Open-Circuit Voltage (VOC) Enhancement in TiO2-Based DSSCs: Incorporation of ZnO Nanoflowers and Au Nanoparticles

An important reason for the relatively low efficiency of dye-sensitized solar cells (DSSCs) is the low open-circuit voltage (VOC) of about 0.7 V for a standard solar cell with a dye that has an absorption onset at 1.6 eV. We report an enhancement of the VOC of about 0.10 V with respect to a TiO2-based DSSC modified with ZnO nanoflowers that we prepared by a new and facile method. An additional increase of the VOC of about 0.08 V was achieved by modifying the ZnO nanoflowers with Au nanoparticles, resulting in a DSSC with an efficiency of 2.79%, highlighted by a high VOC of 0.89 V. Detailed analysis with electrochemical impedance spectroscopy and intensity-modulated photovoltage and photocurrent spectroscopies (IMVS and IMPS) reveal that the main reason for the increase of VOC is related to the shift of the band edges upon coupling TiO2 with ZnO nanoflowers, even though the electron lifetime at the same charge density actually decreases. These results show the intricate interplay between band edge shift, recombination kinetics, and DSSC performance and illustrate that a higher voltage DSSC can be fabricated by modification of the photoanode materials

High throughput fabrication of mesoporous carbon perovskite solar cells

The screen printed mesoporous carbon perovskite solar cell has great potential for commercialisation due to its scalable deposition processes and use of inexpensive materials. However, each layer requires long high temperature heating steps to achieve the necessary sintering and porosity, which is very time and energy intensive for large scale production. Near infrared processing is demonstrated here to reduce the heating time of mesoporous layers within a fully printed lead halide perovskite solar cell from 2 hours to 30 seconds. A stabilised efficiency of 11% was achieved by processing in 30 seconds, identical to that of devices heated in 2 hours. For the first time the effect of residual binder in the carbon electrode on the electron lifetime and charge transfer within devices has been investigated. Furthermore cross section EDX mapping of perovskite infiltration provides a greater understanding into the processing requirements of these devices vital to enable commercialisation

Characterization of Photochromic Dye Solar Cells Using Small-Signal Perturbation Techniques

Photochromic dye-sensitized solar cells (DSSCs) are novel semi-transparent photovoltaic devices that self-adjust their optical properties to the irradiation conditions, a feature that makes them especially suitable for building integrated photovoltaics. These novel solar cells have already achieved efficiencies above 4%, and there are multiple pathways to improve the performance. In this work, we conduct a full characterization of DSSCs with the photochromic dye NPI, combining electrical impedance spectroscopy (EIS) and intensity-modulated photocurrent spectroscopy (IMPS). We argue that the inherent properties of the photochromic dye, which result in a modification of the functioning of the solar cell by the optical excitation that also acts as a probe, pose unique challenges to the interpretation of the results using conventional models. Absorption of light in the visible range significantly increases when the NPI dye is in the activated state; however, the recombination rate also increases, thus limiting the efficiency. We identify and quantify the mechanism of enhanced recombination when the photochromic dye is activated using a combination of EIS and IMPS. From the comparison to a state-of-the-art reference dye (RK1), we were able to detect a new feature in the IMPS spectrum that is associated with the optical activation of the photochromic dye, providing a useful tool for assessing the electronic behavior of the device under different conditions of light excitation. This study provides guidelines to adequate characterization protocols of photochromic solar cells and essential insights on the interfacial electronic processes.Universidad Pablo de Olavide / CEA Grenobl

Homogeneous and highly controlled deposition of low viscosity inks and application on fully printable perovskite solar cells

The fully printed, hole-transporter-free carbon perovskite solar cell structure incorporating a triple mesoscopic layer has emerged as a possible frontrunner for early industrialisation. It is an attractive structure because it can be fabricated by the simple sequential screen printing and sintering of titania, zirconia, and carbon. The device is finalised by manual dropping of a perovskite precursor solution onto the carbon which subsequently infiltrates. This stage in device fabrication is inhomogeneous, ineffective for large areas, and prone to human error. Here we introduce an automated deposition and infiltration system using a robotic dispenser and mesh which delivers the perovskite precursor uniformly to the carbon surface over a large area. It has been successfully used to prepare perovskite solar cells with over 9% efficiency. Cells, prepared by this robotic mesh deposition, showed comparable performance to reference cells, made by standard drop deposition, confirming this approach to be effective and reliable. X-ray diffraction and Raman spectroscopy were used to confirm the uniformity of the deposition over a large area

Influence of Redox Couple on the Performance of ZnO Dye Solar Cells and Minimodules with Benzothiadiazole-Based Photosensitizers

ZnO-based dye-sensitized solar cells exhibit lower efficiencies than TiO2-based systems despite advantageous charge transport dynamics and versatility in terms of synthesis methods, which can be primarily ascribed to compatibility issues of ZnO with the dyes and the redox couples originally optimized for TiO2. We evaluate the performance of solar cells based on ZnO nanomaterial prepared by microwave-assisted solvothermal syn- thesis, using three fully organic benzothiadiazole-based dyes YKP- 88, YKP-137, and MG-207, and alternative electrolyte solutions with the I−/I3−, Co(bpy)32+/3+, and Cu(dmp)21+/2+ redox couples. The best cell performance is achieved for the dye−redox couple combination YKP-88 and Co(bpy)32+/3+, reaching an average −− efficiency of 4.7% and 5.0% for the best cell, compared to 3.7% and 3.9% for the I /I3 couple with the same dye. Electrical impedance spectroscopy highlights the influence of dye and redox couple chemistry on the balance of recombination and regeneration kinetics. Combined with the effects of the interaction of the redox couple with the ZnO surface, these aspects are shown to determine the solar cell performance. Minimodules based on the best systems in both parallel and series configurations reach 1.5% efficiency for an area of 23.8 cm2.Área de Química Físic

Inverted Hysteresis in n-i-p and p-i-n Perovskite Solar Cells

A combination of experimental studies and drift-diffusion modeling has been used to investigate the appearance of inverted hysteresis, where the area under the J-V curve for the reverse scan is lower than in the forward scan, in perovskite solar cells. It is found that solar cells in the p-i-n configuration show inverted hysteresis at a sufficiently high scan rate, whereas n-i-p solar cells tend to have normal hysteresis. By examining the influence of the composition of charge transport layers, the perovskite film crystallinity and the preconditioning treatment, the possible causes of the presence of normal and inverted hysteresis are identified. Simulated current-voltage measurements from a coupled electron-hole-ion driftdiffusion model that replicate the experimental hysteresis trends are presented. It is shown that during current-voltage scans, the accumulation and depletion of ionic charge at the interfaces modifies carrier transport within the perovskite layer and alters the injection and recombination of carriers at the interfaces. Additionally, it is shown that the scan rate dependence of the degree of hysteresis has a universal shape, where the crossover scan rate between normal and inverted hysteresis depends on the ion diffusion coefficient and the nature of the transport layers.Universidad Pablo Olavide. Departamento de Sistemas Físicos, Químicos y Naturale

What difference does a thiophene make? Evaluation of a 4,4′-bis(thiophene) functionalised 2,2′-bipyridyl copper(I) complex in a dye-sensitized solar cell

AbstractThe synthesis of a 4,4′-bis(2-thienyl-5-carboxylic acid) functionalised 2,2′-bipyridine ligand and corresponding copper(I) complex is described and its application in a dye-sensitized solar cell (DSSC) is studied. The positioning of the thiophene groups appears favourable from DFT analysis and a best efficiency of 1.41% was obtained with this dye, for a 0.3 cm2 cell area DSSC. Two absorbance bands are observed in the electronic absorption spectrum of the copper(I) complex at 316 nm and 506 nm, with ε values of 50,000 M−1 cm−1 and 9030 M−1 cm−1, respectively. Cyclic voltammetry and electrochemical impedance spectroscopy are also used to provide a detailed analysis of the dye and assess its functionality in a DSSC