Bulk heterojunction (BHJ) polymer solar cells (PSCs) based on composites of conjugated polymers (electron donor) and fullerene derivatives (electron acceptor) have attracted attention due to their potential as renewable energy sources. [1 -7 ] The major challenges for BHJ solar cells are the achievement of competitive power conversion effi ciencies (PCEs) and the demonstration of long-term air stability. [8][9][10][11][12][13][14][15][16] BHJ solar cells are typically fabricated with a transparent conductive anode (e.g. indium tin oxide, ITO), a low-work-function metal cathode (e.g., Al, Ca), and an active layer (a mixture of conjugated polymer and fullerene derivative) sandwiched between the anode and cathode. The BHJ layer and cathode dramatically affect the stability. In particular, the cathode is susceptible to degradation by oxygen and water vapor. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is often used as an anode buffer layer. Long-term stability is a problem because PEDOT:PSS is hygroscopic and acidic. [17][18][19][20][21] In order to circumvent these problems, inverted polymer solar cells have been developed; air-stable high-work-function metals (e.g., Au, Ag) are used as the anode to collect holes and ITO is used as the cathode to collect electrons. In the inverted architecture, n-type metal oxides such as titanium oxide (TiO x ), zinc oxide (ZnO), and cesium carbonate (Cs 2 CO 3 ) are deposited onto the ITO electrode to break the symmetry. [ 22 − 24 ] The elimination of the PEDOT:PSS layer improves the device stability. Moreover, in the inverted cell, the anode is a highwork-function metal such as Ag, which can be formed using coating or printing technology to simplify and lower the cost of manufacturing. [ 25 ] Among the n-type metal oxides used in inverted cells, ZnO is a promising candidate due to its relatively high electron mobility, environmental stability, and high transparency. A variety of fabrication methods have been employed to grow thin fi lms of ZnO. Sol-gel method has been extensively investigated as a solution-based thin-fi lm deposition process. [ 26 ] Sol-gelderived ZnO fi lm is widely used in inverted solar cells. However, a high annealing temperature, usually over 200 ° C and incompatible with fl exible substrates, is used to promote crystallization and removal of residual organic compounds. [27][28][29] Although solution-processed ZnO nanoparticles have been shown to be easily processed into thin fi lms via spin coating or roll-to-roll printing at room temperature, [ 23 , 30 , 31 ] ZnO nanoparticles are not very stable in solution and a ligand is usually used to stabilize them. [ 32 ] We report here that uniform sol-gel-derived ZnO fi lms can be obtained at relatively low annealing temperatures ( ≤ 200 ° C) and they can function as the effi cient electron transporting layer in inverted solar cells.Despite a dramatic improvement of stability, inverted solar cells suffer from relatively lower PCEs compared to conventional solar cells, mainly due to the ...

Organic photovoltaic devices that can be fabricated by simple processing techniques are under intense investigation in academic and industrial laboratories because of their potential to enable mass production of flexible and cost-effective devices. Most of the attention has been focused on solution-processed polymer bulk-heterojunction (BHJ) solar cells. A combination of polymer design, morphology control, structural insight and device engineering has led to power conversion efficiencies (PCEs) reaching the 6-8% range for conjugated polymer/fullerene blends. Solution-processed small-molecule BHJ (SM BHJ) solar cells have received less attention, and their efficiencies have remained below those of their polymeric counterparts. Here, we report efficient solution-processed SM BHJ solar cells based on a new molecular donor, DTS(PTTh(2))(2). A record PCE of 6.7% under AM 1.5 G irradiation (100 mW cm(-2)) is achieved for small-molecule BHJ devices from DTS(PTTh(2))(2):PC(70)BM (donor to acceptor ratio of 7:3). This high efficiency was obtained by using remarkably small percentages of solvent additive (0.25% v/v of 1,8-diiodooctane, DIO) during the film-forming process, which leads to reduced domain sizes in the BHJ layer. These results provide important progress for solution-processed organic photovoltaics and demonstrate that solar cells fabricated from small donor molecules can compete with their polymeric counterparts.

The thermoelectric properties of a highperformance electron-conducting polymer, (P(NDIOD-T2), extrinsically doped with dihydro-1H-benzoimidazol-2-yl (NDBI) derivatives, are reported. The highest thermoelectric power factor that has been reported for a solution-processed n-type polymer is achieved; and it is concluded that engineering polymerdopant miscibility is essential for the development of organic thermoelectrics.

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Reversible high voltage redox chemistry is an essential component of many electrochemical technologies, from (electro)catalysts to lithium-ion batteries. Oxygen anion redox has garnered intense interest for such applications, particularly lithium ion batteries, as it offers substantial redox capacity at > 4 V vs. Li/Li + in a variety of oxide materials. However, oxidation of oxygen is almost universally correlated with irreversible local structural transformations, voltage hysteresis, and voltage fade, which currently preclude its widespread use. By comprehensively studying the Li 2-x Ir 1-y Sn y O 3 model system, which exhibits tunable oxidation state and structural evolution with y upon cycling, we reveal that this structure-redox coupling arises from the local stabilization of short ~ 1.8 Å metal-oxygen π bonds and ~ 1.4 Å O-O dimers during oxygen 42 redox, which occurs in Li 2-x Ir 1-y Sn y O 3 through ligand-to-metal charge transfer. Crucially, formation of these oxidized oxygen species necessitates the decoordination of oxygen to a single covalent bonding partner through formation of vacancies at neighboring cation sites, driving cation disorder. These insights establish a point defect explanation for why anion redox often occurs alongside local structural disordering and voltage hysteresis during cycling. Our findings offer an explanation for the unique electrochemical properties of lithium-rich layered oxides, with implications generally for the design of materials employing oxygen redox chemistry. 3 Main Text: Reversible redox chemistry in solids under highly oxidizing conditions (e.g. vs H 2 /H + , Li/Li + , or 52 O) is a powerful tool in (electro)chemical systems, increasing the catalytic activity of oxygenevolution and methane-functionalization (electro)catalysts as well as the energy and power densities of lithium-ion batteries (LIBs). 1 In LIBs in particular, employing high-voltage redox has been identified as a promising avenue to meeting the energy density demands of nextgeneration technologies such as plug-in electric vehicles. Recently, anionic oxygen redox has been shown to offer access to substantial high-voltage (de)intercalation capacity in a range of electrode materials, 2-7 spurring an intense research effort to understand this phenomenon. While many oxygen-redox-active materials have been developed, they almost universally exhibit a host of irreversible electrochemical behaviors such as voltage hysteresis and voltage fade. 8 This is most notable in the anion-redox-active Li-rich 62 layered oxides, Li 1+x M 1-x O 2 (M = a transition metal (TM) or non-transition metal such as Al, Sn, Mg, etc.), which exhibit capacities approaching 300 mAh g-1 but have yet to achieve commercial success due to such electrochemical behaviors. 5, 9 It has been shown both experimentally 10-12 and 65 from first-principles thermodynamics 13 that the migration of M into empty Li sites 9-creating structural disorder in the form of M Li /V M antisite/cation vacancy point defect pairs-is at the root of voltage profile...