Achilleas Savva

Methods for Improving the Lifetime Performance of Organic Photovoltaics with Low‐Costing Encapsulation

Recent years have seen considerable advances in organic photovoltaics (OPVs), most notably a significant increase in their efficiency, from around 4 % to over 10 %. The stability of these devices, however, continues to remain an issue that needs to be resolved to enable their commercialization. This review discusses the main degradation processes of OPVs and recent methods that help to increase device stability and lifetime. One of the most effective steps that can be taken to increase the lifetime of OPVs is their encapsulation, which protects them from atmospheric degradation. Efficient encapsulation is essential for long-term device performance, but it is equally important for the commercialization of OPVs to strike a balance between achieving the maximum device protection possible and using low-cost processing for their encapsulation. Various encapsulation techniques are discussed herein, with emphasis on their cost effectiveness and their overall suitability for commercial applications.

Highly efficient indium tin oxide-free organic photovoltaics using inkjet-printed silver nanoparticle current collecting grids

We report an in-depth investigation of an inkjet-printed silver (Ag) nanoparticle grid combined with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) of different conductivities as an alternative to an indium tin oxide (ITO)-based transparent anode for organic solar cell applications. The reported measurements revealed higher transparency of the inkjet-printed Ag nanoparticle-based grid when compared to different thicknesses of ITO on glass substrates. Based on the proposed current collecting grid, a record power conversion efficiency of 2% is achieved for ITO-free organic solar cells.

Cesium-doped zinc oxide as electron selective contact in inverted organic photovoltaics

Water based sol-gel processed Cesium-doped Zinc oxide (CZO) with low processing annealing temperature is introduced as an efficient electron selective contact in inverted Organic Photovoltaics (OPVs). The corresponding inverted OPVs not only demonstrate similar performance compared to the well-established sol-gel processed ZnO inverted devices but also maintain their functionality when thick layers of CZO, suitable for the up scaling scenario of OPVs have been used. The three orders of magnitude higher conductivity of CZO than ZnO in combination with the high transmittance above 80%, makes this doped oxide a suitable electron selective contact for the low-cost, roll-to-roll printing process of OPVs.

The effect of hole transporting layer in charge accumulation properties of p-i-n perovskite solar cells

The charge accumulation properties of p-i-n perovskite solar cells were investigated using three representative organic and inorganic hole transporting layer (HTL): (a) Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS, Al 4083), (b) copper-doped nickel oxide (Cu:NiOx), and (c) Copper oxide (CuO). Through impedance spectroscopy analysis and modelling, it is shown that charge accumulation is decreased in the HTL/perovskite interface, between PEDOT:PSS to Cu:NiOx and CuO. This was indicative from the decrease in double layer capacitance (Cdl) and interfacial charge accumulation capacitance (Cel), resulting in an increase to recombination resistance (Rrec), thus decreased charge recombination events between the three HTLs. Through AFM measurements, it is also shown that the reduced recombination events (followed by the increase in Rrec) are also a result of increased grain size between the three HTLs, thus reduction in the grain boundary area. These charge accumulation properties of the thre...

Linking the HOMO-LUMO gap to torsional disorder in P3HT/PCBM blends

The electronic structure of [6,6]-phenyl C61 butyric acid methyl ester (PCBM), poly(3-hexylthiophene) (P3HT), and P3HT/PCBM blends is studied using soft X-ray emission and absorption spectroscopy and density functional theory calculations. We find that annealing reduces the HOMO-LUMO gap of P3HT and P3HT/PCBM blends, whereas annealing has little effect on the HOMO-LUMO gap of PCBM. We propose a model connecting torsional disorder in a P3HT polymer to the HOMO-LUMO gap, which suggests that annealing helps to decrease the torsional disorder in the P3HT polymers. Our model is used to predict the characteristic length scales of the flat P3TH polymer segments in P3HT and P3HT/PCBM blends before and after annealing. Our approach may prove useful in characterizing organic photovoltaic devices in situ or even in operando.

High performance indium tin oxide-free solution-processed organic light emitting diodes based on inkjet-printed fine silver grid lines

We report on the grid design requirements and inkjet-printing processing conditions of well-defined silver nanoparticles combined with poly(3,4-ethylenedioxylthiophene):poly(styrenesulfonate) PEDOT:PSS as indium tin oxide (ITO) replacement for ITO-free organic light emitting diodes (OLEDs). Solution-processed ITO-free OLEDs based on the 5BTF8 blend of poly(9,9-dioctylfluorene-alt-benzothiadiazole (F8BT) and poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) light-emitting layers, processed in ambient conditions, showed comparable luminance efficiency and power efficiency values to reference devices based on ITO and near identical efficiencies at low luminance values.

Influence of the Hole Transporting Layer on the Thermal Stability of Inverted Organic Photovoltaics Using Accelerated-Heat Lifetime Protocols

High power conversion efficiency (PCE) inverted organic photovoltaics (OPVs) usually use thermally evaporated MoO3 as a hole transporting layer (HTL). Despite the high PCE values reported, stability investigations are still limited and the exact degradation mechanisms of inverted OPVs using thermally evaporated MoO3 HTL remain unclear under different environmental stress factors. In this study, we monitor the accelerated lifetime performance under the ISOS-D-2 protocol (heat conditions 65 °C) of nonencapsulated inverted OPVs based on the thiophene-based active layer materials poly(3-hexylthiophene) (P3HT), 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), and thieno[3,2-b]thiophene-diketopyrrolopyrrole (DPPTTT) blended with [6,6]-phenyl C71-butyric acid methyl ester (PC[70]BM). The presented investigation of degradation mechanisms focus on optimized P3HT:PC[70]BM-based inverted OPVs. Specifically, we present a systematic study on the thermal stability of inverted P3HT:PC[70]BM OPVs using solution-processed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and evaporated MoO3 HTL. Using a series of measurements and reverse engineering methods, we report that the P3HT:PC[70]BM/MoO3 interface is the main origin of failure of the P3HT:PC[70]BM-based inverted OPVs under intense heat conditions, a trend that is also observed for the other two thiophene-based polymers used in this study.

The Role of the Side Chain on the Performance of N-type Conjugated Polymers in Aqueous Electrolytes

We report a design strategy that allows the preparation of solution processable n-type materials from low boiling point solvents for organic electrochemical transistors (OECTs). The polymer backbone is based on NDI-T2 copolymers where a branched alkyl side chain is gradually exchanged for a linear ethylene glycol-based side chain. A series of random copolymers was prepared with glycol side chain percentages of 0, 10, 25, 50, 75, 90, and 100 with respect to the alkyl side chains. These were characterized to study the influence of the polar side chains on interaction with aqueous electrolytes, their electrochemical redox reactions, and performance in OECTs when operated in aqueous electrolytes. We observed that glycol side chain percentages of >50% are required to achieve volumetric charging, while lower glycol chain percentages show a mixed operation with high required voltages to allow for bulk charging of the organic semiconductor. A strong dependence of the electron mobility on the fraction of glycol chains was found for copolymers based on NDI-T2, with a significant drop as alkyl side chains are replaced by glycol side chains.

Room temperature nanoparticulate interfacial layers for perovskite solar cells via solvothermal synthesis

We present a solvothermal synthetic route to produce monodisperse CuO nanoparticles (NPs) in the range of 5–10 nm that can be used as a hole selective interfacial layer between indium tin oxide (ITO) and the perovskite active layer for p–i–n perovskite solar cells by spin casting the dispersions at room temperature. The bottom electrode interface modification provided by spherical CuO-NPs at room temperature promotes the formation of high quality perovskite photoactive layers with a large crystal size and strong optical absorption. Furthermore, it is shown that the nanoparticulate nature of the CuO hole transporting interfacial layer can be used to improve light manipulation within the perovskite solar cell device structure. The corresponding p–i–n CH3NH3PbI3-based solar cells show high Voc values of 1.09 V, which is significantly higher compared to the Voc values obtained with conventional PEDOT:PSS hole selective contact based perovskite solar cells.

Evaporation-free inverted organic photovoltaics using a mixture of silver nanoparticle ink formulations for solution-processed top electrodes

We report an investigation of inkjet-printed silver (Ag) nanoparticle inks combined with a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) formulation for solution-processed top electrodes in inverted organic photovoltaics (OPVs) employing the poly(3-hexylthiopehene):phenyl-C61-butyric acid methyl ester material system. We propose a suitable mixture of Ag nanoparticle inks to control the printability and electrical conductivity of the solution-processed top electrode. Based on the proposed solution-processed hole-selective contact, a power conversion efficiency in the range of 3% is reported for evaporation-free inverted OPVs.