Formerly the "Solar and Photovoltaic Engineering Research Center (SPERC)"

Recent Submissions

• Defect Passivation in Perovskite Solar Cells by Cyano-Based π-Conjugated Molecules for Improved Performance and Stability

(Advanced Functional Materials, Wiley, 2020-07-09) [Article]
Defects at the surface and grain boundaries of metal–halide perovskite films lead to performance losses of perovskite solar cells (PSCs). Here, organic cyano-based π-conjugated molecules composed of indacenodithieno[3,2-b]thiophene (IDTT) are reported and it is found that their cyano group can effectively passivate such defects. To achieve a homogeneous distribution, these molecules are dissolved in the antisolvent, used to initiate the perovskite crystallization. It is found that these molecules are self-anchored at the grain boundaries due to their strong binding to undercoordinated Pb2+. On a device level, this passivation scheme enhances the charge separation and transport at the grain boundaries due to the well-matched energetic levels between the passivant and the perovskite. Consequently, these benefits contribute directly to the achievement of power conversion efficiencies as high as 21.2%, as well as the improved environmental and thermal stability of the PSCs. The surface treatment provides a new strategy to simultaneously passivate defects and enhance charge extraction/transport at the device interface by manipulating the anchoring groups of the molecules.
• Systematic Study on Morphological Development of Blade-Coated Conjugated Polymer Thin Films via In Situ Measurements

(ACS Applied Materials & Interfaces, American Chemical Society (ACS), 2020-07-07) [Article]
The morphology of conjugated polymer thin films, determined by the kinetics of film drying, is closely correlated with their electrical properties. Herein, we focused on dramatic changes in thin film morphology of blade-coated poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} caused by the effect of solvent and coating temperature. Through in situ measurements the evolution of polymer aggregates and crystallites, which plays a decisive role in the formation of the charge transporting pathway, was observed in real-time. By combining in situ ultraviolet-visible spectroscopy and in situ grazing-incidence wide-angle X-ray scattering analysis, we could identify five distinct stages during the blade-coating process; these stages were observed irrespective of the solvent and coating temperature used. The five stages are described in detail with a proposed model of film formation. This insight is an important step in understanding the relationship between the morphology of thin polymer films and their charge-transport properties as well as optimizing the structural evolution of thin films.
• A Highly Conductive Titanium Oxynitride Electron-Selective Contact for Efficient Photovoltaic Devices.

(Advanced materials (Deerfield Beach, Fla.), Wiley, 2020-07-03) [Article]
High-quality carrier-selective contacts with suitable electronic properties are a prerequisite for photovoltaic devices with high power conversion efficiency (PCE). In this work, an efficient electron-selective contact, titanium oxynitride (TiOx Ny ), is developed for crystalline silicon (c-Si) and organic photovoltaic devices. Atomic-layer-deposited TiOx Ny is demonstrated to be highly conductive with a proper work function (4.3 eV) and a wide bandgap (3.4 eV). Thin TiOx Ny films simultaneously provide a moderate surface passivation and enable a low contact resistivity on c-Si surfaces. By implementation of an optimal TiOx Ny -based contact, a state-of-the-art PCE of 22.3% is achieved for a c-Si solar cell featuring a full-area dopant-free electron-selective contact. Simultaneously, conductive TiOx Ny is proven to be an efficient electron-transport layer for organic photovoltaic (OPV) devices. A remarkably high PCE of 17.02% is achieved for an OPV device with an electron-transport TiOx Ny layer, which is superior to conventional ZnO-based devices with a PCE of 16.10%. Atomic-layer-deposited TiOx Ny ETL on a large area with a high uniformity may help accelerate the commercialization of emerging solar technologies.
• Temperature-resilient solid-state organic artificial synapses for neuromorphic computing

(Science Advances, American Association for the Advancement of Science (AAAS), 2020-07-03) [Article]
<jats:p>Devices with tunable resistance are highly sought after for neuromorphic computing. Conventional resistive memories, however, suffer from nonlinear and asymmetric resistance tuning and excessive write noise, degrading artificial neural network (ANN) accelerator performance. Emerging electrochemical random-access memories (ECRAMs) display write linearity, which enables substantially faster ANN training by array programing in parallel. However, state-of-the-art ECRAMs have not yet demonstrated stable and efficient operation at temperatures required for packaged electronic devices (~90°C). Here, we show that (semi)conducting polymers combined with ion gel electrolyte films enable solid-state ECRAMs with stable and nearly temperature-independent operation up to 90°C. These ECRAMs show linear resistance tuning over a >2× dynamic range, 20-nanosecond switching, submicrosecond write-read cycling, low noise, and low-voltage (±1 volt) and low-energy (~80 femtojoules per write) operation combined with excellent endurance (>10$^\{9}$\$ write-read operations at 90°C). Demonstration of these high-performance ECRAMs is a fundamental step toward their implementation in hardware ANNs.</jats:p>
• Structurally Tunable Two-Dimensional Layered Perovskites: From Confinement and Enhanced Charge Transport to Prolonged Hot Carrier Cooling Dynamics.

(The journal of physical chemistry letters, American Chemical Society (ACS), 2020-06-24) [Article]
Two-dimensional (2D) layered metal halide perovskites are potential alternatives to three-dimensional perovskites in optoelectronic applications owing to their improved photostabilities and chemical stabilities. Recent investigations of 2D metal halide perovskites have demonstrated interesting optical and electronic properties of various structures that are controlled by their elemental composition and organic spacers. However, photovoltaic devices that utilize 2D perovskites suffer from poor device efficiency due to inefficient charge carrier separation and extraction. In this Perspective, we shed light on confinement control and structural variation strategies that provide better parameters for the efficient collection of charges. The influence of these strategies on the exciton binding energies, charge-carrier mobilities, hot-carrier dynamics, and electron-phonon coupling in 2D perovskites is thoroughly discussed; these parameters highlight unique opportunities for further system optimization. Beyond the tunability of these fundamental parameters, we conclude this Perspective with the most notable strategies for attaining 2D perovskites with reduced bandgaps to better suit photovoltaic applications.
• Correlating the Structural and Photophysical Properties of Ortho , Meta , and Para -Carboranyl–Anthracene Dyads

(Advanced Electronic Materials, Wiley, 2020-06-23) [Article]
The role of the carborane isomer is investigated on the structural and photophysical properties of molecules comprising a carborane cluster and a conjugated organic moiety is investigated by synthesizing isomeric o -, m -, and p -carboranyl-anthracene donor–acceptor dyads. While appending a carborane leads to emission from a low energy intramolecular charge transfer state for the o -isomer, as well as emission from an excited state localized on the anthracene, this is not the case for the m - and p -carborane derivatives. This difference is attributed to a lower electron affinity for the latter two isomers. However, adding both m - and p - deforms the aromatic backbone and increases its structural rigidity, reducing non-radiative decay pathways and hence enhancing photoluminescence quantum efficiency relative to anthracene.
• Ultrafast Charge Dynamics in Dilute-Donor versus Highly Intermixed TAPC:C60 Organic Solar Cell Blends.

(The journal of physical chemistry letters, American Chemical Society (ACS), 2020-06-23) [Article]
Elucidating the interplay between film morphology, photophysics, and device performance of bulk heterojunction (BHJ) organic photovoltaics remains challenging. Here, we use the well-defined morphology of vapor-deposited di-[4-(N,N-di-p-tolyl-amino)-phenyl]cyclohexane (TAPC):C60 blends to address charge generation and recombination by transient ultrafast spectroscopy. We gain relevant new insights to the functioning of dilute-donor (5% TAPC) fullerene-based BHJs compared to molecularly intermixed systems (50% TAPC). First, we show that intermolecular charge-transfer (CT) excitons in the C60 clusters of dilute BHJs rapidly localize to Frenkel excitons prior to dissociating at the donor:acceptor interface. Thus, both Frenkel and CT excitons generate photocurrent over the entire fullerene absorption range. Second, we selectively monitor interfacial and bulk C60 clusters via their electro-absorption, demonstrating an energetic gradient that assists free charge generation. Third, we identify a fast (<1 ns) recombination channel, whereby free electrons recombine with trapped holes on isolated TAPC molecules. This can harm the performance of dilute solar cells, unless the electrons are rapidly extracted in efficient devices.
• Rapid Photonic Processing of High-Electron-Mobility PbS Colloidal Quantum Dot Transistors.

(ACS applied materials & interfaces, American Chemical Society (ACS), 2020-06-23) [Article]
Recent advances in solution-processable semiconducting colloidal quantum dots (CQDs) have enabled their use in a range of (opto)electronic devices. In most of these studies, device fabrication relied almost exclusively on thermal annealing to remove organic residues and enhance inter-CQD electronic coupling. Despite its widespread use, however, thermal annealing is a lengthy process, while its effectiveness to eliminate organic residues remains limited. Here, we exploit the use of xenon flash lamp sintering to post-treat solution-deposited layers of lead sulfide (PbS) CQDs and their application in n-channel thin-film transistors (TFTs). The process is simple, fast, and highly scalable and allows for efficient removal of organic residues while preserving both quantum confinement and high channel current modulation. Bottom-gate, top-contact PbS CQD TFTs incorporating SiO2 as the gate dielectric exhibit a maximum electron mobility of 0.2 cm2 V-1 s-1, a value higher than that of control transistors (≈10-2 cm2 V-1 s-1) processed via thermal annealing for 30 min at 120 °C. Replacing SiO2 with a polymeric dielectric improves the transistor's channel interface, leading to a significant increase in electron mobility to 3.7 cm2 V-1 s-1. The present work highlights the potential of flash lamp annealing as a promising method for the rapid manufacture of PbS CQD-based (opto)electronic devices and circuits.
• Efficient Hybrid Mixed Ion Perovskite Photovoltaics: In Situ Diagnostics of the Roles of Cesium and Potassium Alkali Cation Addition

(Solar RRL, Wiley, 2020-06-19) [Article]
Perovskite photovoltaics have made extraordinary progress in power conversion efficiency (PCE) and stability owing to process and formulation development. Perovskite cell performance benefits from the addition of alkali metal cations, such as cesium (Cs+) and potassium (K+) in mixed ion systems, but the underlying reasons are not fully understood. Here, we study the solidification of perovskite layers incorporating 5, 10, to 20% of Cs+ and K+ using in situ grazing incidence wide-angle X-ray scattering. We found that K+-doped solutions yield non-perovskite 4H phase rather than the 3C perovskite phase. For Cs+-doped formulations, both 4H and 3C phases are present at 5% Cs+, while the 3C perovskite phase forms in 10% Cs+-doped formulations, with undesirable halide segregation occurring at 20% Cs+. Post-deposition thermal annealing converts the intermediate 4H phase to the desirable 3C perovskite phase. Importantly, perovskite layers containing 5% of Cs+ or K+ exhibit reduced concentration of trap states, enhanced carrier mobility and lifetime. By carefully adjusting the Cs+ or K+ concentration to 5%, we demonstrate perovskite cells with a ≈5% higher average PCE than cells utilizing a higher cation concentrations. The study provides unique insights into the crystallization pathways towards perovskite phase engineering and improved cell performance.
• Micron Thick Colloidal Quantum Dot Solids

(Nano Letters, American Chemical Society (ACS), 2020-06-16) [Article]
Shortwave infrared colloidal quantum dots (SWIRCQDs) are semiconductors capable of harvesting across the AM1.5G solar spectrum. Today’s SWIR-CQD solar cells rely on spin-coating; however, these films exhibit cracking once thickness exceeds ∼500 nm. We posited that a blade-coating strategy could enable thick QD films. We developed a ligand exchange with an additional resolvation step that enabled the dispersion of SWIRCQDs. We then engineered a quaternary ink that combined highviscosity solvents with short QD stabilizing ligands. This ink, bladecoated over a mild heating bed, formed micron-thick SWIR-CQD films. These SWIR-CQD solar cells achieved short-circuit current densities (Jsc) that reach 39 mA cm−2 , corresponding to the harvest of 60% of total photons incident under AM1.5G illumination. External quantum efficiency measurements reveal both the first exciton peak and the closest Fabry−Perot resonance peak reaching approximately 80%this is the highest unbiased EQE reported beyond 1400 nm in a solution-processed semiconductor.
• Fully Inkjet-Printed, Ultrathin and Conformable Organic Photovoltaics as Power Source Based on Cross-Linked PEDOT:PSS Electrodes

(Advanced Materials Technologies, Wiley, 2020-06-15) [Article]
Ultra-lightweight solar cells have attracted enormous attention due to their ultra-conformability, flexibility, and compatibility with applications including electronic skin or miniaturized electronics for biological applications. With the latest advancements in printing technologies, printing ultrathin electronics is becoming now a reality. This work offers an easy path to fabricate indium tin oxide (ITO)-free ultra-lightweight organic solar cells through inkjet-printing while preserving high efficiencies. A method consisting of the modification of a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) ink with a methoxysilane-based cross-linker (3-glycidyloxypropyl)trimethoxysilane (GOPS)) is presented to chemically modify the structure of the electrode layer. Combined with plasma and solvent post-treatments, this approach prevents shunts and ensures precise patterning of solar cells. By using poly(3-hexylthiophene) along rhodanine-benzothiadiazole-coupled indacenodithiophene (P3HT:O-IDTBR), the power conversion efficiency (PCE) of the fully printed solar cells is boosted up to 4.73% and fill factors approaching 65%. All inkjet-printed ultrathin solar cells on a 1.7 µm thick biocompatible parylene substrate are fabricated with PCE reaching up to 3.6% and high power-per-weight values of 6.3 W g−1. After encapsulation, the cells retain their performance after being exposed for 6 h to aqueous environments such as water, seawater, or phosphate buffered saline, paving the way for their integration in more complex circuits for biological systems.
• Large-Area Uniform Polymer Transistor Arrays on Flexible Substrates: Towards High-Throughput Sensor Fabrication

(Advanced Materials Technologies, Wiley, 2020-06-15) [Article]
Solution-processable organic semiconductors can serve as the basis for new products including rollable displays, tattoo-like smart bandages for real-time health monitoring, and conformable electronics integrated into clothing or even implanted in the human body. For such exciting commercial applications to become a reality, good device performance and uniformity over large areas are necessary. The design of new materials has progressed at an astonishing pace, but accessing their intrinsic, efficient electrical properties in large-area flexible device arrays is difficult. The development of protocols that allow integration with industrial-scale processing for high-throughput manufacturing, without the need to compromise on performance, is the key for transitioning these materials to real-life applications. In this work, large-area arrays of organic thin-film transistors obtained by spray-coating the high-mobility polymer indacenodithiophene-co-benzothiadiazole (IDTBT) are demonstrated. A maximum charge carrier mobility of 2.3 cm2 V−1 s−1, with a very narrow performance distribution, is obtained over surface areas of 10 cm × 10 cm. The devices retain their electrical properties when bent multiple times and at different curvatures. In addition, large arrays of highly sensitive (0.25% change in mobility for 1% humidity variation), reusable, near-identical humidity sensors are produced in a one-step fabrication and calibrated from 0% to 94% relative humidity.
• GIWAXS-SIIRkit: scattering intensity, indexing and refraction calculation toolkit for grazing-incidence wide-angle X-ray scattering of organic materials

(Journal of Applied Crystallography, International Union of Crystallography (IUCr), 2020-06-12) [Article]
<jats:p>Grazing-incidence wide-angle X-ray scattering (GIWAXS) has become an increasingly popular technique for quantitative structural characterization and comparison of thin films. For this purpose, accurate intensity normalization and peak position determination are crucial. At present, few tools exist to estimate the uncertainties of these measurements. Here, a simulation package is introduced called <jats:italic>GIWAXS-SIIRkit</jats:italic>, where SIIR stands for scattering intensity, indexing and refraction. The package contains several tools that are freely available for download and can be executed in MATLAB. The package includes three functionalities: estimation of the relative scattering intensity and the corresponding uncertainty based on experimental setup and sample dimensions; extraction and indexing of peak positions to approximate the crystal structure of organic materials starting from calibrated GIWAXS patterns; and analysis of the effects of refraction on peak positions. Each tool is based on a graphical user interface and designed to have a short learning curve. A user guide is provided with detailed usage instruction, tips for adding functionality and customization, and exemplary files.</jats:p>
• Highly Transparent and Conductive Electrodes Enabled by Scalable Printing-and-Sintering of Silver Nanowires

(Nanotechnology, IOP Publishing, 2020-06-12) [Article]
Silver nanowires (Ag NWs) have promised well for flexible and transparent electronics. However, It remains an open question on how to achieve large-scale printing of Ag NWs with high optical transparency, electrical conductivity, and mechanical durability for practical applications, though extensive research for more than one decade. In this work, we propose a possible solution that integrates screen printing of Ag NWs with flash-light sintering (FLS). We demonstrate that the use of low-concentration, screen-printable Ag NW ink enables large-area and high-resolution patterning of Ag NWs. A critical advantage comes from the FLS process that allows low-temperature processing, short operational time, and high output rate - characteristics that fit the scalable manufacturing. Importantly, we show that the resultant Ag NW patterns feature low sheet resistance (1.1-9.2 Ohm/sq), high transparency (75.2-92.6%), and thus a remarkable figure of merit comparable to state of the art. These outstanding properties of Ag NW patterns, together with their scalable fabrication method we proposed, would facilitate many Ag NW-based applications, such as transparent heater, stretchable displays, and wearable devices; here, we demonstrate the novel design of flexible and transparent radio frequency 5G antennas.
• Water stable molecular n-doping produces organic electrochemical transistors with high transconductance and record stability

(Nature Communications, Springer Science and Business Media LLC, 2020-06-12) [Article]
From established to emergent technologies, doping plays a crucial role in all semiconducting devices. Doping could, theoretically, be an excellent technique for improving repressively low transconductances in n-type organic electrochemical transistors – critical for advancing logic circuits for bioelectronic and neuromorphic technologies. However, the technical challenge is extreme: n-doped polymers are unstable in electrochemical transistor operating environments, air and water (electrolyte). Here, the first demonstration of doping in electron transporting organic electrochemical transistors is reported. The ammonium salt tetra-nbutylammonium fluoride is simply admixed with the conjugated polymer poly(N,N’-bis(7-glycol)-naphthalene-1,4,5,8 bis(dicarboximide)-co-2,2’-bithiophene-co-N,N’-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide), and found to act as a simultaneous molecular dopant and morphology-additive. The combined effects enhance the n-type transconductance with improved channel capacitance and mobility. Furthermore, operational and shelflife stability measurements showcase the first example of water-stable n-doping in a polymer. Overall, the results set a precedent for doping/additives to impact organic electrochemical transistors as powerfully as they have in other semiconducting devices.
• Monitoring supported lipid bilayers with n-type organic electrochemical transistors

(Materials Horizons, Royal Society of Chemistry (RSC), 2020-06-11) [Article]
<p>An n-type, accumulation mode, microscale organic electrochemical transistor monitors the activity of a pore-forming protein integrated into a lipid bilayer.</p>
• Monolayer Perovskite Bridges Enable Strong Quantum Dot Coupling for Efficient Solar Cells

(Joule, Elsevier BV, 2020-06-09) [Article]
Solution-processed colloidal quantum dots (CQDs) are promising optoelectronic materials; however, CQD solids have, to date, exhibited either excellent transport properties but fusion among CQDs or limited transport when QDs are strongly passivated. Here, we report the growth of monolayer perovskite bridges among quantum dots and show that this enables the union of surface passivation with improved charge transport. We grow the perovskite layer after forming the CQD solid rather than introducing perovskite precursors into the quantum dot solution: the monolayer of perovskite increases interdot coupling and decreases the distance over which carriers must tunnel. As a result, we double the diffusion length relative to reference CQD solids and report solar cells that achieve a stabilized power conversion efficiency (PCE) of 13.8%, a record among Pb chalcogenide CQD solar cells.
• Engineering Optically Switchable Transistors with Improved Performance by Controlling Interactions of Diarylethenes in Polymer Matrices.

(Journal of the American Chemical Society, American Chemical Society (ACS), 2020-06-03) [Article]
The integration of photochromic molecules into semiconducting polymer matrices via blending has recently attracted a great deal of attention, as it provides the means to reversibly modulate the output signal of electronic devices by using light as a remote control. However, the structural and electronic interactions between photochromic molecules and semiconducting polymers are far from being fully understood. Here we perform a comparative investigation by combining two photochromic diarylethene moieties possessing similar energy levels yet different propensity to aggregate with five prototypical polymer semiconductors exhibiting different energy levels and structural order, ranging from amorphous to semicrystalline. Our in-depth photochemical, structural, morphological, and electrical characterization reveals that the photoresponsive behavior of thin-film transistors including polymer/diarylethenes blends as the active layer is governed by a complex interplay between the relative position of the energy levels and the polymer matrix microstructure. By matching the energy levels and optimizing the molecular packing, high-performance optically switchable organic thin-film transistors were fabricated. These findings represent a major step forward in the fabrication of light-responsive organic devices.
• Colloidal quantum dot photovoltaics using ultra-thin, solution-processed bilayer In2O3/ZnO electron transport layers with improved stability

(ACS Applied Energy Materials, American Chemical Society (ACS), 2020-05-28) [Article]
Solution-processed colloidal quantum dot (CQD) photovoltaics (PVs) continue to mature with improvements in device architectures and ligand exchange strategies. Carrier selective contacts extract photogenerated charge carriers from the CQD absorber; however, the role of the electron-transporting layer (ETL) in stability remains unclear. Herein, we find that the typically used >100 nm thick ZnO ETL suffers from parasitic absorption and carrier recombination resulting in unstable n–i–p solar cells with faster UV-degradation. We address this by developing an ultrathin (ca. 20 nm), quantum-confined, solution-processed In2O3/ZnO ETL. This bilayer ETL results in solar cells with significantly improved overall stability without compromising performance, with an 11.1% power conversion efficiency hero device.
• Modulation of Broadband Emissions in Two-dimensional <100>-oriented Ruddlesden-Popper Hybrid Perovskites

(ACS Energy Letters, American Chemical Society (ACS), 2020-05-28) [Article]
Two-dimensional (2D) Ruddlesden−Popper (RP) perovskites are emerging materials for light-emitting applications. Unfortunately, their desirable narrowband emission coexists with broadband emissions, which limits the color quality and performance of the light source. However, the origin of such broadband emission in ⟨100⟩-oriented perovskites is still under debate. Here, we experimentally and theoretically demonstrate that unlike ⟨110⟩-oriented RP perovskites, the broadband emission of the 2D ⟨100⟩-oriented RP (PEA)2PbI4 (PEA = C6H5C2H4NH3 +) perovskites originates from defect-related luminescence centers. We find that the broadband emission of this prototype 2D structure can be largely suppressed by using excess PEAI treatment. Density functional theory (DFT) calculations indicate that iodine (I) vacancies both in the bulk and on the surface are responsible for the broadband emission. We attribute the decreased broadband emission after PEAI treatment to the passivation of both under coordinated Pb2+ ions on the surface and I vacancies in the bulk through I− ion migration.