Multifunctional Hybrid Interfacial Layers for High-Performance Inverted Perovskite Solar Cells.

Challenges remain hindering the performance and stability of inverted perovskite solar cells (PSCs), particularly for the nonstable interface between lead halide perovskite and charge extraction metal oxide layer. Herein, a simple yet scalable interfacial strategy to facilitate the assemble of high-performance inverted PSCs and scale-up modules is reported. The hybrid interfacial layer containing self-assembly triphenylamine and conjugated poly(arylamine) simultaneously improves the chemical stability, charge extraction, and energy level alignment of hole-selective interface, meanwhile promoting perovskite crystallization. Consequently, the correspondent inverted PSCs and modules achieve remarkable power conversion efficiencies (PCEs) of 24.5% and 20.7% (aperture area of 19.4 cm2 ), respectively. The PSCs maintain over 80% of its initial efficiency under one-sun equivalent illumination of 1200 h. This strategy is also effective to perovskite with various bandgaps, demonstrating the highest PCE of 19.6% for the 1.76-eV bandgap PSCs. Overall, this work provides a simple yet scalable interfacial strategy for obtaining state-of-the-art inverted PSCs and modules.

High-Performance Organic Solar Modules via Bilayer-Merged-Annealing Assisted Blade Coating.

Although encouraging progress is being made on spin-coated prototype cells, organic solar cells (OSCs) still face significant challenges, yet to be explored, for upscaling the multi-stacked photoactive layers in the construction of large-area modules. Herein, high-performance opaque and semitransparent organic solar modules are developed via a bilayer-merged-annealing (BMA)-assisted blade-coating strategy, achieving impressive efficiencies of 14.79% and 12.01% with respect to active area of 18.73 cm2 , which represent the best organic solar minimodules so far. It is revealed that the BMA strategy effectively resolves the de-wetting issues between polar charge transport layer solution and non-polar bulk heterojunction blends, hence improving the film coverage, along with electronic and electric contacts of multi-stacked photoactive layers. As result, organic solar modules coated under ambient conditions successfully retain the high-efficiency of small-area cells upon 312 times area scaling-up. Overall, this work provides a facile and effective method to fabricate high-performance organic solar modules under ambient conditions.

Seed-Assisted Growth of Methylammonium-Free Perovskite for Efficient Inverted Perovskite Solar Cells.

The traditional way to stabilize α-phase formamidinium lead triiodide (FAPbI3 ) perovskite often involves considerable additions of methylammonium (MA) and bromide into the perovskite lattice, leading to an enlarged bandgap and reduced thermal stability. This work shows a seed-assisted growth strategy to induce a bottom-up crystallization of MA-free perovskite, by introducing a small amount of α-CsPbBr3 /DMSO (5%) as seeds into the pristine FAPbI3 system. During the initial crystalization period, the typical hexagonal α-FAPbI3 crystals (containing α-CsPbBr3 seeds) are directly formed even at ambient temperature, as observed by laser scanning confocal microscopy. It indicates that these seeds can promote the formation and stabilization of α-FAPbI3 below the thermodynamic phase-transition temperature. After annealing not beyond 100 °C, CsPbBr3 seeds homogeneously diffused into the entire perovskite layer via an ions exchange process. This work demonstrates an efficiency of 22% with hysteresis-free inverted perovskite solar cells (PSCs), one of the highest performances for MA-free inverted PSCs. Despite absented passivation processes, open-circuit voltage is improved by 100 millivolts compared to the control devices with the same stoichiometry, and long-term operational stability retained 92% under continuous full sun illumination. Going MA-free and low-temperature processes are a new insight for compatibility with tandems or flexible PSCs.

Self-Assembled Donor-Acceptor Dyad Molecules Stabilize the Heterojunction of Inverted Perovskite Solar Cells and Modules.

The heterointerface between a semiconducting metal oxide and a perovskite critically impacts on the overall performance of perovskite solar cells (PVSCs). Herein, we report a feasible yet effective strategy to suppress the interfacial reaction between nickel oxide and the perovskite via chemical passivation with self-assembled dyad molecules, which leads to the simultaneous improvement of the power conversion efficiencies (PCEs) and operational lifetimes of inverted PVSCs. As a result, inverted PVSCs consisting of simple methylammonium iodide perovskites have achieved an excellent PCE of 20.94% and decent photostability with 93% of the initial value after 600 h of 1 sun equivalent illumination. Moreover, this strategy can be readily translated into slot-die fabrication of perovskite modules, achieving a high PCE of 14.90% with an area of 19.16 cm2 (no shade in the interconnecting area) and a geometrical fill factor of 93%. Overall, this work provides an effective strategy to stabilize the vulnerable heterointerface in PVSCs.

Improving the Performances of Perovskite Solar Cells via Modification of Electron Transport Layer.

The commonly used electron transport material (6,6)-phenyl-C61 butyric acid methyl ester (PCBM) for perovskite solar cells (PSC) with inverted planar structures suffers from properties such as poor film-forming. In this manuscript, we demonstrate a simple method to improve the film-forming properties of PCBM by doping PCBM with poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) as the electron transport layer (ETL), which effectively enhances the performance of CH3NH3PbI3 based solar cells. With 5 wt % F8BT in PCBM, the short circuit current (JSC) and fill factor (FF) of PSC both significantly increased from 17.21 ± 0.15 mA·cm-2 and 71.1 ± 0.07% to 19.28 ± 0.22 mA·cm-2 and 74.7 ± 0.21%, respectively, which led to a power conversion efficiency (PCE) improvement from 12.6 ± 0.24% to 15 ± 0.26%. The morphology investigation suggested that doping with F8BT facilitated the formation of a smooth and uniform ETL, which was favorable for the separation of electron-hole pairs, and therefore, an improved performance of PSC.

Understanding the Effect of Delay Time of Solvent Washing on the Performances of Perovskite Solar Cells.

Uniform and dense perovskite films were realized by the one-step solution-processing method combined with toluene washing. The influence of the delay time applied for toluene washing on the film quality of CH3NH3PbI3 (MAPbI3) was investigated in a comprehensive manner. The optimal delay time was experimentally observed at the critical point when the color of the film changes from transparent to hazy. A detailed X-ray diffraction study suggested that such a color change was caused by the emergence of the MAPbI3 crystal nucleus. This finding provides a convenient method to determine the optimal time accurately. With the optimal delay time, the most uniformly distributed MAPbI3 grains with the largest average grain size and the smoothest surface were obtained. Owing to the realization of homogeneous MAPbI3 films combined with full coverage of perovskite on the substrate achieved by toluene washing at the critical point, open-circuit voltage, short-circuit current, fill factor, and power conversion efficiency of 1.11 V, 18.24 mA/cm2, 77.47, and 15.54% were obtained.

Experimental and theoretical optical properties of methylammonium lead halide perovskites.

The optical constants of methylammonium lead halide single crystals CH3NH3PbX3 (X = I, Br, Cl) are interpreted with high level ab initio calculations using the relativistic quasiparticle self-consistent GW approximation (QSGW). Good agreement between the optical constants derived from QSGW and those obtained from spectroscopic ellipsometry enables the assignment of the spectral features to their respective inter-band transitions. We show that the transition from the highest valence band (VB) to the lowest conduction band (CB) is responsible for almost all the optical response of MAPbI3 between 1.2 and 5.5 eV (with minor contributions from the second highest VB and the second lowest CB). The calculations indicate that the orientation of [CH3NH3](+) cations has a significant influence on the position of the bandgap suggesting that collective orientation of the organic moieties could result in significant local variations of the optical properties. The optical constants and energy band diagram of CH3NH3PbI3 are then used to simulate the contributions from different optical transitions to a typical transient absorption spectrum (TAS).

Polaron pair mediated triplet generation in polymer/fullerene blends.

Electron spin is a key consideration for the function of organic semiconductors in light-emitting diodes and solar cells, as well as spintronic applications relying on organic magnetoresistance. A mechanism for triplet excited state generation in such systems is by recombination of electron-hole pairs. However, the exact charge recombination mechanism, whether geminate or nongeminate and whether it involves spin-state mixing is not well understood. In this work, the dynamics of free charge separation competing with recombination to polymer triplet states is studied in two closely related polymer-fullerene blends with differing polymer fluorination and photovoltaic performance. Using time-resolved laser spectroscopic techniques and quantum chemical calculations, we show that lower charge separation in the fluorinated system is associated with the formation of bound electron-hole pairs, which undergo spin-state mixing on the nanosecond timescale and subsequent geminate recombination to triplet excitons. We find that these bound electron-hole pairs can be dissociated by electric fields.