PTB7:PC$_{\text{71}}$BM-Based Solar Cells Fabricated With the Eutectic Alloy Field's Metal as an Alternative Cathode and the Influence of an Electron Extraction Layer

In this work, we study the performance of organic photovoltaic (OPV) cells based on the PTB7:PC
₇₁
BM blend and the use of an alternative metal cathode called field's metal (FM) (Bi/In/Sn: 32.5%, 51%, 16.5%), which can be easily deposited through a vacuum-free process at regular atmosphere and low temperature (90 °C). The charge extraction and losses involved with the use of FM, in combination with an electron extraction layer, in this case the widely used polymer PFN, were fully analyzed using equivalent circuit models for the architecture ITO/PEDOT:PSS/PTB7:PC
₇₁
BM/PFN/FM. Power conversion efficiencies (PCEs) higher than 6% were reached with this architecture, which exhibited J
_sc
of 14.3 mA/cm
²
, V
_oc
of 0.76 V, and FF of 0.6. The single-diode equivalent circuit model demonstrates that devices comprising FM as counterelectrode are characterized by a low series resistance (1.0 Ω · cm
²
), large shunt resistance (1.1 × 10
⁵
Ω · cm
²
), and low saturation current (1.4 × 10
^-7
mA/cm
²
). Additional electrical characterization, through impedance spectroscopy, was also carried out. The obtained PV parameters are similar to those exhibited by devices comprising standard counterelectrodes, i.e., Al, deposited through a high-vacuum evaporation process. Thus, FM, as nonconventional and alternative counterelectrode, in combination with an electron extraction layer, can be used to fabricate OPVs cells with very acceptable performance through a vacuum-free process.