Vertical Stratification Engineering for Organic Bulk-Heterojunction Devices

Abstract

High-efficiency organic solar cells (OSCs) can be produced through optimization of component molecular design, coupled with interfacial engineering and control of active layer morphology. However, vertical stratification of the bulk-heterojunction (BHJ), a spontaneous activity that occurs during the drying process, remains an intricate problem yet to be solved. Routes toward regulating the vertical separation profile and evaluating the effects on the final device should be explored to further enhance the performance of OSCs. Herein, we establish a connection between the material surface energy, absorption, and vertical stratification, which can then be linked to photovoltaic conversion characteristics. Through assessing the performance of temporary, artificial vertically stratified layers created by the sequential casting of the individual components to form a multilayered structure, optimal vertical stratification can be achieved. Adjusting the surface energy offset between the substrate results in donor and acceptor stabilization of that stratified layer. Further, a trade-off between the photocurrent generated in the visible region and the amount of donor or acceptor in close proximity to the electrode was observed. Modification of the substrate surface energy was achieved using self-assembled small molecules (SASM), which, in turn, directly impacted the polymer donor to acceptor ratio at the interface. Using three different donor polymers in conjunction with two alternative acceptors in an inverted organic solar cell architecture, the concentration of polymer donor molecules at the ITO (indium tin oxide)/BHJ interface could be increased relative to the acceptor. Appropriate selection of SASM facilitated a synchronized enhancement in external quantum efficiency and power conversion efficiencies over 10.5%.

Miscibility Tuning for Optimizing Phase Separation and Vertical Distribution toward Highly Efficient Organic Solar Cells

“…For the binary blend systems, the EQE values were improved between 500 and 650 nm as blend film thickness was increased from 100 to 250 nm, which corresponds with the absorption spectra in Figure S1b in the Supporting Information. 2019, 6,1900565 gradient distribution of different components in the blend film, [21] which will be explained in the later section. For the ternary blend systems with blend film thickness increasing from 100 to 270 nm, the absorption enhancement from 500 to 800 nm plays a great role in the improvement of EQE value, which is contributed by DRCN5T and PTB7-Th.…”

“…[1][2][3] It has been recognized as a great promising substitute for siliconbased photovoltaic with advantages of light weight, inexpensive cost, and largearea flexible preparation with roll-to-roll (R2R) process. [14,[21][22][23][24][25][26][27] As an efficient and new type of device structure, ternary OSCs generally consist of multidonor and one acceptor or one donor and multiacceptor. [14,[21][22][23][24][25][26][27] As an efficient and new type of device structure, ternary OSCs generally consist of multidonor and one acceptor or one donor and multiacceptor.…”

“…[42] There are multiple reasons for the poor performance of ternary OSCs with thicker active layer. [21,44] Weather the substrate character is hydrophobic or not, it is still unknown that what determines the morphological stability of thick active layer in the vertical direction. [38] Furthermore, as the blend film thickness increased, the effect of surface energy of substrates on the vertical distribution of donor and acceptor lessened, leading to the unfavorable enrichment of donors or acceptors at cathode and anode, which is harmful to the charge collection in the respective electrodes.…”

Sequential Blade‐Coated Acceptor and Donor Enables Simultaneous Enhancement of Efficiency, Stability, and Mechanical Properties for Organic Solar Cells

“…BHJ structure is a combination of donor‐rich, acceptor‐rich, mixed amorphous or D/A domains because of the partial miscibility of the photoactive materials. As a result, undesired vertical composition profiles are often formed in the active layer during the film‐drying process . Therefore, the optimization of morphological parameters and composition distribution within the BHJ often posed a serious challenge for researchers .…”

High‐Performance Pseudoplanar Heterojunction Ternary Organic Solar Cells with Nonfullerene Alloyed Acceptor

“…However, such a method is unable to separate the donors and the acceptors, the vertical phase separation of the active layers are still not well regulated. There are also related literature reporting the use of orthogonal solvents or simultaneously citing a third solvent to increase the donor and the acceptor contact areas, and achieve higher efficiency whereas the choice of orthogonal solvents and device preparation process are more complicated 31–35. By summarizing the advantages and disadvantages of the above literature, we can find that when donor and acceptor materials were dissolved using nonorthogonal solvents, the processing of the films were well completed which can also regulate vertical phase separation well 36.…”