L-3,4-dihydroxyphenylalanine and Dimethyl Sulfoxide Codoped PEDOT:PSS as a Hole Transfer Layer: towards High-Performance Planar p-i-n Perovskite Solar Cells

HUANG Peng , YUAN Ligang , LI Yaowen , ZHOU Yi , SONG Bo

Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, Jiangsu Province, P. R. China.

Abstract: In the past decade, perovskite solar cells (Pero-SCs) have attracted a great deal of attention owing to their soaring power conversion efficiency (PCE), up to 22.7% in 2017. In p-i-n type Pero-SCs, one of the most commonly used hole transport layer (HTL) materials is poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS), which possesses a high coverage and an extremely smooth surface. However, the inferior electrical conductivity (or large series resistance) and lower work function (WF) of PEDOT:PSS relative to many other HTL materials limits the open-circuit voltages of Pero-SCs. Furthermore, the hygroscopic property and the acidic nature of PEDOT:PSS can readily cause the degradation of perovskite, and thereby affect the long-term stability of Pero-SCs. The abovementioned disadvantages can hinder the application of PEDOT:PSS in high-performance and stable Pero-SCs;therefore, many efforts have been made to modify PEDOT:PSS to prevent these disadvantages, for instance, adding various organic solvents, surfactants, salts, or acids to PEDOT:PSS as dopants. In this paper, we report a simple codoping method to modify PEDOT:PSS, i.e., employing L-3,4-dihydroxyphenylalanine (DOPA) and dimethyl sulfoxide (DMSO) as codopants in PEDOT:PSS, and applying it as a HTL in p-i-n type Pero-SCs. Herein, DOPA and DMSO were mixed separately with PEDOT:PSS to obtain HTLs for comparison. The DMSO-doped PEDOT:PSS improved the conductivity of the PEDOT:PSS film, while the DOPA-doped PEDOT:PSS tuned the WF of the PEDOT:PSS film. Hence, codoping of DMSO and DOPA not only allows for a good match of the energy levels between PEDOT:PSS and the perovskite but also leads to an improvement in the conductivity of PEDOT:PSS. The champion PCE of the Pero-SCs increased from 13.35%to 17.54% after DOPA and DMSO were codoped in PEDOT:PSS. Owing to their aligned energy levels and enhanced charge transportation, the detailed photovoltaic parameters were greatly improved. Scanning electron microscope and X-ray diffraction were used to characterize the morphological change and crystallinity of the perovskite films. Morphological characterization also revealed that the density of grain boundaries in the perovskite films decreased, which should alleviate the charge recombination occurring in the photoactive layer. Both steady-state photoluminescence (PL) and time-resolved PL characterizations were carried out, and they indicated that nonradiative recombination increased for the perovskite films prepared on the doped PEDOT:PSS films. This result explains the improved short-circuit current density. Electrochemical impedance spectroscopy was employed to determine the resistances of the solar cells. The results are consistent with device performance and that reflected in the PL spectra.

Key Words: DOPA; DMSO; Doping; PEDOT:PSS; Perovskite solar cell

1 引言

近年來，鈣鈦礦太陽能電池受到越來越多的關注，主要是由于鈣鈦礦材料具有低成本1、長的載流子壽命2、低的激子束縛能3,4、寬的吸收光譜和高的光吸收系數，這使得它能夠充分的吸收太陽光，并且降低在光電轉換過程中的能量損耗5。從2009年首次報道以來，鈣鈦礦太陽能電池的制備通過調節材料組分、界面工程和優化制備工藝等方法，使其光電轉換效率(PCE)從3.9%迅速提高到22%以上6–9。其中，通過調控電子/空穴傳輸層可以有效促進載流子發生分離并傳輸到對應的電極，在制備高效的鈣鈦礦太陽能電池起了至關重要的作用。常用的空穴傳輸材料有：聚[雙(4-苯基)(2,4,6-三甲基苯基)胺] (PTAA)10，2,2’,7,7’-四[N,N-二(4-甲氧基苯基)氨基]-9,9’-螺二芴(Spiro-OMeTAD)11，聚(9-乙烯基咔唑)(PVK)12，4,4’-環己基二[N,N-二(4-甲基苯基)苯胺] (TAPC)13，聚 3,4-乙烯二氧噻吩:聚苯乙烯磺酸鹽(PEDOT:PSS)14–16等等。在p-i-n型鈣鈦礦太陽能電池中，PEDOT:PSS因其擁有優異的光學透過性、成膜性和對基底的覆蓋度高等優點而成為最常使用的空穴傳輸層材料之一17–19。但是其自身仍然存在的一些缺點限制了進一步使用。首先，PEDOT:PSS的酸性會腐蝕透明導電電極和促使鈣鈦礦晶體發生降解20,21，從而降低鈣鈦礦太陽能電池的穩定性。其次，PEDOT:PSS導電性較差，不利于光生載流子的快速傳輸。此外，PEDOT:PSS較低的功函與鈣鈦礦層會形成一個比較大的能級勢壘，阻礙電荷的傳輸，使得基于PEDOT:PSS的鈣鈦礦太陽能電池開路電壓(Voc)較低(Voc＜ 1 V)22,23。為此已有研究人員報道采用溶劑處理或摻雜的方法來修飾PEDOT:PSS改進其性能24–26。例如，Taylor等27采用 N,N-二甲基亞砜(DMSO)摻雜 PEDOT:SS來提高薄膜的電導率，基于DMSO摻雜PEDOT:PSS的器件效率(15.8%)比未摻雜的器件效率(11.9%)提高到了24.7%；……