No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Naphthodithiophene‐Based Nonfullerene Acceptor for High‐Performance Organic Photovoltaics: Effect of Extended Conjugation
Abstract
Naphtho[1,2-b:5,6-b′]dithiophene is extended to a fused octacyclic building block, which is end capped by strong electron-withdrawing 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile to yield a fused-ring electron acceptor (IOIC2) for organic solar cells (OSCs). Relative to naphthalene-based IHIC2, naphthodithiophene-based IOIC2 with a larger π-conjugation and a stronger electron-donating core shows a higher lowest unoccupied molecular orbital energy level (IOIC2: −3.78 eV vs IHIC2: −3.86 eV), broader absorption with a smaller optical bandgap (IOIC2: 1.55 eV vs IHIC2: 1.66 eV), and a higher electron mobility (IOIC2: 1.0 × 10−3 cm2 V−1 s−1 vs IHIC2: 5.0 × 10−4 cm2 V−1 s−1). Thus, IOIC2-based OSCs show higher values in open-circuit voltage, short-circuit current density, fill factor, and thereby much higher power conversion efficiency (PCE) values than those of the IHIC2-based counterpart. In particular, as-cast OSCs based on FTAZ: IOIC2 yield PCEs of up to 11.2%, higher than that of the control devices based on FTAZ: IHIC2 (7.45%). Furthermore, by using 0.2% 1,8-diiodooctane as the processing additive, a PCE of 12.3% is achieved from the FTAZ:IOIC2-based devices, higher than that of the FTAZ:IHIC2-based devices (7.31%). These results indicate that incorporating extended conjugation into the electron-donating fused-ring units in nonfullerene acceptors is a promising strategy for designing high-performance electron acceptors.
... Because of the bridging moiety of the tiny organic FNA molecules, prolonged conjugation can improve the efficiency of solar cells. The extended conjugation has been shown to lower the LUMO energy, resulting in a narrow bandgap and a larger shift in the maximum values [63]. Lower excitation energy has occurred from the redshift in k max value and significant light absorption, enhancing the molecules' electron-acceptor feature [64]. ...
The main objective of this research was to design non-fullerene acceptors A-D-A framework, using carbazole and benzothiazole derivatives. Density functional theory (DFT) was used to calculate the geometry optimized structures and electronic properties at B3LYP functional with a 6-311G basis set in the gas and solvent phase.The frontier molecular orbital’s (FMO), bandgap, Open-circuit voltage (VOC) and dipole moments of these developed acceptors have been calculated. The theoretical UV absorption spectrawere calculated from Time-Dependent Density Functional Theory (TD-DFT) with the same level of theory used DFT method.They show a suitable bandgap (2.24–2.93 eV) and dipole moment (1.8–10.8 Debye). The maximum wavelength (λmax) for all studied molecules in the range is 665.17–679.97 in both gas and solvent. A slight redshift was observed in all acceptors selected for chlorobenzene compared to gas phase absorption.The non-fullerene acceptor A11 has the lowest bandgap energy (2.24 eV), gas-phase excitation energy (1.86 eV), and chlorobenzene excitation energy (1.86 eV) (1.86 eV). As a result, A11 is predicted to be a good contender for organic non-fullerene acceptors in the future. The open-circuit voltage (Voc) values range from 1.53 to 2.56 eV. Consequently, the optoelectronic, molecular orbital distribution, and A11 and A12 molecules were suitable acceptors for non-fullerene acceptors.
... Fused ring electron acceptors are being extensively studied because of advantages such as the ability to tune energy levels, stronger absorption in the UV−visible region, and greater thermal stability. 20,21 Small-molecule donors are preferable to polymer donors possessing a well-defined structure, easier purification, and perfect batch-to-batch replicability, and these are important parameters in the commercialization of all-small-molecule organic SCs (ASM-OSCs). For achieving a higher PCE of ASM-OSCs, several criteria should be met such as the complementary absorption of donors and acceptors, a welloptimized structure of the blended film, and matched molecular energy levels of donors with that of acceptors. ...
With the aim of upgrading the power conversion efficiency of organic solar cells (OSCs), four novel non-fullerene, A1–A2–D–A2–A1-type small molecules were designed that are derivatives of a recently synthesized molecule SBDT-BDD reported for its efficient properties in all-small-molecule OSCs (ASM-OSCs). Optoelectronic properties of the designed molecules were theoretically computed with a selected CAM-B3LYP functional accompanied by the 6-31G(d,p) basis set of density functional theory (DFT), and excited-state calculations were performed through the time-dependent self-consistent field. The parameters of all analyzed molecules describing the charge distribution (frontier molecular orbitals, density of states, molecular electrostatic potential), absorption properties (UV–vis absorption spectra), exciton dynamics (transition density matrix), electron–hole mobilities (reorganization energies), and exciton binding energies were computed and compared. All the designed molecules were found to be superior regarding the aforesaid properties to the reference molecule. Among all molecules, SBDT1 has the smallest band gap (3.88 eV) and the highest absorption maxima with broad absorption in the visible region. SBDT3 has the lowest binding energy (1.51 eV in chloroform solvent) ensuring easier and faster dissociation of excitons to produce free charge-carriers and has the highest open-circuit voltage (2.46 eV) with PC61BM as the acceptor. SBDT1 possesses the highest hole mobility because it has the lowest value of λ+ (0.0148 eV), and SBDT4 exhibits the highest electron mobility because it has the lowest value of λ– (0.0146 eV). All the designed molecules are good candidates for ASM-OSCs owing to their superior and optimized properties.
... The extended conjugation has been found to decrease the LUMO energy which results in the low band-gap and higher shift in λ max values 36 . The red shift in λ max value along with strong light absorption has resulted to lower the excitation energy, and ultimately reinforce the electron-acceptor property of the molecules 37 . ...
The non-fullerene acceptors A1 – A5 with diflourobenzene or quinoline core (bridge) unit, donor cyclopenta[1,2-b:3,4-b′]dithiophene unit and 2-(2-methylene-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile as acceptor unit with additional phenyl, fulvene or thieno[3,2-d]pyrimidinyl 5-oxide groups have been designed through DFT calculations. The optimization of molecular geometries were performed with density functional theory (DFT) at B3LYP 6-31G (d,p) level of theory. The frontier molecular orbital (FMO) energies, band gap energies and dipole moments (ground and excited state) have been calculated to probe the photovoltaic properties. The band gap (1.42–2.01 eV) and dipole moment values (5.5–18. Debye) showed that these designed acceptors are good candidates for organic solar cells. Time-Dependent Density Functional Theory (TD-DFT) results showed λ max (wave length at maximum absorption) value (611–837 nm), oscillator strength ( f ) and excitation energies (1.50–2.02 eV) in gas phase and in CHCl 3 solvent (1.48–1.89 eV) using integral equation formalism variant (IEFPCM) model. The λ max in CHCl 3 showed marginal red shift for all designed acceptors compared with gas phase absorption. The partial density of states (PDOS) has been plotted by using multiwfn which showed that all the designed molecules have more electronic distribution at the donor moiety and lowest at the central bridge. The reorganization energies of electron ( λ e ) (0.0007 eV to 0.017 eV), and the hole reorganization energy values (0.0003 eV to − 0.0403 eV) were smaller which suggested that higher charged motilities. The blends of acceptors A1 – A5 with donor polymer D1 provided open circuit voltage (V oc ) and ∆HOMO off-set of the HOMO of donor and acceptors. These blends showed 1.04 to 1.5 eV values of V oc and 0 to 0.38 eV ∆HOMO off set values of the donor–acceptor bends which indicate improved performance of the cell. Finally, the blend of D1 – A4 was used for the study of distribution of HOMO and LUMO. The HOMO were found distributed on the donor polymer ( D1 ) while the A4 acceptor was found with LUMO distribution. Based on λ max values, and band gap energies (E g ), excitation energies (E x ), reorganization energies; the A3 and A4 will prove good acceptor molecules for the development of organic solar cells.
... In terms of photovoltaic properties ( Table 2, entry 3), a5 showed much higher J sc and FF with respect to a0, which the authors claim to be a result of higher electron mobility and a lower level of charge recombination. Zhu et al. [58] reported the synthesis of a novel NFA with a naphthalene core (a6); the optoelectronic properties are essentially the same as those of a0, apart from higher HOMO and LUMO levels, probably better matching the donor polymer, resulting in better performance in terms of photovoltaic properties. An anthracene moiety was also proposed as a core by Yao et al. [59] and Feng et al. [60] (compounds a7 and a8). ...
The introduction of the IDIC/ITIC families of non-fullerene acceptors has boosted the photovoltaic performances of bulk-heterojunction organic solar cells. The fine tuning of the photophysical, morphological and processability properties with the aim of reaching higher and higher photocurrent efficiencies has prompted uninterrupted worldwide research on these peculiar families of organic compounds. The main strategies for the modification of IDIC/ITIC compounds, described in several contributions published in the past few years, can be summarized and classified into core modification strategies and end-capping group modification strategies. In this review, we analyze the more recent advances in this field (last two years), and we focus our attention on the molecular design proposed to increase photovoltaic performance with the aim of rationalizing the general properties of these families of non-fullerene acceptors.
Efficient and durable seawater evaporation technologies are vital for expanding access to freshwater and developing novel irrigation systems. Here, a photothermal material termed as Y7, characterized by its fused‐ring, small‐molecule structure, and strong electron‐withdrawing end groups, is combined with an economical wood sponge. This combination is used to create a 3D solar desalination evaporator and to design a self‐circulating evaporative irrigation system. Mirroring the 3D light‐capture porous structure and underwater suspension of diatoms, this hydrophilic evaporator demonstrates efficient light absorption in the range of 300–1100 nm and excellent resistance to salt and photobleaching. With a mass of only 3 mg/3.14 cm ² , the Y7‐evaporator realizes a solar‐energy‐to‐vapor conversion efficiency ( η ) of 91.7% and solar water evaporation rate ( ṁ ) of 1.62 kg m ⁻² h ⁻¹ , marking a new record for purely organic evaporators. After the desalination process, the concentration of primary ions is reduced by five to six orders of magnitude. Based on these findings, an in situ self‐irrigation system is developed using seawater, which efficiently cultivates several plant species. The results present a novel seawater evaporation strategy rooted in enhanced photothermal effects and durability, suggesting potential applications in future self‐sustaining marine cultivation.
ITIC-series nonfullerene organic photovoltaics (NF OPVs) have realized the simultaneous increases of the short-circuit current density (JSC) and open-circuit voltage (VOC), called the positive correlation between JSC and VOC, which could improve the power conversion efficiency (PCE). However, it is complicated to predict the formation of positive correlation in devices through simple calculations of single molecules due to their dimensional differences. Here, a series of symmetrical NF acceptors blended with the PBDB-T donor were chosen to establish an association framework between the molecular modification strategy and positive correlation. It can be found that the positive correlation is modification site-dependent following the energy variation at the different levels. Furthermore, to illustrate a positive correlation, the energy gap differences (ΔEg) and the energy level differences of the lowest unoccupied molecular orbitals (ΔELUMO) between the two changed acceptors were proposed as two molecular descriptors. Combined with the machine learning model, the accuracy of the proposed descriptor is more than 70% for predicting the correlation, which verifies the reliability of the prediction model. This work establishes the relative relationship between two molecular descriptors with different molecular modification sites and realizes the prediction of the trend of efficiency. Therefore, future research should focus on the simultaneous enhancement of photovoltaic parameters for high-performance NF OPVs.
Theoretical simulation of film morphology with bulk heterojunction (BHJ) has become a technological breakthrough point for material and performance development in recent years. Stemming from the precision of calculation and the boom of materials, the unified process for predicting the relevant properties of BHJ through monomers needs to be further updated. This study analyzed the sensibilities of environmental changes (monomers, the acceptors of D/A and A/A in morphology) on structures and properties from fullerene to non‐fullerene materials. The results showed that it is inaccurate to predict morphology only by optimizing the monomer through employing a unified process. Due to material specificity, excessive environmental sensitivity could lead to prediction errors. In different environments, the properties of C60 with rigid structure vary greatly. The stability of fused ring electron acceptors (FREAs) is determined by the type and number of bridge bonds (single and ethylenic bonds) in main backbones, where a small number and strong conjugation are favorable. As a result, flexible ITIC is insensitive to the environment, which could be performed using a unified process. This may be the main reason for their excellent properties. This work puts forward higher requirements for the accuracy of a theoretical calculation process, and also provides an updated theoretical account for an accurate design of bulk heterojunction. Considering the accuracy of theoretical calculation, the sensitivity of its structure and properties to the environment was explored by taking fullerenes to non‐fullerenes as the research gradient. The results showed that the properties of rigid fullerene are highly sensitive to the environment (not directly predictable), while RFEAs should be specifically judged by the type (single and ethylenic bond) and number of bridges in main backbone.
In this computational work, with the aim of boosting the ultimate efficiency of organic photovoltaic cells, seven small acceptors (IDST1-IDST7) were proposed by altering the terminal-acceptors of reference molecule IDSTR. The optoelectronic characteristics of the IDSTR and IDST1-IDST7 molecules were investigated using the MPW1PW91/6-31G(d,p) level of theory, and solvent-state computations were examined using time-dependent density functional theory (TD-DFT) simulation. Nearly all the investigated photovoltaic aspects of the newly proposed molecules were found to be better than those of the IDSTR molecule e.g. in comparison to IDSTR, IDST1-IDST7 exhibit a narrower bandgap (E gap), lower first excitation energy (E x), and a significant red-shift in the absorbance maxima (λ max). According to the findings, IDST3 has the lowest E x (1.61 eV), the greatest λ max (770 nm), and the shortest E gap (2.09 eV). IDST1-IDST7 molecules have higher electron mobility because their RE of electrons is less than that of IDSTR. Hole mobility of IDST2-IDST7 is higher than that of the reference owing to their lower RE for hole mobility than IDSTR. By coupling with the PTB7-Th donor, the open circuit voltage (V OC) of the investigated acceptor molecules (IDSTR and IDST1-IDST7) was calculated and investigation revealed that IDST4-IDST6 molecules showed higher V OC and fill factor (FF) values than IDSTR molecules. Accordingly, the modified molecules can be seriously evaluated for actual use in the fabrication of OSCs with enhanced photovoltaic and optoelectronic characteristics in light of the findings of this study.
Near infrared (NIR) organic photodetector (OPD) has exhibited tremendous potential in research due to mechanical flexibility, optoelectronic tunability, and low cost. However, the limited NIR light absorption, low carrier mobility, and relatively low photon‐to‐electron conversion efficiency in organic photoactive layer have restricted further development of OPD. To address these challenges, localized surface‐plasmon resonance (LSPR) enhanced D18:Y6 OPD is systematically designed and fabricated with uniform Ag nanocubes (AgNCs) of 60 nm deposited right on D18:Y6 organic photoactive layers, realizing a 25‐fold enhancement in responsivity and detectivity which reach 2200 mA W−1 and 2.8 × 1012 Jones, respectively, upon 850 nm laser excitation compared with pristine device without AgNCs. The detailed COMSOL Physics simulation of as‐obtained devices with AgNCs indicates that electric filed is strongly amplified and concentrated within the D18:Y6 film. The enhancement of photoluminescence intensity in D18:Y6 films with AgNCs further verifies the resonant enhancement of light absorption in the D18:Y6 film embedded with AgNCs. To get in‐depth knowledge of the LSPR enhancement effect on OPD, the photocurrent enhancement of devices with different laser power densities and plasmonic modes have been systematically explored. Therefore, this work has opened a new window to enhance the photoresponse for next generation NIR OPDs. Localized surface‐plasmon resonance enhanced D18:Y6 organic photodetector is systematically designed and fabricated with uniform Ag nanocubes of 60 nm, realizing a 25‐fold enhancement in responsivity and detectivity.
Organic solar cells (OSCs) have become one of the most rapidly developing research fields in the past two decades due to their low cost, light weight, and suitability for large-area preparation. OSCs have been using fullerenes and their derivatives as acceptors in the past years, but the shortcomings of fullerenes and their derivatives limit the further improvement of photovoltaic performance. Non-fullerene acceptor materials have been developed due to their modifiable molecular structure, adjustable energy level and regulated blend film morphology, which has set off a research upsurge in the field of OSCs. In this perspective, we mainly summarized the recent advances in innovation of small molecule non-fullerene electron acceptors (SM-NFAs) and structure-property relationships and compatibility of donors and acceptors, as well as the guidelines for future structure designs, morphology control and the direction for the development of SM-NFAs.
Solution processable thin-film organic solar cells (TFOSCs) have attracted a lot of research attention in the past few decades, in the search for low-cost, flexible, and portable solar panels. In light of this, tremendous progress has been made in terms of improving materials design, device architecture, and ease of device fabrication processes. The choice of the donor and acceptor materials in the preparation of a polymer blend bulk heterojunction (BHJ) solar absorber medium is of great importance to the overall performance of organic solar cells (OSCs). The limitation in tuning the energy band structures of fullerene molecules poses significant challenges to the progress in BHJ organic solar cells. In an attempt to cater to the challenges, small-molecule non-fullerene acceptors (NFAs) came into the OSC research space with the possibility of altering the optoelectronic properties of the polymer molecules that bring OSCs closer to the realization of full-scale commercialization. This review discusses the role of NFAs in improving film morphology and reducing energy loss in TFOSCs. The recent development in engineering non-fullerene acceptors for solar energy conversion is presented and discussed in terms of reducing energy loss, improving solar cell performances.
The progress in all-polymer solar cells (all-PSCs) is often accompanied with morphology evolution of the light-harvesting layer. However, the regulation of the bulk-heterojunction (BHJ) morphology of the active layer often encounters obstacles, mainly because of the undesired miscibility of the polymer donor and acceptor. Here, we regulated the BHJ morphology of all-PSCs based on an electron-donating PBDB-T (poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithio-phene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione)]) and an electron-accepting polymer PFA1 via selection of solvents, and found that the PBDB-T:PFA1 film processed with chlorobenzene revealed a smaller phase separation scale than the chloroform processed film, mainly because of the significantly improved miscibility in the PBDB-T:PFA1 blend. Benefitting from the optimized BHJ morphology, the PBDB-T:PFA1 device processed by chlorobenzene achieved a satisfactory power conversion efficiency (PCE) of 15%, obviously higher than that of 12.25% for the chloroform processed device. Meanwhile, the voltage loss of the chlorobenzene processed device is well suppressed comparing with the chloroform processed device, demonstrating unexpected advances of morphology evolution for all-PSCs.
Comprehensive Summary
Fullerene derivatives are classic electron acceptor materials for organic solar cells (OSCs) but possess some intrinsic drawbacks such as weak visible light absorption, limited optoelectronic property tunability, difficult purification and photochemical/morphological instability. Fullerene acceptors are a bottleneck restricting further development of this field. Our group pioneered the exploration of novel nonfullerene acceptors in China in 2006, and initiated the research of two representative acceptor systems, rylene diimide polymer and fused‐ring electron acceptor (FREA). FREA breaks the theoretical efficiency limit of fullerene‐based OSCs (~13%) and promotes the whole field to an unprecedented prosperity with efficiency of 20%, heralding a nonfullerene era for OSCs. In this review, we revisit 15‐year nonfullerene exploration journey, summarize the design principles, molecular engineering strategies, physical mechanisms and device applications of these two nonfullerene acceptor systems, and propose some possible research topics in the near future.
What is the most favorite and original chemistry developed in your research group?
Fused‐ring electron acceptor for photovoltaics.
How do you get into this specific field? Could you please share some experiences with our readers?
In 2006 when I came back to ICCAS from USA and started my independent academic career, I was interested in organic photovoltaics (OPV) which I had never touched before since I believed it is useful and should have a bright future. OPV converts renewable solar energy to clean electricity through photovoltaic effect. The active layer in OPV device consists of electron donor and electron acceptor. Although fullerenes were prevailing acceptor materials in OPV at that time, I doubted if this choice is correct considering their fatal flaws such as weak absorption. I was curious about why no research groups in China explored fullerene alternatives before 2006. In 2006 our group initiated the nonfullerene OPV
research of China. In 2015 we invented the milestone molecule ITIC and pioneered fused‐ring electron acceptor (FREA). Around 300 research groups in >20 countries have utilized the FREA to fabricate high‐efficiency OPV devices with champion efficiency of >20%. The FREA has subverted previously predominant fullerenes, and is inaugurating a new era of the OPV field.
How do you supervise your students?
I expect that my students should have some essential personalities such as curiosity, creativity, devotion, persistence and diligence. I encourage them to do original, unique and leading research. I endow them with cheerful academic atmosphere such as self‐motivation, open‐mindedness and cross‐cooperation. I would like to provide them with warm human solicitude when they need help or suffer from bitterness.
What is the most important personality for scientific research?
Curiosity; uniqueness; persistence.
Organic solar cells are on the dawn of the next era. The change of focus toward non-fullerene acceptors has introduced an enormous amount of organic n-type materials and has drastically increased the power conversion efficiencies of organic photovoltaics, now exceeding 18%, a value that was believed to be unreachable some years ago. In this Review, we summarize the recent progress in the design of ladder-type fused-ring non-fullerene acceptors in the years 2018-2020. We thereby concentrate on single layer heterojunction solar cells and omit tandem architectures as well as ternary solar cells. By analyzing more than 700 structures, we highlight the basic design principles and their influence on the optical and electrical structure of the acceptor molecules and review their photovoltaic performance obtained so far. This Review should give an extensive overview of the plenitude of acceptor motifs but will also help to understand which structures and strategies are beneficial for designing materials for highly efficient non-fullerene organic solar cells.
Considering the increased demand and potential of photovoltaic devices in clean, renewable electrical and hi-tech applications, non-fullerene acceptor (NFA) chromophores have gained significant attention. Herein, six novel NFA molecules IBRD1-IBRD6 have been designed by structural modification of the terminal moieties from experimentally synthesized A2-A1-D-A1-A2 architecture IBR for better integration in organic solar cells (OSCs). To exploit the electronic, photophysical and photovoltaic behavior, density functional theory/time dependent-density functional theory (DFT/ TD-DFT) computations were performed at M06/6-311G(d,p) functional. The geometry, electrical and optical properties of the designed acceptor molecules were compared with reported IBR architecture. Interestingly, a reduction in bandgap (2.528-2.126 eV), with a broader absorption spectrum, was studied in IBR derivatives (2.734 eV). Additionally, frontier molecular orbital findings revealed an excellent transfer of charge from donor to terminal acceptors and the central indenoindene-core was considered responsible for the charge transfer. Among all the chromophores, IBRD3 manifested the lowest energy gap (2.126 eV) with higher λ max at 734 and 745 nm in gaseous phase and solvent (chloroform), respectively due to the strong electron-withdrawing effect of five end-capped cyano groups present on the terminal acceptor. The transition density matrix map revealed an excellent charge transfer from donor to terminal acceptors. Further, to investigate the charge transfer and open-circuit voltage (V oc), PBDBT donor polymer was blended with acceptor chromophores, and a significant V oc (0.696-1.854 V) was observed. Intriguingly, all compounds exhibited lower reorganization and binding energy with a higher exciton dissociation in an excited state. This investigation indicates that these designed chromophores can serve as excellent electron acceptor molecules in organic solar cells (OSCs) that make them attractive candidates for the development of scalable and inexpensive optoelectronic devices. Solar energy is the most poised amongst the renewable sources to avert the climate crisis 1,2. Until now Silicon-based solar cells (SCs) have been used frequently due to their low toxicity, thermal stability, and comparatively impressive power conversion efficiency (PCE); but suffer from certain drawbacks, including the high cost of production, heavy weight (20-30 kg m −2), rigidity, as well as defined and unalterable HOMO-LUMO levels 3. To overcome these drawbacks, significant research effort has been put into more flexible, lightweight (0.5 kg m −2) organic solar cells (OSCs) 4. These show great promise but traditionally suffered from low efficiencies. OSCs are generally bulk heterojunction (BHJ) units where absorption layers blend donor and acceptor molecules. Over the OPEN 1
Renewable energy sources are promising long-term solution to solve the energy supply crisis due to the excessive use of non-renewable fossil fuels. One of the options is solar energy, which can be harvested directly from sunlight using photovoltaic (PV) technology. In recent years, organic solar cells (OSCs) as the building blocks of organic PV technology have emerged in the PV field, enabling the realization of environmental-friendly and low-cost PV technology. However, issues related to efficiency performance still posed a major setback to commercialization of OSCs. In view of this, this study is conducted to present comprehensive understandings on how OSCs’ performance in terms of optical, electrical, morphological and mechanical properties can be improved through device engineering strategy (interface and electrode engineering strategy). In addition, the potential applications of OSCs achieved via device engineering strategy are also being explored. In summary, the studies conducted can be divided into three main parts. The first part focuses on improving OSCs’ performance through interface engineering strategy for the realization of high-performing OSCs. Interface engineering on sol-gel zinc oxide (ZnO) electron-transporting layer (ETL) was conducted by introducing additional oxadiazole-based electron-transporting material called 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD) between ZnO ETL and photoactive layer. The significance of incorporating PBD on ZnO was demonstrated by investigating the change in optical, electrical and morphological properties of pristine ZnO ETL. The findings shown that additional PBD layer could improve pristine ZnO film’s conductivity, create better energy level alignment with the photoactive layer, smoothen ZnO film’s morphology and improve ZnO film’s hydrophobicity. All those factors crucially influenced the charge extraction, transport and recombination processes in OSCs, which were conducive for the enhancement in photovoltaic performance of ZnO/PBD-based device. In fact, through interface engineering strategy, inverted OSCs based on poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2-ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione)] (PBDB-T donor) and 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6/7-methyl)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene (IT-M acceptor) could demonstrate ~7% increment in the photovoltaic performance from 10.8% (ZnO-based device) to 11.6% (ZnO/PBD-based device). The second part focuses on improving OSCs’ performance through electrode engineering strategy for the realization of high-performing flexible OSCs. Electrode engineering on poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) electrode was conducted by utilizing polyhydroxy compound dopant and gentle acid post-treatment method, specifically xylitol dopant and methanesulfonic acid (MSA) treatment. The significance of xylitol dopant and MSA treatment on PEDOT:PSS electrode was demonstrated by investigating the change in optical, electrical, morphological and mechanical properties of pristine PEDOT:PSS electrode. The findings shown that both doping and acid treatment on PEDOT:PSS electrode could improve the optical transparency of electrode, enhance electrode’s conductivity and modify electrode’s morphology. In addition, such treatment could also provide electrode a stronger adhesion ability with the substrate, which were effective for improving the mechanical stability of electrode against extreme mechanical deformation. All those factors promoted the realization of high-performing flexible OSCs based on PEDOT:PSS electrode. In fact, through electrode engineering strategy, conventional OSCs based on poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-fluoro)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2-ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione)] (PBDB-T-2F/PM6 donor) and 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2",3’':4’,5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (BTP-4F/Y6 acceptor) could demonstrate an excellent photovoltaic performance of 14.2% with remarkable mechanical robustness against bending and folding. The last part focuses on the application of device engineering, specifically electrode engineering as a continuation of study from the previous part. The desirable mechanical and optical properties of the engineered PEDOT:PSS could make PEDOT:PSS a great candidate for usage in foldable-flexible semi-transparent OSCs (FST-OSCs). FST-OSCs were fabricated similarly using engineered PEDOT:PSS electrode and PBDB-T-2F:Y6 photoactive layer system. As a result, high-performing FST-OSCs with over 10% efficiency and 21% average visible light transmittance, as well as excellent mechanical stability were obtained. The potential of such FST-OSCs for greenhouse application was investigated by incorporating them as part of roofs in the simulated greenhouse. Comparisons between plants grown under direct sunlight with FST-OSCs roof and those under direct sunlight yielded remarkably similar results in terms of branch sturdiness and hypertrophic leaves, proving the significance of electrode engineering strategy in realizing high-performing FST-OSCs for practical greenhouse applications.
Ionic liquids (ILs) are important media to effect various kinds of polymerizations. Ionic polymerizations were developed in IL solvents and that atom transfer radical polymerization catalysts are attached to ILs to make them more easily recoverable in living polymerizations. Conjugated polymers with metallic electrical transport were introduced around 1977. The rapidly expanding field of the usage of polymers for organic solar cells has been reviewed with respect to materials, processes and devices that significantly deviate from standard approaches. In the inverted architecture, the metal electrode collects holes, and the polymer‐rich layer that forms actually improves the performance of those solar cells. A triboelectric nanogenerator is a newly developed technique for harvesting mechanical energy from ambient environment with sparkly high output and extremely flexible structural designs. Although a polymer electrolyte fuel cell is a superior power source for electric vehicles, the high cost of this technology has become a barrier to its large‐scale commercialization.
Two small-molecule non-fullerene acceptors (NFAs) CPDT-Me-2Eh and CPDT-4Cl-2Eh based on 4H-cyclopenta[1,2-b:5,4-b′] dithiophene as the electron-donating unit 2-(5/6-methyl-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile and 2-(5,6-dichloro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile as the end group respectively were developed. Compared with CPDT-Me-2Eh, CPDT-4Cl-2Eh possesses red-shift absorption spectrum, lower HOMO and LUMO energy level, and higher electron mobility. Non-fullerene polymer solar cells based on PBDB-T: CPDT-4Cl-2Eh gave a power conversion efficiency (PCE) of 7.52%, with a fill factor (FF) of 64.46%, and a short-circuit current (Jsc) of 16.90 mA/cm². As contrast, the cell based on PBDB-T: CPDT-Me-2Eh got a PCE value of 4.39%. Our results indicate that the small molecule acceptor containing the CPDT core can effectively extend the absorption spectrum to the near-infrared (NIR) region by chlorination of the capping group. Our work provides an easy routine to broaden the field of new NIR NFAs with high PCE based on non-fullerene OSCs.
We synthesized three fused-ring electron acceptors (m-ITBr, o-ITBr and IT-2Br) based on indacenodithieno[3,2-b]thiophene as the core unit and 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile with 1 or 2 bromine substituents as the end-capped unit. The effects of bromine atom number and substituted position on molecular film absorption, energy level, photovoltaic performance, charge transport and film morphology were systematically carried out. Compared with ITIC, m-ITBr, o-ITBr and IT-2Br all show red-shifted film absorption (550–750 nm) and downshifted highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. The PM6:IT-2Br based devices exhibited power conversion efficiency (PCE) of up to 12.92%, which was much higher than the reference devices based on ITIC (8.98%). In addition, all three brominated devices achieved high efficiency over 11% (11.84% for m-ITBr devices and 11.14% for o-ITBr devices). Herein, it is also concluded that bromination on terminal groups plays a vital role in enhancing the photovoltaic performance of IDTT-based FREAs.
The performance of organic solar cells based on nonfullerene acceptors is improving rapidly and nowadays the power conversion efficiencies of single-junction devices have achieved over 18%. Nonfullerene acceptors usually consist of electron-rich units and electron-deficient units, and the interaction between these two parts can widen the optical absorption, tune the energy levels and improve the film morphologies of acceptor materials. Among the various electron-withdrawing groups, cyano groups have been most used in the construction of electron-deficient units for nonfullerene acceptors. In this review, the electron-deficient units containing cyano groups for nonfullerene electron acceptors have been summarized, and the effects of these electron-deficient units on the photoelectric properties and photovoltaic performance of acceptor materials have been elaborated to extract some effective molecular design strategies for high-performance acceptor materials. This journal is
To develop new central core structure, we designed and synthesized a naphthalene-fused octacyclic electron-rich block, named as NITT. With the NITT ladder-type core and two distinct terminal groups (IC-2F and CPTCN-Cl), two novel A-D-A-type non-fullerene acceptors (NFAs), namely NITT-BF and NITT-ThCl, were constructed. Two NFAs exhibit broad and intense absorption and high electron mobility due to the coplanar architecture. When blended with polymer donor PM6, NITT-BF-based device acquires more favorable morphology, higher exciton dissociation and charge collection efficiency, and higher and more balanced hole/electron mobilities, resulting in the significantly improved short-circuit current density (JSC) of 18.08 mA cm⁻² and fill factor (FF) of 72.38%. Additionally, both NITT-BF- and NITT-ThCl-based devices deliver very high open-circuit voltages (VOCs) (0.94 V for NITT-BF-based device and 0.97 V for NITT-ThCl-based device). Therefore, PM6:NITT-BF-based device yields an optimal PCE of up to 12.30%, which is much higher than that of PM6:NITT-ThCl-based device (9.58%). Notably, the high PCE of 12.30% is one of the best values for naphthalene-fused NFAs in organic solar cells (OSCs). These results indicate that the linear ladder-type NITT is a promising electron-donating core for constructing high-performance NFAs and terminal strategy is a good way to further boost the photovoltaic performance of OSCs.
π‐Conjugated organic/polymer materials‐based solar cells have attracted tremendous research interest in the fields of chemistry, physics, materials science, and energy science. To date, the best‐performance polymer solar cells (PSCs) have achieved power conversion efficiencies exceeding 18%, mostly driven by the molecular design and device structure optimization of the photovoltaic materials. This review article provides a comprehensive overview of the key advances and current status in aggregated structure research of PSCs. Here, we start by providing a brief tutorial on the aggregated structure of photovoltaic polymers. The characteristic parameters at different length scales and the associated characterization techniques are overviewed. Subsequently, a variety of effective strategies to control the aggregated structure of photovoltaic polymers are discussed for polymer:fullerene solar cells and polymer:nonfullerene small molecule solar cells. Particularly, the control strategies for achieving record efficiencies in each type of PSCs are highlighted. More importantly, the in‐depth structure–performance relationships are demonstrated with selected examples. Finally, future challenges and research prospects on understanding and optimizing the aggregated structure of photovoltaic polymers and their blends are provided. A comprehensive overview of the key advances and current status in aggregated structure research of photovoltaic polymers was presented. This review provided a tutorial on the multi‐level aggregated structure and the characteristic parameters as well as the associated characterization techniques. To in‐depth understand the structure‐performance relationships, effective strategies to control the aggregated structure of photovoltaic polymers are summarized, with an emphasis on the systems of record‐high power conversion efficiencies.
With the significant progress of low bandgap non‐fullerene acceptors, the development of wide bandgap (WBG) donors possessing ideal complementary absorption is of crucial importance to further enhance the photovoltaic performance of organic solar cells. An ideal strategy to design WBG donors is to down‐shift the highest occupied molecular orbital (HOMO) and up‐shift the lowest unoccupied molecular orbital (LUMO). A properly low‐lying HOMO of the donor is favorable to obtaining a high open‐circuit voltage, and a properly high‐lying LUMO of the donor is conductive to efficient exciton dissociation. This work provides a new strategy to enlarge the bandgap of a polymer with simultaneously decreased HOMO and increased LUMO by increasing the polymer backbone curvature. The polymer PIDT‐fDTBT with a large molecular backbone curvature shows a decreased HOMO of −5.38 eV and a prominently increased LUMO of −3.35 eV relative to the linear polymer PIDT‐DTBT (EHOMO = −5.30 eV, ELUMO = −3.55 eV). The optical bandgap of PIDT‐fDTBT is obviously broadened from 1.75 to 2.03 eV. This work demonstrates that increasing the polymer backbone curvature can effectively broaden the bandgap by simultaneously decreasing HOMO and increasing LUMO, which may guide the design of WBG conjugated materials. In this work, an effective strategy is provided to design a large bandgap polymer by increasing the polymer backbone curvature. This strategy can reduce the effective conjugation length of the polymer, leading to a decreased highest occupied molecular orbital and a prominently increased lowest unoccupied molecular orbital, which has great potential in designing wide bandgap conjugated materials.
A comprehensive study on the relationship between the structure and the photophysical and photovoltaic properties of acceptor-donor-acceptor (A-D-A) type nonfullerene acceptors (NFAs) is of pivotal importance to obtain design guidelines for high-performance organic photovoltaics (OPVs). In this study, we synthesized a D unit, NTT, in which the central benzene core, indacenodithieno[3,2-b]thiophene (IT), is replaced by naphthalene. Notably, NTTIC, an A-D-A type NFA with NTT as the D unit, showed a longer singlet exciton lifetime in the film than the IT-based benchmark NFA, ITIC. When paired with a conjugated polymer donor, PBDB-T, the NTTIC-based OPV device exhibited a higher power conversion efficiency (PCE = 9.95%) than the ITIC-based device (9.71%). Furthermore, due to the larger domain size of NTTIC in PBDB-T:NTTIC, the exciton diffusion (ED) at the donor/acceptor interface and the charge transfer (CT) were slower (14 ps in total) than those of PBDB-T:ITIC (7 ps in total). Nevertheless, the total efficiency of the ED and CT in PBDB-T:NTTIC was similar to that in PBDB-T:ITIC (95%) owing to the longer singlet exciton lifetime of the NTTIC film. These results demonstrate the high potential of naphthalene-cored D units of A-D-A type NFAs to achieve a long singlet exciton lifetime and a resultant high PCE in NFA-based OPVs.
A new fused‐ring electron acceptor FNIC3 with dynamics controlled aggregation behavior was synthesized. FNIC3 shows strong absorption in 600–900 nm, HOMO/LUMO energy levels of −5.59/−4.04 eV, and electron mobility of 1.2 × 10⁻³ cm² V⁻¹ s⁻¹. The aggregation of FNIC3 shows strong dependency on film formation time. Prolongation of film formation time promotes the crystallization of FNIC3, leading to improved crystallinity and enlarged aggregate sizes. Aggregation of FNIC3 significantly influences the photovoltaic device parameters. Appropriate aggregation red‐shifts the absorption and improves the mobilities of the blend, which contributes to high photocurrent and fill factor thus high power conversion efficiency (PCE). Overaggregation leads to increased nonradiative energy loss and insufficient charge generation, resulting in decreased open‐circuit voltage and short‐circuit current density. The blends based on PM6:FNIC3 fabricated under proper film formation time exhibit a PCE of 12.3%, higher than those fabricated under short and long film formation time (10.0–10.5%).
A thin film encapsulation layer was fabricated through two-sequential chemical vapor deposition processes for organic light emitting diodes (OLEDs). The fabrication process consists of laser assisted chemical vapor deposition (LACVD) for the first silicon nitride layer and laser assisted plasma enhanced chemical vapor deposition (LAPECVD) for the second silicon nitride layer. While SiNx thin films fabricated by LAPECVD exhibits remarkable encapsulation characteristics, OLEDs underneath the encapsulation layer risk being damaged during the plasma generation process. In order to prevent damage from the plasma, LACVD was completed prior to the LAPECVD as a buffer layer so that the laser during LACVD did not damage the devices because there was no direct irradiation to the surface. This two-step thin film encapsulation was performed sequentially in one chamber, which reduced the process steps and increased fabrication time. The encapsulation was demonstrated on green phosphorescent OLEDs with I–V-L measurements and a lifetime test. The two-step encapsulation process alleviated the damage on the devices by 19.5% in external quantum efficiency compared to the single layer fabricated by plasma enhanced chemical vapor deposition. The lifetime was increased 3.59 times compared to the device without encapsulation. The composition of the SiNx thin films was analyzed through Fourier-transform infrared spectroscopy (FTIR). While the atomic bond in the layer fabricated by LACVD was too weak to be used in encapsulation, the layer fabricated by the two-step encapsulation did not reveal a Si–O bonding peak but did show a Si–N peak with strong atomic bonding.
As an effective molecular modification strategy, side chain engineering has been widely used in promoting the photovoltaic performance of non-fullerene acceptors. Herein, a novel non-fullerene small molecular acceptor i-IEOSi-4F comprising siloxane-terminated alkoxyl side chain was successfully designed and synthesized. The molecule shows an optical band gap of 1.53 eV, with large extinction coefficient of 2.36 × 10⁵ M⁻¹ cm⁻¹ in solution. Two fluorobenzotriazole based polymers J52 and PBZ-2Si with the same backbone units but different side chains were employed as the donor to construct the active layers that all can demonstrate suitable energy levels and complementary absorptions with i-IEOSi-4F. Relative to J52 only bearing alkyl side chain, PBZ-2Si with siloxane-terminated side chain could induce more balanced carrier transports and more favorable morphology, leading to a higher power conversion efficiency (PCE) of 12.66% with a good fill factor of 71.45%. The efficiency is 21% higher than that of 10.46% for the J52 based devices. Our results not only indicate that siloxane-terminated alkoxyl side chain is valuable for efficient non-fullerene acceptors, but also demonstrate that siloxane-terminated side chain on both polymer donor and small molecular acceptor is a useful combination to realize more efficient polymer solar cells.
In pursuit of an alternative renewable and sustainable energy source to fossil fuels, polymer based photovoltaics has attracted considerable interests among researchers because of their lightweight, low cost and flexibility compared to the inorganic counterparts. The concept of bulk heterojunction (BHJ) device structure by blending both acceptor and donor materials as a one single active layer between the electrodes pioneered the polymer photovoltaics. However, the slow but steady enhancement of power conversion efficiency (PCE) with BHJ device having fullerene as the acceptor material was revolving around 5% until non-fullerene based materials were used as the acceptor material which led to ~16% PCE – mainly due to their wide range of visible and NIR absorbance capabilities. In addition, the recent trend of BHJ based polymer photovoltaics, with an organic perovskite and fullerene as donor and acceptor respectively, exhibit ~18% PCE under sunlight illumination which has raised the potential of polymer photovoltaics by thousand folds as the cheapest form of global energy source in coming years. However, this short review highlights the major advances in polymer based photovoltaics in terms of the choice of acceptors and donors, and discusses future directions and promising perspectives of polymer based photovoltaics.
Two FREAs, IUIC-O and IUIC-T, based on an undecacyclic core were developed. IUIC-T having higher extinction coefficient, affords aligned energy levels with PBDB-T, finer nanoscale morphology and more orderly molecular...
The ring-expanded strategy in nonfullerene acceptors (NFAs) with the acceptor–donor–acceptor backbone has been reported to be an effective method to improve the fill factor (FF), open circuit voltage (VOC), and short circuit current (JSC) simultaneously in organic photovoltaics. However, design control is still missing in the ring-expanded strategy, and is urgently needed to further develop the origins and rules. To give insight into this strategy, a detailed theoretical study of the ring-expanded mechanism is performed on the systems comprising different 9,9′-bifluorenylidene-based cores and 1,1-dicyanomethylene-3-indanone group. Some main parameters involved in photoelectric conversion mechanism under the donor excitation (DE) and/or acceptor excitation (AE) are assessed by changing the position and size of ring-expanded modes. The results show that the external ring-expanded modes can not only maintain the original advantage as much as possible, variations in sizes and positions also offer them an opportunity to regulate the aforementioned parameters systematically, leading to better improvement regardless of AE or DE. Thus, the steady improvement in performance mentioned previously is the key to overcoming the negative correlation among FF, VOC, and JSC. This insight and discovery of the ring-expanded strategy provides new design approaches for the next generation of NFAs.
Alkylthienyl decorated two-dimensional (2D) conjugated electron-donating (D) unit such as 4,8-bis(2-alkylthienyl)benzo[1,2-b:4,5-b’]dithiophene has been widely used for designing highly efficient photovoltaic materials. However, the thienyl side chains are less involved in the backbone Frontier molecular orbitals (FMOs). In this paper, we replace the side-chains from tri(n-propyl)silylthienyl to tri(n-propyl)silylethynyl and report two alkynyl-decorated 2D conjugated narrow-bandgap (~1.4 eV) nonfullerene acceptors, named ISI-4F and ISI-4Cl. In a striking contrast to the thienyl-linked counterpart, the ethylnyl is significantly involved in the backbone FMOs of the new acceptor molecules, enhancing the absorptivity and reducing the energy levels. With PM6 as the donor polymer, 12.5% and 12.1% efficiencies are obtained. The VOC are as high as 0.87 – 0.88 V. The tetrachlorinated acceptor supplies a larger fill factor (67.9% vs. 62.2%) while a smaller short-circuit current-density (22.8 vs. 20.5 mA/cm2) than the tetrafluorinated counterpart does. When introduced as acceptor guests of PM6:Y6, 16.1% and 15.9% efficiencies are achieved. These results demonstrate the trialkylsilylalkynyl can be potential side-chain candidates for designing highly efficient acceptor materials.
Porphyrin and its derivatives play important roles in natural energy conversion application. In this work, we designed and synthesized three novel acceptor-donor-acceptor (A-D-A) type small molecules (namely Por-Cu-IC, Por-Cu-ICF and Por-Cu-ICFF) with Cu(II)-porphyrin as central electron-donating core and fluorinated 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile as the electron-withdrawing end groups. These new molecules show broad absorption spectra from 300 to 900 nm with strong intramolecular charge transfer absorption spectra around at 800 nm, and optical bandgap of about 1.4 eV. Organic solar cells (OSCs) based on these new molecules as non-fullerene electron acceptors achieved power conversion efficiencies from 0.5% to 2.44%. The OSCs were characterized by several techniques, including density functional theory (DFT), space-charge limited current (SCLC), photoluminescence spectra (PL) and atomic force microscopy (AFM). These results demonstrate a systematic study of Cu(II)-porphyrin molecules, which could be used to design molecules for use in organic electronics.
ConspectusEmerging solar cells that convert clean and renewable solar energy to electricity, such as organic solar cells (OSCs) and perovskite solar cells (PSCs), have attracted increasing attention owing to some merits such as facile fabrication, low cost, flexibility, and short energy payback time. The power conversion efficiencies (PCEs) of OSCs and PSCs have exceeded 18% and 25%, respectively.Fullerene derivatives have high electron affinity and mobility with an isotropic transport feature. Fullerene-based OSCs yielded superior PCEs to other acceptors and have dominated electron acceptor materials from 1995 to 2015. However, some drawbacks of fullerenes, such as weak visible absorption, limited tunability of electronic properties, laborious purification, and morphological instability, restrict further development of OSCs toward higher PCEs and practical applications. The theoretical PCE of fullerene-based OSCs is limited to ∼13% due to the relatively large energy losses. Many efforts have been dedicated to developing new acceptor systems beyond fullerenes, and some successful systems such as rylene diimides have achieved PCEs up to ca. 11%.In 2015, our group pioneered a new class of electron acceptors, fused-ring electron acceptor (FREA), as represented by the star molecule ITIC. The chemical features of FREAs include: (1) a modular structure, consisting of an electron-donating core, electron-withdrawing end groups, π-bridges, and side chains, which benefits molecular tailoring; (2) facile synthesis, purification, and scalability. The physical features of FREAs include: (1) a broad modulation range of absorption and energy levels; (2) strong absorption, especially in the 700-1000 nm region; (3) high electron mobility. The device features of FREAs include: (1) low voltage loss; (2) high efficiency; (3) good stability. The FREAs boosted PCEs of the OSCs up to 18% and initiated the transformation from the fullerene to nonfullerene era of this field. FREAs can also be used in PSCs as interfacial layers, electron transport layers, or active layers, improving both efficiency and stability of the devices. Beyond photovoltaic applications, FREAs can also be used in photodetectors, field-effect transistors, two-photon absorption, photothermal therapy, solar water splitting, etc.In this Account, we review the development of the FREAs and their applications in OSCs, PSCs, and other related fields. Molecular design, device engineering, photophysics, and applications of FREAs are discussed in detail. Future research directions toward performance optimization and commercialization of FREAs are also proposed.
We designed and synthesized a series of fused-ring electron acceptors (FREAs) based on naphthalene-fused octacyclic cores end-capped by 3-(1,1-dicyanomethylene)-5,6-difluoro-1- indanone (NOICs) using a bottom-up approach. The NOIC series shares the same end groups and side chains, as well as similar fused-ring cores. The butterfly effects, arising from different methoxy positions in the starting materials, impact the design of the final FREAs, as well as their molecular packing, optical and electronic properties, charge transport, film morphology, and performance of organic solar cells. The binary-blend devices based on this NOIC series show power conversion efficiencies varying from 7.15% to 14.1%, due to the different intrinsic properties of the NOIC series, morphologies of blend films, and voltage losses of devices.
In the present study, initially, the new rhodanine-based Bis(Rh)-Ph and Bis(Rh)-TPE were synthesized by a green approach. Following synthesis, Al/Bis(Rh)-Ph/p-Si (D1) and Al/Bis(Rh)-TPE/p-Si (D2) heterojunctions were fabricated by using spin coating method and thermal evaporation technique. The electrical characterizations of fabricated D1 and D2 devices were investigated and compared with each other by using the reverse and forward bias C–V measurements at room temperature and I–V measurements at the three different temperatures and distinct illumination intensities. Additionally, the AFM images of D1 and D2 were examined for surface properties. The crucial parameters such as ideality factor (n), saturation current (I0) and barrier height (ΦB) of D1 and D2 devices were calculated as 2.72, 4.26 × 10⁻¹⁰A, 0.80eV and 1.85, 2.33 × 10⁻⁹A, 0.76eV, respectively. Rectifier rate (RR) for D2 device is ~5.3 times higher than D1. While the ideality factor increased with effect of exposure light, the barrier height decreased. In addition, DFT calculations supported the non-planar structures or propeller structures. The results supported that these devices could be used in optoelectronic applications, especially photodiodes and photo detectors.
Semitransparent organic solar cells (ST-OSCs) have attracted attention for use in building integrated photovoltaics because of their large range tunability in colors, transparency, and high efficiency. However, the development of semitransparent devices based on fullerene acceptors remained almost stagnant in the early period. This was due to the weak absorption of fullerene small molecules in the visible and near-infrared regions as well as the large non-radiative energy loss, resulting in drastic open-circuit voltage loss. In addition, the energy level and chemical structure of fullerene molecules cannot be easily regulated, and the strong aggregation characteristics of fullerenes greatly limit the development of OSCs. In contrast, the designability of the chemical structures and controllability of the energy levels of non-fullerene electron acceptors has encouraged researchers to explore high-performance organic solar cells while and simultaneously stimulating the development of ST-OSCs. In this review, the recent progress in non-fullerene small molecule acceptors for ST-OSCs is summarized. The article focuses on ST-OSCs from the aspects of device structures and active layers. In view of the semitransparent device structure, except for replacing the traditional electrodes with semitransparent electrodes, researchers have introduced suitable interface layers to regulate the absorption and reflection of sunlight. The interface layers mainly contain a reflective layer (evaporated on the top electrode to reflect near-infrared light); an anti-reflection layer (located below ITO (indium tin oxide)) to mitigate light reflection at the air-glass interface and thus enhance the absorption of sunlight); and an optical outcoupling layer (simultaneously increasing reflection and transmission). From the active layer, it is mainly divided into two categories. First, researchers have optimized the photovoltaic performance of semitransparent devices from the perspective of molecular structures, mainly by broadening the absorption window of non-fullerene small molecule acceptors, thus improving the crystallinity and charge mobility of small molecules, and regulating the stacking behavior and orientation of molecules in the films. Second, regarding the active layer processing, much effort has been undertaken to optimize the light absorption, morphology, and charge carrier transport channels of blended films.
The past three years have witnessed rapid growth in the field of organic solar cells (OSCs) based on non‐fullerene acceptors (NFAs), with intensive efforts being devoted to material development, device engineering, and understanding of device physics. The power conversion efficiency of single‐junction OSCs has now reached high values of over 18%. The boost in efficiency results from a combination of promising features in NFA OSCs, including efficient charge generation, good charge transport, and small voltage losses. In addition to efficiency, stability, which is another critical parameter for the commercialization of NFA OSCs, has also been investigated. This review summarizes recent advances in the field, highlights approaches for enhancing the efficiency and stability of NFA OSCs, and discusses possible strategies for further advances of NFA OSCs.
The development of A-D-A type heteroacenes for bulk-heterojunction solar cells has gained immense interest in the last 5-6 years due to synthetic accessibility via new molecular design, facile functionalization, tunable optoelectronic properties, cost-effective device fabrications and high efficiencies. Specifically, they have reached power conversion efficiencies (PCE) up to 18% as electron acceptors, demonstrating their effectiveness to replace fullerene derivatives in OSCs. This review explores the important aspects of the structural modifications of A-D-A-type heteroacenes and their application in high performance binary, ternary and tandem OSCs. The modulation of functional groups and their influence on the frontier orbital tuning, blend morphology, charge transport properties have been comprehensively discussed as a tool to correlate molecular structure-properties with device performance beneficial for new material design. Finally, their applications and prospects in semitransparent OSCs are discussed as a potential for future technology.
Comprehensive design ideas on the fused-ring donor-core in state-of-the-art acceptor-donor-acceptor (A-D-A) nonfullerene acceptors (NFAs) are still of great importance for regulating the electron push-pull effect for the sake of optimal light-harvesting, frontier molecular orbital levels, and finally their photovoltaic properties. Herein, thieno[3,2-b]thiophenes were fused in bay-area of naphthalene via six-member-ring connection, resulting in the formation of dihydropyrenobisthieno[3,2-b]thiophene based octacyclic ladder-type donor core, which was flanked by two 1,1-dicyanomethylene-3-indanone (IC) acceptor motifs with and without 5,6-diflourination, namely PTT-IC and PTT-2FIC, respectively, as novel efficient A-D-A fused-ring electron acceptors (FREAs). Compared with PTT-IC, fluorinated PTT-2FIC possesses narrower optical bandgap of 1.48 eV, better π-π stacking, and its PBDB-T:PTT-2FIC blend film exhibited better morphology, and better hole and electron mobility. As a result, nonfullerene solar cells using PBDB-T:PTT-2FIC as the active layer achieved a decent PCE of 10.40%, with an open-circuit voltage (VOC) of 0.87 V, a fill factor (FF) of 0.65, and a much higher short-circuit current (JSC) of 18.26 mA/cm². Meanwhile, the PBDB-T:PTT-IC cells delivered a lower JSC of 12.58 mA/cm² but a higher VOC of 0.99 V, thus resulting in a PCE of 7.39% due to its wider optical bandgap of 1.58 eV and higher LUMO energy level. These results demonstrated that NFAs based on fused-ring donor core from fusing thieno[3,2-b]thiophenes with naphthalene via six-member-ring connection are promising for organic photovoltaic applications.
The ternary strategy has been widely used in high-efficiency organic solar cells (OSCs). Herein, we successfully incorporated a mid-band-gap star-shaped acceptor, FBTIC, as the third component into the PM6/Y6 binary blend film, which not only achieved a panchromatic absorption but also significantly improved the open-circuit voltage (V OC) of the devices due to the high-lying lowest unoccupied molecular orbital (LUMO) of the FBTIC. Morphology characterizations show that star-shaped FBTIC molecules are amorphously distributed in the ternary system, and the finely tuned ternary film morphology facilitates the exciton dissociation and charge collection in ternary devices. As a result, the best PM6/Y6/FBTIC-based ternary OSCs achieved a power conversion efficiency (PCE) of 16.7% at a weight ratio of 1.0:1.0:0.2.
The precise design of organic photovoltaic materials and the control of morphology in active layer are crucial for achieving high-performance organic solar cells (OSCs). Up to now, however, it remains difficult to fully obtain the intrinsic properties of organic photovoltaic materials, as well as the details of molecular stacking in disordered films and the evolution of the specific morphology of active layers by traditional characterization methods, which is hindering the screening of organic photovoltaic materials and understanding the structure-property relationship. Accordingly, computational chemistry provides a good method and plays a vital role in current scientific research. In this review, we first introduce the theoretical methods used in recent study of OSCs, including density functional theory (DFT), time-dependent DFT (TD-DFT), all atomic molecular dynamics (AAMD) and coarse-grained molecular dynamics (CGMD). Then, the effects of the molecular structure on its conformation, frontier molecular orbital, ultraviolet-visible (UV-Vis) absorption spectrum, dipole moment, electrostatic potential, binding energy, stacking, and morphological evolution are discussed and analyzed. Finally, the intrinsic properties of OSCs are summarized from the molecular structure and the future development and prospects of OSCs are analyzed to accelerate the efficiency over 20% in near future.
Side chain engineering plays a substantial role for high-performance organic solar cells (OSCs). Herein, a series of non-fullerene acceptor (NFA) molecules with A-D-A structures, TTCn-4F, with gradient substituent lengths of terminal side chains (T-SCs) on the molecular backbones have been designed and synthesized. The effects of T-SCs length, ranging from hydrogen atom to n-dodecyl, their optoelectronic properties, thin film molecular packing, blend film morphology, and overall photovoltaic performance have been systematically studied. The results show that among this series of molecules, TTC8-4F with n-octyl substituent, showed the best photovoltaic performance when applied with PM6 as the donor due to its favorable morphology, balanced charge mobility, effective exciton dissociation and less charge recombination. Based on this, its ternary device with F-Br as the secondary acceptor achieved a high PCE of 15.34% with the simultaneously enhanced Voc of 0.938 V, Jsc of 22.66 mA cm−2, and FF of 72.15%. These results indicate that the engineering of T-SCs is an effective strategy for designing high-performance NFAs.
Four fused‐ring electron acceptors consisting of the same indacenobis(dithieno[3,2‐b ;2′,3′‐d ]thiophene) core and 2‐(3‐oxo‐2,3‐dihydroinden‐1‐ ylidene)malononitrile end‐groups without or with fluorination on side‐chains and/or end‐groups are compared to probe the impacts of fluorination position on molecular packing, optical, electronic, and photovoltaic properties of the non‐fullerene acceptors systematically. The fluorination on side‐chains blue‐shifts, while that on end‐groups red‐shifts absorption spectra. Both fluorinations down‐shift energy levels and enhance electron mobility. The single crystal data show that fluorination on side‐chains hardly affects the aggregate structure but induces intermolecular H‐bonding formation between side‐groups, leading to closer molecular stacking; fluorination on end‐groups induces formation of larger π‐orbital plane, closer framework and interpenetrating charge transfer pathways. While both methods of fluorinations decrease open‐circuit voltage, there is an increase in short‐circuit current density, fill factor and power conversion efficiency (PCE) of organic solar cells. Finally, in combination with the polymer donor FTAZ, INIC4/INIC3 with fluorination on side‐chains or end‐groups yield PCEs of 9.6‐11.6%, which is higher than INIC without fluorination (7.8%). FINIC with fluorination on both side‐chains and end‐groups yields the highest PCE of 13.0%.
This article is protected by copyright. All rights reserved.
In recent years, a rapid evolution of organic solar cells has been achieved by virtue of structural design of active layer materials and optimization of film morphology. Along the other characterization techniques, Grazing incidence small‐ and wide angle X‐ray scattering (GISAXS and GIWAXS) have played significant role in deeper understanding of film morphology. In this review, importance of these techniques is explained with examples from various aspect of organic solar cells. Different pre‐and post‐processing conditions such as solvent effect, solvent additive, solvent and thermal annealing were studied in framework of these techniques. Moreover, impact of donor:acceptor ratio and molecular weight of semiconductor on microstructure is also explored. Finally, effect of chemical structure of organic semiconductors (both polymers and small molecules) on the film morphology is discussed. These techniques provide valuable information about crystallinity, phase separation and domain size of nanostructured film morphology, which helps to optimizae the film morphology and enhance the peformance of organic solar cells. In future, role of these techniques will become more important because mystery of film morphology still has to be solved.
This article is protected by copyright. All rights reserved.
A side-chain conjugation strategy in the design of nonfullerene electron acceptors is proposed, with the design and synthesis of a side-chain-conjugated acceptor (ITIC2) based on a 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']di(cyclopenta-dithiophene) electron-donating core and 1,1-dicyanomethylene-3-indanone electron-withdrawing end groups. ITIC2 with the conjugated side chains exhibits an absorption peak at 714 nm, which redshifts 12 nm relative to ITIC1. The absorption extinction coefficient of ITIC2 is 2.7 × 10(5) m(-1) cm(-1) , higher than that of ITIC1 (1.5 × 10(5) m(-1) cm(-1) ). ITIC2 exhibits slightly higher highest occupied molecular orbital (HOMO) (-5.43 eV) and lowest unoccupied molecular orbital (LUMO) (-3.80 eV) energy levels relative to ITIC1 (HOMO: -5.48 eV; LUMO: -3.84 eV), and higher electron mobility (1.3 × 10(-3) cm(2) V(-1) s(-1) ) than that of ITIC1 (9.6 × 10(-4) cm(2) V(-1) s(-1) ). The power conversion efficiency of ITIC2-based organic solar cells is 11.0%, much higher than that of ITIC1-based control devices (8.54%). Our results demonstrate that side-chain conjugation can tune energy levels, enhance absorption, and electron mobility, and finally enhance photovoltaic performance of nonfullerene acceptors.
A fused hexacyclic electron acceptor, IHIC, based on strong electron-donating group dithienocyclopentathieno[3,2-b]thiophene flanked by strong electron-withdrawing group 1,1-dicyanomethylene-3-indanone, is designed, synthesized, and applied in semitransparent organic solar cells (ST-OSCs). IHIC exhibits strong near-infrared absorption with extinction coefficients of up to 1.6 × 10(5) m(-1) cm(-1) , a narrow optical bandgap of 1.38 eV, and a high electron mobility of 2.4 × 10(-3) cm(2) V(-1) s(-1) . The ST-OSCs based on blends of a narrow-bandgap polymer donor PTB7-Th and narrow-bandgap IHIC acceptor exhibit a champion power conversion efficiency of 9.77% with an average visible transmittance of 36% and excellent device stability; this efficiency is much higher than any single-junction and tandem ST-OSCs reported in the literature.
A new polymer donor (PBDB-T-SF) and a new small molecule acceptor (IT-4F) for fullerene-free organic solar cells (OSCs) were designed and synthesized. The influences of fluorination on the absorption spectra, molecular energy levels and charge mobilities of the donor and acceptor were systematically studied. The PBDB-T-SF:IT-4F-based OSC device showed a record high efficiency of 13.1%, and an efficiency of over 12% can be obtained with a thickness of 100–200 nm, suggesting the promise of fullerene-free OSCs in practical applications.
Fabricating organic solar cells (OSCs) with a tandem structure has been considered an effective method to overcome the limited light absorption spectra of organic photovoltaic materials. Currently, the most efficient tandem OSCs are fabricated by adopting fullerene-derivatives as acceptors. Here, we design a new NF-acceptor with an E(opt)g of 1.68 eV for the front sub-cells and optimize the phase separation morphology of a fullerene-free active layer with an E(opt)g of 1.36 eV to fabricate the rear sub-cell. The two sub-cells show a low Eloss and high external quantum efficiency (EQE), and their photo response spectra are complementary. In addition, an interconnection layer composed of ZnO and a pH-neutral self-doped conductive polymer PCP-Na and with a high light transmittance in the near-IR range was developed. Based on the highly optimized sub-cells and ICL, solution-processed fullerene-free tandem OSCs with an average PCE over 13% are demonstrated.
To achieve efficient non-fullerene organic solar cells, it is important to reduce the voltage loss from the optical bandgap to the open-circuit voltage of the cell. Here we report a highly efficient non-fullerene organic solar cell with a high open-circuit voltage of 1.08 V and a small voltage loss of 0.55 V. The high performance was enabled by a novel wide-bandgap (2.05 eV) donor polymer paired with a narrow-bandgap (1.63 eV) small-molecular acceptor (SMA). Our morphology characterizations show that both the polymer and the SMA can maintain high crystallinity in the blend film, resulting in crystalline and small domains. As a result, our non-fullerene organic solar cells realize an efficiency of 11.6%, which is the best performance for a non-fullerene organic solar cell with such a small voltage loss.
Inspired by the remarkable promotion of power conversion efficiency (PCE), commercial applications of organic photovoltaics (OPVs) can be foreseen in near future. One of the most promising applications is semitransparent (ST) solar cells that can be utilized in value-added applications such as energy-harvesting windows. However, the single-junction STOPVs utilizing fullerene acceptors show relatively low PCEs of 4%-6% due to the limited sunlight absorption because it is a dilemma that more photons need to be harvested in UV-vis-near-infrared (NIR) region to generate high photocurrent, which leads to the significant reduction of device transparency. This study describes the development of a new small-bandgap electron-acceptor material ATT-2, which shows a strong NIR absorption between 600 and 940 nm with an Eg(opt) of 1.32 eV. By combining with PTB7-Th, the as-cast OPVs yield PCEs of up to 9.58% with a fill factor of 0.63, an open-circuit voltage of 0.73 V, and a very high short-circuit current of 20.75 mA cm(-2) . Owing to the favorable complementary absorption of low-bangap PTB7-Th and small-bandgap ATT-2 in NIR region, the proof-of-concept STOPVs show the highest PCE of 7.7% so far reported for single-junction STOPVs with a high transparency of 37%.
A new acceptor-donor-acceptor (A-D-A) type small molecule, namely NFBDT, with a heptacyclic benzodi(cyclopentadithiophene) (FBDT) unit based on BDT as the central building block, was designed and synthesized. Its optical, electrical, thermal properties and photovoltaic performances were systematically investigated. NFBDT exhibits a low optical bandgap of 1.56 eV resulting in wide and efficient absorption in the range from 600 to 800 nm, and suitable energy levels as an electron acceptor. With the widely used and successful wide bandgap polymer PBDB-T selected as the donor material, an optimized PCE of 10.42% was obtained for the PBDB-T:NFBDT-based device with an outstanding short-circuit current density of 17.85 mA cm-2 under AM 1.5G irradiation (100 mW cm-2), which is so far among the highest performance of NF-OSC devices. These results demonstrate that BDT unit could also be applied for designing NF-acceptors, and the fused-ring benzodi(cyclopentadithiophene) unit is a promising block for designing new and high performance NF-acceptors.
A new fluorinated nonfullerene acceptor, ITIC-Th1, has been designed and synthesized by introducing fluorine (F) atoms onto the end-capping group 1,1-dicyanomethylene-3-indanone (IC). On the one hand, incorporation of F would improve intramolecular interaction, enhance the push–pull effect between the donor unit indacenodithieno[3,2-b]thiophene and the acceptor unit IC due to electron-withdrawing effect of F, and finally adjust energy levels and reduce bandgap, which is beneficial to light harvesting and enhancing short-circuit current density (JSC). On the other hand, incorporation of F would improve intermolecular interactions through CF···S, CF···H, and CF···π noncovalent interactions and enhance electron mobility, which is beneficial to enhancing JSC and fill factor. Indeed, the results show that fluorinated ITIC-Th1 exhibits redshifted absorption, smaller optical bandgap, and higher electron mobility than the nonfluorinated ITIC-Th. Furthermore, nonfullerene organic solar cells (OSCs) based on fluorinated ITIC-Th1 electron acceptor and a wide-bandgap polymer donor FTAZ based on benzodithiophene and benzotriazole exhibit power conversion efficiency (PCE) as high as 12.1%, significantly higher than that of nonfluorinated ITIC-Th (8.88%). The PCE of 12.1% is the highest in fullerene and nonfullerene-based single-junction binary-blend OSCs. Moreover, the OSCs based on FTAZ:ITIC-Th1 show much better efficiency and better stability than the control devices based on FTAZ:PC71BM (PCE = 5.22%).
We design and synthesize four fused-ring electron acceptors based on 6,6,12,12-tetrakis(4-hexylphenyl)-indacenobis(dithieno[3,2-b;2',3'-d]thiophene) as the electron-rich unit and 1,1-dicyanomethylene-3-indanones with 0 to 2 fluorine substituents as the electron-deficient units. These four molecules exhibit broad (550-850 nm) and strong absorption with high extinction coefficients of (2.1-2.5) ×10(5) M(-1) cm(-1). Fluorine substitution down shifts LUMO energy level, red shift absorption spectrum, and enhance electron mobility. The polymer solar cells based on the fluorinated electron acceptors exhibit power conversion efficiencies as high as 11.5%, much higher than that of their nonfluorinated counterpart (7.7%). We investigate the effects of the fluorine atom number and position on electronic properties, charge transport, film morphology, and photovoltaic properties.
Simutaneously high open circuit voltage and high short circuit current density is a big challenge for achieving high efficiency polymer solar cells due to the excitonic nature of organic semdonductors. Herein, we developed a trialkylsilyl substituted 2D-conjugated polymer with the highest occupied molecular orbital level down-shifted by Si–C bond interaction. The polymer solar cells obtained by pairing this polymer with a non-fullerene acceptor demonstrated a high power conversion efficiency of 11.41% with both high open circuit voltage of 0.94 V and high short circuit current density of 17.32 mA cm−2 benefitted from the complementary absorption of the donor and acceptor, and the high hole transfer efficiency from acceptor to donor although the highest occupied molecular orbital level difference between the donor and acceptor is only 0.11 eV. The results indicate that the alkylsilyl substitution is an effective way in designing high performance conjugated polymer photovoltaic materials.
Nonfullerene acceptor FDICTF (2,9-bis(2methylene-(3-(1,1-dicyanomethylene)indanone))-7, 12- dihydro- 4, 4, 7, 7, 12, 12- hexaoctyl- 4H- cyclopenta[2″, 1″:5, 6;3″, 4″:5', 6'] diindeno[1, 2- b:1', 2'- b']dithiophene) modified by fusing the fluorene core in a precursor, yields 10.06% high power conversion efficiency, and demonstrates that the ladder and fused core backbone in A-D-A structure molecules is an effective design strategy for high-performance nonfullerene acceptors.
A thieno[3,4-b]thiophene-based electron acceptor (ATT-1) is designed and synthesized. ATT-1 exhibits planar conjugated framework, broad absorption with large absorption coefficient, and slightly high LUMO energy level. Bulk-heterojunction (BHJ) solar cells based on PTB7-Th electron donor and ATT-1 electron acceptor delivered PCEs of up to 10.07%, which is so far highest for non-fullerene BHJ solar cells using PTB7-Th donor.
Solution-processed organic photovoltaics (OPV) offer the attractive prospect of low-cost, light-weight and environmentally benign solar energy production. The highest efficiency OPV at present use low-bandgap donor polymers, many of which suffer from problems with stability and synthetic scalability. They also rely on fullerene-based acceptors, which themselves have issues with cost, stability and limited spectral absorption. Here we present a new non-fullerene acceptor that has been specifically designed to give improved performance alongside the wide bandgap donor poly(3-hexylthiophene), a polymer with significantly better prospects for commercial OPV due to its relative scalability and stability. Thanks to the well-matched optoelectronic and morphological properties of these materials, efficiencies of 6.4% are achieved which is the highest reported for fullerene-free P3HT devices. In addition, dramatically improved air stability is demonstrated relative to other high-efficiency OPV, showing the excellent potential of this new material combination for future technological applications.
Organic solar cells have desirable properties, including low cost of materials, high-throughput roll-to-roll production, mechanical flexibility and light weight. However, all top-performance devices are at present processed using halogenated solvents, which are environmentally hazardous and would thus require expensive mitigation to contain the hazards. Attempts to process organic solar cells from non-halogenated solvents lead to inferior performance. Overcoming this hurdle, here we present a hydrocarbon-based processing system that is not only more environmentally friendly but also yields cells with power conversion efficiencies of up to 11.7%. Our processing system incorporates the synergistic effects of a hydrocarbon solvent, a novel additive, a suitable choice of polymer side chain, and strong temperature-dependent aggregation of the donor polymer. Our results not only demonstrate a method of producing active layers of organic solar cells in an environmentally friendly way, but also provide important scientific insights that will facilitate further improvement of the morphology and performance of organic solar cells.
A nonfullerene electron acceptor (IEIC) based on indaceno[1,2-b:5,6-b′]dithiophene and 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile was designed and synthesized. IEIC exhibited good thermal stability, strong absorption in the 500–750 nm region with an extinction coefficient of 1.1 × 105 M−1 cm−1 at 672 nm, deep LUMO energy level (−3.82 eV) close to those of fullerenes, and a relatively high electron mobility of 2.1 × 10−4 cm2 V−1 s−1. Fullerene-free polymer solar cells (PSCs) based on the blends of the IEIC acceptor and a low-bandgap polymer donor PTB7-TH, using a perylene diimide derivative as a cathode interlayer, showed power conversion efficiencies (PCEs) of up to 6.31%, which is among the best PCEs reported for fullerene-free PSCs.
Recent progress in the development of polymer solar cells has improved
power-conversion efficiencies from 3% to almost 9%. Based on
semiconducting polymers, these solar cells are fabricated from
solution-processing techniques and have unique prospects for achieving
low-cost solar energy harvesting, owing to their material and
manufacturing advantages. The potential applications of polymer solar
cells are broad, ranging from flexible solar modules and semitransparent
solar cells in windows, to building applications and even photon
recycling in liquid-crystal displays. This Review covers the scientific
origins and basic properties of polymer solar cell technology, material
requirements and device operation mechanisms, while also providing a
synopsis of major achievements in the field over the past few years.
Potential future developments and the applications of this technology
are also briefly discussed.
We present the development and characterization of a dedicated resonant soft x-ray scattering facility. Capable of operation over a wide energy range, the beamline and endstation are primarily used for scattering from soft matter systems around the carbon K-edge (∼285 eV). We describe the specialized design of the instrument and characteristics of the beamline. Operational characteristics of immediate interest to users such as polarization control, degree of higher harmonic spectral contamination, and detector noise are delineated. Of special interest is the development of a higher harmonic rejection system that improves the spectral purity of the x-ray beam. Special software and a user-friendly interface have been implemented to allow real-time data processing and preliminary data analysis simultaneous with data acquisition.
The photocurrent in conjugated polymer-fullerene blends is dominated by the dissociation efficiency of bound electron-hole pairs at the donor-acceptor interface. A model based on Onsager's theory of geminate charge recombination explains the observed field and temperature dependence of the photocurrent in PPV:PCBM blends. At room temperature only 60% of the generated bound electron-hole pairs are dissociated and contribute to the short-circuit current, which is a major loss mechanism in photovoltaic devices based on this material system.
A wide bandgap small molecular acceptor, SFBRCN, containing a 3D spirobifluorene core flaked with a 2,1,3-benzothiadiazole (BT) and end-capped with highly electron-deficient (3-ethylhexyl-4-oxothiazolidine-2-yl)dimalononitrile (RCN) units, has been successfully synthesized as a small molecular acceptor (SMA) for nonfullerene polymer solar cells (PSCs). This SMA exhibits a relatively wide optical bandgap of 2.03 eV, which provides a complementary absorption to commonly used low bandgap donor polymers, such as PTB7-Th. The strong electron-deficient BT and RCN units afford SFBRCN with a low-lying LUMO (lowest unoccupied molecular orbital) level, while the 3D structured spirobifluorene core can effectively suppress the self-aggregation tendency of the SMA, thus yielding a polymer:SMA blend with reasonably small domain size. As the results of such molecular design, SFBRCN enables nonfullerene PSCs with a high efficiency of 10.26%, which is the highest performance reported to date for a large bandgap nonfullerene SMA.
An effective way to improve the power conversion efficiency of organic solar cells is to use a tandem architecture consisting of two subcells, so that a broader part of the solar spectrum can be used and the thermalization loss of photon energy can be minimized. For a tandem cell to work well, it is important for the subcells to have complementary absorption characteristics and generate high and balanced (matched) currents. This requires a rather challenging effort to design and select suitable active materials for use in the subcells. Here, we report a high-performance solution-processed, tandem solar cell based on the small molecules DR3TSBDT and DPPEZnP-TBO, which offer efficient, complementary absorption when used as electron donor materials in the front and rear subcells, respectively. Optimized devices achieve a power conversion efficiency of 12.50% (verified 12.70%), which represents a new level of capability for solution-processed, organic solar cells.
Technological deployment of organic photovoltaic modules requires improvements in device light-conversion efficiency and stability while keeping material costs low. Here we demonstrate highly efficient and stable solar cells using a ternary approach, wherein two non-fullerene acceptors are combined with both a scalable and affordable donor polymer, poly(3-hexylthiophene) (P3HT), and a high-efficiency, low-bandgap polymer in a single-layer bulk-heterojunction device. The addition of a strongly absorbing small molecule acceptor into a P3HT-based non-fullerene blend increases the device efficiency up to 7.7 ± 0.1% without any solvent additives. The improvement is assigned to changes in microstructure that reduce charge recombination and increase the photovoltage, and to improved light harvesting across the visible region. The stability of P3HT-based devices in ambient conditions is also significantly improved relative to polymer:fullerene devices. Combined with a low-bandgap donor polymer (PBDTTT-EFT, also known as PCE10), the two mixed acceptors also lead to solar cells with 11.0 ± 0.4% efficiency and a high open-circuit voltage of 1.03 ± 0.01 V.
Five polymer donors with distinct chemical structures and different electronic properties are surveyed in a planar and narrow-bandgap fused-ring electron acceptor (IDIC)-based organic solar cells, which exhibit power conversion efficiencies of up to 11%.
Low bandgap n-type organic semiconductor (n-OS) ITIC has attracted great attention for the application as acceptor with medium bandgap p-type conjugated polymer as donor in non-fullerene polymer solar cells (PSCs) because of its attractive photovoltaic performance. Here we report a modification on the molecular structure of ITIC by side chain isomerization with meta-alkyl-phenyl substitution, m-ITIC, to further improve its photovoltaic performance. Compared with its isomeric counterpart ITIC with para-alkyl-phenyl substitution, m-ITIC shows a higher film absorption coefficient, a larger crystalline coherence and higher electron mobility. These inherent advantages of m-ITIC resulted in a higher power conversion efficiency (PCE) of 11.77% for the non-fullerene PSCs with m-ITIC as acceptor and a medium bandgap polymer J61 as donor, which is significantly improved over that (10.57%) of the corresponding devices with ITIC as acceptor. To the best of our knowledge, the PCE of 11.77% is one of the highest values reported in literatures to date for non-fullerene PSCs. More importantly, m-ITIC-based device shows less thickness-dependent photovoltaic behavior than ITIC-based devices in the active-layer thickness range of 80~360 nm, which is beneficial for large area device fabrication. These results indicate that m-ITIC is a promising low bandgap n-OS for the application as acceptor in PSCs and the side chain isomerization could be an easy and convenient way to further improve the photovoltaic performance of the donor and acceptor materials for high efficiency PSCs.
The structure evolution of oligomer fused-ring electron acceptors (FREAs) toward high efficiency of as-cast polymer solar cells (PSCs) is reported. First, a series of FREAs (IC-(1-3)IDT-IC) based on indacenodithiophene (IDT) oligomers as cores are designed and synthesized, effects of IDT number (1-3) on their basic optical and electronic properties are investigated, and more importantly, the relationship between device performance of as-cast PSCs and donor(D)/acceptor(A) matching (absorption, energy level, morphology, and charge transport) of IC-(1-3)IDT-IC acceptors and two representative polymer donors, PTB7-Th and PDBT-T1 is surveyed. Then, the most promising D/A system (PDBT-T1/IC-1IDT-IC) with the best D/A harmony among the six D/A combinations, which yields a power conversion efficiency (PCE) of 7.39%, is found. Finally, changing the side-chains in IC-1IDT-IC from alkylphenyl to alkyl enhances the PCE from 7.39% to 9.20%.
A planar fused-ring electron acceptor (IC-C6IDT-IC) based on indacenodithiophene is designed and synthesized. IC-C6IDT-IC shows strong absorption in 500-800 nm with extinction coefficient of up to 2.4 × 105 M-1 cm-1 and high electron mobility of 1.1 × 10-3 cm2 V-1 s-1. The as-cast polymer solar cells based on IC-C6IDT-IC without additional treatments exhibit power conversion efficiencies of up to 8.71%.
In the past two years, non-fullerene acceptors including polymers and small molecules have become the focus of many research efforts. Fullerene-free organic solar cells (OSCs) have shown efficiencies of up to 6.8% for solution-processed devices, and even up to 8.4% for vacuum-deposited devices, which have been significantly improved relative to those disclosed 2 years ago (generally <4%). Non-fullerene acceptor materials are a new focus in the OSC field. Tailoring extended fused-rings with electron-deficient groups is an effective strategy for design of acceptors. Here, very recent developments in several systems of fused ring-based electron acceptors, such as halogenated (sub or subna)phthalocyanine, imide-functionalized rylene, and linear fused-rings end capped with electron deficient blocks, are reviewed.
A novel non-fullerene electron acceptor (ITIC) that overcomes some of the shortcomings, for example, weak absorption in the visible spectral region and limited energy level variability, of fullerene acceptors is designed and synthesized. Fullerene-free polymer solar cells (PSCs) based on the ITIC acceptor are demonstrated to exhibit power conversion efficiencies of up to 6.8%, a record for fullerene-free PSCs.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Developing novel materials and device architectures to further enhance the efficiency of polymer solar cells requires a fundamental understanding of the impact of chemical structures on photovoltaic properties. Given that device characteristics depend on many parameters, deriving structure-property relationships has been very challenging. Here we report that a single parameter, hole mobility, determines the fill factor of several hundred nanometer thick bulk heterojunction photovoltaic devices based on a series of copolymers with varying amount of fluorine substitution. We attribute the steady increase of hole mobility with fluorine content to changes in polymer molecular ordering. Importantly, all other parameters, including the efficiency of free charge generation and the coefficient of nongeminate recombination, are nearly identical. Our work emphasizes the need to achieve high mobility in combination with strongly suppressed charge recombination for the thick devices required by mass production technologies.
Although fullerenes and their derivatives, such as PCBM, have been the dominant electron-acceptor materials in organic photovoltaic cells (OPVs), they suffer from some disadvantages, such as weak absorption in the visible spectral region, limited spectral breadth and difficulty in variably tuning the band gap. It is necessary to explore non-fullerene electron acceptors that will not only retain the favorable electron-accepting and transporting properties of fullerenes but also overcome their insufficiencies. After a decade of mediocrity, non-fullerene acceptors are undergoing rapid development and are emerging as a hot area of focus in the field of organic semiconductors. Solution-processed bulk heterojunction (BHJ) OPVs based on non-fullerene acceptors have shown encouraging power conversion efficiencies of over 4%. This article reviews recent developments in several classes of solution-processable non-fullerene acceptors for BHJ OPVs. The remaining problems and challenges along with the key research directions in the near future are discussed.
The status of understanding of the operation of bulk heterojunction (BHJ) solar cells is reviewed. Because the carrier photoexcitation recombination lengths are typically 10 nm in these disordered materials, the length scale for self-assembly must be of order 10-20 nm. Experiments have verified the existence of the BHJ nanostructure, but the morphology remains complex and a limiting factor. Three steps are required for generation of electrical power: i) absorption of photons from the sun; ii) photoinduced charge separation and the generation of mobile carriers; iii) collection of electrons and holes at opposite electrodes. The ultrafast charge transfer process arises from fundamental quantum uncertainty; mobile carriers are directly generated (electrons in the acceptor domains and holes in the donor domains) by the ultrafast charge transfer (≈70%) with ≈30% generated by exciton diffusion to a charge separating heterojunction. Sweep-out of the mobile carriers by the internal field prior to recombination is essential for high performance. Bimolecular recombination dominates in materials where the donor and acceptor phases are pure. Impurities degrade performance by introducing Shockly-Read-Hall decay. The review concludes with a summary of the problems to be solved to achieve the predicted power conversion efficiencies of >20% for a single cell.
A solar park based on polymer solar cells is described and analyzed with respect to performance, practicality, installation speed, and energy payback time. It is found that a high voltage installation where solar cells are all printed in series enables an installation rate in Watts installed per minute that far exceed any other PV technology in existence. The energy payback time for the practical installation of polymer solar cell foil on a wooden 250 square meter platform in its present form is 277 days when operated in Denmark and 180 days when operated in southern Spain. The installation and de-installation rate is above 100 m min−1, which, with the present performance and web width, implies installation of >200 W min−1. In comparison, this also exceeds the overall manufacturing speed of the polymer solar cell foil with a width of 305 mm which is currently 1 m min−1 for complete encapsulated and tested foil. It is also significant that simultaneous installation and de-installation which enables efficient schemes for decommissioning and recycling is possible. It is highlighted where research efforts should most rationally be invested in order to make grid electricity from OPV a reality (and it is within reach).
Organic photovoltaic (OPV) technology has been developed and improved
from a fancy concept with less than 1% power conversion effi ciency (PCE)
to over 10% PCE, particularly through the efforts in the last decade. The
signifi cant progress is the result of multidisciplinary research ranging from
chemistry, material science, physics, and engineering. These efforts include
the design and synthesis of novel compounds, understanding and controlling
the fi lm morphology, elucidating the device mechanisms, developing
new device architectures, and improving large-scale manufacture. All of these
achievements catalyzed the rapid growth of the OPV technology. This review
article takes a retrospective look at the research and development of OPV,
and focuses on recent advances of solution-processed materials and devices
during the last decade, particular the polymer version of the materials and
devices. The work in this fi eld is exciting and OPV technology is a promising
candidate for future thin fi lm solar cells.
We show that rational functionalization of the naphthodithiophene core in copolymers based on naphthodithiophene and naphthobisthiadiazole improves the solubility without an alteration of the electronic structure. Surprisingly, the introduction of linear alkyl chains brings about a drastic change in polymer orientation into the face-on motif, which is beneficial for the charge transport in solar cells. As a result, the present polymers exhibit high power conversion efficiencies of up to ∼8.2% in conventional single-junction solar cells.
The carrier collection efficiency (ηc) and energy conversion efficiency (ηe) of polymer photovoltaic cells were improved by blending of the semiconducting polymer with C60 or its functionalized derivatives. Composite films of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV)
and fullerenes exhibit ηc of about 29 percent of electrons per photon and ηe of about 2.9 percent, efficiencies that are better by more than two orders of magnitude than those that have been achieved
with devices made with pure MEH-PPV. The efficient charge separation results from photoinduced electron transfer from the
MEH-PPV (as donor) to C60 (as acceptor); the high collection efficiency results from a bicontinuous network of internal donor-acceptor heterojunctions.
