Ermioni Polydorou

Annealing-free highly crystalline solution-processed molecular metal oxides for efficient single-junction and tandem polymer solar cells

Polyoxometalate (POM) layers have been used to realize efficient and long-term stable single-junction polymer photovoltaic devices with diverse configuration and donor : acceptor combination in the photoactive blend and polymer tandem cells through functioning as effective hole extraction layers and, also, as recombination layers in the interconnecting unit of the tandem cell. Their unique properties, such as their extremely high work function (WF) of 6.0–6.2 eV, their high degree of crystallinity without any post annealing requirements and, especially, the position of their lowest unoccupied molecular orbital (LUMO), were used to control the characteristics of optoelectronic devices. It was found that POMs having a deep LUMO level lying below the highest occupied molecular orbital (HOMO) of the donor polymer are highly beneficial for device operation due to the interfacial p-doping of the latter. We demonstrated conventional and reverse structure single-junction cells reaching an efficiency of 7.9% in the latter case and a tandem cell with an efficiency of 9.9% using an all solution processed inverted structure, where a POM layer simultaneously offers enhanced hole extraction in the sub-cells and minimal losses in the recombination unit. The specific properties of four POM materials and their role as functional layers in those different types of polymer photovoltaic devices are discussed.

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.

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.

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.

Engineering of Porphyrin Molecules for Use as Effective Cathode Interfacial Modifiers in Organic Solar Cells of Enhanced Efficiency and Stability

In the present work, we effectively modify the TiO2 electron transport layer of organic solar cells with an inverted architecture using appropriately engineered porphyrin molecules. The results show that the optimized porphyrin modifier bearing two carboxylic acids as the anchoring groups and a triazine electron-withdrawing spacer significantly reduces the work function of TiO2, thereby reducing the electron extraction barrier. Moreover, the lower surface energy of the porphyrin-modified substrate results in better physical compatibility between the latter and the photoactive blend. Upon employing porphyrin-modified TiO2 electron transport layers in PTB7:PC71BM-based organic solar cells we obtained an improved average power conversion efficiency up to 8.73%. Importantly, porphyrin modification significantly increased the lifetime of the devices, which retained 80% of their initial efficiency after 500 h of storage in the dark. Because of its simplicity and efficacy, this approach should give tantalizing glimpses and generate an impact into the potential of porphyrins to facilitate electron transfer in organic solar cells and related devices.

Influence of microwave exposure of tungsten oxide hole extraction layers on nanomorphology, optical and electrical properties of organic photovoltaics

In this work, the effect of microwave annealing of tungsten oxide films, which are widely used to enhance hole extraction in organic optoelectronic devices such as organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs), on the nanomorhology and optical properties of bulk heterojuction (BHJ) OPVs, is demonstrated. It is found that a short microwave exposure of under-stoichiometric oxide film enhances the crystallinity/ordering of poly(3-hexylthiophene):[6,6]-phenyl-C71-butyric acid methyl ester (P3HT:PC71BM) blends when coated on microwave annealed tungsten oxide films, as revealed from X-ray diffraction and UV-Vis absorption measurements. The performance of OPVs using microwave annealed tungsten oxides and based on a P3HT:PC71BM blend as the photoactive layer reached values of 3.97%, an increase of about 53% compared with the device using the under-stoichiometric tungsten oxide hole extraction layer not subjected to microwave annealing.