Atomic-Layer-Deposited Insulating Oxides for Surface Passivation of TiO2 and ZnO in High Efficiency Organic Photovoltaics

Maria Vasilopoulou^a, Panagiotis Argitis^a, Dimitris Davazoglou^a, Georgios Papadimitropoulos^a, Anastasia Soultati^a, Dimitra Georgiadou^a, Leonidas Palilis^b, Martha Botzakaki^b, Christoforos Krontiras^b, Stavroula Georga^b, Stella Kennou^c, Theodoros Papadopoulos^d, Dina Fattakhova Rohlfing^e, Thomas Bein^e, Florian Auras^e

^aNational Center for Scientific Research Demokritos, Terma Patriarchou Grigoriou, Athens, 15354, Greece

^bUniversity of Patras, Department of Physics, 26500 Patras, Greece

^cDepartment of Chemical Engineering, University of Patras, 26500 Patras, Greece

^dInstitute of Renewable Energy and Environmental Technologies (IREET) Department of Engineering University of Bolton, Deane Rd., BL3 5AB , Bolton

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Dimitra Georgiadou, 040
Publication date: 5th February 2015

Organic photovoltaic (OPV) cells are becoming more attractive for large-scale, high-throughput and cost effective manufacturing on flexible plastic substrates as they offer a viable alternative to conventional solar cells, which are fabricated via vacuum-based processes. An important step towards the realization of widespread commercialization is the achievement of higher efficiencies in the best cells and improvement of the scale-up process. One of the limiting factors in attaining high efficiencies is the energetic losses at the active layer/electrode interfaces. To overcome this last issue, anode and cathode interlayers have been used for efficient transport of charge carriers after the dissociation of the electron-hole pair (exciton) that is generated in the active layer, acting thus as electron- or hole-selective layers, respectively. Among various electron-selective interlayers, titanium dioxide (TiO2) and zinc oxide (ZnO) have been widely used to modify the bottom cathode contact in OPV cells with an inverted architecture due to their relatively low work function, optical transparency, n-type conductivity and solution processability. However, their application in OPVs is greatly hindered by high recombination rates of photogenerated electron-hole pairs, which results from trap states present at their surface. Therefore, passivation of those surface traps has been raised as a major issue in order to reduce charge recombination and improve the electron extraction rates. Here a strategy to passivate surface trap states of TiO2 and ZnO films used as cathode interlayers in organic photovoltaics (OPVs) through applying insulating oxide nanolayers, such as aluminum, hafnium, germanium and zirconium oxides by thermal atomic layer deposition (ALD), is investigated. The results suggest that the surface traps in TiO2 and ZnO are oxygen vacancies, which cause undesirable recombination and high electron extraction barrier, reducing the open-circuit voltage and the short-circuit current of the complete OPV device. It is found that the ALD insulating oxides enable excellent passivation of the TiO2 and ZnO surfaces. OPV devices based on different photoactive layers and using the passivated electron extraction layers exhibit a significant enhancement of more than 30% in their power conversion efficiencies compared to their reference devices without the insulating metal oxide nanolayers. This is a result of significant suppression of charge recombination at the cathode interface of the device.

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