Side‐Chain Engineering on Dopant‐Free Hole‐Transporting Polymers toward Highly Efficient Perovskite Solar Cells (20.19%)

Abstract

A variety of dopant-free hole-transporting materials (HTMs) is developed to serve as alternatives to the typical dopant-treated ones; however, their photovoltaic performance still falls far behind. In this work, the side chain of a polymeric HTM is engineered by partially introducing diethylene glycol (DEG) groups in order to simultaneously optimize the properties of both the bulk of the HTM layer and the HTM/perovskite interface. The intermolecular π-π stacking interaction in the HTM layer is unexpectedly weakened after the incorporation of DEG groups, whereas the lamellar packing interaction is strengthened. A doubled hole mobility is obtained when 3% of the DEG groups replace the original alkyl side chains, and a champion power conversion efficiency (PCE) of 20.19% (certified: 20.10%) is then achieved, which is the first report of values over 20% for dopant-free organic HTMs. The device maintains 92.25% of its initial PCE after storing at ambient atmosphere for 30 d, which should be due to the enhanced hydrophobicity of the HTM film.yielded the highest PCE so far of 24.2%, [1] however, it requires doping treatment by oxidants to enhance its hole conductivity, which always inevitably leads to the consequences of long fabrication duration and poor long-term stability for the PSC devices. [3][4][5] As a result, numerous dopant-free organic substitutes including small molecules and polymers [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] have been developed, and reports of PCE values of over 19% [29,31,45,46] have been frequently issued. Although PCEs of 20.5 [47] and 23.3% [48] were also released, this high efficiency was obtained through optimizing the ETM layer [47] or the perovskite layer, [48] while the HTM itself was earlier designed/ employed and reported with a PCE of only 18.3 [49] or 0.52% [50] at its first appearance. Therefore, strictly speaking there are no dopant-free organic HTMs reported yet to present a PCE of over 20%. Previously, designing of HTMs was mainly focused on the bulky properties of the HTM layer, especially the hole mobilities, and consequently the well-known face-on molecular orientation where the plane of polymer's main chain is parallel or intermolecular π-π stacking direction is perpendicular to the substrate was widely adopted as a criterion when one designs an HTM. [29,[31][32][33]49] Later, the interfaces between the perovskite and HTM layer were paid attention to be modified by small molecules [51][52][53] and polymers, [54,55] so that the defects on the surface and/or in the bulk of the perovskite layer could be passivated, leading to an enhanced photovoltaic performance.

Dopant‐Free Hole Transporting Molecules for Highly Efficient Perovskite Photovoltaic with Strong Interfacial Interaction

“…Besides, the hydrophilic dopants can bring in the degradation issues, which have negative impacts on the stability of PVSCs . Therefore, much effort has been devoted to develop dopant‐free hole transporting materials (HTMs) applied in either normal or inverted (p‐i‐n) PVSCs (e.g., transparent conductive substrate/HTL/perovskite/electron transporting layer (ETL)/metal electrode) for their simple fabrication process, high PCE over 18%, and low hysteresis …”

Low-temperature carbon-based electrodes in perovskite solar cells

“…73 Therefore, dopant-free HSL becomes a more promising material for stabilizing PSCs. [74][75][76][77] Fabrication Fig. 2 Basic mesoporous n-i-p perovskite cell configuration and the main drawbacks of metallic electrodes and organic HSLs, affecting the performance stability of such cells.…”

Hole Transport Materials in Conventional Structural (n–i–p) Perovskite Solar Cells: From Past to the Future

“…In contrast, the doped, spiro‐OMeTAD device suffered 48% of efficiency loss. Most recently, the same group incorporated 3% diethylene glycol groups into the DTB and achieved 20.19% of device efficiency by optimizing the molecular orientation . Tables 7 and 8 present the properties of the device employing each HTM.…”