ZiQi Sun

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is widely used as hole injection/extraction material in organic optoelectronics. However, there still exist drawbacks for PEDOT:PSS such as low work function (WF), poor structural and electrical homogeneity. To solve these problems, methylnaphthalene sulfonate formaldehyde condensate (MNSF) is applied, which has excellent dispersion property, branched chemical structure, and low cost, as dispersant and dopant instead of linear PSS to prepare PEDOT:MNSF. The hole injection/extraction capability of PEDOT:MNSF is systematically studied in organic optoelectronic devices. PEDOT:MNSF-1:6 exhibits unexpected high device performance with a maxima current efficiency of 33.4 cd A⁻¹ in blue phosphorescent organic light-emitting diode and a power conversion efficiency of 13.1% in CH₃NH₃PbI_xCl_3−x-based inverted perovskite solar cell, respectively. Compared with PEDOT:PSS, the relatively higher efficiency of PEDOT:MNSF-1:6 is attributed mainly to its higher WF of 5.29 eV, structural and electrical homogeneity. Our research displays a promising future of MNSF as a cheap and widely available alternative of PSS. Moreover, a clear map is provided for the design of dopant for PEDOT considering the structure of dopant.

Comparing with poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), methylnaphthalene sulfonate formaldehyde condensate-doped PEDOT with higher work function, excellent homogeneity, but low conductivity exhibits higher hole injection/extraction capabilities in organic light-emitting diodes/perovskite solar cells. The semiconductivity of dopant and structural homogeneity are both key factors. The results demonstrate a panoramic prospect for the future design of dopant for PEDOT.

Properties of hole transporting layers (HTLs) and back electrode are very critical to the stability of inverted bulk heterojunction organic photovoltaic (OPV) modules. Here, various deposition methods for back electrodes and materials of HTLs are examined by applying to inverted organic solar cells with a structure of indium tin oxide/ZnO/photoactive layer/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/Ag. The experiment is performed on encapsulated modules with flexible barrier films under accelerated conditions. The OPV modules with screen-printed Ag electrodes are shown to be electrically unstable with a reduction of the current density under damp heat condition at 85 °C/85% RH. Optical images for the active layer/PEDOT:PSS interface reveal that a reaction between the solvent from the Ag electrode and the underlying layers is the major cause for the degradation. In comparison with materials of the HTLs, the PEDOT:PSS layer shows low stability compared to the MoO₃ layer under the accelerated conditions. Unusual chemical changes in the PEDOT:PSS film are observed through X-ray photoelectron spectroscopy and this is further addressed by correlating the stability of the OPV devices.

Critical impact of hole transporting layer (HTL) and back electrode on the stability of organic photovoltaics (OPVs) is discussed. The active and HTL layers are damaged by solvent of the printed Ag electrode, resulting in abrupt decay of the power conversion efficiency. Unstable features of the OPVs accompanied by chemical change of the PEDOT:PSS is further addressed with X-ray photoelectron spectroscopy (XPS).

A small molecule DRCN5T as the donor with the of 1.60 eV, and a 3D perylene diimide as electronic acceptor are used to fabricate non-fullerene all-small-molecule solar cells. Upon thermal annealing, the efficiency of the device is enhanced from 0.47% to 6.16% with a high V_oc of 1.04 V and a small energy loss of 0.56 eV.

Nonfullerene acceptor FDICTF (2,9-bis(2methylene-(3-(1,1-dicyanomethylene)indanone))-7,​12-​dihydro-​4,​4,​7,​7,​12,​12-​hexaoctyl-​4H-​cyclopenta[2″,​1″:5,​6;3″,​4″:5′,​6′]​diindeno[1,​2-​b:1′,​2′-​b′]dithiophene) modified by fusing the fluorene core in a precursor, yields 10.06% high power conversion efficiency, and demonstrates that the ladder and fused core backbone in A–D–A structure molecules is an effective design strategy for high-performance nonfullerene acceptors.

Fullerene-free organic solar cells show over 11% power conversion efficiency, processed by low toxic solvents. The applied donor and acceptor in the bulk heterojunction exhibit almost the same highest occupied molecular orbital level, yet exhibit very efficient charge creation.

Ultraweak light detection with solid-state and cooling-free photodetectors is important for both fundamental research and practical applications. A general phototransistor architecture for detecting ultraviolet–visible light down to 100 fW cm⁻² at room temperature is demonstrated. The exceptional sensitivity stems from an amplification process triggered by incident light. A responsivity of ≈10⁷ A W⁻¹ is achieved.

A sequential-casting ternary method is developed to create stratified bulk heterojunction (BHJ) solar cells, in which the two BHJ layers are spin cast sequentially without the need of adopting a middle electrode and orthogonal solvents. This method is found to be particularly useful for polymers that form a mechanically alloyed morphology due to the high degree of miscibility in the blend.

Bifunctional electrocatalysts for oxygen evolution/reduction reaction (OER/ORR) are desirable for the development of energy conversion technologies. It is discovered that the manganese quadruple perovskites CaMn₇O₁₂ and LaMn₇O₁₂ show bifunctional catalysis in the OER/ORR. A possible origin of the high OER activity is the unique surface structure through corner-shared planar MnO₄ and octahedral MnO₆ units to promote direct OO bond formations.

Recent progress regarding planar heterojunctions (PHJs) is reviewed, with respect to the fundamental understanding of the photophysical processes at the donor/acceptor interfaces in organic photovoltaic devices (OPVs). The current state of OPV research is summarized and the advantages of PHJs as models for exploring the relationship between organic interfaces and device characteristics described. The preparation methods and the characterization of PHJ structures to provide key points for the appropriate handling of PHJs. Next, we describe the effects of the donor/acceptor interface on each photoelectric conversion process are reviewed by examining various PHJ systems to clarify what is currently known and not known. Finally, it is discussed how we the knowledge obtained by studies of PHJs can be used to overcome the current limits of OPV efficiency.

Heterojunctions of organic semiconductors are the most important element in organic photovoltaic devices (OPVs). The recent progress in research on planar heterojunctions is reviewed with respect to a fundamental understanding of the photophysical processes at the donor/acceptor interfaces in OPVs. This provides valuable information to overcome the current limitations regarding the performance of OPVs.

Solution-processed CsPbBr₃ quantum-dot light-emitting diodes with a 50-fold external quantum efficiency improvement (up to 6.27%) are achieved through balancing surface passivation and carrier injection via ligand density control (treating with hexane/ethyl acetate mixed solvent), which induces the coexistence of high levels of ink stability, photoluminescence quantum yields, thin-film uniformity, and carrier-injection efficiency.

While it has been argued that field-dependent geminate pair recombination (GR) is important, this process is often disregarded when analyzing the recombination kinetics in bulk heterojunction organic solar cells (OSCs). To differentiate between the contributions of GR and nongeminate recombination (NGR) the authors study bilayer OSCs using either a PCDTBT-type polymer layer with a thickness from 14 to 66 nm or a 60 nm thick p-DTS(FBTTh₂)₂ layer as donor material and C₆₀ as acceptor. The authors measure JV-characteristics as a function of intensity and charge-extraction-by-linearly-increasing-voltage-type hole mobilities. The experiments have been complemented by Monte Carlo simulations. The authors find that fill factor (FF) decreases with increasing donor layer thickness (L_p) even at the lowest light intensities where geminate recombination dominates. The authors interpret this in terms of thickness dependent back diffusion of holes toward their siblings at the donor–acceptor interface that are already beyond the Langevin capture sphere rather than to charge accumulation at the donor–acceptor interface. This effect is absent in the p-DTS(FBTTh₂)₂ diode in which the hole mobility is by two orders of magnitude higher. At higher light intensities, NGR occurs as evidenced by the evolution of s-shape of the JV-curves and the concomitant additional decrease of the FF with increasing layer thickness.

Back diffusion of holes is identified to control the fill factor and thus the efficiency of bilayer organic solar cells. The competition between recombination at the donor–acceptor interface and extraction at the electrodes is studied by varying the donor layer thickness and light intensity such as to differentiate between monomolecular, i.e., geminate recombination and bimolecular, nongeminate recombination.

Multicolor bandgap fluorescent carbon quantum dots (MCBF-CQDs) from blue to red with quantum yield up to 75% are synthesized using a solvothermal method. For the first time, monochrome electroluminescent light-emitting diodes (LEDs) with MCBF-CQDs directly as an active emission layer are fabricated. The maximum luminance of blue LEDs reaches 136 cd m⁻², which is the best performance for CQD-based monochrome electroluminescent LEDs.

A formamidinium(FA)-based perovskite showns superior optoelectronic properties including better stability than methylammonium-based counterparts. Pure FA-perovskite-based light-emitting diodes (LEDs) with high efficiency are reported. Interestingly, the LED clearly shows a sub-bandgap emission at 1.7 V (bandgap 2.3 eV). This important discovery provides further insights of the charge transport mechanism in perovskite-based optoelectronic devices.

Quadruple-layered TiO₂ films with controllable macropore size are prepared via a confinement self-assembly method. The inverse opal structure with ordered mesoporous (IOM) presents unique light reflection and scattering ability with different wavelengths. Cyan light (400–600 nm) is reflected and scattered by IOM-195, which is in accord with N719 absorption spectra. By manipulating the macropore size, different light responses are obtained.