Teresa S. Ripolles

Recombination Study of Combined Halides (Cl, Br, I) Perovskite Solar Cells

We report on the preparation of a series of solution-processed perovskite solar cells based on methylammonium (MA) lead halide derivatives, MAPbX3, which show tunable optical properties depending on the nature and ratio of the halides employed (X = Cl, Br, and I). Devices have been prepared with different cell architecture, thin film, and mesoporous scaffold (TiO2 and Al2O3). We have analyzed different sample sets focusing on the characterization of the charge recombination by means of impedance spectroscopy (IS). On the one hand, our study discloses that the insertion of both Cl and Br in the perovskite lattice reduces the charge recombination rates in the light absorber film, thus determining the open circuit voltage (Voc) of the device. The samples prepared on a mesoporous Al2O3 electrode present lower charge recombination rates than those devices prepared on mesoporous TiO2. Furthermore, the addition of Br in the perovskite structure was demonstrated to improve slightly the lifetime of the devices; in fact, the efficiencies of all devices tested remained at least at the 80% of the initial value 1 month after their preparation. These results highlight the crucial role of the charge-recombination processes on the performance of the perovskite solar cells and pave the way for further progress on this field.

Polymer defect states modulate open-circuit voltage in bulk-heterojunction solar cells

Defect states influence the operation of organic solar cells altering transport, recombination, and energetic mechanisms. This work investigates how processing conditions induce morphology-related, electrically active defects in the donor polymer of bulk-heterojunction solar cells. Structural order is inferred from absorption and X-ray diffraction data, while defect density is determined from capacitance methods. A correlation is observed between the polymer nanocrystallite size, the defect concentration, and the output voltage. For the case of poly(3-hexylthiophene), processing that promote crystallinity is beneficial for the device performance as it decreases the defect density (energy disorder) that finally enlarges the maximum achievable open-circuit voltage. Defect states within the effective bandgap modulate the downshift of the hole Fermi level upon illumination that in turn establishes the achievable open-circuit voltage.

Colloidal Synthesis of Air-Stable Alloyed CsSn1–xPbxI3 Perovskite Nanocrystals for Use in Solar Cells

Organic-inorganic hybrid perovskite solar cells have demonstrated unprecedented high power conversion efficiencies in the past few years. Now, the universal instability of the perovskites has become the main barrier for this kind of solar cells to realize commercialization. This situation can be even worse for those tin-based perovskites, especially for CsSnI3, because upon exposure to ambient atmosphere the desired black orthorhombic phase CsSnI3 would promptly lose single crystallinity and degrade to the inactive yellow phase, followed by irreversible oxidation into metallic Cs2SnI6. By alloying CsSnI3 with CsPbI3, we herein report the synthesis of alloyed perovskite quantum dot (QD), CsSn1-xPbxI3, which not only can be phase-stable for months in purified colloidal solution but also remains intact even directly exposed to ambient air, far superior to both of its parent CsSnI3 and CsPbI3 QDs. Ultrafast transient absorption spectroscopy studies reveal that the photoexcited electrons in the alloyed QDs can be injected into TiO2 nanocrystals at a fast rate of 1.12 × 1011 s-1, which enables a high photocurrent generation in solar cells.

Tunable Open Circuit Voltage by Engineering Inorganic Cesium Lead Bromide/Iodide Perovskite Solar Cells

Perovskite solar cells based on series of inorganic cesium lead bromide and iodide mixture, CsPbBr3-xIx, where x varies between 0, 0.1, 0.2, and 0.3 molar ratio were synthesized by two step-sequential deposition at ambient condition to design the variations of wide band gap light absorbers. A device with high overall photoconversion efficiency of 3.98 % was obtained when small amount of iodide (CsPbBr2.9I0.1) was used as the perovskite and spiro-OMeTAD as the hole transport material (HTM). We investigated the origin of variation in open circuit voltage, Voc which was shown to be mainly dependent on two factors, which are the band gap of the perovskite and the work function of the HTM. An increment in Voc was observed for the device with larger perovskite band gap, while keeping the electron and hole extraction contacts the same. Besides, the usage of bilayer P3HT/MoO3 with deeper HOMO level as HTM instead of spiro-OMeTAD, thus increased the Voc from 1.16 V to 1.3 V for CsPbBr3 solar cell, although the photocurrent is lowered due to charge extraction issues. The stability studies confirmed that the addition of small amount of iodide into the CsPbBr3 is necessarily to stabilize the cell performance over time.

Highly Efficient 17.6% Tin–Lead Mixed Perovskite Solar Cells Realized through Spike Structure

Frequently observed high Voc loss in tin-lead mixed perovskite solar cells is considered to be one of the serious bottle-necks in spite of the high attainable Jsc due to wide wavelength photon harvesting. An amicable solution to minimize the Voc loss up to 0.50 V has been demonstrated by introducing an n-type interface with spike structure between the absorber and electron transport layer inspired by highly efficient Cu(In,Ga)Se2 solar cells. Introduction of a conduction band offset of ∼0.15 eV with a thin phenyl-C61-butyric acid methyl ester layer (∼25 nm) on the top of perovskite absorber resulted into improved Voc of 0.75 V leading to best power conversion efficiency of 17.6%. This enhancement is attributed to the facile charge flow at the interface owing to the reduction of interfacial traps and carrier recombination with spike structure as evidenced by time-resolved photoluminescence, nanosecond transient absorption, and electrochemical impedance spectroscopy measurements.

Annealing effects on CsPbI3-based planar heterojunction perovskite solar cells formed by vacuum deposition method

Cesium iodide (CsI) is attracting attention as a substitute for organic materials such as CH3NH3I. In this work, we fabricated sequential-vacuum-deposited planar heterojunction (PHJ) cesium lead iodide (CsPbI3) perovskite solar cells with enhanced efficiencies by varying the annealing time (0.5, 1, 5, and 10 min). The effect of performance enhancement was investigated as a function of varying annealing time at 350 °C employing a hot plate. The best-performing device was obtained with an annealing time of 1 min, delivered photocurrent density (J SC) of 12.06 mA/cm2, voltage (V OC) of 0.71 V, and fill factor (FF) of 0.67, leading to a power conversion efficiency (PCE) of 5.71% at standard AM 1.5G solar illumination.

Slow hot carrier cooling in cesium lead iodide perovskites

Lead halide perovskites are attracting a great deal of interest for optoelectronic applications such as solar cells, LEDs, and lasers because of their unique properties. In solar cells, heat dissipation by hot carriers results in a major energy loss channel responsible for the Shockley–Queisser efficiency limit. Hot carrier solar cells offer the possibility to overcome this limit and achieve energy conversion efficiency as high as 66% by extracting hot carriers. Therefore, fundamental studies on hot carrier relaxation dynamics in lead halide perovskites are important. Here, we elucidated the hot carrier cooling dynamics in all-inorganic cesium lead iodide (CsPbI3) perovskite using transient absorption spectroscopy. We observe that the hot carrier cooling rate in CsPbI3 decreases as the fluence of the pump light increases and the cooling is as slow as a few 10 ps when the photoexcited carrier density is 7 × 1018 cm−3, which is attributed to phonon bottleneck for high photoexcited carrier densities. Our findings suggest that CsPbI3 has a potential for hot carrier solar cell applications.

Ultrafast Electron Injection from Photoexcited Perovskite CsPbI3 QDs into TiO2 Nanoparticles with Injection Efficiency near 99

Photoexcited electron injection dynamics from CsPbI3 quantum dots (QDs) to wide gap metal oxides are studied by transient absorption spectroscopy. Experimental results show under a low excitation intensity that ∼99% of the photoexcited electrons in CsPbI3 QDs can be injected into TiO2 with a size-dependent rate ranging from 1.30 × 1010 to 2.10 × 1010 s-1, which is also ∼2.5 times faster than that in the case of ZnO. A demonstration QD-sensitized solar cell based on a CsPbI3/TiO2 electrode is fabricated that delivers a power conversion efficiency of 5%.

Improved Reproducibility and Intercalation Control of Efficient Planar Inorganic Perovskite Solar Cells by Simple Alternate Vacuum Deposition of PbI2 and CsI

Vacuum deposition is a simple and controllable approach that aims to form higher-quality perovskite films compared with those formed using solution-based deposition processes. Herein, we demonstrate a novel method to promote the intercalation control of inorganic cesium lead iodide (CsPbI3) perovskite thin films via alternate vacuum deposition. We also investigated the effect of layer-by-layer deposition of PbI2/CsI to fabricate efficient planar heterojunction CsPbI3 thin films and solar cells. This procedure is comparatively simple when compared with commonly used coevaporation techniques; further, precise intercalation control of the CsPbI3 thin films can be achieved by increasing the number of layers in the layer-by-layer deposition of PbI2/CsI. The best control and the highest reproducibility were achieved for the deposition of four double layers owing to the precise intercalation control during the deposition of the CsPbI3 thin film. A power conversion efficiency of 6.79% was obtained via alternating vacuum deposition of two double layers with a short-circuit current density (Jsc) of 12.06 mA/cm2, an open-circuit voltage (Voc) of 0.79 V, and a fill factor (FF) of 0.72. Our results suggest a route for inorganic precursors to be used for efficient perovskite solar cells via alternating vacuum deposition.

Near IR sensitive Sn based perovskite solar cells with high current density reaching 30mA/cm 2

Enhancing Jsc of solar cells consisting of mixed metal perovskite, MASn_0.5Pb_0.5I₃ (SnPb PVK), is reported. The SnPb PVK was prepared from Pbt/SnL· mixture or SnI₂(DMSO)₂/PbI₂(DMSO)₂ mixture, where these PVK are abbreviated as SnPb-A PVK and SnPb-B PVK, respectively. Urbach energys sharpness factor (σ) for SnPb-B PVK was 1.5 which was higher than 0.5 for SnPb-A PVK, suggesting that the SnPb-B PVK has less crystal disordering than the SnPb-A PVK. The Jsc of the SnPb-B PVK cell was improved to 30 mA/cm², compared to 20 mA/cm² for SnPb-A PVK cell. The carrier dynamics is discussed by transient spectroscopy studies.