《An Electron-Deficient Building Block Based on the B←N Unit: An Electron Acceptor for All-Polymer Solar Cells》
A double B←N bridged bipyridyl (BNBP) is a novel electron-deficient building block for polymer electron acceptors in all-polymer solar cells. The B←N bridging units endow BNBP with fixed planar configuration and low-lying LUMO/HOMO energy levels. As a result, the polymer based on BNBP units (P-BNBP-T) exhibits high electron mobility, low-lying LUMO/HOMO energy levels, and strong absorbance in the visible region, which is desirable for polymer electron acceptors. Preliminary all-polymer solar cell (all-PSC) devices with P-BNBP-T as the electron acceptor and PTB7 as the electron donor exhibit a power conversion efficiency (PCE) of 3.38 %, which is among the highest values of all-PSCs with PTB7 as the electron donor.
built-in voltages;electroabsorption spectroscopy;interlayers;internal quantum efficiency;organic solar cells
The influence of different electron extracting interlayers such as calcium (Ca), zirconium acetylacetonate (ZrAcac), and a polyfluorene derivative (PFN) in combination with an aluminum (Al) cathode is investigated on the performance of bulk-heterojuntion solar cells. Two different photoactive systems, P3HT:PC61BM and PDPP:PC71BM, are selected for this study. The electroabsorption measurements have been carried out for obtaining the built-in voltage (Vbi) and transfer matrix simulations for the determination of parasitic absorption. The solar cell performance is influenced by different parameters such as diode turn-on voltage, leakage currents, built-in voltages, and parasitic absorption. The small diode turn-on voltage and high parasitic absorption in Ca contact devices limit the open circuit voltage and short circuit current, respectively. Likewise, high leakage currents using ZrAcac contact limit the fill factor in P3HT:PC61BM solar cell devices. However, the PFN-based devices with small parasitic absorption, smaller leakage currents, and a relatively high Vbi show maximum performance with both material systems. This work highlights the importance of choosing the suitable interlayers in device optimization and clearly demonstrates that not only the low work function of an electron extracting interlayer but also its optical properties and charge selectivity significantly influence the final solar cell performance.
Organic optoelectronics are promising technologies for energy conversion. However, the electrode interlayer, a key material between active layers and conducting electrodes that controls the transport of charge carriers in and out of devices, is still a chemical challenge. Herein, we report a class of porous organic polymers with tunable work function as hole- and electron-selective electrode interlayers. The network with organoborane and carbazole units exhibits extremely low work-function-selective electron flow; while upon ionic ligation and electro-oxidation, the network significantly increases the work function and turns into hole conduction. We demonstrate their outstanding functions as anode and cathode interlayers in energy-converting solar cells and light-emitting diodes.