Here, we investigated the separate and combined effects of guanidinium (Gua⁺) and/or thiocyanate (SCN⁻) ions on the opto-electronic properties of mixed Sn–Pb perovskites, which are used as absorber layers in photovoltaics. Therefore, we spin-coated Cs_0.25FA_0.75Sn_0.5Pb_0.5I₃ thin films with GuaI, Pb(SCN)₂ or GuaSCN, in the absence or presence of SnF₂. By comparing the (micro)structural and opto-electronic properties of the perovskite films, we elucidated the functions of both ions. We found that SCN⁻ suppresses tin oxidation and doping, reduces crystal defects and improves the carrier transport properties, regardless of SnF₂ addition. We demonstrate that this is due to coordination with Sn²⁺ and scavenging of Sn⁴⁺ in the spin-coating solution, resulting in a pile-up of SnO_x at the film surface. Gua⁺ is incorporated to a limited extent into the 3D cubic perovskite structure. Gua⁺ cannot suppress tin oxidation and doping, making this additive useless without SnF₂. Conversely, when combined with SnF₂, Gua⁺ enhances the carrier transport properties. Combining Gua⁺ and SCN⁻ until a maximum addition of 4 mol% and SnF₂ results in large grains and pinhole-free films with superior charge carrier transport properties, leading to a substantial increase in the pseudo-open circuit voltage of 50 mV. Addition of >4 mol% GuaSCN leads to the formation of Gua-based 2D perovskites, including Gua_xFA_2−xSn_yPb_1−yI₄ and Gua_xFA_3−xSn_yPb_2−yI₇, which do not improve the carrier dynamics. In short, we observe a synergistic effect on addition of Gua⁺ and SCN⁻ ions, which leads to improved structural and opto-electronic properties, which is promising for implementation in solar cells.

Hybrid organic-inorganic perovskites (PVKs) offer exceptional optoelectronic performance, yet reproducible and scalable co-evaporation remains challenging. This study examines the interplay of factors affecting compositional control during three-source PVK deposition. We identify chamber pressure, precursor cross-contamination, and flux instability – especially from organic salts such as formamidinium iodide (FAI) – as major sources of variability. A critical influence is the occurrence of cross-reading, where omnidirectional evaporation of FAI contributes to the reading on the quartz crystal microbalance (QCM) sensors monitoring the inorganic precursors like caesium bromide (CsBr) and lead iodide (PbI₂) even though shielding is present. This effect, strongly dependent on FAI load, deposition rate, and QCM sensor position, erroneously inflates measured fluxes, leading to inaccurate rate control and unintentional compositional drift. Maintaining A-, B- and X-site stoichiometry therefore requires dynamic adjustment of precursor rates, particularly at higher deposition speeds where mean free path limitations come into play. We demonstrate the successful deposition of perovskite layers at a deposition speed of 27.8 nm min⁻¹ as the practical ceiling for the investigated Cs_xFA_1-xPb(I_1-xBr_x)₃ composition within our experimental framework. These findings highlight the delicate balance between deposition speed, precursor stability, and film quality, underscoring the need for improved delivery systems - such as continuous precursor feedthrough, multiple organic sources, alternative vapor transport or flash evaporation methods – to achieve reproducible, fast and large-scale fabrication of high-performance PVK films.

Indium phosphide (InP) quantum dots (QDs) are considered the most promising alternative for Cd and Pb-based QDs for lighting and display applications. However, while core-only QDs of CdSe and CdTe have been prepared with near-unity photoluminescence quantum yield (PLQY), this is not yet achieved for InP QDs. Treatments with HF have been used to boost the PLQY of InP core-only QDs up to 85%. However, HF etches the QDs, causing loss of material and broadening of the optical features. Here, we present a simple postsynthesis HF-free treatment that is based on passivating the surface of the InP QDs with InF_3. For optimized conditions, this results in a PLQY as high as 93% and nearly monoexponential photoluminescence decay. Etching of the particle surface is entirely avoided if the treatment is performed under stringent acid-free conditions. We show that this treatment is applicable to InP QDs with various sizes and InP QDs obtained via different synthesis routes. The optical properties of the resulting core-only InP QDs are on par with InP/ZnSe/ZnS core-shell QDs, with significantly higher absorption coefficients in the blue, and with potential for faster charge transport. These are important advantages when considering InP QDs for use in micro-LEDs or photodetectors.

Mixed Sn-Pb halide perovskites are promising absorber materials for solar cells due to the possibility of tuning the bandgap energy down to 1.2-1.3 eV. However, tin-containing perovskites are susceptible to oxidation affecting the optoelectronic properties. In this work, we investigated qualitatively and quantitatively metastable oxygen-induced doping in isolated ASn_xPb_1-xI₃ (where A is methylammonium or a mixture of formamidinium and cesium) perovskite thin films by means of microwave conductivity, structural and optical characterization techniques. We observe that longer oxygen exposure times lead to progressively higher dark conductivities, which slowly decay back to their original levels over days. Here oxygen acts as an electron acceptor, leading to tin oxidation from Sn²⁺ to Sn⁴⁺ and creation of free holes. The metastable oxygen-induced doping is enhanced by exposing the perovskite simultaneously to oxygen and light. Next, we show that doping not only leads to the reduction in the photoconductivity signal but also induces long-term effects even after loss of doping, which is thought to derive from consecutive oxidation reactions leading to the formation of defect states. On prolonged exposure to oxygen and light, optical and structural changes can be observed and related to the formation of SnO_x and loss of iodide near the surface. Our work highlights that even a short-term exposure to oxygen immediately impairs the charge carrier dynamics of the perovskite, while structural perovskite degradation is only noticeable upon long-term exposure and accumulation of oxidation products. Hence, for efficient solar cells, exposure of mixed Sn-Pb perovskites to oxygen during production and operation should be rigorously blocked.

To increase the open-circuit voltage in solar cells based on triple cation, mixed halide perovskites, reducing recombination processes at the interfaces with transport layers (TLs) is key. Here, we investigated the charge carrier dynamics in bilayers and trilayers of Cs_0.05MA_0.10FA_0.85Pb(I_0.97Br_0.03)₃ (CsMAFA) combined with TLs using time-resolved microwave conductance (TRMC) measurements without and with bias illumination (BI). In the bilayers, we find balanced mobilities for electrons and holes in CsMAFA and nearly quantitative carrier extraction. The small, rapidly decaying TRMC signals for n-i-p- and p-i-n triple layers indicate both carriers are extracted. Applying BI leads to the charging of the TLs and the corresponding electric field prevents additional charge extraction, which demonstrates long-lived charge separation over the CsMAFA/TLs. Most importantly, for all bilayer combinations showing long-lived charge separation, an increase of the quasi-Fermi level splitting with respect to that of the CsMAFA layer is found.

Indium phosphide colloidal quantum dots (CQDs) are the main alternative for toxic and restricted Cd based CQDs for lighting and display applications. Here we systematically report on the size-dependent optical absorption, ensemble, and single particle photoluminescence (PL) and biexciton lifetimes of core-only InP CQDs. This systematic study is enabled by improvements in the synthesis of InP CQDs to yield a broad size series of monodisperse core-only InP CQDs with narrow absorption and PL line width and significant PL quantum yield.