ZiQi Sun

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 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.

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.

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.

The best performing modern optoelectronic devices rely on single-crystalline thin-film (SC-TF) semiconductors grown epitaxially. The emerging halide perovskites, which can be synthesized via low-cost solution-based methods, have achieved substantial success in various optoelectronic devices including solar cells, lasers, light-emitting diodes, and photodetectors. However, to date, the performance of these perovskite devices based on polycrystalline thin-film active layers lags behind the epitaxially grown semiconductor devices. Here, a photodetector based on SC-TF perovskite active layer is reported with a record performance of a 50 million gain, 70 GHz gain-bandwidth product, and a 100-photon level detection limit at 180 Hz modulation bandwidth, which as far as we know are the highest values among all the reported perovskite photodetectors. The superior performance of the device originates from replacing polycrystalline thin film by a thickness-optimized SC-TF with much higher mobility and longer recombination time. The results indicate that high-performance perovskite devices based on SC-TF may become competitive in modern optoelectronics.

Improvement of optoelectronic device performance by using a single-crystalline perovskite thin film is demonstrated by a photodetector. The simultaneously optimized perovskites thickness and crystallinity lead to a record detector performance of a 70 GHz gain-bandwidth product and a 200-photon detection limit.

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.

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.

State-of-the-art light-emitting diodes (LEDs) are made from high-purity alloys of III–V semiconductors, but high fabrication cost has limited their widespread use for large area solid-state lighting. Here, efficient and stable LEDs processed from solution with tunable color enabled by using phase-pure 2D Ruddlesden–Popper (RP) halide perovskites with a formula (CH₃(CH₂)₃NH₃)₂(CH₃NH₃)_n−1Pb_nI_3n+1 are reported. By using vertically oriented thin films that facilitate efficient charge injection and transport, efficient electroluminescence with a radiance of 35 W Sr⁻¹ cm⁻² at 744 nm with an ultralow turn-on voltage of 1 V is obtained. Finally, operational stability tests suggest that phase purity is strongly correlated to stability. Phase-pure 2D perovskites exhibit >14 h of stable operation at peak operating conditions with no droop at current densities of several Amperes cm⁻² in comparison to mixtures of 2D/3D or 3D perovskites, which degrade within minutes.

Phase-pure Ruddlesden–Popper layered perovskites (RPLPs) are investigated for light-emitting diode application. This work demonstrates that RPLPs represent a promising candidate for optoeletronics and the crystal orientation of RPLPs plays a vital role in light-emitting diode devices, resulting in external quantum efficiency ≈1% with ultralow turn-on voltage.

Hybrid halide perovskite is one of the promising light absorber and is intensively investigated for many optoelectronic applications. Here, the first prototype of a self-powered inorganic halides perovskite for chemical gas sensing at room temperature under visible-light irradiation is presented. These devices consist of porous network of CsPbBr₃ (CPB) and can generate an open-circuit voltage of 0.87 V under visible-light irradiation, which can be used to detect various concentrations of O₂ and parts per million concentrations of medically relevant volatile organic compounds such as acetone and ethanol with very quick response and recovery time. It is observed that O₂ gas can passivate the surface trap sites in CPB and the ambipolar charge transport in the perovskite layer results in a distinct sensing mechanism compared with established semiconductors with symmetric electrical response to both oxidizing and reducing gases. The platform of CPB-based gas sensor provides new insights for the emerging area of wearable sensors for personalized and preventive medicine.

The first prototype of a self-powered inorganic halides perovskite is demonstrated for chemical gas sensing at room temperature under visible-light irradiation. These devices consist of a porous network of CsPbBr₃ and can generate an open-circuit voltage of 0.87 V under visible-light irradiation, which can be used to detect O₂, and medically relevant acetone and ethanol with very quick response and recovery time.

In this work, a new benzo[1,2-b:4,5-b′]dithiophene (BDT) building block containing alkylthio naphthyl as a side chain is designed and synthesized, and the resulting polymer, namely PBDTNS-BDD, shows a lower HOMO energy level than that of its alkoxyl naphthyl counterpart PBDTNO-BDD. An optimized photovoltaic device using PBDTNS-BDD as a donor exhibits power conversion efficiencies (PCE) of 8.70% and 9.28% with the fullerene derivative PC₇₁BM and the fullerene-free small molecule ITIC as acceptors, respectively. Surprisingly, ternary blend devices based on PBDTNS-BDD and two acceptors, namely PC₇₁BM and ITIC, shows a PCE of 11.21%, which is much higher than that of PBDTNO-BDD based ternary devices (7.85%) even under optimized conditions.

Ternary blend devices containing a new building block show much better power conversion efficiencies than their corresponding binary counterparts. These new building blocks contain alkylthio naphthyl as a side chain. Blending with two acceptors, PC₇₁BM and ITIC, results in PBDTNS-BDD devices with a power conversion efficiency of 11.21%, which is much higher than that of the PBDTNO-BDD (7.85%) analogue under optimized conditions.

Multijunction solar cells are designed to improve the overlap with the solar spectrum and to minimize losses due to thermalization. Aside from the optimum choice of photoactive materials for the respective sub-cells, a proper interconnect is essential. This study demonstrates a novel all-oxide interconnect based on the interface of the high-work-function (WF) metal oxide MoO_x and low-WF tin oxide (SnO_x). In contrast to typical p-/n-type tunnel junctions, both the oxides are n-type semiconductors with a WF of 5.2 and 4.2 eV, respectively. It is demonstrated that the electronic line-up at the interface of MoO_x and SnO_x comprises a large intrinsic interface dipole (≈0.8 eV), which is key to afford ideal alignment of the conduction band of MoO_x and SnO_x, without the requirement of an additional metal or organic dipole layer. The presented MoO_x/SnO_x interconnect allows for the ideal (loss-free) addition of the open circuit voltages of the two sub-cells.

A novel all-oxide recombination interconnect for organic tandem solar cells is reported. A large interface dipole between the high-work-function (WF) metal oxide MoO_x and low-WF tin oxide (SnO_x) affords ideal alignment of the conduction band of the two n-type metal oxides. The actual recombination of electrons with holes occurs at the interface of organic/MoO_x of the lower sub-cell.

The fabrication of a new type of solar cell encapsulation architecture comprising a periodic array of step-index waveguides is reported. The materials are fabricated through patterning with light in a photoreactive binary blend of crosslinking acrylate and urethane, wherein phase separation induces the spontaneous, directed formation of broadband, cylindrical waveguides. This microstructured material efficiently collects and transmits optical energy over a wide range of entry angles. Silicon solar cells comprising this encapsulation architecture show greater total external quantum efficiencies and enhanced wide-angle light capture and conversion. This is a rapid, straightforward, and scalable approach to process light-collecting structures, whereby significant increases in cell performance may be achieved.

Broadband waveguide array architectures are inscribed into polymer films as a new encapsulant material for solar cells. The architectures are grown in a binary-component, photocurable resin through light-induced self-writing, which elicits spontaneous formation of the core–cladding waveguide profile. Their light-collecting and light-guiding functions are inherited by the film, thereby enabling large-scale enhanced and wide-angle optical energy collection and conversion.

Most nonfullerene acceptors developed so far for high-performance organic solar cells (OSCs) are designed in planar molecular geometry containing a fused-ring core. In this work, a new nonfullerene acceptor of DF-PCIC is synthesized with an unfused-ring core containing two cyclopentadithiophene (CPDT) moieties and one 2,5-difluorobenzene (DFB) group. A nearly planar geometry is realized through the F···H noncovalent interaction between CPDT and DFB for DF-PCIC. After proper optimizations, the OSCs with DF-PCIC as the acceptor and the polymer PBDB-T as the donor yield the best power conversion efficiency (PCE) of 10.14% with a high fill factor of 0.72. To the best of our knowledge, this efficiency is among the highest values for the OSCs with nonfullerene acceptors owning unfused-ring cores. Furthermore, no obvious morphological changes are observed for the thermally treated PBDB-T:DF-PCIC blended films, and the relevant devices can keep ≈70% of the original PCEs upon thermal treatment at 180 °C for 12 h. This tolerance of such a high temperature for so long time is rarely reported for fullerene-free OSCs, which might be due to the unique unfused-ring core of DF-PCIC. Therefore, the work provides new idea for the design of new nonfullerene acceptors applicable in commercial OSCs in the future.

A new nonfullerene acceptor (DF-PCIC) is designed and synthesized by utilizing noncovalent interactions. Organic solar cells (OSCs) with DF-PCIC as the acceptor exhibit the best efficiency of 10.14% with a high fill factor of 0.72. More importantly, excellent morphological stability is achieved for DF-PCIC-based devices, which is meaningful for the future practical applications of OSCs.