Chen Weijie

The formation of an inversion layer within n‐Si near the interface with poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) based conductive thin films is evidenced. High power conversion efficiency in solar cells is correlated with a large contact‐induced band bending in Si, high polymer conductivity, and proper Si interfacial passivation.

Abstract

Heterojunctions formed by ultrathin conductive polymer [poly(3,4‐ethylenedioxythiophene): poly(styrenesulfonate)—PEDOT:PSS] films and n‐type crystalline silicon are investigated by photoelectron spectroscopy. Large shifts of Si 2p core levels upon PEDOT:PSS deposition provide evidence that a dopant‐free p–n junction, i.e., an inversion layer, is formed within Si. Among the investigated PEDOT:PSS formulations, the largest induced band bending within Si (0.71 eV) is found for PH1000 (high PEDOT content) combined with a wetting agent and the solvent additive dimethyl sulfoxide (DMSO). Without DMSO, the induced band bending is reduced, as is also the case with a PEDOT:PSS formulation with higher PSS content. The interfacial energy level alignment correlates well with the characteristics of PEDOT:PSS/n‐Si solar cells, where high polymer conductivity and sufficient Si‐passivation are also required to achieve high power conversion efficiency.

Recent case studies demonstrate how probing of local heterogeneities and ensemble averaged properties of perovskites by surface science techniques can help build connections between material properties and perovskite solar cell (PSC) performance. How the generation/healing of electronic defects within the semiconductor band‐gap influences PSC efficiency, lifetime, as well reproducibility is also the central focus of this review article.

Abstract

ABX₃ type metal halide perovskite solar cells (PSCs) have shown efficiencies over 25%, rocketing toward their theoretical limit. To gain the full potential of PSCs relies on the understanding of the device working mechanisms and recombination, the material quality, and the match of energy levels in the device stacks. In this review, the importance of designing PSCs from the viewpoint of surface/interface science studies is presented. For this purpose, recent case studies are discussed to demonstrate how probing of local heterogeneities (e.g., grains, grain boundaries, atomic structure, etc.) in perovskites by surface science techniques can help correlate material properties and PSC device performance. At the solar cell device level with active areas larger than millimeter scale, the ensemble average measurement techniques can characterize the overall average properties of perovskite films as well as their adjacent layers and provide clues to understand better the solar cell parameters. How generation and healing of electronic defects in perovskite films limit the device efficiency, reproducibility, and stability, and induce the time‐dependent transient behavior in the current‐voltage curves are also the central focus of this review. On the basis of these studies, strategies to further improve efficiency and stability, as well as reducing hysteresis are presented.

A stable quantum dot (QD) ink is reported by using ammonium iodide for the liquid‐state ligand exchange, and improved photovoltaic performance of QD solar cell is obtained by using the ink for the deposition of QD solid film. Experimental studies and theoretical calculations reveal that the enhanced photovoltaic performance is attributed to the improved passivation on the QD surface.

Abstract

Liquid‐state ligand exchange provides an efficient approach to passivate a quantum dot (QD) surface with small binding species and achieve a QD ink toward scalable QD solar cell (QDSC) production. Herein, experimental studies and theoretical simulations are combined to establish the physical principles of QD surface properties induced charge carrier recombination and collection in QDSCs. Ammonium iodide (AI) is used to thoroughly replace the native oleic acid ligand on the PbS QD surface forming a concentrated QD ink, which has high stability of more than 30 d. The ink can be directly applied for the preparation of a thick QD solid film using a single deposition step method and the QD solid film shows better characteristics compared with that of the film prepared with the traditional PbX₂ (X = I or Br) post‐treated QD ink. Infrared light‐absorbing QDSC devices are fabricated using the PbS‐AI QD ink and the devices give a higher photovoltaic performance compared with the devices fabricated with the traditional PbS‐PbX₂ QD ink. The improved photovoltaic performance in PbS‐AI‐based QDSC is attributed to diminished charge carrier recombination induced by the sub‐bandgap traps in QDs. A theoretical simulation is carried out to atomically link the relationship of QDSC device function with the QD surface properties.

This study reports a systematic study in terms of efficiency and stability by post‐treatment with alkyl ammonium iodides of different alkyl lengths on (FAPbI₃)_0.95(MAPbBr₃)_0.05 perovskite surface. As the length of the alkyl chain increases, the electron‐blocking ability and humidity stability increase, but the highest efficiency is obtained at the optimal alkyl length.

Abstract

Recently, two‐dimensional (2D) structure on three‐dimensional (3D) perovskites (graded 2D/3D) has been reported to be effective in significantly improving both efficiency and stability. However, the electrical properties of the 2D structure as a passivation layer on the 3D perovskite thin film and resistance to the penetration of moisture may vary depending on the length of the alkyl chain. In addition, the surface defects of the 2D itself on the 3D layer may also be affected by the correlation between the 2D structure and the hole conductive material. Therefore, systematic interfacial study with the alkyl chain length of long‐chained alkylammonium iodide forming a 2D structure is necessary. Herein, the 2D interfacial layers formed are compared with butylammonium iodide (BAI), octylammonium iodide (OAI), and dodecylammonium iodide (DAI) iodide on a 3D (FAPbI₃)_0.95(MAPbBr₃)_0.05 perovskite thin film in terms of the PCE and humidity stability. As the length of the alkyl chain increased from BA to OA to DA, the electron‐blocking ability and humidity resistance increase significantly, but the difference between OA and DA is not large. The PSC post‐treated with OAI has slightly higher PCE than those treated with BAI and DAI, achieving a certified stabilized efficiency of 22.9%.

A nonfullerene acceptor based active layer with high halogen contents is designed to fabricate efficient thick‐film organic solar cells. The conventional structure device using chlorinated acceptor F–2Cl and fluorinated donor PM6 exhibits a power conversion efficiency over 10% with an active layer thickness of 600 nm.

Abstract

Developing efficient organic solar cells (OSCs) with relatively thick active layer compatible with the roll to roll large area printing process is an inevitable requirement for the commercialization of this field. However, typical laboratory OSCs generally exhibit active layers with optimized thickness around 100 nm and very low thickness tolerance, which cannot be suitable for roll to roll process. In this work, high performance of thick‐film organic solar cells employing a nonfullerene acceptor F–2Cl and a polymer donor PM6 is demonstrated. High power conversion efficiencies (PCEs) of 13.80% in the inverted structure device and 12.83% in the conventional structure device are achieved under optimized conditions. PCE of 9.03% is obtained for the inverted device with active layer thickness of 500 nm. It is worth noting that the conventional structure device still maintains the PCE of over 10% when the film thickness of the active layer is 600 nm, which is the highest value for the NF‐OSCs with such a large active layer thickness. It is found that the performance difference between the thick active layer films based conventional and inverted devices is attributed to their different vertical phase separation in the active layers.

An electron‐deficient unit containing B←N bonds, namely BNIDT, is developed to construct polymer acceptors for photovoltaic applications. Desirable optoelectronic properties such as broad absorption profiles, low‐lying energy levels, ambipolar charge transport properties, and strong electron‐affinity are found for these polymers. All‐polymer solar cells using these B←N embedded polymers as acceptor materials exhibit an enhanced efficiency of 8.78%.

Abstract

In the field of all‐polymer solar cells (all‐PSCs), all efficient polymer acceptors that exhibit efficiencies beyond 8% are based on either imide or dicyanoethylene. To boost the development of this promising solar cell type, creating novel electron‐deficient units to build high‐performance polymer acceptors is critical. A novel electron‐deficient unit containing B←N bonds, namely, BNIDT, is synthesized. Systematic investigation of BNIDT reveals desirable properties including good coplanarity, favorable single‐crystal structure, narrowed bandgap and downshifted energy levels, and extended absorption profiles. By copolymerizing BNIDT with thiophene and 3,4‐difluorothiophene, two novel conjugated polymers named BN‐T and BN‐2fT are developed, respectively. It is shown that these polymers possess wide absorption spectra covering 350–800 nm, low‐lying energy levels, and ambipolar film‐transistor characteristics. Using PBDB‐T as the donor and BN‐2fT as the acceptor, all‐PSCs afford an encouraging efficiency of 8.78%, which is the highest for all‐PSCs excluding the devices based on imide and dicyanoethylene‐type acceptors. Considering that the structure of BNIDT is totally different from these classical units, this work opens up a new class of electron‐deficient unit for constructing efficient polymer acceptors that can realize efficiencies beyond 8% for the first time.

An ideal materials combination based on the electron donor BSFTR and acceptor Y6 is selected to construct small‐molecule solar cells (SMSCs). By morphology optimization, an extraordinary power conversion efficiency of 13.69% with a remarkably low energy loss of 0.48 eV is achieved, which is beneficial from the matched photoelectric properties, the favorable blend morphology, and is the best binary SMSC performance reported so far.

Abstract

Compared with the quick development of polymer solar cells, achieving high‐efficiency small‐molecule solar cells (SMSCs) remains highly challenging, as they are limited by the lack of matched materials and morphology control to a great extent. Herein, two small molecules, BSFTR and Y6, which possess broad as well as matched absorption and energy levels, are applied in SMSCs. Morphology optimization with sequential solvent vapor and thermal annealing makes their blend films show proper crystallinity, balanced and high mobilities, and favorable phase separation, which is conducive for exciton dissociation, charge transport, and extraction. These contribute to a remarkable power conversion efficiency up to 13.69% with an open‐circuit voltage of 0.85 V, a high short‐circuit current of 23.16 mA cm⁻² and a fill factor of 69.66%, which is the highest value among binary SMSCs ever reported. This result indicates that a combination of materials with matched photoelectric properties and subtle morphology control is the inevitable route to high‐performance SMSCs.

The rapid development in large‐area organic solar cells (OSCs) is reviewed. Materials requirements, modular designs, and printing methods for large‐area OSCs are discussed. By combining thick‐film material systems with efficient modular designs, and then by employing the right printing methods, the fabrication of large‐area OSCs will be successfully realized in the near future.

Abstract

The printing of large‐area organic solar cells (OSCs) has become a frontier for organic electronics and is also regarded as a critical step in their industrial applications. With the rapid progress in the field of OSCs, the highest power conversion efficiency (PCE) for small‐area devices is approaching 15%, whereas the PCE for large‐area devices has also surpassed 10% in a single cell with an area of ≈1 cm². Here, the progress of this fast developing area is reviewed, mainly focusing on: 1) material requirements (materials that are able to form efficient thick active layer films for large‐area printing); 2) modular designs (effective designs that can suppress electrical, geometric, optical, and additional losses, leading to a reduction in the PCE of the devices, as a consequence of substrate area expansion); and 3) printing methods (various scalable fabrication techniques that are employed for large‐area fabrication, including knife coating, slot‐die coating, screen printing, inkjet printing, gravure printing, flexographic printing, pad printing, and brush coating). By combining thick‐film material systems with efficient modular designs exhibiting low‐efficiency losses and employing the right printing methods, the fabrication of large‐area OSCs will be successfully realized in the near future.

Interfaces between the photoactive layer and electrodes play a critical role in ultimate device behaviors in organic bulk heterojunction solar cells (OSCs) and hybrid halide perovskite solar cells (PSCs). Recent progress in interface modification for OSCs and PSCs aimed at improving interfacial charge extraction and mitigating surface recombination, and at enhancing trap passivation and device stability is presented.

Abstract

Organic bulk heterojunction solar cells (OSCs) and hybrid halide perovskite solar cells (PSCs) are two promising photovoltaic techniques for next‐generation energy conversion devices. The rapid increase in the power conversion efficiency (PCE) in OSCs and PSCs has profited from synergetic progresses in rational material synthesis for photoactive layers, device processing, and interface engineering. Interface properties in these two types of devices play a critical role in dictating the processes of charge extraction, surface trap passivation, and interfacial recombination. Therefore, there have been great efforts directed to improving the solar cell performance and device stability in terms of interface modification. Here, recent progress in interfacial doping with biopolymers and ionic salts to modulate the cathode interface properties in OSCs is reviewed. For the anode interface modification, recent strategies of improving the surface properties in widely used PEDOT:PSS for narrowband OSCs or replacing it by novel organic conjugated materials will be touched upon. Several recent approaches are also in focus to deal with interfacial traps and surface passivation in emerging PSCs. Finally, the current challenges and possible directions for the efforts toward further boosts of PCEs and stability via interface engineering are discussed.

Solution‐processed MoO₃ (SM), synthesized by a simple low‐temperature process, is utilized as an efficient and stable anode interfacial layer for organic solar cells (OSCs). The ultrasmooth SM film, without pinholes, exhibits excellent photovoltaic performance and device stability in OSCs, maintaining ≈92% of its initial solar cell efficiency over 2500 h storage in inert conditions.

The interfacial layer (IL) in organic solar cells (OSCs) can be an important boosting factor for improving device efficiency and stability. Herein, a facile and cost‐effective approach to form a uniform molybdenum oxide (MoO₃) film with desirable stability is provided, based on solution processing at low temperatures by simplified precursor solution synthesis. The solution‐processed MoO₃ (SM) film, with oxygen vacancies induced by the hydroxyl group, functions as an efficient anode IL in conventional OSCs. The hole‐transporting performance of SM is well demonstrated in nonfullerene‐based OSCs exhibiting over 10% of power conversion efficiency. The enhanced device performance of SM‐based OSCs over that of poly(3,4‐ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is investigated by analyzing the morphology, electronic state, and electrical conductivity of such a hole‐transporting layer, as well as the charge dynamics in the completed devices. Furthermore, the high stability of the SM films in OSCs is examined under various environmental conditions, including long‐term and thermal stability. In particular, fullerene‐based OSCs with SM maintain over 90% of their initial cell performance over 2500 h under inert conditions. It is shown that solution‐processed metal oxides can be viable ILs with high functionality and versatility, overcoming the drawbacks of conventionally adopted conducting polymer interlayers.

High performance flexible organic solar cells with efficiency above 12% for 1 cm² cells are fabricated using a Ag/Cu composite grid electrode. The excellent optical and electrical properties of the Ag/Cu electrode contribute to the high performance and good mechanical resistance of the flexible organic solar cell.

Abstract

With the rapid progress of organic solar cells (OSCs), improvement in the efficiency of large‐area flexible OSCs (>1 cm²) is crucial for real applications. However, the development of the large‐area flexible OSCs severely lags behind the growth of the small‐area OSCs, with the electrical loss due to the large sheet resistance of the electrode being a main reason. Herein, a high conductive and high transparent Ag/Cu composite grid with sheet resistance <1 Ω sq⁻¹ and an average visible light transparency of 84% is produced as the transparent conducting electrode of flexible OSCs. Based on this Ag/Cu composite grid electrode, a high efficiency of 12.26% for 1 cm² flexible OSCs is achieved. The performances of large‐area flexible OSCs also reach 7.79% (4 cm²) and 7.35% (9 cm²), respectively, which are much higher than those of the control devices with conventional flexible indium tin oxide electrodes. Surface planarization using highly conductive PEDOT:PSS and modification of the ZnO buffer layer by zirconium acetylacetonate (ZrAcac) are two necessary steps to achieve high performance. The flexible OSCs employing Ag/Cu grid have excellent mechanical bending resistance, maintaining high performance after bending at a radius of 2 mm.

Publication date: Available online 7 November 2019

Source: Nano Energy

Author(s): Chao Ding, Feng Liu, Yaohong Zhang, Daisuke Hirotani, Xing Rin, Shuzi Hayase, Takashi Minemoto, Taizo Masuda, Ruixiang Wang, Qing Shen

Graphical abstract

Both hot and cold photoexcited carrier (electron and hole) dynamics including relaxation and transfer at the heterojunction interfaces between FAPbI₃ QDs and two kinds of well used charge acceptors, i.e., TiO₂ and NiO_x are investigated systematically. The hot carriers in the FAPbI₃ QDs are cooled to cold carriers with a cooling time of more than 60 ps. The cold-electron and -hole injection rates are size dependent and are 2.01∼2.29 × 10⁹ s⁻¹ and 1.55∼1.96 × 10⁹ s⁻¹ at the two types of FAPbI₃ QD/MO (metal oxide) heterojunctions, respectively. Prototypes of the two types of heterojunction-based QDSCs, i.e., normal-structure solar cells based on FAPbI₃ QD/TiO₂ and inverted-structure solar cells based on FAPbI₃ QD/NiO_x, were developed and the power conversion efficiencies of more than 9% and 5% were obtained, respectively.

A mixed antisolvent treatment by spraying acetonitrile (ACN)‐added chlorobenzene (CBZ) for the fabrication of high‐quality perovskite films is reported. It is found that while the lower‐boiling‐point ACN preserves the morphology of the perovskite film, it has a significant impact on perovskite crystallization dynamics. Moreover, the method developed herein is scalable for future large‐area devices.

Herein, a mixed anti‐solvent treatment by spraying acetonitrile (ACN)‐added chlorobenzene (CBZ) for the fabrication of high‐quality perovskite films is proposed. While the lower‐boiling‐point ACN preserves the morphology of the perovskite film, it has a significant impact on perovskite crystallization dynamics. While CBZ is responsible for facilitating nucleation, ACN performs two functions. ACN as a weak polarity solvent allows the organic salt in the perovskite complex to be redissolved for perovskite formation and loosens the dimethyl sulfoxide (DMSO)–PbI₂ bond for more rapid perovskite crystallization. For the mixed anti‐solvent treatment to be successful, an appropriate amount of ACN is the key and the use of spraying to dispense the mixed anti‐solvent is crucial. This is due to the more instant and rapid reaction caused by the ACN which is faster than spreading the ACN across the substrate by the spinning motion. The resultant film using an appropriate mixed anti‐solvent treatment is a pinhole‐free high‐quality perovskite film with larger grain and enhanced crystallinity compared with CBZ‐only treatment film. The best solar cell using this mixed anti‐solvent treatment with 20% of ACN and 80% of CBZ achieves a PCE of 20.1% due to the reduced recombination.

A series of wide bandgap donor polymers are designed and synthesized by incorporating a monothiophene functionalized with both a fluorine atom and an ester group. Fabricated from nonhalogenated solvent, power conversion efficiencies of 11.39% and 12.11% are achieved for binary and ternary nonfullerene solar cells, respectively.

Abstract

Significant progress has been made in nonfullerene small molecule acceptors (NF‐SMAs) that leads to a consistent increase of power conversion efficiency (PCE) of nonfullerene organic solar cells (NF‐OSCs). To achieve better compatibility with high‐performance NF‐SMAs, the direction of molecular design for donor polymers is toward wide bandgap (WBG), tailored properties, and preferentially ecofriendly processability for device fabrication. Here, a weak acceptor unit, methyl 2,5‐dibromo‐4‐fluorothiophene‐3‐carboxylate (FE‐T), is synthesized and copolymerized with benzo[1,2‐b:4,5‐b′]dithiophene (BDT) to afford a series of nonhalogenated solvent processable WBG polymers P1‐P3 with a distinct side chain on FE‐T. The incorporation of FE‐T leads to polymers with a deep highest occupied molecular orbital (HOMO) level of −5.60−5.70 eV, a complementary absorption to NF‐SMAs, and a planar molecular conformation. When combined with the narrow bandgap acceptor ITIC‐Th, the solar cell based on P1 with the shortest methyl chain on FE‐T achieves a PCE of 11.39% with a large V _oc of 1.01 V and a J _sc of 17.89 mA cm⁻². Moreover, a PCE of 12.11% is attained for ternary cells based on WBG P1, narrow bandgap PTB7‐Th, and acceptor IEICO‐4F. These results demonstrate that the new FE‐T is a highly promising acceptor unit to construct WBG polymers for efficient NF‐OSCs.

In the process of finding high-performance materials for organic photovoltaics (OPVs), it is meaningful if one can establish the relationship between chemical structures and photovoltaic properties even before synthesizing them. Here, we first establish a database containing over 1700 donor materials reported in the literature. Through supervised learning, our machine learning (ML) models can build up the structure-property relationship and, thus, implement fast screening of OPV materials. We explore several expressions for molecule structures, i.e., images, ASCII strings, descriptors, and fingerprints, as inputs for various ML algorithms. It is found that fingerprints with length over 1000 bits can obtain high prediction accuracy. The reliability of our approach is further verified by screening 10 newly designed donor materials. Good consistency between model predictions and experimental outcomes is obtained. The result indicates that ML is a powerful tool to prescreen new OPV materials, thus accelerating the development of the OPV field.

Device model simulation is a macroscopic computer‐assisted tool for modeling organic and organic–inorganic hybrid perovskite solar cells. It simulates the underlying physical mechanisms of the electrical characteristics, such as space‐charge‐limited current, injection‐limited current, ohmic contact, short‐circuit current density, open‐circuit voltage, J–V hysteresis phenomena, power conversion efficiency in the present of surface recombination, trap/defect dependent recombination, or direct band recombination.

Abstract

Device model simulation is one of the primary tools for modeling thin film solar cells from organic materials to organic–inorganic perovskite materials. By directly connecting the current density–voltage (J–V) curves to the underlying device physics, it is helpful in revealing the working mechanism of the heatedly discussed organic–inorganic hybrid perovskite solar cells. Some distinctive optoelectronic features need more phenomenological models and accurate simulations. Herein, the application of the device model method in the simulation of organic and organic–inorganic perovskite solar cells is reviewed. To this end, the ways of the device model are elucidated by discussing the metal–insulator–metal picture and the equations describing the physics. Next, the simulations on J–V curves of organic solar cells are given in the presence of the space charge, interface, charge injection, traps, or exciton. In the perovskite section, the effects of trap states, direct band recombination, surface recombination, and ion migration on the device performance are systematically discussed from the perspective of the device model simulation. Suggestions for designing perovskite devices with better performance are also given.

An ultrathin functional polymer layer is used to enhance the nucleation of atomic layer deposited (ALD) SnO₂ contacts in metal‐halide perovskite solar cells. These nucleation‐enhanced ALD layers act as “built‐in” barriers to both internal and external degradation pathways, significantly improving the long‐term operational stability of high efficiency unencapsulated devices (>18%) in air.

Abstract

Metal‐halide perovskites show promise as highly efficient solar cells, light‐emitting diodes, and other optoelectronic devices. Ensuring long‐term stability is now a major priority. In this study, an ultrathin (2 nm) layer of polyethylenimine ethoxylated (PEIE) is used to functionalize the surface of C₆₀ for the subsequent deposition of atomic layer deposition (ALD) SnO₂, a commonly used electron contact bilayer for p–i–n devices. The enhanced nucleation results in a more continuous initial ALD SnO₂ layer that exhibits superior barrier properties, protecting Cs_0.25FA_0.75Pb(Br_0.20I_0.80)₃ films upon direct exposure to high temperatures (200 °C) and water. This surface modification with PEIE translates to more stable solar cells under aggressive testing conditions in air at 60 °C under illumination. This type of “built‐in” barrier layer mitigates degradation pathways not addressed by external encapsulation, such as internal halide or metal diffusion, while maintaining high device efficiency up to 18.5%. This nucleation strategy is also extended to ALD VO _x films, demonstrating its potential to be broadly applied to other metal oxide contacts and device architectures.

Herein, the dipolar polarization in a quasi‐2D organic–inorganic hybrid‐perovskite nanorod network–based solar cell using impedance spectroscopy is studied. Electric field and photoinduced dipole–dipole interaction plays an important role for the solar cell working at steady states.

Layered quasi‐2D organic–inorganic hybrid perovskites (OIHPs) prevent oxygen and moisture permeation, for long‐lifetime photovoltaic performance. Unfortunately, the electrical and photoinduced surface and dipolar polarizations caused due to the presence of the organic cation spacer in the structure remain unclear. Herein, a high‐performance planar quasi‐2D OIHP solar cell comprising (PEA)₂(MA)₃Pb₄I₁₃ (n = 4) is designed. It displays a large area coverage and an interconnected nanorod network, which contributes to efficient light absorption and charge carrier transport. The surface and dipolar polarizations exhibit remarkable light intensity and electric field–dependent characteristics at short‐circuit‐current (J _sc) and steady‐state (i.e., V _oc) conditions. More importantly, Voc exhibits a nonlinear behavior at steady states. Such a unique feature is in accordance with the dipolar polarization measured at the same condition. The phenomenon can be explained by the significant dipole–dipole interaction at lower electric field strengths. At higher field strengths, the screen of the dipoles due to charge accumulation at the surface of the organic cation spacer leads to slower increment of Voc. Thus, carefully designing the quasi‐2D perovskite nanostructure, together with the dielectric property of the organic cation spacer, may play an exceptionally important role for future high‐performance quasi‐2D perovskite solar cells.

Solution‐processed α‐In₂Se₃ is first used as the hole transport layer in polymer solar cells (PSCs) due to its significant photoelectric properties. A high power conversion efficiency of 9.58% is achieved in α‐In₂Se₃‐based devices, which is comparable with that of poly(3,4‐ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)‐based devices. Furthermore, the α‐In₂Se₃ film possesses excellent thermal stability and enhances the long‐term stability of PSCs.

Herein, a 2D α‐In₂Se₃ nanosheet, a binary III–VI group compound semiconductor, is fabricated by liquid‐phase exfoliation method, and the photoelectric properties of α‐In₂Se₃ material are investigated in depth. It is found that α‐In₂Se₃ film exhibits significant conductivity, outstanding optical transmission, and a suitable work function. Combined with its smooth surface and preferable hydrophobicity, α‐In₂Se₃ film can efficiently facilitate hole transporting in the polymer solar cells (PSCs). Due to the aforesaid advantages, a 2D α‐In₂Se₃ nanosheet is used as a hole transport layer (HTL) in conventional PSCs for the first time, and a relatively high power conversion efficiency (PCE) of 9.58% is achieved with the structure of ITO/α‐In₂Se₃/PBDB‐T:ITIC/Ca/Al, which is comparable with poly(3,4‐ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)‐based devices (9.50%). Interestingly, it is demonstrated that the α‐In₂Se₃ film possesses excellent thermal stability in the range from room temperature to 280 °C, and a PCE of 9.35% is achieved without annealing treatment of α‐In₂Se₃ film, which exhibits a great potential of α‐In₂Se₃ for an annealing‐free approach. Furthermore, the incorporation of α‐In₂Se₃ HTL also remarkably enhances the long‐term stability of PSCs compared with PEDOT:PSS‐based devices. So, the results show that 2D α‐In₂Se₃ is a promising candidate to be an efficient and stable hole‐extraction layer.

The formation of an inversion layer within n‐Si near the interface with poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) based conductive thin films is evidenced. High power conversion efficiency in solar cells is correlated with a large contact‐induced band bending in Si, high polymer conductivity, and proper Si interfacial passivation.

Abstract

Heterojunctions formed by ultrathin conductive polymer [poly(3,4‐ethylenedioxythiophene): poly(styrenesulfonate)—PEDOT:PSS] films and n‐type crystalline silicon are investigated by photoelectron spectroscopy. Large shifts of Si 2p core levels upon PEDOT:PSS deposition provide evidence that a dopant‐free p–n junction, i.e., an inversion layer, is formed within Si. Among the investigated PEDOT:PSS formulations, the largest induced band bending within Si (0.71 eV) is found for PH1000 (high PEDOT content) combined with a wetting agent and the solvent additive dimethyl sulfoxide (DMSO). Without DMSO, the induced band bending is reduced, as is also the case with a PEDOT:PSS formulation with higher PSS content. The interfacial energy level alignment correlates well with the characteristics of PEDOT:PSS/n‐Si solar cells, where high polymer conductivity and sufficient Si‐passivation are also required to achieve high power conversion efficiency.

A new synthesis of large PbS quantum dots (QDs) via cation‐exchange from ZnS nanorods (NRs) is developed, which produces PbS QDs with extremely high monodispersity. Infrared solar cells based on these PbS QDs with different bandgaps show excellent performance. The best device exhibits an efficiency of 10.0% under AM 1.5 solar illumination, perovskite‐filtered efficiency of 4.2%, and silicon‐filtered efficiency of 1.1%.

Abstract

Infrared solar cells that utilize low‐bandgap colloidal quantum dots (QDs) are promising devices to enhance the utilization of solar energy by expanding the harvested photons of common photovoltaics into the infrared region. However, the present synthesis of PbS QDs cannot produce highly efficient infrared solar cells. Here, a general synthesis is developed for low‐bandgap PbS QDs (0.65–1 eV) via cation exchange from ZnS nanorods (NRs). First, ZnS NRs are converted to superlattices with segregated PbS domains within each rod. Then, sulfur precursors are released via the dissolution of the ZnS NRs during the cation exchange, which promotes size focusing of PbS QDs. PbS QDs synthesized through this new method have the advantages of high monodispersity, ease‐of‐size control, in situ passivation of chloride, high stability, and a “clean” surface. Infrared solar cells based on these PbS QDs with different bandgaps are fabricated, using conventional ligand exchange and device structure. All of the devices produced in this manner show excellent performance, showcasing the high quality of the PbS QDs. The highest performance of infrared solar cells is achieved using ≈0.95 eV PbS QDs, exhibiting an efficiency of 10.0% under AM 1.5 solar illumination, a perovskite‐filtered efficiency of 4.2%, and a silicon‐filtered efficiency of 1.1%.

An efficient control of the film quality and thickness distribution of alternating cations in the interlayer space of 2D perovskite (GA)(MA) _n Pb _n I_{3 n +1} (〈n〉 = 3) quantum wells via incorporation of methylammonium chloride as an additive is demonstrated. The optimized device leads to more efficient charge transport and suppressed nonradiative charge recombination. Consequently, the optimized perovskite solar cell delivers an efficiency of 18.48%.

Abstract

2D perovskites stabilized by alternating cations in the interlayer space (ACI) represent a very new entry as highly efficient semiconductors for solar cells approaching 15% power conversion efficiency (PCE). However, further improvements will require understanding of the nature of the films, e.g., the thickness distribution and charge‐transfer characteristics of ACI quantum wells (QWs), which are currently unknown. Here, efficient control of the film quality of ACI 2D perovskite (GA)(MA) _n Pb _n I_{3 n +1} (〈n〉 = 3) QWs via incorporation of methylammonium chloride as an additive is demonstrated. The morphological and optoelectronic characterizations unambiguously demonstrate that the additive enables a larger grain size, a smoother surface, and a gradient distribution of QW thickness, which lead to enhanced photocurrent transport/extraction through efficient charge transfer between low‐n and high‐n QWs and suppressed nonradiative charge recombination. Therefore, the additive‐treated ACI perovskite film delivers a champion PCE of 18.48%, far higher than the pristine one (15.79%) due to significant improvements in open‐circuit voltage and fill factor. This PCE also stands as the highest value for all reported 2D perovskite solar cells based on the ACI, Ruddlesden–Popper, and Dion–Jacobson families. These findings establish the fundamental guidelines for the compositional control of 2D perovskites for efficient photovoltaics.

An ideal materials combination based on the electron donor BSFTR and acceptor Y6 is selected to construct small‐molecule solar cells (SMSCs). By morphology optimization, an extraordinary power conversion efficiency of 13.69% with a remarkably low energy loss of 0.48 eV is achieved, which is beneficial from the matched photoelectric properties, the favorable blend morphology, and is the best binary SMSC performance reported so far.

Abstract

Compared with the quick development of polymer solar cells, achieving high‐efficiency small‐molecule solar cells (SMSCs) remains highly challenging, as they are limited by the lack of matched materials and morphology control to a great extent. Herein, two small molecules, BSFTR and Y6, which possess broad as well as matched absorption and energy levels, are applied in SMSCs. Morphology optimization with sequential solvent vapor and thermal annealing makes their blend films show proper crystallinity, balanced and high mobilities, and favorable phase separation, which is conducive for exciton dissociation, charge transport, and extraction. These contribute to a remarkable power conversion efficiency up to 13.69% with an open‐circuit voltage of 0.85 V, a high short‐circuit current of 23.16 mA cm⁻² and a fill factor of 69.66%, which is the highest value among binary SMSCs ever reported. This result indicates that a combination of materials with matched photoelectric properties and subtle morphology control is the inevitable route to high‐performance SMSCs.

Thanks to the strong electron‐donating capability of carbon–oxygen‐bridged (CO‐bridged) ladder‐type building blocks, CO‐bridged nonfullerene acceptors (NFAs) present low bandgaps and strong light‐harvesting capability, delivering high short‐circuit current density (>28 mA cm⁻²) and high power conversion efficiency (>14% for single‐junction and >17% for tandem) in organic solar cells.

Abstract

Recently, acceptor–donor–acceptor (A–D–A) small molecules have emerged as promising nonfullerene acceptors (NFAs) for organic solar cells and have attracted great attention. The carbon‐bridged (C‐bridged) ladder‐type D unit plays a crucial role in developing high‐performance A–D–A NFAs. However, the medium electron‐donating capability of C‐bridged units is unfavorable for making NFAs with strong light‐harvesting capability. In this regard, carbon–oxygen‐bridged (CO‐bridged) ladder‐type units present advantages in developing strong light‐absorbing NFAs. Here, recent progress in the newly emerging CO‐bridged NFAs is highlighted. The synthetic methods for the polycyclic CO‐bridged building blocks are introduced. The photovoltaic performance for CO‐bridged NFAs is summarized and discussed. Perspectives on developing high‐performance CO‐bridged‐NFA‐based solar cells are made.