Overcoming Fill Factor Reduction in Ternary Polymer Solar Cells by Matching the Highest Occupied Molecular Orbital Energy Levels of Donor Polymers

Abstract

Despite the potential of ternary polymer solar cells (PSCs) to improve photocurrents, ternary architecture is not widely utilized for PSCs because its application has been shown to reduce fill factor (FF). In this paper, a novel technique is reported for achieving highly efficient ternary PSCs without this characteristic sharp decrease in FF by matching the highest occupied molecular orbital (HOMO) energy levels of two donor polymers. Our ternary device—made from a blend of wide‐bandgap poly[4,8‐bis(2‐ethylhexyloxy)benzo[1,2‐b:4,5‐b′]dithiophene‐alt‐2,5‐dioctyl‐4,6‐di(thiophen‐2‐yl)pyrrolo[3,4‐c]pyrrole‐1,3(2H,5H)‐dione) (PBDT‐DPPD) polymer, narrow‐bandgap poly[4,8‐bis[5‐(2‐ethylhexyl)‐2‐thienyl]benzo[1,2‐b:4,5‐b′]dithiophene‐alt‐(4‐(2‐ethylhexyl)‐3‐fluorothieno[3,4‐b]thiophene‐)‐2‐carboxylate‐2‐ 6‐diyl)] (PTB7‐Th) polymer, and [6,6]‐phenyl C70‐butyric acid methyl ester (PC70BM)—exhibits a maximum power conversion efficiency of 10.42% with an open‐circuit voltage of 0.80 V, a short‐circuit current of 17.61 mA cm−2, and an FF of 0.74. In addition, this concept is extended to quaternary PSCs made by using three different donor polymers with similar HOMO levels. Interestingly, the quaternary PSCs also yield a good FF (≈0.70)—similar to those of corresponding binary PSCs. This study confirms that the HOMO levels of the polymers used on the photoactive layer of PSCs are a crucial determinant of a high FF.

Ternary Blend Solar Cells Based on a Conjugated Polymer With Diketopyrrolopyrrole and Carbazole Units

“…The decreased R s is probably ascribed to less charge trap sites due to a reduced intermolecular π-π stacking observed in the absorption spectrum. Such improvements in FF have also been reported for other ternary blend polymer solar cells (Gasparini et al, 2016;Lee et al, 2017;Zhang et al, 2017;Zhao F. et al, 2017;Wang et al, 2018a), suggesting that charge transport might be improved in ternary blends. This is consistent with a slightly improved hole mobility observed for ternary blend films even with a similar surface morphology in comparison with those of binary blend films (see the Supporting Informations S2, S3).…”

Relationship of Ionization Potential and Oxidation Potential of Organic Semiconductor Films Used in Photovoltaics

“…Hence, the V oc should depend on the donor hole–acceptor electron charge transfer state, and indeed a linear dependence between the two is typically found . Tuning the energy level alignment at the bulk heterojunction thus is an effective way to reduce voltage loss and enhance PCE, and results suggest the donor–acceptor junction can be tuned for a near‐zero electron‐ or hole‐transfer driving energy while retaining a high photon to free charge generation . Refined tuning of the energy level alignment at the heterojunctions is aided by understanding of the interface energetics at organic heterojunctions, but direct precise measurements of the energy levels at the heterojunctions still are problematic.…”

The role of the third component in ternary organic solar cells

“…We also consider the stability of the active-layer materials and devices as well as the recent progress in the development of ternary blends to help overcome the degradation and/or reliability issues of binary solar cells 35,90 . The scalability of ternary devices that have yielded record device PCEs (>14%) and environmentally friendly processing opportunities are also briefly discussed 65,76,77,[91][92][93][94] . Finally, we provide a critical perspective on the advantages of ternary blends and devices along with opportunities for improvements and suggest design strategies for highly efficient and stable OSCs for commercially viable photovoltaics.…”