lizhendong

Ionic liquids can retard the perovskite crystallization with the aim to form compact films with larger and more uniformly distributed grain size. The ionic liquid driven crystallization is exploited to prepared a record planar perovskite solar cell with stabilized power output of 19.5%.

Anisotropic charge transfer (CT) state absorption originating from the natural transition dipole alignment at planar donor–acceptor interfaces can be exploited to reveal the full CT state absorption spectrum in organic solar cells. Application of this method reveals the existence of previously unobserved, higher-lying CT absorption buried beneath the excitonic background.

A crosslinked organic hole-transporting layer (HTL) is developed to realize highly efficient and stable perovskite solar cells via a facile thiol-ene thermal reaction. This crosslinked HTL not only facilitates hole extraction from perovskites, but also functions as an effective protective barrier. A high-performance (power conversion efficiency: 18.3%) device is demonstrated to show respectable photo and thermal stability without encapsulation.

The photophysical properties and solar cell performance of the classical donor–acceptor copolymer PCDTBT

(poly(N-9′-heptadecanyl-2,7-carbazole-alt -5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole))) in relation to unintentionally formed main chain defects are investigated. Carbazole–carbazole homocouplings (Cbz hc) are found to significant extent in PCDTBT made with a variety of Suzuki polycondensation conditions. Cbz hc vary between 0 and 8 mol% depending on the synthetic protocol used, and are quantified by detailed nuclear magnetic resonance spectroscopy including model compounds, which allows to establish a calibration curve from optical spectroscopy. The results are corroborated by extended time-dependent density functional theory investigations on the structural, electronic, and optical properties of regularly alternating and homocoupled chains. The photovoltaic properties of PCDTBT:fullerene blend solar cells significantly depend on the Cbz hc content for constant molecular weight, whereby an increasing amount of Cbz hc leads to strongly decreased short circuit currents J_SC. With increasing Cbz hc content, J_SC decreases more strongly than the intensity of the low energy absorption band, suggesting that small losses in absorption cannot explain the decrease in J_SC alone, rather than combined effects of a more localized LUMO level on the TBT unit and lower hole mobilities found in highly defective samples. Homocoupling-free PCDTBT with optimized molecular weight yields the highest efficiency up to 7.2% without extensive optimization.

Electronic main chain defects in conjugated polymers caused by carbazole homocouplings (Cbz hc) significantly deteriorate the performance of organic photovoltaics as shown for poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)) (PCDTBT):PC₇₁BM devices. Cbz hc occur frequently, are quantified for a variety of Suzuki conditions, and are correlated with photophysical properties. PCDTBT without main chain defects and optimized molar mass gives the highest power conversion efficiencies exceeding 7%.

The development of effective and stable hole transporting materials (HTMs) is very important for achieving high-performance planar perovskite solar cells (PSCs). Herein, copper salts (cuprous thiocyanate (CuSCN) or cuprous iodide (CuI)) doped 2,2,7,7-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (spiro-OMeTAD) based on a solution processing as the HTM in PSCs is demonstrated. The incorporation of CuSCN (or CuI) realizes a p-type doping with efficient charge transfer complex, which results in improved film conductivity and hole mobility in spiro-OMeTAD:CuSCN (or CuI) composite films. As a result, the PCE is largely improved from 14.82% to 18.02% due to obvious enhancements in the cell parameters of short-circuit current density and fill factor. Besides the HTM role, the composite film can suppress the film aggregation and crystallization of spiro-OMeTAD films with reduced pinholes and voids, which slows down the perovskite decomposition by avoiding the moisture infiltration to some extent. The finding in this work provides a simple method to improve the efficiency and stability of planar perovskite solar cells.

Copper salts (cuprous thiocyanate or cuprous iodide) doped 2,2,7,7-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene is used as the hole transport layer in planar perovskite solar cells by their good film conductivity and hole mobility. As a result, a maximum 18.02% power conversion efficiency is achieved with improved cell stability. 2D grazing incidence X-ray diffraction technique is utilized to probe the cell degradation process.

The incorporation of multiple donors into the bulk-heterojunction layer of organic polymer solar cells (PSCs) has been demonstrated as a practical and elegant strategy to improve photovoltaics performance. However, it is challenging to successfully design and blend multiple donors, while minimizing unfavorable interactions (e.g., morphological traps, recombination centers, etc.). Here, a new Förster resonance energy transfer-based design is shown utilizing the synergistic nature of three light active donors (two small molecules and a high-performance donor–acceptor polymer) with a fullerene acceptor to create highly efficient quaternary PSCs with power conversion efficiencies (PCEs) of up to 10.7%. Within this quaternary architecture, it is revealed that the addition of small molecules in low concentrations broadens the absorption bandwidth, induces cocrystalline molecular conformations, and promotes rapid (picosecond) energy transfer processes. These results provide guidance for the design of multiple-donor systems using simple processing techniques to realize single-junction PSC designs with unprecedented PCEs.

A viable strategy to realize highly efficient quaternary blend solar cells is introduced that breaks efficiency above 10% with complementary squaraine small molecules–low band-gap polymer combinations. Our quaternary design demonstrates several advantages: (i) broader light absorption, (ii) improved surface morphology, (iii) enhanced cocrystallization packing, (iv) multiple energy and charge transfer pathways to reduce recombination, and (v) increased charge mobility.

The origin of hysteresis behavior is probed in perovskite solar cells (PSCs) with simultaneous measurements of cell open circuit voltage (V_oc) and photoluminescence intensity over time following illumination of the cell. It is shown, for the first time, that the transient changes in terminal voltage and luminescent intensity do not follow the relationship that would be predicted by the generalized Plank radiation law. A mechanism is proposed based on the presence of a resistive barrier to majority carrier flow at the interface between the perovskite film and the electron or hole transport layer, in combination with significant interface recombination. This results in a decoupling of the internal quasi-Fermi level separation and the externally measured voltage. A simple numerical model is used to provide in-principle validation for the proposed mechanism and it is confirmed that mobile ionic species are a likely candidate for creating the time-varying majority carrier bottleneck by its reduced conductivity. The findings show that the V_oc of PSCs may be lower than the limit imposed by the cell luminescence efficiency, even under steady-state conditions.

The origin of hysteresis behavior in perovskite solar cells is probed with simultaneous measurements of cell open circuit voltage and photoluminescence intensity over time following illumination of the cell. The findings demonstrate the existence of a resistive barrier to majority carrier flow at the interfaces of these devices, in combination with significant interface recombination.

On page 5400, M. J. Ko and co-workers report that hydrophobic ligand capped pyrite nanoparticles can act as a bi-functional hole transporting layer (HTL) (charge transporting layer and moisture-proof layer) for perovskite solar cells. Perovskite solar cells employing this HTL exhibit very stable performance under moist condition for at least 1000 hours.

All-polymer solar cells with 7.57% power conversion efficiency are achieved via a new perylenediimide-based polymeric acceptor. Furthermore, the device processed in ambient air without encapsulation can still reach a high power conversion efficiency (PCE) of 7.49%, which is a significant economic advantage from an industrial processing perspective. These results represent the highest PCE achieved from perylenediimide-based polymers.

Two 1D–2D asymmetric benzodithiophenes (BDTs) as donor building blocks are designed and synthesized, combining the advantages of both 1D and 2D symmetric BDTs. The photovoltaic properties of the asymmetric BDT-based polymers are improved greatly in comparison with corresponding symmetric BDT-based polymers. This work provides a new approach to design prospective organic optoelectronic materials employing the symmetry-breaking strategy.

A small-molecular acceptor, tetraphenylpyrazine-perylenediimide tetramer (TPPz-PDI₄), which has a reduced extent of intramolecular twisting compared to two other small-molecular acceptors is designed. Benefiting from the lowest extent of intramolecular twisting, TPPz-PDI₄ exhibits the highest aggregation tendency and electron mobility, and therefore achieves a highest power conversion efficiency of 7.1%.

On page 6562, K. T. Nam, H. W. Jang, and co-workers describe an ultralow electric field and high-ON/OFF-ratio resistive-switching behavior of solution-processed organo-metal halide perovskite (OHP) films with the rotational motion of the A-site molecular cation. Metal/OHP/metal cells exhibit electroforming-free resistive switching at an electric field of 3.25 × 10³ V cm⁻¹ for four distinguishable ON-state resistance levels.

On page 6695, X. Y. Gao, L.-S. Liao, and co-workers describe the fabrication of mixed Pb–In perovskite solar cells, using indium (III) chloride and lead (II) chloride with methylammonium iodide. A maximum power conversion efficiency as high as 17.55% is achieved owing to the high quality of the perovskites with multiple ordered crystal orientations. This work demonstrates the possibility of substituting the Pb (II) by using In (III), which opens a broad route to fabricating alloy perovskite solar cells with mitigated ecological impact.

New indoline dyes (RK-1–4) were designed with a planar geometry and high molar extinction coefficient, which provided surprising power conversion efficiency (PCE) with a thin titanium dioxide film in dye-sensitized solar cells (DSCs). They had a difference in only alkyl chain length. Despite the same molecular structure, the performance of the respective DSCs varied significantly. Investigating the dye adsorption processes and charge transfer kinetics, the alkyl chain length was determined to affect the dye surface coverage as well as the recombination between the injected photoelectrons and the oxidized redox mediators. When applied to the DSCs as a light harvester, RK-3 with the dodecyl group exhibited the best photocurrent density, consequently achieving the best PCE of 9.1% with a 1.8 μm active and 2.5 μm scattering layer because of the most favorable charge injection. However, when increasing the active layer thickness, overall device performance deteriorated and the charge collection and regeneration played major roles for determining the PCE. Therefore, RK-2 featuring the highest surface coverage and moderate alkyl chain length obtained the highest PCEs of 8.8% and 7.9% with 3.5 and 5.1 μm active layers, respectively. These results present a promising perspective of organic dye design for thin film DSCs.

New indoline derivatives characterized by good planarity and high molar absorptivity are successfully applied to dye-sensitized solar cells (DSCs). By controlling the hydrocarbon chain length, noticeable photocurrent density is achieved at a very thin TiO₂ film, which is traced elementally, guiding the research of organic dyes for thin film-based DSCs.