Jon-Paul Sun

Perovskite-fullerene hybrid materials suppress hysteresis in planar diodes

Solution-processed planar perovskite devices are highly desirable in a wide variety of optoelectronic applications; however, they are prone to hysteresis and current instabilities. Here we report the first perovskite–PCBM hybrid solid with significantly reduced hysteresis and recombination loss achieved in a single step. This new material displays an efficient electrically coupled microstructure: PCBM is homogeneously distributed throughout the film at perovskite grain boundaries. The PCBM passivates the key PbI3− antisite defects during the perovskite self-assembly, as revealed by theory and experiment. Photoluminescence transient spectroscopy proves that the PCBM phase promotes electron extraction. We showcase this mixed material in planar solar cells that feature low hysteresis and enhanced photovoltage. Using conductive AFM studies, we reveal the memristive properties of perovskite films. We close by positing that PCBM, by tying up both halide-rich antisites and unincorporated halides, reduces electric field-induced anion migration that may give rise to hysteresis and unstable diode behaviour.

Air-stable n-type colloidal quantum dot solids.

Colloidal quantum dots (CQDs) offer promise in flexible electronics, light sensing and energy conversion. These applications rely on rectifying junctions that require the creation of high-quality CQD solids that are controllably n-type (electron-rich) or p-type (hole-rich). Unfortunately, n-type semiconductors made using soft matter are notoriously prone to oxidation within minutes of air exposure. Here we report high-performance, air-stable n-type CQD solids. Using density functional theory we identify inorganic passivants that bind strongly to the CQD surface and repel oxidative attack. A materials processing strategy that wards off strong protic attack by polar solvents enabled the synthesis of an air-stable n-type PbS CQD solid. This material was used to build an air-processed inverted quantum junction device, which shows the highest current density from any CQD solar cell and a solar power conversion efficiency as high as 8%. We also feature the n-type CQD solid in the rapid, sensitive, and specific detection of atmospheric NO2. This work paves the way for new families of electronic devices that leverage air-stable quantum-tuned materials.

Recent advances of non-fullerene, small molecular acceptors for solution processed bulk heterojunction solar cells

Organic, planar, and electron deficient small molecules were utilized as acceptors in the first reported bilayer heterojunction solar cells, however, current state-of-the-art organic photovoltaic (OPV) cells utilize fullerene derivatives as acceptor molecules. Recently, intensive efforts have been directed towards the development and understanding of soluble, non-fullerene, organic small molecules to fabricate bulk heterojunction (BHJ) solar cells. These efforts have been aimed at overcoming the inherent limitations of fullerene compounds such as the limited spectral breadth, air instability, and the typically higher production costs of fullerenes. In this focused review, we have highlighted the most recent progress over the last couple of years towards developing n-type organic small molecules utilized in BHJ devices in order to provide insight towards improving the overall performance of OPVs.

Electron deficient diketopyrrolopyrrole dyes for organic electronics: synthesis by direct arylation, optoelectronic characterization, and charge carrier mobility

Four electron deficient small molecules based on the diketopyrrolopyrrole (DPP) chromophore were synthesized using microwave-assisted direct arylation reactivity. These molecules are based upon an acceptor–donor–acceptor–donor–acceptor (A1–D–A2–D–A1) framework, where DPP is utilized as the central acceptor (A2) unit. We compared the effect of naphthalimide vs. phthalimide terminal acceptors (A1), and different DPP (A2) alkyl groups, on the optical, thermal, electrochemical and electronic properties. A combination of absorption and emission spectroscopy, differential scanning calorimetry, thermal gravimetric analysis, cyclic voltammetry, ultraviolet photoelectron spectroscopy, charge carrier mobility, and DFT calculations were used to characterize the four materials. All compounds were found to have narrow band-gaps, deep HOMO/LUMO levels, and were able to effectively act as electron transport materials.

Phthalimide-based π-conjugated small molecules with tailored electronic energy levels for use as acceptors in organic solar cells

The design, synthesis, and characterization of seven phthalimide-based organic π-conjugated small molecules are reported. The new materials are based on a phthalimide–thiophene–CORE–thiophene–phthalimide architecture. The CORE units utilized were phthalimide (M2), diketopyrrolopyrrole (M3), isoindigo (M4), naphthalene diimide (M5), perylene diimide (M6), and difluorobenzothiadiazole (M7); they were specifically selected to progressively increase the electron affinity of the resulting compound. A small molecule with no core (M1) was synthesized for comparison. Each material was synthesized through optimized direct heteroarylation cross-coupling procedures using bench top solvents in air. Combinations of UV-visible spectroscopy (UV-vis), cyclic voltammetry (CV), differential scanning calorimetry (DSC), ultraviolet photoelectron spectroscopy (UPS) and density functional theory (DFT) were used to characterize each material. The use of various core acceptor building blocks with differing electron affinities resulted in the series M1–M7 having a range of energetically deep LUMO levels and a range of HOMO–LUMO gap energies. Meanwhile, the melting and crystallization temperatures of the molecules M1–M7 were also found to vary according to the change in central acceptor unit. Compounds M1–M7 were employed as acceptors in combination with either the polymeric donor P3HT or small molecule donor DTS(FBTTh2)2 to understand how the LUMO levels of each acceptor influences the open circuit voltage (Voc). It was found that, in general, Voc was only weakly related to the offset between the HOMO energy level of the donor and LUMO level of the acceptor used, with a Voc of up to 1.2 V being achieved for M1.

An Electron‐Deficient Small Molecule Accessible from Sustainable Synthesis and Building Blocks for Use as a Fullerene Alternative in Organic Photovoltaics

An electron-deficient small molecule accessible from sustainable isoindigo and phthalimide building blocks was synthesized via optimized synthetic procedures that incorporate microwave-assisted synthesis and a heterogeneous catalyst for Suzuki coupling, and direct heteroarylation carbon-carbon bond forming reactions. The material was designed as a non-fullerene acceptor with the help of DFT calculations and characterized by optical, electronic, and thermal analysis. Further investigation of the material revealed a differing solid-state morphology with the use of three well-known processing conditions: thermal annealing, solvent vapor annealing and small volume fractions of 1,8-diiodooctane (DIO) additive. These unique morphologies persist in the active layer blends and have demonstrated a distinct influence on device performance. Organic photovoltaic-bulk heterojunction (OPV-BHJ) devices show an inherently high open circuit voltage (Voc ) with the best power conversion efficiency (PCE) cells reaching 1.0 V with 0.4 v/v % DIO as a processing additive.

The structural evolution of an isoindigo-based non-fullerene acceptor for use in organic photovoltaics

The structural evolution of a functional isoindigo-based non-fullerene acceptor led to the development of three new materials to address the deficiencies of the original framework. Owing to the versatility of the structure and the flexibility of the synthetic procedures, these three new materials were accessible from previously optimized reaction conditions and similar precursor materials. The influence of structural modification on the optical, electrochemical and thermal properties were assessed and correlated with DFT calculations to provide compelling evidence for the effect of each substitution and how they relate to each particular adaptation. The structure–property relationships were investigated for their photovoltaic performance in solution processable BHJ devices fabricated in inverted architectures. Evaluation of device performance demonstrated that a single modification did not improve on the efficiency of the original structure, but the combination of both induced non-planarity, and increased electron affinity of the fourth iteration showed the potential of our framework, with PCE reaching 1.9%.

Phthalimide–thiophene-based conjugated organic small molecules with high electron mobility

A series of low-cost phthalimide end-capped oligothiophene small molecules with variations to the terminal alkyl chain and number of thiophene units in the conjugated core have been synthesized and investigated. All molecules exhibit H-aggregation in the solid-state but different crystal structures and electronic properties, showing that subtle chemical modifications can result in dramatic changes to molecular self-assembly. Field-effect transistors display high electron mobilities of up to 0.2 cm2 V−1 s−1.

N-Annulated perylene diimide dimers: acetylene linkers as a strategy for controlling structural conformation and the impact on physical, electronic, optical and photovoltaic properties

The geometry of organic π-conjugated small molecules can impact the morphology of blended-thin films and subsequent performance in opto-electronic devices. In this report, we investigate the role of molecular conformation of perylene diimide (PDI) dimers designed to act as non-fullerene acceptors in organic solar cells. A series of three PDI dimers is presented in which the PDI chromophores are directly linked via the bay position (PDI2, 3) or separated by one (PDI2Ac, 4) or two (PDI2Ac2, 5) acetylene spacers. In all cases, the exo-position of the PDI dimers is N-annulated. New compounds 4 and 5 were synthesized via an optimized and facile synthetic pathway. Directly linked PDI dimers adopted a highly twisted conformation whereas adding two acetylene spacers rendered the PDI chromophores coplanar. 1H NMR spectroscopic analysis of each dimer revealed a highly sensitive electronic structure that is strongly influenced by the acetylene spacers. It was found that compounds 4 and 5 with less twisted structures exhibited similar electron affinities but lower ionization potentials, lower organic solvent solubility, and red-shifted optical absorption spectra when compared to the highly twisted dimer 3. In addition, 4 and 5 showed a stronger tendency to aggregate in both solution and the solid state. This had a large impact on the performance of organic solar cells using these materials as electron acceptors. Bulk-heterojunction solar cells based upon a PTB7-Th:3 active layer could reach high power conversion efficiencies of 5.23%. In contrast, PTB7-Th:4 and PTB7-Th:5 based devices had ∼5 times lower performance owing to the formation of unfavourable active layer morphologies.

The Silicon:Colloidal Quantum Dot Heterojunction

A heterojunction between crystalline silicon and colloidal quantum dots (CQDs) is realized. A special interface modification is developed to overcome an inherent energetic band mismatch between the two semiconductors, and realize the efficient collection of infrared photocarriers generated in the CQD film. This junction is used to produce a sensitive near infrared photodetector.