Dong Hun Sin

A Nonfullerene Small Molecule Acceptor with 3D Interlocking Geometry Enabling Efficient Organic Solar Cells

A new 3D nonfullerene small-molecule acceptor is reported. The 3D interlocking geometry of the small-molecule acceptor enables uniform molecular conformation and strong intermolecular connectivity, facilitating favorable nanoscale phase separation and electron charge transfer. By employing both a novel polymer donor and a nonfullerene small-molecule acceptor in the solution-processed organic solar cells, a high-power conversion efficiency of close to 6% is demonstrated.

Highly crystalline low-bandgap polymer nanowires towards high-performance thick-film organic solar cells exceeding 10% power conversion efficiency

Preparing polymer nanowire (PNW) structures using donor–acceptor (D–A) conjugated polymers is one promising strategy to improve the power conversion efficiencies (PCEs) of bulk-heterojunction (BHJ) polymer solar cells (PSCs). Here, we report that a high PCE of 10.62% was obtained with a single-junction inverted cell with a 350 nm thick active layer containing highly crystalline PNWs based on a D–A conjugated polymer (P4TNTz-2F), which possesses a deep-lying HOMO level (−5.46 eV) and a low-bandgap (1.59 eV) as well as a planar/rigid backbone. The thick active layer in the P4TNTz-2F:PC71BM-based PSC absorbs incident light almost completely, which in turn contributes to a high short-circuit current density of 19.45 mA cm−2. This high PCE is attributed to the continuous and evenly distributed polymer network with narrow PNWs (≈6 nm in width and several hundred nanometers in length) in the thick film blended with PC71BM, which facilitates charge separation (QPL ≈ 98%) and transport (μh = 8.31 × 10−3 cm2 V−1 s−1). Moreover, this PNW structure in the BHJ active layer can be prepared using a facile film-forming process at a mild blending temperature (≈70 °C), which means that high efficiency BHJ PSCs can be fabricated with good reproducibility. These results demonstrate the great promise of polymer nanowire solar cells and provide important scientific insights that facilitate further improvements in the morphologies and performances of organic solar cells through material design and development.

Understanding Solidification of Polythiophene Thin Films during Spin-Coating: Effects of Spin-Coating Time and Processing Additives

Spin-coating has been used extensively in the fabrication of electronic devices; however, the effects of the processing parameters have not been fully explored. Here, we systematically characterize the effects of the spin-coating time on the microstructure evolution during semiconducting polymer solidification in an effort to establish the relationship between this parameter and the performances of the resulting polymer field-effect transistors (FETs). We found that a short spin-coating time of a few seconds dramatically improve the morphology and molecular order in a conjugated polymer thin film because the π-π stacking structures formed by the polymer molecules grow slowly and with a greater degree of order due to the residual solvent present in the wet film. The improved ordering is correlated with improved charge carrier transport in the FETs prepared from these films. We also demonstrated the effects of various processing additives on the resulting FET characteristics as well as on the film drying behavior during spin-coating. The physical properties of the additives are found to affect the film drying process and the resulting device performance.

Decoupling Charge Transfer and Transport at Polymeric Hole Transport Layer in Perovskite Solar Cells

Tailoring charge extraction interfaces in perovskite solar cells (PeSCs) critically determines the photovoltaic performance of PeSCs. Here, we investigated the decoupling of two major determinants of the efficient charge extraction, the charge transport and interfacial charge transfer properties at hole transport layers (HTLs). A simple physical tuning of a representative polymeric HTL, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), provided a wide range of charge conductivities from 10(-4) to 10(3) S cm(-1) without significant modulations in their energy levels, thereby enabling the decoupling of charge transport and transfer properties at HTLs. The transient photovoltaic response measurement revealed that the facilitation of hole transport through the highly conductive HTL promoted the elongation of charge carrier lifetimes within the PeSCs up to 3 times, leading to enhanced photocurrent extraction and finally 25% higher power conversion efficiency.

Critical factors governing vertical phase separation in polymer–PCBM blend films for organic solar cells

In organic bulk-heterojunction solar cells, the vertical distribution of the composition of the active layers as well as the lateral morphology is one of the critical issues that can significantly affect charge transport and recombination characteristics. Here we studied the critical parameters that can affect the formation of vertically stratified bulk heterojunction organic solar cells based on various polymers with different side chains, and investigated the effect of the miscibility of the polymer–fullerene blend and the crystallinity of the polymer on vertical morphology. The major factor that affected the vertical phase separation was the interaction parameter χ between the polymer and phenyl-C61-butyric acid methyl ester (PCBM). Polymer–PCBM blends with high values of χ tended to trigger surface-directed vertical phase separation during rapid solvent evaporation. However, strong aggregation of polymers with low solubility counteracted this surface-directed vertical stratification. Moreover, solvent additives strongly affected the vertical phase separation processes, and caused the composition of the active layer to fluctuate dramatically. We also found the photovoltaic characteristics, including charge recombination time, to be strongly affected by the vertical distribution of the composition. The modulation of the composition in the vertical direction should therefore be optimized to increase the efficiency of charge collection and hence achieve high-efficiency organic solar cells.

Propeller-shaped small molecule acceptors containing a 9,9′-spirobifluorene core with imide-linked perylene diimides for non-fullerene organic solar cells

Three-dimensional (3D) non-fullerene small molecule acceptors with imide-linked perylene diimides (iPDI) on a 9,9′-spirobi[9H-fluorene] core were designed, synthesized and characterized for use in organic solar cells. The best power conversion efficiency of 5.31% is obtained using SF-iPDI2 in a solution-processed bulk heterojunction solar cell. These results demonstrate that PDI derivatives with 3D molecular structures could serve as high-performance electron acceptors in non-fullerene solar cells.

Synergistic effects of an alkylthieno[3,2-b]thiophene π-bridging backbone extension on the photovoltaic performances of donor–acceptor copolymers

The synergistic effects of a thiophene-based π-bridging backbone extension on the intrinsic and photovoltaic properties of electron donor–acceptor (D–A) copolymers were systematically investigated. A series of alternating D–A copolymers (PBTs) based on 4,8-bis(5-ethylhexylselenophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (EHSeBDT) and 5-(2-butyloctyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (BOTPD), which featured thiophene-based π-bridges, were synthesized: PBT without a π-bridge, PBT with a 3-octylthiophene (OT) π-bridge (PBT-OT), and PBT with a 3-octylthieno[3,2-b]thiophene (OTT) π-bridge (PBT-OTT). The light absorption and charge transport properties were significantly enhanced upon incorporation of the OTT π-bridge. The enhancements resulted from the strong π–π intermolecular interactions using the OTT π-bridging backbone extension between neighboring polymer chains. PBT-OTT was most miscible in PC71BM. As a result, the photoactive layers prepared using PBT-OTT and PC71BM formed a well-mixed bulk-heterojunction morphology and yielded organic solar cells (OSCs) with a high power conversion efficiency of 7.21%. Transient absorption analysis suggested that the π-electrons were further delocalized along the copolymer after incorporation of the OTT π-bridge, and the charge separation efficiency increased. These results suggested that incorporating OTT π-bridges into D–A copolymers provides a useful strategy for developing highly efficient OSCs.

Enhancing the Durability and Carrier Selectivity of Perovskite Solar Cells Using a Blend Interlayer

A mechanically and thermally stable and electron-selective ZnO/CH3NH3PbI3 interface is created via hybridization of a polar insulating polymer, poly(ethylene glycol) (PEG), into ZnO nanoparticles (NPs). PEG successfully passivates the oxygen defects on ZnO and prevents direct contact between CH3NH3PbI3 and defects on ZnO. A uniform CH3NH3PbI3 film is formed on a soft ZnO:PEG layer after dispersion of the residual stress from the volume expansion during CH3NH3PbI3 conversion. PEG also increases the work of adhesion of the CH3NH3PbI3 film on the ZnO:PEG layer and holds the CH3NH3PbI3 film with hydrogen bonding. Furthermore, PEG tailors the interfacial electronic structure of ZnO, reducing the electron affinity of ZnO. As a result, a selective electron-collection cathode is formed with a reduced electron affinity and a deep-lying valence band of ZnO, which significantly enhances the carrier lifetime (473 μs) and photovoltaic performance (15.5%). The mechanically and electrically durable ZnO:PEG/CH3NH3PbI3 interface maintains the sustainable performance of the solar cells over 1 year. A soft and durable cathodic interface via PEG hybridization in a ZnO layer is an effective strategy toward flexible electronics and commercialization of the perovskite solar cells.

Recent Advances in Morphology Optimization for Organic Photovoltaics

Organic photovoltaics are an important part of a next-generation energy-harvesting technology that uses a practically infinite pollutant-free energy source. They have the advantages of light weight, solution processability, cheap materials, low production cost, and deformability. However, to date, the moderate photovoltaic efficiencies and poor stabilities of organic photovoltaics impede their use as replacements for inorganic photovoltaics. Recent developments in bulk-heterojunction organic photovoltaics mean that they have almost reached the lower efficiency limit for feasible commercialization. In this review article, the recent understanding of the ideal bulk-heterojunction morphology of the photoactive layer for efficient exciton dissociation and charge transport is described, and recent attempts as well as early-stage trials to realize this ideal morphology are discussed systematically from a morphological viewpoint. The various approaches to optimizing morphologies consisting of an interpenetrating bicontinuous network with appropriate domain sizes and mixed regions are categorized, and in each category, the recent trends in the morphology control on the multilength scale are highlighted and discussed in detail. This review article concludes by identifying the remaining challenges for the control of active layer morphologies and by providing perspectives toward real application and commercialization of organic photovoltaics.

Enhanced Organic Solar Cell Stability through the Effective Blocking of Oxygen Diffusion using a Self‐Passivating Metal Electrode

A new and simple strategy for enhancing the stability of organic solar cells (OSCs) was developed by using self-passivating metal top electrodes. Systematic investigations on O2 permeability of Al top electrodes revealed that the main pathways for oxidation-induced degradation could be greatly suppressed by simply controlling the nanoscale morphology of the Al electrode. The population of nanoscale pinholes among Al grains, which critically decided the diffusion of O2 molecules toward the Al-organic interfaces that are vulnerable to oxidation, was successfully regulated by rapidly depositing Al or promoting lateral growth among the Al grains, accompanied by increasing the deposition thickness. Our observations suggested that the stability of OSCs with conventional architectures might be greatly enhanced simply by controlling the fabrication conditions of the Al top electrode, without the aid of additional secondary treatments.