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Abstract

We compared indacenodithiophene(IDT)-based fused-ring electron acceptor IDIC1 with its counterpart IHIC1 in which the central benzene unit is replaced by naphthalene unit, and investigated effects of benzene/naphthalene core on optical and electronic properties as well as performance of organic solar cells (OSCs). Compared with benzene-cored IDIC1, naphthalene-cored IHIC1 has a larger π-conjugation with stronger intermolecular π-π stacking. Relative to benzene-cored IDIC1, naphthalene-cored IHIC1 shows a higher lowest unoccupied molecular orbital energy level (IHIC1: −3.75 eV, IDIC1: –3.81 eV) and a higher electron mobility (IHIC1: 3.0 × 10−4 cm2 V−1 s−1, IDIC1: 1.5 × 10−4 cm2 V−1 s−1). When paired with the polymer donor FTAZ that has matched energy levels and complementary absorption spectrum, IHIC1-based OSCs show higher values in open-circuit voltage, short-circuit current density, fill factor and power conversion efficiency relative to the IDIC1-based control devices. These results demonstrate that extending benzene in IDT to naphthalene is a promising approach to upshift energy levels, enhance electron mobility, and finally achieve higher efficiency in nonfullerene acceptor-based OSCs.

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... [14][15][16][17][18][19][20][21]27,[31][32][33][34] However, a more in-depth understanding of these requirements is lacking. For example, there have been many studies on the synthesis and performance of new FREAs, 1,12,13,23,[35][36][37][38][39][40][41][42][43] but little work has focused on the molecular packing of these materials (i.e., requirement (d) above). To obtain high efficiency, OPV electron acceptors need a high electron mobility in order to extract electrons from the active layer and transport them to the cathode before they recombine. ...
... The chemical structure for each acceptor material in this study is depicted in Fig. 1b, and the full synthetic route for each of the FREAs (IDTCF and IDIC) is shown in Fig. S2 (ESI †). The indacenodithiophene (IDT) core, INCN, and TCF acceptor end groups were synthesized according to previous literature reports, 11,39,66 and a Knoevenagel condensation between IDT and INCN or TCF afforded the IDIC or IDTCF in 75% and 52% yields, respectively. The structure of each FREA was confirmed by nuclear magnetic resonance (NMR) ( Fig. S3 and S4, ESI †) and mass spectroscopy (see ESI †), and each FREA showed good solubility in common solvents such as chloroform, toluene, and chlorobenzene. ...
... Fig. S7b (ESI †) shows the dimer system for IDIC, and the distance between the INCN end groups was calculated to be 3.58 Å. This value is further corroborated within literature reports, where GIWAXS measurements of films of neat IDIC show an in-plane (IP) p-p stacking distance of 3.45 Å. 39 In the dimer system of IDTCF, shown in Fig. S7d (ESI †), the FREAs show more twisting and an expanded p-p stacking distance of 3.84 Å. It is important to note that this is the closest packing that is possible for the IDTCF acceptors, and the distance between end groups can be even larger in real films. ...
Article
Newly developed fused-ring electron acceptors (FREAs) have proven to be an effective class of materials for extending the absorption window and boosting the efficiency of organic photovoltaics (OPVs). While numerous acceptors have been developed, there is surprisingly little structural diversity among high performance FREAs in literature. Of the high efficiency electron acceptors reported, the vast majority utilize derivatives of 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (INCN) as the acceptor moiety. It has been postulated that the high electron mobility exhibited by FREA molecules with INCN end groups is a result of close π-π stacking between the neighboring planar INCN groups, forming an effective charge transport pathway between molecules. To explore this as a design rationale for electron acceptors, we synthesized a new fused-ring electron acceptor, IDTCF, which has methyl substituents out of plane to the conjugated acceptor backbone. These methyl groups hinder packing and expand the π-π stacking distance by ~ 1 Å, but have little impact on the optical or electrochemical properties of the individual FREA molecule. The extra steric hindrance from the out of plane methyl substituents restricts packing and results in large amounts of geminate recombination, thus degrading the device performance. Our results show that intermolecular interactions (especially π-π stacking between end groups) play a crucial role in performance of FREAs. We demonstrated that the planarity of the acceptor unit is of paramount importance as even minor deviations in end group distance are enough to disrupt crystallinity and cripple device performance.
... Pyrene has a highly planar structure, good thermal stability, 38 a deep highest occupied molecular orbital (HOMO), 39 and high charge mobility 40 and is thus widely used in the synthesis of organic semiconductors. Inspired by our previous work in which we discovered that replacing the central benzene unit in IDT with naphthalene leads to blue-shifted absorption of FREA, 41 to blue-shift the absorption of narrow-bandgap ITIC, we replace the central benzene unit in IDTT using pyrene. Moreover, to improve electron mobility, we replace IC end groups in ITIC using difluorinated IC (2FIC) because we discovered that replacing IC with 2FIC leads to the higher electron mobility of FREA. ...
Article
A new mid-bandgap nonfullerene acceptor, FPIC, is designed and synthesized based on a novel fused-pyrene electron-donating core. FPIC exhibits intense light absorption between 500 and 750 nm, with a maximum molar extinction coefficient of 2.3 × 105 M–1 cm–1 at 645 nm, a medium optical bandgap of 1.63 eV, as well as a high electron mobility of 1.7 × 10–3 cm2 V–1 s–1. The ternary-blend organic solar cells (OSCs) composing of low-bandgap donor PTB7-Th, ultranarrow-bandgap nonfullerene acceptor F8IC and FPIC yield a high power conversion efficiency (PCE) of 13.0%, significantly surpassing the PCE value of the PTB7-Th/F8IC binary-blend OSCs (9.55%). The ternary blend exhibits complementary absorption, effective exciton dissociation, balanced charge transport and reduced charge recombination, leading to the improvement in open-circuit voltage, short-circuit current density and fill factor, relative to PTB7-Th/F8IC counterpart. This work indicates that the mid-bandgap fused-pyrene electron acceptor FPIC is a promising third component to enhance photovoltaic performance of low-bandgap donor/acceptor binary blends.
... However, overall, this modification resulted in a reduced PCE (7.15%) compared to 6-1 due to a significantly decreased FF. 481 Feng et al. used non-fluorinated INCN units and solubilizing para-alkoxy-phenyl side chains leading to structure 6-3 (O-NTIC), 482 which has a slightly increased band gap compared to the phenylhexyl-substituted analogue IHIC1. 502 By combining this donor unit with INCN-2F (6-4, NT-4F) and INCN-2Cl (6-5, NT-4Cl) end groups, NFAs with lower optical band gaps and downshifted energy levels are obtained. With PM6/6-4-based solar cells, PCEs of 9.46% are reported; PM6/6-5 absorber layers lead to PCEs of 11.4%. ...
Article
Full-text available
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.
... Among them, varying the terminal electron-withdrawing group is proved to be one of effective ways to adjust the frontier molecular orbitals (FMO) and bandgaps [32][33][34][35][36][37], by which the highest PCE over 13% from IDTT based material could be achieved [38,39]. Expanding backbone is another way to improve photoelectronic properties of acceptors, such as replacing the interior benzene unit of IDTT by naphtha- lene [40,41]. Besides that, the side-chain engineering is also adopted by indirectly homoconjugative effect to tune the relative energy levels [42][43][44]. ...
Article
Full-text available
Short-axis substitution, as an effective way to change the optical and electronic properties of the organic semiconductors for organic photovoltaics (OPVs), is a readily approach to modify non-fullerene acceptors, especially for the linear fused rings system. Here, two new fused-ring electron acceptors (CBT-IC and SBT-IC) were designed and developed by short-axis modification based on the dithienyl[1,2-b:4,5-b′]benzodithiophene (BDCPDT) system. Combined with a medium bandgap polymer donor J71, both of the OPV devices exhibit high power conversion efficiency (PCE) over 11%, and ~70% external quantum efficiencies. To better understand how this kind of substitution affects the BDCPDT based acceptors, a comparative analysis is also made with the the plain acceptor BDT-IC without this modification. We believe this work could disclose the great potential and the versatility of BDCPDT block and also enlighten other ladder-type series for further optimization.
... A C C E P T E D ACCEPTED MANUSCRIPT exploited. [28][29][30][31][32][33][34][35][36] Apart from these, the intrinsic properties of SMAs can also be greatly affected by the substitution category and quantity of terminal groups. However, up to now, few studies have discussed effect of the substitution quantity of terminal groups on the photoelectric properties as well as crystallinity of SMAs, and thus the performance of OPVs. ...
Article
Two new n-type organic semiconductor (n-OS) acceptors IDTPC-Me and IDTPC-DMe were designed and synthesized, which were based on a five-ring fused core (IDT) and thiophene-fused ending groups with one or two methyl substitution. The introduction of the methyl group significantly elevated the lowest unoccupied molecular orbital (LUMO) level and affected the intermolecular interaction of acceptors. Photovoltaic performance of the acceptors was investigated by fabricating the polymer solar cells (PSCs) with using a medium bandgap conjugated polymer PTQ10 as donor. With the increase of the number of methyl substituent on the end groups, from IDTPC without methyl substituent, to IDTPC-Me with one methyl substituent and to IDTPC-DMe with two methyl substituents, open circuit voltage (Voc) increased while the short-circuit current (Jsc) and fill factor (FF) decreased for the corresponding PSCs. As a result, the IDTPC-DMe-based polymer solar cells (PSCs) delivered maximum power conversion efficiency (PCE) of 9.3% with a high Voc of 1.02 V, which is slightly higher than that (9.2%) of the IDTPC-Me-based device. The results demonstrate that the ordinary methyl group as well as the number of methyl has a great impact on electron energy level tuning, intermolecular interaction and intrinsic properties of acceptors.
... Compared with other FTAZ:NFA containing systems, FTAZ:ITBC showed a higher roughness value, which could also contribute to the relatively low device performance. 24,49,53 ■ CONCLUSIONS In summary, the use of benzothiophene dioxide as the endcapping group for a nonfullerene acceptor has been investigated for the first time with the synthesis of the novel small-molecule acceptor ITBC. The electron-withdrawing benzothiophene dioxide moiety gives ITBC a near IR absorption and low frontier energy levels, providing a new and simple approach to achieve red-shifted absorption and reduced energy levels compared to other end-cap engineering methods such as the introduction of fluorine by multiple step synthesis. ...
Article
To investigate the relationships between molecular structures and their photovoltaic performance in solar cells, molecular engineering of push-pull small molecules from D-A to A-D-π-D-A architectures is proposed here using benzodithiophene, diketopyrrolopyrrole and fluorine-substituted phenyl groups as D, A and conjugated π units, respectively. In contrast to BDT-DPP, A-D-π-D-A structured DBDT-DPP-BF not only exhibits largely improved optical properties and matched energy levels, but also displays much ordered molecular packing and balanced hole/electron mobility in transistors. Therefore, compared with the inferior photovoltaic performance for BDT-DPP (PCE = 0.29%), the maximum PCE value of 5.27% with a significantly improved J sc value of 12.01 mA/cm ² is finally achieved in DBDT-DPP-BF-based solar cells, indicating that manipulating molecular backbone from D-A to A-D-π-D-A architectures in the benzodithiophene and diketopyrrolopyrrole-contained push-pull small molecules here is an efficient method to enhance their photovoltaic performance in solar cells.
Article
Pyran is fused into the planar donor moiety to enhance the extinction coefficient and reduce the bandgap, thus two acceptors PTTIC and PTBT-R are designed and synthesized. Compared to the IDT-based acceptor, these pyran-bridged acceptors show higher the highest occupied molecular orbital level and wider absorption coverage. Devices fabricated with poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene)-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)-benzo[1,2-c:4,5-c′]dithiophene-4,8-dione)] (PBDB-T):PTTIC as the active layer give a power conversion efficiency (PCE) of 7.35%.
Article
Bulk heterojunction morphology in terms of molecular packing and phase separation is one of the most critical factors determining the device performance of organic photovoltaic devices. The emergence of organic non-fullerene acceptors not only boosts the power conversion efficiency but also brings new challenges and opportunities to the morphology control of the active layer. In this perspective, we review previous grazing incidence X-ray scattering results of non-fullerene acceptor–based thin films and summarize the morphology dependence on three important aspects: chemical structure, ternary mixture, and treatments. Special focus has been placed on fused-ring electron acceptors, which have drawn tremendous attention recently because of their impressive device performance.
Article
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Article
Two new star‐shaped fused electron acceptors, TITT‐3IC and TITT‐3ICF have been designed and synthesized, which consist of a C3h‐symetric coplanar trindeno[1, 2‐b: 4, 5‐b': 7,8‐b'']trithiophene (TITT) as the central core and 3‐(dicyanomethylidene)indan‐1‐one and 2‐(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐inden‐1‐ylidene) malononitrile as the peripheral electron‐withdrawing groups, respectively. With the large coplanar configuration and electron‐rich nature of π‐conjugated backbone, these two acceptors exhibit strong intermolecular charge transfer absorption in the region of 500‐650 nm with the optical bandgaps around 1.9 eV. Relative to TITT‐3IC, TITT‐3ICF shows the downshifted LUMO level and the slightly red‐shifted absorption with the higher molar extinction coefficient due to the stronger electron–withdrawing effect of fluorination. When blending with PTB7‐Th, TITT‐3ICF‐based device displays a higher power conversion efficiency (PCE) of 4.26% than TITT‐3IC‐based device (PCE=3.87%). Comparing with TITT‐3IC based device, the increased short circuit current (JSC) and fill factor (FF) are responsible for the higher PCE value of TITT‐3ICF‐based device, which benefits from its strong and redshifted absorption for light harvesting and proper phase separation morphology for effective exciton dissociation and charge transport. This work demonstrates that as an alternative electron‐donating core, TITT will be promising in designing star‐shaped nonfullerene materials.
Article
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A novel strategy involving judiciously fusing one thiophene/thieno[3,2-b]thiophene on only one side of an indacenodithiophene (IDT) unit to extend IDT backbone conjugation was developed, and three A-D-A type non-fullerene small molecules (TPT-2F, TPTT-2F, and TPTTT-2F) were designed and synthesized to investigate the influence of the extent of IDT core conjugation on its photovoltaic properties. Extending the IDT core conjugation could broaden absorption, upshift the lowest unoccupied molecular orbital (LUMO) energy level, enhance electron mobility, and increase intermolecular π-π stacking. When these three non-fullerene acceptors were applied in organic solar cells (OSCs), simultaneous enhancement of the open-circuit voltage (Voc), short-circuit current (Jsc), and fill factor (FF) was obtained, with the degree of enhancement following the order TPT-2F < TPTT-2F < TPTTT-2F. As a result, the TPTTT-2F based OSCs yielded a high PCE of 12.03%. To the best of our knowledge, the PCE of 12.03% is among the highest values for asymmetric non-fullerene acceptor based OSCs so far. These results demonstrate that extending the conjugation of the IDT core is an effective approach to design highly efficient asymmetric non-fullerene acceptors.
Article
Organic photovoltaics (OPVs) represent one of the potential candidates of next-generation solar cells for converting the green and sustainable solar energy into electrical power. An OPV cell utilizes a blend of electron donor (D) and electron acceptor (A) materials as the photo-active layer, where the photogenerated excitons are separated into mobile electrons and holes. Relative to the fullerene acceptors, nonfullerene small-molecule acceptors (NF-SMAs) have several advantages such as the synthesis-facile chemical modifications and straightforward tunability in the absorptivity, spectral coverage, optical band gap, and frontier molecular orbitals. In recent 3 years, the progress in design and synthesis of the fused-ring NF-SMAs with perpendicular side-chains on the electron-rich core, and again, on the design and synthesis of the wide/medium/low band gap polymer donors have led to realizations of over 13% power conversion efficiencies (PCEs). The rapid advances requires timely review articles. In this review article, we will focus on this type of fused-ring NF-SMAs reported in the past 3 years, with sepcial attention on their molecular structure design and structure-property relationship.
Article
A novel A-D-A (acceptor-donor-acceptor) type non-fullerene small molecule, A201, consisting of an asymmetric thieno[1,2-b]indaceno[5,6-b′]thienothiophene (TITT) unit as middle D part and 2-(3-oxo-2,3-dihydroinden-1-ylidene) malononitrile (IC) groups as end-capped A parts was designed and synthesized. The asymmetric TITT building block showed a higher dipole moment of 0.85 Debye (1 Debye = 3.33564 × 10⁻³⁰ cm) compared with the symmetric analogues of indacenodithiophene (IDT) and indacenodithieno[3,2-b]thiophene (IDTT) of 0.098 and 0.13 Debye, respectively. The solution-processed bulk heterojunction solar cells using a benzotriazole (BTA)-based polymer of J71 as donor and A201 as acceptor, showed a power conversion efficiency (PCE) of 9.36% with an open-circuit voltage (Voc) of 0.88 V, a short-circuit current (Jsc) of 13.15 mA cm⁻², and a fill factor (FF) of 0.67, under the illumination of AM 1.5G at 100 mW cm⁻². The high PCE of this material combination could be attributed to its broad absorption spectrum and the high hole mobility (μh) and electron mobility (μe) of 9.56 × 10⁻⁴ and 5.17 × 10⁻⁴ cm² V⁻¹ s⁻¹, respectively. These results indicate that the asymmetric electron-donating segments are promising to construct A-D-A type small molecular acceptors, which could largely enhance the diversity of building blocks to design photovoltaic materials.
Article
In this work, we have designed and synthesized two new nonfullerene acceptors (NFAs) based on thiophene-fused benzothiadiazole (BTT) unit as -bridge to connect an indacenodithiophene (IDT) as a central core and 3-otcylrhodanine (A1) or 2-(1,1-dicyanootcyl)rhodamine (A2) as the terminal group. Compared with the analogue structure of IDT-2BR with benzothiadiazole (BT) unit as the -bridge, the fusion of an aromatic thiophene ring onto BT unit not only extends the conjugation length, but also stabilizes the quiniod conjugation system, which greatly strengthens the intramolecular charge transfer (ICT) effect. Meanwhile, the introduction of an electron-withdrawing ester group at the fused thiophene ring also intensifies the ICT effect, thus lead to more red-shifted absorption and reduce the bandgap. Moreover, because 2-(1,1-dicyanootcyl)rhodamine unit with a strong electron-withdrawing malononitrile group further strengthens ICT and stabilize the quinoid structure more than 3-otcylrhodanine unit, A2 shows a deeper red-shifted absorption compared with A1, which is favourable for improving the light-harvest efficiency and enhancing the short-circuit current density (Jsc). The photovoltaic performances of the organic solar cell (OSC) devices based on the A1 or A2 as an acceptor and PTB7-Th as a donor have been detailedly investigated. It is found that the device based on A1:PTB7-Th only displays the power conversion efficiency (PCE) of 5.79%, whereas the PCE of A2:PTB7-Th based device with 1% DIO reaches 9.07% with Jsc over 20.33 mA/cm2,which is one of the highest Jsc values among the IDT-2BR type NFAs based OSCs. This work provides further insight into the structural design for new deep narrow bandgap NFAs with highly efficiency in OSCs.
Article
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Article
Fused-ring electron acceptors (FREAs) have a donor–acceptor–donor structure comprising an electron-donating fused-ring core, electron-accepting end groups, π-bridges and side chains. FREAs possess beneficial features, such as feasibility to tailor their structures, high property tunability, strong visible and near-infrared light absorption and excellent n-type semiconducting characteristics. FREAs have initiated a revolution to the field of organic solar cells in recent years. FREA-based organic solar cells have achieved unprecedented efficiencies, over 20%, which breaks the theoretical efficiency limit of traditional fullerene acceptors (~13%), and boast potential operational lifetimes approaching 10 years. Based on the original studies of FREAs, a variety of new structures, mechanisms and applications have flourished. In this Review, we introduce the fundamental principles of FREAs, including their structures and inherent electronic and physical properties. Next, we discuss the way in which the properties of FREAs can be modulated through variations to the electronic structure or molecular packing. We then present the current applications and consider the future areas that may benefit from developments in FREAs. Finally, we conclude with the position of FREA chemistry, reflecting on the challenges and opportunities that may arise in the future of this burgeoning field. Fused-ring electron acceptors are excellent n-type organic semiconductors with outstanding optoelectronic conversion and electron transport abilities. This Review highlights the fundamental principles, design strategies and versatile applications of fused-ring electron acceptors in photovoltaics, electronics and photonics. As part of the Springer Nature Content Sharing Initiative, a view-only version of this paper can be accessed and publicly shared through the following SharedIt link: https://rdcu.be/cSNO4
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Article
A new nonfullerene small molecule acceptor, namely DBTIC, based on an octocyclic thieno[3,2-b]thienodi(indenothiophene) unit using [1]benzothieno[3,2-b][1]-benzothiophene as the core unit, was developed. Despite the medium-bandgap of DBTIC (1.71 eV), a power conversion efficiency of 8.64% can be delivered by the solar cells combining DBTIC and a wide-bandgap polymer donor J52. The high open-circuit voltage (Voc) of 0.94 V is also rare for J52 based devices owing to the high-lying lowest unoccupied molecular orbital level of DBTIC. Moreover, using J71 with lower highest occupied molecular orbital level as polymer donor, a higher Voc up to 1.05 V can be achieved.
Article
In the past few decades, polymer solar cells (PSCs) have been intensively investigated in academic fields. The study of non-fullerene polymer acceptors has become a hot research focus due to their excellent opto-electronic properties such as wide light-absorbing ability, appropriate molecular energy levels, and easy chemical modifications. The much higher power conversion efficiencies (PCEs) of non-fullerene PSCs relative to fullerene PSCs revealed the significant potential of non-fullerene acceptors in PSCs. This review systematically summarizes the recent advancements of efficient polymer acceptors, including perylene diimide-based, naphthalene diimide-based, diketopyrrolopyrrole-based, double B←N bridged bipyridyl-based, and other polymer acceptors. Their structure–property relationships were thoroughly analyzed and summarized, which may provide new guidance for the rational structural design of high-performance photovoltaic materials.
Article
Organic solar cells have become a center of attention in the field of research and technology due to its remarkable features. In the current research work, we designed Benzo Thiophene (BT-CIC) based non-fullerene acceptor organic solar cell having A-D-A novel structure. The designed structures D1-D4 were derived from BT-CIC (non-fullerene acceptor) by replacing 2-(5,6-dichloro-2-methylene-3-oxo-2,3-dihydro-1H-inden-1-ylidene)acetonitrile of reference molecule R with different electron withdrawing end-capper acceptor moieties. The effect of end acceptor groups on absorption, energy level, charge transport, morphology, and photovoltaic properties of the designed molecules (D1-D4) were investigated by TD-DFT B3LYP/6-31G basic level of theory and compared with reference molecule R. Among all novel structures, D3 exhibited maximum absorption (λmax) of 701.7nm and 755.2nm in gaseous state anfd chloroform, respectively. The red shift in D3 was due to the presence of strong electron withdrawing acceptor moiety and more extended conjugation as compared to other structures. D3 also displayed lowest values of energy bandgap (1.97 eV), λe (0.0063eV) and λh (0.0099eV) and which signify its ease electron mobility. Lowest value of binding energy 1.20eV of D3 suggested that this molecule could be easily dissociated into charge carriers TDM results revealed that easy exciton dissociation occurred in D3. Overall, designed structure D3 was found to be more effective and efficient acceptor molecule for SMOSCs. The findings provide novel information for the development of non-fullerene acceptors for OPVs.
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Twistacene-modified π-conjugated compounds usually possessing perfect molecular structure and tailorable optoelectronic property can decrease intramolecular and intermolecular π-stacking to a certain extent. Meanwhile, these branched macromolecules might be chosen as active layers for organic electronics. Following this direction, we design and synthesize two novel dendrimer compounds 1 and 2. Both of them emit strong blue and green fluorescence in organic solvents and thin film with high quantum yields. The fabricated OLED devices based on them showed the maximum brightness of 982 cd m⁻² for device-1 and 20801 cd m⁻² for device-2.
Article
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Small‐molecule acceptors (SMAs)‐based organic solar cells (OSCs) have exhibited great potential for achieving high power conversion efficiencies (PCEs). Meanwhile, developing asymmetric SMAs to improve photovoltaic performance by modulating energy level distribution and morphology has drawn lots of attention. In this work, based on the high‐performance SMA (Y6), three asymmetric SMAs are developed by substituting the fluorine atoms on the terminal group with chlorine atoms, namely SY1 (two F atoms and one Cl atom), SY2 (two F atoms and two Cl atoms), and SY3 (three Cl atoms). Y6 (four F atoms) and Y6‐4Cl (four Cl atoms) are synthesized as control molecules. As a result, SY1 exhibits the shallowest lowest unoccupied molecular orbital energy level and the best molecular packing among these five acceptors. Consequently, OSCs based on PM6:SY1 yield a champion PCE of 16.83% with an open‐circuit voltage (VOC) of 0.871 V, and a fill factor (FF) of 0.760, which is the best result among the five devices. The highest FF for the PM6:SY1‐based device is mainly ascribed to the most balanced charge transport and optimal morphology. This contribution provides deeper understanding of applying asymmetric molecule design method to further promote PCEs of OSCs. Three asymmetric small‐molecule acceptors are developed by changing the fluorine atoms on the terminal group of Y6 to chlorine atoms, namely SY1, SY2, and SY3, with Y6, and Y6‐4Cl are utilized as the reference. Organic solar cells based on the PM6:SY1 blend demonstrate a champion power conversion efficiency of 16.83%. This work can provide a deeper and more comprehensive understanding of applying the asymmetric molecule design method.
Article
Two pairs of constitutional isomers of fused-octacyclic nonfullerene acceptors (NFAs) based on a naphthalene-bisthienothiophene core with or without fluorination at the ending groups have been developed. Compared with the axisymmetric NFAs N66-IC and N66-2FIC with two six-member-ring bridges, their asymmetric constitutional isomers N65-IC and N65-2FIC both with one six-member-ring bridge and one five-member-ring bridge exhibit remarkable red-shifted absorption, higher crystallinity, and slightly down-shifted LUMO energy levels. Organic solar cells based on PBDB-T-2F:N65-2FIC achieved a promising power conversion efficiency of 10.19%, which is three times higher than that of its counterpart PBDB-T-2F:N66-2FIC cell (3.46%). While being blended with PBDB-T as the donor material, the asymmetric acceptor analogue N65-IC based solar cell pronounces a PCE of 9.03%, being significantly improved from that of 5.45% for the PBDB-T:N66-IC based cell, which is in consistency with the results from those cells from their both fluorinated donor and acceptor counterparts. Design rules on either both fluorinated, both non-fluorinated, or cross-combined donor/acceptors for device fabrication has been explored. In addition, PBDB-T-2F:N65-2FIC possesses very promising device stability with 85% of its initial PCE after an exposure time of 1500 h under one sun illumination, which is meaningful for their future commercial devices.
Article
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.
Article
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.
Article
Non-fullerene organic solar cells (NFOSCs) have attracted great deal of attention among researchers in last five years owing to the superior optoelectronics properties of non-fullerene acceptors (NFAs) compared to fullerene derivatives, such as structural versability, suitable energy levels, broad absorption and tunable morphology. The emergence of NFAs offer the opportunity to develop high-performance NFOSCs with a power conversion efficiency (PCE) of over 15%, which indicates that designing and applying new conjugated materials is the crucial factor to enhance the photovoltaic performance. NFAs based on indacenodithiophene (IDT) or its extending backbone core indacenodithienothiophene (IDTT) and end-capped with strong electron-deficient groups have several advantages, such as strong absorption in the visible and Near-Infrared (NIR) region and good alignment of energy levels, indicating that they can be attractive candidates as acceptor materials in OSCs. In this perspective, we discuss the recent advancement made in IDT and IDTT-based NFAs and their photovoltaic device performance. We will mainly concentrate our discussion on the benefits of each NFA in conjunction with the matched conjugated polymer donor in the context of donor/acceptor (D/A) blends correlating it to device performance.
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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.
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Two intermediates (dimethyl 3,7‐dibromonaphthalene‐2,6‐dicarboxylate and dimethyl 1,5‐dibromonaphthalene‐2,6‐dicarboxylate) were synthesized to realize the isomerization of naphthalene‐based decacyclic fused‐ring electron acceptors FTIC1 and FTIC2. The linear‐shaped FTIC1 and nonlinear‐shaped FTIC2 share the same terminal groups and side chains but different isomeric central cores. Both of them share similar light absorption spectra in 500−850 nm region and energy bandgaps, while FTIC2 shows higher maximum molar absorptivity and electron mobility than FTIC1. Compared with the blend film of PM6/FTIC1, the active layer film of PM6/FTIC2 exhibits higher and more balanced hole and electron mobilities due to the influence of the nanoscale morphology. In terms of the photovoltaic performance of the organic solar cells, the FTIC2‐based devices afford a higher efficiency of 11.7% with short‐circuit current density (JSC) of 17.2 mA cm−2 than FTIC1‐based devices (8.98%). This article is protected by copyright. All rights reserved.
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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.
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Two new fused‐ring electron acceptor (FREA) isomers with nonlinear and linear molecular conformation, m‐BAIDIC and p‐BAIDIC, are designed and synthesized. Despite the similar light absorption range and energy levels, the two isomers exhibit distinct electron reorganization energies and molecular packing motifs, which are directly related to the molecular conformation. Compared with the nonlinear acceptor, the linear p‐BAIDIC shows more ordered molecular packing and higher crystallinity. Furthermore, p‐BAIDIC‐based devices exhibit reduced nonradiative energy loss and improved charge transport mobilities. It is beneficial to enhance the open‐circuit voltage (VOC) and short‐current current density (JSC) of the devices. Therefore, the linear FREA, p‐BAIDIC yields a relatively higher efficiency of 7.71% in the binary device with PM6, in comparison with the nonlinear m‐BAIDIC. When p‐BAIDIC is incorporated into the binary PM6/BO‐4Cl system to form a ternary system, synergistic enhancements in VOC, JSC, fill factor (FF), and ultimately a high efficiency of 17.6% are achieved.
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A series of new non-fullerene small molecule acceptors (NTIC, NTIC-Me, NTIC-OMe and NTIC-F) based on the acceptor-donor-acceptor (A-D-A) architecture, using hexacyclic naphthalene-(cyclopentadithiophene) as the central unit were designed and synthesized. The non-fullerene OSC device based on PBDB-T:NTIC showed a highest PCE of 8.63%. With the relative high-lying LUMO level of NTIC-OMe, the PBDB-T:NTIC-OMe based device obtained a comparatively high Voc of 0.965 V and a PCE of 8.61% simitanousely. The results demonstrate that naphthalene core is a promising building block for constructing high efficicent non-fullerene accepotors and furthur boost the photovaltaic perfomance of the devices.
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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.
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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.
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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.
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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.
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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.
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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%).
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A new, easy, and efficient approach is reported to enhance the driving force for charge transfer, break tradeoff between open-circuit voltage and short-circuit current, and simultaneously achieve very small energy loss (0.55 eV), very high open-circuit voltage (>1 V), and very high efficiency (>10%) in fullerene-free organic solar cells via an energy driver.
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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.
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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.
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Triarylamine (TAA) and related materials have dramatically promoted the development of organic and hybrid photovoltaics during the past decade. The power conversion efficiencies of TAA-based organic solar cells (OSCs), dye-sensitized solar cells (DSSCs), and perovskite solar cells (PSCs) have exceeded 11%, 14%, and 20%, which are among the best results for these three kinds of devices, respectively. In this review, we summarize the recent advances of the high-performance TAA-based materials in OSCs, DSSCs, and PSCs. We focus our discussion on the structure–property relationship of the TAA-based materials in order to shed light on the solutions to the challenges in the field of organic and hybrid photovoltaics. Some design strategies for improving the materials and device performance and possible research directions in the near future are also proposed.
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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.
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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.
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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.
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In this work, we designed, calculated and synthesized a planar electron acceptor IDT-2BR with an A-D-A structure, using indacenodithiophene (IDT) unit as a core and 5-(benzo[c][1,2,5]thiadiazol-4-ylmethylene)-3-ethyl-2-thioxothiazo -lidin-4-one (BR) unit as end-capping electron-withdrawing groups. Theoretical calculations revealed that IDT-2BR adopted a nearly flat backbone configuration while the hexylphenyl groups on the IDT moiety exhibited a dihedral angle of ca. 115[degree] to the backbone plane. IDT-2BR exhibited excellent thermal stability, broad and strong absorption from 300 to 750 nm with a maximum extinction coefficient of 1.3 [times] 105 M-1 cm-1 at 636 nm, appropriate LUMO (-3.69 eV) and HOMO (-5.52 eV) energy levels matched with those of P3HT, and relatively high electron mobility of 3.4 [times] 10-4 cm2 V-1 s-1. Fullerene-free PSCs based on P3HT: IDT-2BR blended films gave PCEs of up to 5.12%, which is much higher than that of PC61BM-based control devices (3.71%) and is the highest value
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A ladder-type angular-shaped dithienonaphthalene (aDTN), an isomer of ladder-type linear-shaped dithienonaphthalene (DTN), was designed and synthesized as an electron-rich unit to construct donor–acceptor copolymers with deep-lying highest occupied molecular orbital (HOMO) energy levels. Benzo[c][1,2,5]thiadiazole (BT) with various substituents were used as electron deficient units for synthesizing the target copolymers (PaDTNBTO, PaDTNBTH, and PaDTNBTF) via the Stille coupling reaction. Incorporating different substituents onto the BT moiety has significant effects on the photophysical and electrochemical properties of the copolymers, as well as on the roughness of the polymer/PC71BM blends. With four solubilizing alkyl chains on the aDTN unit, all its three copolymers have good solubility in common solvents. The synthesized copolymers exhibit deep-lying HOMO energy levels, leading to high open circuit voltages (Voc ≥ 0.90 V) of the resulting polymer solar cells. The bulk heterojunction solar cell based on the aDTN-containing copolymers (PaDTNBTO) shows an improved efficiency of 6.44% and an increased Voc of 0.92 V compared to that based on the linear-shaped DTN containing counterpart (efficiency = 4.78%, Voc = 0.86 V). Whereas, under the same device fabrication conditions, PaDTNBTH- and PaDTNBTF-based devices exhibit efficiencies of 5.22% and 1.73%, respectively. Our results demonstrate that aDTN is a better building block in constructing p-type copolymers for high open circuit voltage devices compared to the linear-shaped DTN.
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A novel ladder-type dithienonaphthalene (DTN) was designed and synthesized as an electron-rich unit for constructing donor–acceptor copolymers. Different acceptor moieties, including benzo[c][1,2,5]thiadiazole (BT), 5,6-bis(hexyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (TBT), and 2,5-bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (TDPP), were used as electron-deficient units for the target copolymers. These copolymers (PDTNBT, PDTNTBT, and PDTNTDPP) were obtained via the Stille coupling reaction and characterized by 1H NMR spectroscopy, UV–vis absorption spectroscopy, cyclic voltammetry, and gel permeation chromatography (GPC). Owing to the four solubilizing alkyl chains on the DTN unit, all the three copolymers have good solubility in common solvents. Among these polymers, PDTNTBT exhibits the highest space-charge limit current (SCLC) hole mobility of 2.13 × 10–5 cm2 V–1 s–1, which is beneficial for achieving high performance solar cells. Under the simulated AM 1.5G illumination condition (100 mW/cm2), solar cells based on PDTNTBT:PC71BM (1:3, w/w) exhibit a power conversion efficiency (PCE) of 4.8% with a current density of 10.3 mA cm–2, an open-circuit voltage of 0.86 V, and a fill factor of 54%. With the same device fabrication method, PDTNTDPP:PC71BM (1:3, w/w) and PDTNBT:PC71BM (1:3, w/w) based devices exhibit efficiencies of 1.52% and 2.79%, respectively. Furthermore, inverted solar cells based on these copolymer blends are also fabricated. The inverted devices based on PDTNTDPP:PC71BM (1:2, w/w) and PDTNBT:PC71BM (1:2, w/w) exhibit PCEs of 1.60% and 2.89%, respectively, which are similar to their corresponding conventional devices. And the inverted devices based on PDTNTBT:PC71BM (1:2, w/w) show a higher PCE of 5.0%, and more importantly, they are quite stable as demonstrated by the 4.75% PCE after ambient storage for two months.
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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.
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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.
Article
A fused tris(thienothiophene) (3TT) building block is designed and synthesized with strong electron-donating and molecular packing properties, where three thienothiophene units are condensed with two cyclopentadienyl rings. Based on 3TT, a fused octacylic electron acceptor (FOIC) is designed and synthesized, using strong electron-withdrawing 2-(5/6-fluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)-malononitrile as end groups. FOIC exhibits absorption in 600-950 nm region peaked at 836 nm with extinction coefficient of up to 2 × 105 m-1 cm-1 , low bandgap of 1.32 eV, and high electron mobility of 1.2 × 10-3 cm2 V-1 s-1 . Compared with its counterpart ITIC3 based on indacenothienothiophene core, FOIC exhibits significantly upshifted highest occupied molecular orbital level, slightly downshifted lowest unoccupied molecular orbital level, significantly redshifted absorption, and higher mobility. The as-cast organic solar cells (OSCs) based on blends of PTB7-Th donor and FOIC acceptor without additional treatments exhibit power conversion efficiencies (PCEs) as high as 12.0%, which is much higher than that of PTB7-Th: ITIC3 (8.09%). The as-cast semitransparent OSCs based on the same blends show PCEs of up to 10.3% with an average visible transmittance of 37.4%.
Article
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.
Article
Two novel small molecule acceptors (DTNIC6 and DTNIC8) based on a ladder-type dithienonaphthalene (DTN) building block with linear (hexyl) or branched (2-ethylhexyl) alkyl substituents are designed and synthesized. Both acceptors ex-hibit strong and broad absorption in the range from 500 to 720 nm as well as appropriate HOMO and LUMO levels. Replacing the linear hexyl chains with the branched 2-ethylhexyl chains has a large impact on the film morphology of pho-toactive layers. In the blend film based on DTNIC8 bearing the branched alkyl chains, morphology with well-defined phase separation was observed. This optimal phase morphology yields efficient exciton dissociation, reduced bimolecular recombination, and enhanced and balanced charge carrier mobilities. Benefited from these factors, OSCs based on PBDB-T:DTNIC8 delivered a highest PCE of 9.03% with a high FF of 72.84%. This unprecedented high FF of 72.84% is one of the highest FF values reported for non-fullerene OSCs. Our work not only affords a promising electron acceptor for non-fullerene solar cells, but also provides a side-chain engineering strategy towards high performance OSCs.
Article
π-Conjugated polymers are an important class of materials for organic electronics. In the past decade, numerous polymers with donor-acceptor molecular structures have been developed and used as the active materials for organic devices, such as organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). The choice of the building unit is the primary step for designing the polymers. Benzochalcogenadiazoles (BXzs) are one of the most familiar acceptor building units studied in this area. As their doubly fused system, naphthobischalcogenadiazoles (NXzs), i.e., naphthobisthiadiazole (NTz), naphthobisoxadiazole (NOz), and naphthobisselenadiazole (NSz) are emerging building units that provide interesting electronic properties and highly self-assembling nature for π-conjugated polymers. With these fruitful features, π-conjugated polymers based on these building units demonstrate great performances in OFETs and OPVs. In particular, in OPVs, NTz-based polymers have exhibited more than 10% efficiency, which is among the highest values reported so far. In this Progress Report, the synthesis, properties, and structures of NXzs and their polymers is summarized. The device performance is also highlighted and the structure-property relationships of the polymers are discussed.
Article
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.
Article
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%.
Article
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.
Article
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%.
Article
By creating an effective π-orbital hybridization between the fullerene cage and the aromatic anchor (addend), the azafulleroid interfacial modifiers exhibit enhanced electronic coupling to the underneath metal oxides. High power conversion efficiency of 10.3% can be achieved in organic solar cells using open-cage phenyl C61 butyric acid methyl ester (PCBM)-modified zinc oxide layer.
Article
We develop an efficient fused-ring electron acceptor (ITIC-Th) based on indacenodithieno[3,2-b]thiophene core and thienyl side-chains for organic solar cells (OSCs). Relative to its counterpart with phenyl side-chains (ITIC), ITIC-Th shows lower energy levels (ITIC-Th: HOMO = -5.66 eV, LUMO = -3.93 eV; ITIC: HOMO = -5.48 eV, LUMO = -3.83 eV) due to the σ-inductive effect of thienyl side-chains, which can match with high-performance narrow-bandgap polymer donors and wide-bandgap polymer donors. ITIC-Th has higher electron mobility (6.1 X 10-4 cm2 V-1 s-1) than ITIC (2.6 X 10-4 cm2 V-1 s-1) due to enhanced intermolecular interaction induced by sulfur-sulfur interaction. We fabricate OSCs by blending ITIC-Th acceptor with two different low-bandgap and wide-bandgap polymer donors. In one case a power conversion efficiency of 9.57% was observed, which rivals some of the highest efficiencies for single junction OSCs based on fullerene acceptors.
Article
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%.
Article
Solar cells, a renewable, clean energy technology that efficiently converts sunlight into electricity, are a promising long-term solution for energy and environmental problems caused by a mass of production and the use of fossil fuels. Solution-processed organic solar cells (OSCs) have attracted much attention in the past few years because of several advantages, including easy fabrication, low cost, lightweight, and flexibility. Now, OSCs exhibit power conversion efficiencies (PCEs) of over 10%. In the early stage of OSCs, vapor-deposited organic dye materials were first used in bilayer heterojunction devices in the 1980s, and then, solution-processed polymers were introduced in bulk heterojunction (BHJ) devices. Relative to polymers, vapor-deposited small molecules offer potential advantages, such as a defined molecular structure, definite molecular weight, easy purification, mass-scale production, and good batch-to-batch reproducibility. However, the limited solubility and high crystallinity of vapor-deposited small molecules are unfavorable for use in solution-processed BHJ OSCs. Conversely, polymers have good solution-processing and film-forming properties and are easily processed into flexible devices, whereas their polydispersity of molecular weights and difficulty in purification results in batch to batch variation, which may hamper performance reproducibility and commercialization. Oligomer molecules (OMs) are monodisperse big molecules with intermediate molecular weights (generally in the thousands), and their sizes are between those of small molecules (generally with molecular weights <1000) and polymers (generally with molecular weights >10000). OMs not only overcome shortcomings of both vapor-deposited small molecules and solution-processed polymers, but also combine their advantages, such as defined molecular structure, definite molecular weight, easy purification, mass-scale production, good batch-to-batch reproducibility, good solution processability, and film-forming properties. Therefore, OMs are a good choice for solution-processed reproducible OSCs toward scalable commercialized applications. Considerable efforts have been dedicated to developing new OM electron donors and electron acceptors for OSCs. So far, the highest PCEs of solution-processed OSCs based on OM donors and acceptors are 9-10% and 6-7%, respectively. OM materials have become promising alternatives to polymer and/or fullerene materials for efficient and stable OSCs. In this Account, we present a brief survey of the recent developments in solution-processable OM electron donors and acceptors and their application in OSCs. Rational design of OMs with star- and linear-shaped structures based on triphenylamine, benzodithiophene, and indacenodithiophene units and their impacts on device performance are discussed. Structure-property relationships are also proposed. Furthermore, the remaining challenges and the key research directions in the near future are also addressed. In the next years, an interdisciplinary approach involving novel OM materials, especially electron acceptor materials, accurate morphology optimization, and advanced device technologies will probably bring high-efficiency and stable OSCs to final commercialization.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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).
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
In this Feature Article, we report a family of organic semiconductors based on isomeric naphthodithiophenes (NDTs). The small-molecule- and polymer-based organic semiconductors exhibit field-effect mobilities as high as 1.5 cm2 V−1 s−1 and 0.77 cm2 V−1 s−1, respectively, revealing that NDTs are useful building units for organic semiconductors. We also highlight their structure–property relationships in depth by focusing on the HOMO geometry as well as the packing structure, which provide a clear understanding of how the core structures influence the properties of the resulting materials and thus provide new insight into the design of high-performance organic semiconductors.
Article
We have measured the electrical characteristics and the efficiencies of single-layer organic light-emitting diodes based on poly[2-methoxy-5-(2-ethylhexoxy)-1,4-phenylene vinylene] (MEH-PPV), with Au anodes and Ca, Al, and Au cathodes. We show that proper accounting of the built-in potential leads to a consistent description of the current-voltage data. For the case of Au and Al cathodes, the current under forward bias is dominated by holes injected from the anode and is space-charge limited with a field-dependent hole mobility. The Ca cathode is capable of injecting a space-charge-limited electron current.
Article
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.
Article
Four isomeric naphthodithiophenes (NDTs) with linear and angular shapes were introduced into the polythiophene semiconductor backbones, and their field-effect transistor performances were characterized. The polymers bearing naphtho[1,2-b:5,6-b']dithiophene (NDT3), an angular-shaped NDT, exhibited the highest mobilities of ∼0.8 cm(2) V(-1) s(-1) among the four NDT-based polymers, which is among the highest reported so far for semiconducting polymers. Interestingly, the trend of the mobility in the NDT-based polymers was contrary to our expectations; the polymers with angular NDTs showed higher mobilities than those with linear NDTs despite the fact that naphtho[2,3-b:6,7-b']dithiophene (NDT1), a linear-shaped NDT, has shown the highest mobility in small-molecule systems. X-ray diffraction studies revealed that angular-NDT-based polymers gave the highly ordered structures with a very close π-stacking distance of 3.6 Å, whereas linear-NDT-based polymers had a very weak or no π-stacking order, which is quite consistent with the trend of the mobility. The nature of such ordering structures can be well understood by considering their molecular shapes. In fact, a linear NDT (NDT1) provides angular backbones and an angular NDT (NDT3) provides a pseudostraight backbone, the latter of which can pack into the highly ordered structure and thus facilitate the charge carrier transport. In addition to the ordering structure, the electronic structures seem to correlate with the carrier transport property. MO calculations, supported by the measurement of ionization potentials, suggested that, while the HOMOs are relatively localized within the NDT cores in the linear-NDT-based polymers, those are apparently delocalized along the backbone in the angular-NDT-based polymers. The latter should promote the efficient HOMO overlaps between the polymer backbones that are the main paths of the charge carrier transport, which also agrees with the trend of the mobility. With these results, we conclude that angular NDTs, in particular NDT3, are promising cores for high-performance semiconducting polymers. We thus propose that both the molecular shapes and the electronic structures are important factors to be considered when designing high performance semiconducting polymers.
Article
Recent research advances on conjugated polymers for photovoltaic devices have focused on creating low band gap materials, but a suitable band gap is only one of many performance criteria required for a successful conjugated polymer. This work focuses on the design of two medium band gap (~2.0 eV) copolymers for use in photovoltaic cells which are designed to possess a high hole mobility and low highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. The resulting fluorinated polymer PBnDT-FTAZ exhibits efficiencies above 7% when blended with [6,6]-phenyl C(61)-butyric acid methyl ester in a typical bulk heterojunction, and efficiencies above 6% are still maintained at an active layer thicknesses of 1 μm. PBnDT-FTAZ outperforms poly(3-hexylthiophene), the current medium band gap polymer of choice, and thus is a viable candidate for use in highly efficient tandem cells. PBnDT-FTAZ also highlights other performance criteria which contribute to high photovoltaic efficiency, besides a low band gap.
Article
The current-voltage characteristics of ITO/PEDOT:PSS/OC1C10-PPV:PCBM/Al solar cells were measured in the temperature range 125-320 K under variable illumination, between 0.03 and 100 mW cm(-2) (white light), with the aim of determining the efficiency-limiting mechanism(s) in these devices, and the temperature and/or illumination range(s) in which these devices demonstrate optimal performance. (ITO: indium tin oxide; PEDOT:PSS: poly(styrene sulfonate)-doped poly(ethylene dioxythiophene); OC1C10-PPV: poly[2-methoxy-5-(3,7-dimethyl octyloxy)-1,4-phenylene vinylene]; PCBM: phenyl-C-61 butyric acid methyl ester.) The short-circuit current density and the fill factor grow monotonically with temperature until 320 K. This is indicative of a thermally activated transport of photogenerated charge carriers, influenced by recombination with shallow traps. A gradual increase of the open-circuit voltage to 0.91 V was observed upon cooling the devices down to 125 K. This fits the picture in which the open-circuit voltage is not limited by the work-function difference of electrode materials used. The overall effect of temperature on solar-cell parameters results in a positive temperature coefficient of the power conversion efficiency, which is 1.9% at T = 320 K and 100 mW cm(-2) (2.5% at 0.7 mW cm(-2)). The almost-linear variation of the short-circuit current density with light intensity confirms that the internal recombination losses are predominantly of monomolecular type under short-circuit conditions. We present evidence that the efficiency of this type of solar cell is limited by a light-dependent shunt resistance. Furthermore, the electronic transport properties of the absorber materials, e.g., low effective charge-carrier mobility with a strong temperature dependence, limit the photogenerated current due to a high series resistance, therefore the active layer thickness must be kept low, which results in low absorption for this particular composite absorber.
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