The present work details a facile and low-temperature (125C) solution-processed Al-doped ZnO (AZO) buffer layer functioning very effectively as electron accepting/hole blocking layer for a wide range of polymer:fullerene bulk heterojunction systems, and yielding power conversion efficiency in excess of 10% (8%) on glass (plastic) substrates. We show that ammonia addition to the aqueous AZO nanoparticle solution is a critically important step toward producing compact and smooth thin films which partially retain the aluminum doping and crystalline order of the starting AZO nanocrystals. The ammonia treatment appears to reduce the native defects via nitrogen incorporation, making the AZO film a very good electron transporter and energetically matched with the fullerene acceptor. Importantly, highly efficient solar cells are achieved without the need for additional surface chemical passivation or modification, which has become an increasingly common route to improving the performance of evaporated or solution-processed ZnO ETLs in solar cells.
3720 wileyonlinelibrary.com (OPVs), and organic fi eld-effect transistors (OFETs). A crucial parameter in the development of these technologies is the OSC charge-carrier mobility, and how it is related to the different OSC materials, processing, device architectures, and device operating conditions. Many semiconducting polymers and small molecules are amorphous, and such disorder in the physical morphology is expected to refl ect itself in disorder of the energy levels of the hole and electron transport states ( Figure 1 a). Such energetic disorder will directly impact charge transport and mobility. [1][2][3][4][5][6][7][8][9][10][11][12] For such energetically disordered OSCs, one feature predicted by many charge transport models is a charge-carrier density dependent mobility. The original Gaussian disorder model (GDM) developed by Bässler for hopping transport in a Gaussian density of states (DOS) distribution was a single carrier approach, [ 1 ] as were the models based on it which considered both correlated energetic disorder, [ 2 ] with a smoothly varying energy landscape (see Figure 1 a), and the effect of polaronic relaxation. [ 3 ] The percolation model developed by Vissenberg and Matters for transport involving an exponential DOS instead considered multiple carriers, [ 4 ] and this predicted a mobility which has a power-law dependency on the charge-carrier density. The measured transistor mobility falls two to three orders of magnitude below that predicted from the charge-carrier density dependent model, and does not follow the expected power-law relationship. The experimental results for these two amorphous polymers are therefore consistent with a charge-carrier density independent mobility, and this is discussed in terms of polaron-dominated hopping and interchain correlated disorder.
Nanostructured oxide arrays have received significant attention as charge injection and collection electrodes in numerous optoelectronic devices. Zinc oxide (ZnO) nanorods have received particular interest owing to the ease of fabrication using scalable, solution processes with a high degree of control of rod dimension and density. Here we implement vertical ZnO nanorods as electron injection layers in organic light emitting diodes (OLEDs) for display and lighting purposes. Implementing our nanorods into devices with an emissive polymer, poly(9,9-dioctyluorene-altbenzothiadiazole) (F8BT) and poly(9,9-di-n-octylfluorene-alt-N-(4-butylphenyl)dipheny-lamine) (TFB) as an electron blocking layer, brightness and efficiencies up to 8600 cd/m 2 and 1.66 cd/A were achieved. We highlight simple solution processing methodologies combined with post-deposition thermal processing to achieve complete wetting of the nanorod arrays with the emissive polymer. The introduction of TFB to minimize charge leakage and non-radiative exciton decay results in dramatic increases to device yields and provides an insight into the operating mechanism of these devices. We demonstrate the detected emission originates from within the polymer layers with no evidence of ZnO band-2 edge or defect emission. The work represents a significant development for the ongoing implementation of ZnO nanorod arrays into efficient light emitting devices.
Processing temperature is highlighted as a convenient means of controlling the optical and charge transport properties of solution processed electron transport layers (ETLs) in inverted polymer solar cells. Using the well-studied active layer -poly(3-hexylthiophene-2,5-diyl) (P3HT):indene-C 60 bisadduct (ICBA) -we show the influence of ETL processing temperatures from 25 °C -450 °C, reporting the role of crystallinity, structure, charge transport and Fermi level (E F ) on numerous device performance characteristics. We determine that an exceptionally low temperature processed ETL (110 °C) increases that device power conversion efficiency (PCE) by a factor greater than 50% compared with a high temperature (450 °C) processed ETL. Modulations in device series and shunt resistance, induced by changes in the ETL transport properties are observed in parallel to significant changes in device open circuit voltage attributed to changes on the E F of the ETLs. Our work highlights the importance of interlayer control in multilayer photovoltaic devices and presents a convenient material compatible with future flexible and roll-to-roll processes.
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