Parallel Planar Heterojunction Strategy Enables Sb2S3 Solar Cells with Efficiency Exceeding 8 %

Solution-processed solar cells based on inorganic heterojunctions provide a potential approach to the efficient, stable and low-cost solar cells required for the terrestrial generation of photovoltaic energy. Antimony trisulfide (Sb₂S₃) is a promising photovoltaic absorber. Here, an easily solution-processed parallel planar heterojunction (PPHJ) strategy and related principle are developed to prepare efficient multiple planar heterojunction (PHJ) solar cells, and the PPHJ strategy boosts the efficiency of solution-processed Sb₂S₃ solar cells up to 8.32 % that is the highest amongst Sb₂S₃ devices. The Sb₂S₃-based PPHJ device consists of two kinds of conventional planar heterojunction (PHJ) subcells in a parallel connection: Sb₂S₃-based PHJ subcells dominating the absorption and charge generation and CH₃NH₃PbI₃-based PHJ subcells governing the electron transport towards collection electrode, but it belongs to an Sb₂S₃ device in nature. The resulting PPHJ device combines together the distinctive structural features of Sb₂S₃ absorbing layer as a main absorber and the duplexity of well-crystallized/oriented CH₃NH₃PbI₃ layer in charge transportation as an additional absorber, while the presence of perovskite does not affect device stability. The PPHJ strategy maintains the facile preparation by the conventional sequential depositions of multiple layers, but eliminates the normal complexity in both tandem and parallel tandem PHJ systems.     

Controllable Growth of Perovskite Films by Room‐Temperature Air Exposure for Efficient Planar Heterojunction Photovoltaic Cells

A two‐step solution processing approach has been established to grow void‐free perovskite films for low‐cost high‐performance planar heterojunction photovoltaic devices. A high‐temperature thermal annealing treatment was applied to drive the diffusion of CH₃NH₃I precursor molecules into a compact PbI₂ layer to form perovskite films. However, thermal annealing for extended periods led to degraded device performance owing to the defects generated by decomposition of perovskite into PbI₂. A controllable layer‐by‐layer spin‐coating method was used to grow “bilayer” CH₃NH₃I/PbI₂ films, and then drive the interdiffusion between PbI₂ and CH₃NH₃I layers by a simple air exposure at room temperature for making well‐oriented, highly crystalline perovskite films without thermal annealing. This high degree of crystallinity resulted in a carrier diffusion length of ca. 800 nm and a high device efficiency of 15.6 %, which is comparable to values reported for thermally annealed perovskite films.     

Inkjet Printing and Instant Chemical Transformation of a CH3NH3PbI3/Nanocarbon Electrode and Interface for Planar Perovskite Solar Cells

A planar perovskite solar cell that incorporates a nanocarbon hole‐extraction layer is demonstrated for the first time by an inkjet printing technique with a precisely controlled pattern and interface. By designing the carbon plus CH₃NH₃I ink to transform PbI₂ in situ to CH₃NH₃PbI₃, an interpenetrating seamless interface between the CH₃NH₃PbI₃ active layer and the carbon hole‐extraction electrode was instantly constructed, with a markedly reduced charge recombination compared to that with the carbon ink alone. As a result, a considerably higher power conversion efficiency up to 11.60 % was delivered by the corresponding solar cell. This method provides a major step towards the fabrication of low‐cost, large‐scale, metal‐electrode‐free but still highly efficient perovskite solar cells.     

Correlation between Interfacial Structures and Device Performance: The Double-Edged Sword Effect of Lead Iodide in Perovskite Solar Cells

The interfacial electronic structure of perovskite layers and transport layers is critical for the performance and stability of perovskite solar cells (PSCs). The device performance of PSCs can generally be improved by adding a slight excess of lead iodide (PbI₂) to the precursor solution. However, its underlying working mechanism is controversial. Here, we performed a comprehensive study of the electronic structures at the interface between CH₃NH₃PbI₃ and C₆₀ with and without the modification of PbI₂ using in situ photoemission spectroscopy measurements. The correlation between the interfacial structures and the device performance was explored based on performance and stability tests. We found that there is an interfacial dipole reversal, and the downward band bending is larger at the CH₃NH₃PbI₃/C₆₀ interface with the modification of PbI₂ as compared to that without PbI₂. Therefore, PSCs with PbI₂ modification exhibit faster charge carrier transport and slower carrier recombination. Nevertheless, the modification of PbI₂ undermines the device stability due to aggravated iodide migration. Our findings provide a fundamental understanding of the CH₃NH₃PbI₃/C₆₀ interfacial structure from the perspective of the atomic layer and insight into the double-edged sword effect of PbI₂ as an additive.     

The Effect of Humidity upon the Crystallization Process of Two‐Step Spin‐Coated Organic–Inorganic Perovskites

Moisture is shown to activate the reaction between PbI₂ and methylammonium halides. In addition, two activating mechanisms are proposed for the formation of CH₃NH₃PbI₃ and CH₃NH₃PbI_3?xCl_x films from a series of carefully controlled experiments. When these rapidly formed perovskite films are directly fabricated into the devices, poor photovoltaic properties are found, due to heavy surface charge recombination. However, the cell performance can be significantly enhanced to 13.63 % and to over 12 % in the steady state for CH₃NH₃PbI₃ and to 15.50 % and over 14 % in the steady state for CH₃NH₃PbI_3?xCl_x, if the rapidly formed perovskite film is annealed. Thus, it is believed that moisture (below 60 % RH) is not a problem for the fabrication of highly efficient perovskite solar cells.     

Synthesis of hybrid organic-inorganic perovskite platelets by vacuum impregnation

In perovskite solar cells and optoelectronics, perovskite film morphology controls the performance of the device. Various methods have been developed to control the morphology and coverage of the perovskite films. In this article platelet type perovskite morphlogy was synthesized using low temperature vacuum impregnation of the perovskite solution CH₃NH₃PbI₃ resulting in complete coverage on TiO₂ film. Vacuum impregnation synthesis of perovskites has the advantage of low cost and low temperature which faciliates application in flexible electronics and solar cells.     

Retardation of Trap-Assisted Recombination in Lead Halide Perovskite Solar Cells by a Dimethylbiguanide Anchor Layer

Reaching the full potential of solar cells based on photo-absorbers of organic-inorganic hybrid perovskites requires highly efficient charge extraction at the interface between perovskite and charge transporting layer. This demand is generally challenged by the presence of under-coordinated metal or halogen ions, causing surface charge trapping and resultant recombination losses. These problems can be tackled by introducing a small molecule interfacial anchor layer based on dimethylbiguanide (DMBG). Benefitting from interactions between the nitrogen-containing functional groups in DMBG and unsaturated ions in CH₃NH₃PbI₃ perovskites, the electron extraction of TiO₂ is dramatically improved in association with reduced Schottky–Read–Hall recombination, as revealed by photoluminescence spectroscopy. As a consequence, the power conversion efficiency of CH₃NH₃PbI₃ solar cells is boosted from 17.14 to 19.1 %, showing appreciably reduced hysteresis. The demonstrated molecular strategy based on DMBG enables one to achieve meliorations on key figures of merit in halide perovskite solar cells with improved stability.     

Stable and Low‐Cost Mesoscopic CH3NH3PbI2Br Perovskite Solar Cells by using a Thin Poly(3‐hexylthiophene) Layer as a Hole Transporter

Mesoscopic perovskite solar cells using stable CH₃NH₃PbI₂Br as a light absorber and low‐cost poly(3‐hexylthiophene) (P3HT) as hole‐transporting layer were fabricated, and a power conversion efficiency of 6.64 % was achieved. The partial substitution of iodine with bromine in the perovskite led to remarkably prolonged charge carrier lifetime. Meanwhile, the replacement of conventional thick spiro‐MeOTAD layer with a thin P3HT layer has significantly reduced the fabrication cost. The solar cells retained their photovoltaic performance well when they were exposed to air without any encapsulation, presenting a favorable stability. The combination of CH₃NH₃PbI₂Br and P3HT may render a practical and cost‐effective solid‐state photovoltaic system. The superior stability of CH₃NH₃PbI₂Br is also promising for other photoconversion applications.     

Mixed‐Halide CH3NH3PbI3−xXx (X=Cl,Br, I) Perovskites: Vapor‐Assisted Solution Deposition and Application as Solar Cell Absorbers

There have been recent reports on the formation of single‐halide perovskites, CH₃NH₃PbX₃ (X=Cl, Br, I), by means of vapor‐assisted solution processing. Herein, the successful formation of mixed‐halide perovskites (CH₃NH₃PbI_3?xX_x) by means of a vapor‐assisted solution method at ambient atmosphere is reported. The perovskite films are synthesized by exposing PbI₂ film to CH₃NH₃X (X=I, Br, or Cl) vapor. The prepared perovskite films have uniform surfaces with good coverage, as confirmed by SEM images. The inclusion of chlorine and bromine into the structure leads to a lower temperature and shorter reaction time for optimum perovskite film formation. In the case of CH₃NH₃PbI_3?xCl_x, the optimum reaction temperature is reduced to 100 °C, and the resulting phases are CH₃NH₃PbI₃ (with trace Cl) and CH₃NH₃PbCl₃ with a ratio of about 2:1. In the case of CH₃NH₃PbI_3?xBr_x, single‐phase CH₃NH₃PbI₂Br is formed in a considerably shorter reaction time than that of CH₃NH₃PbI₃. The mesostructured perovskite solar cells based on CH₃NH₃PbI₃ films show the best optimal power conversion efficiency of 13.5 %, whereas for CH₃NH₃PbI_3?xCl_x and CH₃NH₃PbI_3?xBr_x the best recorded efficiencies are 11.6 and 10.5 %, respectively.     

Limiting Perovskite Solar Cell Performance by Heterogeneous Carrier Extraction

Although the power conversion efficiency of perovskite solar cells has improved rapidly, a rational path for further improvement remains unclear. The effect of large morphological heterogeneity of polycrystalline perovskite films on their device performance by photoluminescence (PL) microscopy has now been studied. Contrary to the common belief on the deleterious effect of morphological heterogeneity on carrier lifetimes and diffusivities, in neat CH₃NH₃PbI₃(Cl) polycrystalline perovskite films, the local (intra‐grain) carrier diffusivities in different grains are all surprisingly high (1.5 to 3.3 cm² s^?1; comparable to bulk single‐crystals), and the local carrier lifetimes are long (ca. 200 ns) and surprisingly homogenous among grains, and uniform across grain boundary and interior. However, there is a large heterogeneity of carrier extraction efficiency at the perovskite grain–electrode interface. Improving homogeneity at perovskite grain–electrode contacts is thus a promising direction for improving the performance of perovskite thin‐film solar cells.     

Methylamine‐Gas‐Induced Defect‐Healing Behavior of CH3NH3PbI3 Thin Films for Perovskite Solar Cells

We report herein the discovery of methylamine (CH₃NH₂) induced defect‐healing (MIDH) of CH₃NH₃PbI₃ perovskite thin films based on their ultrafast (seconds), reversible chemical reaction with CH₃NH₂ gas at room temperature. The key to this healing behavior is the formation and spreading of an intermediate CH₃NH₃PbI₃?xCH₃NH₂ liquid phase during this unusual perovskite–gas interaction. We demonstrate the versatility and scalability of the MIDH process, and show dramatic enhancement in the performance of perovskite solar cells (PSCs) with MIDH. This study represents a new direction in the formation of defect‐free films of hybrid perovskites.     

In situ Investigations of Interfacial Degradation and Ion Migration at CH3NH3PbI3 Perovskite/Ag Interfaces

Interfacial properties between perovskite layers and metal electrodes play a crucial role in the device performance and the long-term stability of perovskite solar cells. Here, we report a comprehensive study of the interfacial degradation and ion migration at the interface between CH₃NH₃PbI₃ perovskite layer and Ag electrode. Using in situ photoemission spectroscopy measurements, we found that the Ag electrode could induce the degradation of perovskite layers, leading to the formation of PbI₂ and AgI species and the reduction of Pb²⁺ ions to metallic Pb species at the interface. The unconventional enhancement of the intensities of I 3d spectra provides direct experimental evidences for the migration of iodide ions from CH₃NH₃PbI₃ subsurface to Ag electrode. Moreover, the contact of Ag electrode and perovskite layers induces an interfacial dipole of 0.3 eV at CH₃NH₃PbI₃/Ag interfaces, which may further facilitate iodide ion di usion, resulting in the decomposition of perovskite layers and the corrosion of Ag electrode.     

Ultrafast Photogenerated Hole Extraction/Transport Behavior in a CH3NH3PbI3/Carbon Nanocomposite and Its Application in a Metal‐Electrode‐Free Solar Cell

Aligned and flexible electrospun carbon nanomaterials are used to synthesize carbon/perovskite nanocomposites. The free‐electron diffusion length in the CH₃NH₃PbI₃ phase of the CH₃NH₃PbI₃/carbon nanocomposite is almost twice that of bare CH₃NH₃PbI₃, and nearly 95 % of the photogenerated free holes can be injected from the CH₃NH₃PbI₃ phase into the carbon nanomaterial. The exciton binding energy of the composite is estimated to be 23 meV by utilizing temperature‐dependent optical absorption spectroscopy. The calculated free carriers increase with increasing total photoexcitation density, and this broadens the potential of this material for a broad range of optoelectronics applications. A metal‐electrode‐free perovskite solar cell (power conversion efficiency: 13.0 %) is fabricated with this perovskite/carbon composite, which shows great potential for the fabrication of efficient, large‐scale, low‐cost, and metal‐electrode‐free perovskite solar cells.     

Unraveling the Role of Monovalent Halides in Mixed‐Halide Organic–Inorganic Perovskites

The performance of perovskite solar cells is strongly influenced by the composition and microstructure of the perovskite. A recent approach to improve the power conversion efficiencies utilized mixed‐halide perovskites, but the halide ions and their roles were not directly studied. Unraveling their precise location in the perovskite layer is of paramount importance. Here, we investigated four different perovskites by using X‐ray photoelectron spectroscopy, and found that among the three studied mixed‐halide perovskites, CH₃NH₃Pb(I_0.74Br_0.26)₃ and CH₃NH₃PbBr_3?xCl_x show peaks that unambiguously demonstrate the presence of iodide and bromide in the former, and bromide and chloride in the latter. The CH₃NH₃PbI_3?xCl_x perovskite shows anomalous behavior, the iodide content far outweighs that of the chloride; a small proportion of chloride, in all likelihood, resides deep within the TiO₂/absorber layer. Our study reveals that there are many distinguishable structural differences between these perovskites, and that these directly impact the photovoltaic performances.     

A Rutile TiO2 Electron Transport Layer for the Enhancement of Charge Collection for Efficient Perovskite Solar Cells

Interfacial charge collection efficiency has demonstrated significant effects on the power conversion efficiency (PCE) of perovskite solar cells (PSCs). Herein, crystalline phase‐dependent charge collection is investigated by using rutile and anatase TiO₂ electron transport layer (ETL) to fabricate PSCs. The results show that rutile TiO₂ ETL enhances the extraction and transportation of electrons to FTO and reduces the recombination, thanks to its better conductivity and improved interface with the CH₃NH₃PbI₃ (MAPbI₃) layer. Moreover, this may be also attributed to the fact that rutile TiO₂ has better match with perovskite grains, and less trap density. As a result, comparing with anatase TiO₂ ETL, MAPbI₃ PSCs with rutile TiO₂ ETL delivers significantly enhanced performance with a champion PCE of 20.9 % and a large open circuit voltage (V_OC) of 1.17 V.     

Hole transporting electrodeposited PEDOT–polyelectrolyte layers for perovskite solar cells

Thin layers of poly(3,4-ethylenedioxythiophene) complexes with various sulfonated polyelectrolytes were formed electrochemically in a galvanostatic regime. These transparent hole transport layers were used in CH₃NH₃PbI₃ perovskite solar cells and the influence of the polyelectrolyte structure on the device performance was featured.     

Highly Efficient,Reproducible, Uniform (CH3NH3)PbI3 Layer by Processing Additive Dripping for Solution‐Processed Planar Heterojunction Perovskite Solar Cells

A processing additive dripping (PAD) approach to forming highly efficient (CH₃NH₃)PbI₃ (MAPbI₃) perovskite layers was investigated. A MAPbI₃(CB/DIO) perovskite film fabricated by this approach, which included briefly dripping chlorobenzene incorporating a small amount of diiodooctane (DIO) during casting of a MAPbI₃ perovskite precursor dissolved in dimethylformamide, exhibited superior smooth, uniform morphologies with high crystallinity and large grains and revealed completely homogeneous surface coverage. The surface coverage and morphology of the substrate significantly affected the photovoltaic performance of planar heterojunction (PHJ) perovskite solar cells (PrSCs), resulting in a power conversion efficiency of 11.45 % with high open‐circuit voltage of 0.91 V and the highest fill factor of 80.87 %. Moreover, the PAD approach could effectively provide efficient MAPbI₃(CB/DIO) perovskite layers for highly efficient, reproducible, uniform PHJ PrSC devices without performance loss or variation even over larger active areas.     

Pseudohalide‐Induced Moisture Tolerance in Perovskite CH3NH3Pb(SCN)2I Thin Films

Two pseudohalide thiocyanate ions (SCN^?) have been used to replace two iodides in CH₃NH₃PbI₃, and the resulting perovskite material was used as the active material in solar cells. In accelerated stability tests, the CH₃NH₃Pb(SCN)₂I perovskite films were shown to be superior to the conventional CH₃NH₃PbI₃ films as no significant degradation was observed after the film had been exposed to air with a relative humidity of 95 % for over four hours, whereas CH₃NH₃PbI₃ films degraded in less than 1.5 hours. Solar cells based on CH₃NH₃Pb(SCN)₂I thin films exhibited an efficiency of 8.3 %, which is comparable to that of CH₃NH₃PbI₃ based cells fabricated in the same way.     

First-Principles Study of Aziridinium Lead Iodide Perovskite for Photovoltaics

The long-term stability remains one of the main challenges for the commercialization of the rapidly developing hybrid organic-inorganic perovskite solar cells. Herein, we investigate the electronic and optical properties of the recently reported hybrid halide perovskite (CH₂)₂NH₂PbI₃ (AZPbI₃), which exhibits a much better stability than the popular halide perovskites CH₃NH₃PbI₃ and HC(NH₂)₂PbI₃, by using density functional theory (DFT). We find that AZPbI₃ possesses a band gap of 1.31 eV, ideal for single-junction solar cells, and its optical absorption is comparable with those of the popular CH₃NH₃PbI₃ and HC(NH₂)₂PbI₃ materials in the whole visible-light region. In addition, the conductivity of AZPbI₃ can be tuned from efficient p-type to n-type, depending on the growth conditions. Besides, the charge-carrier mobilities and lifetimes are unlikely hampered by deep transition energy levels, which have higher formation energies in AZPbI₃ according to our calculations. Overall, we suggest that the perovskite AZPbI₃ is an excellent candidate as a stable high-performance photovoltaic absorber material.     

The domination of ionic conductivity in tetragonal phase of the organometal halide perovskite CH3NH3PbI3-xClx

Organometal trihalide perovskites have recently gained extreme attention due to their high solar energy conversion in photovoltaic cells. Here, we investigate the contribution of iodide ions to a total conductivity of the mixed lead halide perovskite CH₃NH₃PbI_3−xCl_x with a use of the modified DC Hebb–Wagner polarization method. It has been identified that an ionic conductivity dominates in tetragonal phase which is associated with room temperature. The obtained activation energy for this type of hopping mechanism is equal to (0.87 ± 0.02) eV, which is in a good agreement with previous literature reports. The high contribution of ionic conductivity at room temperature might be a reason of the observed hysteresis in halide perovskite solar cells.