Solar cells based on conjugated polymer and fullerene blends have been developed as a low-cost alternative to silicon. For efficient solar cells, electron-hole pairs must separate into free mobile charges that can be extracted in high yield. We still lack good understanding of how, why and when carriers separate against the Coulomb attraction. Here we visualize the charge separation process in bulk heterojunction solar cells by directly measuring charge carrier drift in a polymer:fullerene blend with ultrafast time resolution. We show that initially only closely separated (o1 nm) charge pairs are created and they separate by several nanometres during the first several picoseconds. Charge pairs overcome Coulomb attraction and form free carriers on a subnanosecond time scale. Numerical simulations complementing the experimental data show that fast three-dimensional charge diffusion within an energetically disordered medium, increasing the entropy of the system, is sufficient to drive the charge separation process.
We have investigated the effects of thickness variation and thermal treatment of the electrode polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) in photovoltaic and photodetector devices using conjugated polymer blends as the photoactive material. By variation of the PEDOT:PSS layer thickness between 25 and 150 nm, we found optimum device performance, in particular low dark current and high external quantum efficiency (EQE) and open-circuit voltage (V oc ), at around 70 nm. This has been observed for two different active layers. Annealing studies on the PEDOT:PSS films, with temperatures varied between 120 and 400 °C, showed an optimum device performance, in particular EQE and V oc at 250 °C. This optimum performance was found to be associated with loss of water from the PSS shell of the PEDOT:PSS grains. For annealing temperatures above 260 °C, device performance was dramatically reduced. This was associated with chemical decomposition leading to loss of sulfonic acid, although this did not significantly affect the in-plane conductivity.
We study charge generation and recombination in organic solar cells that utilize perylene tetracarboxydiimide (PDI) as an electron acceptor and a conjugated polymer as an electron donor. PDI is a promising electron acceptor because of its strong red absorption, LUMO well placed to accept electrons from many conjugated polymers, and good electron mobility. However, we find that, when PDI is finely dispersed in a conjugated polymer, the device efficiency is severely limited by very fast bimolecular charge recombination and that, when the blend is made coarser, the device efficiency becomes limited instead by PDI excitons quickly relaxing into stabilized intermolecular states between PDI molecules rather than undergoing charge transfer. The intramolecular PDI states formed are the same as those observed in PDI blended with poly(styrene) and have lower energy and mobility than the exciton. The two loss channels, that is, bimolecular recombination when charge transfer is fast and reduced charge transfer due to intermolecular state formation when charge transport is better, mean that quantum efficiency may always be low in organic solar cells utilizing PDI unless modification of the PDI can suppress the rate of intermolecular state formation without compromising chargetransport properties. Our results are based on detailed, bias-dependent transient-absorption experiments which also reveal the carrier mobility and internal quantum efficiency (as a function of field) directly in the operating organic solar cells.
The up‐conversion photoluminescence (PL) in films of polyfluorene (PF) doped with metallated porphyrins is reported for the first time. The dependence of the up‐conversion process on the pump laser intensity and wavelength, the central metal moiety of the dopants, and the temperature is presented. Up‐conversion emission is observed at pump intensities as low as 2 kW cm–2. Comparison of the PF integral PL intensities after laser excitation by 532, 543, and 405 nm enables the discussion of the energy‐transfer mechanism and the efficiency of the process.
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