Open Circuit Voltage Losses in Organic Planar and Bulk Heterojunction Solar Cells - The Role of Interfacial Energetics

The introduction of non-fullerene acceptors (NFAs) has led to a surge in the record power conversion efficiency (PCE) of bulk heterojunction (BHJ) organic photovoltaics (OPV) now exceeding 19%.¹ These achievements were made in part due to the greater flexibility in tuning their energetic levels. Narrow bandgap NFAs enable improved absorption extending into the near-infrared region. Particularly successful candidates like Y6 and IT-4F have also been shown to exhibit long exciton diffusion length exceeding 30 nm.² At the same time, the possibility to better match the energy levels with complementary absorption donor polymers helped to decrease the energy offset related voltage losses. Despite this reduction in interfacial energy offsets, OPVs still exhibit larger voltage losses relative to their effective bandgap than other inorganic photovoltaic technologies. A better understanding of the involved voltage losses and how to further reduce them constitutes the main challenge on the quest to breach the 20% PCE threshold.<br/>Up until now, efficient OPVs consisted of a bulk heterojunction (~ 10nm domain sizes) to enable efficient exciton separation while also allowing for thick active layers (~ 100nm) to maximise absorption. However, the large area of the donor/acceptor interface contributes to the observed voltage losses through the increased recombination rate.³ Given the high exciton diffusion length of NFAs like IT-4F and Y6, this raises the question of whether a planar heterojunction (PHJ) architecture could yield reduced voltage losses at the limited well-defined interface while also enabling high photocurrents through tuning the acceptor layer to the exciton diffusion length.<br/>Here, we study the charge generation and voltage loss characteristics of a series of well-defined PHJs (including IT-4F/PM6 and Y6/PM6) produced by a polymer film transfer method⁴ as well as evaporated PHJs. Through optical and optoelectronic measurements, we assess the charge generation and voltage loss characteristics compared to their BHJ counterparts. A special emphasis is put on the comparison of interfacial energetics – especially the observed morphology dependent electrostatic effects due to the high quadrupole moment in high performing NFAs. Our findings have implications for the optimisation of future low voltage loss device structures as well as for the understanding of interfacial electrostatic effects in BHJs and PHJs.<br/><br/>1. Zhu, L. <i>et al.</i> Single-junction organic solar cells with over 19% efficiency enabled by a refined double-fibril network morphology. <i>Nat. Mater.</i> <b>21</b>, 656–663 (2022).<br/>2. Firdaus, Y. <i>et al.</i> Long-range exciton diffusion in molecular non-fullerene acceptors. <i>Nat Commun</i> <b>11</b>, 5220 (2020).<br/>3. Vandewal, K. <i>et al.</i> Increased Open-Circuit Voltage of Organic Solar Cells by Reduced Donor-Acceptor Interface Area. <i>Adv. Mater.</i> <b>26</b>, 3839–3843 (2014).<br/>4. Lee, T. H. <i>et al.</i> Planar Organic Bilayer Heterojunctions Fabricated on Water with Ultrafast Donor-to-Acceptor Charge Transfer. <i>Solar RRL</i> <b>5</b>, 2100326 (2021).