Ionic Influences on Recombination in Perovskite Solar Cells

The origins of recombination processes, particularly those that relate to IV hysteresis, are still unclear in perovskite solar cells. Of particular interest, is the impact different contact materials have on the level of hysteresis observed. This work shows that there is a clear link between ionic movement and interfacial recombination, which have both been shown to be responsible for hysteresis. By performing low temperature transient photovoltage (TPV) measurements over a period in which ions redistribute within the perovskite layer, the dominant recombination mechanism, responsible for hysteresis and other slow dynamic processes, is found to occur at the TiO2/perovskite interface. We observe an anomalous negative transient upon firing the laser pulse which we attribute to interfacial recombination at the TiO2/perovskite interface. The impact of recombination at the perovskite/HTL interface is shown to be negligible by performing TPV measurements using different laser wavelengths to probe different depths into the perovskite layer, as well as by changing the type of HTL used

Simultaneous Energy Harvesting and Hand Gesture Recognition in Large Area Monolithic Dye-Sensitized Solar Cells

Internet of Things (IoT) devices have become prevalent, embedding intelligence into our environment. It is projected that over 75 billion IoT devices will be connected by 2025 worldwide, with the majority being operated indoors. Dye-sensitized solar cells (DSSC) have recently been optimized for ambient light, having the capabilities of providing sufficient energy for self-powered IoT devices. Interaction with digital technologies, termed Human Computer Interaction (HCI), is often achieved via physical mechanisms (e.g. remote controls, cell phones) which can hinder the natural interface between users and IoT devices, a key consideration for HCI. What if the solar cell that is powering the IoT device can also recognize hand gestures which would allow the user to naturally interact with the system? Previous attempts to achieve this have necessarily employed an array of solar cell/photodiodes to detect directionality. In this work, we demonstrate that by monitoring the photocurrent output of an asymmetrically patterned monolithic (i.e., single cell) DSSC, and using machine learning, we can recognize simple hand gestures, achieving an accuracy prediction of 97.71%. This work shows that, DSSCs are the perfect choice for self-powered interactive technologies, both in terms of powering IoT devices in ambient light conditions and having aesthetic qualities that are prioritized by users. As well as powering interactive technologies, they can also provide a means of interactive control.Comment: Main body: 10 pages, 6 figures, 3 tables. Document includes supplementary info: 30 pages, 47 supplementary figure

A Comparison of Different Textured and Non-Textured Anti-Reflective Coatings for Planar Monolithic Silicon-Perovskite Tandem Solar Cells

Multijunction solar cells offer a route to exceed the Shockley–Queisser limit for single-junction devices. In a few short years, silicon-perovskite tandems have significantly passed the efficiency of the best silicon single-junction cells. For scalable solution processing of silicon-perovskite tandem devices, with the avoidance of vacuum processing steps, a flat silicon sub-cell is normally required. This results in a flat top surface that can lead to higher optical reflection losses than conformal deposition on textured silicon bottom cells. To overcome this, textured anti-reflective coatings (ARCs) can be used on top of the finished cell, with textured polydimethylsiloxane (PDMS), a promising candidate. In this work, we vary the texture geometry and film thickness of PDMS anti-reflective foils to understand the effect of these parameters on reflectance of the foil. The best film is selected, and anti-reflective performance is compared with two common planar ARCs─lithium fluoride (LiF) and magnesium fluoride (MgF2) showing considerable reduction in reflectance for a non-textured silicon-perovskite tandem cell. The application of a PDMS film is shown to give a 3–5% increase in integrated JSC in each sub-cell of a silicon-perovskite tandem structure

Beyond Impedance Spectroscopy of Perovskite Solar Cells: Insights from the Spectral Correlation of the Electrooptical Frequency Techniques

Small perturbation techniques have proven to be useful tools for the investigation of perovskite solar cells. A correct interpretation of the spectra given by impedance spectroscopy (IS), intensity-modulated photocurrent spectroscopy (IMPS), and intensity-modulated photovoltage spectroscopy (IMVS) is key for the understanding of device operation. The utilization of a correct equivalent circuit to extract real parameters is essential to make this good interpretation. In this work, we present an equivalent circuit, which is able to reproduce the general and the exotic behaviors found in impedance spectra. From the measurements, we demonstrate that the midfrequency features that may appear to depend on the active layer thickness, and we also prove the spectral correlation of the three techniques that has been suggested theoretically

A combined transient photovoltage and impedance spectroscopy approach for a comprehensive study of interlayer degradation in non-fullerene acceptor organic solar cells

Organic solar cells utilise thin interlayer materials between the active layer and metal electrodes to improve stability and performance. In this work, we combine transient photovoltage (TPV) and impedance spectroscopy (EIS) measurements to study how degradation affects both the active layer and the interlayer. We show that neither technique alone can provide a complete insight into both of these regions: TPV is more suited to studying degradation of the active layer; EIS clearly identifies the properties of the interlayer. By analysing both of these approaches we are able to assess how different interlayers impact the stability of the active layer, as well as how the interlayers themselves degrade and severely limit device performance. EIS measurements are also able to resolve the impact of the interlayer on series resistance even when it is not apparent from standard current–voltage (JV) measurements. The technique could therefore be valuable for the optimisation of all devices

Limited information of impedance spectroscopy about electronic diffusion transport: The case of perovskite solar cells

Impedance Spectroscopy (IS) has proven to be a powerful tool for the extraction of significant electronic parameters in a wide variety of electrochemical systems, such as solar cells or electrochemical cells. However, this has not been the case with perovskite solar cells, which have the particular ionic-electronic combined transport that complicates the interpretation of experimental results due to an overlapping of different phenomena with similar characteristic frequencies. Therefore, the diffusion of electrons is indistinguishable on IS, and there appears the need to use other small perturbation experimental techniques. Here, we show that voltage-modulated measurements do not provide the same information as light-modulated techniques. We investigate the responses of perovskite solar cells to IS, Intensity-Modulated Photocurrent Spectroscopy (IMPS) and Intensity-Modulated Photovoltage Spectroscopy (IMVS). We find that the perturbations by light instead of voltage can uncover the electronic transport from other phenomena, resulting in a loop in the high-frequency region of the complex planes of the IMPS and IMVS spectra. The calculated responses are endorsed by the experimental data that reproduce the expected high frequency loops. Finally, we discuss the requirement to use a combination of small perturbation techniques for successful estimation of diffusion parameters of perovskite solar cells

Mass Manufactured Glass Substrates Incorporating Prefabricated Electron Transport Layers for Perovskite Solar Cells

A commercially available glass substrate which incorporates both a fluorine‐doped tin oxide and compact TiO2 layer deposited through chemical vapor deposition that is commonly used in “solar control products,” is presented. The substrate, known commercially as Pilkington Eclipse Advantage, is designed for use as an infrared radiation control product and this is the first known instance of it being employed and extensively characterized for use as a mass manufactured n‐type contact in perovskite solar cells. Using this substrate with no additional compact TiO2 layer, perovskite solar cells with PCEs of up to 15.9% are achieved. These devices are superior in performance to those where the compact TiO2 is deposited via spray pyrolysis. The reproducibility and large scale manufacturing base already established with this substrate represents significant potential for solving the problem of upscaling a uniform and pinhole free n‐type compact TiO2 blocking layer

Origin of Exceptionally Slow Light Soaking Effect in Mesoporous Carbon Perovskite Solar Cells with AVA Additive

A range of slow dynamic processes occurring in perovskite solar cells have been linked to ionic migration, including J–V hysteresis and long photovoltage rise and decay times. This work demonstrates the remarkably slow response time of triple mesoporous carbon-based cells, containing the additive 5-aminovaleric acid iodide (AVA). The photovoltage rise under illumination is 1–2 orders of magnitude longer than has previously been observed for planar and mesoporous TiO2 based devices. Transient photovoltage measurements during this slow rise in voltage show a strong negative transient feature which demonstrates the presence of fast recombination. By analyzing the rate of Voc rise and the decay of this negative transient, we show a clear link between this recombination process and the limiting of the Voc. The reduction of recombination over time and the resultant rise in Voc are influenced by the movement of ions in the perovskite. From temperature-dependent measurements, an activation energy consistent with previous literature values for iodide ion migration is obtained, although the attempt frequency is found to be many orders of magnitude lower than that in pure MAPI perovskite devices. We attribute this to the presence of the AVA molecule inhibiting the movement of ions. The importance of the TiO2/ZrO2 interface in leading to this slow behavior is revealed by studying devices with different architectures with and without the AVA additive. A significant increase in response time can only be recreated in a device with both of the mesoporous metal oxide layers and the AVA additive present in the perovskite

Enhancing fully printable mesoscopic perovskite solar cell performance using integrated metallic grids to improve carbon electrode conductivity

Carbon based Perovskite Solar cells (C–PSCs) have emerged as the most promising candidates for commercialisation in the field of perovskite photovoltaics, as they are highly stable, low cost and make use of easily scaled manufacturing techniques. However, the limited conductivity of the carbon electrode inhibits performance and represents a significant barrier to commercial application. Τhis work presents a scalable method for enhancing the carbon electrode conductivity through the integration of aluminium and copper grids into prefabricated C–PSCs. Adhered to the cells using an additional low temperature carbon ink, the metallic grids were found to dramatically reduce top electrode series resistance, leading to a large improvement in fill factor and efficiency. After grid integration, the 1 cm2 C–PSCs yielded power conversion efficiency (PCE) of 13.4% and 13% for copper and aluminium respectively, while standard C–PSCs obtained PCE of 11.3%. Performance is also significantly augmented in the case of larger-scale 11.7 cm2 modules, where PCEs went from 7.7% to 10% and 11% for aluminium and copper grids respectively. This technique offers a fast and low temperature route to high-performance, large-area C–PSCs and could therefore have serious potential for application to the high-volume manufacture of perovskite cells and modules