wangzhaowei

A new metal-oxide-based interconnecting layer (ICL) structure of all-solution processed metal oxide/dipole layer/metal oxide for efficient tandem organic solar cell (OSC) is demonstrated. The dipole layer modifies the work function (WF) of molybdenum oxide (MoO _x ) to eliminate preexisted counter diode between MoO _x and TiO₂. Three different amino functionalized water/alcohol soluble conjugated polymers (WSCPs) are studied to show that the WF tuning of MoO _x is controllable. Importantly, the results show that S-shape current density versus voltage (J–V) characteristics form when operation temperature decreases. This implies that thermionic emission within the dipole layer plays critical role for helping recombination of electrons and holes. Meanwhile, the insignificant homotandem open-circuit voltage (V _oc) loss dependence on dipole layer thickness shows that the quantum tunneling effect is weak for efficient electron and hole recombination. Based on this ICL, poly(3-hexylthiophene) (P3HT)-based homotandem OSC with 1.20 V V _oc and 3.29% power conversion efficiency (PCE) is achieved. Furthermore, high efficiency poly(4,8-bis(5-(2-ethylhexyl)-thiophene-2-yl)-benzo[1,2-b54,5-b9]dithiophene-alt alkylcarbonylthieno[3,4-b]thiophene) (PBDTTT-C-T)-based homotandem OSC with 1.54 V V _oc and 8.11% PCE is achieved, with almost 15.53% enhancement compared to its single cell. This metal oxide/dipole layer/metal oxide ICL provides a new strategy to develop other qualified ICL with different hole transporting layer and electron transporting layer in tandem OSCs.

A new all-solution processed metal oxide/dipole layer/metal oxide interconnecting layer (ICL) for efficient tandem organic solar cell (OSC) is demonstrated. The dipole layer modifies the work function of molybdenum oxide to eliminate a pre-existing counter diode between MoO_x and TiO₂. Based on this ICL, homotandem OSCs with doubled open-circuit voltage and 15.53% enhancement of power conversion efficiency from a single OSC are achieved.

A efficient indium tin oxide (ITO)-free transparent electrode based on an improved Ag film is designed by introducing small amount of Al during co-deposition, producing ultrathin and smooth Ag film with low loss. A transparent electrode as thin as 4 nm is achieved by depositing the film on top of Ta₂O₅ layer, and organic solar cells based on such ultrathin electrode are built, producing power conversion efficiency over 7%. The device efficiency can be optimized by simply tuning Ta₂O₅ layer thickness external to the organic photovoltaic (OPV) structure to create an optical cavity resonance inside the photoactive layer. Therefore Ta₂O₅/Al-doped Ag films function as a high-performance electrode with high transparency, low resistance, improved photon management capability and mechanical flexibility.

A high-performance Ta₂O₅/Al-doped Ag electrode is presented by inserting a di­electric layer of Ta₂O₅ underneath an ultrathin Al-doped Ag film. Such an Al-doped Ag can be as thin as 4 nm with excellent transparency. Fine light management by tuning the Ta₂O₅ thickness leads to enhancement of photocurrent and power conversion efficiency for the Ta₂O₅/Al-doped Ag-based organic solar cells due to the optical cavity effect, superior to their indium tin oxide counterparts.

In organic solar cells (OSCs), the necessity of UV activation that comes with the use of ZnO- and TiO_x-based electron extraction layers (EELs) can be avoided by using tin oxide (SnO_x), which can be prepared at temperatures as low as 80 °C. In contrast to devices based on TiO_x and ZnO, OSCs comprising SnO_x as the EEL show well-behaved solar cell characteristics with a high fill factor (FF) and high efficiency, even without the UV spectral range of the AM1.5 solar spectrum.

Significant efficiency improvements are reported in mesoscopic perovskite solar cells based on the development of a low-temperature solution-processed ZnO nanorod (NR) array exhibiting higher NR aspect ratio, enhanced electron density, and substantially reduced work function than conventional ZnO NRs. These features synergistically result in hysteresis-free, scan-independent, and stabilized devices with an efficiency of 16.1%. Electron-rich, nitrogen-doped ZnO (N:ZnO) NR-based electron transporting materials (ETMs) with enhanced electron mobility produced using ammonium acetate show consistently higher efficiencies by one to three power points than undoped ZnO NRs. Additionally, the preferential electrostatic interaction between the ­nonpolar facets of N:ZnO and the conjugated polyelectrolyte polyethylenimine (PEI) has been relied on to promote the hydrothermal growth of high aspect ratio NR arrays and substantially improve the infiltration of the perovskite light absorber into the ETM. Using the same interactions, a conformal PEI coating on the electron-rich high aspect ratio N:ZnO NR arrays is ­successfully applied, resulting in a favorable work function shift and altogether leading to the significant boost in efficiency from <10% up to >16%. These results largely surpass the state-of-the-art PCE of ZnO-based perovskite solar cells and highlight the benefits of synergistically combining mesoscale control with doping and surface modification.

Enhanced power conversion efficiency of 16.1% is reported in hysteresis-free perovskite solar cells when an electron-rich polymer nanolayer is conformally coated on the surface of electron-collecting, nitrogen-doped, high-aspect-ratio ZnO nanorods.

A scaling effort on perovskite solar cells is presented where the device manufacture is progressed onto flexible substrates using scalable techniques such as slot-die roll coating under ambient conditions. The printing of the back electrode using both carbon and silver is essential to the scaling effort. Both normal and inverted device geometries are explored and it is found that the formation of the correct morphology for the perovskite layer depends heavily on the surface upon which it is coated and this has significant implications for manufacture. The time it takes to form the desired layer morphology falls in the range of 5–45 min depending on the perovskite precursor, where the former timescale is compatible with mass production and the latter is best suited for laboratory work. A significant loss in solar cell performance of around 50% is found when progressing to using a fully scalable fabrication process, which is comparable to what is observed for other printable solar cell technologies such as polymer solar cells. The power conversion efficiency (PCE) for devices processed using spin coating on indium tin oxide (ITO)-glass with evaporated back electrode yields a PCE of 9.4%. The same device type and active area realized using slot-die coating on flexible ITO-polyethyleneterphthalate (PET) with a printed back electrode gives a PCE of 4.9%.

Upscaling of perovskite solar cells is the obvious next step to assess the true potential of perovskite solar cells. An upscaling process for both normal and inverted geometry perovskite solar cells is demonstrated, going from nonscalable state-of-the-art laboratory fabrication procedures to complete roll-to-roll compatible fabrication in ambient conditions, using fabrication methods tested for true scalability.

Length of the terminal alkyl chains at dicyanovinyl (DCV) groups of two dithienosilole (DTS) containing small molecules (DTS(Oct)₂-(2T-DCV-Me)₂ and DTS(Oct)₂-(2T-DCV-Hex)₂ ) is investigated to evaluate how this affects the molecular solubility and blend morphology as well as their performance in bulk heterojunction organic solar cells (OSCs). While the DTS(Oct)₂-(2T-DCV-Me)₂ (a solubility of 5 mg mL⁻¹) system exhibits both high short circuit current density (J _sc) and high fill factor, the DTS(Oct)₂-(2T-DCV-Hex)₂ (a solubility of 24 mg mL⁻¹) system in contrast suffers from a poor blend morphology as examined by atomic force morphology and grazing incidence X-ray scattering measurements, which limit the photovoltaic properties. The charge generation, transport, and recombination dynamics associated with the limited device performance are investigated for both systems. Nongeminate recombination losses in DTS(Oct)₂-(2T-DCV-Hex)₂ system are demonstrated to be significant by combining space charge limited current analysis and light intensity dependence of current–voltage characteristics in combination with photogenerated charge carrier extraction by linearly increasing voltage and transient photovoltage measurements. DTS(Oct)₂-(2T-DCV-Me)₂ in contrast performs nearly ideal with no evidence of nongeminate recombination, space charge effects, or mobility limitation. These results demonstrate the importance of alkyl chain engineering for solution-processed OSCs based on small molecules as an essential design tool to overcome transport limitations.

The length of the terminal alkyl chains at dicyanovinyl groups of two dithienosilole containing small molecules is investigated to evaluate how such parameter influences the molecular solubility, blend morphology, and transport limitations as well as their photovoltaic performance in bulk heterojunction solar cells.

In article number 1500204, Aram Amassian and co-workers demonstrate the preparation of highly efficient polymer solar cells on rigid glass and flexible polyethylene terephthalate (PET) substrates using a facile and low-temperature solution processed Al:ZnO (AZO) nanocrystalline buffer layer prepared in a single step, without requiring any surface passivation. Efficiencies of 10.2% and 8.2% are reported for glass and plastic substrates, respectively.

A power conversion efficiency of 11.8% is demonstrated for planar perovskite solar cell based on acid water-free poly(3,4-ethylenedioxythiophene) (PEDOT) as the hole-transporting layer. This efficiency is further improved to 14.2% using a pH neutralized PEDOT, which reduces the sub-gap defects at the surface of perovskite. All the active layers reported are solution-processed at temperatures below 140 °C making it compatible with roll-to-roll production.

To increase the efficiency of bulk heterojunction (BHJ) solar cells beyond 15%, 300 nm thick devices with 0.8 fill factor (FF) and external quantum efficiency (EQE) >90% are likely needed. This work demonstrates that numerical device simulators are a powerful tool for investigating charge-carrier transport in BHJ devices and are useful for rapidly determining what semiconductor pro­perties are needed to reach these performance milestones. The electron and hole mobility in a BHJ must be ≈10⁻² cm² V⁻¹ s⁻¹ in order to attain a 0.8 FF in a 300 nm thick device with the recombination rate constant of poly(3-hexyl­thiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM). Thus, the hole mobility of donor polymers needs to increase from ≈10⁻⁴ to ≈10⁻² cm² V⁻¹ s⁻¹ in order to significantly improve device performance. Furthermore, the charge-carrier mobility required for high FF is directly proportional to the BHJ recombination rate constant, which demonstrates that decreasing the recombination rate constant could dramatically improve the efficiency of optically thick devices. These findings suggest that researchers should prioritize improving charge-carrier mobility when synthesizing new materials for BHJ solar cells and highlight that they should aim to understand what factors affect the recombination rate constant in these devices.

Recombination in bulk heterojunction solar cells is examined using a combination of experiments and a numerical device simulator. Significant improvements in charge-carrier mobility (μ > 10⁻² cm² V⁻¹ s⁻¹) and decreases in the recombination rate constant (k < 10⁻¹³ cm³ s⁻¹) are needed to achieve a single junction bulk heterojunction solar cell with external quantum efficiency >90% and fill factor >0.8.

Long range electromigration of methylammonium ions (MA⁺) in methyl ammonium lead tri-iodide (MAPbI₃) film is observed directly using the photo­thermal induced resonance technique. The electromigration of MA⁺ leads to the formation of a lateral p-i-n structure, which is the origin of the switchable photovoltaic effect in MAPbI₃ perovskite devices.

Organolead halide perovskites currently are the new front-runners as light absorbers in hybrid solar cells, as they combine efficiencies passing already 20% with deposition temperatures below 100 °C and cheap solution-based fabrication routes. Long-term stability remains a major obstacle for application on an industrial scale. Here, it is demonstrated that significant decomposition effects already occur during annealing of a methylammonium lead triiode perovskite at 85 °C even in inert atmosphere thus violating international standards. The observed behavior supports the view of currently used perovskite materials as soft matter systems with low formation energies, thus representing a major bottleneck for their application, especially in countries with high average temperatures. This result can trigger a broader search for new perovskite families with improved thermal stability.

It is shown that significant degradation effects of CH₃NH₃PbI₃ already occur at 85 °C, independent of the atmosphere, where humidity is a catalyst but not the origin of the degradation. This represents an important message for the perovskite community and leads to recommendations for future perovskite developments.

A strategy for developing a novel donor–π–acceptor conducting polymeric hole transport material (TTB–TTQ) based on thiophene and benzothiadiazole as an alternative to spiro-MeOTAD is reported. The resulting polymer is highly soluble in many organic solvents and exhibits excellent film formability. The addition of lithium bis(trifluoromethanesulfonyl) imide salt and tert-butylpyridine to TTB–TTQ results in a rough film surface with a fibril structure and improved charge transport. A perovskite solar cell with the highest power conversion efficiency (η) yet achieved in such cells, 14.1%, which is 22.6% greater than that of a device employing a spiro-MeOTAD is demonstrated. This strategy provides a novel approach to developing solar cell materials for efficient perovskite solar cells.

A strategy for developing a novel donor–π–acceptor conducting polymeric hole transport material (TTB–TTQ) based on thiophene and benzothiadiazole is reported, increasing highest occupied molecular orbital energy level and then increasing solubility by breaking symmetry. The resulting polymer exhibits excellent film formability, showing highest power conversion efficiency (14.1%) in a perovskite solar cell.

Highly efficient, indium tin oxide (ITO)-free and flexible organic photovoltaic cells are demonstrated with 10.4% power conversion efficiency (PCE) through the innovative integration of ultrathin metal film electrode, conductive fullerene interfacial layer, and efficient active blend into a microcavity-embedded device. In addition, a high PCE of 7.21% is achieved for the upsized devices with an active area of 1 cm².

The photoinduced open-circuit voltage (V_oc) loss commonly observed in bulk heterojunction organic solar cells made from amorphous polymers is investigated. It is observed that the total charge carrier density and, importantly, the recombination dynamics are unchanged by photoinduced burn-in. Charge extraction is used to monitor changes in the density of states (DOS) during degradation of the solar cells, and a broadening over time is observed. It is proposed that the V_oc losses observed during burn-in are caused by a redistribution of charge carriers in a broader DOS. The temperature and light intensity dependence of the V_oc losses can be described with an analytical model that contains the amount of disorder broadening in a Gaussian DOS as the only fit parameter. Finally, the V_oc loss in solar cells made from amorphous and crystalline polymers is compared and an increased stability observed in crystalline polymer solar cells is investigated. It is found that solar cells made from crystalline materials have a considerably higher charge carrier density than those with amorphous materials. The effects of a DOS broadening upon aging are suppressed in solar cells with crystalline materials due to their higher carrier density, making crystalline materials more stable against V_oc losses during burn-in.

Crystalline solar cell materials with high charge carrier density are more stable against disorder-induced open-circuit voltage (V_oc) losses. Light-induced traps increase energetic disorder and cause V_oc losses in amorphous materials that have low charge carrier densities. A significant change in recombination dynamics is not present after the formation of light-induced defects.

Inverted polymer solar cells with a maximum efficiency of 9.23% are fabricated using a dual functional fullerene zwitterion as the cathode interlayer. C₆₀-sulfobetaine acts as an electron acceptor and cathode modification layer. Exceptional insensitivity to interlayer thickness is found with efficiencies exceeding 8%. Slot-die coated solar cells with an organic cathode interlayer have efficiencies of 7.38%.

In article number 1500038, Baoquan Sun and co-workers report the development of a novel sandwiched structure of TiOx/gold nanoparticles (Au NPs)/TiOx to remedy the trap states in low-temperature-processed TiOx films. Hot carriers are injected from Au NPs to the TiOx film and reduce the trap states under solar illumination; this results in a champion perovskite solar cell with the power conversion efficiency of 16.2%.

In article number 1402186, Eva Bundgaard and co-workers present a large study via which a library of conjugated polymers is developed to identify lead candidates that are suitable for indium tin oxide (ITO)-free, flexible polymer solar cells that are fully printed under ambient conditions. Based on several characteristics, 13 lead candidates are found, out of the 104 synthesized polymers. The variation in light absorption (color) of the 104 synthesized polymers is shown on the cover. Cover picture by Markus Hösel.

Light soaking and hysteresis phenomena can reveal the bulk and interfacial polarization effects on photovoltaic processes in perovskite solar cells. The frequency-dependent impedance and time-dependent photoluminescence measurements indicate that bulk and interfacial polarizations are internally coupled in developing short-circuit current, open-circuit voltage, and fill factor through charge dissociation, transport, and collection in light soaking and hysteresis phenomena.

Polymer solar cells are conventionally processed by coating a multicomponent mixture containing polymer, fullerene, solvent, and cosolvent. The photovoltaic performance strongly depends on the nanoscale morphology of the blend, which is largely determined by the precise nature of the solvent composition and drying conditions. Here, an alternative processing route is investigated in which the two active layer components are deposited sequentially via spin coating or doctor blading. Spin coating the fullerene from o-dichlorobenzene on top of the polymer provides virtually identical morphologies and photovoltaic performance. Using blade coating, the influence of the second-layer solvent for the fullerene derivative is investigated in further detail. Different aromatic solvents are compared regarding swelling of the polymer layer, fullerene solubility, and evaporation rate. It is found that while swelling of the polymer by the second-layer solvent is a necessity for sequential processing, the solubility of the fullerene derivative in this solvent has the strongest influence on solar cell performance. Homogeneous layers in which a sufficient amount of fullerene can infiltrate the polymer film can only be achieved when solvents are used that have a very high solubility for the fullerene and swell the polymer layer.

Sequential processing of polymer and fullerene layers produces efficient bulk-heterojunction solar cells when the second-layer solvent is able to swell the deposited polymer layer and has a high solubility for the fullerene. Without either of these properties, the efficiency remains low because the amount of fullerene that can infiltrate into the polymer layer is insufficient to create the optimal nanomorphology.