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

Recent Submissions

  • Suppressing Co-Crystallization of Halogenated Non-Fullerene Acceptors for Thermally Stable Ternary Solar Cells

    Hultmark, Sandra; Paleti, Sri Harish Kumar; Harillo, Albert; Marina, Sara; Nugroho, Ferry Anggoro Ardy; Liu, Yanfeng; Ericsson, Leif K. E.; Li, Ruipeng; Martín, Jaime; Bergqvist, Jonas; Langhammer, Christoph; Zhang, Fengling; Yu, Liyang; Campoy-Quiles, Mariano; Moons, Ellen; Baran, Derya; Müller, Christian (Advanced Functional Materials, Wiley, 2020-10-14) [Article]
    While photovoltaic blends based on non-fullerene acceptors are touted for their thermal stability, this type of acceptor tends to crystallize, which can result in a gradual decrease in photovoltaic performance and affects the reproducibility of the devices. Two halogenated indacenodithienothiophene-based acceptors that readily co-crystallize upon mixing are studied, which indicates that the use of an acceptor mixture alone does not guarantee the formation of a disordered mixture. The addition of the donor polymer to the acceptor mixture readily suppresses the crystallization, which results in a fine-grained ternary blend with nanometer-sized domains that do not coarsen due to a high Tg ≈ 200 °C. As a result, annealing at temperatures of up to 170 °C does not markedly affect the photovoltaic performance of ternary devices, in contrast to binary devices that suffer from acceptor crystallization in the active layer. The results indicate that the ternary approach enables the use of high-temperature processing protocols, which are needed for upscaling and high-throughput fabrication of organic solar cells. Further, ternary devices display a stable photovoltaic performance at 130 °C for at least 205 h, which indicates that the use of acceptor mixtures allows to fabricate devices with excellent thermal stability.
  • Metal Halide Perovskites for High-Energy Radiation Detection

    Kakavelakis, George; Gedda, Murali; Panagiotopoulos, Apostolis; Kymakis, Emmanuel; Anthopoulos, Thomas D.; Petridis, Konstantinos (Advanced Science, Wiley, 2020-10-12) [Article]
    Metal halide perovskites (MHPs) have emerged as a frontrunner semiconductor technology for application in third generation photovoltaics while simultaneously making significant strides in other areas of optoelectronics. Photodetectors are one of the latest additions in an expanding list of applications of this fascinating family of materials. The extensive range of possible inorganic and hybrid perovskites coupled with their processing versatility and ability to convert external stimuli into easily measurable optical/electrical signals makes them an auspicious sensing element even for the high-energy domain of the electromagnetic spectrum. Key to this is the ability of MHPs to accommodate heavy elements while being able to form large, high-quality crystals and polycrystalline layers, making them one of the most promising emerging X-ray and γ-ray detector technologies. Here, the fundamental principles of high-energy radiation detection are reviewed with emphasis on recent progress in the emerging and fascinating field of metal halide perovskite-based X-ray and γ-ray detectors. The review starts with a discussion of the basic principles of high-energy radiation detection with focus on key performance metrics followed by a comprehensive summary of the recent progress in the field of perovskite-based detectors. The article concludes with a discussion of the remaining challenges and future perspectives.
  • Sunlight-Driven Biomass Photorefinery for Coproduction of Sustainable Hydrogen and Value-Added Biochemicals

    Wu, Xinxing; Zhao, Heng; Khan, Mohd Adnan; Maity, Partha; Al-Attas, Tareq; Larter, Stephen; Yong, Qiang; Mohammed, Omar F.; Kibria, Md Golam; Hu, Jinguang (ACS Sustainable Chemistry & Engineering, American Chemical Society (ACS), 2020-10-06) [Article]
    We demonstrate a potential pathway of biomass photorefinery (PR) using low-cost CoO/g-C3N4 catalysts for the coproduction of hydrogen and lactic acid under visible light illumination. To do so, we follow a bottom-up approach to systematically investigate the photoreforming performance of glucose, different model celluloses (cellulose I and mercerized and regenerated cellulose II), and raw biomass. Under optimized conditions, the glucose was totally consumed within 3 h of reaction, with nearly 78 wt % carbon conversion to lactic acid. The highest activity observed for cellulose in the PR used phosphoric acid swollen cellulose (PASC, regenerated cellulose II) with a H2 production rate of ∼178 μmol·h−1·gcat−1 , more than 71 wt % cellulose conversion after 12 h, and the formation of ∼617 μmol lactic acid per gram of cellulose. This high activity was mainly attributed to enhanced interaction of the photocatalyst with PASC, as evidenced by quartz crystal microbalance analysis. Based on the knowledge obtained from model cellulose, we took a step further to evaluate the photorefining ability of raw lignocellulosic biomass wheat straw (WS), with/without various biomass pretreatment strategies. The pretreated biomass showed much higher H2 and lactic acid production and cellulose conversions as compared with raw biomass but the degree of improvement is highly dependent on pretreatment strategies. Our results not only demonstrate the potential of using visible light for the coproduction of H2, along with value-added bioproducts from biomass PR, but also shed light on developing pretreatment strategies to achieve a scalable biomass PR.
  • Quantum Confinement and Thickness-Dependent Electron Transport in Solution-Processed In 2 O 3 Transistors

    Isakov, Ivan; Faber, Hendrik; Mottram, Alexander D.; Das, Satyajit; Grell, Max; Regoutz, Anna; Kilmurray, Rebecca; McLachlan, Martyn A.; Payne, David J.; Anthopoulos, Thomas D. (Advanced Electronic Materials, Wiley, 2020-10-05) [Article]
    The dependence of charge carrier mobility on semiconductor channel thickness in field-effect transistors is a universal phenomenon that has been studied extensively for various families of materials. Surprisingly, analogous studies involving metal oxide semiconductors are relatively scarce. Here, spray-deposited In2O3 layers are employed as the model semiconductor system to study the impact of layer thickness on quantum confinement and electron transport along the transistor channel. The results reveal an exponential increase of the in-plane electron mobility (µe) with increasing In2O3 thickness up to ≈10 nm, beyond which it plateaus at a maximum value of ≈35 cm2 V−1 s−1. Optical spectroscopy measurements performed on In2O3 layers reveal the emergence of quantum confinement for thickness <10 nm, which coincides with the thickness that µe starts deteriorating. By combining two- and four-probe field-effect mobility measurements with high-resolution atomic force microscopy, it is shown that the reduction in µe is attributed primarily to surface scattering. The study provides important guidelines for the design of next generation metal oxide thin-film transistors.
  • How Humidity and Light Exposure Change the Photophysics of Metal Halide Perovskite Solar Cells

    Ugur, Esma; Alarousu, Erkki; Khan, Jafar Iqbal; Vlk, Aleš; Aydin, Erkan; de Bastiani, Michele; Albalawi, Ahmed; Gonzalez Lopez, Sandra P.; Ledinský, Martin; De Wolf, Stefaan; Laquai, Frédéric (Solar RRL, Wiley, 2020-09-24) [Article]
    Metal halide perovskites exhibit outstanding optical and electronic properties, but are very sensitive to humidity and light-soaking. In this work, the photophysics of perovskites that have been exposed to such conditions are studied and, in this context, the impact of excess lead iodide (PbI2) is revealed. For exposed samples, the formation of subbandgap states and increased trap-assisted recombination is observed, using highly sensitive absorption and time-resolved photoluminescence (TRPL) measurements, respectively. It appears that such exposure primarily affects the perovskite surface. Consequently, on n–i–p device level, the spiro-OMeTAD/perovskite interface is more rapidly affected than its buried electron-collecting interface. Moreover, both stoichiometric and nonstoichiometric MAPbI3-based solar cells show reduced device performance mainly due to voltage losses. Overall, this study brings forward key points to consider in engineering perovskite solar cells with improved performance and material stability.
  • Nanoporous GaN/n-type GaN: a cathode structure for ITO-free perovskite solar cells

    Lee, Kwangjae; Min, Jung-Wook; Turedi, Bekir; Alsalloum, Abdullah Yousef; Min, Jung-Hong; Kim, Yeong Jae; Yoo, Young Jin; Oh, Semi; Cho, Namchul; Subedi, Ram Chandra; Mitra, Somak; Yoon, Sang Eun; Kim, Jong Hyun; Park, Kwangwook; Chung, Tae-Hoon; Jung, Sung Hoon; Baek, Jong-Hyeob; Song, Young Min; Roqan, Iman S.; Ng, Tien Khee; Ooi, Boon S.; Bakr, Osman (ACS Energy Letters, American Chemical Society (ACS), 2020-09-17) [Article]
    Introducing suitable electron/hole transport layers and transparent conductive layers (TCLs) into perovskite solar cells (PSCs) is key to enhancing the selective extraction of charge carriers and reducing surface recombination losses. Here, we introduce nanoporous gallium nitride (NP GaN)/n-type GaN (n-GaN) as a dual-function cathode structure for PSCs, acting as both the TCL and the electron transport layer (ETL). We demonstrate that the hierarchical NP GaN structure provides an expanded interfacial contact area with the perovskite absorber, while the n-GaN under the NP GaN displays high transmittance in the visible spectrum as well as higher lateral electric conductivity than that of a conventional ITO film. Prototype MAPbI3 PSCs based on this NP GaN/n-GaN cathode structure (without an extra ETL) show a power conversion efficiency of up to 18.79%. The NP GaN/n-GaN platform demonstrated herein paves the way for PSCs to take advantage of the widely available heterostructures of mature III-nitride-based technologies.
  • Ambipolar Deep-Subthreshold Printed-Carbon-Nanotube Transistors for Ultralow-Voltage and Ultralow-Power Electronics.

    Portilla, Luis; Zhao, Jianwen; Wang, Yan; Sun, Liping; Li, Fengzhu; Robin, Malo; Wei, Miaomiao; Cui, Zheng; Occhipinti, Luigi G; Anthopoulos, Thomas D.; Pecunia, Vincenzo (ACS nano, American Chemical Society (ACS), 2020-09-14) [Article]
    The development of ultralow-power and easy-to-fabricate electronics with potential for large-scale circuit integration (i.e., complementary or complementary-like) is an outstanding challenge for emerging off-the-grid applications, e.g., remote sensing, "place-and-forget", and the Internet of Things. Herein we address this challenge through the development of ambipolar transistors relying on solution-processed polymer-sorted semiconducting carbon nanotube networks (sc-SWCNTNs) operating in the deep-subthreshold regime. Application of self-assembled monolayers at the active channel interface enables the fine-tuning of sc-SWCNTN transistors toward well-balanced ambipolar deep-subthreshold characteristics. The significance of these features is assessed by exploring the applicability of such transistors to complementary-like integrated circuits, with respect to which the impact of the subthreshold slope and flatband voltage on voltage and power requirements is studied experimentally and theoretically. As demonstrated with inverter and NAND gates, the ambipolar deep-subthreshold sc-SWCNTN approach enables digital circuits with complementary-like operation and characteristics including wide noise margins and ultralow operational voltages (≤0.5 V), while exhibiting record-low power consumption (≤1 pW/μm). Among thin-film transistor technologies with minimal material complexity, our approach achieves the lowest energy and power dissipation figures reported to date, which are compatible with and highly attractive for emerging off-the-grid applications.
  • Interplay between temperature and bandgap energies on the outdoor performance of perovskite/silicon tandem solar cells

    Aydin, Erkan; Allen, Thomas; de Bastiani, Michele; Xu, Lujia; Ávila, Jorge; Salvador, Michael; Van Kerschaver, Emmanuel; De Wolf, Stefaan (Nature Energy, Springer Science and Business Media LLC, 2020-09-14) [Article]
    Perovskite/silicon tandem solar cells promise power conversion efficiencies beyond the Shockley–Queisser limit of single-junction devices; however, their actual outdoor performance is yet to be investigated. Here we fabricate 25% efficient two-terminal monolithic perovskite/silicon tandem solar cells and test them outdoors in a hot and sunny climate. We find that the temperature dependence of both the silicon and perovskite bandgaps—which follow opposing trends—shifts the devices away from current matching for two-terminal tandems that are optimized at standard test conditions. Consequently, we argue that the optimal perovskite bandgap energy at standard test conditions is <1.68 eV for field performance at operational temperatures greater than 55 °C, which is lower compared with earlier findings. This implies that bromide-lean perovskites with narrower bandgaps at standard test conditions—and therefore better phase stability—hold great promise for the commercialization of perovskite/silicon tandem solar cells.
  • SOILING LOSS RATE MEASUREMENTS OF PHOTOVOLTAIC MODULES IN A HOT AND HUMID DESERT ENVIRONMENT

    Abdullah, Marwan; Khayyat, Ahmad; Basaheeh, Ali; Kotsovos, Konstantinos; Ballard, Ian; AlSaggaf, Ahmed; Gereige, Issam; Theron, Ricardo (Journal of Solar Energy Engineering, ASME International, 2020-09-10) [Article]
    Abstract Power generation from renewable energy sources, in particular solar photovoltaics (PV), has become extremely attractive thanks to its very low levelized cost of electricity (LCoE). In desert-like environments, the energy yield is drastically reduced due to dust accumulation. While effective and affordable cleaning strategies can be implemented in large, MW-size PV power plants, soiling remains an economic and logistic challenge. In this paper, we analyze the soiling loss rates of PV modules for different tilt angles measured during a period of 15 months in the Western Region of Saudi Arabia. We observe a strong correlation between weather parameters like humidity and wind speed, and the mechanism of dust accumulation. Our measurements show that, for specific weather conditions, soiled modules undergo a partial cleaning process. As a consequence, and for the first time, the soiling loss rates are shown to have a clear dependence on the current soiling state of the modules, with clean modules soiling twice as fast as soiled ones. This dependency is key for predicting the correct cleaning frequency of a PV power plant. Finally, the results obtained for vertically mounted modules (90°), where dust accumulation is negligible, point to a favorable case for the use of bifacial PV modules.
  • Ledge-directed epitaxy of continuously self-aligned single-crystalline nanoribbons of transition metal dichalcogenides

    Aljarb, Areej; Fu, Jui-Han; Hsu, Chih-Chan; Chuu, Chih-Piao; Wan, Yi; Hakami, Mariam; Naphade, Dipti R.; Yengel, Emre; Lee, Chien-Ju; Brems, Steven; Chen, Tse-An; Li, Ming-Yang; Bae, Sang-Hoon; Hsu, Wei-Ting; Cao, Zhen; Albaridy, Rehab; Lopatin, Sergei; Chang, Wen-Hao; Anthopoulos, Thomas D.; Kim, Jeehwan; Li, Lain-Jong; Tung, Vincent (Nature Materials, Springer Science and Business Media LLC, 2020-09-07) [Article]
    Two-dimensional transition metal dichalcogenide nanoribbons are touted as the future extreme device downscaling for advanced logic and memory devices but remain a formidable synthetic challenge. Here, we demonstrate a ledge-directed epitaxy (LDE) of dense arrays of continuous, self-aligned, monolayer and single-crystalline MoS2 nanoribbons on β-gallium (iii) oxide (β-Ga2O3) (100) substrates. LDE MoS2 nanoribbons have spatial uniformity over a long range and transport characteristics on par with those seen in exfoliated benchmarks. Prototype MoS2-nanoribbon-based field-effect transistors exhibit high on/off ratios of 108 and an averaged room temperature electron mobility of 65 cm2 V−1 s−1. The MoS2 nanoribbons can be readily transferred to arbitrary substrates while the underlying β-Ga2O3 can be reused after mechanical exfoliation. We further demonstrate LDE as a versatile epitaxy platform for the growth of p-type WSe2 nanoribbons and lateral heterostructures made of p-WSe2 and n-MoS2 nanoribbons for futuristic electronics applications.
  • A Multilayered Electron Extracting System for Efficient Perovskite Solar Cells

    Seitkhan, Akmaral; Neophytou, Marios; Hallani, Rawad; Troughton, Joel; Gasparini, Nicola; Faber, Hendrik; Abou-Hamad, Edy; Hedhili, Mohamed N.; Harrison, George T.; Baran, Derya; Tsetseris, Leonidas; Anthopoulos, Thomas D.; McCulloch, Iain (Advanced Functional Materials, Wiley, 2020-09-04) [Article]
    Power conversion efficiencies of perovskite solar cells (PSCs) have rapidly increased from 3.8% to a certified 25.2% within only a decade. Eliminating possible recombination losses at the interfaces is essential to further improve both efficiency and stability of this class of emerging photovoltaic technology. Herein, a simple approach for improving the electron extraction of the PC60BM electron transport layer (ETL) is presented by sequentially depositing Al:ZnO (AZO) and triphenyl-phosphine oxide (TPPO) on top of it, in a p–i–n device configuration. The efficiency of the resulting CH3NH3PbI3-based solar cell is shown to improve from 14.6%, measured for the control PC60BM-only cell, to 17.9% for double-ETL (PC60BM/AZO) and 19.2% for triple-ETL (PC60BM/AZO/TPPO)-based devices, respectively. Optimized triple-ETL-based cells exhibit high fill factor of 82%. The combination of electrical and quantum mechanical calculations shows that efficiency improvement is attributed to reduced trap-assisted recombination at the interface and better energy level alignment due to chemical interactions between PC60BM, AZO, and TPPO. Moreover, it is shown that the use of multilayer ETL results in better device stability (T80 ≈ 800 h) under constant illumination. This work opens new possibilities for inexpensive highly efficient and stable multilayered contacts for PSCs by combining organic small molecules and metal oxides.
  • Impact of p-type doping on charge transport in blade-coated small-molecule:polymer blend transistors

    Basu, Aniruddha; Niazi, Muhammad Rizwan; Scaccabarozzi, Alberto Davide; Faber, Hendrik; Fei, Zuping; Anjum, Dalaver H.; Paterson, Alexandra; Boltalina, Olga; Heeney, Martin; Anthopoulos, Thomas D. (Journal of Materials Chemistry C, Royal Society of Chemistry (RSC), 2020-09-03) [Article]
    Blade-coating is used to fabricate high hole mobility organic transistors based on a p-doped small-molecule:polymer blend semiconductor.
  • Correlating the Phase Behavior with the Device Performance in Binary P3HT: NFA Blend Using Optical Probes of Microstructure

    Rezasoltani, Elham; Guilbert, Anne A. Y.; Yan, Jun; Rodríguez-Martínez, Xabier; Azzouzi, Mohammed; Eisner, Flurin; Tuladhar, Sachetan M; Hamid, Zeinab; Wadsworth, Andrew; McCulloch, Iain; Campoy-Quiles, Mariano; Nelson, Jenny (Chemistry of Materials, American Chemical Society (ACS), 2020-09-02) [Article]
    The performance of photovoltaic devices based on blends of conjugated polymers with non-fullerene acceptors depends upon the phase behaviour and microstructure of the binary, which in turn depends on the chemical structures of the molecular components and the blend composition. We investigate the correlation between molecular structure, composition, phase behaviour and device performance of a model system comprising semi-crystalline poly-3-hexylthiophene (P3HT) as the donor polymer and three non-fullerene acceptors, two of which (O-IDTBR/EH-IDTBR) have a planar core with different side-chains, and one (O-IDFBR) has a twisted core. We combine differential scanning calorimetry with optical measurements including UV-Vis, photoluminescence, spectroscopic ellipsometry and Raman, and photovoltaic device performance measurements, all at varying blend composition. For P3HT:IDTBR blends, the crystallinity of polymer and acceptor are preserved over a wide composition range and the blend displays a eutectic phase behaviour, with the optimum solar cell composition lying close to the eutectic. For P3HT:IDFBR blends, increasing acceptor content disrupts the polymer crystallinity, and the optimum device composition appears to be limited by polymer connectivity rather than being linked to the eutectic. The optical probes allow us to probe both the crystalline and amorphous phases, clearly revealing the compositions at which component mixing disrupts crystallinity.
  • Photocatalysts Based on Organic Semiconductors with Tunable Energy Levels for Solar Fuel Applications

    Kosco, Jan; Moruzzi, Floriana; Willner, Benjamin; McCulloch, Iain (Advanced Energy Materials, Wiley, 2020-09-02) [Article]
    The photocatalytic synthesis of solar fuels such as hydrogen and methane from water and carbon dioxide is a promising strategy to store abundant solar energy in order to overcome its intermittency. Although this approach has been studied for decades using inorganic semiconductor photocatalysts, organic semiconductors have only recently gained notable attention. The tunable energy levels of organic semiconductors can enable the design of photocatalysts with optimized solar light utilization. However, the solar conversion efficiency of organic semiconductor photocatalysts has so far been limited by their low quantum efficiencies. To address this issue, various photocatalyst design strategies including semiconductor energy level optimization, surface modification, and the fabrication of heterojunctions have been applied, resulting in substantial increases in photocatalytic efficiency. This progress report systematically describes the strategies employed to increase the efficiency of organic semiconductor photocatalysts for the generation of solar fuels from water and carbon dioxide. Particular attention is given to describing strategies to enhance quantum efficiency, and insights are provided on the mechanisms underlying their success to aid the rational design of future organic photocatalysts. Perspectives on the future challenges and promising research directions for the design of efficient organic photocatalysts for the generation of solar fuels are also provided.
  • Printable CsPbI3 Perovskite Solar Cells with PCE of 19% via an Additive Strategy.

    Chang, Xiaoming; Fang, Junjie; Fan, Yuanyuan; Luo, Tao; Su, Hang; Zhang, Yalan; Lu, Jing; Tsetseris, Leonidas; Anthopoulos, Thomas D.; Liu, Shengzhong (Frank); Zhao, Kui (Advanced materials (Deerfield Beach, Fla.), Wiley, 2020-09-01) [Article]
    All-inorganic CsPbI3 holds promise for efficient tandem solar cells, but reported fabrication techniques are not transferrable to scalable manufacturing methods. Herein, printable CsPbI3 solar cells are reported, in which the charge transporting layers and photoactive layer are deposited by fast blade-coating at a low temperature (≤100 °C) in ambient conditions. High-quality CsPbI3 films are grown via introducing a low concentration of the multifunctional molecular additive Zn(C6 F5 )2 , which reconciles the conflict between air-flow-assisted fast drying and low-quality film including energy misalignment and trap formation. Material analysis reveals a preferential accumulation of the additive close to the perovskite/SnO2 interface and strong chemisorption on the perovskite surface, which leads to the formation of energy gradients and suppressed trap formation within the perovskite film, as well as a 150 meV improvement of the energetic alignment at the perovskite/SnO2 interface. The combined benefits translate into significant enhancement of the power conversion efficiency to 19% for printable solar cells. The devices without encapsulation degrade only by ≈2% after 700 h in air conditions.
  • The role of exciton lifetime for charge generation in organic solar cells at negligible energy-level offsets

    Classen, Andrej; Chochos, Christos L.; Lüer, Larry; Gregoriou, Vasilis G.; Wortmann, Jonas; Osvet, Andres; Forberich, Karen; McCulloch, Iain; Heumüller, Thomas; Brabec, Christoph J. (Nature Energy, Springer Science and Business Media LLC, 2020-08-31) [Article]
    Organic solar cells utilize an energy-level offset to generate free charge carriers. Although a very small energy-level offset increases the open-circuit voltage, it remains unclear how exactly charge generation is affected. Here we investigate organic solar cell blends with highest occupied molecular orbital energy-level offsets (∆EHOMO) between the donor and acceptor that range from 0 to 300 meV. We demonstrate that exciton quenching at a negligible ∆EHOMO takes place on timescales that approach the exciton lifetime of the pristine materials, which drastically limits the external quantum efficiency. We quantitatively describe this finding via the Boltzmann stationary-state equilibrium between charge-transfer states and excitons and further reveal a long exciton lifetime to be decisive in maintaining an efficient charge generation at a negligible ∆EHOMO. Moreover, the Boltzmann equilibrium quantitatively describes the major reduction in non-radiative voltage losses at a very small ∆EHOMO. Ultimately, highly luminescent near-infrared emitters with very long exciton lifetimes are suggested to enable highly efficient organic solar cells.
  • Amorphous/Crystalline Silicon Interface Stability: Correlation between Infrared Spectroscopy and Electronic Passivation Properties

    Holovský, Jakub; Martín De Nicolás, Silvia; De Wolf, Stefaan; Ballif, Christophe (Advanced Materials Interfaces, Wiley, 2020-08-27) [Article]
    Ultrathin layers of hydrogenated amorphous silicon (a-Si:H), passivating the surface of crystalline silicon (c-Si), are key enablers for high-efficiency silicon heterojunction solar cells. In this work, the authors apply highly sensitive attenuated total reflectance Fourier-transform infrared spectroscopy, combined with carrier-lifetime measurements and carrier-lifetime imaging, to resolve several fundamental and technology-related questions related to the a-Si:H/c-Si interface. To gain insight, the a-Si:H/c-Si interfacial morphology is intentionally manipulated by applying different surface, annealing and ageing treatments. Changes are observed in the vibrational modes of hydrides (SiHX), oxides (SiHX(SiYOZ)) together with hydroxyl and hydrocarbon surface groups. The effect of unintentional oxidation and contamination is considered as well. Electronic interfacial properties are reviewed and discussed of hydrogen mono-layer passivation of the c-Si surface and from the perspectives of a-Si:H bulk properties. It is found that both models have severe limitations and suggest that a new physical model of the interface, considering both is required.
  • Miscibility-Controlled Phase Separation in Double-Cable Conjugated Polymers for Single-Component Organic Solar Cells with Efficiencies over 8.

    Jiang, Xudong; Yang, Jinjin; Karuthedath, Safakath; Li, Junyu; Lai, Wenbin; Li, Cheng; Xiao, Chengyi; Ye, Long; Ma, Zaifei; Tang, Zheng; Laquai, Frédéric; Li, Weiwei (Angewandte Chemie (International ed. in English), Wiley, 2020-08-21) [Article]
    In this work, a record power conversion efficiency of 8.40% was obtained in single-component organic solar cells (SCOSCs) based on double-cable conjugated polymers. This is realized based on the finding that exciton separation plays the same important role as charge transport in SCOSCs. Herein, we designed two double-cable conjugated polymers with almost the identical conjugated backbones and electron-withdrawing side units, but the extra chlorine (Cl) atoms had different positions on the conjugated backbones. We found that, when Cl atoms were positioned at the main chains, the polymer formed the twist backbones, enabling better miscibility with the naphthalene diimide side units. This could improve the interface contact between conjugated backbones and side units, resulting in efficient conversion of excitons into free charges. These observations were confirmed by systematical studies via several advanced measurements. These findings reveal the importance of charge generation process in SCOSCs and also suggest a strategy to improve this process, that is, controlling the miscibility between conjugated backbones and aromatic side units in double-cable conjugated polymers.
  • Self-assembled Monolayer Enables HTL-free Organic Solar Cells with 18% Efficiency and Improved Operational Stability

    Lin, Yuanbao; Firdaus, Yuliar; Isikgor, Furkan Halis; Nugraha, Mohamad Insan; Yengel, Emre; Harrison, George T; Hallani, Rawad; El Labban, Abdulrahman; Faber, Hendrik; Ma, Chun; Zheng, Xiaopeng; Subbiah, Anand Selvin; Howells, Calvyn Travis; Bakr, Osman; McCulloch, Iain; De Wolf, Stefaan; Tsetseris, Leonidas; Anthopoulos, Thomas D. (ACS Energy Letters, American Chemical Society (ACS), 2020-08-18) [Article]
    We report on bulk-heterojunction (BHJ) organic photovoltaics (OPVs) using the self-assembled monolayer (SAM) 2PACz as a hole-selective interlayer functionalized directly onto the indium tin oxide (ITO) anode. The 2PACz is found to change the work function of ITO while simultaneously affecting the morphology of the BHJ deposited atop. Cells with PM6:N3 BHJ and ITO-2PACz anode exhibit a power conversion efficiency (PCE) of 16.6%, which is higher than that measured for bare ITO (6.45%) and ITO/PEDOT:PSS (15.94%) based devices. The enhanced performance is attributed to lower contact-resistance, reduced bimolecular recombination losses, and improved charge transport within the BHJ. Importantly, the ITO-2PACz-based OPVs show a dramatically improved operational stability when compared with PEDOT:PSS-based cells. When the ITO-2PACz anode is combined with the ternary PM6:BTP-eC9:PC71BM BHJ, the resulting cells exhibit a maximum PCE of 18.03%, further highlighting the potential of engineered SAMs for use in hole-selective contacts in high-performance OPVs.
  • Autonomous MXene-PVDF actuator for flexible solar trackers

    Tu, Shao Bo; Xu, Lujia; El Demellawi, Jehad K.; Liang, Hanfeng; Xu, Xiangming; Lopatin, Sergei; De Wolf, Stefaan; Zhang, Xixiang; Alshareef, Husam N. (Nano Energy, Elsevier BV, 2020-08-15) [Article]
    We report a novel flexible solar tracking system based on a photothermal-thermomechanical (PT-TM) actuator comprised of Ti3C2Tx MXene and polyvinylidene fluoride (PVDF) bilayer. The actuation function of the proposed device originates from photothermal and surface plasmon-assisted effects in MXenes, coupled with thermomechanical deformation of in-plane aligned PVDF polymer. Two types of solar tracking modes are evaluated based on the experimental deformation behavior of the PT-TM actuator. We find that the uniaxial East-West solar tracking option increases the overall energy intensity reaching the solar module by over 30%, in comparison with the optimized tilting-controlled mode. We also demonstrate the thermally driven self-oscillation of the MXene-PVDF device, which may have promising potential for optically and thermally driven soft robotics. The PT-TM actuator devices display robust mechanical strength and durability, with no noticeable degradation in their performance after more than 1000 cycles.

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