Enhancing the performance of ultrathin CIGS solar cells via industrially feasible passivation approaches

Abstract:Renewable energy sources are the key elements in the equation while pursuing a greener world. There is a continuous growth in photovoltaic technology (PV) in both capacity and demand. To keep this growth rate, the production costs should be further lowered, the usage of raw materials should be limited, and the power conversion efficiency (pce) should be enhanced. The increase in pce can be reached by introducing new yet industrially feasible approaches into solar cell structures or overcoming the current issues in the structures. CIGS-based thin-film solar cells have comparable efficiencies with the market leader, i.e., silicon (Si), and have various application areas like building-integrated PV (BIPV) or vehicle integrated PV (VIPV). Hence, considering the current role of CIGS solar cells in PV technology, many researchers are focusing on making them cheaper by lowering the thickness and, at the same time, increasing the pce. Decreasing the thickness of the absorber layer of a solar cell, which is the active part of the solar cell, has severe consequences. For CIGS-based solar cells, some of these consequences are insufficient absorption and the detrimental impact of back surface recombination. Indeed, there are numerous approaches to overcome these problems. The traditional one is the gallium (Ga) grading. However, the thinner the absorber layer, the less effective the Ga-grading, even has unfavorable effects. Furthermore, Ga-grading is applied as an intermediate step during absorber layer deposition, and hence, it makes the process more complicated, i.e., time-consuming and expensive. To avoid these problems, new ways have been sought and found. By mimicking the strategies used in Sibased solar cells, introducing a passivation layer has become the novel approach in this path. There are multiple ways to employ the passivation layer either to the front or rear surfaces, such as direct current (DC)-sputtering, atomic layer deposition (ALD), or spin coating. Facilitating the current flow after the deposition of the passivation layer is the real challenge. Since putting (in most cases) dielectric-based layers in between the rear contact and absorber, or the front contact and absorber, will cut the current flow. Hence, there should be contact openings in these layers. For this purpose, over the years, nanoparticles and lithography techniques have been widely used and successfully created contact openings in the passivation layers. However, these techniques are time-consuming, expensive, and especially for nanoparticle techniques that use cadmium sulfide (CdS), they are not environmentally friendly. This doctoral study focuses on the rear surface passivation of the ultra-thin film (500 nm) CIGSbased solar cells. The novel alkali salt selenization technique is developed to create contact openings in the aluminum and hafnium-based dielectric layers in the framework of this aim. This technique is cost-effective, easy to employ, green, and has the potential to integrate into large-area applications. Hence, it is an industrially feasible approach. Furthermore, with a fragment of chemistry view, the mechanism behind this approach is discussed in a separate chapter. The first chapter (Chapter 0) provides brief information about the photovoltaic (PV) market and the position of CIGS-based solar cells in the market. Moreover, the crystal structure of the CIGS solar cells and a sample device structure with and without rear surface passivation are explained in detail by mentioning the reason behind the need for the rear surface passivation. The detailed fabrication process of the CIGS solar cells, with and without the passivation layer, with the definition of the techniques, i.e., sputtering, atomic layer deposition, co-evaporation, spin-coating, and chemical bath deposition, involved in the process is described. In order to clarify the fundamental solar cell characteristics used in this study, some terms are explained with necessary formulations. This introductory chapter also shares all of the measurement techniques used in this study, i.e., solar cell simulator, capacitance-voltage measurement, spectral response, photoluminescence, scanning electron microscopy, transmission-reflection, thermal desorption-gas chromatography/mass spectrometry, and time-of-flight secondary ion mass spectrometry measurements. Next, (Chapter 1) a brief summary of rear surface passivation of CIGS solar cells, including background technology and the production line has been given. Furthermore, a review paper that includes the state-of-the-art dielectric-based rear surface passivation techniques until 2018, i.e., beginning of this study, has been shared. First, the chemical and field-effect passivation effects are explained in detail to attain the impact of the used strategy. Afterward, the thicknesses of the passivation layers, the contact opening approaches (lithography, nanoparticle-approach, or tunneling), and the solar cell results (both for reference and passivated solar cells) have been shared in a summary table and afterward in detail for more than twenty-eight studies. To compare the effect of the employed passivation strategy over the solar cell characteristics for passivated and reference solar cells with respect to the absorber layer thickness, three different graphs, i.e., Voc vs. absorber thickness, Jsc vs. absorber thickness, and efficiency vs. absorber thickness, has been prepared. As expected, the effect of the rear surface passivation strategies is more visible for thinner absorber layers. Following the reviewed rear surface passivation strategies (Chapter 2), a novel contacting approach named alkali salt selenization for two different dielectric layers, aluminum oxide (AlOx) and hafnium oxide (HfOx), has been shared. This chapter includes three studies utilizing the same approach with alterations (three publications). The first study (Paper I) introduces the developed alkali salt selenization technique to create contact openings in various AlOx-based passivation layer thicknesses. In this study, NaF is chosen to be the alkali salt agent. As a result, a significant increase in open-circuit voltage (13%, relative increase) for 6nm thick passivation layer has been reached in combination with decreased saturation current density, which is the apparent indication for successful passivation application at the rear surface. The second study (Paper II) is the adaptation of the alkali salt selenization approach to relatively thick AlOx-based passivation layers, up to 30 nm. The preliminary investigations have proved that lithium fluoride creates smaller contact openings, i.e., nanoscale, than sodium fluoride. Hence, in this study, NaF is replaced with LiF. Despite the nano-size contact openings, the solar cell performance for passivated samples has not been enhanced. The reason behind deteriorated solar cell performance is discussed in the appendix, and the usual suspect for these results has been found as lithium fluoride diffusion in the absorber layer. HoweveU, UegaUdleVV of loZ VolaU cell UeVXlWV, Whe conWacW oSeningV¶ Vi]eV haYe been UedXced Wo Whe nanoscale with an industrially viable approach. The next study (Paper III) is a follow-up experiment where HfOx replaces the AlOx due to its proven passivation effects and to verify that the alkali selenization approach is applicable for various dielectric-based layers. To show the potential of the HfOx as the rear surface passivation layer, a metal-oxide-semiconductor (MOS) structure has been developed with HfOx. By a CV measurement, the passivation characteristics of the HfOx layer (interface trap density and density of charges) have been measured and calculated. Next, the initial tests have shown that potassium fluoride is more successful than NaF in creating the contact openings for the HfOx based layer. Hence, this study has used KF as the alkali salt agent. As a result, the power conversion efficiency has been enhanced (3%, absolute increase) with decreased saturation current density by employing a 2 nm thick HfOx passivation layer with openings. The following chapter (Chapter 3) reveals the mechanism behind the creation of the openings and the effects of the spin-coating process as NaF pre-deposition technique over solar cell performance. This chapter includes two studies (two papers submitted). The first study (Paper I) is comSUehenViYe UeVeaUch ZheUe Whe mechaniVm of Whe oSeningV¶ cUeaWion dXUing Veleni]aWion of Whe alkali salt has been explained with a touch of the chemistry behind it. For this purpose, several samples have been prepared to determine the necessity of the factors contributing to the alkali salt selenization process. With the help of this test, the need for selenium, alkali salt, the methyl-based gasses trapped in the dielectric layer, and certain environmental temperatures has been proved. By conducting a heat test, it has been revealed that each alkali halide salt requires a different annealing temperature. Moreover, thermal desorption measurement has confirmed the creation of dimethyl diVelenide comSoXnd dXUing Whe oSeningV¶ cUeaWion. The second study (Paper II) explains the potential risk of using the spin-coating technique as the pre-deposition of NaF salt solution. The foundation of this study has been revealed during an unexpected experimental error. For one sample, half of it has not been coated optimally with NaF. Hence, that part of the solar cell has not worked as well as the other half. For a deeper analysis of the effects, hyperspectral imaging has been used in combination with SEM and EDS measurement, and it has been shown the unhomogeneous coating of the surface due to spin coating. As a result, the part with non-optimal NaF has demonstrated a lower PL response and faster decay time. This chapter has been concluded with an appendix discussion regarding the theory's validity shared in Paper I for HfOx. The theory explained the openings' creation in the AlOx-based passivation layer, but the AlOx had been changed with HfOx, and the openings had been created in this layer as well. Hence, to have a valid theory, it should explain the procedure for the HfOx-based layers as well. To do so, the similarities between AlOx and HfOx precursors have been shared, and the validity of the theory has been discussed in this appendix. The last chapter (Chapter 4) provides the achievements throughout four years together with the opportunities and obstacles about the passivation approach shared in this thesis. It continues with the comparison of the used dielectric-alkali combinations throughout this thesis to allow the readers (for future experiments) to choose between the combinations. Furthermore, the most commonly used contacting approaches have been compared regarding their impacts on the environment, cost, complexity, and solar cell performance. In the end, personal comments on the future of CIGS solar cells in the PV market have been given.

Publication year:2022

Accessibility:Embargoed

Review status:Not peer-reviewed