Chen Weijie

Nature Energy, Published online: 08 February 2021; doi:10.1038/s41560-020-00768-4

A dual‐functional method of simultaneously passivating trap defects in both perovskite and electron transport layer (ETL) films is proposed. Europium ions distribute throughout SnO₂ film and diffuse into perovskite, while most of Eu³⁺ remain at the interface. Under the synergistic effect of distributed Eu³⁺, the electron mobility of ETL is improved and the trap density of perovskite is also reduced.

Abstract

So far, most techniques for modifying perovskite solar cells (PSCs) focus on either the perovskite or electron transport layer (ETL). For the sake of comprehensively improving device performance, a dual‐functional method of simultaneously passivating trap defects in both the perovskite and ETL films is proposed that utilizes guidable transfer of Eu³⁺ in SnO₂ to perovskite. Europium ions are distributed throughout the SnO₂ film during the formation process of SnO₂, and they can diffuse directionally through the SnO₂/perovskite interface into the perovskite, while most of the europium ions remain at the interface. Under the synergistic effect of distributed Eu³⁺ in the SnO₂ and aggregated Eu³⁺ at the interface, the electron mobilities of ETLs are evidently improved. Meanwhile, diffused Eu³⁺ ions passivate the perovskite to reduce trap densities at the grain boundaries, which can dramatically elevate the open‐circuit voltage (V _oc) of PSCs. Finally, the mainly PSCs coated on SnO₂:Eu³⁺ ETL achieve a power conversion efficiency of 20.14%. Moreover, an unsealed device degrades by only 13% after exposure to ambient atmosphere for 84 days.

High efficiency and humidity‐resistant flexible perovskite solar cells (FPSCs) are fabricated, using a SnO₂/Al(acac)₃ bilayer as the electron transfer layer. FPSCs present long‐time stability in ambient conditions (>50% relative humidity) without encapsulation, while yielding a power conversion efficiency (PCE) of up to 20.87%. That may open a new way to improve the stability of FPSCs.

Flexible perovskite solar cells (FPSCs) with high efficiency and excellent mechanical flexible properties have attracted enormous interest as a promising photovoltaic technology in recent years. However, the performance or stability of FPSCs is still far inferior to that of conventional glass‐based perovskite solar cells (PSCs). Herein, a cross‐linking agent called aluminum acetylacetonate (Al(acac)₃) is introduced as an interface layer between electron transport layer and perovskite absorber. Due to the well‐matched energy levels and improved grain size and crystallinity of the perovskite, a champion device with the highest power conversion efficiency (PCE) of 20.87% is achieved on the FPSCs. The device retains about 80% of its initial performance after 1000 h under >50% relative humidity without encapsulation. In addition, attributed to the Al(acac)₃ super bending resistance, more than 91% of the original PCE is retained after 1500 bending cycles. This work proposes the substrate side optimization for improving device efficiency and stability which may provide a novel concept for promoting the development of FPSCs.

An electrospray printing technique is developed to continuously print the TiO₂ electron transport layer, perovskite layer, and carbon layer, enabling a cost‐effective device. The electrospray technique is capable of printing uniform, compact, and high adhesion layers with controllable dimensions and patterns. This work demonstrates a fully printed low‐cost solar cell and provides a feasible process for perovskite solar cells to initial industrialization.

Abstract

With the power conversion efficiencies of perovskite solar cells (PSCs) exceeding 25%, the PSCs are a step closer to initial industrialization. Prior to transferring from laboratory fabrication to industrial manufacturing, issues such as scalability, material cost, and production line compatibility that significantly impact the manufacturing remain to be addressed. Here, breakthroughs on all these fronts are reported. Carbon‐based PSCs with architecture fluorine doped tin oxide (FTO)/electron transport layer/perovskite/carbon, that eliminate the need for the hole transport layer and noble metal electrode, provide ultralow‐cost configuration. This PSC architecture is manufactured using a scalable and industrially compatible electrospray (ES) technique, which enables continuous printing of all the cell layers. The ES deposited electron transport layer and perovskite layer exhibit properties comparable to that of the laboratory‐scale spin coating method. The ES deposited carbon electrode layer exhibits superior conductivity and interfacial microstructure in comparison to films synthesized using the conventional doctor blading technique. As a result, the fully ES printed carbon‐based PSCs show a record 14.41% power conversion efficiency, rivaling the state‐of‐the‐art hole transporter‐free PSCs. These results will immediately have an impact on the scalable production of PSCs.

The templated growth of ultra‐thin 2D perovskite with pre‐formed lead iodide at grain boundaries is developed. The 2D perovskite enhances charge‐carrier extraction and transport and can also suppresses interfacial nonradiative recombination and improve phase stability. The 2D‐based inorganic solar cell exhibits an efficiency of 18.82% with a high open‐circuit voltage and excellent stability.

Abstract

It is highly desirable for all‐inorganic perovskite solar cells (PVSCs) to have reduced nonideal interfacial charge recombination in order to improve the performance. Although the construction of a 2D capping layer on 3D perovskite is an effective way to suppress interfacial nonradiative recombination, it is difficult to apply it to all‐inorganic perovskites because of the resistance of Cs⁺ cesium ions in cation exchange reactions. To alleviate this problem, a simple approach using an ultra‐thin 2D perovskite to terminate CsPbI₃ grain boundaries (GBs) without damaging the original 3D perovskite is developed. The 2D perovskite at the GBs not only enhances the charge‐carrier extraction and transport but also effectively suppresses nonradiative recombination. In addition, because the 2D perovskite can prevent the moisture and oxygen from penetrating into the GBs and at the same time suppress the ion migration, the 2D terminated CsPbI₃ films exhibit significantly improved stability against humidity. Moreover, the devices without encapsulation can retain ≈81% of its initial power conversion efficiency (PCE) after being stored at 40 ± 5% relative humidity for 84 h. The 2D‐based champion device exhibits a high PCE of 18.82% with a high open‐circuit voltage of 1.16 V.

Highly efficient CsPbI_1.5Br_1.5 perovskite solar cells (PSCs) are achieved via introducing fluorescein isothiocyanate (FITC) organic dye as passivator. FITC not only reduces the metal ion related trap states but also improves film crystallinity, resulting in an enhancement of device efficiency from 12.3% to 14.05%. In addition, it is demonstrated that CsPbI_1.5Br_1.5 perovskite shows the optimal halide composition for inorganic PSCs.

Abstract

All‐inorganic perovskite solar cells (PSCs) have recently received growing attention as a promising template to solve the thermal instability of organic–inorganic PSCs. However, the thermodynamic phase instability and relatively low device efficiency pose challenges. Herein, highly efficient and stable CsPbI_1.5Br_1.5 compositional perovskite‐based inorganic PSCs are fabricated using an organic dye, fluorescein isothiocyanate (FITC), as a passivator. The carboxyl and thiocyanate groups of FITC not only minimize the trap states by forming interactions with the under‐coordinated Pb²⁺ ions but also significantly increase the grain size and improve the crystallinity of the perovskite films during annealing. Consequently, perovskite films with superior optoelectronic properties, prolonged carrier lifetime, reduced trap density, and improved stability are obtained. The resulting device yields a champion efficiency of 14.05% with negligible hysteresis, which presents the highest reported efficiency for inorganic CsPbI_1.5Br_1.5 solar cells reported thus far. In addition, FITC can be generally adopted as attractive passivator to improve the performance of CsPbI₂Br‐ and CsPbIBr₂‐based PSCs. Furthermore, with a comprehensive comparison of mixed‐halide inorganic perovskites, it is demonstrated that CsPbI_1.5Br_1.5 compositional perovskite is a promising candidate with the optimal halide composition for high‐performance inorganic PSCs.

A new class of polymer acceptors (P _As, P(BDT2BOY5‐X)) consisting of benzodithiophene (BDT) and non‐fullerene small molecule‐accepting units is developed, which shows excellent material compatibility with an efficient BDT‐based polymer donor (P _D). The resulting all‐polymer solar cells show excellent photovoltaic efficiency, thermal stability, and mechanical robustness at the same time, benefitting from the high chemical and molecular compatibilities between P _D and P _A.

Abstract

All‐polymer solar cells (all‐PSCs) are a highly attractive class of photovoltaics for wearable and portable electronics due to their excellent morphological and mechanical stabilities. Recently, new types of polymer acceptors (P _As) consisting of non‐fullerene small molecule acceptors (NFSMAs) with strong light absorption have been proposed to enhance the power conversion efficiency (PCE) of all‐PSCs. However, polymerization of NFSMAs often reduces entropy of mixing in PSC blends and prevents the formation of intermixed blend domains required for efficient charge generation and morphological stability. One approach to increase compatibility in these systems is to design P _As that contain the same building blocks as their polymer donor (P _D) counterparts. Here, a series of NFSMA‐based P _As [P(BDT2BOY5‐X), (X = H, F, Cl)] are reported, by copolymerizing NFSMA (Y5‐2BO) with benzodithiophene (BDT), a common donating unit in high‐performance P _Ds such as PBDB‐T. All‐PSC blends composed of PBDB‐T P _D and P(BDT2BOY5‐X) P _A show enhanced molecular compatibility, resulting in excellent morphological and electronic properties. Specifically, PBDB‐T:P(BDT2BOY5‐Cl) all‐PSC has a PCE of 11.12%, which is significantly higher than previous PBDB‐T:Y5‐2BO (7.02%) and PBDB‐T:P(NDI2OD‐T2) (6.00%) PSCs. Additionally, the increased compatibility of these all‐PSCs greatly improves their thermal stability and mechanical robustness. For example, the crack onset strain (COS) and toughness of the PBDB‐T:P(BDT2BOY5‐Cl) blend are 15.9% and 3.24 MJ m^–3, respectively, in comparison to the PBDB‐T:Y5‐2BO blends at 2.21% and 0.32 MJ m^–3.

Nonfullerene acceptors dominate organic solar cell research due to their promising high device efficiencies. However, key challenges for achieving high stability in commercially viable devices still remain. In this review, recent progress and challenges toward stable organic solar cells are discussed correlating molecular design and device engineering to device stability.

Abstract

Organic solar cells (OSCs) based on nonfullerene acceptors (NFAs) have made significant breakthrough in their device performance, now achieving a power conversion efficiency of ≈18% for single junction devices, driven by the rapid development in their molecular design and device engineering in recent years. However, achieving long‐term stability remains a major challenge to overcome for their commercialization, due in large part to the current lack of understanding of their degradation mechanisms as well as the design rules for enhancing their stability. In this review, the recent progress in understanding the degradation mechanisms and enhancing the stability of high performance NFA‐based OSCs is a specific focus. First, an overview of the recent advances in the molecular design and device engineering of several classes of high performance NFA‐based OSCs for various targeted applications is provided, before presenting a critical review of the different degradation mechanisms identified through photochemical‐, photo‐, and morphological degradation pathways. Potential strategies to address these degradation mechanisms for further stability enhancement, from molecular design, interfacial engineering, and morphology control perspectives, are also discussed. Finally, an outlook is given highlighting the remaining key challenges toward achieving the long‐term stability of NFA‐OSCs.

Bridge‐jointed 2D nanosheets are inserted between the methylammonium‐free perovskite and the dopant‐free hole transport layer (HTL) to form a scalable heterostructure, which preserves p‐type semiconduction of HTL and suppresses nonradiative‐recombination. Further, a perovskite solar module with an area of 35.80 cm² shows a certified efficiency of 15.3% and encapsulated modules retain over 91% of initial efficiency after damp heat test for 1000 h.

Abstract

Perovskite solar cell (PSC) modules employing a hole transport layer (HTL) without unstable dopants possess high potential for improving operational stability. However, the low efficiencies of the devices greatly limit their commercial applications owing to the lower efficacy of the dopant‐free HTL, introduced by the unintentional n‐doping effect of volatile ions from the halide‐rich perovskite surface. Here, a scalable heterostructure integrated by a methylammonium‐free perovskite film with an iodide‐rich surface, an ultrathin interlayer of bridge‐jointed graphene oxide nanosheets (BJ‐GO), and an HTL without additional ionic dopants is developed. In this heterostructure, the iodide ions are physically immobilized by the compact 2D network, and lead defects are chemically passivated by multiple coordination bonds. Moreover, the BJ‐GO with tunable surface energy enables a highly ordered HTL a considerably improved carrier mobility by an order of magnitude. Finally, the PSC module with an area of 35.80 cm² employing this heterostructure shows a certified efficiency of 15.3%. The encapsulated PSC modules retain over 91% of initial efficiency after the damp heat test at 85 °C and ≈85% relative humidity for 1000 h, while maintaining 90% of the initial value for 1000 h at the maximum power point under continuous 1‐Sun illumination at 60 °C.

Inorganic perovskite CsPbI₂Br is applied to prepare high‐performance semitransparent perovskite solar cells (ST‐PSCs). (Chloromethylene)‐dimethylammonium chloride as an additive is introduced into the perovskite precursor to favor high‐quality CsPbI₂Br perovskite films. Through optimizing the perovskite film, interface, and electrode type, the efficiency of the ST‐PSC reaches 14.01% and 10.36% under an average visible transmittance (AVT) of 31.7% and 40.9%, respectively.

Abstract

Thanks to the tunable bandgap and excellent photoelectric characteristics, perovskites have been widely used in semitransparent solar cells (ST‐SCs). Most works present unsatisfactory power conversion efficiencies (PCEs) through reducing the thickness of the perovskite films because there is a trade‐off between PCE and average visible transmittance (AVT). As a consequence, most PCEs are less than 12% when the AVT is higher than 20% due to the limited voltage (V _oc) and short‐circuit current (J _sc). Herein, a strategy of intermediate adduct (IMAT) engineering is developed to improve the film quality of the inorganic perovskite CsPbI₂Br, which is a challenging issue to limit its performance of efficiency and stability. A normal n–i–p‐structured PSC based on the optimal CsPbI₂Br film delivers a PCE of 16.02% with excellent stability. Furthermore, through optimizing the electrode type and interface, the ST‐PSC shows a high V _oc larger than 1.2 V and the PCE reaches 14.01% and 10.36% under an AVT of 31.7% and 40.9%, respectively. This is the first demonstration of a CsPbI₂Br ST‐PSC, and it outperforms most of other types of perovskites.

Over 23% efficiency is achieved using a stabilized phenyl‐C₆₁‐butyric acid methyl ester (PCBM):bathophenanthroline (Bphen) interlayer in SnO₂‐based perovskite solar cells, which can retain over 92% of their initial efficiency after 1000 h continuous illumination of maximum power point tracking at 60 °C.

Abstract

It is crucial to make perovskite solar cells sustainable and have a stable operation under natural light soaking before they become commercially acceptable. Herein, a small amount of the small molecule bathophenanthroline (Bphen) is introduced into [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester and it is found that Bphen can stabilize the C₆₀‐cage well through formation of much more thermodynamically stable charge‐transfer complexes. Such a strengthened complex is used as an interlayer at the in‐light perovskite/SnO₂ side to achieve a champion device with efficiency of 23.09% (certified 22.85%). Most importantly, the stability of the resulting devices can be close to meeting the requirements of the International Electrotechnical Commission 61215 standard under simulated UV preconditioning and light‐soaking testing. They can retain over 95% and 92% of their initial efficiencies after 1100 h UV irradiation and 1000 h continuous illumination of maximum power point tracking at 60 °C, respectively.

A dissymmetric fused‐ring acceptor BTIC‐2Cl‐γCF₃ with chlorine and trifluoromethyl end groups give a power conversion efficiency (PCE) of over 17 % which is the highest among polymer solar cells processed by halogen‐free solvents. Dissymmetric chlorination and trifluoromethylation is a practical approach towards a low band‐gap acceptor for eco‐compatible processed photovoltaic applications.

Abstract

To elevate the performance of polymer solar cells (PSC) processed by non‐halogenated solvents, a dissymmetric fused‐ring acceptor BTIC‐2Cl‐γCF₃ with chlorine and trifluoromethyl end groups has been designed and synthesized. X‐ray crystallographic data suggests that BTIC‐2Cl‐γCF₃ has a 3D network packing structure as a result of H‐ and J‐aggregations between adjacent molecules, which will strengthen its charge transport as an acceptor material. When PBDB‐TF was used as a donor, the toluene‐processed binary device realized a high power conversion efficiency (PCE) of 16.31 %, which improved to 17.12 % when PC71ThBM was added as the third component. Its efficiency of over 17 % is currently the highest among polymer solar cells processed by non‐halogenated solvents. Compared to its symmetric counterparts BTIC‐4Cl and BTIC‐CF₃‐γ, the dissymmetric BTIC‐2Cl‐γCF₃ integrates their merits, and has optimized the molecular aggregations with excellent storage and photo‐stability, and also extending the maximum absorption peak in film to 852 nm. The devices exhibit good transparency indicating a potential utilization in semi‐transparent building integrated photovoltaics (ST‐BIPV).

Publication date: March 2021

Source: Applied Materials Today, Volume 22

Author(s): Anna Z. Szeremeta, Andrzej Nowok, Sebastian Pawlus, Katarzyna Fedoruk, Monika Trzebiatowska, Mirosław Mączka, Joanna Symonowicz, Marian Paluch, Adam Sieradzki

In situ grazing‐incidence wide‐angle X‐ray scattering experiments are carried out to probe the function of a 1,8‐diiodooctane (DIO) additive in manipulating the crystallization kinetics of perovskite. The presence of DIO induces a multi‐stage intermediate phase transformation process and enhances the activation energy E _a for nucleation and crystallization, resulting in a highly controllable crystallization pathway and optimized perovskite thin film morphology.

Abstract

Additive processing is proven to be an effective method to improve the efficiency and stability of perovskite solar cells; however, its intrinsic role in directing the crystallization pathway and thus morphology formation remains unknown. In situ grazing‐incidence wide‐angle x‐ray scattering (GIWAXS) is applied to study the function of a 1,8‐diiodooctane (DIO) additive in manipulating the crystallization behavior of perovskite thin films. It is seen that the DIO additive could induce multi‐stage intermediate crystallization phases and increases the activation energy for nucleation and growth, which postpones the perovskite phase transformation time and broadens the transition zone. The elongated crystallization process affords improved perovskite thin film crystallinity and reduces defect density, which enables a longer carrier diffusion length. As a result, improved device efficiency, moisture, and thermal stability can be achieved. The current study provides a new prospective in understanding the additive function in perovskite thin film morphology control from fundamental parameters, indicating the importance of minor processing conditions in global property management toward high device performance.