Naphthalene Diimide-Based n-Type Polymers: Efficient Rear Interlayers for High-Performance Silicon-Organic Heterojunction Solar Cells

Molecular Structures of Poly(methyl methacrylate) at Different Buried Interfaces Revealed by Sum Frequency Generation Vibrational Spectroscopy

Silica or calcium fluoride (CaF2) substrate-supported poly(methyl methacrylate) (PMMA) thin films as insulating layers are commonly used in photoelectric/photovoltaic devices to improve the efficiency or stability of these devices. However, a comparative investigation of molecular structures at buried PMMA/silica and PMMA/CaF2 interfaces under thermal stimuli remains unexplored. In this study, we qualitatively and quantitatively revealed different molecular orderings and orientations of PMMA at two interfaces before and after annealing using sum frequency generation (SFG) vibrational spectroscopy. SFG vibrations were carefully assigned by using various deuterated PMMAs. SFG results indicated that, at the buried PMMA/silica interface, the side OCH3 groups were prone to lie down before annealing and tended to stand up after annealing. In contrast, the case was the opposite at the buried PMMA/CaF2 interface. The relative hydrophobicity/hydrophilicity of the two substrates and the developed hydrogen bonds upon annealing at the buried PMMA/silica interface, which is absent at the CaF2 surface, are believed to be the driving forces for different interfacial molecular structures. This study benefits the molecular-level understanding of the interfacial local structural relaxation of polymers at buried interfaces and the rational design of photoelectric/photovoltaic devices from the molecular level.

Non-conventional bulk heterojunction nanoparticle photocatalysts for sacrificial hydrogen evolution from water

Photocatalyst systems combining donor polymers with acceptor molecules have shown the highest evolution rates for sacrificial hydrogen production from water for organic systems to date. Here, new donor molecules have been designed and synthesised taking inspiration from the structure-performance relationships which have been established in the development of non-fullerene acceptors. While a conventional bulk heterojunction (BHJ) pairing consists of a donor polymer and acceptor small molecule, here we have successfully reversed this approach by using new p-type small molecules in combination with a n-type conjugated polymer to produce non-conventional BHJ (ncBHJ) nanoparticles. We have applied these ncBHJs as photocatalysts in the sacrificial hydrogen evolution from water, and the best performing heterojunction displayed high activity for sacrificial hydrogen production from water with a hydrogen evolution rate of 22 321 μmol h-1 g-1 which compares well with the state-of-the-art for conventional BHJ photocatalyst systems.

Improving the Performance of Si/PEDOT:PSS Hybrid Solar Cells with More Economical and Environmentally Friendly Alcohol Ether Solvents

The photoelectric characteristics of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) films significantly affect the power conversion efficiency and stability of Si/PEDOT:PSS hybrid solar cells. In this paper, we investigated PEDOT:PSS modification with alcohol ether solvents (dipropylene glycol methyl ether (DPM) and propylene glycol phenyl ether (PPH)). The reduction of PSS content and the transformation of the PEDOT chain from benzene to a quinone structure in PEDOT:PSS induced by doping with DPM or PPH are the reasons for the improved conductivity of PEDOT:PSS films. DPM and PPH doping improves the quality of silicon with the PEDOT:PSS heterojunction and silicon surface passivation, thereby reducing the surface recombination of charge carriers, which improves the photovoltaic performance of Si/PEDOT:PSS solar cells. Comparing the power conversion performance (PCE) and air stability of Si/PEDOT:PSS solar cells with DPM (13.24%), DPH (13.51%), ethylene glycol (EG, 13.07%), and dimethyl sulfoxide (DMSO, 12.62%), it is suggested that doping with DPM and DPH can replace DMSO and EG to enhance the performance of Si/PEDOT:PSS solar cells. The EG and DMSO solvents not only have a certain toxicity to the human body but also are not environmentally friendly. In comparison to DMSO and EG, DPM and DPH are more economical and environmentally friendly, helping to reduce the manufacturing cost of Si/PEDOT:PSS solar cells and making them more conducive to their commercial applications.

Progress and Future Prospects of Wide-Bandgap Metal-Compound-Based Passivating Contacts for Silicon Solar Cells

Advanced doped-silicon-layer-based passivating contacts have boosted the power conversion efficiency (PCE) of single-junction crystalline silicon (c-Si) solar cells to over 26%. However, the inevitable parasitic light absorption of the doped silicon layers impedes further PCE improvement. To this end, alternative passivating contacts based on wide-bandgap metal compounds (so-called dopant-free passivating contacts (DFPCs)) have attracted great attention, thanks to their potential merits in terms of parasitic absorption loss, ease-of-deposition, and cost. Intensive research activity has surrounded this topic with significant progress made in recent years. Various electron-selective and hole-selective contacts based on metal compounds have been successfully developed, and a champion PCE of 23.5% has been achieved for a c-Si solar cell with a MoO_x -based hole-selective contact. In this work, the fundamentals and development status of DFPCs are reviewed and the challenges and potential solutions for enhancing the carrier selectivity of DFPCs are discussed. Based on comprehensive and in-depth analysis and simulations, the improvement strategies and future prospects for DFPCs design and device implementation are pointed out. By tuning the carrier concentration of the metal compound and the work function of the capping transparent electrode, high PCEs over 26% can be achieved for c-Si solar cells with DFPCs.

Recent Advances of Furan and Its Derivatives Based Semiconductor Materials for Organic Photovoltaics

The state-of-the-art bulk-heterojunction (BHJ)-type organic solar cells (OSCs) have exhibited power conversion efficiencies (PCEs) of exceeding 18%. Thereinto, thiophene and its fused-ring derivatives play significant roles in facilitating the development of OSCs due to their excellent semiconducting natures. Furan as thiophene analogue, is a ubiquitous motif in naturally occurring organic compounds. Driven by the advantages of furan, such as less steric hindrance, good solubility, excellent stacking, strong rigidity and fluorescence, biomass derived fractions, more and more research groups focus on the furan-based materials for using in OSCs in the past decade. To systematically understand the developments of furan-based photovoltaic materials, the relationships between the molecular structures, optoelectronic properties, and photovoltaic performances for the furan-based semiconductor materials including single furan, benzofuran, benzodifuran (BDF) (containing thienobenzofuran (TBF)), naphthodifurans (NDF), and polycyclic furan are summarized. Finally, the empirical regularities and perspectives of the development of this kind of new organic semiconductor materials are extracted.

Tuning of the optoelectronic properties of peptide-appended core-substituted naphthalenediimides: the role of self-assembly of two positional isomers

This study demonstrates how the self-assembly pattern of two different and isomeric peptide-appended core-substituted naphthalenediimides (NDIs) affects the modulation of their optoelectronic properties. Two isomeric peptide-attached NDIs were synthesized, purified and characterized. Interchanging the position of attachment of the peptide units and the alkyl chains in the NDI has altered the respective self-assembling patterns of these isomeric molecules in the aggregated states. The isomer having a peptide moiety in the core position and the alkyl chain in the imide position (compound N1) forms face to face stacking or 'H' aggregates in aliphatic solvents including n-hexane, and n-decane, whereas compound N2, in which the peptide moiety is at the imide position and the alkyl chain is attached at the core position of NDI exhibits edge to edge stacking or J aggregates under the same conditions as it is evident from their UV-vis studies. The H aggregated species (obtained from N1) show inter-connected nanofibers, whereas the J aggregated species (obtained from N2) exhibit the morphology of helical nanoribbons. FT-IR and X-ray diffraction studies are in favor of the same aggregation behavior. The individual packing patterns of these two peptide-based isomers have a direct impact on their respective electrical conductivity. Interestingly, the H aggregated species shows 100 times greater current conductivity than that of the J aggregate. Moreover, it is only the H aggregated species that exhibits a photocurrent, and no such photocurrent response is observed with the J aggregates. Computational studies also support that different types of aggregation patterns are formed by these two isomeric molecules in the same solvent system. This unique example of tuning of optoelectronic behavior holds future promise for the development of new peptide-conjugated π-functional materials.

Narrow-Bandgap Single-Component Polymer Solar Cells with Approaching 9% Efficiency

Two narrow-bandgap block conjugated polymers with a (D1-A1)-(D2-A2) backbone architecture, namely PBDB-T-b-PIDIC2T and PBDB-T-b-PTY6, are designed and synthesized for single-component organic solar cells (SCOSCs). Both polymers contain same donor polymer, PBDB-T, but different polymerized nonfullerene molecule acceptors. Compared to all previously reported materials for SCOSCs, PBDB-T-b-PIDIC2T and PBDB-T-b-PTY6 exhibit narrower bandgap for better light harvesting. When incorporated into SCOSCs, the short-circuit current density (J_sc ) is significantly improved to over 15 mA cm^-2 , together with a record-high power conversion efficiency (PCE) of 8.64%. Moreover, these block copolymers exhibit low energy loss due to high charge transfer (CT) states (E_ct ) plus small non-radiative loss (0.26 eV), and improved stability under both ambient condition and continuous 80 °C thermal stresses for over 1000 h. Determination of the charge carrier dynamics and film morphology in these SCOSCs reveals increased carrier recombination, relative to binary bulk-heterojunction devices, which is mainly due to reduced ordering of both donor and acceptor fragments. The close structural relationship between block polymers and their binary counterparts also provides an excellent framework to explore further molecular features that impact the photovoltaic performance and boost the state-of-the-art efficiency of SCOSCs.

Interfacial Engineering of Cu2O Passivating Contact for Efficient Crystalline Silicon Solar Cells with an Al2O3 Passivation Layer

Passivating contacts that simultaneously promote carrier selectivity and suppress surface recombination are considered as a promising trend in the crystalline silicon (c-Si) photovoltaic industry. In this work, efficient p-type c-Si (p-Si) solar cells with cuprous oxide (Cu₂O) hole-selective contacts are demonstrated. The direct p-Si/Cu₂O contact leads to a substoichiometric SiO_x interlayer and diffusion of Cu into the silicon substrate, which would generate a deep-level impurity behaving as carrier recombination centers. An Al₂O₃ layer is subsequently employed at the p-Si/Cu₂O interface, which not only serves as a passivating and tunneling layer but also suppresses the redox reaction and Cu diffusion at the Si/Cu₂O interface. In conjunction with the high work function of Au and the superior optical property of Ag, a power conversion efficiency up to 19.71% is achieved with a p-Si/Al₂O₃/Cu₂O/Au/Ag rear contact. This work provides a strategy for reducing interfacial defects and lowering energy barrier height in passivating contact solar cells.

Recent structural evolution of lactam- and imide-functionalized polymers applied in organic field-effect transistors and organic solar cells

Organic semiconductor materials, especially donor-acceptor (D-A) polymers, have been increasingly applied in organic optoelectronic devices, such as organic field-effect transistors (OFETs) and organic solar cells (OSCs). Plenty of high-performance OFETs and OSCs have been achieved based on varieties of structurally modified D-A polymers. As the basic building block of D-A polymers, acceptor moieties have drawn much attention. Among the numerous types, lactam- and imide-functionalized electron-deficient building blocks have been widely investigated. In this review, the structural evolution of lactam- or imide-containing acceptors (for instance, diketopyrrolopyrrole, isoindigo, naphthalene diimide, and perylene diimide) is covered and their representative polymers applied in OFETs and OSCs are also discussed, with a focus on the effect of varied structurally modified acceptor moieties on the physicochemical and photoelectrical properties of polymers. Additionally, this review discusses the current issues that need to be settled down and the further development of new types of acceptors. It is hoped that this review could help design new electron-deficient building blocks, find a more valid method to modify already reported acceptor units, and achieve high-performance semiconductor materials eventually.

Performance-Enhancing Approaches for PEDOT:PSS-Si Hybrid Solar Cells

The emerging energy crisis has focused significant worldwide attention on solar cells. Although crystalline silicon solar cells are currently widely used, their high cost limits the development of solar power generation. Consequently, hybrid solar cells are becoming increasingly important, especially organic-Si hybrid solar cells (HSCs). Organic-Si HSCs combine a mature technology and high efficiency with the low-temperature manufacturing process and tunable optoelectronic properties of organic solar cells. The organic material can be P3HT, carbon nanotubes, graphene, and PEDOT:PSS. Here we review the performance of PEDOT:PSS/Si HSCs and methods for improving their efficiency, such as PEDOT:PSS modification, optimization of the trapping effect, passivation of the silicon surface, addition of an interface layer, improvement of a back contact, and optimization of the metal top electrode. This Review should help fill the gap in this area and provide perspectives for the future development of the PEDOT:PSS/Si HSCs.

Prominent Heat Dissipation in Perovskite Light-Emitting Diodes with Reduced Efficiency Droop for Silicon-Based Display

Solution-processed perovskite light-emitting diodes (LEDs) possess outstanding optoelectronic properties for potential solid-state display applications. However, poor device stability results in significant efficiency droop partly being ascribed to Joule heating when LEDs are operated at high current densities. Herein, we used monocrystal silicon (c-Si) as the substrate and a charge injection layer to alleviate the thermal affection in perovskite LED (PeLED). By incorporating silicon oxide (SiO_x) and poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4'-(N-(4-butylphenyl) (TFB) layers to tune the charge injection balance in a c-Si-based device, a PeLED achieves an external quantum efficiency of 2.12% with a current efficiency of 6.06 cd A^-1. Benefiting from excellent heat dissipation of c-Si, the PeLEDs display reduced efficiency droop and extended operational lifetime. Furthermore, both electroluminescent (EL) dynamic information and static pattern displays of a c-Si-based PeLED have been successfully demonstrated. These results reveal the feasibility of potential practical c-Si-based PeLEDs with reduced efficiency droop for EL display applications.

Air and temperature sensitivity of n-type polymer materials to meet and exceed the standard of N2200

N-type organic semiconductors are notoriously unstable in air, requiring the design of new materials that focuses on lowering their LUMO energy levels and enhancing their air stability in organic electronic devices such as organic thin-film transistors (OTFTs). Since the discovery of the notably air stable and high electron mobility polymer poly{[N,N'-bis (2-octyldodecyl)- naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,29-bisthiophene)} (N2200), it has become a popular n-type semiconductor, with numerous materials being designed to mimic its structure. Although N2200 itself is well-studied, many of these comparable materials have not been sufficiently characterized to compare their air stability to N2200. To further the development of air stable and high mobility n-type organic semiconductors, N2200 was studied in organic thin film transistors alongside three N2200-based analogues as well as a recently developed polymer based on a (3E,7E)-3,7-bis(2-oxoindolin-3-ylidene)benzo[1,2-b:4,5-b']difuran-2,6(3 H,7 H)-dione (IBDF) core. This IBDF polymer has demonstrated promising field-effect mobility and air stability in drop-cast OTFTs. While N2200 outperformed its analogues, the IBDF-based polymer displayed superior air and temperature stability compared to N2200. Overall, polymers with more heteroatoms displayed greater air stability. These findings will support the development of new air-stable materials, and further demonstrate the persistent need for the development of novel n-type semiconductors.

Vacuum-Free, Room-Temperature Organic Passivation of Silicon: Toward Very Low Recombination of Micro-/Nanotextured Surface Structures

Crystalline silicon (c-Si) solar cells remain dominant in the photovoltaic (PV) market because of their cost-effective advantages. However, the requirement for expensive vacuum equipment and the power-hungry thermal budget for surface passivation technology, which is one of the key enablers of the high performance of c-Si solar cells, impede further reductions of costs. Thus, the omission of the vacuum and high-temperature process without compromising the passivation effect is highly desirable due to cost concerns. Here, we demonstrate a vacuum-free, room-temperature organic Nafion thin-film passivation scheme with an effective minority carrier lifetime (τ_eff) exceeding 9 ms on an n-type c-Si wafer with a resistivity of 1-5 Ω·cm, corresponding to an implied open circuit voltage (i V_oc) of 724 mV and upper-limit surface recombination velocity (SRV) of 1.46 cm/s, which is a level that is in line with the hydrogenated amorphous Si film-passivation scheme used in the current PV industry. We find that the Nafion film passivation of Si can be enhanced in an O₂ atmosphere and that the Nafion/c-Si interface oxidation should be responsible for the passivation mechanism. This highly effective passivation is also achieved on various micro-/nanotextured Si surface structures from actual production, including a pyramidal surface and nanopore-pyramid hybrid structure with nanopores on the inclined plane of the pyramid. We develop an organic Nafion-passivated n-type back-junction Si solar cell to examine application in a real device. The open circuit voltage ( V_oc) of the solar cell with the Nafion passivation layer achieves a clear improvement (30.8 mV) over those without the passivation layer, resulting in an increase (1.5%) in the power conversion efficiency. These results suggest the potential use of these organic electronics with current Si microelectronics and a new strategy for the development of vacuum-free, low-temperature Si-based PVs at low cost.

Lithium and Silver Co-Doped Nickel Oxide Hole-Transporting Layer Boosting the Efficiency and Stability of Inverted Planar Perovskite Solar Cells

In this work, a lithium and silver co-doping strategy has been successfully implied to prepare NiO _x films for high performance inverted planar perovskite solar cells (PSCs). Compared to the pristine and single-doped NiO _x, the Li and Ag co-doping approach exhibits the synergistic effect and can endow NiO _x films with higher electrical conductivity, higher hole mobility and better interface energy band alignment with perovskite active layers. Moreover, the perovskite film with enhanced crystallinity can be obtained induced by the Li,Ag:NiO _x film. The PSC with Li,Ag:NiO _x HTL shows a high power conversion efficiency (PCE) up to 19.24% and less hysteresis effect, which outperforms the devices with the pristine NiO _x or single-doped NiO _x HTLs. Meanwhile, the Li,Ag:NiO _x device can retain 95% of its initial PCE after storage at the relative humidity of 30 ± 2% in 30 days without encapsulation. Our work demonstrates that lithium and silver co-doping is a promising route for realizing efficient p-type NiO _x HTL, which provides a simple way to boost the efficient and stable of inverted planar PSCs.

Highly Conducting Hybrid Silver-Nanowire-Embedded Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) for High-Efficiency Planar Silicon/Organic Heterojunction Solar Cells

Embedding nanowires, such as silver nanowires (AgNWs), in a transparent conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) to enhance its conductivity is technologically important for improving the performances of devices comprising transparent conductive layers. Addition of nanowires in the highly conducting form of cosolvent (ethylene glycol) or mixed-cosolvent (ethylene glycol and methanol) modified PEDOT:PSS could change the nanowire structure and significantly alter the conductivity. Here, we report a simple method to embed AgNWs in PEDOT:PSS efficiently to improve its conductivity. By incorporating nanowires in the mixed cosolvent matrix prior to addition into PEDOT:PSS, this method preserves the structure of the nanowires while enabling conductivity enhancement. In contrast, the addition of AgNWs into cosolvent-premodified PEDOT:PSS leads to breaking of nanowires and conductivity impediment. The hybrid films with efficiently embedded AgNWs and mixed-cosolvent-modified PEDOT:PSS show a sheet resistance of 104 Ω/□, which is among the lowest ever reported for the as-deposited films, with conductivity enhancement of 33% relative to that of mixed-cosolvent-modified PEDOT:PSS. The resulting planar heterojunction solar cell (HSC) based on AgNW-embedded PEDOT:PSS exhibits a power conversion efficiency of greater than 15%. This demonstrates the importance of reducing sheet resistance by integrating nanowires into the PEDOT:PSS matrix as effective charge-transfer conduits interconnecting the highly conducting quinoid chains. The present approach to efficiently embed AgNWs in PEDOT:PSS could be readily extended to other nanowires or nanoparticles for improving the performance of PEDOT:PSS for applications in not just HSCs but indeed other electronic devices that require both transparent and highly conductive layers.