Photoinduced Charge Transfer in Donor–Acceptor (DA) Copolymer: Fullerene Bis-adduct Polymer Solar Cells

Abstract

Polymer solar cells (PSCs) consisting of fullerene bis-adduct and poly(3-hexylthiophene) (P3HT) blends have shown higher efficiencies than P3HT:phenyl C(61)-butyric acid methyl ester (PCBM) devices, because of the high-lying lowest unoccupied molecular orbital (LUMO) level of the fullerene bis-adducts. In contrast, the use of fullerene bis-adducts in donor-acceptor (DA) copolymer systems typically causes a decrease in the device's performance due to the decreased short-circuit current (J(SC)) and the fill factor (FF). However, the reason for such poor performance in DA copolymer:fullerene bis-adduct blends is not fully understood. In this work, bulk-heterojunction (BHJ)-type PSCs composed of three different electron donors with four different electron acceptors were chosen and compared. The three electron donors were (1) poly[(4,8-bis-(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene)-2,6-diyl-alt-(5-octylthieno[3,4-c]pyrrole-4,6-dione)-1,3-diyl] (PBDTTPD), (2) poly[(4,8-bis-(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene)-2,6-diyl] (PBDTTT-C), and (3) P3HT polymers. The four electron acceptors were (1) PCBM, (2) indene-C(60) monoadduct (ICMA), (3) indene-C(60) bis-adduct (ICBA), and (4) indene-C(60) tris-adduct (ICTA). To understand the difference in the performance of BHJ-type PSCs for the three different polymers in terms of the choice of fullerene acceptor, the structural, optical, and electrical properties of the blends were measured by the external quantum efficiency (EQE), photoluminescence, grazing incidence X-ray scattering, and transient absorption spectroscopy. We observed that while the molecular packing and optical properties cannot be the main reasons for the dramatic decrease in the PCE of the DA copolymers and ICBA, the value of the driving force for charge transfer (ΔG(CT)) is a key parameter for determining the change in J(SC) and device efficiency in the DA copolymer- and P3HT-based PSCs in terms of fullerene acceptor. The low EQE and J(SC) in PBDTTPD and PBDTTT-C blended with ICBA and ICTA were attributed to an insufficient ΔG(CT) due to the higher LUMO levels of the fullerene multiadducts. Quantitative information on the efficiency of the charge transfer was obtained by comparing the polaron yield, lifetime, and exciton dissociation probability in the DA copolymer:fullerene acceptor films.

High-performance ternary blend polymer solar cells involving both energy transfer and hole relay processes

“…FF remained larger than 70.0% even when PID2 content was increased to 70% and V oc increased monotonically with increased PID2 content at all ratios because of deeper HOMO energy levels of PID2 than PTB7-Th. This phenomenon was also observed for many other ternary systems reported20303132 and several possible mechanisms are proposed to explain the increased V oc , such as the electronic alloy charge-transfer state reported in ref. 33 and parallel-like BHJ concept demonstrated in ref.…”

Fullerene‐Bisadduct Acceptors for Polymer Solar Cells

“…Janssen and co-workers [48] studied the effect of the fullerene acceptor (PCBM, bisPCBM, and ICBA) on the photovoltaic performance of a D-A copolymer of fluorene and benzothiadiazole (PF10TBT) as the donor. Otherwise, the bisadduct would show poorer photovoltaic performance, owing to the decreased J sc and FF, as in the cases of PBDTTPD and PBDTTT-C. [49] Rumbles and co-workers [50] studied the effect of Gibbs energy (DG, driving force for electron transfer) on the charge-creation yield of the photoinduced electron transfer (PET) in the system with a conjugated polymer donor and a fullerene-based acceptor. The devices with fullerene bisadducts bisPCBM and ICBA as acceptors displayed an even higher V oc of 1.127-1.173 V, but a strong decrease in J sc and FF, thereby resulting in a lower PCE.…”

Diethynylbenzo[1,2‐b:4,5‐b′]dithiophene‐based small molecule and cross‐conjugated copolymers for organic solar cells

“…Note the deep E HOMO of copolymers may lead to high V oc of the fabricated OSCs, since empirically the V oc is proportional to the discrepancy between the E HOMO of the donor materials and the E LUMO of electron acceptors . Moreover, the decreased of E LUMO of the resulting copolymers may lead to insufficient driving force for charge separation and prevent the charge dissociation in the interface of donors/acceptors . The energy level diagrams of all materials are illustrated in Figure (b) and the corresponding electrochemical data were summarized in Table .…”