RESUMO
Solution-processed inverted bulk heterojunction (BHJ) organic solar cells (OSCs) are expected to play a significant role in the future of large-area flexible devices and printed electronics. In order to catch the potential of this inverted BHJ technology for use in devices, a solar cell typically requires low-resistance ohmic contact between the photoactive layers and metal electrodes, since it not only boosts performance but also protects the unstable conducting polymer-based active layer from degradation in the working environment. Interfacial engineering delivers a powerful approach to enhance the efficiency and stability of OSCs. In this study, we demonstrated the surface passivation of the ZnO electron transport layer (ETL) by an ultrathin layer of tetracyanoethylene (TCNE). We show that the TCNE film could provide a uniform and intimate interfacial contact between the ZnO and photo-active layer, simultaneously reducing the recombination of electron and holes and series resistance at the contact interface. After successful insertion of TCNE between the ZnO film and the active layer, the parameters, such as short circuit current density (J sc) and fill factor (FF), greatly improved, and also a high-power conversion efficiency (PCE) of â¼8.59% was achieved, which is â¼15% more than that of the reference devices without a TCNE layer. The devices fabricated were based on a poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b : 4,5-b']dithiophene-2,6-diyl]-[3-fluoro-2[(2-ethylhexyl)-carbonyl]-thieno[3,4-b]thiophenediyl]] (PTB7):(6,6)-phenyl C71 butyric acid methyl ester (PC71BM) blend system. These results suggest that this surface modification strategy could be readily extended in developing large-scale roll-to-roll fabrication of OSCs.
RESUMO
In organic semiconductors, optical absorption is pivotal for the performance of optoelectronic devices. The absorption by the semiconductors generates excitons which dissociate into free charge carriers, resulting in energy conversion. Although high performance has been achieved in non-fullerene organic solar cells, their charge generation behavior is far from being well understood. Keeping this in view, we have employed optical spectroscopic tools to study the charge generation mechanism in FLR (1,6,7,10-tetramethylfluoranthene) as a non-fullerene electron acceptor blended with P3HT (poly(3-hexylthiophene)) as an electron donor in five different solvents. Through steady state UV-visible and photoluminescence spectroscopy, we provide a basic understanding of charge transport by enlightening the influence of solvents on the aggregation behavior and exciton bandwidth. Furthermore, for the first time, by employing ultrafast vis-NIR transient absorption spectroscopy, we address the ultrafast charge generation and charge separation mechanism with systematic variation in solvent polarity by incorporating the time evolution of the transient species under various pump-probe wavelengths in the range of 450 nm to 1600 nm. For the different excitation wavelengths, the lifetime kinetics have been depicted by their multiexponential fits. The results show a fast decay term at a lifetime of a few picoseconds (ps) (â¼1 to 5 ps) and a slow decay term at a lifetime of â¼500 ps. The charge generation in the P3HT:FLR blend proceeds on a ps time scale, which implies good intermixing of the components. It is clearly established that the non-halogenated solvents influence this aggregation behavior and higher conjugation lengths with higher photoluminescence quenching contribute to the higher charge generation. The enhanced polaron population in P3HT with the addition of FLR illustrates the importance of this acceptor material in the blend because a good solvent-material combination is essential to enhance the charge generation. As such, this comprehensive study explicitly shows the role of FLR as an emerging efficient non-fullerene acceptor for further improving the performance of devices.
