Interfacial Engineering of Multi-Junction Polymer Solar Cells

Organic photovoltaic (OPV) technology has drawn much attention during the past few decades due to its role to role processability, compatibility with flexible substrates and low manufacturing cost. However, one of the major drawbacks of OPV technology is narrow absorption width which limits device performance. To absorb solar energy more efficiently, multi-junction polymer solar cells are used where two or more subcells are stacked together. There are several challenges in developing multi-junction polymer solar cells. These include selection of proper absorber layers and fabrication of robust, transparent, conductive interfacial layer to connect the subcells. The goal of this work was to develop a low temperature processable interconnect layer for solution processed tandem and triple junction polymer solar cells with high efficiency. Different characterization techniques were carried out to find the optimum interfacial conditions to connect the top and bottom cells. The experimental results show that surface morphology of PEDOT:PSS in interconnecting layer (ICL) impacts overall tandem device performance. The annealing temperature for the ICL (PEDOT:PSS/AZO/PEIE) was also investigated. The new ICL was used to successfully connect the top and bottom cells in double junction polymer solar cells without major losses. A triple junction polymer solar cell was also fabricated using the same ICL where identical polymer (P3HT) was used in top, middle and bottom layers. Double junction Voc of 1.05 V and triple junction Voc of 1.4 V indicated that the ICLs worked xiv effectively. However, triple junction solar cells exhibited poor device performance. The Jsc of triple junction polymer solar cells was low due to the use of the same polymer in top, middle and bottom cells. FF and Voc of triple junction polymer solar cells were improved by optimizing the thickness of active layer

High-Sensitivity Demodulation of Fiber-Optic Acoustic Emission Sensor Using Self-Injection Locked Diode Laser

We demonstrate the use of a self-injection locked distributed feedback (DFB) diode laser for high-sensitivity detection of acoustic emission (AE) using a fiber-coil Fabry-Perot interferometer (FPI) sensor. The FPI AE sensor is formed by two weak fiber Bragg gratings on the ends of a long span of coiled fiber, resulting in dense sinusoidal fringes in its reflection spectrum that allows the use of a modified phase-generated carrier demodulation method. The demodulation method does not require agile tuning capability of the laser, which makes the self-injection locked laser particularly attractive for the application. Experimental results indicate that the self-injection locked laser increases the signal-to-noise ratio by ∼33 dB compared with the free-running DFB laser. We studied the mode-hopping and laser instability of the self-injection locked laser and their effect on the demodulated signal and found that a mode hopping event causes an abrupt change in the laser intensity after the resonator inside the feedback loop. It manifests itself as a short transient signal in the output of the AE sensor system. With the identification of the mode-hopping events, the associated spurious signal can be identified and discarded in the signal processing without causing significant disruption to the sensor system. Experiment shows that the frequency of locked lasers could oscillate during unstable operations. The fundamental frequency is determined by the time delay of the feedback light and is typically much larger than the AE frequency. Therefore, the laser frequency oscillations have no negative effect on the performance of the sensor system. Finally, we show that the frequency of mode-hopping occurrence is related to the length of the feedback loop and reducing the loop length can effectively reduce the frequency of mode-hopping occurrence

Modeling of organic solar cell using response surface methodology

Polymer solar cells have drawn much attention during the past few decades due to their low manufacturing cost and incompatibility for flexible substrates. In solution-processed organic solar cells, the optimal thickness, annealing temperature, and morphology are key components to achieving high efficiency. In this work, response surface methodology (RSM) is used to find optimal fabrication conditions for polymer solar cells. In order to optimize cell efficiency, the central composite design (CCD) with three independent variables polymer concentration, polymer-fullerene ratio, and active layer spinning speed was used. Optimal device performance was achieved using 10.25 mg/ml polymer concentration, 0.42 polymer-fullerene ratio, and 1624 rpm of active layer spinning speed. The predicted response (the efficiency) at the optimum stationary point was found to be 5.23% for the Poly(diketopyrrolopyrrole-terthiophene) (PDPP3T)/PC60BM solar cells. Moreover, 97% of the variation in the device performance was explained by the best model. Finally, the experimental results are consistent with the CCD prediction, which proves that this is a promising and appropriate model for optimum device performance and fabrication conditions. Keywords: Organic photovoltaics, Performance measures, Response surface, Experimental design, Optimizatio

Efficient CsF interlayer for high and low bandgap polymer solar cell

Low bandgap polymer solar cells have a great deal of importance in flexible photovoltaic market to absorb sun light more efficiently. Efficient wide bandgap solar cells are always available in nature to absorb visible photons. The development and incorporation of infrared photovoltaics (IR PV) with wide bandgap solar cells can improve overall solar device performance. Here, we have developed an efficient low bandgap polymer solar cell with CsF as interfacial layer in regular structure. Polymer solar cell devices with CsF shows enhanced performance than Ca as interfacial layer. The power conversion efficiency of 4.5% has been obtained for PDPP3T based polymer solar cell with CsF as interlayer. Finally, an optimal thickness with CsF as interfacial layer has been found to improve the efficiency in low bandgap polymer solar cells

Performance analysis of high efficiency InxGa1−xN/GaN intermediate band quantum dot solar cells

In this subsistent fifth generation era, InxGa1−xN/GaN based materials have played an imperious role and become promising contestant in the modernistic fabrication technology because of some of their noteworthy attributes. On our way of illustrating the performance, the structure of InxGa1−xN/GaN quantum dot (QD) intermediate band solar cell (IBSC) is investigated by solving the Schrödinger equation in light of the Kronig-Penney model. In comparison with p-n homojunction and heterojunction solar cells, InxGa1−xN/GaN IBQD solar cell manifests larger power conversion efficiency (PCE). PCE strongly depends on position and width of the intermediate bands (IB). Position of IBs can be controlled by tuning the size of QDs and the Indium content of InxGa1−xN whereas, width of IB can be controlled by tuning the interdot distance. PCE can also be controlled by tuning the position of fermi energy bands as well as changing the doping concentration. In this work, maximum conversion efficiency is found approximately 63.2% for a certain QD size, interdot distance, Indium content and doping concentration. Keywords: Solar cell, InxGa1−xN/GaN, Quantum dots, Intermediate band(s), Power conversion efficienc

Efficient Perovskite Solar Cells by Temperature Control in Single and Mixed Halide Precursor Solutions and Films

Thermal annealing and precursor composition play critical roles in crystallinity control and morphology formation of perovskite thin films for achieving higher photovoltaic performance. In this study we have systematically studied the role of annealing temperature on the crystallinity of perovskite (CHNH₃PbI₃) thin films cast from single (without PbCl₂) and mixed (with PbCl₂) halide precursors. Higher annealing temperature leads to agglomeration of perovskite crystals. The effects of annealing temperature on the performance of perovskite solar cells are different in single and mixed halide processed films. It is observed that the perovskite crystallinity and film formation can be altered with the addition of lead chloride in the precursor solution. We report that single halide perovskite solar cells show no change in morphology and crystal size with increase in annealing temperature, which was confirmed by UV–vis absorption spectroscopy, X-ray diffraction (XRD), and atomic force microscopy (AFM). However, mixed halide perovskite (CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i>) solar cells show significant change in crystal formation in the active layer when increasing annealing temperature. In addition, heating perovskite precursor solutions at 150 °C can lead to enhancement in solar cell efficiency for both single and mixed halide systems. Perovskite solar cells fabricated using heated precursor solutions form dense film morphology and thus significantly improved fill factor up to 80% with power conversion efficiency exceeding 13% under AM 1.5 condition

Interfacial Study To Suppress Charge Carrier Recombination for High Efficiency Perovskite Solar Cells

We report effects of an interface between TiO₂–perovskite and grain–grain boundaries of perovskite films prepared by single step and sequential deposited technique using different annealing times at optimum temperature. Nanoscale kelvin probe force microscopy (KPFM) measurement shows that charge transport in a perovskite solar cell critically depends upon the annealing conditions. The KPFM results of single step and sequential deposited films show that the increase in potential barrier suppresses the back-recombination between electrons in TiO₂ and holes in perovskite. Spatial mapping of the surface potential within perovskite film exhibits higher positive potential at grain boundaries compared to the surface of the grains. The average grain boundary potential of 300–400 mV is obtained upon annealing for sequentially deposited films. X-ray diffraction (XRD) spectra indicate the formation of a PbI₂ phase upon annealing which suppresses the recombination. Transient analysis exhibits that the optimum device has higher carrier lifetime and short carrier transport time among all devices. An optimum grain boundary potential and proper band alignment between the TiO₂ electron transport layer (ETL) and the perovskite absorber layer help to increase the overall device performance