Chen Weijie

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.

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%.

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 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.

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.

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.

A low-temperature pseudohalide-induced post-processing technology is developed to improve the crystallinity and compactness of the perovskite active layer. In article number 1704836, Zhaoxin Wu, Alex K.-Y. Jen, and co-workers report that the retreatment process yields a controllable decompostion-to-recrystallization evolution of the perovskite film. Planar heterojunction solar cells assembled from the modified perovskite film are successfully applied in both rigid and flexible devices.

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%.

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.

Highly efficient PbS colloidal quantum dot (QD) solar cells based on an inverted structure have been missing for a long time. The bottlenecks are the construction of an effective p–n heterojunction at the illumination side with smooth band alignment and the absence of serious interface carrier recombination. Here, solution-processed nickel oxide (NiO) as the p-type layer and lead sulfide (PbS) QDs with iodide ligand as the n-type layer are explored to build a p–n heterojunction at the illumination side. The large depletion region in the QD layer at the illumination side leads to high photocurrent. Interface carrier recombination at the interface is effectively prohibited by inserting a layer of slightly doped p-type QDs with 1,2-ethanedithiol as ligands, leading to improved voltage of the device. Based on this graded device structure design, the efficiency of inverted structural heterojunction PbS QD solar cells is improved to 9.7%, one time higher than the highest efficiency achieved before.

An inverted structural QD solar cell with a p–n heterojunction located at the illumination side is constructed and exhibits remarkably high photocurrent above 27 mA cm⁻². The insertion of an intermediate buffer layer effectively reduces interface recombination, giving rise to improved voltage. The power conversion efficiency is improved to 9.7%, which is doubled for inverted structural heterojunction QD solar cells.

Developing low-cost photovoltaic absorbers that can harvest the short-wave infrared (SWIR) part of the solar spectrum, which remains unharnessed by current Si-based and perovskite photovoltaic technologies, is a prerequisite for making high-efficiency, low-cost tandem solar cells. Here, infrared PbS colloidal quantum dot (CQD) solar cells employing a hybrid inorganic–organic ligand exchange process that results in an external quantum efficiency of 80% at 1.35 µm are reported, leading to a short-circuit current density of 34 mA cm⁻² and a power conversion efficiency (PCE) up to 7.9%, which is a current record for SWIR CQD solar cells. When this cell is placed at the back of an MAPbI₃ perovskite film, it delivers an extra 3.3% PCE by harnessing light beyond 750 nm.

PbS colloidal quantum dot solar cells with high quantum efficiency (EQE) in the infrared are reported as a result of optimized surface passivation, band alignment, and optical design. The reported solar cells achieve short-circuit current density over 34 mA cm⁻², external quantum efficiency (EQE) of 80% at 1350 nm, and power conversion efficiency of 7.9% under one sun illumination.

A facile and efficient photoreduction method is employed to synthesize the composite of methylammonium lead iodide perovskite (MAPbI₃) with reduced graphene oxide (rGO). This MAPbI₃/rGO composite is shown to be an outstanding visible-light photocatalyst for H₂ evolution in aqueous HI solution saturated with MAPbI₃. Powder samples of MAPbI₃/rGO (100 mg) show a H₂ evolution rate of 93.9 µmol h⁻¹, which is 67 times faster than that of pristine MAPbI₃, under 120 mW cm⁻² visible-light (λ ≥ 420 nm) illumination, and the composite is highly stable showing no significant decrease in the catalytic activity after 200 h (i.e., 20 cycles) of repeated H₂ evolution experiments. The electrochemiluminescence performance of MAPbI₃ is investigated to explore the charge transfer process, to find that the photogenerated electrons in MAPbI₃ are transferred to the rGO sites, where protons are reduced to H₂.

Through compositing with photoreduced graphene oxide (rGO), the photocatalytic hydrogen evolution activity of CH₃NH₃PbI₃ is greatly elevated by a factor of 67 (93.9 µmol h⁻¹ with 100 mg initial amount, λ ≥ 420 nm, 120 mW cm⁻²), making the CH₃NH₃PbI₃/rGO composite an efficient and stable visible-light photocatalyst in aqueous HI solution saturated with MAPbI₃.

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 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.

In this communication, novel and simplified structure Cu(In,Ga)Se₂ (CIGS) solar cells, which nominally consist of only a CIGS photoabsorber layer sandwiched between back and front contact layers but yet demonstrate high photovoltaic efficiencies, are reported. To realize this accomplishment, Si-doped CIGS films grown by the three-stage coevaporation method, B-doped ZnO transparent conductive oxide front contact layers deposited by chemical vapor deposition, and heat–light soaking treatments are used. Si-doping of CIGS films is found to modify the film surfaces and grain boundary properties and also affect the alkali metal distribution profiles in CIGS films. These effects are expected to contribute to improvements in buffer-free CIGS device performance. Heat–light soaking treatments, which are occasionally performed to improve conventional buffer-based CIGS device performance, are found to be also effective in enhancing buffer-free CIGS photovoltaic efficiencies. This result suggests that the mechanism behind the beneficial effects of heat–light soaking treatments originates from CIGS bulk issues and is independent of the buffer materials. Consequently, over 16.5% efficiencies, including an independently certified value, are demonstrated from completely buffer-free CIGS photovoltaic devices.

Buffer-free simplified structure chalcogenide thin film solar cells with high efficiencies over 16.5% are demonstrated using novel Cu(In,Ga)Se₂ (CIGS) photoabsorber layers grown with Si-doping. Heat–light soaking treatments boost device performance regardless of the use of buffer layers, implying that the light-induced metastable phenomena in CIGS devices originate from bulk CIGS rather than buffer layers or the CIGS/buffer interface.

The solid electrolyte interphase (SEI) on metallic electrodes must be elastic and able to reversibly flex and expand to accommodate changes in electrode volume. In their Communication (DOI: 10.1002/anie.201711598), L. A. Archer et al. used SiCl₄ as an interfacial cross-linking agent in liquid electrolytes to create hybrid SEIs on Li metal anodes that are composed of Si-interlinked OOCOR molecules and host LiCl salt. Such hybrid SEIs exhibit good charge-transfer kinetics and high exchange current densities.