Suppressing the Surface Recombination and Tuning the Open-Circuit Voltage of Polymer/Fullerene Solar Cells by Implementing an Aggregative Ternary Compound

Stretchable Photodiodes with Polymer-Engineered Semiconductor Nanowires for Wearable Photoplethysmography

Healthcare systems worldwide have been stressed to provide sufficient resources to serve the increasing and aging population in our society. The situation became more challenging at the time of pandemic. Technology advancement, especially the adoption of wearable health monitoring devices, has provided an important supplement to current clinical equipment. Most health monitoring devices are rigid, however, human tissues are soft. Such a difference has prohibited intimate contact between the two and jeopardized wearing comfortableness, which hurdles measurement accuracy especially during longtime usage. Here, we report a soft and stretchable photodiode that can conformally adhere onto the human body without any pressure and measure cardiovascular variables for an extended period with higher reliability than commercial devices. The photodiode used a composite light absorber consisting of an organic bulk heterojunction embedded into an elastic polymer matrix. It is discovered that the elastic polymer matrix not only improves the morphology of the bulk heterojunction for obtaining the desired mechanical properties but also alters its electronic band structure and improves the electrical properties that lead to a reduced dark current and enhanced photovoltage in the stretchable photodiode. The work has demonstrated high fidelity measurements and longtime monitoring of heat rate variability and oxygen saturation, potentially enabling next-generation wearable photoplethysmography devices for point-of-care diagnosis of cardiovascular diseases in a more accessible and affordable way.

Ternary Blend Organic Solar Cells: Understanding the Morphology from Recent Progress

Ternary blend organic solar cells (TB-OSCs) incorporating multiple donor and/or acceptor materials into the active layer have emerged as a promising strategy to simultaneously improve the overall device parameters for realizing higher performances than binary devices. Whereas introducing multiple materials also results in a more complicated morphology than their binary blend counterparts. Understanding the morphology is crucially important for further improving the device performance of TB-OSC. This review introduces the solubility and miscibility parameters that affect the morphology of ternary blends. Then, this review summarizes the recent processes of morphology study on ternary blends from the aspects of molecular crystallinity, molecular packing orientation, domain size and purity, directly observation of morphology, vertical phase separation as well as morphological stability. Finally, summary and prospects of TB-OSCs are concluded.

1,10-Phenanthroline as an Efficient Bifunctional Passivating Agent for MAPbI3 Perovskite Solar Cells

Passivation is one of the most promising concepts to heal defects created at the surface and grain boundaries of polycrystalline perovskite thin films, which significantly deteriorate the photovoltaic performance and stability of corresponding devices. Here, 1,10-phenanthroline, known as a bidentate chelating ligand, is implemented between the methylammonium lead iodide (MAPbI₃) film and the hole-transport layer for both passivating the lead-based surface defects (undercoordinated lead ions) and converting the excess/unreacted lead iodide (PbI₂) buried at interfaces, which is problematic for the long-term stability, into "neutralized" and beneficial species (PbI₂(1,10-phen)_x, x = 1, 2) for efficient hole transfer at the modified interface. The defect healing ability of 1,10-phenanthroline is verified with a set of complementary techniques including photoluminescence (steady-state and time-resolved), space-charge-limited current (SCLC) measurements, light intensity dependent JV measurements, and Fourier-transform photocurrent spectroscopy (FTPS). In addition to these analytical methods, we employ advanced X-ray scattering techniques, nano-Fourier transform infrared (nano-FTIR) spectroscopy, and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) to further analyze the structure and chemical composition at the perovskite surface after treatment at nanoscale spatial resolution. On the basis of our experimental results, we conclude that 1,10-phenanthroline treatment induces the formation of different morphologies with distinct chemical compositions on the surface of the perovskite film such that surface defects are effectively passivated, and excess/unreacted PbI₂ is converted into beneficial complex species at the modified interface. As a result, an improved power conversion efficiency (20.16%) and significantly more stable unencapsulated perovskite solar cells are obtained with the 1,10-phenanthroline treatment compared to the MAPbI₃ reference device (18.03%).

Increasing Photostability of Inverted Nonfullerene Organic Solar Cells by Using Fullerene Derivative Additives

Organic solar cells (OSCs) recently achieved efficiencies of over 18% and are well on their way to practical applications, but still considerable stability issues need to be overcome. One major problem emerges from the electron transport material zinc oxide (ZnO), which is mainly used in the inverted device architecture and decomposes many high-performance nonfullerene acceptors due to its photocatalytic activity. In this work, we add three different fullerene derivatives-PC₇₁BM, ICMA, and BisPCBM-to an inverted binary PBDB-TF:IT-4F system in order to suppress the photocatalytic degradation of IT-4F on ZnO via the radical scavenging abilities of the fullerenes. We demonstrate that the addition of 5% fullerene not only increases the performance of the binary PBDB-TF:IT-4F system but also significantly improves the device lifetime under UV illumination in an inert atmosphere. While the binary devices lose 20% of their initial efficiency after only 3 h, this time is increased fivefold for the most promising ternary devices with ICMA. We attribute this improvement to a reduced photocatalytic decomposition of IT-4F in the ternary system, which results in a decreased recombination. We propose that the added fullerenes protect the IT-4F by acting as a sacrificial reagent, thereby suppressing the trap state formation. Furthermore, we show that the protective effect of the most promising fullerene ICMA is transferable to two other binary systems PBDB-TF:BTP-4F and PTB7-Th:IT-4F. Importantly, this effect can also increase the air stability of PBDB-TF:IT-4F. This work demonstrates that the addition of fullerene derivatives is a transferable and straightforward strategy to improve the stability of OSCs.

A Protocol for Fast Prediction of Electronic and Optical Properties of Donor-Acceptor Polymers Using Density Functional Theory and the Tight-Binding Method

The ability of donor-acceptor (D-A) type polymers to control the positions of the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals makes them a popular choice for organic solar cell applications. The alternating D-A pattern in a monomer leads to a weak electronic coupling between the constituent monomers within the polymer chain. Exploiting the weak electronic coupling characteristics, we developed a method to efficiently calculate (1) the electronic properties and (2) the optical gap of such polymer chains. The electronic properties (HOMO and LUMO energies, ionization potential, electron affinity, and quasiparticle gap of an oligomer of any length up to an infinitely long polymer) of the D-A polymers are predicted by combining density functional theory calculation results and a tight-binding model. The weak electronic coupling implies that the optical gap of the polymer is size-independent, and thus, it can be calculated using a monomer. We validated the methods using a set of 104 polymers by checking the consistency where the electronic gap of a polymer is larger than the optical gap. Furthermore, we establish relationships between the results obtained from more accurate, yet slower methods (i.e., B3LYP functional, singlet-ΔSCF) with those obtained from the faster counterparts (i.e., BLYP functional, triplet-ΔSCF). Leveraging the found relationships, we propose a way in which the electronic and optical properties of the polymers can be calculated efficiently while retaining high accuracy. The use of the tight-binding model combined with the approach to estimate more accurate results based on less expensive simulations is crucial in the applications where a large volume of computations needs to be carried out efficiently with sufficiently high accuracy, such as high-throughput computational screening or training a machine-learning model.

Surface Recombination in Ultra-Fast Carrier Dynamics of Perovskite Oxide La0.7Sr0.3MnO3 Thin Films

Aspects of the optoelectronic performance of thin-film ferromagnetic materials are evaluated for application in ultrafast devices. Dynamics of photocarriers and their associated spin polarization are measured using transient reflectivity (TR) measurements in cross linear and circular polarization configurations for La_0.7Sr_0.3MnO₃ films with a range of thicknesses. Three spin-related recombination mechanisms have been observed for thicker films (thickness of d ≥ 20 nm) at different time regimes (τ), which are attributed to the electron-phonon recombination (τ < 1 ps), phonon-assisted spin-lattice recombination (τ ∼ 100 ps), and thermal diffusion and radiative recombination (τ > 1 ns). Density functional theory (DFT+U) based first-principles calculations provide information about the nature of the optical transitions and their probabilities for the majority and the minority spin channels. These transitions are partly responsible for the aforementioned recombination mechanisms, identified through the comparison of linear and circular TR measurements. The same sets of measurements for thinner films (4.4 nm ≤ d < 20 nm) revealed an additional relaxation dynamic (τ ∼ 10 ps), which is attributed to the enhanced surface recombination of charge carriers. Our DFT+U calculations further corroborate this observation, indicating an increase in the surface density of states with decreasing film thickness which results in higher amplitude and smaller time constant for surface recombination as the film thickness decreases.