Formamidinium (FA)-based perovskite materials show an extended absorption spectrum to 840 nm, which enables high power conversion efficiencies of over 20% compared with normal-structure perovskite solar cells (PSCs).

at the surface and grain boundaries, acting as carrier recombination centers and greatly limiting the open-circuit voltage (V oc ) of PSCs. Meanwhile, these trap states can lead to the infiltration of moisture and oxygen into perovskite, and subsequently harm the device stability in ambient environment. [23][24][25][26] These trap states at the surface and grain boundaries are most likely induced by ions migration, oxidization of I − or evaporation of methylammonium iodide (MAI), which are mainly manifested as under-coordinated metal cations or halide anions. [27][28][29] So far, a variety of passivation materials (also known as passivator) have been added into perovskite films to induce defect passivation through forming coordination with under-coordinated metal cations or halide anions. For instance, phenyl-C61-butyric acid methyl ester (PCBM), as a Lewis acid could passivate the trap states by forming coordination with halide ions and thus eliminate the notorious photocurrent hysteresis. [28,30] On the contrary, Snaith and co-workers demonstrated that Lewis base molecules, such as thiophene or pyridine, could heal the trap states by forming coordination with under-coordinated Pb 2+ ions in perovskite films. [31] Since these initial results of defect passivation by reducing the uncoordinated ions in perovskite layer is proven to be effective, a defect passivator in perovskite layer should have more room for improvement. For example, a well-designed passivator in the perovskite layer should take the defect coordination and device air stability into consideration simultaneously. Thus, more effort is required to understand how to choose a suitable passivator in the perovskite layer.Among the large selection of functional groups, carboxyl (COOH) has been effectively used in other photovoltaic device as an indispensable anchoring group due to the strong coordination with metal oxide. [32] In the case of perovskite films, COOH is also found to have the interaction with perovskite films. [33,34] Small molecules such as amino acids, [35] acetate acids [8] were used to crosslink the perovskite boundaries or assist the crystallization process. However, the effect of charge recombination which is critical for the device performance is rarely mentioned in the previous work. It may because of small molecules were used as small amount of additives in the perovskite film which randomly distributed among the crystal boundaries. As the defects of perovskite film mainly locate at the top surface, [28] the controlling of stereochemical configuration is required for the film surface passivation.Organic-inorganic halide perovskites are efficient absorbers for solar cells. Nevertheless, the trap states at the surfaces and grain boundaries are a detri mental factor compromising the device performance. Here, an organic dye (AQ310) is employed as passivator to reduce the trap states of the perovs kites and promote better stability. The results demonstrate that the trap states of perovskite are minimized by the presence of AQ310's CO...

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We study the optoelectronic properties of a type-II heterojunction (HJ) comprising a monolayer of the transition metal dichalcogenide (TMDC), WS, and a thin film of the organic semiconductor, 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA). Both theoretical and experimental investigations of the HJ indicate that Frenkel states in the organic layer and two-dimensional Wannier-Mott states in the TMDC dissociate to form hybrid charge transfer excitons at the interface that subsequently dissociate into free charges that are collected at opposing electrodes. A photodiode employing the HJ achieves a peak external quantum efficiency of 1.8 ± 0.2% at a wavelength of 430 ± 10 nm, corresponding to an internal quantum efficiency (IQE) as high as 11 ± 1% in these ultrathin devices. The photoluminescence spectra of PTCDA and PTCDA/WS thin films show that excitons in the WS have a quenching rate that is approximately seven times higher than in PTCDA. This difference leads to strong wavelength dependence in IQE.

We study the charge transfer (CT) states in small-molecule blend heterojunctions comprising the nonpolar donor, tetraphenyldibenzoperiflanthene (DBP), and the acceptor, C70, using electroluminescence and steady-state and time-resolved photoluminescence spectroscopy along with density functional theory calculations. We find that the CT exciton energy blue shifts as the C70 concentration in the blend is either decreased or increased away from 50 vol %. At 20 K, the increase in CT state lifetime is correlated with the increasing diameter of C70 nanocrystallites in the blends. A quantum confinement model is used to quantitatively describe the dependence of both CT energy and lifetime on the C70 or DBP domain size. Two discrete CT emission peaks are observed for blends whose C70 concentration is >65%, at which point C70 nanocrystallites with diameters >4 nm appear in high-resolution transmission electron micrographs. The presence of two CT states is attributed to coexistence of crystalline C70 and amorphous phases in the blends. Furthermore, analysis of CT dissociation efficiency versus photon energy suggests that the >90% dissociation efficiency of delocalized CT2 states from the crystalline phase significantly contributes to surprisingly efficient photogeneration in highly dilute (>80% C70) DBP/C70 heterojunctions.