Degradation Behavior of PTB7:PC71BM and P3HT:PC71BM Organic Solar Cells with a TiOx Interlayer

Recently, the performance of organic solar cells has dramatically improved by incorporating a functional interfacial layer between active layer and metal electrode, forming bulk heterojunction (BHJ). Many interfacial materials, such as metal oxides, self-assembled monolayers (SAMs) and conjugated polyelectrolytes (CPEs), have successfully enhanced energy conversion efficiency of solar cells [4]. Among these interlayers, titanium oxide (TiO_x) was used for an optical spacer as well as a hole blocking layer which led to an increase in the power conversion efficiency (PCE). In addition TiOx is known to improve the stability of the organic solar cell devices [5].

In this study, the organic photovoltaic with bulk heterojunction (BHJ), thieno(3,4-b)-thiophene/ benzodithiophene copolymer and (6,6)-phenyl C₇₁ butyric acid methyl ester (PTB7/PC₇₁BM) treated with titanium oxide (TiO_x) interlayer was fabricated and compared with the stability of organic solar cells based on poly (3- hexylthiophene) and [6,6]-phenylC₇₁ butyric acid methyl ester (P3HT/PC₇₁BM) with TiO_x interlayer. TiO_x was prepared by sol-gel chemistry method. The crystalline structure of TiO_x was described by X-ray diffraction (XRD) while the absorption spectrum, and surface morphology of the BHJ were studied by UV-Vis spectroscopy and atomic force microscopy (AFM) respectively.

Organic solar cells were fabricated by spin-coating PEDOT:PSS layer on the top of ITO glass substrate. After annealing, the substrates were then transferred into a nitrogen filled glove box for spin-casting photoactive layers. Then TiO_x layer was spin coated in air. Subsequently, aluminum (Al) electrode was deposited, producing the devices active area of ~ 10 mm². Finally after Al deposition, the P3HT/PC₇₁BM device was subjected to annealing treatment in glove box.

The electrical properties of the resultant devices were investigated by measuring the current density–voltage (J–V). Also the normalized efficiency of both devices as a function of time has been measured and stability was analyzed in terms of optical, structural and morphological degradation.

References

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