Organic co-evaporated films of a PPV-pentamer and C60:: model systems for donor/acceptor polymer blends

Organic solar cells based on an active layer of a spincast polymer donor/acceptor blend have proven to be very efficient. We present similar photovoltaic devices with organic layers that are formed using the technique of vacuum deposition. The donor and acceptor materials are, respectively, the five-ring PPV-type oligomer 2-methoxy-5-(2'-ethylhexyloxy)-1,4-bis((4',4-bisstyryl)styrylbenzene) (MEH-OPV5) and C-60. An elevated substrate temperature during deposition of the single MEH-OPV5 layers on ITO-coated glass substrates yielded polycrystalline films with a rough surface. as was determined from AFM and XRD analysis. The co-evaporation of both materials, also at high substrate temperature, resulted in amorphous, but very smooth films exhibiting a good percolation of donor and acceptor. The dark I-V behaviour of single-layer and donor/acceptor-layer devices in an ITO/PEDOT/organic/Al configuration is compared. It was found that the PEDOT/C-60 interface of an ITO/PEDOT/MEH-OPV5:C-60/Al solar cell structure is responsible for the exponential rise of its dark I-V curve under forward bias. Sandblasting of the glass substrate was applied as a way to reduce the reflection of the incoming light and resulted in a significant increase of the short-circuit current. Standardised spectral response measurements confirmed this effect. The sandblasted bulk heterojunction photovoltaic devices were characterised under AM1.5 illumination and reached a power conversion efficiency of 2.2%. 0 2002 Elsevier Science B.V. All rights reserved.