Hydrogen-Bonded Dopant-Free Hole Transport Material Enables Efficient and Stable Inverted Perovskite Solar Cells
2021; Chinese Chemical Society; Volume: 4; Issue: 9 Linguagem: Inglês
10.31635/ccschem.021.202101483
ISSN2096-5745
AutoresRui Li, Chongwen Li, Maning Liu, Paola Vivo, Meng Zheng, Zhicheng Dai, Jingbo Zhan, Benlin He, Haiyan Li, Wenjun Yang, Zhongmin Zhou, Haichang Zhang,
Tópico(s)Organic Electronics and Photovoltaics
ResumoOpen AccessCCS ChemistryRESEARCH ARTICLE5 Sep 2022Hydrogen-Bonded Dopant-Free Hole Transport Material Enables Efficient and Stable Inverted Perovskite Solar Cells Rui Li†, Chongwen Li†, Maning Liu, Paola Vivo, Meng Zheng, Zhicheng Dai, Jingbo Zhan, Benlin He, Haiyan Li, Wenjun Yang, Zhongmin Zhou and Haichang Zhang Rui Li† Key Laboratory for Rubber-Plastics of Ministry of Education/Shandong Province, School of Polymer Science and Technology, Qingdao University of Science and Technology, Qingdao 266042 †R. Li and C. Li contributed equally to this work.Google Scholar More articles by this author , Chongwen Li† Institute of Materials Science and Engineering, Ocean University of China, Qingdao 266100 †R. Li and C. Li contributed equally to this work.Google Scholar More articles by this author , Maning Liu Hybrid Solar Cells, Faculty of Engineering and Natural Sciences, Tampere University, FI-33014 Tampere Google Scholar More articles by this author , Paola Vivo Hybrid Solar Cells, Faculty of Engineering and Natural Sciences, Tampere University, FI-33014 Tampere Google Scholar More articles by this author , Meng Zheng Key Laboratory for Rubber-Plastics of Ministry of Education/Shandong Province, School of Polymer Science and Technology, Qingdao University of Science and Technology, Qingdao 266042 Google Scholar More articles by this author , Zhicheng Dai Key Laboratory for Rubber-Plastics of Ministry of Education/Shandong Province, School of Polymer Science and Technology, Qingdao University of Science and Technology, Qingdao 266042 Google Scholar More articles by this author , Jingbo Zhan Key Laboratory for Rubber-Plastics of Ministry of Education/Shandong Province, School of Polymer Science and Technology, Qingdao University of Science and Technology, Qingdao 266042 Google Scholar More articles by this author , Benlin He Institute of Materials Science and Engineering, Ocean University of China, Qingdao 266100 Google Scholar More articles by this author , Haiyan Li Institute of Materials Science and Engineering, Ocean University of China, Qingdao 266100 Google Scholar More articles by this author , Wenjun Yang Key Laboratory for Rubber-Plastics of Ministry of Education/Shandong Province, School of Polymer Science and Technology, Qingdao University of Science and Technology, Qingdao 266042 Google Scholar More articles by this author , Zhongmin Zhou *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory for Rubber-Plastics of Ministry of Education/Shandong Province, School of Polymer Science and Technology, Qingdao University of Science and Technology, Qingdao 266042 Google Scholar More articles by this author and Haichang Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory for Rubber-Plastics of Ministry of Education/Shandong Province, School of Polymer Science and Technology, Qingdao University of Science and Technology, Qingdao 266042 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202101483 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Although many dopant-free hole transport materials (HTMs) for perovskite solar cells (PSCs) have been investigated in the literature, novel and useful molecular designs for high-performance HTMs are still needed. In this work, a hydrogen-bonding association system (NH⋯CO) between amide and carbonyl is introduced into the pure HTM layer. Our study demonstrates that the hydrogen-bonding association can not only significantly increase the HTM’s hole transport mobility and functionalize the surface passivation to the perovskite layer, but also form Pb–N coordination bonds at the interface to promote the hole extraction while hindering the interfacial charge recombination. As a result, the PSCs based on dopant-free hydrogen-bonded HTMs can achieve a champion power conversion efficiency (PCE) of 21.62%, which is around 32% higher than the pristine PSC without the hydrogen-bonding association. Furthermore, the dopant-free hydrogen-bonded HTMs based device shows remarkable long-term light stability, retaining 87% of its original value after 500 h continuous illumination, measured at the maximum power point. This work not only provides a potential HTM with hydrogen-bonding association in PSCs, but also demonstrates that introducing hydrogen bonding into the materials is a useful and simple strategy for developing high-performance dopant-free HTMs. Download figure Download PowerPoint Introduction The power conversion efficiency (PCE) of perovskite solar cells (PSCs), the leading third-generation low-cost photovoltaic technology, has increased remarkably from 3.8% in 2009 to 25.5% in 2020, which is comparable to that of commercialized crystalline silicon.1 After light absorption in the PSCs, the generated electrons and holes need to be transported through the perovskite layer and collected at the adjacent charge selective interfaces. A recent report2 highlights the fact that the performance of PSCs is dominated by swift hole transport (hole injection rate ∼1 ns) rather than relatively slow electron transfer (electron injection rate ∼11 ns). This suggests that hole transport materials (HTMs) play a key role in the impressive progress of PSCs. Currently, state-of-the-art PSCs with conventional n–i–p structures utilize 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenlamine)-9,9-Spirobifluorene (Spiro-OMeTAD) and poly-triarylamine (PTAA) as standard HTMs. However, Spiro-OMeTAD and PTAA are not only tremendously expensive but suffer from low mobility (<1 × 10−5 cm2 V−1 s−1) and limited conductivity ( 80% in the visible range and sheet resistance of 8−10 Ω−2 were purchased from Techno Print Co., Ltd. (Chiba, Japan). The patterned ITO substrate was first cleaned using a surfactant, then washed with sequential sonication in deionized water, ethanol, and acetone for 10 min, respectively. Finally, it was subjected to UV/ozone treatment for 30 min before utilization. The HTM layers were fabricated by spin-coating them at 3000 rpm for 30 s, then annealing at 100 and 180 °C for 10 and 30 min, respectively. The perovskite films were prepared by dripping 100 μL of the perovskite precursor solution on substrates followed by spin-coating at 500 rpm for 2 s and 4000 rpm for 50 s. 750 μL of diethyl ether was dripped at the 25th second of the second step. Then the films were transferred to a preheated hot plate at 65 °C for 2 min and then to a 100 °C hot plate for 15 min. After the formation of the perovskite film, the C60 electron transporting layer (30 nm) and 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) hole-blocking layer (10 nm) were evaporated successively. A 100 nm thick of Ag cathode was thermally evaporated under a reduced pressure of 2 × 10−5 Torr to achieve a complete device via a metal shadow mask of 0.094 cm2. Results and Discussion Design and synthesis of the TPA-DPP Figure 1 shows the molecular structure of TPADPP-Boc. The synthetic route to the target TPADPP-Boc was straightforward via the Suzuki coupling reaction between dibromominated DPP and 4-broate ester-4-N,N-bis(4-methoxyphenyl)aniline (TPA-Bo) (see Supporting Information). Compared to PTAA or Spiro-OMeTAD HTMs, the cost of the TPADPP-Boc is significantly lower ($12.69/g) ( Supporting Information Tables S2–S8). The two t-Boc units of the TPADPP-Boc enable good solubility in most common organic solvents, rendering their solutions processable. Upon thermal annealing, the t-Boc units were decomposed, while the TPADPP-Boc was converted into TPADPP. TPADPP contains two lactam units that can form intermolecular hydrogen-bonding pairs (NH⋯OC, Figure 1a).23 To validate the formation of the TPADPP with fused hydrogen bonding after the decarboxylation of the t-Boc groups, thermogravimetric analysis (TGA), elemental analysis, and Fourier transform infrared (FT-IR) experiments were conducted. As shown in Figure 1b, the TPADPP-Boc decomposed under 185 °C heating for 10 min with a weight loss of around 18.19%, which matched well with the weight percentage of the t-Boc units in TPADPP-Boc (18.08%). In addition, the elemental analysis of the annealed TPADPP-Boc matched well with the TPADPP element composition. These observations indicate that the t-Boc units could be easily decomposed through the thermal annealing process and the TPADPP-Boc converted into TPADPP. To further confirm the formation of the hydrogen bonding between the neighboring TPADPP molecules in the thin film, the FT-IR spectra were measured under 185 °C for varied periods. As can be seen from Figure 1c, during the thermal annealing at 185 °C for 150 s, the absorption peak at 1753 cm−1 (C=O stretching of t-Boc units) gradually decreased and finally disappeared, which can be ascribed to the decarboxylation. Meanwhile, the N-Boc units changed into N–H groups. Once the N–H group emerged, the hydrogen-bonding association was formed between the N–H units and the C=O units from the neighboring TPADPP. This led to the following observations: (1) A broad absorption peak between 2700 and 3255 cm−1 emerged, typically the peak located at 3128 cm−1, which is ascribed to the hydrogen-bonded NH stretching vibration. (2) The absorption peak of the carbonyl group from the DPP core located at 1673 cm−1 was shifted to 1643 cm−1, indicating that the isolated C=O groups were bonded with NH units (C=O⋯H–N). (3) The amide I signal shifted to lower wavenumbers (C=O stretching, from 1673 to 1643 cm−1), while the amide II signal shifted to higher wavenumbers (N–H bending, from 1563 to 1596 cm−1), which consequently confirmed the formation of a secondary amine.33–36 The FT-IR results agreed well with those reported for published hydrogen-bonded systems. The UV–vis absorption spectra of both molecules in the thin-film state are shown in Supporting Information Figure S1. Both materials exhibited two absorption peaks between 590 and 665 nm. After the decarboxylation, the peaks were significantly red-shifted by 25 nm, which could be ascribed to the fact that the hydrogen-bonding association in the TPADPP thin film resulted in strong aggregation.17 From the absorption onset, the optical bandgaps of 1.68 eV for TPADPP-Boc and 1.52 eV for TPADPP were estimated ( Supporting Information Table S9). The electrochemical properties of the materials were studied using cyclic voltammetry (CV) and ultraviolet photoelectron spectroscopy (UPS). From the onsets of anodic oxidation and cathodic reduction, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of TPADPP-Boc were calculated as −5.25 and −3.72 eV, while the TPADPP showed slightly higher HOMO energy levels (−5.21 eV) and lower LUMO energy levels (−3.70 eV, Figure 1d). The high HOMO energy levels of the two materials can be ascribed to the strong electron donor character of bis(4-methoxyphenyl)amine. The HOMO energy levels calculated from CV curves matched well with the results obtained by UPS (−5.25 and −5.20 eV for TPADPP-Boc and TPADPP, respectively, Supporting Information Figure S2). The FMO energy levels of the two molecules were higher than the valence band of the perovskite, indicating that the two materials are potentially suitable as HTMs for only transferring the holes while blocking the electron transfer from the perovskite layer to HTM layers. Device properties To build an efficient PSC device, the morphology, quality, and contact angle of the HTMs thin films should be carefully considered. The surface morphology of the HTMs was characterized by atomic force microscopy (AFM). The TPADPP-Boc film was annealed at 110 °C for 10 min to remove the residual chlorobenzene. Under this condition, the TPADPP-Boc exhibited thermally stable properties ( Supporting Information Figure S3). To get rid of the two Boc groups and obtaining the TPADPP-Boc film, further thermal treatment under 185 °C for 10 min was used. As shown in Figures 2a and 2b, both film surfaces were fully covered by the HTMs. However, the TPADPP film showed lower roughness and was more uniform than the TPADPP-Boc film. Generally, the perovskite materials are difficult to deposit on organic HTM layers, due to their low wettability. To check the wettability of both films, the contact angles of both films were measured. Figures 2c and 2d show that the TPADPP film exhibits a much lower contact angle than TPADPP-Boc (TPADPP-Boc: 92°; TPADPP-61°), indicating the ease of perovskite film deposition. This might be ascribed to the fact that, after removing the Boc units, the NH functional groups, which are more hydrophilic, emerged. Figure 2 | AFM images of (a) TPADPP-Boc and (b) TPADPP thin films on ITO. Contact angles of (c) TPADPP-Boc and (d) TPADPP films with respect to water droplets. (e) The SCLC measurements of hole-only devices with configuration “ITO/PEDOT:PSS/HTM/Ag”. Download figure Download PowerPoint High hole transport mobility plays a key role in designing high-performance dopant-free HTMs. Normally, HTMs would not need additional doping process if they exhibit hole mobilities up to 10−3 ∼ 10−4 cm2 V−1 s−1.37 In this work, the charge transport properties of both films with and without Boc groups were evaluated by utilizing the space-charge-limited-current (SCLC) method.38 The dark I–V characteristics of the hole-only devices with configuration ‘ITO/poly(3,4-ethylenedioxythiophene) (PEDOT): poly(styrenesulfonate) (PSS)/hole transport layer (HTL)/Ag’ were evaluated (Figure 2e). The hole mobilities were extracted by the Mott–Gurney Law (1): μ = 8 J D L 3 9 ε 0 ε r V 2 (1)where ε0 is the vacuum permittivity, εr is the relative dielectric constant (assuming εr = 3 for organic materials), L is the thickness of HTM, JD is the dark current density, and V is the applied voltage. The determined thickness of both films under optimal concentration was 20 nm. As a result, the hole mobilities of the films with and without Boc groups were calculated as 1.11 × 10−4 and 3.09 × 10−4 cm2 V−1 s−1, respectively, compared to 6.94 × 10−4 cm2 V−1 s−1 of classic PTAA ( Supporting Information Figure S4). This indicates that both pristine films can efficiently perform as hole transport materials in PSCs. Compared to TPADPP-Boc, the hole transport mobility of TPADPP was enhanced by almost three times, which can be ascribed to the hydrogen-bonding formation. Scanning electron microscopy (SEM) was used to characterize the surface morphology of the perovskite films atop the HTMs with and without Boc groups. As shown in Figures 3a and 3b, both perovskite films are uniform and fully covered to prevent direct contact with the cathode and anode and in turn eliminating current leakage.39 Besides, there is no discernible thickness change of perovskite films, as shown in Supporting Information Figure S5. Figure 3c shows the UV–vis absorption spectra of the perovskite samples. The absorption of both the perovskite films is almost the same at 780 nm, which can be attributed to the similar size and thickness of the perovskite grains. To investigate the influence of the HTMs with and without Boc groups on the crystallinity of perovskite films, the X-ray diffraction (XRD) analysis was performed. As shown in Figure 3d, the 2θ angle of all diffraction peaks of the perovskite films, which are independent on the underlayer, is consistent with the previous reports. Both perovskite films showed strong diffraction peaks at 14.11° with full width at half maxima (FWHM) of 0.149° for TPADPP and 0.167° for TPADPP, respectively, which is assigned to the (100) planes. The higher peak of perovskite deposited on the TPADPP film at 14.11° demonstrates its better crystallinity and more effective charge transport along the z-axis compared to the perovskite deposited on the TPADPP-Boc film. This might be ascribed to the fact that, after removing the Boc units, the NH groups emerged and passivated the perovskite surface. Figure 3 | Top-view SEM images of perovskite films based on (a) TPADPP-Boc and (b) TPADPP. (c) UV–vis absorption spectra. (d) XRD pattern spectra. (e) Steady-state PL spectra. (f) TRPL decay transient spectra. Download figure Download PowerPoint After the light absorption, the generated holes need to be transported through the perovskite layer and collected at the adjacent HTM layer. This step plays a key role in achieving high-performance PSCs. To investigate the interfacial hole extraction process at the interface between the perovskite and HTM, steady-state and time-resolved photoluminescence (TRPL) measurements were conducted. Figure 3e shows a clearly enhanced PL quenching when depositing a perovskite film on the TPADPP layer compared to the case of perovskite atop the TPADPP-Boc layer. The calculated quenching efficiencies, that is, hole extraction efficiencies, are 58.4% and 91.0% for TPADPP-Boc and TPADPP, respectively. This suggests that the NH groups of TPADPP can form Pb–N coordination-bonds through the formation of Lewis adducts between undercoordinated Pb atoms and N atoms at the perovskite/HTM interface, which can significantly promote interfacial hole extraction.40 The hole extraction dynamics were also monitored by comparing the TRPL decays in Figure 3f. A clear decay acceleration was observed for glass/perovskite/TPADPP compared to glass/perovskite/TPADPP-Boc, indicating that the holes at the perovskite/HTM interface can be swiftly extracted with the help of the Pb–N bond, which is highly consistent with PL quenching data. We then attempted to quantitatively analyze the PL decay data by using a reported kinetic model, that is, a one-dimensional charge diffusion model (see the analysis method in Supporting Information).41,42 A rate equation (see Supporting Information Equation S1) was used to fit the PL decay data, including first-order recombination (k1) via carrier traps, second-order nongeminate-free charge carrier (electron and hole) recombination (k2), and interfacial hole extraction process (kHT). The resulting k1, k2, and kHT for two HTMs are summarized in Supporting Information Table S9. TPADPP exhibited reduced k1 (8.5 × 106 s−1) and k2 (1.3 × 10−10 s−1 cm3) compared to those (k1 = 2.3 107 s−1 and k2 = 4.1 × 10−10 s−1 cm3) of TPADPP-Boc, suggesting that the interfacial Pb–N bond not only promotes hole extraction but also hinders single-carrier trapping recombination by filling the surface traps of the perovskite. Moreover, the hole extraction rate constant (kHT = 5.4 × 108 s−1) of TPADPP was nearly one order of magnitude higher than that (6.1 × 107 s−1) of TPADPP-Boc, which is also evident by the faster effective lifetime (τ1/e = 11.5 ns) of TPADPP-based film compared to that (τ1/e = 24.7 ns) of TPADPP-Boc case (see Supporting Information Table S9). To investigate the PSC performance based on the designed HTMs, the p–i–n configuration of ITO/HTM/Perovskite/C60/BCP/Ag (Figure 4a) PSCs with an active area of 0.094 cm2 was realized. Figure 4b shows the energy-level alignment of each layer of the device. The optimal thickness of the HTM, evaluated by comparing the device performance with varying concentrations of TPADPP solutions ( Supporting Information Figure S6), was ∼20 nm. Figure 4c shows the current density–voltage (J–V) curves (measuring under AM 1.5G illumination at 100 mW cm−2) of the champion device with negligible hysteresis. TPADPP-based PSC exhibits an open-circuit voltage (VOC) of 1.115 V, a short-circuit current density (JSC) of 23.27 mA cm−2, a fill factor (FF) of 0.833, and a PCE of 21.62% under the reverse-scan direction. The parameters of this champion device under the forward-scan direction include a VOC of 1.118 V, a JSC of 23.24 mA cm−2, an FF of 0.818, and a PCE of 21.26%. This indicates the TPADPP is a promising dopant-free HTM. For the comparison of the hydrogen-bonding association of HTM on the performance of the PSC, the device with TPADPP-Boc film as HTM was fabricated, and the corresponding J–V curves are also shown in Figure 4c. Table 1 summarizes the photovoltaic parameters of the devices with TPADPP and TPADPP-Boc as HTMs, respectively. After removing the Boc units from the HTM structure, the VOC and JSC of the corresponding PSCs were significantly enhanced, which resulted in a PCE boost from 16.36 up to 21.62% (32% enhancement). Such a significant performance improvement can be ascribed to the following: (1) The hydrogen-bonding association for the HTMs improves the hole transport mobility within the HTM layer. (2) The NH units passivate the perovskite surface, leading to a perovskite layer with good quality and crystallinity. (3) The NH groups form Pb-N coordination bonds between the top-coordinated Pb atoms at the interface, which can significantly improve the hole extraction rate.28 In this work, the p–i–n PSCs, employing a hydrogen-bonded dopant-free HTM, achieved a champion PCE of 21.62%, which is comparable with the recent highest PCE records.43 Figure 4 | (a) Illustration of t
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