Improved Understanding of the Electronic and Energetic Landscapes of Perovskite Solar Cells: High Local Charge Carrier Mobility, Reduced Recombination, and Extremely Shallow Traps

The intriguing photoactive features of organic–inorganic hybrid perovskites have enabled the preparation of a new class of highly efficient solar cells. However, the fundamental properties, upon which the performance of these devices is based, are currently under-explored, making their elucidation a vital issue. Herein, we have investigated the local mobility, recombination, and energetic landscape of charge carriers in a prototype CH₃NH₃PbI₃ perovskite (PVK) using a laser-flash time-resolved microwave conductivity (TRMC) technique. PVK was prepared on mesoporous TiO₂ and Al₂O₃ by one or two-step sequential deposition. PVK on mesoporous TiO₂ exhibited a charge carrier mobility of 20 cm² V^–1 s^–1, which was predominantly attributed to holes. PVK on mesoporous Al₂O₃, on the other hand, exhibited a 50% lower mobility, which was resolved into balanced contributions from both holes and electrons. A general correlation between crystal size and mobility was revealed irrespective of the fabrication process and underlying layer. Modulating the microwave frequency from 9 toward 23 GHz allowed us to determine the intrinsic mobilities of each PVK sample (60–75 cm² V^–1 s^–1), which were mostly independent of the mesoporous scaffold. Kinetic and frequency analysis of the transient complex conductivity strongly support the superiority of the perovskite, based on a significant suppression of charge recombination, an extremely shallow trap depth (10 meV), and a low concentration of these trapped states (less than 10%). The transport mechanism was further investigated by examining the temperature dependence of the TRMC maxima. Our study provides a basis for understanding perovskite solar cell operation, while highlighting the importance of the mesoporous layer and the perovskite fabrication process

Dependences of the Optical Absorption, Ground State Energy Level, and Interfacial Electron Transfer Dynamics on the Size of CdSe Quantum Dots Adsorbed on the (001), (110), and (111) Surfaces of Single Crystal Rutile TiO₂

Quantum dots (QDs) provide an attractive alternative sensitizer to organic dyes. However, there have been few reports on QD-sensitized solar cells (QDSCs) that have photovoltaic conversion efficiencies exceeding those of dye-sensitized solar cells. This is because of the lack of fundamental studies of QDs on conventional nanocrystalline metal oxide electrodes which possess much amount of heterogeneity. An important first step is an investigation of the dependences of the optical absorption, the ground state energy level, and the interfacial electron transfer (IET) on the size of QDs deposited on well characterized single crystal oxides. The present study focuses on a system of CdSe QDs adsorbed on the (001), (110), and (111) surfaces of single crystal rutile-TiO₂. The optical absorption spectra, characterized using photoacoustic spectroscopy, were found to be independent of the surface orientation concerning the optical absorption edge. The exponential optical absorption tail (Urbach tail) suggests that the disorder decreases with the increasing size of the QDs and is independent of the surface orientation. The ground state energy levels of the QDs were characterized using photoelectron yield spectroscopy. That on the (001) surface shifts upward, while that on the (110) surface shifts downward with increasing QD size. That on the (111) surface is independent of the QD size, indicating the difference of the influence of the surface orientation on adsorption of the QDs. The IET rate constant and the relaxation component were characterized. The IET rate constant was found to decrease as the size of the QDs increases and depends on the surface orientation, indicating the differences in the decrease of the free energy change and lower coupling between the excited state of CdSe QDs and the Ti 3d orbitals in rutile-TiO₂. The relaxation component increases with increasing QD size and depends on the surface orientation, correlating with the density of states in the conduction band of rutile-TiO₂

Interplay of Orientation and Blending: Synergistic Enhancement of Field Effect Mobility in Thiophene-Based Conjugated Polymers

Trade-off between mechanical flexibility due to amorphicity and highly facile charge transport emanating from molecular crystallinity demands the orientation of conjugated polymers (CPs) for their utilization as active semiconducting material for flexible organic electronics. We have already demonstrated that it is rather easy to orient nonregiocontrolled poly­(3-hexylthiophenes) (NR-P3HT) as compared to their highly regioregular counterparts due to very high alkyl chain interdigitation. To provide an amicable solution, efforts have been directed to orient blends of two CPs such as NR-P3HT (amorphous and flexible) and poly­(2,5-bis­(3-tetradecylthiophen-2-yl)­thieno­[3,2-<i>b</i>]­thiophene) (PBTTT) (crystalline and facile charge transport) using a solution-based procedure floating film and transfer method (FTM). FTM-processed thin films of this blend system exhibited very high field effect transistor (FET) mobility reaching up to 0.1 cm²/V s, which is much higher than the corresponding individual CPs. In spite of only 10% incorporation of PBTTT in blend of NR-P3HT and PBTTT, there was a synergistically enhanced optical dichroic ratio (4.6 to 7.2) and FET mobility (8-fold) as compared to pristine NR-P3HT. At the same time, there was a 5-fold enhancement of FET mobility when 20% NR-P3HT was added in PBTTT as compared to that of PBTTT. This synergistic enhancement of charge carrier transport in the blend system has been explained by formation of oriented self-assembled fibrous domains of NR-P3HT and facile interdomain transport in crystalline PBTTT

Crystal Growth, Exponential Optical Absorption Edge, and Ground State Energy Level of PbS Quantum Dots Adsorbed on the (001), (110), and (111) Surfaces of Rutile-TiO₂

It is important to investigate the dependencies of the optical absorption and the ground state energy level on the size of the semiconductor quantum dots (QDs) on fully studied single crystal TiO₂ surfaces. The present study focuses on the systems comprising PbS QDs on (001), (110), and (111) surfaces of single crystal rutile-TiO₂. By the optical absorption characterization, the average diameter of PbS QDs on a (001) surface is independent of the number of adsorption cycles, although those on (110) and (111) surfaces increase with the number of cycles. The rate of adsorption of PbS QDs on a (001) surface is higher than those grown on (110) and (111) surfaces. The results suggest that the crystal growth is caused by the difference of the surface energy of the substrate. The exponential optical absorption edge suggests that the structural disorder of PbS QDs on (001) and (110) surfaces increases as the number of adsorption cycles increases. On the other hand, that on a (111) surface decreases as the number of adsorption cycles increases. The ground state energy level of the PbS QDs is independent of the surface orientation of the single crystal rutile-TiO₂, but shows negative polarization with the increase of adsorption cycles. It is owing to the possibility of the increase of color centers (electron capture by S^– vacancies) in PbS QDs, corresponding to the increase of structural disorder

Study To Observe the Effect of PbI₂ Passivation on Carbon Electrode for Perovskite Solar Cells by Quartz Crystal Microbalance System

A perovskite solar cell (PSC) utilizing a carbon electrode is a potential candidate for industrially viable, low-cost and highly stable photovoltaics. Therefore, it is important to understand the interface between perovskite layer and carbon electrode to achieve the improved performance of PSCs. We demonstrate an improvised two-step perovskite MAPbI₃ (methylammonium lead iodide) deposition method, involving a pretreatment of PbI₂ on the porous structure of TiO₂/ZrO₂/Carbon, which led to the difference in performance. A PbI₂ passivation layer at the interface between carbon electrode and perovskite resulted in the improved power conversion efficiency (PCE) of 7.30% from 2.21% compared to a one-step perovskite deposition with no pretreatment of PbI₂. This study further explores that an enhanced PCE of 6.55% can be achieved with one-step fabrication while keeping the same perovskite. A fascinating methodology, utilizing quartz crystal microbalance (QCM), which involves the adsorption of PbI₂ on the carbon surface, was employed to unravel this difference. QCM monitored adsorbed mass in real time and revealed that the mass of PbI₂ on carbon layer increased with the increase in concentration of PbI₂ in dimethylformamide (DMF). It was noticed that PbI₂ was still adsorbed on the carbon surface even after rinsing with DMF, suggesting strong bonding of PbI₂ with carbon. PbI₂ presence after rinsing was also verified by X-ray photoelectron spectroscopy (XPS), which indicates that part of the Pb–I reacts with the −OH on the carbon surface forming C–O–Pb linkages. Our study demonstrates that a carbon electrode passivated with PbI₂ could reduce carrier recombination and improve performance of PSCs

Effects of Temperature on Electrochemical Properties of Bismuth Oxide/Manganese Oxide Pseudocapacitor

In this study, a temperature investigation is conducted on a bismuth oxide/manganese oxide (Bi₂O₃/MnO₂) supercapacitor to determine how temperature affects the performances of the supercapacitor. Energy and power densities of 9.5 Wh kg^–1 and 102.6 W kg^–1 are obtained at 60 °C, respectively, which are approximately twice the values for supercapacitors at 0 °C and 1.37-fold higher than those at 30 °C. Additionally, the supercapacitors achieve energy densities of 4.9 and 6.9 Wh kg^–1, and power densities of 53.8 and 74.8 W kg^–1 at 0 and 30 °C, respectively. Interestingly, the hybrid Bi₂O₃/MnO₂ active materials exhibit superior stability and reversibility, retaining 95% of the original capacitance at 30 °C and >75% at the high temperature of 60 °C. Although the cooler supercapacitor exhibits a slightly higher resistive performance, its excellent capacitance retention upon continuous charging/discharging measurement at 0 °C shows its potential for use as an all-weather compatible supercapacitor in the automotive sector

Real-Time Photodynamics of Squaraine-Based Dye-Sensitized Solar Cells with Iodide and Cobalt Electrolytes

A series of dye-sensitized solar cells (DSSCs) has been prepared by using indole-based or quinoline-based squaraines (SQs) as the sensitizer and containing the commonly used I₃^–/I^– redox pair or the lately employed cobalt complexes, [Co­(dimethylbipyridine)₃]^3+/2+, [Co­(bipyridine)₃]^3+/2+, and [Co­(phenanthroline)₃]^3+/2+ redox electrolytes. The photodynamics of the different electron transfer reactions have been investigated by means of the femto- to millisecond pump–probe techniques. In the femtosecond transient absorption experiments, the electron injection rate constants and efficiencies, <i>k</i>_ei and φ_ei, were determined for each cell. Larger values of <i>k</i>_ei and φ_ei for the indole-based (SQ 8) compared to the quinoline-based (SQ 12) squaraines were obtained (13.2 × 10¹⁰ s^–1 and 0.95 × 10¹⁰ vs 6.9 × 10¹⁰ s^–1 and 0.81 for SQ 8 or SQ 12 with the I₃^–/I^– pair, respectively), despite the similar values of the electron injection driving forces (−Δ<i>G</i>⁰_ei = 0.75 vs 0.76 eV). This is due to the lower electron density in the lowest unoccupied molecular orbital at the anchoring group (−COOH) in SQ 12 compared to SQ 8. However, the type of electrolyte did not affect the kinetics of the electron injection processes. In the flash photolysis experiments, the kinetic parameters of the electron recombination via dye or electrolyte and the cation regeneration were calculated from the decays of the transient absorption signals of the electrons (1550 nm) or the SQ cation (570 nm). It was found that the electron recombination with the oxidized redox species is faster with the Co-based compared to the I₃^–/I^– electrolytes for both SQs, τ_rec = 3 versus ∼0.5–1 ms. This proves that the steric hindrance in these SQs is not sufficient to avoid the approach of the Co³⁺ species to the surface of the TiO₂ nanoparticle. Moreover, the regeneration rate constants and efficiencies, <i>k</i>_reg and φ_reg, are considerably smaller for the cells with the different Co-based electrolytes compared to those with the I₃^–/I^– pair (i.e., <i>k</i>_reg = 30 × 10⁴ vs 8 × 10⁴ M^–1s^–1 and φ_reg = 0.96 vs 0.75 with the [Co­(dmb)₃]^3+/2+ for SQ 8). This is explained by the lower regeneration driving force, −Δ<i>G</i>_reg, in the Co-based electrolytes (0.3–0.1 eV). Thus, the use of Co-based electrolytes in these two SQs is detrimental to the overall efficiency of the cell, since −Δ<i>G</i>_reg values below 0.4 eV do not give complete regeneration efficiency. Finally, we have compared the measured photocurrent with the calculated electron injection and regeneration efficiencies, and we found a good correlation between both parameters

Cesium Lead Halide Inorganic-Based Perovskite-Sensitized Solar Cell for Photo-Supercapacitor Application under High Humidity Condition

In shaping a clean and green energy environment, the installation of a self-rechargeable supercapacitor in an electric vehicle has the goal of decreasing the emission of unwanted gases, which can be realized by adopting a perovskite solar cell for self-charging the supercapacitor. In this work, a CsPbBr_2.9I_0.1 perovskite-sensitized solar cell is integrated for the first time with an asymmetrical supercapacitor for a photo-supercapacitor application. Prior to this integration, the performances of the perovskite-sensitized solar cell and supercapacitor are individually examined. The perovskite-sensitized solar cell displays a good efficiency, with the ability to retain 70% of its efficiency after a week of storage in a dark humidity-controlled desiccator and 33% of its efficiency under UV and air exposure at a high relative humidity of more than 80% for 24 h. The asymmetrical supercapacitor exhibits a high areal capacitance of 150 mF cm^–2 with a capacitance loss of only 4% after continuous cyclic performances, which shows its potential for the photo-supercapacitor application. The photo-supercapacitor device is sensitive to light, with the photovoltage and photocurrent plunging to zero in the absence of light, and provides an areal capacitance of 30 mF cm^–2. It thus unlocks opportunities for photo-supercapacitor applications in line with green energy development

Magnesium-Doped MAPbI₃ Perovskite Layers for Enhanced Photovoltaic Performance in Humid Air Atmosphere

Despite the high efficiency of MAPbI₃ perovskite solar cells, the long term stability and degradation in humid atmosphere are issues that still needed to be addressed. In this work, magnesium iodide (MgI₂) was first successfully used as a dopant into MAPbI₃ perovskite prepared in humid air atmosphere. Mg doping decreased the valence band level, which was determined from photoelectron yield spectroscopy. Compared to the pristine MAPbI₃ perovskite film, the 1.0% Mg-doped perovskite film showed increased crystal grain size and formation of pinhole-free perovskite film. Performance of the solar cell was increased from 14.2% of the doping-free solar cell to 17.8% of 1.0% Mg-doped device. Moreover, 90% of the original power conversion efficiency was still retained after storage in 30–40% relative humidity for 600 h

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 (<i>J</i>_sc) of 12.06 mA/cm², an open-circuit voltage (<i>V</i>_oc) 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