ZiQi Sun

Solar cells based on mixed organic–inorganic halide perovskites are promising photovoltaic technologies with low-cost and fantastic power conversion efficiency (PCE). Enhancing the nucleation and regulating the crystallization rate of perovskite films and improving the bendability of brittle hybrid grains are crucial to improving the photovoltaic performance of flexible perovskite solar cells (PVSCs). Here, a simple approach is first introduced for fabricating perovskite films with full coverage and larger crystalline size by incorporating the elastomer polyurethane (PU) into the perovskite precursor solution to both retard the crystallization rate and improve the bendability. Shiny, smooth perovskite films are obtained with compact, micrometer-sized crystalline grains that exhibit excellent photoelectric performances. The PVSCs fabricated by incorporating PU into the perovskite precursor offer an impressive PCE of 18.7% with almost no photocurrent hysteresis and excellent stability in ambient air. More importantly, the elastomer PU additive crosslinks the grain boundaries between neighboring perovskite crystals to form a PU network that effectively improves the bendability of the perovskite films.

Polyurethane (PU) has been used as an effective additive to optimize the performance of perovskite solar cells by retarding crystallization rate and enhancing grain size of perovskite crystals. More importantly, elastomer PU can effectively improve the bendability of perovskite films due to denseness and high elasticity created by crosslinking grain boundaries between neighboring perovskite crystals to form a PU network.

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.

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.

Mixed iodide-bromide organolead perovskites with a bandgap of 1.70–1.80 eV have great potential to boost the efficiency of current silicon solar cells by forming a perovskite-silicon tandem structure. Yet, the stability of the perovskites under various application conditions, and in particular combined light and heat stress, is not well studied. Here, FA_0.15Cs_0.85Pb(I_0.73Br_0.27)₃, with an optical bandgap of ≈1.72 eV, is used as a model system to investigate the thermal-photostability of wide-bandgap mixed halide perovskites. It is found that the concerted effect of heat and light can induce both phase segregation and decomposition in a pristine perovskite film. On the other hand, through a postdeposition film treatment with benzylamine (BA) molecules, the highly defective regions (e.g., film surface and grain boundaries) of the film can be well passivated, thus preventing the progression of decomposition or phase segregation in the film. Besides the stability improvement, the BA-modified perovskite solar cells also exhibit excellent photovoltaic performance, with the champion device reaching a power conversion efficiency of 18.1%, a stabilized power output efficiency of 17.1% and an open-circuit voltage (V _oc) of 1.24 V.

Using a postdeposition film treatment with benzylamine (BA) molecules, the highly defective regions of the wide-bandgap FA_0.15Cs_0.85Pb(I_{1− x} Br _x )₃ films can be well passivated, thus preventing the progression of decomposition or phase segregation in the film during combined heat and light stress. The BA-treated perovskite solar cells exhibit a stabilized power output efficiency of 17.1% and an open-circuit voltage (V _oc) of 1.24 V.

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.

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%.

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.

A new donor (D)–acceptor (A) conjugate, benzodithiophene-rhodanine–[6,6]-phenyl-C₆₁ butyric acid methyl ester (BDTRh–PCBM) comprising three covalently linked blocks, one of p-type oligothiophene containing BDTRh moieties and two of n-type PCBM, is designed and synthesized. A single component organic solar cell (SCOSC) fabricated from BDTRh–PCBM exhibits the power conversion efficiency (PCE) of 2.44% and maximum external quantum efficiency of 46%, which are the highest among the reported efficiencies so far. The SCOSC device shows efficient charge transfer (CT, ≈300 fs) and smaller CT energy loss, resulting in the higher open-circuit voltage of 0.97 V, compared to the binary blend (BDTRh:PCBM). Because of the integration of the donor and acceptor in a single molecule, BDTRh-PCBM has a specific D–A arrangement with less energetic disorder and reorganization energy than blend systems. In addition, the SCOSC device shows excellent device and morphological stabilities, showing no degradation of PCE at 80 °C for 100 h. The SCOSC approach may suggest a great way to suppress the large phase segregation of donor and acceptor domains with better morphological stability compared to the blend device.

Integration of donor and acceptor in a single molecule by a covalent linkage is a promising approach to overcome unfavorably large-phase separation in bulk heterojunction blend solar cells. A new all-in-one system forms a specific molecular arrangement which decreases energetic disorder and facilitates ultrafast charge separation.

Organic–inorganic halide perovskites are promising photodetector materials due to their strong absorption, large carrier mobility, and easily tunable bandgap. Up to now, perovskite photodetectors are mainly based on polycrystalline thin films, which have some undesired properties such as large defective grain boundaries hindering the further improvement of the detector performance. Here, perovskite thin-single-crystal (TSC) photodetectors are fabricated with a vertical p–i–n structure. Due to the absence of grain-boundaries, the trap densities of TSCs are 10–100 folds lower than that of polycrystalline thin films. The photodetectors based on CH₃NH₃PbBr₃ and CH₃NH₃PbI₃ TSCs show low noise of 1–2 fA Hz^−1/2, yielding a high specific detectivity of 1.5 × 10¹³ cm Hz^1/2 W⁻¹. The absence of grain boundaries reduces charge recombination and enables a linear response under strong light, superior to polycrystalline photodetectors. The CH₃NH₃PbBr₃ photodetectors show a linear response to green light from 0.35 pW cm⁻² to 2.1 W cm⁻², corresponding to a linear dynamic range of 256 dB.

Photodetectors based on organic–inorganic halide perovskite thin single crystals (TSCs) are fabricated. Due to the absence of grain-boundaries, very low trap densities, and small thickness (≈10 µm) of the TSCs, the TSC photodetectors with vertical p–i–n structure show low noise (1–2 fA Hz^−1/2), high specific detectivity (≈1.5 × 10¹³ cm Hz^1/2 W⁻¹), and large linear-dynamic-range (256 dB).

Despite great progress in the photovoltaic conversion efficiency (PCE) of inorganic–organic hybrid perovskite solar cells (PSCs), the large-scale application of PSCs still faces serious challenges due to the poor-stability and high-cost of the spiro-OMeTAD hole transport layer (HTL). It is of great fundamental importance to rationally address the issues of hole extraction and transfer arising from HTL-free PSCs. Herein, a brand-new PSC architecture is designed by introducing multigraded-heterojunction (GHJ) inorganic perovskite CsPbBr_xI_3−x layers as an efficient HTL. The grade adjustment can be achieved by precisely tuning the halide proportion and distribution in the CsPbBr_xI_3−x film to reach an optimal energy alignment of the valance and conduction band between MAPbI₃ and CsPbBr_xI_3−x. The CsPbBr_xI_3−x GHJ as an efficient HTL can induce an electric field where a valance/conduction band edge is leveraged to bend at the heterojunction interface, boosting the interfacial electron–hole splitting and photoelectron extraction. The GHJ architecture enhances the hole extraction and conduction efficiency from the MAPbI₃ to the counter electrode, decreases the recombination loss during the hole transfer, and benefits in increasing the open-circuit voltage. The optimized HTL-free PCS based on the GHJ architecture demonstrates an outstanding thermal stability and a significantly improved PCE of 11.33%, nearly 40% increase compared with 8.16% for pure HTL-free devices.

Through energy-band engineering, a brand-new perovskite solar cell architecture with multigraded-heterojunction (GHJ) inorganic perovskite CsPbBr_xI_3−x layers as an efficient hole-transport layer is designed. The GHJ architecture enhances the hole extraction and conduction efficiency, and decreases the recombination loss during the hole transfer. A certified efficiency of 11.33% is obtained and the high-performing devices show outstanding thermal- and humidity-stability.

Lead halide perovskites have recently emerged as promising absorbers for fabricating low-cost and high-efficiency thin-film solar cells. The record power conversion efficiency of lead halide perovskite-based solar cells has rapidly increased from 3.8% in 2009 to 22.1% in early 2016. Such rapid improvement is attributed to the superior and unique photovoltaic properties of lead halide perovskites, such as the extremely high optical absorption coefficients and super-long photogenerated carrier lifetimes and diffusion lengths that are not seen in any other polycrystalline thin-film solar cell materials. In the past a few years, theoretical approaches have been extensively applied to understand the fundamental mechanisms responsible for the superior photovoltaic properties of lead halide perovskites and have gained significant insights. This review article highlights the important theoretical results reported in literature for the understanding of the unique structural, electronic, optical, and defect properties of lead halide perovskite materials. For comparison, we also review the theoretical results reported in literature for some lead-free perovskites, double perovskites, and nonperovskites.

Progress in the theoretical study of metal halide perovskite absorber materials is reviewed with a focus on the understanding of the unique structural, electronic, optical, and defect properties of lead halide perovskites. For comparison, the theoretical results reported for some lead-free perovskites, double perovskites and nonperovskites are also reviewed.

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.