Decreased surface defects and non-radiative recombination via the passivation of the halide perovskite film by 2-thiophenecarboxylic acid in triple-cation perovskite solar cells

Organic-inorganic lead halide perovskite solar cells (PSCs) attract great research interest due to their significant device performance and optoelectronic properties. However, reducing charge recombination and efficiency loss due to surface defects of the perovskite layer are still big issues to overcome for PSCs. Herein, we have employed a simple molecule, 2-thiophenecarboxylic acid (2TiCOOH), via post-treatment to passivate the uncoordinated Pb²⁺ on the perovskite film surface and improve the stability at the perovskite/Spiro-OMeTAD interface. The spectral results illustrate that the 2TiCOOH passivated devices exhibit higher carrier lifetime, charge extraction, and minimized defect induced recombination. Also, solar cells with 2TiCOOH show better charge collection, improved J_SC, FF, and outstanding power conversion efficiency (PCE). In addition, 2TiCOOH passivated solar cells show tremendously stable performance output with less than 1% PCE drop after 100 days. This work provides a facile surface passivation strategy for fabricating highly efficient, low cost, and stable perovskite solar cells, which can be used for large scale technology and commercialization.

Determination of the nonradiative conversion efficiency of lead mixed-halide perovskites using optical and photothermal spectroscopy

Mixed-halide organic-inorganic hybrid perovskites are considered promising light-absorbing materials in the development of solar cells related to the obtained high-power conversion efficiency. Current efforts are focused on the study of the energy-conversion mechanisms, where the nonradiative recombination pathway is the least explored. In this work, a combination of optical and photoacoustic spectroscopies is used to determine the visible spectral light-into-heat conversion efficiency of lead-based mixed-halide organic-inorganic hybrid perovskites in a semicomplete n-i-p mesoscopic perovskite solar cell (PSC). A remarkable average conversion efficiency of about 87% has been found for the nonradiative combination in the perovskite, with the estimated composition ${{\rm FA}_{0.71}}{{\rm MA}_{0.29}}{{\rm PbI}_{2.9}}{{\rm Br}_{0.1}}$FA_0.71MA_0.29PbI_2.9Br_0.1 in the wavelength range of 400 to 800 nm. As a result, 13% of the incident light is transformed in radiative recombination processes and/or photodegradation of the material. Furthermore, the extinction coefficient and refractive index of the material are reported, and it was found that the optical constants and the optical absorption in the short-wavelength range are significantly smaller than previously reported for${{\rm MAPbI}_3}$MAPbI₃.

Enhanced Device Efficiency and Long-Term Stability via Boronic Acid-Based Self-Assembled Monolayer Modification of Indium Tin Oxide in a Planar Perovskite Solar Cell

Interfacial engineering is essential for the development of highly efficient and stable solar cells through minimizing energetic losses at interfaces. Self-assembled monolayers (SAMs) have been shown as a handle to tune the work function (WF) of indium tin oxide (ITO), improving photovoltaic cell performance and device stability. In this study, we utilize a new class of boronic acid-based fluorine-terminated SAMs to modify ITO surfaces in planar perovskite solar cells. The SAM treatment demonstrates an increase of the WF of ITO, an enhancement of the short-circuit current, and a passivation of trap states at the ITO/[poly(3,4ethylenedioxylenethiophene):poly(styrenesulfonic acid)] interface. Device stability improves upon SAM modification, with efficiency decreasing only 20% after one month. Our work highlights a simple treatment route to achieve hysteresis-free, reproducible, stable, and highly efficient (16%) planar perovskite solar cells.

Enhancement of photocurrent extraction and electron injection in dual-functional CH3NH3PbBr3 perovskite-based optoelectronic devices via interfacial engineering

Photocurrent extraction and electron injection in CH₃NH₃PbBr₃ (MAPbBr₃) perovskite-based optoelectronic devices are both significantly increased by improving the contact at the PCBM/MAPbBr₃ interface with an extended solvent annealing (ESA) process. Photoluminescence quenching and x-ray diffraction experiments show that the ESA not only improves the contact at the PCBM/MAPbBr₃ interface but also increases the crystallinity of the MAPbBr₃ thin films. The optimized dual-functional PCBM-MAPbBr₃ heterojunction based optoelectronic device has a high power conversion efficiency of 4.08% and a bright visible luminescence of 1509 cd m^-2. In addition, the modulation speed of the MAPbBr₃ based light-emitting diodes is larger than 14 MHz, which indicates that the defect density in the MAPbBr₃ thin film can be effectively reduced by using the ESA process.

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.

Modulating Excitonic Recombination Effects through One-Step Synthesis of Perovskite Nanoparticles for Light-Emitting Diodes

The primary advantages of halide perovskites for light-emitting diodes (LEDs) are solution processability, direct band gap, good charge-carrier diffusion lengths, low trap density, and reasonable carrier mobility. The luminescence in 3 D halide perovskite thin films originates from free electron-hole bimolecular recombination. However, the slow bimolecular recombination rate is a fundamental performance limitation. Perovskite nanoparticles could result in improved performance but processability and cumbersome synthetic procedures remain challenges. Herein, these constraints are overcome by tailoring the 3 D perovskite as a near monodisperse nanoparticle film prepared through a one-step in situ deposition method. Replacing methyl ammonium bromide (CH₃ NH₃ Br, MABr) partially by octyl ammonium bromide [CH₃ (CH₂ )₇ NH₃ Br, OABr] in defined mole ratios in the perovskite precursor proved crucial for the nanoparticle formation. Films consisting of the in situ formed nanoparticles displayed signatures associated with excitonic recombination, rather than that of bimolecular recombination associated with 3 D perovskites. This transition was accompanied by enhanced photoluminescence quantum yield (PLQY≈20.5 % vs. 3.40 %). Perovskite LEDs fabricated from the nanoparticle films exhibit a one order of magnitude improvement in current efficiency and doubling in luminance efficiency. The material processing systematics derived from this study provides the means to control perovskite morphologies through the selection and mixing of appropriate additives.

Morphology Analysis and Optimization: Crucial Factor Determining the Performance of Perovskite Solar Cells

This review presents an overall discussion on the morphology analysis and optimization for perovskite (PVSK) solar cells. Surface morphology and energy alignment have been proven to play a dominant role in determining the device performance. The effect of the key parameters such as solution condition and preparation atmosphere on the crystallization of PVSK, the characterization of surface morphology and interface distribution in the perovskite layer is discussed in detail. Furthermore, the analysis of interface energy level alignment by using X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy is presented to reveals the correlation between morphology and charge generation and collection within the perovskite layer, and its influence on the device performance. The techniques including architecture modification, solvent annealing, etc. were reviewed as an efficient approach to improve the morphology of PVSK. It is expected that further progress will be achieved with more efforts devoted to the insight of the mechanism of surface engineering in the field of PVSK solar cells.

The Additive Coordination Effect on Hybrids Perovskite Crystallization and High-Performance Solar Cell

The coordination effects of additives during perovskite crystal growth are investigated, and a novel technique to fabricate high-quality perovskite thin films by introduction of weak coordination additives (e.g., acetonitrile) in the precursors is demonstrated.

Cross-Linkable, Solvent-Resistant Fullerene Contacts for Robust and Efficient Perovskite Solar Cells with Increased JSC and VOC

The active layers of perovskite solar cells are also structural layers and are central to ensuring that the structural integrity of the device is maintained over its operational lifetime. Our work evaluating the fracture energies of conventional and inverted solution-processed MAPbI₃ perovskite solar cells has revealed that the MAPbI₃ perovskite exhibits a fracture resistance of only ∼0.5 J/m², while solar cells containing fullerene electron transport layers fracture at even lower values, below ∼0.25 J/m². To address this weakness, a novel styrene-functionalized fullerene derivative, MPMIC₆₀, has been developed as a replacement for the fragile PC₆₁BM and C₆₀ transport layers. MPMIC₆₀ can be transformed into a solvent-resistant material through curing at 250 °C. As-deposited films of MPMIC₆₀ exhibit a marked 10-fold enhancement in fracture resistance over PC₆₁BM and a 14-fold enhancement over C₆₀. Conventional-geometry perovskite solar cells utilizing cured films of MPMIC₆₀ showed a significant, 205% improvement in fracture resistance while exhibiting only a 7% drop in PCE (13.8% vs 14.8% PCE) in comparison to the C₆₀ control, enabling larger V_OC and J_SC values. Inverted cells fabricated with MPMIC₆₀ exhibited a 438% improvement in fracture resistance with only a 6% reduction in PCE (12.3% vs 13.1%) in comparison to those utilizing PC₆₁BM, again producing a higher J_SC.

Efficient perovskite light-emitting diodes by film annealing temperature control

A bright perovskite light-emitting diode has been fabricated through film annealing temperature control.