Abstract
Morphological stability is a key requirement for outdoor operation of organic solar cells. We demonstrate that morphological stability and lifetime of polymer/fullerene based solar cells under thermal stress depend strongly on the substrate interface on which the active layer is deposited. In particular, we find that the stability of benchmark PCDTBT/PCBM solar cells under modest thermal stress is substantially increased in inverted solar cells employing a ZnO substrate compared to conventional devices employing a PEDOT:PSS substrate. This improved stability is observed to correlate with PCBM nucleation at the 50 nm scale, which is shown to be strongly influenced by different substrate interfaces. Employing this approach, we demonstrate remarkable thermal stability for inverted PCDTBT:PC70BM devices on ZnO substrates, with negligible (<2%) loss of power conversion efficiency over 160 h under 85 °C thermal stress and minimal thermally induced “burn-in” effect. We thus conclude that inverted organic solar cells, in addition to showing improved environmental stability against ambient humidity exposure as widely reported previously, can also demonstrate enhanced morphological stability. As such we show that the choice of suitable substrate interfaces may be a key factor in achieving prolonged lifetimes for organic solar cells under thermal stress conditions.
Highlights
Organic photovoltaic (OPV) solar cells attract significant interest due to their potential in ease of processing, low cost and high flexibility
Through a side-by-side comparison of the morphological and device behaviour of the benchmark PCDTBT:PCBM blend system on two substrate types that are extensively used for OPV device fabrication, PEDOT:PSS and ZnO, we show that the thermal “burn-in” process is strongly dependent upon the bottom interface on which the active layer is deposited
We further demonstrate superior thermal stability of devices based on ZnO substrates (Supplementary Fig. S1) which were found to show significantly suppressed nm-scale PCBM crystallisation compared to PEDOT:PSS, thereby providing an attractive approach to control the blend morphology and improve the operating stability of OPV devices by varying their interfacial properties
Summary
Our study is focused on investigating the influence of substrates on the morphological behaviour of OPV blend films and devices under thermal annealing conditions. (See Supplementary Fig. S2 for an illustration of our methodology). The neutron reflectivity scattering length density depth profile of PCDTBT:PCBM on SiOx (in comparison with PEDOT:PSS, see Supplementary Fig. S9) suggests that a surface energy mediated higher local PCBM concentration adjacent to the substrate, coupled with a higher mobility of PCBM within the polymer matrix, may facilitate micron-scale crystallisation. PCBM crystallisation at both the micron- and nano-scale appears to correlate with differences in surface energy (and potentially surface roughness), with ZnO exhibiting minimal crystallisation for both mechanisms within the temperature range studied With this approach, we demonstrate that inverted OPV device structures employing a ZnO electron collection layer can exhibit, in addition to previously reported improved resistance to humidity induced degradation, remarkably improved morphological and device stability under 85 °C thermal stress. This study demonstrates that selection of suitable device electrodes can substantially enhance the morphological stability of the photoactive layer and the thermal stability of organic solar cells
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