ABSTRACT:While recent improvements in the reported peak power conversion efficiency (PCE) of hybrid organic-inorganic perovskite solar cells have been truly astonishing, there are many fundamental questions about the electronic behavior of these materials. Here we have studied a set of electronic devices employing methylammonium lead iodide ((MA)PbI 3 ) as the active material and conducted a series of temperature-dependent measurements. Field-effect transistor, capacitor and photovoltaic cell measurements all reveal behavior consistent with substantial and strongly temperature-dependent polarization susceptibility in (MA)PbI 3 at temporal and spatial scales that significantly impact functional behavior. The relative PCE of (MA)PbI 3 photovoltaic cells is observed to reduce drastically with decreasing temperature, suggesting that such polarization effects could be a prerequisite for high-performance device operation. The pace at which new materials and designs for solar cells emerge is very slow, [1][2][3][4][5][6] and is arguably comparable to the discovery of high T c superconductors. [7][8] The finding that hybrid organic metal halide solar cells based on CH 3 NH 3 PbI 3 can lead to high power conversion efficiency (PCE) using simple coating methods, in a material comprising earth-abundant elements has therefore garnered significant interest. 6, 9-10 Not only has the peak reported PCE exceeded 20% in a short time, [11][12][13] but the processing techniques widely employed suggest that commercial products could be fabricated using low-cost, large-area techniques, potentially compatible with flexible substrates. 14The ABX 3 perovskite crystal structure is characterized by three-dimensionally corner connected network of BX 6/2 octahedra that is filled by the A ions. Perovskite compounds where the A is an organic cation, B is usually a main group element, and X is a group 7 anion (halide) We found that the electrical characteristics of (MA)PbI 3 had a complex temperature dependence. At room temperature these devices exhibit low source-drain currents and no fieldinduced current modulation (see Figure 1(a)). However when the temperature is reduced below 220K, a field-effect is observed and the drain current continues to increase as the temperature is reduced. Where the drain current is observed to be substantially in excess of gate current the gradual-channel approximation 44 has been applied to extract a value of field-effect mobility. The gradual channel approximation for extraction of carrier mobility in field-effect devices assumes a single electronic charge carrier, uniform carrier accumulation in the channel, and timeindependent behavior and consequently does not provide a simple estimation of the carrier motion for measurements with non-ideal effects. This is consistent with our observation of absolute values of electron mobility substantially lower than those previously reported using other techniques. 16 Nonetheless it can be applied as a proxy for relative transistor performance, assuming a high transcondu...

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The additive substituent effect of phenylmercapto derivatives of corannulene are investigated by spectroscopic, electrochemical, and computational techniques. The per-substituted phenylmercaptocorannulene is incorporated as a replacement for [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) in a photovoltaic device. Keywords: photovoltaic, electrochemistry, corannulene, photochemistry IntroductionIn a similar fashion to fullerenes, 1 curved aromatic hydrocarbons, like corannulene, 1, behave as electrondeficient π-systems, 2 the redox properties of which can be tuned by derivatization (Figure 1). 3 Curved aromatic hydrocarbons related to corannulene have been used in various materials applications including the development of fluorescent chemosensors, 4 organic field-effect transistors, 5 and heterojunction photovoltaics. 6 In each case, specific photochemical and redox properties have been important to the success of the application. Tuning the photochemical and redox properties of corannulene cognates corresponds to influencing the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels as well as the HOMO-LUMO gap. Whereas simple alkyl substituents raise HOMO and LUMO levels, 7 fluorinated alkyl groups lower both; 8 in neither case is there a substantial effect on the HOMO-LUMO gap as gauged from absorption spectroscopy. Organothio-substituents raise HOMO and lower LUMO levels such that one can obtain good electron acceptors with smaller HOMO-LUMO gaps. For example, compared to 1, a pale yellow material with first reduction at −2.33 eV in acetonitrile, decasubstitution of 1 with thiophenol (SPh) produces a deep red material with the first reduction potential at −1.22 eV, 10 which is similar to the reported value of −1.17 volts for [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM). Given that many high-performing organic photovoltaic (OPV) devices rely on C 60 and its derivatives to act as electron acceptors, 11 this similarity in electrochemistry between PCBM and decakisphenylmercaptocorannulene motivates a study into the nature of tuning the physical properties (electrochemistry, ultraviolet-visible (UV-Vis) absorption, fluorescence, and phosphorescence) of corannulene by phenylmercapto substitution and the photovoltaic characterization of decakis(phenylthio-)corannulene as an acceptor in an OPV device. Furthermore, the optoelectronic tunability of corannulenes makes them potentially useful as organic light emitting diodes (OLEDs), 12 as organic conductors, 13 or in high density carbon electrodes. Vol. 27, No. 10, 2016 ExperimentalGeneral synthesis 1,3-Dimethyl-2-imidazolidinone (DMI; 5-10 mL, dried over molecular sieve) was added to a round bottom flask equipped with a reflux condenser. Thiophenol (1.5 equiv./ reaction site) and sodium hydride (1.2 equiv./reaction site, 60% in mineral oil) were added and stirred at room temperature for 10 min. The halogenated corannulene derivative (2a-7a, 1 equiv.) was added and the solution was heated at 60 °C for 18 h. The...

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