currently at the forefront of efficient OSCs and outperform binary systems in terms of power conversion efficiency (PCE), largely due to improved light absorption and mor phology, especially if novel nonfullerene acceptors are employed. [2] Ternary solar cells use a single (ternary blend) photo active layer for photon harvesting and thus are less challenging and costly to realize, compared to tandem solar cells, [3] which have the potential of even higher efficien cies. [4] However, in TSCs, the third compo nent of the blend not only improves light harvesting, but also plays an active role in the photophysical processes including exciton and chargecarrier dynamics and can, in fact, also influence the blend's morphology. [5] This is of critical impor tance in small moleculebased devices, where intermolecular charge transport dominates. [6] Often, the third component in ter nary solar cells enhances the PCE by improving the shortcircuit current (J SC ) [5c] and opencircuit voltage (V OC ), which both can be tuned by the donor/ acceptor composition. [2d,5b] The chal lenge here is, however, to improve all device parameters, while maintaining or increasing the fill factor (FF) of the ternary organic solar cell. [7] Here, we study the photophysics of such a highperformance allsmallmolecule ternary solar cell com posed of DR3TBDTT (DR3) [8] as an electron donor in combi nation with a nonfullerene smallmolecule acceptor, namely ICC6IDTIC (or ICC6) [9] and PC 71 BM as the third component, shown to enhance the device performance. The fully opti mized ternary devices (1:1:0.4 wt%, DR3:ICC6:PC 71 BM) yield an average PCE of 10.8% with a FF of 72%, V OC of 0.87 V, and J SC of 16.3 mA cm −2 . Using picosecond-nanosecond (ps-ns) transient absorption (TA) spectroscopy, we selectively excite each component of the blend and probe the processes following photoexcitation. We find that excitation of PC 71 BM molecules results in fast singlet energy transfer to ICC6, sub sequently followed by hole transfer to DR3. We confirm this observation by TA and timeresolved photoluminescence (TRPL) spectroscopy on binary blends of PC 71 BM:ICC6, showing fast energy transfer from PC 71 BM to ICC6. Our TA studies demon strate that the increased external quantum efficiency (EQE) of the ternary blend is due to higher mobility of charge carriers in Ternary organic solar cells (OSCs) are among the best-performing organic photovoltaic devices to date, largely due to the recent development of nonfullerene acceptors. However, fullerene molecules still play an important role in ternary OSC systems, since, for reasons not well understood, they often improve the device performance, despite their lack of absorption. Here, the photophysics of a prototypical ternary small-molecule OSC blend composed of the donor DR3, the nonfullerene acceptor ICC6, and the fullerene derivative PC 71 BM is studied by ultrafast spectroscopy. Surprisingly, it is found that after excitation of PC 71 BM, ultrafast singlet energy transfer to ICC6 competes efficiently with cha...
