退火温度对电子束蒸发沉积Cu_2O薄膜性能的影响

近年来,钙钛矿太阳电池(PSCs)得到了迅猛发展,而无机空穴传输材料(IHTMs)的使用可进一步降低电池的成本,提高电池的稳定性.本文通过电子束蒸发制备了Cu_2O薄膜,研究了空气中退火温度及时间对薄膜组成、结构及光电性能的影响,并构筑了p-i-n反型平面异质结钙钛矿太阳电池.研究发现:由于热解作用,直接通过电子束蒸发制备的薄膜为Cu_2O和Cu的混合物;而在空气中经过退火后,由于氧化作用,随着退火温度的升高,薄膜的组分由混合物转变为纯的Cu_2O,再转变成纯的CuO.通过控制退火温度制备的Cu_2O薄膜的光学带隙约为2.5 eV,载流子迁移率约为30 cm~2·V~(-1)·s~(-1).应用于PSCs,薄膜的最佳厚度为40 nm,但电池性能低于PEDOT:PSS基的PSCs.这主要是由于钙钛矿前驱液在Cu_2O薄膜的润湿性较差,吸收层中有大量微孔洞存在,致使漏电流增强,电池的性能降低.然而,当采用Cu_2O/PEDOT:PSS双HTMs设计时,由于PEDOT:PSS对Cu_2O具有较强的腐蚀作用,使电池性能恶化.

Influence of annealing temperature on properties of Cu2O thin films deposited by electron beam evaporation

Inorganic-organic metal halide perovskite solar cells (PSCs) have drawn tremendous attention as a promising next-generation solar-cell technology because of their high efficiencies and low production cost. Since the first report in 2009, the recorded power conversion efficiency (PCE) of PSCs has rapidly risen to 22.1% by using 2, 2', 7, 7'-tetrakis (N,Ndi-p-methoxyphenyl-amine) 9,9-spirobifluorene (spiro-MeoTAD) as hole transport material (HTM), with the efforts devoted to the device architecture optimization, material compositional engineer and interface engineering. Nevertheless, the synthesis and cost of the organic HTM (OHTM) become a major challenging issue and therefore alternative materials are required. In the past few years, the applications of inorganic HTMs (IHTMs) in PSCs have shown large improvement in PCE and stability. For example, PSCs with CuO_x as IHTM reached a PCE of 19.0% with better stability. Even more exciting, the theoretical PCE of PSC based on Cu₂O HTM reaches 24.4%. So, Cu₂O is a promising IHTM for future optimized PSC and the large area uniform preparation is very important. In this paper, Cu₂O films have been successfully prepared using electron beam evaporation followed by air annealing. The influences of annealing temperature and time on the composition, structure, and photoelectric characteristics of film are investigated in detail. It is found that the as-deposited film is a mixture of Cu₂O and Cu. With the increase of annealing temperature, material composition is transformed from mixture to pure Cu₂O phase, and then to CuO, due to the oxidation in air. In an annealing temperature between 100℃ to 150℃, pure Cu₂O film can be obtained with an average transmission rate over 70%, optical band-gap of 2.5 eV, HOMO level of -5.32 eV, and a carrier mobility of 30 cm²·V^-1·s^-1. When the film is treated with a UV lamp, the structure and composition of the film can be changed more easily because of the enhancement of oxidation. Finally, reverted planar PSCs with the structure of Ag/PCBM/CH₃NH₃PbI₃/HTMs/ITO are constructed and compared carefully based on HTMs of Cu₂O, with poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)(PEDOT:PSS), and Cu2O/PEDOT:PSS layers, respectively. An optimum thickness of 40 nm of Cu₂O HTM is achieved with high carrier extraction rate. However, the performances of all of the PSCs are inferior to those of PEDOT:PSS-based devices, due to the formation of pinholesin absorber layer resulting from the strong hydrophobicity of Cu₂O film. However, the efficiency of PSC based on Cu2O/PEDOT:PSS double-HTM is deteriorated because of the chemical interaction between PEDOT:PSS and Cu₂O. These findings provide some important guidelines for the design of HTMs.