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Charge selective interlayers are of critical importance in order for solar cells based on low mobility materials, such as polymer-fullerene blends, to perform well. Commonly used anode interlayers consist of high work function transition metal oxides, with molybdenum trioxide (MoO3) being arguably the most used. Here, it is shown that a thin interlayer of MoO3 causes unintentional bulk doping in solar cells based on polymers and polymer-fullerene blends. The doping concentrations determined from capacitance–voltage measurements are larger than 1016 cm−3 and are seen to increase closer to the anode, reference devices without MoO3 are undoped. Using time of flight secondary ion mass spectroscopy it is furthermore shown that molybdenum is present on the surface of all films with an interfacial layer of MoO3 beneath the active layer. Doping concentrations of this magnitude are detrimental for device performance, especially for active layers >100 nm.

Inverted organic solar cells generally exhibit a strong s-shaped kink in the current–voltage characteristics (JV curve) that may be removed by exposure to UV light (light-soaking) leading to a drastically improved performance. Using in-device characterization methods the origin of the light-soaking issue in inverted solar cells employing titanium dioxide (TiO2) as an electron selective layer is clarified. An injected hole reservoir accumulated at the TiO2/organic interface of the pristine device is observed from extraction current transients; the hole reservoir increases the recombination and results in an s-shape in the JV curve of pristine devices. The hole reservoir and the s-shape is a result of the energetics at the selective contact in the pristine device; the effect of UV exposure is to decrease the work function of the indium tin oxide/TiO2-contact, increasing the built-in potential. This hinders the build-up of the hole reservoir and the s-shape is removed. The proposed model is in excellent agreement with drift-diffusion simulations.

The impact of trapping on the recombination dynamics in polymer:fullerene blends is clarified using the highly ordered bulk heterojunction (BHJ) blend poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) at different weight ratios as a model system. The recombination dynamics are determined using both transient charge extraction and steady-state techniques. The results show that both the decay of photogenerated charge and the light ideality factor at a polymer:fullerene weight ratio of 1:4 are fully consistent with 2D Langevin recombination; in the 1:1 case the recombination is seen to be affected by electron trapping. The theory of 2D Langevin recombination is extended to the case with high trap density in agreement with the observations in the 1:1 case. The recombination capture coefficients are derived both for trap-assisted and band-to-band recombination and it can be seen that anisotropic charge transport reduces the capture coefficients in both cases resulting in a reduced overall recombination.