typically based on n-type metal oxides, our device is solutionprocessed at room temperature, enabling easy processibility over a large area. Accordingly, the approach is fully amenable to highthroughput roll-to-roll manufacturing techniques, may be used to fabricate vacuum-deposition-free PSCs of large area, and find practical applications in future mass production. Moreover, our discovery overturns a well-accepted belief (the inferior performance of inverted PSCs) and clearly shows that the characteristics of high performance, improved stability and ease of use can be integrated into a single device, as long as the devices are optimized, both optically and electrically, by means of a meticulously designed device structure. We also anticipate that our findings will catalyse the development of new device structures and may move the efficiency of devices towards the goal of 10% for various material systems. Previously, we reported that PFN can be incorporated into polymer light-emitting devices (PLEDs) to enhance electron injection from high-work-function metals such as aluminium (work function w of 4.3 eV) 22,23 and has thus been used to realize high-efficiency, air-stable PLEDs 24 . Furthermore, we also found that efficient electron injection can be obtained even in the most noble metals with extremely high work functions, such as gold (w ¼ 5.2 eV), by lowering the effective work function (for example lowering w in gold by 1.0 eV), which has previously been ascribed to the formation of a strong interface dipole 25 .

/pdf/enhanced-power-conversion-efficiency-in-polymer-solar-cells-4oh1re6qpi.pdf

Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure

Chemically derived graphene oxide (GO) possesses a unique set of properties arising from oxygen functional groups that are introduced during chemical exfoliation of graphite. Large-area thin-film deposition of GO, enabled by its solubility in a variety of solvents, offers a route towards GO-based thin-film electronics and optoelectronics. The electrical and optical properties of GO are strongly dependent on its chemical and atomic structure and are tunable over a wide range via chemical engineering. In this Review, the fundamental structure and properties of GO-based thin films are discussed in relation to their potential applications in electronics and optoelectronics.

Chemically derived graphene oxide: towards large-area thin-film electronics and optoelectronics.

Simultaneous enhancement of open-circuit voltage, short-circuit current density, and fill factor in highly efficient polymer solar cells by incorporating an alcohol/water-soluble conjugated polymer as cathode interlayer is domonstrated. When combined with a low-bandgap polymer PTB7 as the electron donor material, the power efficiency of the devices is improved to a certified 8.370%. Due to the drastic improvement in efficiency and easy utilization, this method opens new opportunities for PSCs from various material systems to improve towards 10% efficiency.

Simultaneous Enhancement of Open-Circuit Voltage, Short-Circuit Current Density, and Fill Factor in Polymer Solar Cells

Organic and printed electronics technologies require conductors with a work function that is sufficiently low to facilitate the transport of electrons in and out of various optoelectronic devices. We show that surface modifiers based on polymers containing simple aliphatic amine groups substantially reduce the work function of conductors including metals, transparent conductive metal oxides, conducting polymers, and graphene. The reduction arises from physisorption of the neutral polymer, which turns the modified conductors into efficient electron-selective electrodes in organic optoelectronic devices. These polymer surface modifiers are processed in air from solution, providing an appealing alternative to chemically reactive low–work function metals. Their use can pave the way to simplified manufacturing of low-cost and large-area organic electronic technologies.

http://science.sciencemag.org/content/sci/336/6079/327.full.pdf

A Universal Method to Produce Low―Work Function Electrodes for Organic Electronics

Although power conversion efficiency (PCE) of state-of-the-art perovskite solar cells has already exceeded 20%, photo- and/or moisture instability of organolead halide perovskite have prevented further commercialization. In particular, the underlying weak interaction of organic cations with surrounding iodides due to eight equivalent orientations of the organic cation along the body diagonals in unit cell and chemically non-inertness of organic cation result in photo- and moisture instability of organometal halide perovskite. Here, a perovskite light absorber incorporating organic–inorganic hybrid cation in the A-site of 3D APbI3 structure with enhanced photo- and moisture stability is reported. A partial substitution of Cs+ for HC(NH2)2+ in HC(NH2)2PbI3 perovskite is found to substantially improve photo- and moisture stability along with photovoltaic performance. When 10% of HC(NH2)2+ is replaced by Cs+, photo- and moisture stability of perovskite film are significantly improved, which is attributed to the enhanced interaction between HC(NH2)2+ and iodide due to contraction of cubo-octahedral volume. Moreover, trap density is reduced by one order of magnitude upon incorporation of Cs+, which is responsible for the increased open-circuit voltage and fill factor, eventually leading to enhancement of average PCE from 14.9% to 16.5%.

Formamidinium and Cesium Hybridization for Photo- and Moisture-Stable Perovskite Solar Cell

The authors report on a spray deposition method as a cost-efficient technique for the fabrication of organic solar cells (OSCs). Active layers of OSCs were fabricated using conventional handheld airbrushes. Although the spray deposited film showed a relatively rougher surface than spin coated ones, pinhole-free and constant thickness films could be obtained. An optimized OSC showed 2.83% of power conversion efficiency and 52% of incident photon to current conversion efficiency even though the device was fabricated in air. The performance of sprayed OSCs was comparable to that of the spin coated devices fabricated in air.

Fabrication of organic bulk heterojunction solar cells by a spray deposition method for low-cost power generation

Polymer-based photovoltaic cells, with periodic sub-micrometer structures as an efficient light-trapping scheme, are investigated to improve the performance of organic solar cells based on poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C61. A soft lithographic approach that uses photoresponsive azo polymer films as masters and poly(dimethylsiloxane) as stamps is used to form surface relief gratings (SRGs) on the active layers. The effect of periodic gratings on solar cell performance is precisely investigated according to various grating conditions such as period, depth, and dimension. The solar cells with 1D and 2D SRGs present improved incident-photon-to-current conversion efficiencies and an overall increase in power conversion efficiencies, primarily resulting from the enhancement of short-circuit current density, indicating that periodic structures induce further photon absorption in the active film.

Efficient Polymer Solar Cells with Surface Relief Gratings Fabricated by Simple Soft Lithography

Novel poly[(9,9-bis((6'-(N,N,N-tirimethylammonium)hexyl)-2,7-fluorene)-alt-(9,9-bis(2-(2-(2-methoxyethoxy)ethoxy)ethyl)-9-fluorene)) dibromide (WPF-6-oxy-F) and poly[(9,9-bis((6'-(N,N,N-trimethylammonium)hexyl)-2,7-fluorene)-alt-(9,9-bis(2-(2-methoxyethoxy)ethyl)-fluorene)] dibromide (WPF-oxy-F) compounds are developed and the use ofthese water-soluble polymers as an interfacial layer for low-cost poly(3-hexylthiophene):phenyl-C 61 butyric acid methyl ester (P3HT:PCBM) organic solar cells (OSCs) is investigated. When WPF-oxy-F or WPF-6-oxy-F is simply inserted between the active layer and the cathode as an interfacial dipole layer by spin-coating water-soluble polyfluorenes, the open-circuit voltage (V oc ), fill factor (FF), and power-conversion efficiency (PCE) of photovoltaic cells with high work-function metal cathodes, such as Al, Ag, Au, and Cu, dramatically increases. For example, when WPF-6-oxy-F is used with Al, Ag, Au, or Cu, regardless of the work-function of the metal cathode, the V oc is 0.64, 0.64, 0.58, and 0.63 V, respectively, approaching the original value of the P3HT:PCBM system because of the formation of large interfacial dipoles through a reduction of the metal work-function. In particular, introducing WPF-6-oxy-F into a low-cost Cu cathode dramatically enhanced the device efficiency from 0.8% to 3.36%.

Water-Soluble Polyfluorenes as an Interfacial Layer Leading to Cathode-Independent High Performance of Organic Solar Cells

Enhanced performance of inverted polymer solar cells (PSCs) is demonstrated by indium tin oxide (ITO) interfacial tuning via a water-soluble polyfluorene (WPF-6-oxy-F). Kelvin probe studies and dark current-voltage curves demonstrated that the WPF-6-oxy-F layer reduces the ITO work-function because of the favorable interfacial dipole formed by the WPF-6-oxy-F interlayer, thereby enhancing the built-in potential and reducing the interface resistance. As a result, introduction of the WPF-6-oxy-F by simple solution processing into the inverted PSCs dramatically enhanced cell-performances. This approach could be very beneficial and an important step for the future development of all-solution-processed or roll-to-roll processed PSCs.

Enhanced performance of inverted polymer solar cells with cathode interfacial tuning via water-soluble polyfluorenes

Colloidal-quantum-dot (CQD) photovoltaic devices are promising candidates for low-cost power sources owing to their low-temperature solution processability and bandgap tunability. A power conversion efficiency (PCE) of >10% is achieved for these devices; however, there are several remaining obstacles to their commercialization, including their high energy loss due to surface trap states and the complexity of the multiple-step CQD-layer-deposition process. Herein, high-efficiency photovoltaic devices prepared with CQD-ink using a phase-transfer-exchange (PTE) method are reported. Using CQD-ink, the fabrication of active layers by single-step coating and the suppression of surface trap states are achieved simultaneously. The CQD-ink photovoltaic devices achieve much higher PCEs (10.15% with a certified PCE of 9.61%) than the control devices (7.85%) owing to improved charge drift and diffusion. Notably, the CQD-ink devices show much lower energy loss than other reported high-efficiency CQD devices. This result reveals that the PTE method is an effective strategy for controlling trap states in CQDs.

Seung-Hwan Oh

Papers

Fabrication of organic bulk heterojunction solar cells by a spray deposition method for low-cost power generation

Efficient Polymer Solar Cells with Surface Relief Gratings Fabricated by Simple Soft Lithography

Water-Soluble Polyfluorenes as an Interfacial Layer Leading to Cathode-Independent High Performance of Organic Solar Cells

Enhanced performance of inverted polymer solar cells with cathode interfacial tuning via water-soluble polyfluorenes

High‐Efficiency Photovoltaic Devices using Trap‐Controlled Quantum‐Dot Ink prepared via Phase‐Transfer Exchange