Microstructural and Electronic Origins of Open‐Circuit Voltage Tuning in Organic Solar Cells Based on Ternary Blends

Abstract

loss compared to the theoretical radiative limit remains a vexing problem. [5][6][7] Within this context, organic ternary heterojunctions are becoming more widely studied as a means to optimize OPV performance. [8][9][10][11][12][13][14][15] In these blends, the use of one acceptor and multiple donors or one donor and multiple acceptors has experimentally been shown to enable improvements over a two-material blend by increasing the open-circuit voltage ( V oc ) without sacrifi cing photocurrent ( J sc ). Furthermore, the use of a second absorbing material often allows a larger portion of the solar spectrum to be harvested, making up for the lessthan-ideal absorption spectra of many common polymer absorbers. [ 16 ] The J sc of the mixed composition will thus be higher than that of the individual subcells due to the expanded external quantum effi ciency (EQE) response of the ternary BHJ. Hence ternary blends may provide a means to circumvent the aforementioned tradeoff between voltage and current when optimizing the optical gap of the solar cell.It is established that the properties of charge transfer (CT) states formed at the donor-acceptor interface are central to the device physics of BHJs. [ 7,[17][18][19] CT states are molecular species where an electron is located in the acceptor phase near the interface leaving a hole behind the donor phase. The extent of delocalization of CT states is still the subject of debate in the organic photovoltaic community. [20][21][22][23] The energy of the CT state ( E CT ) is defi ned as the energy difference between the fully relaxed CT state and the ground state of the blend. This quantity is important as it has been widely observed that E CT correlates well with V oc . [ 18 ] However, the CT energy depends sensitively on the intermolecular distance and relative orientation of the molecules at the interface. [24][25][26] The energy levels of the interfacial molecules are further affected by the relative mesoscale morphology of the phases (e.g., degree of aggregation and order) [27][28][29] and by which molecules surround them (i.e., how intimately mixed the interfacial region is.) [ 30 ] By the very nature of how they form, BHJs exhibit signifi cant variability in molecular confi gurations, which in turn introduces energetic disorder in the distribution of CT states. In ternary systems, all of these effects are compounded, with the added complication that chemical and electronic interactions (i.e., the degree of miscibility and electronic wavefunction overlap) affect the properties of CT states. So far, there is little fundamental Organic ternary heterojunction photovoltaic blends are sometimes observed to undergo a gradual evolution in open-circuit voltage ( V oc ) with increasing amounts of a second donor or an acceptor. The V oc is strongly correlated with the energy of the charge transfer state in the blend, but this value depends on both local and mesoscopic orders. In this work, the behavior of V oc in the presence of a wide range of interfacial electronic states is inv...

Multiple Cases of Efficient Nonfullerene Ternary Organic Solar Cells Enabled by an Effective Morphology Control Method

“…As will be detailed later, the interfacial tension between two organic molecules has been demonstrated to correlate with the extent of mixing. [15,22,63,[71][72][73][74] Second, the FF of the devices increases first and then drops with increasing IEIC-Th content. There are a few clear trends in the J-V characteristics of the PTFB-O:ITIC-Th:IEIC-Th ternary devices.…”

“…[16,63,73,78] Such close contact could potentially lead to the formation of a mechanical alloy, which alone may not be beneficial for device performance. [73] Khlyabich et al propose that similar surface tensions between the polymer donors give rise to the organic alloy. [68] resultant devices may exhibit a tunable V OC , [72,77] which has been observed in recently published high performance ternary OSCs by Wang et al [16] There have been hypothesis about the fundamental material property that could lead to such organic alloy.…”

Trade‐Off between Trap Filling, Trap Creation, and Charge Recombination Results in Performance Increase at Ultralow Doping Levels in Bulk Heterojunction Solar Cells

“…A question arises as to why there Adv. [45,47] based on the Marcus theory (Supporting Information). 2016, 6, 1601149 www.MaterialsViews.com www.advenergymat.de (Figure 6b): we observe a systematic blueshift of the subgap CT band with the doping level, indicating that a morphology change affecting CT state energetics has occurred.…”

Insights into photovoltaic properties of ternary organic solar cells from phase diagrams

“…Enhancing the short circuit current density (J sc ) is possible by adding a component with a complementary absorption profile [15–19]. In addition, several studies showed that also the open circuit voltage (V oc ) can be tuned by the third component [20–22]. …”

“…So far, only few investigations have addressed the correlation of the underlying photophysical processes in ternary blend OSCs to phase morphology [20–22]. Already in binary blend systems, a highly optimized morphology is necessary to efficiently generate and extract charge carriers.…”