Enhanced Efficiency and Long-Term Stability of Perovskite Solar Cells by Synergistic Effect of Nonhygroscopic Doping in Conjugated Polymer-Based Hole-Transporting Layer

Abstract

A face-on oriented and p-doped semicrystalline conjugated polymer, poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole)] (PPDT2FBT), was studied as a hole-transport layer (HTL) in methylammonium lead triiodide-based perovskite solar cells (PVSCs). PPDT2FBT exhibits a mid-band gap (1.7 eV), high vertical hole mobility (7.3 × 10 cm/V·s), and well-aligned frontier energy levels with a perovskite layer for efficient charge transfer/transport, showing a maximum power conversion efficiency (PCE) of 16.8%. Upon doping the PPDT2FBT HTL with a nonhygroscopic Lewis acid, tris(pentafluorophenyl)borane (BCF, 2-6 wt %), the vertical conductivity was improved by a factor of approximately 2, and the resulting PCE was further improved up to 17.7%, which is higher than that of standard PVSCs with 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) as an HTL. After BCF doping, the clearly enhanced carrier diffusion coefficient, diffusion length, and lifetime were measured using intensity-modulated photocurrent and photovoltage spectroscopy. Furthermore, compared to the standard PVSCs with spiro-OMeTAD, the temporal device stability was remarkably improved, preserving the ∼60% of the original PCE for 500 h without encapsulation under light-soaking condition (1 sun AM 1.5G) at 85 °C and 85% humidity, which is mainly due to the highly crystalline conjugated backbone of PPDT2FBT and nonhygroscopic nature of BCF. In addition, formamidinium lead iodide/bromide (FAPbIBr)-based PVSCs with the BCF-doped PPDT2FBT as an HTL was also prepared to show 18.8% PCE, suggesting a wide applicability of PPDT2FBT HTL for different types of PVSCs.

Semitransparent FAPbI3‐xBrx Perovskite Solar Cells Stable under Simultaneous Damp Heat (85 °C/85%) and 1 Sun Light Soaking

“…Although abundant studies for semitransparent PSCs have been done, there are no reports on highly stable semitransparent PSCs satisfying both damp heat and 1 sun light soaking test for commercialization. Hence, here, we demonstrate the 17.3% PCE of a highly stable sandwich‐type semitransparent CH(NH 2 ) 2 PbI 3‐ x Br x (FAPbI 3‐ x Br x ) PSCs, made by a lamination method through conductive hydrophobic poly[(2,5‐bis(2‐hexyldecyloxy)phenylene)‐alt‐(5,6‐difluoro‐4,7‐di(thiophen‐2‐yl)benzo[c][1,2,5]‐thiadiazole)] (PPDT2FBT) doped with nonhygroscopic tris(pentafluorophenyl)borane (BCF) HTGL. In addition, we demonstrated a 14.3% PCE for flexible sandwich‐type semitransparent FAPbI 3‐ x Br x PSCs.…”

“…To measure the conductivity of each sample, we fabricated sandwich type dummy cells composed of glass/ITO/PPDT2FBT:BCF HTGL/ITO/glass by replacing the glass substrate with a patterned ITO/glass substrate (thickness = 1 mm) as shown in the inset of Figure d. From the slope of the I – V plots in Figure d, we can calculate the DC (direct current) conductivity (σ 0 = IdA −1 V −1 , where A is area of overlapped patterned ITO and d is the thickness of the films)[2d,15] of samples 1–4 as 0.0159, 0.0157, 0.0154, and 0.0153 mS cm −1 , respectively, which is consistent with our previous report and is a similar order of magnitude with PTAA (≈0.034 mS cm −1 )[2d] with hygroscopic lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and tert‐butylamine (tBP). Here, it should be noted that the conductivity of PPDT2FBT:BCF HTGL is not significantly changed irrespective of its thickness variations so it is expected that the device performance will not significantly depend on the thickness of PPDT2FBT:BCF HTGL.…”

Analysing the Prospects of Perovskite Solar Cells within the Purview of Recent Scientific Advancements

“…Replacement of Spiro-OMeTAD with other equally efficient HTLs has been realized worldwide. According to a recent study performed by Chang et al a p-type conjugated polymer poly[(2,5-bis(2-hexyldecyloxy)-phenylene)-alt-(5,6-difluoro-4,7-di(thiop-hen-2-yl)benzo[c]-[1,2,5]-thiadiazole)] (PPDT2FBT) doped with a non-hygroscopic Lewis acid, tris(penta-fluorophenyl)borane (BCF, 2−6 wt %) has been proven to be a much more efficient HTL than standard Spiro-OMeTAD [174]. They showed PCE up to 17.7% with standard MAPbI 3 PSC and retaining 60% of the primary PCE upon exposure to 85% humidity for 500 h without any encapsulation.…”

Emerging Characterizing Techniques in the Fine Structure Observation of Metal Halide Perovskite Crystal

“…These observations suggest that the crystal-like perovskite sensitizer in solar cells possesses better performance than the three-dimensional (3D) perovskite structure, which can be utilized as an efficient approach to improve the stability of perovskite crystals. Moreover, external doping with other organics or inorganics can also improve the stability of perovskite [70,71] crystals, as well as the coordination between the perovskite film and the adjacent film [72,73]. Nevertheless, stability is still one of the crucial issues in the development of perovskite solar cells.…”