Hole Transport Material Research Articles

Glycol Monomethyl Ether-Substituted Carbazolyl Hole-Transporting Material for Stable Inverted Perovskite Solar Cells with Efficiency of 25.52 .

Organic self-assembled molecules (OSAMs) based hole-transporting materials play a pivotal role in achieving highly efficient and stable inverted perovskite solar cells (IPSCs). However, the reported carbazol-based OSAMs have serious drawbacks, such as poor wettability for perovskite solution spreading due to the nonpolar surface, worse matched energy arrangement with perovskite, and limited molecular species, which greatly limit the device performance. To address above problems, a novel OSAM [4-(3,6-glycol monomethyl ether-9H-carbazol-9-yl) butyl]phosphonic acid (GM-4PACz) was synthesized as hole-transporting material by introducing glycol monomethyl ether (GM) side chains at carbazolyl unit. GM groups enhance the surface energy of Indium Tin Oxide (ITO)/SAM substrate to facilitate the nucleation and growth of up perovskite film, suppress cation defects, release the residual stress at SAM/perovskite interface, and evaluate energy level for matching with perovskite. Consequently, the GM-4PACz based IPSC achieves a champion PCE of 25.52 %, a respectable open-circuit voltage (VOC) of 1.21 V, a high stability, possessing 93.29 % and 91.75 % of their initial efficiency after aging in air for 2000 h or tracking at maximum power point for 1000 h, respectively.

Porphyrin-Based Hole-Transporting Materials for Perovskite Solar Cells: Boosting Performance with Smart Synthesis.

Perovskite solar cells (PSCs) are becoming a promising and revolutionary advancement within the photovoltaic field globally. Continuous improvement in efficiency, straightforward processing methods, and use of lightweight and cost-effective materials represent superior features, among other notable aspects. Still, long-term stability and durability are issues to address to facilitate widespread commercial adoption and practical application prospects. Research has focused on overcoming these challenges, and charge transport materials play a critical role in determining charge dynamics, photovoltaic performance, and device stability. Conventional hole-transporting materials (HTMs), spiro-OMeTAD and PTAA, contribute to remarkable power conversion efficiencies owing to high thin-film quality and matched energy alignment. However, they often show a high material cost, low carrier mobility, and poor stability, which greatly limit their practical applications. Now, this review outlines recent advances in synthetic approaches to porphyrin-based HTMs to tune the charge dynamics by optimizing their molecular structures and properties. The main structural features comprise porphyrins of A4-type, trans A2B2-type, and photosynthetic pigment analogues. Strategies include well-established routes to provide the required macrocycles, such as condensation of pyrrole or dipyrromethanes with suitable aldehydes, metalation of the porphyrin inner core, and postfunctionalization of peripheral positions. These functionalizations involve conventional procedures (e.g., halogenation, esterification, transesterification, nucleophilic oxidation, reduction, and nucleophilic substitution) as well as metal-catalyzed ones such as Suzuki-Miyaura, Sonogashira, Buchwald-Hartwig, and Ullmann cross-coupling reactions. As HTMs can also protect the perovskite layer from the external environment, porphyrin structures play a pivotal role in chemical, mechanical, and environmental stability, with their high hydrophobicity ability as the most significant parameter. The impact of porphyrins on the hole hopping of other HTMs while acting as an additive or an interlayer, passivating defects, and improving charge transport is also highlighted to provide real insights into ways to develop efficient and stable porphyrin-based materials for PSCs. This perspective aims to guide the scientific community in the design of new porphyrin molecules to place PSCs as an outperformer in photovoltaic technologies.

Low‐Temperature TiO2 Electron Transporting Layer for Planar Hole Transport Material‐Free Carbon Electrode‐CsFA‐Based Perovskite Solar Cells

Carbon electrode‐based perovskite solar cells (C‐PSCs) without a hole transport material (HTM) are cost‐effective and exhibit impressive long‐term stability. The electron transporting layer (ETL) plays a crucial role in planar CsFA‐based HTM‐free C‐PSCs, serving as both an electron transporter and a hole barrier. Herein, the role of low‐TiO2 morphology and thickness on the performance of CsFA‐based HTM‐free C‐PSCs are addressed. Herein, the devices are fabricated with a simple structure fluorine‐doped tin oxide /TiO2 nanoparticles (TiO2 NPs)/Cs0.17FA0.83Pb(I0.83Br0.17)3/carbon, using low‐temperature processes (≤150 °C) under ambient air conditions. By optimizing TiO2 NP layer thickness via spin‐coating speed adjustments, the ETL's coverage and compactness are improved, enhancing the perovskite film's quality, crystallinity, and grain size. An optimal TiO2 ETL at 1500 rpm yields 10.80% efficiency and demonstrates exceptional stability, maintaining 80% efficiency over 120 days in an air environment without encapsulation. The enhancement in device performance is attributed to improved surface properties of the TiO2 NPs ETL, effectively reducing interfacial charge recombination. This straightforwardly supports the development of sustainable, commercial‐ready CsFA HTM‐free C‐PSCs.

Copper naphthalocyanine-based hole-transport material for high-performance and thermally stable perovskite solar cells

Copper naphthalocyanine-based hole-transport material for high-performance and thermally stable perovskite solar cells

Exploring synthesis, characterization, and computational insights into indacenodithiophene-based hole transporting materials for enhanced perovskite solar cell applications

Utilizing hole-transporting materials (HTMs) to extract and transport holes from perovskite materials to the electrode remains essential in most perovskite solar cell (PSC) architectures. Developing cost-effective and efficient HTMs is essential for advancing PSC technology. We have synthesized a novel HTM, TPA-IDT-TPA, which has an extended fused ring as the core moiety called indacenodithiophene (IDT) and p-methoxy triphenylamine (p-mTPA) as terminal groups. TPA-IDT-TPA exhibits appropriate frontier molecular orbital (FMO) energy levels that match perovskite materials. Density functional theory (DFT) simulations were performed to comprehend the electronic, excited-state, and charge transport properties. The DFT results indicate that the extended π-conjugation, rigidity, and the central core ring of the HTM enhanced the π-π stacking, contributing to efficient charge transport. The PSC constructed with TPA-IDT-TPA achieves a device efficiency of 8.34%, with high values of JSC and VOC, which can be further enhanced through molecular optimization.

Chemical polishing strategies to improve flexible perovskite solar cells based on dopant-free hole transport layers

For n-i-p typed flexible perovskite solar cells (fPSCs), the doped hole transport layer significantly impacts the devices' long-term stability. Using dopant-free organic hole transport materials (d-HTMs) is promising for stable fPSCs. However, the low conductivity of d-HTMs limited their thickness, making them sensitive to the surface morphology of the perovskite film's upper surface. Here, we report a chemical polishing strategy using 1-hexyl-3-methylimidazolium acetate (HMIM∙Ac) as the polishing reagent to enhance the upper surface of the perovskite film, which could form a smooth and flat surface. Meanwhile, the treatment can reduce surface defects and smaller grains on top of the surface. Then, we deposite an ultra-thin dopant-free PM6 layer, a typical hole transport layer, on top of the polished perovskite film. The PM6 layer shows an improved face-on orientation and then carrier mobility. Moreover, suppressed non-radiative recombination at the perovskite/PM6 interface is also observed, translating into a higher open-circuit voltage and fill factor of the fPSCs. As a result, a champion power conversion efficiency (PCE) of 17.76 ​%, with an open-circuit voltage of 1.025 ​V and fill factor of 78.2 ​%, is obtained, which is one of the highest PCEs among the reported fPSCs based on d-HTMs. Our strategy demonstrates a facile but effective way of developing high-efficiency and stable fPSCs for future applications.

Thiocarbonyl-Based Hole Transport Materials with Enhanced Defect Passivation Ability for Efficient and Stable Perovskite Solar Cells.

Organic hole transporting materials (HTMs) are extensively studied in perovskite solar cells (PSCs). The HTMs directly contact the underlying perovskite material, and they play additional roles apart from hole transporting. Developing organic HTMs with defect passivation function has been proved to be an efficient strategy to construct efficient and stable PSCs. In this work, new organic molecules with thiocarbonyl (C═S) and carbonyl (C═O) functional groups are synthesized and applied as HTMs (named FN-S and FN-O). FN-S with C═S can be facilely obtained from FN-O containing C═O. Notably, the C═S in FN-S results in superior defect passivation ability compared to FN-O. Moreover, FN-S exhibits excellent hole extraction/transport capability. Conventional PSCs using FN-S as HTM show an impressive power conversion efficiency (PCE) of 23.25%, with excellent long-term stability and operational stability. This work indicates that simply converting C═O to C═S is an efficient way to improve the device performance by strengthening the defect passivation functionality.

Numerical design of non-toxic high-efficiency tandem solar cell with distinct hole transport materials

The metal-halide perovskite is at the forefront of photovoltaic research and has the potential to combine low manufacturing costs with excellent power conversion efficiency. In this study, a solar cell capacitance simulator was used to analyze the performance of a tandem solar cell in which MASnI3 was used as the lower cell active layer, and NaZn0.7Cu0.3Br3 was used as the upper cell active layer. The primary objective of this research is to search for a device structure with enhanced efficiency. Here, it is evaluated how absorber thickness, temperature, hole transport layer, defect density, and metal work function affect solar cell performance to reach this aim. After looking at the data for several different solar cell configurations, it is seen that indium tin oxide (ITO) / titanium dioxide (TiO2)/sodium zinc copper bromide (NaZn0.7Cu0.3Br3)/ methylammonium tin iodide (MASnI3) / copper oxide (CuO)/gold (Au) offers the best efficiency at 32.58 %, fill factor at 82.19 %, short circuit current density at 34.8389 mA/cm2, and open circuit voltage at 1.1375 Vs. Device simulations showed an optimal MASnI3 absorber thickness of about 1 µm. Finally, the simulation results reveal that the device's efficiency declines when the absorber's defect density and cell operation temperature rise. The device topologies are stable at around 300 Kelvin. Last but not least, any conductive material can be used as an anode if it has a work function that is more than or roughly equivalent to 5.10 eV.

Effects of carbazolyl and diphenylamino substituents bearing methoxy groups on the performance of hole-transporting materials in OLEDs

Newly designed and synthesized derivatives of pentaphenylbenzene with methoxy-substituted carbazolyl or diphenylamino moieties were investigated to estimate their applicability as hole transport materials. Both the compounds exhibit high thermal stability. The intramolecular charge transfer is blocked for the film of the compound containing diphenylamino groups. The intermolecular charge transfer is induced in the film of carbazolyl-containing compound. The derivative of pentaphenylbenzene and diphenylamine exhibits higher hole drift mobility (2.4·10−3 cm2/V·s at the electric field of 5.5·105 V/cm) and by 0.1 eV lower ionization potential than the carbazolyl-containing compound. Both the compounds were utilized as hole-transporting materials in a series of organic light emitting diodes (OLEDs) based on of thermally activated delayed fluorescence. With the maximum values of external quantum efficiency of 25.9 % and power efficiency of 43.4 lm/W, OLEDs containing the layers of the synthesized compounds outperformed the device based on TCTA by 4 %, without the change in spectral properties. Variable angle spectroscopic ellipsometry revealed the moderate average roughness of the films of the compound deposited by the thermal vacuum evaporation technique with an arithmetic mean deviation of not more than 0.8 nm. The prominent hole transport characteristics of the compounds make them good candidates for utilization in optoelectronic devices.

Inorganic hole transport layer for lead free bismuth halide perovskite for photovoltaic device

The finest Hole Transport Layer (HTL) with extraordinary optical and electrical properties are significantly influenced the Power Conversion Efficiency (PCE) of the photovoltaic device. Inorganic hole transport materials exhibit better performance than organic hole transport materials. Herein, we demonstrated that optoelectronic properties of inorganic materials of NiO doped with Cu as Hole Transport Layer (HTL) for Lead free Bismuth halide perovskite solar cell. Solution process method of sol – gel was used to synthesize p-type NiO doped with Cu and yield nano particles of high quality. Cu + doped into Ni2+ sites has improved the crystallization of nanoparticles, carrier mobility and carrier electric properties. Results of the study revealed that the Enhancement of electrical conductivity of about 2 % is obtained for NiO doped with Cu which is ∼1.287 × 10−02 compared to the pristine NiO which is ∼2.655 × 10−03.

Stabilizing Perovskite Solar Cells Based on 2,2′,7,7′‐Tetrakis(N,N‐DI‐P‐Methoxyphenylamine)‐9,9′‐Spirobifluoren with Perfluoropolyethers

Organic–inorganic hybrid perovskites in combination with the hole‐transport material (HTM) 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluoren (spiro‐OMeTAD) yield impressive power conversion efficiency (PCE) in n–i–p perovskite solar cells (PSC). To ensure sufficient hole extraction from the perovskite absorber to the metal electrode, the lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) doping is considered indispensable to promote spiro‐OMeTAD oxidization and obtain consequently enhanced conductivity. However, LiTFSI‐doped spiro‐OMeTAD usually leads to a reduced stability of PSCs because of the hygroscopic nature of LiTFSI, which further limits its potential commercialization. Herein, a hydrophobic material, namely perfluoropolyethers (PFPEs), is used as surface modifier of doped HTM to improve both efficiency and stability of PSCs. It is revealed that the introduction of PFPE increases the concentration of positive radicals, enhances charge‐carrier transport, and improves stability of PSCs under wetting stress. Moreover, the device based on PFPE achieves a champion PCE of 21.94%. In these findings, valuable insights are provided for the future commercialization of PSCs.

Theoretical investigation of DMPA-containing carbazole-based hole-transport materials for perovskite solar cells

This study mainly focuses on DFT and TD-DFT modelling and extensive analysis of eight carbazole-based molecules (MM, MPM, MP2.5M, MP2.6M, MP3.5M, MPPM, MP2P2M, and MP3P3M) as Hole Transporting Materials in Perovskite Solar cells. These compounds have in common the arm N3,N3,N6,N6-tetrakis(4-methoxyphenyl)-9H-carbazole-3,6-diamine (M) characterized by its strong ability to carry holes, low cost, and good stability. The molecule MM presents two arms without acceptor core, while other compounds present two arms separated by different acceptor cores: phenyl, biphenyl, pyridine or bipyridine. All under-probe compounds were favored candidates for HTMs in perovskite solar cells due to their excellent HOMO delocalization, reduced hole reorganization energies, lower chemical reactivity, longer radiative lifetimes, and spontaneous solvation processes. Flatness proved beneficial when the core consisted of two cycles, particularly for MP2P2M. The results showed that the HOMO levels were primarily influenced by M fragments in most of the molecules, while the LUMO levels were distributed across both the carbazole units and the cores. All molecules exhibited a more stable LUMO compared to that of Spiro-OMETAD, with MP2P2M showing a notably stable level, resulting in a narrower gap. The study found that, except for MP2P2M, these materials did not interfere with the perovskite layer and effectively filled pores. The incorporation of acceptor core groups improved the conjugation backbone and electronic states, significantly enhancing charge transport properties, particularly in molecules with a double ring structure. Furthermore, adding a pyridine center greatly increased solubility, especially in MP3P3M, suggesting that acceptor core inclusion benefits the solubility of the investigated HTMs. The use of two rings core was shown to enhance light harvesting efficiency, boost photocurrent generation, and improve charge transfer performance, particularly in MP2P2M, which is distinguished by its superior flatness, solubility, and charge transfer properties.

Blending Self-Assembled Monolayers for Enhanced Band Alignment and Improved Morphology in p-i-n Perovskite Photodetectors.

Perovskite photodetectors, devices that convert light to electricity, require good extraction and low noise levels to maximize the signal-to-noise ratio. Self-assembling monolayers (SAMs) have been shown to be effective hole transport materials thanks to their atomic layer thickness, transparency, and energetic alignment with the valence band of the perovskite. While efforts are being made to reduce noise levels via the active layer, little has been done to reduce noise via SAM interfacial engineering. Herein, we report hybrid perovskite photodetectors with high detectivity by blending two different SAMs (2-PACz and Me-4PACz). We find that with a 1:1 2-PACz:Me-4PACz ratio (by weight), the devices achieved a low noise of 1 × 10-13 A Hz-1/2, a high responsivity of 0.41 A W-1 at 710 nm, and a specific detectivity of 6.4 × 1011 Jones at 710 nm at -0.5 V, outperforming its two counterparts. In addition to the improved noise levels in these devices, impedance spectroscopy revealed that higher recombination lifetimes of 0.85 μs were achieved for the 1:1 2-PACz:Me-4PACz-based photodetectors, confirming their low defect density.

Evaluating Fluorinated-Aniline Units with Functionalized Spiro[Fluorene-9,9'-Xanthene] as Hole-Transporting Materials in Perovskite Solar Cells and Light-Emitting Diodes.

In the field of perovskite optoelectronics, developing hole-transporting materials (HTMs) on the spiro[fluorene-9,9'-xanthene] (SFX) platform is one of the current research focuses. The SFX inherits the merits of spirobifluorene in terms of the configuration and property, but it is more easily derivatized and regulated by virtue of its binary structure. In this work, we design and synthesize four isomeric SFX-based HTMs, namely m-SFX-mF, p-SFX-mF, m-SFX-oF, and p-SFX-oF, through varying the positions of fluorination on the peripheral aniline units and their substitutions on the SFX core, and the optoelectronic performance of the resulting HTMs is evaluated in both perovskite solar cells (PSCs) and light-emitting diodes (PeLEDs) by the vacuum thermal evaporating hole-transporting layers (HTLs). The HTM p-SFX-oF exhibits an improved power conversion efficiency of 15.21% in an inverted PSC using CH3NH3PbI3 as an absorber, benefiting from the deep HOMO level and good HTL/perovskite interface contact. Meanwhile, the HTM m-SFX-mF provides a maximum external quantum efficiency of 3.15% in CsPb(Br/Cl)3-based PeLEDs, which is attributed to its perched HOMO level and shrunken band-gap for facilitating charge carrier injection and then exciton combination. Through elucidating the synergistic position effect of fluorination on aniline units and their substitutions on the SFX core, this work lays the foundation for developing low-cost and efficient HTMs in the future.

The optoelectronic properties of acetylene based compounds as hole transporter in perovskite solar cells: A computational study

Hole transport materials (HTMs)are pivotal components inperovskite solar cells (PSCs), significantly influencing theirpower conversion efficiency (PCE). This study explores the potential of variousdiacetylide-triphenylamine (DATPA) derivatives (HTMs 1–8)to function as hole transporters, employingdensity functional theory (DFT)andtime-dependent density functional theory (TD-DFT)calculations.The energy levels of thehighest occupied molecular orbital (HOMO)andlowest unoccupied molecular orbital (LUMO), as well as theband gap, were computed using theB3LYP/6-311G(d)level of theory. The findings reveal that, with the exception of HTM 8, these compounds exhibit suitable HOMO and LUMO levels relative to the perovskite layer and possess a lower band gap energy compared to the commonly usedspiro-OMeTAD.Additionally, the calculation ofhole mobility (Kh)using the Marcus method demonstrated a satisfactory value, substantiating the applicability of these compounds as HTMs. Further calculations of parameters such ashole reorganization energy (λh),absolute hardness (η),ionization potential (IP),electronic affinity (EA),solubility (ΔGsolv), andexciton binding energy (Eb)affirm that these compounds are promising candidates for hole transport in perovskite solar cells. Notably, the compoundHTM 2 (NMe2-DATPA)outperforms the others in hole transfer efficiency.

Advancing optoelectronic characteristics of Diketopyrrolopyrrole-Based molecules as donors for organic and as hole transporting materials for perovskite solar cells

A stable and efficient hole-transport material (HTM) is crucial for high-performance perovskite solar cells (PSCs). A 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (Spiro-MeOTAD) being used widely to prepare highly efficient PSCs. However, Spiro-MeOTAD has some limitations due to its complex synthesis, which increases its cost, and it also requires dopants to improve its performance. Therefore, we designed thirteen unique small-molecule-based HTMs (MK1-MK13), which are easy to synthesize, highly cost-effective, and don’t require dopants to prepare efficient PSCs. Their electrical and optical properties are then investigated theoretically using advanced quantum chemical approaches. The designed molecules showed lower energy gaps and improved optical and optoelectronic characteristics because of the improved phase inversion geometry. The detailed photo-physical and optoelectronic characteristics have been studied using density functional theory (DFT) and time-dependent (TD-DFT) calculations. Moreover, we investigated the impact of holes and electrons and the density of states, open-circuit voltage, frontier molecular orbital, transition density matrix, and other structural and photovoltaic characteristics of these materials. Among these, the MK3 molecule possesses the much narrower optical band gap of 1.04 eV and absorbance (λ max) of 684 nm, respectively. In addition, a profound investigation of the MK3/PC61BM blend shows excellent charge transfer at the acceptor–donor interface. Therefore, our proposed technique is necessary for generating appropriate photovoltaic materials for efficient optoelectronic devices and is helpful in further advancing the field.

Molecular packing regulation of dopant-free hole transport polymers for efficient perovskite solar cells

Spiro-OMeTAD is a primary hole transport material (HTM) employed in most state-of-the-art regular perovskite solar cells (PSCs). The essential reliance on hygroscopic ionic dopants to enhance the conductivity and mobility of Spiro-OMeTAD has dramatically compromised the stability of PSCs. Here, we demonstrated excellent photovoltaic performance of PSCs by developing two dopant-free polymers, namely L1 and L2, using thieno[3,2-b]thiophene as a building block. It is found that the n-hexyl-modified thiophene side chains endow the polymer L2 with favorable crystallinity, unique self-assembly behavior, and a preferable face-on stacking orientation. After the addition of a small amount (10 %) of PM6 to create a polymer alloy named LPA, the above properties were further improved, and the resulting film exhibited a distinct fibrous morphology, resulting in increased hole mobility and effective defect passivation. Consequently, PSCs employing LPA as a dopant-free HTM afforded a high efficiency of 23.81 %. Importantly, LPA-based PSCs exhibit significantly enhanced operational stability with a T80 lifetime of 1572 h at 55 °C. This work provides a crucial guideline for the design of dopant-free polymers, thereby advancing the practical application of PSCs.

A facile method to obtain colloidal dispersions of nickel hydroxide: Improving the processing of nickel oxide and facilitating its upscaling for perovskite-type solar devices

Nickel hydroxide has been successfully employed as a precursor to the widely used, inorganic hole transport material (HTM) nickel oxide (NiOx). However, manufacturing NiOx HTM layers from nickel hydroxide is more complicated than those involving organometallic precursors due to its poor solubility/dispersibility. We report here a substantial increase in nickel hydroxide dispersibility in organic solvents by complexing it with monoethanolamine. These improvements have enabled us to develop a simpler method for processing nickel hydroxide that resemble the known sol-gel method. The new metal complex remains dispersed for months and converts to nickel oxide at a temperature similar to that of nickel hydroxide (270–300 ºC). An extensive characterisation of NiOx films obtained from the deposited precursor has been carried out. Perovskites solar cells have also been built with these films as a proof of concept, showing promising results for the layers sintered at low (270 ºC) and high (500 ºC) temperatures. The pixel with highest efficiency for both sintering temperatures were 14.7 % and 16.7 %, respectively, which are close to or surpass the ones of the control samples (15.4 % and 15.7 %, respectively). The applied unpaired t-test statistical method showed that the mean efficiency values for our thick samples prepared at 270 °C are not statistically different from those of the control cells. Furthermore, the samples prepared at 500 °C presented a significant statistical difference with the control cells, showing higher average efficiencies (12.8 % and 13.3 % versus 11.4 % and 11.7 %, reverse and forward measurements, respectively). The simplicity of the manufacturing method developed, together with the use of non-toxic organic compounds for its preparation and the promising results observed in solar devices, makes it suitable for being upscaled.

In-depth analysis of γ-CuI as an HTM for perovskite solar cells: A comprehensive DFT study of structural, elastic, mechanical, charge density, and optoelectronic properties

Herein, we employed Density Functional Theory (DFT) to comprehensively investigate pristine γ-CuI properties under two computational schemes: Generalized Gradient Approximation (GGA) and GGA + Hubbard correction. Structural stability is rigorously assessed through ground state energy optimization, complemented by a meticulous examination of elastic constants to confirm mechanical stability. Later on, we analyzed the electronic properties, as well as the optical properties, incorporating parameters such as the extinction coefficient that was low and refractive index that was high in the IR and visible regions. Notably, γ-CuI exhibits low reflectivity and absorption in the critical regions of interest. Our findings demonstrate that γ-CuI can serve as a cost-effective Hole Transporting Material (HTM), effectively reducing optical losses in perovskite solar cells.

Degassing 4-tert-Butylpyridine in the Spiro-MeOTAD Film Improves the Thermal Stability of Perovskite Solar Cells.

The organic molecular 2,2',7,7'-tetrakis(4,4'-dimethoxy-3-methyldiphenylamino)-9,9'-spirobifluorene (Spiro-MeOTAD) is known as a typical hole transport material in the development of an all-solid-state perovskite solar cell (PSC). Spiro-MeOTAD requires additives of lithium bifurflimide (LiTFSI) and 4-tert-butylpyridine (tBP) to increase the conductivity and solubility for enhancing the photovoltaic performance of PSCs. However, those additives have an adverse effect on the thermal stability. We report on the origin of instability of additive-containing Spiro-MeOTAD at 85 °C and the methodology to solve the thermal instability. We have found that the interaction of LiTFSI with the underneath perovskite surface facilitated by diffusive tBP is responsible for thermal degradation. Degasification of tBP from the Spiro-MeOTAD film is found to be the key to achieving thermally stable PSCs, where the optimal degassing process achieves 90% of the initial power conversion efficiency (PCE) at 85 °C after 1000 h.