVSK may be the ideal candidate for the commercialization of PVSK solar cells since the tetragonal black phase of MA PVSKs are stable at low temperatures.
However, the intrinsically volatile nature of the MA organic cation will lead to the rapid decomposition of PVSK and precipitate trigonal PbI2 at elevated temperatures.
For example, I− ions will easily migrate through the polycrystalline PVSK grains and even out of the PVSK layer to interfere with the metal electrode when exposed to thermal energy.
Also, the randomly oriented PVSK crystallites may result in poor charge transport in the vertical direction, a consequence of the fast and uncontrollable growth process of the PVSK film.
Crystallization kinetics and morphology control of formamidinium–cesium mixed-cation lead mixed-halide perovskite via tunability of the colloidal precursor solution.
Perovskite solar cells with inorganic electron- and hole-transport layers exhibiting long-term (≈500 h) stability at 85 °C under continuous 1 sun illumination in ambient air.
Increasing the activation energy for thermal decomposition of the PVSK film is significant to prolong the long-term thermal stability of PVSK solar cells.
On the other hand, many attempts have been made to obtain high-quality PVSK films that mitigate these challenges by utilizing various specific functional groups that can interact with PVSK. Park and coworkers introduced dimethyl sulfoxide (DMSO) with S=O group to retard the crystal growth via the Lewis base-acid adduct method.
There are also several studies that incorporated volatile or non-volatile small molecules or polymer additives with similar functional groups to passivate the trap defects and improve the device performance.
However, it remains difficult to slow down the crystal growth and control the orientated crystals for the purpose of simultaneously boosting the device PCE and long-term stability without sacrificing its electronic properties.
Herein, we introduced a small molecule 1,3,7-trimethylxanthine (Figure 1A), popularly known as caffeine, into the MA-based PVSK. By utilizing the carboxyl groups in the different chemistry environment,
Determination of caffeine content in coffee using Fourier transform infra-red spectroscopy in combination with attenuated total reflectance technique: a bioanalytical chemistry experiment for biochemists.
caffeine served as a “molecular lock” that interacted strongly with Pb2+ ions to slow down the PVSK crystal growth and induced a preferred orientation by increasing the activation energy. The superior crystallinity of the PVSK films with caffeine showed a reduced defect density and better vertical charge transport, achieving a champion PCE as high as 20.25%. In parallel, the excellent film quality suppressed ion migration, and the non-volatile caffeine interacted with the PVSK again during the degradation process to improve the thermal stability of the device. Ultimately, the caffeine-based devices were shown to be thermally stable at 85°C for over 1,300 h.
Figure 1Interaction between the Caffeine and Perovskite
(A) Lewis chemical structure and 3D structure of 1,3,7-trimethylxanthine (caffeine).
(B) FTIR spectra and fingerprint regions of pure caffeine, caffeine-PVSK, and the pristine PVSK films. The triangles indicate the stretching vibration peaks of C=O in the two films.
(C) FTIR spectra and fingerprint regions of pure caffeine and PbI2-MAI-DMSO-caffeine adduct film. The triangles indicate the stretching vibration peaks of C=O in the two films.
Effects of Caffeine on the PVSK Film Crystal Growth
Figure 1B shows the Fourier transform infrared spectroscopy (FTIR) spectra of caffeine, pristine MAPbI3, and MAPbI3 with caffeine. The stretching vibrations related to the two C=O bonds stretching in pure caffeine appear at 1,652 cm−1 and 1,699 cm−1, respectively.
Determination of caffeine content in coffee using Fourier transform infra-red spectroscopy in combination with attenuated total reflectance technique: a bioanalytical chemistry experiment for biochemists.
Upon adding caffeine into MAPbI3 film, it is observed that only the C=O stretching with lower frequency (due to the conjugation with C=C bond resulting in the electron delocalization) shifted from 1,652 to 1,657 cm−1, while the vibration mode of C=O at 1,699 cm−1 maintains its original value. This indicates the existence of caffeine in the PVSK film after annealing, and caffeine likely formed an adduct with MAPbI3 via the interaction between Pb2+ in PVSK and one of the C=O bonds in caffeine.
The FTIR spectra of pure PbI2 and PbI2: caffeine were also measured to further verify that caffeine interacted with the Pb2+ to form a strong Lewis acid-base adduct
(Figure S1). The shifts of the two characteristic C=O stretches followed the same trend as that of the PVSK case. This interaction was further confirmed by the shift of the Pb 4f orbital in the X-ray photoelectron spectroscopy (XPS) (Figure S2). To study the role of caffeine in PVSK crystal growth, we additionally conducted the FTIR on the PbI2-MAI-DMSO-caffeine adduct and observed that the same C=O stretching vibration shifted from 1,652 to 1,643 cm−1 (Figure 1C). Therefore, this strong interaction of C=O in caffeine with Pb2+ ions was expected to serve as a molecular lock to increase the activation energy of nucleation, which retarded the perovskite PVSK crystal growth and improved the crystallinity of the PVSK films with a preferred orientation. More importantly, the residual molecular lock possibly interacted with the amorphized PVSK again upon heating, which can play a crucial role in prohibiting the thermal-induced decomposition.
Figure 2A shows the cross-sectional scanning electron microscopy (SEM) of the caffeine-incorporated PVSK film. Then, we conducted steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) decay measurements to study the film quality and charge recombination dynamics, as shown in Figures 2B and 2C, respectively. The PL intensity of the caffeine-incorporated PVSK film was enhanced by six times compared to that of the pristine PVSK film. Simultaneously, the peak position was blue-shifted from 770 to 763 nm, which further confirmed that the number of trap states was reduced upon the addition of caffeine (Figure S3). Bi-exponential rate law model was employed to fit the PL decay. Both the pristine MAPbI3 and the caffeine-incorporated MAPbI3 films showed relatively faster decay time (τ1) and relatively slower decay time (τ2).
The faster decay is likely related to non-radiative recombination induced by charge-trapping defect states. On the other hand, τ2 represents the bi-molecular recombination in the bulk film. With the addition of caffeine, the fraction of the fast decay phase lifetime decreased from 57.8% to 41.9%, and the lifetime increased from 7.4 to 21.98 ns. Notably, the slow decay phase lifetime enhanced from 51.9 to 114.3 ns. These results further demonstrate the lower trap density and better electronic properties of the caffeine-incorporated PVSK film.
Figure 2Effects of Caffeine on the PVSK Film Crystal Growth
(A) Cross-sectional SEM of caffeine-containing perovskite film.
(B and C) Photoluminescence (PL) (B) and time-resolved PL spectra (C) of PVSK films without and with caffeine.
(D) X-ray diffraction patterns of as-cast PVSK films with or without caffeine.
(E) Radially integrated intensity plots along (110) crystal plane from the 2D grazing incidence wide-angle X-ray diffraction (GIWAXS) patterns in MAPbI3 and MAPbI3: caffeine films.
To examine the crystal structure, we conducted thin-layer X-ray diffraction (XRD) measurements for the PVSK films deposited on indium tin oxide (ITO) substrate (Figure 2D). The diffraction peak at 12.5°, assigned to the (001) planes of the hexagonal PbI2, was not detected for both the MAPbI3 and MAPbI3: caffeine films. Both films showed the same tetragonal PVSK phase with the dominant (110) lattice reflection at 13.9°, which is the preferred orientation for the PVSK films. The ratio of the (110) peak intensity at 13.9° to the (222) peak intensity at 31.8° increased from 2.00 to 2.43 upon incorporation of caffeine. This suggests that the (110) grains grew faster by consuming the randomly oriented neighboring crystals. We calculated the crystallite size using the Scherrer’s equation and by taking the full-width half-maximum (FWHM) of the (110) peak. At the optimized ratio of caffeine added, the crystallite size increased from 37.97 to 55.99 nm, consistent with the surface SEM images of the PVSK films (Figure S4). Obviously, the caffeine-incorporated PVSK showed an overall crystallinity enhancement, which was further confirmed by two-dimensional (2D) grazing incidence wide-angle X-ray diffraction (GIWAXS) analysis. Figure 2E shows the normalized azimuth angle plots along the (110) plane of pure MAPbI3 and caffeine-incorporated MAPbI3 (1 wt %) films, which were integrated from the 2D GIWAXS patterns (Figure S5). At the azimuth angle of 90°, the caffeine-incorporated PVSK film shows a very sharp peak compared with the pristine PVSK film. The narrower FWHM suggests that the incorporation of caffeine assisted the growth of the PVSK grains along the in-plane direction, which would enhance the charge transport of the device.
were employed as the electron-transporting layer. Pure MAPbI3 and MAPbI3: caffeine at various concentrations were employed as the active layer. Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) doped with 4-isopropyl-4′-methyldiphenyliodonium tetrakis (pentafluorophenyl) borate (TPFB)
was employed as the hole-transporting layer (HTL). Silver (Ag) was used as the cathode. Figure 3A shows the J-V curves of the champion devices based on pure MAPbI3 and MAPbI3: caffeine measured under reverse scanning, as illuminated by an AM 1.5G solar simulator with an intensity of 100 mW cm−2. The ratio of caffeine added was varied from 0 to 2 wt %. With increasing amounts of caffeine from 0 (reference) to 1 wt %, the open-circuit voltage (VOC), short-circuit current (Jsc), fill factor (FF), and reproducibility were systematically enhanced (Figure S6). The highest PCE achieved for the pure MAPbI3 devices was 17.59% (Voc: 1.074 V, Jsc: 22.29 mA/cm2, and FF: 73.46%). In contrast, a PCE of 20.25% (Voc: 1.143 V, Jsc: 22.97 mA/cm2, and FF: 77.13%) was achieved for the optimized devices with 1 wt % caffeine. The enhanced Voc and FF may be associated with decreased non-radiative recombination and crystal defects owing to the passivation effect induced by the incorporation of caffeine, consistent with the PL and TRPL results. The Jsc was also enhanced from 22.29 to 22.97 mA/cm2, which was confirmed by the external quantum efficiency (EQE) spectra of devices based on pure MAPbI3 and MAPbI3: caffeine (Figure 3B). The calculated Jsc values obtained from the integration of the EQE spectra were close to the measured values with the AM 1.5G reference spectrum (average mismatch is below 5%, Table 1). The quantum efficiency enhancement in the long wavelength region was consistent with the ultraviolet-visible (UV-vis) absorption spectra (Figure S7). The absorption enhancement may have resulted from the larger crystal size and better crystallinity of the caffeine-incorporated devices, which has been proven to enhance the light-harvesting efficiency caused by enhanced light scattering. The steady-state PCEs of the best-performing devices were 17.04% and 19.76% for the devices without and with caffeine, respectively (Figure S8). J-V hysteresis between the reverse and forward scan directions was also decreased with the addition of caffeine. The hysteresis index decreased from 0.157 to 0.097 upon addition of 1 wt % caffeine (Figure S9; Table S1). To demonstrate the universality of caffeine, we examined the device performances of the devices based on mixed A-site cations and mixed halide PVSK (CsFAMAPbIxBr3-x). The J-V curves under both reverse and forward scan directions and the steady-state output efficiency are shown in Figure S10 and Table S2. The PVSK solar cells with caffeine showed enhanced photovoltaic performance compared to the reference, which indicates this approach could be effective and universal.
Figure 3Enhanced Photovoltaic Performances and TPC and TPV Analysis
(A) J-V curves of the champion PSC of pristine PVSK and caffeine-containing PVSK in reverse scan direction.
(B) EQE spectra and integrated current densities from the EQE spectra of PVSK devices with or without adding caffeine.
(C and D) Normalized transient photovoltage (TPV) decay (C) and normalized transient photocurrent (TPC) decay (D) in MAPbI3 and MAPbI3: caffeine-based perovskite solar cells.
To gain further insight into the performance enhancement resulting from the use of caffeine, we characterized the charge-transfer kinetics and charge recombination in the pure MAPbI3 and MAPbI3: caffeine solar cells.
We used transient photovoltage decay (TPV) under the open-circuit condition to characterize the solar cells and found that the charge-recombination lifetime (τr) of the MAPbI3: caffeine-based device was substantially longer than that of the device based on pure MAPbI3 (285 μs versus 157 μs) (Figure 3C), consistent with the slowed charge recombination in the MAPbI3: caffeine film, as concluded from the TRPL measurement. This indicates a lower defect concentration and hence superior electronic quality in the caffeine-incorporated PVSK bulk film, consistent with the higher Voc of the corresponding devices. Meanwhile, transient photocurrent decay (TPC) under the short-circuit condition was conducted to investigate the influence of caffeine on the charge transfer in the devices. Although the addition of caffeine did not affect the band structure of the PVSK (Figure S11), the charge transfer lifetime (τt) of the caffeine-containing PVSK decreased from 2.67 to 2.08 μs. The enhanced charge transfer might have resulted from the reduced interfacial defects and the better crystal orientation of the MAPbI3: caffeine films.
Thermal Stability and TGA Analysis
To prove the molecular locking effect of caffeine on the corresponding PVSK devices during the thermal degradation process, we conducted a continuous thermal stress stability test for the devices based on bare MAPbI3 and MAPbI3: caffeine at 85°C in a nitrogen atmosphere. The device with caffeine showed excellent thermal stability, preserving 86% of its original PCE after 1,300 h. The represented J-V curves before and after being subjected to the thermal stability test are shown in Figures S12, S18, and S19. The controlled device only retained 60% of its original PCE after 175 h, which may be due to ion migration, poor crystal quality, and phase instability of bare MAPbI3 at high temperatures. Furthermore, to evaluate the operational stability, all the devices were encapsulated under a nitrogen atmosphere and exposed to continuous illumination (90 ± 10 mW, without UV filter) under open-circuit conditions (Figure S13). Caffeine can also effectively enhance the operational stability of the PVSK solar cell. To understand the mechanism of the enhanced thermal stability of the caffeine-incorporated devices in the context of ion migration and phase decomposition, we first conducted XRD analysis on the devices after the thermal stability test (Figure 4B). For the reference device, there was a strong peak at 12.5°, attributed to the (001) planes of hexagonal PbI2. The extremely weak diffraction at 13.9° suggests a complete degradation of the PVSK crystal. Not surprisingly, there was a relatively strong diffraction signal at 38.5°, assigned to the (003) plane of PbI2. Although there was a peak at 12.5° in the target device, the (110) plane signal was still strong. The superior crystallinity of the caffeine-incorporated PVSK might have resulted in the suppression of ion migration during heating. Thermogravimetric analysis (TGA) on caffeine and the adduct powders was conducted to analyze the phase stability and thermal properties of caffeine and the intermediate adduct phase. Figures 4C and 4D indicate the weight loss and heat flow of the powders, respectively, based on caffeine, pure PVSK, and caffeine-incorporated PVSK. Caffeine decomposed completely at around 285°C, demonstrating its superior thermal stability at temperatures below 200°C (Figure S14). From Figure 4C, there are three main steps of the weight loss at around 70°C, 340°C, and 460°C for the pure PVSK powders. These three steps are correlated with the sublimation of DMSO, MAI, and PbI2, respectively.
For the caffeine-containing PVSK adduct powders, the sublimation temperature of MAI and PbI2 were higher than that of the bare PVSK powders, which indicates that more energy is required to break the interaction between caffeine and the PVSK precursors. This is further confirmed by the heat flow diagrams as shown in Figure 4D. The strong interaction formed by the caffeine molecular lock may increase the activation energy for the decomposition upon heating.
(A) Devices thermal stability upon 85°C continuous annealing in nitrogen box.
(B) XRD patterns of aged device based on pure MAPbI3 and MAPbI3 with caffeine.
(C and D) Thermogravimetric analysis (TGA) of weight loss (C) and heat flow (D) of pristine caffeine, MAI-PbI2-DMSO adduct powder, and MAI-PbI2-DMSO-caffeine adduct powder.
Microstructure Analysis via Cross-Sectional STEM and In Situ HRTEM
To further investigate the role of caffeine in suppressing ion migration and thermal decomposition, microstructure analysis was carried out. We first conducted cross-sectional scanning transmission electron microscopy (STEM) and energy-dispersive X-ray (EDX) spectroscopy analysis (Figure 5). The samples were directly collected from the devices after the 1,300-h thermal stability test via focused ion beam (FIB). Figure S15 shows the EDX mapping of selective regions on both the control and target devices. The spatial distribution of the Pb and I elements determined the PVSK active layer region. The Ag electrode was above the active layer, separated by the PTAA HTL. For the control sample, there were significant Ag signals (Ag clusters) with a similar intensity as that in the electrode region detected at the interface between the HTL and the active layer region. It is likely that the Ag diffused through the whole PVSK region, as confirmed by the observation that the Ag signals were detected even in the ITO electrode region. More importantly, the iodine signals were clearly detected in the Ag electrode region. Iodine could accumulate at the electrode and interface. It easily reacted with Ag to form AgI, which will negatively impact the device performance. In contrast, there is no obvious indication of such similar ion migration in the caffeine-incorporated PVSK device. To further confirm the result quantitively, line-scanning profiles were also measured. As shown in Figure 5G, the thickness of the Ag electrode was 50 nm, two sharp peaks at both the electrode and interface were observed, and the Ag signal was detected almost throughout the entire device. However, the thickness of the Ag electrode in the caffeine-incorporated PVSK device maintained its original value of 100 nm. More importantly, a sharp iodine peak was detected in the electrode region of the control device, in agreement with the previous conclusion. Hence, from the STEM results, the suppression of ion migration in the caffeine-containing films ensured the high thermal resistance of the devices. We also conducted real-time high-resolution transmission electron spectroscopy (HRTEM) to study the effect of caffeine on the phase transformation of the PVSK. The electron beam (E-beam) of the HRTEM instrument was utilized as the source of the thermal energy. Figures 6A–6D show the HRTEM images and the corresponding fast Fourier transforms (FFTs) of the diffraction patterns of both the caffeine-incorporated and pristine PVSKs. MAPbI3 layers with various crystallographic orientations were observed in both samples. The representative spot diffractions (yellow circles) with an interplanar spacing of 3.1 Å, which are well matched with the (110) diffraction of MAPbI3, are shown in Figures 6B and 6D. After exposure to the E-beam for 5 min 30 s, the environmental temperature of the samples was elevated to around 135°C.
Figures 6E–6H present the HRTEM images and the corresponding FFTs of the diffraction patterns of the aged caffeine-incorporated and pristine PVSKs. Although the intensity of the (110) diffraction spots of the caffeine-incorporated PVSK became weak, no new diffraction peaks appeared. Notably, a critical alteration of the MAPbI3 layer was observed in the control sample with the (110) diffraction spots observed to split (red circle). On the other hand, there was a new broad ring that appeared in the FFT at 3.9 nm−1, and new diffraction spots were observed at the same place. These morphological characteristics suggest that some crystallized PVSK phase had been transferred to the amorphized phase with precipitated trigonal PbI2 grains at this region, which agrees with a previous study that the thermal degradation of PVSK is often considered as the reverse process of the PVSK film growth.
We also conducted in situ TEM measurement on the other regions of the films, and the results followed a similar trend as described previously (Figures S16 and S17). From these results, we speculate that the existence of the caffeine additive serves as a molecular lock to interact with the amorphized PVSK phase again to increase the decomposition activation energy of the PVSK, which locks the amorphized phase of the PVSK, thereby preventing the degradation of the PVSK when exposed to high temperatures.
(A–F) Energy-dispersive X-ray spectra (EDX) mapping of the aged pure PVSK device (A) Ag, (B) I, and (C) Pb, and the aged caffeine-containing PVSK device (D) Ag, (E) I, and (F) Pb.
(G and H) EDX line scans of (G) aged pure PVSK device and (H) aged caffeine-containing PVSK device.
High resolution transmittance electron microscopy (HRTEM) of (A) fresh caffeine containing PVSK, (C) fresh pure PVSK, (E) aged (5 min 30 s) caffeine containing PVSK, and (G) aged (5 min 30 s) pure PVSK. Corresponding fast Fourier transforms (FFTs) of (B) fresh caffeine containing PVSK, (D) fresh pure PVSK, (F) aged (5 min 30 s) caffeine containing PVSK, and (H) aged (5 min 30 s) pure PVSK.
In conclusion, we employed caffeine, a conjugated Lewis base with two carboxyl groups, to serve as a molecular lock on PVSK. The strong interaction between caffeine and the Pb2+ ions increased the growth activation energy of the PVSK film, which facilitated the growth of high-quality films that showed preferred orientation and superior electronic properties. Consequently, a PCE as high as 20.25% was achieved for the champion device. In the meantime, the superior film quality suppressed the migration of ions. Also, the residual conjugated molecular lock effectively prevented any form of thermal degradation (the reverse of the PVSK synthesis process), which realized PVSK: caffeine-based solar cells that were thermally stable for over 1,300 h at 85°C.
Experimental Procedures
Materials
Unless stated otherwise, solvents and chemicals were obtained commercially and used without further purification. N, N-dimethylformamide (DMF), DMSO, chlorobenzene (CB), caffeine, toluene, water, and Ag were obtained from Sigma-Aldrich Inc. TPFB and PbI2 were obtained from TCI Inc. MAI was obtained from One Materials. PTAA was obtained from Xi'an Polymer Light Technology Corp and Sigma-Aldrich Inc (Lot# MKCD5161). Tin oxide (SnO2) nanoparticle was obtained from Alfa-Aesar Inc.
Device Fabrication
PVSK solar cells were fabricated with the following structures: indium tin oxide (ITO), SnO2, MAPbI3, PTAA, and Ag. The ITO glass was pre-cleaned in an ultrasonic bath of acetone and isopropanol, and treated in ultraviolet-ozone for 20 min. A thin layer (∼30 nm) of SnO2 was spin-coated onto the ITO glass and baked at 150°C for 30 min. SnO2 was diluted in water (1.67 mg mL−1). After cooling at room temperature, the glass, ITO, and SnO2 substrates were transferred into a nitrogen glove box. PVSK solution was prepared by dissolving 159 mg MAI and 461 mg PbI2 and 71 μL DMSO in 600 μL DMF. For the optimized caffeine containing PVSK, 6 mg caffeine was added into the solution. The PVSK solution was spin-coated on the substrate at 2,500 rpm for 25 s, to which 0.1 mL of CB was dropped after 10 s. The spin-coated film was heat treated at 65°C for 1 min followed by 100°C for 20 min. The PTAA solution (30 mg mL−1, in CB with 10% TPFB) was spun onto the PVSK film as a hole conductor. The devices were completed by evaporating 100 nm Ag in a vacuum chamber (base pressure, 5 × 10−4 Pa).
Synthesis of Adduct Powders
For MAI·PbI2·DMSO and MAI·PbI2·DMSO·caffeine adducts,1 mmol of PbI2, MAI, and DMSO with or without 6 mg caffeine were dissolved in 600 μL of DMF, to which 10 mL of diethyl ether was added to precipitate the corresponding adduct. The precipitates were collected and dried under vacuum.
Device Characterization
J-V characteristics of photovoltaic cells were taken using a Keithley 2400 source measure unit under a simulated AM 1.5G spectrum, with an Oriel 9600 solar simulator. Typically, the devices were measured in reverse scan (1.2 V → 0 V, step 0.02 V, 100 mV/s) and forward scan (0 V → 1.2 V, step 0.02 V, 100 mV/s). All the devices were measured without pre-conditioning such as light-soaking and applied bias voltage. Steady-state power conversion efficiency was calculated by measuring stabilized photocurrent density under constant bias voltage. EQEs were measured using an integrated system (Enlitech, Taiwan) and a lock-in amplifier with a current preamplifier under short-circuit conditions. For TPV and current (TPC) measurements, a white light bias was generated from an array of diodes (Molex 180081-4320) to simulate 0.5 sun bias light working condition. A pulsed red dye laser (Rhodamine 6G, 590 nm) pumped by a nitrogen laser (LSI VSL-337ND-S) was used as the perturbation source, with a pulse width of 4 ns and a repetition frequency of 10 Hz. The intensity of the perturbation laser pulse was controlled to maintain the amplitude of transient VOC below 5 mV so that the perturbation assumption of excitation light holds. The voltages under open circuit and currents under short-circuit conditions were measured over a 1 MΩ and a 50 Ω resistor and were recorded on a digital oscilloscope (Tektronix DPO 4104B).
Materials Characterization
UV-vis absorption spectra of the PVSK films were obtained using a U-4100 spectrophotometer (Hitachi) equipped with integrating sphere in which monochromatic light was incident to the substrate side. XRD patterns of the films were recorded by X-ray diffractometer (PANalytical) with Cu kα radiation at a scan rate of 4°/min. Surface and cross-sectional microscopic images of the films and devices were acquired by SEM (Nova Nano 230). ∼1-nm-thick gold was sputtered on cross-sectional samples to enhance the conductivity. Transmission Fourier transform infrared (FTIR) spectroscopic analysis was performed using FT/IR-6100 (Jasco). The chamber was purged with nitrogen gas during the measurement. Steady-state PL measurement was carried out using Horiba Jobin Yvon system in which a 640 nm monochromatic laser was used as an excitation fluorescence source. Time-resolved PL decay profiles of the PVSK films were investigated by a Picoharp 300 with time-correlated single-photon counting capabilities. A 640 nm monochromatic pulsed laser with a repetition frequency of 100 kHz was generated from a picosecond laser diode head (PLD 800B, PicoQuant). The energy density of the excitation light was controlled to be ∼1.4 nJ/cm2.
GIWAXS test was performed at the BL14B1 beamline of China Shanghai Synchrotron Radiation Facility (SSRF). 2D GIWAXS patterns were obtained by a MarCCD 225 detector mounted vertically at around 256.401 mm from the sample with an exposure time of 50 s at a grazing incidence angle of 0.15°. The x axis of GIWAXS patterns were represented by the diffraction vector with q = 4πsin(θ)/λ, where θ is half of the diffraction angle and λ is the wavelength of incident X-ray. XPS measurements were carried out on an XPS AXIS Ultra DLD (Kratos Analytical). An Al Kα (1,486.6 eV) X-ray was used as the excitation source. UPS measurements were carried out to determine the work function and the position of valence band maximum of materials. A He discharge lamp, emitting ultraviolet energy at 21.2 eV, was used for excitation. All UPS measurements were performed using standard procedures with a −9 V bias applied between the samples and detectors. Clean gold was used as a reference. High angle annular dark field scanning transmission electron microscopy (HAADF STEM) images, EDS maps, and line-scan profiles were taken on a FEI Titan STEM operated at 300 kV. The device glass, ITO, SnO2, PVSK, PTAA, and Ag was used for the TEM sample. The focused ion beam technique was used for cross-TEM sample preparation. HRTEM analysis was performed by Titan (FEI). The PVSK film was scratched off from the substrate and dispersed in toluene by sonication for 10 min, which was dropped on a copper grid. Accelerating voltage of 300 kV was used for the measurement. TGA was conducted using SDT Q600 (TA instruments) under dry air with a heating rate of 10°C/min.
Acknowledgments
Y. Yang acknowledges the Air Force Office of Scientific Research (AFOSR) (grant no. FA2386-18-1-4094), the Office of Naval Research (ONR) (grant no. N00014-17-1-2,484), the UC Solar Program (grant no. MRPI 328368), and Jinzhou Solargiga Energy in China. Z.-K.W. acknowledges the Natural Science Foundation of China (no. 91733301). This project is also funded by the Collaborative Innovation Center of Suzhou Nano Science and Technology and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
Author Contributions
R.W. and J.X. contributed equally to this work. R.W. and Y. Yang conceived the idea. R.W. and L.M. carried out solar cell fabrication and characterization. J.X. carried out the FTIR measurement and analyzed the data. J.-W.L., Z.W., and S.T. carried out the XRD measurement. P.S. and T.H. carried out the TPC and TPV measurements. Z.Z. and Y.H. carried out the TEM measurement. J.-W.L. carried out the TGA measurement. Z.-K.W. carried out the GIWAXS measurement. Y. Yuan provided advice on the thermal stability measurement. R.W., J.X., L.M., and Y. Yang analyzed the data and wrote the manuscript. L.C., J.L.Y., and Y.D. provided helpful discussions during the project and helped with the manuscript preparation. All the authors thank Dr. Yingguo Yang for assistance with GIWAXS measurement. All the authors discussed the results and commented on the manuscript. Y. Yang supervised the project.
Efficient planar heterojunction perovskite solar cells by vapour deposition many different photovoltaic technologies are being developed for large-scale solar energy conversion.
Crystallization kinetics and morphology control of formamidinium–cesium mixed-cation lead mixed-halide perovskite via tunability of the colloidal precursor solution.
Perovskite solar cells with inorganic electron- and hole-transport layers exhibiting long-term (≈500 h) stability at 85 °C under continuous 1 sun illumination in ambient air.
Determination of caffeine content in coffee using Fourier transform infra-red spectroscopy in combination with attenuated total reflectance technique: a bioanalytical chemistry experiment for biochemists.
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decreased from 0.157 to 0.097 upon addition of 1 wt % caffeine (Figure S9; Table S1). To demonstrate the universality of caffeine, we examined the device performances of the devices based on mixed A-site cations and mixed halide PVSK (CsFAMAPbIxBr3-x). The J-V curves under both reverse and forward scan directions and the steady-state output efficiency are shown in Figure S10 and Table S2. The PVSK solar cells with caffeine showed enhanced photovoltaic performance compared to the reference, which indicates this approach could be effective and universal.
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