Fine-Tuning Contact via Complexation for High-Performance Organic Solar Cells

Open AccessCCS ChemistryRESEARCH ARTICLE1 Mar 2022Fine-Tuning Contact via Complexation for High-Performance Organic Solar Cells Yawen Li†, Zhenzhen Zhang†, Xiaona Han†, Tao Li and Yuze Lin Yawen Li† Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 , Zhenzhen Zhang† Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 , Xiaona Han† Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 , Tao Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Center for Spintronics and Quantum Systems, State Key Laboratory for Mechanical Behavior of Materials, Department of Materials Science and Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049 and Yuze Lin *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 https://doi.org/10.31635/ccschem.021.202100832 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail For organic optoelectronic devices, precise tuning of the electrical property of both active layers and interfaces is crucial to achieve enhanced device performance. Herein, we developed a facile method using complexation to modify the work function and energy levels of cathode contact layers in organic solar cells (OSCs) to achieve suitable work function and energy levels while retaining relatively good conductivity. Compared with the control devices with neat (N,N-dimethyl-ammonium N-oxide)propyl perylene diimide (PDINO) contacts, the tris(pentafluorophenyl)borane (BCF)-complexed PDINO cathode contacts showed enhanced power conversion efficiencies (PCEs), which is independent of the composition of the active layer. More specifically, single-junction OSCs employing PDINO cathode contact with 2 wt % BCF-additive achieved an average PCE of 17.7%. Based on experimental data and theoretical modeling, we found that the boron cores of BCF coordinate with the amino N-oxide terminal substituent of PDINO after generating BCF–H2O/methanol complexes. BCF segments with a strong electron-withdrawing property can effectively reduce the energy levels of the PDINO–BCF complex, and thus enhance device PCE when it is used as a cathode contact in OSCs. This strategy can be extended to other types of photovoltaic devices, photodetectors, and light-emitting diodes. Download figure Download PowerPoint Introduction Precisely manipulating the electrical properties, such as charge carrier density, conductivity, energy level, and work function, of semiconductors is one of the underpinnings of inorganic and organic electronics, such as solar cells, field-effect transistors, light-emitting diodes, and others.1–5 Organic solar cells (OSCs) are promising cost-effective, light-weight alternatives to silicon-based solar cells for certain applications.5–16 Plentiful active layer materials, such as PM6,17 PM7,18 PM6-Tz20,19 PTQ10,20 D18,21 M34,22 Y6,23 BTP-4Cl,24 and their analogues, have been explored to boost the efficiency of OSCs to over 16%.25 In contrast, it is more challenging to develop efficient contact materials (interface materials) with all requirements of high conductivity, proper work function and energy levels, orthogonal solubility, and so forth. Very few types of efficient organic semiconducting contacts have been employed for high-performance OSCs.26–28 In addition to the design and synthesis of new efficient contact materials, it is necessary to develop facile strategies to modify currently available materials for optimized energy level alignment, reduced series resistance, and finally improved device performance in OSCs and other optoelectronic applications.29,30 Herein, we developed a convenient method of introducing tris(pentafluorophenyl)borane (BCF) into the cathode layer to tune energy level alignment within devices. OSCs employing (N,N-dimethyl-ammonium N-oxide)propyl perylene diimide (PDINO) with BCF additive (Figure 1a) as cathode contacts show enhanced device power conversion efficiencies (PCEs) relative to the control devices with neat PDINO contacts, independent of the composition of the active layer. Among them, D18:Y6-based OSCs with the PDINO–BCF contact show average PCE of 17.7%, which is one of the highest values reported for single-junction OSCs to date.21 The complex of PDINO–BCF–H2O/methanol is formed in the PDINO–BCF blend layers, in which the BCF segment with a strong electron-withdrawing property reduces the electron density of the π-system of the PDINO segment, resulting in the lowering of cathode layer energy levels and thus enhancing device efficiency of OSCs. Figure 1 | Work function and energy level characterizations. (a) Chemical structures of PDINO and BCF. (b) Setup scheme of KPFM measurement. (c) Morphology and CPD distribution images of the PDINO, PDINO:BCF (50:1, w/w), and PDINO:BCF (1:1, w/w) films. (d) Histograms of CPD images. (e) UPS spectra of the PDINO, PDINO:BCF (50:1, w/w), and PDINO:BCF (1:1, w/w) films. (f) Energy diagram of the PDINO, PDINO:BCF (50:1, w/w), and PDINO:BCF (1:1, w/w) films. Dash lines indicate work function. Download figure Download PowerPoint Experimental Methods Fabrication of solar cell devices OSCs were fabricated with the structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) (PEDOT):poly(styrenesulphonate) (PSS)/active layer/PDINO with and without BCF/Al or Ag. The patterned ITO glass was cleaned by sonication for 30 min independently in deionized water, acetone, and isopropanol. After patterned ITO glass was treated in an UV-ozone chamber (SunMonde Company, Shanghai, China) for 5 min, a hole-transporting layer of PEDOT:PSS (Clevios VP AI 4083, Heraesus, Leverkusen, Germany) was deposited by the spin-coating solution at 4000 rpm for 30 s, followed by thermal annealing at 150 °C for 20 min. A solution of donor:acceptor blend was subsequently spin-coated on the PEDOT:PSS layer to form a photosensitive layer. Then PDINO with and without BCF additive in methanol solution (1 mg mL−1) was coated on the top of the active layer by spin coating at 3000 rpm for 30 s. The BCF first was dissolved in methanol, and then added to the PDINO methanol solution using micropipette. The solution was mixed for at least 30 min and then used. Finally, Al or Ag was evaporated onto the surface under vacuum (ca. 10−5 Pa). The active area of the device was 4 mm2. The current density–voltage (J–V) curves were measured under AM 1.5G light at 100 mW cm−2. The details of characterization are presented in the Supporting Information. Results and Discussion PDINO has been one of the most widely used solution-processed cathode contacts for high-performance OSCs.20,31–35 However, the lowest unoccupied molecular orbital (LUMO) energy level of PDINO of approximately −3.7 eV26 is higher than those (−3.85 to −4.0 eV) of the popular electron acceptors like PC71BM, (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene) (ITIC), and Y6.13,23,36 These mismatched energy levels between PDINO and the widely used electron acceptors indicate that the device performance of OSCs based on PDINO cathode contacts could be further enhanced by eliminating the LUMO energy barrier between PDINO and the active layer. BCF, as a typical and widely used p-type dopant, is able to move down the energy levels and work function of many organic molecules and polymer semiconductors for organic electronics,37–44 which drives us to investigate how BCF additive affects the electronic property of contact layers, like PDINO. To investigate the effect of BCF additive on the electronic property of the PDINO thin film, the work function and energy levels of PDINO thin film with and without BCF additive were acquired by using Kelvin probe force microscopy (KPFM), and UV photoelectron spectroscopy (UPS). KPFM is a surface potential microscopy that is a noncontact variant of atomic force microscopy (AFM). By KPFM, the surface potential and effective work function of metal or semiconductor surfaces can be observed at atomic or molecular scales.45 The measured effective work function relates to many surface phenomena, including doping and band bending of semiconductors.46 KPFM can reflect information about the composition and electronic state of the local structures on the surface of a solid by mapping the effective work function of the detected surface.47 Here, the setup scheme of KPFM measurement is shown in Figure 1b. In Figure 1c, the PDINO films with and without BCF additive are composed of nanograins as determined by KPFM. The root-mean-square roughness values of PDINO neat films, PDINO:BCF (50:1, w/w) blend film, and PDINO:BCF (1:1, w/w) blend film are 7.5, 7.8, and 8.6 nm, respectively. The spatial distributions of contact potential difference (CPD) of the three samples are rather uniform, but the absolute average CPD values differ dramatically among them. The CPD reflects the relative work function with respect to that of the tip (Pt/Ir-coated conductive silicon probe), which is defined as (Φtip – Φsample)/e (Φtip and Φsample are the tip and sample work functions, respectively, and e is the elementary charge).48 Taking the work function of the tip as a reference, the work function increases with the content of BCF. The work function of PDINO:BCF blended film with a weight ratio of 50∶1 and 1∶1 is approximately 0.1 and 0.6 V, respectively, higher than that of the pure PDINO thin film (Figure 1d). UPS measurement shows that BCF additive increases the work function and moves down the highest occupied molecular orbital (HOMO) energy level of PDINO. The neat PDINO thin film shows a work function of 3.93 eV and HOMO level of −6.02 eV (Figures 1e and 1f). PDINO:BCF blend film with 2 wt % BCF additive shows a higher work function of 4.01 eV with a lower HOMO level of −6.09 eV. As the BCF content in PDINO increases to 1:1, the work function of the PDINO:BCF blend film further increases to 4.76 eV with a much lower HOMO level of −6.80 eV. This result from UPS observation is consistent with the KPFM results. These results indicate that BCF can efficiently move down the work function and HOMO energy levels of the PDINO film. Then the LUMO energy of the PDINO film should also be lower as an accompanying effect of the moving down HOMO because of the relatively fixed optical bandgap of PDINO ( Supporting Information Figure S1). After we demonstrated that BCF moves down both work function and energy levels of PDINO film, the conductivity of PDINO film with and without BCF additive was investigated by two probe measurements on lateral devices, in which the varied channel lengths were used to exclude the effect of contact resistances, respectively. The lateral device structure is illustrated in Figure 2a and consisted of two gold electrodes deposited on insulated SiO2 substrates covered by PDINO and PDINO:BCF blend film. The length of electrodes was 1400 μm, and varied channel widths of 30, 40, and 50 μm were used. In Figure 2b, the calculated conductivity of neat PDINO film is 6.1 × 10−6 S cm−1; the PDINO:BCF blend film (50:1, w/w) showed similar conductivity of 5.4 × 10−6 S cm−1; and the PDINO:BCF blend film (5:1, w/w) showed over one order lower conductivity of 1.6 × 10−7 S cm−1. Although 2 wt % BCF additive decreases conductivity, the dark current of lateral devices based on PDINO:BCF (50:1, w/w) blend film is higher than the device based on neat PDINO film (Figure 2c). This phenomenon can be explained by reduced contact resistance between the PDINO:BCF film and metal electrodes, due to their better energy alignment. The electron mobility within blend films was calculated from the space–charge-limited current (SCLC) method employing an electron-only device structure of ITO/ZnO/PDINO or PDINO:BCF blend film/Al. The neat PDINO thin film exhibits electron mobility of 1.7 × 10−5 cm2 V−1 s−1, and the PDINO:BCF blend film with a weight ratio of 50∶1 shows increased electron mobility of 3.4 × 10−5 cm2 V−1 s−1 ( Supporting Information Figure S2). Here, the increased charge-carrier mobility of PDINO induced by 2% BCF additive is consistent with what is observed in literature.37,43 Figure 2 | Charge-transporting property of PDINO with and without BCF. (a) Scheme of the device setup for conductivity measurement. (b) dV/dI vs d/S plots and calculated conductivity of PDINO and PDINO:BCF blend films (50:1, 5:1 w/w). (c) I–V characteristics of PDINO and PDINO:BCF blend films (50:1, 5:1 w/w) with channel widths of 30 μm. (d) Setup scheme of C-AFM measurement. (e) Vertical dark-current images of C-AFM. (f) Dark current extracted from C-AFM images. Download figure Download PowerPoint We also used conductive AFM (C-AFM) to investigate the vertical conductivity of PDINO film with and without BCF additive. The C-AFM can simultaneously measure the surface topography of the sample and the electric current flow across the sample thickness.49 The C-AFM of PDINO film with and without 2 wt % BCF was measured by contact mode (Figure 2d), in which the ITO is the bottom electrode and the platinum-coated tip works as the top electrode. In Figure 2e, both the films show high dark currents except for a few grain boundaries. Figure 2f reveals that PDINO film with 2 wt % BCF exhibits a slightly higher dark current than that of the neat PDINO thin film at the same bias of ±1 V. This result indicates that the small quantity of BCF additive did not obviously change the conductivity of PDINO film on a vertical direction to the substrate but did tune the work function and energy level of PDINO causing a lower contact resistance within devices. Since PDINO with optimized BCF amounts shows optimized energy level and work function while maintaining good conductivity, one of its promising applications is the use in high-performance solar cells as a cathode contact layer to modify the contact between active layer and electrodes. To demonstrate its potential, we fabricated OSCs with a device structure of ITO/PEDOT:PSS/organic bulk heterojunction (BHJ) photoactive layer/PDINO with and without BCF/metal cathode (Al or Ag) (Figure 3a). First, we selected the typical polymer donor PTB7-Th and fullerene acceptor PC71BM as the BHJ layer ( Supporting Information Figure S3). The corresponding photovoltaic data, as well as device efficiencies of the OSC devices with optimized PTB7-Th:PC71BM blends using PDINO with varied amounts of BCF additive as cathode contacts under the illumination of AM 1.5G, 100 mW cm−2, are listed in Supporting Information Table S1 ( Supporting Information Figure S4). The devices with neat PDINO as the cathode interfacial materials deliver a PCE of 5.9% with an open-circuit voltage (Voc) of 0.800 V, short-circuit current density (Jsc) of 13.6 mA cm−2, and fill factor (FF) of 53.5%. The PDINO with an addition of 2 wt % BCF displays an increased PCE of 6.7% (Voc = 0.809 V, Jsc = 14.2 mA cm−2, FF = 58.2%). We found that the Jsc increases from 13.6 to 14.2 mA cm−2 and FF increases from 53.5% to 58.2%, which can be attributed to the better electron extraction of cathode electrodes. To test the universality of the PDINO with BCF cathode contact for OSCs fabricated with other active layers, devices were fabricated with high-performance nonfullerene BHJ absorbers consisting of PM6:Y6:IDIC50 as the ternary component and D18:Y6 as binary component ( Supporting Information Figure S3). Like PTB7-Th:PC71BM-based OSCs, the PDINO:BCF cathode contact improves the efficiencies of PM6:Y6:IDIC and D18:Y6-based devices. Relative to the control device with neat PDINO contact, 2 wt % BCF increases the PCE from 15.4% to 15.8% in PM6:Y6:IDIC-based OSCs ( Supporting Information Figure S4 and Table S1). D18:Y6-based binary OSCs with 2 wt % BCF added PDINO contact shows an average PCE of 17.7% for eight devices, with the best value of 18.1% (Figure 3b), a Voc of 0.865 V, Jsc of 27.5 mA cm−2, and FF of 75.9%. In contrast, the control device based on D18:Y6 active layer with neat PDINO contact shows a relatively low PCE of 17.1% (with an average value of 16.9%). The PCEs achieved in our devices with the PDINO:BCF contact are among the best values reported for single-junction OSCs to date.21,24,51–54 It should be noted that the BCF additive in PDINO film may slightly affect the morphology and electronic property on the surface layer of the BHJ active layer55,56 because of likely direct contact between the active layer and proximate BCF. Figure 3c shows that the external quantum efficiency (EQE) spectrum of the devices based on D18:Y6 active layer and PDINO contact with 2 wt % BCF additive is higher than that of the control devices without BCF additive. The photocurrent density calculated from EQE was <5% mismatch from those of the J–V measurement, which is among the reasonable mismatched values reported for high-performance OSCs.57,58 Figure 3d shows D18:Y6-based OSCs employed with PDINO layer with 2 wt % BCF yield lower dark current density relative to the control devices based on neat PDINO cathode contact. These results are consistent with PDINO with 2 wt % BCF additive being a better cathode contact than the neat PDINO layer. We also measured the shelf-, thermal-, and photostability of devices with PDINO or PDINO:BCF (50:1, w/w) contact and found that BCF did not obviously affect device stabilities ( Supporting Information Figure S5). Figure 3 | Photovoltaic performance of the OSCs employing PDINO with and without BCF. (a) Device structure of the OSCs. (b) J–V curves of the best OSCs based on D18:Y6 blend using PDINO film with and without 2 wt % BCF as cathode contact layer under the illumination of AM 1.5G, 100 mW cm−2. (c) EQE spectra of the best OSCs based on D18:Y6 blend using PDINO film with and without 2 wt % BCF as a cathode contact layer. (d) Dark currents of the best OSCs based on D18:Y6 blend using PDINO film with and without 2 wt % BCF as a cathode contact layer. Download figure Download PowerPoint The next question to be addressed is how the BCF interacts with PDINO to tune the electronic property of the PDINO and finally improves the device performance when acting as cathode contacts. Very recently, a comprehensive mechanism of BCF p-doping thiophene-related polymer and organic semiconductors has been well demonstrated where p-type doping occurred between polymers and BCF–H2O complex. The BCF–H2O complex as a Brønsted acid protonates the polymer backbone followed by electron transfers from a neutral chain segment to a positively charged, protonated one, forming a Lewis acid-base adduct.37 This protonic acid doping by BCF–H2O complex generally occurs in organic and polymeric semiconductors in the presence of a thiophene-related moiety, which usually behaves as a p-type and Lewis basic property. However, it is still unknown how the BCF interacts with other types of organic semiconductors that do not consist of any thiophene-related moiety, including the PDINO we used here. To reveal the mechanism of interaction between PDINO and BCF, the electrospray ionization mass spectrometry (ESI-MS), 19F nuclear magnetic resonance (19F-NMR), UV–vis absorption spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) measurements were conducted to analyze the PDINO–BCF mixture. ESI-MS is a MS technique that produces ions using an electrospray in which a high voltage is applied to transform a liquid to an aerosol. This soft ionization technique overcomes the propensity of molecules to fragment when ionized, thus it is especially useful in producing ions from macromolecules and coordination compounds. ESI-MS of BCF from methanol solution showed a very weak molecule weight peak of BCF molecule, which indicates most BCF molecules reacted or strongly interacted with others in solution. Instead, we attribute two m/z values with a single negative charge (M−) at 529 and 543 (Figure 4a) to a BCF–H2O complex and BCF–methanol complex, respectively. This result indicates BCF can form a complex with methanol, along with an unavoidable trace of moisture or water in air or solvent. Then the 19F NMR spectra of BCF in methanol–D4 solution show two independent groups of peaks (Figure 4b), which are consistent with that of the BCF–methanol complex and BCF–H2O complex formed. Then ESI-MS from PDINO:BCF mixed methanol solution shows m/z of 593 (M2+), 1123 (M+), and 1137(M+), which are assigned to PDINO dimer, complex, and complex, respectively. These results indicate that PDINO with BCF–H2O/methanol complex, and the interaction between PDINO and BCF–H2O/methanol is relatively strong so that ions by MS 19F NMR spectra of BCF in PDINO:BCF mixed methanol–D4 solution show new of and three of at the on the and on the BCF are to with PDINO. the with PDINO, it is the molecular of BCF is to an of the new of three of in PDINO:BCF not is the more likely of the BCF with PDINO. Figure 4 | of the interaction between PDINO and BCF. (a) ESI-MS of BCF and PDINO:BCF w/w). (b) 19F NMR of BCF and PDINO:BCF w/w). (c) UV–vis absorption spectroscopy of PDINO:BCF in the relative of vs from PDINO:BCF from to (d) spectroscopy of BCF and PDINO:BCF (e) of BCF and PDINO:BCF (1:1, w/w). Download figure Download PowerPoint Then we used UV–vis absorption and spectroscopy to investigate which of PDINO with the boron cores of BCF. to in a of BCF and thiophene-related the addition of BCF in a in the optical bandgap of those polymers and induced new absorption at by The UV–vis absorption spectra of PDINO in methanol solution with and without a varied of BCF additive were In Figure PDINO methanol show three typical absorption peaks which are by Compared with thiophene-related BCF not obviously the optical bandgap change the peak of PDINO in solution. However, the PDINO solution with BCF addition shows an change in peak Relative to the neat PDINO methanol the BCF in PDINO, the absorption peak first decreases and then increases as the weight ratio of PDINO to BCF These in absorption and of PDINO indicate the of the interaction between PDINO and BCF affect the of PDINO in Then both the perylene and perylene diimide with ( Supporting Information Figure were induced as molecules to investigate UV–vis absorption and after BCF. Compared with the PDINO BCF additive has a effect on the absorption peak and of perylene or in which indicates is interaction between Lewis acid BCF and the Lewis We further added BCF methanol in which a Brønsted acid complex with BCF into which still could not any absorption By the structures of and PDINO, N-oxide groups of PDINO are the potential to with BCF or BCF–H2O/methanol complex. the in Figure we found a new peak at the of in PDINO:BCF with a weight ratio of which is attributed to to the The results further the PDINO:BCF complex and the N-oxide groups of PDINO and of BCF are the with which are consistent with the results of UV–vis absorption and the NMR of PDINO:BCF mixture. the peak value of of BCF from of BCF to of PDINO:BCF and the effect of electron from PDINO to the of The results to further electron effect from PDINO to BCF. In Figure PDINO:BCF (1:1, w/w) blend shows energy of relative to that of the neat BCF film along with increased energy of the N-oxide terminal relative to that of neat PDINO film the energy attributed to the of the eV) after BCF to PDINO film. These results further that the N-oxide on PDINO are the with BCF. from PDINO to BCF the complexation is to to the lower energy levels and work function of PDINO film by BCF. Finally, we the of BCF electronic property of PDINO. As shown in Figure the BCF with water and methanol to form the Brønsted acid and then with N-oxide of PDINO. PDINO to BCF the within the PDINO–BCF complex. The density was applied to the and HOMO levels of PDINO and complex. Relative to that of neat PDINO, the PDINO segment in the complex shows lower electron density, which is attributed to the strong electron-withdrawing property of the BCF segment (Figure As a the calculated LUMO and HOMO of complex are and eV which are lower than that of PDINO eV for LUMO and eV for (Figure The of molecular of PDINO–BCF complex is consistent with what we observed in our experimental measurements and further our acting of BCF with PDINO to its energy Figure 5 | mechanism between PDINO and BCF, and density (a) of the interaction between PDINO and BCF. (b) Molecular on the electron density of PDINO and complex by at the (c) energy level of PDINO and complex by at the Download figure Download PowerPoint We the work function and energy levels of cathode contact a convenient complexation to energy alignment, and thus the device efficiency of OSCs. Relative to the neat PDINO film, the PDINO by 2 wt % BCF lower energy levels while maintaining relatively good conductivity. PDINO:BCF blend was used as a cathode contact in OSCs, device PCEs were improved with devices with neat PDINO contact, independent of the photoactive