Liuyanfeng

Naphthalenediimide-based random copolymers (PNDI-TVTx) with different π-conjugated dithienylvinylene (TVT) versus π-nonconjugated dithienylethane (TET) unit ratios (x = 1000%) are investigated. The PNDI-TVTx-transistor electron/hole mobilities are affected differently, a result rationalized by molecular orbital topologies and energies, with hole mobility vanishing but electron mobility decreasing only by ≈2.5 times when going from x = 100% to 40%.

Hybrid organic–inorganic halide perovskites have emerged at the forefront of solution-processable photovoltaic devices. Being the perovskite precursor mixture a complex equilibrium of species, it is very difficult to predict/control their interactions with different substrates, thus the final film properties and device performances. Here the wettability of CH₃NH₃PbI₃ (MAPbI₃) onto poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole transporting layer is improved by exploiting the cooperative effect of graphene oxide (GO) and glucose inclusion. The glucose, in addition, triggers the reduction of GO, enhancing the conductivity of the PEDOT:PSS+GO+glucose based nanocomposite. The relevance of this approach toward photovoltaic applications is demonstrated by fabricating a hysteresis-free MAPbI₃ solar cells displaying a ≈37% improvement in power conversion efficiency if compared to a device grown onto pristine PEDOT:PSS. Most importantly, V_OC reaches values over 1.05 V that are among the highest ever reported for PEDOT:PSS p-i-n device architecture, suggesting minimal recombination losses, high hole-selectivity, and reduced trap density at the PEDOT:PSS along with optimized MAPbI₃ coverage.

The synergistic effect of graphene oxide and glucose in improving the conduction properties of polymer electrolyte poly(3,4-ethyl­enedioxythiophene):poly­(styrenesul­fonate) and modifying the sensible interface of perovskite solar cells is reported. This method allows obtaining hysteresis-free and high V_OC CH₃NH₃PbI₃ devices displaying a ≈37% improvement in power conversion efficiency, evidencing minimal recombination losses and very efficient charge extraction at the electrodes.

Well-defined small molecule (SM) donors can be used as alternatives to π-conjugated polymers in bulk-heterojunction (BHJ) solar cells with fullerene acceptors (e.g., PC₆₁/₇₁BM). Taking advantage of their synthetic tunability, combinations of various donor and acceptor motifs can lead to a wide range of optical, electronic, and self-assembling properties that, in turn, may impact material performance in BHJ solar cells. In this report, it is shown that changing the sequence of donor and acceptor units along the π-extended backbone of benzo[1,2-b:4,5-b′]dithiophene–6,7-difluoroquinoxaline SM donors critically impacts (i) molecular packing, (ii) propensity to order and preferential aggregate orientations in thin-films, and (iii) charge transport in BHJ solar cells. In these systems (SM1-3), it is found that 6,7-difluoroquinoxaline ([2F]Q) motifs directly appended to the central benzo[1,2-b:4,5-b′]dithiophene (BDT) unit yield a lower-bandgap analogue (SM1) with favorable molecular packing and aggregation patterns in thin films, and optimized BHJ solar cell efficiencies of ≈6.6%. ¹H-¹H DQ-SQ NMR analyses indicate that SM1 and its counterpart with [2F]Q motifs substituted as end-group SM3 possess distinct self-assembly patterns, correlating with the significant charge transport and BHJ device efficiency differences observed for the two analogous SM donors (avg. 6.3% vs 2.0%, respectively).

Changing the sequence of donor and acceptor units along the π-extended backbone of benzo[1,2-b:4,5-b′]dithiophene–6,7-difluoroquinoxaline small molecule (SM) donors critically impacts (i) molecular packing, (ii) propensity to order and preferential aggregate orientations in thin-films, and (iii) charge transport in bulk-heterojunction (BHJ) solar cells. The lower-bandgap analogue (SM1) achieves distinct local packing and aggregation patterns in thin films, and optimized BHJ solar cell efficiencies of ≈6.6%.

Donor–acceptor–acceptor′ small-molecule donors are synthesized to investigate regioisomeric effects on organic photovoltaic device performance. Cross-conjugation in 2-((7-(N-(2-ethylhexyl)-benzothieno[3,2-b]thieno[3,2-d]pyrrol-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)methylene)malononitrile leads to an increased open-circuit voltage compared with its isomer 2-((7-(N-(2-ethylhexyl)-benzothieno[3,2-b]thieno[2,3-d]pyrrol-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)methylene)malononitrile. A correlation is then established between molecular conjugation length and orbital energies, and hence open-circuit voltage.

Simple organic molecules with permanent dipole moments – amino acids and heterocycles – have been successfully employed in bulk-heterojunction organic solar cells as interlayer between photoactive material and electron contact. A large increase of open-circuit voltage and fill factor can be observed for four different polymers as donor material in the photoactive layer. A combination of current–voltage curves, scanning Kelvin-probe atomic force microscopy, ultraviolet photoelectron spectroscopy, and electroluminescence measurements as well as numerical simulations are carried out to clarify in detail the underlying mechanisms. All results fully confirm the hypothesis that the main effect is an accumulation of electrons and a depletion of holes in the photoactive layer in the vicinity of the electron contact induced by a decrease of its effective work function. Further, density functional theory calculations and literature reports of the energy levels of the dipole molecules strongly suggest that the charge carriers tunnel through the thin dipole layer which does however not limit the current. This represents a versatile, simple, and cheap method to realize highly selective contacts which may also be beneficial for other types of solar cells and devices where contact selectivity is crucial.

Simple organic molecules with permanent dipole moments significantly enhance the selectivity of the electron contact in organic solar cells. They modify the work function of Indium Tin Oxide strongly which is investigated by means of scanning Kelvin-probe force microscopy, photoelectron spectroscopy, and electroluminescence in combination with numerical simulations. No preferential tunneling of electrons is needed to consistently explain all results.

Semitransparent perovskite solar cells based on smooth perovskite films and ultrathin Cu (1 nm)/Au (7 nm) metal electrode demonstrate an efficiency of 16.5%. When illuminated through the semitransparent perovskite cell, a near-infrared-enhanced silicon heterojunction solar cell operates with 6.5% efficiency, leading to a total perovskite/silicon four-terminal tandem efficiency of 23.0%.

In article number 1600386, Jeffrey G. Tait and co-workers develop non-hazardous ink systems, based on solvent-alcohol-acid, for coating perovskite photovoltaic films in a single step. Coating was further developed from spin to blade, while the high yield process produced efficient 4 cm² modules.

Taking the π-conjugated polymers PBDT[2X]T (X = H, F) as model systems, the effects of fluorine substitution on main-chain conformations, packing, and electronic couplings are examined. This combination of molecular dynamics simulations and solid-state NMR shows that a higher propensity for backbone planarity in PBDT[2F]T leads to more pronounced, yet staggered, chain stacking, which generally leads to higher electronic couplings and binding energy between neighboring chains.

Uniform perovskite films are achieved by HCl-assisted one-step spin-coating at room temperature. By this method, a highest power conversion efficiency of 17.9% is obtained for perovskite solar cells (PSCs). The devices retain ≈95% of their original efficiency after storage in air for two months. The highest efficiency obtained for large-area PSCs (0.86 cm²) is 15.7%.

Organic photovoltaics (OPVs) hold great promise for next-generation photovoltaics in renewable energy because of the potential to realize low-cost mass production via large-area roll-to-roll printing technologies on flexible substrates. To achieve high-efficiency OPVs, one key issue is to overcome the insufficient photon absorption in organic photoactive layers, since their low carrier mobility limits the film thickness for minimized charge recombination loss. To solve the inherent trade-off between photon absorption and charge transport in OPVs, the optical manipulation of light with novel micro/nano-structures has become an increasingly popular strategy to boost the light harvesting efficiency. In this Review, we make an attempt to capture the recent advances in this area. A survey of light trapping schemes implemented to various functional components and interfaces in OPVs is given and discussed from the viewpoint of plasmonic and photonic resonances, addressing the external antireflection coatings, substrate geometry-induced trapping, the role of electrode design in optical enhancement, as well as optically modifying charge extraction and photoactive layers.

Recent advances in light trapping for organic photovoltaics are reviewed in terms of photon management induced by dielectric or metallic micro/nanostructures. Implementing photonic structures into various functional layers or interfaces is highlighted to lead to the redistribution of optical field in the cells and thus the enhanced light harvesting.