Impact of band-gap graded structures artificially implemented in Mg–ZnO epitaxial films on photoelectrochemical properties

The impact of band gap-graded structures artificially implemented in a photocatalyst on the photoelectrochemical properties was investigated in a model system of epitaxial thin film Mg–ZnO.

Ionic liquid/ZnO(0001[combining macron]) single crystal and epitaxial film interfaces studied through a combination of electrochemical measurements and a pulsed laser deposition process under vacuum

O-Polar ZnO(0001[combining macron]) single crystals and ZnO and Mg-doped ZnO (MgZnO) films which were subsequently deposited on the ZnO crystals by a pulsed laser deposition (PLD) method were electrochemically investigated through the interfaces with ionic liquid (IL) in a vacuum. The sample surfaces were confirmed to be atomically clean and flat by reflection high energy electron diffraction (RHEED) observation, prior to their electrochemical measurements. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were then performed, and the donor density, flat band potential of these ZnO samples, and the electric double layer capacitance at the IL/ZnO interfaces were successfully evaluated. The flat band potentials of ZnO and MgZnO films were found to shift to more negative potentials relative to those of the single crystal ZnO, with different values for thicker films, respectively. Some possible origins of the different flat band potentials between ZnO and MgZnO films, and their film thickness dependence of the flat band potential will be discussed in this paper.

Understanding charge transfer and recombination by interface engineering for improving the efficiency of PbS quantum dot solar cells

In quantum dot heterojunction solar cells (QDHSCs), the QD active layer absorbs sunlight and then transfers the photogenerated electrons to an electron-transport layer (ETL). It is generally believed that the conduction band minimum (CBM) of the ETL should be lower than that of the QDs to enable efficient charge transfer from the QDs to the collection electrode (here, FTO) through the ETL. However, by employing Mg-doped ZnO (Zn_1-xMg_xO) as a model ETL in PbS QDHSCs, we found that an ETL with a lower CBM is not necessary to realize efficient charge transfer in QDHSCs. The existence of shallow defect states in the Zn_1-xMg_xO ETL can serve as additional charge-transfer pathways. In addition, the conduction band offset (CBO) between the ETL and the QD absorber has been, for the first time, revealed to significantly affect interfacial recombination in QDHSCs. We demonstrate that a spike in the band structure at the ETL/QD interface is useful for suppressing interfacial recombination and improving the open-circuit voltage. By varying the Mg doping level in ZnO, we were able to tune the CBM, defect distribution and carrier concentration in the ETL, which play key roles in charge transfer and recombination and therefore the device performance. PbS QDHSCs based on the optimized Zn_1-xMg_xO ETL exhibited a high power conversion efficiency of 10.6%. Our findings provide important guidance for enhancing the photovoltaic performance of QD-based solar cells.

Great photoluminescence enhancement in Al-sputtered Zn0.78Mg0.22O films

Zn_0.78Mg_0.22O thin films were grown on a-plane sapphire substrates by plasma-assisted molecular beam epitaxy. Compared with ZnO, the crystal quality of Zn_0.78Mg_0.22O thin films degrades significantly, which results in low internal quantum efficiency (η_int). Besides improving the quality of Zn_0.78Mg_0.22O, an effective method has been used to enhance the internal quantum efficiency and the UV emission of Zn_0.78Mg_0.22O by sputtering Al nanoparticles. Taking advantage of the resonant coupling between UV emission of Zn_0.78Mg_0.22O film and Al nanoparticle surface plasmons (SPs), a 59-fold enhancement of the UV emission and a 3.5-fold enhancement of η_int has been achieved under the optimized sputtering time. Moreover, the enhancement ratio is stable after two months. It paves a facile way in fabricating high-efficiency UV optoelectronic devices.

Composition dependent band offsets of ZnO and its ternary alloys

We report the calculated fundamental band gaps of wurtzite ternary alloys Zn_1-xM_xO (M = Mg, Cd) and the band offsets of the ZnO/Zn_1-xM_xO heterojunctions, these II-VI materials are important for electronics and optoelectronics. Our calculation is based on density functional theory within the linear muffin-tin orbital (LMTO) approach where the modified Becke-Johnson (MBJ) semi-local exchange is used to accurately produce the band gaps, and the coherent potential approximation (CPA) is applied to deal with configurational average for the ternary alloys. The combined LMTO-MBJ-CPA approach allows one to simultaneously determine both the conduction band and valence band offsets of the heterojunctions. The calculated band gap data of the ZnO alloys scale as E_g = 3.35 + 2.33x and E_g = 3.36 - 2.33x + 1.77x² for Zn_1-xMg_xO and Zn_1-xCd_xO, respectively, where x being the impurity concentration. These scaling as well as the composition dependent band offsets are quantitatively compared to the available experimental data. The capability of predicting the band parameters and band alignments of ZnO and its ternary alloys with the LMTO-CPA-MBJ approach indicate the promising application of this method in the design of emerging electronics and optoelectronics.

Enhanced internal quantum efficiency in non-polar ZnO/Zn0.81Mg0.19O multiple quantum wells by Pt surface plasmons coupling

Non-polar-oriented ZnO/Zn0.81Mg0.19O multiple quantum wells (MQWs) were grown on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy. The internal quantum efficiency (η(int)) of the non-polar MQWs was only 1.8%. The degraded quality of non-polar MQWs is the main factor for the low η(int). Besides improving the quality of non-polar MQWs, an effective way to enhance the UV emission of the non-polar MQWs by sputtering Pt nanoparticles has been used. Employing the resonant coupling between UV emission from the MQWs and Pt nanoparticle surface plasmons (SPs), a 20-fold enhancement of the UV emission has been achieved under the optimized sputtering time. Moreover, the η(int) value of the non-polar MQWs has been strongly improved with the help of Pt. 6.7-fold enhancement of η(int) has been achieved due to SPs coupling. It paves a new way in designing highly efficient non-polar LEDs.

Efficient light emission from inorganic and organic semiconductor hybrid structures by energy-level tuning

The fundamental limits of inorganic semiconductors for light emitting applications, such as holographic displays, biomedical imaging and ultrafast data processing and communication, might be overcome by hybridization with their organic counterparts, which feature enhanced frequency response and colour range. Innovative hybrid inorganic/organic structures exploit efficient electrical injection and high excitation density of inorganic semiconductors and subsequent energy transfer to the organic semiconductor, provided that the radiative emission yield is high. An inherent obstacle to that end is the unfavourable energy level offset at hybrid inorganic/organic structures, which rather facilitates charge transfer that quenches light emission. Here, we introduce a technologically relevant method to optimize the hybrid structure's energy levels, here comprising ZnO and a tailored ladder-type oligophenylene. The ZnO work function is substantially lowered with an organometallic donor monolayer, aligning the frontier levels of the inorganic and organic semiconductors. This increases the hybrid structure's radiative emission yield sevenfold, validating the relevance of our approach.