Hydrogen-Bonded Dopant-Free Hole Transport Material Enables Efficient and Stable Inverted Perovskite Solar Cells

Abstract

Open 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 (<3 × 10−7 S cm−1).3 These materials thus need dopants such as bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI) to increase their hole mobility and conductivity that may lead to device degradation due to the (1) sophisticated oxidation process associated with undesired ion migration and (2) chemical interaction with the down-lying perovskite layer.4 Hence, to overcome the drawbacks of doped HTMs, during the last several years a wide range of novel low-cost dopant-free HTMs have been reported as alternatives to the state-of-the-art HTMs.5–11 A good dopant-free HTM is expected to simultaneously display high hole transport mobility and well-matched frontier molecular orbitals (FMOs) with the perovskite valence band edge. Furthermore, to reduce the energy losses at the interface, the HTM needs to passivate the unavoidable defects present in the perovskite layer. In HTMs, holes need to be efficiently transferred within and between individual molecules (intra-/intermolecular conjugation hole transport). To enhance the intramolecular conjugation hole transport, donor–acceptor (D–A) type, D–D type, strong planarity, and large π-conjugation molecular designs have mainly been used for dopant-free HTM.12–16 Charge-carrier mobility between the adjacent molecules significantly increases if the materials exhibit self-assembling properties that can be exploited to generate ordered structures.17,18 However, well-organized nanostructures are difficult to obtain with classic solution processing methods due to the disorder packing. Introducing a hydrogen-bonding functionality in the materials, resulting in hydrogen-bonded material superstructures, can be a suitable method to fulfil the above-mentioned critical requirements. Molecules with fused hydrogen-bonding have been reported to exhibit high charge-carrier mobility and conductivity in organic field-effect transistors (OFETs), as demonstrated by our previous work and by other groups as well.19–23 Materials with high hole mobility in OFETs might be suitable as dopant-free HTMs for PSCs. To the best of our knowledge, there are only two articles that claim the application of hydrogen-bonding functionalized HTMs in PSCs.24,25 Kaneko et al.16 proposed small molecular dopant-free HTMs to fabricate n–i–p type PSCs with PCE of 14.5%, but the authors did not focus on the inverted p–i–n structure. In addition, they synthesized the molecules containing multilarge size alkyl chains to enhance the solubility of the small molecules. In most cases, large alkyl-chains result in poor packing and low hole mobility. Later, Más-Montoya et al.25 synthesized hydrogen-bonded small molecules with no alkyl chain for p–i–n PSCs with PCE of 15.9%, which remained stable for more than 1200 s with a variation of less than 1%. However, due to the poor solubility of the molecules, the device fabrication procedure relied on the thermal evaporation technique, which increases the complexity and cost of the device fabrication. Thus, making dopant-free HTMs by the vapor method cannot be easily and widely used by most research groups ( Supporting Information Table S1). Herein, we report dopant-free D–A–D HTMs with tert-butoxylcarbonyl (t-Boc)-substituted diketopyrrolopyrrole (DPP) as acceptors, capped at both ends with electron-rich units of triphenylamine (TPA), named TPADPP-Boc. DPP is the most widely used chromophore in OFETs, which enables extremely high hole mobility,19,26 and TPA is a popular unit to build high-performance HTMs.27–31 In the designed molecule, the t-Boc units could be easily decomposed into carbon dioxide and isobutylene gas upon the thermal annealing process.32 Meanwhile, the N–H units emerge, and the hydrogen-bonded 3,6-di(5-N,N-bis(4-methoxyphenyl)aniline-thiophene-2-yl)pyrrolo[3,4-c]pyrrole-2,5(1H,4H)-dicarboxylate (TPADPP) was formed between the N–H units and the C=O units from the neighboring DPP core (Figure 1). The application of solution-processable hydrogen-bonded dopant-free HTMs is studied in this work. The results showed that hydrogen-bonding association increases the hole mobility of HTMs from 1.11 × 10−4 to 3.09 × 10−4 cm2 V−1 s−1, resulting in a significantly enhanced PCE of PSCs from 16.36% to 21.62% (32% enhancement), which indicates that hydrogen-bonded materials are promising candidates as dopant-free HTMs for efficient PSCs. Figure 1 | (a) Chemical structures of materials, illustration of decarboxylation of PADPP-Boc and hydrogen-bonding formation. (b) TGA spectrum of TPADPP-Boc from room temperature to 185 °C (heating rate 50 °C/min). Once the temperature reached 110 °C, the material was kept at this condition for 800 s. (c) FT-IR spectra of TPADPP-Boc at 185 °C for 150 s. (d) Cyclic voltammograms of the materials as thin films deposited on ITO. Solution: 0.1 M TBAPF6/Acetonitrile. Potentials calculated versus ferrocene. Scan rate: 100 mV s−1; T = 20 °C. Download figure Download PowerPoint Experimental Methods Synthesis of TPADPP-Boc TPA-Bo (0.188 g, 0.42 mmol), DPP-Boc-Br (0.131 g, 0.2 mmol), and potassium carbonate aqueous solution (2 M/L 5 mL) were added into freshly distilled toluene (15 mL) (Scheme 1). The mixture was degassed with nitrogen, followed by the addition of tetrakis(triphenylphosphine)palladium (0.0115 g, 0.01 mmol). The mixture reacted for 36 h at 100 °C under N2 protection. After cooling the solution to room temperature, it was extracted with dichloromethane and deionized water twice and dried over anhydrous MgSO4. The crude product was purified by column chromatography (silica gel, dichloromethane) to afford compound TPADPP-Boc as a blue solid (0.195 g, yield: 88%). 1H NMR (500 MHz, d1-CHCl3, δ) (ppm): 8.31–8.32 (d, 2H), 7.42–7.44 (d, 2H), 7.24–7.25 (d, 2H), 7.08–7.10 (d, 2H), 6.85–6.90 (q, 4H), 3.81 (s, 12H), 1.63 (s, 18H). 13C NMR (125 MHz, d1-CHCl3, δ) (ppm): 159.31, 156.44, 151.67, 149.56, 149.14, 140.02, 136.71, 135.74, 127.16, 126.97, 124.63, 122.72, 119.50, 114.85, 109.62, 85.71, 55.51, 55.45, 27.75. Microanalysis found C, 69.45%; H, 5.27%; N, 5.05%; S, 5.79% (C, 69.42%; H, 5.28%; N, 5.06%; S, 5.79%). Scheme 1 | Synthetic route of TPADPP-Boc. The synthesis route and NMR spectrum of the starting product of TPA-DPP are shown in Supporting Information Figures S9–S13 and Schemes S1–S4. Download figure Download PowerPoint Synthesis of TPADPP TPADPP-Boc (0.055 g, 0.05 mmol) was thermally annealed on a hot plate at 185 °C for 10 min (Scheme 2). The dark blue product of TPADPP was obtained (0.045 g, yield: 100%). Due to the poor solubility of the TPADPP, the NMR spectra were not measured. Microanalysis found C, 71.49%; H, 4.69%; N, 6.18%; S, 7.05% (C, 71.50%; H, 4.67%; N, 6.18%; S, 7.07%). Scheme 2 | Synthetic route of TPADPP. Download figure Download PowerPoint Perovskite (FA0.8Cs0.2PbI2.96Br0.04) solution The 1.35 M FA0.8Cs0.2PbI2.96Br0.04 perovskite precursor solution was obtained by dissolving 137.6 mg of formamidinium iodide, 52 mg of cesium iodide, 447.2 mg of lead iodide, 11 mg of lead bromide, and 5.2 mg of lead thiocyanate in a mixed solvent of N,N-dimethylformamide and dimethyl sulfoxide with a ratio of 3:1. The precursor solution was stirred at 60 °C for 3 h before use. Device fabrication The indium-doped tin oxide (ITO) glass substrates with an optical transmission of >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 Figure 2 | of (a) TPADPP-Boc and (b) TPADPP thin films on ITO. angles of (c) TPADPP-Boc and (d) TPADPP films with to water The of with Download figure Download PowerPoint hole transport mobility a key role in high-performance dopant-free HTMs. HTMs not need process if they exhibit hole to 10−4 cm2 V−1 In this work, the charge transport properties of both films with and without Boc groups were by the The dark of the with transport layer were (Figure The hole were extracted by the = 3 2 is the is the = 3 for organic is the of is the dark and is the The of both films under was 20 nm. As a result, the hole 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, to × 10−4 cm2 V−1 of classic PTAA ( Supporting Information Figure This indicates that both pristine films can efficiently as hole transport materials in PSCs. Compared to the hole transport mobility of TPADPP was enhanced by which can be ascribed to the hydrogen-bonding formation. electron microscopy was used to the surface morphology of the perovskite films the HTMs with and without Boc groups. As shown in Figures and both perovskite films are uniform and fully covered to contact with the cathode and and in there is no of perovskite films, as shown in Supporting Information Figure Figure shows the UV–vis absorption spectra of the perovskite The absorption of both the perovskite films is the at nm, which can be to the size and of the perovskite To the of the HTMs with and without Boc groups on the of perovskite films, the analysis was As shown in Figure the angle of peaks of the perovskite films, which are on the is with the previous Both perovskite films showed strong peaks at with at of for TPADPP and for TPADPP, respectively, which is to the The higher peak of perovskite deposited on the TPADPP film at demonstrates its and more charge transport the 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 and the perovskite Figure 3 | of perovskite films based on (a) TPADPP-Boc and (b) TPADPP. (c) UV–vis absorption (d) Download figure Download PowerPoint After the light the generated holes need to be transported through the perovskite layer and collected at the adjacent HTM layer. This a key role in high-performance PSCs. To the interfacial hole extraction process at the interface between the perovskite and and were conducted. Figure shows a enhanced a perovskite film on the TPADPP layer to the of perovskite the TPADPP-Boc layer. The calculated that hole extraction are and for TPADPP-Boc and TPADPP, respectively. This suggests that the NH groups of TPADPP can form Pb–N through the formation of between and at the interface, which can significantly promote interfacial hole The hole extraction were also by the in Figure A was for to indicating that the holes at the interface can be extracted with the of the Pb–N which is with then to the by using a reported that a charge (see the analysis method in Supporting A rate (see Supporting Information was used to the via charge (electron and and interfacial hole extraction process The resulting and for two HTMs are in Supporting Information Table TPADPP exhibited reduced × s−1) and × to those = and = × of that the interfacial Pb–N not only hole extraction but also by the surface of the the hole extraction rate = × s−1) of TPADPP was of higher than that × s−1) of which is also by the = ns) of film to that = ns) of TPADPP-Boc (see Supporting Information Table S9). To the PSC performance based on the designed HTMs, the p–i–n of (Figure PSCs with an of 0.094 cm2 was Figure shows the of layer of the The of the by the device performance with of TPADPP solutions ( Supporting Information Figure was nm. Figure shows the curves under at 100 of the champion device with PSC exhibits an of a of a of and a PCE of 21.62% under the The of this champion device under the a of a of an of and a PCE of This indicates the TPADPP is a promising dopant-free the of the hydrogen-bonding association of HTM on the performance of the the device with TPADPP-Boc film as HTM was and the curves are also shown in Figure Table 1 the photovoltaic of the with TPADPP and TPADPP-Boc as HTMs, respectively. After removing the Boc units from the HTM the and of the PSCs were significantly which resulted in a PCE from to 21.62% (32% a performance can be ascribed to the (1) The hydrogen-bonding association for the HTMs the hole transport mobility within the HTM layer. (2) The NH units passivate the perovskite leading to a perovskite layer with good and (3) The NH groups form coordination bonds between the at the interface, which can significantly the hole extraction In this work, the p–i–n PSCs, a hydrogen-bonded dopant-free a champion PCE of 21.62%, which is comparable with the recent highest PCE Figure | (a) of