Active Layer-Incorporated, Spectrally Tuned Au/SiO₂ Core/Shell Nanorod-Based Light Trapping for Organic Photovoltaics

We demonstrate that incorporation of octadecyltrimethoxysilane (OTMS)-functionalized, spectrally tuned, gold/silica (Au/SiO₂) core/shell nanospheres and nanorods into the active layer of an organic photovoltaic (OPV) device led to an increase in photoconversion efficiency (PCE). A silica shell layer was added onto Au core nanospheres and nanorods in order to provide an electrically insulating surface that does not interfere with carrier generation and transport inside the active layer. Functionalization of the Au/SiO₂ core/shell nanoparticles with the OTMS organic ligand was then necessary to transfer the Au/SiO₂ core/shell nanoparticles from an ethanol solution into an OPV polymer-compatible solvent, such as dichlorobenzene. The OTMS-functionalized Au/SiO₂ core/shell nanorods and nanospheres were then incorporated into the active layers of two OPV polymer systems: a poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCB₆₀M) OPV device and a poly[2,6-4,8-di(5-ethylhexylthienyl)benzo[1,2-<i>b</i>;3,4-<i>b</i>]dithiophene-<i>alt</i>-5-dibutyloctyl-3,6-bis(5-bromothiophen-2-yl)pyrrolo[3,4-<i>c</i>]pyrrole-1,4-dione] (PBDTT-DPP:PC₆₀BM) OPV device. For the P3HT:PC₆₀BM polymer with a band edge of ∼700 nm, the addition of the core/shell nanorods with an aspect ratio (AR) of ∼2.5 (extinction peak ∼670 nm) resulted in a 7.1% improvement in PCE, while for the PBDTT-DPP:PC₆₀BM polymer with a band edge of ∼860 nm, the addition of core/shell nanorods with an AR of ∼4 (extinction peak ∼830 nm) resulted in a 14.4% improvement in PCE. The addition of Au/SiO₂ core/shell nanospheres to the P3HT:PC₆₀BM polymer resulted in a 2.7% improvement in PCE, while their addition to a PBDTT-DPP:PC₆₀BM polymer resulted in a 9.1% improvement. The PCE and <i>J</i>_sc enhancements were consistent with external quantum efficiency (EQE) measurements, and the EQE enhancements spectrally matched the extinction spectra of Au/SiO₂ nanospheres and nanorods in both OPV polymer systems

Interface Engineering of High-Performance Perovskite Photodetectors Based on PVP/SnO₂ Electron Transport Layer

Hybrid organic–inorganic perovskites have attracted intensive interest as active materials for high-performance photodetectors. However, studies on the electron transport layer (ETL) and its influence on the response time of photodetectors remain limited. Herein, we compare the performances of perovskite photodetectors with TiO₂ and SnO₂ ETLs, especially on the response time. Both photodetectors exhibit a high on/off current ratio of 10⁵, a large detectivity around 10¹² Jones, and a linear dynamic range over 80 dB. The SnO₂-based perovskite photodiodes show ultrahigh response rates of 3 and 6 μs for the rise and decay times, respectively. However, photodetectors with TiO₂ ETLs have low responsivity and long response time at low driving voltage, which is attributed to the electron extraction barrier at the TiO₂/perovskite interface and the charge traps in the TiO₂ layer. Furthermore, the dark current of SnO₂-based perovskite photodiodes is effectively suppressed by inserting a poly­(vinylpyrrolidone) interlayer, and then the on/off current ratio increases to 1.2 × 10⁶, corresponding to an improvement of 1 order of magnitude. Such low-cost, solution-processable perovskite photodetectors with high performance show promising potential for future optoelectronic applications

Systematic Investigation of Benzodithiophene- and Diketopyrrolopyrrole-Based Low-Bandgap Polymers Designed for Single Junction and Tandem Polymer Solar Cells

The tandem solar cell architecture is an effective way to harvest a broader part of the solar spectrum and make better use of the photonic energy than the single junction cell. Here, we present the design, synthesis, and characterization of a series of new low bandgap polymers specifically for tandem polymer solar cells. These polymers have a backbone based on the benzodithiophene (BDT) and diketopyrrolopyrrole (DPP) units. Alkylthienyl and alkylphenyl moieties were incorporated onto the BDT unit to form BDTT and BDTP units, respectively; a furan moiety was incorporated onto the DPP unit in place of thiophene to form the FDPP unit. Low bandgap polymers (bandgap = 1.4–1.5 eV) were prepared using BDTT, BDTP, FDPP, and DPP units via Stille-coupling polymerization. These structural modifications lead to polymers with different optical, electrochemical, and electronic properties. Single junction solar cells were fabricated, and the polymer:PC₇₁BM active layer morphology was optimized by adding 1,8-diiodooctane (DIO) as an additive. In the single-layer photovoltaic device, they showed power conversion efficiencies (PCEs) of 3–6%. When the polymers were applied in tandem solar cells, PCEs over 8% were reached, demonstrating their great potential for high efficiency tandem polymer solar cells

Selective Direct Growth of Atomic Layered HfS₂ on Hexagonal Boron Nitride for High Performance Photodetectors

Hafnium disulfide (HfS₂) has attracted significant interest because of the predicted excellent electronic properties superior to group VIB transition metal dichalcogenides. On the other hand, atomically thin hexagonal boron nitride (h-BN) is an ideal dielectric substrate for optoelectronic applications of other 2D materials. Here, for the first time, we report the direct growth of high-quality atomic layered HfS₂ on few-layer h-BN transferred on SiO₂/Si substrates by chemical vapor deposition. It was found that the HfS₂ layers are selectively grown on h-BN rather than on SiO₂/Si. Density functional theory calculations are performed to help understand the mechanism of selective growth of HfS₂. Furthermore, the photodetectors based on the HfS₂/h-BN heterostructures exhibit excellent visible-light sensing performance, such as a high on/off ratio of more than 10⁵, an ultrafast response rate of about 200 μs, a high responsivity of 26.5 mA W^–1, and a competitive detectivity exceeding 3 × 10¹¹ Jones, superior to the vast majority of the reported 2D materials based photodetectors. The remarkable performance of the HfS₂/h-BN photodetector is attributed to the unique features of 2D h-BN substrate

A Selenophene Containing Benzodithiophene-<i>alt</i>-thienothiophene Polymer for Additive-Free High Performance Solar Cell

A selenophene-modified PTB7, PBDTSe-TT, is reported. The structure adjustment carried out by alkylselenophene substitution on the BDT building block is shown to slightly affect the polymer’s electronic property, and an enlarged <i>V</i>_OC of the resulting photovoltaic device is observed. More importantly, the PBDTSe-TT:PC₇₁BM bulk-heterojunction thin film morphology can be optimized through this modification. As a result, an efficient PCE of 8.8% is achieved without using any solvent additive or special interfacial layer. In addition, the PBDTSe-TT-based device is relatively stable under thermal stress, making it a good candidate for fabricating stacking cells. Finally, a ∼10% PCE tandem device is demonstrated by using identical PBDTSe-TT:PC₇₁BM subcells

Low-Temperature Solution-Processed Perovskite Solar Cells with High Efficiency and Flexibility

Perovskite compounds have attracted recently great attention in photovoltaic research. The devices are typically fabricated using condensed or mesoporous TiO₂ as the electron transport layer and 2,2′7,7′-tetrakis-(<i>N</i>,<i>N</i>-dip-methoxy­phenyl­amine)9,9′-spiro­bi­fluorene as the hole transport layer. However, the high-temperature processing (450 °C) requirement of the TiO₂ layer could hinder the widespread adoption of the technology. In this report, we adopted a low-temperature processing technique to attain high-efficiency devices in both rigid and flexible substrates, using device structure substrate/ITO/PEDOT:PSS/CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i>/PCBM/Al, where PEDOT:PSS and PCBM are used as hole and electron transport layers, respectively. Mixed halide perovskite, CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i>, was used due to its long carrier lifetime and good electrical properties. All of these layers are solution-processed under 120 °C. Based on the proposed device structure, power conversion efficiency (PCE) of 11.5% is obtained in rigid substrates (glass/ITO), and a 9.2% PCE is achieved for a polyethylene terephthalate/ITO flexible substrate

Integrated Perovskite/Bulk-Heterojunction toward Efficient Solar Cells

We successfully demonstrated an integrated perovskite/bulk-heterojunction (BHJ) photovoltaic device for efficient light harvesting and energy conversion. Our device efficiently integrated two photovoltaic layers, namely a perovskite film and organic BHJ film, into the device. The device structure is ITO/TiO₂/perovskite/BHJ/MoO₃/Ag. A wide bandgap small molecule DOR3T-TBDT was used as donor in the BHJ film, and a power conversion efficiency (PCE) of 14.3% was achieved in the integrated device with a high short circuit current density (<i>J</i>_SC) of 21.2 mA cm^–2. The higher <i>J</i>_SC as compared to that of the traditional perovskite/HTL (hole transporting layer) device (19.3 mA cm^–2) indicates that the BHJ film absorbs light and contributes to the current density of the device. Our result further suggests that the HTL in traditional perovskite solar cell, even with good light absorption capability, cannot contribute to the overall device photocurrent, unless this HTL becomes a BHJ layer (by adding electron transporting material like PC₇₁BM)