Decoupling Interfacial Charge Transfer from Bulk Diffusion Unravels Its Intrinsic Role for Efficient Charge Extraction in Perovskite Solar Cells

In a perovskite solar cell, the overall photoinduced charge-transfer (CT) process comprises both charge diffusion through the bulk to perovskite/electrode interfaces and interfacial electron and hole transfer to electrodes. In this study, we decoupled these two entangled processes by investigating the film thickness-dependent CT dynamics from CH₃NH₃PbI₃ perovskites to [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) (electron acceptor) and spiro-OMeTAD (hole acceptor). By fitting ultrafast transient absorption kinetics to an explicit “diffusion-coupled charge-transfer” model, we found that the charge diffusion from the film interior to perovskite/electrode interfaces took ∼200 ps to a few nanoseconds, depending on the thickness of perovskite film; the subsequent interfacial charge transfer was ultrafast, ∼6 ps for electron transfer to PCBM and ∼8 ps for hole transfer to spiro-OMeTAD, and led to efficient charge extraction (>90%) to electrodes in a 400 nm thick film. Our results indicate that the picosecond interfacial charge transfer is a key to high-performance perovskite solar cells

Long-Distance Charge Carrier Funneling in Perovskite Nanowires Enabled by Built-in Halide Gradient

The excellent charge carrier transportation in organolead halide perovskites is one major contributor to the high performance of many perovskite-based devices. There still exists a possibility for further enhancement of carrier transportation through nanoscale engineering, owing to the versatile wet-chemistry synthesis and processing of perovskites. Here we report the successful synthesis of bromide-gradient CH₃NH₃PbBr_<i>x</i>I_3–<i>x</i> single-crystalline nanowires (NWs) by a solid-to-solid ion exchange reaction starting from one end of pure CH₃NH₃PbI₃ NWs, which was confirmed by local photoluminescence (PL) and energy dispersive X-ray spectroscopy (EDS) measurements. Due to the built-in halide gradient, the long-distance carrier transportation was driven by the energy funnel, rather than the spontaneous carrier diffusion. Indeed, local PL kinetics demonstrated effective charge carrier transportation only from the high-bandgap bromide-rich region to the low-bandgap iodine-rich region over a few micrometers. Therefore, these halide gradient NWs might find applications in various optoelectronic devices requiring long-distance and directional delivery of excitation energy

Observation of Internal Photoinduced Electron and Hole Separation in Hybrid Two-Dimentional Perovskite Films

Two-dimensional (2D) organolead halide perovskites are promising for various optoelectronic applications. Here we report a unique spontaneous charge (electron/hole) separation property in multilayered (BA)₂(MA)_<i>n</i>−1Pb_<i>n</i>I_3<i>n</i>+1 (BA = CH₃(CH₂)₃NH₃⁺, MA = CH₃NH₃⁺) 2D perovskite films by studying the charge carrier dynamics using ultrafast transient absorption and photoluminescence spectroscopy. Surprisingly, the 2D perovskite films, although nominally prepared as “<i>n</i> = 4”, are found to be mixture of multiple perovskite phases, with <i>n</i> = 2, 3, 4 and ≈ ∞, that naturally align in the order of <i>n</i> along the direction perpendicular to the substrate. Driven by the band alignment between 2D perovskites phases, we observe consecutive photoinduced electron transfer from small-<i>n</i> to large-<i>n</i> phases and hole transfer in the opposite direction on hundreds of picoseconds inside the 2D film of ∼358 nm thickness. This internal charge transfer efficiently separates electrons and holes to the upper and bottom surfaces of the films, which is a unique property beneficial for applications in photovoltaics and other optoelectronics devices

“Intact” Carrier Doping by Pump–Pump–Probe Spectroscopy in Combination with Interfacial Charge Transfer: A Case Study of CsPbBr₃ Nanocrystals

Carrier doping is important for semiconductor nanocrystals (NCs) as it offers a new knob to tune NCs’ functionalities, in addition to size and shape control. Also, extensive studies on NC devices have revealed that under operating conditions NCs are often unintentionally doped with electrons or holes. Thus, it is essential to be able to control the doping of NCs and study the carrier dynamics of doped NCs. The extension of previously reported redox-doping methods to chemically sensitive materials, such as recently introduced perovskite NCs, has remained challenging. We introduce an “intact” carrier-doping method by performing pump–pump–probe transient absorption spectroscopy on NC–acceptor complexes. The first pump pulse is used to trigger charge transfer from the NC to the acceptor, leading to NCs doped with a band edge carrier; the following pump–probe pulses measure the dynamics of carrier-doped NCs. We performed this measurement on CsPbBr₃ NCs and deduced positive and negative trion lifetimes of 220 ± 50 and 150 ± 40 ps, respectively, for 10 nm diameter NCs, both dominated by Auger recombination. It also allowed us to identify randomly photocharged excitons in CsPbBr₃ NCs as positive trions

Comparison between IQSeq and Cufflinks.

<p>Comparison between IQSeq and Cufflinks.</p

Classification of different isoform composition between stages.

<p>Classification of different isoform composition between stages.</p

FasterShotgunFIM (<i>I,</i>Θ<i>,Samp_m,p,q</i>)

<p>FasterShotgunFIM (<i>I,</i>Θ<i>,Samp_m,p,q</i>)</p

Total time used by brute-force simulation vs. FIM based heuristic to estimate in simplified genes.

<p>Total time used by brute-force simulation vs. FIM based heuristic to estimate in simplified genes.</p

Computations on gene TCF7.

<p>(a) Known isoforms (b) Splicing graph (c) Simulation schema.</p

A simple shotgun read generation model.

<p>A simple shotgun read generation model.</p