Stacking Sequence and Acceptor Dependence of Photocurrent Spectra and Photovoltage in Organic Two-Junction Devices

Both single-junction and tandem organic photovoltaic cells have been well developed. A tandem cell contains two junctions with a charge recombination layer (CRL) inserted between the two junctions. So far, there is no detailed report on how the device will perform that contains two junctions but without a CRL in between. In this work, we report the photocurrent spectra and photovoltage output of the devices that contains two bulk-heterojunctions (BHJ) stacked directly on top of each other without a CRL. The top active layer is prepared by transfer printing. The photocurrent response spectra and photovoltage are found to be sensitive to stacking sequence and the selection of electron acceptors. The open-circuit voltage of the devices (up to 1.09 V) can be higher than the devices containing either junction layer. The new phenomenon in the new device architecture increases the versatility of the optoelectronic devices based on organic semiconductors

Flexible and Transparent Organic–Inorganic Hybrid Thermoelectric Modules

Light-weight, mechanically flexible, transparent thermoelectric modules are promising as portable and easy-to-integrate energy sources. Here, we demonstrate flexible, transparent thermoelectric modules by using a conducting polymer poly­(3,4-ethylenedioxythiophene):poly­(styrenesulfonate) (PEDOT:PSS) as the p-type leg and indium tin oxide (ITO)-PEDOT:PSS as the n-type leg. Main observations include the following: (1) the bilayer combination of ITO-PEDOT:PSS (PEDOT:PSS coated on top of the ITO) displays a negative Seebeck coefficient (<i>S</i>) and the value is similar to that of the ITO single layer; (2) the <i>S</i> value of the ITO-PEDOT:PSS is almost not dependent on the area ratio of the stacked PEDOT:PSS and ITO; and (3) the conducting polymer PEDOT:PSS deposition on top of ITO helps the ITO not to generate cracks during bending, which enhances the mechanical flexibility of the ITO. On the basis of these observations, thermoelectric modules with eight pairs of junctions are fabricated and the thermoelectric modules’ Δ<i>V</i>/Δ<i>T</i> (modules’ generated thermovoltage per temperature difference) is nearly the addition of <i>S</i> values of all legs. Thermoelectric modules show good mechanical flexibility and air stability. Applications of thermoelectric modules have also been demonstrated to produce thermovoltage via the temperature difference produced by a human hand or warm water

Universal Strategy To Reduce Noise Current for Sensitive Organic Photodetectors

Low noise current is critical for achieving high-detectivity organic photodetectors. Inserting charge-blocking layers is an effective approach to suppress the reverse-biased dark current. However, in solution-processed organic photodetectors, the charge-transport material needs to be dissolved in solvents that do not dissolve the underneath light-absorbing layer, which is not always possible for all kinds of light-absorbing materials developed. Here, we introduce a universal strategy of transfer-printing a conjugated polymer, poly­(3-hexylthiophene) (P3HT), as the electron-blocking layer to realize highly sensitive photodetectors. The transfer-printed P3HT layers substantially and universally reduced the reverse-biased dark current by about 3 orders of magnitude for various photodetectors with different active layers. These photodetectors can detect the light signal as weak as several picowatts per square centimeter, and the device detectivity is over 10¹² Jones. The results suggest that the strategy of transfer-printing P3HT films as the electron-blocking layer is universal and effective for the fabrication of sensitive organic photodetectors

Nonreduction-Active Hole-Transporting Layers Enhancing Open-Circuit Voltage and Efficiency of Planar Perovskite Solar Cells

Inverted planar perovskite solar cells using poly­(3,4-ethylenedioxythiophene):poly­(styrenesulfonate) (PEDOT:PSS) as the hole-transporting layer (HTL) are very attractive because of their low-temperature and easy processing. However, the planar cells with the PEDOT:PSS HTL typically display lower open-circuit voltage (<i>V</i>_OC) (about 0.90 V) than that of devices with TiO₂-based conventional structure (1.0–1.1 V). The underlying reasons are still not clear. In this work, we report the PEDOT:PSS that is intrinsically p-doped can be chemically reduced by methylamine iodide (MAI) and MAPbI₃. The reaction reduces the work function (WF) of PEDOT:PSS, which suppresses the efficient hole collection and yields lower <i>V</i>_OC. To overcome this issue, we adopt undoped semiconducting polymers that are intrinsically nonreduction-active (NRA) as the HTL for inverted planar perovskite solar cells. The cells display enhanced <i>V</i>_OC from 0.88 ± 0.04 V (PEDOT:PSS HTL, reference cells) to 1.02 ± 0.03 V (P3HT HTL) and 1.04 ± 0.03 V (PTB7 and PTB-Th HTL). The power conversion efficiency (PCE) of the devices with these NRA HTL reaches about 17%

The Influence of the Work Function of Hybrid Carbon Electrodes on Printable Mesoscopic Perovskite Solar Cells

In printable mesoscopic perovskite solar cells (PSCs), carbon electrodes play a significant role in charge extraction and transport, influencing the overall device performance. The work function and electrical conductivity of the carbon electrodes mainly affect the open-circuit voltage (<i>V</i>_OC) and series resistance (<i>R</i>_s) of the device. In this paper, we propose a hybrid carbon electrode based on a high-temperature mesoporous carbon (m-C) layer and a low-temperature highly conductive carbon (c-C) layer. The m-C layer has a high work function and large surface area and is mainly responsible for charge extraction. The c-C layer has a high conductivity and is responsible for charge transport. The work function of the m-C layer was tuned by adding different amounts of NiO, and at the same time, the conductivities of the hybrid carbon electrodes were maintained by the c-C layer. It was supposed that the increase of the work function of the carbon electrode can enhance the <i>V</i>_OC of printable mesoscopic PSCs. Here, we found the <i>V</i>_OC of the device based on hybrid carbon electrodes can be enhanced remarkably when the insulating layer has a relatively small thickness (500–1000 nm). An optimal improvement in <i>V</i>_OC of up to 90 mV could be achieved when the work function of the m-C was increased from 4.94 to 5.04 eV. When the thickness of the insulating layer was increased to ∼3000 nm, the variation of <i>V</i>_OC as the work function of m-C increased became less distinct

Efficient Colorful Perovskite Solar Cells Using a Top Polymer Electrode Simultaneously as Spectrally Selective Antireflection Coating

Organometal halide perovskites have shown excellent optoelectronic properties and have been used to demonstrate a variety of semiconductor devices. Colorful solar cells are desirable for photovoltaic integration in buildings and other aesthetically appealing applications. However, the realization of colorful perovskite solar cells is challenging because of their broad and large absorption coefficient that commonly leads to cells with dark-brown colors. Herein, for the first time, we report a simple and efficient strategy to achieve colorful perovskite solar cells by using the transparent conducting polymer (poly­(3,4-ethylenedioxythiophene):poly­(styrenesulfonate), PEDOT:PSS) as a top electrode and simultaneously as an spectrally selective antireflection coating. Vivid colors across the visible spectrum are attained by engineering optical interference effects among the transparent PEDOT:PSS polymer electrode, the hole-transporting layer and the perovskite layer. The colored perovskite solar cells display power conversion efficiency values from 12.8 to 15.1% (from red to blue) when illuminated from the FTO glass side and from 11.6 to 13.8% (from red to blue) when illuminated from the PEDOT:PSS side. The new approach provides an advanced solution for fabricating colorful perovskite solar cells with easy processing and high efficiency

Vertical Stratification Engineering for Organic Bulk-Heterojunction Devices

High-efficiency organic solar cells (OSCs) can be produced through optimization of component molecular design, coupled with interfacial engineering and control of active layer morphology. However, vertical stratification of the bulk-heterojunction (BHJ), a spontaneous activity that occurs during the drying process, remains an intricate problem yet to be solved. Routes toward regulating the vertical separation profile and evaluating the effects on the final device should be explored to further enhance the performance of OSCs. Herein, we establish a connection between the material surface energy, absorption, and vertical stratification, which can then be linked to photovoltaic conversion characteristics. Through assessing the performance of temporary, artificial vertically stratified layers created by the sequential casting of the individual components to form a multilayered structure, optimal vertical stratification can be achieved. Adjusting the surface energy offset between the substrate results in donor and acceptor stabilization of that stratified layer. Further, a trade-off between the photocurrent generated in the visible region and the amount of donor or acceptor in close proximity to the electrode was observed. Modification of the substrate surface energy was achieved using self-assembled small molecules (SASM), which, in turn, directly impacted the polymer donor to acceptor ratio at the interface. Using three different donor polymers in conjunction with two alternative acceptors in an inverted organic solar cell architecture, the concentration of polymer donor molecules at the ITO (indium tin oxide)/BHJ interface could be increased relative to the acceptor. Appropriate selection of SASM facilitated a synchronized enhancement in external quantum efficiency and power conversion efficiencies over 10.5%