Chen Weijie

Publication date: 17 April 2019

Source: Joule, Volume 3, Issue 4

Author(s): Anita Ho-Baillie, Meng Zhang, Cho Fai Jonathan Lau, Fa-Jun Ma, Shujuan Huang

Context & Scale

Summary

Graphical Abstract

All‐bromide perovskite solar cells with gradient bandgap are constructed by vapor deposition procedure accompanying with the derivative‐phase to boost the hole extraction efficiency and stability. An impressive power conversion efficiency of 10.17% is obtained via a vapor deposition method for a hole transfer layer‐free inorganic PSC. The device also exhibits an excellent humidity and thermal stability for more than 3000 h in RH ≈45% environment and 700 h at 100 °C. These results pave a great advancement in all inorganic PSCs and also open the window of perovskite derivative‐phase.

Inorganic perovskite materials have demonstrated outstanding performance in the field of photovoltaic devices due to their superior charge carrier transport properties and excellent thermal stability. In particular, the inorganic perovskite derivative phases show special properties in terms of phase stability and optoelectronic application, especially in the phase transition investigation. However, their commercial applications still face challenges due to the large recombination at the interface, resulting in poor efficiency and metastable phases such as iodide perovskite existing in the film. Herein, an all‐bromide inorganic perovskite solar cell has been developed by introducing the derivative phases (CsPb₂Br₅ and Cs₄PbBr₆) to construct gradient bandgap architecture. This graded heterojunction device is realized with a controllable sequential vapor deposition procedure. The valance band maximum elevates gradually with the presence of derivative phases and effectively blocks electrons and boosts the hole extraction efficiency at the counter electrode, which promotes charge separation and reduces the interface recombination. Ultimately, an impressive power conversion efficiency of 10.17% is achieved through a CsPbBr₃/CsPbBr₃‐CsPb₂Br₅/CsPbBr₃‐Cs₄PbBr₆ architecture strategy with excellent stability above 3000 h (85% of initial performance) in a humid environment (@RH ≈45%) and 700 h (83% of initial efficiency) under thermal conditions (@ 100 °C).

Recently, lead halide‐based perovskites have become one of the hottest topics in photovoltaic research due to their excellent optoelectronic properties. Among them, organic‐inorganic hybrid perovskite solar cells (PSCs) have made very rapid progress with their power conversion efficiency (PCE) now at 23.7%. However the intrinsically unstable nature of these materials, particularly to moisture and heat, may be a problem for their long‐term stability. Replacing the fragile organic group with more robust inorganic Cs+ cations forms the cesium lead halide system (CsPbX3, X is halide) as all‐inorganic perovskites which are much more thermally stable and often more stable to other factors. From the first report in 2015 to now, the PCE of CsPbX3‐based PSCs has abruptly increased from 2.9% to 17.1% with much enhanced stability. In this review, we aim to summarize the field up to now, propose solutions in terms of development bottlenecks, and attempt to boost further research in CsPbX3 PSCs. After a general introduction, we discuss the various CsPbX¬3 materials and cells beginning with CsPbI3, followed by CsPbBr3 and then the mixed halide CsPb(I,Br)3 materials and cells. Finally, challenges and perspectives for the future development of CsPbX3 PSCs are presented.

Publication date: May 2019

Source: Nano Energy, Volume 59

Author(s): Chen Dong, Xiuxun Han, Wenhui Li, Qingqing Qiu, Jinqing Wang

Graphical abstract

The molecular orientation and charge extraction of PEDOT:PSS‐based hole‐transporting layers are effectively modulated through fine tuning of the surface energy by introducing poly(styrene sulfonic acid) sodium salts or nickel formate dihydrate, which boosts the fill factor and eventual efficiency of organic solar cells based on fullerene and nonfullerene acceptors.

Abstract

Interface properties are of critical importance for high‐performance bulk‐heterojunction (BHJ) organic solar cells (OSCs). Here, a universal interface approach to tune the surface free energy (γ_S) of hole‐transporting layers (HTLs) in a wide range through introducing poly(styrene sulfonic acid) sodium salts or nickel formate dihydrate into poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is reported. Based on the optimal γ_S of HTLs and thus improved face‐on molecular ordering in BHJs, enhanced fill factor and power conversion efficiencies in both fullerene and nonfullerene OSCs are achieved, which is attributed to the increased charge carrier mobility and sweepout with reduced recombination. It is found that the face‐on orientation‐preferred BHJs (PBDB‐TF:PC₇₁BM, PBDB‐T:PC₇₁BM, and PBDB‐TF:IT‐4F) favor HTLs with higher γ_S while the edge‐on orientation‐preferred BHJs (PDCDT:PC₇₁BM, P3HT:PC₇₁BM and PDCBT:ITIC) are partial to HTLs with lower γ_S. Based on the surface property–morphology–device performance correlations, a suggestion to select a suitable HTL in terms of γ_S for a specific BHJ with favored molecular arrangement is provided. This work enriches the fundamental understandings on the interface characteristics and morphological control toward high‐efficiency OSCs based on a wide range of BHJ materials.

The efficiency limits in non‐fullerene organic solar cells are examined using a numerical simulator. Power conversion efficiency (PCE) of over 18% using recently reported carrier mobility values and voltage losses, are predicted. Increasing the mobility to >10⁻³ cm² V⁻¹ s⁻¹ and decreasing the recombination constant to <10⁻¹² cm³ s⁻¹ is shown to yield a single‐junction and 2T‐tandem cell with PCEs of >20% and >25%, respectively.

Abstract

The reported power conversion efficiencies (PCEs) of nonfullerene acceptor (NFA) based organic photovoltaics (OPVs) now exceed 14% and 17% for single‐junction and two‐terminal tandem cells, respectively. However, increasing the PCE further requires an improved understanding of the factors limiting the device efficiency. Here, the efficiency limits of single‐junction and two‐terminal tandem NFA‐based OPV cells are examined with the aid of a numerical device simulator that takes into account the optical properties of the active material(s), charge recombination effects, and the hole and electron mobilities in the active layer of the device. The simulations reveal that single‐junction NFA OPVs can potentially reach PCE values in excess of 18% with mobility values readily achievable in existing material systems. Furthermore, it is found that balanced electron and hole mobilities of >10⁻³ cm² V⁻¹ s⁻¹ in combination with low nongeminate recombination rate constants of 10⁻¹² cm³ s⁻¹ could lead to PCE values in excess of 20% and 25% for single‐junction and two‐terminal tandem OPV cells, respectively. This analysis provides the first tangible description of the practical performance targets and useful design rules for single‐junction and tandem OPVs based on NFA materials, emphasizing the need for developing new material systems that combine these desired characteristics.

Publication date: May 2019

Source: Nano Energy, Volume 59

Author(s): Yanbo Gao, Yanjie Wu, Hongbin Lu, Cong Chen, Yue Liu, Xue Bai, Lili Yang, William W. Yu, Qilin Dai, Yu Zhang

Graphical abstract

In this work, the CsPbBr₃ NPs are used to serve as nucleation centers to improve the quality of the perovskite films. In addition, the introduction of CsPbBr₃ NPs also leads to the formation of Cs_1-yMAyPbI_3-xBr_x between MAPbI₃ layer and the hole transporting layer. The optimized device exhibits an PCE of 20.46% and excellent long-term stability (90% of initial PCE remains after 500 h).

Publication date: May 2019

Source: Nano Energy, Volume 59

Author(s): Seong-Cheol Yun, Sunihl Ma, Hyeok-Chan Kwon, Kyungmi Kim, Gyumin Jang, Hyunha Yang, Jooho Moon

Abstract

Graphical abstract

The addition of rigid molecular crosslinker p-aminobenzoic acid iodide (PABA∙HI) enhanced the stability of perovskites against moisture by forming quasi-two-dimensional perovskite along grain boundaries. Perovskite solar cells with PABA∙HI retained 91% of their initial power-conversion efficiency of 17.4% under a relative humidity of 75% at 25 °C after 312 h of exposure under dark condition.

In article no. 1800284, Qunwei Tang and co‐workers apply SnO₂ QDs and CsMBr₃ (M = Sn, Bi, Cu) QDs as electron transporting and hole transporting materials for all‐inorganic CsPbBr₃ PSCs, respectively. Owing to the high optical transmittance and electron mobility of the SnO₂ QDs electron transport layer as well as the hole extraction of CsMBr₃ QD hole transport layer, the device achieves a champion power conversion effi ciency of 10.60% and improved stability.

Passivation

Moisture penetration through surface defects into the active layer is responsible for degradation of device performance in humid environments. In article no. 1800327, Zhijie Wang, Huiqiong Zhou, Shengchun Qu, and coworkers show that the interaction of the perovskite material with DRCN5T increases the durability of the device in ambient conditions, because the passivated defect sites on the film surface suppresses the transit of moisture or oxygen through the defects.

PBDFFPD possesses a facile synthetic route, presents a large conjugated plane, and depicts a deep HOMO energy level. These admirable properties generate a remarkable PCE of 9.58% with a large FF of 70.1% when solar cells are fabricated based on PBDFFPD:ITIC. Obviously, PBDFFPD accords with the PSCs design philosophy of low cost, is adaptable for the mass production of PSCs, and deserves further research.

Non‐fullerene polymer solar cells (NF‐PSCs) have achieved tremendous progress in power conversion efficiency (PCE), which is mainly attributed to the well absorption complementation and the admirable energy‐level alignment between donor polymers and fullerene‐free acceptors. However, the development of efficient donor polymers pairing with fullerene‐free acceptors relatively lags behind that of fullerene‐free acceptors in terms of number and diversity for fabricating NF‐PSCs. In this work, a two‐dimensional medium bandgap copolymer (PBDFFPD) based on benzo[1,2‐b:3,4‐b′]difuran (BDF) and furo[3,4‐c]pyrrole‐4,6‐dione (FPD), is firstly designed and synthesized. The as‐prepared polymer possesses a large conjugated plane with negligible torsion, strong intermolecular and intramolecular interaction, and deep highest occupied molecular orbital (HOMO) energy level. The optimized photovoltaic device based on PBDFFPD:ITIC wins a remarkable PCE of 9.58% with a large FF of 70.1%, the highest values ever reported for FPD‐based polymers. In addition, the statistical data from different batches of devices shows that PSCs based on PBDFFPD:ITIC at optimized conditions depict an excellent reproducibility of PCE with a deviation of 2.29%. The results demonstrate that PBDFFPD possesses great potential for constructing highly efficient NF‐PSCs.

A new methodology is presented for preparing a dopant‐free Spiro‐OMeTAD film by dynamic spin‐coating the pristine Spiro‐OMeTAD solution from a halogen‐free green solvent THF, which yields a record efficiency of 17% as along with negligible hysteresis in planar PSCs. Importantly, the strategy brings the field a step closer toward cost‐effective and environmental friendly production of PSCs with enhanced longevity.

In this paper, highly efficient (17%) perovskite solar cells (PSCs) based on a hole‐transporting layer (HTL) made of dopant‐free Spiro‐OMeTAD processed from a non‐halogenated solvent (THF) are reported for the first time. In addition to the high efficiency, a negligible hysteresis effect is observed for the devices with dopant‐free Spiro‐OMeTAD hole‐transporting material (HTM), which is often a problem for planar n‐i‐p type PSCs. By eliminating the hydroscopic dopants, the ambient stability of the completed PSC devices are much improved. Another advantage of using THF as a solvent is that much less of the Spiro‐OMeTAD material is required (5 mg ml⁻¹) to coat the HTL compared to that used in a conventional chlorobenzene solvent (70 mg ml⁻¹). Our result provides a simple yet effective method to fabricate dopant‐free PSCs toward cost‐effective and environmental friendly production of PSCs with enhanced stability.

The power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) are already higher than that of other thin film technologies, but laboratory cell-fabrication methods are not scalable. Here, we report an additive strategy to enhance the efficiency and stability of PSCs made by scalable blading. Blade-coated PSCs incorporating bilateral alkylamine (BAA) additives achieve PCEs of 21.5 (aperture, 0.08 cm²) and 20.0% (aperture, 1.1 cm²), with a record-small open-circuit voltage deficit of 0.35 V under AM1.5G illumination. The stabilized PCE reaches 22.6% under 0.3 sun. Anchoring monolayer bilateral amino groups passivates the defects at the perovskite surface and enhances perovskite stability by exposing the linking hydrophobic alkyl chain. Grain boundaries are reinforced by BAA and are more resistant to mechanical bending and electron beam damage. BAA improves the device shelf lifetime to >1000 hours and operation stability to >500 hours under light, with 90% of the initial efficiency retained.

A dithienobenzodithiophene‐based π‐conjugated polymer with fluorinated benzotriazole is applied through an anti‐solvent process to passivate the defects of the perovskite film. The fluorinated polymer interacts with undercoordinated Pb²⁺ ions to form a Pb‐F bond in the perovskite crystals, resulting in a reduced trap density, fast charge transfer, and enhanced performance and stability of the perovskite solar cell.

A dithienobenzodithiophene‐based π‐conjugated polymer consisting of fluorinated benzotriazole and benzothiadiazole is successfully applied through anti‐solvent method to passivate the defects of perovskite crystals. The fluorinated polymer interacts with under coordinated Pb²⁺ ions in the perovskite crystals to form Pb‐F bond which effectively passivates the defects. The trap density is reduced and the charge carrier transfer between the perovskite film and Spiro‐OMeTAD is also improved after passivation with the polymer. As a result, a power conversion efficiency (PCE) of 18.03% is achieved in the champion cell. After storing in an ambient environment with 60% relative humidity for 1000 h, the device still retains 90% of the original PCE. These results demonstrate that dithienobenzodithiophene‐based π‐conjugated polymers are promising materials for passivation of perovskite films to further improve the performance and stability of perovskite solar cells.

A near‐infrared non‐fullerene acceptor BTTIC is well compatible with different bandgap polymers, i.e., J71 (1.92 eV), PBDB‐T (1.80 eV), and PTB7‐Th (1.58 eV), which achieves power conversion efficiencies (PCEs) as high as 12.8%, 13.2%, and 10.4%, with fill factors all over 70%, suggesting BTTIC is a promising non‐fullerene acceptor for polymers selectivity.

Owing to their good polymer compatibility, fullerene derivatives, such as PC₆₁BM and PC₇₁BM, have been the dominant electron acceptors to pair with various polymer donors in polymer solar cells (PSCs). The recent surge of non‐fullerene materials leads to several high‐performance molecular acceptors. Despite their high performance in a given polymer/acceptor system, the generality of these acceptors, i.e., their compatibility with different donor polymers remains uncertain. Here, a high‐performance small molecule acceptor (SMA), BTTIC, is designed and synthesized to combine with three polymers with different bandgaps, namely J71 (1.92 eV), PBDB‐T (1.80 eV), and PTB7‐Th (1.58 eV). Complementary absorption, compatible energy levels, and particularly the favorable morphologies between BTTIC and the three polymers enable high power conversion efficiencies (PCEs), which are 12.8%, 13.2%, and 10.4% for J71:BTTIC‐, PBDB‐T:BTTIC‐, and PTB7‐Th:BTTIC‐based PSCs, respectively, significantly higher than the PCEs of the fullerene‐ or other non‐fullerene‐based counterparts. Moreover, another famous p‐type polymer donor PffBT4T‐2OD, which shows poor solubility in chloroform and has not yet been studied in non‐fullerene PSCs, is also investigated. Processing by dissolving PffBT4T‐2OD and BTTIC in boiling chloroform enables PffBT4T‐2OD:BTTIC‐based PSCs with a PCE of 10.18%, which is significantly higher than that of PSCs (4.78%) before using boiling chloroform processing. The good compatibility of BTTIC with polymers that have either large, moderate, or small bandgaps makes it a promising non‐fullerene acceptor for next‐generation non‐fullerene PSCs.

A very simple rod‐shaped bithiophene‐based small molecule, T2‐ORH, is synthesized in only two steps to obtain a nonfullerene acceptor for use in efficient organic photovoltaic cells. The additive‐free inverted PTB7‐Th:T2‐ORH single‐junction device exhibits a power conversion efficiency of 9.33%, with a remarkably low E _loss of 0.51 eV due to a smooth homogeneous film morphology and vertical and parallel charge transport.

Abstract

The introduction of rigid and extended ladder‐type fused‐ring cores, such as indacenodithiophene, has enabled the synthesis of a variety of nonfullerene small molecules for use as electron acceptors in high‐performance organic photovoltaic cells. Contrasting with recent trends, a very simple‐structured nonfullerene acceptor (NFA), T2‐ORH, consisting of a bithiophene core and octyl‐substituted rhodanine ends, is synthesized in two steps from inexpensive commercially available raw materials. Its relatively short π‐conjugation results in a wide bandgap and a blue‐shifted UV–vis absorption profile complementary to those of poly[4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene‐co‐3‐fluorothieno[3,4‐b]thiophene‐2‐carboxylate] (PTB7‐Th). Despite a sufficient offset between T2‐ORH and PTB7‐Th, the lowest unoccupied molecular orbital (LUMO) energy level of T2‐ORH is still higher than the LUMOs of other NFAs (e.g., ITIC). Therefore, the PTB7‐Th:T2‐ORH blend film exhibits an efficiency of 9.33% with a high open‐circuit voltage of 1.07 V and a short‐circuit current of 14.72 mA cm⁻² in an additive‐free single‐junction cell. Importantly, the optimized device displays a remarkably low energy loss of 0.51 eV, in which bimolecular and monomolecular charge recombination is effectively suppressed by solvent vapor annealing treatment. The blend film has a very smooth and homogeneous morphology, providing both vertical and parallel charge transport in the devices.

A novel method for defects passivation in perovskite solar cells via controlling the electron density distribution of D‐π‐A molecule is proposed. As the polarity of the passivated molecule increases, the passivation effect on the under‐coordinated Pb²⁺ defects will be more obvious, leading to an increase of 80 mV in the open circuit voltage of the devices.

Abstract

Organic–inorganic hybrid perovskite solar cells (PSCs) are a promising photovoltaic technology that has rapidly developed in recent years. Nevertheless, a large number of ionic defects within perovskite absorber can serve as non‐radiative recombination center to limit the performance of PSCs. Here, organic donor‐π‐acceptor (D‐π‐A) molecules with different electron density distributions are employed to efficiently passivate the defects in the perovskite films. The X‐ray photoelectron spectroscopy (XPS) analysis shows that the strong electron donating N,N‐dibutylaminophenyl unit in a molecule causes an increase in the electron density of the passivation site that is a carboxylate group, resulting in better binding with the defects of under‐coordinated Pb²⁺ cations. Carrier lifetime in the perovskite films measured by the time‐resolved photoluminescence spectrum is also prolonged by an increase in donation ability of the D‐π‐A molecules. As a consequence, these benefits contribute to an increase of 80 mV in the open circuit voltage of the devices, enabling a maximum power conversion efficiency (PCE) of 20.43%, in comparison with PCE of 18.52% for the control device. The authors' findings provide a novel strategy for efficient defect passivation in the perovskite solar cells based on controlling the electronic configuration of passivation molecules.

Impacts of alkaline on the defects property and crystallization kinetics in perovskite solar cells

Impacts of alkaline on the defects property and crystallization kinetics in perovskite solar cells, Published online: 07 March 2019; doi:10.1038/s41467-019-09093-1

Nickel oxide based perovskite solar cells are reviewed comprehensively in the present paper. Particularly, the fabrication method for NiO _x films, surface modification, and doping strategies are discussed in detail with special attention paid to the relationship between the optoelectronic properties of NiO _x films and the performance of resulting perovskite solar cell devices.

Organic–inorganic halide perovskite solar cells (PSCs) have achieved great success in recent years with a demonstrated power conversion efficiency (PCE) increasing rapidly from 3.8% to 22.3% for single junction devices. Most high‐performance PSCs consist of a perovskite absorber sandwiched between an electron transport layer (ETL) and a hole transport layer (HTL), which extracts electrons (holes) and blocks holes (electrons) from the absorber efficiently. Inorganic hole transport materials have extracted extensive attention due to their higher mobility and better stability. Particularly, the excellent hole selective transport property of nickel oxide (NiO _x ) has been highlighted by its recent application in organometallic halide PSCs, due to the favorable band alignment formed between the halide perovskite absorber and NiO _x HTL. This comprehensive review summarizes the recent progress in the fabrication of NiO _x films and their application in PSCs. Special attention is paid to the optoelectronic properties of NiO _x films, which strongly depend on the synthesis methods and post‐treatment conditions, as well as the resulting photovoltaic device performance. Surface modification and doping strategies that are used to improve the optoelectronic properties of NiO _x films and the resulting device performance are discussed with emphasis. Finally, a short perspective of NiO _x ‐based PSCs is also provided.

The elemental sulfur (S₈) added to the perovskite precursor solution ((FAPbI₃)_0.95(MAPbBr₃)_0.05 in dimethylformamide/dimethyl sulfoxide) not only increases the stability of the solution owing to amine–sulfur coordination but also significantly improves the thermalstability and photostability due to the increase in chemical stability of the perovskite material itself without compromising the power conversion efficiency of the resulting perovskite solar cells.

Abstract

Efficient perovskite solar cells (PSCs) are mainly fabricated by a solution coating processes. However, the efficiency of such devices varies significantly with the aging time of the precursor solution used to fabricate them, which includes a mixture of perovskite components, especially methylammonium (MA), and formamidinium (FA) cations. Herein, how the inorganic–organic hybrid perovskite precursor solution of (FAPbI₃)_0.95(MAPbBr₃)_0.05 degrades over time and how such degradation can be effectively inhibited is reported on, and the associated mechanism of degradation is discussed. Such degradation of the precursor solution is closely related to the loss of MA cations dissolved in the FA solution through the deprotonation of MA to volatile methylamine (CH₃NH₂). Addition of elemental sulfur (S₈) drastically stabilizes the precursor solution owing to amine–sulfur coordination, without compromising the power conversion efficiency (PCE) of the derived PSCs. Furthermore, sulfur introduced to stabilize the precursor solution results in improved PSC stability.