Simple fabrication of perovskite solar cells using lead acetate as lead source at low temperature

We reported a simple one-step, low-temperature solution-processed technique to fabricate perovskite solar cells using lead acetate as the lead source. Solvent annealing was applied for grain growth to obtain better morphology. Uniform perovskite films without pinholes can be obtained by solvent annealing for 5 min at 100 °C. Planar perovskite solar cells based on the high quality perovskite films deliver power conversion efficiency up to 12.71% with negligible hysteresis and good reproducibility. In addition, the substrate surfaces have little effect on the crystallization of perovskite when lead acetate was used, leading to uniform films on different substrates, which can provide a wide choice of substrates and interfacial materials.     

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.     

Efficient electron-blocking layer-free planar heterojunction perovskite solar cells with a high open-circuit voltage

Perovskite solar cells (PSCs) with a simple device structure are particularly attractive due to their low cost and convenient fabrication process. Herein, highly efficient, electron-blocking layer (EBL)-free planar heterojunction (PHJ) PSCs with a structure of ITO/CH₃NH₃PbI₃/PCBM/Al were fabricated via low-temperature, solution-processed method. The power conversion efficiency (PCE) of over 11% was achieved in EBL-free PHJ-PSCs, which is closed to the value of PSC devices with the PEDOT:PSS as the EBL. It is impressed that the open-circuit voltage (V_oc) up to 1.06 V, an average value of 1.0 V for 43 devices, was obtained in EBL-free PHJ-PSCs. The electrochemical impedance spectroscopy (EIS) results suggested that the high PCE and V_oc are attributed to the relatively large recombination resistance and low contact resistance in EBL-free PHJ-PSCs. The solution-processed, EBL-free PHJ structure paves a boulevard for fabricating high-efficiency and low-cost PSCs.     

Efficient planar heterojunction perovskite solar cells fabricated by in-situ thermal-annealing doctor blading in ambient condition

The in-situ thermal-annealing doctor blading was developed to fabricate high-quality perovskite CH₃NH₃PbI₃ thin film and efficient planar heterojunction perovskite solar cells (PHJ-PSCs) in ambient condition with humidity of ∼45%. The morphology of CH₃NH₃PbI₃ thin film fabricated by in-situ thermal-annealing doctor blading varied from random nanowires to oriented domains as increasing the substrate temperature, and the domain size became larger and larger with increasing substrate temperature. The PHJ-PSCs with a structure of ITO/PEDOT:PSS/CH₃NH₃PbI₃/PCBM/Ag was fabricated based on in-situ thermal-annealing doctor-bladed CH₃NH₃PbI₃ thin film in ambient condition, resulting in the power conversion efficiency up to 11.29% without obvious hysteresis under different scanning directions and speeds. The performance is comparable to that of PHJ-PSCs fabricated by spin-coating deposition in glovebox with the same structure. The research results suggested that efficient PHJ-PSCs could be fabricated by large-scale in-situ thermal-annealing doctor blading in ambient condition, which is matchable with large-scale, roll-to-roll process and shows potential application in industrial production.     

Efficient and hysteresis-less pseudo-planar heterojunction perovskite solar cells fabricated by a facile and solution-saving one-step dip-coating method

Rough dense sol-gel-derived titanium dioxide (TiO₂) electron-transport layers (ETLs) and smooth organolead halide perovskite (PVK) films for pseudo-planar heterojunction perovskite solar cells (P-PH PVKSCs) were fabricated by a facile one-step dip-coating method. The highly compact TiO₂ ETLs and uniform PVK films endow the device a high power conversion efficiency (PCE) of over 11%, which was nearly identical to that of a reference device (12%) fabricated by conventional spin-coating. Furthermore, the device showed no pronounced hysteresis when tested by scanning the voltage in a forward and backward direction, showing the potential of facile and waste-free dip-coating in replacing of spin-coating for large area perovskite solar cells preparation. Lastly, the hysteresis was compared and discussed and models regarding the abnormal hysteresis, roll-over and current peak phenomena were proposed as well.     

High-quality CH3NH3PbI3 thin film fabricated via intramolecular exchange for efficient planar heterojunction perovskite solar cells

The high-quality CH₃NH₃PbI₃ perovskite thin film with excellent coverage and uniformity was prepared using an intramolecular exchange technology via a low-temperature, two-step sequential deposition process. The PbI₂(DMSO) complex was synthesized at room temperature without any additives and was deposited, then the CH₃NH₃I solution was deposited subsequently. The further controllable thermal annealing process resulted in the complete formation of flat and uniform CH₃NH₃PbI₃ thin film with large-size grains and (110) preferred crystallographic orientation. The perovskite solar cells (PSCs) with a very simple inverted planar heterojunction structure of ITO/PEDOT:PSS/CH₃NH₃PbI₃/PCBM/Al and without other buffer layers, e.g., C₆₀, LiF, BCP, etc., were fabricated, resulting in a power conversion efficiency (PCE) as high as 14.26%. The results suggest that the low-temperature, two-step sequential deposition process with intramolecular exchange technology provides a good route to fabricate high-quality perovskite thin film and efficient PSCs, which would match with large-scale, high-output roll-to-roll (R2R) printing/coating techniques.     

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.     

In-situ synthesis of metal nanoparticle-polymer composites and their application as efficient interfacial materials for both polymer and planar heterojunction perovskite solar cells

A new approach for the synthesis of gold nanoparticles (Au NPs) via a simple and fast in-situ generation method using an amine-containing polymer (PN4N) as both stabilizer and reducing agent is reported. The application of the Au NPs-PN4N hybrid material as efficient interfacial layer in different types of solar cells was also explored. The synthesized Au NPs show good uniformity in size and shape and the Au NPs doped PN4N hybrid composites exhibit high stability. Amine-containing polymers are good cathode interfacial materials (CIMs) in polymer solar cells (PSCs) and planar heterojunction perovskite solar cells (PVKSCs). The performance of the PSCs with Au NPs doped PN4N CIMs is largely improved when compares to devices with pristine PN4N CIM due to the enhanced electronic properties of the doped PN4N. Furthermore, by incorporating larger Au NPs into PEDOT:PSS to enhance absorption of the light harvesting layer, power conversion efficiencies (PCEs) of 6.82% and 13.7% are achieved for PSC with PCDTBT/PC₇₁BM as the light harvesting materials and PVKSC with a ∼280 nm-thick CH₃NH₃PbI_3−xCl_x perovskite layer, respectively. These results indicate that Au NPs doped into both PEDOT:PSS and PN4N interlayers exhibited a synergistic effect in performance improvement of PSCs and PVKSCs.     

Efficient and non-hysteresis CH3NH3PbI3/PCBM planar heterojunction solar cells

Highly efficient and non-hysteresis organic/perovskite planar heterojunction solar cells was fabricated by low-temperature, solution-processed method with a structure of ITO/PEDOT:PSS/CH₃NH₃PbI₃/PCBM/Al. The high-quality perovskite thin film was obtained using a solvent-induced-fast-crystallization deposition involving spin-coating the CH₃NH₃PbI₃ solution followed by top-dropping chlorobenzene with an accurate control to induce the crystallization, which results in highly crystalline, pinhole-free, and smooth perovskite thin film. Furthermore, it was found that the molar ratio of CH₃NH₃I to PbI₂ greatly influence the properties of CH₃NH₃PbI₃ film and the device performance. The equimolar or excess PbI₂ was facile to form a flat CH₃NH₃PbI₃ film and produced relatively uniform perovskite crystals. Perovskite solar cells (PSCs) with high-quality CH₃NH₃PbI₃ thin film showed good performance and excellent repeatability. The power conversion efficiency (PCE) up to 13.49% was achieved, which is one of the highest PCEs obtained for low-temperature, solution-processed planar perovskite solar cells based on the structure ITO/PEDOT:PSS/CH₃NH₃PbI₃/PC₆₁BM/Al. More importantly, PSCs fabricated using this method didn’t show obvious hysteresis under different scan direction and speed.     

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%.     

Efficient perovskite solar cells using trichlorosilanes as perovskite/PCBM interface modifiers

Interfaces are crucial for high-performance perovskite solar cells. Here, phenyltrichlorosilane (PTS) and octadecyltrichlorosilane (OTS) were used to modify the interface between perovskite layer and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) layer in an inverted layered perovskite device. Such treatments facilitated the formation of a high-quality PCBM film and effectively decreased the density of surface traps that induce undesirable electron-hole recombination. As a result, the average power conversion efficiency of PTS (and OTS) modified devices was improved from 9.60% to 11.96% (and 11.08%), with a highest value of 12.63% (and 11.87%). Therefore, this study provides an attractive mothed to improve the quality of PCBM film on top of perovskite layer and finally the performance of inverted perovskite solar cells.     

Cathode modification with solution-processed hybrid electron extraction layer for improved charge collection of planar heterojunction perovskite solar cells

Optimal interface modification of perovskite solar cells is critical to achieve efficient and balanced charge transport and collection. We herein demonstrated that a solution-processed hybrid cathode interfacial layer composed of polyetyleneimine ethoxylate and a lithium quinolate complex improves photovoltaic performance of the planar heterojunction perovskite solar cells. The hybrid cathode modifier effectively lowered work function of ITO cathode, which afforded efficient electron transport and collection at ITO. Furthermore, surface roughness of ITO was significantly decreased, leading to enlarged grain size in densely-packed perovskite thin film. Consequently, the perovskite solar cells with hybrid electron extraction layer generated maximum power conversion efficiency up 15.21%, which is 25% improved value than that without hybrid electron extraction layer. Furthermore, highly-flexible flexible devices with a hybrid electron extraction layer exhibited a promising efficiency of 14.41%, demonstrating its potential for high performance perovskite solar cells.     

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.     

Low temperature fabrication of formamidinium based perovskite solar cells with enhanced performance by chlorine incorporation

Recently formamidinium (FA) based perovskite solar cell was demonstrated to show high performance and better stability upon partial substitution of FA with Cs cation. However, the fabrication of device required high-temperature processing on TiO₂ electrode and thus limits the use of flexible polymeric substrates. Here, we present a low temperature approach for the fabrication of p-i-n perovskite solar cells based on Cs_0.15FA_0.85PbI₃. Furthermore, we investigated the effects of chlorine on the morphology and crystallinity of the perovskite films and the corresponding photovoltaic performance. Chlorine incorporation can significantly enlarge the size of grains and improve the crystallinity of perovskite films with full surface coverage. A best power conversion efficiency of 14.5% was realized for planar perovskite solar cells with negligible hysteresis and remarkable reproducibility.     

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.     

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.     

Grain structure control and greatly enhanced carrier transport by CH3NH3PbCl3 interlayer in two-step solution processed planar perovskite solar cells

We report that the use of a CH₃NH₃PbCl₃ interlayer onto the PEDOT:PSS layer in the two-step solution deposition of CH₃NH₃Pbl₃ for planar p-i-n type perovskite solar cells (PSCs) can lead to a dramatic enhancement of short-circuit current density (J_sc) by 52.8% from 13.07 mA cm⁻² to 19.98 mA cm⁻². While the absorption and thus the composition of the perovskite layers remain unchanged, Incident photon-to-current efficiency (IPCE) measurement results reveal much enhanced carrier transport, which in turn can be correlated to the larger and more columnar grain structure in the perovskite layer with the use of the CH₃NH₃PbCl₃ interlayer. On the other hand, the two-step solution processed perovskite layers without the CH₃NH₃PbCl₃ interlayer exhibit smaller and more cross-hatching grain structure and yield significantly smaller J_sc. Therefore our results revealed clearly that the insertion of CH₃NH₃PbCl₃ interlayer, which affects the nucleation dynamics, may control the grain structure of the two-step solution processed perovskite layers and improve dramatically the photovoltaic performance of the resultant planar p-i-n type PSCs. Our CH₃NH₃PbCl₃ interlayer may thus serve as an effective method for p-i-n PSCs to achieve high J_sc with thicker perovskite layer.     

Bottom-contact passivation for high-performance perovskite solar cells using TaCl5-doped SnO2 as electron-transporting layer

Organic-inorganic hybrid perovskite solar cells (PSCs) possess the promising potential to substitute photovoltaic technologies in the traditional model. The modified SnO₂ as an electron-transporting layer (ETL) has been studied extensively because of its excellent properties. Herein, we implemented the TaCl₅ doped SnO₂ ETL in the n-i-p structure perovskite solar cells. The doped SnO₂ solution was demonstrated the characterization of neutral power of value and hydrophobicity. The conduction band of changed ETLs shifted downward by 0.26 eV resulting in the efficient electron transfer. Furthermore, the doped SnO₂ films affect the perovskite crystallite size with passivated traps and reduced nonradiative recombination loss. After employing TaCl₅-doped SnO₂ ETL, the open-circuit voltage enhances from 0.97 to 1.08 V and a united power conversion efficiency increases from 16.38% to 18.23% achieved when adopted 1.0 wt% doped TaCl₅–SnO₂ TEL. The developed doping method provides an effective method to passivate SnO₂ for fabricating high-performance perovskite solar cells.     

Enhancing current density of perovskite solar cells using TiO2-ZrO2 composite scaffold layer

A novel scaffold layer composed of TiO₂-ZrO₂ composite was fabricated for perovskite solar cell. Compared with pure TiO₂ nanoparticles (NPs), the relatively larger ZrO₂ NPs could increase film roughness and enhance light-scattering effect in TiO₂-ZrO₂ composite films. The device exhibited outstanding power conversion efficiency (PCE) of 11.41%. The morphology and aggregation of particles, three-dimensional roughness, as well as the ingredient and micro-structure of FTO/compact TiO₂/TiO₂-ZrO₂ was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscope (AFM), energy dispersive spectrometer (EDS), and X-ray diffraction (XRD), respectively. Moreover, the optical property of TiO₂-ZrO₂ films for visible light was characterized by UV–visible absorption spectroscopy (UV–vis), and its influence on quantum yield of the device was further demonstrated by incident photon-to-electron conversion efficiency (IPCE). Owing to the inert oxide, the short-circuit current density of perovskite solar cell using TiO₂-ZrO₂ composition as scaffold layer increased by 21% compared to the one employing pure TiO₂ mesoporous film.     

Synergistic effect of MACl and DMF towards efficient perovskite solar cells

The performance of perovskite solar cells (PSCs) is extremely dependent on morphology and crystallinity of perovskite film. One of the most effective methods to achieve high performance perovskite solar cells has been to introduce additives that serve as dopants, crystallization or passivation agents. Herein a facile strategy by introducing methylammonium chloride (MACl) and polar solvent N,N-Dimethylformamide (DMF) as co-additives in two-step sequential method is proposed to realize high quality perovskite film. It is demonstrated that DMF facilitates methylammonium iodide (MAI) penetrating easily into PbI₂ layer to form highly crystalized perovskite film with uniform morphology which is essential to achieve high V_OC. While MACl induces MAPbI₃ to crystallize in a pure α-phase and suppress non-photovoltaic phase, which guarantees high FF. Pure α-phase perovskite film with uniform morphology can be achieved by adopting MACl and DMF together and the corresponding solar cell illustrates a power conversion efficiency (PCE) of 19.02% with substantially promoted durability. Moreover, A V_OC as high as 1.181 V is succeeded for MAPbI₃ based solar cell benefiting from the synergistic effect.