Organic Solar Cells with an Efficiency Approaching 15%

In this work, we focus on the fine-tuning of absorption spectra to achieve well-matched light response and reducing the energy loss (E-loss = E-g(opt) - e*V-OC, where E-g(opt) is the optical bandgap, and V-OC is the open circuit voltage) for the sub-cells of tandem organic solar cells (OSCs) to improve photovoltaic performance. We select PTB7-Th:IEICO-4F with a band gap of 1.24 eV as the rear cell and J52-2F:IT-M with a band gap of 1.59 eV as the front cell to fabricate the tandem OSCs. The absorption spectra of PTB7-Th:IEICO-4F is mainly located in the region of 600 - 950 nm, while that of J52-2F:IT-M is mainly located in the region of 450 - 750 nm. Their response spectra are very complementary, and both of the sub-cells show high external quantum efficiency, prompting the tandem OSCs to realize high-efficiency utilization of solar spectrum in the range of 300 - 1000 nm. In this tandem OSC, the E-loss in both front and rear cells are also effectively controlled to be 0.64 and 0.53 eV, respectively. In addition, an efficient interconnection layer (ICL) of ZnO/PCP-Na is used to connect the front and the rear sub-cells, which shows outstanding diode characteristics and high light transmittance in the long wavelength direction. In view of these excellent properties, the tandem solar cells manufactured in this work possess the advantages of high short-circuit current density (J(SC)) and V-OC, reaching 13.3 mA/cm(2) and 1.65 V, respectively. The highest power conversion efficiency (PCE) of the tandem OSC reaches 14.9%. The device parameters decrease slightly after encapsulation due to the intense ultraviolet irradiation, and a certified PCE of 14.0%, with a V-OC of 1.64 V, a J(SC) of 12.9 mA/cm(2), and a fill factor (FF) of 0.664, recorded by National Institute of Metrology (NIM), China. This result is the best obtained in the organic photovoltaic field.