Chen Weijie

Defect tolerance of metal‐halide perovskites is a commonly evoked concept to explain the development of high efficiency solar cells upon solution processing. However, moving the attention to solar cell stability, these materials seem to be defect intolerant. Further material engineering is needed to obtain a 100% defect tolerant materials platform.

Abstract

Metal‐halide perovskites present exceptional optoelectronic properties such as large light absorption coefficients, long free charge carrier diffusion lengths with ambipolar character. They are apparently protected by what is often described as a “defect tolerance” which has allowed to achieve, relatively quickly, highly performing devices. Nevertheless, there also exists a “defect intolerance” when it is dealt with stability. Further rationalization of the passivation strategies, especially for complex chemical systems, will be beneficial to achieve a full materials library which can be the platform for an efficient and reliable technology.

High‐efficiency solution‐processed hybrid tandem photovoltaic devices, employing inorganic perovskite and organic bulk‐heterojunction as the photoactive layers, are demonstrated. A PCE of 18.04% in the hybrid tandem device is achieved, which is significantly higher than the comparable single‐junction devices, owing to a near‐optimal absorption spectral match.

Abstract

Although the power conversion efficiency (PCE) of inorganic perovskite‐based solar cells (PSCs) is considerably less than that of organic‐inorganic hybrid PSCs due to their wider bandgap, inorganic perovskites are great candidates for the front cell in tandem devices. Herein, the low‐temperature solution‐processed two‐terminal hybrid tandem solar cell devices based on spectrally matched inorganic perovskite and organic bulk heterojunction (BHJ) are demonstrated. By matching optical properties of front and back cells using CsPbI₂Br and PTB7‐Th:IEICO‐4F BHJ as the active materials, a remarkably enhanced stabilized PCE (18.04%) in the hybrid tandem device as compared to that of the single‐junction device (9.20% for CsPbI₂Br and 10.45% for PTB7‐Th:IEICO‐4F) is achieved. Notably, the PCE of the hybrid tandem device is thus far the highest PCE among the reported tandem devices based on perovskite and organic material. Moreover, the long‐term stability of inorganic perovskite devices under humid conditions is improved in the hybrid tandem device due to the hydrophobicity of the PTB7‐Th:IEICO‐4F back cell. In addition, the potential promise of this type of hybrid tandem device is calculated, where a PCE of as much as ≈28% is possible by improving the external quantum efficiency and reducing energy loss in the sub‐cells.

A novel BDT‐based random polymeric hole‐transporting layer (asy‐ranPBTBDT) is developed with irregularity from asymmetric substitution and random copolymerization. The resulting low crystallinity from the irregularity leads to superior solubility capacity and suppressed charge recombination and morphological changes. Therefore, the colloidal quantum dot solar cells using asy‐ranPBTBDT‐based device show highly efficient power conversion efficiency of 13.2% with superior operational stability.

Abstract

Next‐generation solution‐processed solar cells will hopefully be processed using green solvents, and will unite high performance with operating stability. Colloidal quantum dot/polymer hybrid solar cells are of interest for their harvest of the visible as well as the near infrared; however, today's best polymer hole‐transporting layers (HTLs) rely on processing using hazardous solvents such as chlorobenzene. This stems from the strong polymer–polymer attraction in polymeric p‐type materials, which accounts for their limited solubility. Here, a new random polymeric HTL (asy‐ranPBTBDT) is reported that is soluble in green solvents such as 2‐methylanisole without compromising ultimate device power conversion efficiency. The new polymer structure induces a strong π–π stacking face‐on orientation and less lateral grain growth compared to control asy‐PBTBDT, leading to reduced charge recombination and improved device stability. The resulting device exhibits a power conversion efficiency (PCE) of 13.2% and retains 89% of its initial efficiency after 120 h of continuous device operation at the maximum power point, compared to a PCE of 11.4% and 71% degradation for control devices.

This study reports the deposition of a TiO₂ electron transporting layer for perovskite solar cells by spray coating using a stable TiO₂ colloidal aqueous solution, which is synthesized via the self‐condensation of a titanium peroxide complex under hydrothermal conditions. Although the whole fabrication process for the cells is performed at 100 °C, 22.7% efficiency is achieved.

Abstract

TiO₂ is one of the most efficient and widely used materials for electron‐transporting layer (ETLs) in perovskite solar cells (PSCs). The formation of efficient TiO₂ layers is generally carried out at high temperature by baking at a temperature >400 °C or by vacuum deposition (e.g., atomic layer deposition and E‐beam). In this study, the preparation of a TiO₂ ETL for PSCs is reported with excellent properties at low temperatures based on the synthesis of a stable TiO₂ colloidal aqueous solution and spray coating. The prepared TiO₂ colloids are able to produce a dense and uniform ETL even if it is simply dried at 100 °C after spray coating. It is believed that this is owing to the peroxo functional group remaining on the surface of the TiO₂ colloids. The TiO₂ ETLs, combined with the TiO₂ underlayer formed by chemical bath deposition, and the sprayed TiO₂ colloids allowed the fabrication of PSCs with performance similar to those of PSCs produced by annealing at 450 °C with a TiO₂ paste. The PSCs fabricated entirely at 100 °C demonstrated power conversion efficiency of 22.7% in small cells, and 19.0% in mini‐modules.

To generate cost‐efficient and high‐performanced polymers, a simple chemical steric effect (SE) is introduced to benzothiophene (BDT)‐based side chains. The polymeric crystallinity and miscibility are rebalanced and a power conversion efficiency (PCE) of 14.53% is achieved. Thus, the SE applied in crystalline polymer pave an easier and cheaper route to realize the coordination of low‐cost fabrication and high‐performance.

Abstract

Low synthetic cost and high performance are becoming a new challenge in designing polymer donors for large‐scaled polymer solar cells (PSCs) fabrication; however, complicated synthetic routes and high material costs hamper the widespread commercial application of OPVs. Here, a simple and low‐cost chemical steric effect (SE) is introduced to BDT‐based side chains. Through adjusting alkyl side chains, the polymeric crystallinity and miscibility are rebalanced and subsequently the photovoltaic device based on the meta‐positioned alkyl polymer outperforms its para‐positioned counterpart. The champion device based on the polymer with the meta‐positioned side chains affords a PCE of 14.53% without sacrificing its high fill factor of 0.77, which could be attributed to a more balanced charge‐carrier transport ability and optimized morphology. This is the highest PCE value reported in BTZ based polymer donors to date. Thus, it shows that the SE applied in high crystalline polymer could pave an easier and cheaper chemical route to realize the coordination of low‐cost fabrication and high‐performance.

Metal halide perovskite (MHP)‐based tandem solar cells, including MHP/silicon, MHP/CuInGa, MHP/organic photovoltaic, MHP/quantum dot, and all‐perovskite tandem cells, which are boosting the development of cost‐effective and high‐performance, next‐generation solar cells than can compete with fossil fuels, are reviewed.

Abstract

Metal halide perovskite (MHP)‐based tandem solar cells are a promising candidate for use in cost‐effective and high‐performance solar cells that can compete with fossil fuels. To understand the research trends for MHP‐based tandem solar cells, a general introduction to single‐junction and multiple‐junction MHP solar cells and the configuration of tandem devices is provided, along with an overview of the recent progress regarding various MHP‐based tandem cells, including MHP/crystalline silicon, MHP/CuInGaS, MHP/organic photovoltaic, MHP/quantum dot, and all‐perovskite tandem cell. Future research directions for MHP‐based tandem solar cells are also discussed.

The whole crystallization pathways and mechanism of two‐step‐fabricated perovskites are unveiled by in situ grazing‐incidence wide‐angle X‐ray scattering measurements and density functional theory calculations. Sequential A‐site doping of Cs⁺ and GA⁺ is found to alter the crystallization kinetics and improves the perovskite film morphology, giving rise to device efficiency as high as 23.5%.

Abstract

Two‐step‐fabricated FAPbI₃‐based perovskites have attracted increasing attention because of their excellent film quality and reproducibility. However, the underlying film formation mechanism remains mysterious. Here, the crystallization kinetics of a benchmark FAPbI₃‐based perovskite film with sequential A‐site doping of Cs⁺ and GA⁺ is revealed by in situ X‐ray scattering and first‐principles calculations. Incorporating Cs⁺ in the first step induces an alternative pathway from δ‐CsPbI₃ to perovskite α‐phase, which is energetically more favorable than the conventional pathways from PbI₂. However, pinholes are formed due to the nonuniform nucleation with sparse δ‐CsPbI₃ crystals. Fortunately, incorporating GA⁺ in the second step can not only promote the phase transition from δ‐CsPbI₃ to the perovskite α‐phase, but also eliminate pinholes via Ostwald ripening and enhanced grain boundary migration, thus boosting efficiencies of perovskite solar cells over 23%. This work demonstrates the unprecedented advantage of the two‐step process over the one‐step process, allowing a precise control of the perovskite crystallization kinetics by decoupling the crystal nucleation and growth process.

A monolithic organic/colloidal quantum dots hybrid tandem solar cell (TSC) is developed by introducing dual near‐infrared absorbers. In addition, the interconnecting layer is engineered to use an organic subcell as a front cell to increase the matching current. Therefore, the hybrid TSC shows the highest power conversion efficiency (PCE) of 13.7% in comparision with previously reported monolithic PbS colloidal quantum dot (CQD)‐based hybrid TSCs.

Abstract

Monolithically integrated hybrid tandem solar cells (TSCs) that combine solution‐processed colloidal quantum dot (CQD) and organic molecules are a promising device architecture, able to complement the absorption across the visible to the infrared. However, the performance of organic/CQD hybrid TSCs has not yet surpassed that of single‐junction CQD solar cells. Here, a strategic optical structure is devised to overcome the prior performance limit of hybrid TSCs by employing a multibuffer layer and a dual near‐infrared (NIR) absorber. In particular, a multibuffer layer is introduced to solve the problem of the CQD solvent penetrating the underlying organic layer. In addition, the matching current of monolithic TSCs is significantly improved to 15.2 mA cm⁻² by using a dual NIR organic absorber that complements the absorption of CQD. The hybrid TSCs reach a power conversion efficiency (PCE) of 13.7%, higher than that of the corresponding individual single‐junction cells, representing the highest efficiency reported to date for CQD‐based hybrid TSCs.

Organic‐inorganic hybrid perovskite materials have emerged as promising photovoltaic candidates. Herein, a supramolecular complex [(C₈H₁₇)₄N]₄[SiW₁₂O₄₀] is synthesized and introduced into SnO₂ to produce a mutifunctionalized electron transport layer (ETL). Suppressed trap state density and improved band alignment are attained in modified perovskite solar cells. Devices with [(C₈H₁₇)₄N]₄[SiW₁₂O₄₀] show a champion efficiency of 22.84% and stable performance under irradiation.

Recently, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has been developed to exceed 25%, and charge transport layer optimization is a promising strategy for further efficiency improvement in PSCs. Herein, a supramolecular complex [(C₈H₁₇)₄N]₄[SiW₁₂O₄₀] (TASiW‐12) is synthesized and its doped form in SnO₂ (hereafter S‐SnO₂) is used as a charge transport layer (electron transport layer, ETL). This study demonstrates that S‐SnO₂ introduction is a practical and effective way to improve the bulk ETL and those of the ETL/perovskite interface. S‐SnO₂ leads to improved band alignment, suppressed trap‐assisted charge recombination, and enhanced electron mobility. In addition, an enhanced open‐circuit voltage (V _oc) of 1.16 V and an efficiency of 22.8% are successfully achieved in n–i–p planar PSCs. Meanwhile, S‐SnO₂ acts as a crucial agent to reduce charge accumulation at the S‐SnO₂/perovskite interface. The device possesses superior stability for 3072 h with only a 5.65% loss of the initial PCE. These results indicate that high‐efficiency PSCs can be easily attained by introducing a TASiW‐12‐doped ETL with integrated functions.

The optical modeling of substrate‐configuration metal‐halide perovskite solar cells on opaque substrates such as planarized steel quantitatively explains the short‐circuit current density and identifies that it is limited by absorption and reflection by the dielectric–metal–dielectric top electrode and absorption by the organic hole transport layer.

To date, substrate‐configuration metal‐halide perovskite solar cells (PSCs) fabricated on opaque substrates such as metal foils provide inferior efficiencies compared with superstrate‐configuration cells on transparent substrates such as glass. Herein, a substrate‐configuration PSC on planarized steel is presented. To quantify the differences between the two configurations, a 15.6%‐efficient n–i–p superstrate‐configuration PSC is transformed step wise into a substrate‐configuration cell. Guided by optical modeling, the opaque Au electrode is replaced by a transparent MoO₃/thin Au/polystyrene dielectric–metal–dielectric electrode. The semitransparent device affords efficiencies of 15.4% and 11.4% for bottom and top illumination, respectively. Subsequently, substrate‐configuration PSCs with a metal bottom electrode are fabricated on glass and planarized steel, using a thin MoO₃ interlayer between the Au bottom electrode and the SnO₂ electron transport layer. The glass‐based substrate‐configuration cell provides 14.0% efficiency with identical open‐circuit voltage and fill factor as the superstrate cell. The cell on planarized steel reaches 11.5% efficiency due to a lower fill factor. For both substrate‐configuration cells, the lower short‐circuit current density limits the efficiency. Optical modeling explains this quantitatively to be due to absorption and reflection by the top electrode and absorption by the organic hole transport layer.

A solution‐processed calcium acetylacetone film is used as an electron‐selective contact in crystalline silicon solar cells to reduce the rear‐side contact resistance loss. Combined with an intrinsic amorphous silicon passivating layer, a stable power conversion efficiency of 21.6% is realized with full area rear contact.

Abstract

Crystalline silicon (c‐Si) solar cells featuring carrier‐selective passivating contacts have become a prominent path to develop highly efficient photovoltaic devices. Development of electron‐selective materials that can provide excellent surface passivation and low contact resistivity to c‐Si substrates while presenting good environmental stability is crucial for practical implementation. Here, an easy approach is demonstrated to achieve low resistivity Ohmic contacts between slightly doped n‐type c‐Si and aluminum electrodes via simple spin‐coating of metal acetylacetone (MAcac) film on a c‐Si surface. Contact resistivity of 1.3 mΩ cm² (18.2 mΩ cm² with an a‐Si:H(i) passivating layer) is realized when a thin calcium acetylacetone (CaAcac) interlayer is introduced between c‐Si and Al. An n‐Type c‐Si solar cell with a full area rear a‐Si:H(i)/CaAcac/Al electron‐selective contact is demonstrated with a power conversion efficiency of 21.6%. This work not only demonstrates an approach to develop highly efficient n‐type c‐Si solar cells with effective electron‐selective passivating contacts, but also contributes toward accomplishing a simplified fabrication process for photovoltaic devices, from vacuum to solution processing.

Nature Energy, Published online: 14 September 2020; doi:10.1038/s41560-020-00687-4

A generally applicable sequential deposition (SD) strategy is developed to construct high‐performance organic solar cells (OSCs) without involving complicated procedures for morphological control. The SD‐processed OSCs via simple adjustment of the acceptor layer to impact the blend phase can afford higher efficiencies than their conventional OSC counterparts, providing an avenue toward promoting better photovoltaic performance and reducing production requirements.

Abstract

Bulk‐heterojunction (BHJ) organic solar cells (OSCs) are prepared by a common one‐step solution casting of donor‐acceptor blends often encounter dynamic morphological evolution which is hard to control to achieve optimal performance. To overcome this hurdle, a generally applicable, sequential processing approach has been developed to construct high‐performance OSCs without involving tedious processes. The morphology of photoactive layers comprising a polymer donor (PM6) and a nonfullerene acceptor (denoted as Y6‐BO) can be precisely manipulated by tuning Y6‐BO layer with a small amount of 1‐chloronaphthalene additive to induce the structural order of Y6‐BO molecules to impact the blend phase. The results of a comparative investigation elucidate that such two‐step procedure can afford more favorable BHJ microstructure than that achievable with the single blend‐casting route. This translates into improved carrier generation and transport, and suppressed charge recombination. Consequently, the devices based on sequential deposition (SD) deliver a remarkable efficiency up to 17.2% (the highest for SD OSCs to date), outperforming that from the conventional BHJ devices (16.4%). The general applicability of this approach has also been tested on several other nonfullerene acceptors which show similar improvements. These results highlight that SD is a promising processing alternative to promote better photovoltaic performance and reduce production requirements.

As a key factor affecting the performance and stability of perovskite solar cells, the application of the interface layer is crucial. This review summarizes that ultra‐dense and stable metal oxide layers can be prepared by atomic layer deposition for application in perovskite solar cells, and their commercial feasibility is prospected.

Abstract

In recent years, the development of perovskite solar cells (PSCs) is advancing along the way, and the efficiency is comparable to traditional silicon‐based solar cells. However, as crucial factors in the road to commercialization, stability and upscaling manufacture have not been fully investigated yet. To solve these problems, the exploration of charge transport layer (CTL) is clearly imminent, which is critical to the stability of PSCs. Among them, inorganic metal oxides have better stability than organic CTL. Particularly, the atomic layer deposition (ALD) process can fabricate dense and scalable metal oxides based on the self‐limiting surface reaction. This perspective focuses on the recent progress of ALD‐grown metal oxides in PSCs: both of electron and hole transport layer; connection layer in tandem architectures; application in semi‐transparent perovskite solar cells (ST‐PSCs); prospective of commercialization feasibility of the ALD‐grown metal oxides in ST‐PSCs.

The inorganic salt of potassium thiocyanate plays a triple passivation role at the interface between NiO _x hole transport layer and perovskite by strong covalent band and electrostatic force. It can reduce the interface defects and promote carrier extraction and transportation, which leads to high efficiency and good stability. This provides a promising multiple passivation strategy for improving perovskite solar cells performance.

Abstract

Inverted perovskite solar cells (PSCs) are still suffering low power conversion efficiency because of hole accumulation and trap‐assisted non‐radiative recombination at the interface originating from the large energy offset, interface defects, and rough contact. Here, a triple passivation of the two in‐between surfaces of the hole transport layer (HTL) and perovskite is proposed. The inorganic salt of potassium thiocyanate (KSCN) is introduced to simultaneously cross‐link NiO _x , HTL, and methylammonium lead iodide (MAPbI₃), which can significantly improve both device performances and stability. In addition to potassium passivation, the thiocyanate shows two good passivation effects on perovskite and NiO _x to achieve the triple passivation. The strong NiN bonding exhibits strong polar covalent bond properties to make the electron deviate from the Ni side. Meanwhile, the strong electrostatic force between S and Pb in MAPbI₃ makes the Pb atomic layer closer to perovskite to restrain the I atom. Meanwhile, the KSCN modification leads to better valence band alignment. Eventually, the KSCN meditated PSCs exhibit both high efficiency of 21.23% with open‐circuit voltage of 1.14 V and improved operational stability. The demonstration of triple interface passivation contributes to establishing promising multiple passivation strategies for improving the demanding PSC performances and stability.

The incorporation of various lanthanides ions in perovskite films (perovskite solar cells (PSCs)) and Ce³⁺ doping achieves the best performance, with a champion power conversion efficiency of 21.67% in contrast to 18.50% for pristine PSCs and outstanding long‐term and UV stability that originates from special Ce³⁺/Ce⁴⁺ redox pair and the unique 4f‐5d absorption in the UV region.

Abstract

Since Yan's work, incorporation of some lanthanide elements, such as Eu and Nd, into MAPbI₃ layer has been proven to be a powerful strategy on improving the permanence of the perovskite solar cells (PSCs). However, a comprehensive configuration has not been given for different lanthanide elements doping while the mechanism has not been clarified. Herein, the incorporation of various lanthanides ions (Ln³⁺ = Ce³⁺, Eu³⁺, Nd³⁺, Sm³⁺, or Yb³⁺) into perovskite films to largely enhance the performance of PSCs is presented. Arising from the enlarged grain size and crystallinity of perovskite film upon Ln³⁺ ions doping, the efficiency and stability of PSCs are significantly improved. Extraordinarily, PSCs with Ce³⁺ doping achieve the best performance, with a champion power conversion efficiency (PCE) of 21.67% in contrast to 18.50% for pristine PSCs, and outstanding long‐term and UV irradiation stability. Such high performance of PSCs after Ce³⁺ doping originates from special Ce³⁺/Ce⁴⁺ redox pair and the unique 4f‐5d absorption in the UV region. Finally, the flexible PSCs with low‐temperature preparation are explored. Considering the richer deposition of cerium element in the earth and lower price, the findings may provide new opportunities for developing low‐cost, highly efficient, air/UV stable, and flexible PSCs.

Publication date: December 2020

Source: Nano Energy, Volume 78

Author(s): Malin B. Johansson, Ling Xie, Byeong Jo Kim, Jakob Thyr, Timo Kandra, Erik M.J. Johansson, Mats Göthelid, Tomas Edvinsson, Gerrit Boschloo

Herein, the correlation between chemical structures and physicochemical properties of organic hole transporting materials, which plays fundamental roles in supervising future molecular design and synthesis for perovskite solar cells, is outlined. To move in the practical line of perovskite solar marketing, interfacial contact materials are key parts in case of stability and efficiency.

Perovskite solar cells (PSCs) with advantages of exceptional photovoltaic performance and facile solution‐processed fabrication have shown great potential in future scalable application. After about a decade of rapid development, this new PSCs technology demonstrates over 25% efficiency, a comparable performance with traditional silicon solar cells. Further, the development of PSCs in the direction of scalable production still highly relies on designing innovative materials with low cost and high efficiency. Recently, a great number of functional organic molecules as hole transport materials (HTMs) have been designed, synthesized, and studied in PSCs, including molecules with planar structure, 3D geometry, or different core units. Discovering the correlation between their chemical structures and physicochemical properties plays a fundamental role in supervising future molecular design and synthesis. Herein, recent advances in organic molecular HTMs with various structures in typical and reverse PSCs device configuration are summarized, including doped and doping‐free materials. By evaluating the structural modification and analyzing their effects on photovoltaic performance, the goal is to generate universal strategies for preparing low‐cost and efficient HTMs, paving the way for future scalable application of PSCs.

A bipolar organic material 1,4‐bis(perfluorophenyl)‐2,5‐di(pyridin‐4‐yl)‐1,4‐dihydropyrrolo [3,2‐b] pyrrole (PFPPY) is utilized to passivate perovskite surface and boundary defects via solvent engineering approach, generating an impressive power conversion efficiency (PCE) of 19.62% and greatly enhanced stability.

At the device operating conditions, defects such as interstitials, vacancies, and impurities at the grain boundary and surface of photoactive layer have great impact on the power conversion efficiency (PCE) and device stability. To better passivate the surface and boundary defects, and further enhance the PCE and device stability, herein, a bipolar organic material termed 1,4‐bis(perfluorophenyl)‐2,5‐di(pyridin‐4‐yl)‐1,4‐dihydropyrrolo [3,2‐b] pyrrole (PFPPY) is introduced as modifier in antisolvent. The effects of PFPPY on perovskite film quality, photovoltaic performance, and charge transfer properties are systematically investigated. Under the optimized conditions, the PFPPY‐treated device shows an impressive PCE of 19.62%, which is 12% higher than the reference device (17.59%), and greatly enhanced stability, maintaining 95% of its initial efficiency under room temperature (RT) and relative humidity (RH) 30% condition for 650 h without encapsulation.

Exposing metal halide perovskite films to humid air under illumination induces subbandgap defect states and increases trap‐assisted carrier recombination, while in the dark, neither are the carrier dynamics changed nor are subbandgap defect states formed. Thus, light‐activated defect formation is the origin of photovoltage losses, caused by degradation of the perovskite/spiro‐OMeTAD interface in n–i–p metal halide perovskite solar cells.

Metal halide perovskites exhibit outstanding optical and electronic properties, but are very sensitive to humidity and light‐soaking. In this work, the photophysics of perovskites that have been exposed to such conditions are studied and, in this context, the impact of excess lead iodide (PbI₂) is revealed. For exposed samples, the formation of subbandgap states and increased trap‐assisted recombination is observed, using highly sensitive absorption and time‐resolved photoluminescence (TRPL) measurements, respectively. It appears that such exposure primarily affects the perovskite surface. Consequently, on n–i–p device level, the spiro‐OMeTAD/perovskite interface is more rapidly affected than its buried electron‐collecting interface. Moreover, both stoichiometric and nonstoichiometric MAPbI₃‐based solar cells show reduced device performance mainly due to voltage losses. Overall, this study brings forward key points to consider in engineering perovskite solar cells with improved performance and material stability.