Impact of charge carrier recombination and energy disorder on the open-circuit voltage of polymer solar cells

Abstract

Charge carrier recombination and energy disorder in organic solar cells both have a profound impact on the open-circuit voltage of the device. In this paper, both traditional fullerene-(PC<sub>71</sub>BM) and nonfullerene-(O-IDTBR) based solar cells were fabricated using the same electron donor material (PTB7-Th). The room-temperature current density–voltage characteristics showed that despite the values of their power conversion efficiencies were very close, there was a huge open circuit voltage (<i>V</i><sub>oc</sub>) difference between the PC<sub>71</sub>BM and O-IDTBR devices. To understand the sources of the <i>V</i><sub>oc</sub> variation, characterization techniques such as impedance spectra, low temperature electrical characterization method, transient photovoltage, and electroluminescent spectra were carried out. Temperature-dependent <i>V</i><sub>oc</sub> of the devices were measured in a large temperature range between 120 K and 300 K. The charge transfer state energy (<i>E</i><sub>CT</sub><italic/>) of the fullerene and the nonfullerene cells were determined to be 1.13 V and 1.34 V, respectively. Furthermore, the Mott-Schottky equation was applied to analyze the capacitance- voltage curves of the fabricated devices. Results showed that the built-in voltage (<i>V</i><sub>bi</sub>) of the O-IDTBR based cell (1.38 V) was much higher than that of the PC<sub>71</sub>BM cell (1.15 V). By analyzing the above data, it was easy to speculate that charge carrier recombination loss in the PC<sub>71</sub>BM device was more serious since the net electric field was relatively weak. Impedance spectra were used to measure the charge carrier recombination process in both devices. Fitting results through the equivalent circuit stated clearly that values of the recombination resistance in the O-IDTBR device were much higher in the test range, indicating that the charge carrier was less easy to recombine in the nonfullerene device. This speculation could be verified by the transient photovoltage (TPV) measurements since the carrier lifetime in the O-IDTBR device was much longer. The excited states in the devices were investigated by the electroluminescence spectra. Since the full width at half maximum (FWHM) of the O-IDTBR emission spectrum was narrower, the excited state energy distribution in the O-IDTBR device was more uniform. Based on the above analyses, the higher <i>V</i><sub>oc</sub> in the O-IDTBR device was attributed to the mild charge carrier recombination and low energy disorder.