Performance Of Organic Solar Cells Research Articles

Oligothiophene Additive‐Assisted Morphology Control and Recombination Suppression Enable High‐Performance Organic Solar Cells

AbstractTuning the morphology through processing additives represents one of the most promising strategies to boost the performance of organic solar cells (OSCs). However, it remains unclear how oligothiophene‐based solid additives influence the molecular packing and performance of OSCs. Here, two additives namely 2T and 4T, are introduced into state‐of‐the‐art PM6:Y6‐based OSCs to understand how they influence the film formation process, nanoscale morphology, and the photovoltaic performance. It is found that the 2T additive can improve the molecular packing of both donor polymer and non‐fullerene acceptor, resulting in lower Urbach energy and reduced energy loss. Furthermore, the blend film with 2T treatment displays enhanced domain purity and a more favorable distribution of the acceptor and donor materials in the vertical direction, which can enhance charge extraction efficiency while simultaneously suppressing charge recombination. Consequently, OSCs processed with 2T additive realize a promising efficiency of 18.1% for PM6:Y6‐based devices. Furthermore, the general applicability of the additive is demonstrated, and an impressive efficiency of 18.6% for PM6:L8‐BO‐based OSCs is achieved. These findings highlight that the uncomplicated oligothiophenes have excellent potential in fine‐adjustment of the active layer morphology, which is crucial for the future development of OSCs.

Central core substitution optimized molecular aggregation and photovoltaic performance in A-DA′D-A type acceptors

With the development of A-DA′D-A (D: electron-donating unit; A: electron-withdrawing unit) type acceptors, the performances of organic solar cells (OSCs) have reached a new stage. The central core of A-DA′D-A molecules exhibits a significant impact on molecular planarity and stacking. In this research, the conjugated central core of A-DA′D-A type molecules was extended to the two-dimensional (2D) direction, and the influence of alkyl chains on the 2D-central cores on molecular structure and device performances was explored. Compared with TPTC6 containing alkyl chains, TPTC0 without alkyl chains showed stronger intermolecular interactions and crystallinity, which realized advantageous phase separation in the blend film, leading to more effective charge separation and transport. Therefore, devices based on TPTC0 exhibited an excellent power conversion efficiency (PCE) of 17.70%. In contrast, the blend film exhibited a high energetic disorder due to the poor miscibility between PM6 and TPTC0, resulting in decreased VOC compared with TPTC6. In this work, the influence of alkyl chains on molecular stacking and crystallinity in 2D-expanding conjugated materials was discussed in detail, which would promote the development of related research.

Graphene and Graphene Derived Nanomaterials as Versatile Candidates for Organic Solar Cells and Smart Windows Applications – A Review

AbstractGraphene and related materials have been employed in several applications till date due to the unique set of properties. Research in the area of organic solar cells and smart windows are important to meet the increasing global energy demands and to realize the future goal of zero energy buildings. Here, we present a summary of the important research progress in application of graphene and related materials in organic solar cells and smart windows. This review starts by introducing the need of photovoltaic technology including organic solar cells and smart windows. The remaining sections are broadly divided into two sections – the first one briefly introduces organic cells and their different types, and the next section focuses on the different chromogenic technologies used in smart windows. After a brief overview of graphene, the application of graphene and related materials in organic cells and smart windows are presented. In the end, some of the major challenges and possible research directions are also outlined. Hopefully, this review will shed light on the recent trends and directions to achieve enhancement in performance of organic solar cells and smart windows with integration of graphene and provide overview for researchers about this novel field.

Synergistically boosting performances of organic solar cells from dithieno[3,2-b]benzo[1,2-b;4,5-b′]dithiophene-based copolymers via side chain engineering and radical polymer additives

A wise strategy to improve the performance of NFA-OSCs from DTBDT-based polymer donors via synergistic employment of side-chain engineering and radical conjugated polymer additives is presented.

Employing a Similar Acceptor Material as Third Component to Enhance the Performance of Organic Solar Cells

We synthesized two derivatives of Y6, namely Y-TNF and Y-TN. Compared to Y6, these two derivatives possess fluorinated and non-fluorinated extended terminal group, respectively. Y-TNF exhibits a red-shifted absorption compared...

Enhancing the performance of organic solar cells by using PDINN-doped PEDOT:PSS as the hole transport layer

PDINN, a perylene diimide derivative, enhances the electrical conductivity of the hole transport layer in PEDOT:PSS-based organic solar cells. This results in improved power conversion efficiencies compared to the control ones.

Imide-Free Electron-Deficient Electrolytes with Adjustable Cathode-Modifying Capabilities Achieved by Side Chains Engineering for Efficient Organic Solar Cells

Cathode interlayers (CILs) play crucial roles in boosting the performance of organic solar cells (OSCs). Herein, a class of novel electron-deficient electrolytes, namely BDOPV-1 and BDOPV-2, based on benzodifurandione-based oligo(p-phenylene...

Using Ag nanoparticles in the electron transport layer of perovskite solar cells to improve efficiency

The incorporation of metal nanoparticles within various types of photovoltaic technologies has been shown to increase the performance of organic solar cells, dye sensitized solar cells, and more recently perovskite solar cells. Using this type of nanostructured composite transport layer could help improve the efficiency and stability of perovskite solar cells whilst avoiding the use of dopants that can damage the perovskite layer. In this work, Ag nanoparticles are synthesized and used to form a SnO2:Ag nanoparticle composite transport layer for the first time. On its own, SnO2 is in one of the most efficient transport layers for perovskite solar cells. Upon inclusion of the Ag nanoparticles the recombination rate is increased (detrimental for device performance) as shown by impedance spectroscopy, and the charge carrier transfer and extraction is enhanced (beneficial for device performance) as shown by photoluminescence measurements. In order to balance these opposing factors, the nanoparticle concentration was optimized at an intermediate concentration with a corresponding power conversion efficiency increase from 13.4 ± 0.7 % for reference solar cells without nanoparticles to 14.3 ± 0.3 % for those with nanoparticles. These devices are one of the first examples of, and exhibit the highest reported efficiency for, perovskite solar cells fabricated completely in air with nanocomposite oxide layers. The protocol developed and reported here to improve the nanocomposite transport layer has general applicability in other fields, including LEDs, FETs, and electronic devices where transparency and conductivity are also required.

Chlorine-mediated strategy for organic photovoltaics.

Organic solar cells (OSCs), a nascent technology in the photovoltaic field, have attracted considerable research interest. Recently, the power conversion efficiency (PCE) of OSCs has significantly improved, thereby demonstrating substantial potential for commercialization. To achieve this, it is crucial to enhance the performance and stability of OSCs, necessitating the development of novel materials and devices. This feature article presents a review of chlorine-mediated photovoltaic materials in our group. By carefully controlling energy levels, molecular stacking and aggregation behavior, significantly improved performance was achieved. Furthermore, single-crystal analysis facilitated a profound comprehension of the influence of chlorine-mediated interactions on molecular stacking. This has enabled the design and synthesis of a series of high-performance non-fullerene acceptors (NFAs) with three-dimensional network stacking structures. Building upon these materials, we developed quasi-planar heterojunction (Q-PHJ) devices with a significant stability advantage. To sum up, the chlorine-mediated materials and the Q-PHJ devices provide valuable guidance and reference for the development of efficient and stable organic solar cells.

Enhancing the performance of non-fullerene organic solar cells through side-chain engineering of asymmetrical non-fused-ring electron acceptors

In organic solar cells (OSCs), the intricate synthesis and lower yield of ladder-type fused ring small molecule receptors (FR-SMAs) have prompted the quest for simpler, higher yielding non-fused ring (NFR)-SMAs.

Optical enhancement of perovskite solar cells by metallic nano-patterns

The integration of metallic nanoparticles (MNPs) with plasmonic effects is an alternate approach for managing photons and charge carriers, and is considered a promising method for advancing solar cell technologies. Plasmonic-enhanced solar energy harvesting involves three mechanisms: hot-electron injection, light trapping, and modulation of energy flow direction through dipole-dipole coupling. These phenomena have been observed to significantly enhance the performance of silicon, gallium arsenide, dye-sensitized, and organic solar cells. However, for emerging perovskite solar cells, the light trapping effect, specifically, the far-field scattering via MNPs, has been seldom reported. The anomalous phenomenon is primarily attributed to the size constraints imposed on MNPs by the thickness of the functional layers in cell devices. According to the theory of localized surface plasmon resonance (SPR), the characteristic size of the MNPs needs to be larger than 90 nm to achieve optimal photon scattering. Conversely, charge transport layers like NiO<sub>x</sub> and SnO<sub>2</sub> in perovskite solar cells are typically thin, ranging from a few to several tens of nanometers in thickness. Therefore, the community of perovskite solar cells still faces a great challenge in harvesting light through plasmonic scattering.<br>Compared to MNPs, the shape, size, periodicity, and other characteristic parameters of two-dimensional metal patterns within the horizontal plane are not limited by the thickness of the device's functional layer, thus enabling a more flexible regulation of the SPR response band, vibration intensity, and dissipation method of plasmonic energy. In this study, based on the finite-difference time-domain (FDTD) method and rigorous coupled-wave analysis (RCWA), we systematically investigated the SPR spectra of different metal patterns. The results demonstrate that by optimizing characteristic parameters such as pattern shape, thickness, and periodicity, a significant SPR phenomenon can be observed in the near-infrared region, with scattering dominating over extinction. For the optimal metal ring pattern, the SPR peak corresponds to a wavelength of 772 nm, with relative absorption, scattering, and extinction cross-sections of 0.54, 1.39, and 1.93, respectively. The weighted average absorption of the perovskite response layer in the range of 700-850 nm increased from 53.61% to 65.36%. Correspondingly, the photocurrent density of the device increased from 20.39 to 22.72 mA/cm<sup>2</sup>, and the photoelectric conversion efficiency was relatively improved by 11.45%. This research provides a novel path for light trapping design in perovskite solar cells in the near-infrared region, and serves as a "spectrum-based" reference for SPR regulation in other similar devices.

Trap suppression in ordered organic photovoltaic heterojunctions.

The high trap density (generally 1016-1018 cm-3) in organic solar cells (OSCs) brings about the localization of charge carriers and reduced charge carrier lifetime, mainly due to the weak intermolecular interactions of organic semiconductors resulting in their relatively poor crystallinity, which leads to low charge carrier mobilities and intense non-radiative recombination, thus impeding the further improvement of power conversion efficiencies (PCEs). Therefore, trap suppression is crucial to boost the performance of OSCs, and improving the crystallinity of donor/acceptor materials and enhancing the molecular order in devices can contribute to the trap suppression in OSCs. In this feature article, we summarize the recent advances of trap suppression in OSCs by material design and device engineering, and further outline possible development directions for trap suppression to enhance PCEs of OSCs.

Enhancing performance and scalability of HTL-free organic solar cells with keggin polyoxometalates-type additive

Interfacial layers play a crucial role in enhancing the performances and stabilities of organic solar cells (OSCs). Despite achieving high efficiencies from OSCs, the challenges associated with large-area manufacturing and the need for nanoscale uniformity during coating have hindered the large-scale production of these devices. To address these issues, we propose the concept of hole transport layer (HTL)-free OSCs to simplify the fabrication processes and improve the efficiency of production. In this study, we explore the potential of phosphotungstic acid (PTA), a Keggin polyoxometalate, both as a promising HTL and as an additive in HTL-free OSCs to enhance device performances and scalabilities in cell-to-module OSCs. Remarkably, incorporating PTA as an annealing-free HTL and as an additive in the photoactive layer (PM6:Y6:PC71BM) significantly increases power conversion efficiency (PCE) up to 17.28 % and 16.33 %, respectively, with respect to that of the reference device with PEDOT:PSS (15.88 %). We successfully fabricate a large-area OSC module, exhibiting an impressive PCE of 15.18 % and an active area of 54 cm2, achieving approximately 85 % of the effectiveness of small-area OSCs. The improved performance is attributed to the enhanced transmittance, excellent carrier-dynamic capacity, suppressed carrier recombination, and reduced trap-states in the ternary bulk-heterojunction OSCs. This study demonstrates the promising versatility of PTA for application in OSCs, offering potential enhancements in device performance and scalability for future applications.

Dual-Use Self-Assembled Monolayer Controlling Charge Carrier Extraction in Organic Solar Cells.

The development and use of interface materials are essential to the continued advancement of organic solar cells (OSCs) performance. Self-assembled monolayer (SAM) materials have drawn attention because of their simple structure and affordable price. Due to their unique properties, they may be used in inverted devices as a modification layer for modifying ZnO or as a hole transport layer (HTL) in place of typical poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) in conventional devices. In this work, zinc oxide (ZnO) is modified using five structurally similar SAM materials. This resulted in a smoother surface, a decrease in work function, a suppression of charge recombination, and an increase in device efficiency and photostability. In addition, they can introduced asfor hole extraction layer between the active layer and MoO3 , enabling the use of the same material at several functional layers in the same device. Through systematic orthogonal evaluation, it is shown that some SAM/active layer/SAM combinations still offered device efficiencies comparable to ZnO/SAM, but with improved device' photostability. This study may provide recommendations for future SAM material's design and development as well as a strategy for boosting device performance by using the same material across both sides of the photoactive layer in OSCs.

A Water-Processed Mesoscale Structure Enables 18.5% Efficient Binary Layer-by-Layer Organic Solar Cells.

The two-step layer-by-layer (LBL) deposition of donor and acceptor films enables desired vertical phase separation and high performance in organic solar cells (OSCs), which becomes a promising technology for large-scale printing devices. However, limitations including the use of toxic solvents and unpredictable infiltration between donor and acceptor still hinder the commercial production of LBL OSCs. Herein, we developed a water-based nanoparticle (NP) ink containing donor polymer to construct a mesoscale structure that could be infiltrated with an acceptor solution. Using non-halogen o-xylene for acceptor deposition, the LBL strategy with a mesoscale structure delivered outstanding efficiencies of 18.5% for binary PM6:L8-BObased LBL OSCs. Enhanced charge carrier mobility and restricted trap states were observed in the meso-LBL devices with optimized vertical morphology. It is believed that the findings in this work will bring about more research interest and effort on eco-friendly processing in preparation for the industrial production of OSCs.

Enhancing Efficiency and Stability in Organic Photovoltaics through Miscibility of High‐Tg Insulating Material with Y‐Series Nonfullerene Acceptors

Organic solar cells (OSCs) hold significant promise as candidates for next‐generation solar technology. Nevertheless, in state‐of‐the‐art OSCs using Y‐series nonfullerene acceptors, it remains challenging to achieve a high power conversion efficiency (PCE) while simultaneously addressing the inherent thermal instability. This study shows a straightforward additive strategy that can effectively accomplish both goals. By introducing 2 wt% commercially available insulating polyphenylene oxide (SA9000), the PCE and thermal stability of Y6‐based solar cells can be simultaneously improved. Atomic force microscopy confirms a slower coarsening process in the film with SA9000 under thermal stress. The investigation indicates that SA9000's success lies in its excellent miscibility with Y6, which can selectively alter electron mobility, resulting in a balance between hole and electron mobility, and also prevents excessive Y6 aggregation at elevated temperatures. This work reveals a selection criterion for commercial and low‐cost insulating materials as an additive to effectively improve the performance and stability of OSCs simultaneously.

Biaxially Conjugated Materials for Organic Solar Cells.

Organic solar cells (OSCs) represent one of the most important emerging photovoltaic technologies that can implement solar energy conversion efficiently. The chemical structure of organic semiconductors deployed in the active layer of OSCs plays a critical role in the photovoltaic performance and chemical/physical stability of relevant devices. With the structure innovation of organic semiconductors, especially nonfullerene acceptors (NFAs), the performance of OSCs have been promoted rapidly in recent years, with state-of-the-art power conversion efficiencies (PCEs) exceeding 19.5%. Compared with other photovoltaics like perovskite, the shortcoming of OSCs mainly lies in the high nonradiative recombination loss. However, the photocurrent density is superior in OSCs owing to the easy modulation of the NFA band gap toward the near-infrared region. In these regards, the effort to further boost the PCE of OSCs to achieve a milestone >21% should be devoted to reducing the nonradiative loss while further broadening the absorption band. Developing organic semiconductors with biaxially extended conjugated structures has provided a potential solution to achieve these goals. Herein, we summarize the design rules and performance progress of biaxially extended conjugated materials for OSCs. The descriptions are divided into two major categories, i.e., polymers and NFAs. For p-type polymers, we focus on the biaxial conjugation on some representative building blocks, e.g., polythiophene, triphenylamine, and quinoxaline. Whereas for n-type polymers, some structures with large conjugated planes in the normal direction are presented. We also elaborate on the biaxial conjugation strategies in NFAs with modification site at either the π-core or side-group. The general structure-property relationships are further retrieved within these materials, with focus on the short-wavelength absorption and nonradiative energy loss. Finally, we provide an outlook for the further structure modification strategies of biaxially conjugated materials toward highly efficient, stable, and industry-compatible OSCs.

(Invited) Performance Improvement and Potential Applications of Ultrathin Organic Solar Cells

Flexible and thin photovoltaics can supply a variety of applications such as power sources for wearable electronics and soft robots. We reduce the total thickness of such organic photovoltaics down to several μm, which enables extreme bendability and lightweight properties. The important parameters include high efficiency and stability in ambient air conditions. Here we show our recent progress with ultra-thin organic solar cells and potential applications for wearable electronics and soft robots.The current best power conversion efficiency (PCE) for ultrathin organic solar cells is 15.8% reported by ourselves1). We have solved the specific issues behind the usage of ultrathin substrate films to achieve almost comparable PCE with those using normal flexible organic solar cells. One specific technology is the usage of polymer films having good thermal stability and smooth surface roughness2). Another important technology is the development of stable charge carrier injection layers and interfaces at the active layer. The state-of-the-art active layer materials such as PM6:Y6 is known not to be stable in ambient air and other environmental stresses. We verified the degradation occurred by the diffusion of Zn atoms from the conventional electron-transport layer (ETL) of ZnO obtained from the sol-gel process to the active layer, and solved it by replacing the ETL with more stable organic/inorganic hybrid material1,3). Such improved ETL and interface achieved highly balanced performance and stability, enabling good PCE on ultrathin substrates.We improved both PCE and environmental stabilities simultaneously. The stabilities include mechanical robustness3), water-proof property4), thermal stability2), and storage stability in ambient air5). Material development and process engineering enable such improved stabilities. In addition, such environmental stability can offer another practical usage. Our ultrathin solar cells have excellent thermal stability up to 120 ℃. This is capable of the hot-melt process which is a mature technology for the apparel industry. We can adhere our ultrathin organic solar cells directly onto textiles with such hot-melt films without any degraded performances. Apart from the improvement of the performance of ultrathin organic solar cells, we are focusing on how to implement such flexible devices onto three-dimensional surfaces5).We demonstrated some potential applications using our ultrathin organic solar cells used as a power source for wearable sensor applications7,8). We fabricated ultrathin organic solar cell modules and sensors. The sensor was powered by the solar cells under light illumination and monitored electrocardiogram signals with a high signal-to-noise ratio. In addition to this, we also demonstrated rechargeable cyborg insects9). In this research, we adhered our ultrathin organic solar cells onto a living insect abdomen and demonstrated a recharging wireless locomotion control system.We are focusing on both performance improvements of ultrathin organic solar cells and the integration of these cells to supply power to wearable electronics. By accelerating system-level integration research in the next decade to meet the required specifications of appropriate target applications, ultrathin organic solar cells will solve the problems associated with the supply of power for various electronics and will contribute to the overall development of society.1) S. Xiong et al., Adv. Sci. 2022, 9, 2105288.2) X. Xu et al., Proc. Natl. Acad. Sci. 2018, 115, 4589.3) F. Qin et al., Nat. Commun. 2020, 11, 4508 .4) H. Jinno et al., Nat. Energy 2017, 2, 780.5) Z. Jiang et al., Proc. Natl. Acad. Sci. 2020, 117, 6391.6) S. I. Rich, S. Lee, K. Fukuda, and T. Someya, 2022, 34, 2106683.7) S. Park et al., Nature 2018, 561, 516.8) H. Jinno et al., Nat. Commun. 2021, 12, 2234.9) Y. Kakei et al., npj Flex. Electron. 2022, 6, 78. Figure 1

Morphology optimization induced by a highly volatile solid additive contributes to efficient organic solar cells with enhanced photostability

The morphology of the active layer plays a crucial role in the device performance and stability of organic solar cells (OSCs). To obtain the optimized blend film morphology, numerous solvent additives have been developed and utilized. Nevertheless, the commonly used high boiling point solvent additives have strict volume control and residual problems, which are not conducive to the development of OSCs. To alter the situation, it is urgent to develop highly volatile additives to minimize the unfavorable effect. Here, a volatile aromatic small molecule, benzoylacetate (BA), is used as a solid additive to control the morphology of the active layer during the post-treatment procedure. More importantly, BA has an apparent positive interaction with Y6, leading to more favorable molecular arrangement in the blend film and enhanced charge transfer properties. Finally, the BA-treated PM6:Y6 device exhibits improved device performance and photostability. Moreover, BA could be used as a universal solid additive for different OSC systems. Specifically, a superior PCE of 18.5 % is achieved in BA-processed PM6:L8-BO binary system with good photostability. Our work demonstrates an approach of applying a volatile solid additive to improve the device performance and photostability.

Revealing the Effect of Halogenation Strategy on the Regulation of Crystallization Kinetics and Molecular Packing for High‐Performance Organic Solar Cells

AbstractHalogenation of non‐fused ring electron acceptors (NFREAs) plays an important role in regulating their optoelectronic properties. However, the underlying mechanisms and their impact on the performance of organic solar cells (OSCs) have remained unclear. Herein, a series of halogenated NFREAs incorporating F, Cl, and Br, are prepared to study their effect on crystallization kinetics, phase separation, molecular packing, and charge transport. Among various halogenation strategies, chlorination minimizes the Coulomb attractive energy between donor and acceptor, thereby facilitating exciton dissociation. In situ UV–vis absorption tests reveal that chlorinated acceptors exhibit a longer crystallization time, effectively suppressing excessive molecular aggregation and enhancing overall crystallinity. Additionally, chlorinated acceptors exhibit a longer exciton diffusion length, which promotes exciton dissociation while mitigating charge recombination in the devices. Consequently, two chlorinated NFREAs, TCN‐Cl, and PCN‐Cl, yield an impressive power conversion efficiency (PCE) of 14.85% and 15.30%, respectively, when blended with PM6 and J52 donors. These values represent the highest reported PCEs to date for NFREAs with A‐π‐A’‐π‐A and A‐π‐D‐π‐A structures. The study elucidates the crucial role of chlorination in extending exciton diffusion length and crystallization time. These effects significantly benefit phase separation within the active layers, enhance charge separation, and suppress recombination for achieving high‐efficiency OSCs.