Incorporating Graphitic Carbon Nitride (g‐C₃N₄) Quantum Dots into Bulk‐Heterojunction Polymer Solar Cells Leads to Efficiency Enhancement

Abstract

Graphitic carbon nitride (g‐C 3 N 4 ) has been commonly used as photocatalyst with promising applications in visible‐light photocatalytic water‐splitting. Rare studies are reported in applying g‐C 3 N 4 in polymer solar cells. Here g‐C 3 N 4 is applied in bulk heterojunction (BHJ) polymer solar cells (PSCs) for the first time by doping solution‐processable g‐C 3 N 4 quantum dots (C 3 N 4 QDs) in the active layer, leading to a dramatic efficiency enhancement. Upon C 3 N 4 QDs doping, power conversion efficiencies (PCEs) of the inverted BHJ‐PSC devices based on different active layers including poly(3‐hexylthiophene‐2,5‐diyl):[6,6]‐phenyl‐C61‐butyric acid methyl ester (P3HT:PC 61 BM), poly(4,8‐bis‐alkyloxybenzo(l,2‐b:4,5‐b′)dithiophene‐2,6‐diylalt‐(alkyl thieno(3,4‐b)thiophene‐2‐carboxylate)‐2,6‐diyl):[6,6]‐phenyl C 71 ‐butyric acid methyl ester (PBDTTT‐C:PC 71 BM), and poly[4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene‐co‐3‐fluorothieno [3,4‐b]thiophene‐2‐carboxylate] (PTB7‐Th):PC 71 BM reach 4.23%, 6.36%, and 9.18%, which are enhanced by ≈17.5%, 11.6%, and 11.8%, respectively, compared to that of the reference (undoped) devices. The PCE enhancement of the C 3 N 4 QDs doped BHJ‐PSC device is found to be primarily attributed to the increase of short‐circuit current ( J sc ), and this is confirmed by external quantum efficiency (EQE) measurements. The effects of C 3 N 4 QDs on the surface morphology, optical absorption and photoluminescence (PL) properties of the active layer film as well as the charge transport property of the device are investigated, revealing that the efficiency enhancement of the BHJ‐PSC devices upon C 3 N 4 QDs doping is due to the conjunct effects including the improved interfacial contact between the active layer and the hole transport layer due to the increase of the roughness of the active layer film, the facilitated photoinduced electron transfer from the conducting polymer donor to fullerene acceptor, the improved conductivity of the active layer, and the improved charge (hole and electron) transport.