Plasmonically sensitized metal-oxide electron extraction layers for organic solar cells

Abstract

ZnO and TiOx are commonly used as electron extraction layers (EELs) in organic solar cells (OSCs). A general phenomenon of OSCs incorporating these metal-oxides is the requirement to illuminate the devices with UV light in order to improve device characteristics. This may cause severe problems if UV to VIS down-conversion is applied or if the UV spectral range (λ < 400 nm) is blocked to achieve an improved device lifetime. In this work, silver nanoparticles (AgNP) are used to plasmonically sensitize metal-oxide based EELs in the vicinity (1–20 nm) of the metal-oxide/organic interface. We evidence that plasmonically sensitized metal-oxide layers facilitate electron extraction and afford well-behaved highly efficient OSCs, even without the typical requirement of UV exposure. It is shown that in the plasmonically sensitized metal-oxides the illumination with visible light lowers the WF due to desorption of previously ionosorbed oxygen, in analogy to the process found in neat metal oxides upon UV exposure, only. As underlying mechanism the transfer of hot holes from the metal to the oxide upon illumination with hν < Eg is verified. The general applicability of this concept to most common metal-oxides (e.g. TiOx and ZnO) in combination with different photoactive organic materials is demonstrated.

Direct observation of UV-induced charge accumulation in inverted-type polymer solar cells with a TiOx layer: Microscopic elucidation of the light-soaking phenomenon

“…[3][4][5][6][7] One of the fundamental issues is the light-soaking phenomenon with respect to UV-light irradiation, which has been commonly observed in IOSCs with electron-collecting layers such as TiO x . 2,[8][9][10][11][12][13][14][15] Although high-temperature annealing treatments on the TiO x layer can prevent the phenomenon, such treatments are undesirable for low-temperature device fabrication using flexible plastic substrates. 15 Additionally, long-term UV-light irradiation on IOSCs can cause device degradation.…”

“…Although the light-soaking phenomenon has been explained on the basis of charge-trap fillings in the electron-collecting layers using phenomenological methods, the mechanism is not yet completely understood, and a microscopic investigation using a direct research method is greatly desired. [9][10][11][12][13][14]16 To fully elucidate the light-soaking phenomenon, electron spin resonance (ESR) spectroscopy is the most suitable technique because ESR is a highly sensitive and nondestructive technique for directly observing the charges in organic semiconductors and their devices at the molecular level. [17][18][19][20][21][22][23][24] ESR studies have demonstrated a clear correlation between performance deterioration and charge accumulation (or trappings) in a normal-type OSC, 18 and have clarified degradation mechanism due to charge formation during device fabrication.…”

Solvothermal synthesis of ZnO nanoparticles at low temperatures as cathode buffer layers for polymer solar cells with an inverted device structure

“…In the inverted structure the interface of ITO/PEDOT:PSS can be avoided by using various high band gap n-type of oxides [9][10][11][12], such as ZnO, TiO 2 and SnOx. Among these materials, ZnO is more attractive for its high optical transparency, environmentally friendly nature, relatively higher electron mobility, Ohmic contact with the organic layer and blocking capability for the UVlight induced photodegradation of organic materials.…”

Plasmon-Electrical Effects on Organic Solar Cells by Incorporation of Metal Nanostructures

“…(b) In addition, the incorporation of plasmonic metal nanomaterials into carrier transport layer (CTL) has also been studied to enhance performance of OSCs [11], [36]- [43]. The mechanisms of morphology modification, hot carriers transfer [41], [42] and charge-accumulation effects [43] have been reported to improve the performance of OSCs. (c) The introduction of metal nanopatterned structure as electrode was also reported, the excitation of a bounded surface wave along the interface of metal nanopatterned electrode and the surrounding materials by SPR effect and confinement of near field in active layer by quasi-guide modes were attributed to the improvement of active layer absorption [44]- [52].…”