Natural passivation of the perovskite layer by oxygen in ambient air to improve the efficiency and stability of perovskite solar cells simultaneously

For the hybrid organic-inorganic halide perovskite (HHPs) material, oxygen passivation could enhance the photoluminescence intensity, while it also deteriorates the stability of HHPs film or device. It is a challenge how to use oxygen passivation to improve the efficiency and stability of the HHPs solar cells simultaneously. Here we reported a novel and simple method for natural passivation of the perovskite layer with oxygen in ambient air by exclusion of light illumination. The HHPs solar cells were fabricated by deposition of hole transport layer after oxygen passivation. By optimizing the passivation conditions, the power conversion efficiency of the solar cells increases to 20.1% from 17.3% for those without passivation. Meanwhile, the stability of the HHPs solar cells with passivation was improved. This work is the first report applying oxygen passivation to improve the efficiency and stability of HHPs solar cells simultaneously.     

Self-encapsulated semi-transparent perovskite solar cells with water-soaked stability and metal-free electrode

Semi-transparent and self-encapsulated perovskite solar cells could be fabricated by simply laminating the front sub-cell (ITO/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/CH₃NH₃PbI₃) and back sub-cell (FTO/compact-TiO₂/mesoporous-TiO₂/CH₃NH₃PbI₃), without vacuum-evaporating metal electrode. The addition of chlorobenzene (CB) between two perovskite layers accelerated perovskite crystals interfusion and close interfacial contact from two separated sub-cells, which contributed to perovskite film with high crystallinity and light absorption in laminated cells. The self-encapsulated perovskite solar cell (device area of 0.39 cm²) with CB treatment not only showed power conversion efficiency of 6.9%, but also existed excellent stability even if soaking in water for 24 h. This novel approach to fabricate semi-transparent, solution-processible, cost-effective and high-stable perovskite solar cells may provide a reliable royal road for realizing commercial application in exterior building window, with the combination of large-area roll-to-roll printing technique, etc.     

Improved morphology and enhanced stability via solvent engineering for planar heterojunction perovskite solar cells

Solvent engineering technique for planar heterojunction perovskite solar cells is an efficient way to achieve uniformly controlled grain morphology for perovskite films. In this report, diethyl ether solvent engineering technique was used for Methyl ammonium lead triiodide (CH₃NH₃PbI₃) perovskite thin films for planar heterojunction solar cells which exhibited a PCE of 9.20%. Morphological improvements and enhanced grain sizes leads to enhanced absorption of CH₃NH₃PbI₃. Moreover solar cells have showed an excellent environmental stability of more than 100 days. This increase in efficiency is due to improved film morphology of perovskite layer after solvent treatment which has been revealed under UV–Vis spectroscopy, SEM images, X-ray diffraction and impedance spectroscopy.     

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.     

Synthesis and photovoltaic performance of a non-fullerene acceptor comprising siloxane-terminated alkoxyl side chain

As an effective molecular modification strategy, side chain engineering has been widely used in promoting the photovoltaic performance of non-fullerene acceptors. Herein, a novel non-fullerene small molecular acceptor i-IEOSi-4F comprising siloxane-terminated alkoxyl side chain was successfully designed and synthesized. The molecule shows an optical band gap of 1.53 eV, with large extinction coefficient of 2.36 × 10⁵ M⁻¹ cm⁻¹ in solution. Two fluorobenzotriazole based polymers J52 and PBZ-2Si with the same backbone units but different side chains were employed as the donor to construct the active layers that all can demonstrate suitable energy levels and complementary absorptions with i-IEOSi-4F. Relative to J52 only bearing alkyl side chain, PBZ-2Si with siloxane-terminated side chain could induce more balanced carrier transports and more favorable morphology, leading to a higher power conversion efficiency (PCE) of 12.66% with a good fill factor of 71.45%. The efficiency is 21% higher than that of 10.46% for the J52 based devices. Our results not only indicate that siloxane-terminated alkoxyl side chain is valuable for efficient non-fullerene acceptors, but also demonstrate that siloxane-terminated side chain on both polymer donor and small molecular acceptor is a useful combination to realize more efficient polymer solar cells.     

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.     

Side chain engineering: The effect on the properties of isoindigo-based conjugated polymers contain different length and structure alkyl chains on nitrogen atom

A series of donor-acceptor (D-A) conjugated polymers based on benzo[1,2-b:4,5-b’]dithiophene (BDT) and isoindigo with different alkyl side chains were designed and synthesized. These polymers named PBDT-TT-IID_O, PBDT-TT-IID_EH, PBDT-TT-IID_BO, PBDT-TT-IID_HD, and PBDT-TT-IID_OD have different length or structure of the alkyl side chains on isoindigo unit. As the length of the alkyl chain increases, the solubility of the polymer enhances. The results indicate that the alkyl side chains have little effect on the optical properties of individual polymer molecules, but have a significant impact on optical, electrochemical, film-forming and photovoltaic properties of the polymers in the aggregated state. PBDT-TT-IID_HD with a sixteen carbon branched chain achieves the best power conversion efficiencies (PCEs) of 6.83% when blending with [6,6]-phenyl C71 butyric acid methyl ester (PC₇₁BM) as acceptor. The influence of the alkyl side chain on the short-circuit current is much greater than that of the open circuit voltage. Atomic force microscopy (AFM) reveals that the side chains changed the morphology of the active layers and the size of the phase region. For isoindigo based polymers, hexyl-decyl group in the commonly used alkyl chains appear to be a good choice.     

Synthesis and characterization of the conjugated polymers tethered with dipolar side chains containing a benzothiadiazole entity for bulk heterojunction solar cells

New conjugated copolymers (P1?P3) containing dipolar side chains connected to the main chain via triphenylamine donors have been synthesized and characterized. The side chains of these polymers have an electron deficient benzothiadiazole moiety in the spacer, but with different acceptors at the end. By changing the acceptor moieties of the side chain, the absorption spectra and HOMO/LUMO gaps of the polymers can be fine-tuned, ranging from 1.86 to 1.59 eV. Solution processed bulk heterojunction (BHJ) solar cells using these polymers as the donor and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as the acceptor were fabricated and measured under 100 mW cm^?2 of AM 1.5 illumination. The cell based on the blend of P1/PCBM (1:1, w/w) exhibited the highest power conversion efficiency of 1.78%, with open circuit voltage (V_oc) = 0.79 V, short circuit current (J_sc) = 6.63 mA cm^?2 and fill factor (FF) = 0.34, respectively.     

Enhanced efficiency and stability of polymer solar cells with TiO2 nanoparticles buffer layer

TiO₂ sols synthesized with a facile solution-based method were used as a buffer layer between the active layer and the cathode Al in conventional structure polymer solar cells (PSCs). Using transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray diffraction (XRD) and atomic force microscopy (AFM), the morphological and crystallographic properties of synthesized TiO₂ nanoparticles (TiO₂ NPs) as well as the buffer layer were studied in detail. It was observed that by increasing H₂O in the process of peptization both the crystallinity and particle size of TiO₂ NPs were enhanced, while the particles in sol showed a narrower size distribution conformed by dynamic light scattering. Inserting TiO₂ NPs as a buffer layer in conventional structure PSCs, both the power conversion efficiency (PCE) and stability were improved dramatically. PSCs based on the structure of ITO/PEDOT:PSS/P3HT:PCBM/TiO₂ NPs/Al showed the short-circuit current (J_sc) of 12.83 mA/cm² and the PCE of 4.24%, which were improved by 31% and 37%, respectively comparing with the reference devices without a TiO₂ buffer layer. The stability measurement showed that PSC devices with a TiO₂ NPs buffer layer could retain 80% of the original PCEs after exposed in air for 200 h, much better than the devices without such a buffer layer. The effect can be attributed to the protection by the buffer layer against oxygen and H₂O diffusion into the active layers. The observations indicate that TiO₂ NPs synthesized by facile solution-based method have great potential applications in PSCs, especially for large-area printed PSCs.     

Crystallization process of perovskite modified by adding lead acetate in precursor solution for better morphology and higher device efficiency

The power conversion efficiency of 15.20% is achieved in this study for planar perovskite solar cells fabricated in air from one-step spin-coating lead chloride (PbCl₂) based precursor modified by additional adding 1% lead acetate (PbAc₂), much higher than the reference one from pure PbCl₂ precursor without modification. A higher quality perovskite film with increased coverage is the reason for this improvement. The perovskite nucleation rate and start time of nucleation are key parameters of perovskite crystallization kinetics. By adding 1% PbAc₂ to the precursor, the density of perovskite crystal nucleuses is optimized to achieve the best film and then the highest device performance.     

Ternary organic solar cells with improved efficiency and stability enabled by compatible dual-acceptor strategy

To further elevate the power conversion efficiency (PCE) of organic solar cells (OSCs), ternary strategy is one of the most efficient methods via simply incorporating a suitable third component. Here, a nonfullerene small molecule acceptor MOITIC was incorporated into the state-of-art PM6:Y6 binary system to further enhance the photovoltaic performance. Detailed investigation revealed that MOITIC exhibited a good miscibility and compatibility with Y6, forming alloy-like acceptors in the ternary blends. The alloy-like phase promoted the phase separation and optimized the morphology of ternary blend, which afforded higher and more balanced carrier mobility and reduced charge recombination in devices. Moreover, the larger energy offset between PM6 and MOITIC:Y6 acceptor alloy is beneficial to enhance open-circuit voltage (V_oc) of corresponding devices. As a consequence, the optimized ternary OSC (PM6:Y6:MOITIC = 1:1:0.1) showed a significantly increased PCE of 17.1% with simultaneously enhanced V_oc of 0.882 V, short-circuit current density (J_sc) of 25.6 mA cm⁻², and fill factor (FF) of 75.7%, which has about 9% enhancement compared to the control binary PM6:Y6 (15.7%). In addition, the optimized ternary device exhibited better stability. This work indicates that ternary strategy via combining two compatible small molecule acceptors is effective to simultaneously improve the efficiency and stability of OSCs.     

胆甾液晶应用于P3HT∶PCBM聚合物光伏器件研究

本研究利用液晶材料的自组装特性,将胆甾醇油酸酯3β-Hydroxy-5-cholestene 3-oleate掺杂有机聚合物太阳能电池活性层P3HT:PC61BM内,制备出不同掺杂比例的光伏器件。实验结果表明,液晶掺杂质量比为0.3%时,器件的光电转换效率最高。说明液晶分子可诱导活性层材料分子在结晶过程中有序排列,减少层内分子团簇,减少活性层薄膜缺陷,形成有效的载流子传输通道。适当的掺杂比例,增大了器件并联电阻和填充因子,器件性能得到改善。     

A rapid annealing technique for efficient perovskite solar cells fabricated in air condition under high humidity

A rapid annealing technique for fabricating perovskite materials via microwave radiation in air condition is presented. A planar-heterojunction perovskite device via microwave radiation within 6 min exhibits an efficiency of 10.29%, compared to 11.08% for a 90 min heating-annealed device in inert atmosphere, which is higher than that (8.04%) of a heating-annealed device in air condition under high humidity (∼60%). We believe that the microwave annealing technique provides a fast and less energy-intensive process for fabricating ideal perovskite active layers for high performance solar cells.     

Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate films with low conductivity and low acidity through a treatment of their solutions with probe ultrasonication and their application as hole transport layer in polymer solar cells and perovskite solar cells

Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) has been widely used as the hole transport material in optoelectronic devices. To avoid the cross talk among different crossbars, PEDOT:PSS with low conductivity is required. It thus has a high loading of the non-conductive PSSH. The PSSH-to-PEDOT weight ratio is 6 for Clevios P VP Al 4083 that is the most popular polymer as the hole transport layer. However, the acidic PSSH brings severe problems to the device stability and performance. Here, PEDOT:PSS solutions with low acidity can be prepared through a facile treatment of PEDOT:PSS solution by probe ultrasonication. Two grades of PEDOT:PSS, Clevios PH1000 and Clevios P, with a PSSH-to-PEDOT weight ratio of 2.5 were treated by probe ultrasonication. The ultrasonication can lower the viscosity and the colloidal sizes of PEDOT:PSS solutions and conductivity of PEDOT:PSS films. The pH value of probe-ultrasonicated Clevios P was 2.12, higher than that (1.77) of pristine Clevios P VP Al 4083. The ultrasonication-treated PEDOT:PSS solutions were used as hole transport layer in polymer solar cells and perovskite solar cells. The photovoltaic performances of these solar cells are comparable to that of control devices employing Clevios P VP Al 4083 PEDOT:PSS as the hole transport layer.     

A low-cost and low-temperature processable zinc oxide-polyethylenimine (ZnO:PEI) nano-composite as cathode buffer layer for organic and perovskite solar cells

Nanocomposite buffer layer based on metal oxide and polymer is merging as a novel buffer layer for organic solar cells, which combines the high charge carrier mobility of metal oxide and good film formation properties of polymer. In this work, a nanocomposite of zinc oxide and a commercialized available polyethylenimine (PEI) was developed and used as the cathode buffer layer (CBL) for the inverted organic solar cells and p-i-n heterojunction perovskite solar cells. The cooperation of PEI in nano ZnO offers a good film forming ability of the composite material, which is an advantage in device fabrication. In addition, power conversion efficiency (PCE) of the ZnO:PEI CBL based device was also improved when compared to that of ZnO-only and PEI-only devices. The highest PCE of P3HT:PC₆₁BM and PTB7-Th:PC₆₁BM devices reached to 3.57% and 8.16%, respectively. More importantly, there is no obvious device performance loss with the increase of the layer thickness of ZnO:PEI CBL to 60 nm in organic solar cells, which is in contrast to the PEI based devices, whose device performance decreases dramatically when the PEI layer thickness is higher than 6 nm. Such a nano composite material is also applicable in inverted heterojunction perovskite solar cells. A PCE of 11.76% was achieved for the perovskite solar cell with a thick ZnO:PEI CBL (150 nm) CBL, which is around 1.71% higher than that of the reference cell without CBL, or with ZnO CBL. In addition, stability of the organic and perovskite solar cells having ZnO:PEI CBL was also found to be improved in comparison with that of PEI based device.     

Highly conductive PEDOT:PSS film by doping p-toluenesulfonic acid and post-treatment with dimethyl sulfoxide for ITO-free polymer dispersed liquid crystal device

In this paper, the highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) film realized by applying the doping technique and the post-treatment process is demonstrated. The conductivity of the spin coated PEDOT:PSS film enhanced greatly from 0.7 S/cm to 736 S/cm after 1.25% of p-toluenesulfonic acid solution (50 wt%) was doped into the PEDOT:PSS aqueous dispersion. The post-treatment using dimethyl sulfoxide further improved the conductivity to 1549 S/cm. The highly conductive PEDOT:PSS film was used as transparent electrode to fabricate ITO-free polymer dispersed liquid crystal (PDLC) cell. The experimental results showed that the electro-optical properties of the PDLC cell fabricated by the highly conductive PEDOT:PSS film were comparable to those of the PDLC cell constructed by ITO. This study reveals that the highly conductive PEDOT:PSS film is a prospective material for manufacturing ITO-free liquid crystal devices.     

A host material containing tetraphenylsilane for phosphorescent OLEDs with high efficiency and operational stability

A host material containing tetraphenylsilane moiety, 9-(4-triphenylsilanyl-(1,1′,4,1′′)-terphenyl-4′′-yl)-9H-carbazole (TSTC), was synthesized for green phosphorescent organic light emitting diodes. The tetraphenylsilane moiety was introduced to provide high triplet energy level, thermal and chemical stability, and glassy properties leading to high efficiency and operational stability of the devices. Ir(ppy)₃ based OLEDs using the TSTC host and DTBT (2,4-diphenyl-6-(4′-triphenylsilanyl-biphenyl-4-yl)-1,3,5-triazine) hole blocking layer (HBL) resulted in the maximum external quantum efficiency of 19.8% and the power efficiency of 59.4 lm/W. High operational stability with a half lifetime of 160,000 h at an initial luminance of 100 cd/m² was achieved from an electrophosphorescent device using TSTC host and BAlq HBL.     

A methylammonium iodide healing method for CH3NH3PbI3 perovskite solar cells with high fill factor over 80%

Repressing the thermal decomposition during the process of heat treatment plays an indispensable part in the preparation of perovskite films. Here, a methylammonium iodide healing method was applied to prevent the volatilization of the organic component inside the perovskite structure during the heat treatment. High-quality CH₃NH₃PbI₃ film with a much larger grain size over 800 nm was successfully fabricated via this healing method. Besides, the absorption and photoluminescence intensity were also both improved. Finally, the best power conversion efficiency of 18.89% with a fill factor over 80% was realized in an n–i–p configuration while possessing outstanding stability. This work suggests that methylammonium iodide healing method is a reliable way to promote crystal growth and improve the photovoltaic performance and humidity stability of the CH₃NH₃PbI₃ solar cells.     

Acetylene-bridged D–A–D type small molecule comprising pyrene and diketopyrrolopyrrole for high efficiency organic solar cells

To explore effects of acetylene-incorporation, acetylene-bridged D–A–D type small molecules ((HD/OD)-DPP-A-PY) using pyrene as a donor and diketopyrrolopyrrole as an acceptor were successfully synthesized and characterized. (HD/OD)-DPP-A-PY exhibited planar back-bone, conjugation extension, enhanced light absorption, and low HOMO energy level. Combined with the advanced properties, solution-processed OSCs based on a blend of HD-DPP-A-PY as a donor and [6,6]-phenyl-C₇₁-butyric-acid-methyl-ester (PC₇₀BM) as an acceptor exhibited PCEs as high as 3.15%.     

Silicon germanium active layer with graded band gap and µc-Si:H buffer layer for high efficiency thin film solar cells

We investigated solar cells with graded band gap hydrogenated amorphous silicon germanium active layer and hydrogenated microcrystalline silicon buffer layer at the interface of intrinsic and n-type doped layer. A significantly improved, 10.4% device efficiency was observed in this type of single junction solar cell. The intrinsic type microcrystalline silicon buffer layer is thought to play dual roles in the device; as a crystalline seed-layer for growth of n-type hydrogenated microcrystalline silicon layer and helping efficient electron collection across the i/n interface. Based on these, an enhancement in cell parameters such as the open-circuit voltage (V_oc), and fill factor (FF) was observed, where the FF and V_oc reaches up to 69% and 0.85 V respectively. Our investigation shows a simple way to improve device performance with narrow-gap silicon germanium active layer in solar cells in comparison to the conventionally constant band gap device structure.