Chen Weijie

Visual transparent photovoltaics offers an opportunity to generate electricity through see-through glass in an integrated application. Herein, a thorough simulation study of transparent tandem perovskite solar cells/organic solar cells is used to demonstrate this. Using an optimized design and proper light management techniques, it is possible to reach an efficiency of 15% with a visual transparency of 50%.

Solar cells transparent in the visible range are highly requested for integration in see-through photovoltaic (PV) applications such as building glass façades or greenhouse roofs. The development of advanced transparent PV can fully exploit the tandem technology where the top cell absorbs the near-ultraviolet solar spectrum while the bottom one absorbs the near-infrared part. Herein, a possible implementation of this tandem PV paradigm, namely, the tandem structure composed of a high-bandgap halide perovskite solar cell and a low-bandgap organic solar cell, is considered. Electro-optical simulation results based on parameters calibrated on experimental data show that an efficiency of 15% can be achieved with an average visible transmittance above 50%. This can be obtained considering the halide perovskite with mixed chlorine and bromine anions, a nonfullerene-based bulk heterojunction, a well-calibrated light management, and a three-terminal configuration of the tandem.

Nanomorphology and crystallinity of solvent vapor annealed DRCN5T:PC₇₁BM bulk heterojunction solar cells has been systematically studied using advanced electron microscopy and correlated to their efficiency. An optimum domain size and single-crystalline nature of DRCN5T fibers are found crucial for their performance. Donor solubility controls crystallization and coarsening resulting in an inverse relationship between optimal annealing time and donor solubility.

Solvent vapor annealing (SVA) has been shown to significantly improve the device performance of organic bulk-heterojunction solar cells, yet the mechanisms linking nanomorphology, crystallinity of the active layer, and performance are still largely missing. Here, the mechanisms are tackled by correlating the evolution of nanomorphology, crystallinity, and performance with advanced transmission electron microscopy methods systematically. Model system of DRCN5T:PC₇₁BM blends are SVA treated with four solvents differing in their donor and acceptor solubilities. The choice of solvent drastically influences the rate at which the maximum device efficiency establishes, though similar values can be achieved for all solvents. The donor solubility is identified as a key parameter that controls the kinetics of diffusion and crystallization of the blend molecules, resulting in an inverse relationship between optimal annealing time and donor solubility. For the highest efficiency, optimum domain size and single-crystalline nature of DRCN5T fibers are found to be crucial. Moreover, the π–π stacking orientation of the crystallites is directly revealed and related to the nanomorphology, providing insight into the charge carrier transport pathways. Finally, a qualitative model relating morphology, crystallinity, and device efficiency evolution during SVA is presented, which may be transferred to other light-harvesting blends.

A novel method for the deposition of amorphous tin oxide layers for use as the electron transport layer (ETL) of perovskite solar cells is described. This reactive electron-beam process involves using a tin source material and an oxygen partial pressure to oxidize the tin en route to the substrate. Devices using this ETL demonstrate efficiencies up to 19.3%.

Tin oxide (SnO _x ) electron-extraction layers are fabricated via a reactive electron-beam evaporation process from a metal source under a partial pressure of oxygen. These are then used in standard (n-i-p) architecture perovskite solar cells and achieve power conversion efficiencies up to 19.3%. The SnO _x deposition process is performed onto substrates maintained at low temperature compared to similar techniques, with films not requiring any subsequent high-temperature post-deposition annealing. This demonstrates the potential compatibility of reactive electron-beam evaporation with roll-to-roll processing onto flexible polymeric substrates.

A new ionic liquid with multifunctional cations and Cl anion is developed and is successfully incorporated to stabilize the black phase and passivate the surface and boundary defects of a FAPbI₃ film, achieving a power conversion efficiency of 22.01%.

FAPbI₃ is considered one of the most ideal perovskite materials to construct highly efficient perovskite solar cell (PSC), but the poor phase stability and film-forming properties hinder further performance improvements. Herein, an ionic liquid 3-(2-amino-2-oxoethyl)-1-methyl-1H-imidazol-3-ium chloride (AOMCl) with multifunctional cations and Cl anion to assist crystallization, reduce defects, and stabilize α-FAPbI₃ is reported. Results show that the perovskite film's nonradiative recombination and phase transition are suppressed after AOMCl treatment. The PSCs with AOMCl treatment obtain a power conversion efficiency (PCE) up to 22.01% with negligible hysteresis. In addition, the large-area (1 cm²) device achieves a PCE of 17.11%. The AOMCl-treated unencapsulated device exhibits impressive stability, maintaining an initial efficiency of over 94% after 1600 h in air environment. This work provides a strategy for designing FAPbI₃ additive ideally.

It is demonstrated that phthalimide (2-N) molecules as antisolvent additives can achieve crystal growth regulation, including rapid nucleation and slow crystal growth process. The perovskite films treated with 2-N exhibit the best crystallinity, largest grain size, and lowest trap density, while the corresponding device efficiency reaching 20.14% under AM 1.5G illumination and the highest indoor efficiency of 40.14% under light-emitting diode conditions (1062 lux, @2956 K).

Abstract

The main reason for large energy loss in all-inorganic perovskites is ascribed to the slow nucleation and fast crystallization of all-inorganic perovskite films. Herein, a manipulating strategy is demonstrated to simultaneously realize rapid nucleation and slow crystal growth of CsPbI₃ perovskite films by employing solvent molecular sieves in the antisolvent. First, the antisolvent treatment of mixed chlorobenzene and ethyl alcohol can induce the instantaneous supersaturation of perovskites to achieve rapid nucleation. Subsequently, the molecular layer of phthalimide (2-N) molecules on the perovskite surface can be used as solvent molecular sieves to precisely control the evaporation of the solvent through molecule–solvent interactions. In addition, the molecules remaining on the surface can also effectively passivate the surface defects and improve the device performance. By this strategy, a synchronous regulation of rapid nucleation and slow crystal growth of perovskite films is realized for the first time. As a result, the CsPbI₃ film with 2-N treatment presents high-quality crystallinity with large grains and less defects. The champion device exhibits an outdoor power conversion efficiency (PCE) up to 20.14% under AM1.5G illumination, and an indoor PCE up to 40.07% (P _out:133.9 µW cm^–2) under a commonly used light-emitting diode light source (2956 K, 1062 lux).

A comprehensive overview of fully stretchable organic solar cells (f-SOSCs), including essential studies to make each layer of an f-SOSC stretchable and efficient is provided. Various strategies to simultaneously enhance the photovoltaic and mechanical properties of the active layer, ranging from material design to fabrication control, are emphasized.

Abstract

Recent advances in the power conversion efficiency (PCE) of organic solar cells (OSCs) have greatly enhanced their commercial viability. Considering the technical standards (e.g., mechanical robustness) required for wearable electronics, which are promising application platforms for OSCs, the development of fully stretchable OSCs (f-SOSCs) should be accelerated. Here, a comprehensive overview of f-SOSCs, which are aimed to reliably operate under various forms of mechanical stress, including bending and multidirectional stretching, is provided. First, the mechanical requirements of f-SOSCs, in terms of tensile and cohesion/adhesion properties, are summarized along with the experimental methods to evaluate those properties. Second, essential studies to make each layer of f-SOSCs stretchable and efficient are discussed, emphasizing strategies to simultaneously enhance the photovoltaic and mechanical properties of the active layer, ranging from material design to fabrication control. Key improvements to the other components/layers (i.e., substrate, electrodes, and interlayers) are also covered. Lastly, considering that f-SOSC research is in its infancy, the current challenges and future prospects are explored.

Sequential casting (SC) processing is practical and universal for device performance improvement in both fullerene- and nonfullerene-based systems of organic solar cells (OSCs). A swelling–intercalation phase-separation model is proposed to interpret the morphology evolution during SC processing. Notably, a champion efficiency of 18.86% (certified as 18.44%) is reached from SC processing, representing the highest value among binary OSCs.

Abstract

Forming an ideal bulk heterojunction (BHJ) morphology is a critical issue governing the photon to electron process in organic solar cells (OSCs). Complementary to the widely-used blend casting (BC) method for BHJ construction, sequential casting (SC) can also enable similar or even better morphology and device performance for OSCs. Here, BC and SC methods on three representative donor:acceptor (D:A) blends are utilized, that is, PM6:PC₇₁BM, PM6:IT-4F and PM6:L8-BO. Higher power conversion efficiencies (PCEs) in all cases by taking advantage of beneficial morphology from SC processing are achieved, and a champion PCE of 18.86% (certified as 18.44%) based on the PM6:L8-BO blend is reached, representing the record value among binary OSCs. The observations on phase separation and vertical distribution inspire the proposal of the swelling–intercalation phase-separation model to interpret the morphology evolution during SC processing. Further, the vertical phase segregation is found to deliver an improvement of device performance via affecting the charge transport and collection processes, as evidenced by the D:A-ratio-dependent photovoltaic properties. Besides, OSCs based on SC processing show advantages on device photostability and upscale fabrication. This work demonstrates the versatility and efficacy of the SC method for BHJ-based OSCs.

By simultaneous control of the intermediate phase during crystallization of the perovskite film and surface passivation using a single molecule additive N,N-dimethylimidodicarbonimidic diamide hydroiodide (DIAI), which has multiple functional groups, an impressive device photoelectric conversion efficiency as high as 24.13% with negligible hysteresis is obtained. The bare device without any encapsulation maintains 94.1% of its initial efficiency after ambient exposure for over 1000 h.

Abstract

With its power conversion efficiency surpassing those of all other thin-film solar cells only a few years after its invention, the perovskite solar cell has become a superstar. Controlling the intermediate phase of crystallization is a key to obtaining high-quality perovskite films. Herein, a single molecule additive, N,N-dimethylimidodicarbonimidic diamide hydroiodide (DIAI), is incorporated into the perovskite precursor to eliminate the influence of intermediate phases. By taking advantage of the interaction of DIAI and dimethyl sulfoxide (DMSO), the intermediate phase FAI-PbI₂-DMSO complex is eliminated, and δ-FAPbI₃ is entirely converted to the desired α-FAPbI₃ during the crystallization step, resulting in enlarged grain size and improved crystalline quality. This is the first observation in the solution method that FAPbI₃ can be obtained without an intermediate phase for high-performance perovskite solar cells. Furthermore, DIAI is effective at passivating surface defects, resulting in reduced defect density, increased carrier lifetime, and improved device efficiency and stability. The champion device achieves an efficiency of 24.13%. Furthermore, the bare device without any encapsulation maintains 94.1% of its initial efficiency after ambient exposure over 1000h. This work contributes a strategy of synergistic crystallization and passivation to directly form α-FAPbI₃ from the precursor solution without the influence of intermediate impurities for high-performance perovskite applications.

Publication date: 20 July 2022

Source: Joule, Volume 6, Issue 7

Author(s): Jueming Bing, Laura Granados Caro, Harsh P. Talathi, Nathan L. Chang, David R. Mckenzie, Anita W.Y. Ho-Baillie

A spin-forced (SF) method as a facile approach is proposed to control the crystallization kinetics of CsPbI₂Br crystal growth, achieving a uniform, and high-quality film. The perovskite film formation during spin-coating and annealing is evaluated by optical in situ characterization. As a result, an efficiency of 17.0% with excellent thermal stability at 300 °C is achieved.

Abstract

All-inorganic perovskites have emerged as promising photovoltaic materials due to their superior thermal stability compared to their heat-sensitive hybrid organic–inorganic counterparts. In particular, CsPbI₂Br shows the highest potential for developing thermally-stable perovskite solar cells (PSCs) among all-inorganic compositions. However, controlling the crystallinity and morphology of all-inorganic compositions is a significant challenge. Here, a simple, thermal gradient- and antisolvent-free method is reported to control the crystallization of CsPbI₂Br films. Optical in situ characterization is used to investigate the dynamic film formation during spin-coating and annealing to understand and optimize the evolving film properties. This leads to high-quality perovskite films with micrometer-scale grain sizes with a noteworthy performance of 17% (≈16% stabilized), fill factor (FF) of 80.5%, and open-circuit voltage (V _OC) of 1.27 V. Moreover, excellent phase and thermal stability are demonstrated even after extreme thermal stressing at 300 °C.

A series of polymerized small molecular acceptors are developed by rational fluorination on the end groups and π-spacers to fine-tune the photoelectronic properties, molecular crystallinity, and miscibility with the polymer donor, which enables a record power conversion efficiency of 17.41% in layer-by-layer processed all-polymer solar cells.

Abstract

Despite remarkable breakthrough made by virtue of “polymerized small-molecule acceptor (PSMA)” strategy recently, the limited selection pool of high-performance polymer acceptors and long-standing challenge in morphology control impede their further developments. Herein, three PSMAs of PYDT-2F, PYDT-3F, and PYDT-4F are developed by introducing different fluorine atoms on the end groups and/or bithiophene spacers to fine-tune their optoelectronic properties for high-performance PSMAs. The PSMAs exhibit narrow bandgap and energy levels that match well with PM6 donor. The fluorination promotes the crystallization of the polymer chain for enhanced electron mobility, which is further improved by following n-doping with benzyl viologen additive. Moreover, the miscibility is also improved by introducing more fluorine atoms, which promotes the intermixing with PM6 donor. Among them, PYDT-3F exhibits well-balanced high crystallinity and miscibility with PM6 donor; thus, the layer-by-layer processed PM6/PYDT-3F film obtains an optimal nanofibril morphology with submicron length and ≈23 nm width of fibrils, facilitating the charge separation and transport. The resulting PM6/PYDT-3F devices realizes a record high power conversion efficiency (PCE) of 17.41% and fill factor of 77.01%, higher than the PM6/PYDT-2F (PCE = 16.25%) and PM6/PYDT-4F (PCE = 16.77%) devices.

In this paper, a new method termed hydrazine hydrate (N₂H₄) assisted solution treatment (HHST) is first introduced to improve CdS electron transport layers (ETLs) quality, which enables over 10.30% planar Sb₂(S,Se)₃ solar cells, achieves a smoother CdS and Sb₂(S,Se)₃ surface, tailors the elemental composition, optimizes the defect properties and improves interfacial quality.

Abstract

Antimony selenosulfide (Sb₂(S,Se)₃), a simple alloyed compound containing earth-abundant constituents, with a tunable bandgap and high absorption coefficient has attracted significant attention in high-efficiency photovoltaic applications. Optimizing interfacial defects and absorber layers to a high standard is essential in improving the efficiency of Sb₂(S,Se)₃ solar cells. In particular, the electron transport layer (ETL) greatly affects the final device performance of the superstrate structure. In this study, a simple and effective hydrazine hydrate (N₂H₄) solution post-treatment is proposed to modify CdS ETL in order to enhance Sb₂(S,Se)₃ solar cell efficiency. By this process, oxides and residual chlorides, caused by CdCl₂ treated CdS under a high temperature over 400 °C in air, are appropriately removed, rendering smoother and flatter CdS ETL as well as high-quality Sb₂(S,Se)₃ thin films. Furthermore, the interfacial energy band alignment and recombination loss are both improved, resulting in an as-fabricated FTO/CdS-N₂H₄/Sb₂(S,Se)₃/spiro-OMeTAD/Au solar cell with a high PCE of 10.30%, placing it in the top tier of Sb-based solar devices. This study provides a fresh perspective on interfacial optimization and promotes the future development of antimony chalcogenide-based planar solar cells.

High-performance inverted perovskite solar cells are fabricated with KBF₄ additive in perovskite layers. The introduction of KBF₄ can enlarge grain size, manipulate microstrain, and suppress the formation of deep trap states in the perovskite layers. The synergistic effects greatly elongate carrier lifetime and enable power conversion efficiencies over 23% and 21% for rigid and flexible devices.

Abstract

Triple-cation mixed perovskites have attracted much attention recently owing to their prominent optoelectronic properties and good stability for perovskite solar cells. However, the introduction of those cations with different sizes in the perovskite materials will drive the perovskite lattice away from ideal cubic structure and lead to microstrain in the resultant films. Herein,a small amount of KBF₄ as an additive to elevate the quality of triple-cation mixed perovskite thin films is introduced. It is found that KBF₄ can enhance the crystallinity and alleviate microstrain of the perovskite thin films. Moreover, KBF₄ can passivate defects in perovskite grains, leading to much longer carrier lifetimes. Consequently, the resultant devices show improved fill factor, enhanced device efficiency, and better device stability. Under optimum fabrication conditions, triple-cation mixed perovskite solar cells with an inverted structure show power conversion efficiency over 23% as well as excellent stability under different conditions.

To enhance the efficiency and stability of perovskite solar cells (PSCs), a bifunctional strategy for simultaneously passivating defects in both the TiO₂ and the perovskite layer with fluoride passivation is developed. The PSC efficiency can be significantly increased from 21.3% to 23.7%. Moreover, the long-term stability of PSCs, including humidity, light, and thermal stability, can be significantly improved.

Abstract

Due to the low formation energy, surface defects are more likely to form on the surface of TiO₂ films, resulting in a decline in the efficiency and stability of perovskite solar cells (PSCs). Additionally, defects on the bottom surface of the perovskite layer in contact with TiO₂ play a key role in V _oc (open circuit voltage) loss and the PSC degradation process. Therefore, to improve the efficiency and stability of PSCs, it is critical to develop a reproducible and low-cost method for passivating the defects on both the TiO₂ surface and on the bottom surface of the perovskite layer. In this work, fluoride is utilized as a bifacial contact passivation agent for decreasing the number of defects on the TiO₂ surface and the bottom surface of the perovskite layer. PSC efficiency can be significantly increased from 21.3% to 23.7% with fluoride passivation. In addition, the long-term stability of PSCs, especially light irradiation stability, can be markedly improved. The passivation effects of fluoride treatment on TiO₂ films are studied by theoretical calculation and experimental characterization. This work provides a thorough understanding of the TiO₂/perovskite interface and demonstrates an approach for improving the efficiency and stability of PSCs.

A new hybrid electron-transport layer (ETL) ZnO/NMA was developed, when combined with D18 : N3, the highest power conversion efficiency (18.20 %) among inverted single-junction organic solar cells was achieved with an operational lifetime of 5 years. The hybrid ETL approach represents an important forward step for the commercial application of OSCs.

Abstract

Although organic solar cells (OSCs) have delivered an impressive power conversion efficiency (PCE) of over 19 %, most of them demonstrated rather limited stability. So far, there are hardly any effective and universal strategies to improve stability of state-of-the-art OSCs. Herein, we developed a hybrid electron-transport layer (ETL) in inverted OSCs using ZnO and a new modifying agent (NMA), and significantly improved the stability and PCEs for all the tested devices. In particular, when applied in the D18 : N3 system, its inverted OSC exhibits so far the highest PCE (18.20 %) among inverted single-junction OSCs, demonstrating an extrapolated T₈₀ lifetime of 7572 h (equivalent to 5 years under outdoor exposure). This is the first report with T₈₀ over 5000 h among OSCs with over 18 % PCE. Furthermore, a high PCE of 16.12 % can be realized even in a large-area device (1 cm²). This hybrid ETL strategy provides a strong stimulus for highly prospective commercialization of OSCs.

Highly oriented quasi-2D perovskite with reverse-graded structure is obtained by bifacial stamping. The bulky cations are transferred from the 2D supply to 3D perovskite substrates during stamping, inducing transformation into quasi-2D perovskite. The solvent evaporation inhibited by stamping retards crystallization, resulting in the vertical alignment of the quasi-2D perovskite with reverse-graded structure.

Abstract

Quasi 2D perovskite solar cells (PSCs) are promising light absorbers that overcome the inherent instabilities of 3D perovskites. High-performance and stable 2D PSCs require careful control over the crystallographic orientation and phase distribution. This study introduces a simple and universal bifacial stamping method to obtain highly oriented perovskite crystals with a reverse-graded structure, where the low-n-value 2D perovskite phases are located mainly at the film surfaces. Bifacial stamping of 3D perovskite films atop the 2D films enables incorporation of 2D spacer cations into the 3D film surfaces, forming reverse-graded quasi-2D perovskite films. During stamping, suppressed evaporation of the precursor solvent induces heterogeneous nucleation from the contact interface between the 2D and 3D films, resulting in well-crystallized perovskite films having out-of-plane alignments with respect to the substrate. Thus, a highly oriented and reverse-graded quasi-2D perovskite with an average n value of 18 is obtained with power conversion efficiency exceeding 17% and high open-circuit voltage of 1.11 V for iso-butylammonium (iso-BA)-based (iso-BA₂MA _{n −1}Pb _n I_{3 n +1}) PSCs. The unencapsulated device retains 92% of its initial efficiency after aging at 40 ± 5% relative humidity for 1200 h. This work provides a new strategy for fabricating highly oriented and phase-controlled quasi-2D PSCs.

Potassium L-aspartate (PL-A) is introduced in buried interface of perovskite solar cells. The diffusion of potassium cations on PL-A can form a gradient n-doping in perovskite while the anions remain at the SnO₂ surface modulates dipole moment towards ETL. As a result, the energy level structure near SnO₂/perovskite interface is aligned, restraining the carrier's recombination and improving their transport.

Abstract

The under-coordinated defects within perovskite and its relevant interfaces always attract and trap the free carriers via the electrostatic force, significantly limiting the charge extraction efficiency and the intrinsic stability of perovskite solar cells (PSCs). Herein, self-diffusion interfacial doping by using ionic potassium L-aspartate (PL-A) is first reported to restrain the carrier trap induced recombination via the reconstruction of energy level structure at SnO₂/perovskite interface in conventional n-i-p structured PSCs. Experiments and theories are consistent with the PL-A anions that can remain at the SnO₂ surface due to strong chemical adsorption. During the spin-coating of the perovskite film, the cations gradually diffuse into perovskite and endow an n-doping effect, which provides higher force and a better energy level alignment for the carrier transport. As a result, they obtained 23.74% power conversion efficiency for the PL-A modified small-area devices, with dramatically improved open-circuit voltage of 1.19 V. The corresponding large-area devices (1.05 cm²) achieved an efficiency of 22.23%. Furthermore, the modified devices exhibited negligible hysteresis and enhanced ambient air stability exceeding 1500 h.

Bulk organic heterojunctions broaden the spectral response range of perovskite solar cells to 930 nm. Black phosphorus quantum dots and double hole transport layer modulate carrier transport in perovskite solar cells.

Abstract

Light management through organic bulk heterojunction (BHJ) has been widely reported to push up the performance of lead-based perovskite solar cells (PSCs) by extending the spectral response. However, the development of integrated perovskite/organic bulk heterojunction solar cells (IPSCs) encounters a bottleneck problem that the poor carrier extraction capability of perovskite and BHJ leads to the severe loss of open-voltage (V _oc) and fill factor (FF). Herein, the strategy of introducing black phosphorous quantum dots (BPQDs) and cuprous oxide (CuO _x ) into IPSCs is adopted, which not only successfully extends the single-component PSCs light response to 930 nm, but also significantly reduces the V _oc and FF loss of IPSCs. BPQDs with bipolar charge transport and high mobility characteristics improves the electron/hole transport behaviors of perovskite and BHJ films. CuO _x with matching energy levels is introduced between BHJ and Spiro-OMeTAD as a buffer layer, which provides good driving force for the transportation of holes. The champion device achieves a power conversion efficiency of 23.52%. The IPSCs devices also display an excellent long-term and humidity stability. This work demonstrates an approach to solve the carrier extraction key issues that limits the performance of IPSCs, which achieves an instructive result in the development of PSCs light management.

This work innovatively introduces a multiple-ring aromatic ammonium, 1-naphthylamine (1-NA) spacer. 2D RP CsPbI₃ perovskite using 1-NA spacer with extended π-conjugation lengths not only reduce the exciton binding energy and facilitate the efficient separation of excitons, but also promotes electronic coupling between organic and inorganic layers, improving interlayer charge transport. Importantly, (1-NA)₂(Cs)₃Pb₄I₁₃ device exhibits record 16.62% performance with enhanced stability.

Abstract

Two-dimensional (2D) Ruddlesden–Popper (RP) CsPbI₃ perovskite possesses superior phase stability by introducing steric hindrance. However, due to the quantum and dielectric confinement effect, 2D structures usually exhibit large exciton binding energy, and the charge tunneling barrier across the organic interlayer is difficult to eliminate, resulting in poor charge transport and performance. Here, a multiple-ring aromatic ammonium, 1-naphthylamine (1-NA) spacer is developed for 2D RP CsPbI₃ perovskite solar cell (PSC). Theoretical simulations and experimental characterizations demonstrate that the 2D RP CsPbI₃ perovskite using 1-NA spacer with extended π-conjugation lengths reduces the exciton binding energy and facilitates the efficient separation of excitons. In addition, its cations have a significant contribution to the conduction band, which can reduce the bandgap, promote electronic coupling between organic and inorganic layers, and improve interlayer charge transport. Importantly, the strong π–π conjugation of 1-NA spacer can enhance intermolecular interactions and hydrogen bonding, and prepare high-quality films with preferred vertical orientation, resulting in lower defect density, and directional charge transport. As a result, the (1-NA)₂(Cs)₃Pb₄I₁₃ PSC exhibits a record 16.62% performance with enhanced stability. This work provides an efficient approach to improve charge transport and device performance by developing multiple-ring aromatic spacers.

A room temperature sol-gel processed MoO _x is developed for highly efficient and stable inverted organic photovoltaics via an anion-induced catalytic reaction (ACR). The ACR-derived MoO _x thin film exhibits a defect-free and highly oriented metal-oxygen network without post-treatment, enabling outstanding physical and electrical properties on top of the organic photoactive layer.

Abstract

Solution-processed transition metal oxides (TMOs) prepared from complex ion precursors are developed as promising scalable interfacial layers for non-fullerene organic photovoltaics (OPVs); however, challenges remain in achieving defect-free and highly oriented metal-oxygen networks without post-deposition treatments due to the presence of residual organic metal-binding ligands in films. Herein, the novel strategy that the problematic organic metal-binding ligands in TMO precursors can be successfully eliminated by an anion-induced catalytic reaction (ACR) at room temperature is demonstrated, in which the low-level anions induce electron redistribution and instability of TMO precursors, expediting binding ligand removal during the hydrolysis reaction. The subsequent condensation process facilitates a dimensionally confined and continuous metal-oxygen network with a 20-fold increase in electrical conductivity (from 8.4 × 10⁻⁴ to 1.8 × 10⁻² S m⁻¹) and superior work function tunability (from 5.1 to 5.3 eV) compared to the pristine film. The ACR-derived TMO thin film on top of a ternary PBDB-TF:Y6:PC₇₁BM photoactive layer enables an inverted device configuration with improved efficiency of 17.6%, as well as enhanced stability over 70% of the initial efficiency for up to 100 h AM 1.5G illumination.

Field effect passivation is identified as the origin of the voltage enhancement upon inserting a LiF interlayer at the electron selective contact of perovskite solar cells by high sensitivity near-UV photoelectron spectroscopy. LiF increases the defect density in the C₆₀, however, the minority charge carrier density in the vicinity of the interface is lowered, resulting in an overall reduction in the non-radiative recombination.

Abstract

The fullerene C₆₀ is commonly applied as the electron transport layer in high-efficiency metal halide perovskite solar cells and has been found to limit their open circuit voltage. Through ultra-sensitive near-UV photoelectron spectroscopy in constant final state mode (CFSYS), with an unusually high probing depth of 5–10 nm, the perovskite/C₆₀ interface energetics and defect formation is investigated. It is demonstrated how to consistently determine the energy level alignment by CFSYS and avoid misinterpretations by accounting for the measurement-induced surface photovoltage in photoactive layer stacks. The energetic offset between the perovskite valence band maximum and the C₆₀ HOMO-edge is directly determined to be 0.55 eV. Furthermore, the voltage enhancement upon the incorporation of a LiF interlayer at the interface can be attributed to originate from a mild dipole effect and probably the presence of fixed charges, both reducing the hole concentration in the vicinity of the perovskite/C₆₀ interface. This yields a field effect passivation, which overcompensates the observed enhanced defect density in the first monolayers of C₆₀.

Understanding the molecular origin of photodegradation is necessary to improve organic photovoltaic stability. Here, it is demonstrated that small changes in the molecular structure of ITIC derivatives result in very different acceptor and blend photostability. This is rationalized by the different intermolecular interactions of acceptors driven by their different quadrupole moment.

Abstract

Understanding degradation mechanisms of organic photovoltaics (OPVs) is a critical prerequisite for improving device stability. Herein, the effect of molecular structure on the photostability of non-fullerene acceptors (NFAs) is studied by changing end-group substitution of ITIC derivatives: ITIC, ITIC-2F, and ITIC-DM. Using an assay of in situ spectroscopy techniques and molecular simulations, the photodegradation product of ITIC and the rate of product formation are identified, which correlates excellently to reported device stability, with ITIC-2F being the most stable and ITIC-DM the least. The choice of acceptor is found to affect both the donor polymer (PBDB-T) photostability and the morphological stability of the bulk heterojunction blend. Molecular simulations reveal that NFA end-group substitution strongly modulates the electron distribution within the molecule and thus its quadrupole moment. Compared to unsubstituted-ITIC, end-group fluorination results in a stronger, and demethylation a weaker, molecular quadrupole moment. This influences the intermolecular interactions between NFAs and between the NFA and the polymer, which in turn affects the photostability and morphological stability. This hypothesis is further tested on two other high quadrupole acceptors, Y6 and IEICO-4F, which both show impressive photostability. The strong correlation observed between NFA quadrupole moment and photostability opens a new synthetic direction for photostable organic photovoltaic materials.

Publication date: September 2022

Source: Nano Energy, Volume 100

Author(s): Yangjie Lan, Yang Wang, Yue Lai, Zheren Cai, Mingquan Tao, Yuduan Wang, Mingzhu Li, Xia Dong, Yanlin Song

High-performance binary photovoltaic devices based on D18 and L8-BO are constructed via manipulating the vertical component distribution in a sequential deposition (SD) process. After tuning the spin-coating speeds of film deposition, the optimal SD device affords a record power conversion efficiency of 19.05% (certified, 18.9%) for binary single-junction organic solar cells, much higher than that of the corresponding blend casting device (18.14%).

Abstract

The variation of the vertical component distribution can significantly influence the photovoltaic performance of organic solar cells (OSCs), mainly due to its impact on exciton dissociation and charge-carrier transport and recombination. Herein, binary devices are fabricated via sequential deposition (SD) of D18 and L8-BO materials in a two-step process. Upon independently regulating the spin-coating speeds of each layer deposition, the optimal SD device shows a record power conversion efficiency (PCE) of 19.05% for binary single-junction OSCs, much higher than that of the corresponding blend casting (BC) device (18.14%). Impressively, this strategy presents excellent universality in boosting the photovoltaic performance of SD devices, exemplified by several nonfullerene acceptor systems. The mechanism studies reveal that the SD device with preferred vertical components distribution possesses high crystallinity, efficient exciton splitting, low energy loss, and balanced charge transport, resulting in all-around enhancement of photovoltaic performances. This work provides a valuable approach for high-efficiency OSCs, shedding light on understanding the relationship between photovoltaic performance and vertical component distribution.

A facile top-down strategy to control the all-polymer solar cell (all-PSC) morphology is developed by layer-by-layer processing. Optimal intermixing of polymer components and improved molecular packing of bottom layer and top layer can be achieved by choosing a suitable top-layer processing solvent. Consequently, a favorable morphology with gradient vertical composition distribution is realized, affording a high all-PSC efficiency of 17.0%.

Abstract

A major challenge hindering the further development of all-polymer solar cells (all-PSCs) employing polymerized small-molecule acceptors is the relatively low fill factor (FF) due to the difficulty in controlling the active-layer morphology. The issues typically arise from oversized phase separation resulting from the thermodynamically unfavorable mixing between two macromolecular species, and disordered molecular orientation/packing of highly anisotropic polymer chains. Herein, a facile top-down controlling strategy to engineer the morphology of all-polymer blends is developed by leveraging the layer-by-layer (LBL) deposition. Optimal intermixing of polymer components can be achieved in the two-step process by tuning the bottom-layer polymer swelling during top-layer deposition. Consequently, both the molecular orientation/packing of the bottom layer and the molecular ordering of the top layer can be optimized with a suitable top-layer processing solvent. A favorable morphology with gradient vertical composition distribution for efficient charge transport and extraction is therefore realized, affording a high all-PSC efficiency of 17.0% with a FF of 76.1%. The derived devices also possess excellent long-term thermal stability and can retain >90% of their initial efficiencies after being annealed at 65 °C for 1300 h. These results validate the distinct advantages of employing an LBL processing protocol to fabricate high-performance all-PSCs.

Introduction of Cl atoms into the backbone in small-molecule donors (SMDs) enhances the molecular planarity and rigidity through intramolecular Cl−S interactions. Additionally, asymmetric isomerization of side chains balances crystallinity and miscibility to achieve ideal morphology for simultaneously high short-circuit current density and fill factor to yield a record power conversion efficiency of 15.73 % among halogenated SMD-based binary all-small-molecule organic solar cells.

Abstract

Intramolecular Cl−S non-covalent interaction is introduced to modify molecular backbone of a benzodithiophene terthiophene rhodamine (BTR) benchmark structure, helping planarize and rigidify the molecular framework for improving charge transport. Theoretical simulations and temperature-variable NMR experiments clearly validate the existence of Cl−S non-covalent interaction in two designed chlorinated donors and explain its important role in enhancing planarity and rigidity of the molecules for enhancing their crystallinity. The asymmetric isomerization of side-chains further optimizes the molecular orientation and surface energy to strike a balance between its crystallinity and miscibility. This carefully manipulated molecular design helps result in increased carrier mobility and suppressed charge recombination to obtain simultaneously enhanced short-circuit current (J _sc) and fill factor (FF) and a very high efficiency of 15.73 % in binary all-small-molecule organic solar cells.