Outstanding Thermal Stability of Perovskite Solar Cells Based on Zn(TFSI)₂-Doped Spiro-MeOTAD

Abstract

Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is the most general dopant for 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-MeOTAD), which has been dominantly used as a hole transport material to achieve record efficiencies of perovskite solar cells (PSCs) in spite of resultant poor thermal stability. Recently, zinc(II) bis(trifluoromethanesulfonyl)imide (Zn(TFSI)2) was proposed as an excellent candidate to replace LiTFSI. In this study, the use of Zn(TFSI)2 as a dopant for spiro-MeOTAD is explored to investigate the thermal stability of PSCs. A complexation of Zn(TFSI)2 along with tert-butylpyridine during the doping process of spiro-MeOTAD is favorable for the device operation at 85 °C, while PSCs with LiTFSI demonstrate a rapid performance degradation. Differential scanning calorimetry, X-ray diffraction, and morphology confirm that LiTFSI-doped spiro-MeOTAD is prone to involving higher crystallinity at the elevated temperature, compared to the one with Zn(TFSI)2, directly affecting the photovoltaic performance by aggravating the charge recombination at the interface.

Effect of Zn(TFSI)2 on the performance-aging time of perovskite solar cells

“…Such a long aging time for Zn(TFSI) 2 to optimize device performance would be troublesome in many aspects (e.g. unreliable initial performance, time-inefficiency, strict atmosphere control during the doping process, etc), which could mean avoiding the use of Zn(TFSI) 2 in spite of its strong advantage of long-term stable high performance [10,16]. Therefore, control of energy alignment was attempted by interface engineering at the perovskite/HTL interface to shorten the device aging time for performance optimization.…”

“…The intrinsically low hole mobility of spiro-MeOTAD strongly demands an additional dopant to reach a high PCE over 25%; lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is the most popular dopant from the perspective of the PCE [6][7][8][9]. However, LiTFSI can act as a plasticizer and can be responsible for lowering the thermal stability of PSCs [9][10][11][12][13]. While various metal complexes with TFSI have been adopted as dopants for the spiro-MeOTAD in order to improve the long-term stability of PSCs, such as Cu(TFSI) 2 [14], Ca(TFSI) 2 [14,15], Sc(TFSI) 3 [14], and Mg(TFSI) 2 [15], the enhanced stability was inevitably obtained at the expense of losing considerable PCE [14,15].…”

Recent Progress of Urea-Based Deep Eutectic Solvents as Electrolytes in Battery Technology: A Critical Review

“…Lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) and zinc bis(trifluoromethane sulfonyl)imide (ZnTFSI) are inorganic salts with the chemical formulas of LiC 2 F 6 NO 4 S 2 and Zn(C 2 F 6 NO 4 S 2 ) 2 , respectively [55]. Despite sharing similar physicochemical properties, LiTFSI exhibits several advantages due to its use in Li batteries and its use in combination with urea [56]. Both are white hygroscopic powders [57,58].…”

Irreversible phase back conversion of α‐FAPbI3 driven by lithium‐ion migration in perovskite solar cells

“…However, these dopants were found to be highly hygroscopic and can induce damage to the underlying perovskite layer 9 . Thus, replacing these dopants with other alternatives has been widely investigated to achieve more stable PSCs 10,11 . Nevertheless, most state‐of‐the‐art PSCs, including record‐setting devices, still incorporate these conventional dopants 3 …”

“…9 Thus, replacing these dopants with other alternatives has been widely investigated to achieve more stable PSCs. 10,11 Nevertheless, most state-of-the-art PSCs, including record-setting devices, still incorporate these conventional dopants. 3 Besides participating in undesirable chemical reactions, Li + ions tend to penetrate the underlying perovskite and ETL depending on the aging time.…”