Cover Picture: Solution‐Processed Organic Tandem Solar Cells (Adv. Funct. Mater. 14/2006)

The fabrication of a solution‐processed polymer tandem cell by stacking two single cells in series is reported by de Boer and co‐workers on p. 1897. The bottom and top cell are complementary with respect to their absorption spectra and the layer thickness of the bottom cell was optimized in order to create an optical cavity that efficiently transmits the required wavelength for the top cell. The combination of this tandem architecture with more efficient small‐bandgap materials will enable the realization of highly efficient organic solar cells. A solution‐processed polymer tandem cell fabricated by stacking two single cells in series is demonstrated. The two bulk‐heterojunction subcells have complementary absorption maxima at λ_max ～ 850 nm and λ_max ～ 550 nm, respectively. A composite middle electrode is applied that serves both as a charge‐recombination center and as a protecting layer for the first cell during spin‐coating of the second cell. The subcells are electronically coupled in series, which leads to a high open‐circuit voltage of 1.4 V, equal to the sum of each subcell. The layer thickness of the first (bottom) cell is tuned to maximize the optical absorption of the second (top) cell. The performance of the tandem cell is presently limited by the relatively low photocurrent generation in the small‐bandgap polymer of the top cell. The combination of our tandem architecture with more efficient small‐bandgap materials will enable the realization of highly efficient organic solar cells in the near future.     

Organic Tandem and Multi‐Junction Solar Cells

The emerging field of stacked layers (double‐ and even multi‐layers) in organic photovoltaic cells is reviewed. Owing to the limited absorption width of organic molecules and polymers, only a small fraction of the solar flux can be harvested by a single‐layer bulk heterojunction photovoltaic cell. Furthermore, the low charge‐carrier mobilities of most organic materials limit the thickness of the active layer. Consequently, only part of the intensity of the incident light at the absorption maximum is absorbed. A tandem or multi‐junction solar cell, consisting of multiple layers each with their specific absorption maximum and width, can overcome these limitations and can cover a larger part of the solar flux. In addition, tandem or multi‐junction solar cells offer the distinct advantage that photon energy is used more efficiently, because the voltage at which charges are collected in each sub‐cell is closer to the energy of the photons absorbed in that cell. Recent developments in both small‐molecule and polymeric photovoltaic cells are discussed, and examples of photovoltaic architectures, geometries, and materials combinations that result in tandem and multi‐junction solar cells are presented.     

Accurate Measurement and Characterization of Organic Solar Cells

Methods to accurately measure the current–voltage characteristics of organic solar cells under standard reporting conditions are presented. Four types of organic test cells and two types of silicon reference cells (unfiltered and with a KG5 color filter) are selected to calculate spectral‐mismatch factors for different test‐cell/reference‐cell combinations. The test devices include both polymer/fullerene‐based bulk‐heterojunction solar cells and small‐molecule‐based heterojunction solar cells. The spectral responsivities of test cells are measured as per American Society for Testing and Materials Standard E1021, and their dependence on light‐bias intensity is reported. The current–voltage curves are measured under 100 mW cm^–2 standard AM 1.5 G (AM: air mass) spectrum (International Electrotechnical Commission 69094‐1) generated from a source set with a reference cell and corrected for spectral error.     

Nanoscale Phase Separation and High Photovoltaic Efficiency in Solution‐Processed,Small‐Molecule Bulk Heterojunction Solar Cells

Research relating to organic solar cells based on solution‐processed, bulk heterojunction (BHJ) films has been dominated by polymeric donor materials, as they typically have better film‐forming characteristics and film morphology than their small‐molecule counterparts. Despite these morphological advantages, semiconducting polymers suffer from synthetic reproducibility and difficult purification procedures, which hinder their commercial viability. Here, a non‐polymeric, diketopyrrolopyrrole‐based donor material that can be solution processed with a fullerene acceptor to produce good quality films is reported. Thermal annealing leads to suitable phase separation and material distribution so that highly effective BHJ morphologies are obtained. The frontier orbitals of the material are well aligned with those of the fullerene acceptor, allowing efficient electron transfer and suitable open‐circuit voltages, leading to power conversion efficiencies of 4.4 ± 0.4% under AM1.5G illumination (100 mW cm^?2). Small molecules can therefore be solution processed to form high‐quality BHJ films, which may be used for low‐cost, flexible organic solar cells.     

Film Morphology of High Efficiency Solution‐Processed Small‐Molecule Solar Cells

Morphological control over the bulk heterojunction (BHJ) microstructure of a high‐efficiency small molecule photovoltaic system is demonstrated using both thermal treatment and solvent additive processing. Single crystal X‐ray diffraction is utilized to understand molecular interactions in the solid state and the BHJ morphology is examined using bright field, high‐resolution, and cross‐section transmission electron microscopy techniques. Controlling the domain size, while maintaining good molecular order within the semiconducting donor material, is found to be crucial in achieving high performance and over 90% internal quantum efficiency exhibited under the optimized conditions.     

Nongeminate Recombination and Charge Transport Limitations in Diketopyrrolopyrrole‐Based Solution‐Processed Small Molecule Solar Cells

Charge transport and nongeminate recombination are investigated in two solution‐processed small molecule bulk heterojunction solar cells consisting of diketopyrrolopyrrole (DPP)‐based donor molecules, mono‐DPP and bis‐DPP, blended with [6,6]‐phenyl‐C71‐butyric acid methyl ester (PCBM). While the bis‐DPP system exhibits a high fill factor (62%) the mono‐DPP system suffers from pronounced voltage dependent losses, which limit both the fill factor (46%) and short circuit current. A method to determine the average charge carrier density, recombination current, and effective carrier lifetime in operating solar cells as a function of applied bias is demonstrated. These results and light intensity measurements of the current‐voltage characteristics indicate that the mono‐DPP system is severely limited by nongeminate recombination losses. Further analysis reveals that the most significant factor leading to the difference in fill factor is the comparatively poor hole transport properties in the mono‐DPP system (2 × 10^?5 cm² V^?1 s^?1 versus 34 × 10^?5 cm² V^?1 s^?1). These results suggest that future design of donor molecules for organic photovoltaics should aim to increase charge carrier mobility thereby enabling faster sweep out of charge carriers before they are lost to nongeminate recombination.     

Solution‐Processed Metallic Nanowire Electrodes as Indium Tin Oxide Replacement for Thin‐Film Solar Cells

Solution processed silver nanowire (Ag NW) films are introduced as transparent electrodes for thin‐film solar cells. Ag NW electrodes were processed by doctor blade‐coating on glass substrates at moderate temperatures (less than 100 °C). The morphological, optical, and electrical characteristics of these electrodes were investigated as a function of processing parameters. For solar‐cell application, Ag NW electrodes with an average transparency of 90% between 450 and 800 nm and a sheet resistivity of ≈10 Ω per square were chosen. The performance of poly(3‐hexylthiophen‐2,5‐diyl):[6,6]‐phenyl‐C61‐butyric acid methyl ester (P3HT:PCBM) solar cells on Ag NW electrodes was found to match the performance of otherwise identical cells on indium tin oxide. Overall, P3HT:PCBM solar cells with an efficiency of 2.5% on transparent Ag NW electrodes have been realized.     

Highly Efficient and Bendable Organic Solar Cells with Solution‐Processed Silver Nanowire Electrodes

Highly efficient and bendable organic solar cells (OSCs) are fabricated using solution‐processed silver nanowire (Ag NW) electrodes. The Ag NW films were highly transparent (diffusive transmittance ≈ 95% at a wavelength of 550 nm), highly conductive (sheet resistance ≈ 10 Ω sq^?1), and highly flexible (change in resistance ≈ 1.1 ± 1% at a bending radius of ≈200 μm). Power conversion efficiencies of ≈5.80 and 5.02% were obtained for devices fabricated on Ag NWs/glass and Ag NWs/poly(ethylene terephthalate) (PET), respectively. Moreover, the bendable devices fabricated using the Ag NWs/PET films decrease slightly in their efficiency (to ≈96% of the initial value) even after the devices had been bent 1000 times with a radius of ≈1.5 mm.     

Crystallization‐Induced Phase Separation in Solution‐Processed Small Molecule Bulk Heterojunction Organic Solar Cells

The driving forces and processes associated with the development of phase separation upon thermal annealing are investigated in solution‐processed small molecule bulk heterojunction (BHJ) organic solar cells utilizing a diketopyrrolopyrrole‐based donor molecule and a fullerene acceptor (PCBM). In‐situ thermal annealing X‐ray scattering is used to monitor the development of thin film crystallization and phase separation and reveals that the development of blend phase separation strongly correlates with the nucleation of donor crystallites. Additionally, these morphological changes lead to dramatic increases in blend electron mobility and solar cell figures of merit. These results indicate that donor crystallization is the driving force for blend phase separation. It is hypothesized that donor crystallization from an as‐cast homogeneous donor:acceptor blend simultaneously produces donor‐rich domains, consisting largely of donor crystallites, and acceptor‐rich domains, formed from previously mixed regions of the film that have been enriched with acceptor during donor crystallization. Control of donor crystallization in solution‐processed small molecule BHJ solar cells employing PCBM is thus emphasized as an important strategy for the engineering of the nanoscale phase separated, bicontinuous morphology necessary for the fabrication of efficient BHJ photovoltaic devices.     

Low‐Work‐Function Surface Formed by Solution‐Processed and Thermally Deposited Nanoscale Layers of Cesium Carbonate

Nanostructured layers of Cs₂CO₃ are shown to function very effectively as cathodes in organic electronic devices because of their good electron‐injection capabilities. Here, we report a comprehensive study of the origin of the low work function of nanostructured layers of Cs₂CO₃ prepared by solution deposition and thermal evaporation. The nanoscale Cs₂CO₃ layers are probed by various characterization methods including current–voltage (I–V) measurements, photovoltaic studies, X‐ray photoelectron spectroscopy (XPS), UV photoelectron spectroscopy (UPS), and impedance spectroscopy. It is found that thermally evaporated Cs₂CO₃ decomposes into CsO₂ and cesium suboxides. The cesium suboxides dope CsO₂, yielding a heavily doped n‐type semiconductor with an intrinsically low work function. As a result, devices fabricated using thermally evaporated Cs₂CO₃ are relatively insensitive to the choice of the cathode metal. The reaction of thermally evaporated Cs₂CO₃ with Al can further reduce the work function to 2.1 eV by forming an Al–O–Cs complex. Solution‐processed Cs₂CO₃ also reduces the work function of Au substrates from 5.1 to 3.5 eV. However, devices prepared using solution‐processed Cs₂CO₃ exhibit high efficiency only if a reactive metal such as Al or Ca is used as the cathode metal. A strong chemical reaction occurs between spin‐coated Cs₂CO₃ and thermally evaporated Al. An Al–O—Cs complex is formed as a result of this chemical reaction at the interface, and this layer significantly reduces the work function of the cathode. Finally, impedance spectroscopy results prove that this layer is highly conductive.     

Solution‐Processed Metal Oxide Nanocrystals as Carrier Transport Layers in Organic and Perovskite Solar Cells

There has been rapid progress in solution‐processed organic solar cells (OSCs) and perovskite solar cells (PVSCs) toward low‐cost and high‐throughput photovoltaic technology. Carrier (electron and hole) transport layers (CTLs) play a critical role in boosting their efficiency and long‐time stability. Solution‐processed metal oxide nanocrystals (SMONCs) as a promising CTL candidate, featuring robust process conditions, low‐cost, tunable optoelectronic properties, and intrinsic stability, offer unique advantages for realizing cost‐effective, high‐performance, large‐area, and mechanically flexible photovoltaic devices. In this review, the recent development of SMONC‐based CTLs in OSCs and PVSCs is summarized. This paper starts with the discussion of synthesis approaches of SMONCs. Then, a broad range of SMONC‐based CTLs, including hole transport layers and electron transport layers, are reviewed, in which an emphasis is placed on the improvement of the efficiency and device stability. Finally, for the better understanding of the challenges and opportunities on SMONC‐based CTLs, several strategies and perspectives are outlined.     

A Versatile Fluoro‐Containing Low‐Bandgap Polymer for Efficient Semitransparent and Tandem Polymer Solar Cells

The versatility of a fluoro‐containing low band‐gap polymer, poly[2,6‐(4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b’]dithiophene)‐alt‐4,7‐(5‐fluoro‐2,1,3‐benzothia‐diazole)] (PCPDTFBT) in organic photovoltaics (OPVs) applications is demonstrated. High boiling point 1,3,5‐trichlorobenzene (TCB) is used as a solvent to manipulate PCPDTFBT:[6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) active layer morphology to obtain high‐performance single‐junction devices. It promotes the crystallization of PCPDTFBT polymer, thus improving the charge‐transport properties of the active layer. By combining the morphological manipulation with interfacial optimization and device engineering, the single‐junction device exhibits both good air stability and high power‐conversion efficiency (PCE, of 6.6%). This represents one of the highest PCE values for cyclopenta[2,1‐b;3,4‐b’]dithiophene (CPDT)‐based OPVs. This polymer is also utilized for constructing semitransparent solar cells and double‐junction tandem solar cells to demonstrate high PCEs of 5.0% and 8.2%, respectively.     

Diphenylmethanofullerenes: New and Efficient Acceptors in Bulk‐Heterojunction Solar Cells

A novel fullerene derivative, 1,1‐bis(4,4′‐dodecyloxyphenyl)‐(5,6) C₆₁, diphenylmethanofullerene (DPM‐12), has been investigated as a possible electron acceptor in photovoltaic devices, in combination with two different conjugated polymers poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐para‐phenylene vinylene] (OC₁C₁₀‐PPV) and poly[3‐hexyl thiophene‐2,5‐diyl] (P3HT). High open‐circuit voltages, V_OC = 0.92 and 0.65 V, have been measured for OC₁C₁₀‐PPV:DPM‐12‐ and P3HT:DPM‐12‐based devices, respectively. In both cases, V_OC is 100 mV above the values measured on devices using another routinely used fullerene acceptor, [6,6]‐phenyl‐C₆₁ butyric acid methyl ester (PCBM). This is somewhat unexpected when taking into account the identical redox potentials of both acceptor materials at room temperature. The temperature‐dependent V_OC reveals, however, the same effective bandgap (HOMO_Polymer–LUMO_Fullerene; HOMO = highest occupied molecular orbital, LUMO = lowest unoccupied molecular orbital) of 1.15 and 0.9 eV for OC₁C₁₀‐PPV and P3HT, respectively, independent of the acceptor used. The higher V_OC at room temperature is explained by different ideality factors in the dark‐diode characteristics. Under white‐light illumination (80 mW cm^–2), photocurrent densities of 1.3 and 4.7 mA cm^–2 have been obtained in the OC₁C₁₀‐PPV:DPM‐12‐ and P3HT:DPM‐12‐based devices, respectively. Temperature‐dependent current density versus voltage characteristics reveal a thermally activated (shallow trap recombination limited) photocurrent in the case of OC₁C₁₀‐PPV:DPM‐12, and a nearly temperature‐independent current density in P3HT:DPM‐12. The latter clearly indicates that charge carriers traverse the active layer without significant recombination, which is due to the higher hole‐mobility–lifetime product in P3HT. At the same time, the field‐effect electron mobility in pure DPM‐12 has been found to be μ_e = 2 × 10^–4 cm² V^–1 s^–1, that is, forty‐times lower than the one measured in PCBM (μ_e = 8 × 10^–3 cm² V^–1 s^–1).     

The Locus of Free Charge‐Carrier Generation in Solution‐Cast Zn1–xMgxO/Poly(3‐hexylthiophene) Bilayers for Photovoltaic Applications

The photoconductivity of solution‐cast Zn_1–xMg_xO (x=0‐0.4) and poly(3‐hexylthiophene) (P3HT) thin films, and Zn_1‐xMg_xO/P3HT bilayers is investigated using Time‐Resolved Microwave Conductivity (TRMC) with the aim of determining the locus of free charge carrier generation in the bilayer system. The photoconductivity of Zn_1–xMg_xO thin films, under illumination with 300 nm laser pulses, is limited by the formation of stable excitons and by scattering of the carriers at grain boundaries. The electron mobility in Zn_1–xMg_xO films decreases exponentially with Mg concentration, up to x=0.4. In agreement with previous work, free carriers are observed in the P3HT film under illumination with 500 nm pulses in the absence of an acceptor. Under illumination with 500 nm pulses, where only the polymer absorbs, the TRMC signal for the Zn_1–xMg_xO/P3HT bilayers for x≥0.2 is the same as that of pure P3HT, indicating that free carrier generation in these bilayers occurs predominately by exciton dissociation in the polymer bulk, and not at the interface between the polymer and the solution‐cast oxide. At lower Mg concentrations (x<0.2) the TRMC signal increases with decreasing x following the dependence of the electron mobility in the oxide but its light intensity dependence remains consistent with free carrier generation in the polymer bulk. To explain these results and previously published photovoltaic device data (Adv. Funct. Mater. 2007 , 17, 264) we propose that free carrier generation in the bilayers predominantly occurs in the bulk of P3HT, and is followed by electron injection to the oxide to yield photocurrent in photovoltaic cells. The dependence of the TRMC signal of the bilayers on Mg concentration is explained in terms of the yield for free carrier generation in the polymer and the relative contributions of electrons in the oxide and holes in the polymer.     

Over 16.7% Efficiency Organic‐Silicon Heterojunction Solar Cells with Solution‐Processed Dopant‐Free Contacts for Both Polarities

Realization of synchronous improvement in optical management and electrical engineering is necessary to achieve high‐performance photovoltaic device. However, inherent challenges are faced in organic‐silicon heterojunction solar cells (HSCs) due to the poor contact property of polymer on structured silicon surface. Herein, a remarkable efficiency boost from 12.6% to over 16.7% in poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate)/n‐silicon (PEDOT:PSS/n‐Si) HSCs by independent optimization of hole‐/electron‐selective contacts only relying on solution‐based processes is realized. A bilayer PEDOT:PSS film with different functionalizations is utilized to synchronously realize conformal contact and effective carrier collection on textured Si surface, making the photogenerated carriers be well separated at heterojunction interface. Meanwhile, fullerene derivative is used as electron‐transporting layer at the rear n‐Si/Al interface to reduce the contact barrier. The study of carriers' transport and independent optimization on separately contacted layers may lead to an effective and simplified path to fabricate high‐performance organic‐silicon heterojunction devices.     

High Photoconductive Responsivity in Solution‐Processed Polycrystalline Organic Composite Films

We demonstrate a novel approach for enhancing photoconductive responsivity (R) using a solution‐based organic semiconductor composite that yields R approaching 25 AW^?1, which is two to three orders of magnitude higher than the R in films comprising a single molecular component. We present extensive studies of photoconductivity, photoluminescence, and crystalline structural order that elucidate the mechanisms underlying this high photoconductive responsivity. The high R is found to arise from high photoconductive gain (82) due to a long mobile hole lifetime stemming from a prolonged occupation of electrons in deep traps generated at interfacial regions between the molecular crystallites.     

Interface Modification to Improve Hole‐Injection Properties in Organic Electronic Devices

The performance of organic electronic devices is often limited by injection. In this paper, improvement of hole injection in organic electronic devices by conditioning of the interface between the hole‐conducting layer (buffer layer) and the active organic semiconductor layer is demonstrated. The conditioning is performed by spin‐coating poly(9,9‐dioctyl‐fluorene‐co‐N‐ (4‐butylphenyl)‐diphenylamine) (TFB) on top of the poly(3,4‐ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) buffer layer, followed by an organic solvent wash, which results in a TFB residue on the surface of the PEDOT:PSS. Changes in the hole‐injection energy barriers, bulk charge‐transport properties, and current–voltage characteristics observed in a representative PFO‐based (PFO: poly(9,9‐dioctylfluorene)) diode suggest that conditioning of PEDOT:PSS surface with TFB creates a stepped electronic profile that dramatically improves the hole‐injection properties of organic electronic devices.     

Imaging the Electric Potential within Organic Solar Cells

The charge transport in organic solar cells is investigated by surface potential measurements via scanning Kelvin probe microscopy. Access to the solar cell's cross‐section is gained by milling holes with a focused ion beam which enables the direct scan along the charge transport path. In a study of poly(3‐hexylthiophene):1‐(3‐methoxycarbonyl)propyl‐1‐phenyl[6,6]C61 (P3HT:PCBM) bulk heterojunction solar cells, the open circuit voltage is built up at the top contact. A comparison of the potential distribution within normal and inverted solar cells under operation exhibits strongly different behaviors, which can be assigned to a difference in interface properties.     

Solution‐Processed Bulk‐Heterojunction Solar Cells Based on Monodisperse Dendritic Oligothiophenes

A novel family of soluble conjugated dendritic oligothiophenes (DOTs) as monodisperse 3D macromolecular architectures was characterized with respect to optical and redox properties in solution and in solid films. Band gaps of 2.5–2.2 eV, typical for organic semiconductors, were determined as well as HOMO/LUMO energy levels ideal for efficient electron transfer to acceptors such as [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) identifying them as suitable materials for solar cell applications. Solution‐processed bulk‐heterojunction solar cells using DOTs as electron donor and PCBM as acceptor were prepared and investigated. High open‐circuit voltages V_OC of 1.0 V and power‐conversion efficiencies up to 1.72% were obtained for the DOT‐based devices. The higher generations DOTs provide the highest efficiencies. Based on the monodispersity of the DOTs, an analysis of the molar ratio between donor and acceptor in the blended film was possible leading to an optimal value of five to six thiophene units per PCBM.