Non-fullerene Bulk Heterojunction Research Articles

Enhanced Self-Stimulated Dissociation in Non-fullerene Layer-by-Layer Organic Solar Cells through Superimposed and Oriented Dipolar Polarization

A non-fullerene layer-by-layer (LBL) structure can establish effective donor/acceptor (D/A) interfaces and lead to remarkable photovoltaic efficiencies based on facile solution processing properties, as compared with non-fullerene bulk heterojunction devices. However, the operating mechanisms have been remaining as an unaddressed issue to understand the role of D/A interfaces toward developing photovoltaic processes in non-fullerene LBL where the high electrically polarizable non-fullerene molecules exist. Here, we address the operating mechanisms of photovoltaic actions by in situ monitoring the dynamic D/A interfaces upon selectively exciting the non-fullerene acceptor layer to address the effects of optically induced superimposed and oriented dipoles on electron–hole pair self-stimulated dissociation in non-fullerene LBL organic solar cells [ITO/PEDOT:PSS/PM6/Y6/PDIN/Al]. Essentially, the self-stimulated dissociation at the D/A interfaces can be monitored by magneto-photocurrent while separately exciting the donor and acceptor components with two different laser beams. We observed an interesting phenomenon that optically exciting the non-fullerene Y6 acceptor component leads to easier electron–hole dissociation at D/A interfaces and consequently an increase on electron–hole pair self-stimulated dissociation to generate photocurrent in non-fullerene LBL solar cells. This phenomenon presents a hypothesis that optically exciting the compactly stacked non-fullerene Y6 molecules induces superimposed and oriented electrical dipoles within A–D–A–D–A structures to increase the self-stimulated dissociation at D/A interfaces toward developing the photovoltaic actions in non-fullerene LBL solar cells. This hypothesis is supported by our photoinduced capacitance result: photoexcitation increases bulk polarization in the dipolar polarization zone in non-fullerene LBL solar cells. Furthermore, optically generated dipoles are verified by photoinduced magneto-capacitance, solely generated by photoinduced polarization rather than photogenerated carriers, in non-fullerene LBL PM6/Y6. Clearly, the optically generating superimposed and oriented electrical dipoles become a critical parameter to operate the photovoltaic actions at D/A interfaces in non-fullerene LBL solar cells.

Insights into Magneto-Photocurrent and Coherent Spin Mixing for Binary and Ternary Nonfullerene Bulk Heterojunction Organic Solar Cells

Insights into Magneto-Photocurrent and Coherent Spin Mixing for Binary and Ternary Nonfullerene Bulk Heterojunction Organic Solar Cells

Influence of Acid-Doped Poly(ethylene imine) Interlayers on the Performance of Polymer:Nonfullerene Solar Cells.

This work reports that ultrathin polymeric films doped with organic acid molecules can act as an electron-transporting interfacial layer in polymer:nonfullerene solar cells. The polymeric interfacial layers, which consist of poly(ethylene imine) (PEI) doped with 3-hydroxypropane-1-sulfonic acid (HPSA) at various HPSA molar ratios, are introduced between transparent indium-tin oxide (ITO) electrodes and polymer:nonfullerene bulk heterojunction layers. The HPSA-doped PEI (PEI:HPSA) films are optically translucent in the wavelength range of ≈300-800nm, while the acidity of PEI solutions reached ≈pH = 7 at HPSA = 30 mol%. The power conversion efficiency of solar cells is improved by doping 20 mol% HPSA due to the increased short circuit current density without open circuit voltage reduction. The improvement in solar cell performances is attributed to an adequate control of HPSA doping ratios, which spares undoped amine units of PEI for making sufficient net dipole layers with ITO surfaces and makes permanent charges for high electrical conductivity in the layers. The surface morphology and doped states are characterized with atomic force microscopy and X-ray photoelectron spectroscopy.

Photoinduced spin-orbital coupling effect at donor: acceptor interface in non-fullerene organic solar cells

Non-fullerene based organic solar cells show excellent optoelectronic properties. In this study, firstly, we measured the magneto-photocurrent (MPC) effect to monitor the spin orbit coupling (SOC) action of electron-hole pairs at the donor: acceptor (D:A) interfaces, introduced by the dipole-dipole interaction in non-fullerene ITIC molecules. The MPC of non-fullerene based bulk heterojunctions show a broader line-shape, which implies a stronger SOC in electron-hole pairs at D:A interface as compared to the fullerene based bulk heterojunctions under photo-excitation. Secondly, the light-induced electron paramagnetic resonance (LEPR) results further give a direct evidence for stronger SOC in electron-hole pairs at D:A interface in non-fullerene bulk-heterojunction. Theoretically, increasing SOC provides a pathway for singlet electron-hole pairs to convert into triplet electron-hole pairs, leading to more spin states ready for dissociation. Therefore, the increased SOC will increase the magneto-photocurrent, and improve the performance of non-fullerene solar cells, we consider that photoexcitation can present a new approach to strengthen the SOC effect in organic solar cells toward better photovoltaic actions. Non-fullerene based organic solar cells show excellent optoelectronic properties and device performance. We think these excellent properties come from a stronger spin orbit coupling effect in electron-hole pairs at D: A interface under photoexcitation as compared to the fullerene bulk-heterojunctions, leading to increase the magneto-photocurrent. In addition, the capacitance-frequency is also reveal that the stronger SOC for non-fullerene bulk-heterojunctions originates from the larger electron cloud deformation electron-hole pairs caused by photoinduced dipole-dipole interactions. Therefore, we think the photoexcitation present a new approach to strengthen the SOC effect in organic solar cells toward better photovoltaic actions. • Non-fullerene based solar cell of ITO/ZnO/Active layer/MoO 3 /Ag shows a higher PCE of 11.30%. • A strong SOC exists in electron-hole pairs at D:A interface in the non-fullerene under lighting. • The strong SOC comes from the larger electron cloud deformation. • Photo-excitation can present a new approach to strengthen the SOC effect in organic solar cells.

Revealing conflicting effects of solvent additives on morphology and performance of non-fullerene organic photovoltaics under different illuminance conditions

The use of solvent additives is effective for controlling the morphology and efficiency of non-fullerene bulk-heterojunction (BHJ) organic photovoltaics (OPVs) under both indoor and outdoor conditions. In this study, we examined the effects of solvent additives on the morphology and photovoltaic operation of non-fullerene OPVs under different illuminance conditions. Solvent additives (e.g., 1,8-diiodooctane) improved molecular packing and charge transport in non-fullerene BHJ layers, increasing the power conversion efficiency (PCE) under 1-sun illumination. However, incorporating the solvent additives promoted the formation of a crystallization-induced roughened surface and an isolated-non-fullerene domain in the intermixing region. These unexpected morphological features increased the degree of trap-assisted charge recombination and leakage current, significantly reducing the PCE of non-fullerene OPVs at low light levels corresponding to indoor lighting. Understanding the effect of solvent additives inspires us to further control the morphological features of the non-fullerene OPVs by fine-tuning the solvent-additive concentration beyond the globally accepted level. This was effective for optimizing the indoor PCE of non-fullerene OPVs (21.71%), which exceeded that of the conventionally accepted reference (18.02%). This study provides guidelines for understanding the critical roles of solvent additives and optimizing the morphology of non-fullerene BHJs for high-performance indoor OPV applications.

Improving Thermal and Photostability of Polymer Solar Cells by Robust Interface Engineering.

As the power conversion efficiency (PCE) of organic photovoltaics (OPVs) approaches 19%, increasing research attention is being paid to enhancing the device's long-term stability. In this study, a robust interface engineering of graphene oxide nanosheets (GNS) is expounded on improving the thermal and photostability of non-fullerene bulk-heterojunction (NFA BHJ) OPVs to a practical level. Three distinct GNSs (GNS, N-doped GNS (N-GNS), and N,S-doped GNS (NS-GNS)) synthesized through a pyrolysis method are applied as the ZnO modifier in inverted OPVs. The results reveal that the GNS modification introduces passivation and dipole effects to enable better energy-level alignment and to facilitate charge transfer across the ZnO/BHJ interface. Besides, it optimizes the BHJ morphology of the photoactive layer, and the N,S doping of GNS further enhances the interaction with the photoactive components to enable a more idea BHJ morphology. Consequently, the NS-GNS device delivers enhanced performance from 14.5% (control device) to 16.5%. Moreover, the thermally/chemically stable GNS is shown to stabilize the morphology of the ZnO electron transport layer (ETL) and to endow the BHJ morphology of the photoactive layer grown atop with a more stable thermodynamic property. This largely reduces the microstructure changes and the associated charge recombination in the BHJ layer under constant thermal/light stresses. Finally, the NS-GNS device is demonstrated to exhibit an impressive T80 lifetime (time at which PCE of the device decays to 80% of the initial PCE) of 2712 h under a constant thermal condition at 65°Cin a glovebox and an outstanding photostability with a T80 lifetime of 2000 h under constant AM1.5G 1-sun illumination in an N2 -controlled environment.

Hole Transfer Prompted by Viscous Oligomer Solid Additives in Non-Fullerene Bulk-Heterojunction Layers

Hole Transfer Prompted by Viscous Oligomer Solid Additives in Non-Fullerene Bulk-Heterojunction Layers

N-doping of nonfullerene bulk-heterojunction organic solar cells strengthens photogeneration and exciton dissociation

N-doping of the bulk-heterojunction active layer to enlarge polymeric donor (D) and nonfullerene acceptor (A) domains, dilate D/A interspace and reduce acceptor exciton binding energy to enhance light harvesting and suppress germinate recombination.

Manipulating Crystallization Kinetics of Conjugated Polymers in Nonfullerene Photovoltaic Blends toward Refined Morphologies and Higher Performances

The introduction of solvent additives has become a general approach in optimizing the active layer morphology in organic photovoltaics (OPV) to achieve high power conversion efficiency. It is of general interest to understand the mechanism of how additives optimize the thin-film nanostructure formation such as crystallization kinetics and phase separation. In the current manuscript, state-of-the-art nonfullerene bulk heterojunction blends, PM6:IT-4F mixture, are used in a slot-die coating experiment. The 1,8-octanedithiol (DIO)-aided fabrication leads to a significant increase in the power conversion efficiency (PCE) from 10.11 to 12.67%. Exciton dissociation and carrier transport have also been improved, largely associated with morphology improvement. Time-resolved crystallization kinetics during PM6:IT-4F film formation under different processing conditions was studied by in situ grazing-incidence wide-angle X-ray scattering (GIWAXS), in which a detailed polymer fibril morphology formation was seen. The Johnson–Mehl–Avrami–Kolmogorov (JMAK) crystallization analysis affords insights that the addition of the DIO additive in a small amount would not only increase the nucleation rate during the nucleation and growth stage but also introduce secondary fibril crystal perfection via burst nucleation-mediated crystallization to a grain boundary-induced crystallization stage change. These observations can be of general interest to the OPV community in manipulating the printed solar cell morphology and efficiency toward the commercial application.

Wide Band-gap Two-dimension Conjugated Polymer Donors with Different Amounts of Chlorine Substitution on Alkoxyphenyl Conjugated Side Chains for Non-fullerene Polymer Solar Cells

In this study, wide bandgap (WBG) two-dimensional (2D) copolymer donors (DZ1, DZ2, and DZ3) based on benzodithiophene (BDT) on alkoxyphenyl conjugated side chains without and with different amounts of chlorine atoms and difluorobenzotriazole (FBTZ) are designed and synthesized successfully for efficient non-fullerene polymer solar cells (PSCs). Three polymer donors DZ1, DZ2, and DZ3 display similar absorption spectra at 300–700 nm range with optional band-gap (Egopt) of 1.84, 1.92, and 1.97 eV, respectively. Compared with reported DZ1 without chlorine substitution, it is found that introducing chlorine atoms into the meta-position of the alkoxyphenyl group affords polymer possessing a deeper the highest occupied molecular orbital (HOMO) energy level, which can increase open circuit voltage (VOC) of PSCs, as well as improve hole mobility. Non-fullerene bulk heterojunction PSCs based on DZ2:MeIC demonstrate a relatively high power conversion efficiency (PCE) of 10.22% with a VOC of 0.88 V, a short-circuit current density (JSC) of 17.62 mA/cm2, and a fill factor (FF) of 68%, compared with PSCs based on DZ1:MeIC (a PCE of 8.26%) and DZ3:MeIC (a PCE of 6.28%). The results imply that adjusting chlorine atom amount on alkoxyphenyl side chains based on BDT polymer donors is a promising approach of synthesizing electron-rich building block for high performance of PSCs.

Synergistic Effects of Polymer Donor Backbone Fluorination and Nitrogenation Translate into Efficient Non-Fullerene Bulk-Heterojunction Polymer Solar Cells.

State-of-the-art non-fullerene bulk-heterojunction (BHJ) polymer solar cells outperform the more extensively studied polymer-fullerene BHJ solar cells in terms of efficiency, thermal-, and photostability. Considering the strong light absorption in the near-infrared region (600-1000 nm) for most of the efficient acceptors, the exploration of high-performing large band gap (LBG) polymer donors with complementary optical absorption ranging from 400 to 700 nm remains critical. In this work, the strategy of concurrently incorporating fluorine (-F) and unsaturated nitrogen (-N) substituents along the polymer backbones is used to develop the LBG polymer donor PB[N][F]. Results show that the F- and N-substituted polymer donor PB[N][F] realizes up to 14.4% efficiency in BHJ photovoltaic devices when paired with a benchmark molecule acceptor Y6, which largely outperforms the analogues PB with an efficiency of only 3.6% and PB[N] with an efficiency of 11.8%. Systematic examinations show that synergistic effects of polymer backbone fluorination and nitrogenation can significantly increase ionization potential values, improve charge transport, and reduce bimolecular recombination and trap-assisted recombination in the PB[N][F]:Y6 BHJ system. Importantly, our study shows that the F- and N-substituted conjugated polymers are promising electron-donor materials for solution-processed non-fullerene BHJ solar cells.

Understanding the Interplay of Transport‐Morphology‐Performance in PBDB‐T‐Based Polymer Solar Cells

Polymer–polymer blends have been reported to exhibit exceptional thermal and ambient stability. However, power conversion efficiencies (PCEs) from devices using polymeric acceptors have been recorded to be significantly lower than those based on conjugated molecular acceptors. Herein, two organic nonfullerene bulk heterojunction (BHJ) blends ITIC:PBDB‐T and N2200:PBDB‐T, together with their fullerene counterpart, PCBM:PBDB‐T, are adopted to understand the effect of electron acceptors on device performance. Free charge carrier properties using time‐resolved microwave conductivity (TRMC) measurements are comprehensively investigated. The nonfullerene devices show an improved PCE of 10.06% and 6.65% in the ITIC‐ and N2200‐based cells, respectively. In comparison, the PCBM:PBDB‐T‐based devices yield a PCE of 5.88%. The optimal N2200:PBDB‐T produced the highest TRMC mobility, longest lifetime, and greatest free‐carrier diffusion length. It is found that such phenomena can be associated with the unfavorable morphology of the all‐polymer BHJ microstructure. In contrast, the solar cells using either the PCBM or ITIC acceptors display a more balanced donor and acceptor phase separation, leading to more efficient free‐carrier separation and transport in the operating device. By sacrificing efficiency for superior stability, it is shown that the improved structure in all‐polymer blend can deliver a more stable morphology under thermal stress.

A Benzo[1,2-b:4,5-c']Dithiophene-4,8-Dione-Based Polymer Donor Achieving an Efficiency Over 16.

It is of great significance to develop efficient donor polymers during the rapid development of acceptor materials for nonfullerene bulk-heterojunction (BHJ) polymer solar cells. Herein, a new donor polymer, named PBTT-F, based on a strongly electron-deficient core (5,7-dibromo-2,3-bis(2-ethylhexyl)benzo[1,2-b:4,5-c']dithiophene-4,8-dione, TTDO), is developed through the design of cyclohexane-1,4-dione embedded into a thieno[3,4-b]thiophene (TT) unit. When blended with the acceptor Y6, the PBTT-F-based photovoltaic device exhibits an outstanding power conversion efficiency (PCE) of 16.1% with a very high fill factor (FF) of 77.1%. This polymer also shows high efficiency for a thick-film device, with a PCE of ≈14.2% being realized for an active layer thickness of 190 nm. In addition, the PBTT-F-based polymer solar cells also show good stability after storage for ≈700 h in a glove box, with a high PCE of ≈14.8%, which obviously shows that this kind of polymer is very promising for future commercial applications. This work provides a unique strategy for the molecular synthesis of donor polymers, and these results demonstrate that PBTT-F is a very promising donor polymer for use in polymer solar cells, providing an alternative choice for a variety of fullerene-free acceptor materials for the research community.

Bithieno[3,4-c]pyrrole-4,6-dione-Mediated Crystallinity in Large-Bandgap Polymer Donors Directs Charge Transportation and Recombination in Efficient Nonfullerene Polymer Solar Cells

Solution-processed nonfullerene bulk-heterojunction (BHJ) polymer solar cells (PSCs), which are composed of polymer donors and organic acceptors, are proven to manifest promising performance and lo...

Internal nanoscale architecture and charge carrier dynamics of wide bandgap non-fullerene bulk heterojunction active layers in organic solar cells

The nanoscale architecture of active layer based on wide bandgap non-fullerene solar cells exhibits pronounced influence by the solvent additive.

5H-Fluoreno [3,2- b:6,7- b’] Dithiophene Based Non-fullerene Small Molecular Acceptors for Polymer Solar Cell Application

Two novel non-fullerene small molecule acceptors were prepared with the conjugated backbone of 5H-fluoreno[3, 2- b:6, 7- b’] dithiophene carrying the electron deficient unit of dicyanomethylene indanone (DICTFDT) and rhodanine (TFDTBR), respectively. The two acceptors exhibited excellent thermal stability and strong absorption in the visible region. The LUMO level is estimated to be at -3.89 eV for DICTFDT and -3.77 eV for TFDTBR. When utilized as the acceptor in bulk heterojunction polymer solar cells with the polymer donor of PBT7-Th, the optimized maximum power conversion efficiency of 5.12% and 3.95% was obtained for the device with DICTFDT and TFDTBR, respectively. The research demonstrates that 5H-fluoreno[3, 2- b:6, 7- b’] dithiophene can be an appealing candidate for constructing small molecular electron acceptor towards efficient polymer:non-fullerene bulk heterojunction solar cells.

Energy level gamut-a wide-angle lens to look at photoelectronic properties of diketopyrrolopyrrole-benzothiadiazole-based small molecules.

Demands in the field of molecular design for optimized bandgap and proper energy levels to obtain high efficiencies are growing progressively in organic electronics. In the present work, we designed a series of molecules based on diketopyrrolopyrrole (DPP) and benzothiadiazoles (BT). We also studied the efeect of the presence and position of the nitrogen atom as an effective heteroatom. Finally, we optimized the energy levels of the designed structures to find the most favorable donor properties along with fullerene and non-fullerene (NF) acceptors in bulk heterojunction (BHJ) solar cell systems. To shed new light on the electronic characteristics of the designed structures, we developed a correction gamut of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. The gamut is a span that predicts the occurrence of practical HOMO or LUMO with high probability from density functional theory computations in the gas phase. The model was validated using experimental energy level values of a similar structure as reference material. The results obtained by the new pathway of combining the idea of energy level gamuts with the modified Scharber model for NF BHJ suggested that the designed structures can afford power conversion efficiencies (PCE) for NF-BHJ of 8.5-10.5%. Graphical abstract Improved approach for predicting power conversion efficiencies (PCE) of designed molecules.

Subphthalocyanine Triimides: Solution Processable Bowl-Shaped Acceptors for Bulk Heterojunction Solar Cells.

Ten subphthalocyanine triimides (SubPcTI) with different substituents at imide sites and B atoms were designed and synthesized. These compounds with low-lying lowest unoccupied molecular orbital energy levels (from -3.91 to -3.98 eV), strong absorption in the range of 450-650 nm, and adjustable solubility are expected to be excellent electron acceptors. Non-fullerene bulk heterojunction organic solar cells based on acceptor 8c showed power conversion efficiency of 4.92%, which is the highest value among subphthalocyanine derivatives.

Multistacked Detectors with Transparency-Controlled Polymer:Nonfullerene Bulk Heterojunction Sensing Layers for Visible Light Communications.

Here, we report multistacked organic photodiodes (OPDs) for potential light-fidelity (Li-Fi) detectors in visible light communications. The multistacked detectors were fabricated by integrating semitransparent inverted-type OPDs with polymer:nonfullerene bulk heterojunction (BHJ) layers. The thickness of BHJ sensing layers in the inverted-type OPDs was controlled to 30, 60, and 100 nm for top, middle, and bottom cells in the multistacked detectors, respectively, while 30 nm thick metal (silver) electrodes were used for securing both electrical conductivity and optical transparency. Results showed that each OPD cell in the multistacked detectors could deliver exact current signals upon illumination with a modulated white light from inorganic light-emitting diode lamp. The current signal from each OPD cell was linearly decreased as the modulation frequency of white light increased, supporting a stable operation of the multistacked detectors. Finally, the present multistacked detectors were able to exactly detect the programmed Li-Fi signals with a particular digitized information.

Rationalizing device performance of perylenediimide derivatives as acceptors for bulk-heterojunction organic solar cells

Abstract This study examines the correlation between the structure of four perylenediimide (PDI) derivatives and their optoelectronic behaviors. Three of them with one, two or four PDI moieties conjugated peripherally to porphyrin core by phenylene linker, denoted as nPDI-ZnP (n = 1,2,4), are investigated as acceptors on the charge transport property and device performance in non-fullerene (NF) bulk-heterojunction (BHJ) organic photovoltaic cells with a commercial polymer PTB7-Th as the donor material. Besides, the parent PDI molecule is studied as a control. Detailed charge carrier transport analysis is carried out on these PTB7-Th:Acceptor BHJs. Among all acceptor molecules, PTB7-Th:4PDI-ZnP film possesses the most desirable transport properties among the NF BHJs, and the best device performance with a power conversion efficiency (PCE) of 8.35%. To quantify the transport behaviors, the charge imbalance factor Δ is introduced to describe the global electron and hole balance in different electric fields. We discover that PTB7-Th:4PDI-ZnP has the least Δ and thereby gives rise to a BHJ cells with the highest fill factor and PCE.