18.9% Efficient Organic Solar Cells Based on n‐Doped Bulk‐Heterojunction and Halogen‐Substituted Self‐Assembled Monolayers as Hole Extracting Interlayers

Abstract

The influence of halogen substitutions (F, Cl, Br, and I) on the energy levels of the self‐assembled hole‐extracting molecule [2‐(9H‐Carbazol‐9‐yl)ethyl]phosphonic acid (2PACz), is investigated. It is found that the formation of self‐assembled monolayers (SAMs) of [2‐(3,6‐Difluoro‐9H‐carbazol‐9‐yl)ethyl]phosphonic acid (F‐2PACz), [2‐(3,6‐Dichloro‐9H‐carbazol‐9‐yl)ethyl]phosphonic acid (Cl‐2PACz), [2‐(3,6‐Dibromo‐9H‐carbazol‐9‐yl)ethyl]phosphonic acid (Br‐2PACz), and [2‐(3,6‐Diiodo‐9H‐carbazol‐9‐yl)ethyl]phosphonic acid (I‐2PACz) directly on indium tin oxide (ITO) increases its work function from 4.73 eV to 5.68, 5.77, 5.82, and 5.73 eV, respectively. Combining these ITO/SAM electrodes with the ternary bulk‐heterojunction (BHJ) system PM6:PM7‐Si:BTP‐eC9 yields organic photovoltaic (OPV) cells with power conversion efficiency (PCE) in the range of 17.7%–18.5%. OPVs featuring Cl‐2PACz SAMs yield the highest PCE of 18.5%, compared to cells with F‐2PACz (17.7%), Br‐2PACz (18.0%), or I‐2PACz (18.2%). Data analysis reveals that the enhanced performance of Cl‐2PACz‐based OPVs relates to the increased hole mobility, decreased interface resistance, reduced carrier recombination, and longer carrier lifetime. Furthermore, OPVs featuring Cl‐2PACz show enhanced stability under continuous illumination compared to ITO/PEDOT:PSS‐based cells. Remarkably, the introduction of the n‐dopant benzyl viologen into the BHJ further boosted the PCE of the ITO/Cl‐2PACz cells to a maximum value of 18.9%, a record‐breaking value for SAM‐based OPVs and on par with the best‐performing OPVs reported to date.

A 19% efficient and stable organic photovoltaic device enabled by a guest nonfullerene acceptor with fibril-like morphology

“…So, Br-2PACz SAM, which has a similar surface energy and energy levels to Cl-2PACz (Fig. S15, ESI †), 32 was used as the hole-extracting interlayer on ITO for the sole purpose of ToF-SIMS measurements.…”

In Situ Generation of n‐Type Dopants by Thermal Decarboxylation

“…[ 5,6 ] Progress in this area has been particularly valuable for the development of efficient OLED devices, and the use of dopants has also led to some of the highest efficiency OPV devices reported to date. [ 7–10 ]…”

18.73% efficient and stable inverted organic photovoltaics featuring a hybrid hole-extraction layer

“…Recently, self-assembled monolayers (SAMs) of [2-(9Hcarbazol-9-yl)ethyl]phosphonic acid (2PACz) and its derivatives were used as nm-thin HELs on ITO for OPVs. [38][39][40][41][42][43] The resulting cells exhibited higher PCEs and better operational lifetime than control OPVs made with the commercial hole-conducting polymer poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). Although the operational lifetime of SAM-based OPVs was longer than that of control cells with PEDOT:PSS, their operational stability remains inferior to that achieved in state-of-the-art inverted OPVs.…”

“…In an effort to address this timely goal, we developed a hybrid HEL composed of (2-((3,6-dibromo-9H-carbazol-9-yl)ethyl)phosphonic acid) (Br-2PACz) 38,42 and MoO 3 (Br-2PACz/ MoO 3 ). The performance characteristics of best-in-class OPVs featuring the hybrid HEL were then compared against cells featuring the same BHJ but using MoO 3 as the HEL.…”

“…S1, ESI †). 42,46 To form the hybrid HEL, Br-2PACz was first spincoated onto the BHJ, followed by the processing of a 7 nm-thin MoO 3 layer via thermal vacuum sublimation (see ESI †). We chose Br-2PACz because of its recently demonstrated superior HEL functionality in high-performance OPVs based on the standard architecture.…”