Size-Controllable Metal Chelates as Both Light Scattering Centers and Electron Collection Layer for High-Performance Polymer Solar Cells

Open AccessCCS ChemistryRESEARCH ARTICLE1 Oct 2021Size-Controllable Metal Chelates as Both Light Scattering Centers and Electron Collection Layer for High-Performance Polymer Solar Cells Hao Liu†, Runnan Yu†, Yiming Bai, Yan Zeng, Yuanping Yi, Jun Lin, Jianhui Hou and Zhan’ao Tan Hao Liu† State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing 102206 †H. Liu and R. Yu contributed equally to this work.Google Scholar More articles by this author , Runnan Yu† Beijing Advanced Innovation Center for Soft Matter Science and Engineering, State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029 †H. Liu and R. Yu contributed equally to this work.Google Scholar More articles by this author , Yiming Bai State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing 102206 Google Scholar More articles by this author , Yan Zeng Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 Google Scholar More articles by this author , Yuanping Yi Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 Google Scholar More articles by this author , Jun Lin State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing 102206 Google Scholar More articles by this author , Jianhui Hou Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 Google Scholar More articles by this author and Zhan’ao Tan *Corresponding author: E-mail Address: [email protected] Beijing Advanced Innovation Center for Soft Matter Science and Engineering, State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202000550 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail An electron collection layer (ECL) between a photoactive overlay and an electrode plays a crucial role in optimizing the light field and charge extraction in bulk-heterojunction (BHJ) polymer solar cells (PSCs). However, the typical thickness of the photoactive layer is thinner than its optical path lengths, limiting further improvement of light absorption and device performance. Herein, we modulated the conjugated length of acetylacetonate-based ligands to synthesize a series of metal chelates [Hf(Acac)4, Hf(ACB1)4, and Hf(ACB)4] and used them as light-scattering functionalized ECLs for PSCs. Benefitting from the strong and different self-assembly effects of these chelates, size-controllable nanoparticles were formed and well distributed on the entire surface of the photoactive layer, which acted as scattering centers to modulate the light field of the active layers effectively. Among these chelates, due to the most suitable aggregation size of Hf(ACB1)4 nanoparticles, a preferable light-harvesting framework was achieved in PSCs using Hf(ACB1)4 as ECL that eventually yielded a high PCE of >17%. Thus, our work demonstrates a flexible design strategy for obtaining a series of size-controllable metal chelates as efficient ECL for facilitating light trapping in PSCs. Download figure Download PowerPoint Introduction Polymer solar cells (PSCs) with great advantages of light-weight, flexibility, and high potentials for low-cost and large-scale industrialization have been studied extensively in the last decades.1 The innovation came from countless efforts in material design, device engineering, and theoretical investigation, which dramatically improved the power conversion efficiencies (PCEs) of the single-junction PSCs to >16%.2–6 In addition to the efforts used in developing the active layer materials, the design and application of appropriate electrode buffer layers also play a vital role in achieving highly efficient devices. Typically, there are two main types of electrode buffer layers, including the electron collection layer (ECL) and hole collection layer (HCL), that could optimize the energy level alignment of the devices, decrease the energy band barriers, and adjust the light distribution within the photoactive layer.7–13 A variety of materials such as metal oxide, small organic molecules, and conjugated polymer have been designed and used as ECL in PSCs.14–27 Among these materials, the n-type metal oxides have exhibited good conductivity and optical transparency. However, metal oxides’ properties are sensitive to oxygen and UV-irradiation, and their complicated fabrication process along with high annealing temperature limit their further application in PSC industrialization. Recent advances in conjugated polymer synthesis have paved the way for obtaining varieties of alcohol-soluble conjugated materials to apply as efficient ECLs in PSCs via simple solution-processing technology.14–16 Nevertheless, the polymeric ECL materials suffer from some inevitable problems such as batch-to-batch variations and complicated synthetic routes.28 Hence, more promising alternative ECLs need to be developed to meet the requirements of further production and application for highly-effective PSCs. Bulk-heterojunction (BHJ) PSCs are limited by the short exciton diffusion distance and low intrinsic mobility of organic semiconductors due to the thickness of the photoactive layer, typically ranging from 100 to 200 nm. Such thickness is much leaner than the absorption length of its active layer, thereby difficult to further enhance the light absorption property without compromising the efficiency of PSCs.22 Hence, several approaches, like scattering elements, photonic crystals, optical spacers, and plasmonic nanoparticles, have been demonstrated to realize the light trapping in PSCs.29–31 We reported previously that CeOx and Ta-OMe aggregated on the surface of the active layer to form “island” scattering centers, and thus, optimized the light-field distribution of PSCs.32,33 Unfortunately, it was challenging to manipulate the size of these scattering centers to improve their properties further. Moreover, Au, Ag, and other metal oxide nanoparticles with well-defined sizes could also be synthesized to enhance the light scattering at the front surface of photovoltaic devices.34 However, it is difficult for such ECL to achieve uniform interface contact due to the possibility of microscopic short circuits, pinholes, and other defects; meanwhile, the cost and complicated processing are still problems to overcome.35–38 Therefore, to make the best of the light-scattering effect, it is essential to develop a simple preparation approach to fabricate efficient ECL materials with adequate charge transport capacity to improve the PSC photovoltaic performance. Hafnium(IV) acetylacetonate [Hf(Acac)4] with the merits of simple preparation and good adaptability has been applied successfully as an effective ECL in PSCs.39 It has been demonstrated that the acetylacetone-composed six-membered ring has a self-assembly effect through fusing with aromatic or other π-delocalized chelate rings.40 The self-assembly interaction is more significant than the CH-π interaction, implying that introducing an aromatic ring into the ligand structure expands the π-delocalization and render them condense stacking in a parallel direction.41 Since the chelate ring with a delocalized π bond can interact noncovalently, similar to the aromatic organic molecule, the fused chelates could be stacked on each other to form a specific nanostructure. Based on this principle, we manipulated the electronic and morphological properties of Hf(Acac)4 analogs by extending the π-conjugation of the chelates via simple chemical modifications. In this contribution, we designed the fusion of two acetylacetonate ligand molecules, 1-phenylbutane-1,3-dione (ACB1) and 1,3-diphenylpropane-1,3-dione (ACB), by introducing a benzene ring to the structure of acetylacetone. The chelates obtained, based on hafnium, Hf(ACB1)4 and Hf(ACB)4, exhibited a much stronger aggregation effect and tended to self-assemble into larger nanoparticles than Hf(Acac)4, and could distribute evenly on the entire surface of the photoactive layer. The highly efficient polymer donor PM6 and nonfullerene acceptor Y6 or BTP-eC9 were selected as the photoactive materials to characterize the optical, electrical, and morphological properties of Hf(Acac)4, Hf(ACB1)4, and Hf(ACB)4. Several measurements were used to study the photoelectric properties of different chelates and their effects on the device performance. Since the scattered light-induced enhancement in the optical path length the light-harvesting within the photoactive layer, the short-circuit current density (Jsc), and PCE of the PSCs using the chelate as ECLs, were increased significantly, especially for Hf(ACB1)4, which had the most suitable nanoscale structure. Also, the simulation calculation results were highly consistent with the experimental results relative to the light-scattering effect of different ligand chelates on enhanced external quantum efficiency (EQE) selectivity. Consequently, a high PCE of 17.13% was achieved in the PM6:BTP-eC9-based PSCs with Hf(ACB1)4 as ECL. Experimental Methods Indium tin oxide (ITO) glass was purchased from CSG Holding Co. (Shenzhen, China). poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (Clevious PVP AI4083) was purchased from H.C. Starck GmbH (Taicang City, China). PM6, BTP-eC9, and Y6 were purchased from Solarmer Energy Inc. (Beijing, China). Chloronaphthalene was supplied by Sigma-Aldrich (Shanghai, China ). Acetophenone, sodium amide, HfCl4, acetylacetone zirconium, and 1,3-diphenylpropane-1,3-dione (ACB) were purchased from Alfa Aesar (Beijing, China). All materials are to be used directly after purchase. Synthesis of ACB1 Acetophenone (5 g, 0.042 mol) was dissolved in a flask containing 50 mL ethyl acetate and placed in an ice-water bath. Then sodium amide (4.06 g, 0.1 mol) was slowly added, allowing the reaction to proceed for 3 h; the precipitate formed was separated by filtration and washed with diethyl ether to isolate a solid white powder. The solid was dissolved in water, and 50% HCl was added to adjust the pH to 5. The precipitate obtained (ACB1) was isolated by filtration, recrystallized from n-hexane, and dried. The ACB1 crystals were characterized, as follows: Proton nuclear magnetic resonance (1H NMR; 600 MHz, CDCl3, δ): 7.88 (s, 2H), 7.52 (t, J = 7.4 Hz, 1H), 7.45 (t, J = 7.6 Hz, 2H), 6.18 (s, 1H), 2.20 (s, 3H). Anal. Calcd for ACB1: C, 74.06; H, 6.22. Found: C, 74.22; H, 6.14 (Scheme 1). Scheme 1 | Synthesis route of ACB1. Download figure Download PowerPoint Synthesis of chelates The ligand (ACB1 and ACB) and HfCl4 were dissolved in ethanol at a ratio of 4∶1 and stirred at 70 °C overnight to obtain a white solid, which was washed with methanol and n-hexane to obtain a metal complex. Anal. Calcd for Hf (ACB1)4: C, 58.36; H, 4.41. Found: C, 58.08; H,4.18. Anal. Calcd for Hf (ACB)4: C, 67.26; H, 4.14. Found: C, 69.39; H, 3.94. Fabrication and characterization of polymer solar cell devices We designed two kinds of PSCs based on the device structure listed as follows: (1) ITO/PEDOT:PSS/PM6:Y6/Hf (Acac)4/Al, (2) ITO/PEDOT:PSS/PM6:Y6/Hf(ACB1)4/Al, (3) ITO/PEDOT:PSS/PM6:Y6/Hf(ACB)4/Al, and (4) ITO/PEDOT:PSS/PM6:BTP-eC9/Hf(ACB1)4/Al. The production process employed was as follows: The ITO glass substrates were washed sequentially by ultrasonic treatment in cleanser essence, water, ultrapure water, acetone, isopropyl alcohol, and subsequently, dried in an oven at 150 °C for 15 min in air. The substrates were then placed into a UV-ozone (UVO) chamber (Ultraviolet Ozone Cleaner; Jelight Co. Inc., Irvine, CA) for 15 min. When the temperature was down to 25 °C, the ITO glass substrates were spin-coated with a PEDOT:PSS, a polymer mixture layer of two ionomers, at 3000 rpm for 40 s, and then baked at 150 °C for 15 min in air. Subsequently, the ITO substrates were transferred into a glove box and spin-coated with a photoactive layer, consisting of chloroform solution of PM6:Y6 or PM6:BTP-eC9 (1∶1.2 w/w, polymer concentration of 8 mg·mL−1) and 0.3% volume ratio of chloronaphthalene additive solution (for refractive index determination) at 2000 rpm for 60 s under a nitrogen atmosphere. The Hf(Acac)4, Hf(ACB1)4, and Hf(ACB)4 solutions were made by adding chelate powder into the ethanol solvent to obtain a colorless solution at a concentration of 1 mg·mL−1. The ECL solution was directly spin-coated on the active metal chelates layer at 3000 rpm for 40 s to obtain a cathode buffer layer without further thermal annealing or other post-treatment. Finally, a 100 nm Al metal electrode was thermally deposited in the vacuum chamber. The current density–voltage (J–V) measurements of the devices were performed inside a nitrogen-filled glove box using a Keithley 2400 source measure unit (SMU) under simulated AM1.5G irradiation (100 mW·cm−2) and a xenon-lamp-based solar simulator (SAN-EI, AAA grade). An AC Mode III atomic force microscopy (AFM; Agilent Technologies, Shanghai, China) operated in the tapping mode at room temperature, under a standard atmosphere, to measure the active layer’s surface morphologies. The EQE measurements were conducted on QE-R systems (Enli Tech Co. Ltd., Beijing, China) with the standard single-crystal Si photovoltaic cell calibrated at each wavelength. The transmittance and absorbance spectra of the devices were measured by a LAMBDA 950 UV/vis/near-infrared (NIR) spectrophotometer (PerkinElmer, Shanghai, China). Time-resolved fluorescence device by Edinburgh Instruments Ltd. (Livingston, United Kingdom) F900 was used for steady/transient-state fluorescence spectrometry measurements. 1H and 13C NMR spectra were measured on a Bruker Arx-400 spectrometer (Bruker, Shanghai, China) using chloroform-d as a solvent and tetramethylsilane as an internal standard. The surface morphology of the samples was observed by scanning electron microscopy (SEM) of FEI Quanta 200 F (HE FEI YINHAO Chemical Co. Ltd., Hefei, Anhui, China) at an accelerating voltage of 30 kV. A JEOL JEM 2100 transmission electron microscope (TEM; JEOL Ltd., Shanghai, China) was used to investigate the surface morphologies. The contact angle images were analyzed using a profilometer of Dektak XT (Bruker) under ambient conditions. Results and Discussion The molecular structures of Hf(Acac)4, Hf(ACB1)4, and Hf(ACB)4 are shown in Figure 1a. As described in the synthesis subsection of Experimental Methods, the ACB1 ligands were obtained with a one-step synthetic route, while the ACB ligand was commercially purchased and used as received. Then ACB1 and ACB were coordinated with HfCl4 to synthesize the metal chelates Hf(ACB1)4 and Hf(ACB)4, respectively. First, TEM and energy-dispersive spectrometer (EDS) measurements were employed to investigate the structure and morphology of the nanoscale morphologies of Hf(Acac)4, Hf(ACB1)4, or Hf(ACB)4 on the ITO substrate. As shown in Figure 1b, a relatively regular circular aggregation morphology with an average diameter of 80 nm is observable in the surface morphology of the Hf(Acac)4. In the EDS map (see Supporting Information Figure S1), the O and Hf atoms representing Hf(Acac)4 were uniformly distributed on the surface of the film without any prominent aggregation structures. Similar spherical nanoparticles with a larger average surface diameter of 110 nm were formed by Hf(ACB1)4, and a significant aggregation effect was confirmed by the signals of O and Hf elements in the corresponding EDS map. For Hf(ACB)4, which had the most extended conjugated length, significant aggregations occurred, and the average size of the Hf(ACB)4 nanoparticles was increased further to 160 nm. These results indicated that the acetylacetone-based chelates with different conjugated lengths facilitated self-assembly into hemispherical nanoparticles of different aggregation sizes, forming continuous film structures. Figure 1 | (a) Synthesis route of the metal complexes, including Hf(Acac)4, Hf(ACB1)4, and Hf(ACB)4. (b) Plane-view TEM images of the spin-coated films of Hf(Acac)4, Hf(ACB1)4, and Hf(ACB)4. (c) ESP maps and the modeled bimolecular aggregation configurations of the three chelates, marked with the lowest binding energy. Download figure Download PowerPoint Density functional theory (DFT) calculations were performed to explain the formation of the different-size spherical nanoparticles by the three metal chelates at the molecular level.42–45 Before investigating the combination configuration of each compound, the electrostatic potential (ESP) was calculated to specify the combination sites. As displayed in Figure 1c, the oxygen atoms around the central metal presented the most negative area due to its strong intrinsic electronegativity, whereas the hydrogen atoms in the ambient ligands exhibited the most intense positive area, suggesting a physical interaction between the corresponding areas. Toward determining the most thermodynamically stable configuration, we modeled three kinds of common dimer geometries, including the parallel, T-type, and stagger configurations, and the binding energy (Eb) for each configuration of all three compounds, as summarized in Supporting Information Table S1. Compared with the parallel and T-type configurations of the three chelates, the stagger type demonstrates the lowest Eb, which manifested that the chelate molecules tended to form a cambered aggregated structure rather than regularly arranged and tiled in the nanofilms, accounting for the formed spherical nanoparticles. Moreover, with the increased conjugated system, the gradually extended molecular dimension and the enhanced Eb of the optimal aggregation configuration (from −6.99, −16.07 to −23.96 kcal·mol−1) rendered the formation of the progressively enlarged nanoparticles in the Hf(Acac)4, Hf(ACB1)4, and Hf(ACB)4 films, consistent with the experimental observations. Subsequently, we performed optical simulations along with theoretical calculations on this series of chelate ECLs to study the optical properties of the formed nanoparticle films. Mie theory was used to simulate the optical properties of nanoparticles formed by the metal chelates.46 The small particles covered the photoactive BHJ layer uniformly with an aggregation of some large-sized spheres (50–250 nm). According to Mie theory, the extinction properties of nanoparticles formed by metal chelates are determined by scattering and absorption. In Supporting Information Figure S2, the peak positions of the Hf(Acac)4, Hf(ACB1)4, and Hf(ACB)4 extinction curves were calculated to be 298, 336, and 358 nm, with the full width at half maximum of 42, 74, and 78 nm, respectively. Thus, compared with Hf(Acac)4, a broader range of light could be reflected by Hf(ACB1)4 and Hf(ACB)4 to improve the light absorption of the active layer. Besides, the intensity of extinction for Hf(ACB1)4 was much higher than the other two chelates, suggesting the best extinction capacity of Hf(ACB1)4 among this metal chelates series, which is beneficial for obtaining higher JSC values in PSCs. To evaluate the properties of Hf(Acac)4, Hf(ACB1)4, and Hf(ACB)4, as ECLs, polymer donor PM6, and nonfullerene acceptor Y6 (Figure 2a) were selected as the active layer materials for the fabrication of PSCs. The absorption spectra of the ECLs and the active layer materials were measured and plotted, as shown in Figure 2b. The three ECL types showed the main absorption peaks in the short wavelength region without overlapping with the active layer materials. As the ligand conjugation was extended gradually, the peak positions of the three chelates redshifted constantly, as indicated by the absorption peaks of Hf(Acac)4, Hf(ACB1)4, and Hf(ACB)4 located at 304, 334, and 358 nm, respectively. According to the energy levels reported in the previous article5,39 and the UV photoelectron spectroscopy (UPS) test, the energy levels of the active layer materials and chelates are shown in Supporting Information Figure S3. The onset of the photoemission energies (PEE) for Hf(ACB1)4 and Hf(ACB)4 were 17.09 and 17.12 eV, respectively, and the calculated work functions of Hf(ACB1)4 and Hf(ACB)4 were 4.13 and 4.10 eV.47 The low Fermi levels of Hf(ACB1)4 and Hf(ACB)4, (−4.13 and −4.10 eV), which lie close to the lowest unoccupied molecular orbital (LUMO) energy level of Y6, are energetically favorable for electron extraction from the LUMO of Y6 to the Al electrode through the ECLs. Besides, the low-lying highest occupied molecular orbital (HOMO) energy levels of Hf(ACB1)4 and Hf(ACB)4 (−6.51 and −6.36 eV) demonstrated excellent hole-blocking ability. Through the above tests, we found that Hf(ACB1)4 and Hf(ACB)4, possessing suitable optical and electrical properties, were good ECL candidates for preparing PSCs. Figure 2 | (a) Molecular structures of active layer donor material PM6, acceptor material Y6, and acceptor material BTP-eC9. (b) The absorption spectrum of pure PM6, Y6, Hf(Acac)4, Hf(ACB1)4, and Hf(ACB)4 films. (c) Plane and (d) cross-sectional view SEM images of the active layer film and those spin-coated with Hf(Acac)4, Hf(ACB1)4, and Hf(ACB)4, respectively. (e) The corresponding 3D AFM surface topography. Download figure Download PowerPoint SEM analysis was carried out to determine the morphological structure and the exact distribution of the chelate molecules on the BHJ photoactive layer films. From the plane and cross-sectional view of SEM images in Figures 2c and 2d, it is apparent that the BHJ photoactive layer film without any ECLs showed a smooth surface topography. Then we prepared the other three samples by directly spin-coating the ethanol solution of the different chelates onto the active layer films. Compared with the flat nanoparticles formed by Hf(Acac)4, the island-like nanoparticles of Hf(ACB1)4 were distributed uniformly on the BHJ photoactive layer. As the conjugate length of the ligands extended further, much larger size half-spherical nanoparticles with a mean radius of >100 nm could be formed by Hf(ACB)4 chelates, and its strongest aggregation effect was demonstrated by a cross-sectional SEM image of the BHJ/Hf(ACB)4 film. As summarized in Table 1, in the SEM patterns of 20 μm2, the counted number of nanoparticles formed by Hf(Acac)4, Hf(ACB1)4, and Hf(ACB)4 were 140, 105, and 36, respectively, and the total areas of the three nanoparticles are 2.22, 2.66, and 1.51 μm2, occupied 11.3%, 13.5%, and 7.7% of the entire area. As the conjugation of the organic ligand extended, the number of nanoparticles per unit area decreased, and the average area of the individual nanoparticles enlarged gradually from 0.016 to 0.042 μm2. A similar phenomenon was noticeable in the SEM images of the corresponding films of ITO substrates ( Supporting Information Figure S4), while the size of the self-assembled nanoparticles of the three chelates on the BHJ substrates was larger than those formed on the ITO, which might also relate to the molecular polarity and solubility.48 As shown in Supporting Information Figure S5, the contact angles of the ethanol solution of ITO/Hf(Acac)4, ITO/Hf(ACB1)4, and ITO/Hf(ACB)4 were 60.3°, 63.5°, and 72.7°, respectively, and those on the BHJ/chelate films showed much higher values of 83.6°, 92.6°, and 96.0°, respectively. Thus, we concluded that the contact angle of the chelates enlarged as the conjugation of the ligand increased. Also, the hydrophobic effect was one of the critical driving forces affecting the self-assembly process of metal chelates. Table 1 | Surface Statistics of Hf(Acac)4, Hf(ACB1)4, and Hf(ACB)4 ECL Count Total Area (μm2) Average Size (μm2) Area Fraction (%) Hf(Acac)4 140 2.22 0.016 11.3 Hf(ACB1)4 105 2.66 0.025 13.5 Hf(ACB)4 36 1.51 0.042 7.7 Tapping-mode AFM was also employed to characterize the changes in topography morphologies and surface area (SA) of the PM6:Y6-based BHJ film and those films coated with Hf(Acac)4, Hf(ACB1)4, and Hf(ACB)4, respectively.32 As shown in Figure 2e, the BHJ film showed the root mean square (RMS) roughness of 2.8 nm and a SA of 25.17 μm2 in a 5 μm × 5 μm scanning size. After it was spin-coated with Hf(Acac)4, the BHJ film displayed a smooth and uniform surface morphology with a SA of 25.20 μm2. Compared with the BHJ/Hf(Acac)4 film, the surface topography of the BHJ/Hf(ACB1)4 or BHJ/Hf(ACB)4 films demonstrated the bigger undulations with higher RMS of 7.9 and 8.5 nm, respectively, due to the larger aggregation and discrete distribution of Hf(ACB1)4 or Hf(ACB)4. Besides, the increased SA of the BHJ/Hf(ACB1)4 (25.33 μm2) and BHJ/Hf(ACB)4 films (25.70 μm2) might have potentially decreased the contact resistances with the following vacuum-deposited metal electrode.49 Finite-difference time-domain (FDTD) model and optical transfer matrix formalism simulation have been utilized to simulate the optical field distribution of various devices to affect their different chelates in nanoparticles eventually.34,35,50 Hence, we used the three-dimensional (3D)-FDTD method to build a simulation model, setting a plane-wave light source under the entire structure, with an analog device structure of ITO glass (170 nm)/PEDOT:PSS (30 nm)/active layer (100 nm)/ECL/Al (100 nm). The heights of Hf(Acac)4, Hf(ACB1)4, and Hf(ACB)4 nanoparticles of 10, 30, and 80 nm, respectively, were obtained from the SEM images and used as the radius of the simulated nanoparticles to set the corresponding hemisphere model, which was arranged periodically on the surface of the active layer (Figures 3a–3h). The incident light from the +Z direction is reflected by the back electrode and then scattered by the nanoparticles. Compared with the control device, the light-field intensity inside the PSCs with ECL metal chelates was enhanced significantly. Among the three types, the internal light-field intensity of the device with Hf(ACB1)4 was much stronger than that of the devices with Hf(Acac)4 or Hf(ACB)4 as ECL. However, since the FDTD model could only simulate monochromatic light as the light source model, optical transfer matrix model formalism of simulations have been established to investigate the wavelength selectivity of such device structures.19,51 The structure of the analog device was the same as the parameters of the FDTD simulation. As exhibited in Supporting Information Figure S6, the active layer of Hf(Acac)4-processed device had a stronger light field in the region from 350 to 500 nm, while Hf(ACB1)4 as ECL made the light field of the active layer more concentrated at 500–700 nm, and the light-field intensity was significantly larger than Hf(Acac)4. When Hf(ACB)4 was used as ECL, the light-field distribution of the active layer concentrated mainly in the region of >700 nm, and the light-field intensity in the shortwave direction was significantly weaker than the other two chelates. It also indicated that the light-scattering effect of chelates with different conjugations had wavelength selectivity to enhance the optical field of PSCs. Figure 3 | (a–d) Device structures with Hf(Acac)4, Hf(ACB1)4, Hf(ACB)4, and no ECL in light-field simulation. (e–h) The light-field intensity distribution of the device with Hf(Acac)4, Hf(ACB1)4, Hf(ACB)4 nanoparticles, and no ECL calculated by FDTD simulation. Download figure Download PowerPoint The devices with ITO/PEDOT:PSS/active layer/ECL/Al structures (Figure 4a) were fabricated and tested under AM 1.5G (100 mW·cm−2) illumination. The ethanol solution of metal chelates was spin-coated directly on the photoactive layer, without any post-treatment. For comparison, devices without ECL were also prepared as controls under the same fabrication conditions. As depicted in Figure 4b and summarized in Table 2, the control device delivered a JSC of 21.74 mA·cm−2, an open-circuit voltage (VOC) of 0.79 V, and a fill factor (FF) of 65.89%, obtaining a moderate PCE of 11.42%. The PSC with Ca as ECL exhibited a maximum PCE of 14.03% with a JSC of 24.11 mA·cm2, a Voc of 0.82 V, and an FF of 71.09%. When using Hf(Acac)4 as ECL, the photoelectric performance has been improved, where the JSC raised to 24.75 mA·cm−2, the VOC incr