Enhancement in performance of ternary blend-polymer solar cells using a PEDOT:PSS–graphene oxide hole transport layer via Förster resonance energy transfer and balanced charge transport

Abstract

In this work, we report high performance ternary blend polymer solar cells (TPSCs) fabricated by employing poly((3-hexyl)thiophene) (P3HT): poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene)-2-(carboxylate-2-6-diyl)] (PTB7-Th): PC₇₁BM ternary blend as the active layer, and poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)–graphene oxide (GO) composite as the hole transport layer (HTL). The power conversion efficiency (PCE) of the best TPSC is enhanced to ∼7.1% as compared to that of P3HT:PC₇₁BM (PCE ∼ 3.7%) and PTB7-Th:PC₇₁BM (PCE ∼ 4%) reference binary blend based devices. The enhancement in PCE is attributed to the synergistic effect of superior photovoltaic properties of P3HT:PTB7-Th:PC₇₁BM (0.3 : 0.7 : 1) ternary blend as well as better hole-transport properties of PEDOT:PSS–GO (1 : 1) composite HTL. External quantum efficiency spectra showed a broad spectral response with photon-to-electron conversion approaching up to 67–75% in the extended wavelength range of 350–780 nm covering the Vis-NIR range, potentially due to the complementary absorption from the P3HT and PTB7-Th molecules. Photoluminescence (PL) quenching and time-resolved PL studies demonstrated a significant reduction in photo-exciton lifetime τ₁ from 129 ps (in P3HT) to 17 ps (in P3HT:PTB7-Th:PC₇₁BM), which signifies quick and strong non-radiative relaxation of the photo-excited excitons. This is primarily dominated by both Förster resonance energy transfer and charge transfer mechanisms owing to cascade energy level alignment, and subsequently lead to effective exciton dissociation in the ternary blend. Balanced hole and electron mobilities (μ_e/μ_h = 1.81) facilitate the charge transport, and is governed by the trap-free space charge limited current with weak bimolecular recombination. Also, the conductivity and ultraviolet-photoelectron measurements revealed that the addition of GO nanosheets in PEDOT:PSS imparts high electrical conductivity and increases the work function of HTL such that its HOMO level match perfectly with the HOMO level of the polymers in the active layer. This reduces the potential barrier at the HTL/active layer interface and promotes both the hole extraction and transport. Moreover, the incorporation of  GO nanosheets results in increase of LUMO level of the PEDOT:PSS HTL which effectively block the electrons and facilitate the transport of holes in HTL without recombination. The simultaneous existence of all these beneficial properties of the ternary blend collectively boost the performance of TPSCs by enhancing the short-circuit current to 12.3 mA cm⁻², open-circuit voltage to 0.74 V, and fill-factor to 0.78. These findings show that (P3HT:PTB7-Th:PC₇₁BM) and PEDOT:PSS–GO composites can be the promising ternary blend active layer and hole transporting material, respectively, for achieving high performance PSCs. This work provides a new platform for the development of the ternary blend approach of PSC technology.