Photovoltaics technology, or solar cell, has been developed for more than a half century. This idea that utilizes energy from an infinite resource is indeed fascinating and drives scientists and engineers to devote themselves to this field. It is one of the major renewable energy types which accounts for 2% of the global total electricity consumption in 2018 and this percentage is still growing in a steady trend. According to a publicized prediction, the solar power could supply 20% of the total electricity demand in 2030 because of drastically dropping price per watt. Although a brilliant future of solar cell is projected, there are still many unknown physical phenomena and engineering difficulties nonetheless. These challenges in photovoltaics are thus of our interest to work on.
The state of art highest efficiency photovoltaics is multi-junction inorganic solar cell, which is used in astronautic application. Yet the complicated fabrication processes and high cost limit its usage. It is the reason that silicon solar cell with moderate efficiency and process rules the whole solar cell market. However, the world still needs a different type of solar cell to fulfill the requirements of low weight, flexible, transparent, non-toxic and solution process compatibility to accustom to the other environment conditions. Organic solar cell is one of the strong candidates for reaching this target.
In this dissertation, I aim to achieve high efficiency organic solar cells in single-junction and multi-junction structure with different strategies. There are the modification of charge transport layer and interconnecting layer for a better Ohmic contact and fermi level pining as well as the sub-cell current matching by third photo-active component.
In Chap. 3, the project for improving the transport properties of single junction devices has been explored. A high mobility Indium Oxide In2O3 as an universal electron transport layer was chosen and improved the original device efficiencies for 10%. The physical fundamentals for efficiency improvement have been found that the crystalline In2O3 with highly aligned nanocrystallites can induce the crystallization of polymer into a preferential molecular packing for the charge transport and the charge extraction in the crystalline In2O3 devices is significantly faster than its counterpart devices.
Based on the work principle of Fermi level pinning, organic tandem solar cells with a tunnel junction consisted of zirconium (IV) acetylacetonate (Zr‐Acac) has been developed and discussed in Chap. 4. The Zr‐Acac has a suitable low work function to pin the Fermi level of the interconnecting layer and hence, can effectively behave as the charge recombination function for the tandem device. Furthermore, Zr-Acac requires a least harmful preparation process method in the tandem devices. Comparing to the reference device, the Zr‐Acac interconnecting layer can effectively enhance the efficiency by 20%.
The other strategies to improve the tandem organic solar cells with the ternary structure have been discussed in Chap. 5 and Chap. 6. Broadening the light absorption spectrum needs the optimization of current matching among different sub-cells with good interconnecting layer. Herein, it is firstly discovered that using ternary blend sub-cell in tandem device can finely optimize the current matching via tuning the materials composition in the ternary blend. The voltage-current trade-off in the tandem devices can be achieved. Due to the amorphous nature of the used small molecular, the phase separation in this ternary blend upon heating is sufficiently suppressed, resulting in a good thermal stability and viable for a better environment of interconnecting layer preparation.
The device efficiency measured in the lab has reached 13% and the certified device efficiency is 11.5% by NREL, which achieves the world record as of October 2018.
My conclusion and future perspective of these works on organic solar cells are summarized in Chap. 7.