Probing fluorescence resonance energy transfer and hole transfer in organic solar cells using a tandem structure

With the emergence of nonfullerene acceptors (NFAs), single junction organic solar cells (OSCs) have achieved power conversion efficiencies (PCEs) of over 19%. Due to the intrinsic efficiency limits in low-bandgap NFA OSCs, the donor-to-acceptor energy transfer and acceptor-to-donor hole transfer have considerable impacts on the performance of the OSCs. However, time-resolved ultrafast spectroscopy is often used to prove the energy transfer and hole transfer processes. Herein, we designed thin films and active layers with a tandem structure composed of donors/electron blocking layers/acceptors to demonstrate the occurrence of donor-to-acceptor fluorescence resonance energy transfer (FRET) and acceptor-to-donor hole transfer. By analyzing the steady state photoluminescence peak position and intensity with varied substrate temperatures, FRET from PTO2 to IDTT-IC or IDTT-4FIC and hole transfer from IDTT-IC or IDTT-4FIC to PTO2 were demonstrated. In addition, we monitored devices with a tandem structure active layer using short/long pass filters to excite the donor or acceptor selectively and further confirmed the occurrence of FRET and hole transfer during device operation. Our results proved the effect of FRET and hole transfer directly in thin films and devices, providing a feasible yet practical approach with easily accessible measurements other than ultrafast spectroscopy.