14.2% Efficiency Dye‐Sensitized Solar Cells by Co‐sensitizing Novel Thieno[3,2‐b]indole‐Based Organic Dyes with a Promising Porphyrin Sensitizer

A new series of 4‐hexyl‐4H‐thieno[3,2‐b]indole (HxTI) based organic chromophores is developed by structural engineering of the electron donor (D) group in the D–HxTI–benzothiadiazole‐phenyl‐acceptor platform with different fluorenyl moieties, such as unsubstituted fluorenyl (SGT‐146) and hexyloxy (SGT‐147), decyloxy (SGT‐148) and hexyloxy‐phenyl substituted (SGT‐149) fluorenyl moieties. In comparison to a reference dye SGT‐137 with a biphenyl‐based donor, the effects of the donating ability and bulkiness of the fluorenyl based donor in this D–π–A‐structured platform on molecular properties and photovoltaic performance are investigated to establish the structure–property relationship. The photovoltaic performance of dye‐sensitized solar cells (DSSCs) is improved according to the bulkiness of the donor groups. As a result, the DSSCs based on SGT‐149 show high power conversion efficiencies (PCEs) of 11.7% and 10.0% with a [Co(bpy)₃]^2+/3+ (bpy = 2,2′‐bipyridine) and an I^?/I₃^? redox electrolyte, respectively. Notably, the co‐sensitization of SGT‐149 with a SGT‐021 porphyrin dye by utilizing a simple “cocktail” method, exhibit state‐of‐the‐art PCEs of 14.2% and 11.6% with a [Co(bpy)₃]^2+/3+ and an I^?/I₃^? redox electrolyte, respectively.     

Thieno[3,2‐b][1]benzothiophene Derivative as a New π‐Bridge Unit in D–π–A Structural Organic Sensitizers with Over 10.47% Efficiency for Dye‐Sensitized Solar Cells

Three new thieno[3,2‐b][1]benzothiophene ( TBT )‐based donor–π–acceptor (D–π–A) sensitizers, coded as SGT ‐ 121 , SGT ‐ 129 , and SGT ‐ 130 , have been designed and synthesized for dye‐sensitized solar cells (DSSCs), for the first time. The TBT , prepared by fusing thiophene unit with the phenyl unit of triphenylamine donor, is utilized as the π‐bridge for all sensitizers with good planarity. They have been molecularly engineered to regulate the highest occupied molecular orbital (HOMO)‐lowest unoccupied molecular orbital (LUMO) energy levels and extend absorption range as well as to control the electron‐transfer process that can ensure efficient dye regeneration and prevent undesired electron recombination. The photovoltaic performance of SGT‐sensitizer‐based DSSCs employing Co(bpy)₃^2+/3+ (bpy = 2,2′‐bipyridine) redox couple is systematically evaluated in a thorough comparison with Y123 as a reference sensitizer. Among them, SGT ‐ 130 with benzothiadiazole‐phenyl ( BTD ‐ P ) unit as an auxiliary acceptor exhibits the highest power‐conversion efficiency (PCE) of 10.47% with J_sc = 16.77 mA cm^?2, V_oc = 851 mV, and FF = 73.34%, whose PCE is much higher than that of Y123 (9.5%). It is demonstrated that the molecular combination of each fragment in D–π–A organic sensitizers can be a pivotal factor for achieving the higher PCEs and an innovative strategy for strengthening the drawbacks of the π‐bridge.     

Ligand‐Assisted Assembly Approach to Synthesize Large‐Pore Ordered Mesoporous Titania with Thermally Stable and Crystalline Framework

A novel ligand‐assisted assembly approach is demonstrated for the synthesis of thermally stable and large‐pore ordered mesoporous titanium dioxide with a highly crystalline framework by using diblock copolymer poly(ethylene oxide)‐b‐polystyrene (PEO‐b‐PS) as a template and titanium isopropoxide (TIPO) as a precursor. Small‐angle X‐ray scattering, X‐ray diffraction (XRD), transmission electron microscopy (TEM), high‐resolution scanning electron microscopy, and N₂‐sorption measurements indicate that the obtained TiO₂ materials possess an ordered primary cubic mesostructure with large, uniform pore diameters of about 16.0 nm, and high Brunauer–Emmett–Teller surface areas of ～112 m² g^?1, as well as high thermal stability (～700 °C). High resolution TEM and wide‐angle XRD measurements clearly illustrate the high crystallinity of the mesoporous titania with an anatase structure in the pore walls. It is worth mentioning that, in this process, in addition to tetrahydrofuran as a solvent, acetylacetone was employed as a coordination agent to avoid rapid hydrolysis of the titanium precursor. Additionally, stepped evaporation and heating processes were adopted to control the condensation rate and facilitate the assembly of the ordered mesostructure, and ensure the formation of fully polycrystalline anatase titania frameworks without collapse of the mesostructure. By employing the obtained mesoporous and crystallized TiO₂ as the photoanode in a dye‐sensitized solar cell, a high power‐conversion efficiency (5.45%) can be achieved in combination with the N719 dye, which shows that this mesoprous titania is a great potential candidate as a catalyst support for photonic‐conversion applications.     

Solid‐State Dye‐Sensitized Solar Cells Using a Novel Class of Ullazine Dyes as Sensitizers

Here we present the photovoltaic performance of solid‐state dye‐sensitized solar cells (DSCs) using a series of ullazine‐based metal‐free organic sensitizers and spiro‐MeOTAD as a hole‐transport material. A maximum of 4.95% power conversion efficiency measured under standard AM 1.5G illumination (100 mW cm^?2) was achieved with the best performing ullazine dye, and was further improved to 5.40% through co‐sensitization with the triphenylamine‐based organic sensitizer, D35. This study investigates the effect of the molecular structure of the ullazine sensitizer on the performance in solid‐state DSCs.     

Mesoporous TiO2 Beads Offer Improved Mass Transport for Cobalt‐Based Redox Couples Leading to High Efficiency Dye‐Sensitized Solar Cells

Overcoming ionic diffusion limitations is essential for the development of high‐efficiency dye‐sensitized solar cells based on cobalt redox mediators. Here, improved mass transport is reported for photoanodes composed of mesoporous TiO₂ beads of varying pore sizes and porosities in combination with the high extinction YD2‐o‐C8 porphyrin dye. Compared to a photoanode made of 20 nm‐sized TiO₂ particles, electrolyte diffusion through these films is greatly improved due to the large interstitial pores between the TiO₂ beads, resulting in up to 70% increase in diffusion‐limited current. Simultaneously, transient photocurrent measurements reveal no mass transport limitations for films of up to 10 μm thickness. In contrast, standard photoanodes made of 20 nm‐sized TiO₂ particles show non‐linear behavior in photocurrent under 1 sun illumination for a film thickness as low as 7 μm. By including a transparent thin mesoporous TiO₂ underlayer in order to reduce optical losses at the fluorine‐doped tin oxide (FTO)‐TiO₂ interface, an efficiency of 11.4% under AM1.5G 1 sun illumination is achieved. The combination of high surface area, strong scattering behavior, and high porosity makes these mesoporous TiO₂ beads particularly suitable for dye‐sensitized solar cells using bulky redox couples and/or viscous electrolytes.     

Donor‐Acceptor Interfacial Interactions Dominate Device Performance in Hybrid P3HT‐ZnO Nanowire‐Array Solar Cells

The adsorption of self‐assembled monolayers (SAMs) on metal oxide surfaces is a promising route to control electronic characteristics and surface wettability. Here, arylphosphonic acid derivatives are used to modulate the surface properties of vertically oriented ZnO nanowire arrays. Arylphosphonate‐functionalized ZnO nanowires are incorporated into hybrid organic‐inorganic solar cells in which infiltrated poly(3‐hexylthiophene) (P3HT) serves as the polymer donor. Strong correlations between device short‐circuit current density (J _sc) and power conversion efficiencies (PCEs) with ZnO surface functionalization species are observed and a weak correlation in the open‐circuit voltage (V _oc) is observed. Inverted solar cells fabricated with these treated interfaces exhibit PCEs as high as 2.1%, primarily due to improvements in J _sc. Analogous devices using untreated ZnO arrays having efficiencies of 1.6%. The enhancement in J _sc is attributed to surface passivation of ZnO by SAMs and enhanced wettability from P3HT, which improve charge transfer and reduce carrier recombination at the organic‐inorganic interface in the solar cells.     

Nanowire‐Based Three‐Dimensional Transparent Conducting Oxide Electrodes for Extremely Fast Charge Collection

A 3D transparent conducting oxide (3D‐TCO) has been fabricated by growing Sn‐doped indium oxide (ITO) nanowire arrays on glass substrates via a vapor transport method. The 3D TCO charge‐collection properties have been compared to those of conventional two‐dimensional TCO (2D‐TCO) thin films. For use as a photoelectrode in dye‐sensitized solar cells, ITO‐TiO₂ core‐shell nanowire arrays were prepared by depositing a 45 nm‐thick mesoporous TiO₂ shell layer consisting of ～6 nm anatase nanoparticles using TiCl₄ treatments. Dye‐sensitized solar cells fabricated using these ITO‐TiO₂ core‐shell nanowire arrays show extremely fast charge collection owing to the shorter electron paths across the 45 nm‐thick TiO₂ shell compared to the 2D TCO. Interestingly, the charge‐collection time does not increase with the overall electrode thickness, which is counterintuitive to conventional diffusion models. This result implies that, in principle, maximum light harvesting can be achieved without hindering the charge collection. The proposed new 3D TCO should also be attractive for other photovoltaic applications where the active layer thickness is limited by poor charge collection.     

Titanium Dioxide Mesoporous Electrodes for Solid‐State Dye‐Sensitized Solar Cells: Cross‐Analysis of the Critical Parameters

We report a comparative study on the use of four different mesoporous titanium dioxide (TiO₂) photo‐electrodes for the fabrication of solid‐state dye‐sensitized solar cells (sDSSCs). The photovoltaic parameters of the device correlate with several intrinsic properties of the film, based not only on its morphological features, as commonly considered in standard characterizations, but also on the transport and the electronic properties of the photo‐electrode. These properties differ significantly for TiO₂ electrodes processed using different colloidal pastes, and are decisive for the photovoltaic efficiency, ranging from 3.7% up to 5.1%. In particular, the dielectric permittivity of each mesoporous layer (ε_eff) and the number of traps (N_t) determined by the space‐charge‐limited current (SCLC) theory are found to be a bottle‐neck for the charge transport, greatly influencing the fill factor (FF) and open circuit voltage (V_oc) of the cells. In addition, a direct correlation between TiO₂ surface potential with the V_oc was established. Cross‐analysis of key macroscopic parameters of the films prior to integration in the devices, in particular focusing on the determination of the capacitance and surface potential shift of the TiO₂ mesoporous anode, represents a straightforward yet powerful method to screen and select the most suitable TiO₂ for applications in sDSSCs.     

N‐Annulated Perylene as a Coplanar π‐Linker Alternative to Benzene as a Low Energy‐Gap,Metal‐Free Dye in Sensitized Solar Cells

Perylenes are well‐known pigments with excellent chemical, thermal, and photochemical stabilities and have been used in various optical and electronic fields. Although for sensitized mesoscopic solar cells there is rapid progress of metal‐free thiophene dyes, which now reach over 11.5% power conversion efficiency (PCE) at air mass 1.5 global (AM1.5G) conditions, the so far reported highest PCE of a perylene dye is only 6.8%. Here, a new metal‐free organic donor‐acceptor (D‐A) dye ( C261 ) featuring a bisarylamino functionalized N‐annulated perylene electron‐releasing segment and a cyanoacrylic acid electron‐withdrawing unit is synthesized. Combining a mesoporous titania film grafted by this structurally simple perylene dye with a non‐corrosive cobalt redox shuttle, an 8.8% PCE is achieved at an irradiance of the AM1.5G sunlight. By selecting the model dye G221 as a reference, theoretical calculations, steady‐state and time‐resolved spectroscopies, and electrical measurements are used to compare the energy‐levels, light absorptions, and mutichannel charge transfer dynamics that contribute to the photovoltaic behavior.     

TiO2 Nanocrystals Synthesized by Laser Pyrolysis for the Up‐Scaling of Efficient Solid‐State Dye‐Sensitized Solar Cells

A crucial issue regarding emerging nanotechnologies remains the up‐scaling of new functional nanostructured materials towards their implementation in high performance applications on a large scale. In this context, we demonstrate high efficiency solid‐state dye‐sensitized solar cells prepared from new porous TiO₂ photoanodes based on laser pyrolysis nanocrystals. This strategy exploits a reduced number of processing steps as well as non‐toxic chemical compounds to demonstrate highly porous TiO₂ films. The possibility to easily tune the TiO₂ nanocrystal physical properties allows us to demonstrate all solid‐state dye‐sensitized devices based on a commercial benchmark materials (organic indoline dye and molecular hole transporter) presenting state‐of‐the‐art performance comparable with reference devices based on a commercial TiO₂ paste. In particular, a drastic improvement in pore infiltration, which is found to balance a relatively lower surface area compared to the reference electrode, is evidenced using laser‐synthesized nanocrystals resulting in an improved short‐circuit current density under full sunlight. Transient photovoltage decay measurements suggest that charge recombination kinetics still limit device performance. However, the proposed strategy emphasizes the potentialities of the laser pyrolysis technique for up‐scaling nanoporous TiO₂ electrodes for various applications, especially for solar energy conversion.     

Recent Progress in Dye‐Sensitized Solar Cells Using Nanocrystallite Aggregates

Nanocrystallite aggregates are spherical assemblies of nanometer‐sized crystallites and feature a size on the order of sub‐micrometers. This paper reports and summarizes recent progress in nanocrystallite aggregates for applications in dye‐sensitized solar cells. It emphasizes that nanocrystallite aggregates are a promising class of materials with the capability to generate light scattering, enhance electron transport, retain high specific surface area for dye adsorption, and facilitate electrolyte diffusion while serving as the photoelectrode film of a dye‐sensitized solar cell. In the Perspectives section, it is suggested that optimization of the porosity of the aggregates, the facets of nanocrystallites forming the aggregates, and the structure of photoelectrode film could possibly lead to breakthroughs in improving the power conversion efficiency of the current state‐of‐the‐art dye‐sensitized solar cells.     

Cobalt Polypyridyl Complexes as Transparent Solution‐Processable Solid‐State Charge Transport Materials

Charge transport materials (CTMs) are traditionally inorganic semiconductors or metals. However, over the past few decades, new classes of solution‐processable CTMs have evolved alongside new concepts for fabricating electronic devices at low cost and with exceptional properties. The vast majority of these novel materials are organic compounds and the use of transition metal complexes in electronic applications remains largely unexplored. Here, a solution‐processable solid‐state charge transport material composed of a blend of [Co(bpyPY4)](OTf)₂ and Co(bpyPY4)](OTf)₃ where bpyPY4 is the hexadentate ligand 6,6′‐bis(1,1‐di(pyridin‐2‐yl)ethyl)‐2,2′‐bipyridine and OTf^? is the trifluoromethanesulfonate anion is reported. Surprisingly, these films exhibit a negative temperature coefficient of conductivity (dσ/dT) and non‐Arrhenius behavior, with respectable solid‐state conductivities of 3.0 S m^?1 at room temperature and 7.4 S m^?1 at 4.5 K. When employed as a CTM in a solid‐state dye‐sensitized solar cell, these largely amorphous, transparent films afford impressive solar energy conversion efficiencies of up to 5.7%. Organic–inorganic hybrid materials with negative temperature coefficients of conductivity generally feature extended flat π‐systems with strong π–π interactions or high crystallinity. The lack of these features promotes [Co(bpyPY4)](OTf)_{2+ x} films as a new class of CTMs with a unique charge transport mechanism that remains to be explored.     

Towards Long‐Term Photostability of Solid‐State Dye Sensitized Solar Cells

The solid‐state dye‐sensitized solar cell (DSSC) was introduced to overcome inherent manufacturing and instability issues of the electrolyte‐based DSSC and progress has been made to deliver high photovoltaic efficiencies at low cost. However, despite 15 years research and development, there still remains no clear demonstration of long‐term stability. Here, solid‐state DSSCs are subjected to the severe aging conditions of continuous illumination at an elevated temperature. A fast deterioration in performance is observed for devices encapsulated in the absence of oxygen. The photovoltaic performance recovers when re‐exposed to air. This reversible behavior is attributed to three related processes: i) the creation of light and oxygen sensitive electronic shunting paths between TiO₂ and the top metal electrode, ii) increased recombination at the TiO₂/organic interface, and iii) the creation of deep electron traps that reduce the photocurrent. The device deterioration is remedied by the formation of an insulating alumino‐silicate shell around the TiO₂ nanocrystals, which reduces interfacial recombination, and the introduction of an insulating mesoporous SiO₂ buffer layer between the top electrode and TiO₂, which acts as a permanent insulating barrier between the TiO₂ and the metal electrode, preventing shunting.     

Evaporation‐Free Nonfullerene Flexible Organic Solar Cell Modules Manufactured by An All‐Solution Process

To ensure laboratory‐to‐industry transfer of next‐generation energy harvesting organic solar cells (OSCs), it is necessary to develop flexible OSC modules that can be produced on a continuous roll‐to‐roll basis and to apply an all‐solution process. In this study, nonfullerene acceptors (NFAs)‐based donor polymer, SMD2, is newly designed and synthesized to continuously fabricate high‐performance flexible OSC modules. Also, multifunctional hole transport layers (HTLs), WO₃/HTL solar bilayer HTLs, are developed and applied via an all‐solution process called “ProcessOne” into inverted structure. SMD2, the donor terpolymer, has a deep highest occupied molecular orbital (HOMO) level and can achieve a power conversion efficiency (PCE) of 11.3% with NFAs without any pre‐/post‐treatment because of its optimal balance between crystallinity and miscibility. Furthermore, the integration of multifunctional HTLs enables the recovery of the drop in open circuit voltage (V_OC) caused by a mismatch in energy levels between the deep HOMO level of the NFAs‐based bulk‐heterojunction layer and the solution‐processed HTLs. Also, the photostability under ultraviolet‐exposure necessary for “ProcessOne” is greatly improved because of the integration of multifunctional HTLs. Consequently, because of the synergistic effects of these approaches, the flexible OSC modules fabricated in an industrial production line have a PCE of 5.25% (P_max = 419.6 mW) on an active area of 80 cm².     

Spray Deposition of Silver Nanowire Electrodes for Semitransparent Solid‐State Dye‐Sensitized Solar Cells

Transparent top electrodes for solid‐state dye‐sensitized solar cells (ssDSCs) allow for fabrication of mechanically stacked ssDSC tandems, partially transparent ssDSCs for building integration, and ssDSCs on metal foil substrates. A solution‐processed, highly transparent, conductive electrode based on PEDOT:PSS [poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate)] and spray‐deposited silver nanowires (Ag NWs) is developed as an effective top contact for ssDSCs. The electrode is solution‐deposited using conditions and solvents that do not damage or dissolve the underlying ssDSC and achieves high performance: a peak transmittance of nearly 93% at a sheet resistance of 18 Ω/square – all without any annealing that would harm the ssDSC. The role of the PEDOT:PSS in the electrode is twofold: it ensures ohmic contact between the ssDSC 2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenylamine)9,9′‐spirobifluorene (Spiro‐OMeTAD) overlayer and the silver nanowires and it decreases the series resistance of the device. Semitransparent ssDSCs with D35 dye fabricated using this Ag NW/PEDOT:PSS transparent electrode show power conversion efficiencies of 3.6%, nearly as high as a reference device using an evaporated silver electrode (3.7%). In addition, the semitransparent ssDSC shows high transmission between 700–1100 nm, a necessity for use in efficient tandem devices. Such an electrode, in combination with efficient ssDSCs or hybrid perovskite‐sensitized solar cells, can allow for the fabrication of efficient, cost‐effective tandem photovoltaics.     

Electron‐Beam‐Evaporated Nickel Oxide Hole Transport Layers for Perovskite‐Based Photovoltaics

High‐quality charge carrier transport materials are of key importance for stable and efficient perovskite‐based photovoltaics. This work reports on electron‐beam‐evaporated nickel oxide (NiO_x) layers, resulting in stable power conversion efficiencies (PCEs) of up to 18.5% when integrated into solar cells employing inkjet‐printed perovskite absorbers. By adding oxygen as a process gas and optimizing the layer thickness, transparent and efficient NiO_x hole transport layers (HTLs) are fabricated, exhibiting an average absorptance of only 1%. The versatility of the material is demonstrated for different absorber compositions and deposition techniques. As another highlight of this work, all‐evaporated perovskite solar cells employing an inorganic NiO_x HTL are presented, achieving stable PCEs of up to 15.4%. Along with good PCEs, devices with electron‐beam‐evaporated NiO_x show improved stability under realistic operating conditions with negligible degradation after 40 h of maximum power point tracking at 75 °C. Additionally, a strong improvement in device stability under ultraviolet radiation is found if compared to conventional perovskite solar cell architectures employing other metal oxide charge transport layers (e.g., titanium dioxide). Finally, an all‐evaporated perovskite solar mini‐module with a NiO_x HTL is presented, reaching a PCE of 12.4% on an active device area of 2.3 cm².     

Physically Stable Polymer‐Membrane Electrolytes for Highly Efficient Solid‐State Dye‐Sensitized Solar Cells with Long‐Term Stability

A 3D polymer‐network‐membrane (3D‐PNM) electrolyte is described for highly stable, solid‐state dye‐sensitized solar cells (DSCs) with excellent power‐conversion efficiency (PCE). The 3D‐PNM electrolyte is prepared by using one‐pot in situ cross‐linking polymerization on the surface of dye‐sensitized TiO₂ particles in the presence of redox species. This method allows the direct connection of the 3D‐PNM to the surface of the TiO₂ particles as well as the in situ preparation of the electrolyte gel during device assembly. There are two junction areas (liquid and solid‐state junctions) in the DSCs that employ conventional polymer electrolytes, and the major interface is at the liquid‐state junction. The solid‐state junction is dominant in the DSCs that employ the 3D‐PNM electrolyte, which exhibit almost constant performance during aging at 65 °C for over 700 h (17.0 to 17.2 mA cm^–2). The best cell performance gives a PCE of 9.1%; this is slightly better than the performance of a DSC that employs a liquid electrolyte.     

Dominating Energy Losses in NiO p‐Type Dye‐Sensitized Solar Cells

Nickel oxide based p‐type dye‐sensitized solar cells (DSCs) are limited in their efficiencies by poor fill factors (FFs). This work explores the origins of this limitation. Transient absorption spectroscopy identifies fast recombination between the injected hole and the dye anion under applied load as one of the predominant reasons for the poor FF of NiO‐based DSCs. A reduced hole injection efficiency, η_INJ, under applied load is found to play an equally important role. Both, the dye regeneration yield, Φ_REG, and η_INJ decrease by approximately 40%–50% when moving from short‐ to open‐circuit conditions. Spectroelectrochemical measurements reveal that the electrochromic properties of NiO are a further limiting factor for the device performance leading to variable light‐harvesting efficiencies, η_LH, under applied load. The peak light‐harvesting efficiency decreases from 63% at short circuit to 57% at 600 mV reducing the FF of NiO DSCs by 5%. This effect is expected to be more pronounced for future devices with higher operating voltages. Incident, photon‐to‐electron conversion efficiency front–back analysis at applied bias is utilized to characterize the interfacial charge recombination. It is found that the recombination between the injected hole and the redox mediator has a surprisingly small effect on the FF.     

MoS2‐Based All‐Purpose Fibrous Electrode and Self‐Powering Energy Fiber for Efficient Energy Harvesting and Storage

Here an all‐purpose fibrous electrode based on MoS₂ is demonstrated, which can be employed for versatile energy harvesting and storage applications. In this coaxial electrode, ultrathin MoS₂ nanofilms are grown on TiO₂ nanoparticles coated carbon fiber. The high electrochemical activity of MoS₂ and good conductivity of carbon fiber synergistically lead to the remarkable performances of this novel composite electrode in fibrous dye‐sensitized solar cells (showing a record‐breaking conversion efficiency of 9.5%) and high‐capacity fibrous supercapacitors. Furthermore, a self‐powering energy fiber is fabricated by combining a fibrous dye‐sensitized solar cell and a fibrous supercapacitor into a single device, showing very fast charging capability (charging in 7 s under AM1.5G solar illumination) and an overall photochemical‐electricity energy conversion efficiency as high as 1.8%. In addition, this wire‐shaped electrode can also be used for fibrous Li‐ion batteries and electrocatalytic hydrogen evolution reactions. These applications indicate that the MoS₂‐based all‐purpose fibrous electrode has great potential for the construction of high‐performance flexible and wearable energy devices.     

Overcoming the “Light‐Soaking” Issue in Inverted Organic Solar Cells by the Use of Al:ZnO Electron Extraction Layers

A common phenomenon of organic solar cells (OSCs) incorporating metal‐oxide electron extraction layers is the requirement to expose the devices to UV light in order to improve device characteristics – known as the so‐called “light‐soaking” issue. This behaviour appears to be of general validity for various metal‐oxide layers, various organic donor/acceptor systems, and regardless if single junction devices or multi stacked cells are considered. The requirement of UV exposure of OSCs may impose severe problems if substrates with limited UV transmission, UV blocking filters or UV to VIS down‐conversion concepts are applied. In this paper, we will demonstrate that this issue can be overcome by the use of Al doped ZnO (AZO) as electron extraction interlayer. In contrast to devices based on TiO_x and ZnO, the AZO devices show well‐behaved solar cell characteristics with a high fill factor (FF) and power conversion efficiency (PCE) even without the UV spectral components of the AM1.5 solar spectrum. As opposed to previous claims, our results indicate that the origin of s‐shaped characteristics of the OSCs is the metal‐oxide/organic interface. The electronic structures of the TiO_x/fullerene and AZO/fullerene interfaces are studied by photoelectron spectroscopy, revealing an electron extraction barrier for the TiO_x/fullerene case and facilitated electron extraction for AZO/fullerene. These results are of general relevance for organic solar cells based on various donor acceptor active systems.