One-Step Synthesis of ZnO Nanocrystals in <i>n</i>‑Butanol with Bandgap Control: Applications in Hybrid and Organic Photovoltaic Devices

Solution-grown ZnO nanocrystals (NCs) offer a viable path for large-area, low-temperature processing required for optoelectronics on flexible substrates. They can function as the acceptor in hybrid photovoltaic (HPV) devices or the electron transport layer (ETL) in organic photovoltaic (OPV) devices. We demonstrate a one-step synthesis, using diethanolamine (DEA) and water as additives, to produce ZnO NCs in <i>n</i>-butanol with controlled bandgap. We found that the addition of DEA enhances ZnO NC dispersion, which facilitates the formation of smooth films. For a given DEA concentration, varying the water volume fraction in precursor solution controls the ZnO NC grain size, hence its bandgap. We studied the impact of ZnO bandgap on HPV and OPV device performance. While ZnO NCs with higher bandgap increase the open circuit voltage of bilayer polymer/ZnO HPV devices, ETLs of ZnO NCs with different bandgaps result in identical inverted OPV device performance. The ZnO NC suspensions in <i>n</i>-butanol also allow the fabrication of conventional OPV devices, as they form uniform thin films on top of organics without additional processing. Compared to devices with Ca/Al contacts, these conventional devices exhibit equivalent performance but superior stability in air. These ZnO NC <i>n</i>-butanol suspensions could be readily applied to other optoelectronic and photovoltaic applications, for which type II heterojunction is desired and low-temperature solution processing is required

Solution Synthesized <i>p</i>‑Type Copper Gallium Oxide Nanoplates as Hole Transport Layer for Organic Photovoltaic Devices

<i>p</i>-Type metal-oxide hole transport layer (HTL) suppresses recombination at the anode and hence improves the organic photovoltaic (OPV) device performance. While NiO_<i>x</i> has been shown to exhibit good HTL performance, very thin films (<10 nm) are needed due to its poor conductivity and high absorption. To overcome these limitations, we utilize CuGaO₂, a <i>p</i>-type transparent conducting oxide, as HTL for OPV devices. Pure delafossite phase CuGaO₂ nanoplates are synthesized via microwave-assisted hydrothermal reaction in a significantly shorter reaction time compared to via conventional heating. A thick CuGaO₂ HTL (∼280 nm) in poly­(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) devices achieves 3.2% power conversion efficiency, on par with devices made with standard HTL materials. Such a thick CuGaO₂ HTL is more compatible with large-area and high-volume printing process

Gallium Nitride Based Logpile Photonic Crystals

We demonstrate a nine-layer logpile three-dimensional photonic crystal (3DPC) composed of single crystalline gallium nitride (GaN) nanorods, ∼100 nm in size with lattice constants of 260, 280, and 300 nm with photonic band gap in the visible region. This unique GaN structure is created through a combined approach of a layer-by-layer template fabrication technique and selective metal organic chemical vapor deposition (MOCVD). These GaN 3DPC exhibit a stacking direction band gap characterized by strong optical reflectance between 380 and 500 nm. By introducing a “line-defect” cavity in the fifth (middle) layer of the 3DPC, a localized transmission mode with a quality factor of 25–30 is also observed within the photonic band gap. The realization of a group III nitride 3DPC with uniform features and a band gap at wavelengths in the visible region is an important step toward realizing complete control of the electromagnetic environment for group III nitride based optoelectronic devices

Effects of Contact-Induced Doping on the Behaviors of Organic Photovoltaic Devices

Substrates can significantly affect the electronic properties of organic semiconductors. In this paper, we report the effects of contact-induced doping, arising from charge transfer between a high work function hole extraction layer (HEL) and the organic active layer, on organic photovoltaic device performance. Employing a high work function HEL is found to increase doping in the active layer and decrease photocurrent. Combined experimental and modeling investigations reveal that higher doping increases polaron–exciton quenching and carrier recombination within the field-free region. Consequently, there exists an optimal HEL work function that enables a large built-in field while keeping the active layer doping low. This value is found to be ∼0.4 eV larger than the pinning level of the active layer material. These understandings establish a criterion for optimal design of the HEL when adapting a new active layer system and can shed light on optimizing performance in other organic electronic devices

Quantitative Analyses of Competing Photocurrent Generation Mechanisms in Fullerene-Based Organic Photovoltaics

The performance of fullerene-based organic photovoltaic devices (OPVs) with low donor concentrations is not limited by the trade-off between short-circuit current density (<i>J</i>_sc) and open-circuit voltage (<i>V</i>_oc), unlike bulk heterojunction OPVs. While the high <i>V</i>_oc in this novel type of OPVs has been studied, here we investigate the mechanisms that govern <i>J</i>_sc, which are not well understood. Three mechanisms, diffusion limited exciton relaxation, geminate recombination during exciton dissociation, and nongeminate recombination during charge transport, are studied analytically by combining various experimental techniques and transfer matrix simulation. We find that exciton dissociation at donor/acceptor interfaces is the dominant factor to produce high <i>J</i>_sc, and at low P3HT concentrations exciton relaxation limits photocurrent generation. With more P3HT inclusion, the creation of interfaces promotes exciton dissocation but also reduces fullerene crystallinity, weakening the driving force for charge separation, and introduces nongeminate recombination sites. Quantitative analyses show that the magnitude of measured <i>J</i>_sc and the donor concentration dependence are well accounted for by these three competing mechanisms