Toward More Efficient Organic Solar Cells: A Detailed Study of Loss Pathway and Its Impact on Overall Device Performance in Low‐Offset Organic Solar Cells

Abstract

Low-offset organic solar cell systems have attracted great interest since nonfullerene acceptors came into the picture. While numerous studies have focused on the charge generation process in these low-offset systems, only a few studies have focused on the details of each loss channel in the charge generation process and their impact on the overall device performance. Here, several nonfullerene acceptors are blended with the same polymer donor to form a series of low-offset organic solar cell systems where significant variation in device performance is observed. Through detailed analyses of loss pathways, it is found that: i) the donor:acceptor interfaces of PM6:Y6 and PM6:TPT10 are close to the optimum energetic condition, ii) energetics at the donor:acceptor interface are the most important factor to the overall device performance, iii) exciton dissociation yield can be field-dependent owing to the sufficiently small energetic offset at the donor:acceptor interface, and iv) the change in substituents in the terminal group of Y-series acceptors in this work mainly affects energetics at the donor:acceptor interface instead of the interface density in the active layer. In general, this work presents a path toward more efficient organic solar cells.

Origin of electric field-dependent charge generation in organic photovoltaics with planar and bulk heterojunctions

“…Recent studies provide evidence that the insufficient energetic difference between the S 1 and CT states decreases the charge generation efficiency and makes the generation process electric field dependent. 13,14 These results imply that the efficient CT process from the S 1 to CT states requires a sufficient energy difference between these states, assuming that Marcus electron transfer theory applies to the molecules. 8,15 In addition, this first CT process from the S 1 to CT states may be more important than the transition from the CT to CS states.…”

“…To answer this question, we evaluated the electronic state energies of these devices because an insufficient energetic difference between the S 1 and CT states can cause eld-dependent charge generation in OPVs. 8,13 The S 1 state energy was evaluated using EQE spectra (Fig. 3a).…”

What We have Learnt from PM6:Y6

“…[176] As an important photovoltaic parameter, the FF of OSCs stands for the effectiveness of charge generation and collection, which plays a critical role in power conversion efficiency. [177,178] Under idealized conditions, the current under illumination is expected to obey the Shockley diode equation, according to which. [179]…”

“…As an important photovoltaic parameter, the FF of OSCs stands for the effectiveness of charge generation and collection, which plays a critical role in power conversion efficiency. [ 177,178 ] Under idealized conditions, the current under illumination is expected to obey the Shockley diode equation, according to which. [ 179 ] $$$$\begin{equation}J\ = \ {J}_R - \ \ {J}_G = {J}_0\ \left[ {\exp \left( {\frac{{qV}}{{{n}_{{\mathrm{id}}}kT}}} \right) - 1} \right] - \ {J}_G\end{equation}$$$$Here, J G is the photogeneration current, J 0 is the associated dark saturation current, and n id is the diode ideality factor ( n id = 1 when bimolecular recombination dominates, and 2 when trap‐assisted recombination dominates).…”

Additive-Free All-Green Solvent-Processed Efficient and Stable Pseudo-Bilayer Bulk Heterojunction Ternary Organic Solar Cells

“…Moreover, bulk heterojunction (BHJ) films are usually faced with complex dynamics and kinetics during the morphological evolution process. Consequently, the uncertain morphology phase separation of active layers is frequently associated with severe charge recombination loss. − In addition, it is an arduous task to form a good vertical component distribution in the BHJ structure due to the similar molecular structure and good miscibility between donor and SMAs. The pseudo-bilayer planar heterojunction (PPHJ) device, by contrast, offers complementary advantages, making it a potential alternative for device fabrication and enabling the different vertical phase separation in heterojunction films.…”