Performance Of Organic Solar Cells Research Articles

Unveiling the influence of ferromagnetic Gd-doped ZnO films on the performance of organic solar cells

Organic solar cells (OSCs) have emerged as a promising technology for renewable energy generation due to their low cost, lightweight, flexibility, and compatibility with roll-to-roll manufacturing. However, OSCs still face challenges in achieving high power conversion efficiency (PCE) due to various factors, including recombination loss. In this study, we investigated the effect of introducing a layer of eight atomic percent gadolinium-doped zinc oxide (Zn0.92Gd0.08O) between the poly(3-hexylthiophene) (P3HT): [6,6]-phenyl butyric acid methyl ester (PCBM) active layer and the fluorine-doped tin oxide (FTO) electrode of the OSC. The reference cell, which has an Au/P3HT:PCBM/ZnO/FTO structure, exhibits a PCE of 0.52%. Remarkably, when the Zn0.92Gd0.08O layer was inserted (Au/P3HT:PCBM/Zn0.92Gd0.08O/FTO), the PCE increased significantly to 3.42%, which is more than six times the increase. Through further analysis, we present that the insertion of the Zn0.92Gd0.08O layer induces spin polarization in the P3HT:PCBM layer, leading to enhanced charge transport and reducing the recombination rate. Based on the findings, it can be concluded that the Zn0.92Gd0.08O film can potentially improve OSC performance.

Correlation of Functional Coumarin Dye Structure with Molecular Packing and Organic Solar Cells Performance

Nowadays, ternary organic solar cells (OSCs) based on all‐small molecules have the convenient strategy toward high‐performance and stable devices. Herein, two coumarin‐based dyes C4 and C4–CN are designed and synthesized with a simple synthetic route. The molecules display different intra‐ and intermolecular interactions in the solid state originating from acceptor substitution, as revealed by the single‐crystal structure analysis. The introduction of dicyanomethylene acceptor group in C4–CN plays a synergistic role in molecular packing and orientation in the solid state, thus fostering the elongation of exciton lifetime and improved charge transport in photoactive layer. The optimized C4–CN:AQx–3‐based binary OSCs show a power conversion efficiency (PCE) of 15.07%, which is appreciably higher than the C4:AQx–3‐based OSC (11.39%) due to the appropriate energy‐level alignment, balanced charge transport, favorable phase separation, and lower charge recombination. Furthermore, by introduction of C4–CN as the third component into C4:AQx–3 blend, the ternary OSC shows an improved PCE of 15.34%, which is linked with the enhanced crystallization, optimized morphology, balanced charge carrier mobility, and suppressed recombination with low energy loss.

Building-up relations between intra- and intermolecular interactions, miscibility, and performance for low-cost, efficient fully non-fused acceptor-based organic solar cells

Although the molecular electronic forces (e.g., intra- and intermolecular interactions) within active layers largely govern the device performance of organic solar cells (OSCs), they are complicated and less understood. In this study, we have synthesized two low-cost isomeric non-fused acceptors (TT-Naph1 and TT-Naph2) with 1-naphthyl and 2-naphthyl aromatic chains, respectively and quantified the molecular interaction−photovoltaic performance relationship. Benefiting from the enhanced dipole moment, TT-Naph2 possesses a strong dipole–dipole intermolecular interaction, while the improved backbone planarity endows TT-Naph1 with a strengthened intramolecular charge-transfer effect, which can regulate the desired blend morphology with the D18 donor polymer as a result of its low miscibility with D18. Less miscible nanostructures are more pronounced in the layer-by-layer (LBL) systems than bulk heterojunction (BHJ) ones, increasing the power conversion efficiencies (PCEs) in the sequence of TT-Naph2 BHJ < TT-Naph1 BHJ < TT-Naph2 LBL < TT-Naph1 LBL. Notably, a ternary LBL OSC based on TT-Naph1 achieved remarkable PCE of 18.41%, one of the top values for LBL-type OSCs. Our findings provide insights into the controlling effect of intra- and intermolecular interactions on the active layers for efficient non-fused acceptor-based OSCs.

Effects of Acceptors on the Charge Photogeneration Dynamics of PM6-Based Solar Cells

In this work, we investigated the effects of different acceptors (IT−4F and PC71BM) on the charge dynamics in PM6-based solar cells. The correlation between different acceptors and the performance of organic solar cells was studied by atomic force microscope, steady-state absorption spectrum, transient absorption spectrum, and electrical measurements. Optical absorption exhibited that IT−4F has strong absorption in the near-infrared region for the active layer. Transient absorption measurements showed that different acceptors (IT−4F and PC71BM) had a significant influence on the behaviors of PM6 excitons and charge dynamics. That is, the exciton dissociation rate and delocalized polaron transport in the PM6:IT−4F active layer were significantly faster than that in the PM6:PC71BM active layer. The lifetime of localized polaron in the PM6:PC71BM active layer was longer than that in the PM6:IT−4F active layer. Conversely, the lifetime of delocalized polaron in the PM6:IT−4F active layer was longer than that in the PM6:PC71BM active layer. Electrical measurement analysis indicated that lower bimolecular recombination, higher charge transport, and charge collection ability were shown in the PM6:IT−4F device compared with the PM6:PC71BM device. Therefore, PM6:IT−4F solar cells achieved a higher power conversion efficiency (12.82%) than PM6:PC71BM solar cells (8.78%).

(Invited) Making Stacks of Nitrogenated Polycyclic Aromatic Hydrocarbons with Hydrogen Bonds

Architectures constituted of stacked aromatics play a crucial role in the development of function in biomacromolecules. Recreating this feature on synthetic systems would not only allow understanding and reproducing biological functions but also developing new functions. For instance, the performance of organic field-effect transistors, light-emitting diodes, and solar cells critically depends on the charge transport efficiency of stacks of organic semiconductors. Among different packing configurations, lamellar π-stacking has been identified as an optimal configuration for electronic transport, because of the efficient intermolecular electronic coupling.Nitrogenated polycyclic aromatic hydrocarbons and nanographenes have attracted considerable attention in recent years. Besides the effect of nitrogen doping on the electronic structure, sp2 nitrogens can be engaged in hydrogen bonding interactions as acceptors. We have developed the synthesis of novel architectures constituted of stacked nitrogenated polycyclic aromatic hydrocarbons held together by hydrogen bonds, which include new families of pseudorotaxanes, rotaxanes, foldamers and supramolecular polymers. The synthesis and self-assembling properties of these systems will be discussed together with their charge transporting properties.

(Invited) Non-Polymer Organic Solar Cells: Microscopic Phonon Control to Suppress Non-Radiative Voltage Loss Via Charge-Separated State

Recent remarkable developments on non-fullerene solar cells have reached photoelectric conversion efficiency (PCE) of 18% by tuning the band energy levels in small molecular acceptors. In this respect, development of the small donor molecule is essential. Although several methods have been applied to elucidate the elementary processes of charge carriers in BHJ-OSCs, mechanistic details of non-radiative voltage loss are poorly understood originating from interfacial heterogeneous electron-hole pairs.A recent time-resolved electron paramagnetic resonance (TREPR) study suggested an impact of the recombination process generating the triplet excitons in non-fullerene acceptor solar cells,[1] while geometries and motions of charge-separated (CS) states were elucidated in details by electron spin polarization (ESP).[2-4] Furthermore, although effects of low-frequency nuclear motions (phonon) on the photocarrier generations have been discussed in several literatures,[2,5,6] microscopic origin of the phonon playing a role on the OPV performance is unclear including impact of the phonon assist on primary recombination processes. Here we systematically investigated mechanisms of solar cell performance with diketopyrrolopyrrole (DPP)–tetrabenzoporphyrin (BP) conjugates of C4-DPP–H2BP and C4-DPP–ZnBP where C4 represents butyl group substituted at the DPP unit as small p-type molecules in non-polymer organic solar cells (OSC) with an acceptor of [6,6]-phenyl-C61-buthylic acid methyl ester. We clarified microscopic origins of the photocarrier caused by phonon-assisted 1D electron-hole dissociations at donor:acceptor interface. Using a time-resolved electron paramagnetic resonance, we have characterized controlled charge-recombination by manipulating disorders in π-π donor stacking, ensuring carrier transport through face-on molecular conformations to suppress non-radiative voltage loss via capturing specific interfacial radical pairs separated by 1.8 nm in bulk-heterojunction solar cells. While disordered lattice motions from the π-π stackings via zinc ligation are essential to enhance the entropy for charge-dissociations at the interface, too much ordered crystallinity causes backscattering phonon to reduce OSC performances.

A DFT study for improving the photovoltaic performance of organic solar cells by designing symmetric non-fullerene acceptors by quantum chemical modification on pre-existed LC81 molecule

Minimizing the energy loss and improving the open circuit voltage of organic solar cells is still a primary concern for scientists working in this field. With the aim to enhance the photovoltaic performance of organic solar cells by minimizing energy loss and improving open circuit voltage, seven new acceptor molecules (LC1-LC7) are presented in this work. These molecules are designed by modifying the terminal acceptors of pre-existed “LC81” molecule based on an indacinodithiophene (IDT) fused core. The end-group modification approach is very fruitful in ameliorating the efficacy and optoelectric behavior of OSCs. The newly developed molecules presented remarkable improvements in performance-related parameters and optoelectronic properties. Among all designed molecules, LC7 exhibited the highest absorption maxima (λmax = 869 nm) with the lowest band-gap (1.79 eV), lowest excitation energy (Ex = 1.42 eV), lowest binding energy, and highest excited state lifetime (0.41 ns). The newly designed molecules LC2, LC3, and LC4 exhibited remarkably improved Voc that was 1.84 eV, 1.82 eV, and 1.79 eV accordingly, compared to the LC81 molecule with Voc of 1.74 eV LC2 molecule showed significant improvement in fill factor compared to the previously presented LC81 molecule. LC2, LC6, and LC7 showed a remarkable reduction in energy loss by showing Eloss values of 0.26 eV, 0.18 eV, and 0.25 eV than LC81 molecule (0.37 eV). These findings validate the supremacy of these developed molecules (especially LC2) as potential components of future OSCs.

Synthetic Strategy for the Development of Conjugated Polyelectrolytes as Cathode Interfacial Layers: Toward Sustainable Organic Devices

AbstractThe use of conjugated polyelectrolytes (CPEs) as interfacial layers (ILs) to increase the efficiency of organic light‐emitting diodes (OLEDs), organic solar cells (OSCs), and organic transistors is a well‐established strategy. Here, the rational process is reported that led to develop the bifunctional poly[(2,7‐(9,9’‐bis(6’‐diethoxylphosphorylhexyl)‐fluorene)‐alt‐(2,7‐(9,9′‐bis(6″‐trimethylammonium bromide)hexyl)‐fluorene)] (PF‐NBr‐EP). PF‐NBr‐EP is successfully incorporated as a cathode IL (CIL) in diverse electronic and optoelectronic devices. The properties of two fluorene‐based CPEs containing, respectively, phosphonate (EP) and ammonium (NBr) moieties used as building blocks for the synthesis of the PF‐NBr‐EP copolymer and their mixtures are investigated to evaluate the combined effect of the two moieties and therefore to gain insight into the behavior of PF‐NBr‐EP chemical structure in the devices. Additionally, the performance of OLEDs and OSCs is analyzed based on established active layers, incorporating all these neat and mixed CILs.

Enhanced performance of organic solar cells through incorporation of MoS2 nanoparticles in bulk heterojunction layer

In this work, MoS2 nanoparticles were incorporated in P3HT:PC61BM bulk heterojunction (BHJ) layers and the effect of their concertation on BHJ morphology and structure, its optical absorption spectra and photovoltaic characteristics of P3HT:PC61BM BHJ solar cells were investigated in detail. MoS2 nanoparticles were deposited by laser ablation of MoS2 powders in chlorobenzene. AFM and optical absorption study indicates that an addition of MoS2 nanoparticles in BHJ results in an improved crystallinity and a decreased defect density. The volt-ampere measurements of BHJ solar cells shows that the increase in the MoS2 concertation up to 0.5 wt% leads to the boost of photovoltaic characteristics and a champion device, based on BHJ with MoS2 nanoparticles at concertation 0.5 wt %, generated the power conversion efficiency about 3%, which 3 times higher than an efficiency of a device based on pristine BHJ. The impedance spectroscopy (IS) technique was used to understand the effect of MoS2 nanoparticles on the charge transport mechanism. IS study revealed that a resistance of the BHJ layer slightly and steadily grows by increasing the concentration of MoS2 nanoparticles, whereas a recombination resistance determining the charge recombination rate increases reaching maximum value at the MoS2 concertation of 0.5 wt %. A further growth in the MoS2 concertation results in a decrease of the recombination resistance indicating an acceleration of charge recombination rate. The influence of conductivity nature of MoS2 nanoparticles and their distribution in the BHJ layer on charge transport mechanism is also discussed.

Benzotriazole-based 3D 4-Arm Small Molecules Enable 19.1% Efficiency for PM6:Y6-based Ternary Organic Solar Cells.

The third component featuring a planar backbone structure similar to the binary host molecule has been the empirical formula for improving the photovoltaic performance of ternary organic solar cells (OSCs). In this work, we explored a new avenue that introduces 3-dimensional structured molecules as guest acceptors. Spirobifluorene (SF) is chosen as the core to combine with three different terminal-modified (rhodanine, thiazolidinedione, and dicyano-substituted rhodanine) benzotriazole (BTA) units, affording three 4-arm molecules, SF-BTA1, SF-BTA2, and SF-BTA3, respectively. After adding these three materials into the classical system PM6: Y6, the resulting ternary devices obtained ultra-high power conversion efficiencies (PCEs) of 19.1%, 18.7%, and 18.8%, respectively, compared with the binary OSCs (PCE = 17.4%). SF-BTA1-3 can work as energy donors to increase charge generation via energy transfer. In addition, the charge transfer between PM6 and SF-BTA1-3 also acts to enhance charge generation. Introducing SF-BTA1-3 could form acceptor alloys to modify the molecular energy level and inhibit the self-aggregation of Y6, thereby reducing energy loss and balancing charge transport. Our success in 3D multi-arm materials as the third component shows good universality and brings a new perspective. The further functional development of multi-arm materials could make OSCs more stable and efficient.

Benzotriazole‐Based 3D Four‐Arm Small Molecules Enable 19.1 % Efficiency for PM6 : Y6‐Based Ternary Organic Solar Cells

AbstractA third component featuring a planar backbone structure similar to the binary host molecule has been the preferred ingredient for improving the photovoltaic performance of ternary organic solar cells (OSCs). In this work, we explored a new avenue that introduces 3D‐structured molecules as guest acceptors. Spirobifluorene (SF) is chosen as the core to combine with three different terminal‐modified (rhodanine, thiazolidinedione, and dicyano‐substituted rhodanine) benzotriazole (BTA) units, affording three four‐arm molecules, SF‐BTA1, SF‐BTA2, and SF‐BTA3, respectively. After adding these three materials to the classical system PM6 : Y6, the resulting ternary devices obtained ultra‐high power‐conversion efficiencies (PCEs) of 19.1 %, 18.7 %, and 18.8 %, respectively, compared with the binary OSCs (PCE=17.4 %). SF‐BTA1‐3 can work as energy donors to increase charge generation via energy transfer. In addition, the charge transfer between PM6 and SF‐BTA1‐3 also acts to enhance charge generation. Introducing SF‐BTA1‐3 could form acceptor alloys to modify the molecular energy level and inhibit the self‐aggregation of Y6, thereby reducing energy loss and balancing charge transport. Our success in 3D multi‐arm materials as the third component shows good universality and brings a new perspective. The further functional development of multi‐arm materials could make OSCs more stable and efficient.

Charge Carrier Dynamics in Non-Fullerene Acceptor-Based Organic Solar Cells: Investigating the Influence of Processing Additives Using Transient Absorption Spectroscopy.

In this study, we present a comprehensive investigation into the charge generation mechanism in bulk-heterojunction organic solar cells employing non-fullerene acceptors (NFAs) both with and without the presence of processing additives. While photovoltaic devices based on Y6 or BTP-eC9 have shown remarkable power conversion efficiencies, the underlying charge generation mechanism in polymer:NFA blends remains poorly understood. To shed light on this, we employ transient absorption (TA) spectroscopy to elucidate the charge transfer pathway within a blend of the donor polymer PM6 and NFAs. Interestingly, the charge carrier lifetimes of neat Y6 and BTP-eC9 are comparable, both reaching up to 20 ns. However, the PM6:BTP-eC9 blend exhibits substantially higher charge carrier generation and a longer carrier lifetime compared to PM6:Y6 blend films, leading to superior performance. By comparing TA data obtained from PM6:Y6 or PM6:BTP-eC9 blend films with and without processing additives, we observe significantly enhanced charge carrier generation and prolonged charge carrier lifetimes in the presence of these additives. These findings underscore the potential of manipulating excited species as a promising avenue for further enhancing the performance of organic solar cells. Moreover, this understanding contributes to the advancement of NFA-based systems and the optimization of charge transfer processes in polymer:NFA blends.

D-A-D type small molecule donors based on BODIPY skeleton for bulk heterojunction organic solar cells

In recent years, structural modification for boron-dipyrromethene (BODIPY) derivatives has been proven to be an effective way to improve their performance of organic solar cells (OSCs). Here, three D-A-D type small molecule donors BDP-1, BDP-2 and BDP-3 were synthesized based on 3,5,8-trimethyl BODIPY dye as the core by the introduction of 4-(diethylamino)phenyl, 4-diethylamino-2-methoxyphenyl and 4-(diphenylamino)phenyl at the 3, 5 and 8 positions, respectively, and used for the construction of organic solar cells. BDP-1, BDP-2, and BDP-3 are synthesized. Due to the strong electron-donating ability of the substituted groups at 3, 5, and 8 positions of BODIPY core, three donors exhibit extended π-conjugation and effective intramolecular charge transfer, resulting in the strong absorption in the range of 500–––900 nm with narrow band gaps below 1.52 eV. BDP-1, BDP-2 and BDP-3 display complementary absorption spectra and suitable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital levels (LUMO) with fullerene derivative PC71BM. These molecules (BDP-1, BDP-2 or BDP-3) as electron donor were applied into BHJ solar cells by using PC71BM as electron acceptor. The performance of BHJ organic solar cells is systematically investigated and compared. The optimized solar cells based on BDP-1, BDP-2 and BDP-3 show high power conversion efficiencies (PCEs) of 9.52%, 11.83% and 5.27%, respectively. The OSCs based on BDP-2: PC71BM exhibited excellent short-circuit current density (Jsc), high electron mobility and open-circuit voltage (Voc). This work demonstrates that BODIPY derivatives with simple-structured show great potential for high performance OSCs.

A Review of the Improvements in the Performance and Stability of Ternary Semi-Transparent Organic Solar Cells: Material and Architectural Approaches

In the past decade, considerable efforts have been made to develop semi-transparent organic solar cells (ST-OSCs). Different materials and architectures were examined with the aim of commercializing these devices. Among these, the use of ternary active layers demonstrated great promise for the development of efficient semi-transparent organic solar cells with the potential for future applications, including but not limited to self-powered greenhouses and powered windows. Researchers seek alternative solutions to trade-off between the power conversion efficiency (PCE) and average visible transmittance (AVT) of ST-OSCs, with photoactive materials being the key parameters that govern both (PCE) and (AVT), as well as device stability. Several new organic materials, including polymers and small molecules, were synthesized and used in conjunction with a variety of techniques to achieve semi-transparent conditions. In this review paper, we look at the working principle and key parameters of semi-transparent organic solar cells, as well as the methods that have been used to improve the performance and stability of ternary-based semi-transparent organic solar cells. The main approaches were concluded to be spectral enhancement and increments in the transparency of the active layer through band gap tuning, utilizing novel organic semi-conductors, optical engineering, and the design architecture of the active layers.

An effective strategy to design high-performance wide bandgap polymer donors for organic solar cells by increasing the backbone curvature

Although wide bandgap (WBG) polymer donors are widely used in non-fullerene organic solar cells (OSCs), only a few WBG polymers exhibited high power conversion efficiencies (PCEs). Developing WBG donors with ideal complementary absorption with narrow bandgap non-fullerene acceptors is critical to further improving the photovoltaic performance of OSCs. Here, we provide an effective strategy to widen the bandgap (Eg) of polymers by increasing the curvature of the polymer donor backbone structure and compare it with the fluorination strategy. Both experimental and theoretical results demonstrated that the fluorination strategy reduced the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of PBDT-DT2FBT with little effect on the Eg. In contrast, the polymer PBDT-DTfBT, with a large backbone curvature, exhibited a reduced HOMO energy level and a significantly increased LUMO energy level, resulting in a significant increase in the optical Eg of 0.19 eV. The PBDT-DTfBT-based device achieved a high PCE of 15.89%, which far exceeded the PCEs of the PBDT-DTBT (9.84%) and PBDT-DT2FBT-based devices (8.02%). This work could guide the design of high-performance WBG donors.

Ternary Organic Solar Cells with Power Conversion Efficiency Approaching 15% by Fine-Selecting the Third Component.

Nonfullerene acceptors with mediate bandgap play a crucial role in ternary devices as the third component, further boosting the performance of organic solar cells (OSCs). Herein, three F-series acceptors (F-H, F-Cl, and F-2Cl) with mediate bandgap are selected and introduced into the PM6:BDT-Br binary system as third component to find the detailed influence of end groups with chlorine (Cl) atom substitution on the performance of ternary organic solar cells. Due to the increased substitution of Cl atoms on the end groups, F-Cl and F-2Cl as guest acceptors reveal a superior ability to regulate the morphology of blend films, contributing to the ordered packing properties and high crystallinity. As a result, F-Cl and F-2Cl based ternary OSCs achieve significantly improved PCEs of 13.89% and 14.67%, respectively, compared with the binary devices (12.70%). On the contrary, F-H without Cl atom displays a poor compatibility with the host system, resulting in an inferior ternary device with a low PCE of 10.79%. This work indicates that F-series acceptors with mediate bandgap are a promising class of third component for high-performance ternary OSCs. And introducing more Cl atoms substitution on the end groups, especially F-2Cl, will own a broad applicability for other binary devices.

Understanding the Role of Triplet‐Triplet Annihilation in Non‐Fullerene Acceptor Organic Solar Cells

AbstractNon‐fullerene acceptors (NFAs) have enabled power conversion efficiencies exceeding 19% in organic solar cells (OSCs). However, the open‐circuit voltage of OSCs remains low relative to their optical gap due to excessive non‐radiative recombination, and this now limits performance. Here, an important aspect of OSC design is considered, namely management of the triplet exciton population formed after non‐geminate charge recombination. By comparing the blends PM6:Y11 and PM6:Y6, it is shown that the greater crystallinity of the NFA domains in PM6:Y11 leads to a higher rate of triplet‐triplet annihilation (TTA). This is attributed to the four times larger ground state dipole moment of Y11 versus Y6, which improves the long range NFA out‐of‐plane ordering. Since TTA converts a fraction of the non‐emissive triplet states into bright singlet states, it has the potential to reduce non‐radiative voltage losses. Through a kinetic analysis of the recombination processes under 1‐Sun illumination, a framework is provided for determining the conditions under which TTA may improve OSC performance. If these could be satisfied, TTA has the potential to reduce non‐radiative voltage losses by up to several tens of millivolts.

Molecular design of D-π-A-π-D small molecule donor materials with narrow energy gap for organic solar cells applications.

Developing novel materials present a great challenge to improve the photovoltaic performance of organic solar cells (OSCs). In this paper, we designed a series of the donor-π bridge-acceptor-π bridge-donor (D-π-A-π-D) structure molecules. These molecules consist of diketopyrrolopyrrole (DPP) moiety as core, 9-hexyl-carbazole moiety as terminal groups, and different planar electron-rich aromatic groups as π-bridges. The density functional theory (DFT) and time-dependent DFT (TD-DFT) computations showed that the frontier molecular orbital (FMO) energy levels, energy gaps, electron-driving forces (ΔEL-L), open-circuit voltage (Voc), fill factor (FF), reorganization energy (λ), exciton binding energy (Eb), and absorption spectra of the designed molecules can be effectively adjusted by the introduction of different π-bridges. The designed molecules have narrow energy gap and strong absorption spectra, which are beneficial for improving the photoelectric conversion efficiency of organic solar cells. In addition, the designed molecules possess large ΔEL-L, large Voc, and FF values and low Eb when the typical fullerene derivatives are used as acceptors. The FMO energy levels of the designed molecules can provide match well with the typical fullerene acceptors PC61BM, bisPC61BM, and PC71BM. Our results suggest that the designed molecules are expected to be promising donor materials for OSCs. All DFT and TD-DFT calculations were carried out using the Gaussian 09 code. The computational technique chosen was the hybrid functional B3LYP and the 6-31G(d,p) basis set. The benzene and chloroform solvent effects have been considered using the polarized continuum model (PCM) at the TD-DFT level. The simulated absorption spectra of designed molecules were plotted by using the GaussSum 1.0 program.

Manipulation of the spacing between the donor and acceptor by gradual side-chain modification to reduce energy loss of the organic solar cells

As one of the simple but effective molecular modify strategies, the side chain engineering based on siloxane-terminated alkyl side chain has been proved to be a promising design strategy for facilitating the formation of optimal film morphology, and further improving the photovoltaic performance of organic solar cells (OSCs). Herein, two polymer donors named 2FP-1EH-1Si and 2FP-2Si with symmetrical and asymmetrical siloxane-terminated alkyl side chain substitution were designed and synthesized. The results demonstrate that compared with the alkyl side chain substituted polymer 2FP-2EH, the introduction of siloxane-terminated alkyl side chain not only regulates the miscibility between the polymer donor and small molecular acceptor (SMA), but also affects the spacing between the donor (D) and the acceptor (A) molecules. As a result, the asymmetrical and symmetrical siloxane-terminated alkyl side chain substituted polymer 2FP-1EH-1Si and 2FP-2Si exhibit lower surface energy, stronger self-aggregation ability and higher hole mobility. More importantly, the OSCs based on 2FP-1EH-1Si:IT-4F and 2FP-2Si:IT-4F exhibited gradually decreased energy loss, which is suppressed by controlling the D-A spacing, along with the increased open-circuit voltage (VOC) from 0.77 V to 0.80 V and then to 0.86 V. This work further reveals the regulation mechanism of siloxane-terminated alkyl chain on the photovoltaic performance of active layer materials for efficient OSCs with enhanced stability.

Impact of π–bridge unit in fully nonfused ring electron acceptor on the photovoltaic performance of organic solar cells

Due to the advantages of simple synthesis and low cost, fully nonfused ring electron acceptors (FN-FREAs) have been developed in recent years. Almost all reported FN-FREAs use thiophene derivatives as π-bridging unit, while other π-bridging units have not yet been developed. Here, an FN-FREA M01 with 2,5-bis(alkyloxy)phenylene as the π-bridge unit is designed and synthesized to compare its photovoltaic property with an analogue M05 which uses 3,4-bis(alkyloxy)thiophene as the π-bridge unit. Although the only variation between M01 and M05 is the π-bridge unit, these two FN-FREAs demonstrate significantly different photovoltaic performances. Organic solar cells (OSCs) based on M01 exhibit a high power conversion efficiency of 13.70%, while M05 based one shows a much lower efficiency of 3.96% with a markedly reduced short-circuit current density. Fluorescence, Raman, electron paramagnetic resonance, and femtosecond time-resolved transient absorption spectroscopy prove that M05 tends to be oxidized to form radical cations which aggravates exciton quenching and charge carrier recombination in the active layer, while in M01 such radical cations are absent. All in all, we have demonstrated that the π-bridging unit of FN-FREAs is pivotal to the photovoltaic performance and provided a useful design strategy to low-cost, high-efficiency, and chemically stable acceptor materials.