The incorporation of thermionic emission and work function tuning layer into intermediate connecting layer for high performance tandem organic solar cells

Abstract

There are two conventional methods to eliminate counter-diode in intermediate connecting layer (ICL) for tandem organic solar cell (OSC), namely forming tunneling junction by using heavily doped electron (ETL) and hole transport layer (HTL) and establishing recombination center of metal layer in between ETL and HTL of ICL. Here, a new method to solve the typical S-shape current density (J)–voltage (V) characteristics of tandem OSC resulted from quasi-Fermi level splitting in ICL has been proposed and demonstrated. Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) and amino-functionalized alcohol soluble conjugated polymer (PF6N25Py) are effective HTL and ETL respectively in single OSC. Their direct combination as ICL would result a typical S-shape J–V curve due to mis-aligned quasi-Fermi levels. Our results show that workfunction of PEDOT:PSS can be widely tuned by changing thickness of as-prepared zirconium acetylacetonate (a-ZrAcac). With an 11.1nm thickness of a-ZrAcac, the WF of PEDOT:PSS has been considerably decreased from 5.0eV to 3.49eV. Based on PEDOT:PSS/ a-ZrAcac/ PF6N25Py ICL, a high efficiency tandem OSC with 8.21% PCE, ignorable Voc loss and no S-shape J–V curve has been achieved. The removal of S-shape in J–V curve is also demonstrated when a-ZrAcac is inserted between PEDOT:PSS/ UV-inactivated zinc oxide (ZnO) as ICL in tandem OSC. Consequently, this method contributes towards a general strategy to develop other qualified ICL with different HTL and ETL for tandem OSC.