Theoretical Study On The Properties Of Organic Small Molecule Transport Materials In Perovskite Solar Cells

In recent years,perovskite solar cells have stood out among many solar cells and have been rapidly improving the photoelectric conversion efficiency.The transport materials in perovskite solar cells are divided into hole transport materials and electron transport materials.The function of the hole transport material is to transport holes and avoid charge recombination.Spiro-OMeTAD is currently the most typical hole transport material,and its use has significantly improved the photoelectric conversion efficiency.However,commercial applications are limited due to the tedious synthesis steps and complex purification processes.Electron transport materials play an important role in the extraction and transmission of photogenerated electrons.As a typical fullerene derivative electron transport material,PCBM is often used in inverted perovskite solar cells.However,its disadvantages are high production costs and limited energy level tunability.Therefore,it is of great significance to develop efficient and inexpensive hole transport materials and new non-fullerene electron transport materials.This paper uses density functional theory to study the electronic and transport properties of a series of organic small molecule hole transport materials and electron transport materials.The relationship between molecular structure and properties is discussed in detail,and several methods to improve the mobility of transport materials are proposed and explained.The research content is divided into the following aspects:1.Based on the density functional theory and Marcus theory,we have calculated a series of Z26 molecular derivatives（Z26-2,Z26-3 and Z26-4）,and studied the effect ofπ-bridge size and intermolecular packing on hole mobility in hole transport materials.The calculation results show that Z26-2（7.7′10^–44 cm2 V-¹ s-¹）and Z26-3(1.3′10^–3 cm2V-¹ s-¹)have larger hole mobility than Z26(5.60′10^-4 cm² V-¹ s-¹)due to their appropriate conjugated length leading to effective face to face packing.The smallest hole mobility of Z26-4(4.20′10^-5 cm² V-¹ s-¹)is attributed to its overlong conjugation with four double bonds on each side,which produces a long centroid-to-centroid distance and small electronic coupling.This theoretical study of the relationship between theπ-bridgesize and the intermolecular packing will provide a theoretical basis for the design of new hole transport materials in the future.2.In view of the current low mobility of electron transport materials in the field of perovskite solar cells and their development lags far behind hole transport materials,we have turned our attention to electron transport materials.Based on a high mobility organic small molecule transport material（4Cl-TAP）,we designed four new molecules（P1,P2,P3,P4）and applied density functional theory in combination with Marcus theory to calculate their front-line molecular orbital energy levels,recombination energy,andelectron mobility.The electron transport properties of these molecules were evaluated from the front molecular orbital,electron mobility,solubility and stability.At the same time,we also studied the effects of changing side chains on electronic properties and electron transport properties.The results show that changing the side chain to a furan ring and a thiophene ring can increase the conjugation of the molecule and form a good face-to-faceπ-πpacking,thus increasing the electron mobility.This work reveals the greatpotential of new non-fullerene small molecules based on 4Cl-TAP as electron transport materials.3.Based on triphenylamine as the core organic small molecule electron transport material TPA-3CN,we designed four new organic small molecules（TPA-1,TPA-2,TPA-3,TPA-4）.Based on density functional theory and Marcus theory,the effects of different degree of bridging on molecular stacking and electron mobility were studied.The calculation results show that TPA-3CN has the lowest electron mobility due to its head-to-head stacking and large centroid-to-centroid distance.After the newly designed molecule bridge the triphenylamine,the stacking model of the molecule changed to face-to-face stacking with a smaller centroid-to-centroid distance,which promoted the electron transport.Therefore,a greater electron mobility was obtained.TPA-4 has the largestelectron mobility and several orders of magnitude higher than the experimental molecule because it has the best planarity and can form a more efficient face-to-face stacking.Thiswork shows that non-fullerene organic small molecules based on fully-bridged triphenylamine cores are expected to become a potential electron transport material for perovskite solar cells.