Exploring the Feasibility of an Annealing-Free ZnO-Based Electron Transport Layer for Flexible Indoor Organic Photovoltaics, Sustainable in Ambient Conditions

Inverted flexible devices with bulk-hetero junction (BHJ) structures were fabricated mainly for indoor applications. The devices had the structure of PET/ITO/ETL/Active Material/MoO3/Ag, wherein D18 was used as the donor and Y12 as the acceptor, as they cover the visible spectrum, which is advantageous for indoor photovoltaics (IPV). For the ascribed inverted structure, zinc oxide (ZnO) is the prime candidate for the electron transport layer (ETL) owing to its suitable energy levels, transparency, and adhesion to the active layer. Sol–gel synthesis of ZnO is generally preferred due to its ease of synthesis and transparency. High annealing temperature is required for the above-mentioned process, which hinders usage in flexible substrates as they degrade. Herein, we prepare ZnO nanoparticles using the particle precipitation method to bypass annealing, facilitating the fabrication of flexible organic solar cells (OSCs). Furthermore, we dope the ZnO with cesium (Cs), enhancing the charge transport properties and thus increasing efficiency. The devices are illuminated under 1 sun conditions (AM1.5G) and a cool white LED, and their photovoltaic parameters are investigated. The devices (Cs-doped) achieved power conversion efficiency (PCE) values of 8.15% (AM1.5G) and 18.74% (under LED). Bending tests were also performed to compare J–V characteristics and the retention of the device properties. This work establishes the fact that Cs-doped ZnO performs well over pristine ZnO for its application as an ETL, especially in flexible OSCs. All of the device characterizations were performed in ambient conditions without a glovebox, demonstrating the highly stable nature of the devices. The work emphasizes exploiting viabilities to produce highly stable OSCs in a normal atmosphere, especially for indoor energy harvesting that can be integrated into the Internet of Things (IoT) in the future.