Showing papers on "Fullerene published in 2022"

Recent Progress in Organic Solar Cells Based on Non-Fullerene Acceptors: Materials to Devices

Abstract: Harnessing energy from the sun is attracting increasing attention as the traditional fossil-based energy sources are being depleted. Photovoltaics (PV) is now an established technology and the most promising method...

Non-fullerene acceptors with hetero-dihalogenated terminals induce significant difference in single crystallography and enable binary organic solar cells with 17.5% efficiency

Abstract: A synergistic hetero-dihalogenated terminals strategy was systematically employed for the first time to enhance single-crystal packing, boosting the device performance of a Y-BO-FCl:PM6 device with a remarkable PCE of 17.52%.

Achieving Record-Efficiency Organic Solar Cells upon Tuning the Conformation of Solid Additives

Abstract: Volatile solid additives (SADs) are considered as a simple yet effective approach to tune the film morphology for high-performance organic solar cells (OSCs). However, the structural effects of the SADs on the photovoltaic performance are still elusive. Herein, two volatilizable SADs were designed and synthesized. One is SAD1 with twisted conformation, while the other one is planar SAD2 with the S···O noncovalent intramolecular interactions (NIIs). The theoretical and experimental results revealed that the planar SAD2 with smaller space occupation can more easily insert between the Y6 molecules, which is beneficial to form a tighter intermolecular packing mode of Y6 after thermal treatment. As a result, the SAD2-treated OSCs exhibited less recombination loss, more balanced charge mobility, higher hole transfer rate, and more favorable morphology, resulting in a record power conversion efficiency (PCE) of 18.85% (certified PCE: 18.7%) for single-junction binary OSCs. The universality of this study shed light on understanding the conformation effects of SADs on photovoltaic performances of OSCs.

Reducing Energy Disorder in Perovskite Solar Cells by Chelation.

Abstract: In inverted perovskite solar cells (PSCs), the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is a widely used electron transport material. However, a high degree of energy disorder and inadequate passivation of PCBM limit the efficiency of devices, and severe self-aggregation and unstable morphology limit the lifespan of devices. Here, we design a series of fullerene dyads FP-Cn (n = 4, 8, 12) to replace PCBM as an electron transport layer, where [60]fullerene is linked with a terpyridine chelating group via a flexible alkyl chain of different lengths as a spacer. Among three fullerene dyads, FP-C8 shows the most enhanced molecule ordering and adhesion with the perovskite surface due to the balanced decoupling between the chelation effect from terpyridine and the self-assembly of fullerene, leading to lower energy disorder and higher morphological stability relative to PCBM. The FP-C8/C60-based devices using Cs0.05FA0.90MA0.05PbI2.85Br0.15 as a light absorber show a power conversion efficiency of 21.69%, higher than that of PCBM/C60 (20.09%), benefiting from improved electron extraction and transport as well as reduced charge recombination loss. When employing FAPbI3 as a light absorber, the FP-C8/C60-based devices exhibit an efficiency of 23.08%, which is the champion value of inverted PSCs with solution-processed fullerene derivatives. Moreover, the FP-C8/C60-based devices show better moisture and thermal stability than PCBM/C60-based devices and maintain 96% of their original efficiency after 1200 h of operation, while their counterpart PCBM/C60 maintains 60% after 670 h.

Probing the Reactions of Thiourea (CH4N2S) with Metals (X = Au, Hf, Hg, Ir, Os, W, Pt, and Re) Anchored on Fullerene Surfaces (C59X)

Abstract: Upon various investigations conducted in search for a nanosensor material with the best sensing performance, the need to explore these materials cannot be overemphasized as materials associated with best sensing attributes are of vast interest to researchers. Hence, there is a need to investigate the adsorption performances of various metal-doped fullerene surfaces: C59Au, C59Hf, C59Hg, C59Ir, C59Os, C59Pt, C59Re, and C59W on thiourea [SC(NH2)2] molecule using first-principles density functional theory computation. Comparative adsorption study has been carried out on various adsorption models of four functionals, M06-2X, M062X-D3, PBE0-D3, and ωB97XD, and two double-hybrid (DH) functionals, DSDPBEP86 and PBE0DH, as reference at Gen/def2svp/LanL2DZ. The visual study of weak interactions such as quantum theory of atoms in molecule analysis and noncovalent interaction analysis has been invoked to ascertain these results, and hence we arrived at a conclusive scientific report. In all cases, the weak adsorption observed is best described as physisorption phenomena, and CH4N2S@C59Pt complex exhibits better sensing attributes than its studied counterparts in the interactions between thiourea molecule and transition metal-doped fullerene surfaces. Also, in the comparative adsorption study, DH density functionals show better performance in estimating the adsorption energies due to their reduced mean absolute deviation (MAD) and root-mean-square deviation (RMSD) values of (MAD = 1.0305, RMSD = 1.6277) and (MAD = 0.9965, RMSD = 1.6101) in DSDPBEP86 and PBE0DH, respectively.

Printing fabrication of large-area non-fullerene organic solar cells

Abstract: Organic solar cells (OSCs) based on a bulk heterojunction structure exhibit inherent advantages, such as low cost, light weight, mechanical flexibility, and easy processing, and they are emerging as a potential renewable energy technology. However, most studies are focused on lab-scale, small-area ( 1 cm2) OSCs still exhibit considerable efficiency loss during upscaling from small-area to large-area, which is a big challenge. In recent years, along with the rapid development of high-performance non-fullerene acceptors, many researchers have focused on developing large-area non-fullerene-based devices and modules. There are three essential issues in upscaling OSCs from small-area to large-area: fabrication technology, equipment development, and device component processing strategy. In this review, the challenges and solutions in fabricating high-performance large-area OSCs are discussed in terms of the abovementioned three aspects. In addition, the recent progress of large-area OSCs based on non-fullerene electron acceptors is summarized.

Achieving 16% Efficiency for Polythiophene Organic Solar Cells with a Cyano‐Substituted Polythiophene

Abstract: Polythiophenes (PTs) are promising electron donors in organic solar cells (OSCs) due to their simple structures and excellent synthetic scalability. However, the device performance of PT‐based OSCs is rather poor due mainly to large photon energy losses and an unfavorable active layer morphology. Herein, the authors report a new PT, which is abbreviated as P5TCN‐2F and features cyano‐group substituents for high‐efficiency OSCs. The cyano‐group endows P5TCN‐2F with a deep‐lying highest occupied molecular orbital energy level, which thereby contributed to high open‐circuit voltage in OSCs as a result of reduced non‐radiative recombination energy loss. Moreover, the cyano‐group leads to strong interchain interaction, improved polymer crystallinity, and appropriate miscibility with the prevailing non‐fullerene acceptors. Consequently, P5TCN‐2F offers over 15% power conversion efficiency when blended with various Y‐series non‐fullerene acceptors (Y6, Y6‐BO, eC9, and L8‐BO). Particularly, a champion efficiency of 16.1% is obtained by the P5TCN‐2F:Y6 blend, which is largely higher than that of any previous PT‐based OSCs. Moreover, the average figure of merit of the active layer based on P5TCN‐2F is much superior to that of benzodithiophene‐based polymers. These results suggest the renaissance of PT‐based OSCs and have opened an avenue to access high‐performance materials for the large‐scale production of OSC modules.

Small reorganization energy acceptors enable low energy losses in non-fullerene organic solar cells

Abstract: Abstract Minimizing energy loss is of critical importance in the pursuit of attaining high-performance organic solar cells. Interestingly, reorganization energy plays a crucial role in photoelectric conversion processes. However, the understanding of the relationship between reorganization energy and energy losses has rarely been studied. Here, two acceptors, Qx-1 and Qx-2, were developed. The reorganization energies of these two acceptors during photoelectric conversion processes are substantially smaller than the conventional Y6 acceptor, which is beneficial for improving the exciton lifetime and diffusion length, promoting charge transport, and reducing the energy loss originating from exciton dissociation and non-radiative recombination. So, a high efficiency of 18.2% with high open circuit voltage above 0.93 V in the PM6:Qx-2 blend, accompanies a significantly reduced energy loss of 0.48 eV. This work underlines the importance of the reorganization energy in achieving small energy losses and paves a way to obtain high-performance organic solar cells.

Printing fabrication of large-area non-fullerene organic solar cells

Abstract: Organic solar cells (OSCs) based on a bulk heterojunction structure exhibit inherent advantages, such as low cost, light weight, mechanical flexibility, and easy processing, and they are emerging as a potential renewable energy technology. However, most studies are focused on lab-scale, small-area (<1 cm2) devices. Large-area (>1 cm2) OSCs still exhibit considerable efficiency loss during upscaling from small-area to large-area, which is a big challenge. In recent years, along with the rapid development of high-performance non-fullerene acceptors, many researchers have focused on developing large-area non-fullerene-based devices and modules. There are three essential issues in upscaling OSCs from small-area to large-area: fabrication technology, equipment development, and device component processing strategy. In this review, the challenges and solutions in fabricating high-performance large-area OSCs are discussed in terms of the abovementioned three aspects. In addition, the recent progress of large-area OSCs based on non-fullerene electron acceptors is summarized.

Non-Fullerene Acceptors with Direct and Indirect Hexa-fluorination Afford >17% Efficiency in Polymer Solar Cells

Abstract: The rational molecular design of non-fullerene acceptors (NFAs) in organic solar cells (OSCs) can profoundly influence photovoltaic (OPV) performance. NFA fluorination has to date proven beneficial to cell performance. However,...

Non‐Radiative Recombination Energy Losses in Non‐Fullerene Organic Solar Cells

Abstract: Impressive short‐circuit current density and fill factor have been achieved simultaneously in single‐junction organic solar cells (OSCs) with the emergence of high‐performance non‐fullerene acceptors. However, the power conversion efficiencies (PCEs) of OSCs still lag behind those of inorganic and perovskite solar cells, mainly due to the modest open‐circuit voltage (VOC) imposed by relatively large energy loss (Eloss). Generally, Eloss in solar cells can be divided into three parts. Among them, ΔE1 is inevitable for all photovoltaic cells and depends on the optical bandgap of solar cells, while radiative recombination energy loss, ΔE2, in OSCs can approach the negligible value via finely matching donor with acceptor material in the blend. The relatively large non‐radiative recombination energy loss, ΔE3, becomes the main barrier to further reduce Eloss and thus enhance PCE in non‐fullerene acceptor‐based OSCs. In this review, the recent studies and achievements about ΔE3 in non‐fullerene acceptor‐based OSCs have been summarized from the aspects of material design, morphology manipulation, ternary strategy, mechanism, and theoretical study. It is hoped that this review helps to get a deep understanding and boost the advance of ΔE3 study in OSCs.

Fibrillization of Non‐Fullerene Acceptors Enables 19% Efficiency Pseudo‐Bulk Heterojunction Organic Solar Cells

Abstract: The structural order and aggregation of non‐fullerene acceptors (NFA) are critical toward light absorption, phase separation, and charge transport properties of their photovoltaic blends with electron donors, and determine the power conversion efficiency (PCE) of the corresponding organic solar cells (OSCs). In this work, the fibrillization of small molecular NFA L8‐BO with the assistance of fused‐ring solvent additive 1‐fluoronaphthalene (FN) to substantially improve device PCE is demonstrated. Molecular dynamics simulations show that FN attaches to the backbone of L8‐BO as the molecular bridge to enhance the intermolecular packing , inducing 1D self‐assembly of L8‐BO into fine fibrils with a compact polycrystal structure. The L8‐BO fibrils are incorporated into a pseudo‐bulk heterojunction (P‐BHJ) active layer with D18 as a donor, and show enhanced light absorption, charge transport, and collection properties, leading to enhanced PCE from 16.0% to an unprecedented 19.0% in the D18/L8‐BO binary P‐BHJ OSC, featuring a high fill factor of 80%. This work demonstrates a strategy for fibrillating NFAs toward the enhanced performance of OSCs.

Adsorption properties of metal functionalized fullerene (C59Au, C59Hf, C59Ag, and C59Ir) nanoclusters for application as a biosensor for hydroxyurea (HXU): insight from theoretical computation

Abstract: Abstract This theoretical study was conducted to evaluate the efficiency of fullerene C60 and its metal functionalized nano clusters (C59Au, C59Hf, C59Ag and C59Ir) as a sensor for hydroxyurea (HXU). The various conclusions concerning the adsorption and sensing properties of the studied nano surfaces were achieved using density functional theory (DFT) at the M062X-D3/gen/LanL2DZ/def2svp level of theory. Among the nano clusters studied for this interaction, analysis of the HOMO–LUMO energy differences (Eg) showed that HXU@C59Hg (H2) reflects the least energy gap of 3.042 eV, indicating its greater reactivity, sensitivity and conductivity. Also, the adsorption phenomenon in this current study is best described as chemisorptions owing to the negative adsorption enthalpies observed. Thus, the adsorption energy (EAd) follows an increasing pattern of: HXU@C60 (C1) (−0.218 eV) < HXU@C59Ir (I1) (−1.361 eV) < HXU@C59Au (A1) (−1.986 eV) < HXU@C59Hf (H1) (−2.640 eV) < HXU@C59Hg (H2) (−3.347 eV). Least Eg, highest EAd and non-covalent nature of interaction attributed to C59Hg surface are sufficient to show that, among all studied surfaces, C59Hg surface emerged as the most suitable adsorbent for the adsorption of HXU. Hence, it can be used in modeling future adsorbent material for hydroxyurea.

The Renaissance of Oligothiophene‐Based Donor–Acceptor Polymers in Organic Solar Cells

Abstract: The power conversion efficiencies (PCEs) of organic solar cells (OSCs) have increased rapidly owing to the development of non‐fullerene acceptors (NFAs). However, the development of polymer donors lags behind significantly. Currently, the polymer donors are dominated by a handful of thiophene‐substituted benzo[1,2‐b:4,5‐b']dithiophene (BDTT) polymers, which suffer from lengthy synthesis and high production cost. Compared with BDTT‐based polymers, oligothiophene‐based donor‐acceptor polymers feature much easier synthesis, which were the prevailing polymer donors in fullerene‐based OSCs, but almost disappeared in non‐fullerene OSCs. Herein, two oligothiophene‐based donor‐acceptor polymers (PTTz‐3HD and PTTz‐4HD) are reported to re‐evaluate this kind of polymer in non‐fullerene OSCs. Benefiting from the exquisite alkyl chain design, the polymer PTTz‐3HD exhibits more planar conformation, stronger aggregation, and higher crystallinity, which in turn contributes to the formation of an optimal active layer morphology when blended with NFA. As a result, a PCE of 16.1% and 16.7% is achieved by PTTz‐3HD in binary and ternary OSCs, respectively. Of particular note, the product of short‐circuit current density and fill factor of PTTz‐3HD is fully comparable to those of BDTT‐based polymers. These results suggest the renaissance of oligothiophene‐based donor‐acceptor polymers in OSCs and demonstrate a promising avenue to access high‐efficiency OSCs from low‐cost materials.

Fullerene C60 derivatives as antimicrobial photodynamic agents

Abstract: • The antimicrobial photoinactivation activity of fullerene C 60 derivatives is discussed. • The design and development of the fullerene structures are examined. • The spectroscopic and photodynamic properties of fullerenes are analyzed. • Applications of fullerenes to eradicate microorganisms are argued. Functionalized fullerenes have shown interesting biomedical applications as potential phototherapeutic agents. The hydrophobic carbon sphere of fullerene C 60 can be substituted by cationic groups to obtain amphiphilic structures. These compounds absorb mainly UV light, but absorption in the visible region can be enhanced by anchoring light-harvesting antennas to the C 60 core. Upon photoexcitation, fullerenes act as spin converters by effective intersystem crossing. From this excited state, they can react with ground state molecular oxygen and other substrates to form reactive oxygen species. This process leads to the formation of singlet molecular oxygen by energy transfer or superoxide anion radical by electron transfer. Photodynamic inactivation experiments indicate that cationic fullerenes are highly effective photosensitizers with applications as broad-spectrum antimicrobial agents. In these structures, the hydrophobic character of C 60 improves membrane penetration, while the presence of positive charges increases the binding of the fullerene derivatives with microbial cells. Herein, we summarize the progress of antimicrobial photodynamic inactivation based on substituted fullerenes specially designed to improve the photodynamic activity.

Fullerene C60 derivatives as antimicrobial photodynamic agents

Abstract: Functionalized fullerenes have shown interesting biomedical applications as potential phototherapeutic agents. The hydrophobic carbon sphere of fullerene C60 can be substituted by cationic groups to obtain amphiphilic structures. These compounds absorb mainly UV light, but absorption in the visible region can be enhanced by anchoring light-harvesting antennas to the C60 core. Upon photoexcitation, fullerenes act as spin converters by effective intersystem crossing. From this excited state, they can react with ground state molecular oxygen and other substrates to form reactive oxygen species. This process leads to the formation of singlet molecular oxygen by energy transfer or superoxide anion radical by electron transfer. Photodynamic inactivation experiments indicate that cationic fullerenes are highly effective photosensitizers with applications as broad-spectrum antimicrobial agents. In these structures, the hydrophobic character of C60 improves membrane penetration, while the presence of positive charges increases the binding of the fullerene derivatives with microbial cells. Herein, we summarize the progress of antimicrobial photodynamic inactivation based on substituted fullerenes specially designed to improve the photodynamic activity.

End-capped group modification on cyclopentadithiophene based non-fullerene small molecule acceptors for efficient organic solar cells; a DFT approach.

Abstract: Four acceptor-donor-acceptor (A-D-A) type cyclopentadithiophene core-based non-fullerene small acceptor molecules were designed with the objective to improve the proficiency of photovoltaic cells. A comprehensive density functional theory (DFT) analysis was done by employing B3LYP functional with 6-31G(d,p) basis set to study optoelectronic properties of R as well as M1-M4 molecules, while the time-dependent self-consistent field (TDSCF) was utilized to analyze their excited state calculations. Several essential characteristics must be refined in order to enhance the efficiency of small molecular acceptors, i.e., the density of states (DOS), HOMO-LUMO band gap, transition density matrix (TDM), dipole moment, reorganization energy, light-harvesting efficiency, and open-circuit voltage, etc. In comparison to the R molecule, all the derived molecules show better maximum absorption (in chloroform solvent) with a range of 886-951 nm and a smaller band gap with a range of 1.65-1.55 eV M2 retains the least exciton binding energy of 0.24 eV, and amongst all the investigated molecules M3 molecule has the least interaction coefficient values so, it possesses better charge transport probability. The reorganization energy values in eV for both electron (0.00579) and hole (0.00737) are the least for M3 molecule, so this molecule exhibits better charge mobility for electron and hole. VOC of R and M1-M4 molecule was calculated by theoretically computing the values of their complexes with PTB7-Th donor molecule.

Recent Progress in Organic Solar Cells: A Review on Materials from Acceptor to Donor

Abstract: In the last few decades, organic solar cells (OSCs) have drawn broad interest owing to their advantages such as being low cost, flexible, semitransparent, non-toxic, and ideal for roll-to-roll large-scale processing. Significant advances have been made in the field of OSCs containing high-performance active layer materials, electrodes, and interlayers, as well as novel device structures. Particularly, the innovation of active layer materials, including novel acceptors and donors, has contributed significantly to the power conversion efficiency (PCE) improvement in OSCs. In this review, high-performance acceptors, containing fullerene derivatives, small molecular, and polymeric non-fullerene acceptors (NFAs), are discussed in detail. Meanwhile, highly efficient donor materials designed for fullerene- and NFA-based OSCs are also presented. Additionally, motivated by the incessant developments of donor and acceptor materials, recent advances in the field of ternary and tandem OSCs are reviewed as well.

Regiospecific N-alkyl substitution tunes the molecular packing of high-performance non-fullerene acceptors

Abstract: The rapid development of non-fullerene acceptors (NFAs) with strong near-infrared absorption has led to remarkably enhanced short-circuit current density (Jsc) values in organic solar cells (OSCs). NFAs based on the benzotriazole (Bz) fused-ring π-core have great potential in delivering both high Jsc and decent open-circuit voltage values due to their strong intramolecular charge transfer with reasonably low energy loss. In this work, we have designed and synthesized a series of Bz-based NFAs, PN6SBO-4F, AN6SBO-4F and EHN6SEH-4F, via regiospecific N-alkyl engineering based on the high-performance NFA mBzS-4F that was reported previously. The molecular packing of mBzS-4F, AN6SBO-4F, and EHN6SEH-4F single crystals was analyzed using X-ray crystallography in order to provide a comprehensive understanding of the correlation between the molecular structure, the charge-transporting properties, and the solar cell performance. Compared with the typical honeycomb single-crystal structure of Y6 derivatives, these NFAs exhibit distinctly different molecular packing patterns. The strong interactions of terminal indanone groups in mBzS-4F and the J-aggregate-like packing in EHN6SEH-4F lead to the formation of ordered 3D networks in single-crystals with channels for efficient charge transport. Consequently, OSCs based on mBzS-4F and EHN6SEH-4F show efficient photon-to-current conversions, achieving the highest power conversion efficiency of 17.48% with a Jsc of 28.83 mA cm−2.

Regiospecific <i>N</i>-alkyl substitution tunes the molecular packing of high-performance non-fullerene acceptors

Abstract: The rapid development of non-fullerene acceptors (NFAs) with strong near-infrared absorption has led to remarkably enhanced short-circuit current density (Jsc) values in organic solar cells (OSCs). NFAs based on the benzotriazole (Bz) fused-ring π-core have great potential in delivering both high Jsc and decent open-circuit voltage values due to their strong intramolecular charge transfer with reasonably low energy loss. In this work, we have designed and synthesized a series of Bz-based NFAs, PN6SBO-4F, AN6SBO-4F and EHN6SEH-4F, via regiospecific N-alkyl engineering based on the high-performance NFA mBzS-4F that was reported previously. The molecular packing of mBzS-4F, AN6SBO-4F, and EHN6SEH-4F single crystals was analyzed using X-ray crystallography in order to provide a comprehensive understanding of the correlation between the molecular structure, the charge-transporting properties, and the solar cell performance. Compared with the typical honeycomb single-crystal structure of Y6 derivatives, these NFAs exhibit distinctly different molecular packing patterns. The strong interactions of terminal indanone groups in mBzS-4F and the J-aggregate-like packing in EHN6SEH-4F lead to the formation of ordered 3D networks in single-crystals with channels for efficient charge transport. Consequently, OSCs based on mBzS-4F and EHN6SEH-4F show efficient photon-to-current conversions, achieving the highest power conversion efficiency of 17.48% with a Jsc of 28.83 mA cm-2.

Efficient and Stable Tin Perovskite Solar Cells by Pyridine‐Functionalized Fullerene with Reduced Interfacial Energy Loss

Abstract: In tin perovskite solar cells (PSCs), fullerene (C60) and fullerene derivative [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) are commonly utilized electron transport materials. However, the energetic disorder, inadequate passivation, and energy level mismatch of C60 and PCBM limit the improvement of power conversion efficiency (PCE) and lifespan of tin PSCs. In this work, a multifunctional interface manipulation strategy is developed by introducing a pyridine‐functionalized fullerene derivative, fullerene‐n‐butyl‐pyridine (C60‐BPy), into the interface between the tin perovskite and the electron transport layer (ETL) to improve the photovoltaic performance and stability of tin PSCs. The C60‐BPy can strongly anchor on the perovskite surface via coordination interactions between the pyridine moiety and the Sn2+ ion, which not only reinforces the passivation of the trap‐state within the tin perovskite film, but also regulates the interface energy level alignment to reduce non‐radiative recombination. Moreover, the improved interface binding and carrier transport properties of C60‐BPy contribute to superior device stability. The resulting devices have achieved the highest PCE of 14.14% with negligible hysteresis, and are maintained over 95% of their initial PCE under continuous one‐sun illumination for 1000 h.

Machine‐Learning Analysis of Small‐Molecule Donors for Fullerene Based Organic Solar Cells

Abstract: In recent years, development in organic solar cells speeds up and performance continuously increases. From the last few years, machine learning gains fame among scientists who are researching on organic solar cells. Herein, machine learning is used to screen the small-molecule donors for organic solar cells. Molecular descriptors are used as input to train machine models. A variety of machine-learning models are tested to find the suitable one. Random forest model shows best predictive capability (Pearson's coefficient = 0.93). New small-molecule donors are also designed from easily synthesizable building units. Their power conversion efficiencies (PCEs) are predicted. Potential candidates with PCE > 11% are selected. The approach presented herein helps to select the efficient materials in short time with ease.

Mapping the energy level alignment at donor/acceptor interfaces in non-fullerene organic solar cells

Abstract: Energy level alignment (ELA) at donor (D) -acceptor (A) heterojunctions is essential for understanding the charge generation and recombination process in organic photovoltaic devices. However, the ELA at the D-A interfaces is largely underdetermined, resulting in debates on the fundamental operating mechanisms of high-efficiency non-fullerene organic solar cells. Here, we systematically investigate ELA and its depth-dependent variation of a range of donor/non-fullerene-acceptor interfaces by fabricating and characterizing D-A quasi bilayers and planar bilayers. In contrast to previous assumptions, we observe significant vacuum level (VL) shifts existing at the D-A interfaces, which are demonstrated to be abrupt, extending over only 1-2 layers at the heterojunctions, and are attributed to interface dipoles induced by D-A electrostatic potential differences. The VL shifts result in reduced interfacial energetic offsets and increased charge transfer (CT) state energies which reconcile the conflicting observations of large energy level offsets inferred from neat films and large CT energies of donor - non-fullerene-acceptor systems.

Understanding the Role of Order in Y‐Series Non‐Fullerene Solar Cells to Realize High Open‐Circuit Voltages

Abstract: Non‐fullerene acceptors (NFAs) as used in state‐of‐the‐art organic solar cells feature highly crystalline layers that go along with low energetic disorder. Here, the crucial role of energetic disorder in blends of the donor polymer PM6 with two Y‐series NFAs, Y6, and N4 is studied. By performing temperature‐dependent charge transport and recombination studies, a consistent picture of the shape of the density of state distributions for free charges in the two blends is developed, allowing an analytical description of the dependence of the open‐circuit voltage VOC on temperature and illumination intensity. Disorder is found to influence the value of the VOC at room temperature, but also its progression with temperature. Here, the PM6:Y6 blend benefits substantially from its narrower state distributions. The analysis also shows that the energy of the equilibrated free charge population is well below the energy of the NFA singlet excitons for both blends and possibly below the energy of the populated charge transfer manifold, indicating a down‐hill driving force for free charge formation. It is concluded that energetic disorder of charge‐separated states has to be considered in the analysis of the photovoltaic properties, even for the more ordered PM6:Y6 blend.