Defect passivation in methylammonium/bromine free inverted perovskite solar cells using charge-modulated molecular bonding.

Molecular passivation is a prominent approach for improving the performance and operation stability of halide perovskite solar cells (HPSCs). Herein, we reveal discernible effects of diammonium molecules with either an aryl or alkyl core onto Methylammonium-free perovskites. Piperazine dihydriodide (PZDI), characterized by an alkyl core-electron cloud-rich-NH terminal, proves effective in mitigating surface and bulk defects and modifying surface chemistry or interfacial energy band, ultimately leading to improved carrier extraction. Benefiting from superior PZDI passivation, the device achieves an impressive efficiency of 23.17% (area ~1 cm2) (low open circuit voltage deficit ~0.327 V) along with superior operational stability. We achieve a certified efficiency of ~21.47% (area ~1.024 cm2) for inverted HPSC. PZDI strengthens adhesion to the perovskite via -NH2I and Mulliken charge distribution. Device analysis corroborates that stronger bonding interaction attenuates the defect densities and suppresses ion migration. This work underscores the crucial role of bifunctional molecules with stronger surface adsorption in defect mitigation, setting the stage for the design of charge-regulated molecular passivation to enhance the performance and stability of HPSC.

Chemical and Electronic Investigation of Buried NiO1-δ, PCBM, and PTAA/MAPbI3-xClx Interfaces Using Hard X-ray Photoelectron Spectroscopy and Transmission Electron Microscopy.

Identification and profiling of molecular fragments generated over the lifespan of halide perovskite solar cells are needed to overcome the stability issues associated with these devices. Herein, we report the characterization of buried CH3NH3PbI3-xClx (HaP)-transport layer (TL) interfaces. By using hard X-ray photoelectron spectroscopy in conjunction with transmission electron microscopy, we reveal that the chemical decomposition of HaP is TL-dependent. With NiO1-δ, phenyl-C61-butyric acid methyl ester (PCBM), or poly(bis(4-phenyl) (2,4,6-trimethylphenyl)amine) (PTAA) as TLs, probing depth analysis shows that the degradation takes place at the interface (HaP/TL) rather than the HaP bulk area. From core-level data analysis, we identified iodine migration toward the PCBM- and PTAA-TLs. Unexpected diffusion of nitrogen inside NiO1-δ-TL was also found for the HaP/NiO1-δ sample. With a HaP/PCBM junction, HaP is dissociated to PbI2, whereas HaP/PTAA contact favored the formation of CH3I. The low stability of HaP solar cells in the PTAA-TL system is attributed to the formation of CH3I and iodide ion vacancies. Improved stability observed with NiO1-δ-TL is related to weak dissociation of stoichiometric HaP. Here, we provide a new insight to further distinguish different mechanisms of degradation to improve the long-term stability and performance of HaP solar cells.

Photoinduced ion-redistribution in CH3NH3PbI3 perovskite solar cells.

We use photoinduced absorption spectroscopy (PAS) to study the ionic motion in CH3NH3PbI3 perovskite solar cells, consisting of indium tin oxide (ITO)/NiOx/perovskite/phenyl-C61-butyric-acid-methyl ester (PCBM)/aluminum-doped zinc oxide (AZO)/ITO. We observed a slow (∼50 mHz) spectral blue shift (∼10-4 eV) under modulated 520 nm illumination, which we interpreted in terms of the modulation in the bulk ion density. Numerical simulation shows that the mobile ion moves in and out from the double layers at the perovskite/charge transport layer interfaces in order to recover the bulk charge neutrality tipped off-balance by the photocarriers. The diffusion coefficient of the ion is 10-10 to 10-11 cm2 s-1, when we assume that the characteristic time constant of the ion motion is governed by the diffusion.

Unraveling the Impacts Induced by Organic and Inorganic Hole Transport Layers in Inverted Halide Perovskite Solar Cells.

The carrier transport layers (CTLs) have exhibited the influence on performance and stability of halide perovskite solar cells (HaPSCs). The exploration of characteristic impacts on HaPSCs induced by the CTL unveils the key factors underlying the device physics. In this work, we investigate the impacts of the organic or inorganic hole transport layer (HTL) in HaPSCs by analyzing the elemental distribution, the current-voltage characteristics, and the capacitance spectroscopy. The organic HTL device shows the lower activation energy ( EA < Eg) indicating a dominant interface-mediated recombination. The defect analysis reveals that the device with the inorganic HTL induces rather deep antisite defects with slightly higher trap densities. This is attributed to the diffusion of metal cations into the halide perovskite (HaP) during crystallization of HaP layer grown on the inorganic HTLs. Our results suggest that the passivation of deep defect and suppression of trap densities in the HaP either using ideal CTLs or optimizing the fabrication route is crucial to improving the device parameters approaching the theoretical limit.

Tailoring the Open-Circuit Voltage Deficit of Wide-Band-Gap Perovskite Solar Cells Using Alkyl Chain-Substituted Fullerene Derivatives.

Wide-band-gap (WB) perovskite devices are promising as the top cell of silicon-perovskite tandem devices to boost the efficiency beyond the Shockley-Queisser limit. Here, we tailor the performance parameters of WB mixed-halide perovskite solar cell with long alkyl chain-substituted fullerene derivatives as an electron transport layer (ETL). The device with C60-fused N-methylpyrrolidine- meta-dodecyl phenyl (C60MC12) demonstrates an enhanced power conversion efficiency of 16.74% with the record open circuit voltage ( VOC) of 1.24 V, an increase by 70 mV with concomitant VOC deficit reduction to 0.47 V. This is achieved by mitigating the recombination loss through the use of highly crystalline C60MC12 film compared to amorphous [6,6]-phenyl-C61-butyric acid methyl ester layer. The device analysis reveals the soothing of the defect activities with shallower defect states and passivation of the interface recombination centers for the device with C60MC12. We ascribe this property to the crystallinity of fullerene derivatives as ETL, which is also important for the optimization of device parameters, besides the band alignment matching of WB perovskite devices.