Dekun Zhang

Elucidating the role of chlorine in perovskite solar cells

It has been proposed that introducing the chlorine anion into a CH3NH3PbI3 perovskite material can substantially improve the materials properties as well as the solar cell performance. To elucidate the role of chlorine in perovskite solar cells (PSCs), here we introduced PbCl2 into the precursor, and studied the chlorine configuration evolution during perovskite film formation and the associated influence on PSC performance in detail. We found that chlorine could be successfully incorporated into the precursor film in the form of PbICl or PbCl2 through a properly designed preparation, and it was conserved in the final perovskite film with a configuration of MAPbCl3, PbICl or PbCl2 depending on the fabrication process. However, no evidence of a MAPbI3−xClx phase was observed, and it is considered that MAPbI3−xClx might be metastable or possesses a higher formation energy. In addition, we demonstrate that the formation of a porous PbICl scaffold in the precursor film plays a key role in high quality perovskite film realization, benefiting from an effective stress release during structure expansion after methylammonium iodide dripping. Moreover, we propose that residual amorphous PbCl2 can effectively passivate defects in perovskite film, and dramatically improve the film’s electrical properties. Finally, n–i–p type planar PSCs with efficiencies up to 19.45% were achieved. It should be mentioned that the whole process for the formation of the PSCs is performed at less than 100 °C, which is beneficial for a wide range of applications, such as flexible and tandem solar cells.

High efficiency and high open-circuit voltage quadruple-junction silicon thin film solar cells for future electronic applications

Conversion of clean and renewable solar energy into electricity with photovoltaic (PV) devices, based on earth-abundant silicon elements to meet increasing global energy demands and environmental sustainability, has motivated various potential industrial and domestic applications. In addition to large-scale electricity production of market-dominant crystalline silicon PVs, the unique properties of silicon-based thin-film solar-cells (TFSCs) make them very attractive as affordable clean and safe energy devices. Herein, with large-scale and mature plasma-enhanced chemical vapor deposition (PECVD) process that can efficiently fabricate high-performing a-SiC:H, a-SiGe:H, a-Si:H, and μc-Si:H single- and various multi-junction TFSCs, we report a highly-efficient and flexibly tunable monolithic quadruple-junction silicon TFSC with a high photovoltage above 3.0 V and power conversion efficiency of 15.03% (NREL measured 14.58%). Our proposed high-voltage silicon TFSCs, with excellent performance, can further enrich the toolbox for functional photoelectrical devices and inspire possible future applications as highly promising power supply sources in charging electronics, splitting and disinfecting water, powering household electronic devices, solar to CO2 reduction, and other possible applications.

Optical properties of conductive silver-nanowire films with different nanowire lengths

Transparent electrodes made of silver nanowires (Ag NWs) exhibit a higher flexibility than conventional indium tin oxide electrodes. For this reason, Ag NWs may find applications in future flexible electronic and optoelectronic devices. However, different optoelectronic devices have different specific requirements for Ag NWs. For example, the optical transmittance haze is an important but rarely studied aspect of Ag NW films. In this study, the optical transmittance and optical scattering of long (5–50 μm, L-NWs) and short (1–20 μm, S-NWs) Ag NW films were investigated. The L-NWs exhibited better optical transmission than the S-NWs, whereas the S-NWs exhibited better light-scattering properties than the L-NWs. Our results indicate that the L-NWs are suitable for touch-screen displays, whereas the S-NWs are better suited as transparent conductive films for solar cells. We analyzed the scattering ratio of forward-scattered light to backscattered light for both the L-NWs and S-NWs and discovered that the mesh size affected the scattering ratio. For longer wavelengths, a larger mesh yielded a higher backscattering ratio, whereas a smaller mesh yielded a lower backscattering ratio. We formulated an equation for calculating the reflection haze using the total reflection (Ag NWs/glass), R and the reflection of glass, R0. The reflection haze of the S-NWs and L-NWs exhibited different trends in the visible–near-infrared region. An omnidirectional scattering model for the Ag NWs was used to evaluate the Ag NW scattering properties. The results of this study have great significance for the evaluation of the performance of Ag NWs in optoelectronic devices.

Hydrogenated TiO2 Thin Film for Accelerating Electron Transport in Highly Efficient Planar Perovskite Solar Cells

Abstract Intensive studies on low‐temperature deposited electron transport materials have been performed to improve the efficiency of n‐i‐p type planar perovskite solar cells to extend their application on plastic and multijunction device architectures. Here, a TiO2 film with enhanced conductivity and tailored band edge is prepared by magnetron sputtering at room temperature by hydrogen doping (HTO), which accelerates the electron extraction from perovskite photoabsorber and reduces charge transfer resistance, resulting in an improved short circuit current density and fill factor. The HTO film with upward shifted Fermi level guarantees a smaller loss on V OC and facilitates the growth of high‐quality absorber with much larger grains and more uniform size, leading to devices with negligible hysteresis. In comparison with the pristine TiO2 prepared without hydrogen doping, the HTO‐based device exhibits a substantial performance enhancement leading to an efficiency of 19.30% and more stabilized photovoltaic performance maintaining 93% of its initial value after 300 min continuous illumination in the glove box. These properties permit the room‐temperature magnetron sputtered HTO film as a promising electron transport material for flexible and tandem perovskite solar cell in the future.

Origin of Photovoltage Enhancement via Interfacial Modification with Silver Nanoparticles Embedded in an a-SiC:H p-Type Layer in a-Si:H Solar Cells

We used silver nanoparticles (Ag-NPs) embedded in the p-type semiconductor layer of hydrogenated amorphous silicon (a-Si:H) solar cells in the Schottky barrier contact design to modify the interface between aluminum-doped ZnO (ZnO:Al, AZO) and p-type hydrogenated amorphous silicon carbide (p-a-SiC:H) without plasmonic absorption. The high work function of the Ag-NPs provided a good channel for the transport of photogenerated holes. A p-type nanocrystalline SiC:H layer was used to compensate for the real surface defects and voids on the surface of Ag-NPs to reduce recombination at the AZO/p-type layer interface, which then enhanced the photovoltage of single-junction a-Si:H solar cells to values as high as 1.01 V. The Ag-NPs were around 10 nm in diameter and thermally stable in the p-type a-SiC:H film at the solar-cell process temperature. We will also show that a wide range of photovoltages between 1.01 and 2.89 V could be obtained with single-, double-, and triple-junction solar cells based on the single-junction a-Si:H solar cells with tunable high photovoltage. These solar cells are suitable photocathodes for solar water-splitting applications.

Microstructure and lateral conductivity control of hydrogenated nanocrystalline silicon oxide and its application in a-Si:H/a-SiGe:H tandem solar cells*

Phosphorous-doped hydrogenated nanocrystalline silicon oxide (n-nc-SiO x :H) films are prepared via radio frequency plasma enhanced chemical vapor deposition (RF-PECVD). Increasing deposition power during n-nc-SiO x :H film growth process can enhance the formation of nanocrystalline and obtain a uniform microstructure of n-nc-SiO x :H film. In addition, in 20s interval before increasing the deposition power, high density small grains are formed in amorphous SiO x matrix with higher crystalline volume fraction (I c) and have a lower lateral conductivity. This uniform microstructure indicates that the higher I c can leads to better vertical conductivity, lower refractive index, wider optical band-gap. It improves the back reflection in a-Si:H/a-SiGe:H tandem solar cells acting as an n-nc-SiO x :H back reflector prepared by the gradient power during deposition. Compared with the sample with SiO x back reflector, with a constant power used in deposition process, the sample with gradient power SiO x back reflector can enhance the total short-circuit current density (J sc) and the initial efficiency of a-Si:H/a-SiGe:H tandem solar cells by 8.3% and 15.5%, respectively.

Perovskite/silicon-based heterojunction tandem solar cells with 14.8% conversion efficiency via adopting ultrathin Au contact*

A rising candidate for upgrading the performance of an established narrow-bandgap solar technology without adding much cost is to construct the tandem solar cells from a crystalline silicon bottom cell and a high open-circuit voltage top cell. Here, we present a four-terminal tandem solar cell architecture consisting of a self-filtered planar architecture perovskite top cell and a silicon heterojunction bottom cell. A transparent ultrathin gold electrode has been used in perovskite solar cells to achieve a semi-transparent device. The transparent ultrathin gold contact could provide a better electrical conductivity and optical reflectance-scattering to maintain the performance of the top cell compared with the traditional metal oxide contact. The four-terminal tandem solar cell yields an efficiency of 14.8%, with contributions of the top (8.98%) and the bottom cell (5.82%), respectively. We also point out that in terms of optical losses, the intermediate contact of self-filtered tandem architecture is the uppermost problem, which has been addressed in this communication, and the results show that reducing the parasitic light absorption and improving the long wavelength range transmittance without scarifying the electrical properties of the intermediate hole contact layer are the key issues towards further improving the efficiency of this architecture device.

Interfacial modification for improving inverted organic solar cells by poly(N-vinylpyrrolidone)

The effect of the thickness of the poly(N-vinylpyrrolidone) interface modifier on the photovoltaic performance of inverted organic solar cells was investigated. Superior interface properties provided efficient charge transport and decreased the charge recombination due to PVP interlayer, which reduced the energy barrier for electron extraction by lowering the hydroxide radical amount. We obtained an enhanced efficiency of 4.55% (for the P3HT:PCBM device) and 6.18% (for the PTB7:PC71BM device).

Mechanism insight into the effect of I/P buffer layer on the performance of NIP-type hydrogenated microcrystalline silicon solar cells

A simulation and experimental study on the effect of the buffer layer at the I/P interface on the performance of NIP-type hydrogenated microcrystalline silicon (μc-Si:H) single-junction solar cells is presented. Device-quality hydrogenated amorphous silicon (a-Si:H) material as a buffer layer at the I/P interface obviously improves the performance of NIP-type μc-Si:H single-junction solar cells. In addition to the well-known mechanism that an a-Si:H I/P buffer layer can reduce the recombination current density at I/P interfaces, the optically and electrically calibrated simulations and supporting experimental results in this study illustrate that the performance improvement also originates from the mitigation of the electric screening effect due to the reduced defect density at the I/P interfaces, which reinforces the bulk electric field. Integrating an optimized hydrogen profiling strategy and adding a-Si:H I/P buffer layer yielded an initial efficiency of 9.20% for μc-Si:H single-junction solar cells wi...

Novel Insight into the Function of PC 61 BM in Efficient Planar Perovskite Solar Cells

We introduced a PC61BM layer between the compact TiO2 layer and the perovskite absorber, which forms a porous precursor film, and thus promotes uniform perovskite films with large grain size and improves the device efficiency.