Systematic Investigation of Benzodithiophene- and Diketopyrrolopyrrole-Based Low-Bandgap Polymers Designed for Single Junction and Tandem Polymer Solar Cells

The tandem solar cell architecture is an effective way to harvest a broader part of the solar spectrum and make better use of the photonic energy than the single junction cell. Here, we present the design, synthesis, and characterization of a series of new low bandgap polymers specifically for tandem polymer solar cells. These polymers have a backbone based on the benzodithiophene (BDT) and diketopyrrolopyrrole (DPP) units. Alkylthienyl and alkylphenyl moieties were incorporated onto the BDT unit to form BDTT and BDTP units, respectively; a furan moiety was incorporated onto the DPP unit in place of thiophene to form the FDPP unit. Low bandgap polymers (bandgap = 1.4–1.5 eV) were prepared using BDTT, BDTP, FDPP, and DPP units via Stille-coupling polymerization. These structural modifications lead to polymers with different optical, electrochemical, and electronic properties. Single junction solar cells were fabricated, and the polymer:PC₇₁BM active layer morphology was optimized by adding 1,8-diiodooctane (DIO) as an additive. In the single-layer photovoltaic device, they showed power conversion efficiencies (PCEs) of 3–6%. When the polymers were applied in tandem solar cells, PCEs over 8% were reached, demonstrating their great potential for high efficiency tandem polymer solar cells

Synthesis of 5<i>H</i>‑Dithieno[3,2‑<i>b</i>:2′,3′‑<i>d</i>]pyran as an Electron-Rich Building Block for Donor–Acceptor Type Low-Bandgap Polymers

We describe the detailed synthesis and characterization of an electron-rich building block, dithienopyran (DTP), and its application as a donor unit in low-bandgap conjugated polymers. The electron-donating property of the DTP unit was found to be the strongest among the most frequently used donor units such as benzodithiophene (BDT) or cyclopentadithiophene (CPDT) units. When the DTP unit was polymerized with the strongly electron-deficient difluorobenzothiadiazole (DFBT) unit, a regiorandom polymer (PDTP–DFBT, bandgap = 1.38 eV) was obtained. For comparison with the DTP unit, polymers containing alternating benzodithiophene (BDT) or cyclopentadithiophene (CPDT) units and the DFBT unit were synthesized (PBDT–DFBT and PCPDT–DFBT). We found that the DTP based polymer PDTP–DFBT shows significantly improved solubility and processability compared to the BDT or CPDT based polymers. Consequently, very high molecular weight and soluble PDTP–DFBT can be obtained with less bulky side chains. Interestingly, PDTP–DFBT shows excellent performance in bulk-heterojunction solar cells with power conversion efficiencies reaching 8.0%, which is significantly higher than PBDT–DFBT and PCPDT–DFBT based devices. This study demonstrates that DTP is a promising building block for high-performance solar cell materials

Low-Temperature Solution-Processed Perovskite Solar Cells with High Efficiency and Flexibility

Perovskite compounds have attracted recently great attention in photovoltaic research. The devices are typically fabricated using condensed or mesoporous TiO₂ as the electron transport layer and 2,2′7,7′-tetrakis-(<i>N</i>,<i>N</i>-dip-methoxy­phenyl­amine)9,9′-spiro­bi­fluorene as the hole transport layer. However, the high-temperature processing (450 °C) requirement of the TiO₂ layer could hinder the widespread adoption of the technology. In this report, we adopted a low-temperature processing technique to attain high-efficiency devices in both rigid and flexible substrates, using device structure substrate/ITO/PEDOT:PSS/CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i>/PCBM/Al, where PEDOT:PSS and PCBM are used as hole and electron transport layers, respectively. Mixed halide perovskite, CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i>, was used due to its long carrier lifetime and good electrical properties. All of these layers are solution-processed under 120 °C. Based on the proposed device structure, power conversion efficiency (PCE) of 11.5% is obtained in rigid substrates (glass/ITO), and a 9.2% PCE is achieved for a polyethylene terephthalate/ITO flexible substrate

A Selenophene Containing Benzodithiophene-<i>alt</i>-thienothiophene Polymer for Additive-Free High Performance Solar Cell

A selenophene-modified PTB7, PBDTSe-TT, is reported. The structure adjustment carried out by alkylselenophene substitution on the BDT building block is shown to slightly affect the polymer’s electronic property, and an enlarged <i>V</i>_OC of the resulting photovoltaic device is observed. More importantly, the PBDTSe-TT:PC₇₁BM bulk-heterojunction thin film morphology can be optimized through this modification. As a result, an efficient PCE of 8.8% is achieved without using any solvent additive or special interfacial layer. In addition, the PBDTSe-TT-based device is relatively stable under thermal stress, making it a good candidate for fabricating stacking cells. Finally, a ∼10% PCE tandem device is demonstrated by using identical PBDTSe-TT:PC₇₁BM subcells