Development of highly efficient organic and perovskite solar cells
Date of Issue2018-01-30
School of Electrical and Electronic Engineering
With rapid industrial development and global population growth in 21st century, the search for renewable energy source becomes more important. Among various renewable energy sources, solar energy is the most plentiful one. Great efforts have been made to develop highly efficient and low-cost photovoltaic (PV) technologies. So far, the dominant PV technology is based on inorganic semiconductor materials, such as Silicon (Si) and Gallium arsenide (GaAs). Despite the high power conversion efficiency, these PV technologies require strict manufacturing processes and have very high materials and fabrication costs, thus preventing them from being widely accepted. To reduce the PV technology price, researchers have been searching for new generations of PV technologies based on cheaper materials and low-cost manufacturing processes. In recent years, organic solar cells and perovskite solar cells using organic or organic-inorganic hybrid materials in photoactive layers have attracted lots of attentions as a new generation of PV technologies. They are attractive because of their low-cost fabrication processes, mechanical flexibility, solution processability, etc. Flexible organic solar cells and perovskite solar cells can be fabricated by a simply printing process like newspapers and have potential to be applied on various substrates, such as curved roofs, uneven roads and clothes. The objective of this work is to develop low temperature processed interfacial organic materials and novel fabrication techniques to improve the performance and reduce fabrication costs of organic solar cells and perovskite solar cells. First, a newly synthesized n-type conjugated polyelectrolyte, poly [9,9-bis((60-N,N,N-trimethylamino)hexyl)-fluorene-alt-cobenzoxadia zole dibr- omide] (PFBD), was applied into organic solar cells with an inverted structure as electron transporting layer (ETL) to improve the organic solar cells’ performance. The organic solar cells using PFBD as the ETL exhibited a highest power conversion efficiency (PCE) of 7.21% and improved stability compared with the commonly used another ETL, PFN. The improved stability should come from the neutral nature of the PFBD solution because the good solubility of PFBD in methanol without the requirement of acetic acid. Besides the good photovoltaic performance of PFBD, we observed a light-soaking effect that the organic solar cells’ performance kept improving with increasing the sun light illumination time during the measurement of organic solar cells with PFBD ETL. To solve this problem, we proposed a UV pre-treatment method and demonstrated that the UV pre-treatment could successfully eliminate the light-soaking effect. Through Kelvin Probe measurement and impedance analysis, we found that the change of work function of ITO electrodes modified by PFBD plays an important role in influencing the organic solar cells’ performance. UV-vis absorption measurement results revealed that UV light had a great influence on benzoxadiazole (BD) unites on polymer backbone of PFBD, thus leading to the light-soaking effect. Our results demonstrate that PFBD is a promising material for highly efficient and stable organic solar cells. Second, we developed and applied a series of newly synthesized p-type polymers into perovskite solar cells as hole transporting layers (HTLs). The perovskite solar cells based on these polymer HTLs exhibited good stability and PCE. Besides, these polymer HTLs can achieve a good photovoltaic performance without requiring a complex doping process which is often needed in other commonly used small molecular or polymer HTLs in perovskite solar cells. Without the requirement for doping process, the manufacturing costs of perovskite solar cells based on our polymer HTLs can be further reduced and the devices’ stability was improved. Besides we also studied the factors that influence the performance of the polymer HTLs. Through molecular design, these polymer HTLs have almost the same band energy levels but different crystallinity, thus demonstrating different performance when applied into perovskite solar cells. So except for band alignment between HTLs and photoactive layers, crystallinity of the HTLs is also very important. The polymer HTL with the highest crystallinity has the best performance when used in perovskite solar cells with a highest PCE of 14.02%. The best perovskite solar cells exhibited excellent stability that the PCE of the device still retained 80% of the original value after storing in a nitrogen filled glove box for 113 days. Last, we designed and fabricated organic solar cells and perovskite solar cells by using a transfer printing technique to deposit the top Au electrodes. The top Au electrodes are normally deposited by vacuum deposition methods, such as thermal evaporator and E-beam evaporator. The vacuum deposition methods increase the fabrication costs of organic solar cells and perovskite solar cells and are not compatible with roll-to-roll manufacturing process. To reduce the manufacturing costs, we used a PDMS stamp coated with Au film to transfer print the Au onto the top of organic solar cells and perovskite solar cells as top electrodes. Compared with the vacuum deposition methods, our transfer printing method improved the performance and stability of organic solar cells and perovskite solar cells. The perovskite solar cells with transfer printed Au electrodes have a highest PCE of 13.72%, which is higher than the highest PCE of the control devices with the thermally evaporated Au electrodes. The organic solar cells with transfer printed and thermally evaporated Au electrodes have comparable PCE. Both perovskite solar cells and organic solar cells with transfer printed top electrodes have good stability. The good stability of the solar cells with transfer printed electrodes was demonstrated to come from less diffused Au atoms into the organic interfacial layers as indicated by the SCLC analysis and XPS measurement results.
DRNTU::Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics