Chemical Linkage and Passivation at Buried Interface for Thermally Stable Inverted Perovskite Solar Cells with Efficiency over 22%

Open AccessCCS ChemistryRESEARCH ARTICLES6 Oct 2022Chemical Linkage and Passivation at Buried Interface for Thermally Stable Inverted Perovskite Solar Cells with Efficiency over 22% Ying Tan, Xueqing Chang, Jun-Xing Zhong, Wenhuai Feng, Meifang Yang, Tian Tian, Li Gong and Wu-Qiang Wu Ying Tan MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-sen University, Guangzhou 510006 Google Scholar More articles by this author , Xueqing Chang MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-sen University, Guangzhou 510006 Google Scholar More articles by this author , Jun-Xing Zhong MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-sen University, Guangzhou 510006 Google Scholar More articles by this author , Wenhuai Feng MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-sen University, Guangzhou 510006 Google Scholar More articles by this author , Meifang Yang MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-sen University, Guangzhou 510006 Google Scholar More articles by this author , Tian Tian MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-sen University, Guangzhou 510006 Google Scholar More articles by this author , Li Gong Instrumental Analysis Research Center, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author and Wu-Qiang Wu *Corresponding author: E-mail Address: wuwq3[email protected] MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-sen University, Guangzhou 510006 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202202154 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Poly[bis(4-phenyl)-(2,4,6-trimethylphenyl)amine] (PTAA) has been developed as one of the most popular hole transport layer (HTL) materials in inverted perovskite solar cells (PSCs). However, the efficiency, thermal stability, and reproducibility of PTAA-based devices are still largely limited by underoptimized chemical interaction, energy level alignment, and contact affinity at the PTAA/perovskite interface. To this end, we introduced a bilateral chemical linker to simultaneously achieve favorable chemical interaction with the PTAA underlayer and form robust coordination bonding with the buried perovskite bottom layer, which beneficially improved the contact affinity, facilitated the hole extraction, well-passivated the interfacial defects, and relieved the nonradiative charge recombination at the HTL/perovskite buried interface. The inverted PSCs modified with interfacial chemical linker exhibited consistently higher power conversion efficiencies and performance reproducibility than that of the PTAA-only devices. Combined with the blade-coated FA0.4MA0.6PbI3 perovskite layer, a champion efficiency of 22.23% has been achieved, which is one of the highest reported values for the inverted PSCs based on the bilayer HTL. The targeted device showed enhanced thermal stability under continuous heating at 85 °C owing to suppressed composition segregation with robust interfacial linkage and consolidation. This work offers a new insight towards making efficient, thermally stable, and reproducible perovskite photovoltaics. Download figure Download PowerPoint Introduction Over the past decade, metal halide perovskite solar cells (PSCs) have attracted intensive research attention owing to their fascinating advantages, including high efficiency, low cost, and solution processability, rendering them one of the most promising next-generation photovoltaic technologies.1–4 With strenuous efforts made on composition optimization, crystallization regulation, interface engineering, and device architecture design, the power conversion efficiencies (PCEs) of PSCs have been progressively lifted and certified to be as high as 25.7%, which is already on par with or even surpassing their silicon-based competitors.5 Among various types of PSCs, the inverted devices with a p–i–n configuration are of particular interest due to their low-temperature fabrication processability, which is compatible for use in flexible or tandem solar cells with greater practical application value.6,7 For the inverted PSCs, the p-type hole transport layer (HTL) served as a hole-selective contact between the transparent conducting oxide substrate and the perovskite layer, which is responsible for extracting the photogenerated holes and inevitably affects the crystallization kinetics of the adjacent perovskite layer. Among the various types of reported HTLs, the poly[bis(4-phenyl)-(2,4,6-trimethylphenyl)amine] (PTAA) has been demonstrated as one of the most popular and successful HTL materials in the inverted PSCs.8–11 Though promising, there is still a plenty of room to further optimize the PTAA-based PSCs, especially considering several fatal drawbacks that significantly limit the enhancement of device performance. First, the PTAA HTL is notorious for its hydrophobic property, which shows poor wettability to the perovskite ink, thus increasing the difficulties of realizing uniform coating of the perovskite layer on top of the entire PTAA layer, resulting in poor interfacial contact.12 Second, the PTAA molecules have negligible interaction with perovskites, which minimally affect the crystallization kinetics of perovskite films. Third, it has been reported that the defective PTAA/perovskite interface with a large number of randomly oriented nanocrystals cause serious interfacial nonradiative recombination, thus significantly limiting the device performance, especially the open-circuit voltage (VOC).13 Finally, the energy-level alignment between PTAA and the perovskite is still underoptimized, leading to moderate hole extraction at the HTL/perovskite interface.14 Modifying the hydrophobic PTAA surface with hydrophilic molecules15–19 or improving the electrical properties of the PTAA layer via doping20–23 have been developed to partially solve the above-mentioned problems. However, the reported strategies do not always work effectively, and the repetition of device performance is largely affected by the PTAA materials of different molecular weights.24,25 In addition, thermally stable PTAA-based PSCs operating at 85 °C for a long time are rarely reported, considering the heat-resistant capability of polymer-based HTL is not that good, and the fixation of intact perovskites is not ideal. Hence, it is highly desirable and of great significance to explore a universal and effective way to modify the PTAA/perovskite interface to maximize the interfacial charge extraction and minimize the interfacial charge recombination while simultaneously enhancing the thermal stability of resultant devices. In this work, we introduced an “all-in-one” [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz) interlayer between the PTAA and perovskite layer, which could achieve favorable energy-level alignment with underlying PTAA and form robust coordination bonding with adjacent perovskite for facilitated interfacial hole extraction and suppressed interfacial charge recombination. Encouragingly, an impressive efficiency over 22.23% with a high VOC of 1.14 V and negligible photocurrent density-photovoltage (J–V) hysteresis was obtained for the inverted PSCs modified with 2PACz interlayer. This, to the best of our knowledge, is one of the highest reported values for the bilayered HTL-based inverted PSCs. In addition, the devices without encapsulation exhibited extended lifespan (<2000 h) against ambient storage and thermal stress. Experimental Methods Materials Formamidinium iodide (99.99%), methylammonium iodide (99.99%), and methyl ammonium bromide (99.99%) were purchased from GreatCell Solar Limited (Australia). N,N-dimethylmethanamide (DMF, 99.8%), dimethyl-sulfoxide (DMSO, 99.9%), chlorobenzene (CB, 99.8%), and bathocuproine (BCP) were obtained from Sigma-Aldrich (Shanghai, China). PTAA (99%, Mw < 6000), lead bromide (PbBr2, 99%) and C60 was purchased from Xi’an Polymer Light Technology Corp. (Xi’an, China). CsI (99.999%) and PbI2 (99.999%) were obtained from Alfa Aesar (Shanghai, China). 2PACz was purchased from TCI (Shanghai, China) Development Co., Ltd. All chemicals were employed as received without further modification. Perovskite precursor preparation For the Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 perovskite, the precursor solution (1.5 M) was prepared in mixed solvents of DMF and DMSO with a volume ratio of 4:1. For the FA0.4MA0.6PbI3 perovskite, the precursor solution (1.2 M) was prepared in 2-ME solution. Before use, small amounts of additives, that is, methylammonium hypophosphite (0.15 wt %) and l-α-phosphatidylcholine (0.05 wt %), were incorporated to achieve uniform coating of the perovskite film. Device fabrication The PSCs were fabricated with an inverted structure of indium tin oxide (ITO)/PTAA with or without 2PACz/perovskite/C60/BCP/Cu. The ITO substrates were cleaned by sequential ultrasonication in deionized water, acetone, and isopropanol, respectively, and then dried in the oven. Next, the substrate underwent UV ozone treatment for 20 min. PTAA (5.0 mg mL−1 in CB) was dynamically spin-coated onto ITO glass at 5000 rpm for 30 s and then annealed at 100 °C for 10 min. For the device with 2PACz interlayer, the 2PACz solution (10 mg mL−1 in DMF) was dynamically spin-coated on top of the PTAA layer at 5000 rpm for 30 s and then annealed at 100 °C for 5 min. Then, the perovskite ink was spin-coated onto the ITO/HTL substrates at 2000 rpm for 10 s and 6000 rpm for 30 s, in which 200 μL of CB was dropped on the spinning substrate 15 s prior to the end of the second step. The substrates were then annealed on hotplate at 100 °C for 10 min to obtain crystalline perovskite film. For the FA0.4MA0.6PbI3 devices, the precursor solution was blade-coated onto the ITO/HTL substrates with a gap of 100 μm at a blade speed of 10 mm s−1. The N2 knife worked at 0.2 MPa during blade-coating. 8 μL perovskite ink was used for each coating, followed by thermal annealing at 100 °C for 10 min to obtain the perovskite film. Finally, 20 nm C60, 7 nm BCP and 80 nm Cu were sequentially thermally evaporated on top of the perovskite films to complete the device. Characterization The Fourier transform infrared (FT-IR) spectrum was measured with a PerkinElmer Frontier FT-IR spectrometer (PerkinElmer, United States) in the range 4000–400 cm−1. The surface wettability was measured using an optical contact angle analyzer (KRÜSS-DSA100, Kruss, Germany). The X-ray diffraction (XRD) pattern was obtained from a Miniflex600 X-ray diffractometer (Rigaku, Japan). The morphological features of the samples were characterized using a field emission scanning electron microscope (SEM, Hitachi-SU8010, Hitachi, Japan). Conductive atomic force microscopy (c-AFM) measurements were performed on a scanning probe microscope (Bruker Dimension Fastscan, Bruker, Germany). The Raman spectrum was measured with microconfocal Raman spectrometer (InVia, United Kingdom). J–V characteristics of the PSCs were measured using a Keithley 2400 SourceMeter under the simulated AM1.5G irradiation generated by the Oriel Sol3A Class AAA Solar simulator based on Xenon lamps. The light intensity was calibrated by a silicon reference cell equipped with a SCHOTT visible color KG5 glass filter (Newport 91150V). Nonreflective shadow masks with an aperture of 0.08 cm2 were used to define the active working areas of the PSCs. For the space-charge limited current (SCLC) test, the J–V characteristics were measured using a CH Instruments CHI660 electrochemical (ChenHua, China) workstation in a shield box with a structure of ITO/HTL/perovskite/Spiro-OMeTAD/Cu, where the HTL is PTAA or PTAA/2PACz. External quantum efficiency (EQE) was measured by the solar cell quantum efficiency measurement system (SCS10, Zolix Instruments Co., Ltd., China). An Edinburgh Instruments Ltd FLSP980 (Edinburgh, United Kingdom) was used to test the steady-state photoluminescence (PL) and timeresolved photoluminescence (TRPL) with a pulsed excitation laser of 400 nm. The electrochemical impedance spectroscopy (EIS) analysis was carried out using an electrochemical workstation (Zahner Zennium, Zahner, Germany) with the frequency range from 10 Hz to 1 MHz without illumination and under an applied bias of −0.8 V. Transient absorption was measured by a regeneratively amplified Ti: sapphire laser source (Coherent Legend, 800 nm, 150 fs, 5 mJ per pulse, and 1 kHz repetition rate) and Helios (Ultrafast Systems LLC, United States) spectrometer. Results and Discussion In a preliminary study, the PTAA materials with a molecular weight less than 6 kDa and Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 perovskites were employed to assemble inverted PSCs. The polymeric PTAA molecule has a polytriarylamine skeleton (Figure 1a), and the hydrophobicity of solution-processed PTAA film is heavily dependent on the aggregation degree and molecular weight of PTAA. Similar to the dilemma commonly encountered by other researchers, the perovskite film deposited on pristine PTAA film showed very poor surface coverage owing to the poor wettability of the perovskite ink on the hydrophobic surface of PTAA film. Treating the PTAA surface with DMF solvent only partially solved the problem.15,20 The resultant perovskite film still showcased incomplete surface coverage, which is not desirable for large-scale deployment of PSCs. To this end, a thin layer of 2PACz, featuring a structure of carbazole and phosphonic acid (PA) group connected by a C2H4 chain (Figure 1b), has been deposited on a PTAA layer, prior to the perovskite layer deposition. We can obtain the uniform coating and full surface coverage of perovskite film on PTAA/2PACz substrates ( Supporting Information Figure S1), which can be attributed to the enhanced wettability of the PTAA surface with 2PACz modification, as is evidenced by the decreased contact angle from 35.7° to 13.1° (Figure 1c,d). In this case, 2PACz with polar and hydrophilic PA groups plays a critical role in improving the contact affinity between HTL substrate and perovskite ink. In general, enhanced wettability of HTL is a result of higher nucleation density of perovskites, which is conducive to the densification growth of perovskite film with improved quality and better coverage.19 Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were also used to investigate the surface topography of the PTAA films with or without 2PACz modification. The 2PACz-treated PTAA layer exhibited a smoother surface than the pristine PTAA film, which was manifested by the smaller root-mean-square value (i.e. 2.66 vs 4.09 nm, Supporting Information Figure S2). This result was also consistent with the morphological observation from SEM images ( Supporting Information Figure S3). Reduced surface roughness of HTL is highly desirable in planar heterojunction PSCs, which is beneficial to improve the interfacial contact between HTL and perovskite layer, thus ensuring high device performance.17,26 Figure 1 | Chemical structure of (a) PTAA and (b) 2PACz. Contact angle images for a droplet of perovskite precursor on (c) PTAA and (d) PTAA/2PACz. (e) FT-IR spectra of pristine 2PACz and 2PACz-PbI2 blend. (f) Schematic illustration of chemical interaction and defect passivation of PTAA/perovskite buried interface by 2PACz molecules. Download figure Download PowerPoint The functions of 2PACz interlayer far exceed enhancing the wettability of the HTL surface and improving the HTL/perovskite contact affinity. It is anticipated that the 2PACz containing a carbazole structure and PA groups is able to chemically interact with both underlying PTAA and upper perovskite layer. The interaction between 2PACz and PTAA has been investigated by Raman spectroscopy ( Supporting Information Figure S4), which has been widely utilized to investigate the potential chemical interaction between different molecules with specific functional groups and/or organic moieties.27–29 For the PTAA sample, the peak located at 1293 cm−1 corresponded to the C–N bond stretching vibration of the triphenylamine structure. For the 2PACz sample, the peaks located at 1214 and 1233 cm−1 corresponded to the stretching vibration of C=C bond of the disubstituted benzene ring, and the peaks located at 1230 and 1356 cm−1 corresponded to the C–N bond stretching vibration of the pyridine ring. Obviously, considerable red-shifts of these Raman peaks were observed in the PTAA-2PACz sample, indicating decreased electron cloud density of the corresponding chemical bonds. It can be interpreted that such electron delocalization effect is caused by the weak chemical interaction between PTAA and 2PACz molecules. Considering the relatively weak interaction between 2PACz and the underlying PTAA layer, one could assume that the 2PACz can be partially washed away during the subsequent deposition of the perovskite layer. We adopted AFM step height measurement to determine the thickness of different HTLs and their combinations. As a result, the thickness of the pristine PTAA HTL layer was determined to be ∼8 nm, and the thickness of the PTAA/2PACz bilayered HTL after DMF washing was determined to be ∼11 nm ( Supporting Information Figure S5). Hence, one could conclude that the thickness of 2PACz interlayer between the PTAA layer and perovskite layer is approximately 3 nm. 2PACz itself is a p-type material, which has an excellent hole extraction capability.11,30,31 It is reported that the hole mobility of the 2PACz derivatives was almost one magnitude higher than that of the undoped PTAA materials.25,32 In our case, the conductivity of the PTAA and PTAA/2PACz HTLs was evaluated by the SCLC technique, and the device structure of ITO/HTL/Cu was used for characterization ( Supporting Information Figure S6). Typically, the direct current (DC) conductivity (σ0) can be calculated from the slope of the log(J–V) plot in the ohmic region ( Supporting Information Figure S6b) by using the equation:33 J = σ 0 ( A / d ) V (1)where A is the area of sample (0.08 cm2) and d is the thickness of sample (i.e. 8 nm for PTAA and 11 nm for PTAA/2PACz), respectively. As a result, the conductivity of PTAA/2PACz HTL is 1.35 × 10−5 S cm−1, which is much higher than that of the pristine PTAA HTL (9.95 × 10−6 S cm−1). The increased conductivity of bilayered HTL can be attributed to the intrinsically high hole mobility of 2PACz itself, as well as reduced hole injection barrier due to more matched interfacial energy level alignment between PTAA and 2PACz.27,34 The pristine 2PACz also showcased the vibration signals for P–OH groups at 1004 and 924 cm−1. Mixing the 2PACz with PbI2 red-shifted the vibration peaks of P–OH groups to 1013 and 928 cm−1, respectively (Figure 1e). Such an obvious shift of FT-IR peaks could be ascribed to the chemical interaction between PA groups and metal halides, which is consistent with the previous reports.30,35–37 Specifically, 2PACz could function as a Lewis base due to its PA groups, which could form coordination bonds with the empty 6p orbital of the Pb2+ in the perovskites.38,39 As schematically illustrated in Figure 1f, the 2PACz molecule serves as a two-way bridge to chemically couple with both PTAA and perovskite, which can modify the surface properties of PTAA and passivate the defects at buried HTL/perovskite interface via robust coordination bonding, which is beneficial to facilitate the interfacial charge extraction and suppress the interfacial charge recombination. The improved contact affinity and chemical interaction effect will be in favor of forming high-quality perovskite films with improved crystallinity and optimized morphologies.40 We then investigated the effect of the 2PACz modification layer on the crystallinity and morphologies of perovskite films. The 2PACz interfacial linkage slightly enhanced the crystallinity of perovskite film without changing the crystal structure of the perovskite films ( Supporting Information Figure S7). Compared with the PTAA-based sample, the perovskite film grown on PTAA/2PACz substrate was smoother and the was relatively ( Supporting Information Figure In general, the improved wettability of PTAA/2PACz substrate to the perovskite solution result in higher nucleation density and thus smaller the of the perovskite film deposited on PTAA/2PACz substrate can be by the one of 2PACz could be washed the perovskite layer and chemically with the perovskites, by forming coordination bonding between PA groups and metal which can the between metal and organic ammonium halides, thus the crystallization and ensuring the growth of large the other the smoother surface of underlying PTAA/2PACz substrate such as which is beneficial to facilitate the thus resulting in the of perovskite films with large The surface is favorable to the subsequent coating of a uniform electron transport layer and the large are characterized by less and reduced defect which are for better device performance. We fabricated the inverted PSCs with a device architecture of in which the HTL is pristine PTAA or bilayered PTAA/2PACz (Figure The SEM images of the corresponding devices are in Supporting Information Figure which shows that the thickness of perovskite film is nm for both devices. can better interfacial contact at devices, perovskite with both HTL and ( Supporting Information Figure which is for extraction and transport the Figure shows the J–V of the PTAA and PSCs fabricated with Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 perovskite films. The corresponding photovoltaic such as photocurrent density open-circuit and are in to PTAA-based PSCs, the photovoltaic of the devices improved the from to by a negligible J–V hysteresis with reduced hysteresis ( Supporting Information Figure and Such a performance enhancement can be ascribed to the of facilitated hole extraction and suppressed charge recombination at HTL/perovskite buried interface with 2PACz modification, which will be in the The J–V hysteresis can be attributed to the optimized with reduced charge at the HTL/perovskite In addition, devices reduced current as to the PTAA-based PSCs ( Supporting Information Figure the of both the bilayered HTL film and perovskite The was measured to the of the enhanced photocurrent ( Supporting Information Figure and the device exhibited higher values over the region from to nm, to the PTAA-based which is in with the J–V Supporting Information Figure the of photocurrent density and as a function of time for the PSCs operating at the power as A of is for the device based on PTAA/2PACz which is higher than that of its PTAA-based The PSCs exhibited better reproducibility than the reference PTAA-only (Figure Specifically, more than of devices over while only of PTAA-based PSCs showed over The reproducibility of device performance the of bilayered PTAA/2PACz HTL. Figure | (a) Schematic illustration of inverted with a device architecture of (b) J–V characteristics and (c) of PSCs fabricated with or without 2PACz interlayer. (d) J–V characteristics of the devices measured under (e) J–V and (f) spectrum and value of the champion blade-coated based on PTAA/2PACz HTL and FA0.4MA0.6PbI3 perovskite film. Download figure Download PowerPoint 1 | of the Inverted PSCs on PTAA or PTAA-2PACz HTL under Sun VOC PTAA Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 PTAA/2PACz PTAA/2PACz FA0.4MA0.6PbI3 1.14 / was calculated from a of 30 devices for each Typically, the enhancement of VOC by for the PSCs can be attributed to the reduced defect and suppressed recombination the devices and/or at The defect of the perovskite films were determined by the J–V of the devices under The limit voltage can be obtained and the defect density can be calculated to the = 0 V the is the of perovskite for Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 is the thickness of perovskite film nm, Supporting Information Figure and the As to the PTAA-based device = = × the device a decreased of V and a reduced of × (Figure The of defect density can be ascribed to improved quality of the perovskite film and robust passivation of interfacial at the buried HTL/perovskite interface with 2PACz modification. was employed to the charge recombination the devices. the with the one can conclude the targeted device with 2PACz passivation the smaller and higher recombination ( Supporting Information Figure further better charge and largely suppressed charge recombination We also the PTAA/2PACz HTL with the blade-coated FA0.4MA0.6PbI3 perovskite for the of further improving the device performance. A champion of 22.23% was achieved, by a of a VOC of 1.14 and a of (Figure an illumination area of We measured the J–V of the champion device under different scanning ( Supporting Information Figure which showed negligible J–V The spectrum of the champion device showed an of (Figure which is in with the obtained from J–V a of only This result the and of 2PACz modification at the buried HTL/perovskite interface to improve the efficiency and stability of inverted PSCs,