Surface versus Bulk Currents and Ionic Space-Charge Effects in CsPbBr3 Single Crystals.

CsPbBr3 single crystals have potential for application in ionizing-radiation detection devices due to their optimal optoelectronic properties. Yet, their mixed ionic-electronic conductivity produces instability and hysteretic artifacts hindering the long-term device operation. Herein, we report an electrical characterization of CsPbBr3 single crystals operating up to the time scale of hours. Our fast time-of-flight measurements reveal bulk mobilities of 13-26 cm2 V-1 s-1 with a negative voltage bias dependency. By means of a guard ring (GR) configuration, we separate bulk and surface mobilities showing significant qualitative and quantitative transport differences. Our experiments of current transients and impedance spectroscopy indicate the formation of several regimes of space-charge-limited current (SCLC) associated with mechanisms similar to the Poole-Frenkel ionized-trap-assisted transport. We show that the ionic-SCLC seems to be an operational mode in this lead halide perovskite, despite the fact that experiments can be designed where the contribution of mobile ions to transport is negligible.

Looking beyond the Surface: The Band Gap of Bulk Methylammonium Lead Iodide.

Despite the intense research on photovoltaic lead halide perovskites, reported optical properties as basic as the absorption onset and the optical band gap vary significantly. To unambiguously answer the question whether the discrepancies are a result of differences between bulk and "near-surface" material, we perform two nonlinear spectroscopies with drastically different information depths on single crystals of the prototypical (CH3NH3)PbI3 methylammonium lead iodide. Two-photon absorption, detected via the resulting generation of carriers and photocurrents (2PI-PC), probes the interband transitions with an information depth in the millimeter range relevant for bulk (single-crystal) material. In contrast, the transient magneto-optical Kerr effect (trMOKE) measured in a reflection geometry determines the excitonic transition energies in the region near (hundreds of nm) the surface which also determine the optical properties in typical thin films. To identify differences between structural phases, we sweep the sample temperature across the orthorhombic-tetragonal phase transition temperature. In the application-relevant room-temperature tetragonal phase (at 170 K), we find a bulk band gap of 1.55 ± 0.01 eV, whereas in the near-surface region excitonic transitions occur at 1.59 ± 0.01 eV. The latter value is consistent with previous reflectance measurements by other groups and considerably higher than the bulk band gap. The small band gap of the bulk material explains the extended infrared absorption of crystalline perovskite solar cells, the low-energy bands which carry optically driven spin-polarized currents, and the narrow bandwidth of crystalline perovskite photodetectors making use of the spectral filtering at the surface.

Impurity Tracking Enables Enhanced Control and Reproducibility of Hybrid Perovskite Vapor Deposition.

Metal halide perovskite semiconductors have the potential to enable low-cost, flexible, and efficient solar cells for a wide range of applications. Physical vapor deposition by co-evaporation of precursors is a method that results in very smooth and pinhole-free perovskite thin films and allows excellent control over film thickness and composition. However, for a deposition method to become industrially scalable, reproducible process control and high device yields are essential. Unfortunately, to date, the control and reproducibility of evaporating organic precursors such as methylammonium iodide (MAI) have proved extremely challenging. We show that the established method of controlling the evaporation rate of MAI with quartz microbalances (QMBs) is critically sensitive to the concentration of the impurities MAH2PO3 and MAH2PO2 that are usually present in MAI after synthesis. Therefore, controlling the deposition rate of MAI with QMBs is unreliable since the concentration of such impurities typically varies from one batch of MAI to another and even during the course of a deposition. However once reliable control of MAI deposition is achieved, we find that the presence of precursor impurities during perovskite deposition does not degrade the solar cell performance. Our results indicate that as long as precursor deposition rates are well controlled, physical vapor deposition will allow high solar cell device yields even if the purity of precursors changes from one run to another.

Structural fluctuations cause spin-split states in tetragonal (CH3NH3)PbI3 as evidenced by the circular photogalvanic effect.

Lead halide perovskites are used in thin-film solar cells, which owe their high efficiency to the long lifetimes of photocarriers. Various calculations find that a dynamical Rashba effect could significantly contribute to these long lifetimes. This effect is predicted to cause a spin splitting of the electronic bands of inversion-symmetric crystalline materials at finite temperatures, resulting in a slightly indirect band gap. Direct experimental evidence of the existence or the strength of the spin splitting is lacking. Here, we resonantly excite photocurrents in single crystalline ([Formula: see text])[Formula: see text] with circularly polarized light to clarify the existence of spin splittings in the band structure. We observe a circular photogalvanic effect, i.e., the photocurrent depends on the light helicity, in both orthorhombic and tetragonal ([Formula: see text])[Formula: see text] At room temperature, the effect peaks for excitation photon energies [Formula: see text] meV below the direct optical band gap. Temperature-dependent measurements reveal a sign change of the effect at the orthorhombic-tetragonal phase transition, indicating different microscopic origins in the two phases. Within the tetragonal phase, both [Formula: see text] and the amplitude of the circular photogalvanic effect increase with temperature. Our findings support a dynamical Rashba effect in this phase, i.e., a spin splitting caused by thermally induced structural fluctuations which break inversion symmetry.

Improved charge carrier dynamics in polymer/perovskite nanocrystal based hybrid ternary solar cells.

Here, brand new ternary hybrid solar cells comprising perovskite nanocrystals (NCs) with a complementary absorption profile of the organic host matrix are reported. In particular, NH2CH[double bond, length as m-dash]NH2PbI3 (FAPbI3) perovskite NCs are implemented in bulk heterojunction organic solar cells based on the pDPP5T-2 electron donating polymer and a [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) acceptor at various loading amounts and the fabricated hybrid photovoltaics are thoroughly studied by employing different optoelectrical characterization methods. Current-voltage measurements and photoinduced charge carrier extraction by linear increasing voltage (photo-CELIV) reveal improved charge generation and charge transport properties upon incorporation of perovskite NCs into the photo-active layer of the hybrid solar cell. The power conversion efficiency (PCE) of the hybrid solar cell comprising 5% perovskite NCs is 10% enhanced compared to the organic reference, mainly due to the enlarged light harvesting and increased short circuit current density (Jsc). However, results suggest that introducing a higher amount of perovskite content induces bimolecular and trap-assisted recombination in the ternary devices. We perform a comprehensive transient absorption study of the charge transfer/transport mechanisms by employing femto-second pump-probe transient absorption spectroscopy (fs-TAS). fs-TAS measurements demonstrate a slower charge carrier recombination rate due to the introduction of perovskite NCs into the photoactive layer. Results reveal that DPP injects electrons from the singlet excited state into the perovskite NCs, which causes the desired cascading charge carrier transfer. In ternary blends, a small amount of FAPbI3 NCs provides an additional pathway in favor of the charge-separated state via the NCs, which, despite accelerating the depopulation of DPP's singlet excited state slightly slows down the charge-separation process between DPP and PC61BM. Interestingly, the loss processes are slowed down; an effect that is more important and, hence, explains the improved solar cell efficiency.

A generic interface to reduce the efficiency-stability-cost gap of perovskite solar cells.

A major bottleneck delaying the further commercialization of thin-film solar cells based on hybrid organohalide lead perovskites is interface loss in state-of-the-art devices. We present a generic interface architecture that combines solution-processed, reliable, and cost-efficient hole-transporting materials without compromising efficiency, stability, or scalability of perovskite solar cells. Tantalum-doped tungsten oxide (Ta-WO x )/conjugated polymer multilayers offer a surprisingly small interface barrier and form quasi-ohmic contacts universally with various scalable conjugated polymers. In a simple device with regular planar architecture and a self-assembled monolayer, Ta-WO x -doped interface-based perovskite solar cells achieve maximum efficiencies of 21.2% and offer more than 1000 hours of light stability. By eliminating additional ionic dopants, these findings open up the entire class of organics as scalable hole-transporting materials for perovskite solar cells.

Brightly Luminescent and Color-Tunable Formamidinium Lead Halide Perovskite FAPbX3 (X = Cl, Br, I) Colloidal Nanocrystals.

In the past few years, hybrid organic-inorganic and all-inorganic metal halide perovskite nanocrystals have become one of the most interesting materials for optoelectronic applications. Here, we report a facile and rapid room temperature synthesis of 15-25 nm formamidinium CH(NH2)2PbX3 (X = Cl, Br, I, or mixed Cl/Br and Br/I) colloidal nanocrystals by ligand-assisted reprecipitation (LARP). The cubic and platelet-like nanocrystals with their emission in the range of 415-740 nm, full width at half-maximum (fwhm) of 20-44 nm, and radiative lifetimes of 5-166 ns enable band gap tuning by halide composition as well as by their thickness tailoring; they have a high photoluminescence quantum yield (up to 85%), colloidal and thermodynamic stability. Combined with surface modification that prevents degradation by water, this nanocrystalline material is an ideal candidate for optoelectronic devices and applications. In addition, optoelectronic measurements verify that the photodetector based on FAPbI3 nanocrystals paves the way for perovskite quantum dot photovoltaics.

Ligand-assisted thickness tailoring of highly luminescent colloidal CH3NH3PbX3 (X = Br and I) perovskite nanoplatelets.

Quantum size-confined CH3NH3PbX3 (X = Br and I) perovskite nanoplatelets with remarkably high photoluminescence quantum yield (up to 90%) were synthesized by ligand-assisted re-precipitation. Thickness-tunability was realized by varying the oleylamine and oleic acid ligand ratio. This method allows tailoring the nanoplatelet thickness by adjusting the number of unit cell monolayers. Broadly tunable emission wavelengths (450-730 nm) are achieved via the pronounced quantum size effect without anion-halide mixing.

Giant Rashba Splitting in CH_{3}NH_{3}PbBr_{3} Organic-Inorganic Perovskite.

As they combine decent mobilities with extremely long carrier lifetimes, organic-inorganic perovskites open a whole new field in optoelectronics. Measurements of their underlying electronic structure, however, are still lacking. Using angle-resolved photoelectron spectroscopy, we measure the valence band dispersion of single-crystal CH_{3}NH_{3}PbBr_{3}. The dispersion of the highest energy band is extracted applying a modified leading edge method, which accounts for the particular density of states of organic-inorganic perovskites. The surface Brillouin zone is consistent with bulk-terminated surfaces both in the low-temperature orthorhombic and the high-temperature cubic phase. In the low-temperature phase, we find a ring-shaped valence band maximum with a radius of 0.043 Å^{-1}, centered around a 0.16 eV deep local minimum in the dispersion of the valence band at the high-symmetry point. Intense circular dichroism is observed. This dispersion is the result of strong spin-orbit coupling. Spin-orbit coupling is also present in the room-temperature phase. The coupling strength is one of the largest ones reported so far.

Coloring Semitransparent Perovskite Solar Cells via Dielectric Mirrors.

While perovskite-based semitransparent solar cells for window applications show competitive levels of transparency and efficiency compared to organic photovoltaics, the color perception of the perovskite films is highly restricted because band gap engineering results in losses in power conversion efficiencies. To overcome the limitation in visual aesthetics, we combined semitransparent perovskite solar cells with dielectric mirrors. This approach enables one to tailor the device appearance to almost any desired color and simultaneously offers additional light harvesting for the solar cell. In the present work, opto-electrical effects are investigated through quantum efficiency and UV-to-visible spectroscopic measurements. Likewise, a detailed chromaticity analysis, featuring the transmissive and reflective color perception of the device including the mirror, from both sides and in different illumination conditions, is presented and analyzed. Photocurrent density enhancement of up to 21% along with overall device transparency values of up to 31% (4.2% efficiency) is demonstrated for cells showing a colored aesthetic appeal. Finally, a series of simulations emulating the device chromaticity, transparency, and increased photocurrent density as a function of the photoactive layer thickness and the design wavelength of the dielectric mirror are presented. Our simulations and their experimental validation enabled us to establish the design rules that consider the color efficiency/transparency interplay for real applications.