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In the fast-evolving field of halide perovskite semiconductors, the 2D perovskites (A')2(A) n-1M n X3n+1 [where A = Cs+, CH3NH3+, HC(NH2)2+; A' = ammonium cation acting as spacer; M = Ge2+, Sn2+, Pb2+; and X = Cl-, Br-, I-] have recently made a critical entry. The n value defines the thickness of the 2D layers, which controls the optical and electronic properties. The 2D perovskites have demonstrated preliminary optoelectronic device lifetime superior to their 3D counterparts. They have also attracted fundamental interest as solution-processed quantum wells with structural and physical properties tunable via chemical composition, notably by the n value defining the perovskite layer thickness. The higher members (n > 5) have not been documented, and there are important scientific questions underlying fundamental limits for n To develop and utilize these materials in technology, it is imperative to understand their thermodynamic stability, fundamental synthetic limitations, and the derived structure-function relationships. We report the effective synthesis of the highest iodide n-members yet, namely (CH3(CH2)2NH3)2(CH3NH3)5Pb6I19 (n = 6) and (CH3(CH2)2NH3)2(CH3NH3)6Pb7I22 (n = 7), and confirm the crystal structure with single-crystal X-ray diffraction, and provide indirect evidence for "(CH3(CH2)2NH3)2(CH3NH3)8Pb9I28" ("n = 9"). Direct HCl solution calorimetric measurements show the compounds with n > 7 have unfavorable enthalpies of formation (ΔHf), suggesting the formation of higher homologs to be challenging. Finally, we report preliminary n-dependent solar cell efficiency in the range of 9-12.6% in these higher n-members, highlighting the strong promise of these materials for high-performance devices.

In the fast-evolving field of halide perovskite semiconductors, the 2D perovskites (A′)2(A)n-1MnX3n+1 [where A = Cs+, CH3NH3 +, HC(NH2)2 +; A′ = ammonium cation acting as spacer; M = Ge2+, Sn2+, Pb2+; and X = Cl-, Br-, I-] have recently made a critical entry. The n value defines the thickness of the 2D layers, which controls the optical and electronic properties. The 2D perovskites have demonstrated preliminary optoelectronic device lifetime superior to their 3D counterparts. They have also attracted fundamental interest as solution-processed quantum wells with structural and physical properties tunable via chemical composition, notably by the n value defining the perovskite layer thickness. The higher members (n > 5) have not been documented, and there are important scientific questions underlying fundamental limits for n. To develop and utilize these materials in technology, it is imperative to understand their thermodynamic stability, fundamental synthetic limitations, and the derived structure-function relationships. We report the effective synthesis of the highest iodide n-members yet, namely (CH3(CH2)2NH3)2(CH3NH3)5Pb6I19 (n = 6) and (CH3(CH2)2NH3)2(CH3NH3)6Pb7I22 (n = 7), and confirm the crystal structure with single-crystal X-ray diffraction, and provide indirect evidence for "(CH3(CH2)2NH3)2(CH3NH3)8Pb9I28" ("n = 9"). Direct HCl solution calorimetric measurements show the compounds with n > 7 have unfavorable enthalpies of formation (ΔHf), suggesting the formation of higher homologs to be challenging. Finally, we report preliminary n-dependent solar cell efficiency in the range of 9-12.6% in these higher n-members, highlighting the strong promise of these materials for high-performance devices. [ABSTRACT FROM AUTHOR]

Despite rapid advances in conversion efficiency (>22%), the environmental stability of perovskite solar cells remains a substantial barrier to commercialization. Here, we show a significant improvement in the stability of inverted perovskite solar cells against liquid water and high operating temperature (100 °C) by integrating an ultrathin amorphous oxide electron extraction layer via atomic layer deposition (ALD). These unencapsulated inverted devices exhibit a stable operation over at least 10 h when subjected to high thermal stress (100 °C) in ambient environments, as well as upon direct contact with a droplet of water without further encapsulation. [ABSTRACT FROM AUTHOR]

Perovskite-containing solar cells were fabricated in a two-step procedure in which PbI2 is deposited via spin-coating and subsequently converted to the CH3NH3PbI3 perovskite by dipping in a solution of CH3NH3I. By varying the dipping time from 5 s to 2 h, we observe that the device performance shows an unexpectedly remarkable trend. At dipping times below 15 min the current density and voltage of the device are enhanced from 10.1 mA/cm² and 933 mV (5 s) to 15.1 mA/cm² and 1036 mV (15 min). However, upon further conversion, the current density decreases to 9.7 mA/cm² and 846 mV after 2 h. Based on X-ray diffraction data, we determined that remnant PbI2 is always present in these devices. Work function and dark current measurements showed that the remnant PbI2 has a beneficial effect and acts as a blocking layer between the TiO2 semiconductor and the perovskite itself reducing the probability of back electron transfer (charge recombination). Furthermore, we find that increased dipping time leads to an increase in the size of perovskite crystals at the perovskite-hole-transporting material interface. Overall, approximately 15 min dipping time (~2% unconverted PbI2) is necessary for achieving optimal device efficiency. [ABSTRACT FROM AUTHOR]

Lead halide perovskites present a versatile class of solution-processable semiconductors with highly tunable bandgaps that span ultraviolet, visible, and near-infrared portions of the spectrum. We explore phase-separated chloride and iodide lead perovskite mixtures as candidate materials for intermediate band applications in future photovoltaics. X-ray diffraction and scanning electron microscopy reveal that deposition of precursor solutions across the MAPbCl 3 /MAPbI 3 composition space affords quasi-epitaxial cocrystallized films, in which the two perovskites do not alloy but instead remain phase-segregated. First-principle calculations further support the formation of an epitaxial interface and predict energy offsets in the valence band and conduction band edges that could result in intermediate energy absorption. The charge dynamics of variable mixtures of the relatively narrow bandgap (1.57 eV) MAPbI 3 perovskite and wide bandgap (3.02 eV) MAPbCl 3 are probed to map charge and energy flow direction and kinetics. Time-resolved photoluminescence and transient absorption measurements reveal charge transfer of photoexcited carriers in MAPbCl 3 to MAPbI 3 in tens of picoseconds. The rate of quenching can be further tuned by replacing MAPbI 3 with two-dimensional Ruddlesden-Popper (BA) 2 (MA) n-1 Pb n I 3 n+1 ( n = 3, 2, and 1) perovskites, which also remain phase-separated.

A low-temperature (<120 °C) route to pinhole-free amorphous TiO2 compact layers may pave the way to more efficient, flexible, and stable inverted perovskite halide device designs. Toward this end, we utilize low-temperature thermal atomic layer deposition (ALD) to synthesize ultrathin (12 nm) compact TiO2 underlayers for planar halide perovskite PV. Although device performance with as-deposited TiO2 films is poor, we identify room-temperature UV-O3 treatment as a route to device efficiency comparable to crystalline TiO2 thin films synthesized by higher temperature methods. We further explore the chemical, physical, and interfacial properties that might explain the improved performance through X-ray diffraction, spectroscopic ellipsometry, Raman spectroscopy, and X-ray photoelectron spectroscopy. These findings challenge our intuition about effective electron selective layers as well as point the way to a greater selection of flexible substrates and more stable inverted device designs.