The application of liquid‐exfoliated 2D transition metal disulfides (TMDs) as the hole transport layers (HTLs) in nonfullerene‐based organic solar cells is reported. It is shown that solution ...processing of few‐layer WS2 or MoS2 suspensions directly onto transparent indium tin oxide (ITO) electrodes changes their work function without the need for any further treatment. HTLs comprising WS2 are found to exhibit higher uniformity on ITO than those of MoS2 and consistently yield solar cells with superior power conversion efficiency (PCE), improved fill factor (FF), enhanced short‐circuit current (JSC), and lower series resistance than devices based on poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) and MoS2. Cells based on the ternary bulk‐heterojunction PBDB‐T‐2F:Y6:PC71BM with WS2 as the HTL exhibit the highest PCE of 17%, with an FF of 78%, open‐circuit voltage of 0.84 V, and a JSC of 26 mA cm−2. Analysis of the cells' optical and carrier recombination characteristics indicates that the enhanced performance is most likely attributed to a combination of favorable photonic structure and reduced bimolecular recombination losses in WS2‐based cells. The achieved PCE is the highest reported to date for organic solar cells comprised of 2D charge transport interlayers and highlights the potential of TMDs as inexpensive HTLs for high‐efficiency organic photovoltaics.
The use of liquid exfoliated 2D WS2 and MoS2 as hole‐transporting layers (HTLs) in ultrahigh efficiency organic solar cells is reported. WS2 yields cells with higher power conversion efficiency (PCE), fill‐factor, and short‐circuit current than MoS2 and poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate). When WS2 is introduced as HTL in PBDB‐T‐2F:Y6:PC71BM organic solar cells, a maximum PCE value of 17% is achieved.
The performance of organic photovoltaic (OPV) material systems are hypothesized to depend strongly on the intermolecular arrangements at the donor:fullerene interfaces. A review of some of the most ...efficient polymers utilized in polymer:fullerene PV devices, combined with an analysis of reported polymer donor materials wherein the same conjugated backbone was used with varying alkyl substituents, supports this hypothesis. Specifically, the literature shows that higher-performing donor–acceptor type polymers generally have acceptor moieties that are sterically accessible for interactions with the fullerene derivative, whereas the corresponding donor moieties tend to have branched alkyl substituents that sterically hinder interactions with the fullerene. To further explore the idea that the most beneficial polymer:fullerene arrangement involves the fullerene docking with the acceptor moiety, a family of benzo1,2-b:4,5-b′dithiophene–thieno3,4-cpyrrole-4,6-dione polymers (PBDTTPD derivatives) was synthesized and tested in a variety of PV device types with vastly different aggregation states of the polymer. In agreement with our hypothesis, the PBDTTPD derivative with a more sterically accessible acceptor moiety and a more sterically hindered donor moiety shows the highest performance in bulk-heterojunction, bilayer, and low-polymer concentration PV devices where fullerene derivatives serve as the electron-accepting materials. Furthermore, external quantum efficiency measurements of the charge-transfer state and solid-state two-dimensional (2D) 13C{1H} heteronuclear correlation (HETCOR) NMR analyses support that a specific polymer:fullerene arrangement is present for the highest performing PBDTTPD derivative, in which the fullerene is in closer proximity to the acceptor moiety of the polymer. This work demonstrates that the polymer:fullerene arrangement and resulting intermolecular interactions may be key factors in determining the performance of OPV material systems.
Self‐assembled monolayers (SAMs) based on Br‐2PACz (2‐(3,6‐dibromo‐9H‐carbazol‐9‐yl)ethylphosphonic acid) 2PACz (2‐(9H‐Carbazol‐9‐yl)ethylphosphonic acid) and MeO‐2PACz ...(2‐(3,6‐dimethoxy‐9H‐carbazol‐9‐yl)ethylphosphonic acid) molecules were investigated as hole‐extracting interlayers in organic photovoltaics (OPVs). The highest occupied molecular orbital (HOMO) energies of these SAMs were measured at −6.01 and −5.30 eV for Br‐2PACz and MeO‐2PACz, respectively, and found to induce significant changes in the work function (WF) of indium‐tin‐oxide (ITO) electrodes upon chemical functionalization. OPV cells based on PM6 (poly(2,6‐(4,8‐bis(5‐(2‐ethylhexyl‐3‐fluoro)thiophen‐2‐yl)‐benzo1,2‐b:4,5‐b’dithiophene))‐alt‐(5,5‐(1’,3’‐di‐2‐thienyl‐5’,7’‐bis(2‐ethylhexyl)benzo1’,2’‐c:4’,5’‐c’dithiophene‐4,8‐dione)) : BTP‐eC9 : PC71BM (6,6‐phenyl‐C71‐butyric acid methyl ester) using ITO/Br‐2PACz anodes exhibited a maximum power conversion efficiency (PCE) of 18.4 %, outperforming devices with ITO/MeO‐2PACz (14.5 %) and ITO/poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT : PSS) (17.5 %). The higher PCE was found to originate from the much higher WF of ITO/Br‐2PACz (−5.81 eV) compared to ITO/MeO‐2PACz (4.58 eV) and ITO/PEDOT : PSS (4.9 eV), resulting in lower interface resistance, improved hole transport/extraction, lower trap‐assisted recombination, and longer carrier lifetimes. Importantly, the ITO/Br‐2PACz electrode was chemically stable, and after removal of the SAM it could be recycled and reused to construct fresh OPVs with equally impressive performance.
Super SAM: Two self‐assembled monolayers (SAMs; Br‐2PACz and MeO‐2PACz) are investigated as hole‐extracting interlayer in organic photovoltaics and compared against the widely used PEDOT : PSS. Cells based on the ternary bulk‐heterojunction blend PM6 : BTP‐eC9 : PC71BM and ITO/Br‐2PACz as the anode exhibit the highest power conversion efficiency of 18.4 %, outperforming devices with ITO/MeO‐2PACz (14.5 %) and even ITO/PEDOT : PSS (17.5 %).
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)ethylphosphonic acid (2PACz), is investigated. It is ...found that the formation of self‐assembled monolayers (SAMs) of 2‐(3,6‐Difluoro‐9H‐carbazol‐9‐yl)ethylphosphonic acid (F‐2PACz), 2‐(3,6‐Dichloro‐9H‐carbazol‐9‐yl)ethylphosphonic acid (Cl‐2PACz), 2‐(3,6‐Dibromo‐9H‐carbazol‐9‐yl)ethylphosphonic acid (Br‐2PACz), and 2‐(3,6‐Diiodo‐9H‐carbazol‐9‐yl)ethylphosphonic 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.
The conventional hole‐extracting polymer poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate in organic bulk‐heterojunction (BHJ) photovoltaics is replaced with engineered self‐assembled monolayers (SAMs). Combining the nanometre‐thin SAM interlayers with an n‐doped BHJ, organic photovoltaics with improved stability and a maximum power conversion efficiency of 18.9% are demonstrated.
Benzo1,2‐b:4,5‐b'difuran–thieno3,4‐cpyrrole‐4,6‐dione (PBDFTPD) polymers prepared by microwave‐assisted synthesis can achieve power conversion efficiencies (PCEs) >7% in bulk‐heterojunction solar ...cells with phenyl‐C61/71‐butyric acid methyl ester (PCBM). In “as‐cast” PBDFTPD‐based devices solution‐processed without a small‐molecule additive, high PCEs can be obtained in spite of the weak propensity of the polymers to self‐assemble and form π‐aggregates in thin films.
Metalenses are one of the most promising metasurface applications. However, all-dielectric reflective metalenses are rarely studied, especially regarding their off-axis focusing performance. After ...experimentally studying the material optical properties in this work, we propose reflective metalens based on titanium dioxide (TiO 2 ) and silicon dioxide (SiO 2 ), which operate at a visible wavelength of 0.633 µm. Unlike conventional reflective metalenses based on metallic mirrors, the proposed device was designed based on a modified parabolic phase profile and was integrated onto a dielectric distributed Bragg reflector periodic structure to achieve high reflectivity with five dielectric pairs. The focusing efficiency characteristics of the metalens were experimentally studied for beam angles of incidence between 0 ∘ and 30 ∘ . The results reveal that the focusing efficiency for the modified metalens design remains higher than 54%, which is higher than 50%, making it promising for photonic miniaturization and integration.
Chemical bath deposition (CBD) of tin oxide (SnO2) thin films as an electron-transport layer (ETL) in a planar-heterojunction n–i–p organohalide lead perovskite and organic bulk-heterojunction (BHJ) ...solar cells is reported. The amorphous SnO2 (a-SnO2) films are grown from a nontoxic aqueous bath of tin chloride at a very low temperature (55 °C) and do not require postannealing treatment to work very effectively as an ETL in a planar-heterojunction n–i–p organohalide lead perovskite or organic BHJ solar cells, in lieu of the commonly used ETL materials titanium oxide (TiO2) and zinc oxide (ZnO), respectively. Ultraviolet photoelectron spectroscopy measurements on the glass/indium–tin oxide (ITO)/SnO2/methylammonium lead iodide (MAPbI3)/2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene device stack indicate that extraction of photogenerated electrons is facilitated by a perfect alignment of the conduction bands at the SnO2/MAPbI3 interface, while the deep valence band of SnO2 ensures strong hole-blocking properties. Despite exhibiting very low electron mobility, the excellent interfacial energetics combined with high transparency (E gap,optical > 4 eV) and uniform substrate coverage make the a-SnO2 ETL prepared by CBD an excellent candidate for the potentially low-cost and large-scale fabrication of organohalide lead perovskite and organic photovoltaics.
Poly(benzo1,2‐b:4,5‐b′dithiophene–alt–thieno3,4‐cpyrrole‐4,6‐dione) (PBDTTPD) polymer donors with linear side‐chains yield bulk‐heterojunction (BHJ) solar cell power conversion efficiencies (PCEs) of ...about 4% with phenyl‐C71‐butyric acid methyl ester (PC71BM) as the acceptor, while a PBDTTPD polymer with a combination of branched and linear substituents yields a doubling of the PCE to 8%. Using transient optical spectroscopy it is shown that while the exciton dissociation and ultrafast charge generation steps are not strongly affected by the side chain modifications, the polymer with branched side chains exhibits a decreased rate of nongeminate recombination and a lower fraction of sub‐nanosecond geminate recombination. In turn the yield of long‐lived charge carriers increases, resulting in a 33% increase in short circuit current (J
sc). In parallel, the two polymers show distinct grazing incidence X‐ray scattering spectra indicative of the presence of stacks with different orientation patterns in optimized thin‐film BHJ devices. Independent of the packing pattern the spectroscopic data also reveals the existence of polymer aggregates in the pristine polymer films as well as in both blends which trap excitons and hinder their dissociation.
The polymer side chain pattern determines the efficiency of PBDTTPD:phenyl‐C61/71‐butyric acid methyl ester solar cells because it changes the yield of free charges and the nongeminate recombination dynamics, as demonstrated using broadband transient pump–probe spectroscopy. Tuning of the side chains leads to a doubling of the power conversion efficiency from 4% up to 8%.
The bulk heterojunction (BHJ) solar cell performance of many polymers depends on the polymer molecular weight (M
n) and the solvent additive(s) used for solution processing. However, the mechanism ...that causes these dependencies is not well understood. This work determines how M
n and solvent additives affect the performance of BHJ solar cells made with the polymer poly(di(2‐ethylhexyloxy)benzo1,2‐b:4,5‐b′dithiophene‐co‐octylthieno3,4‐cpyrrole‐4,6‐dione) (PBDTTPD). Low M
n PBDTTPD devices have exceedingly large fullerene‐rich domains, which cause extensive charge‐carrier recombination. Increasing the M
n of PBDTTPD decreases the size of these domains and significantly improves device performance. PBDTTPD aggregation in solution affects the size of the fullerene‐rich domains and this effect is linked to the dependency of PBDTTPD solubility on M
n. Due to its poor solubility high M
n PBDTTPD quickly forms a fibrillar polymer network during spin‐casting and this network acts as a template that prevents large‐scale phase separation. Furthermore, processing low M
n PBDTTPD devices with a solvent additive improves device performance by inducing polymer aggregation in solution and preventing large fullerene‐rich domains from forming. These findings highlight that polymer aggregation in solution plays a significant role in determining the morphology and performance of BHJ solar cells.
The performance of poly(di(2‐ethylhexyloxy)benzo1,2‐b:4,5‐b′dithiophene‐co‐octylthieno3,4‐cpyrrole‐4,6‐dione) (PBDTTPD) bulk heterojunction solar cells strongly depends on the polymer molecular weight, and processing these bulk heterojunctions with a solvent additive preferentially improves the performance of low molecular weight devices. It is demonstrated that polymer aggregation in solution significantly impacts the thin‐film bulk heterojunction morphology and is vital for high device performance.
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