| dc.description.abstract | Solar energy is one of the essential green energy resources owing to its naturally massive abundance, universal accessibility, and long-term sustainability. The evolution of the solar photovoltaic industry has been remarkable in recent years. The main limitations of current well-developed photovoltaic devices (Silicon technology, CIGS, and CdTe) are the high cost and/or the toxic element. The earth-abundant thin-film solar cells (TFSCs) for approaching both environmentally friendly and cost-effective, therefore, are the ideal solution for harvesting solar energy. Among current earth-abundant materials, tin(II) monosulfide (SnS) is considered a promising cost-effective semiconductor. However, the device performance of SnS-based solar cells remains quite low owing to the lack of understanding of SnS properties. This dissertation is attempting to address the main strategies which might play a key role in boosting the device performance of SnS TFSCs. The works mainly focus on presenting a novel experimental design methodology to overcome the SnS growth strategies and developing an alternative eco-friendly buffer layer to obtain favorable band alignment at SnS/buffer layer interface.
With low-dimensional crystal structural materials (particularly SnS), the crystallographic orientation plays a key role in manipulating the charge transport, carrier recombination, and eventually device characteristics. Many deposition methods have been developed to grow SnS thin-film and recently vapor transport deposition (VTD) has shown a great increase in both SnS quality and device performance. However, up to now, not much report provides direct evidence about the effect of crystallographic orientation on the charged transport and device performance, or how to control the crystallographic orientation in SnS thin-film. Herein, we proposed an effective experimental setup/geometry and a multi-step annealing process during the VTD process to intentionally tailor the crystal orientation. These two approaches have directly modified the growth behavior and tailored the crystallographic orientation. All observed results supported that (040) plane is harmful to charge transport, and caused revere recombination in SnS devices (bulk and/or interface). Therefore, the suppression of the (040) plane in SnS thin films led to a dramatic improvement in PCE from 0.11% to almost 2%.
A photovoltaic device is a stack of multiple layers which have their own importance and effect on the final device performance. Therefore, not only the absorber layer, it is essential to take into account some important layers such as the buffer layer, back contact, front contact, etc. Among them, selecting a buffer layer is substantial because the unfavorable band alignment at p-n heterojunction might cause acute recombination at the interface and degrade device efficiency. Therefore, we developed an eco-friendly and wide bandgap buffer layer Zinc-Tin-Oxide (ZTO) with a tunable band offset. The conduction band offset (CBO) switch from the “cliff-type” (CBO = -0.41 eV with CdS buffer layer) to the “spike-type” (CBO = +0.23 eV with ZTO11 buffer layer) by controlling the Zn-to-Sn ratio. The favorable CBO led to the great suppression of the interfacial recombination which was proved by the increase of activation energy from 0.72 eV to 0.85 eV. As expectable result, the dramatical enhancement in the device performance was attained from PCE = 1.67%, Voc = 0.24 V, and Jsc = 13.57 mA/cm2 to PCE = 3.0%, Voc = 0.34 V and Jsc = 18.7 mA/cm2. | en_US |