1 Scientific RepoRts | 6:30868 | DOI: 10.1038/srep30868 www.nature.com/scientificreports Highly stable tandem solar cell monolithically integrating dye-sensitized and CIGs solar cells sang Youn Chae 1,2,* , se Jin park 1,3,* , oh-shim Joo 1 , Yongseok Jun 4 , Byoung Koun Min 1,5 & Yun Jeong Hwang 1 A highly stable monolithic tandem solar cell was developed by combining the heterogeneous photovoltaic technologies of dye-sensitized solar cell (DssC) and solution-processed CuIn x Ga 1-x se y s 1-y (CIGS) thin flm solar cells. The durability of the tandem cell was dramatically enhanced by replacing the redox couple from (I /I ) - 3 - to [Co(bpy) 3 ] 2+ /[Co(bpy) 3 ] 3+ ), accompanied by a well-matched counter electrode (PEDOT:PSS) and sensitizer (Y123). A 1000 h durability test of the DSSC/CIGS tandem solar cell in ambient conditions resulted in only a 5% decrease in solar cell efciency. Based on electrochemical impedance spectroscopy and photoelectrochemical cell measurement, the enhanced stability of the tandem cell is attributed to minimal corrosion by the cobalt-based polypyridine complex redox couple. Te development of solar cells with tandem architecture has attracted attention due to the possibility of overcom- ing the Shockley-Queisser limit of single junction devices 1,2 . Te power conversion efciency of tandem solar cells can be improved by mechanically stacking or monolithically integrating two or more sub-cells with complemen- tary absorption characteristics 3 . Mechanically stacked architecture has the advantage of manufacturing simplicity, but it potentially sufers from optical loss due to the presence of superfuous substrate within the two sub-cells 4 . In this context, monolithically integrated tandem architecture is more suitable for the ultimate goal of a tandem device, which is to facilitate the efcient absorbance of a broader range of wavelengths. However, there are still many issues to be resolved before highly efcient monolithic tandem solar cells can be mass produced, such as lattice and bandgap matching, tunnel junction fabrication, and recombination layers 5,6 . To date, various tandem structures have been suggested based on a combination of inorganic/inorganic, organic/organic, or inorganic/organic solar sub-cells. A world record efciency of 37% has been achieved by triple-junction solar cells based on III–V compound semiconductor materials (the InGaP/GaAs/InGaAs tandem structure). In addition, amorphous and microcrystalline silicon (a-Si/μc-Si) based inorganic tandem cells have exhibited a solar cell efciency of 13.6% 7 . Organic/organic triple-junction solar cells have also been successfully manufactured using diferent band-gap polymers, with a solar cell efciency of 11% 8 . Various forms of inorganic/ organic solar cells (known as hybrid tandem solar cells) have been studied, such as dye sensitized solar cells (DSSC)/Si, DSSC/GaAs, and perovskite/μc-Si 3,9–11 . Models suggest that the optimal bandgap for tandem solar cells is 1.7 eV and 1.1 eV for the top and bottom cells, respectively. Copper chalcopyrite semiconductors – Cu(In, Ga)(S, Se) 2 (CIGS) – are especially promising candidates for tandem cells because the band gap can be tuned from 1.0 to 2.4 eV in accordance with the com- position ratios 12 . However, it is difcult to achieve high efciency with monolithic CIGS/CIGS tandem cells due to damage to the sub-cell during construction of the top CIGS solar cell and the low efciency of this sub-cell 13 . In addition to tandem architecture involving similar classes of CIGS materials, substantially diferent types of single cells have also been combined with CIGS cells. Of these, a CIGS-based tandem solar cell constructed 1 Clean Energy Research Center, Korea Institute of Science and Technology, Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul, 02792, Republic of Korea. 2 Department of Chemistry, College of Science, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul, 02841, Republic of Korea. 3 Department of Materials Science and Engineering, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul, 02841, Republic of Korea. 4 Department of Materials Chemistry and Engineering, Konkuk University, 120 Neungdong-ro, Gwangjin-gu, Seoul, 143-701, Republic of Korea. 5 Green School, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul, 02841, Republic of Korea. * These authors contributed equally to this work. Correspondence and requests for materials should be addressed to B.K.M. (email: bkmin@kist.re.kr) or Y.J.H. (email: yjhwang@kist.re.kr) received: 02 April 2016 Accepted: 29 June 2016 Published: 04 August 2016 opeN