Low temperature fabrication of hybrid solar cells using co-sensitizer of perovskite and lead sulfide nanoparticles

Our cost-effective approach for hybridizing methylammonium lead iodide and PbS nanoparticles at low temperature (≤100 °C) for photovoltaic devices is introduced. As employed into a perovskite based solar cell platform, effects of PbS on the device performance were investigated. Through experimental observations under simulated air-mass 1.5G illumination (irradiation intensity of 100 mWcm⁻²), the efficiency of a perovskite:PbS device is 11% higher than that of a pristine perovskite solar cell under the same fabrication conditions as a result of the broadened absorption range in the infrared region. The highest photovoltaic performance was observed at a PbS concentration of 2% with an open-circuit voltage, short-circuit current density, fill factor, and power-conversion efficiency of 0.557 V, 22.841 mA cm⁻², 0.55, and 6.99%, respectively. Furthermore, PbS NPs could induce hydrophobic modification of the perovskite surface, leading to an improvement of the device stability in the air. Finally, the low-temperature and cost-effective fabrication process of the hybrid solar cells is a good premise for developing flexible/stretchable cells as well as future optoelectronic devices.     

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.     

Efficient lead acetate sourced planar heterojunction perovskite solar cells with enhanced substrate coverage via one-step spin-coating

In planar heterojunction (PHJ) perovskite solar cells (PerSCs) without mesoporous metal oxide skeleton, there is challenge of formation perovskite film with full coverage to the conductive substrate through solution-process the lead halide precursors. Selecting a lead source with more volatile byproducts is an effective approach to obtain much smoother films with smaller and fewer pinholes. Herein, we demonstrate efficient CH₃NH₃PbI₃/PCBM PHJ PerSCs by using lead acetate (Pb(Ac)₂) as lead precursor. The morphology of the perovskite thin films were investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively, and the crystalline quality of the perovskite films were investigated by X-ray diffraction (XRD) spectroscopy. Time-resolved photoluminescence (TRPL) was used to investigate the PL lifetime of the perovskite film. The perovskite film derived from Pb(Ac)₂ shows enhanced surface coverage and improved photoluminescence lifetime in comparison with PbI₂ sourced perovskite film. Averaged over 20 individual devices, the power conversion efficiency (PCE) of devices derived from Pb(Ac)₂ reaches 14.81%, much higher than PbI₂ sourced devices by one-step (8.23%) or two-step (10.58%) spin-coating.     

Influence of TiO2 compact layer precursor on the performance of perovskite solar cells

The optimization of the hole-blocking layer in perovskite solar cells (PSC), typically based on TiO₂, is crucial, as it strongly affects the device performance. In this work, we thoroughly characterize the thickness, roughness, and crystal structure of a set of TiO₂ compact layers produced by spin coating of different precursor sols and correlate the choice of the TiO₂ precursor to the photovoltaic performance of the PSC. By replacing the commonly used titanium isopropoxide (TTIP) blocking layer precursor with titanium tetrachloride (TiCl₄), a clear enhancement in the PSC performance was observed, particularly in the hysteresis behavior and stability. The results from the morphological/structural analysis and transient photoluminescence studies clarify the different behavior of the compact layers in PSCs.     

Bifunctional-based small molecule as multifunctional additive for improved performance of perovskite solar cells

Hybrid perovskites have great potential as light-absorbing materials, while its degradation of poor crystallization and ion migration have been the major obstacle in perovskite solar cells (PSCs). Herein, the bifunctional-based small molecule dicyandiamide (DCD) was applied as additive in the PSCs. The amino and cyano group of DCD could effectively control crystallization and passivating grain defects, resulting in the high-quality perovskite films with large grain size. In addition, the adding of DCD increases the film conductivity of perovskite active layer, which is beneficial for charge transport in perovskite film. The DCD added PSCs shows an optimal power conversion efficiency (PCE) of 19.08% with negligible hysteresis. Furthermore, the long-term stability of PSCs is significantly enhanced. The results indicate that the device's integrative performance could be efficiently improved by the synergistic effect of amino and cyano functional groups, meaning that the addition of DCD into perovskite precursor could enable this optimization.     

Tuning the work function of indium-tin-oxide electrodes for low-temperature-processed,titanium-oxide-free perovskite solar cells

In this work, low-temperature-processed, titanium-oxide-free perovskite solar cells (PSCs) with power conversion efficiency (12.2%) comparable to that of titanium-oxide-based PSCs were fabricated. The fabricated PSCs exhibited a small hysteresis owing to the two-step preparation method employed for perovskite films. A poly [(9,9-bis(3’-(N,N-dimethylamino) propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN)-tuned indium-tin-oxide (ITO) electrode was employed as a cathode and phenyl-C61-butyric acid methyl ester (PCBM) was used as an electron transport layer. Owing to the formation of surface dipoles, the work function of the ITO electrode could be tuned from −4.75 eV to −4.12 eV, which is more efficient for electron collection. Our proposed method significantly lowers the PSC processing temperature from 500 °C to 100 °C, which is advantageous for commercialization of PSCs and fabrication of flexible PSCs.     

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.     

The effect of external electric field on the performance of perovskite solar cells

Planar heterojunction perovskite solar cells were fabricated through a low temperature approach. We find that the device performance significantly depends on the external bias before and during measurements. By appropriate optimization of the bias conditions, we could achieve an 8-fold increase in the power conversion efficiency. The significant improvement in device performance might be caused by the ion motion in the perovskite under the external electric field.     

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.     

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.     

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.     

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.     

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.     

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.     

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%.     

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.     

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.     

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.     

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.     

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.