Clean and renewable energy has been a considerable issue in the last decade. For this reason, organic photovoltaic cells (OPCs) have been attractive devices as next-generation substitute energy sources [1–4]. At present, the performance of OPCs has been reported up to power conversion efficiency (PCE) of 10% and above [5, 6]. There have been reports that polymer solar cells have many advantages of cost effectiveness in the fabrication process, and the mechanical flexibility and polymeric materials provide a wide field of applications. Furthermore, the advantage of organic photovoltaic cells has a high potential to be manufactured using continuous coating technology capable of producing large areas at a low cost [7, 8]. Poly(3,4-ethylenedioxythiophene:poly(4-styrenesulfonate)) (PEDOT:PSS) is the most widely utilized as hole-conducting layer material in organic light-emitting diodes and photovoltaic cells . The advantages of PEDOT:PSS include low temperature, excellent stability, large area processing, low cost, and flexibility. However, the efficiency of this material is limited by their low carrier mobility . Therefore, hole mobility is a key parameter for photovoltaic devices with respect to their adaption in device applications.
ZnO has received much attention over the past few years because of its wide range of properties that depend on doping, including a range of conductivity from metallic to insulating (including n-type and p-type conductivity), high transparency, piezoelectricity, wide-bandgap semiconductivity, room-temperature ferromagnetism, and huge magneto-optic and chemical-sensing effects. Without much effort, it can be grown into many different nanoscale forms, thus allowing various novel devices to be achieved . ZnO, a II-VI semiconductor, is now recognized as a promising candidate for blue and ultraviolet light-emitting diodes or laser diodes due to its wide bandgap of 3.37 eV and large exciton binding energy of 60 meV [12–17]. Its large exciton binding energy allows excitonic absorption and recombination even at room temperature, which makes this material appealing . A lot of methods have been extensively used for oriented ZnO film synthesis, including laser molecular beam epitaxy, pulsed laser deposition, metal-organic chemical vapor deposition, sputtering , cathodic magnetron sputtering and reactive electron beam evaporation, spray pyrolysis, and electrodeposition. However, sol-gel processes are particularly adapted to produce ZnO colloids and films in a simple, low-cost, and highly controlled way. The sol-gel process, also called soft chemistry (‘chimie douce’), allows elaboration of a solid material from a solution by using a sol or a gel as an intermediate step and at much lower temperatures than is possible by traditional methods of preparation . It enables the powderless processing of glasses, ceramics, and thin films or fibers directly from a solution. The synthesis of solid materials via chimie douce often involves wet chemistry reactions and sol-gel chemistry based on the transformation of molecular precursors into an oxide network by hydrolysis and condensation reactions [19, 20].
Recently, poly(3-hexylthiophene) (P3HT) has been used as a hole transporter in combination with ZnO nanostructures. These devices have an efficiency of approximately 0.5% under standard solar conditions (AM 1.5, 100 mW/cm2) and show a current density of Jsc = 2.2 mA/cm2, an open-circuit voltage of Voc = 440 mV, and a fill factor of 0.56. This cell performance can be significantly improved to Jsc = 10.0 mA/cm2, Voc = 475 mV, and a fill factor of 0.43, leading to an efficiency of 2% by using a blend of P3HT and (6,6)-phenyl-C61-butyric acid methyl ester. The low open-circuit voltage in hybrid solar cells using ZnO as the electrode material is not yet fully understood. Certainly, more investigation is necessary to find the leakage, and then higher cell efficiencies can be expected .
In this work, we have investigated the structural, morphological, and optical properties of ZnO nanostructured fibrous film spin coated on indium-tin oxide (ITO) glass. We fabricated polymer solar cells that have the structure of ITO/ZnO/PEDOT:PSS/active layer (P3HT:ICBA)/Al. Poly(3-hexylthiophene-2,5-diyl) (P3HT) and indene-C60 bisadduct (ICBA) were blended and used as an active layer in polymer bulk heterojunction (BHJ) photovoltaic cells. The performance characteristics of polymer photovoltaic cells using ZnO nanostructured fibrous film as a hole-conducting layer have been investigated.