Jae Ho Yun

Due to the wide-spread use of photovoltaic (PV) systems, interest regarding economic benefit and energy-sharing from surplus electricity has been raised. In this study, decentralized thermal and electric convergence energy prosumers for an energy-sharing community are proposed. For the convergence energy system, a simultaneous heating and cooling heat pump (SHCHP) system integrated with a thermal network is proposed, and the energy performance and operating energy savings of the proposed system were investigated. A smart village located in the Busan Eco Delta Smart City was selected as a case study for the simulation analysis. Experimental data of the heat pump system were used to analyze the SHCHP. The analysis results showed that the proposed system could provide over 53% and 86% of the load cover and supply cover factors, respectively. The proposed system can earn economic benefits, such as energy trading from the surplus electricity of PV systems and thermal energy produced by an SHCHP, more than 30.5 times those of conventional air-source heat pump systems. These benefits mainly originate that a conventional system can trade the surplus electricity from a PV system but the proposed system can trade produced thermal energy from SHCHPs and the surplus electricity from PV systems.

The potential-induced degradation (PID) mechanism in Cu(In,Ga)(Se,S)₂ (CIGS) thin-film solar cells, which are alternative energy sources with a high efficiency (>23%) and upscaling possibilities, remains unclear. Therefore, the cause of PID in CIGS solar cells was investigated in this study at the cell level. First, an appropriate PID experiment structure at the cell level was determined. Subsequently, PID and recovery tests were conducted to confirm the PID phenomenon. Light current–voltage (I–V), dark I–V, and external quantum efficiency (EQE) analyses were conducted to determine changes in the cell characteristics. In addition, capacitance–voltage (C–V) measurements were carried out to determine the doping concentration and width of the space charge region (SCR). Based on the results, the causes of PID and recovery of CIGS solar cells were explored, and it was found that PID occurs due to changes in the bulk doping concentration and built-in potential at the junction. Furthermore, by distinguishing the effects of temperature and voltage, it was found that PID phenomena occurred when potential difference was involved.

Due to increased grid problems caused by renewable energy systems being used to realize zero energy buildings and communities, the importance of energy sharing and self-sufficiency of renewable energy also increased. In this study, the energy performance of an energy-sharing community was investigated to improve its energy efficiency and renewable energy self-sufficiency. For a case study, a smart village was selected via detailed simulation. In this study, the thermal energy for cooling, heating, and domestic hot water was produced by ground source heat pumps, which were integrated with thermal energy storage (TES) with solar energy systems. We observed that the ST system integrated with TES showed higher self-sufficiency with grid interaction than the PV and PVT systems. This was due to the heat pump system being connected to thermal energy storage, which was operated as an energy storage system. Consequently, we also found that the ST system had a lower operating energy, CO₂ emissions, and operating costs compared with the PV and PVT systems.

Kesterite-structured Cu₂ZnSnSe₄ (CZTSe) is considered as one of the Earth-abundant and non-toxic photovoltaic materials. CZTSe films have been prepared using a single-step co-evaporation method at a relatively low temperature (i.e., below 500 °C). Due to the volatile nature of tin-selenide, the control over substrate temperature (i.e., growth temperature) is very important in terms of the deposition of high-quality CZTSe films. In this regard, the effects of growth temperatures on the CZTSe film morphology were investigated. The suitable temperature range to deposit CZTSe films with Cu-poor and Zn-rich compositions was 380–480 °C. As the temperature increased, the surface roughness of the CZTSe film decreased, which could improve p/n junction properties and associated device performances. Particularly, according to capacitance-voltage (C-V) and derived-level capacitance profiling (DLCP) measurements, the density of interfacial defects of CZTSe film grown at 480 °C showed the lowest value, of the order of ~3 × 10¹⁵ cm⁻³. Regardless of applied growth temperatures, the formation of a MoSe₂ layer was rarely observed, since the growth temperature was not high enough to have a reaction between Mo back contact layers and CZTSe absorber layers. As a result, the photovoltaic (PV) device with CZTSe film grown at 480 °C yielded the best power conversion efficiency of 6.47%. It is evident that the control over film growth temperature is a critical factor for obtaining high-quality CZTSe film prepared by one-step process.

Flexible Cu(In,Ga)Se₂ (CIGS) solar cells are developed on polyimide (PI) foil by using a three‐stage co‐evaporation method. To deposit CIGS on a PI substrate, low temperature deposition process (below 440 °C) is investigated. By optimizing the three‐stage process, power conversion efficiency (PCE) values of 12.1% and 13.6% are obtained at maximum process temperatures of 400 °C and 440 °C, respectively (without anti‐reflection coating). The CIGS absorber deposited at 400 °C is grown with (220)/(204) rather than (112) as its dominant growth orientation, accompanied by highly homogeneous crystal structure and phases. Admittance spectroscopy reveals that the defect levels were lower in the CIGS absorber with (220)/(204) dominant growth orientation, which could have contributed to the PCE of 12.1%. The CIGS solar cell deposited at a temperature of 440 °C exhibits worse defect characteristics compared to that fabricated at 400 °C. However, the CIGS solar cell fabricated below 440 °C exhibited marginal double‐grading of Ga/(Ga + In) in the absorber layer, which resulted in an improved PCE of 13.6%. This result implies that at a temperature below 440 °C, the double‐grading of Ga/(Ga + In) is the more dominant factor causing the PCE improvement above 13%.

Different from conventional photovoltaics, building-integrated photovoltaics needs not only high performance but also a high degree of transparency. Nevertheless, the Cu(In,Ga)Se₂ (CIGS) solar cell has advantages in terms of the highest conversion efficiency and stability among all thin-film-based solar cells. The semitransparent (ST) CIGS solar cell using an ultrathin CIGS absorber on a transparent conducting oxide (TCO) experiences loss in fill factor and open circuit voltage due to the poor grain morphology and CIGS/TCO interface. Thus, these issues must be addressed to realize ST CIGS solar cells. Wide-bandgap (1.5 eV) submicron CIGS-based solar cells were prepared in this study unlike conventional CIGS with a bandgap of 1.2 eV, in order to enhance see-throughness. But such cells demonstrated low conversion efficiency due to the poor grain morphology and absence of back grading. Therefore, for the sake of improving grain morphology and to create back grading, a sulfurized-AgGa (AGS) layer was deposited between CIGS and the indium tin oxide (ITO) back contact. Ag from the AGS layer diffused throughout the absorber and thus ameliorated the grain morphology. However, Ga and S in the AGS layer remained confined near the back contact, therefore resulting in the creation of back grading. Consequently, a solar cell based on 230 nm thick CIGS modified with a 45 nm thick AGS layer exhibited efficiency of 5.94% with averaged visible transmittance over 25%. This is the highest reported efficiency for a ST CIGS solar cell with over 20% visible transparency. The CIGS solar cell based on this novel approach can be a competent candidate for building-integrated semitransparent photovoltaics applications.

The net electricity savings in dwellings and commercial edifices can be substantially enhanced by using BISTPV shrewdly that can pave the way to realize net zero-energy buildings.

An investigation into the effects of Se flux on absorber thin film growth at each step of a 3‐stage co‐evaporation process was conducted to further optimize the performance of CIGS solar cells. In ‘step I’ forming an In‐Ga‐Se precursor thin film during the 3‐stage process, Se flux affected the preferred orientation of the CIGS crystal structure, but not the film morphology. In ‘step II’, no correlation was found between Se flux and the crystal structure, although excessively high Se flux employed throughout the 3‐stage process degraded the solar cell performance. A CIGS thin film, with a (220/204) crystal orientation, minor physical surface defects and ∼20 nm thick MoSe₂ at CIGS/Mo interface, was obtained by fine control of Se flux conditions (high Se flux at ‘step I’ and low Se flux at ‘step II’) at optimum substrate temperatures. The solar cell fabricated using the aforementioned CIGS thin film showed the highest conversion efficiency of 20.02 %.