Anastasia Soultati

The Influence of Hydrogenation and Oxygen Vacancies on Molybdenum Oxides Work Function and Gap States for Application in Organic Optoelectronics

Molybdenum oxide is used as a low-resistance anode interfacial layer in applications such as organic light emitting diodes and organic photovoltaics. However, little is known about the correlation between its stoichiometry and electronic properties, such as work function and occupied gap states. In addition, despite the fact that the knowledge of the exact oxide stoichiometry is of paramount importance, few studies have appeared in the literature discussing how this stoichiometry can be controlled to permit the desirable modification of the oxides electronic structure. This work aims to investigate the beneficial role of hydrogenation (the incorporation of hydrogen within the oxide lattice) versus oxygen vacancy formation in tuning the electronic structure of molybdenum oxides while maintaining their high work function. A large improvement in the operational characteristics of both polymer light emitting devices and bulk heterojunction solar cells incorporating hydrogenated Mo oxides as hole injection/extraction layers was achieved as a result of favorable energy level alignment at the metal oxide/organic interface and enhanced charge transport through the formation of a large density of gap states near the Fermi level.

Porphyrin oriented self-assembled nanostructures for efficient exciton dissociation in high-performing organic photovoltaics

Herein we report on enhanced organic solar cell performance through the incorporation of cathode interfacial layers consisting of self-organized porphyrin nanostructures with a face-on configuration. In particular, a water/methanol-soluble porphyrin molecule, the free base meso-tetrakis(1-methylpyridinium-4-yl)porphyrin chloride, is employed as a novel cathode interlayer in bulk heterojunction organic photovoltaics. It is demonstrated that the self-organization of this porphyrin compound into aggregates in which molecules adopt a face-to-face orientation parallel to the organic semiconducting substrate induces a large local interfacial electric field that results in a significant enhancement of exciton dissociation. Consequently, enhanced photocurrent and open circuit voltage were obtained resulting in overall device efficiency improvement in organic photovoltaics based on bulk heterojunction mixtures of different polymeric donors and fullerene acceptors, regardless of the specific combination of donor–acceptor employed. To highlight the impact of molecular orientation a second porphyrin compound, the Zn-metallated meso-tetrakis(1-methylpyridinium-4-yl)porphyrin chloride, was also studied and it was found that it forms aggregates with an edge-to-edge molecular configuration inducing a smaller increase in the device performance.

Correction: Hydrogenated under-stoichiometric tungsten oxide anode interlayers for efficient and stable organic photovoltaics

Correction for ‘Hydrogenated under-stoichiometric tungsten oxide anode interlayers for efficient and stable organic photovoltaics’ by M. Vasilopoulou et al., J. Mater. Chem. A, 2014, 2, 1738–1749.

Surface passivation effect by fluorine plasma treatment on ZnO for efficiency and lifetime improvement of inverted polymer solar cells

Zinc oxide (ZnO) is an important material for polymer solar cells (PSCs) where the characteristics of the interface can dominate both the efficiency and lifetime of the device. In this work we study the effect of fluorine (SF6) plasma surface treatment of ZnO films on the performance of PSCs with an inverted structure. The interaction between fluorine species present in the SF6 plasma and the ZnO surface is also investigated in detail. We provide fundamental insights into the passivation effect of fluorine by analyzing our experimental results and theoretical calculations and we propose a mechanism according to which a fluorine atom substitutes an oxygen atom or occupies an oxygen vacancy site eliminating an electron trap while it may also attract hydrogen atoms thus favoring hydrogen doping. These multiple fluorine roles can reduce both the recombination losses and the electron extraction barrier at the ZnO/fullerene interface improving the selectivity of the cathode contact. Therefore, the fabricated devices using the fluorine plasma treated ZnO show high efficiency and stable characteristics, irrespective of the donor : acceptor combinations in the photoactive blend. Inverted polymer solar cells, consisting of the P3HT:PC71BM blend, exhibited increased lifetime and high power conversion efficiency (PCE) of 4.6%, while the ones with the PCDTBT:PC71BM blend exhibited a PCE of 6.9%. Our champion devices with the PTB7:PC71BM blends reached a high PCE of 8.0% and simultaneously showed exceptional environmental stability when using the fluorine passivated ZnO cathode interlayers.

Fast Recovery of the High Work Function of Tungsten and Molybdenum Oxides via Microwave Exposure for Efficient Organic Photovoltaics.

In this work, we use microwave exposure of tungsten and molybdenum oxides to improve hole extraction in organic photovoltaics (OPVs). This is a result of fast recovery of the high work function of metal oxides occurring within a few seconds of microwave processing. Using the space-charge-limited current model, we verified the formation of an anode contact that facilitates hole extraction, while Mott-Schottky analysis revealed the enhancement of the device built-in field in the devices with the microwave-exposed metal oxides. Both were attributed to the formation of large interfacial dipoles at the ITO/microwave-exposed metal oxide interface. The power conversion efficiency (PCE) of OPVs using microwave-exposed metal oxides and based on blends of poly[(9-(1-octylnonyl)-9H-carbazole-2,7-diyl)-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) with ([6,6]-phenyl-C71 butyric acid methyl ester, PC71BM) reached values of 7.2%, which represents an increase of about 30% compared with the efficiency of 5.7% of devices using metal oxides not subjected to microwave exposure.

Sol–gel synthesized, low-temperature processed, reduced molybdenum peroxides for organic optoelectronics applications

Reduced molybdenum peroxides with varying degrees of reduction were synthesized following a modified sol–gel peroxo method and the respective films were employed as anode interfacial layers in organic optoelectronics applications, such as organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs). The degree of reduction was controlled through both the synthesis route and the thermal treatment protocol of the obtained films. The films were thoroughly investigated with a variety of spectroscopic, diffraction, and electron microscopy methods (UV-Vis, FT-IR, XPS, UPS, Raman, XRD, SEM, and TEM). These films were found to be considerably sub-stoichiometric with a relatively high content of hydrogen. When they were used as anode interfacial layers in OLED and OPV devices, high efficiencies and adequate temporal stability were achieved. The enhanced hole injection/extraction properties of the reduced molybdenum peroxide films were attributed to the improved charge transport facilitated through the gap states present in these materials.

A silanol-functionalized polyoxometalate with excellent electron transfer mediating behavior to ZnO and TiO 2 cathode interlayers for highly efficient and extremely stable polymer solar cells

Combining high efficiency and long lifetime under ambient conditions still poses a major challenge towards commercialization of polymer solar cells. Here we report a facile strategy that can simultaneously enhance the efficiency and temporal stability of inverted photovoltaic architectures. Inclusion of a silanol-functionalized organic–inorganic hybrid polyoxometalate derived from a PW9O34 lacunary phosphotungstate anion, namely (nBu4N)3[PW9O34(tBuSiOH)3], significantly increases the effectiveness of the electron collecting interface, which consists of a metal oxide such as titanium dioxide or zinc oxide, and leads to a high efficiency of 6.51% for single-junction structures based on poly(3-hexylthiophene):indene-C60 bisadduct (P3HT:IC60BA) blends. The above favourable outcome stems from a large decrease in the work function, an effective surface passivation and a decrease in the surface energy of metal oxides which synergistically result in the outstanding electron transfer mediating capability of the functionalized polyoxometalate. In addition, the insertion of a silanol-functionalized polyoxometalate layer significantly enhances the ambient stability of unencapsulated devices which retain nearly 90% of their original efficiencies (T90) after 1000 hours.

Highly conductive, optically transparent, low work-function hydrogen-doped boron-doped ZnO electrodes for efficient ITO-free polymer solar cells

In this work, highly conductive, optically transparent and low work-function hydrogen-doped boron-doped ZnO (BZO:H) cathode electrodes were prepared by a hydrogen post annealing treatment of the as-deposited boron-doped ZnO (BZO) samples. It was found that hydrogen post annealing at temperatures around 200 °C resulted in the formation of electrode materials which exhibited higher conductivity and carrier concentration, reduced sheet resistance and significantly increased optical transparency compared with their non-annealed BZO counterparts. In addition, hydrogen incorporation in the material lattice caused a significant reduction in their work function which may be beneficial for device operation. As a result, polymer solar cells using BZO:H films as transparent cathode electrodes exhibited higher efficiencies compared to those obtained for devices using the non-annealed counterparts. In particular, devices based on the poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C71butyric acid methyl ester (PC71BM) system as the photoactive layer exhibited a PCE of 3.90%, whereas those based on the poly[(9-(1-octylnonyl)-9H-carbazole-2,7-diyl)-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT):PC71BM and poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7):PC71BM as the active components reached high PCE values of 5.90 and 7.25%, respectively, which are comparable or even higher than reported efficiencies obtained for devices using doped ZnO-based transparent electrodes.

Dehydration of molybdenum oxide hole extraction layers via microwave annealing for the improvement of efficiency and lifetime in organic solar cells

A significant contribution to the improvement of efficiency and lifetime of organic solar cells is due to the successful engineering of the metal contact/organic interface by introducing appropriate interlayers. In the current work we show that a short microwave post-annealing treatment in air of an under-stoichiometric molybdenum oxide (MoOx) hole transport layer significantly enhanced the performance and lifetime of an organic solar cell based on a poly(3-hexylthiophene):[6,6]-phenyl-C71-butyric acid methyl ester (P3HT:PC71BM) blend. The enhanced performance is mainly driven by improvement in the short circuit current (Jsc) and the fill factor (FF), caused by, except for an increase of the anode work function, reduced series resistance, and increased shunt resistance and also higher charge generation efficiency, reduced recombination losses and improved hole transport towards the anode contact. In addition, the lifetime of the devices with microwave annealed MoOx interlayers was also significantly improved compared to those with as-deposited MoOx and, especially, those with the PEDOT-PSS interlayer. The above were attributed to effective dehydration which was also followed by structural transformation and crystallization of the MoOx layer during microwave annealing. The removal of absorbed water molecules led to alterations of the structure and microstructure of the MoOx films, visible in the X-ray diffraction patterns, infrared and Raman spectra and atomic force microscopy images recorded on their surface without influencing the oxides chemical composition as evidenced by X-ray photoelectron spectroscopy. During microwave annealing the substrate remains practically at room temperature, so the method is applicable for films deposited on plastics or other temperature-sensitive substrates.

Insights into the passivation effect of atomic layer deposited hafnium oxide for efficiency and stability enhancement in organic solar cells

Atomic layer deposited hafnium oxide is inserted between the zinc oxide electron transport material and the photoactive blend to serve as an ultra-thin passivation interlayer in organic solar cells with an inverted architecture. The deposition of hafnium oxide significantly improves the surface properties of zinc oxide via effective surface passivation and beneficial modification of surface energy; the latter leads to improved nanomorphology of the photoactive blend. As a result, lower recombination losses and improved electron transport/collection at the cathode interface are achieved. A simultaneous increase in open-circuit voltage, short-circuit current density and fill factor is obtained leading to a power conversion efficiency of 6.30% in the ALD-modified cell using a poly(3-hexylthiophene):indene-C60-bisadduct blend as the photoactive layer; this represents a 25% improvement compared to 5.04% of the reference device. Moreover, the incorporation of the passivation interlayer yields a significant stability enhancement in the fabricated solar cells which retain more than 80% of their initial efficiency (T80 lifetime) after 750 hours while the reference cell exhibits a T80 equal to 250 hours.