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

### Recent Submissions

• #### Effects of Vertical Molecular Stratifications and Microstructures on the Properties of Fullerene-Free Organic Solar Cells

(Advanced Photonics Research, Wiley, 2022-01-18) [Article]
From the past years, the most commonly reported state-of-the-art binary bulk heterojunction organic solar cells (OSCs) are mostly based on mixtures of polymer donors and fullerene-free acceptors (polymer:NFA). However, along with it are a number of contradictory propositions, including (but not limited to) strategies to reduce energy loss and improve photocurrent generation through energy level alignments. Due to the resulting high similarity of molecular fragments from polymer:NFA heterojunctions, the effects of vertical molecular stratification are not yet well studied. Herein, the time-of-flight secondary ion mass spectrometry (ToF-SIMS) molecular depth profiling reveals a vertical stratification in PM6:IT-4Cl and illustrates how it can significantly influence the photovoltaic properties. The said inhomogeneity is also bound to introduce microstructure variations within device active layers. Consequently, it is systematically demonstrated how thin-film microstructures can influence optoelectronic properties, wherein important metrics (e.g., energy losses and molecular energy offsets) are highly dependent. Thus, the understanding from this work provides foundations for more precise development of strategies to further advance OSC technology in future studies.
• #### Self-Assembly and Regrowth of Metal Halide Perovskite Nanocrystals for Optoelectronic Applications

(Accounts of Chemical Research, American Chemical Society (ACS), 2022-01-16) [Article]
Conspectus Over the past decade, the impressive development of metal halide perovskites (MHPs) has made them leading candidates for applications in photovoltaics (PVs), X-ray scintillators, and light-emitting diodes (LEDs). Constructing MHP nanocrystals (NCs) with promising optoelectronic properties using a low-cost approach is critical to realizing their commercial potential. Self-assembly and regrowth techniques provide a simple and powerful “bottom-up” platform for controlling the structure, shape, and dimensionality of MHP NCs. The soft ionic nature of MHP NCs, in conjunction with their low formation energy, rapid anion exchange, and ease of ion migration, enables the rearrangement of their overall appearance via self-assembly or regrowth. Because of their low formation energy and highly dynamic surface ligands, MHP NCs have a higher propensity to regrow than conventional hard-lattice NCs. Moreover, their self-assembly and regrowth can be achieved simultaneously. The self-assembly of NCs into close-packed, long-range-ordered mesostructures provides a platform for modulating their electronic properties (e.g., conductivity and carrier mobility). Moreover, assembled MHP NCs exhibit collective properties (e.g., superfluorescence, renormalized emission, longer phase coherence times, and long exciton diffusion lengths) that can translate into dramatic improvements in device performance. Further regrowth into fused MHP nanostructures with the removal of ligand barriers between NCs could facilitate charge carrier transport, eliminate surface point defects, and enhance stability against moisture, light, and electron-beam irradiation. However, the synthesis strategies, diversity and complexity of structures, and optoelectronic applications that emanate from the self-assembly and regrowth of MHPs have not yet received much attention. Consequently, a comprehensive understanding of the design principles of self-assembled and fused MHP nanostructures will fuel further advances in their optoelectronic applications. In this Account, we review the latest developments in the self-assembly and regrowth of MHP NCs. We begin with a survey of the mechanisms, driving forces, and techniques for controlling MHP NC self-assembly. We then explore the phase transition of fused MHP nanostructures at the atomic level, delving into the mechanisms of facet-directed connections and the kinetics of their shape-modulation behavior, which have been elucidated with the aid of high-resolution transmission electron microscopy (HRTEM) and first-principles density functional theory calculations of surface energies. We further outline the applications of assembled and fused nanostructures. Finally, we conclude with a perspective on current challenges and future directions in the field of MHP.
• #### Air-Processable and Thermally Stable Hole Transport Layer for Non-Fullerene Organic Solar Cells

(ACS Applied Energy Materials, American Chemical Society (ACS), 2022-01-10) [Article]
Power conversion efficiencies (PCEs) of organic solar cells (OSCs) have now surpassed 19%. This has led to an increased focus on developing devices using methods and materials that are scalable, processable under ambient air atmospheres, and stable. However, current materials fall short of the essential requirements for stability and processability needed for cost-effective large-scale fabrication of high-performing OSCs. Here, we report a hybrid solution-processable hole transport layer (HTL) based on tantalum-doped tungsten oxide (TaWOx) nanoparticles and poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) demonstrating good wettability over the hydrophobic active layer. N-i-p-type OSCs that are processed fully under ambient conditions, based on a polymer donor and a non-fullerene acceptor incorporating a combined TaWOx-PEDOT:PSS layer as HTL deliver a power conversion efficiency of 8.6%. OSCs utilizing the TaWOx-PEDOT:PSS HTL demonstrate improved thermal stability compared to devices based on the previously reported solution-processed MoOx-PEDOT:PSS HTL, which was found to severely degrade upon thermal treatment at 85 °C. Photoelectron spectroscopy and secondary ion mass spectrometry (SIMS) reveal that the MoOx-PEDOT:PSS HTL is prone to thermally induced intermixing with the underlying active layer, resulting in unfavorable changes in the interfacial energetics. No significant heat-induced changes are observed in the case of the TaWOx-PEDOT:PSS HTL when annealed up to 120 °C, imparting enhanced thermal stability to the devices. Improved wettability on hydrophobic surfaces, combined with air processability and enhanced thermal stability makes TaWOx-PEDOT:PSS a promising HTL material for fabricating stable NFA solar cells using roll-to-roll compatible printing and coating methods.
• #### A Universal Co-Solvent Evaporation Strategy Enables Direct Printing of Perovskite Single Crystals for Optoelectronic Device Applications

Solution-processed metal halide perovskite single crystals (SCs) are in high demand for a growing number of emerging device applications due to their superior optoelectronic properties compared to polycrystalline thin films. However, the historical focus on thin film optoelectronic and photovoltaic devices explains the absence of methods suitable for facile, scalable and high throughput fabrication of precision-engineered and positioned SCs and arrays. Here, we present a universal co-solvent evaporation (CSE) strategy by which perovskite SCsand arrays are produced directly on substrates from individual drying droplets in a single step within minutes at room temperature. The CSE strategy successfully guides supersaturation of drying droplets to suppress all unwanted crystallization pathways and is shown to produce SCsof a wide variety of three-dimensional (3D), quasi-two dimensional (2D), and mixed cation/halideperovskites. The drying droplet approach works with commonly used solvents, making it universal. Importantly, the CSE strategy ensures the SC consumes the precursor in its entirety, leaving little to no residue on substrates, which is crucial for enabling fabrication of SC arrays on large areas via printing and coating techniques. We go on to demonstrate direct on-chip fabrication of 3D and quasi-2D perovskite photodetector devices with outstanding performance. Our approach shows that metal halide perovskite SCs can now be produced on substrates from a drying solution via a wide range of solution processing methods, including microprinting and scalable, high throughput coating methods.

• #### Flexible, large-area, multi-layered graphene/cellulose composite for dye filtration applications

(Materials Today Communications, Elsevier BV, 2022-01-06) [Article]
Textile and clothing industry effluents are the primary source of water pollution, affecting aquatic animals and plants. The need of the hour is to filter these effluents to avoid contamination. Nanofiltration using carbon-based membranes is an effective method in dye filtration. The present work describes graphene oxide (GO) and reduced graphene oxide (RGO) coated cellulose surfaces for dye filtration. These sheets exhibited high porosity, large surface area, high flexibility, inexpensive, easy to fabricate, and successfully employed for efficient nano dye filtration. The novelty of the present work is the filtration of most industrially used dyes such as methylene blue and rose bengal. Results confirm the filtration up to 98% and 94% for both Methylene Blue (MB) and Rose Bengal (RB) dyes, respectively, and the eco-friendly mode of treating industrial dye-contaminated water.
• #### Direct Visualization of a Gold Nanoparticle Electron Trapping Effect

(Journal of the American Chemical Society, American Chemical Society (ACS), 2022-01-05) [Article]
A new atomic-scale anisotropy in the photoreaction of surface carboxylates on rutile TiO<sub>2</sub>(110) induced by gold clusters is found. STM and DFT+U are used to study this phenomenon by monitoring the photoreaction of a prototype hole-scavenger molecule, benzoic acid, over stoichiometric (s) s-TiO<sub>2</sub>, Au<sub>9</sub>/s-TiO<sub>2</sub>, and reduced (r) Au<sub>9</sub>/r-TiO<sub>2</sub>. STM results show that benzoic acid adsorption displaces a large fraction of Au clusters from the terraces toward their edges. DFT calculations explain that Au<sub>9</sub> clusters on stoichiometric TiO<sub>2</sub> are distorted by benzoic acid adsorption. The influence of sub-monolayers of Au on the UV/visible photoreaction of benzoic acid was explored at room temperature, with adsorbate depletion taken as a measure of activity. The empty sites, observed upon photoexcitation, occurred in elongated chains (2 to 6 molecules long) in the [11̅0] and [001] directions. A roughly 3-fold higher depletion rate is observed in the [001] direction. This is linked to the anisotropic conduction of excited electrons along [001], with subsequent trapping by Au clusters leaving a higher concentration of holes and thus an increased decomposition rate. To our knowledge this is the first time that atomic-scale directionality of a chemical reaction is reported upon photoexcitation of the semiconductor.
• #### A flexible capacitive photoreceptor for the biomimetic retina.

(Light: Science & Applications, Springer Science and Business Media LLC, 2022-01-02) [Article]
Neuromorphic vision sensors have been extremely beneficial in developing energy-efficient intelligent systems for robotics and privacy-preserving security applications. There is a dire need for devices to mimic the retina's photoreceptors that encode the light illumination into a sequence of spikes to develop such sensors. Herein, we develop a hybrid perovskite-based flexible photoreceptor whose capacitance changes proportionally to the light intensity mimicking the retina's rod cells, paving the way for developing an efficient artificial retina network. The proposed device constitutes a hybrid nanocomposite of perovskites (methyl-ammonium lead bromide) and the ferroelectric terpolymer (polyvinylidene fluoride trifluoroethylene-chlorofluoroethylene). A metal-insulator-metal type capacitor with the prepared composite exhibits the unique and photosensitive capacitive behavior at various light intensities in the visible light spectrum. The proposed photoreceptor mimics the spectral sensitivity curve of human photopic vision. The hybrid nanocomposite is stable in ambient air for 129 weeks, with no observable degradation of the composite due to the encapsulation of hybrid perovskites in the hydrophobic polymer. The functionality of the proposed photoreceptor to recognize handwritten digits (MNIST) dataset using an unsupervised trained spiking neural network with 72.05% recognition accuracy is demonstrated. This demonstration proves the potential of the proposed sensor for neuromorphic vision applications.
• #### Spray deposited gallium doped zinc oxide (GZO) thin film as the electron transport layer in inverted organic solar cells

(SOLAR ENERGY, Elsevier BV, 2022) [Article]
The electric field-assisted spray deposition of highly conducting and transparent gallium doped zinc oxide (GZO) films, with noticeable improvement in optical, structural, and electrical properties, is reported for application in organic photovoltaics with superior device performance. The GZO films deposited with applied voltages of 0 and 2000V to the nozzle exhibited an improved electrical resistivity of ∼1.8 × 10−3 Ω-cm and a transmittance (in the visible region) of over 80% for the films deposited under electric field, which can be used as electron transport layer (ETL) in inverted organic solar cells (IOSCs- ITO/GZO/PTB7:PC71BM/MoO3/Ag). Improved photovoltaic performance (power conversion efficiency: 6.5%) is obtained for the IOSC devices fabricated GZO ETL with the applied voltage of 2000V in comparison to the device fabricated using GZO ETL with the applied voltage = 0V (power conversion efficiency: 5.8%).
• #### Efficient generation of remarkably long-lived charges in organic semiconductor heterojunction nanoparticles enables high photocatalytic efficiency

(Accepted by Nature Energy, Springer Nature, 2022) [Article]
There is a growing interest in developing organic semiconductor photocatalysts for the synthesis of solar fuels. Most organic photocatalysts rely on exciton dissociation by sacrificial reagents to drive charge separation. We demonstrate that organic semiconductor nanoparticle (NP) photocatalysts comprising the conjugated polymer PM6 matched with PCBM or Y6 electron acceptors can intrinsically generate remarkably long-lived reactive charges, which enable them to efficiently drive the H2 evolution reaction (HER). The optimized PM6:Y6 NPs achieved external quantum efficiencies (EQEs) of 1.0% to 5.0% at 400 to 900 nm and the optimized PM6:PCBM NPs achieved EQEs of 8.7% to 2.6% at 400 to 700 nm. Selective Pt photodeposition was observed on the PCBM domain in the PM6:PCBM NPs and its location was rationalized based on the location of photogenerated electrons in the heterojunction. Employing transient and operando photoinduced optical absorption spectroscopies on ps-s timescales, we find that the heterojunction structure in these NPs greatly enhances the generation of long-lived charges (ms-s timescale) even in the absence of electron/hole scavengers or Pt. The generation of such long-lived reactive charges is striking and opens potential applications for driving kinetically slow and technologically desirable oxidations, or in water splitting Z-schemes.
• #### Aqueous Aluminum-Carbon Rechargeable Batteries

(Advanced Materials Interfaces, Wiley, 2021-12-31) [Article]
Carbon cathodes have shown excellent electrochemical behavior in aluminum batteries based on non-aqueous electrolytes. By contrast, their use in Al systems operating in a salt-water medium is plagued by poor and unstable performance. Herein, it is sustained that a successful C cathode for rechargeable aqueous Al batteries requires surface customization to enable hydrophilicity and grafting of charged Al molecules. Employing a freeze-dried reduced graphene oxide (rGO) as the active electrode material, an aqueous Al-C battery is assembled with a high energy density (136 Wh kg−1 per cathode mass) and one of the best capacity retentions reported (≈60% across a range of current densities and constant Coulombic efficiencies close to unit). Furthermore, the rGO cathode more than doubles the benchmark for life cycles (to ≈200 cycles) and can be charged rapidly (<5 min). To explain this response, a charge storage mechanism is proposed wherein the [Al(H2O)6]3+ ions do not get desolvated when inserted into the cathode. The guest Al ions (surface adsorbed or intercalated) act as proton donors and may get anchored on the oxygen moieties of the rGO, further promoting the formation of an electrochemical double layer. A mixed charge-storage regime follows that stabilizes the carbon cathode and enables an unprecedented response.
• #### N-type polymer semiconductors incorporating para, meta, and ortho-carborane in the conjugated backbone

(Polymer, Elsevier BV, 2021-12-27) [Article]
We report on three novel n-type conjugated polymer semiconductors incorporating carborane in the polymer backbone and demonstrate their applicability in optoelectronic devices. Comparing the optoelectronic properties of para-, meta-, and ortho-carborane isomers revealed similar energetic characteristics between the different polymers, with the carborane unit acting as a “conjugation breaker”, confining electron delocalisation to the conjugated moieties. The fabrication of all-polymer organic photovoltaic (OPV) devices and thin-film transistors (TFTs) revealed some differences in device performance between the polymers, with the meta-carborane based polymer exhibiting superior performance in both OPV and TFT devices.
• #### Conjugated polymers with controllable interfacial order and energetics enable tunable heterojunctions in organic and colloidal quantum dot photovoltaics

(JOURNAL OF MATERIALS CHEMISTRY A, Royal Society of Chemistry (RSC), 2021-12-24) [Article]
Conjugated polymers are widely used as photoactive and transport layers in organic and hybrid photovoltaics (PV), where the energetics of polymers are a key design criterion. Here, we show that significant variations in terminal molecular ordering between top and bottom surfaces of a wide range of conjugated polymer films can result in sizable interfacial ionization energy (IE) differences by as much as 0.33 eV, which has significant impact on organic and hybrid PV devices. Such tunability is surprisingly seen even in nominally amorphous polymers. We devise a strategy leveraging wet and dry laminations to form donor–acceptor planar heterojunction (PHJ) devices using exposed and buried surfaces of donor polymers and demonstrate meaningful influence over the open circuit voltage (V$_OC$) by up to 0.32 V. We use this insight to devise a controlled intermixing approach which yields superior V$_OC$ and J$_SC$ to conventional bulk heterojunction devices by leveraging the disordered interface to maximize V$_OC$ and the greater aggregation of the donor to increase the J$_SC$. We go on to demonstrate how judicious control of polymer surface IE benefits charge extraction in colloidal quantum dot PV devices in the role of hole transport layers. Our results show that polymer interfacial and bulk properties are both critical to the functionality of optoelectronic devices and should both be given prime consideration when designing heterojunction devices.
• #### Backbone-driven host–dopant miscibility modulates molecular doping in NDI conjugated polymers

(Materials Horizons, Royal Society of Chemistry (RSC), 2021-12-20) [Article]
Molecular doping is the key to enabling organic electronic devices, however, the design strategies to maximize doping efficiency demands further clarity and comprehension. Previous reports focus on the effect of the side chains, but the role of the backbone is still not well understood. In this study, we synthesize a series of NDI-based copolymers with bithiophene, vinylene, and acetylenic moieties (P1G, P2G, and P3G, respectively), all containing branched triethylene glycol side chains. Using computational and experimental methods, we explore the impact of the conjugated backbone using three key parameters for doping in organic semiconductors: energy levels, microstructure, and miscibility. Our experimental results show that P1G undergoes the most efficient n-type doping owed primarily to its higher dipole moment, and better host–dopant miscibility with N-DMBI. In contrast, P2G and P3G possess more planar backbones than P1G, but the lack of long-range order, and poor host–dopant miscibility limit their doping efficiency. Our data suggest that backbone planarity alone is not enough to maximize the electrical conductivity (σ) of n-type doped organic semiconductors, and that backbone polarity also plays an important role in enhancing σ via host–dopant miscibility. Finally, the thermoelectric properties of doped P1G exhibit a power factor of 0.077 μW m−1 K−2, and ultra-low in-plane thermal conductivity of 0.13 W m−1K−1 at 5 mol% of N-DMBI, which is among the lowest thermal conductivity values reported for n-type doped conjugated polymers.
• #### Near-IR Absorbing Molecular Semiconductors Incorporating Cyanated Benzothiadiazole Acceptors for High-Performance Semitransparent n-Type Organic Field-Effect Transistors

(ACS Materials Letters, American Chemical Society (ACS), 2021-12-17) [Article]
Small band gap molecular semiconductors are of interest for the development of transparent electronics. Here we report two near-infrared (NIR), n-type small molecule semiconductors, based upon an acceptor-donor-acceptor (A-D-A) approach. We show that the inclusion of molecular spacers between the strong-electron-accepting end group, 2,1,3-benzothiadiazole-4,5,6-tricarbonitrile, and the donor core affords semiconductors with very low band gaps down to 1 eV. Both materials were synthesized by a one-pot, 6-fold nucleophilic displacement of a fluorinated precursor by cyanide. Significant differences in solid-state ordering and charge carrier mobility are observed depending on the nature of the spacer, with a thiophene spacer resulting in solution processed organic field-effect transistors (OFETs) exhibiting excellent electron mobility up to 1.1 cm2 V-1 s-1. The use of silver nanowires as the gate electrode enables the fabrication of a semitransparent OFET device with an average visible transmission of 71% in the optical spectrum.
• #### Organic neuromorphic electronics for sensorimotor integration and learning in robotics

(Science Advances, American Association for the Advancement of Science (AAAS), 2021-12-10) [Article]
In living organisms, sensory and motor processes are distributed, locally merged, and capable of forming dynamic sensorimotor associations. We introduce a simple and efficient organic neuromorphic circuit for local sensorimo-tor merging and processing on a robot that is placed in a maze. While the robot is exposed to external environ-mental stimuli, visuomotor associations are formed on the adaptable neuromorphic circuit. With this on-chip sensorimotor integration, the robot learns to follow a path to the exit of a maze, while being guided by visually indicated paths. The ease of processability of organic neuromorphic electronics and their unconventional form factors, in combination with education-purpose robotics, showcase a promising approach of an affordable, versatile, and readily accessible platform for exploring, designing, and evaluating behavioral intelligence through decen-tralized sensorimotor integration.
• #### Room-temperature multiple ligands-tailored SnO2 quantum dots endow in situ dual-interface binding for upscaling efficient perovskite photovoltaics with high VOC.

(Light: Science & Applications, Springer Science and Business Media LLC, 2021-12-03) [Article]
The benchmark tin oxide (SnO2) electron transporting layers (ETLs) have enabled remarkable progress in planar perovskite solar cell (PSCs). However, the energy loss is still a challenge due to the lack of "hidden interface" control. We report a novel ligand-tailored ultrafine SnO2 quantum dots (QDs) via a facile rapid room temperature synthesis. Importantly, the ligand-tailored SnO2 QDs ETL with multi-functional terminal groups in situ refines the buried interfaces with both the perovskite and transparent electrode via enhanced interface binding and perovskite passivation. These novel ETLs induce synergistic effects of physical and chemical interfacial modulation and preferred perovskite crystallization-directing, delivering reduced interface defects, suppressed non-radiative recombination and elongated charge carrier lifetime. Power conversion efficiency (PCE) of 23.02% (0.04 cm2) and 21.6% (0.98 cm2, VOC loss: 0.336 V) have been achieved for the blade-coated PSCs (1.54 eV Eg) with our new ETLs, representing a record for SnO2 based blade-coated PSCs. Moreover, a substantially enhanced PCE (VOC) from 20.4% (1.15 V) to 22.8% (1.24 V, 90 mV higher VOC, 0.04 cm2 device) in the blade-coated 1.61 eV PSCs system, via replacing the benchmark commercial colloidal SnO2 with our new ETLs.