Tetrapropyl-substituted palladium phthalocyanine used as an efficient hole transport material in perovskite solar cells

This report presents two tetrapropyl-substituted metal phthalocyanines (PdPrPc and ZnPrPc) used as dopant-free hole transport materials (HTMs) for perovskite solar cells (PSCs). The substitution of Pd atom did not significantly reduce the mobility of the material but increased its lowest unoccupied molecular orbital (LUMO) level. Owing to spin-orbit coupling, the PdPrPc is thought to have a longer carrier diffusion length than that of the ZnPrPc. The higher LUMO level together with the longer carrier diffusion length of the PdPrPc reduced the hole-electron recombination, which led to a higher FF value of its PSC, giving rise to a higher PCE of 18.09% than that of the ZnPrPc-based device. Further, the PdPrPc-based PSC exhibited an increased stability compared with the ZnPrPc-based one. The result indicates the potential application of MPcs containing heavier atoms in efficient and stable PSCs.     

Black phosphorus doped Poly(triarylamine) as hole transport layer for highly efficient perovskite solar cells

Charge transport layer plays a critical role in high-performance perovskite solar cells (PSCs). Herein, few-layered 2D black phosphorus (BP) nanosheet doped poly(triarylamine) (PTAA) is employed as hole transport layer for PSCs. The BP:PTAA significantly improves charge extraction at perovskite/BP:PTAA interface together with the smaller energy barrier, the increased conductivity of the PTAA film, and the formation of the high-quality perovskite film with enlarged crystal gain size, which suppress the interfacial charge recombination and trap-assisted recombination. As a result, the champion device using BP:PTAA produces the higher power conversion efficiency of 20.49% than the control device of 18.26%. Moreover, the remarkable improvement in device stability has been demonstrated attributed to the more hydrophobicity of BP:PTAA and the perovskite layer with less defect states. This work provides an effective hole transport layer for PSCs, which is comparable with the commonly used 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ):PTAA.     

Effect of dimethylamino substituent on tetraphenylethylene-based hole transport material in perovskite solar cells

A N,N-dimethylamino substituted tetraphenylethylene derivative (TPE-NMe) was synthesized and characterized, and was successfully applied as hole transport material (HTM) in perovskite solar cells. The methoxy-substituted analogue TPE-4DPA was also studied for comparison. The effect of replacing the para-methoxy substituent with N,N-dimethylamino on photophysical properties, energy levels, and hole transport properties is investigated. Photovoltaic performances of the corresponding devices using the two HTMs are studied. Compared to the methoxy substituent, the N,N-dimethylamino groups in TPE-NMe generates a lower V_oc (0.87 V), yet it provides higher J_sc (21.69 mA/cm²) and FF (0.73) values, resulting in an overall power conversion efficiency of 13.78%.     

Novel D-A-D type small-molecular hole transport materials for stable inverted perovskite solar cells

Hole transport materials (HTMs), as a critical role in the hole extraction and transportation processes, highly influence the efficiency and stability of perovskite solar cells (PSCs). Despite that several efficient dopant-free HTMs have been reported, there is still no clear structure-property relationship that could give instructions for the rational molecular design of efficient HTMs. Thus, in this work, a series of donor–acceptor-donor (D–A–D) type carbazole-based small molecules, TM-1 to TM-4, have been carefully designed and synthesized. By varing the electron acceptor unit from benzene to pyridine, pyrazine and diazine, their packing structure in single crystals, optical and electronic properties have shown a great difference. While as dopant-free HTM in p-i-n type PSCs, TM-2 improved the device photovoltaic performance with a power conversion efficiency from 15.02% (based on PEDOT:PSS) to 16.13%. Moreover, the unencapsulated device based on TM-2 retains about 80% of its initial efficiency after 500 h storage in ambient environment, showing the superior stability.     

Moderately reduced graphene oxide/PEDOT:PSS as hole transport layer to fabricate efficient perovskite hybrid solar cells

Graphene oxide (GO) with single layer was moderately reduced at 200 °C for 4 h under N₂. Then the moderately reduced graphene oxide (rGO) water solution was employed as an additive to tune the properties of conventional poly(ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) solution. It's found that the incorporation of rGO into PEDOT:PSS nearly did not change its transparency, hydrophilic property, or the surface roughness. So, the rGO/PEDOT:PSS composite was used as a hole transport layer (HTL) to fabricate perovskite solar cells (PSCs). As a result, PSCs with rGO/PEDOT:PSS as HTL exhibit improved power conversion efficiency than that of PSCs with PEDOT:PSS as HTL. Our findings show that moderately reduced rGO/PEDOT:PSS could be an efficient HTL to improve power conversion efficiency of PSCs.     

F4TCNQ-doped DEPT-SC as hole transporting material for stable perovskite solar cells

The CH₃NH₃PbI₃-based perovskite solar cells using α, α′-diethoxyethyl-oligothiophenes (DEPT-SC) doped with 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquino-dimethane (F4TCNQ) as hole transport material (HTM) exhibited a power conversion efficiency of 11.52%. Compared to the pristine devices, the perovskite solar cells using the new synthesized HTM showed an increased efficiency by about 18% and exhibited better photo-stability, indicating that the organic dopant is an effective method for DEPT-SC toward stable perovskite solar cells.     

A charge-separated interfacial hole transport semiconductor for efficient and stable perovskite solar cells

Perovskite solar cells (PSCs) with high efficiency and high stability are still a challenge to produce although remarkable successes have been achieved since they were first reported in 2009. One strategy to effectively improve both the performance as well as the stability is to introduce an interfacial layer between perovskite and hole transport material. Herein, we report a charge-separated (CS) organic semiconductor as the interfacial layer that forms cascaded energy levels between perovskite and hole transportation material. This CS semiconductor displays high hole and electron mobilities by converting long-lived CS states in solution into permanent polarons (charged carriers) in films. Doping with iodinehydride is able to improve the surface morphology of the CS semiconductor layer. Our devices with an iodinehydride-doped CS semiconductor layer exhibit an efficiency of 17.87%, which is increased by ~25% in comparison with 14.24% of the reference devices that have no interfacial layer. This additional CS semiconductor layer also enhances the unsealed device stability by maintaining 90% of initial PCE, while the reference devices degraded by 35% at a relative humidity of 20–30%, temperature of 25 °C and ambient light for 240 h. This result reveals that the utilization of CS states is an alternative approach to construct high charge transport organic semiconductors. An interfacial semiconductor with proper energy level and a matching hole transport mobility can improve the hole extraction, speed up hole transport and suppress charge recombination of PSCs, and thus may be an effective strategy to improve their efficiency and stability.     

Benzodithiophene-based small molecules with various termini as hole transporting materials in efficient planar perovskite solar cells

In this study we prepared four benzodithiophene (BDT)-based small organic molecules presenting bithiophene (TT), thiophene (FT), carbazole (CB), and triphenylamine (TPA) units, respectively, as termini, and used them as hole transporting materials for perovskite solar cells (PSCs). The high degrees of planarity of these BDT-based small molecules imparted them with high degrees of stacking and charge transport. These small molecules had suitable optical properties and energy level alignments for use in PSCs based on MAPbI₃, with compact-TiO₂ as the electron transporting layer and a BDT-based material as the hole transporting layer, in a n–i–p structure. Among our tested BDT-based materials, the PSC incorporating BDT-TT had the best performance, with an average power conversion efficiency of 13.63%.     

Undercoordinated Pb2+ defects passivation via tetramethoxysilane-modified for efficient and stable perovskite solar cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have developed rapidly in recent years, and the instability limits its commercialization. Non-radiative recombination caused by defects and water stability affect the device stability. Here we introduce an organic silane additive, tetramethoxysilane (TMOS), which can reduce the non-radiative recombination and prevent the water erosion. The methoxy group in TMOS can combine with Pb²⁺ of perovskite to passivate undercoordinated Pb²⁺ defects and reduce non-radiative recombination. Under a certain humidity, the hydrolyzed product SiO₂ can occupy the grain boundary sites to prevent the erosion of water molecules, slow down the degradation of perovskite, and improve the crystal phase stability of perovskite. The PCE of the device increases from 17.13% to 20.12%. After 400 h at 50% relative humidity (RH), the PSC with 2% TMOS can maintain the efficiency of 90%, while the efficiency of the control group quickly dropped to only 70% of the initial.     

Design new hole transport materials for efficient perovskite solar cells by suitable combination of donor and core groups

The electronic, optical, and hole transporting mobility of three organic hole transporting materials (HTMs), X59, X59-P, and X59-T, are investigated by combination of first principle and molecular dynamics associated with Marcus theory and Einstein equation. As compared with the experimental reported X59, the new designed X59-P has more stable HOMO energy level. Moreover, the latter has smaller reorganization energy and larger hole transfer integral resulting in the larger hole transporting mobility. Besides the hole transporting mobility, the solubility and stability of two designed molecules are also evaluated by comparison with X59, which are two important items to determine the cost and performance in real application of solar cell. More importantly, they would be synthesized in a benign condition without expensive materials. Our studies introduce a possible pathway to explore the efficient HTMs by suitable combination mode rather than development of new groups.     

Efficiency enhancement in perovskite solar cell utilizing solution-processable phthalocyanine hole transport layer with thermal annealing

We demonstrate efficiency enhancement in perovskite solar cells (PSCs) utilizing a free-dopant hole transporting material (HTM), non-peripherally substituted octapentyl phthalocyanine (C5PcH₂) with thermal annealing. Particularly, by using thermal annealing approach, the external quantum efficiency at around 480 nm increase from 78 to 84%. Hence, the fill factor and short-circuit current density are markedly improved from 0.35 ± 0.02 to 0.55 ± 0.05 and from 18 ± 1 to 18.8 ± 0.3 mA cm^-2, respectively. Finally, the best device is achieved with power conversion efficiencies of 12.2% by annealing at 130 °C for 10 min. The photoluminescence and photo-induced charge carrier extraction in linearly increasing voltages measurements indicate that the charge carrier mobility in C5PcH₂ increases, and thereby the hole extraction and transportation from the perovskite layer to the Au anode as well the photovoltaic performance of PCS is improved by using thermal annealing processing.     

Boosting the performance and stability of perovskite solar cells with phthalocyanine-based dopant-free hole transporting materials through core metal and peripheral groups engineering

Three novel dopant-free hole-transporting materials (HTMs) based on phthalocyanine core containing (4-methyl formate) phenoxy or (4-butyl formate) phenoxy as the peripheral groups with cupper or zinc as the core metals (CuPcNO₂-OMFPh, CuPcNO₂-OBFPh, ZnPcNO₂-OBFPh) were designed and synthesized. All of the phthalocyanine complexes show excellent thermal stabilities, appropriate energy levels and suitable hole mobilities. The potential of three HTMs were tested in perovskite solar cells (PSCs) and ZnPcNO₂-OBFPh based PSC obtained power conversion efficiency (PCE) of 15.74% under 100 mA cm⁻² standard AM 1.5G solar illumination. Most important of all, PSC based on ZnPcNO₂-OBFPh shows better stability than that of the other two phthalocyanines and Spiro-OMeTAD under continuous light irradiation at 60 °C and maximum power point tracking in ambient air without encapsulation after 500 h. The results show that the introduction of appropriate peripheral groups and core metals can improve the performance and stability of PSCs dramatically, which provides an alternative way to develop HTMs for efficient and stable PSCs.     

Graphene-oxide doped PEDOT:PSS as a superior hole transport material for high-efficiency perovskite solar cell

Inverted perovskite solar cells have attracted a great deal of attention due to its high power conversion efficiency, simple configuration, and low-cost processing. The hole transport material (HTM) is a crucial factor in high performance inverted perovskite solar cell. However, the hole mobility for most common of HTM is too low to matching perovskite materials. Herein, we report a superior HTM with high hole mobility to significantly improve solar cell efficiency. Upon doing the commonly used PEDOT:PSS HTM by graphene oxide (GO), its hole mobility is increased from 5.55 × 10⁻⁵ to 1.57 × 10⁻⁴ cm² V⁻¹ s⁻¹, leading to efficient hole extraction and low current leakage, therefore 20% higher power conversion efficiency comparing to the control device without the GO doping. The development open the opportunities for efficient HTMs based on the two-dimensional materials in the perovskite solar cells.     

An ammonia modified PEDOT: PSS for interfacial engineering in inverted planar perovskite solar cells

Poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) is one of the most widely used hole transport layers (HTL) in inverted perovskite solar cells (PSCs) due to its simple solution-processed ability, high transparency, and conductivity. However, PEDOT:PSS-based devices suffer a lower open-circuit voltage (V_oc) than devices with the conventional structure. To address this issue, we fabricated ammonia-modified PEDOT:PSS films by simply doping PEDOT:PSS solution with different ratio of ammonia. The acidity of PEDOT:PSS can be neutralized by the doped ammonia, which inhibits the ion-exchange reaction between PSS-H and CH₃NH₃I, thus retarding the reduction of the work function for PEDOT:PSS to some extent. As a result, a superior power conversion efficiency (PCE) of 15.5% was obtained for the device based on the ammonia-doped PEDOT:PSS HTL than that of the pristine PEDOT:PSS-based device. We ascribe the PCE enhancement to the increased V_oc and fill factor (FF), which is attributed not only to the better energy-level alignment between the ammonia-modified PEDOT:PSS film and perovskite layer but also to the increased grain size and crystallinity of perovskite film.     

Solution-processed vanadium oxide thin film as the hole extraction layer for efficient hysteresis-free perovskite hybrid solar cells

In this study, we report a simple way to fabricate VO_x thin film from pure-water solution, as the hole extraction layer (HEL) for perovskite hybrid solar cells (pero-HSCs). Furthermore, an aminopropanoic acid (APPA) interfacial layer is used to modify VO_x thin film for reducing the charge carrier recombination rate. As a result, the pero-HSCs with the VO_x/APPA HEL exhibits better device performance than that of the pero-HSCs with the VO_x HEL and the pero-HSCs with poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate) HEL. Moreover, the pero-HSCs with the VO_x/APPA HEL exhibits hysteresis-free characteristics. All these results indicate that we report a simple approach to realize high performance of perovskite hybrid solar cells.     

High-performance hole transport layer based on WS2 doped PEDOT:PSS for organic solar cells

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All-solid solar cells with hybrid perovskite absorbers and graphene electron transport layers

This work reports the perovskite/titanium dioxide (TiO₂)heterojunction solid state solar cells (SSSCs) with a hole transport material (HTM) and graphene electron transport layer. The effects of a nanostructure CH₃NH₃PbI₃ perovskite thin film, the HTM, and graphene electron transport layer in SSSC structure were examined. The SSSCs prepared with the optimal parameter exhibited a short-circuit current density (J_SC), open-circuit voltage (V_OC), and power conversion efficiency (η) of 17.89 mA/cm², 0.89 V, and 6.91%, respectively. Obvious improvements in power conversion efficiency of the SSSCs were observed by using the HTM and graphene electron transport layer. The HTM and graphene thin films provide a great hole and electron transfer channel for the photogenerated carriers to external circuit, respectively.     

Copolymers based on thiazolothiazole-dithienosilole as hole-transporting materials for high efficient perovskite solar cells

The interlayers, including hole transporting layer (HTL) and electron transporting layer (ETL), segregating photoactive layer and the electrodes play an important role in charge extraction and transportation in perovskite solar cells (pero-SCs). Two novel copolymers, PDTSTTz and PDTSTTz-4, for the first time were applied as HTL in the n-i-p type pero-SCs, with the device structure of ITO/compact TiO₂/CH₃NH₃PbI_3-xCl_x/HTL/MoO₃/Ag. The highest occupied molecular orbitals (HOMO) levels of PDTSTTz and PDTSTTz-4 exhibit a suitable band alignment with the valence band edge of the perovskite. Both of them lead to improved device performances compared with reference pero-SCs based on P3HT as HTL. To further balance the charge extraction and the diffusion length of charge carriers, pristine C₆₀ was introduced at the cathode side of the pero-SCs, working together with TiO₂ as ETL. With insertion of both the HTL and ETL, the performance of pero-SCs was greatly enhanced. The optimized devices exhibited impressive PCEs of 14.4% and 15.8% for devices based on PDTSTTz and PDTSTTz-4. The improved performance is attributed to better light harvest ability, decreased interface resistance and faster decay time due to the introduction of the interlayers.     

Anti-solvent engineering via potassium bromide additive for highly efficient and stable perovskite solar cells

Anti-solvent assisted crystallization is commonly employed method to achieve high-quality perovskites attributed to its great operability. Herein, we report an anti-solvent engineering approach via simply using potassium bromide (KBr) additive with commonly used chlorobenzene (CB) in triple-cation perovskite solar cells (PSCs). We show that the KBr additive in the CB anti-solvent not only increases the crystallinity and passivates the perovskite top surface defects, but also leads to suppressed nonradiative recombination and facilitates charge extraction at interfaces. Interestingly, due to the halide vacancies filling with K⁺ ions, hysteresis behavior in the treated perovskite layer was suppressed. Consequently, a champion power conversion efficiency (PCE) of 18.29% was yielded for anti-solvent engineering employing KBr (an 20% improvement in PCE compared to the CB-only anti-solvent device). Furthermore, the optimized device based on KBr demonstrated improved stability, maintaining 80% of its original efficiency after aging in an environment with a relative humidity of 30–50% for 1080 h. Our study reports the significant role of anti-solvent engineering in improving perovskite's quality for efficient PSCs and develops the potential for PSC commercialization.     

Fused-ring electron acceptor as an efficient interfacial material for planar and flexible perovskite solar cells

Perovskite solar cell (PSC) has attracted great attention due to its high power conversion efficiency (PCE), low cost and solution processability. The well-designed interface and the modification of electron transport layer (ETL) are critical to the PCE and long-term stability of PSCs. In this article, a fused-ring electron acceptor is employed as the interfacial material between TiO₂ and the perovskite in rigid and flexible PSCs. The modification improves the surface of TiO₂, which decreases the defects of ETL surface. Moreover, the modified surface has lower hydrophilicity, and thus is beneficial to the growth of perovskite with large grain size and high quality. As a result, the interfacial charge transfer is promoted and the interfacial charge recombination can be suppressed. The highest PCE of 19.61% is achieved for the rigid PSCs after the introduction of ITIC, and the hysteresis effect is significantly reduced. Flexible PSC with ITIC obtains a PCE of 14.87%, and the device stability is greatly improved. This study provides an efficient candidate as the interfacial modifier for PSCs, which is compatible with low-temperature solution process and has a great practical potential for the commercialization of PSCs.