Semiconducting Polymer Thin Films Used in Organic Solar Cells: A Scanning Tunneling Microscopy Study

Abstract

The widely used semiconducting polymers poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) (PH1000), poly(3‐hexylthiophene‐2,5‐diyl) (P3HT), and Poly[[4,8‐bis[(2‐ethylhexyl)oxy]benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl][3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thiophenediyl]] (PTB7) for organic photovoltaic (OPV) cells, are deposited on highly ordered pyrolytic graphite substrates and investigated by scanning tunneling microscopy (STM) under the liquid/solid interface technique. PEDOT:PSS film morphology shows an ellipsoidal shape of PEDOT surrounded by PSS with 4 nm size. STM images of P3HT reveal that films tend to form well‐defined crystalline domains with an average interchain distance around 1.41 nm and chain length ≈41 nm. PTB7 chains are self‐aggregated by using chlorobenzene as solvent; however, when using 1‐phenyloctane with 3% of 1,8‐diiodooctane, these chains show a worm‐like pattern with a distance of ≈2 nm between backbone chains and a chain length of ≈90 nm. For P3HT and PTB7 based OPVs, local J–V plots are determined from photoconductive atomic force microscopy (pc‐AFM) measurements and fitted with the modified Mott−Gurney equation, the reached parameters are correlated with those achieved from the macroscopic J–V graphs. Active films of OPVs cells based on PTB7 are also analyzed by using AFM phase imaging showing domains size of 80–120 nm. This current STM analysis can elucidate some new important insights on polymer film formation and morphology and therefore, their influence on the OPV performance.

Expanding the carbo‐Benzene Chemical Space for Electron‐Accepting Ability: Trifluorotolyl/Tertiobutyl Substitution Balance

“…In a few regions, 'horizontal' molecules are however observed, the planar C 18 core stacking face-to-face with the substrate via van der Waals interactions (Figure 3,c). [26,27] Figures 3,b and 3,d suggest that the molecular alignment is oriented by the CF 3 groups. [28] The autocorrelation [a] See reference [12].…”

Interfacial Energetic Level Mapping and Nano-Ordering of Small Molecule/Fullerene Organic Solar Cells by Scanning Tunneling Microscopy and Spectroscopy

“…Although previous reports [6,8] have analyzed the morphological control of the bulk heterojunction by using different post-processing strategies (TA, SVA and TA + SVA) and techniques (AFM, TEM, GIWAXS) of DRCN5T:[70]PCBM blend, to the best of our knowledge, no previous studies have been carried out in order to directly observe the morphology evolution process of DRCN5T molecule and DRCN5T:[70]PCBM blend, as well as the energetic level alignment at the D-A interface upon different annealing treatments; further, these studies were carried out by using STS and STM techniques; moreover, these measures were performed at the liquid/solid interface approach (room environmental conditions) [17,25], this approach is cheaper, easier and faster of performing than that of the UHV STM/STS systems (however, with less quality/resolution). All these stated reasons could help to better understand the PV performance in OSC devices.…”

“…Samples were then removed from the Petri dish and left on a plate at room temperature again for 10 min for proper drying of the thin films (remember that chloroform rapidly evaporates because its ebullition point is just 61 • C, further, SVA treatment is mainly for the sample surface). Finally, a thin film of PFN polyelectrolyte (spin-coated, thickness~8 nm; Ossila Ltd., Sheffield, UK) as electron extraction layer (ETL) and Field's metal (FM, drop-coated; RotoMetals, CA, USA) as an alternative top electrode, were deposited at regular atmosphere conditions as described in our previous works [17][18][19]; the active area was 0.04 cm 2 by the drop casting method (thickness <700 pm). The solid thin films were thermally annealed on a hot plate at 80 °C and 120 °C for 10 min, a lot of care was taken for not destroying the films due to their thinness.…”

“…700 pm) of DRCN5T and DRCN5T:[70]PCBM blend, were analyzed only for TA at different temperatures (room temperature = WO, 80, and 120 • C) by STM/STS because these very thin organic films (necessary fact required by these used techniques) are destroyed by applying the SVA process due to their thinness. STM/STS measurements provide the nano-morphology conformation and local density of states (LDOS) information of the organic samples, respectively [16,17]. Gaining knowledge over the morphology mechanisms and evolution in the blend, and correlating this information with energy levels between donor and acceptor compounds, it is essential to provide a clear pathway to further improve the overall device performance.…”

Nanoscale Mapping of Photo‐Induced Charge Carriers Generated at Interfaces of a Donor/Acceptor 2D‐Assembly by Light‐Assisted‐Scanning Tunneling Microscopy

“…On the one hand, the observation of the electroluminescent phenomenon under the tip of a scanning tunneling Bulk-heterojunction-OPV-cells based on the PTB7/PCBM couple are known for achieving high power conversion efficiencies. [18] As the interfacial area of the donor/acceptor domains has been pointed out as one of the key factors to improve the efficiency of organic photovoltaic devices, [19][20][21][22][23][24] we have first carried out a systematic analysis of the morphology of the bicomponent film that we obtained from a solution deposition approach. Then, we have compared the behavior of the PTB7/ PC 71 BM system under light irradiation with that of a non-photoactive self-assembled monolayer (SAM) of adamantanethiol (AT).…”