An effective way to improve polymer solar cell efficiency is to use a tandem structure, as a broader part of the spectrum of solar radiation is used and the thermalization loss of photon energy is minimized. In the past, the lack of high-performance low-bandgap polymers was the major limiting factor for achieving high-performance tandem solar cell. Here we report the development of a high-performance low bandgap polymer (bandgap <1.4 eV), poly[2,7-(5,5-bis-(3,7-dimethyloctyl)-5H-dithieno[3,2-b:2′,3′-d]pyran)-alt-4,7-(5,6-difluoro-2,1,3-benzothia diazole)] with a bandgap of 1.38 eV, high mobility, deep highest occupied molecular orbital. As a result, a single-junction device shows high external quantum efficiency of >60% and spectral response that extends to 900 nm, with a power conversion efficiency of 7.9%. The polymer enables a solution processed tandem solar cell with certified 10.6% power conversion efficiency under standard reporting conditions (25 °C, 1,000 Wm−2, IEC 60904-3 global), which is the first certified polymer solar cell efficiency over 10%.

Tandem solar cells have the potential to improve photon conversion efficiencies (PCEs) beyond the limits of single-junction devices. In this study, a triple-junction tandem design is demonstrated by employing three distinct organic donor materials having bandgap energies ranging from 1.4 to 1.9 eV. Through optical modeling, balanced photon absorption rates are achieved and, thereby, the photo-currents are matched among the three subcells. Accordingly, an efficient triple-junction tandem organic solar cell can exhibit a record-high PCE of 11.5%.

Polymer tandem solar cells with 10.2% power conversion efficiency are demonstrated via stacking two PDTP-DFBT:PC₇₁ BM bulk heterojunctions, connected by MoO₃/PEDOT:PSS/ZnO as an interconnecting layer. The tandem solar cells increase the power conversion efficiency of the PDTP-DFBT:PC₇₁ BM system from 8.1% to 10.2%, successfully demonstrating polymer tandem solar cells with identical sub-cells of double-digit efficiency.

We describe the detailed synthesis and characterization of an electron-rich building block, dithienopyran (DTP), and its application as a donor unit in lowbandgap 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.

No abstract