Ligang Yuan

Suppression of carrier recombination is critically important in realizing high-efficiency polymer solar cells. Herein, it is demonstrated difluoro-substitution of thiophene conjugated side chain on donor polymer can suppress triplet formation for reducing carrier recombination. A new medium bandgap 2D-conjugated D–A copolymer J91 is designed and synthesized with bi(alkyl-difluorothienyl)-benzodithiophene as donor unit and fluorobenzotriazole as acceptor unit, for taking the advantages of the synergistic fluorination on the backbone and thiophene side chain. J91 demonstrates enhanced absorption, low-lying highest occupied molecular orbital energy level, and higher hole mobility, in comparison with its control polymer J52 without fluorination on the thiophene side chains. The transient absorption spectra indicate that J91 can suppress the triplet formation in its blend film with n-type organic semiconductor acceptor m-ITIC (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(3-hexylphenyl)-dithieno[2,3-d:2,3′-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene). With these favorable properties, a higher power conversion efficiency of 11.63% with high V_OC of 0.984 V and high J_SC of 18.03 mA cm⁻² is obtained for the polymer solar cells based on J91/m-ITIC with thermal annealing. The improved photovoltaic performance by thermal annealing is explained from the morphology change upon thermal annealing as revealed by photoinduced force microscopy. The results indicate that side chain engineering can provide a new solution to suppress carrier recombination toward high efficiency, thus deserves further attention.

Suppression of carrier recombination is critically important for efficient polymer solar cells. Herein, it is demonstrated that difluoro-substitution of thiophene-conjugated side chains on the medium-bandgap polymer donor can suppress triplet formation for reducing carrier recombination and improving photovoltaic performance.

2D Ruddlesden–Popper (RP) perovskites have recently emerged as promising candidates for hybrid perovskite photovoltaic cells, realizing power-conversion efficiencies (PCEs) of over 10% with technologically relevant stability. To achieve solar cell performance comparable to the state-of-the-art 3D perovskite cells, it is highly desirable to increase the conductivity and lower the optical bandgap for enhanced near-IR region absorption by increasing the perovskite slab thickness. Here, the use of the 2D higher member (n = 5) RP perovskite (n-butyl-NH₃)₂(MeNH₃)₄Pb₅I₁₆ in depositing highly oriented thin films from dimethylformamide/dimethylsulfoxide mixtures using the hot-casting method is reported. In addition, they exhibit superior environmental stability over thin films of their 3D counterpart. These films are assembled into high-efficiency solar cells with an open-circuit voltage of ≈1 V and PCE of up to 10%. This is achieved by fine-tuning the solvent ratio, crystal growth orientation, and grain size in the thin films. The enhanced performance of the optimized devices is ascribed to the growth of micrometer-sized grains as opposed to more typically obtained nanometer grain size and highly crystalline, densely packed microstructures with the majority of the inorganic slabs preferentially aligned out of plane to the substrate, as confirmed by X-ray diffraction and grazing-incidence wide-angle X-ray scattering mapping.

Controllable tuning of the thin film properties of high-n member layered Ruddlesden–Popper perovskites, BA₂MA₄Pb₅I₁₆, is achieved via a hot-casting method using dimethylformamide (DMF)/dimethylsulfoxide (DMSO) processing solvent. Unlike the polycrystalline films grown from DMF, the optimized 3:1 DMF:DMSO films are essentially single-crystalline with regularly stacked inorganic slabs, and deliver solar cell power conversion efficiencies up to 10%.

Tremendous progress has recently been achieved in the field of perovskite solar cells (PSCs) as evidenced by impressive power conversion efficiencies (PCEs); but the high PCEs of >20% in PSCs has so far been mostly achieved by using the hole transport material (HTM) spiro-OMeTAD; however, the relatively low conductivity and high cost of spiro-OMeTAD significantly limit its potential use in large-scale applications. In this work, two new organic molecules with spiro[fluorene-9,9′-xanthene] (SFX)-based pendant groups, X26 and X36, have been developed as HTMs. Both X26 and X36 present facile syntheses with high yields. It is found that the introduced SFX pendant groups in triphenylamine-based molecules show significant influence on the conductivity, energy levels, and thin-film surface morphology. The use of X26 as HTM in PSCs yields a remarkable PCE of 20.2%. In addition, the X26-based devices show impressive stability maintaining a high PCE of 18.8% after 5 months of aging in controlled (20%) humidity in the dark. We believe that X26 with high device PCEs of >20% and simple synthesis show a great promise for future application in PSCs, and that it represents a useful design platform for designing new charge transport materials for optoelectronic applications.

The importance of the pendant groups on triphenylamine-based hole transport materials (HTMs) in perovskite solar cells is investigated. A new HTM X26 with optimal spiro[fluorene-9,9′-xanthene]-based pendant groups shows an efficiency of over 20%. This work demonstrates that the pendant groups in HTMs play important roles in determining the molecular property, solar cell performance, and stability.

Here, conjugated polymer is added as third component to tune the solution viscosity, morphology, and function of small molecule (SM) based bulk-heterojunction (BHJ) solar cells, which are fabricated using blade coating. Novel information about the effect of blade coating speed on the nanoscale morphology and function of ternary blend solar cells is provided. The crystal sizes increase with an increase of coating speed for both binary and ternary blends, while the addition of the third component tends to favor smaller SM crystal grains and improves the connectivity of SM crystals. Small angle neutron scattering experiments provide the first clear experimental evidence that the addition of the third component would significantly impact the fullerene phase separation, which is crucial for bimolecular recombination and charge transport. It shows that for both binary and ternary blends, the concentration and sizes of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) aggregates increase with an increase of coating speed, while addition of third component does not affect the volume fraction of PCBM aggregates but impacts the size of PCBM aggregates. It is demonstrated that the judicious selection of blade coating speed and addition of conjugated polymer optimize the morphology of SM-BHJ, providing guidelines for high performance SM-BHJs from roll-to-roll production.

The crystallization of small molecules and fullerene phase separation in SM-BHJ blends are significantly impacted by the blade coating speed as well as the addition of a third component. The judicious selection of blade coating speed and the addition of a conjugated polymer optimizes the morphology of SM-BHJ, which will provide guidelines for high performance SM-BHJ from roll-to-roll production.

In article number 1602812, Jung-Yong Lee, Bumjoon J. Kim, and co-workers investigate nonconjugated polymer additives (nPAs) for highly efficient and stable polymer solar cells (PSCs). The P2VP nPA self-assembles vertically on the ZnO surface via a single coating process for the deposition of active materials. The self-assembled P2VP reduces the work function and surface defect density of ZnO, which leads to efficient and stable PSCs with up to 11.14% efficiency.

In article number 1701458, Heeman Choe, Yung-Eun Sung, and co-workers, develop a multidimensional Ti photoelectrode designed for enhanced light harvesting and charge collection using freeze casting and electrochemical anodization methods. The successfully produced photoelectrode architecture with 1D TiO₂ nanotube arrays on a 3D porous metallic Ti current collector generates a remarkably high photocurrent density in mesoscopic solar cell applications.

In article number 1700314, Mukundan Thelakkat and co-workers focus on the correlation of structures, morphology and chain orientation as a function of molecular weight, dispersity, intramolecular and intermolecular interactions and processing techniques of conjugated polymers and their photovoltaic blends. The tools for elucidating fundamental information of structure formation and orientation consist of optical spectroscopy and scattering techniques.

Low-temperature-processed perovskite solar cells (PSCs), which can be fabricated on rigid or flexible substrates, are attracting increasing attention because they have a wide range of potential applications. In this study, the stability of reduced graphene oxide and the ability of a poly(triarylamine) underlayer to improve the quality of overlying perovskite films to construct hole-transport bilayer by means of a low-temperature method are taken advantage of. The bilayer is used in both flexible and rigid inverted planar PSCs with the following configuration: substrate/indium tin oxide/reduced graphene oxide/polytriarylamine/CH₃NH₃PbI₃/PCBM/bathocuproine/Ag (PCBM = [6,6]-phenyl-C₆₁-butyric acid methyl ester). The flexible and rigid PSCs show power conversion efficiencies of 15.7 and 17.2%, respectively, for the aperture area of 1.02 cm². Moreover, the PSC based the bilayer shows outstanding light-soaking stability, retaining ≈90% of its original efficiency after continuous illumination for 500 h at 100 mW cm⁻².

Low-temperature-processed hole-transport bilayer (reduced graphene oxide/polytriarylamine) is constructed to fabricate inverted perovskite solar cells (PSCs), which based on flexible and rigid substrates show power conversion efficiencies of 15.7 and 17.2% on the cells area of 1.02 cm², respectively. In addition, the PSCs with the hole-transport bilayer show outstanding light-soaking stability.

Perovskite solar cells (PSCs) are of great interest in current photovoltaic research due to their extraordinary power conversion efficiency of ≈20% and boundless potentialities. The high efficiency has been mostly obtained from TiO₂-based PSCs, where TiO₂ is utilized as a hole-blocking, mesoporous layer. However, trapped charges and the light-induced photocatalytic effect of TiO₂ seriously degrade the perovskite and preclude PSCs from being immediately commercialized. Herein, a simplified PSC is successfully fabricated by eliminating the problematic TiO₂ layers, using instead a fluorine-doped tin oxide (FTO)/perovskite/hole–conductor/Au design. Simultaneously, the sluggish charge extraction at the FTO/perovskite interface is overcome by modifying the surface of the FTO to a porous structure using electrochemical etching. This surface engineering enables a substantial increase in the photocurrent density and mitigation of the hysteretic behavior of the pristine FTO-based PSC; a remarkable 19.22% efficiency with a low level of hysteresis is obtained. This performance is closely approaching that of conventional PSCs and may facilitate their commercialization due to improved convenience, lower cost, greater stability, and potentially more efficient mass production.

Electrochemically etched fluorine-doped tin oxide (FTO) provides large surfacial area compared with commercial FTO and quickly extracts photoexcited electrons at the FTO/perovskite interface. Accordingly, the photocurrent density and performance of hole-blocking layer-free planar-type perovskite solar cell are improved, where the remarkable power conversion efficiency of 19.22% is achieved.

The electron transport layer (ETL) plays a fundamental role in perovskite solar cells. Recently, graphene-based ETLs have been proved to be good candidate for scalable fabrication processes and to achieve higher carrier injection with respect to most commonly used ETLs. Here, the effects of different graphene-based ETLs in sensitized methylammonium lead iodide (MAPI) solar cells are experimentally studied. By means of time-integrated and picosecond time-resolved photoluminescence techniques, the carrier recombination dynamics in MAPI films embedded in different ETLs is investigated. Using graphene doped mesoporous TiO₂ (G+mTiO₂) with the addition of a lithium-neutralized graphene oxide (GO-Li) interlayer as ETL, it is found find that the carrier collection efficiency is increased by about a factor two with respect to standard mTiO₂. Taking advantage of the absorption coefficient dispersion, the MAPI layer morphology is probed, along the thickness, finding that the MAPI embedded in the ETL composed by G+mTiO₂ plus GO-Li brings to a very good crystalline quality of the MAPI layer with a trap density about one order of magnitude lower than that found with the other ETLs. In addition, this ETL freezes MAPI at the tetragonal phase, regardless of the temperature. Graphene-based ETLs can open the way to significant improvement of perovskite solar cells.

The effects of different graphene-based electron transport layers (ETLs) in perovskite methylammonium lead iodide (MAPI) solar cells are experimentally investigated. Using graphene-doped mesoporous TiO₂ (mTiO₂) with the addition of a lithium-neutralized graphene oxide interlayer as the ETL, the carrier collection efficiency is increased by approximately a factor two with respect to standard mTiO₂.

Two novel narrow bandgap π-conjugated polymers based on naphtho[1,2-c:5,6-c′]bis([1,2,5]thiadiazole) (NT) unit are developed, which contain the thiophene or benzodithiophene flanked with alkylthiophene as the electron-donating segment. Both copolymers exhibit strong aggregations both in solution and as thin films. The resulting copolymers with higher molecular weight show higher photovoltaic performance by virtue of the enhanced short-circuit current densities and fill factors, which can be attributed to their higher absorptivity and formation of more favorable film morphologies. Polymer solar cells (PSCs) fabricated with the copolymer PNTT achieve remarkable power conversion efficiencies (PCEs) > 11% based on both conventional and inverted structures at the photoactive layer thickness of 280 nm, which is the highest value so far observed from NT-based copolymers. Of particular interest is that the device performances are insensitive to the thickness of the photoactive layer, for which the PCEs > 10% can be achieved with film thickness ranging from 150 to 660 nm, and the PCE remains >9% at the thickness over 1 µm. These findings demonstrate that these NT-based copolymers can be promising candidates for the construction of thick film PSCs toward low-cost roll-to-roll processing technology.

Two novel conjugated polymers based on naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazol (NT) as the electron-deficient unit are developed for polymer solar cells (PSCs). The fabricated PSCs based on the high molecular weight copolymer and the fullerene acceptor ([6,6]-phenyl-C71-butyric acid methyl ester) present remarkable power conversion efficiencies over 10% with the bulk-heterojunction film thickness ranging from 150 to 660 nm.

Semitransparent perovskite solar cells (st-PSCs) have received remarkable interest in recent years because of their great potential in applications for solar window, tandem solar cells, and flexible photovoltaics. However, all reported st-PSCs require expensive transparent conducting oxides (TCOs) or metal-based thin films made by vacuum deposition, which is not cost effective for large-scale fabrication: the cost of TCOs is estimated to occupy ≈75% of the manufacturing cost of PSCs. To address this critical challenge, this study reports a low-temperature and vacuum-free strategy for the fabrication of highly efficient TCO-free st-PSCs. The TCO-free st-PSC on glass exhibits 13.9% power conversion efficiency (PCE), and the four-terminal tandem cell made with the st-PSC top cell and c-Si bottom cell shows an overall PCE of 19.2%. Due to the low processing temperature, the fabrication of flexible st-PSCs is demonstrated on polyethylene terephthalate and polyimide, which show excellent stability under repeated bending or even crumbing.

Fully solution-processed transparent conducting oxide-free semitransparent perovskite solar cells are reported to allow low-cost fabrication of highly efficient tandem solar cells and flexible solar cells. Nitric acid annealed poly(3,4-ethylenedioxythiophene): polystyrene sulfonate is incorporated in the fabrication process to realize high-throughput printing of highly conductive transparent electrodes.

Bulk heterojunction (BHJ) morphologies are vital to the device performance of organic solar cells (OSCs), including phase separation in lateral and vertical directions. However, the morphology developed from the blend solution is not easily predicted and controlled, especially in the vertical direction, because the BHJ morphology is kinetically frozen during the rapid solvent evaporation process. Here, a simple approach to control BHJ morphologies with optimized phase distribution for small molecule:[6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁ BM) blends by enhancing the substrate temperature during the spin-coating process. Three molecules with various fluorine atoms in the end acceptor units are selected. The relationship among molecular structures, substrate temperature effects on the morphology, and device performances are symmetrically investigated. Low temperature induces a multiple-sublayer-like architecture with significantly varied distributions of composition, morphology, and localized state energy, while high processing temperature induces more uniform film. The short-circuit current, open-circuit voltage, and fill factor of the devices are tuned with synergic improvement of efficiency toward over 10% and 11% for conventional and inverted devices. This work reveals the origination of vertical phase segregation, and provides a facile strategy to optimize the hierarchical phase separation for enhancing the performance of OSCs.

Vertical phase segregation in small molecule photovoltaic devices is manipulated via substrate temperature tuning. Low temperature induces multiple-sublayer-like architecture with significantly varied distributions of composition, morphology, and localized state energy, while high processing temperature induces more uniform film. The parameters of devices are largely tuned with synergic improvement of efficiency toward over 10% and 11% for conventional and inverted devices.

In planar perovskite solar cells, it is vital to engineer the extraction and recombination of electron–hole pairs at the electron transport layer/perovskite interface for obtaining high performance. This study reports a novel titanium oxide (TiO₂) bilayer with different Fermi energy levels by combing atomic layer deposition and spin-coating technique. Energy band alignments of TiO₂ bilayer can be modulated by controlling the deposition order of layers. The TiO₂ bilayer based perovskite solar cells are highly efficient in carrier extraction, recombination suppression, and defect passivation, and thus demonstrate champion efficiencies up to 16.5%, presenting almost 50% enhancement compared to the TiO₂ single layer based counterparts. The results suggest that the bilayer with type II band alignment as electron transport layers provides an efficient approach for constructing high-performance planar perovskite solar cells.

TiO₂ bilayers with different energy band alignments are fabricated by combing atomic layer deposition and a spin-coating technique. Perovskite solar cells based on TiO₂ electron transport bilayers are highly efficient in carrier extraction, recombination suppression, and defect passivation, and thus demonstrate champion efficiency up to 16.5%, which is almost 50% enhancement compared to the TiO₂ single layer based counterparts.