Internet use has become an integral part of youth’s everyday life. While the Internet brings convenience for information access, communication, and entertainment (e.g., Thorsteinsson & Davey, 2014; Wallace, 2014), when the mechanism of Internet use becomes maladaptive, a phenomenon called problematic Internet use (PIU) arises. PIU among youth has been defined as excessive or maladaptive use of online activities, which interferes with youth’s development, academic performance, and social life in negative ways (Beard & Wolf, 2001; Ceyhan, 2008; Ioannidis et al., 2019). Across the world, China was the first country that recognized PIU as an official mental health concern, given the significantly high and increasing trend of PIU prevalence rates among Chinese youth (Jiang & Leung, 2011). Previous research has well-documented the detrimental outcomes of PIU among Chinese adolescents. However, there has been a lack of investigation on the etiology of PIU from a social and cognitive perspective. In Study 1, structural equation modeling (SEM) was used to empirically identify how PIU differed between students’ sex, family income level, and left-behind status as well as to examine the relations between SEL competencies and PIU. Results yielded that male students endorsed higher levels of PIU and time management on the Internet than female students, whereas no differences were observed across family income levels and left-behind status. In addition, students with higher levels of SEL competency and all five core domains of SEL competencies (i.e., responsible decision-making, social awareness, self-management, self-awareness, and social relationship) reported less frequent PIU, illustrating the preventative roles of SEL competencies on PIU. Based upon Study 1, Study 2 considered social factors additional to the cognitive factors when studying the etiology of PIU. Similar to Study 1, SEM was used to examine the associations between traditional bullying and cyberbullying victimization and PIU, and the moderating effects of SEL competency and five core SEL domains on the associations between traditional bullying and cyberbullying victimization and PIU. Upon controlling for students’ demographic variables, the study found that students who experienced more frequent traditional bullying and cyberbullying victimization reported a higher level of PIU. SEL competency, social-awareness, relationship skills, and self-awareness exacerbated the positive relationships between traditional bullying victimization and PIU. Taken together, the findings from both studies supported the preventive roles of SEL competencies and detrimental roles of bullying victimization on PIU and revealed the promoting effects of both overall and domain-specific social-emotional attributes in PIU development when youth faced bullying, which intended to inform and facilitate the preventative and interventive efforts on Chinese adolescents’ bullying victimization and PIU.

In the past decade, research on organic solar cells has gone through an important development stage leading to major enhancements in power conversion efficiency, from 4% to 9% in single-junction devices. During this period, there are many novel processing techniques and device designs that have been proposed and adapted in organic solar-cell devices. One well-known device architecture that helps maximize the solar cell efficiency is the multi-junction tandem solar-cell design. Given this design, multiple photoactive absorbers as subcells are stacked in a monolithic fashion and assembled via series connection into one complete device, known as the tandem solar cell. Since multiple absorbers with different optical energy bandgaps are being applied in one tandem solar-cell device, the corresponding solar cell efficiency is maximized through expanded absorption spectrum and reduced carrier thermalization loss.

In Chapter 3, the architecture of solution-processible, visibly transparent solar cells is introduced. Unlike conventional organic solar-cell devices with opaque electrodes (such as silver, aluminum, gold and etc.), the semi-transparent solar cells rely on highly transparent electrodes and visibly transparent photoactive absorbers. Given these two criteria, we first demonstrated the visibly transparent single-junction solar cells via the polymer absorber with near-infrared absorption and the top electrode based on solution-processible silver nanowire conductor. The highest visible transparency (400 ~ 700 nm) of 65% was achieved for the complete device structure. More importantly, power conversion efficiency of 4% was also demonstrated.

In Chapter 4, we stacked two semi-transparent photoactive absorbers in the tandem architecture in order to realize the semi-transparent tandem solar cells. A noticeable performance improvement from 4% to 7% was observed. More importantly, we modified the interconnecting layers with the incorporation of a thin conjugated polyelectrolyte layer functioning as the surface dipole formation layer to provide better electrical contact with the photoactive layer. Due to the effectiveness of the conjugated polyelectrolyte layer, performance improvement was also observed. Furthermore, other issues regarding the semi-transparent tandem solar cells (e.g., photocurrent matching, exterior color tuning, and transparency tuning) are all explored to optimize best performance.

In Chapter 5 and 6, the architectures of double- and triple-junction tandem solar cells are explored. Theoretically, triple-junction tandem solar cells with three photoactive absorbers with cascaded energy bandgaps have the potential to achieve higher performance, in comparison with double-junction tandem solar cells. Such expectations can be ascribed to the minimized carrier thermalization loss and further improved light absorption. However, the design of triple-junction solar cells often involves sophisticated multiple layer deposition as well as substantial optimization. Therefore, there is a lack of successful demonstrations of triple-junction solar cells outperforming the double-junction counterparts.

To solve the incompatible issues related to the layer deposition in the fabrication, we proposed a novel architecture of inverted-structure tandem solar cells with newly designed interconnecting layers. Our design of interconnecting layers does not only focus on maintaining the orthogonal solution processing advantages, but also provides an excellent compatibility in the energy level alignment to allow different absorber materials to be used. Furthermore, we also explored the light management inside the double- and triple-junction tandem solar cells. The study of light management was carried out through optical simulation method based transfer matrix formalism. The intention is to obtain a balanced photocurrent output from each subcells inside the tandem solar cell, thus the minimal recombination loss at the contact of interconnecting layers and the optimal efficiency can be expected. With help from simulations, we were able to calibrate the thickness of each photoactive layer as well as the thickness of interconnecting layers to achieve the optimized processing conditions. With the highest power conversion efficiency, 11.5%, triple-junction tandem solar cells outperform the double-junction tandem solar cells at 10.5%.

In summary, this dissertation has provided practical solutions to the current demand of high-performance and easily manufactured organic solar cells from the solar cell industry. Particularly, triple-junction tandem solar cells with efficiencies over 11% should have great potential to contribute to high-efficiency solar-cell applications, whereas semi-transparent tandem solar cells with efficiency at 7% should be applicable to building-integrated applications.