•A non-fullerene electron acceptor, SF(DPPFB)4, is designed and synthesized.•SF(DPPFB)4 has four fluorine atoms as the terminal groups.•The introduction of fluorine atoms decreases the energy levels of SF(DPPFB)4.•The fluorination also enhances the electron mobility of SF(DPPFB)4.•The OSCs with SF(DPPFB)4 as the electron acceptor show the improved currents.In this paper, a non-fullerene electron acceptor, SF(DPPFB)4, which owns a spirobifluorene core and four diketopyrrolopyrrole (DPP) arms end capped by 4-fluorobenzene, is designed and synthesized for solution-processable organic solar cells (OSCs). SF(DPPFB)4 shows similar absorption bands as those of its non-fluorinated parent compound, SF(DPPB)4. However, the terminal fluorine atoms reduce the energy levels of SF(DPPFB)4, especially, its lowest unoccupied molecular orbital level decreases by 0.04 eV than that of SF(DPPB)4, which enlarges the energy offset between the electron donor and acceptor, favorable for the dissociation of excitons in OSCs. Moreover, the fluorination improves the electron mobility of SF(DPPFB)4. Thus, the OSCs with poly(3-hexylthiophene) as the electron donor and SF(DPPFB)4 as the electron acceptor can provide a maximum power conversion efficiency of 4.42%, with a lifted short-circuit current (Jsc) of 8.48 mA cm−2.

Nowadays, organic solar cells (OSCs) with efficiencies over 10% have been achieved through the elaborate design of electron donors and fullerene acceptors. However, the drawbacks of fullerene acceptors, like poor absorption, limited chemical and energetic tunabilities, high-cost purification and morphological instability, have become the bottlenecks for the further improvement of OSCs. To overcome the mentioned shortages from fullerene, research studies on non-fullerene electron acceptors have boomed. To date, the highest efficiency of fullerene-free OSCs has been pushed to be 12%, which surpasses that of fullerene-based OSCs. In this perspective, we focus on summarizing the development of small molecule electron acceptors designed to replace the fullerene derivatives. Since it has been revealed that the search for matched donor:acceptor pairs is important for accomplishing high efficiencies, we therefore divide electron acceptors into several categories according to the donors used in fullerene-free OSCs. After the introduction of these acceptors, we outline the designing rules as well as perspectives for the development of non-fullerene acceptors. We believe that the development of non-fullerene electron acceptors will make organic photovoltaics closer to practical applications.

In this study, we design and synthesize a new non-fullerene electron acceptor, SF(DPPB)4, in which a spirobifluorene (SF) core is installed with four benzene endcapped diketopyrrolopyrrole (DPP) arms. SF(DPPB)4 exhibits energy levels matching perfectly with those of the commonly used poly(3-hexyl thiophene) (P3HT) donor in polymer solar cells (PSCs). Furthermore, a designed cross-shaped molecular geometry helps in suppressing strong intermolecular aggregation in the P3HT:SF(DPPB)4 blend, leading to efficient non-fullerene PSCs. The resultant devices give a maximum power conversion efficiency (PCE) of 5.16% with an extremely high open-circuit voltage (Voc) of 1.14 V. In contrast, the devices based on P3HT:PC61BM blends provide a PCE of 3.18% with a Voc of 0.62 V. Finally, we observe that the P3HT:SF(DPPB)4 devices exhibit significantly improved thermal stability from that of the P3HT:PC61BM devices; upon thermal treatment at 150 °C for 3 h, the PCEs of P3HT:SF(DPPB)4 devices remain unchanged, whereas those of the P3HT:PC61BM devices drop drastically to below 1%. The abovementioned results demonstrate that the new design strategy of employing a high-performance non-fullerene acceptor, SF(DPPB)4, is promising for the future practical application of PSCs.

Perylene diimide (PDI), which features intense absorption, a low-lying energy level, and high electron mobility, is a promising building block for electron acceptors in organic solar cells (OSCs). However, this planar molecule has a strong tendency to form large aggregates during film formation which strongly limits its OSC performance. Herein, we report a new and simple PDI derivative, B(PDI)3, in which a central benzene unit is employed to connect three PDI arms. This compact arrangement of sterically bulky PDI moieties leads to a twisted molecular geometry of the resultant structure. This suppresses the strong crystallization tendency of PDI chromophores, owing to the broken molecular coplanarity and symmetry. Therefore, B(PDI)3 is applied as a non-fullerene acceptor in OSCs, providing a good power conversion efficiency of 5.65% when blended with the PTB7-Th donor.

Effective electron acceptor materials usually have a deep lowest unoccupied molecular orbital (LUMO) energy level that can split excitons and generate current. A non-fullerene electron acceptor (F8-DPPTCN) was developed, using fluorene as the core with arms of diketopyrrolopyrrole (DPP) having thiophene-2-carbonitrile as the terminal units. The new molecule had a LUMO of −3.65 eV and a narrow bandgap (Eg) of 1.66 eV, owing to the electronegativity of the thiophene-2-carbonitrile group and its conjugation with DPP units. Organic solar cells (OSCs) with F8-DPPTCN as the acceptor and poly(3-hexylthiophene) (P3HT) as the donor were fabricated. A power conversion efficiency (PCE) of 2.37% was obtained with an open-circuit voltage (Voc) of 0.97 V, a short-circuit current (Jsc) of 6.25 mA cm−2, and a fill factor (FF) of 0.39. Structural characterization showed that P3HT and F8-DPPTCN were kinetically trapped in a weakly separated state whereas thermal annealing led to the crystallization of P3HT and the formation of a network structure with a mesh-size of several hundred nanometers. When a solvent additive, diiodooctane, was used and the mixture was thermally annealed, both P3HT and F8-DPPTCN crystallized and a multi-length scale network was formed. Though the PCEs were low, the changes in the PCE could be correlated with the morphological changes, opening pathways to increase performance further.

•Sequential activation of CH bonds for accessing complicate multi-DPP-based oligomers.•The integration of easily accessible acceptors with economical donor for fullerene-free OPVs.•High VOC of fullerene-free OPVs using P3HT as donor.•Accessing renewable solar energy via sustainable chemistry.Exploring sustainable chemistry for renewable energy plays a key role in meeting the ever increasing energy demand without sacrificing the environment. In this study, two novel diketopyrrolopyrrol(DPP)-based π-conjugated oligomers (named as TPE-DPP4 and BP-DPP4) have been readily synthesized via a ligand-free Pd-catalyzed sequential activation of CH bond in two steps with good yields starting from simple building blocks. Poly(3-hexylthiophene) is employed as an electron donor to blend with the new DPP-derived electron acceptors for the fullerene-free bulk heterojunction organic photovoltaics. The power conversion efficiency of 2.49% has been achieved, corresponding with an open-circuit voltage of 1.16 V, which is among the highest open-circuit voltages for the single-junction organic photovoltaics. The facilely accessible electron acceptors blended with cost-effective poly(3-hexylthiophene) donor for fullerene-free organic photovoltaics opens a new pathway to access renewable solar energy via sustainable chemistry.

•Molecule DPP(C2T)2 end-capped with thiophene-2,3-dicarboxylate was synthesized.•Introducing ester groups to both α and β-position of thiophene ring was achieved.•Energy levels were manipulated by introducing of extra ester groups.•DPP(C2T)2 can serve as both donor and acceptor in OSCs.It is a primary strategy to manipulate the energy levels of organic semiconductor molecules for getting better performances in organic photovoltaic devices. In this paper, we designed and synthesized a diketopyrrolopyrrole (DPP)-based small molecule, 3,6-bis(5-[(diethyl thiophene-2,3-dicarboxylate)-2-yl]thiophene-2-yl)- 2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP(C2T)2), by introducing ester groups to both α and β-position of the end-capped thiophene ring. Due to the electron-withdrawing effect of ester groups, the resulting molecule exhibits low-lying the highest occupied molecular orbital (HOMO) energy level of −5.37 eV and the lowest unoccupied molecular orbital (LUMO) energy level of −3.78 eV. Therefore, DPP(C2T)2 can be used as either electron donor or acceptor for solution processed organic solar cells and shows a power conversion efficiency (PCE) of 1.66% when blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) and 1.08% with poly(3-hexylthiophene) (P3HT). The crystallinity, carrier mobilities and film morphology are systematically investigated and discussed, in order to study the influence of extra ester group on the properties of the resulting molecule.

•An AlQ3 derivative Al(4CAQ)3 is designed and synthesized in 6-step reactions.•Al(4CAQ)3 has three 2-ethylhexyl cyanoacrylate groups at C-4 of the quinoline ring.•Al(4CAQ)3 owns an appropriate LUMO of −3.7 eV to pair with donors in OSCs.•Al(4CAQ)3 shows excellent solubility and good UV–visible absorptions.•Al(4CAQ)3 would be a promising non-fullerene acceptor for solution-processed OSCs.In this Letter, a new AlQ3 derivative is designed and synthesized through introducing 2-ethylhexyl cyanoacrylate at C-4 of the quinoline ring. The obtained Al(4CAQ)3 shows excellent solubility (>50 mg/ml) in common solvents. Furthermore, the LUMO energy level of Al(4CAQ)3 is greatly lowered to −3.70 eV, which matches those of donors used in organic solar cells (OSCs). Together with its octahedral molecular geometry and good UV–visible absorptions, Al(4CAQ)3 would be a promising solution-processed electron acceptor for OSCs.

A new diketopyrrolopyrrole derivative with appropriate energy levels and bipolar charge-transport properties is designed and synthesized. When this molecule is used as either electron donor or acceptor, the resulting organic solar cells both give the power conversion efficiencies over 3%.

•Two diketopyrrolopyrrole based non-fullerene acceptors are designed and synthesized.•They have similar chemical components but different molecular geometries.•Distinctions are shown between two acceptors on physical and photovoltaic properties.•The molecule with the more twisted conformation shows a better PCE of 0.65% in OSCs.The non-fullerene acceptors with different geometric structures have great impact on light absorption, exciton dissociation, and charge transportation in the active layer of organic solar cells (OSCs). In this paper, we designed and synthesized two diketopyrrolopyrrole based non-fullerene acceptors, Ph(DPP)2 and PhDMe(DPP)2 with similar chemical components but different molecular geometries. Due to its more twisted molecular conformation, PhDMe(DPP)2 shows more blue-shifted absorption bands, higher electron mobility, and better miscibility with the polymer donor poly(3-hexylthiophene) (P3HT) while compared to Ph(DPP)2. Therefore, the resulting P3HT:PhDMe(DPP)2 based OSCs shows a better power conversion efficiency (PCE) of 0.65%, higher than that from P3HT:Ph(DPP)2 based OSCs (0.48%), which can be ascribed to more efficient exciton dissociation and electron transportation in the active layer of P3HT:PhDMe(DPP)2.

We designed and synthesized a diketopyrrolopyrrole (DPP) molecule with a fully-planar molecular geometry, 3,6-bis{5-[(ethylfuran-2-carboxylate)-2-yl]thiophene-2-yl}-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP(CF)2), for solution-processable organic solar cells (OSCs). It is theoretically calculated that the dihedral angles between the two furan-2-carboxylate end-groups and the DPP core are both only 0.56°. Due to this negligible steric distortion, the molecular conformation of DPP(CF)2 can be considered fully coplanar, leading to a higher crystallinity for the DPP(CF)2 film. As a result, the hole mobility of DPP(CF)2 is one order of magnitude higher than that of the DPP derivative with thiophene-2-carboxylate as the end-group (DPP(CT)2). DPP(CF)2 exhibits both a low optical band gap (Eg) of 1.60 eV and a low-lying highest occupied molecular orbital (HOMO) energy level of −5.33 eV, implying that DPP(CF)2 is a promising electron donor for OSCs. OSCs with DPP(CF)2 or DPP(CT)2 as the electron donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the electron acceptor were fabricated. It is found that the DPP(CF)2-based devices exhibit much better photovoltaic performance than the DPP(CT)2-based devices, with the highest power conversion efficiency of 5.37% and a short-circuit current density of 11.4 mA cm−2. This phenomenon can be ascribed to the superior charge-transporting ability of DPP(CF)2 due to its fully-planar molecular geometry.

Two novel solution-processable acceptor–donor–acceptor (A–D–A)-structured organic small molecules with diketopyrrolopyrrole (DPP) as terminal acceptor units and pentathiophene (PTA) or pyrrole-modified pentathiophene (NPTA) as the central donor unit, namely, DPP2(PTA) and DPP2(NPTA), were designed and synthesized. We examined the effects of changing the central bridging heteroatoms of the five-ring-fused thienoacene core identity from sulfur [DPP2(PTA)] to nitrogen [DPP2(NPTA)] in the small-molecule donor material. Replacement of the bridging atom with a different electronic structure has a visible effect on both the optical and electrical properties: DPP2(NPTA), which contains much more electron-rich pyrrole in the central thienoacene unit, possesses red-shifted absorption and a higher HOMO level relative to DPP2(PTA) with the less electron-rich thiophene in the same position. More importantly, substitution of the bridging atoms results in a change of the substituting alkyl chains due to the nature of the heteroatoms, which significantly tailored the crystallization behavior and the ability to form an interpenetrating network in thin-film blends with an electron acceptor. Compared to DPP2(PTA) with no alkyl chain substituting on the central sulfur atom of the PTA unit, DPP2(NPTA) exhibits improved crystallinity and better miscibility with PC71BM probably because of a dodecyl chain on the central nitrogen atom of the NPTA unit. These features endow the DPP2(NPTA)/PC71BM blend film higher hole mobility and better donor/acceptor interpenetrating network morphology. Optimized photovoltaic device fabrication based on DPP2(NPTA)/PC71BM (1.5:1, w/w) has resulted in an average power conversion efficiency (PCE) as high as 3.69% (the maximum PCE was 3.83%). This study demonstrates that subtle changes and tailoring of the molecular structure, such as simply changing the bridging heteroatom in the thienoacene unit in D/A-type small molecules, can strongly affect the physical properties that govern their photovoltaic performances.Keywords: cycle extension; diketopyrrolopyrrole; heterocycle-modified pentathiophene; organic solar cell; solution-processable small molecule; structure−property relationship;

•Two new triphenylamine (TPA) modified bis-diketopyrrolopyrrole molecular donor materials were synthesized.•Introducing TPA as end-capping donor units induce a significant red-shift (60 nm) of absorption onset.•Unlike most of TPA based molecules, strong aggregation was found in the solid state for both molecules.•A relatively high power conversion efficiency of 4.04% was achieved.Two new solution-processable enlarged π-conjugated donor–acceptor (D–A) organic small molecules consisting of dialkoxysubstituted benzo[1,2-b:4,5-b′]dithiophene (BDT) or dioctyltertthiophene (3T) as the central donor units, diketopyrrolopyrrole (DPP) as the acceptor unit and triphenylamine (TPA) as the terminal conjugated segment, TPA–DPP–BDT and TPA–DPP–3T, were designed and synthesized. Both small molecules possess broad absorption ranging from 300 to 800 nm with an optical band at approximately 1.50 eV and relatively low HOMO energy levels from −5.12 to approximately −5.16 eV. Expectedly, the UV–Vis absorption onset (810 nm) of TPA–DPP–BDT is largely red-shifted (60 nm) relative to that (750 nm) of previously reported BDT(TDPP)2, which consists of BDT and DPP units. Unlike most of the TPA based molecules, strong molecular aggregation was observed in the solid state for both small molecules. In addition, atomic force microscopy (AFM) and X-ray diffraction (XRD) investigations indicated that TPA–DPP–3T and TPA–DPP–BDT exhibit good miscibility with fullerene derivatives. The organic solar cells based on TPA–DPP–BDT/PC61BM(1:1) demonstrated power conversion efficiencies as high as 4.04% with a short-circuit current density (Jsc) of 11.40 mA cm−2 and a fill factor (FF) of 53.2% when the active layer of the cell was annealed at 130 °C for 10 min.Graphical abstract

•Three DPP derivatives with different end-groups were designed and synthesized.•Three DPP derivatives exhibit similar energy structures but different photovoltaic properties.•End-groups influence photovoltaic properties of three molecules through the inducing of different morphologies.In this work, a diphenyl substituted diketopyrrolopyrrole (DPP) and its two derivatives end-capped with fluorine and n-butyl respectively, namely PDPPP, FPDPPPF, and RPDPPPR, are designed and synthesized. The resulting molecules exhibit similar energy structures, i.e. both relatively narrow optical band gaps (1.75–1.79 eV) and deep highest occupied molecular orbital (HOMO) energy levels (−5.18 to −5.25 eV), implying that all of them are potentially good electron donors for organic solar cells (OSCs). However, three molecules show different photovoltaic performances when they are blended with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) to fabricate OSCs: the RPDPPPR-based device gives the highest power conversion efficiency (PCE) of 1.59%, whereas the PCEs of PDPPP and FPDPPPF-based OSCs are 0.46% and 0.55%, respectively. Through atomic force microscopy (AFM), X-ray diffraction (XRD) and space charge limited current (SCLC) characterizations, the prominent role of end-groups in the photovoltaic properties of DPP derivatives is disclosed: terminal alkyl chains in RPDPPPR can promote molecular crystallization and lead to the formation of finer phase-separation domains in the blended film, which are in favor of charge generation and transportation in the photovoltaic devices. Thus, RPDPPPR provides the best photovoltaic property among three DPP molecules.

For highly efficient organic solar cells (OSCs), the electron donor should possess not only a narrow band gap (Eg) but also a low highest occupied molecular orbital (HOMO) energy level. To achieve it, in this paper, we designed and synthesized a diketopyrrolopyrrole (DPP) derivative end capped with an ethyl thiophene-2-carboxylate moiety, 3,6-bis{5-[(ethyl thiophene-2-carboxylate)-2-yl]thiophene-2-yl}-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP(CT)2). Through UV-vis absorption and cyclic voltammetry (CV) measurements, we demonstrated that the resulting molecule exhibits both a low optical Eg of 1.65 eV and a lower-lying HOMO energy level of −5.33 eV owing to the electronegativity of the ester group and the conjugation effect of the thiophene ring. Therefore, when DPP(CT)2 is used as the electron donor to blend with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) for solution processable OSCs, a power conversion efficiency (PCE) of 4.02% combined with an open-circuit voltage (VOC) as high as 0.94 V and a broad photovoltaic response range extending to around 750 nm is obtained.

Diketopyrrolopyrrole (DPP) derivatives are an important class of high-performance pigment used in inks, paints, plastics, and organic electronics. Until now, DPP derivatives containing sophisticated aryl units at the DPP core have usually been obtained via Suzuki, Stille, or Negishi cross-coupling reactions, which require organometallic precursors. In this work, a series of DPP-based π-conjugated molecules bearing diverse aryl substituents on the thiophene- or benzene-DPPs were facilely synthesized in moderate to excellent yields through the Pd-catalyzed direct arylation of C–H bonds. The synthetic procedures feature advantages over traditional C–C cross-coupling reactions such as: (1) avoidance of the use of organometallic reagents in the starting materials leading to simpler byproducts and higher atom economy, (2) fewer synthetic steps, (3) higher yields, (4) better compatibility with chemically sensitive functional groups, and (5) simpler catalytic systems free of phosphine ligands. These advantages make the present protocol an ideal and versatile strategy for the synthesis of DPP derivatives, especially for structurally complicated DPPs that may possess chemically sensitive functionalities. The optical and electrochemical properties of the synthesized DPPs (17 compounds) were systematically investigated using UV-vis spectroscopy, steady-state fluorescence spectroscopy, and cyclic voltammetry (CV).

Three star-shaped D–A small molecules, (P-DPP)3TPA, (4-FP-DPP)3TPA, and (4-BuP-DPP)3TPA were designed and synthesized with triphenylamine (TPA) as the core, diketopyrrolopyrrole (DPP) as the arm, and unsubstituted or substituted benzene rings (phenyl, P; 4-fluoro-phenyl, 4-FP; 4-n-butyl-phenyl, 4-BuP) as the end-group. All the three small molecules show relatively narrow optical band gaps (1.68–1.72 eV) and low-lying highest occupied molecular orbital (HOMO) energy levels (−5.09∼−5.13 eV), implying that they are potentially good electron donors for organic solar cells (OSCs). Then, photovoltaic properties of the small molecules blended with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as electron acceptor were investigated. Among three small molecules, the OSC based on (P-DPP)3TPA:PCBM blend exhibits a best power conversion efficiency (PCE) of 2.98% with an open-circuit voltage (Voc) of 0.72 V, a short-circuit current density (Jsc) of 7.94 mA/cm2, and a fill factor (FF) of 52.2%, which may be ascribed to the highest hole mobility of (P-DPP)3TPA.Keywords: diketopyrrolopyrrole; organic solar cells; small molecule; star-shaped; triphenylamine;

•A ligand exchange process by acetic acid in the hybrid film of P3HT: CdSe QDs was achieved.•Hybrid solar cells with PCE of nearly 2% at low annealing temperature (120 °C) after ligand exchange were achieved.•The mechanism of performance enhancement was investigated.•Acetic acid shows its advantages of environmental friendliness, non-toxicity and mild odors, comparing to the thiols that are widely used in hybrid solar cells.We utilize an adsorption/desorption process between the excess acetic acid in solution and the bound oleate ions on the surface of CdSe quantum dots to achieve ligand exchange process in the hybrid film of poly (3-hexylthione) (P3HT): CdSe quantum dots (QDs). By this post-deposition ligand exchange method, we achieved hybrid solar cells with power conversion efficiency of nearly 2% at low annealing temperature (120 °C). Compared to the thiols that are widely used in hybrid solar cells, acetic acid shows its advantages of environmental friendliness, non-toxicity and mild odors. As such, our method is attractive for future hybrid solar cell applications.

High-performance hybrid solar cells (HSCs) based on P3HT:CdSe QD blends are achieved through post-deposition ligand exchange by n-butanethiol (n-BT) with a high power conversion efficiency of 3.09%. The mechanism by which n-BT modifies the surface structures of CdSe QDs and thus improves the HSCs performance is investigated.

To lower the HOMO (highest occupied molecular orbital) energy level of polythieno[3,4-b]thiophene (∼−4.5 eV), a series of ester-functionalized polythieno[3,4-b]thiophene derivatives (P1–P3) were designed and synthesized by Stille cross coupling reaction. The resulting copolymers exhibited broad and strong absorption bands from visible to near infrared region with low optical band gaps of 1.23–1.42 eV. Through cyclic voltammetry measurements, it was found that the HOMO energy levels of the copolymers gradually decreased with increasing the content of the thiophene-3,4-dicarboxylate moiety, i.e. −4.91 eV for P1, −5.00 eV for P2, and −5.11 eV for P3. Preliminary photovoltaic properties of the copolymers blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as electron acceptor were investigated. Among the three copolymers, P1 exhibited the best photovoltaic performance with an open circuit voltage (Voc) of 0.54 V, a short circuit current density (Isc) of 3.3 mA/cm2, a fill factor (FF) of 0.57, and a power conversion efficiency (PCE) of 1.02%. A high Voc up to 0.71 V was achieved in the solar cell based on a P3:PCBM blend.Graphical abstractHighlights► We introduce thiophene-3,4-dicarboxylate into polythieno[3,4-b]thiophene's backbone. ► The HOMO gradually decreased with increasing the content of thiophene-3,4-dicarboxylate. ► Photovoltaic properties of copolymers were investigated. ► A relatively high Voc of 0.71 V was achieved.

This paper reports how the morphology of a polymer–fullerene derivative blend is tuned via the different aggregate states of the polymer in solutions. Based on a copolymer with benzodiothiophene and thiophene-3-carboxylate as alternating units (PBDTCT), we explored the polymer aggregation (i.e., organo-gels) behavior as a function of steric hindrance of aromatic solvents imposed by substituents. We showed that the size of organo-gels decreased as the substituents of solvents got larger. Also, the phase separation and domain size of the subsequent spin-coated films increased monotonically with that of the organo-gels in solution. Through this knowledge, we eventually achieve controlled morphology and optimized organic solar cells (OSCs) performance. Our results present a significant step forward for understanding the self-assembly behavior of conjugated polymers, control of their morphology and optimization of OSC performance.

We report the synthesis of Cu2S nanocrystalline thin film directly on indium tin oxide (ITO) substrate by a facile hydrothermal method using glutathione as the capping agent. The morphology and the phase composition of the obtained Cu2S nanostructure film are characterized by X-ray diffraction and field emission scanning electron microscopy. A capping agent directed two-step growth mechanism is proposed based on investigation of the reaction condition dependent product morphology. Optical study shows that Cu2S crystalline film has a broad spectrum response over visible and near-infrared region. The current–voltage results from ITO/Cu2S/Al devices indicate that Cu2S film with larger crystal size has better conduction capacity.Highlights► We use a hydrothermal process to synthesize Cu2S nanocrystalline films. ► Glutathione content and reaction time have great effects on film morphologies. ► The Cu2S films show broad spectrum response in the near-infrared region. ► Morphology influences both optical and electrical properties of the thin films.

Fabrication of organic light-emitting diodes (OLEDs) and lasers on silicon substrates is a feasible route to integrate microelectronic chips with optical devices for telecommunications. However, the efficiency of Si-anode based OLEDs is restricted by the imbalance of hole–electron injection because a p-type Si anode owns better hole injection ability than ITO. We have used fluorinated tris-(8-hydroxy-quinolinato) aluminum (FAlq3) derivatives to prepare Si-anode based OLEDs. We observed that, when tris-(5-fuloro-8-hydroxyquinolinato) aluminum (5FAlq3) is used as the electron-transporting material instead of Alq3, the cathode electron injection is enhanced due to its lower lowest unoccupied molecular orbital (LUMO) compared to the Alq3. The device can keep the relative carrier balance even when a Si anode capable of stronger hole injection was used. Further optimization of the device structure by introducing 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) as a hole blocking layer showed significant increase in the device power efficiency from 0.029 to 0.462 lm/W. This indicates that use of fluorinated Alq3 derivatives is an effective way to improve the performance of Si-anode based OLEDs.

The lower the highest occupied molecular orbital (HOMO) energy level of the conjugated polymer is, the higher the open-circuit voltage (VOC) of the obtained polymer solar cell (PSC) is. To achieve this goal, a new conjugated polymer (PDTPTPD) alternating dithienopyrrole (DTP) and thienopyrroledione (TPD) units was designed and synthesized by Stille coupling reaction. Through UV–vis absorption and cyclic voltammetry (CV) measurements, it was found that the resulting copolymer exhibited both a low optical band gap of 1.62 eV and a low HOMO energy level of −5.09 eV owing to the electronegativity of TPD moiety. Preliminary photovoltaic study disclosed that the PSC based on PDTPTPD:PCBM ([6,6]-phenyl-C61 butyric acid methyl ester) blend showed a power conversion efficiency (PCE) of 1.9%, with a VOC of 0.70 V, and a short circuit current (ISC) of 6.97 mA/cm2, suggesting that PDTPTPD is a promising photovoltaic polymer.

To investigate the relationship between π−π stacking and charge transport property of organic semiconductors, a highly soluble violanthrone derivative, 16,17-bis(2-ethylhexyloxy)anthra[9,1,2-cde-]benzo[rst]pentaphene-5,10-dione (3), is designed and synthesized. The π−π stacking behavior and the aggregation of compound 3 in both solution and thin film were studied in detail by 1H nuclear magnetic resonance (NMR) spectroscopy, ultraviolet−visible (UV−vis) absorption, X-ray diffraction (XRD), and atomic force microscopy (AFM). When 1H NMR spectroscopy and theoretical modeling results were combined, the arrangements of compound 3 molecules in the aggregates are demonstrated, where the dipole moments of the two adjacent molecules are nearly reversed to achieve efficient intermolecular π−π overlapping. Furthermore, it is interesting to find that the π−π stacking of compound 3, in both solution and thin films, can be enhanced by introducing a poor solvent n-hexane into the dilute chloroform solution. The resulting film exhibits more red-shifted absorption and higher crystallinity than the film made from pure chloroform solvent, suggesting that π−π interactions in the solid state are intensified by the poor solvent. Organic field-effect transistors (OFETs) with compound 3 film as the transportation layer were fabricated. It is disclosed that the compound 3 film obtained from the chloroform/n-hexane mixed solvents exhibits 1 order of magnitude higher hole mobility than that from the pure chloroform solvent because of the enhanced π−π interactions and the higher crystallinity in the former film. This work provided us valuable information in the improvement of electronic and optoelectronic performances of organic semiconductors by tuning their aggregate structures.

We report the synthesis of 3D structural CdS nanocrystals by a simple biomolecule-assisted hydrothermal process. The CdS nanocrystals are composed of many branched nanorods with the diameter of about 50 nm, and the length of about 250 nm. The phase and crystallographic properties are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffractometry (XRD). The composites based on CdS nanocrystals and poly[2-methoxy-5-(2-ethylhexyloxy-p-phenylenevinylene)] (MEH-PPV) have been prepared by spin-coating of the mixture in the common solvent. The optical properties of the composites are investigated using ultraviolet–visible (UV–Vis) absorption and photoluminescence (PL) spectroscopies. A significant fluorescence quenching of MEH-PPV in the composites is observed at high CdS nanocrystals/MEH-PPV ratios, indicating that the photo-induced charge transfer occurred due to the energy level offset between the donor MEH-PPV and the acceptor CdS nanocrystals. The obvious photovoltaic behavior of the solar cell made from this composite further demonstrates the mentioned photo-induced charge transfer process.

Three novel conjugated polymers have been designed and synthesized via the alternative copolymerization of the electron-donating monomer benzodithiophene (BDT) and three different electron-accepting monomers: perylene diimide (PDI), naphthalene diimide (NDI), and phthalimide (PhI). All obtained copolymers show good solubility in common organic solvents as well as broader absorptions in visible region and narrower optical band gaps compared to homopolymers from BDT units. It is found that the absorptions of the copolymers are red-shifted with increasing the electron-withdrawing ability of the co-monomer. In particular, the absorption edge of P(BDT-NDI) film extends to 760 nm, whereas that of P(BDT-PhI) film is only at 577 nm. Cyclic voltammograms of the three polymers disclose that P(BDT-PDI) and P(BDT-NDI) are typical n-type materials because PDI and NDI are strong electron-accepting groups, while P(BDT-PhI) is a stable p-type material where the weak electron-withdrawing monomer (PhI) is introduced. The results suggest that the absorption range and the electrochemical properties of the conjugated polymers can be tuned by appropriate molecule-tailoring, which will help exploring ideal conducting polymers for potential applications in polymer optoelectronics, especially in polymer solar cells.

An asymmetrical perylene diimide 3, N-(4-methoxyphenyl)-N′-(4-nitrophenyl)-perylene-3,4,9,10-tetracarboxylic diimide, was synthesized, and its self-assembly and dissociation behaviors in chloroform was studied in detail by UV–vis and fluorescence spectroscopies. The resulting unique helical nanostructures from 3 were proposed to be self-assembled via the cooperative actions of π–π stacking, steric hindrance and electrophile–nucleophile type pairing.

This communication reports that not only the emission colour but also the photoluminescence quantum yield of Alq3 can be tuned by introducing fluorine atoms at different positions; with fluorination at C-5 the emission is red-shifted with a tremendously decreased intensity, fluorination at C-6 causes a blue-shift with a significantly increased intensity, and fluorination at C-7 has a minor effect on both the colour and intensity of Alq3's emission.

We report the synthesis of 3D structural CdS nanocrystals by a simple biomolecule-assisted hydrothermal process. The CdS nanocrystals are composed of many branched nanorods with the diameter of about 50 nm, and the length of about 250 nm. The phase and crystallographic properties are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffractometry (XRD). The composites based on CdS nanocrystals and poly[2-methoxy-5-(2-ethylhexyloxy-p-phenylenevinylene)] (MEH-PPV) have been prepared by spin-coating of the mixture in the common solvent. The optical properties of the composites are investigated using ultraviolet–visible (UV–Vis) absorption and photoluminescence (PL) spectroscopies. A significant fluorescence quenching of MEH-PPV in the composites is observed at high CdS nanocrystals/MEH-PPV ratios, indicating that the photo-induced charge transfer occurred due to the energy level offset between the donor MEH-PPV and the acceptor CdS nanocrystals. The obvious photovoltaic behavior of the solar cell made from this composite further demonstrates the mentioned photo-induced charge transfer process.

We designed and synthesized a diketopyrrolopyrrole (DPP) molecule with a fully-planar molecular geometry, 3,6-bis{5-[(ethylfuran-2-carboxylate)-2-yl]thiophene-2-yl}-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP(CF)2), for solution-processable organic solar cells (OSCs). It is theoretically calculated that the dihedral angles between the two furan-2-carboxylate end-groups and the DPP core are both only 0.56°. Due to this negligible steric distortion, the molecular conformation of DPP(CF)2 can be considered fully coplanar, leading to a higher crystallinity for the DPP(CF)2 film. As a result, the hole mobility of DPP(CF)2 is one order of magnitude higher than that of the DPP derivative with thiophene-2-carboxylate as the end-group (DPP(CT)2). DPP(CF)2 exhibits both a low optical band gap (Eg) of 1.60 eV and a low-lying highest occupied molecular orbital (HOMO) energy level of −5.33 eV, implying that DPP(CF)2 is a promising electron donor for OSCs. OSCs with DPP(CF)2 or DPP(CT)2 as the electron donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the electron acceptor were fabricated. It is found that the DPP(CF)2-based devices exhibit much better photovoltaic performance than the DPP(CT)2-based devices, with the highest power conversion efficiency of 5.37% and a short-circuit current density of 11.4 mA cm−2. This phenomenon can be ascribed to the superior charge-transporting ability of DPP(CF)2 due to its fully-planar molecular geometry.

Perylene diimide (PDI), which features intense absorption, a low-lying energy level, and high electron mobility, is a promising building block for electron acceptors in organic solar cells (OSCs). However, this planar molecule has a strong tendency to form large aggregates during film formation which strongly limits its OSC performance. Herein, we report a new and simple PDI derivative, B(PDI)3, in which a central benzene unit is employed to connect three PDI arms. This compact arrangement of sterically bulky PDI moieties leads to a twisted molecular geometry of the resultant structure. This suppresses the strong crystallization tendency of PDI chromophores, owing to the broken molecular coplanarity and symmetry. Therefore, B(PDI)3 is applied as a non-fullerene acceptor in OSCs, providing a good power conversion efficiency of 5.65% when blended with the PTB7-Th donor.

Effective electron acceptor materials usually have a deep lowest unoccupied molecular orbital (LUMO) energy level that can split excitons and generate current. A non-fullerene electron acceptor (F8-DPPTCN) was developed, using fluorene as the core with arms of diketopyrrolopyrrole (DPP) having thiophene-2-carbonitrile as the terminal units. The new molecule had a LUMO of −3.65 eV and a narrow bandgap (Eg) of 1.66 eV, owing to the electronegativity of the thiophene-2-carbonitrile group and its conjugation with DPP units. Organic solar cells (OSCs) with F8-DPPTCN as the acceptor and poly(3-hexylthiophene) (P3HT) as the donor were fabricated. A power conversion efficiency (PCE) of 2.37% was obtained with an open-circuit voltage (Voc) of 0.97 V, a short-circuit current (Jsc) of 6.25 mA cm−2, and a fill factor (FF) of 0.39. Structural characterization showed that P3HT and F8-DPPTCN were kinetically trapped in a weakly separated state whereas thermal annealing led to the crystallization of P3HT and the formation of a network structure with a mesh-size of several hundred nanometers. When a solvent additive, diiodooctane, was used and the mixture was thermally annealed, both P3HT and F8-DPPTCN crystallized and a multi-length scale network was formed. Though the PCEs were low, the changes in the PCE could be correlated with the morphological changes, opening pathways to increase performance further.

For highly efficient organic solar cells (OSCs), the electron donor should possess not only a narrow band gap (Eg) but also a low highest occupied molecular orbital (HOMO) energy level. To achieve it, in this paper, we designed and synthesized a diketopyrrolopyrrole (DPP) derivative end capped with an ethyl thiophene-2-carboxylate moiety, 3,6-bis{5-[(ethyl thiophene-2-carboxylate)-2-yl]thiophene-2-yl}-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP(CT)2). Through UV-vis absorption and cyclic voltammetry (CV) measurements, we demonstrated that the resulting molecule exhibits both a low optical Eg of 1.65 eV and a lower-lying HOMO energy level of −5.33 eV owing to the electronegativity of the ester group and the conjugation effect of the thiophene ring. Therefore, when DPP(CT)2 is used as the electron donor to blend with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) for solution processable OSCs, a power conversion efficiency (PCE) of 4.02% combined with an open-circuit voltage (VOC) as high as 0.94 V and a broad photovoltaic response range extending to around 750 nm is obtained.

High-performance hybrid solar cells (HSCs) based on P3HT:CdSe QD blends are achieved through post-deposition ligand exchange by n-butanethiol (n-BT) with a high power conversion efficiency of 3.09%. The mechanism by which n-BT modifies the surface structures of CdSe QDs and thus improves the HSCs performance is investigated.

Diketopyrrolopyrrole (DPP) derivatives are an important class of high-performance pigment used in inks, paints, plastics, and organic electronics. Until now, DPP derivatives containing sophisticated aryl units at the DPP core have usually been obtained via Suzuki, Stille, or Negishi cross-coupling reactions, which require organometallic precursors. In this work, a series of DPP-based π-conjugated molecules bearing diverse aryl substituents on the thiophene- or benzene-DPPs were facilely synthesized in moderate to excellent yields through the Pd-catalyzed direct arylation of C–H bonds. The synthetic procedures feature advantages over traditional C–C cross-coupling reactions such as: (1) avoidance of the use of organometallic reagents in the starting materials leading to simpler byproducts and higher atom economy, (2) fewer synthetic steps, (3) higher yields, (4) better compatibility with chemically sensitive functional groups, and (5) simpler catalytic systems free of phosphine ligands. These advantages make the present protocol an ideal and versatile strategy for the synthesis of DPP derivatives, especially for structurally complicated DPPs that may possess chemically sensitive functionalities. The optical and electrochemical properties of the synthesized DPPs (17 compounds) were systematically investigated using UV-vis spectroscopy, steady-state fluorescence spectroscopy, and cyclic voltammetry (CV).

A new diketopyrrolopyrrole derivative with appropriate energy levels and bipolar charge-transport properties is designed and synthesized. When this molecule is used as either electron donor or acceptor, the resulting organic solar cells both give the power conversion efficiencies over 3%.

Nowadays, organic solar cells (OSCs) with efficiencies over 10% have been achieved through the elaborate design of electron donors and fullerene acceptors. However, the drawbacks of fullerene acceptors, like poor absorption, limited chemical and energetic tunabilities, high-cost purification and morphological instability, have become the bottlenecks for the further improvement of OSCs. To overcome the mentioned shortages from fullerene, research studies on non-fullerene electron acceptors have boomed. To date, the highest efficiency of fullerene-free OSCs has been pushed to be 12%, which surpasses that of fullerene-based OSCs. In this perspective, we focus on summarizing the development of small molecule electron acceptors designed to replace the fullerene derivatives. Since it has been revealed that the search for matched donor:acceptor pairs is important for accomplishing high efficiencies, we therefore divide electron acceptors into several categories according to the donors used in fullerene-free OSCs. After the introduction of these acceptors, we outline the designing rules as well as perspectives for the development of non-fullerene acceptors. We believe that the development of non-fullerene electron acceptors will make organic photovoltaics closer to practical applications.