Hongmei Dang

Nanowire CdS-CdTe solar cells with molybdenum oxide as contact

Using a 10 nm thick molybdenum oxide (MoO3−x) layer as a transparent and low barrier contact to p-CdTe, we demonstrate nanowire CdS-CdTe solar cells with a power conversion efficiency of 11% under front side illumination. Annealing the as-deposited MoO3 film in N2 resulted in a reduction of the cell’s series resistance, from 9.97 Ω/cm2 to 7.69 Ω/cm2, and increase in efficiency from 9.9% to 11%. Under illumination from the back, the MoO3−x/Au side, the nanowire solar cells yielded Jsc of 21 mA/cm2 and efficiency of 8.67%. Our results demonstrate use of a thin layer transition metal oxide as a potential way for a transparent back contact to nanowire CdS-CdTe solar cells. This work has implications toward enabling a novel superstrate structure nanowire CdS-CdTe solar cell on Al foil substrate by a low cost roll-to roll fabrication process.

CdS nanowire layers of enhanced transmittance for window layer applications in thin film solar cells

Arrays of n-CdS nanowires were investigated as replacement for the planar n-CdS film; the latter is presently used as a window layer in important photovoltaic devices like CdS-CdTe and CdS-CIGS. CdS nanowire arrays continued to exhibit substantial transmission even when the wavelength of incident radiation was well below 512 nm, which is the wavelength corresponding to the energy band gap of single crystal CdS. Theoretical calculation involving the number of excess photons transmitted through the CdS nanowire window layer indicated a potential enhancement in photocurrent by as much as 38% over the planar CdS film window layer device. Next, Au/CdS Schottky diodes were formed by depositing thin films of gold on the nanowire arrays. Analysis of the current-voltage characteristics of these Schottky diodes showed smaller diode ideality factors and higher carrier concentrations in devices with CdS nanowires than in devices with planar CdS films; these are desirable junction characteristics when the designer is trying to maximize the open circuit voltage and the power output.

Wide spectral response and ambient air-stable nanowire window-absorber type solar cells

Use of embedded CdS nanowires is demonstrated for partially overcoming the spectral transmission loss and the low mechanical and electrical robustness of planar CdS window layer and thus enhancing the quantum efficiency and reliability of CdS-CdTe solar cells. Power conversion efficiency of 12% is achieved, and aging and reliability are improved by approximately 3 times in un-encapsulated cells, over their traditional planar CdS counterparts. A nearly ideal spectral response of quantum efficiency at a wide spectrum range provides evidence for enhanced light absorption and carrier generation and collection in the absorber layer. Our results prove the efficacy of this nanowire approach for enhancing the quantum efficiency and the reliability in window-absorber type solar cells like CdS-CdTe, CdS-CIGS and CdS-CZTSSe.

Nanotube photovoltaic configuration for enhancement of carrier generation and collection

We report on the growth of geometric feature tuned semiconductor nanotubes on a transparent substrate through the application of an anodic aluminum oxide membrane-assisted method. Three-dimensional nanotube solar cells are developed in which semiconductor absorbers are not only used to fill the inner core of the nanotubes, but also to replace the membrane and to fill the intertube space between the nanotubes. The nanotube solar cells generate and separate carriers in three dimensions, namely, inside the cores of the nanotubes, in the intertube space between the nanotubes along the radial direction, and above the nanotubes along the axial direction. In preliminary experiments conducted to demonstrate the potential of this approach, nanotube CdS-CdTe solar cells were fabricated. CdS nanotubes with an inner diameter, wall thickness and intertube spacing of 35, 20, and 35 nm, respectively, were grown; the porosity and CdS nanotube density were 36.5% and 2.26 × 1010 nanotubes/cm2, respectively. These features of CdS nanotubes enable more efficient carrier collection because of the reduced recombination, especially in those cases in which the minority carrier lifetime is short, thus resulting in a diffusion length of less than 100 nm. Nanotube CdS-CdTe solar cells exhibit a wide and strong spectral response and quantum efficiency, indicating enhanced light absorption and carrier generation and collection. Without the benefit of an antireflection coating, the cells exhibited a wide and strong spectral response of quantum efficiency, and a short current density of 25.5 mA/cm2, an open circuit voltage of 750 mV, and a power conversion efficiency of 10.7% under 1-sun illumination. The materials and electro-optical characterizations indicated well-defined junction and interface behavior in these 3D nanotube solar cell configurations.

Comprehensive study of the two-step solution processes in ambient air for lead iodide perovskite solar cells

Organic-inorganic lead halide perovskite solar cells have attracted a great deal of interest because of potential low cost and high efficiency. However, in most publications, solar cells were fabricated in a highly controlled environment (glove boxes). Only a few papers reported fabrication of solar cells in air with the maximum efficiency of ~15%. We carried out comprehensive study of the two-step solution process parameters for fabrication of perovskite solar cells in ambient air including PbI2 spin coating speed, mesoporous TiO2 spin coating speed, hole transport material (HTM) spin coating speed, and HTM exposure to air etc. We found that the higher spin speed of PbI2 is critical for high-quality PbI2 film formation. The HTM exposure to air is also important to fabricate solar cells with larger current density and better fill factors.

Molybdenum oxide contacts for nanowire CdS-CdTe solar cells

Using a 10nm thick molybdenum oxide (MoO_3-x) layer as a transparent and low barrier contact to p-CdTe, we demonstrate nanowire CdS-CdTe solar cells with a power conversion efficiency of 11% under front side illumination. Annealing the as-deposited MoO₃ film in N₂ resulted in a reduction of the cells series resistance from 10 Ω/cm² to 7.7 Ω/cm², and efficiency improvement from 9.9% to 11%. Under illumination from the back, the MoO_3-x/Au side, the nanowire solar cells yielded J_sc of 20.9mA/cm² and efficiency of 8.7%. This work has implications toward enabling a novel superstrate structure nanowire CdS-CdTe solar cell on Al foil substrate by a low cost roll-to roll fabrication process.

Effects of anodic aluminum oxide membrane on performance of nanostructured solar cells

Three nanowire solar cell device configurations have been fabricated to demonstrate the effects of the host anodized aluminum oxide (AAO) membrane on device performance. The three configurations show similar transmittance spectra, indicating that AAO membrane has negligible optical absorption. Power conversion efficiency (PCE) of the device is studied as a function of the carrier transport and collection in cell structures with and without AAO membrane. Free standing nanowire solar cells exhibit PCE of 9.9%. Through inclusion of AAO in solar cell structure, interface defects and traps caused by humidity and oxygen are reduced, and direct contact of CdTe tentacles with SnO2 and formation of micro shunt shorts are prevented; hence PCE is improved to 11.1%–11.3%. Partially embedded nanowire solar cells further reduce influence of non-ideal and non-uniform nanowire growth and generate a large amount of carriers in axial direction and also a small quantity of carriers in lateral direction, thus becoming a promising solar cell structure. Thus, including AAO membrane in solar cell structure provides favorable electro-optical properties as well as mechanical advantages.

Electro-optical characterization of n-CdS nanowires/p-CdTe heterojunction solar cell devices

Arrays of 100 nm long n-cadmium sulfide (CdS) nanowires were investigated as window layers for solar cells. These arrays exhibited enhanced transmission especially for wavelengths below 512 nm. Next, films of CdTe were deposited by CSS and the solar cell devices completed. I-V and C-V characteristics were measured in the temperature range of 100-300 °K. Analysis indicated that interface recombination and tunneling played dominant roles in electron transport across the heterojunction. The highest power conversion efficiency (PCE) nanowire cell to date had Voc, Jsc, FF and PCE values of 766 mV, 25 mA/cm2, 58% and 11.1% respectively.

Preparation and evaluation of perovskite solar cells in the absolute atmospheric environment

Recently solid state hybrid organometal halide perovskite solar cells have become the research hotspot. We fabricated perovskite solar cells by a simple two-step method completely in the regular ambient condition without a glove box, and all materials were just stored in ambient air. The cross-section SEM image of the perovskite solar cells exhibits a well-defined layer-by-layer structure with clear interfaces. XRD pattern shows main diffraction peaks centred at 14.2° (110) and 28.5° (220), which can be assigned to the CH3NH3PbI3 phase, suggesting a crystal structure, and the peak centred at 12.7° is attributed to PbI2. Our best Jsc is 19.2 mA/cm2, the best Voc is 1.0 V, the best FF is 0.65, and the best PCE is 10.2%.