Moritz Riede

Abstract:

We investigate the dominating recombination mechanisms in bulk heterojunction solar cells, using a blend of ZnPc and C as model system. Analyzing the open-circuit voltage (V oc) as a function of illumination intensity, we find that trap-assisted recombination dominates for low light intensities, whereas at 1 sun, direct/bimolecular recombination becomes important. The recombination parameters are not significantly influenced by the blend mixing ratio and are also valid for injected charges. By changing the hole transport layer, recombination at the contact is separately identified as further mechanism reducing V oc at higher light intensities. © 2013 AIP Publishing LLC.

Abstract:

Trap states are known to be one of the key parameters limiting charge transport in organic semiconductors and hence the performance of organic solar cells. Here, small-molecule organic solar cells based on a bulk heterojunction between zinc-phtalocyanine (ZnPc) and the fullerene C60 are characterized according to their trapping nature by noninvasive methods and under ambient conditions. We show how impedance spectroscopy, applied to systematically varied device structures, reveals the trap localization as well as its occupation mechanisms. Further insight is given from investigations of different device working points and illumination intensities. Thus, we find the traps to be bulk states in the active layer with an electron-trapping nature. They can be described by a Gaussian energy distribution of 55 meV width, centered at 0.46 eV below the electron transport level and with a concentration of 3.5 × 1016 cm-3. Moreover, the trap states act as recombination centers in the presence of injected or photogenerated charge carriers. The results are confirmed by electrical simulations. © 2013 American Physical Society.

Abstract:

Efficient organic electronic devices require a detailed understanding of the relation between molecular structure, thin film growth, and device performance, which is only partially understood at present. Here, we show that small changes in molecular structure of a donor absorber material lead to significant changes in the intermolecular arrangement within organic solar cells. For this purpose, phenyl rings and propyl side chains are fused to the diindenoperylene (DIP) molecule. Grazing incidence X-ray diffraction and variable angle spectroscopic ellipsometry turned out to be a powerful combination to gain detailed information about the thin film growth. Planar and bulk heterojunction solar cells with C60 as acceptor and the DIP derivatives as donor are fabricated to investigate the influence of film morphology on the device performance. Due to its planar structure, DIP is found to be highly crystalline in pristine and DIP:C60 blend films while its derivatives grow liquid-like crystalline. This indicates that the molecular arrangement is strongly disturbed by the steric hindrance induced by the phenyl rings. The high fill factor (FF) of more than 75% in planar heterojunction solar cells of the DIP derivatives indicates excellent charge transport in the pristine liquid-like crystalline absorber layers. However, bulk heterojunctions of these materials surprisingly result in a low FF of only 54% caused by a weak phase separation and thus poor charge carrier percolation paths due to the lower ordered thin film growth. In contrast, crystalline DIP:C60 heterojunctions lead to high FF of up to 65% as the crystalline growth induces better percolation for the charge carriers. However, the major drawback of this crystalline growth mode is the nearly upright standing orientation of the DIP molecules in both pristine and blend films. This arrangement results in low absorption and thus a photocurrent which is significantly lower than in the DIP derivative devices, where the liquid-like crystalline growth leads to a more horizontal molecular alignment. Our results underline the complexity of the molecular structure-device performance relation in organic semiconductor devices. © 2013 Elsevier B.V. All rights reserved.