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      Polyethylenimine Interfacial Layers in Inverted Organic Photovoltaic Devices: Effects of Ethoxylation and Molecular Weight on Efficiency and Temporal Stability.

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          Abstract

          We report a comparative study of polyethylenimine (PEI) and ethoxylated-polyethylenimine (PEIE) cathode buffer layers in high performance inverted organic photovoltaic devices. The work function of the indium-tin oxide (ITO)/zinc oxide (ZnO) cathode was reduced substantially (Δφ = 0.73-1.09 eV) as the molecular weight of PEI was varied from 800 g mol(-1) to 750 000 g mol(-1) compared with the observed much smaller reduction when using a PEIE thin film (Δφ = 0.56 eV). The reference inverted polymer solar cells based on the small band gap polymer PBDTT-FTTE (ITO/ZnO/PBDTT-FTTE:PC70BM/MoO3/Ag), without a cathode buffer layer, had an average power conversion efficiency (PCE) of 6.06 ± 0.22%. Incorporation of a PEIE cathode buffer layer in the same PBDTT-FTTE:PC70BM blend devices gave an enhanced performance with a PCE of 7.37 ± 0.53%. In contrast, an even greater photovoltaic efficiency with a PCE of 8.22 ± 0.10% was obtained in similar PBDTT-FTTE:PC70BM blend solar cells containing a PEI cathode buffer layer. The temporal stability of the inverted polymer solar cells was found to increase with increasing molecular weight of the cathode buffer layer. The results show that PEI is superior to PEIE as a cathode buffer layer in high performance organic photovoltaic devices and that the highest molecular weight PEI interlayer provides the highest temporal stability.

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          Author and article information

          Journal
          ACS Appl Mater Interfaces
          ACS applied materials & interfaces
          American Chemical Society (ACS)
          1944-8252
          1944-8244
          Dec 02 2015
          : 7
          : 47
          Affiliations
          [1 ] Department of Chemical Engineering and Department of Chemistry, University of Washington , Seattle, Washington 98195-1750, United States.
          Article
          10.1021/acsami.5b08147
          26550983
          0e477b46-a13b-4562-93ed-3634155fec7b
          History

          work function reduction,cathode buffer layers,device stability,organic photovoltaic devices,polyethylenimine,polymer solar cells

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