Research On Surface Modification Of ZnO Electron Transport Layers And Performance Of Organic Solar Cells

Organic polymer solar cells are widely explored for the advantages of flexibility,crimpability,simple preparation process and low cost.The performance of the conventional device is poor because the ITO anode will be corroded by PEDOT:PSS and Al electrodes are easy to be oxidized.Inverted structure devices can be used to solve the above problems,where the Zn O can be used to replace PEDOT:PSS to avoid ITO corrosion and Ag can be used to replace Al to avoid metal electrode oxidation.The Zn O thin films prepared by sol-gel method usually introduce the surface defects,which affect the morphology of the deposited active layer and act as the recombination centers of photoinduced charges in the process of charge transfer,resulting in significant current losses and thus affecting device performance.In order to improve the performance of inverted organic solar cells with Zn O as electron transport layer,the surface of Zn O thin films was first modified by three kinds of diol molecule with two alcohol hydroxyl groups:glycol（EG）,1,2-propanediol（PG）and 1,3-propanediol（PDO）.The properties of Zn O films and active layers were characterized by X-ray diffraction spectrum,infrared spectrum,contact angle measuring instrument and atomic force microscope.The inverted devices with Zn O as electron transport layer（ETL）and P3HT:PC₆₁BM as active layer were prepared to explore the performance changes before and after Zn O modification.The results showed that the energy level of Zn O films were decreased after diol molecular modification.The hydrophobicity and the surface roughness of m-Zn O films were increased.The surface roughness of the active layer deposited on m-Zn O was decreased and the degree of phase separation was increased,the interpenetrating network structure was better,and the crystallinity of the active layer film was increased.Among them,the morphology of the active layer deposited on Zn O/EG was appropriate,the crystallinity was the highest,and the morphology and interpenetrating network structure were better.The difference in crystallinity and surface morphology of the active layer deposited on surface-modified Zn O was mainly caused by the different chemical structure of diol molecules.In addition,it may also be related to the properties of the active layer material.Among them,the Zn O/EG devices exhibited the best performance with the efficiency increased from 2.71%of Zn O based device to 3.46%by28%and the EQE increased from 60%to 73%.The efficiency of the Zn O-based device is equivalent to the value of the standard device of P3HT:PC₆₁BM system（2.89%）.And the efficiency of the modified-device was improved.It is concluded that the surface modification of Zn O with diol molecules can improve the performance of P3HT:PC₆₁BM inverted organic solar cells.Then the method of improving the device performance by modifying Zn O was extend to PTB7-Th:PC₆₁BM inverted organic solar cells.And the modification materials were extended from diols to monoalcohols according to the properties of the surface modification materials.The Zn O films were modified by various alcohol molecules,such as methanol,ethylene glycol,propylene glycol and isopropanol.And the changes of morphology and photoelectric properties of Zn O,the morphology of active layer and the performance of inverted organic solar cells with PTB7-Th:PC₆₁BM as active layer were investigated.Among them,the efficiency of Zn O-based devices is 5.52%,which is close to that the value of standard devices（5.97%）.The device with Zn O/PDO as the electron transport layer possessed the best performance:the efficiency increased from 5.52%to 7.05%by 28%,the J_sc increased from 13.91 m A/cm² to 15.74 m A/cm²with an increase of 13%,the FF increased from 46%to 53%by 15%and the EQE increased from 60%to 79%by 22%.The results indicated that the surface modification on Zn O electron transport layer with alcohol molecules can improve the performance of PTB7-Th:PC₆₁BM inverted organic solar cells.It is proved that the surface modification material of Zn O with alcohol molecules has expansibility.And the method of surface modification on Zn O with alcohol molecules to improve the performance of devices is also suitable for PTB7-Th:PC₆₁BM inverted organic solar cells.In the end of this paper,the inverted organic solar cells with the non-fullerene material system of PTB7-Th:SDTF-FPDI₂ as the active layer were prepared for the first time,where the novel non-fullerene material of SDTF-FPDI₂ was synthesized and used as acceptor.The result showed that the device exhibited the efficiency of 1.59%,the EQE of 21%and the J_sc of 4.22m A/cm².The method of improving the device performance with modified-Zn O as the electron transport layer was applied to the PTB7-Th:SDTF-FPDI₂ inverted organic solar cells.The device with Zn O/EG as the electron transport layer was prepared and the efficiency of the device was improved to 3.61%,which was twice as high as the unmodified device and 28%higher than the traditional structure device.The EQE of the device was also increased from 21%of the unmodified-device to 55%,which is 31%higher than the traditional structure device.The improvement of the device performance are mainly due to the significant increase of J_sc of the modified-device,which was greatly improved to more than 10 m A/cm²,30%higher than the traditional structure device of 7.74 m A/cm².It is concluded that the non-fullerene material system of PTB7-Th:SDTF-FPDI₂ can be applied to inverted structure devices with Zn O as electron transport layer.The surface modification of Zn O film by EG can improve the device performance of PTB7-Th:SDTF-FPDI₂ inverted organic solar cells.We realized a new breakthrough in the device performance of this non fullerene material system,which provides a reference for the application of other new material systems and the optimization of device performance.In this paper,the interpenetrating network structure of the active layer deposited on modified Zn O with different alcohol molecules and the performance of the inverted device are different,which may be caused by the different chemical structure of alcohol molecules and the different properties of active layer materials.It is a simple and effective method that modifying Zn O with alcohol molecules to improve the performance of inverted organic solar cells,which provides an idea to improve the performance of inverted organic solar cells.