Chen Weijie

A simple method for obtaining highly efficient and stable inverted perovskite solar cells (PeSCs) is suggested. A defect‐free perovskite film with large‐sized grains is achieved by adding an organic conjugated molecule, which improves the charge extraction and reduces defect sites in perovskite crystals, resulting in highly efficient and stable PeSCs.

Abstract

The addition of chemical additives is considered as a promising approach for obtaining high‐quality perovskite films under mild conditions, which is essential for both the efficiency and the stability of organic–inorganic hybrid perovskite solar cells (PeSCs). Although such additive engineering yields high‐quality films, the inherent insulating property of the chemical additives prevents the efficient transport and extraction of charge carriers, thereby limiting the applicability of this approach. Here, it is shown that organic conjugated molecules having rhodanine moieties (i.e., SA‐1 and SA‐2) can be used as semiconducting chemical additives that simultaneously yield large‐sized perovskite grains and improve the charge extraction. Using this strategy, a high power conversion efficiency of 20.3% as well as significantly improved long‐term stability under humid air conditions is achieved. It is believed that this approach can provide a new pathway to designing chemical additives for further improving the efficiency and stability of PeSCs.

Sn dopants trigger extrinsic self‐trapping of excitons in bulk 2D perovskite crystals, and afford broadband red‐to‐near‐infrared emission, with luminescence quantum yield increase from 0.7% to 6.0% (8.6‐fold). Random potential wells that the Sn dopants create preferentially localize excitons through the fast (sub‐picosecond) exciton diffusion, suppressing the original weak emissions from free and bound excitons.

Abstract

As emerging efficient emitters, metal‐halide perovskites offer the intriguing potential to the low‐cost light emitting devices. However, semiconductors generally suffer from severe luminescence quenching due to insufficient confinement of excitons (bound electron–hole pairs). Here, Sn‐triggered extrinsic self‐trapping of excitons in bulk 2D perovskite crystal, PEA₂PbI₄ (PEA = phenylethylammonium), is reported, where exciton self‐trapping never occurs in its pure state. By creating local potential wells, isoelectronic Sn dopants initiate the localization of excitons, which would further induce the large lattice deformation around the impurities to accommodate the self‐trapped excitons. With such self‐trapped states, the Sn‐doped perovskites generate broadband red‐to‐near‐infrared (NIR) emission at room temperature due to strong exciton–phonon coupling, with a remarkable quantum yield increase from 0.7% to 6.0% (8.6 folds), reaching 42.3% under a 100 mW cm⁻² excitation by extrapolation. The quantum yield enhancement stems from substantial higher thermal quench activation energy of self‐trapped excitons than that of free excitons (120 vs 35 meV). It is further revealed that the fast exciton diffusion involves in the initial energy transfer step by transient absorption spectroscopy. This dopant‐induced extrinsic exciton self‐trapping approach paves the way for extending the spectral range of perovskite emitters, and may find emerging application in efficient supercontinuum sources.

Publication date: March 2019

Source: Nano Energy, Volume 57

Author(s): Luozheng Zhang, Xianyong Zhou, Xiongwei Zhong, Chun Cheng, Yanqing Tian, Baomin Xu

Abstract

Graphical abstract

A conjugated polyelectrolyte with organic cations CH₃NH₃⁺ as the hole-transporting layer presents a power conversion efficiency of up to 19.76% for inverted perovskite solar cells.

Perovskite solar cells have experienced a rapid development since the first report in 2012 with the power conversion efficiency approaching the theoretical limit. Device stability is still one of the remaining challenges for commercialisation. In this Review, the authors address the important role the charge selective contacts play in the long‐term stability of perovskite solar cells.

Abstract

Lead halide perovskite solar cells have rapidly achieved high efficiencies comparable to established commercial photovoltaic technologies. The main focus of the field is now shifting toward improving the device lifetime. Many efforts have been made to increase the stability of the perovskite compound and charge‐selective contacts. The electron and hole selective contacts are responsible for the transport of photogenerated charges out of the solar cell and are in intimate contact with the perovskite absorber. Besides the intrinsic stability of the selective contacts themselves, the interfaces at perovskite/selective contact and metal/selective contact play an important role in determining the overall operational lifetime of perovskite solar cells. This review discusses the impact of external factors, i.e., heat, UV‐light, oxygen, and moisture, and measured conditions, i.e., applied bias on the overall stability of perovskite solar cells (PSCs). The authors summarize and analyze the reported strategies, i.e., material engineering of selective contacts and interface engineering via the introduction of interlayers in the aim of enhancing the device stability of PSCs at elevated temperatures, high humidity, and UV irradiation. Finally, an outlook is provided with an emphasis on inorganic contacts that is believed to be the key to achieving highly stable PSCs.

A fast, nondestructive, camera‐based method to capture optical bandgap images of perovskite solar cells with micrometer‐scale spatial resolution is developed. This technique allows for the probing of relative variations in optical bandgaps across various solar cells, and for the resolution of bandgap inhomogeneity within the same device due to material degradation and impurities. The results are independently confirmed with other optical‐based techniques.

Abstract

A fast, nondestructive, camera‐based method to capture optical bandgap images of perovskite solar cells (PSCs) with micrometer‐scale spatial resolution is developed. This imaging technique utilizes well‐defined and relatively symmetrical band‐to‐band luminescence spectra emitted from perovskite materials, whose spectral peak locations coincide with absorption thresholds and thus represent their optical bandgaps. The technique is employed to capture relative variations in optical bandgaps across various PSCs, and to resolve optical bandgap inhomogeneity within the same device due to material degradation and impurities. Degradation and impurities are found to both cause optical bandgap shifts inside the materials. The results are confirmed with micro‐photoluminescence spectroscopy scans. The excellent agreement between the two techniques opens opportunities for this imaging concept to become a quantified, high spatial resolution, large‐area characterization tool of PSCs. This development continues to strengthen the high value of luminescence imaging for the research and development of this photovoltaic technology.

A new class of inorganic photovoltaic materials, silver bismuth iodide sulfides, is introduced. The optoelectronic properties of these compounds can be tuned by variation of the sulfide to iodide ratio. Thin solar cells based on Ag _a Bi _b I _{a +3 b −2 x} S _x light harvesters exhibit power conversion efficiencies above 5%, are stable under ambient conditions and can be produced on the centimeter‐size scale.

Abstract

Silver bismuth iodides (Ag _a Bi _b I _a+3b ) are nontoxic and comparatively cheap photovoltaic materials, but their wide bandgaps and downshifted valence band edges limit their performance as light absorbers in solar cells. Herein, a strategy is introduced to tune the optoelectronic properties of Ag _a Bi _b I _a+3b by partial anionic substitution with the sulfide dianion. A consistent narrowing of the bandgap by 0.1 eV and an upshift of the valence band edge by 0.1–0.3 eV upon modification with sulfide are demonstrated for AgBiI₄, Ag₂BiI₅, Ag₃BiI₆, and AgBi₂I₇ compositions. Solar cells based on silver bismuth sulfoiodides embedded into a mesoporous TiO₂ electron‐transporting scaffold, and a poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine] hole‐transporting layer significantly outperform devices based on sulfide‐free materials, mainly due to enhancements in the photocurrent by up to 48%. A power conversion efficiency of 5.44 ± 0.07% (J _sc = 14.6 ± 0.1 mA cm⁻²; V _oc = 569 ± 3 mV; fill factor = 65.7 ± 0.3%) under 1 sun irradiation and stability under ambient conditions for over a month are demonstrated. The results reported herein indicate that further improvements should be possible with this new class of photovoltaic materials upon advances in the synthetic procedures and an increase in the level of sulfide anionic substitution.

A Z‐scheme mimic based on photosystem II wired to a quantum‐dot‐modified inverse opal TiO₂ electrode allows H₂O oxidation at very low potential. The sandwich‐like arrangement of this photobioanode and a transparent ATO electrode hosting bilirubin oxidase are used for the construction of a H₂O/O₂ photobioelectrochemical cell, which can be illuminated through the transparent biocathode.

Abstract

A biohybrid photobioanode mimicking the Z‐scheme has been developed by functional integration of photosystem II (PSII) and PbS quantum dots (QDs) within an inverse opal TiO₂ architecture giving rise to a rather negative water oxidation potential of about −0.55 V vs. Ag/AgCl, 1 m KCl at neutral pH. The electrical linkage between both light‐sensitive entities has been established through an Os‐complex‐modified redox polymer (P_Os), which allows the formation of a multi‐step electron‐transfer chain under illumination starting with the photo‐activated water oxidation at PSII followed by an electron transfer from PSII through P_Os to the photo‐excited QDs and finally to the TiO₂ electrode. The photobioanode was coupled to a novel, transparent, inverse‐opal ATO cathode modified with an O₂‐reducing bilirubin oxidase for the construction of a H₂O/O₂ photobioelectrochemical cell reaching a high open‐circuit voltage of about 1 V under illumination.

Increasing the power conversion efficiency of silicon (Si) photovoltaics is a key enabler for continued reductions in the cost of solar electricity. Here, we describe a two-terminal perovskite/Si tandem design that increases the Si cell’s output in the simplest possible manner: by placing a perovskite cell directly on top of the Si bottom cell. The advantageous omission of a conventional interlayer eliminates both optical losses and processing steps and is enabled by the low contact resistivity attainable between n-type TiO₂ and Si, established here using atomic layer deposition. We fabricated proof-of-concept perovskite/Si tandems on both homojunction and passivating contact heterojunction Si cells to demonstrate the broad applicability of the interlayer-free concept. Stabilized efficiencies of 22.9 and 24.1% were obtained for the homojunction and passivating contact heterojunction tandems, respectively, which could be readily improved by reducing optical losses elsewhere in the device. This work highlights the potential of emerging perovskite photovoltaics to enable low-cost, high-efficiency tandem devices through straightforward integration with commercially relevant Si solar cells.

Al(OH)₃ nanolayers are employed for the interface modification of Cu₂ZnSn(S,Se)₄/CdS hetero‐junction of kesterite solar cells. Considerable open circuit voltage and fill factor increment are observed, which is ascribed to reduced interface recombination and shunt paths by an epitaxial relationship of Al(OH)₃ with kesterite and CdS.

A strategy for interface engineering of hetero‐junctions in kesterite solar cells by using Al(OH)₃ is demonstrated. The hydroxide nanolayers are prepared via a facile and fast wet chemical route, based on an aqueous solution of aluminum chlorides and thioacetamide. Considerable enhancement of open circuit voltage (V _oc) (30–60 mV) and fill factor (FF) (10–20%) after this chemical treatment are observed, achieving a champion conversion efficiency of 9.1% and a champion FF of 70% (among the best FF in kesterite solar cells). The functional mechanism is systematically studied by current‐voltage, capacitance‐voltage, temperature dependence of current–voltage and photoluminescence measurements, which reveal that Al(OH)₃ nanolayers can effectively reduce the interface recombination and largely improve the shunt resistance. Furthermore, atomic resolution high angle annular dark field scanning transmission electron microscopy (HAADF‐STEM) evidences the epitaxial relationship of Al(OH)₃ with kesterite and CdS, indicating the benign and effective interface passivation achieved by this chemical treatment. Finally, based on HAADF‐STEM and electron energy loss spectroscopy mappings, insights into the efficiency limiting and beneficial factors for CZTSSe solar cells, as well as suggestions to further improve both the bulk and related interfaces are presented.

Ultra‐low concentration of 2–3 nm gold nanoparticles embedded with an Ohmic contact into a NiO_x film triples its hole concentration. This leads to a significant conductivity increase of the NiO_x hole transport layer and the consequent decrease of the series resistance of the HTL based perovskite solar cells, pronouncedly improving the efficiency of PVSCs from 17.8 to 20.2%.

NiO_x is a promising hole transport material for p‐i‐n perovskite solar cells (PVSCs) on account of its high mobility, excellent stability and prospect for large‐scale fabrication. However, the typical conductivity of NiO_x is low because of the low carrier concentration, which consequently compromises its photovoltaic performance. Herein, we show that the carrier concentration of NiO_x can be improved by as much as three times through embedding a small concentration (0.11 At%) of gold nanoparticles (Au‐NPs), 2–3 nm in diameter, into the NiO_x thin film. The enhancement is due to the formation of Ohmic contact between Au and NiO_x while avoiding direct contact between Au and perovskite. All key parameters of the PVSC, namely J _sc, V _oc, and FF have improved, and the overall efficiency shows a significant improvement from 17.8% to 20.2%. The small size, low concentration, and Ohmic contact nature of the embedded Au‐NPs, together with this simple method point to a new promising direction for developing high performance PVSCs.

Two phthalimide‐based high mobility polymers with a D‐A₁‐D‐A₂ backbone motif are synthesized. A remarkable power conversion efficiency of 12.74 and 13.31% is achieved from fluorinated phthalimide‐difluorobenzothiadiazole and phthalimide‐difluorobenzothiadiazole‐based nonfullerene polymer solar cells, respectively. The results demonstrate that phthalimides are excellent building blocks for enabling polymer semiconductors with outstanding solar cell performances.

Abstract

Highly efficient nonfullerene polymer solar cells (PSCs) are developed based on two new phthalimide‐based polymers phthalimide‐difluorobenzothiadiazole (PhI‐ffBT) and fluorinated phthalimide‐ffBT (ffPhI‐ffBT). Compared to all high‐performance polymers reported, which are exclusively based on benzo[1,2‐b:4,5‐b′]dithiophene (BDT), both PhI‐ffBT and ffPhI‐ffBT are BDT‐free and feature a D‐A₁‐D‐A₂ type backbone. Incorporating a second acceptor unit difluorobenzothiadiazole leads to polymers with low‐lying highest occupied molecular orbital levels (≈−5.6 eV) and a complementary absorption with the narrow bandgap nonfullerene acceptor IT‐4F. Moreover, these BDT‐free polymers show substantially higher hole mobilities than BDT‐based polymers, which are beneficial to charge transport and extraction in solar cells. The PSCs containing difluorinated phthalimide‐based polymer ffPhI‐ffBT achieve a substantial PCE of 12.74% and a large V _oc of 0.94 V, and the PSCs containing phthalimide‐based polymer PhI‐ffBT show a further increased PCE of 13.31% with a higher J _sc of 19.41 mA cm⁻² and a larger fill factor of 0.76. The 13.31% PCE is the highest value except the widely studied BDT‐based polymers and is also the highest among all benzothiadiazole‐based polymers. The results demonstrate that phthalimides are excellent building blocks for enabling donor polymers with the state‐of‐the‐art performance in nonfullerene PSCs and the BDT is not necessary for constructing such donor polymers.

The molecular order of nonfullerene electron acceptor INPIC‐4F is manipulated by varying the self‐organization time during solution casting. With the presence of solvent vapor, INPIC‐4F grows into spherulites with poor efficiency. On the contrary, casting on hot substrates promotes face‐on π−π stacking, which improves absorption as well as efficient exciton dissociation and balanced charge mobility for a maximum efficiency of 13.1%.

Abstract

Developing a fundamental understanding of the molecular order within the photoactive layer, and the influence therein of solution casting conditions, is a key factor in obtaining high power conversation efficiency (PCE) polymer solar cells. Herein, the molecular order in PBDB‐T:INPIC‐4F nonfullerene solar cells is tuned by control of the molecular organization time during film casting, and the crucial role of retarding the crystallization of INPIC‐4F in achieving high performance is demonstrated. When PBDB‐T:INPIC‐4F is cast with the presence of solvent vapor to prolong the organization time, INPIC‐4F molecules form spherulites with a polycrystalline structure, resulting in large phase separation and device efficiency below 10%. On the contrary, casting the film on a hot substrate is effective in suppressing the formation of the polycrystalline structure, and encourages face‐on π−π stacking of INPIC‐4F. This molecular transformation of INPIC‐4F significantly enhances the absorption ability of INPIC‐4F at long wavelengths and facilitates a fine phase separation to support efficient exciton dissociation and balanced charge transport, leading to the achievement of a maximum PCE of 13.1%. This work provides a rational guide for optimizing nonfullerene polymer solar cells consisting of highly crystallizable small molecular electron acceptors.

Rapid crystallization is demonstrated to be necessary in achieving high‐quality 2DRP perovskite films by comparing dimethylacetamide (DMAC), N,N‐dimethylformamide, and dimethyl sulfoxide solvents. The improved stability and efficiency are observed using DMAC due to the accelerating crystallization rate of 2DRP perovskite crystals.

Abstract

Due to the additional introduction of bulky organic ammonium and the competition between bulky organic ammonium and methyl ammonium in 2D Ruddlesden‐Popper (2DRP) perovskite, the crystallization process becomes complicated. Here, it is demonstrated that the rapid crystallization controlled by processing solvents plays an important role in achieving high‐quality 2DRP perovskite films. It is found that the processing solvents, e.g., dimethylacetamide (DMAC), N,N‐dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), with a different polarity and boiling point, have almost no effect on crystal structure and phase distribution but have a remarkable effect on crystallization kinetics, crystal growth orientation, and crystallinity of 2DRP perovskite. Compared to polar aprotic solvent DMF and DMSO with a high boiling point, DMAC with low polarity and a suitable boiling point has a weak coordination to lead and ammonium salts and is easy to escape during solution processing, which is able to accelerate the crystallization rate of 2DRP perovskite. Benefitting from the rapid crystallization enabled high‐quality 2DRP perovskite films, the best‐performing device with improved stability and a power conversion efficiency of 12.15% is obtained using DMAC solvent. These findings may give guidance for solvent engineering for highly efficient 2DRP perovskite solar cells in the future.

Scalable room‐temperature sputtering deposition of the SnO₂ electron transport layer (ETL) with reduced gap states has been demonstrated. Perovskite solar cells using a SnO₂ ETL show an efficiency up to 20.2% and a T₈₀ lifetime of 625 h. Mini‐modules with a 22.8 cm₂ aperture area show efficiencies over 12% and a T₈₀ lifetime of 515 h, which indicates the upscalability of our method.

Abstract

Stability and scalability have become the two main challenges for perovskite solar cells (PSCs) with the research focus in the field advancing toward commercialization. One of the prerequisites to solve these challenges is to develop a cost‐effective, uniform, and high quality electron transport layer that is compatible with stable PSCs. Sputtering deposition is widely employed for large area deposition of high quality thin films in the industry. Here the composition, structure, and electronic properties of room temperature sputtered SnO₂ are systematically studied. Ar and O₂ are used as the sputtering and reactive gas, respectively, and it is found that a highly oxidizing environment is essential for the formation of high quality SnO₂ films. With the optimized structure, SnO₂ films with high quality have been prepared. It is demonstrated that PSCs based on the sputtered SnO₂ electron transport layer show an efficiency up to 20.2% (stabilized power output of 19.8%) and a T₈₀ operational lifetime of 625 h. Furthermore, the uniform and thin sputtered SnO₂ film with high conductivity is promising for large area solar modules, which show efficiencies over 12% with an aperture area of 22.8 cm² fabricated on 5 × 5 cm² substrates (geometry fill factor = 91%), and a T₈₀ operational lifetime of 515 h.

Two novel spiro‐type hole‐transporting materials (HTMs) SCZF‐5 and SAF‐5 are designed based on different spiro‐cores, SZCF and SAF, respectively, and are applied in the perovskite solar cells. An impressive power conversion efficiency of 20.10% is achieved in the SCZF‐5‐based device, which is obviously higher than that of commercial HTM spiro‐OMeTAD (19.11%) and SAF‐5 (13.93%).

Abstract

Hole‐transporting materials (HTMs) play a significant role in hole transport and extraction for perovskite solar cells (PeSCs). As an important type of HTMs, the spiro‐architecture‐based material is widely used as small organic HTM in PeSCs with good photovoltaic performances. The skeletal modification of spiro‐based HTMs is a critical way of modifying energy level and hole mobility. Thus, many spiro alternatives are developed to optimize the spiro‐type HTMs. Herein, a novel carbazole‐based single‐spiro‐HTM named SCZF‐5 is designed and prepared for efficient PeSCs. In addition, another single‐spiro HTM SAF‐5 with reported 10‐phenyl‐10H‐spiro[acridine‐9,9′‐fluorene] (SAF) core is also synthesized for comparison. Through varying from SAF core to SCZF core as well as comparing with the classic 9,9′‐spiro‐bifluorene, it is found that the new HTM SCZF‐5 exhibits more impressive power conversion efficiency (PCE) of 20.10% than SAF‐5 (13.93%) and the commercial HTM spiro‐OMeTAD (19.11%). On the other hand, the SCZF‐5‐based device also has better durability in lifetime testing, indicating the newly designed SCZF by integrating carbazole into the spiro concept has good potential for developing effective HTMs.

A high dipole moment cation as a large organic spacer reduces the dielectric confinement effect and hence promotes separation of photogenerated electron–hole pairs in layered 2D perovskite materials, which leads to more efficient and stable perovskite solar cells.

Abstract

Layered 2D organic–inorganic hybrid perovskite is appearing as a rising star in the photovoltaic field, thanks to its superior moisture resistance by the organic spacer cations. Unfortunately, these cations lead to high exciton binding energy in the 2D perovskites, which suffers from lower efficiency in the devices. It thus requires a clear criterion to select/design appropriate organic spacer cations to improve the device efficiency based on this class of materials. Here, 2,2,2‐trifluoroethylamine (F₃EA⁺) is introduced to combine with butylammonium (BA⁺) cations as mixed spacers. While BA⁺ enables self‐assembly of 2D perovskite crystals by van der Waals interaction, the introduction of F₃EA⁺ spacers with a high dipole moment suppress nonradiative recombination and promote separation of photogenerated electron–hole pairs by taking the advantage of electronegativity of fluorine. The resultant solar cells based on [(BA)_{1– x} (F₃EA) _x ]₂(MA)₃Pb₄I₁₃ exhibit substantially increased open circuit voltage and fill factor compared with that of (BA)₂(MA)₃Pb₄I₁₃. The champion [(BA)_0.94(F₃EA)_0.06]₂(MA)₃Pb₄I₁₃ solar cell yields a power conversion efficiency of 12.51%, which is among the best performances so far. These findings suggest an effective strategy to design organic spacer cations in layered perovskite for solar cells and other optoelectronic applications.

Multi-inch single-crystalline perovskite membrane for high-detectivity flexible photosensors

Multi-inch single-crystalline perovskite membrane for high-detectivity flexible photosensors, Published online: 13 December 2018; doi:10.1038/s41467-018-07440-2