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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.

We report on a set of two analogous, F-substituted oligothiophene derivatives (D5T6F-M and D7T8F-M), in which systematic fluorination at the thienyl moieties is shown to induce a sufficient increase in electron affinity (EA; i.e. suppressed LUMO level) and in ionization potential (IP; i.e. deeper HOMO) to (i) impart the molecules with electron-accepting and -transporting characteristics, and (ii) yield high open-circuit voltages ＞1 V in BHJ solar cells when combined with a low-bandgap polymer donor (PCE10) commonly used with fullerenes. The nonfullerene BHJ devices with the SM acceptor D5T6F-M achieve power conversion efficiencies (PCEs) of up to ca. 4.5% (vs. ca. 2% for D7T8F-M-based devices). Our study shows that –F substitutions in SM systems – otherwise used as donors – is an effective approach in the design of nonfullerene acceptors for efficient BHJ solar cells with polymer donors. [ABSTRACT FROM AUTHOR]

Abstract: Achieving efficient bulk‐heterojunction (BHJ) solar cells from blends of solution‐processable small‐molecule (SM) donors and acceptors is proved particularly challenging due to the complexity in obtaining a favorable donor–acceptor morphology. In this report, the BHJ device performance pattern of a set of analogous, well‐defined SM donors—DR3TBDTT (DR3), SMPV1, and BTR—used in conjunction with the SM acceptor IDTTBM is examined. Examinations show that the nonfullerene “All‐SM” BHJ solar cells made with DR3 and IDTTBM can achieve power conversion efficiencies (PCEs) of up to ≈4.5% (avg. 4.0%) when the solution‐processing additive 1,8‐diiodooctane (DIO, 0.8% v/v) is used in the blend solutions. The figures of merit of optimized DR3:IDTTBM solar cells contrast with those of “as‐cast” BHJ devices from which only modest PCEs <1% can be achieved. Combining electron energy loss spectrum analyses in scanning transmission electron microscopy mode, carrier transport measurements via “metal‐insulator‐semiconductor carrier extraction” methods, and systematic recombination examinations by light‐dependence and transient photocurrent analyses, it is shown that DIO plays a determining role—establishing a favorable lengthscale for the phase‐separated SM donor–acceptor network and, in turn, improving the balance in hole/electron mobilities and the carrier collection efficiencies overall. [ABSTRACT FROM AUTHOR]

Solution-processable small molecule (SM) donors are promising alternatives to their polymer counterparts in bulk-heterojunction (BHJ) solar cells. While SM donors with favorable spectral absorption, self-assembly patterns, optimum thin-film morphologies, and high carrier mobilities in optimized donor-acceptor blends are required to further BHJ device efficiencies, material structure governs each one of those attributes. As a result, the rational design of SM donors with gradually improved BHJ solar cell efficiencies must concurrently address: (i) bandgap tuning and optimization of spectral absorption (inherent to the SM main chain) and (ii) pendant-group substitution promoting structural order and mediating morphological effects. In this paper, the rational pendant-group substitution in benzo[1,2- b:4,5- b′]dithiophene-6,7-difluoroquinoxaline SMs is shown to be an effective approach to narrowing the optical gap ( Eopt) of the SM donors ( SM1 and SM2), without altering their propensity to order and form favorable thin-film BHJ morphologies with PC71BM. Systematic device examinations show that power conversion efficiencies >8% and open-circuit voltages ( VOC) nearing 1 V can be achieved with the narrow-gap SM donor analog ( SM2, Eopt = 1.6 eV) and that charge transport in optimized BHJ solar cells proceeds with minimal, nearly trap-free recombination. Detailed device simulations, light intensity dependence, and transient photocurrent analyses emphasize how carrier recombination impacts BHJ device performance upon optimization of active layer thickness and morphology. [ABSTRACT FROM AUTHOR]

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 cm2 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 large‐scale 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 PC71BM 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 cm2 V‐1 s‐1), ii) longer carrier lifetimes (2.4 µs), and iii) reduced geminate recombination within BHJ active layers to which PC71BM has been added as ternary component. Optically thick (up to ≈500 nm) devices are shown to maintain PCEs >8%, and optimized DR3:ICC6:PC71BM solar cells demonstrate long‐term shelf stability (dark) for >1000 h, in 55% humidity air environment. Small‐molecule‐based ternary BHJ solar cells with the SM donor DR3, the SM acceptor ICC6, and the fullerene PC71BM yield high power conversion efficiencies nearing 11% for active layer thicknesses >200 nm. With low geminate and nongeminate recombination, long carrier lifetimes, and high/balanced carrier mobilities, the ternary system maintains PCEs >8% over a wide range of active layer thicknesses within 200–500 nm. [ABSTRACT FROM AUTHOR]