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.     

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.     

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.     

Polymer selection toward efficient polymer/PbSe planar heterojunction hybrid solar cells

By using a series of polymers in the polymer/PbSe planar heterojunction hybrid solar cells (HSCs), we found that the open circuit voltage of HSCs showed a great improvement compared to that of PbSe Schottky junction solar cells, which might be attributed to the formation of interface dipole, resulting in decreased interfacial resistance, increased built-in electrical field, as well as reduced exciton recombination at interface. Meanwhile, polymers with higher PL quenching have more favorable hole transfer which lead to better device performance. In addition, the energy levels and surface energy of the polymers might largely affect their interaction with PbSe NCs, leading to different interfacial morphologies and influencing the charge transfer efficiency. Furthermore, the optimized HSCs showed a remarkable PCE of 5.31% which was the highest efficiency reported for polymer/PbSe based HSCs. We believe this HSC efficiency can be further improved by selecting polymers with rationally designed structures.     

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 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.     

Interface degradation of perovskite solar cells and its modification using an annealing-free TiO2 NPs layer

Interface is one of the most important factors to influence the device stability, which directly determines the commercialization of perovskite solar cells (PSCs). The research disclosed the degradation process and mechanism of planar heterojunction (PHJ) PSCs with a structure of ITO/PEDOT:PSS/CH₃NH₃PbI₃/PCBM/Al using in-situ experiments. The degradation of PHJ-PSCs is mainly attributed to the interface decay of perovskite/cathode. Large amount of bubbles formed quickly at the interface and grew up as PHJ-PSCs exposed to air. The cathode electrode easily peeled off from the devices that led to lose the efficiency completely after only 1 h exposure to air. On the other hand, the degradation driven by intrinsic decomposition of perovskite itself under atmosphere (humidity ∼ 45 RH%) was not obvious and the power conversion efficiency (PCE) could retain almost the same when only the perovskite layer was exposed to air for 200 h. Furthermore, annealing-free TiO₂ nanocrystalline particles (TiO₂ NPs) as an interface modification layer was inserted into PHJ-PSCs and dramatically improved the stability, of which the PCEs retained over 75% of its initial values after exposure to air for 200 h. The results provide important information to understand the degradation of PSCs and the improvement of the stability, which may potentially accelerate the development and commercialization of PSCs.     

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.     

Flexible,hole transporting layer-free and stable CH3NH3PbI3/PC61BM planar heterojunction perovskite solar cells

Hole transporting layer (HTL)-free CH₃NH₃PbI₃/PC₆₁BM planar heterojunction perovskite solar cells were fabricated with the configuration of ITO/CH₃NH₃PbI₃/PC₆₁BM/Al. The devices present a remarkable power conversion efficiency (PCE) of 11.7% (12.5% best) under AM 1.5G 100 mW cm⁻² illumination. Moreover, the HTL-free perovskite solar cells on flexible PET substrates are first demonstrated, achieving a power conversion efficiency of 9.7%. The element distribution in the HTL-free perovskite solar cell was further investigated. The results indicated that the PbI₂ enriched near the PC₆₁BM side for chlorobenzene treatment via the fast deposition crystallization method. Without using HTL on the ITO, the device is stable with comparison to that with poly(3,4-ethylenedioxylenethiophene): poly(styrene sulfonate) (PEDOT:PSS) as HTL. In addition, the fabricating time of the whole procedure from ITO substrate cleaning to device finishing fabrication only cost about 3 h for our mentioned devices, which is much more rapid than other structure devices containing other transporting layer. The high efficient and stable HTL-free CH₃NH₃PbI₃/PC₆₁BM planar heterojunction perovskite solar cells with the advantage of saving time and cost provide the potential for commercialization printing electronic devices.     

TiO2/SnOxCly double layer for highly efficient planar perovskite solar cells

Recently, perovskite solar cells have attracted tremendous research interest due to their amazing light to electric power conversion efficiency (PCE). However, most high performance devices usually use mesoporous TiO₂ as the electron transport layer (ETL), which increases cost for practical application. Here, TiO₂/SnO_xCl_y double layer was employed as the ETL for planar perovskite solar cells. Compared with bare TiO₂, perovskite solar cell based on TiO₂/SnO_xCl_y shows drastically improved power conversion efficiency and reduced hysteresis. These improvements are attributed to TiO₂/SnO_xCl_y which could enhance electron extraction and reduce surface trap-state.     

Large-area and high-performance CH3NH3PbI3 perovskite photodetectors fabricated via doctor blading in ambient condition

Organic-inorganic hybrid halide perovskites have attracted much research interest in optoelectronic field due to their excellent photoelectric properties. Herein, we report large-area and high-performance perovskite CH₃NH₃PbI₃ photodetectors fabricated via in-situ thermal-treatment doctor blading technique in ambient condition (humidity ∼45%). As compared with spin-coating deposition technique, the doctor-bladed CH₃NH₃PbI₃ films have larger grain size, as well as good reliability and reproducibility in large area. The doctor-bladed CH₃NH₃PbI₃ photodetectors exhibited high detectivity (D*) of 2.9 × 10¹² Jones and high responsivity (R) of 8.95 A/W, as well as the fast response time of less than 7.7 ms. The results indicate that doctor-bladed CH₃NH₃PbI₃ film is a very promising candidate for fabricating large-scale and high-performance optoelectronic devices.     

Designing novel thin film polycrystalline solar cells for high efficiency: sandwich CIGS and heterojunction perovskite

Heterojunction and sandwich architectures are two new-type structures with great potential for solar cells. Specifically, the heterojunction structure possesses the advantages of efficient charge separation but suffers from band offset and large interface recombination; the sandwich configuration is favorable for transferring carriers but requires complex fabrication process. Here, we have designed two thin-film polycrystalline solar cells with novel structures:sandwich CIGS and heterojunction perovskite, referring to the advantages of the architectures of sandwich perovskite (standard) and heterojunction CIGS (standard) solar cells, respectively. A reliable simulation software wxAMPS is used to investigate their inherent characteristics with variation of the thickness and doping density of absorber layer. The results reveal that sandwich CIGS solar cell is able to exhibit an optimized efficiency of 20.7%, which is much higher than the standard heterojunction CIGS structure (18.48%). The heterojunction perovskite solar cell can be more efficient employing thick and doped perovskite films (16.9%) than these typically utilizing thin and weak-doping/intrinsic perovskite films (9.6%). This concept of structure modulation proves to be useful and can be applicable for other solar cells.     

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.     

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.     

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.     

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.     

Substrate effect on the interfacial electronic structure of thermally-evaporated CH3NH3PbI3 perovskite layer

The impact of substrate work function on the interfacial electronic structure of thermally-evaporated CH₃NH₃PbI₃ perovskite films on various substrates have been systematically investigated using in-situ ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). On substrates with work function lower than ∼4.43 eV, a Fermi level pinning effect of the lowest unoccupied molecular orbital (LUMO) is observed, resulting in the near zero electron extraction barrier for the CH₃NH₃PbI₃ perovskite solar cells. On the other hand, when substrates with high work function are used, even exceed the highest occupied molecular orbital (HOMO) of CH₃NH₃PbI₃, an almost constant hole extraction barrier of ∼0.88 eV is observed, indicating that the efficiency of hole extraction at these interfaces are low. In order to understand the low hole extraction efficiency at interfaces between CH₃NH₃PbI₃ and these high work function electrodes, the evolution of electronic structures at the interface between CH₃NH₃PbI₃ and MoO₃ is further investigated. The charge transfer and dipole formation between CH₃NH₃PbI₃ and MoO₃ are deduced from the UPS and XPS results, and the energy level alignment between CH₃NH₃PbI₃ and MoO₃ is discussed.     

Viscosity effect of ionic liquid-assisted controlled growth of CH3NH3PbI3 nanoparticle-based planar perovskite solar cells

A precise control of the morphology and crystallization of perovskite thin-films is well-correlated to higher perovskite solar cells performances. Ionic liquids (ILs) can retard perovskite crystallization to aid the formation of films with uniform morphology to realize highly efficient perovskite solar cells. Herein, we attempt to control the nanostructural growth of CH₃NH₃PbI₃ thin films by adding ILs to the perovskite spin-coating solution and investigate the effect of IL viscosity on the resulting CH₃NH₃PbI₃ nanoparticle (NP) thin films. NPs with desirable morphology were obtained using ILs with a low viscosity that completely dissolved in the CH₃NH₃PbI₃ solution. In particular, the IL tetrabutylammonium chloride yielded NPs with a diameter of 500 nm and controllable morphology, crystallinity, and absorption behavior, which led to improved photovoltaic performance compared with that of solar cells containing NPs produced using other ILs. Our findings revealed a pathway to obtain uniformly distributed CH₃NH₃PbI₃ NP thin films for use in perovskite solar cells. The developed method is well suited for large-scale production of perovskite thin films on flexible substrates.