We describe the use of few-layer graphene nanoribbons, either attached to counter electrodes or dispersed into electrolyte, to induce optical transparency of an iodide/triiodide redox couple in a dye-sensitized solar cell (DSSC). We then evaluate the effect of reversible bleaching of the electrolyte on the DSSC performance. This bleaching effect is related to an energy transfer from photoexcited quantum-dot-like regions to the triiodide (I 3 À ) radical ions in the electrolyte, saturating their absorption in the visible optical range. DSSC power conversion efficiency using few-layer graphene nanoribbons at the counter electrode (5.8%) did not deteriorate when the electrolyte became optically transparent. The increased transparency of the electrolyte resulted in a decreased photocurrent density (from 17.6 to 14.2 mA/cm 2 ), an unchanged open circuit voltage of 750 mV, and a slightly increased fill factor (from 0.45 to 0.55). When the few-layer graphene nanoribbons were introduced into the electrolyte directly by ultrasonication, a semitransparent DSSC was found to have increased its power conversion efficiency in an optically inverted setup from 5.75% to 7.01%, arising from an increase in photocurrent from 9.9 to 12.1 mA/cm 2 . This significant photocurrent increase demonstrates that the effect of electrolyte bleaching can be used for further improving power conversion efficiency for inverted and tandem DSSCs, in which light has to pass through the electrolyte to generate photocurrent on one or more photocells.
We report a novel method for stable doping of carbon nanotubes (CNT) based on methods of molecular self assembly. A conformal growth of a self-assembled monolayer of fluoroalkyl trichloro-silane (FTS) at CNT surfaces results in a strong increase of the sheet conductivity of CNT electrodes by 60–300%, depending on the CNT chirality and composition. The charge carrier mobility of undoped partially aligned CNT films was independently estimated in a field-effect transistor geometry (∼100 cm2V−1s−1). The hole density induced by the FTS monolayer in CNT sheets is estimated to be ∼1.8 × 1014 cm−2. We also show that FTS doping of CNT anodes greatly improves the performance of organic solar cells. This large and stable doping effect, easily achieved in large-area samples, makes this approach very attractive for applications of CNTs in transparent and flexible electronics.
Organic light emitting devices, in particular, properties of polymer light-emitting transistors with printed electrodes and bilayer printed devices with in-plane emission have been investigated and discussed. The semitransparent device based on poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) with Ag-nanowire source/drain and transparent carbon nanotube gate electrodes exhibits ambipolar and light-emitting characteristics. For the devices with oriented poly(9,9-dioctylfluorene) (F8) films, enhanced electron and hole field-effect mobilities have been achieved by aligning the polymer chains parallel to the transport direction. The bilayer device using F8BT lower layer and oriented F8 upper layer with the channel direction parallel to the polymer orientation exhibits improved EL intensity and higher external quantum efficiency than that with the channel direction perpendicular to the polymer chains orientation. The optical pulses of more than 100 Hz frequency are generated by directly modulating a bilayer device with an in-plane emission pattern.
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