The effect of multilayer barrier materials on the lifetime of organic photovoltaic cells has been investigated. For thin film encapsulated cells a protective layer was used to prevent damage during barrier layer deposition. No post deposition effects developed after dry box storage. In accelerated temperature and humidity lifetime testing the degradation of the encapsulated cells can be related to the loss of effective cell area. An extrapolation of the lifetime at room conditions has been quantitatively determined by comparing the cell degradation with the loss of Ca in a Ca-oxidation test. The results indicate a barrier permeation rate of 10<sup>-4</sup> gr/[m<sup>2</sup>* day] for these samples, corresponding to a lifetime of greater than 5000 hours. Routes to improvement of the OPV cell lifetime are discussed.
The challenge to reversing the layer sequence of organic photovoltaics (OPVs) is to prepare a selective contact
bottom cathode and to achieve a suitable morphology for carrier collection in the inverted structure. We report the
creation of an efficient electron selective bottom contact based on a solution-processed Titania layer on top of Indium
Tin Oxide. The use of o-xylene as the casting solvent creates an efficient carrier collection network with little vertical
phase segregation, providing sufficient performance for both regular as well as inverted solar cells. We demonstrate
inverted layer sequence OPVs with AM 1.5-calibrated power conversion efficiencies of over 3%.
Different material combinations of two conjugated polymers, each blended with the methanofullerene acceptor phenyl-C<sub>61</sub> butyric acid methyl ester (PCBM) have been evaluated focusing on their potential for application as absorber material in polymer-fullerene bulk-heterojunction solar cells. Devices based on these solution processable composite materials have been studied by means of temperature dependent profiling of the photocurrent. In combination with measurements of the incident photon conversion efficiency, this technique probes the charge carrier recombination losses within the absorber material. Samples based on material composites with a low mobility-lifetime (μτ) product of the charge carriers (OC<sub>1</sub>C<sub>10</sub>-PPV: PCBM) exhibit a thermally activated photocurrent throughout the temperature range from 100 K to 350 K. The latter issue is attributed to the presence of shallow traps inside the bulk of the absorber limiting the photocurrent by recombination and scattering of the charge carriers with defects. Accordingly, the active layer thickness must be kept low at the expense of optical absorption. In contrast, the photocurrent in devices based on absorber materials with a high μτ product, P3HT: PCBM, saturates at a certain temperature and becomes constant, reflecting that all photogenerated charge carriers are efficiently extracted within their lifetime prior to recombination. Thus, solar cells with absorber materials demonstrating a high μτ product, have the potential to be designed with relatively thick absorber films above 100 nm. A large active layer thickness is a prerequisite for industrial deposition techniques, e.g., screen-printing, and improves the mechanical stability of large area flexible solar cells. As consequence of a high μt product the increase of the active layer thickness to L=350 nm in P3HT: PCBM photovoltaic devices results in a higher density of photogenerated charge carriers due to improved light absorption. Consequently, a strongly increased short-circuit current density of up to 15.2 mA/cm<sup>2</sup> was obtained for devices with absorber thickness of 350 nm which rising the power conversion efficiency up to 3.1 %.