The intensifying energy crisis and increasing electricity costs and their hazardous impact on the environment have attained a substantial amount of attention in research and development due to the requirement for more efficient and viable alternative energy resources (Chakraborty et al., 2021). This makes renewable and energy harvesting systems a necessity for addressing the energy crisis around the globe. Energy harvesting can be performed from various techniques and sources, depending on the type of technology used to generate the clean energy, and includes thermoelectric energy harvesting, photovoltaic energy harvesting, piezoelectric energy harvesting, pyroelectric energy harvesting, electromagnetic energy harvesting, wind energy harvesting, and vibration energy harvesting (Davidson and Mo, 2014; Akinaga, 2020; Zhou et al., 2021). Solar energy generation has gained momentum in the last few decades due to its higher efficiency and high availability. However, the associated cost of solar panels makes it a less cost-effective energy generation method (Pastuszak and Węgierek, 2022). This opens up an opportunity for low-cost advanced-generation solar cells, which are currently being synthesized in laboratories and are considered to be the future of low-cost and efficient solar energy generation. Organic solar cells, perovskite solar cells, and dye-sensitized solar cells (DSSCs) are considered the future of the solar cell industry. However, the high cost of perovskite materials, stability issues, and low lifetime of organic and perovskite solar cells are considered the biggest drawbacks of these solar cells. Among these solar cells are dye-sensitized solar cells, which are easy to fabricate, low-cost flexible, lightweight, and reliable solar cells. DSSCs are gaining momentum due to their capability of efficiency improvement by modification of their cell layers (Alizadeh et al., 2022). Currently, the highest reported efficiency of DSSCs is 14.1% but researchers are working towards improving the efficiency with easy fabrication techniques and low cost of cell preparation (Singh and Shougaijam, 2022). The efficiency of energy conversion in a dye-sensitized solar cell is determined by the semiconductor, sensitizer, electrolyte, and counter electrode. The efficiency of DSSCs can be improved by carefully determining the optimal importance of several manufacturing process factors (Castán et al., 2022; Richhariya et al., 2022; Ari et al., 2023). ZnO-based semiconductor photo anode has been used in DSSCs under different morphologies including nanoparticles, nanorods, and nanowires. Chung et al. reported that an improvement in the fill factor of the DSSC is observed when annealed nanorods are used as photoanodes due to better crystallinity (Chung et al., 2010). Similarly, a power conversion efficiency of 2.35% has been reported using hedgehog-like ZnO-nanorods on the ZnO nanoparticle seed layer (Yuliasari et al., 2022). Moreover, several doped ZnO nanostructures are reported in the literature as photoanodes for DSSCs with improved photovoltaic efficiencies. For example, Cd-doped ZnO nanostructures show a power conversion efficiency of 0.5%, Cu-doped ZnO nanostructures show a power conversion efficiency of 2.03%, and Pd-doped ZnO nanostructures show a power conversion efficiency of 11.92% (Esgin et al., 2022; Esakki et al., 2023; Mujahid and Al-Hartomy, 2023). However, the study of the second group of highly conductive metals and their doping with ZnO has not been reported in the literature. Since the second group elements are extremely stable and conductive, they are considered an excellent additive in ZnO nanocrystals for the improvement of the efficiency of solar cells.

On the other hand, microelectromechanical systems (MEMS)-based energy harvesters have been used due to their small size and complete package on a single chip (Fang et al., 2006; Zorlu et al., 2013; Sun et al., 2018; Toshiyoshi et al., 2019). MEMS-based energy harvesters have advantages over the conventionally used systems, including low cost, easy maintenance, relatively easier operation, higher efficiency, low environmental impacts, easier synthesis and fabrication, and wireless systems for better usage (Liu et al., 2012; Bounouh and Bélières, 2014). These devices use ambient mechanical energy, converting it into useful electrical energy. Among different types of MEMS-based energy harvesting, piezoelectric energy harvesters utilize mechanical energy in the form of stress and pressure and convert it into electrical energy. The mechanical energy applied can be in the form of stress including vibrational stress, noise, strain, sonic waves, applied temperature gradient, energy generated due to gas and liquid flow as well as other physical motions (Rajeev et al., 2021). Different MEMS-based energy harvesters, including triboelectric nanogenerators and piezoelectric nanogenerators, have gained popularity in terms of generating clean energy (Maamer et al., 2019; Anand et al., 2021; Zhang et al., 2021). Among these devices, piezoelectric nanogenerators use piezoelectric materials that are capable of converting mechanical energy (vibration and stress) into useful electrical energy. Some useful and commonly used piezoelectric materials include zinc oxide (ZnO), lead zirconate titanate, barium titanate, lead titanate, and lithium tantalite (Bestley Joe and Shaby, 2021; Iqbal et al., 2021; Poon et al., 2021; Tiwari et al., 2021; Debnath and Kumar, 2022). ZnO has been considered as the most suitable material piezoelectric energy harvester (Karumuthil et al., 2019; Rajeev et al., 2020). ZnO exhibits excellent band gap tenability and piezoelectric properties owing to its structural properties of transparency, n-type dipole structure for cutting-edge piezoelectric properties, its ability to alter nanostructural properties, and its chemical stability as well as no lead-based toxic impurity, which damages energy harvesters (Gullapalli et al., 2010; Vaseem et al., 2010; Cherumannil Karumuthil et al., 2017). With these excellent properties, ZnO has been extensively used as a piezoelectric MEMS energy harvester. Tao reported the use of ZnO as a 2DOF MEMS energy harvester with a reported voltage output of 10 and 15 mV (Tao et al., 2019). Similarly, Wang reported the use of dual ZnO thin film with a highly improved voltage output of 1.77 V. The research was performed with ANSYS simulation and the accuracy of the simulation still needs to be checked (Wang and Du, 2015a). Doped ZnO nanostructures for application in MEMS energy harvesters still require further research to study their impact on the current density and voltage output.

Owing to the superior properties of ZnO, the structural and optical properties of ZnO can be altered by doping it with other materials. The group II elements, including calcium, magnesium, strontium, and barium, are considered the most conductive materials that can be easily incorporated into the atomic structure of ZnO, resulting in altered structural and optical properties, including the grain size and band-gap, of ZnO structures produced as a result of doping (Sheikh et al., 2013; Afzal et al., 2020; Baig et al., 2020; Sarwar et al., 2021). Adding second group element doping in ZnO nanostructures results in the alternation of the band gap as well as the structural morphology of ZnO nanostructures (Mahdhi et al., 2018; Sarwar and Ashraf, 2020; Tayyaba et al., 2020). Various ways can be used to prepare the ZnO nanostructures including chemical and physical methods (Dobrzański and Pakuła, 2005; An et al., 2020; Boing et al., 2020). The chemical techniques are considered low-cost techniques; however, their reproducibility is relatively lower as compared to physical techniques. Among different chemical techniques, chemical bath deposition is considered one of the most efficient methods to generate ZnO thin film with different types of nanostructures (Wu and Liu, 2002; Hodes, 2007; Mugle and Jadhav, 2016; Wasim et al., 2020).

There is a research gap that lies within the study of the small-scale energy harvesting methods (DSSCs and MEMs energy harvesters) fabricated using second group element doped ZnO. Among second group elements, including Mg, Ca, Be, and Sr, Strontium has a similar ionic radius to zinc, which minimizes lattice distortion and defect formation, leading to enhanced electrical properties in ZnO. As the ionic radius of strontium is large as compared to the other elements of group II, it is expected to alter the properties of ZnO more as compared to other group elements including Mg and Ca. Whereas Mg, Ca, and Be doping can introduce more significant lattice disruptions and defects due to differences in ionic radii. In this study, simulation and experimental work related to strontium-doped ZnO has been conducted for use in a dye-sensitized solar cell and MEMS energy harvester. The study included the fabrication of an anodic aluminum oxide (AAO) template, which was used as a substrate. The optical, structural, photovoltaic, and piezoelectric properties of Sr-doped ZnO nanostructures were analyzed. We have conducted a comparative study between solar cell and MEMS energy harvesting by developing complete solar cell and MEMS-based energy harvesters based on Sr-doped ZnO nanostructures and predicting the voltage and current of the energy harvesters.

A fuzzy rule-based system was used to predict the voltage and current output of the ZnO used as an anode of solar cells and the piezoelectric material in energy harvesters. Anodic aluminum oxide was used as a template, and Sr-doped zinc oxide nanostructures were deposited on the substrate to study energy generation properties. Figure 1 shows the fuzzy logic interface for the simulation. The input used includes anodic aluminum oxide pore size, Sr doping concentration in zinc oxide, and the process temperature for the deposition of the Sr-doped ZnO nanostructures. The band gap of the prepared film, current, and voltage for the energy harvesters were taken as output.

Based on the literature, the ranges of the input and output parameters were selected in order to predict the possible solution for the output of the energy harvesters. The input pore size of the AAO membrane varied from 1 to 1,000 nm, the Sr-doping concentration in ZnO ranged from 2% to 8%, and the process temperature for the process varied from 10 to 95°C. The output parameters were assigned the ranges of 2–8 mV for output voltage, 0.3–1.8 uA for output current, and 3.1–3.4 eV for band gap value. The rules were then selected based on real life scenarios and the literature. A total of 27 rules were selected and based on these rules, a rule viewer graph was studied and analyzed with the variation of inputs. The rule viewer result was then compared with the Mamdani model. Calculated values were compared for error between the simulated and calculated values.

Figure 2 shows the rule viewer of the inputs and outputs based on the rules selected. It was observed that at an AAO membrane with a pore size of 300 nm, with 4% doping of Sr in ZnO nanostructures, and at 90°C process temperature, generated an output voltage of 7.08 mV, an output current of 1.12 uA, and a band gap of 3.25 eV, as shown in Figure 2.

Based on the simulated values from the rule viewer, the values were compared with the Mamdani calculated values using the Mamdani model formula as shown below in Equation 1. P = ∑ R i   x   S i ∑ R i (1) where Ri is equal to the minimum membership function value and Si is known as the singleton value.

The differences between the simulated and calculated values along with the differences between the values are shown in Table 1. The values show that the simulated and calculated values had a very small error that led to the accuracy of the designed system.

All materials were analytically cleaned. The synthesis of Sr-doped ZnO nanostructure with 4% doping was performed on the anodic aluminum oxide substrate. The porous anodic aluminum oxide substrate was fabricated via an anodization process that is similar to an electrochemical system. The Sr-doped ZnO nanorods were then deposited on the porous AAO substrate. Then, contacts were sputtered on the designed MEMS energy harvester to analyze the current and voltage of the energy harvester. The Pt counter electrode, dye, and electrolyte were used for solar cells, and a solar simulator was used for the complete solar cell analysis.

This study presents an investigation into Sr-doped ZnO nanorods utilized as an AAO template for dye-sensitized solar cells and MEMS energy harvesters. Notably, the calculated band gap was found to increase from 3.25 eV in pristine ZnO to 3.27 eV in Sr-doped ZnO, a phenomenon attributed to the incorporation of strontium ions affecting the electronic band structure. The utilization of a porous AAO template facilitated the fabrication of smoother nanorods, owing to the templating effect provided by the uniform pores. These controlled growth conditions resulted in more ordered and uniform nanorod surfaces, enhancing the overall performance of the nanostructures. The subsequent evaluation of the solar cell’s efficiency yielded promising results, including a 2.2% conversion efficiency, 0.67 V open circuit voltage, 5.1 mA/cm² short circuit current density, and a 0.62 fill factor. When compared to conventional ZnO nanorods, the Sr-doped ZnO nanorods on the AAO template demonstrated significant improvements in power conversion efficiency, primarily due to the reduced band gap, which facilitates efficient electron transport from the dye to the external circuit. Additionally, the porous nature of the AAO template enhanced electron mobility. Furthermore, the MEMS energy harvester exhibited enhanced performance, with open circuit voltages ranging from 7.0 to 7.2 mV and short circuit currents ranging between 1.05 and 1.15 uA for both polarities. These findings underscore the potential of the fabricated film for applications in both solar energy conversion and piezoelectric energy harvesting.