Decreasing Charge Losses in Perovskite Solar Cells Through mp-TiO₂/MAPI Interface Engineering

On the basis of our experience in controlling the recombination kinetics in Dye Sensitized Solar Cells (DSSC) by modifying the mesoporous TiO₂ (mp-TiO₂) interface, we modified the methylammonium lead iodide (MAPI) /mp-TiO₂ interface with a nanoscopic layer of insulating Al₂O₃. The effects on device efficiency, the open-circuit voltage (<i>V</i>_oc), device reproducibility, and the relationship between the increase in <i>V</i>_oc, and the presence of the Al₂O₃ layer is thoroughly discussed and explained. Although in DSSC there is a TiO₂ conduction band edge shift for Al₂O₃ coated mp-TiO₂ films, in MAPI perovskite solar cells the charge vs voltage measurements carried out under sun-simulated irradiation conditions show a negligible shift of the exponential charge distribution whether it is measured using PICE (Photo Induced Charge Extraction) or PIDC (Photo Induced Differential Charging). Furthermore, the charge recombination lifetime decreases considerably in the Al₂O₃-treated samples, which improves the overall efficiency of the device because of the slower rate in the back-electron transfer reactions

The Redox Pair Chemical Environment Influence on the Recombination Loss in Dye-Sensitized Solar Cells

Reduction of recombination losses in dye-sensitized solar cells (DSC) is vital to fabricate efficient devices. The electron recombination lifetime depends on the relative energetics of the semiconductor and the redox pair and on the chemical nature of the electrolyte (hole conductor). In this work, the behavior of the electron lifetime in DSC devices prepared with various solvents (acetonitrile, valeronitrile, ethylene carbonate, pure ionic liquids), additives (lithium ions, TBP), and redox pairs (iodide/iodine, Co­(II)/Co­(III)) is thoroughly studied using high-extinction dyes. Lifetimes were extracted by means of small-perturbation electrochemical techniques (impedance spectroscopy, intensity-modulated photovoltage spectroscopy) and open-circuit voltage decays. To ensure a safe inner comparison and a proper interpretation, all devices were constructed using the same type of TiO₂ electrode and the same dyes (C101 and Z907 for iodide/iodine and cobalt-based electrolytes, respectively). Furthermore, small-perturbation techniques and voltage decay provided consistent results. The lifetime shows a clear change of behavior when iodide/iodine electrolytes in organic solvents are compared to iodide/iodine in ionic liquids and with cobalt electrolytes. In the first case, the lifetime–voltage semilogarithmic plot exhibits a curvature, whereas in the second case the behavior is purely exponential. This observation is consistent with previous theoretical predictions based on the multiple-trapping model and the Marcus–Gerischer theory, which predict an exponential law for large reorganization energies and a curvature for small ones. The obtained results show that solvents or ligands that interact strongly with the redox mediator originate larger reorganization energies and lead to devices with shorter lifetimes. This can be interpreted as an enhancement of extra routes for electron recombination as a consequence of a wider overlap in energies between donor and acceptor states for strongly interacting chemical environments

Selective Organic Contacts for Methyl Ammonium Lead Iodide (MAPI) Perovskite Solar Cells: Influence of Layer Thickness on Carriers Extraction and Carriers Lifetime

We have fabricated MAPI solar cells using as selective contacts PEDOT:PSS polymer for holes and PCBM-C70 fullerene derivative for electrons. The thickness of MAPI, PCBM-C70, and PEDOT:PSS layers has been varied in order to evaluate the contribution of each layer to the final device performance. We have measured the devices capacitance under illumination and the charge carrier’s lifetime using photoinduced time-resolved techniques. The results show that in this kind of devices the limiting layer is the PCBM-C70 due to its relative reduced mobility compared to PEDOT:PSS that makes the control of the fullerene thickness crucial for device optimization. Moreover, capacitive measurements show differences for the devices having different PCBM-C70 layer thicknesses in contrast with the measurements on the different PEDOT:PSS thickness. These give indications about holes and electrons storage and their distribution

Measurements of Efficiency Losses in Blend and Bilayer-Type Zinc Phthalocyanine/C₆₀ High-Vacuum-Processed Organic Solar Cells

Losses of charge carriers, due to the interfacial charge recombination processes, in small molecule organic solar cells (SMOSCs) have been investigated under operating conditions. The devices consist of zinc phthalocyanine (ZnPc) as electron donor material and C60 as electron acceptor. The results obtained by using time-resolved techniques such as charge extraction (CE) and photoinduced transient photovoltage (TPV) have been compared to the measurements carried out with impedance spectroscopy (IS) and show good agreement. Significantly, much difference is observed in either the charge density distribution versus the device voltage or the charge carriers lifetime when comparing bulk heterojunction versus bilayer-type ZnPc:C₆₀ devices. The implications of the faster charge carrier recombination with the device fill factor (FF) and the open circuit voltage (<i>V</i>_OC) are discussed

Advances in the Synthesis of Small Molecules as Hole Transport Materials for Lead Halide Perovskite Solar Cells

ConspectusOver hundreds of new organic semiconductor molecules have been synthesized as hole transport materials (HTMs) for perovskite solar cells. However, to date, the well-known <i>N</i>²,<i>N</i>²,<i>N</i>^2′,<i>N</i>^2′,<i>N</i>⁷,<i>N</i>⁷,<i>N</i>^7′, octakis-(4-methoxyphenyl)-9,9-spirobi-[9,9′-spirobi­[9<i>H</i>-fluorene]-2,2′,7,7′-tetramine (<b>spiro-OMeTAD</b>) is still the best choice for the best perovskite device performance. Nevertheless, there is a consensus that <b>spiro-OMeTAD</b> by itself is not stable enough for long-term stable devices, and its market price makes its use in large-scale production costly.Novel synthetic routes for new HTMs have to be sought that can be carried out in fewer synthetic steps and can be easily scaled up for commercial purposes. On the one hand, synthetic chemists have taken, as a first approach, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the <b>spiro-OMeTAD</b> molecule as a reference to synthesize molecules with similar energy levels, although these HOMO and LUMO energy levels often have been measured indirectly in solution using cyclic voltammetry. On the other hand, the “spiro” chemical core has also been studied as a structural motif for novel HTMs. However, only a few molecules incorporated as HTMs in complete functional perovskite solar cells have been capable of matching the performance of the best-performing perovskite solar cells made using <b>spiro-OMeTAD</b>.In this Account, we describe the advances in the synthesis of HTMs that have been tested in perovskite solar cells. The comparison of solar cell efficiencies is of course very challenging because the solar cell preparation conditions may differ from laboratory to laboratory. To extract valuable information about the HTM molecular structure–device function relationship, we describe those examples that always have used <b>spiro-OMeTAD</b> as a control device and have always used identical experimental conditions (e.g., the use of the same chemical dopant for the HTM or the lack of it).The pioneering work was focused on well-understood organic semiconductor moieties such as arylamine, carbazole, and thiophene. Those chemical structures have been largely employed and studied as HTMs, for instance, in organic light-emitting devices. Interestingly, most research groups have reported the hole mobility values for their novel HTMs. However, only a few examples have been found that have measured the HOMO and LUMO energy levels using advanced spectroscopic techniques to determine these reference energy values directly. Moreover, it has been shown that those molecules, upon interacting with the perovskite layer, often have different HOMO and LUMO energies than the values estimated indirectly using solution-based electrochemical methods.Last but not least, porphyrins and phthalocyanines have also been synthesized as potential HTMs for perovskite solar cells. Their optical and physical properties, such as high absorption and good energy transfer capabilities, open new possibilities for HTMs in perovskite solar cells

D‑π‑A Porphyrin Employing an Indoline Donor Group for High Efficiency Dye-Sensitized Solar Cells

Dye-sensitized solar cell (DSC) devices were fabricated using a novel donor-(π bridge)-acceptor (D-π-A) porphyrin sensitizer, <b>VC-70</b>, in which an indoline is linked directly to the porphyrin core and functions as the donor group. The best efficiencies of <b>VC-70</b> and reference <b>YD2-</b><i><b>o</b></i><b>-C8</b> devices were found to be 7.31 and 7.60%, respectively, and AMG 1.5 illumination and device properties were fully characterized using transient absorption, charge extraction, and transient photovoltage techniques. A notable effect on TiO₂ conduction band energetics and electron lifetime was observed following light soaking of <b>VC-70</b> devices under AMG 1.5 illumination. Upon cosensitization of <b>VC-70</b> with the organic dye <b>D205</b> an improved efficiency of 8.10% was obtained

Understanding the Effect of Donor Layer Thickness and a MoO₃ Hole Transport Layer on the Open-Circuit Voltage in Squaraine/C₆₀ Bilayer Solar Cells

Small molecule organic solar cells are becoming increasingly efficient through improved molecular design. However, there is still much to be understood regarding device operation. Here we study bilayer solar cells employing a 2,4-bis­[4-(<i>N,N</i>-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ) donor and fullerene acceptor to probe the effect of donor layer thickness and a MoO₃ electron transport layer on device performance. The thickness of SQ is seen to drastically affect the open-circuit voltage (<i>V</i>_OC) and fill factor (FF), while the short circuit current is not altered significantly. The fact that the <i>V</i>_OC of the bilayers with thin (6 nm) donor layers shows a strong dependence on the material and workfunction of the anode cannot be explained with a model for a perfect bilayer. Recombination of electrons from C₆₀ at the anode contact has to be possible to understand the strong effect of the anode workfunction. Using numerical simulations and a simple two-diode model we show that the most likely interpretation of the observed effects is that for thin SQ layers, the roughness of the interface is high enough to allow electrons in the C₆₀ to tunnel through the SQ to recombine directly at the anode. Thicker SQ layers will block most of these recombination pathways, which explains the drastic dependence of <i>V</i>_OC on thickness. Bulk-heterojunction devices were also fabricated to illustrate the effect of anode material on the <i>V</i>_OC

Improving CdSe Quantum Dot/Polymer Solar Cell Efficiency Through the Covalent Functionalization of Quantum Dots: Implications in the Device Recombination Kinetics

Novel quantum dot capping ligands based on fullerene derivatives were attached through click-chemistry to the surface of semiconductor CdSe nanocrystals (C₇₀–CdSe). Steady-state and time-correlated luminescence studies in solution show efficient quenching of the quantum dot (QD) emission in C₇₀–CdSe. When this material was blended with the polymer poly-3-hexyl thiophene (P3HT) to fabricate bulk-heterojunction solar cells, P3HT/C₇₀–CdSe devices doubled the light-to-energy conversion efficiency when compared to P3HT/Py–CdSe reference devices prepared using pyridine as the capping agent. This is due to an increase in both photocurrent and fill factor showing the beneficial efficient effect of fullerene to improve light harvesting and charge transport in these devices. However, C₇₀ also appears to increase recombination in these devices as evidenced by both transient absorption spectroscopy and transient photovoltage measurements. This work also discusses the effects on the CdSe functionalization with C₇₀ over the device charge recombination kinetics that limit the efficiency in CdSe QDs/polymer solar cells

Influence of the Molecular Weight and Size Dispersion of the Electroluminescent Polymer on the Performance of Air-Stable Hybrid Light-Emitting Diodes

The influence of the chain length and the molecular weight distribution of the electroluminescent polymer on the carrier transport properties and morphology of air stable hybrid light-emitting diodes is reported. It is found that variations between diverse as-received commercial batches play a major role in the performance of the devices, whose maximum luminance can differ up to 2 orders of magnitude. Through complementary optoelectronic, structural, and morphological characterization techniques, we provide insights into the relationship between charge dynamics and the structure of polymeric electroluminescent materials. The carrier dynamics are found to be dominated by both the polymeric chain length and the hole transport, which in turn is dependent on the concentration of trap states. Furthermore, the chain length is seen to affect the morphology of the active layer

Benzothiadiazole Substituted Semiconductor Molecules for Organic Solar Cells: The Effect of the Solvent Annealing Over the Thin Film Hole Mobility Values

We have synthesized and characterized two low molecular weight organic molecules, namely, <b>CS01</b> and <b>CS03</b> having the benzo­[<i>c</i>]­[1,2,5]­thiadiazole-4,7-diamino core but differing in the number of aromatic rings at the amino groups. The molecules, when processed to make thin organic films, display absorbance up to the near-IR region (∼750 nm) and good hole mobility values. Upon mixing each organic semiconductor molecule with the fullerene derivative PC₇₁BM, we monitored a strong quenching of the fluorescence emission. We assigned such a process to efficient charge transfer from the <b>CS01</b> and <b>CS03</b> molecules to the fullerenes. Moreover, fueled by this observation, we prepared organic solar cells and obtained, as a first attempt, efficiencies over 2% under 1 sun light simulated solar radiation. Furthermore, the film optimization through a careful solvent annealing process increased further the efficiencies up to 4.80% for <b>CS01</b> and 5.12% for <b>CS03</b>. The observed increase in efficiency is due to a better morphology obtained through solvent annealing of the thin films. However, an in-depth analysis reveals that the solvent annealing led to a better hole mobility, but the electron mobility remains similar