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A Ladder-Type Heteroheptacene 12H-Dithieno[2',3':4,5]thieno[3,2-b:2',3'-h]fluorene Based D-A Copolymer with Strong Intermolecular Interactions towards Efficient Polymer Solar Cells

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Abstract

Ladder-type electron-donating units for D-A copolymers applied in polymer solar cells usually comprise multiple tetrahedral carbon bridges bonded with out-of-plane alkyl chains for desirable solubility for device processing. However, molecular packing of resultant copolymers in the solid state and charge transport within devices are also impeded in spite of with multiple fused aromatic backbones. To mitigate this issue, a structurally well-defined ladder-type electron donating heteroheptacene, 12H-dithieno[2',3':4,5]thieno[3,2-b:2',3'-h]fluorene (DTTF) with extended conjugated backbone and a single tetrahedral carbon bridge attached with two bulky alkyl chains, was designed and synthesized. The copolymerization of DTTF with 4,7-bis(4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTBT) afforded a soluble D-A copolymer (PDTTF-DTBT) with medium optical bandgap of 1.72 eV and low-lying HOMO level at −5.36 eV. PDTTF-DTBT unprecedentedly exhibits strong intermolecular stacking ability and presents preferential face-on orientation on both ZnO and PEDOT:PSS layer. The improved packing order and appropriate phase separation of both the copolymer and PC71BM in the bulk heterojunction blend on ZnO layer over on PEDOT:PSS layer lead to much improved power conversion efficiency of ~ 8.2% in the inverted solar cell device, among the highest for reported ladder-type D-A copolymers. The research demonstrate that it is an effective method to incorporate a single tetrahedral carbon bridge to the molecular center of a ladder-type heteroacenes with heavily extended π-conjugation to prepare D-A copolymers towards highly efficient PSCs.

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A novel non-fullerene acceptor, 2,2′-((2Z,2′Z)-((5,5′-(4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(4-((2-ethylhexyl)oxy)thiophene-5,2-diyl))bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IEICO), possessing a very low bandgap of 1.34 eV and a high-lying lowest unoccupied molecular orbital level of −3.95 eV, is designed and synthesized by introducing electron-donating alkoxy groups to the backbone of a conjugated small molecule. Impressive power conversion efficiencies of 8.4% and 10.7% are obtained for the IEICO-based single and tandem polymer solar cells.
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Advances in the design and application of highly efficient conjugated polymers and small molecules over the past years have enabled the rapid progress in the development of organic photovoltaic (OPV) technology as a promising alternative to conventional solar cells. Among the numerous OPV materials, benzodithiophene (BDT)-based polymers and small molecules have come to the fore in achieving outstanding power conversion efficiency (PCE) and breaking 10% efficiency barrier in the single junction OPV devices. Remarkably, the OPV device featured by BDT-based polymer has recently demonstrated an impressive PCE of 11.21%, indicating the great potential of this class of materials in commercial photovoltaic applications. In this review, we offered an overview of the organic photovoltaic materials based on BDT from the aspects of backbones, functional groups, alkyl chains, and device performance, trying to provide a guideline about the structure-performance relationship. We believe more exciting BDT-based photovoltaic materials and devices will be developed in the near future.
Article
Unlabelled: Herein, a successful application of V2O5·nH2O film as hole transporting layer (HTL) instead of Pedot: PSS in polymer solar cells is demonstrated. The V2O5·nH2O layer was spin-coated from V2O5·nH2O sol made from melting-quenching sol-gel method by directly using vanadium oxide powder, which is readily accessible and cost-effective. V2O5·nH2O (n ≈ 1) HTL is found to have comparable work function and smooth surface to that of Pedot: PSS. For the solar cell containing V2O5·nH2O HTL and the active layer of the blend of a novel polymer donor (PBDSe-DT2PyT) and the acceptor of PC71BM, the PCE was significantly improved to 5.87% with a 30% increase over 4.55% attained with Pedot: PSS HTL. Incorporation of V2O5·nH2O as HTL in the polymer solar cell was found to enhance the crystallinity of the active layer, electron-blocking at the anode and the light-harvest in the wavelength range of 400-550 nm in the cell. V2O5·nH2O HTL improves the charge generation and collection and suppress the charge recombination within the PBDSe-DT2PyT:PC71BM solar cell, leading to a simultaneous enhancement in Voc, Jsc, and FF. The V2O5·nH2O HTL proposed in this work is envisioned to be of great potential to fabricate highly efficient PSCs with low-cost and massive production.
Article
Unlabelled: A new n-type polymer, PF3N-2TNDI, with high electron mobility, is developed as efficient cathode interfacial material and interconnecting layer (ICL) for constructing high-performance tandem organic solar cells. Tandem cells employing the ICL with structure of PF3N-2TNDI/Ag/ Pedot: PSS achieve a high power conversion efficiency (PCE) of 11.35%. Moreover, flexible tandem cells with PCE over 10% are also demonstrated.
Article
A series of ladder-type thienoacenes based on benzodithiophene (BDT) has been synthesized and characterized. They were shown to be p-type of semiconductors with wide band gaps and able to support multiple, stable cationic states. As the conjugation lengthens, these oligomers become more emissive, showing high quantum yields. They were shown to be good two photon absorber, exhibiting high two photon absorption coef-ficients.
Article
Harvesting solar energy from sunlight to generate electricity is considered as one of the most important technologies to address the future sustainability of humans. Polymer solar cells (PSCs) have attracted tremendous interest and attention over the past two decades due to their potential advantage to be fabricated onto large area and light-weight flexible substrates by solution processing at a lower cost. PSCs based on the concept of bulk heterojunction (BHJ) configuration where an active layer comprises a composite of a p-type (donor) and an n-type (acceptor) material represents the most useful strategy to maximize the internal donor-acceptor interfacial area allowing for efficient charge separation. Fullerene derivatives such as [6,6]-phenyl-C61 or 71-butyric acid methyl ester (PCBM) are the ideal n-type materials ubiquitously used for BHJ solar cells. The major effort to develop photoactive materials is numerously focused on the p-type conjugated polymers which are generally synthesized by polymerization of electron-rich donor and electron-deficient acceptor monomers. Compared to the development of electron-deficient comonomers (acceptor segments), the development of electron-rich donor materials is considerably flourishing. Forced planarization by covalently fastening adjacent aromatic and heteroaromatic subunits leads to the formation of ladder-type conjugated structures which are capable of elongating effective conjugation, reducing the optical bandgap, promoting intermolecular π-π interactions and enhancing intrinsic charge mobility. In this review, we will summarize the recent progress on the development of various well-defined new ladder-type conjugated materials. These materials serve as the superb donor monomers to prepare a range of donor-acceptor semi-ladder copolymers with sufficient solution-processability for solar cell applications.
Article
Charge transport and nongeminate recombination are investigated in two solution-processed small molecule bulk heterojunction solar cells consisting of diketopyrrolopyrrole (DPP)-based donor molecules, mono-DPP and bis-DPP, blended with [6,6]-phenyl-C71-butyric acid methyl ester (PCBM). While the bis-DPP system exhibits a high fill factor (62%) the mono-DPP system suffers from pronounced voltage dependent losses, which limit both the fill factor (46%) and short circuit current. A method to determine the average charge carrier density, recombination current, and effective carrier lifetime in operating solar cells as a function of applied bias is demonstrated. These results and light intensity measurements of the current-voltage characteristics indicate that the mono-DPP system is severely limited by nongeminate recombination losses. Further analysis reveals that the most significant factor leading to the difference in fill factor is the comparatively poor hole transport properties in the mono-DPP system (2 × 10−5 cm2 V−1 s−1 versus 34 × 10−5 cm2 V−1 s−1). These results suggest that future design of donor molecules for organic photovoltaics should aim to increase charge carrier mobility thereby enabling faster sweep out of charge carriers before they are lost to nongeminate recombination.
Article
Tandem solar cells have the potential to improve photon conversion efficiencies (PCEs) beyond the limits of single-junction devices. In this study, a triple-junction tandem design is demonstrated by employing three distinct organic donor materials having bandgap energies ranging from 1.4 to 1.9 eV. Through optical modeling, balanced photon absorption rates are achieved and, thereby, the photo-currents are matched among the three subcells. Accordingly, an efficient triple-junction tandem organic solar cell can exhibit a record-high PCE of 11.5%.
Article
Solution processed single junction polymer solar cells (PSCs) has been developed from less than 1% power conversion efficiency (PCE) to beyond 9% PCE in the last decade. The significant efficiency improvement comes from progress in both rational design of donor polymers and innovation of device architectures. Among all the novel high efficient donor polymers, PTB7 stands out as the most widely used one for solar cell studies. Herein the recent development of PTB7 solar cells is reviewed. Detailed discussion of basic property, structure property relationship, morphology study, interfacial engineering, and inorganic nanomaterials incorporation is provided. We concluded with possible future directions for further increasing the performance of PTB7 solar cells.
Article
This Progress Report highlights recent advances in polymer solar cells with special attention focused on the recent rapid-growing progress in methods that use a thin layer of alcohol/water-soluble conjugated polymers as key component to obtain optimized device performance, but also discusses novel materials and device architectures made by major prestigious institutions in this field. We anticipate that due to drastic improvements in efficiency and easy utilization, this method opens up new opportunities for PSCs from various material systems to improve towards 10% efficiency, and many novel device structures will emerge as suitable architectures for developing the ideal roll-to-roll type processing of polymer-based solar cells.
Article
Solution deposited bulk heterojunction organic solar cells are viewed as one of the most promising alternative energy sources because of their ease of processing and their potential to be produced using large scale techniques such as roll-to-roll, newspaper style, coating. Since organic materials have a relatively low dielectric constant the dissociation of an excited electron–hole pair into free collectable charge carriers is inefficient in many cases. Often the excited electron–hole pairs recombine back to the ground state in a process known as geminate recombination before they ever fully dissociate into free charge carriers. Even after dissociation, free holes and electrons can encounter each other once more and subsequently recombine back to the ground state in a process known as nongeminate recombination. In both cases the incident photon energy is lost and fewer carriers are collected at the electrodes. Hence, charge carrier recombination is one of the key loss mechanisms in organic solar cells. In this review the latest on geminate and nongeminate recombination is discussed.
Article
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.
Article
We demonstrate that the power conversion efficiency can be significantly improved in solution-processed small-molecule solar cells by tuning the thickness of the active layer and inserting an optical spacer (ZnO) between the active layer and the Al electrode. The enhancement in light absorption in the cell was measured with UV-vis absorption spectroscopy and by measurements of the photo-induced carriers generation rate. The ZnO layer used to improve the light-harvesting increases the charge collection efficiency, serves as a blocking layer for holes, and reduces the recombination rate. The combined optical and electrical improvements raise the power conversion efficiency of solution-processed small-molecule solar cells to 8.94%; i.e. comparable to that of polymer counterparts.
Article
Solution-processed small molecule p-DTS(FBTTh2)2:PC71BM bulk-heterojunction (BHJ) solar cells with power conversion efficiency of 8.01% are demonstrated. The fill factor (FF) is sensitive to the thickness of a Calcium (Ca) layer between the BHJ layer and the Al cathode; for 20nm Ca thickness, the FF ~ 73 %, the highest value reported for an organic solar cell. The maximum external quantum efficiency exceeds 80%. After correcting for the total absorption in the cell through normal incidence reflectance measurements, the internal quantum efficiency approaches 100 % in the spectral range 600-650 nm and well over 80 % across the entire spectral range from 400 - 700 nm. Analysis of the current-voltage (J-V) characteristics at various light intensities provides information on the different recombination mechanisms in the BHJ solar cells with different thicknesses of the Ca layer. Our analysis reveals that the J-V curves are dominated by first-order recombination from the short circuit condition to the maximum power point and evolve to bimolecular recombination in the range of voltage from the maximum power point to the open circuit condition in the optimized device with Ca thickness of 20 nm. In addition, the normalized photocurrent density curves reveal that the charge collection probability remains high; about 90% of charges are collected even at the maximum power point. The dominance of bimolecular recombination only when approaching open circuit, the lack of Shockley-Read-Hall recombination at open circuit, and the high charge collection probability (97.6% at the short circuit and constant over wide range of applied voltage) leads to the high fill factor.
Article
Conjugated polymers based on a heteroacene, 3,7-dialkyl-dithieno[2,3-d:2',3'-d']benzo[1,2-b:4,5-b']dithiophene (DBD), are synthesized. These polymers show broad UV-vis absorption with energy bandgaps below 1.7 eV. PTDBD2, showing good miscibility in a polymer/phenyl-C71-butyric acid methyl ester (PC(71) BM) blend film, achieves a power conversion efficiency (PCE) of 7.6%. The results indicate that copolymers containing DBD are promising candidates for high-performance organic solar cells.
Article
The research on the polymer-based solar cells (PSCs) has attracted an increasing amount of attention in recent years because PSCs pose potential advantages over mainstream inorganic-based solar cells, such as significantly reduced material/fabrication costs, flexible substrates, and light weight of finished solar cells. The research community has made great progress in the field of bulk heterojunction (BHJ) polymer solar cells since its inception in 1995. The power conversion efficiency (PCE), a key parameter to assess the performance of solar cells, has increased from 1% in the 1990s to over 8% just recently. These great advances are mainly fueled by the development of conjugated polymers used as the electron-donating materials in BHJ solar cells. In this Perspective, we first briefly review the progress on the design of conjugated polymers for polymer solar cells in the past 16 years. Since a conjugated polymer can be arbitrarily divided into three constituting components-the conjugated backbone, the side chains, and the substituents-we then focus on the rational design of conjugated polymers by separately discussing the influence of each component on the physical and photovoltaic (PV) properties of these polymers. Special attention is paid to the design of donor-acceptor type low-band-gap polymers because this approach is prevailing in the literature with its unique features. In doing so, we strive to extract useful rules for the rational design of conjugated polymers with predictable properties. We conclude by proposing future research opportunities to achieve even higher PCEs for PSCs.
Article
Bulk heterojunction (BHJ) polymer solar cells (PSCs) sandwich a blend layer of conjugated polymer donor and fullerene derivative acceptor between a transparent ITO positive electrode and a low work function metal negative electrode. In comparison with traditional inorganic semiconductor solar cells, PSCs offer a simpler device structure, easier fabrication, lower cost, and lighter weight, and these structures can be fabricated into flexible devices. But currently the power conversion efficiency (PCE) of the PSCs is not sufficient for future commercialization. The polymer donors and fullerene derivative acceptors are the key photovoltaic materials that will need to be optimized for high-performance PSCs.
Article
The prospect of using low cost, high throughput material deposition processes to fabricate organic circuitry and solar cells continues to drive research towards improving the performance of the semiconducting materials utilized in these devices. Conjugated aromatic polymers have emerged as a leading candidate semiconductor material class, due to their combination of their amenability to processing and reasonable electrical and optical performance. Challenges remain, however, to further improve the charge carrier mobility of the polymers for transistor applications and the power conversion efficiency for solar cells. This optimization requires a clear understanding of the relationship between molecular structure and both electronic properties and thin film morphology.
Article
A flexible solar device showing exceptional air and mechanical stability is produced by simultaneously optimizing molecular structure, active layer morphology, and interface characteristics. The PFDCTBT-C8-based devices with inverted architecture exhibited excellent power conversion efficiencies of 7.0% and 6.0% on glass and flexible substrates, respectively.
Article
We have developed an improved small-angle X-ray scattering (SAXS) model and analysis methodology to quantitatively evaluate the nanostructures of a blend system. This method has been applied to resolve the various structures of self-organized poly(3-hexylthiophene)/C61-butyric acid methyl ester (P3HT/PCBM) thin active layer in a solar cell from the studies of both grazing-incidence small-angle X-ray scattering (GISAXS) and grazing-incidence X-ray diffraction (GIXRD). Tuning the various length scales of PCBM-related structures by a different annealing process can provide a flexible approach and better understanding to enhance the power conversion of the P3HT/PCBM solar cell. The quantitative structural characterization by this method includes (1) the mean size, volume fraction, and size distribution of aggregated PCBM clusters, (2) the specific interface area between PCBM and P3HT, (3) the local cluster agglomeration, and (4) the correlation length of the PCBM molecular network within the P3HT phase. The above terms are correlated well with the device performance. The various structural evolutions and transformations (growth and dissolution) between PCBM and P3HT with the variation of annealing history are demonstrated here. This work established a useful SAXS approach to present insight into the modeling of the morphology of P3HT/PCBM film. In situ GISAXS measurements were also conducted to provide informative details of thermal behavior and temporal evolution of PCBM-related structures during phase separation. The results of this investigation significantly extend the current knowledge of the relationship of bulk heterojunction morphology to device performance.
Article
Solar cells based on the polymer−fullerene bulk heterojunction (BHJ) concept are an attractive class of low-cost solar energy harvesting devices. Because the power conversion efficiency (PCE) of these solar cells is still significantly lower than that of their inorganic counterparts, however, materials design and device engineering efforts are directed toward improving their output. A variety of factors limit the performance of BHJ solar cells, but the properties of the materials in the active layer are the primary determinant of their overall efficiency.