Improvement of open-circuit voltage and photovoltaic properties of 2D-conjugated polymers by alkylthio substitution

Improvement of red light harvesting ability and open circuit voltage of Cu:NiOx based p-i-n planar perovskite solar cells boosted by cysteine enhanced interface contact

As researchers continue to develop new organic materials for solar cells, benzo[1,2-b:4,5-b']dithiophene (BDT)-based polymers have come to the fore. To improve the photovoltaic properties of BDT-based polymers, researchers have developed and applied various strategies leading to the successful molecular design of highly efficient photovoltaic polymers. Novel polymer materials composed of two-dimensional conjugated BDT (2D-conjugated BDT) have boosted the power conversion efficiency of polymer solar cells (PSCs) to levels that exceed 9%. In this Account, we summarize recent progress related to the design and synthesis of 2D-conjugated BDT-based polymers and discuss their applications in highly efficient photovoltaic devices. We introduce the basic considerations for the construction of 2D-conjugated BDT-based polymers and systematic molecular design guidelines. For example, simply modifying an alkoxyl-substituted BDT to form an alkylthienyl-substituted BDT can improve the polymer hole mobilities substantially with little effect on their molecular energy level. Secondly, the addition of a variety of chemical moieties to the polymer can produce a 2D-conjugated BDT unit with more functions. For example, the introduction of a conjugated side chain with electron deficient groups (such as para-alkyl-phenyl, meta-alkoxyl-phenyl, and 2-alkyl-3-fluoro-thienyl) allowed us to modulate the molecular energy levels of 2D-conjugated BDT-based polymers. Through the rational design of BDT analogues such as dithienobenzodithiophene (DTBDT) or the insertion of larger π bridges, we can tune the backbone conformations of these polymers and modulate their photovoltaic properties. We also discuss the influence of 2D-conjugated BDT on polymer morphology and the blends of these polymers with phenyl-C61 (or C71)-butyric acid methyl ester (PCBM). Finally, we summarize the various applications of the 2D-conjugated BDT-based polymers in highly efficient PSC devices. Overall, this Account correlates the molecular structures of the 2D-conjugated BDT-based polymers with their photovoltaic properties. As a result, this Account can guide the molecular design of organic photovoltaic materials and the development of organic materials for other types of optoelectronic devices.

Synthesis and photovoltaic properties of 2D-conjugated polymers with alkylsilyl-substituted thieno[3,2-b]thiophene conjugated side chains

Photovoltaic conjugated polymer materials are composed of π-conjugated backbone and flexible side chains. Their potential advantages of cost-effective production, fabrication on flexible and light weight substrates by roll-to-roll solution processing, capability to be fabricated into flexible devices and reduced environment impact have made them receive consid- erable attention in both academia and industry. In recent years, most of the interest has been directed into the optimization of main chain, while research on side-chain engineering of polymer is relatively few. Initially, side chains have been primarily utilized as solubilizing groups in organic photovoltaic conjugated polymers. However, roles which side chains play are far beyond. Accordingly, it has been found that side chains of organic conjugated polymer have a different impact on polymer absorption, emission, energy level, carrier mobility, nanoscale morphology and interfacial contact. Certainly enough, these findings have profound guiding significance on molecular structure design and optimization, blend microstructure, interfacial morphology, preparation methods and processing technology of device and so on. Moreover, based on ameliorative prepara- tion and processing technology of device, side-chain engineering is deserved to be deeply researched due to their remarkably improved ability to regulate and control blend microstructure, interfacial morphology so as to obtain ideal photovoltaic proper- ties and device performance, the ultimate goal. Thus, as for resent progress in application, in this perspective article, we will present a review on side chain engineering and assess different side chain on the basis of molecular structure design and opti- mization, as well as summarize relevant issues to be solved and related work in our laboratory. Keywords side chain engineering; photovoltaic properties; polymer solar cells; nanoscale morphology

Several research groups are currently working on n‐ZnO/p‐Si heterojunction solar cell, and recently, Pietruszka et al [Sol. Energ. Mat. Sol. Cells 147 (2016) 164‐170] has reported the highest efficiency of 7.1% for this structure. The main challenge is to enhance the open circuit voltage up to theoretically predicted value of >0.6 V. This paper reports >20% improvement in open circuit voltage of n‐ZnO/p‐Si solar cell by depositing amorphous‐ZnO at the interface at room temperature that possibly improves the passivation and/or avoids oxide formation at the interface during ZnO deposition. Two other materials, aluminum nitride and amorphous‐Si, have also been used as buffer layers to evaluate their effect on suppression of interface states. Furthermore, additional advantage of ZnO as an antireflector has been experimentally verified for different thicknesses of ZnO film.

Tetrathiophene-based fully non-fused ring electron acceptors via asymmetric side chain engineering

Recent Advances in Molecular Design of Organic Thermoelectric Materials

ABSTRACTA series of donor‐acceptor conjugated polymers incorporating benzodithiophene (BDT) as donor unit and phenanthrenequnioxaline as acceptor unit with different side chains have been designed and synthesized. For polymer P1 featuring the BDT unit and alkoxy chains substituted phenanthrenequnioxaline unit in the backbone, serious steric hindrance resulted in quite low molecular weight. The implementation of thiophene ring spacer in polymer P2 greatly suppressed the interannular twisting to extend the effective conjugation length and consequently gave rise to improved absorption property and device performance. In addition, utilizing the alkylthienyl side chains to replace the alkyl side chains at BDT unit in polymer P3 further enhanced the photovoltaic performance due to the increased conjugation length. For polymer P4, translating the alkoxy side chains at the phenanthrenequnioxaline ring into the alkyl side chains at thiophene linker group enhanced molecular planarity and strengthened π−π stacking. Consequently improved absorption property and increased hole mobility were achieved for polymer P4. Our results indicated that side chain engineering not only can influence the solubility of polymer but also can determine the polymer backbone planarity and hence the photovoltaic properties. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1915–1926

Back surface reflectors have the potential to improve thermophotovoltaic (TPV) device performance though the recirculation of infrared photons. The ''hybrid'' back-surface reflector (BSR) TPV cell approach allows one to construct BSRs for TPV devices using conventional, high efficiency, GaInAsSb-based TPV material. The design, fabrication, and measurements of hybrid BSR-TPV cells are described. The BSR was shown to provide a 4 mV improvement in open-circuit voltage under a constant shortcircuit current, which is comparable to the 5 mV improvement theoretically predicted. Larger improvements in open-circuit voltage are expected in the future with materials improvements.

ConspectusSolution-processable conjugated polymers are typically composed of two distinct structural components: rigid conjugated backbones and flexible side chains, each with unique roles and properties. The conjugated backbone forms the core framework of the polymer and is directly responsible for its optoelectronic properties, such as light absorption, emission, and charge transport. Meanwhile, the conjugated backbone can undergo chemical doping, where molecular dopants introduce charge carriers to modulate the carrier density and electrical conductivity. Therefore, the conjugated backbone is the critical determinant of the resulting optoelectronic performance. However, on the other hand, the flexible side chains, originally introduced to improve solution processability, were long considered chemically inert to the doping reaction. Recent advances have shown that the role of side chains is more than just improving solubility, demonstrating the significant impact of side chains on the packing of the conjugated backbone, film morphology, and electronic properties of conjugated polymers. Side chain engineering has become an essential design strategy for creating high-performance conjugated polymers in various applications.In this Account, we aim to emphasize the importance of side chain engineering toward controllable chemical doping of conjugated polymers, where side chain engineering allows us to tune the molecular packing, doping efficiency, and film morphology, thereby enhancing charge transport and optoelectronic performance. Specifically, the length, branching structures, and functional groups of the side chains can be systematically varied to control the solubility, miscibility, and interactions of conjugated polymers with dopants. For example, longer or branched side chains can improve solubility but may disrupt the π-π stacking between the conjugated backbones, thereby reducing the charge transport efficiency of the polymer. Shorter or linear side chains may enhance backbone packing and electronic coupling, though at the expense of reduced solubility. The impact of side chains on the doping process is particularly noteworthy. Although side chains are chemically inert to doping reactions, their design influences all three critical steps of the doping process: mixing, ionization, and carrierization. Side chains affect the spatial distribution of dopants during mixing, modulate the local environment to facilitate charge transfer during ionization, and influence the dissociation of ion pairs into free charge carriers during carrier generation. Functional side chains with polar groups, for example, can enhance dopant-polymer compatibility, while those with functional groups can modulate the dielectric environment to weaken ion pairing and promote free carrier generation. The interplay between side chains and the conjugated backbone is critical to achieving optimal optoelectronic performance in applications such as organic photovoltaics, field-effect transistors, and thermoelectrics. Rational side chain engineering provides a powerful tool to address these challenges in doping, morphology control, and charge transport, bringing more opportunities to design advanced conjugated polymers and chemical dopants tailored to specific applications.

Alkyl chains engineering plays an important role in photovoltaic materials for organic solar cells. Herein, three A‐DA'D‐A (acceptor–donor–acceptor'–donor–acceptor) type acceptors named Y6, Y6‐C4, and Y6‐C5 with different branching position on the pyrrole motif are discussed and the relationship between molecular aggregation, crystalline, and device performance are systematically investigated. The distance between the branching position and the main backbone affects their optical absorption and energy levels. Y6‐C4 and Y6‐C5 with the branching position at the fourth and fifth carbon of the alkyl chain show blue‐shifted absorption and increased electrochemical bandgaps, compared with Y6 with the branching position at the second carbon of the alkyl side chain. In addition, this distance influences the molecular aggregation and crystalline behavior of the donor/acceptor blends. Compared with Y6‐C4, Y6‐C5 possesses a stronger crystalline and aggregate ability in the blends with a lower non‐radiative energy loss, which results in a higher open circuit voltage (Voc) of 0.88 V. Finally, Y6‐C5‐based binary device achieved a high power conversion efficiency up to 16.73% with afill factor (FF) of 0.78. These results demonstrate that the side chain engineering is an effective strategy for tuning the molecular aggregation and crystalline to improve photovoltaic performance of the A‐DA'D‐A type acceptors.

Hammer throw-liked hybrid cyclic and alkyl chains: A new side chain engineering for over 18 % efficiency organic solar cells

In this manuscript, conventional and inverted organic solar cells based on P3HT:PC[60]BM have been explored to understand the effect of deep defect states on the open circuit voltage. The enhancement in the open circuit voltage in the inverted structure compared to the conventional structure has been comprehensively discussed in terms of density of defect states. To comply with the investigation, DC and AC measurements (impedance spectroscopy) at various temperatures have been performed extensively. Enhancement in open circuit voltage at low temperature is observed which is described by the shifting of hole and electron quasi-Fermi levels. The important observation from the defect density of states profile is that the center of Gaussian distribution is shifted to high energy as the temperature is increased which is an indication of the creation of shallow traps in polymers. In the inverted device, the disorder parameter (σ) is 33 meV, whereas in the conventional device it becomes 75 meV. This implies that the energetic disorder is reduced in an inverted device which helps in the improvement of open circuit voltage.

This paper identifies the contributions of p—a—SiC:H layers and i—a—Si:H layers to the open circuit voltage of p—i—n type a—Si:H solar cells deposited at a low temperature of 125 °C. We find that poor quality p—a—SiC:H films under regular conditions lead to a restriction of open circuit voltage although the band gap of the i-layer varies widely. A significant improvement in open circuit voltage has been obtained by using high quality p—a—SiC:H films optimized at the “low-power regime" under low silane flow rates and high hydrogen dilution conditions.

Donor-acceptor (D-A) copolymers doped with n-type dopants are widely sought after for their potential in organic thermoelectric devices. However, the existing structural disorder significantly hampers their charge transport and thermoelectric performance. In this Letter, we propose a mechanism to mitigate this disorder through side chain engineering. Utilizing molecular dynamics simulations, we demonstrate that strong Coulomb interactions between counterions and charged polymer backbones induce a transition in the stacking arrangement of the polymer backbones from a slipped to a vertical configuration. However, the presence of side chain steric hindrance impedes the formation of closely packed and ordered vertical stacking arrangements, resulting in greater distances between adjacent backbones and a higher level of structural disorder in the doped films. Therefore, we propose minimizing side chain steric hindrance to enhance the structural order in doped films. Our findings provide essential insights for advancing high-performance thermoelectric polymers.