Ligang Yuan

The commercialization of nonfullerene organic solar cells (OSCs) critically relies on the response under typical operating conditions (for instance, temperature and humidity) and the ability of scale-up. Despite the rapid increase in power conversion efficiency (PCE) of spin-coated devices fabricated in a protective atmosphere, the efficiencies of printed nonfullerene OSC devices by blade coating are still lower than 6%. This slow progress significantly limits the practical printing of high-performance nonfullerene OSCs. Here, a new and relatively stable nonfullerene combination is introduced by pairing the nonfluorinated acceptor IT-M with the polymeric donor FTAZ. Over 12% efficiency can be achieved in spin-coated FTAZ:IT-M devices using a single halogen-free solvent. More importantly, chlorine-free, blade coating of FTAZ:IT-M in air is able to yield a PCE of nearly 11% despite a humidity of ≈50%. X-ray scattering results reveal that large π–π coherence length, high degree of face-on orientation with respect to the substrate, and small domain spacing of ≈20 nm are closely correlated with such high device performance. The material system and approach yield the highest reported performance for nonfullerene OSC devices by a coating technique approximating scalable fabrication methods and hold great promise for the development of low-cost, low-toxicity, and high-efficiency OSCs by high-throughput production.

A new nonfullerene combination composed of a high-performance polymer and a nonfluorinated small molecule is presented. It holds great potential for additive-free and halogen-free processing. Small and pure domains and face-on molecular packing collectively enable the first demonstration of ≈11% efficiency air-processed and stable nonfullerene solar cells by blade-coating techniques. Additionally, complete solvent–morphology–performance relations are established for further improvements.

The use of a ternary component is a facile and effective method to further improve the device performances of binary organic solar cells (OSCs) comprising one donor and one acceptor. Recently, the rapid progress of highly efficient nonfullerene acceptor materials has offered a new opportunity for studying ternary OSCs because of the flexibility of ternary components, and the photovoltaic performance of ternary OSCs has been promoted quickly. In this research news article, some strategies for materials selection of the ternary components are concisely summarized and the recent progress in ternary OCSs based on nonfullerene acceptors, and the challenges and perspectives of ternary OSCs are also presented.

Fullerene-free ternary organic solar cells integrating complementary absorptions are greatly inspired by the prosperous development of nonfullerene acceptors with their great performance and abundant varieties. This research news demonstrates the main functions of third components and some design guidelines for ternary composition and summarizes the recent advanced progress for high-performance ternary organic solar cells based on nonfullerene acceptors.

Perovskite solar cells have delivered power conversion efficiency beyond 22% in less than seven years, implying the potential for the paradigm shift of low-cost photovoltaics with high efficiency and low embedded energy. Besides the “perovskite fever,” the development of new hole transport materials (HTM), especially dopant-free HTMs, is another research hotspot. This is because the currently used HTMs, such as spiro-OMeTAD derivatives, require additional chemical doping process to ensure sufficient conductivity and proper ionic potential level for efficient hole transport and collection. However, the commonly used dopants are volatile and hygroscopic which not only increase the complexity and cost of device fabrication but also deteriorate the device stability. So far, there have been several reviews on new HTMs, but review or analysis on dopant-free HTMs is scarce. In this review, all reported dopant-free HTMs are categorized into four primary different types and lessons will be learned during the separate discussions. The stability test behavior of all the intrinsic HTMs will be evaluated directly. In the end, the correlations between the properties of the intrinsic HTMs and parameters of the devices will be plotted to shed light on the future direction of development of this field.

Chemical dopants inside organic semiconductor are however not chemically bonded to the matrix. Their hydrophilic and mobile nature plays a significant role in the degradation of the perovskite devices. Dopant free HTMs are of great importance for the final application of this new PV technology.

The surface composition of perovskite films is very sensitive to film processing and can deviate from the optimal, which generates unfavorable defects and results in efficiency loss in solar cells and slow response speed in photodetectors. An argon plasma treatment is introduced to modify the surface composition by tuning the ratio of organic and inorganic components as well as defect type before deposition of the passivating layer. It can efficiently enhance the charge collection across the perovskite–electrode interface by suppressing charge recombination. Therefore, perovskite solar cells with argon plasma treatment yield enhanced efficiency to 20.4% and perovskite photodetectors can reach their fastest respond speed, which is solely limited by the carrier mobility.

An argon plasma treatment is introduced to modify the surface trap types of halide perovskite, which improves the efficiency of a solar cell to 20.4%. The plasma treatment is shown as being also applicable to modify the perovskite single crystals, which enable the fastest response speed for single-crystal photodetectors.

Self-healing, where a modification in some parameter is reversed with time without any external intervention, is one of the particularly interesting properties of halide perovskites. While there are a number of studies showing such self-healing in perovskites, they all are carried out on thin films, where the interface between the perovskite and another phase (including the ambient) is often a dominating and interfering factor in the process. Here, self-healing in perovskite (methylammonium, formamidinium, and cesium lead bromide (MAPbBr₃, FAPbBr₃, and CsPbBr₃)) single crystals is reported, using two-photon microscopy to create damage (photobleaching) ≈110 µm inside the crystals and to monitor the recovery of photoluminescence after the damage. Self-healing occurs in all three perovskites with FAPbBr₃ the fastest (≈1 h) and CsPbBr₃ the slowest (tens of hours) to recover. This behavior, different from surface-dominated stability trends, is typical of the bulk and is strongly dependent on the localization of degradation products not far from the site of the damage. The mechanism of self-healing is discussed with the possible participation of polybromide species. It provides a closed chemical cycle and does not necessarily involve defect or ion migration phenomena that are often proposed to explain reversible phenomena in halide perovskites.

Direct proof of self-healing inside lead bromide perovskite crystals is provided. Two-photon excitation, from high-intensity sub-bandgap (800 nm) pulsed illumination, allows damage to and monitoring deep (>100 μm) inside bromide perovskite single crystals via photoluminescence. Complete or partial recovery of photodamage is observed in minutes to hours. A complete chemical mechanism involving ABr₃ species is proposed to explain the phenomenon.

All inorganic cesium lead halide (CsPbX₃, X = Cl, Br, I) perovskite nanocrystals (PeNCs) are synthesized by employing polar solvent controlled ionization (PCI) method in precursors. The new strategy can be easily carried out at room temperature and allow to employ smaller amount of weaker polarity and a broader range of low-boiling low-toxic solvents. The as prepared CsPbX₃ PeNCs reveal tunable emission spectra from 380 to 700 nm and high quantum yields over 80% with narrow full width at half maximum (FWHM). Meanwhile, larger “effective Stokes shifts” of PeNCs in PCI method, which enlarges 200% more than other PeNCs in regular methods, are observed. Most interestingly, the PeNCs growth process is coupling with some typical crystals formations. The main morphologies of CsPbI₃ PeNCs are hybrid of nanorods and nanoparticles. The primary morphologies of CsPbBr_xI_3-x and CsPbBr₃ PeNCs are nanowires, which are supposed to have great potentials for applying in laser arrays and highly sensitive photodetector applications. Furthermore, such superior optical is endowed to fabricate white light emitting diodes, which has wide color gamut covering up to 120% of the National Television Systems Committee color standard.

All inorganic cesium lead halideperovskite nanocrystals (PeNCs) are synthesized by a new strategy employing a polar solvent controlled ionization method. The as prepared PeNCs reveal tunable emission spectra, high quantum yields, narrow emission, and large “effective Stokes shifts.” Most interestingly, the PeNCs growth process is coupling with some typical crystals formations, including nanoparticles, nanorods, and nanowires.

Planar perovskite solar cells (PSCs) based on low-temperature-processed (LTP) SnO₂ have demonstrated excellent photovoltaic properties duo to the high electron mobility, wide bandgap, and suitable band energy alignment of LTP SnO₂. However, planar PSCs or mesoporous (mp) PSCs based on high-temperature-processed (HTP) SnO₂ show much degraded performance. Here, a new strategy with fully HTP Mg-doped quantum dot SnO₂ as blocking layer (bl) and a quite thin SnO₂ nanoparticle as mp layer are developed. The performances of both planar and mp PSCs has been greatly improved. The use of Mg-SnO₂ in planar PSCs yields a high-stabilized power conversion efficiency (PCE) of close to 17%. The champion of mp cells exhibits hysteresis free and stable performance with a high-stabilized PCE of 19.12%. The inclusion of thin mp SnO₂ in PSCs not only plays a role of an energy bridge, facilitating electrons transfer from perovskite to SnO₂ bl, but also enhances the contact area of SnO₂ with perovskite absorber. Impedance analysis suggests that the thin mp layer is an “active scaffold” selectively collecting electrons from perovskite and can eliminate hysteresis and effectively suppress recombination. This is an inspiring advance toward high-performance PSCs with HTP mp SnO₂.

A fully high-temperature (HT)-processed Mg-incorporated quantum dot (QD) SnO₂ blocking layer (bl)/mesoporous (mp) SnO₂ layer is used for the fabrication of perovskite solar cells (PSCs). Optimized fully HT mp SnO₂ PSCs can be achieved using an Mg-incorporated QD SnO₂ bl/100-nm-thick mp SnO₂ layer and its champion cell harvests a high stabilized power conversion efficiency of 19.2% with hysteresis free and stable performance.

Organometal halide perovskites have attracted widespread attention as the most favorable prospective material for photovoltaic technology because of their high photoinduced charge separation and carrier transport performance. However, the microstructural aspects within the organometal halide perovskite are still unknown, even though it belongs to a crystal system. Here direct observation of the microstructure of the thin film organometal halide perovskite using transmission electron microscopy is reported. Unlike previous reports claiming each phase of the organometal halide perovskite solely exists at a given temperature range, it is identified that the tetragonal and cubic phases coexist at room temperature, and it is confirmed that superlattices composed of a mixture of tetragonal and cubic phases are self-organized without a compositional change. The organometal halide perovskite self-adjusts the configuration of phases and automatically organizes a buffer layer at boundaries by introducing a superlattice. This report shows the fundamental crystallographic information for the organometal halide perovskite and demonstrates new possibilities as promising materials for various applications.

Coexistence of cubic and tetragonal phases at room temperature and the existence of self-assembled superlattices are confirmed in organometal halide perovskite by transmission electron microscopy. The superlattices are composed of a mixture of tetragonal and cubic phases without compositional change. Based on the phase coexistence and the superlattices, the organometal halide perovskite self-adjusts its microstructural configuration.

The effects of alkyl chain regiochemistry on the properties of donor polymers and performances of non-fullerene organic solar cells are investigated. Two donor polymers (PfBTAZ and PfBTAZS) are compared that have nearly identical chemical structures except for the regiochemistry of alkyl chains. The optical properties and crystallinity of two polymers are nearly identical yet the PfBTAZ:O-IDTBR blend exhibits nearly double domain size compared to the blend based on PfBTAZS:O-IDTBR. To reveal the origins of the very different domain size of two blends, the morphology of neat polymer films is characterized, and it is found that PfBTAZ tends to aggregate into much larger polymer fibers without the presence of O-IDTBR. This indicates that it is not the polymer:O-IDTBR interactions but the intrinsic aggregation properties of two polymers that determine the morphology features of neat and blend films. The stronger aggregation tendency of PfBTAZ could be explained by its more co-planar geometry of the polymer backbone arising from the different alkyl chain regiochemistry. Combined with the similar trend observed in another set of donor polymers (PTFB-P and PTFB-PS), the results provide an important understanding of the structure–property relationships that could guide the development of donor polymers for non-fullerene organic solar cells.

The effects of alkyl chain regiochemistry on the properties of donor polymers and the performance of non-fullerene organic solar cells are investigated. It is found that the alkyl chain regiochemistry has great impacts on the morphology features of the neat and blend films, and the PfBTAZS:O-IDTBR-based cells with small domain size can achieve a high efficiency of 10.4%.

Fully solution-processed semi-transparent perovskite solar cells with a highest PCE of 14.17% are realized by incorporating a thin lay of polyethylenimine (PEI), which acts as the interfacial work function modification layer as well as the protection layer of the printed AgNW electrode by suppressing the chemical corrosion of AgNW electrode during printing.

The collection efficiency of photogenerated charges in polymer solar cells (PSCs) is strongly influenced by the built-in field (E_in) that develops across the photoactive materials. Here, by investigating the E_in-development regimes in PSCs by introducing two types of interlayers, electric dipole layers (EDLs) and charge transport layers (CTLs), the device architecture is optimized to result in a larger E_in. By incorporating a pair of EDLs on both sides of the photoactive layer, the E_in is modulated by shifting the vacuum energy at each metal–semiconductor interface, providing a larger E_in than that in conventional PSCs using typical CTLs, such as metal oxides and/or conducting polymers. These devices using paired EDLs exhibit an average PCE of 9.8%, which far surpasses the average PCE of ≈8.5% for paired CTLs.

Polymer solar cells with a new device architecture for reinforcing built-in electric field is demonstrated. A pair of strong electric dipole layers on both sides of the photoactive layer maximizes internal electric field in the operating state under short circuit condition, which permits an efficient “sweep out” of photo generated charges compared to those of typical charge transport layers.

A new synthetic route, to prepare an alkylated indacenodithieno[3,2-b]thiophene-based nonfullerene acceptor (C8-ITIC), is reported. Compared to the reported ITIC with phenylalkyl side chains, the new acceptor C8-ITIC exhibits a reduction in the optical band gap, higher absorptivity, and an increased propensity to crystallize. Accordingly, blends with the donor polymer PBDB-T exhibit a power conversion efficiency (PCE) up to 12.4%. Further improvements in efficiency are found upon backbone fluorination of the donor polymer to afford the novel material PFBDB-T. The resulting blend with C8-ITIC shows an impressive PCE up to 13.2% as a result of the higher open-circuit voltage. Electroluminescence studies demonstrate that backbone fluorination reduces the energy loss of the blends, with PFBDB-T/C8-ITIC-based cells exhibiting a small energy loss of 0.6 eV combined with a high J_SC of 19.6 mA cm⁻².

The synthesis of a novel alkylated indacenodithioeno[3,2-b]thiophene (C₈-IDTT) based nonfullerene acceptor (C₈-ITIC), is reported. Compared to ITIC with phenylalkyl side chains, the acceptor exhibits a redshifted absorption with increased absorptivity. Solar cell power conversion efficiencies (PCEs) of up to 13.2 % are achieved, with the high PCE attributed to the broad absorption, high crystallinity of C8-ITIC and low voltage loss.

Two series of new polymers with medium and wide bandgaps to match fullerene (PC₇₁BM) and fullerene-free 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC) acceptors are designed and synthesized, respectively. For constructing the key donor building blocks, the effective symmetry-breaking strategy is employed. Two common aromatic rings (thiophene and benzene) are chosen as one side substituted groups in the asymmetric benzodithiophene (BDT) monomers. In addition, another rigid benzene ring is inserted between aryl and thioether in the side chains, which results in larger twisting and destroying the aggregation and forming longer lever arms. As a result, highly ordered polymers (PBDTsTh-FBT and PBDTsPh-FBT) with strong aggregation properties can blend well with roughly spherical PC₇₁BM, while amorphous polymers (PBDTsThPh-BDD and PBDTsPhPh-BDD) with long and rigid aryl rings show good miscibility with elongated ITIC, and finally, both devices exhibit excellent power conversion efficiencies over 10%. Thus, it clearly shows that the asymmetric BDT unit is an excellent donor building block to construct highly efficient photovoltaic polymers. Meanwhile, this work demonstrates that it is not necessary that high-performance fullerene-free polymer solar cells (PSCs) require highly ordered microstructures in the blending films, different from the fullerene-based PSCs.

Two series of new asymmetric benzodithiophene building block based polymers with medium and wide bandgaps to match fullerene and fullerene-free acceptors are designed and synthesized, respectively. The significant function of the benzene rings in controlling morphology is revealed, which would be a promising strategy to further design prospective light-harvesting polymers for high-performance polymer solar cells.

In article number 1702415, Xiaochen Ren, Wenping Hu, and co-workers present an overview of 2D organic materials for optoelectronic applications. After systematically elaborating the large-area 2D crystal-growth strategies and patterning techniques, the unique operating mechanism resulting from a 2D configuration, and their applications in the context of electronic and optoelectronic devices are introduced.

The tunnel junction (TJ) intermediate connection layer (ICL), which is the most critical component for high-efficient tandem solar cell, generally consists of hole conducting layer and polyethyleneimine (PEI) polyelectrolyte. However, because of the nonconducting feature of pristine PEI, photocurrent is open-restricted in ICL even with a little thick PEI layer. Here, high-efficiency homo-tandem solar cells are demonstrated with enhanced efficiency by introducing carbon quantum dot (CQD)-doped PEI on TJ–ICL. The CQD-doped PEI provides substantial dynamic advantages in the operation of both single-junction solar cells and homo-tandem solar cells. The inclusion of CQDs in the PEI layer leads to improved electron extraction property in single-junction solar cells and better series connection in tandem solar cells. The highest efficient solar cell with CQD-doped PEI layer in between indium tin oxide (ITO) and photoactive layer exhibits a maximum power conversion efficiency (PCE) of 9.49%, which represents a value nearly 10% higher than those of solar cells with pristine PEI layer. In the case of tandem solar cells, the highest performing tandem solar cell fabricated with C-dot-doped PEI layer in ICL yields a PCE of 12.13%; this value represents an ≈15% increase in the efficiency compared with tandem solar cells with a pristine PEI layer.

High-efficiency homo-tandem solar cells with enhanced power conversion efficiency (PCE) are demonstrated by introducing carbon quantum dot (CQD)-doped PEI on a tunnel junction intermediate connection layer. The inclusion of CQDs in the PEI layer leads to improved electron extraction properties and better series connection. The best tandem solar cell fabricated with the CQD-doped PEI layer yields a PCE of 12.13%.