A novel organic interface layer material to improve the efficiency of solar cells

Compared with the mature inorganic silicon solar cells[1] and perovskite solar cells[2], OSC is slightly obviously immature in terms of short service life and bad power conversion efficiency (PCE)[3]. Therefore, how to improve its durability and PCE to promote its development has become the main research direction. For solar cells, one of the most effective methods to improve their performance is to introduce different materials to optimize the electron transport layer to improve the charge transport characteristics inside the cell[4]. For organic solar cells, it is to modify the existing materials, introducing organic[5] (e.g. conductive polymer or polyelectrolyte) and inorganic[6][7][8] (e.g. metal oxide) interface modification layers to optimize this layer, so as to realize the regulation of interface energy level and improve PCE. In this work, the method of introducing an organic interface modified layer and reasonable experimental steps is used to synthesize non-conjugated polymer polyimide derivatives (BDI-RO). Combined with various photoelectric test methods and according to the work function and optical band gap of the modified interface layer, the effect of using BDI-RO as a cathode interlayer on the performance of OSC is studied and discussed.


Development History of OSCs
In 1986, Dr. Deng first proposed to use copper phthalocyanine (CuPc) and derivatives of perylene (PV) as donor and acceptor materials, introducing the concept of heterojunction, to prepare a double-layer polymer solar cell (PSC) with a PCE of 1% through continuous deposition method [9] (as shown in Figure 1), which is the milestone in the development of OSCs.Nowadays, heterojunctions have become a widely studied object in OSCs, and almost all efficient OSCs use BHJ structures.

Purpose of Design
This study uses the donor receptor heterojunction structure (Figure 1) To improve the device's energy conversion efficiency, it is necessary to ensure that the contact formed between the two electrodes and the active layer is ohmic.This design adopts the method of introducing an organic interface layer to achieve the interface layer's modification of the electron transport layer.
Figure 1 Schematic diagram of orthostatic structure of body heterojunction [10] .

Preparation of OSCs
First, ITO was cleaned by ultrasonic and dried by N 2 .After drying, ultraviolet ozone was used for 15 min.Second, PEDOT: PSS was filtered by a filter head and then dripped onto ITO glass.The solution was spun into a film by rotating the substrate of the spinning coating machine at a high speed.Third, the cathode interface layer and the active layer were spun in a glove box with a uniform nitrogen environment.Lastly, cathode metal evaporation plating: The cathode metal used in this experiment is Ag.

3.1.1
UV-vis Spectrogram.The absorption of BDI-RO is concentrated in the ultraviolet region (as shown in Figure 3), which means the light absorption of the active layer material will not be affected.The absorption cut-off edge is 351 nm, and then the optical band gap of the material is obtained: Figure 3 UV-vis spectrogram of BDI-RO.

Cyclic Voltammetry.
The resulting curve is as follows (as shown in Figure 4):

16eV
The LUMO level of the material can be well matched with the LUMO level of the accepter Y6 (-4.10eV), which is conducive to electron transport in the device and reduces the charge recombination that occurs at the interface.

Scanning Kelvin Probe (SKP).
There is a significant difference between the metalwork function before and after modification, indicating that a -0.83 eV dipole is formed at the interface.SKP test shows that BDI-RO can significantly reduce the work function of Ag (from 5.20 eV to 4.37 eV), which is conducive to increasing the built-in electric field of the device and improving the efficiency of the cells (as shown in Figure 5).

Device Performance Characterization
The device was made by spinning BDI-RO solution with 2 mg/mL concentration on the active layer at a speed of 4000 r/min, and the J-V curve of the device is shown as follows (as shown in Figure 6): In the figure, the black curve is the control group with the bare silver electrode, and the red curve is the study group with the best-modified device.The specific photovoltaic parameters are shown in Table 1: Table 1  According to the table, it can be considered that the BDI-RO modified electronic transport layer can improve the efficiency of organic solar cell devices (from 11.44% to 14.26%) and improve the performance of the devices.

Conclusion
This study focuses on the effect of modifying the electron transport layer with novel non-conjugated polymer polyimide derivatives (BDI-RO) on the performance of OSCs.According to the tests, the data show that the BDI-RO film can increase the electric field built in the device, promote the electron transport layer to receive electrons, reduce the recombination rate of electrons and holes, reduce the charge loss inside the device, improve the photoelectric conversion efficiency of the cells, and then achieve the effect of improving the performance of solar cell devices.
In future research and study, I will pay more attention to the integrity of the research, consider more adverse factors that may affect the experimental results, think about solutions, and strive to make the research results more referential.

Figure 2
Figure 2 Synthesis equation of BDI-RO.

Figure 4 C
Figure 4 C-V Curve of BDI-RO As measured by the figure, the initial reduction potential of the electrode is -0.64 eV, then its LUMO energy level is: E e E 4.80 4.16eVThe LUMO level of the material can be well matched with the LUMO level of the accepter Y6 (-4.10eV), which is conducive to electron transport in the device and reduces the charge recombination that occurs at the interface.

Figure 5
Figure 5 Comparison of metal work function before and after interface modification.

Figure 6 J
Figure 6 J-V curve comparison diagram of the device before and after modification.
Specific Photovoltaic Parameters of OSCs with Bare Ag and BDI-RO modified