Research Article Hydrothermal Synthesis of Nitrogen and Phosphorus Codoped Carbon Dot Interfacial Modification Layer for Efficient Charge Transfer between ZnO Electron Transport Layer and PTB7:PC 70 BM Active Layer Bisrat Nigusie Tafese , 1 Fekadu Gochole Aga , 1 Thothadri Ganesh, 1 and Chernet Amente Geffe 2 1 Department of Materials Science and Engineering, School of Mechanical, Chemical, and Materials Engineering (SoMCME), Adama Science and Technology University (ASTU), P.O. Box 1888, Adama, Ethiopia 2 Department of Physics, College of Computational and Natural Science, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia Correspondence should be addressed to Bisrat Nigusie Tafese; bisrat.nigusie@astu.edu.et and Fekadu Gochole Aga; fekadu.gochole@astu.edu.et Received 12 January 2023; Revised 24 February 2023; Accepted 1 March 2023; Published 24 March 2023 Academic Editor: Arun Thirumurugan Copyright © 2023 Bisrat Nigusie Tafese et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The interface between the active layer and the charge-transporting layer is critical for performance improvement in polymer solar cells (PSCs). The use of zinc oxide (ZnO) as an electron transport layer (ETL) in PSCs was limited due to inherent defects in the surface of ZnO prepared by the sol-gel method, mismatched energy bands with the photoactive layer, and incompatibility between the photoactive layer and ZnO ETL. In this study, nitrogen and phosphorus codoped carbon dots (N & P-CDs) were prepared from Ensete ventricosum (false banana) and used as an interfacial modification layer for ZnO ETL. The inverted devices with structures ITO/ZnO/N & P-CD/PTB7:PC70BM/Al were fabricated to investigate the charge transfer dynamic between the active layer and ETL interface modification with N & P-CDs. We have observed that the interfacial modification between the ZnO ETL and the active layer, using N & P-CDs, improves the charge transfer between ZnO ETL and PTB7:PC 70 BM active layer. The obtained result shows that the ETL/BHJ interface resistance of the devices with ZnO:undoped CDs, ZnO:N-CDs, ZnO:P-CDs, and ZnO:N & P-CD ETLs decreases dramatically from 103.4 to 84.04, 78.16, 37.88, and 28.9 Ω, respectively. This is due to the improvement of charge extraction efficiency by smoothing ZnO surface defects and minimizing the band mismatch between the active layer and ZnO using N & P-CDs. The results indicate that the water-soluble N & P-CDs developed in this study have the potential to be used for efficient free charge carrier extraction for PSCs. 1. Introduction The challenge of energy supply with an ever-increasing population [1] and significant drawbacks with traditional energy sources push researchers to find ecofriendly and renewable energy sources for the aforementioned problems [2, 3]. One alternative renewable source of energy is the conversion of solar energy into electricity using solar cell devices through the general steps of photon absorption, exciton creation, electron-hole separation, transport, and collection at electrodes [4]. Among the various solar cells, polymer solar cells (PSCs) have received a lot of attention in the scientific community due to the potential of low- cost materials, simple fabrication processes, and large- area manufacturing with lightweight and flexible properties [5–9]. Hindawi International Journal of Energy Research Volume 2023, Article ID 8847653, 15 pages https://doi.org/10.1155/2023/8847653