High-Hole-Mobility Metal-Organic Framework as Dopant-Free Hole Transport Layer for Perovskite Solar Cells

Tuning the morphology and energy levels in organic solar cells with metal-organic framework nanosheets

Metal-organic framework nanosheets (MONs) have proved themselves to be useful additives for enhancing the performance of a variety of thin film solar cell devices. However, to date only isolated examples have been reported. In this work we take advantage of the modular structure of MONs in order to resolve the effect of their different structural and optoelectronic features on the performance of organic photovoltaic (OPV) devices. Three different MONs were synthesized using different combinations of two porphyrin-based ligands meso-tetracarboxyphenyl porphyrin (TCPP) or tetrapyridyl-porphyrin (TPyP) with either zinc and/or copper ions and the effect of their addition to polythiophene-fullerene (P3HT-PC71BM) OPV devices was investigated. The power conversion efficiency (PCE) of devices was found to approximately double with the addition of MONs of Zn2(ZnTCPP) -4.7% PCE, 10.45 mA/cm2 short-circuit current density (JSC), 0.69 open-circuit voltage (VOC), 64.20% fill-factor (FF), but was unchanged with the addition of Cu2(ZnTPyP) (2.6% PCE, 3.68 mA/cm2 JSC, 0.59 VOC, 46.27% FF) and halved upon the addition of Cu2(CuTCPP) (1.24% PCE, 6.72 mA/cm2 JSC, 0.59 VOC, 56.24% FF) compared to devices without nanosheets (2.6% PCE, 6.61 mA/cm2 JSC, 0.58 VOC, 56.64% FF). Our analysis indicates that there are three different mechanisms by which MONs can influence the photoactive layer - light absorption, energy level alignment, and morphological changes. Analysis of external quantum efficiency, UV-vis and photoelectron spectroscopy data found that MONs have similar effects on light absorption and energy level alignment. However, atomic force and Raman microscopy studies revealed that the nanosheet thickness and lateral size are crucial parameters in enabling the MONs to act as beneficial additives resulting in an improvement of the OPV device performance. We anticipate this study will aid in the design of MONs and other 2D materials for future use in other light harvesting and emitting devices.

Air/liquid interfacial formation process of conductive metal-organic framework nanosheets

The air/liquid interface is a superior platform to create nanosheets of materials by promoting spontaneous two-dimensional growth of components. Metal-organic frameworks (MOFs)-intrinsically porous crystals-with π-conjugated triphenylene-based ligands show high electrical conductivities. Forming nanosheets of such conductive MOFs should enable their use in electronic devices. Although highly conductive MOF nanosheets have been created at the air/liquid interface, direct control of their continuity, morphology, thickness, crystallinity, and orientation directly influencing device performance remains as an issue to be addressed. Here, we present detailed insights into the formation process of electrically conductive MOF nanosheets composed of 2,3,6,7,10,11-hexaiminotriphenylene (HITP) and Ni2+ ions (HITP-Ni-NS) at the air/liquid interface. The morphological and structural features of HITP-Ni-NS strongly depend on the standing time-the time without any external actions involved, but leaving the interface undisturbed after setting the ligand solution onto the metal-ion solution. We find that the fundamental features of HITP-Ni-NS are determined by the standing time with conductivity sensitively influenced by such pre-determined HITP-Ni-NS characteristics. These findings will lead towards the establishment of a rational strategy for creating MOF nanosheets at the air/liquid interface with desired properties, thereby accelerating their use in diverse potential applications.

Improvement of Open-Circuit Voltage Deficit via Pre-Treated NH4 + Ion Modification of Interface between SnO2 and Perovskite Solar Cells

Passivation is a popular method to increase power conversion efficiency (PCE), reduce hysteresis related to surface traps and defects, and adjust mismatched energy levels. In this paper, an approach is reported using ammonium chloride (AC) to enhance passivation effects by controlling chlorine (Cl) and ammonium ions (NH₄ ⁺ ) on the front and back side of tin oxides (SnO₂ ). AC pre-treatment is applied to indium tin-oxide (ITO) prior to SnO₂ deposition to advance the passivation approaches and compare the completely separated NH₄ ⁺ and Cl passivation effects, and sole NH₄ ⁺ is successfully isolated on the SnO₂ surface, the counterpart of AC-post-treatment, generating ammonia (NH₃ ) and Cl. It is demonstrated that multifunctional healing effects of NH₄ ⁺ are ascribed from AC-pre-treatment being the basis of SnO₂ crystallization and adjusting bifacial interface energy levels at ITO/SnO₂ and SnO₂ /perovskite to enhance photo-carrier transport. As calculated by density functional theory, how the change of the passivation agent from Cl to NH₄ ⁺ more effectively suppresses non-radiative recombination ascribed to hydrated SnO₂ surface defects is explained. Consequently, enhancement of photo-carrier transport significantly improves a superior open-circuit voltage of 1.180 V and suppresses the hysteresis, which leads to the PCE of 22.25% in an AC-pre-treated device 3.000% higher than AC-post-treated devices.