In this work, we present a new strategy to use fused and twisted conjugated backbones to construct conjugated small molecules and polymers as electron acceptors for non-fullerene solar cells. The new conjugated materials containing binary perylene bisimide (PBI) units linked with coplanar thieno[3,2-b]thiophene (as trans-PBI) or twisted thieno[2,3-b]thiophene (as cis-PBI) and the corresponding conjugated polymer cis-polyPBI were developed. A fused conjugated backbone ensures good charge transport, in which the twisted polymer cis-polyPBI was found to show the highest electron mobility of 1.2 × 10−2 cm2 V−1 s−1 in field-effect transistors. Meanwhile, a twisted conjugated backbone effectively prevents the aggregation and crystallization of PBI units, resulting in isotropic charge transport and finely tuned micro-phase separation in bulk-heterojunction thin films. The high electron mobility and isotropic crystallinity make the fused and twisted electron acceptors achieve high power conversion efficiencies above 6% in non-fullerene solar cells, while the coplanar molecule trans-PBI as the electron acceptor shows a very low efficiency of 0.13%.

Ein sternförmiges porphyrinbasiertes …… Molekül mit vier Perylenbisimid-Armen (PBI-Por) wurde als Nichtfulleren-Elektronenakzeptor für Solarzellen entworfen. In der Zuschrift auf S. 2738 ff. zeigen Z. Wang, W. Li et al., dass die Kombination eines Donorpolymers mit PBI-Por in einer Solarzelle eine Photoantwort im Bereich λ=300 bis 850 nm auslöst, die eine maximale externe Quanteneffizienz (EQE) von fast 0.70 und eine vielversprechende Umwandlungseffizienz von 7.4 % aufweist.

AbstractA star-shaped electron acceptor based on porphyrin as a core and perylene bisimide as end groups was constructed for application in non-fullerene organic solar cells. The new conjugated molecule exhibits aligned energy levels, good electron mobility, and complementary absorption with a donor polymer. These advantages facilitate a high power conversion efficiency of 7.4 % in non-fullerene solar cells, which represents the highest photovoltaic performance based on porphyrin derivatives as the acceptor.

Spectroscopic photodetection is a powerful tool in disciplines such as medical diagnosis, industrial process monitoring, or agriculture. However, its application in novel fields, including wearable and biointegrated electronics, is hampered by the use of bulky dispersive optics. Here, solution-processed organic donor–acceptor blends are employed in a resonant optical cavity device architecture for wavelength-tunable photodetection. While conventional photodetectors respond to above-gap excitation, the cavity device exploits weak subgap absorption of intermolecular charge-transfer states of the intercalating poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bimolecular crystal. This enables a highly wavelength selective, near-infrared photoresponse with a spectral resolution down to 14 nm, as well as dark currents and detectivities comparable with commercial inorganic photodetectors. Based on this concept, a miniaturized spectrophotometer, comprising an array of narrowband cavity photodetectors, is fabricated by using a blade-coated PBTTT:PCBM thin film with a thickness gradient. As an application example, a measurement of the transmittance spectrum of water by this device is demonstrated.

In this work, we provide systematic studies on the non-fullerene solar cells based on diketopyrrolopyrrole (DPP) polymers as electron donors and a well-known electron acceptor ITIC. ITIC has been widely reported in non-fullerene solar cells with high power conversion efficiencies (PCEs) above 10%, when it is combined with a wide band gap conjugated polymer, while its application in small band gap DPP polymers has never been reported. Herein, we select four DPP polymers containing different thienyl linkers, resulting in distinct absorption spectra, energy levels and crystalline properties. Non-fullerene solar cells based on DPP polymers as donors and ITIC as an acceptor show PCEs of 1.9–4.1% and energy loss of 0.55–0.82 eV. The PCEs are much lower than those of cells based on fullerene derivatives due to the poor miscibility between the DPP polymers and ITIC, as confirmed by the morphology and charge transport investigation. The results indicate that it is important to tune the miscibility between the donor and acceptor in order to realize optimized micro-phase separation, which can further enhance the performance of DPP polymer based non-fullerene solar cells.

We report a systematic study of the efficiency limitations of non-fullerene organic solar cells that exhibit a small energy loss (Eloss) between the polymer donor and the non-fullerene acceptor. To clarify the impact of Eloss on the performance of the solar cells, three thieno[3,4-c]pyrrole-4,6-dione-based conjugated polymers (PTPD3T, PTPD2T, and PTPDBDT) are employed as the electron donor, which all have complementary absorption spectra compared with the ITIC acceptor. The corresponding photovoltaic devices show that low Eloss (0.54 eV) in PTPDBDT:ITIC leads to a high open-circuit voltage (Voc) of 1.05 V, but also to a small quantum efficiency, and in turn photocurrent. The high Voc or small energy loss in the PTPDBDT-based solar cells is a consequence of less non-radiative recombination, whereas the low quantum efficiency is attributed to the unfavorable micro-phase separation, as confirmed by the steady-state and time-resolved photoluminescence experiments, grazing-incidence wide-angle X-ray scattering, and resonant soft X-ray scattering (R-SoXS) measurements. We conclude that to achieve high performance non-fullerene solar cells, it is essential to realize a large Voc with small Eloss while simultaneously maintaining a high quantum efficiency by manipulating the molecular interaction in the bulk-heterojunction.

A series of halogenated conjugated molecules, containing F, Cl, Br and I, were easily prepared via Knoevenagel condensation and applied in field-effect transistors and organic solar cells. Halogenated conjugated materials were found to possess deep frontier energy levels and high crystallinity compared to their non-halogenated analogues, which is due to the strong electronegativity and heavy atom effect of halogens. As a result, halogenated semiconductors provide high electron mobilities up to 1.3 cm2 V−1 s−1 in transistors and high efficiencies over 9% in non-fullerene solar cells.

Perylene bisimide (PBI) unit has been widely used to design conjugated materials, which can be used as electron acceptor in organic solar cells due to its strong electron-deficient ability. In this work, a conjugated polymer based on PBI dimer as monomer was designed, synthesized, and compared to the conjugated polymer containing single PBI as repeating units. The two conjugated polymers were found to have similar molecular weight, absorption spectra and energy levels. Density functional theory calculation revealed that the PBI dimer-based polymer exhibited highly twisted conjugated backbone due to the large dihedral angle between the two PBI units. The PBI-based polymers as electron acceptor were applied into polymer-polymer solar cells, in which PBI dimer-based polymer solar cells were found to show a high short circuit current density (Jsc = 11.2 mA∙cm−2 and a high power conversion efficiency (PCE) of 4.5%. In comparison, the solar cells based on PBI-based polymer acceptor only provided a Jsc of 7.2 mA∙cm−2 and PCE of 2.5%. The significantly enhanced PCE in PBI dimer-based solar cells was attributed to the mixed phase in blended thin films, as revealed by atom force microscopy. This study demonstrates that PBI dimer can be used to design polymer acceptors for high performance polymerpolymer solar cells.

•Four diketopyrrolopyrrole-based polymer acceptors were designed for polymer-polymer solar cells.•Polymer acceptors with large-sized aromatic units show small LUMO offset and low energy loss below 0.60 eV.•The PCEs of cells were enhanced from 0.14% to 3.1% when increasing the size of aromatic units.In this work, we develop four diketopyrrolopyrrole-based polymer acceptors for application in polymer-polymer solar cells. The polymer acceptors contain different-sized aromatic units, from small thiophene to benzodithiophene and large alkylthio-benzodithiophene units. Although the polymer acceptor with large-sized groups shows small LUMO offset and low energy loss when blended with the donor polymer PTB7-Th, the corresponding solar cells can achieve a high power conversion efficiency (PCE) of 3.1% due to high photocurrent. In contrast, the polymer acceptor with small thiophene units only provides a low PCE of 0.14% in solar cells. These results indicate that polymer acceptors with large-sized aromatic units can be potentially used into high performance non-fullerene solar cells.Download high-res image (179KB)Download full-size image

Conjugated polymers have been extensively studied for application in organic solar cells. In designing new polymers, particular attention has been given to tuning the absorption spectrum, molecular energy levels, crystallinity, and charge carrier mobility to enhance performance. As a result, the power conversion efficiencies (PCEs) of solar cells based on conjugated polymers as electron donor and fullerene derivatives as electron acceptor have exceeded 10% in single-junction and 11% in multijunction devices. Despite these efforts, it is notoriously difficult to establish thorough structure–property relationships that will be required to further optimize existing high-performance polymers to their intrinsic limits.In this Account, we highlight progress on the development and our understanding of diketopyrrolopyrrole (DPP) based conjugated polymers for polymer solar cells. The DPP moiety is strongly electron withdrawing and its polar nature enhances the tendency of DPP-based polymers to crystallize. As a result, DPP-based conjugated polymers often exhibit an advantageously broad and tunable optical absorption, up to 1000 nm, and high mobilities for holes and electrons, which can result in high photocurrents and good fill factors in solar cells. Here we focus on the structural modifications applied to DPP polymers and rationalize and explain the relationships between chemical structure and organic photovoltaic performance. The DPP polymers can be tuned via their aromatic substituents, their alkyl side chains, and the nature of the π-conjugated segment linking the units along the polymer chain. We show that these building blocks work together in determining the molecular conformation, the optical properties, the charge carrier mobility, and the solubility of the polymer. We identify the latter as a decisive parameter for DPP-based organic solar cells because it regulates the diameter of the semicrystalline DPP polymer fibers that form in the photovoltaic blends with fullerenes via solution processing. The width of these fibers and the photon energy loss, defined as the energy difference between optical band gap and open-circuit voltage, together govern to a large extent the quantum efficiency for charge generation in these blends and thereby the power conversion efficiency of the photovoltaic devices. Lowering the photon energy loss and maintaining a high quantum yield for charge generation is identified as a major pathway to enhance the performance of organic solar cells. This can be achieved by controlling the structural purity of the materials and further control over morphology formation. We hope that this Account contributes to improved design strategies of DPP polymers that are required to realize new breakthroughs in organic solar cell performance in the future.

Conjugated polymers with LUMO levels of −4.00 eV and a perylene bisimide derivative with a LUMO level of −4.56 eV were used in non-fullerene solar cells in which power conversion efficiencies up to 1.4% were achieved.

Low dark current is critical to realize high-performance near-infrared organic photodetectors (NIR-OPDs). In general, organic photodetectors (OPDs) are with vacuum-deposited metals as the top electrode. The deposition of such metal would inevitably form doping to the organic active layer and thus yield high dark current. Herein, we employ transfer-printed conducting polymer (tp-CP) as the top electrode instead of the vacuum-deposited metal electrode. The photodetector with tp-CP electrode exhibits over two orders of magnitude lower dark current density than the device with the vacuum-deposited metal electrode. The photodetector with tp-CP electrode displays a responsivity of 0.37 A W−1 at 850 nm and a low dark current density of 3.0 nA cm−2 at −0.2 V based on a near-infrared (NIR) active layer of PMDPP3T:PC61BM that absorbs photons up to 1000 nm. The detectivity of the NIR photodetector reaches as high as over 1013 Jones. Furthermore, the NIR photodetector is double-side responsive to incident light, either from the bottom or the top electrode, because the top tp-CP electrode shows similar transparency as the bottom indium-tin oxide electrode.

A series of six diketopyrrolopyrrole (DPP) based conjugated polymers with a varying content of solubilizing perfluoroalkyl chains were synthesized. Based on a systematic investigation of the influence of the solvent on the photovoltaic performance, it is found that 1,6-diiodoperfluorohexane (IC6F12I) is an effective solvent additive to enhance the power conversion efficiency (PCE) of DPP polymers with perfluoroalkyl side chains. The polymers consist of thiazole-flanked DPP units that alternate along the main chain with varying ratios of thiophene (T) and perfluoroalkyl benzodithiophene (FBDT) units. The polymers possess high molecular weights, narrow band gaps and good crystalline properties. The DPP polymers were used as electron acceptors in bulk heterojunction solar cells with another DPP polymer as the electron donor. A solvent mixture of CHCl3:1-chloronaphthalene (1-CN) is found to provide the best PCE of 2.9% in non-fluorine based DPP polymer solar cells, but yields a low PCE of 0.52% for perfluoroalkyl-containing polymer solar cells. Perfluoroalkyl-containing polymer solar cells fabricated from CHCl3 with IC6F12I as the processing additive show a significantly improved PCE of 2.1%. The morphology analysis of the blend films reveals that IC6F12I as an additive improves the micro-phase separation between the polymer donor and acceptor, which results in enhanced charge generation.

In this work, solution-processed organic solar cells with conjugated small molecules both as electron donors and electron acceptors were studied, where the influence of the chemical structures of the donor and acceptor on the device performance was systematically investigated. A small molecular donor incorporating binary electron-deficient units, diketopyrrolopyrrole and pentacyclic aromatic bislactam, was synthesized to provide a low band gap of 1.65 eV and low-lying energy levels. Three molecules, from a fullerene derivative to non-fullerene perylene bisimide-based acceptors, were selected as electron acceptors to construct organic solar cells. The results showed that fullerene-based solar cells provided power conversion efficiencies (PCEs) of up to 4.8%, while the non-fullerene solar cells also exhibited promising PCEs of 2.4% and 3.5%, with a photoresponse of up to 750 nm. Further analysis of the bulk-heterojunction systems between donors and acceptors revealed that the relatively low carrier mobilities of the non-fullerene acceptors and the large phase separations are mainly responsible for the less efficient solar cells. Our results demonstrate that molecules containing several electron-deficient units can effectively reduce the band gap of small molecules, and thus offer great potential for realizing high performance fullerene and non-fullerene solar cells.

Conjugated polymers consisting of diketopyrrolopyrrole (DPP) units have been successfully applied in field-effect transistors (FETs) and polymer solar cells (PSCs), while most of the DPP polymers were designed as symmetric structures containing identical aromatic linkers. In this manuscript, we design a new asymmetric DPP polymer with varied aromatic linkers in the backbone for application in FETs and PSCs. The designation provides the chance to finely adjust the energy levels of conjugated polymers so as to influence the device performance. The asymmetric polymer exhibits highly crystalline properties, high hole mobilities of 3.05 cm2 V–1 s–1 in FETs, and a high efficiency of 5.9% in PSCs with spectra response from 300 to 850 nm. Morphology investigation demonstrates that the asymmetric polymer has a large crystal domain in blended thin films, indicating that the solar cell performance can be further enhanced by optimizing the microphase separation. The study reveals that the asymmetric design via adjusting the aromatic linkers in DPP polymers is a useful route toward flexible electronic devices.Keywords: asymmetric polymer; conjugated polymer; crystalline; diketopyrrolopyrrole; field-effect transistors; polymer solar cells

A semi-crystalline conjugated polymer based on two electron-deficient units, pentacyclic lactam (PCL) and diketopyrrolopyrrole (DPP), was designed and synthesized for application in field-effect transistors (FETs) and polymer solar cells (PSCs). The polymer has a high molecular weight, near-infrared absorption up to 900 nm and good solubility in toluene. When the polymer thin films were solution-processed from toluene with diphenyl ether as an additive, the FET devices achieved a high hole mobility of 0.81 cm2 V−1 s−1. With the same solution-processing solvents, bulk-heterojunction solar cells based on this polymer as an electron donor provided a power conversion efficiency of 4.7% with an optimal energy loss of 0.65 eV due to its deep lowest unoccupied molecular orbital level. Further study on the morphology of the pure polymer or blend thin films by atomic force microscopy, transmission electron microscopy and 2D grazing-incidence wide-angle X-ray scattering reveals that the new polymer has good crystalline property, which is mainly due to the coplanar nature of the conjugated backbone. This work demonstrates that conjugated polymers incorporating several electron-deficient units can be potentially used in high performance FETs and PSCs.

Four typical diketopyrrolopyrrole (DPP)-based conjugated polymers were used as electron donors in all-polymer solar cells (PSCs) with a naphthalenediimide-based polymer N2200 as the electron acceptor. The four DPP polymers have near-infrared absorption spectra up to 1000 nm and suitable energy levels for charge separation from donor to acceptor. DPP polymer:N2200 cells were found to have high open circuit voltages in comparison to fullerene-based solar cells but with low short circuit current densities and fill factors, so that the power conversion efficiencies of these cells were relatively low (0.45–1.7%). These blends relatively had balanced but low hole and electron mobilities from space charge limit current measurements, small surface roughness, and highly quenched photoluminescence (PL) from steady-state PL. These studies show that the low photocurrent and performance arise from the miscibility of the DPP and N2200 polymers, which enhances the charge recombination. The finding was further confirmed by grazing incidence X-ray diffraction and resonant soft X-ray scattering. All the PSCs based on DPP polymers were investigated, opening further studies based on these systems due to the broad absorption, high carrier mobilities and good crystalline properties of DPP polymers.

•Diketopyrrolopyrrole-based polymers as donor and a perylene bisimide derivative as acceptor were applied in non-fullerene solar cells.•The power conversion efficiencies of 1.6–2.6% were achieved with broad photo-response from 300 nm to 1000 nm.•The modest PCEs of these DPP polymers based non-fullerene solar cells were attributed to the high charge recombination.Diketopyrrolopyrrole (DPP)-based conjugated polymers have been successfully applied in high performance field-effect transistors and fullerene-based solar cells, but show limited application in non-fullerene solar cells. In this work, we use four DPP polymers as electron donor and a perylene bisimide dye as electron acceptor to construct non-fullerene solar cells. The donors and acceptor have complementary absorption spectra in visible and near-infrared region, resulting in broad photo-response from 300 nm to 1000 nm. The solar cells were found to provide relatively low power conversion efficiencies of 1.6–2.6%, which was mainly due to low photocurrent and fill factor. Further investigation reveals that the low performance is originated from the high charge recombination in photo-active layers. Our systematical studies will help better understand the non-fullerene solar cells based on DPP polymers and inspire new researches toward efficient non-fullerene solar cells with broad photo-response.

•Methylated polymers based on diketopyrrolopyrrole and dithienothiophene were designed for field-effect transistors.•Methylated polymers were found to show high hole mobilities up to 5.32 cm2 V−1 s−1.•Asymmetrically methylated polymers with highest hole mobility show “face-on” orientation in thin films.In this work, a series of conjugated polymers based on diketopyrrolopyrrole (DPP) and dithienothiophene were designed for application in field-effect transistors (FETs). Owing to the synthetic nature of DPP units, the DPP polymers here contain different aromatic linkers with thiophene and methylthiophene, resulting in non-methylated and methylated DPP polymers. Methylated DPP polymers were found to show good crystalline properties and provide high hole mobilties up to 5.32 cm2 V−1 s−1 in FETs, while non-methylated polymer exhibits a hole mobility of 3.16 cm2 V−1 s−1. Especially, the polymer containing asymmetric linkers presents “face-on” orientation in thin films but provides the highest mobility. Our results reveal that the polymers incorporating methyl units as side chains can be used to realize high carrier mobility in FETs.

Fluorinated conjugated polymers have been widely used in high performance polymer solar cells, but they showed limited application in field-effect transistors (FETs). In this paper, we focus on the influence of fluorine atoms upon charge transport of conjugated polymers in FET devices. Two series of conjugated polymers without or with fluorine atoms were designed and applied into FETs. Nonfluorinated conjugated polymers show high hole mobilties up to 11.16 cm2 V–1 s–1, while fluorinated polymers exhibit low hole mobilities below 1.80 cm2 V–1 s–1. Further investigation by differential scanning calorimetry (DSC) and 2D grazing-incidence wide-angle X-ray scattering (2D-GIWAXS) reveal that fluorinated conjugated polymers show low crystallinity and “face-on” orientation in thin films, explaining their poor hole mobilities in FET devices. Our results clearly show how the chemical structures influence the charge transport properties, which can be used to design new conjugated polymers toward high performance FETs.

The influence of the chemical structure of conjugated polymers on the nanophase separation and device performance in fullerene-based solar cells has been widely studied, while this is less investigated in non-fullerene solar cells. In this work, we design three conjugated polymers with different length of side chains, and we find that the length of side chains has little influence on the quantum efficiencies of non-fullerene solar cells. As a comparison, the length of side chains has a significant effect on the quantum efficiencies of fullerene-based solar cells. This indicates that morphology of the blended thin films in non-fullerene solar cells is rather independent of the length of the donor side chains, and the mechanism for morphology evolution in the non-fullerene system is completely different from that in the fullerene system. Our conclusion is confirmed by a variety of advanced characterization techniques. The studies reveal that in blended thin films based on the non-fullerene material the donor polymers with different side chains have a similar coherence length of π–π stacking, crystal size and domain purity, giving rise to similar internal quantum efficiency and power conversion efficiency of the solar cells.

Diketopyrrolopyrrole-based conjugated polymers bridged with thiazole units and different donors have been designed for polymer solar cells. Quantum efficiencies above 50% have been achieved with energy loss between optical band gap and open-circuit voltage below 0.6 eV.

Three thiazole-bridged DPP polymers with deep lowest unoccupied molecular orbital (LUMO) levels were designed for field-effect transistors (FETs) and polymer–polymer solar cells. By introducing thiazole–thiazole coupled segments, perfluoroalkyl side chains or strong electron-deficient naphthalenediimide units into the conjugated backbone the three thiazole-bridged DPP polymers have LUMO levels of −4.0 to −4.4 eV. The three DPP polymers exhibit optical absorption in the near-infrared region, crystallinity and an electron mobility of around 0.01 cm2 V−1 s−1 in bottom contact FETs. The polymers were applied as electron acceptors in polymer–polymer solar cells to provide PCEs of around 0.4%. The low PCEs are mainly due to low short-circuit currents (Jsc) and attributed to large phase separation. Our results demonstrate several efficient strategies to lower the energy levels of conjugated polymers in order to be used as universal acceptors for photovoltaic cells.

Five wide or medium band gap diketopyrrolopyrrole (DPP)-based conjugated polymers with pyridine as bridges were developed for organic field-effect transistors (OFETs) and polymer solar cells (PSCs). By introducing copolymerized aromatic building blocks from strong electron-donating units to electron-deficient units into the conjugated backbone, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the DPP polymers were tailored to the low-lying position. Therefore, the polarity of charge transport in OFETs can be switched from p-type to n-type. The DPP polymer with a low-lying LUMO of −3.80 eV provides a hole-only mobility of 2.95 × 10−2 cm2 V−1 s−1, while an electron-only mobility of 1.24 × 10−3 cm2 V−1 s−1 is found in the DPP polymer with a LUMO of −4.22 eV. Further investigation of photovoltaic cells based on these DPP polymers shows a modest power conversion efficiency (PCE) of around 2%. Our results demonstrate that wide band gap pyridine-bridged DPP polymers have potential application in OFETs and OSCs by adjusting their energy level with alternated units on the conjugated backbone.

In this work, solution-processed organic solar cells with conjugated small molecules both as electron donors and electron acceptors were studied, where the influence of the chemical structures of the donor and acceptor on the device performance was systematically investigated. A small molecular donor incorporating binary electron-deficient units, diketopyrrolopyrrole and pentacyclic aromatic bislactam, was synthesized to provide a low band gap of 1.65 eV and low-lying energy levels. Three molecules, from a fullerene derivative to non-fullerene perylene bisimide-based acceptors, were selected as electron acceptors to construct organic solar cells. The results showed that fullerene-based solar cells provided power conversion efficiencies (PCEs) of up to 4.8%, while the non-fullerene solar cells also exhibited promising PCEs of 2.4% and 3.5%, with a photoresponse of up to 750 nm. Further analysis of the bulk-heterojunction systems between donors and acceptors revealed that the relatively low carrier mobilities of the non-fullerene acceptors and the large phase separations are mainly responsible for the less efficient solar cells. Our results demonstrate that molecules containing several electron-deficient units can effectively reduce the band gap of small molecules, and thus offer great potential for realizing high performance fullerene and non-fullerene solar cells.

Three thiazole-bridged DPP polymers with deep lowest unoccupied molecular orbital (LUMO) levels were designed for field-effect transistors (FETs) and polymer–polymer solar cells. By introducing thiazole–thiazole coupled segments, perfluoroalkyl side chains or strong electron-deficient naphthalenediimide units into the conjugated backbone the three thiazole-bridged DPP polymers have LUMO levels of −4.0 to −4.4 eV. The three DPP polymers exhibit optical absorption in the near-infrared region, crystallinity and an electron mobility of around 0.01 cm2 V−1 s−1 in bottom contact FETs. The polymers were applied as electron acceptors in polymer–polymer solar cells to provide PCEs of around 0.4%. The low PCEs are mainly due to low short-circuit currents (Jsc) and attributed to large phase separation. Our results demonstrate several efficient strategies to lower the energy levels of conjugated polymers in order to be used as universal acceptors for photovoltaic cells.

Low dark current is critical to realize high-performance near-infrared organic photodetectors (NIR-OPDs). In general, organic photodetectors (OPDs) are with vacuum-deposited metals as the top electrode. The deposition of such metal would inevitably form doping to the organic active layer and thus yield high dark current. Herein, we employ transfer-printed conducting polymer (tp-CP) as the top electrode instead of the vacuum-deposited metal electrode. The photodetector with tp-CP electrode exhibits over two orders of magnitude lower dark current density than the device with the vacuum-deposited metal electrode. The photodetector with tp-CP electrode displays a responsivity of 0.37 A W−1 at 850 nm and a low dark current density of 3.0 nA cm−2 at −0.2 V based on a near-infrared (NIR) active layer of PMDPP3T:PC61BM that absorbs photons up to 1000 nm. The detectivity of the NIR photodetector reaches as high as over 1013 Jones. Furthermore, the NIR photodetector is double-side responsive to incident light, either from the bottom or the top electrode, because the top tp-CP electrode shows similar transparency as the bottom indium-tin oxide electrode.

Conjugated polymers with LUMO levels of −4.00 eV and a perylene bisimide derivative with a LUMO level of −4.56 eV were used in non-fullerene solar cells in which power conversion efficiencies up to 1.4% were achieved.

A series of six diketopyrrolopyrrole (DPP) based conjugated polymers with a varying content of solubilizing perfluoroalkyl chains were synthesized. Based on a systematic investigation of the influence of the solvent on the photovoltaic performance, it is found that 1,6-diiodoperfluorohexane (IC6F12I) is an effective solvent additive to enhance the power conversion efficiency (PCE) of DPP polymers with perfluoroalkyl side chains. The polymers consist of thiazole-flanked DPP units that alternate along the main chain with varying ratios of thiophene (T) and perfluoroalkyl benzodithiophene (FBDT) units. The polymers possess high molecular weights, narrow band gaps and good crystalline properties. The DPP polymers were used as electron acceptors in bulk heterojunction solar cells with another DPP polymer as the electron donor. A solvent mixture of CHCl3:1-chloronaphthalene (1-CN) is found to provide the best PCE of 2.9% in non-fluorine based DPP polymer solar cells, but yields a low PCE of 0.52% for perfluoroalkyl-containing polymer solar cells. Perfluoroalkyl-containing polymer solar cells fabricated from CHCl3 with IC6F12I as the processing additive show a significantly improved PCE of 2.1%. The morphology analysis of the blend films reveals that IC6F12I as an additive improves the micro-phase separation between the polymer donor and acceptor, which results in enhanced charge generation.

A series of halogenated conjugated molecules, containing F, Cl, Br and I, were easily prepared via Knoevenagel condensation and applied in field-effect transistors and organic solar cells. Halogenated conjugated materials were found to possess deep frontier energy levels and high crystallinity compared to their non-halogenated analogues, which is due to the strong electronegativity and heavy atom effect of halogens. As a result, halogenated semiconductors provide high electron mobilities up to 1.3 cm2 V−1 s−1 in transistors and high efficiencies over 9% in non-fullerene solar cells.

In this work, we provide systematic studies on the non-fullerene solar cells based on diketopyrrolopyrrole (DPP) polymers as electron donors and a well-known electron acceptor ITIC. ITIC has been widely reported in non-fullerene solar cells with high power conversion efficiencies (PCEs) above 10%, when it is combined with a wide band gap conjugated polymer, while its application in small band gap DPP polymers has never been reported. Herein, we select four DPP polymers containing different thienyl linkers, resulting in distinct absorption spectra, energy levels and crystalline properties. Non-fullerene solar cells based on DPP polymers as donors and ITIC as an acceptor show PCEs of 1.9–4.1% and energy loss of 0.55–0.82 eV. The PCEs are much lower than those of cells based on fullerene derivatives due to the poor miscibility between the DPP polymers and ITIC, as confirmed by the morphology and charge transport investigation. The results indicate that it is important to tune the miscibility between the donor and acceptor in order to realize optimized micro-phase separation, which can further enhance the performance of DPP polymer based non-fullerene solar cells.