Field of Expertise: Advanced Material Science
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Field of Expertise: Advanced Material Science | |
Exploring polymer/nanoparticle hybrid solar cells in tandem architecture Organic tandem solar cells have attracted much attention in recent years as the efficiencies of tandem devices can exceed the efficiencies which are possible with single junction solar cells. Tandem solar cells offer the possibility to harvest a broader part of the solar spectrum which already led to record efficiencies above 10% prepared by organic small molecules but also with polymer/fullerene composites. In this contribution we present our results on nanocomposite tandem solar cells, where the active layers consist of mixtures of semiconductor nanostructures and a semiconducting conjugated polymer. In contrast to PCBM/polymer solar cells it is, therefore, not only possible to tune the absorption properties by those of the polymer but also by the choice of the inorganic semiconductor. We realized such hybrid tandem solar cells following a recently developed in-situ preparation route using tailored metal xanthate precursors to guarantee high solubility in organic solvents and to form the pure metal sulfides at low temperatures. Thus, a common solution of these metal xanthates with the conjugated polymer is coated directly on the transparent electrodes (e.g. ITO, ITO/PEDOT:PSS) and subsequently converted to the corresponding metal sulfides within the polymer matrix using temperatures below 200 °C. Therefore, no capping ligand is needed to stabilize the nanoparticles during the formation. In a first series, we investigated a hybrid–organic tandem solar cell, with a hybrid solar cell consisting of the silafluorene containing low band gap polymer (PSiF-DBT) and copper indium sulphide (CIS)nanoparticles as the bottom cell, and a low band gap polymer (PTB7)/fullerene derivative (PC61BM) organic solar cell as the top cell in order to study different recombination layers. Tandem devices with open circuit voltages nearly reaching the sum of the individual cells have been realised. The short circuit current is equal to the value of the hybrid single cell and a fill factor above 50% is obtained, leading to power conversion efficiencies of about 4.1%. Furthermore, the first results on hybrid–hybrid tandem solar cells consisting of two PSiF-DBT/CIS solar cells are presented. Although the preparation of these double hybrid devices is challenging because of the necessity of two thermal annealing steps, the resulting multilayer stack reveals smooth and homogeneous layers with sharp interfaces. The first working hybrid–hybrid tandem solar cells still exhibited 81% of the sum of the open circuit voltages of the single junction solar cells. |