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A new procedure for the cosensitization with quantum dots (QDs) and dyes for sensitized solar cells is reported here. Cascade cosensitization of TiO₂ electrodes is obtained by the sensitization with CdS QDs and zinc phthalocyanines (ZnPcs), in which ZnPcs containing a sulfur atom are specially designed to produce a cascade injection by direct attachment to QDs. This strategy causes a double synergetic interaction. This is the differentiating point of cascade cosensitization in comparison with other approaches in which dyes with conventional functionalization are anchored to TiO₂ electrodes. Cosensitization produces a panchromatic response from the visible to near-IR region already observed with other sensitization strategies. However, cascade cosensitization produces in addition a synergistic interaction between QDs and dye, that it is not merely limited to the complementary light absorption, but dye enhances the efficiency of QD sensitization acting as a passivating agent. The cascade cosensitization concept is demonstrated with using [Co(phen)₃]^3+/2+ redox electrolyte. The TiO₂/CdS QD-ZnPc/[Co(phen)₃]^3+/2+ sensitized solar cell shows a large improvement of short-circuit photocurrent and open-circuit voltage in comparison with samples just sensitized with QDs. The advent of such cosensitized QD-ZnPc solar cells paves the way to extend the absorbance region of the promising QD-based solar cells and the development of a new family of molecules designed for this purpose.

The cascade co-sensitization concept is demonstrated by the sensitization of TiO₂ electrodes with CdS quantum dots (QDs) covalently linked to zinc phthalocyanines (ZnPcs) via a sulfur atom. The efficiency of co-sensitized CdS QD-SZnPc cells, using Co(phen)₃]³⁺/[Co(phen)₃]²⁺ as electrolyte, is 212% higher than that of a solar cell sensitized just with CdS QD.

To achieve high-performance perovskite solar cells, especially with mesoscopic cell structure, the design of the electron transport layer (ETL) is of paramount importance. Highly branched anatase TiO₂ nanowires (ATNWs) with varied orientation are grown via a facile one-step hydrothermal process on a transparent conducting oxide substrate. These films show good coverage with optimization obtained by controlling the hydrothermal reaction time. A homogeneous methyl­ammonium lead iodide (CH₃NH₃PbI₃) perovskite thin film is deposited onto these ATNW films forming a bilayer architecture comprising of a CH₃NH₃PbI₃ sensitized ATNW bottom layer and a CH₃NH₃PbI₃ capping layer. The formation, grain size, and uniformity of the perovskite crystals strongly depend on the degree of surface coverage and the thickness of the ATNW film. Solar cells constructed using the optimized ATNW thin films (220 nm in thickness) yield power conversion efficiencies up to 14.2% with a short-circuit photocurrent density of 20.32 mA cm⁻², an open-circuit photovoltage of 993 mV, and a fill factor of 0.70. The dendritic ETL and additional perovskite capping layer efficiently capture light and thus exhibit a superior light harvesting efficiency. The ATNW film is an effective hole-blocking layer and efficient electron transport medium for excellent charge separation and collection within the cells.

A facile solution-based route to fabricate thin films of dendritic anatase TiO₂nano­wires on TCO substrates is developed. Solar cells containing the perovskite-infiltrated nanowire layer and uniform perovskite capping layer yield impressive power conversion efficiencies (>14%) due to efficient light harvesting and charge collection in the bilayer structure.

A facile, one step, roll-to-roll compatible laser patterning technique to improve and simultaneously tune the optoelectronic properties of graphene based transparent conductive electrodes (TCE) is demonstrated by E. Stratakis, E. Kymakis, and colleagues on page 2213. In order to overcome the trade-off between the sheet resistance and transparency, reduced graphene oxide micromeshes are laser-patterned on plastic substrate and incorporated in flexible organic photovoltaic devices as the TCE.

Spin-coated film of poly(vinylidenefluoride-hexafluoropropylene) (P(VDF-HFP)) acts as a cathode/anode buffer layer in polymer solar cells (PSCs) with conventional/inverted device structures. Such devices show optimized performances comparable with the controlled device, making P(VDF-HFP) a good substitute for LiF/MoO₃ as a cathode/anode buffer layer. Ultraviolet photoelectron spectroscopy (UPS) and Kelvin force microscope (KFM) measurements show that increased surface potential of active layers improves cathode contact. In piezoresponse force microscopy (PFM) measurement, P(VDF-HFP) responds to applied bias in phase curve, showing tunable dipole. This tunable dipole renders surface potential under applied bias. As a result, open-circuit voltage of devices alters instantly with poling voltage. Moreover, positive poling of P(VDF-HFP) together with simultaneous oxidation of Ag gradually improves performance of inverted structure device. Integer charge transfer (ICT) model elucidates improved electrode contacts by dipole tuning, varying surface potential and vacuum level shift. Understanding the function of dipole makes P(VDF-HFP) a promising and versatile buffer layer for PSCs.

Poly(vinylidenefluoride-hexafluoropropylene) (P(VDF-HFP)) is demonstrated as an efficient buffer layer in cathode/anode interface of conventional/inverted solar cells, with tunable dipole rendering surface potential of active layers. Integer charge transfer (ICT) model is employed to unveil the effect of surface potential on electrode contact and device performance. Understanding the function of dipole makes P(VDF-HFP) a versatile buffer layer for PSCs.

Energy levels of the first monolayer are manipulated at donor/acceptor interfaces in planar heterojunction organic photovoltaics by using molecular self-organization. A “cascade” energy landscape allows thermal-activation-free charge generation by photoirradiation, destabilizes the energy of the interfacial charge-transfer state, and suppresses bimolecular charge recombination, resulting in a higher open-circuit voltage and fill factor.

The observed magneto-photocurrent and magneto-photoluminescence indicate that charge recombination and dissociation are spin-dependent in organo­metal halide perovskites. A magnetic field suppresses spin mixing between the antiparallel and parallel spin states in electron–hole pairs, decreasing the antiparallel/parallel ratio. This decreases exciton formation available for light emission but increases electron–hole pairs ready for charge dissociation, decreasing photoluminescence and increasing photocurrent.

A novel wide-bandgap copolymer (PDBT-T1) is developed and applied in organic solar cells, which yield a high efficiency of 9.74% and a high fill factor of 75%. The high photovoltaic performance is due to efficient photogenerated exciton dissociation and charge collection in PDBT-T1-based solar cells. The results show that PDBT-T1 is an outstanding candidate as a wide-bandgap material for tandem (or multi-junction) organic solar cells.

On page 1955 P. Kordt, D. Andrienko, and colleagues illustrate different approaches for modeling the charge transfer of an amorphous phase of DPBIC, a compound used in hole-conducting and electron-blocking layers in blue phosphorescent organic light-emitting diodes. In particular, it is shown how material properties such as density, radial distribution functions, ionization potentials and electron affinities, energetic disorder, charge mobility, and current–voltage characteristics can be extracted from simulations.

To develop high-capacitance flexible solid-state supercapacitors and explore its application in self-powered electronics is one of ongoing research topics. In this study, self-stacked solvated graphene (SSG) films are reported that have been prepared by a facile vacuum filtration method as the free-standing electrode for flexible solid-state supercapacitors. The highly hydrated SSG films have low mass loading, high flexibility, and high electrical conductivity. The flexible solid-state supercapacitors based on SSG films exhibit excellent capacitive characteristics with a high gravimetric specific capacitance of 245 F g⁻¹ and good cycling stability of 10 000 cycles. Furthermore, the flexible solid-state supercapacitors are integrated with high performance perovskite hybrid solar cells (pero-HSCs) to build self-powered electronics. It is found that the solid-state supercapacitors can be charged by pero-HSCs and discharged from 0.75 V. These results demonstrate that the self-powered electronics by integration of the flexible solid-state supercapacitors with pero-HSCs have great potential applications in storage of solar energy and in flexible electronics, such as portable and wearable personal devices.

Self-powered electronics is demonstrated by integration of high-performance perovskite hybrid solar cells with flexible solid-state supercapacitors, which is based on self-stacked solvated graphene films and possess high capacitance and excellent mechanical properties. The self-powered electronics is further demonstrated to have great potential applications in storage of solar energy.

On page 1912, J. Huang, and co-workers develop a solution-processed organolead trihalide perovskite photodetector that combines a high photoconductive gain with a broad spectral response across the ultraviolet (UV) to the near-infrared (NIR). The hole traps at the top surface of the perovskites are exploited to boost the performance with an ingenious device design at extremely low driving voltage, which enables a compact integration with low-voltage circuits.

Hybrid organometal halide perovskites have been demonstrated to have outstanding performance as semiconductors for solar energy conversion. Further improvement of the efficiency and stability of these devices requires a deeper understanding of their intrinsic photophysical properties. Here, the structural and optical properties of high-quality single crystals of CH₃NH₃PbI₃ from room temperature to 5 K are investigated. X-ray diffraction reveals an extremely sharp transition at 163 K from a twinned tetragonal I4/mcm phase to a low-temperature phase characterized by complex twinning and possible frozen disorder. Above the transition temperature, the photoluminescence is in agreement with a band-edge transition, explaining the outstanding performances of the solar cells. Whereas below the transition temperature, three different excitonic features arise, one of which is attributed to a free-exciton and the other two to bound excitons (BEs). The BEs are characterized by a decay dynamics of about 5 μs and by a saturation phenomenon at high power excitation. The long lifetime and the saturation effect make us attribute these low temperature features to bound triplet excitons. This results in a description of the room temperature recombination as being due to spontaneous band-to-band radiative transitions, whereas a diffusion-limited behavior is expected for the low-temperature range.

Low-temperature photophysical investigations of CH₃NH₃PbI₃ single crystals indicate that the recombination in these perovskites is due to spontaneous band-to-band radiative transition at room temperature and to singlet-free-exciton and bound-triplet excitons below the phase transition temperature. The bound-triplet excitons are characterized by a decay dynamics of about 5 μs and by a saturation phenomenon due to many-body interactions.

A series of novel acceptor–donor–acceptor oligothiophenes terminally substituted with the 1-(1,1-dicyanomethylene)-cyclohex-2-ene (DCC) acceptor has been synthesized. Structural, thermal, optoelectronic, and photovoltaic properties of the π-extended DCCnTs (n = 1–4) are characterized and contrasted to the trends found for the series of parent dicyanovinyl (DCV)-substituted oligothiophenes DCVnT. The optoelectronic properties reveal the influence of the additional exocyclic, sterically fixed double bonds in trans-configuration in the novel DCCnT derivatives. A close correspondence for derivatives with equal number of double bonds, that is, DCCnTs and DCV(n + 1)Ts, is identified. Despite having the same energy gap, the energy levels of the frontier orbitals, HOMO and LUMO, for the DCC-derivatives are raised and more destabilized due to the aromatization energy of a thiophene ring versus two exocyclic double bonds indicating improved donor and reduced acceptor strength. DCC-terthiophenes give good photovoltaic performance as donor materials in vacuum-processed solar cells (power conversion efficiencies ≤ 4.4%) clearly outperforming all comparable DCV4T derivatives.

A new series of acceptor-substituted oligothiophenes (DCCnT) is investigated. Structural, thermal, optoelectronic, and photovoltaic properties are contrasted to dicyanovinylene-capped oligothiophenes (DCVnT). Melting temperatures and solubilities are significantly enhanced for the DCCnTs versus DCVnTs. Oligomers with equal numbers of double bonds, show very similar absorption profiles. In vacuum-processed planar heterojunction solar cells, DCC-terthiophenes DCC3T and DCC3T-Me show superior photovoltaic parameters compared to conjugated corresponding DCV-quaterthiophenes.

The charge photogeneration and recombination processes in organic photovoltaic solar cells based on blend of the low bandgap copolymer, PTB7 (fluorinated poly-thienothiophene-benzodithiophene) with C60-PCBM using optical, electrical, and magnetic measurements in thin films and devices is studied. A variety of steady state optical and magneto-optical techniques were employed, such as photoinduced absorption (PA), magneto-PA, doping-induced absorption, and PA-detected magnetic resonance (PADMR); as well as picosecond time-resolved PA. The charge polarons and triplet exciton dynamics in films of pristine PTB7, PTB7/fullerene donor–acceptor (D–A) blend is followed. It is found that a major loss mechanism that limits the power conversion efficiency (PCE) of PTB7-based solar cell devices is the “back reaction” that leads to triplet exciton formation in the polymer donor from the photogenerated charge-transfer excitons at the D–A interfaces. A method of suppressing this “back reaction” by adding spin½ radicals Galvinoxyl to the D–A blend is presented; this enhances the cell PCE by ≈30%. The same method is not effective for cells based on PTB7/C70-PCBM blend, where high PCE is reached even without Galvinoxyl radical additives.

Charge transfer process in an organic photovoltaic (OPV) cell is studied in thin films and devices of a low bandgap polymer. Major loss in copolymer-based OPV devices is the formation of triplet excitons in the polymer donor from ³CT at the donor–acceptor interfaces. A method is presented to circumvent this process by incorporating spin ½ additives.

Photocharge generation mechanisms in perovskite solar cells are examined by T.-W. Ng, C.-S. Lee, and co-workers on page 1213. It is found that perovskites/C60 is in fact an inert N–N junction providing no driving force for charge separation. Photovoltaic effects in perovskite solar cells are attributed to direct free-carrier generation within the perovskite film.

Motivated by the possibility of modifying energy levels of a molecule without substantially changing its band gap, the impact of gradual fluorination on the optical and structural properties of zinc phthalocyanine (F_nZnPc) thin films and the electronic characteristics of F_nZnPc/C₆₀ (n = 0, 4, 8, 16) bilayer cells is investigated. UV–vis measurements reveal similar Q- and B-band absorption of F_nZnPc thin films with n = 0, 4, 8, whereas for F₁₆ZnPc a different absorption pattern is detected. A correlation between structure and electronic transport is deduced. For F₄ZnPc/C₆₀ cells, the enhanced long range order supports fill factors of 55% and an increase of the short circuit current density by 18%, compared to ZnPc/C₆₀. As a parameter being sensitive to the organic/organic interface energetics, the open circuit voltage is analyzed. An enhancement of this quantity by 27% and 50% is detected for F₄ZnPc- and F₈ZnPc-based devices, respectively, and is attributed to an increase of the quasi-Fermi level splitting at the donor/acceptor interface. In contrast, for F₁₆ZnPc/C₆₀ a decrease of the open circuit voltage is observed. Complementary photoelectron spectroscopy, external quantum efficiency, and photoluminescence measurements reveal a different working principle, which is ascribed to the particular energy level alignment at the interface of the photoactive materials.

The impact of gradually fluorinated zinc phthalocyanine molecules on the photophysical properties of F_n ZnPc/C₆₀ solar cells is investigated. Upon increasing fluorination, distinct variations of the cell parameters such as the open circuit voltage are observed. By combining complementary structural and electron spectroscopy analyses, this yields a detailed picture of the relevant D/A interface energetics and processes on microscopic length scales.

The use of metal oxide interlayers in polymer solar cells has great potential because metal oxides are abundant, thermally stable, and can be used in flexible devices. Here, a layer-by-layer (LbL) protocol is reported as a facile, room-temperature, solution-processed method to prepare electron transport layers from commercial ZnO nanoparticles and polyacrylic acid (PAA) with a controlled and tunable porous structure, which provides large interfacial contacts with the active layer. Applying the LbL approach to bulk heterojunction polymer solar cells with an optimized ZnO layer thickness of ≈25 nm yields solar cell power-conversion efficiencies (PCEs) of ≈6%, exceeding the efficiency of amorphous ZnO interlayers formed by conventional sputtering methods. Interestingly, annealing the ZnO/PAA interlayers in nitrogen and air environments in the range of 60–300 °C reduces the device PCEs by almost 20% to 50%, indicating the importance of conformational changes inherent to the PAA polymer in the LbL-deposited films to solar cell performance. This protocol suggests a new fabrication method for solution-processed polymer solar cell devices that does not require postprocessing thermal annealing treatments and that is applicable to flexible devices printed on plastic substrates.

A unique approach to fabricating bulk heterojunction polymer solar cells consisting of an electron transport layer of ZnO nanoparticles and polyacrylic acid prepared by a layer-by-layer technique is described. With spin-coated active layers of poly(benzo[1,2-b:4,5-b′]dithiophene-thieno[3,4-c]pyrrole-4,6-dione and [6,6]-phenyl C₆₁ butyric acid methyl ester on a 25-nm-thick ZnO/polyacrylic acid layer, the power-conversion efficiency of the solar cell is ≈6%, exceeding that of ZnO interlayers formed by sputtering.