Theoretical and experimental investigation of multiple quantum wells for photovoltaic applications

Abstract

There are certain losses associated with the single junction solar cell such as, thermalization loss, transmission loss, recombination loss and blackbody radiation loss, most of which can be minimized by using multiple energy bands technologies, i.e. multiple quantum well solar cell (MQWSC). Alternatively, this MQWSC technology can be integrated with the multiple junction solar cell (MJSC) to absorb the different part of the solar spectrum more efficiently. As, in MJSC minimum of the current from all the sub-cells will be delivered. Therefore, by inserting multiple quantum well in the sub-cells which is generating low current, this issue in MJSCs can be resolved up to some extent. In MQWSC, transport mechanism of charge carrier is mainly thermally activated, thus these are estimated to show a better conversion efficiency at high temperature than the conventional cells and therefore, makes them more feasible for concentrated solar cell applications. Before the actual experimental fabrication of the MQWSC, it is vital to accomplish theoritcal study of the performance parameters of the MQWSC under different conditions. Therefore, an analytical and simulation study has to be commenced to evaluate the performance parameters that will be expected from MQWSC under various conditions. First, analytical model is developed for the p-i-n structure, as MQWSC is just the extension of p i-n structure in which MQWs are inserted in the intrinsic region. Then, an analytical model is developed for the MQWSC. In the developed models, significantly the spectral irradiance available from American Society for Testing and Materials (ASTM) standards data sheets is used for realising photon flux density instead of commonly used Planck’s blackbody radiation law. Further, the photon flux density is utilized to evaluate the performance parameters of MQWSC and bulk p-i-n solar cell. The developed model of MQWSC is further compared with the published experimental data obtained by other reasearchers on the same technology. In comparision, the material and structural properties of the published experimental researches are utilized to obtain the results from analytical model. The close aggrement of the analytically obtained results with that of the published experimental results by other researches validate the analytical model. Next, based on the developed models, the analysis has been performed for different composition of indium in InxGa1-xN well layers on performance parameters of InGaN/GaN-based MQWSC since InGaN is a well established material for the high bandgap device technology. As the temperature of operation also has a substantial influence on solar cell performance, an extensive discussion on the temperature dependence of the performance parameters of the p-i-n and MQW solar cell is performed. The analysis has also been performed to study the impact of number of quantum well variation on device performance parameters. Results suggest that by incorporating quantum wells in the intrinsic region (x = 0.1 in InxGa1-xN), ~27 % increment in the conversion efficiency can be achieved as compared to that from the bulk solar cell. Moreover, the rise in temperature leads to the increase in short circuit current density; however, open circuit voltage and conversion efficiency decreases. A decrement of ~9.7 % in the conversion efficiency of MQWSC is observed with the rise in temperature from 200 to 400 K as compared to ~11.6 % decline in p-i-n solar cell. Additionally, the CdZnO/ZnO-based MQWSC has been proposed and its performance is studied under various conditions such as cadmium composition variation and operation temperature variation. The results show that, for the proposed CdZnO/ZnO-based MQWSC, the open-circuit voltage (Voc) has a negative temperature coefficient (-2.63 mV/˚C), and short-circuit current density (Jsc) and conversion efficiency (η) have positive temperature coefficients of 2.43×10-3 mA/cm2 .˚C and 2.91×10-3 %/˚C, respectively. Furthermore, the influence of solar irradiance on the performance parameters and J-V characteristics of MQWSC is examined by modifying the developed model i.e. by including the effect of solar irradiance in the previously developed model. For CdZnO/ZnO-based MQWSC, the short circuit current density increases from 0.12 to 57.98 mA/cm2 , open circuit voltage rises from 2.60 to 2.77 V and photon conversion efficiency from 2.85 to 3.04 %, as solar irradiance increases from 0.1 to 50 suns. Next, optimization of CdZnO/ZnO-based Multiple quantum wells (MQWs) realized for the first time by dual ion beam sputtering (DIBS) at different deposition conditions in terms of ion beam power, substrate temperature, and time cessation between deposition of successive layers is performed. The effects of DIBS deposition conditions are analyzed by spectroscopy ellipsometry (SE), secondary ion mass spectroscopy (SIMS) and high resolution transmission electron microscopy (HRTEM) and discussed systematically. The SIMS and HRTEM analysis have been used for depth profiling at high resolution of CdZnO/ZnO-based MQWs structure. The deposition of CdZnO/ZnO-based MQW structure performed at 100 °C, with time cessation of 30 min between successive layer growth and ion beam power of 14 W has displayed the best results in terms of distinct well and barrier layers formation.