Organic-inorganic halide perovskites incorporating two-dimensional (2D) structures have shown promise for enhancing the stability of perovskite solar cells (PSCs). However, the bulky 2D cations often limit charge transport. Here, we report on a simple approach based on molecular design of the organic 2D spacer to improve the transport properties of 2D perovskites, and we use phenethylammonium (PEA) as an example. We demonstrate that by fluorine substitution on the para position in PEA to form 4-fluoro-phenethylammonium (F-PEA), the average phenyl ring centroid-centroid distances in the organic layer become shorter with aligned stacking of perovskite sheets. The impact is enhanced orbital interactions and charge transport across adjacent inorganic layers as well as increased carrier lifetime and reduced trap density. Using a simple perovskite deposition at room temperature without using any additives, we obtained power conversion efficiency >13% for (F-PEA)2MA4Pb5I16 based PSCs. In addition, the thermal stability of 2D PSCs based on F-PEA is significantly enhanced compared to those based on PEA.

During the past decade, inorganic CQDs, namely the lead chalcogenides (e.g., PbS), have attracted tremendous attention in solution-processed solar cells. Due to the great efforts on CQDs synthesis modification, [7][8][9] surface passivation, [10][11][12] and device fabrication optimization, [13][14][15][16] PbS QD solar cells continue to progress at an extraordinary rate, improving overall efficiencies by ≈1% per year and currently have a certified power conversion efficiency (PCE) exceeding 12%. [17] Meanwhile, the past decade has witnessed unprecedented success of organicinorganic hybrid perovskites in PV applications, with the reported PCE of perovskite solar cells exceeding 23%. [18][19][20][21][22][23][24][25][26][27][28] However, the challenging stability issues of these hybrid perovskites further motivate the research of all-inorganic perovskites (CsPbX 3 , X = Cl − , Br − , I − or mixed halides) without any volatile organic components. [29][30][31][32][33][34][35][36][37][38] Among these all-inorganic perovskite materials, α-CsPbI 3 exhibits an ideal optical bandgap (E g ) of 1.73 eV for PV applications. However, the nonphotoactive orthorhombic phase (E g = 2.82 eV) is more thermodynamically preferred at low temperature. [29] Therefore, the perovskite phase of CsPbI 3 usually requires complex annealing processes at high temperature to achieve satisfactory film quality. As mentioned above, QD technology offers colloidal synthesis of conventional bulk materials, which Surface manipulation of quantum dots (QDs) has been extensively reported to be crucial to their performance when applied into optoelectronic devices, especially for photovoltaic devices. In this work, an efficient surface passivation method for emerging CsPbI 3 perovskite QDs using a variety of inorganic cesium salts (cesium acetate (CsAc), cesium idodide (CsI), cesium carbonate (Cs 2 CO 3 ), and cesium nitrate (CsNO 3 )) is reported. The Cs-salts post-treatment can not only fill the vacancy at the CsPbI 3 perovskite surface but also improve electron coupling between CsPbI 3 QDs. As a result, the free carrier lifetime, diffusion length, and mobility of QD film are simultaneously improved, which are beneficial for fabricating high-quality conductive QD films for efficient solar cell devices. After optimizing the post-treatment process, the short-circuit current density and fill factor are significantly enhanced, delivering an impressive efficiency of 14.10% for CsPbI 3 QD solar cells. In addition, the Cs-salt-treated CsPbI 3 QD devices exhibit improved stability against moisture due to the improved surface environment of these QDs. These findings will provide insight into the design of high-performance and low-trap-states perovskite QD films with desirable optoelectronic properties. Perovskite Quantum DotsThe ORCID identification number(s) for the author(s) of this article can be found under https://doi.Solution-processed colloidal quantum dots (CQDs) are promising candidates for the next generation photovoltaics (PVs) due to the excellent tuna...

Metal halide perovskite semiconductors possess outstanding characteristics for optoelectronic applications including but not limited to photovoltaics. Low-dimensional and nanostructured motifs impart added functionality which can be exploited further. Moreover, wider cation composition tunability and tunable surface ligand properties of colloidal quantum dot (QD) perovskites now enable unprecedented device architectures which differ from thin-film perovskites fabricated from solvated molecular precursors. Here, using layer-by-layer deposition of perovskite QDs, we demonstrate solar cells with abrupt compositional changes throughout the perovskite film. We utilize this ability to abruptly control composition to create an internal heterojunction that facilitates charge separation at the internal interface leading to improved photocarrier harvesting. We show how the photovoltaic performance depends upon the heterojunction position, as well as the composition of each component, and we describe an architecture that greatly improves the performance of perovskite QD photovoltaics.

We present a cation-exchange approach for tunable A-site alloys of cesium (Cs+) and formamidinium (FA+) lead triiodide perovskite nanocrystals that enables the formation of compositions spanning the complete range of Cs1–x FA x PbI3, unlike thin-film alloys or the direct synthesis of alloyed perovskite nanocrystals. These materials show bright and finely tunable emission in the red and near-infrared range between 650 and 800 nm. The activation energy for the miscibility between Cs+ and FA+ is measured (∼0.65 eV) and is shown to be higher than reported for X-site exchange in lead halide perovskites. We use these alloyed colloidal perovskite quantum dots to fabricate photovoltaic devices. In addition to the expanded compositional range for Cs1–x FA x PbI3 materials, the quantum dot solar cells exhibit high open-circuit voltage (V OC) with a lower loss than the thin-film perovskite devices of similar compositions.