Intrinsic efficiency limits in low-bandgap non-fullerene acceptor organic solar cells. Nature Materials.

Two dimensional inorganic–organic hybrid perovskites (2D perovskites) suffer from not only quantum confinement, but also dielectric confinement, hindering their application perspective in devices involving the conversion of an optical input into current. In this report, we theoretically predict that an extremely low exciton binding energy can be achieved in 2D perovskites by using high dielectric-constant organic components. We demonstrate that in (HOCH2CH2NH3)2PbI4, whose organic material has a high dielectric constant of 37, the dielectric confinement is largely reduced, and the exciton binding energy is 20-times smaller than that in conventional 2D perovskites. As a result, the photo-induced excitons can be thermally dissociated efficiently at room temperature, as clearly indicated from femtosecond transient absorption measurements. In addition, the mobility is largely improved due to the strong screening effect on charge impurities. Such low dielectric-confined 2D perovskites show excellent carrier extraction efficiency, and outstanding humidity resistance compared to conventional 2D perovskites.

currently at the forefront of efficient OSCs and outperform binary systems in terms of power conversion efficiency (PCE), largely due to improved light absorption and mor phology, especially if novel nonfullerene acceptors are employed. [2] Ternary solar cells use a single (ternary blend) photo active layer for photon harvesting and thus are less challenging and costly to realize, compared to tandem solar cells, [3] which have the potential of even higher efficien cies. [4] However, in TSCs, the third compo nent of the blend not only improves light harvesting, but also plays an active role in the photophysical processes including exciton and chargecarrier dynamics and can, in fact, also influence the blend's morphology. [5] This is of critical impor tance in small moleculebased devices, where intermolecular charge transport dominates. [6] Often, the third component in ter nary solar cells enhances the PCE by improving the shortcircuit current (J SC ) [5c] and opencircuit voltage (V OC ), which both can be tuned by the donor/ acceptor composition. [2d,5b] The chal lenge here is, however, to improve all device parameters, while maintaining or increasing the fill factor (FF) of the ternary organic solar cell. [7] Here, we study the photophysics of such a highperformance allsmallmolecule ternary solar cell com posed of DR3TBDTT (DR3) [8] as an electron donor in combi nation with a nonfullerene smallmolecule acceptor, namely ICC6IDTIC (or ICC6) [9] and PC 71 BM as the third component, shown to enhance the device performance. The fully opti mized ternary devices (1:1:0.4 wt%, DR3:ICC6:PC 71 BM) yield an average PCE of 10.8% with a FF of 72%, V OC of 0.87 V, and J SC of 16.3 mA cm −2 . Using picosecond-nanosecond (ps-ns) transient absorption (TA) spectroscopy, we selectively excite each component of the blend and probe the processes following photoexcitation. We find that excitation of PC 71 BM molecules results in fast singlet energy transfer to ICC6, sub sequently followed by hole transfer to DR3. We confirm this observation by TA and timeresolved photoluminescence (TRPL) spectroscopy on binary blends of PC 71 BM:ICC6, showing fast energy transfer from PC 71 BM to ICC6. Our TA studies demon strate that the increased external quantum efficiency (EQE) of the ternary blend is due to higher mobility of charge carriers in Ternary organic solar cells (OSCs) are among the best-performing organic photovoltaic devices to date, largely due to the recent development of nonfullerene acceptors. However, fullerene molecules still play an important role in ternary OSC systems, since, for reasons not well understood, they often improve the device performance, despite their lack of absorption. Here, the photophysics of a prototypical ternary small-molecule OSC blend composed of the donor DR3, the nonfullerene acceptor ICC6, and the fullerene derivative PC 71 BM is studied by ultrafast spectroscopy. Surprisingly, it is found that after excitation of PC 71 BM, ultrafast singlet energy transfer to ICC6 competes efficiently with cha...

Reaching device efficiencies that can rival those of polymer-fullerene BHJ solar cells (>10%) remains challenging with the "All-Small-Molecule" (All-SM) approach, in part because of (i) the morphological limitations that prevail in the absence of polymer and (ii) the difficulty to raise and balance out carrier mobilities across the active layer. In this report, we show that blends of the SM donor DR3TBDTT (DR3) and the nonfullerene SM acceptor O-IDTBR are conducive to "All-SM" BHJ solar cells with high open-circuit voltages (VOC) >1.1 V and PCEs as high as 6.4% (avg. 6.1%) when the active layers are subjected to a post-processing solvent vapor-annealing (SVA) step with dimethyl disulfide (DMDS)-a low-partial-pressure solvent found to markedly impact the structural development of the BHJ network. Combining electron energy loss spectroscopy (EELS) analyses and systematic carrier recombination examinations, This article is protected by copyright. All rights reserved. 3 we show that SVA treatments with DMDS play a determining role in improving charge transport and reducing non-geminate recombination for the DR3:O-IDTBR system. Correlating our experimental results and device simulations, we find that substantially higher BHJ solar cell efficiencies of >12% could be achieved if the IQE and carrier mobilities of the active layer could be increased to >85% and >10-4 cm 2 V-1 s-1 , respectively, while suppressing the recombination rate constant k to <10-12 cm 3 s-1. This publication is based upon work supported by the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under Award No. CRG_R2_13_BEAU_KAUST_1. The authors acknowledge concurrent support under Baseline Research Funding from KAUST. The authors thank KAUST ACL for technical support in the mass spectrometry analyses. V. S. acknowledges financial support from the NDSEG fellowship. Use of

shown to reach power conversion efficiencies (PCEs) >10%, [2] the synthesis of π-conjugated polymers with specific optical gaps and little-to-no batch-to-batch variation remains a practical limitation in the scaling-up of high-efficiency polymerfullerene BHJ devices. [3] In parallel, the narrow spectral absorption of fullerene acceptors (e.g., [6,6]-phenyl-C 61 (or C 71 )butyric acid methyl ester, namely, PC 61 BM and PC 71 BM) falling in the short-wavelengths region (≈300-450 nm) inherently limits photonic absorption, since most of the photon flux occurs in the visible range of the solar spectrum (>450 nm). By contrast, all-SM BHJ solar cells can involve two visible light absorbers-, i.e., π-extended SM donor and SM acceptorthus, all-SM devices could in principle produce higher photocurrents and outperform their polymer-fullerene BHJ counterparts. [1g,4] SM materials feature on the properties of being well-defined, size-monodispersed, and scalable. "Nonfullerene" SM acceptors add the benefit of being synthetically accessible in controlled purities, while these may also be readily produced on large scales. Nevertheless, reaching high PCE values with the all-SM device approach has remained challenging thus far, mainly owing to i) morphology limitations prevailing in the absence of polymer (e.g., lack of control over domain sizes, crystallinity, among other important aspects) and ii) the difficulty to reach the enhancement and balance of charge transport in active layers. [3a,4b,5] With the use of processing additives [6] or solvent vapor annealing (SVA) Solution-processed small molecule (SM) solar cells have the prospect to outperform their polymer-fullerene counterparts. Considering that both SM donors/acceptors absorb in visible spectral range, higher expected photocurrents should in principle translate into higher power conversion efficiencies (PCEs). However, limited bulk-heterojunction (BHJ) charge carrier mobility (<10-4 cm 2 V -1 s -1 ) and carrier lifetimes (<1 µs) often impose active layer thickness constraints on BHJ devices (≈100 nm), limiting external quantum efficiencies (EQEs) and photocurrent, and making largescale processing techniques particularly challenging. In this report, it is shown that ternary BHJs composed of the SM donor DR3TBDTT (DR3), the SM acceptor ICC6 and the fullerene acceptor PC 71 BM can be used to achieve SM-based ternary BHJ solar cells with active layer thicknesses >200 nm and PCEs nearing 11%. The examinations show that these remarkable figures are the result of i) significantly improved electron mobility (8.2 × 10 -4 cm 2 V -1 s -1 ), ii) longer carrier lifetimes (2.4 µs), and iii) reduced geminate recombination within BHJ active layers to which PC 71 BM has been added as ternary component. Optically thick (up to ≈500 nm) devices are