Design of (X-DADAD)_n Type Copolymers for Efficient Bulk Heterojunction Organic Solar Cells

Abstract

We show that extended TBTBT structure (T = thiophene, B = benzothiadiazole) can be used as an electrondeficient building block for designing conjugated polymers with deeply lying HOMO energy levels and narrow band gaps. The first carbazole−TBTBT copolymer P2 demonstrated power conversion efficiencies exceeding 6% in bulk heterojunction solar cells in combination with advanced operational stability, unlike conventional donor polymers such as PTB7, PBDTTT-CF, etc.

Solubilized 5,6-bis(octyloxy)benzoxadiazole as a versatile acceptor block for designing novel (-X-DADAD-)n and (-X-DADADAD-)n electron donor copolymers for bulk heterojunction organic solar cells

“…4; the corresponding data for the polymer-PC 61 BM composite systems are presented in ESI. It is seen from Table 2 that the solar cells based on P1-P3 exhibit high open circuit voltages outperforming significantly devices based on the previously reported benzothiadiazole analogues (V OC ¼775 mV) [12,13,14]. This result confirmed our initial hypothesis that the material HOMO energy can be lowered and the open circuit voltage of photovoltaic cells can be improved by replacing benzothiadiazole acceptor (A) units with benzoxadiazole in the (-X-DADAD-) n polymers.…”

“…In particular, conjugated polymers comprising carbazole (X) moieties bearing solubilizing alkyl chains, thiophene (T) donor units (D) and benzothiadiazole (BT) acceptor fragments (A) were reported [12,13]. The application of the extended T-BT-T-BT-T building block (as compared to the conventional T-BT-T) allowed us to reduce significantly the energy band gap of the resulting polymer (from 1.90 to 1.65 eV) while keeping the HOMO level almost unchanged ( À 5.50 eV vs. À 5.44 eV).…”

“…It is notable that the designed carbazole-(fluorene)-thiophenebenzothiadiazole copolymers demonstrated an excellent photochemical stability in thin films and in devices [12]. Additionally, novel materials enabled fabrication of flexible plastic solar cells with the efficiencies of 5-6% [15].…”

Ultrafast electron and hole transfer in bulk heterojunctions of low-bandgap polymers

“…3. Generally, the infrared response of the BHJ blends consists of the responses of ICT excitons (dominating at early times) and polarons (dominating at long times) [19,20,39,50]. The ICT exciton response was recorded from the films of the pristine polymers at short delay times (~1 ps).…”

“…Typically, the [70]PCBM exciton dissociation is diffusion-delayed, which is observed as a gradual in-crease of the PIA signal [25,30,31,39,50,52]. In the case of BTT-BPP-based blends, however, the growth is most probably hidden by the signal decay due to the ICT exciton recombination as they take place at similar timescales.…”