Zhaolai Chen

From planar-heterojunction to n–i structure: an efficient strategy to improve short-circuit current and power conversion efficiency of aqueous-solution-processed hybrid solar cells

In this paper, inverted hybrid solar cells (HSCs) with planar heterojunction (PHJ) and n–i structure are fabricated from an aqueous-solution-processed poly(p-phenylenevinylene) (PPV) precursor and 2-mercaptoethylamine (MA)-stabilized CdTe nanocrystals (NCs). Firstly, one densely stacked, smooth CdTe film with a wide absorption range and sufficient water resistance is obtained and used for fabricating the PHJ device. A PCE of 3.75% is obtained, which is comparable to the bulk-heterojunction (BHJ) devices. Secondly, the n–i structure is constructed by replacing the PPV layer with a mixed solution consisting of the PPV precursor and CdTe NCs. The n–i structure shows improved photocurrent with lowered fill factor (FF). The low FF is attributed to the severe phase separation of the PPV:CdTe layer, which is caused by the hydrophobicity of the annealed CdTe layer. Subsequently, the hydrophilicity of the annealed CdTe layer is improved by spin-coating an aqueous solution of CdCl2, which greatly increases the FF. Eventually, a record power conversion efficiency of 4.76% is achieved and the maximum Jsc can reach 16.08 mA cm−2.

Conducting the Temperature-Dependent Conformational Change of Macrocyclic Compounds to the Lattice Dilation of Quantum Dots for Achieving an Ultrasensitive Nanothermometer

We report a ligand decoration strategy to enlarge the lattice dilation of quantum dots (QDs), which greatly enhances the characteristic sensitivity of a QD-based thermometer. Upon a multiple covalent linkage of macrocyclic compounds with QDs, for example, thiolated cyclodextrin (CD) and CdTe, the conformation-related torsional force of CD is conducted to the inner lattice of CdTe under altered temperature. The combination of the lattice expansion/contraction of CdTe and the stress from CD conformation change greatly enhances the shifts of both UV-vis absorption and photoluminescence (PL) spectra, thus improving the temperature sensitivity. As an example, β-CD-decorated CdTe QDs exhibit the 0.28 nm shift of the spectra per degree centigrade (0.28 nm/°C), 2.4-fold higher than those of monothiol-ligand-decorated QDs.

Aqueous-Processed Inorganic Thin-Film Solar Cells Based on CdSexTe1–x Nanocrystals: The Impact of Composition on Photovoltaic Performance

Aqueous processed nanocrystal (NC) solar cells are attractive due to their environmental friendliness and cost effectiveness. Controlling the bandgap of absorbing layers is critical for achieving high efficiency for single and multijunction solar cells. Herein, we tune the bandgap of CdTe through the incorporation of Se via aqueous process. The photovoltaic performance of aqueous CdSexTe1-x NCs is systematically investigated, and the impacts of charge generation, transport, and injection on device performance for different compositions are deeply discussed. We discover that the performance degrades with the increasing Se content from CdTe to CdSe. This is mainly ascribed to the lower conduction band (CB) of CdSexTe1-x with higher Se content, which reduces the driving force for electron injection into TiO2. Finally, the performance is improved by mixing CdSexTe1-x NCs with conjugated polymer poly(p-phenylenevinylene) (PPV), and power conversion efficiency (PCE) of 3.35% is achieved based on ternary NCs. This work may provide some information to further optimize the aqueous-processed NC and hybrid solar cells.

Polymer-Passivated Inorganic Cesium Lead Mixed-Halide Perovskites for Stable and Efficient Solar Cells with High Open-Circuit Voltage over 1.3 V.

Cesium-based trihalide perovskites have been demonstrated as promising light absorbers for photovoltaic applications due to their superb composition stability. However, the large energy losses (Eloss ) observed in inorganic perovskite solar cells has become a major hindrance impairing the ultimate efficiency. Here, an effective and reproducible method of modifying the interface between a CsPbI2 Br absorber and polythiophene hole-acceptor to minimize the Eloss is reported. It is demonstrated that polythiophene, deposited on the top of CsPbI2 Br, can significantly reduce electron-hole recombination within the perovskite, which is due to the electronic passivation of surface defect states. In addition, the interfacial properties are improved by a simple annealing process, leading to significantly reduced energy disorder in polythiophene and enhanced hole-injection into the hole-acceptor. Consequently, one of the highest power conversion efficiency (PCE) of 12.02% from a reverse scan in inorganic mixed-halide perovskite solar cells is obtained. Modifying the perovskite films with annealing polythiophene enables an open-circuit voltage (VOC ) of up to 1.32 V and Eloss of down to 0.5 eV, which both are the optimal values reported among cesium-lead mixed-halide perovskite solar cells to date. This method provides a new route to further improve the efficiency of perovskite solar cells by minimizing the Eloss .

Efficient aqueous-processed hybrid solar cells from a polymer with a wide bandgap

In this work, MPPV with a wide bandgap is synthesized and combined with CdTe NCs for aqueous-processed polymer–nanocrystal hybrid solar cells (HSCs). A PCE of 5.18% is achieved, which is the highest for solar cells via an aqueous process. The function of MPPV is deeply investigated which shows that MPPV can fill the voids between CdTe NCs to decrease the leakage current. Furthermore, hole transfer from CdTe NCs to MPPV occurs, thus leading to increased carrier lifetime. Finally, MPPV can promote the interfacial carrier injection. We demonstrate that efficient HSCs can be achieved from polymers with a wide bandgap. In this case, key factors limiting traditional HSCs may be eliminated. As a consequence, this work may provide an alternative way to develop HSCs.

Thin single crystal perovskite solar cells to harvest below-bandgap light absorption

The efficiency of perovskite solar cells has surged in the past few years, while the bandgaps of current perovskite materials for record efficiencies are much larger than the optimal value, which makes the efficiency far lower than the Shockley–Queisser efficiency limit. Here we show that utilizing the below-bandgap absorption of perovskite single crystals can narrow down their effective optical bandgap without changing the composition. Thin methylammonium lead triiodide single crystals with tuned thickness of tens of micrometers are directly grown on hole-transport-layer covered substrates by a hydrophobic interface confined lateral crystal growth method. The spectral response of the methylammonium lead triiodide single crystal solar cells is extended to 820 nm, 20 nm broader than the corresponding polycrystalline thin-film solar cells. The open-circuit voltage and fill factor are not sacrificed, resulting in an efficiency of 17.8% for single crystal perovskite solar cells.Thin films of halide perovskites are promising for solar cell technology but they do not perform well at the band edge due to the low optical absorption. Herein, Chen et al. fabricate a high efficiency single crystal perovskite solar cell with thicker single crystals to harvest the below-bandgap photons.

Low-Noise and Large-Linear-Dynamic-Range Photodetectors Based on Hybrid-Perovskite Thin-Single-Crystals

Organic-inorganic halide perovskites are promising photodetector materials due to their strong absorption, large carrier mobility, and easily tunable bandgap. Up to now, perovskite photodetectors are mainly based on polycrystalline thin films, which have some undesired properties such as large defective grain boundaries hindering the further improvement of the detector performance. Here, perovskite thin-single-crystal (TSC) photodetectors are fabricated with a vertical p-i-n structure. Due to the absence of grain-boundaries, the trap densities of TSCs are 10-100 folds lower than that of polycrystalline thin films. The photodetectors based on CH3 NH3 PbBr3 and CH3 NH3 PbI3 TSCs show low noise of 1-2 fA Hz-1/2 , yielding a high specific detectivity of 1.5 × 1013 cm Hz1/2 W-1 . The absence of grain boundaries reduces charge recombination and enables a linear response under strong light, superior to polycrystalline photodetectors. The CH3 NH3 PbBr3 photodetectors show a linear response to green light from 0.35 pW cm-2 to 2.1 W cm-2 , corresponding to a linear dynamic range of 256 dB.

High efficiency aqueous-processed MEH-PPV/CdTe hybrid solar cells with a PCE of 4.20%

Due to their low cost, environmental friendliness and efficiency, aqueous-processed polymer/nanocrystal hybrid solar cells (HSCs) have attracted much attention in recent years. To promote the development of aqueous-processed HSCs, the design and synthesis of new water soluble conjugated polymers (WSCPs) are required. In this work, aqueous-processed MEH-PPV/CdTe HSCs are firstly fabricated. A PCE of 4.20% is achieved, which is comparable to the oil-processed MEH-PPV/NC HSCs. The water-soluble MEH-PPV precursor could increase the miscibility between polymers and aqueous CdTe NCs, and decrease their phase size. Furthermore, by adjusting MEH-PPV/CdTe ratio and annealing temperature, the nanoscale morphology of MEH-PPV/CdTe HSCs will change, which impacts charge transfer and transport. These results provide an important approach for efficient aqueous-processed MEH-PPV/NC HSCs.

Improvement in Open-Circuit Voltage of Thin Film Solar Cells from Aqueous Nanocrystals by Interface Engineering.

In this work, improved solar cells from aqueous CdTe NCs is achieved by replacing evaporated MoOx with spiro-OMeTAD as a hole transfer layer. The increased Voc and Jsc can be attributed to interfacial dipole effect and reduced back recombination loss, respectively. A high PCE of 6.56% for solar cells from aqueous NCs is obtained by optimizing the microstructure further.

High-Efficiency Aqueous-Solution-Processed Hybrid Solar Cells Based on P3HT Dots and CdTe Nanocrystals

Without using any environmentally hazardous organic solution, we fabricated hybrid solar cells (HSCs) based on the aqueous-solution-processed poly(3-hexylthiophene) (P3HT) dots and CdTe nanocrystals (NCs). As a novel aqueous donor material, the P3HT dots are prepared through a reprecipitation method and present an average diameter of 2.09 nm. When the P3HT dots are mixed with the aqueous CdTe NCs, the dependence of the device performance on the donor-acceptor ratio shows that the optimized ratio is 1:24. Specifically, the dependence of the device performance on the active-layer thermal annealing conditions is investigated. As a result, the optimized annealing temperature is 265 °C, and the incorporation of P3HT dots as donor materials successfully reduced the annealing time from 1 h to 10 min. In addition, the transmission electron microscopy and atomic force microscopy measurements demonstrate that the size of the CdTe NCs increased as the annealing time increased, and the annealing process facilitates the formation of a smoother interpenetrating network in the active layer. Therefore, charge separation and transport in the P3HT dots:CdTe NCs layer are more efficient. Eventually, the P3HT dots:CdTe NCs solar cells achieved 4.32% power conversion efficiency. The polymer dots and CdTe NCs based aqueous-solution-processed HSCs provide an effective way to avoid a long-time thermal annealing process of the P3HT dots:CdTe NCs layer and largely broaden the donor materials for aqueous HSCs.