Polyelectrolyte interlayers with a broad processing window for high efficiency inverted organic solar cells towards mass production

Neutral polyelectrolyte interfacial layers in organic solar cells are well-known for their ability to tailor the work function of electrodes, improve charge carrier extraction and maximize open circuit voltage. However, they also suffer from low charge carrier conductivity, and therefore the interlayer must be kept thin, which in turn requires very precise deposition. This prerequisite significantly reduces the robustness of the fabrication process and makes such structures difficult to up-scale for roll-to-roll mass production. Herein, we find that by washing the polyelectrolyte layer with N,N-dimethylformamide (DMF) after deposition, solar cell efficiency jumps to near optimum levels, no matter what the original thickness of the polyelectrolyte layer. Subsequent characterization of the DMF-washed ZnO/PEI interlayer reveals a changed surface structure, passivated surface trap states, and thus improved transport properties and lower recombination losses. We demonstrate the general applicability of the method to other state-of-the-art material systems, namely P3HT:ICBA, PTB7:PC71BM and PTB7-Th:PC71BM. We find that the more efficient the material system, the larger the improvement in efficiency after DMF washing. Thus, this method represents a general way to relax the fabrication criteria for high efficiency organic solar cells. We anticipate that this method could be of use in other classes of devices such as OTFTs and OLEDs

Passivation agent with dipole moment for surface modification towards efficient and stable perovskite solar cells

Recently, there has been renewed interest in interface engineering as a means to further push the performance of perovskite solar cells closer to the Schockly-Queisser limit. Herein, for the first time we employ a multi-functional 4-chlorobenzoic acid to produce a self-assembled monolayer on a perovskite surface. With this interlayer we observe passivation of perovskite surface defects and a significant suppression of non-radiative charge recombination. Furthermore, at the surface of the interlayer we observe, charge dipoles which tune the energy level alignment, enabling a larger energetic driving force for hole extraction. The perovskite surface becomes more hydrophilic due to the presence of the interlayer. Consequently, we observe an improvement in open-circuit voltage from 1.08 to 1.16 V, a power conversion efficiency improvement from 18% to 21% and an improved stability under ambient conditions. Our work highlights the potential of SAMs to engineer the photo-electronic properties and stability of perovskite interfaces to achieve high-performance light harvesting devices

Linking vertical bulk-heterojunction composition and transient photocurrent dynamics in organic solar cells with solution-processed MoOx contact layers

It is demonstrated that a combination of microsecond transient photocurrent measurements and fi lm morphology characterization can be used to identify a charge-carrier blocking layer within polymer:fullerene bulk-heterojunction solar cells. Solution-processed molybdenum oxide (s-MoO x ) interlayers are used to control the morphology of the bulk-heterojunction. By selecting either a low- or high-temperature annealing (70 C or 150 C) for the s-MoO x layer, a well-performing device is fabricated with an ideally interconnected, high-efficiency morphology, or a device is fabricated in which the fullerene phase segregates near the hole extracting contact preventing efficient charge extraction. By probing the photocurrent dynamics of these two contrasting model systems as a function of excitation voltage and light intensity, the optoelectronic responses of the solar cells are correlated with the vertical phase composition of the polymer:fullerene active layer, which is known from dynamic secondary-ion mass spectroscopy (DSIMS). Numerical simulations are used to verify and understand the experimental results. The result is a method to detect poor morphologies in operating organic solar cells

Combining plasmonic trap filling and optical backscattering for highly efficient third generation solar cells

© The Royal Society of Chemistry. Metal oxide contact layers such as ZnO and TiOx are commonly used in third generation solar cells as they can be solution processed and have a relatively high conductivity. It is well known that by ultraviolet (UV) light-soaking such devices, their overall device efficiency can be boosted. This improvement in efficiency is due to high energy UV light exciting hot carriers which then fill the trap states in the metal oxide film. Unfortunately, UV causes degradation of the active layer and thus must be filtered out if long lifetimes are to be achieved. In this work, we use plasmonically excited metal nano-structures embedded in a ZnO metal oxide layer to generate hot charge carriers from visible light alone, thus removing the need for UV light soaking. Using this approach, the solar cells also exhibit better charge transport/recombination properties as well as enhanced light trapping behavior. We demonstrate that the power conversion efficiency of a low-bandgap thieno[3,4-b]thiophene/benzodithiophene (PTB7) based solar cell can be increased from 7.91% to 9.36%

Beyond the 2D Field‐Effect Charge Transport Paradigm in Molecular Thin‐Film Transistors

Organic field-effect transistors (OFETs) are considered almost purely interfacial devices with charge current mainly confined in the first two semiconducting layers in contact with the dielectric with no active role of the film thickness exceeding six to eight monolayers (MLs). By a combined electronic, morphological, structural, and theoretical investigation, it is demonstrated that the charge mobility and source–drain current in 2,20-(2,20-bithiophene-5,50-diyl)bis(5-butyl-5H-thieno[2,3-c]pyrrole-4,6)-dione (NT4N) organic transistors directly correlate with the out-of-plane domain size and crystallite orientation in the vertical direction, well beyond the dielectric interfacial layers. Polycrystalline films with thickness as high as 75 nm (≈30 MLs) and 3D molecular architecture provide the best electrical and optoelectronic OFET characteristics, highlighting that the molecular orientational order in the bulk of the film is the key-enabling factor for optimum device performance. X-ray scattering analysis and multiscale simulations reveal the functional correlation between the thickness-dependent molecular packing, electron mobility, and vertical charge distribution. These results call for a broader view of the fundamental mechanisms that govern field-effect charge transport in OFETs beyond the interfacial 2D paradigm and demonstrate the unexpected role of the out-of-plane domain size and crystallite orientation in polycrystalline films to achieve optimum electronic and optoelectronic properties in organic transistors

In Situ Visualization and Quantification of Electrical Self‐Heating in Conjugated Polymer Diodes Using Raman Spectroscopy

Self-heating in organic electronics can lead to anomalous electrical performance and even accelerated degradation. However, in the case of disordered organic semiconductors, self-heating effects are difficult to quantify using electrical techniques alone due to complex transport properties. Therefore, more direct methods are needed to monitor the impact of self-heating on device performance. Here, self-heating in poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b′] dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) diodes is visualized using Raman spectroscopy, and thermal effects due to self-heating are quantified by exploiting temperature-dependent shifts in the polymer vibrational modes. The temperature increases due to self-heating are quantified by correlating the Raman shifts observed in electrically biased diodes with temperature-dependent Raman measurements. Temperature elevations up to 75 K are demonstrated in the PCPDTBT diodes at moderate power of about 2.6–3.3 W cm−2. Numerical modeling rationalizes the significant role of Joule and recombination heating on the diode current–voltage characteristics. This work demonstrates a facile approach for in situ monitoring of self-heating in organic semiconductors for a range of applications, from fundamental transport studies to thermal management in devices

Investigation of the s-shape caused by the hole selective layer in bulk heterojunction solar cells

A common failure mechanism of organic solar cells is the development of an s-shaped current voltage curve. Herein, we investigated the origin of this degradation mechanism by replacing the commonly used hole selective layer poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) with a co-evaporated layer of N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)benzidine (TPD) and Dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HATCN). By varying the ratio of TPD to HATCN we are able to tune both the mobility and work function of the hole selective layer. Using a combination of field effect mobility measurements, Kelvin-Probe work function measurements, and numerical modeling we demonstrate that a degraded mobility of the hole selective layer leads to a buildup of charge within the device and reduction of fill factor

Using Ligand Engineering to Produce Efficient and Stable Pb–Sn Perovskite Solar Cells with Antioxidative 2D Capping Layers

Pb–Sn binary halide perovskites are a promising photovoltaic material due to their low toxicity and optical absorption spectrum well matched to the solar spectrum. However, the ready oxidation of Sn2+ to Sn4+ makes the material system currently too unstable to commercialize. Herein, ligand engineering based on antioxidative tyramine (hydrochloride, TACl) is presented for the first time to increase the stability of this material system. Using this strategy, we generate a two-dimensional (2D) capping layer on top of a standard three-dimensional Pb–Sn film. After capping, the surface defects can be passivated and the TACl-based 2D perovskite effectively protected Sn2+ from oxidation, which stabilized the Sn–Pb perovskite composition, avoiding the Pb-based perovskite formation. It is further found that the TACl treatment suppressed the halide segregation and improved the perovskite film photostability. Cell efficiency increases from 16.25 to 18.28% and device lifetime (T80) increases from less than 100 to over 1000 h. Our finding suggests that tuning ligand form/function represents a potentially highly productive direction to explore when trying to produce stable tin-based perovskite devices

Differing Impacts of Blended Fullerene Acceptors on the Performance of Ternary Organic Solar Cells

Organic photovoltaic (OPV) devices offer the ability to tune the electronic and optical properties of the active layer by selection of a wide range of molecules; however, their power conversion efficiencies currently lag other competing photovoltaic technologies. One method to enhance their performance is to add a further active material into the absorber layer, resulting in a ternary OPV. However, selecting appropriate ternary blend components to yield an improvement in performance is challenging due to the multitude of materials properties and physical processes that ternary blends can display. Here, we perform a systematic set of experiments on OPV ternary blends incorporating either of the donor polymers P3HT or PTB7 and the fullerene acceptors PCBM and ICBA. Some combinations of ternary blends are shown to outperform the reference binaries in terms of open-circuit voltage or short-circuit current; however, this was not observed for all combinations. Improvements in internal quantum efficiency of the order of 25% were observed for PTB7-based ternaries compared to the reference binary, which is attributed to a reduction in charge recombination. All blends showed some improvement in open-circuit voltage with addition of ICBA due to alloying of the fullerene components, but to differing degrees which is argued to be due to molecular morphology. These findings demonstrate that the benefits one can obtain using a ternary OPV approach vary depending on the materials system to an extent that depends upon the ternary blend morphology

Decarbonising electrical grids using photovoltaics with enhanced capacity factors

Many scenarios for Net Zero anticipate substantial growth of Solar PV generation to satisfy 30% of our electricity needs. However, this scale of deployment introduces challenges as supply may not meet demand, thereby necessitating energy storage and demand-side management. Here we demonstrate a different, complementary approach to resolving this challenge in which Solar PV generation can be made intrinsically less variable than commercial PV. Proof-of-concept dye-sensetised PVs for which the power conversion efficiency increases as light intensity reduces are demonstrated. Modelling of the UK mainland energy network predicts that these devices are more effective at displacing high carbon generation from coal and gas than commercial PV. The capacity factor of these PV devices are controlled by their design, and capacity factors >60% greater than silicon are predicted based on experimental data. These data demonstrate a new approach to designing PV devices in which minimising variability in generation is the goal. This new design target can be realised in a range of emerging technologies, including Perovskite PV and Organic PV, and is predicted to be more effective at delivering carbon reductions for a given energy network than commercial PV