Performance Of Organic Solar Cells Research Articles

19.0% Efficiency Binary Organic Solar Cells Enabled by Using a Building Block as Solid Additive

The morphology regulation of active layer has an important role in the development of organic solar cells (OSCs), which can improve device performance and prolong device stability. Herein, we propose the commonly used donors building block, benzo[1,2-b:4,5-b']dithiophene (BDT), as a novel volatile solid additive into PM6:Y-series solar cells. BDT can be well mixed with accepter Y6 by molecular interaction. Therefore, compared with the control film, the BDT-processed film achieves improved aggregation and enhanced crystallinity. Owing to the synergetic effects on the phase separation and molecular arrangement, the device processed with BDT exhibits the significantly enhanced exciton dissociation, charge transfer and collection along with the suppressed bimolecular recombination. Consequently, the device based on PM6:Y6 with BDT-processing obtains the 17.91% power conversion efficiency (PCE) and T90 ＞ 3000h shelf-life. Meantime, the universality of additive strategy has also been verified in several Y-series accepter OSCs. Impressively, an outstanding PCE of 19.01% is achieved for the BDT-processed PM6:L8BO OSCs, which is one of the highest reported values of PM6:L8BO system. Eventually, this work provides a facile and efficacious approach to manipulate the active layer morphology, improve the photovoltaic performance and stability of OSCs.

Developing Efficient Benzene Additives for 19.43% Efficiency of Organic Solar Cells by Crossbreeding Effect of Fluorination and Bromination

AbstractEmploying volatile solid additives have emerged as a promising method to optimize the morphology and improve the performance of organic solar cells (OSCs). However, principles governing the efficient design of solid additives remain elusive. Herein, the programmed fluorination and/or bromination on benzene core to develop efficient additives for OSCs is reported. The programmed fluorination and/or bromination endow the five halogen benzene derivatives, 1,3,5‐trifluorobenzene, hexafluorobenzene, 1,3,5‐tribromo‐2,4,6‐trifluorobenzene (TFTB), 1,3,5‐tribromobenzene, and hexabromobenzene, with different melting and boiling points, volatility, as well as interactions with the host blend. Studies indicate that the additives with extremely high and low volatility are almost powerless and even detrimental to the morphology evolution. Among them, the combination of fluorine and bromine atoms on TFTB not only enables the more appropriate m.p./b.p. and volatility, but also exerts stronger molecular interactions with the host blend, giving rise to higher ordered molecular packing and more favorable morphology. Importantly, TFTB exhibits good universality to optimize the performances of OSCs with high power conversion efficiencies (PCEs; over 18%) in a group of binary blend systems, and an impressive PCE of 19.43% in the ternary PBTz‐F:PM6:L8‐BO system.

PC71BM as Morphology Regulator for Highly Efficient Ternary Organic Solar Cells with Bulk Heterojunction or Layer-by-Layer Configuration.

The ternary strategy is one of the effective methods to regulate the morphology of the active layer in organic solar cells (OSCs). In this work, the ternary OSCs with bulk heterojunction (BHJ) or layer-by-layer (LbL) active layers are prepared by using the polymer donor PM6 and the non-fullerene acceptor L8-BO as the main system and the fullerene acceptor PC71BM as the third component. The power conversion efficiencies (PCEs) of BHJ OSCs and LbL OSCs are increased from 17.10% to 18.02% and from 17.20% to 18.20% by introducing PC71BM into the binary active layer, respectively. The in situ UV-vis absorption spectra indicate that the molecular aggregation and crystallization process can be prolonged by introducing PC71BM into the PM6:L8-BO or PM6/L8-BO active layer. The molecular orientation and molecular crystallinity in the active layer are optimized by introducing the PC71BM into the binary BHJ or LbL active layers, which can be confirmed by the experimental results of grazing incidence wide-angle X-ray scattering. This study demonstrates that the third component PC71BM can be used as a morphology regulator to regulate the morphology of BHJ or LbL active layers, thus effectively improving the performance of BHJ and LbL OSCs.

Exploiting the donor‐acceptor‐additive interaction's morphological effect on the performance of organic solar cells

AbstractOrganic solar cells (OSCs) have demonstrated over 19% power conversion efficiency (PCE) with the help of material innovation and device optimization. Co‐working with newly designed materials, traditional solvent additives, 1‐chloronaphthalene (CN), and 1,8‐diodooctane (DIO) are still powerful in morphology modulation towards satisfying efficiencies. Here, we chose recently reported high‐performance polymer donors (PM6 & D18‐Fu) and small molecular acceptors (Y6 & L8‐BO) as active layer materials and processed them by different conditions (CN or DIO or none). Based on corresponding 12 groups of device results, and their film morphology characterizations (both ex‐situ and in‐situ ones), the property‐performance relationships are revealed case by case. It is thereby supposed to be taken as a successful attempt to demonstrate the importance and complexity of donor‐acceptor‐additive interaction, since the device performance and physics analyses are also tightly combined with morphology variation. Furthermore, ternary blend construction for PCE improvement provides an approaching 19% level and showcases the potential of understanding‐guided‐optimization (UGO) in the future of OSCs.

Synergistic direct and indirect central unit fluoridation of non-fullerene acceptor enables high-efficiency organic solar cells

The fluorination strategy for non-fullerene acceptors (NFAs), involving direct and indirect fluorination such as trifluoromethylation, has been established as an effective method to improve the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Although end-group fluorination has achieved significant success, the fluorination of central units in NFAs has received limited exploration owing to the lack of substitution sites in conventional Y-series acceptors. To systematically investigate the effect of different central unit fluorination strategies on molecular stacking and subsequently the performance of OSCs, we utilized recently developed phenazine (CH)-series acceptors to design a series of NFAs, namely CH-F, CH-CF, and CH-FCF, which are featured with central unit mono-fluorination (CH-F), mono-trifluoromethylation (CH-CF), and synergistic fluorination and trifluoromethylation (CH-FCF), respectively. Among them, CH-FCF-based blend films (PM6/CH-FCF) demonstrate strengthened molecular interactions and crystallinity, an excellent fibrillar network morphology and bettered charge generation/transport performance. Consequently, the binary OSCs based on CH-FCF achieved the best PCE of 18.41 %, which outperforms its counterparts based on CH-F (17.34 %) and CH-CF (17.62 %). These results emphasize the significance of synergistically regulating the central unit of small molecule acceptors through different fluorination strategies to manipulate molecular packing and thus enhance the photovoltaic performance of OSCs.

Improved Short-Circuit Current and Fill Factor in PM6:Y6 Organic Solar Cells through D18-Cl Doping

Based on the PM6:Y6 binary system, a novel non-fullerene acceptor material, D18-Cl, was doped into the PM6:Y6 blend to fabricate the active layer. The effects of different doping ratios of D18-Cl on organic solar cells were investigated. The best-performing organic solar cell was achieved when the doping ratio of D18-Cl reached 20 wt%. It exhibited a short-circuit current of 28.13 mA/cm2, a fill factor of 70.25%, an open-circuit voltage (Voc) of 0.81 V, and a power conversion efficiency of 16.08%. The introduction of an appropriate amount of D18-Cl expanded the absorption spectrum of the active layer, improved the morphology of the active layer, reduced large molecular aggregation and defects, minimized bimolecular recombination, and optimized the collection efficiency of charge carriers. These results indicate the critical importance of selecting an appropriate third component in binary systems and optimizing the doping ratio to enhance the performance of ternary organic solar cells.

Fluorenyl-based polyurethane efficiently improves the flexibility and photovoltaic performance of organic solar cells

Introducing proper insulating polymer in the active layer can improve the photovoltaic and mechanical properties of organic solar cells. Here, we design and synthesize a polyurethane insulating material K-BA20 (or 50) containing PDMS and 2D aryl block 9,9-diphenyl-9H-fluorene (DPF). Isophorone diisocyanate (IPDI) is selected as the connection unit and hydrogen bond is introduced to reduce the chain slippage caused by excessive one-dimensional structure. This strategy can enhance the adhesion between components, which can successfully improve the photovoltaic performance and mechanical properties of the devices. When 5 and 15 wt% K-BA20 was introduced into the PM6:Y6-based system, the short-circuit current density (JSC) of 26.71 and 26.65 mA cm−2, and the power conversion efficiency (PCE) of 16.67% and 16.05% were obtained. More interestingly, the active layer introduced K-BA20 with low DPF ratio has good stretchable properties, while K-BA50 with high DPF ratio has more stable flexibility. The breaking elongation of the blend film with introducing 15 wt% K-BA20 was increased 3.2-fold, from 5.81% to 18.46%, and the PM6:Y6-based flexible devices after doping 5 wt% K-BA50 can still maintain half of the initial efficiency under 600 bending fatigues. Hence, this work shows that introducing K-BA20 (or 50) into PM6:Y6 -based devices can bring out a series of positive feedbacks in terms of charge recombination, crystallinity and molecular interaction. It can be seen that constructing new insulating polyurethane materials with rigid and flexible segments is an effective strategy for development of flexible organic solar cells.

Theoretical Insights into the Effect of Conjugated Cores on Optical and Photovoltaic Properties of Small Molecule Acceptors for Organic Solar Cells

AbstractThe research of organic photovoltaic materials based on small molecules is receiving high attention from researchers of organic solar cells (OSCs) to continuously improve their power conversion efficiency (PCE). To enhance the PCE, we have designed four small molecule acceptors P3–P6 based on reported IDT(TCV)2 (P1) and IDTT(TCV)2 (P2), with different central aromatic fused rings, and tricyanoethylene (TCV) as the terminal acceptor unit. The ground‐state properties, excited‐state properties and photovoltaic properties of donor/acceptor blends are calculated by density functional theory and time‐dependent density functional theory. Also, the relationship between their photoelectric and structural properties has been discussed. The calculation results show that P5 and P6 can simultaneously increase open‐circuit voltages (Voc), fill‐factors (FF), and short circuit current (Jsc) due to their high lowest unoccupied molecular orbital, small energy gap and high charge collection efficiency. In terms of charge‐transfer mechanism, PTB7‐Th/P1–P6 can generate CT states through direct excitation and hot exciton, meanwhile there exist more opportunities of producing CT states via intermolecular electric field mechanism. Our results will provide theoretical guidance for further designing and synthesizing of acceptors to improve the performance of OSCs.

Non-fullerene acceptors with thiazole π-bridge for organic solar cells

Intramolecular noncovalent conformational locks (NoCLs) have been found to be effective in increasing the rigidity and planarity of the molecular backbone. This, in turn, leads to reduced recombination energy and improved intramolecular charge transport mobility, resulting in significant enhancements in the photovoltaic performance of organic solar cell (OSC) devices. To take advantage of this effect, we designed and synthesized two small molecule non-fullerene acceptors (NFAs) with A-π-D-π-A structures using thiazole (Tz): IDTz-IC4F and IDTz-IC4Cl. With the synergistic assistance of NoCLs and halogenated end groups, the NFAs IDTz-IC4F and IDTz-IC4Cl exhibited power conversion efficiencies (PCEs) of 6.20 % and 6.06 %, respectively. This study presents a strategy for enhancing the PCE of OSCs by incorporating cooperative NoCLs and halogen end groups, providing an exciting avenue for designing high-performance non-fullerene materials in the future.

Recent Advances of Benzodithiophene-Based Donor Materials for Organic Solar Cells.

Recently, the power conversion efficiency (PCE) of organic solar cells (OSCs) has increased dramatically, making a big step toward the industrial application of OSCs. Among numerous OSCs, benzodithiophene (BDT)-based OSCs stand out in achieving efficient PCE. Notably, single-junction OSCs using BDT-based polymers as donor materials have completed a PCE of over 19%, indicating a dramatic potential for preparing high-performance large-scale OSCs. This paper reviews the recent progress of OSCs based on BDT polymer donor materials (PDMs). The development of BDT-based OSCs is concisely summarized. Meanwhile, the relationship between the structure of PDMs and the performance of OSCs is further described in this review. Besides, the development and prospect of single junction OSCs are also discussed.

An Approach Towards Low Energy Loss by End-capped Modification of A2–D–A1–D–A2-type Molecules for Tuning the Photovoltaic Properties of Organic Solar Cells

In this project, the study is focused on the substitution at determined sites of reference molecule (R) to enhance optoelectronic properties, which might affect the photovoltaic performance of organic solar cells. Herein, seven new molecules (M1 to M7) were designed by the modification of the terminal acceptors. The density functional theory was employed to explore optoelectronic properties. Majority of the molecules own exceptional optical properties than reference molecule, for instance, narrow bandgap, low excitation energy, lower binding energies, better oscillator strength and progressive light harvesting efficiency. The density of state, frontier molecular orbitals and transition density matrix diagrams indicated that the charge density transfer occurs from donor to acceptor. Moreover, charge transfer investigations of designed molecules with PTB7-Th complex were performed by analyzing the concentration of charge transfer over molecular orbitals, i.e., highest occupied to lowest unoccupied molecular orbitals. Thus, these computed molecules may be employed to develop efficient organic solar cell devices with promising photovoltaic prospects in the near future.

Investigation of Hole-Transfer Dynamics through Simple EL De-Convolution in Non-Fullerene Organic Solar Cells.

In conventional fullerene-based organic photovoltaics (OPVs), in which the excited electrons from the donor are transferred to the acceptor, the electron charge transfer state (eECT) that electrons pass through has a great influence on the device's performance. In a bulk-heterojunction (BHJ) system based on a low bandgap non-fullerene acceptor (NFA), however, a hole charge transfer state (hECT) from the acceptor to the donor has a greater influence on the device's performance. The accurate determination of hECT is essential for achieving further enhancement in the performance of non-fullerene organic solar cells. However, the discovery of a method to determine the exact hECT remains an open challenge. Here, we suggest a simple method to determine the exact hECT level via deconvolution of the EL spectrum of the BHJ blend (ELB). To generalize, we have applied our ELB deconvolution method to nine different BHJ systems consisting of the combination of three donor polymers (PM6, PBDTTPD-HT, PTB7-Th) and three NFAs (Y6, IDIC, IEICO-4F). Under the conditions that (i) absorption of the donor and acceptor are separated sufficiently, and (ii) the onset part of the external quantum efficiency (EQE) is formed solely by the contribution of the acceptor only, ELB can be deconvoluted into the contribution of the singlet recombination of the acceptor and the radiative recombination via hECT. Through the deconvolution of ELB, we have clearly decided which part of the broad ELB spectrum should be used to apply the Marcus theory. Accurate determination of hECT is expected to be of great help in fine-tuning the energy level of donor polymers and NFAs by understanding the charge transfer mechanism clearly.

A Theoretical Study on the Underlying Factors of the Difference in Performance of Organic Solar Cells Based on ITIC and Its Isomers.

Recently, non-fullerene-based organic solar cells (OSCs) have made great breakthroughs, and small structural differences can have dramatic impacts on the power conversion efficiency (PCE). We take ITIC and its isomers as examples to study their effects on the performance of OSCs. ITIC and NFBDT only differed in the side chain position, and they were used as models with the same donor molecule, PBDB-T, to investigate the main reasons for the difference in their performance in terms of theoretical methods. In this work, a detailed comparative analysis of the electronic structure, absorption spectra, open circuit voltage and interfacial parameters of the ITIC and NFBDT systems was performed mainly by combining the density functional theory/time-dependent density functional theory and molecular dynamics simulations. The results showed that the lowest excited state of the ITIC molecule possessed a larger ∆q and more hybrid FE/CT states, and PBDB-T/ITIC had more charge separation paths as well as a larger kCS and smaller kCR. The reason for the performance difference between PBDB-T/ITIC and PBDB-T/NFBDT was elucidated, suggesting that ITIC is a superior acceptor based on a slight modulation of the side chain and providing a guiding direction for the design of superior-performing small molecule acceptor materials.

High efficiency ternary organic solar cells based on two nonfused ring electron acceptors with similar molecular structure

Organic solar cells (OSCs) based on nonfused ring electron acceptors (NFREAs) have shown great potential for future commercial applications due to their reduced synthesis complexity and low cost, and ternary strategy is a promising path for further boosting the development of NFREAs-based OSCs. Herein, two NFREAs FO-4Cl and FO-N with similar molecular structure are employed to fabricate high-performance ternary OSCs via combining a commonly used polymer donor PBDB-T. The PBDB-T:FO-4Cl and PBDB-T:FO-N binary devices can exhibit PCEs of 12.05% and 11.33%, respectively. Incorporating FO-N into the binary blend PBDB-T:FO-4Cl, the PBDB-T:FO-4Cl:FO-N ternary device can achieve a significantly improved PCE of 13.15% with a Voc of 0.87 V, a Jsc of 20.59 mA/cm2 and an FF of 73.02%, which is 9% and 16% higher than the PBDB-T:FO-4Cl and PBDB-T:FO-N based binary devices, respectively. The enhanced Jsc and FF can be ascribed to the enhanced photon harvesting, improved film morphology and high and balanced charge transport. Meanwhile, the Voc of ternary devices are gradually improved after the introduction of the high-lying LUMO component FO-N, which demonstrates that the two acceptors may form alloy state in the blend system. Our results demonstrate that ternary strategy is an effective path for further boosting the performance of NFREA-based OSCs by incorporating isogenous acceptors.

Unraveling the Impact of Thickness on Active Layer Morphology and Device Performance of Semitransparent Organic Solar Cells: A Comprehensive Study

Unraveling the Impact of Thickness on Active Layer Morphology and Device Performance of Semitransparent Organic Solar Cells: A Comprehensive Study

Surface Modification of ITO with N-Heterocyclic Carbene Precursors Results in Electron Selective Contacts in Organic Photovoltaic Devices.

Surface modification of indium tin oxide (ITO) electrodes with organic molecules is known to tune their work function which results in higher charge carrier selectivity in corresponding organic electronic devices and hence influences the performance of organic solar cells. In recent years, N-heterocyclic carbenes (NHCs) have also been proven to be capable to modify the work function of metals and semimetals compared to the unfunctionalized surface via the formation of strong covalent bonds. In this report, we have designed and performed the modification of the ITO surface with NHC by using the zwitterionic bench stable IPr-CO2 as the NHC precursor, applied via spin coating. Upon modification, the work function of ITO electrodes was reduced significantly which resulted in electron selective contacts in corresponding organic photovoltaic devices. In addition, various characterization techniques and analytical methods are used to elucidate the nature of the bound species and the corresponding binding mechanism of the material to the ITO surface.

Improving the Performance of Layer-by-Layer Organic Solar Cells by n-Doping of the Acceptor Layer.

Molecular dopants can effectively improve the performance of organic solar cells (OSCs). Here, PM6/BTP-eC9-4Cl-based OSCs are fabricated by a layer-by-layer (LbL) deposition method, and the electron acceptor BTP-eC9-4Cl layer is properly doped by n-type dopant benzyl viologen (BV) or [4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl]dimethyl-amine (N-DMBI-H). The power conversion efficiency (PCE) of OSCs increases from 16.80 to 17.61 or 17.84% when the acceptor layer is doped by BV (0.01 wt %) or N-DMBI-H (0.01 wt %), respectively. At the optimal doping concentration, the device exhibits more balanced charge transport, fewer bimolecular recombinations, faster charge separation and transfer, and better stability. This doping strategy has good universality; when the acceptor layer L8-BO of LbL OSCs is doped by 0.01 wt % BV or 0.01 wt % N-DMBI-H, the PCE increases from 17.49 to 18.35 or 18.25%, respectively. All in all, our studies have demonstrated that the doping strategy is effective in enhancing the performance of OSCs.

Enhanced Performance and Stability of Q‐PHJ Devices through Strategic Placement of Dimerized Acceptors

AbstractOligomeric acceptors are an effective π‐conjugate extension for small molecules because they effectively combine the advantages of polymeric and small molecular acceptors. The choice of linker group and oligomer position is however crucial for the efficient expression of π‐expansion advantages. Here, vinylene is chosen as the linker group and three oligomeric acceptors are produced with different connecting positions or end‐groups. Under illumination, the dimer acceptor dBTIC‐γV‐BO achieves the best power conversion efficiencies of 17.14% and a T80 lifetime of 2150 h amongst quasi‐planar heterojunction devices, which are much higher and more stable than the directly connected dimers dBTICγ‐EH or dBTIC‐δV‐BO. These results indicate that the oligomerization of small molecules with an appropriate linker group in the γ‐position is an effective strategy with which to improve the photovoltaic performance and stability of organic solar cells, and significantly promote their commercialization.

Molecular Stacking and Aggregation Optimization of Photoactive Layer through Solid Additive Enables High‐Performance Organic Solar Cells

AbstractRegulating molecular packing and aggregation of photoactive layer is a critical but challenging issue in developing high‐performance organic solar cells. Herein, two structurally similar analogues of anthra[2,3‐b : 6,7‐b′]dithiophene (ADT) and naphtho[1,2‐b : 5,6‐b′]dithiophene (NDT) are developed as solid additive to exploit their effect in regulating the molecular aggregation and π‐stacking of photoactive layer. We clarify that the perpendicular arrangements of NDT can enlarge the molecular packing space and improve the face‐on stacking of Y6 during the film formation, favoring a more compact and ordered long‐range π‐π stacking in the out‐of‐plane direction after the removal of NDT under thermal annealing. The edge‐to‐face stacked herringbone‐arrangement of ADT along with its non‐volatilization under thermal annealing can induce the coexistence of face‐on and edge‐on stacking of blend film. As a result, the NDT treatment shows encouraging effect in improving the photovoltaic performance of devices based on various systems. Particularly, a remarkable PCE of 18.85 % is achieved in the PM6 : L8‐BO‐based device treated by NDT additive, which is a significant improvement with regard to the PCE of 16.41 % for the control device. This work offers a promising strategy to regulate the molecular packing and aggregation of photoactive layer towards significantly improved performance and stability of organic solar cells.

Molecular Stacking and Aggregation Optimization of Photoactive Layer through Solid Additive Enables High-Performance Organic Solar Cells.

Regulating molecular packing and aggregation of photoactive layer is a critical but challenging issue in developing high-performance organic solar cells. Herein, two structurally similar analogues of anthra[2,3-b:6,7-b']dithiophene (ADT) and naphtho[1,2-b:5,6-b']dithiophene (NDT) are developed as solid additive to exploit their effect in regulating the molecular aggregation and π-stacking of photoactive layer. We clarify that the perpendicular arrangements of NDT can enlarge the molecular packing space and improve the face-on stacking of Y6 during the film formation, favoring a more compact and ordered long-range π-π stacking in the out-of-plane direction after the removal of NDT under thermal annealing. The edge-to-face stacked herringbone-arrangement of ADT along with its non-volatilization under thermal annealing can induce the coexistence of face-on and edge-on stacking of blend film. As a result, the NDT treatment shows encouraging effect in improving the photovoltaic performance of devices based on various systems. Particularly, a remarkable PCE of 18.85% is achieved in the PM6:L8-BO-based device treated by NDT additive, which is a significant improvement with regard to the PCE of 16.41% for the control device. This work offers a promising strategy to regulate the molecular packing and aggregation of photoactive layer towards significantly improved performance and stability of organic solar cells.