Subtle Polymer Donor and Molecular Acceptor Design Enable Efficient Polymer Solar Cells with a Very Small Energy Loss

Abstract

A new wide bandgap polymer donor, PNDT‐ST, based on naphtho[2,3‐b:6,7‐b′]dithiophene (NDT) and 1,3‐bis(thiophen‐2‐yl)‐5,7‐bis(2‐ ethylhexyl)benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione (BDD) is developed for efficient nonfullerene polymer solar cells. To better match the energy levels, a new near infrared small molecule of Y6‐T is also developed. The extended π‐conjugation and less twist of PNDT‐ST provides it with higher crystallinity and stronger aggregation than the PBDT‐ST counterpart. The higher lowest occupied molecular orbital level of Y6‐T than Y6 favors the better energy level match with these polymers, resulting in improved open circuit voltage (Voc) and power conversion efficiency (PCE). The high crystallinity and strong aggregation of PNDT‐ST also induces large phase separation with poorer morphology, leading to lower fill factor and reduced PCE than PBDT‐ST. To mediate the crystallinity and optimize the morphology, PNDT‐ST and PBDT‐ST are blended together with Y6‐T, forming the ternary blend devices. As expected, the two compatible polymers allow continual optimization of the morphology by varying the blend ratio. The optimized ternary blend devices deliver a champion PCE as high as 16.57% with a very small energy loss (Eloss) of 0.521 eV. Such small Eloss is the best record for polymer solar cells with PCEs over 16% to date.

Understanding the Effect of End Group Halogenation in Tuning Miscibility and Morphology of High‐Performance Small Molecular Acceptors

“…Organic solar cells (OSCs) have great prospects for many industrial applications thanks to their flexibility, semitransparency, and light weight. [ 1–6 ] Nonfullerene small molecular acceptors (SMAs) [ 7–29 ] are the main driving force for the recent development of the field, with power conversion efficiencies (PCEs) over 17% reported by different teams. [ 30–43 ] SMAs have many attractive properties including their strong and tunable absorption, the easily adjustable energy levels and the enhanced chemical and device stability.…”

Developing Wide Bandgap Polymers Based on Sole Benzodithiophene Units for Efficient Polymer Solar Cells

“…The chlorinated PBDTCl had slightly lower HOMO level than the fluorinated PBDTCl, owing to the empty 3d orbitals of chlorine which could accommodate pelectrons from BDT backbone. [48] These polymers exhibited good energy level matchw ith Y6 (HOMO/LUMO = À5.65/ À4.10 eV). [46] The lowered HOMO levels by side chain sulfuration, fluorination and chlorination would be expectedt oi mprove the V oc values of the resulting PSCs.…”

“…In addition, the charge dissociation could be studied by using photocurrent density (J ph , J ph = J sc À J d ,w here J sc and J d are the current density of the devices under illumination and dark, respectively) versus the internal voltage (V eff , V eff = V 0 À V, where V 0 is the voltage when J ph = 0, while V is the applied voltage) plots. [48] The charge dissociation efficiency (P E,T = J ph / J sat ,i nw hich J ph is the photocurrent density when V = 0) could be calculated to be 0.87, 0.91, 0.93 and 0.95 for PBDTT:Y6, PBDTS:Y6, PBDTF:Y6, PBDTCl:Y6, respectively.A mong them, PBDTCl:Y6 devices had the highest charge dissociation efficiency and lowest charger ecombination probability,w hich would contribute to the best deviceperformance.…”

Recent Advances Toward Highly Efficient Tandem Organic Solar Cells

“…Fusing one more benzene ring onto the BDT skeleton afforded naphthodithiophene (NDT) with the more extended π-conjugation length, which could improve the crystallinity of the polymer for enhancing the charge transport properties. [16,93,94] Our group developed a new DPP-based polymer of PNDTDPP by Stille-coupling with NDT block. [93] PNDTDPP exhibited a very low bandgap of 1.36 eV, which contributed to a high J sc of 13.86 mA cm À2 in top subcell and a high PCE of 9.40% in tandem devices.…”