shuhui

Publication date: 16 January 2019

Source: Joule, Volume 3, Issue 1

Author(s): David Kiermasch, Lidón Gil-Escrig, Henk J. Bolink, Kristofer Tvingstedt

Context & Scale

Summary

Graphical Abstract

The chloride coated‐PbSe quantum dots are passivated using CsPbX3 (X = Br and I) perovskite quantum dots via halide ion exchange in solution. Solar cells using PbSe quantum dots passivated by CsPbBr_0.5I_2.5 mixed halide perovskites achieve a 9.2% power conversion, the highest reported efficiency for a PbSe QD device to date.

PbSe quantum dots (QDs) have stronger electronic coupling resulting from a large Bohr exciton radius, suggesting PbSe QDs may be able to achieve superior charge separation and transport in optoelectronic devices compared with PbS QDs. However, PbS QDs solar cell have achieved a certified 12.01% power conversion efficiency (PCE), whereas PbSe QD photovoltaics lag behind at 8.2% PCE. One reason for this difference is that there has been significantly less work done on surface passivation of PbSe QDs. Here, the surface passivation of chlorinated PbSe QDs is optimized via a halide ion exchange treatment using mixed halide CsPb(Br/I)₃ perovskite nanocrystals. Champion devices made from treated QDs achieved a PCE of 9.2%, V _oc of 0.56 V, J _sc of 25.7 mA cm⁻², and fill factor of 64%. Average PCEs for optimized cells are 8.9%. Detailed physical characterizations including capacitance‐voltage (C‐V), V _oc, and J _sc as a function of light intensity, transient photovoltage, and photocurrent measurements are all carried out to investigate the mechanism of the improvement in the PCE and to understand the role of the mixed halide perovskites in providing superior surface passivation for PbSe solar cells. At this time, 9.2% is the highest PCE yet reported for PbSe QDs solar cells.

Design and application of volatilizable solid additives in non-fullerene organic solar cells

Design and application of volatilizable solid additives in non-fullerene organic solar cells, Published online: 07 November 2018; doi:10.1038/s41467-018-07017-z

A novel thermionic emission–based interconnecting layer (ICL) for all‐perovskite tandem solar cells is demonstrated with solution‐processed light absorbers, wide absorption, and high efficiency. The novel ICL structure employs a new hybrid system of fluoride silane–incorporated polyethylenimine ethoxylated for solvent resistance and defect passivation.

Abstract

All‐perovskite tandem cells have been considered a potential candidate for bringing the power conversion efficiency (PCE) beyond the Shockley–Queisser limit of single‐junction device while retaining the advantages of earth‐abundant materials and solution processability. However, a challenging issue with regard to realizing such solution‐processed devices is the fulfillment of complex and coupled requirements of the interconnecting layer (ICL), including solvent resistance to protect underlying perovskite film, high electrical properties for carrier transport and recombination, and high optical transmission. In this work, a new thermionic emission–based ICL with enhanced solvent resistance features is demonstrated. Fundamentally, the thermionic emission plays a critical role in the electron transport process in the ICL, which is confirmed through both experimental and theoretical studies. Besides achieving high optical transmission and electrical properties, the new ICL chemically protects the underlying perovskite film by introducing a fluoride silane–incorporated polyethylenimine ethoxylated hybrid system that also passivates the surface defects to reduce electrical loss. The monolithic all‐perovskite tandem cells demonstrate highest PCE of 17.9% (from current density–voltage scan) and the highest steady‐state efficiency is 16.1% for a typical device. Consequently, this work contributes to not only understanding the fundamental mechanism of ICLs but also promotes robust and low‐cost photovoltaics.

Previously unreported nanoscale defects are observed in solution‐processed methylammonium lead tri‐iodide (MAPI) perovskite solar cells. A novel methodology is introduced that eliminates these defects, modifies MAPI crystallinity and enhances power conversion efficiency >30%. In‐situ optoelectronic characterization correlates performance enhancements to improvements in charge collection efficiency, reduced electron–hole recombination, and an overall decrease of trap‐state density.

Abstract

Here, previously unobserved nanoscale defects residing within individual grains of solution‐processed methylammonium lead tri‐iodide (CH₃NH₃PbI₃, MAPI) thin films are identified. Using scanning transmission electron microscopy (STEM), the defects inherently associated with the established solution‐processing methodology are identified, and a facile processing modification to eliminate these defects is introduced. Specifically, defect elimination is achieved by coannealing the as‐deposited MAPI layer with the electron transport layer (phenyl‐C₆₁‐butyric acid methyl, PCBM) resulting in devices that significantly outperform devices prepared using the established methodology—with power conversion efficiencies increasing from 13.6% to 17.4%. The use of transmission electron microscopy allows the correlation of performance enhancements to improved intragrain crystallinity and shows that highly coherent crystallographic orientation results within individual grains when processing is modified. Detailed optoelectronic characterization reveals that the improved intragrain crystallinity drives an improvement of charge collection and a reduction of PEDOT:PSS/perovskite interfacial recombination. The study suggests that the microstructural defects in MAPI, owing to a lack of structural coherence throughout the thickness of thin film, are a significant cause of interfacial recombination.

A novel cryogenic process, described by Charles Surya and co‐workers in article number 1804402 has universal applicability to grow organometal halide perovskites. A cryogenic temperature inhibits premature coalescence of nuclei into large crystallites, decoupling nucleation and crystallization phases. The enhanced control over the growth process yields excellent film and high‐performance solar cells.

Publication date: 16 January 2019

Source: Joule, Volume 3, Issue 1

Author(s): Weijie Chen, Haiyang Chen, Guiying Xu, Rongming Xue, Shuhui Wang, Yaowen Li, Yongfang Li

Context & Scale

Summary

Graphical Abstract

The theoretical power conversion efficiency of all‐inorganic CsPbI₂Br perovskite solar cells is predicted to be 22.1%, and only by taking both material chemistry and device physics into consideration can researchers achieve this goal.

Cesium‐based all‐inorganic perovskite solar cells (PSCs), especially for CsPbI₂Br component‐based devices, have attracted increasing attention due to its advantage of superior thermal and phase stability. Since the pioneering study reported in 2016, more than 30 papers have been published, reporting the rapid boost in the power conversion efficiency (PCE) of PSCs to 14.81%. The CsPbI₂Br PSC is one of the most remarkable research hotspots in the field of perovskite photovoltaics. In this progress report, the recent advances in CsPbI₂Br PSCs are systematically reviewed, which in turn introduces the basic property and stability of active layers, and the performance improvements in these devices. The challenges as well as the possible solutions toward better‐performing CsPbI₂Br PSCs are also discussed. The theoretical calculation results point out that there is much room for further device performance enhancement, particularly in open‐circuit voltages. This progress report focuses on CsPbI₂Br material properties and summarizes recent strategies to improve the corresponding device's PCE, in order to open new perspectives toward commercial utility of PSCs.

A graded bilayered inorganic hole‐transporting layer (including compact NiO _x and mesoporous CuGaO₂) is developed for inverted perovskite solar cells. The resulting devices demonstrate both high efficiency, with the champion one giving a stabilized efficiency of ≈20% and superior thermal stability with >80% of the initial efficiency being retained subject to 1000 hours' thermal aging at 85 °C.

Abstract

The unstable feature of the widely employed organic hole‐transporting materials (HTMs) (e.g., spiro‐MeOTAD) significantly limits the practical application of perovskite solar cells (PSCs). Therefore, it is desirable to design new structured PSCs with stable HTMs presenting excellent carrier extraction and transfer properties. This work demonstrates a new inverted PSC configuration. The new PSC has a graded band alignment and bilayered inorganic HTMs (i.e., compact NiO_x and mesoporous CuGaO₂). In comparison with planar‐structured PSCs, the mesoporous CuGaO₂ can effectively extract holes from perovskite due to the increased contact area of the perovskite/HTM. The graded energy alignment constructed in the ultrathin compact NiO_x, mesoporous CuGaO₂, and perovskite can facilitate carrier transfer and depress charge recombination. As a result, the champion device based on the newly designed mesoscopic PSCs yields a stabilized efficiency of ≈20%, which is considered one of the best results for inverted PSCs with inorganic HTMs. Additionally, the unencapsulated PSC device retains more than 80% of its original efficiency when subjected to thermal aging at 85 °C for 1000 h in a nitrogen atmosphere, thus demonstrating superior thermal stability of the device. This study may pave a new avenue to rational design of highly efficient and stable PSCs.

A novel SnO₂‐in‐polymer matrix is demonstrated to be an excellent electron‐selective layer in perovskite solar cells. The polymer is uniformly dispersed in SnO₂ colloidal ink and promotes the nanoparticle disaggregation in the ink. Planar‐structure perovskite solar cells based on this SnO₂‐in‐polymer matrix show a high efficiency of 20.8% with negligible hysteresis and superior reproducibility.

Abstract

Understanding interfacial loss and the ways to improving interfacial property is critical to fabricate highly efficient and reproducible perovskite solar cells (PSCs). In SnO₂‐based PSCs, nonradiative recombination sites at the SnO₂–perovskite interface lead to a large potential loss and performance variation in the resulting photovoltaic devices. Here, a novel SnO₂‐in‐polymer matrix (i.e., polyethylene glycol) is devised as the electron transporting layer to improve the film quality of the SnO₂ electron transporting layer. The SnO₂‐in‐polymer matrix is fabricated through spin‐coating a polymer‐incorporated SnO₂ colloidal ink. The polymer is uniformly dispersed in SnO₂ colloidal ink and promotes the nanoparticle disaggregation in the ink. Owing to polymer incorporation, the compactness and wetting property of SnO₂ layer is significantly ameliorated. Finally, photovoltaic devices based on Cs_0.05FA_0.81MA_0.14PbI_2.55Br_0.45 perovskite sandwiched between SnO₂ and Spiro‐OMeTAD layer are fabricated. Compared with the averaging power conversion efficiency of 16.2% with 1.2% deviation for control devices, the optimized devices exhibit an improved averaging efficiency of 19.5% with 0.25% deviation. The conception of polymer incorporation in the electron transporting layer paves a way to further increase the performance of planar perovskite solar cells.

High‐efficiency (21.06%) and durable 2D–3D vertical aligned perovskite solar cells (PSCs) with phase pure 2D perovskite are demonstrated. The phase pure 2D perovskite minimizes photo‐generated charge‐carrier localization in the low‐dimensional perovskite; the dominant vertical alignment does not affect charge‐carrier extraction. The traditional constraint of trade‐off between efficiency and stability in PSC is overcome.

Abstract

Three‐dimensional (3D) metal‐halide perovskite solar cells (PSCs) have demonstrated exceptional high efficiency. However, instability of the 3D perovskite is the main challenge for industrialization. Incorporation of some long organic cations into perovskite crystal to terminate the lattice, and function as moisture and oxygen passivation layer and ion migration blocking layer, is proven to be an effective method to enhance the perovskite stability. Unfortunately, this method typically sacrifices charge‐carrier extraction efficiency of the perovskites. Even in 2D–3D vertically aligned heterostructures, a spread of bandgaps in the 2D due to varying degrees of quantum confinement also results in charge‐carrier localization and carrier mobility reduction. A trade‐off between the power conversion efficiency and stability is made. Here, by introducing 2D C₆H₁₈N₂O₂PbI₄ (EDBEPbI₄) microcrystals into the precursor solution, the grain boundaries of the deposited 3D perovskite film are vertically passivated with phase pure 2D perovskite. The phases pure (inorganic layer number n = 1) 2D perovskite can minimize photogenerated charge‐carrier localization in the low‐dimensional perovskite. The dominant vertical alignment does not affect charge‐carrier extraction. Therefore, high‐efficiency (21.06%) and ultrastable (retain 90% of the initial efficiency after 3000 h in air) planar PSCs are demonstrated with these 2D–3D mixtures.

Here, an AI treatment is developed that provides a general method for optimizing the interfacial properties of inorganic perovskite solar cells, which leads to proper band edge bending, decreased surface defects, and a high‐quality quantum dots–modified layer. These changes prove effective at decreasing recombination loss and improving hole extraction efficiency. As a result, the FAI‐treated champion device achieves long‐term stabilized power conversion efficiencies above 14%.

Abstract

Recently, inorganic CsPbI₂Br perovskite is attracting ever‐increasing attention for its outstanding optoelectronic properties and ambient phase stability. Here, an efficient CsPbI₂Br perovskite solar cell (PSC) is developed by: 1) using a dimension‐grading heterojunction based on a quantum dots (QDs)/bulk film structure, and 2) post‐treatment of the CsPbI₂Br QDs/film with organic iodine salt to form an ultrathin iodine‐ion–enriched perovskite layer on the top of the perovskite film. It is found that the above procedures generate proper band edge bending for improved carrier collection, resulting in effectively decreased recombination loss and improved hole extraction efficiency. Meanwhile, the organic capping layer from the iodine salt also surrounds the QDs and tunes the surface chemistry for further improved charge transport at the interface. As a result, the champion device achieves long‐term stabilized power conversion efficiency beyond 14%.

All-inorganic cesium lead iodide perovskite solar cells with stabilized efficiency beyond 15%

All-inorganic cesium lead iodide perovskite solar cells with stabilized efficiency beyond 15%, Published online: 31 October 2018; doi:10.1038/s41467-018-06915-6

Management of transition dipoles in organic hole-transporting materials under solar irradiation for perovskite solar cells

Management of transition dipoles in organic hole-transporting materials under solar irradiation for perovskite solar cells, Published online: 31 October 2018; doi:10.1038/s41467-018-06998-1

Phase transition effects on thermal stability of planar perovskite solar cells are illuminated. Large carrier trap densities are observed in the methylammonium lead triiodide‐based solar cells aged under high operating temperatures. These carrier traps are detrimental to long‐term stability. Perovskite alloys with mixed both cations and anions could effectively avoid the formation of carrier traps and result in better device stability.

Abstract

A power conversion efficiency of over 20% has been achieved in CH₃NH₃PbI₃‐based perovskite solar cells (PSC), however, low thermal stability associated with the presence of a phase transition between tetragonal and cubic structures near room temperature is a major issue that must be overcome for future practical applications. Here, the influence of the phase transition on the thermal stability of PSCs is investigated in detail by comparing four kinds of perovskite films with different compositions of halogen atoms and organic components. Thermally stimulated current measurements reveal that a large number of carrier traps are generated in solar cells with the perovskite CH₃NH₃PbI₃ as a light absorber after operation at 85 °C, which is higher than the phase‐transition temperature. Electrochemical impedance spectroscopy measurements further exclude effects of a possible morphology change on the formation of carrier traps. These carrier traps are detrimental to the thermal stability. The thermogravimetric analysis does not show a decomposition for any of the materials in the temperature range relevant for operation. The perovskite alloys do not have this phase transition, resulting in effectively suppressed formation of carrier traps. PSCs with improved thermal stability under the standard thermal cycling test are demonstrated.

The sp²‐nitrogen positions in the n‐type (D–A₁–D–A₂) conjugated polymers have a significant impact on the photovoltaic properties of p–i–n perovskite solar cells when they are used as an electron transporting layer. pBTTz with the HOMO and LUMO levels well‐matched with the valence and conduction bands of the perovskite layer, respectively, shows excellent power conversion efficiency and high stability.

Abstract

It is highly desirable to employ n‐type polymers as electron transporting layers (ETLs) in inverted perovskite solar cells (PSCs) due to their good electron mobility, high hydrophobicity, and simplicity of film forming. In this research, the capability of three n‐type donor–acceptor₁–donor–acceptor₂ (D–A₁–D–A₂) conjugated polymers (pBTT, pBTTz, and pSNT) is first explored as ETLs because these polymers possess electron mobilities as high as 0.92, 0.46, and 4.87 cm² (Vs)⁻¹ in n‐channel organic transistors, respectively. The main structural difference among pBTT, pBTTz, and pSNT is the position of sp²‐nitrogen atoms (sp²‐N) in the polymer main chains. Therefore, the effect of different substitution positions on the PSC performances is comprehensively studied. The as‐fabricated p–i–n PSCs with pBTT, pBTTz, and pSNT as ETLs show the maximum photoconversion efficiencies of 12.8%, 14.4%, and 12.0%, respectively. To be highlighted, pBTTz‐based device can maintain 80% of its stability after ten days due to its good hydrophobicity, which is further confirmed by a contact angle technique. More importantly, the pBTTz‐based device shows a neglected hysteresis. This study reveals that the n‐type polymers can be promising candidates as ETLs to approach solution‐processed highly‐efficient inverted PSCs.

A nearly formamidinium (FA) lead–tin (Pb–Sn) mixed perovskite FAPb_0.75Sn_0.25I₃ is exploited to fabricate a low‐bandgap perovskite solar cell. By combination with a NiO _x hole transport layer, a power conversion efficiency of 17.25% is obtained. This low‐bandgap perovskite solar cell maintains about 91% of its original efficiency at 80 °C for 20 h, which demonstrates good thermal stability.

Abstract

Low bandgap lead–tin (Pb–Sn) mixed perovskite solar cells have achieved high power conversion efficiency in excess of 17%. However, methylammonium (MA) cation is usually contained, and the thermal stability of MA is always a great concern. In this work, according to composition engineering, a nearly formamidinium (FA) based low‐bandgap Pb–Sn mixed perovskite FAPb_0.75Sn_0.25I₃ is being tried to explore as the absorber layer. Combined with interface engineering by replacing poly(3,4‐ethylenedioxythiophene)‐polystyrenesulfonic acid (PEDOT:PSS), layer with NiO _x as hole transport layer, a power conversion efficiency of 17.25% is obtained. This low‐bandgap perovskite solar cell maintains about 91% of its original efficiency at 80 °C for 20 h, and 92% of its initial performance after 46 days storage at the room temperature. The good thermal stability of nearly FA based low‐bandgap perovskite could be good for delivering efficient and stable perovskite‐perovskite tandem solar cells.