Yinhua Zhou

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

A Sturdy Electrode Coating To operate efficiently, organic devices—such as light-emitting diodes—require electrodes that emit or take up electrons at low applied voltages (that is, have low work functions). Often these electrodes are metals, such as calcium, that are not stable in air or water vapor and have to be protected from environmental damage. Zhou et al. (p. 327; see the Perspective by Helander) report that a coating polymer containing aliphatic amine groups can lower the work functions of various types of electrodes by up to 1.7 electron volts and can be used in a variety of devices. Air-stable, physisorbed polymers containing aliphatic amine groups can improve the efficiency of organic electronic devices. 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.

Recyclable organic solar cells on cellulose nanocrystal substrates

Solar energy is potentially the largest source of renewable energy at our disposal, but significant advances are required to make photovoltaic technologies economically viable and, from a life-cycle perspective, environmentally friendly, and consequently scalable. Cellulose nanomaterials are emerging high-value nanoparticles extracted from plants that are abundant, renewable, and sustainable. Here, we report on the first demonstration of efficient polymer solar cells fabricated on optically transparent cellulose nanocrystal (CNC) substrates. The solar cells fabricated on the CNC substrates display good rectification in the dark and reach a power conversion efficiency of 2.7%. In addition, we demonstrate that these solar cells can be easily separated and recycled into their major components using low-energy processes at room temperature, opening the door for a truly recyclable solar cell technology. Efficient and easily recyclable organic solar cells on CNC substrates are expected to be an attractive technology for sustainable, scalable, and environmentally-friendly energy production.

Indium tin oxide-free and metal-free semitransparent organic solar cells

We report on indium tin oxide (ITO)-free and metal-free semitransparent organic solar cells with a high-conductivity poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (PH1000) as both the bottom and the top electrodes. The PH1000 film showed a conductivity of 680±50u2002S/cm. A ZnO layer was used as an interlayer to produce an electron-selective electrode. The semitransparent devices with a structure of glass/PH1000/ZnO/poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester/PEDOT:PSS (CPP 105 D)/PH1000 exhibited an average power conversion efficiency of 1.8% estimated for 100u2002mW/cm2 air mass 1.5 global illumination. This geometry alleviates the need of vacuum deposition of a top electrode.

High performance polymeric charge recombination layer for organic tandem solar cells

We report on inverted polymer tandem solar cells wherein the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), modified at one interface with ethoxylated polyethylenimine (PEIE), acts as an efficient charge recombination layer. This recombination layer shows very low optical absorption, high electrical conductivity, and a large work function contrast of 1.3 eV between its top and bottom interfaces. Its use yields tandem cells in which the open-circuit voltage is the sum of that of individual cells. The fill factor of tandem cells connected in series is found to be larger than that of single-junction cells. Its simple polymeric composition and its unprecedented performance make it a promising component for emerging organic photovoltaic technologies.

A vertically integrated solar-powered electrochromic window for energy efficient buildings.

A solution-processed self-powered polymer electrochromic/photovoltaic (EC/PV) device is realized by vertically integrating two transparent PV cells with an ECD. The EC/PV cell is a net energy positive dual functional device, which can be reversibly switched between transparent and colored states by PV cells for regulating incoming sunlight through windows. The two PV cells can individually, or in pairs, generate electricity.

All-plastic solar cells with a high photovoltaic dynamic range

We report on semitransparent air-processed all-plastic solar cells, fabricated from vacuum-free processes, comprising two polymer electrodes, a polymeric work-function modification layer and a polymer:fullerene photoactive layer. The active layer and the top PEDOT:PSS electrode were prepared by sequential film-transfer lamination on polyethylenimine-modified PEDOT:PSS bottom electrodes. The transferring of films offers ease of layer patterning and the misalignment of defects in the different layers resulting from the additive film transfer lamination process yields high shunt resistance values of 108 ohm cm2. Consequently, all-plastic solar cells fabricated with this process exhibit very low reverse bias dark current and can operate in the photovoltaic quadrant with light irradiance varying over five orders of magnitude. The analysis of the values of the open-circuit voltage as a function of light irradiance over that wide dynamic range points toward an ideality factor of n = 1.82 and a reverse saturation current density of 6.2 × 10−11 A cm−2 for solar cells with an active layer comprised of a blend of poly(3-hexylthiophene) and an indene fullerene bis-adduct.

Organic Photovoltaic Cells with Stable Top Metal Electrodes Modified with Polyethylenimine

Efficient organic photovoltaic cells (OPV) often contain highly reactive low-work-function calcium electron-collecting electrodes. In this work, efficient OPV are demonstrated in which calcium electrodes were avoided by depositing a thin layer of the amine-containing nonconjugated polymer, polyethylenimine (PEIE), between the photoactive organic semiconductor layer and stable metal electrodes such as aluminum, silver, or gold. Devices with structure ITO/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/poly(3-hexylthiophene):indene-C60-bis-adduct (P3HT:ICBA)/PEIE/Al demonstrated overall photovoltaic device performance comparable to devices containing calcium electron-collecting electrodes, ITO/PEDOT:PSS/P3HT:ICBA/Ca/Al, with open-circuit voltage of 775±6 mV, short-circuit current density of 9.1±0.5 mA cm(-2), fill factor of 0.65±0.01, and power conversion efficiency of 4.6±0.3%, averaged over 5 devices at 1 sun.

Roles of thermally-induced vertical phase segregation and crystallization on the photovoltaic performance of bulk heterojunction inverted polymer solar cells

Brief 160 °C annealing treatments dramatically enhanced the performance of bulk heterojunction inverted polymer solar cells with an ITO/ZnO/P3HT:PCBM/MoO3/Ag structure. The influence of such treatments on cell performance has been correlated to vertical phase segregation and crystallization within the photoactive layer of such cells. The photoactive layer, comprised of a mixture of P3HT and PCBM deposited on ZnO, was annealed for 10–30 min at 160 °C. Depth profiling with X-ray photoelectron spectroscopy (XPS) revealed that such annealing resulted in enrichment of the P3HT concentration near the ZnO layer, particularly after 20 and 30 min of annealing. Crystallization of P3HT was detected by X-ray diffraction (XRD) analyses after 10 to 30 min of such annealing, with little difference in the extent of crystallization detected over this time frame. It was found that vertical segregation does not seem to play a role as significant as that of crystallization on cell performance.

Polyvinylpyrrolidone-modified indium tin oxide as an electron-collecting electrode for inverted polymer solar cells

We report on the photovoltaic properties of inverted polymer solar cells that use a polyvinylpyrrolidone (PVP) modified indium tin oxide (ITO) layer as the electron-collecting electrode. An ultrathin PVP layer, prepared by spin-coating, on top of ITO, was used to induce a reduction of its work function, allowing it to act as an electron-collecting electrode. Devices made on pristine ITO showed s-shape current-voltage characteristics, which were removed after exposure to ultraviolet radiation due to a reduction of the work function of ITO. Inverted solar cells with ITO/PVP electrodes yield efficiencies comparable to devices with ITO/ZnO electron-selective electrodes.

Inverted polymer solar cells with amorphous indium zinc oxide as the electron-collecting electrode

We report on the fabrication and performance of polymer-based inverted solar cells utilizing amorphous indium zinc oxide (a-IZO) as the electron-collecting electrode. Amorphous IZO films of 200 nm thickness were deposited by room temperature sputtering in a high-purity argon atmosphere. The films possessed a high optical transmittance in the visible region (≥ 80%), a low resistivity (3.3 × 10⁻⁴ Ωcm), a low surface roughness (root mean square = 0.68 nm), and a low work function (4.46 ± 0.02 eV). Inverted solar cells with the structure a-IZO/P3HT: PCBM/PEDOT:PSS/Ag exhibited a power conversion efficiency of 3% estimated for AM 1.5G, 100 mW/cm² illumination.