Low-temperature solution-processed high-efficiency colloidal quantum dot (CQD) photovoltaic devices are developed by improving the interfacial properties of p-n heterojunctions. A unique conjugated polyelectrolyte, WPF-6-oxy-F, is used as an interface modification layer for ZnO/PbS-CQD heterojunctions. With the insertion of this interlayer, the device performance is dramatically improved. The origins of this improvement are determined and it is found that the multifunctionality of the WPF-6-oxy-F interlayer offers the following essential benefits for the improved CQD/ZnO junctions: (i) the dipole induced by the ionic substituents enhances the quasi-Fermi level separation at the heterojunction through favorable energy band-bending, (ii) the ethylene oxide groups containing side chains can effectively passivate the interfacial defect sites of the heterojunction, and (iii) these effects occur without deterioration in the intrinsic depletion region or the series resistance of the device. All of the figures-of-merit of the devices are improved as a result of the enhanced built-in potential (electric field) and the reduced interfacial charge recombination at the heterojunction. The benefits due to the WPF-6-oxy-F interlayer are generally applicable to various types of PbS/ZnO heterojunctions. Finally, CQD photovoltaic devices with a power conversion efficiency of 9% are achievable, even by a solution process at room temperature in an air atmosphere. The work suggests a useful strategy to improve the interfacial properties of p-n heterojunctions by using polymeric interlayers.
Bibliographical noteFunding Information:
The authors gratefully acknowledge support from The New and Renewable Energy Core Technology Program of Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry and Energy, Republic of Korea (S.-Y.J., Grant No. 20133030000210), and Global Scholarship Program for Foreign Graduate Students at Kookmin University in Korea (R.A.). A portion of this research was supported by National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (S.-H.O., Grant No. 2012M2A2A6013183).
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- conjugated polyelectrolytes
- low-temperature process
- p-n heterojunction
- quantum dots
- solar cells