Nanostructured Al-ZnO/CdSe/Cu2O ETA solar cells on Al-ZnO film/quartz glass templates
© Wang et al; licensee Springer. 2011
Received: 19 August 2011
Accepted: 4 December 2011
Published: 4 December 2011
The quartz/Al-ZnO film/nanostructured Al-ZnO/CdSe/Cu2O extremely thin absorber solar cell has been successfully realized. The Al-doped ZnO one-dimensional nanostructures on quartz templates covered by a sputtering Al-doped ZnO film was used as the n-type electrode. A 19- to 35-nm-thin layer of CdSe absorber was deposited by radio frequency magnetron sputtering, coating the ZnO nanostructures. The voids between the Al-ZnO/CdSe nanostructures were filled with p-type Cu2O, and therefore, the entire assembly formed a p-i-n junction. The cell shows the energy conversion efficiency as high as 3.16%, which is an interesting option for developing new solar cell devices.
PACS: 88.40.jp; 73.40.Lq; 73.50.Pz.
Extremely thin absorber [ETA] solar cells have attracted much attention because of their probability to be low-cost solar cells. It consists of a nano- or microstructured layer which also serves as an n-type window layer to the cell, an absorber (1.1 < E g < 1.8 eV) that is conformably deposited on the layer, and a void-filling p-type material with a metallic back contact. Oxide semiconducting materials have potential applications in ETA solar cells as p- and n-type widow layers due to their many excellent physical properties and economic values, such as having a wide bandgap, good thermal stability, and being a low-cost and environment-friendly material. However, it is difficult to achieve p- and n-type oxide semiconductors simultaneously. One-dimensional [1-D] nanostructured zinc oxide [ZnO] semiconducting materials, which are intrinsic n-type wide-bandgap semiconductors with a direct bandgap energy of 3.37 eV and exciton bounding energy of 60 meV , are specially suitable for n-type electrode materials of solar cells [2–8]. When 1-D ZnO is used as an n-type electrode material for solar cells, it has a number of advantages, such as a high transmittance in the visible wavelength region, a larger surface area, a high electron mobility along the growth direction, and a highly efficient electron transport . However, the p-type ZnO electrode material has to be replaced by other semiconducting materials due to the difficulty in the growth of p-type ZnO. ETA solar cells with a ZnO nanostructure as n-type window material have reached the efficiency of 2.3% to 3.4% [2–4], but the p-type window material is CuSCN in these reported n-ZnO/absorber/p-CuSCN-structured ETA solar cells. In the process of such ETA solar cell fabrication, CuSCN is in the upper part of the absorber layer. When the fabrication of the absorber layer was finished, the synthesis of CuSCN encountered unexpected problems, such as homogeneity and reproducibility problems, during solution deposition. Cuprous oxide [Cu2O] is a natural p-type direct gap semiconductor with a bandgap energy of 2.1 eV . It has been predicated that Cu2O is promising for photovoltaic applications, with a theoretical energy conversion efficiency of 20% . Many papers have reported the fabrication of solar cells used in ZnO and Cu2O semiconducting materials as n- and p-type window layers [11–13], but the energy conversion efficiency is less than 1.5%. The realizations of such solar cells favor the fact that Cu2O is potentially useful for the application in the p-type layer of ETA solar cells.
In this study, we report the preparation and characterization of nanostructured Al-ZnO/CdSe/Cu2O solar cells on a Al-ZnO film/quartz glass template with the energy conversion efficiency as high as 3.16%.
Materials and methods
The morphology and structure of the as-synthesized samples were characterized using field-emission scanning electron microscopy (NoVaTM Nano SEM 430, Shanghai NanoVis Electronics Technologies Ltd., Co., Shanghai, China) and X-ray diffraction [XRD] (X'Pert Pro with CuKα1 of 1.5406 Å; X'Pert, PANalytical B.V., Almelo, The Netherlands). Optical transmission was carried out using a UV-3101PC spectrometer (Shimadzu Corporation, Nakagyo-ku, Kyoto, Japan). Photo conversion was measured using the Newport model 69907; the light source was a xenon lamp with a 100-mW/cm2 power at an air mass [AM] of 1.5. The light illuminated the samples from the glass substrate side with a contact area of 0.71 cm × 0.52 cm to 0.369 cm2. Electrodes were fabricated by depositing metal indium on the Al-ZnO film/quartz templates and on the gold/nickel on Cu2O film by conventional vacuum evaporation and sintering in vacuum (≤ 10-5 Torr).
Results and discussion
Figure 1a shows the morphologies of the quartz/Al-ZnO film/Al-ZnO nanostructures, indicating that the highly densified Al-ZnO hierarchical nanostructures are quasi-perpendicular to the substrate, and the typical density of the nanostructures is approximately 6 × 109 cm-2. The Al-ZnO nanostructures comprised three segments, namely a long single nanorod at the bottom, a thin cap in between, and several nanorods on top, respectively. The single nanorod is about 3 μm in length. The diameter of the nanorod is uneven and becomes larger and larger near the thin cap. The thin cap is about 50 nm in thickness with a diameter of approximately 400 nm. The nanorods on top have a hexagonal morphology with a length of approximately 600 nm. An absorber layer of CdSe was evenly deposited on the surface of the quartz/Al-ZnO film/Al-ZnO nanostructure by RF magnetron sputtering. The thickness of CdSe was estimated as half the difference value between the diameter of CdSe-coating ZnO nanostructure in the largest part and the diameter of the pristine ZnO nanostructure. The thickness of the CdSe layer was estimated in the range of 19 to 35 nm based on multimetering. The diameter of a single ZnO nanostructure was measured by the size label system of NoVaTM Nano SEM 430 (Shanghai NanoVis Electronics Technologies Ltd., Co., Shanghai, China), as shown in the insets of Figure 1a, b. The Cu2O film was subsequently deposited on the surface of the quartz/Al-ZnO film/nanostructured Al-ZnO/CdSe/by RF magnetron sputtering; the cross-sectional view shows that the sample is compact and dense, as shown in Figure 1c.
The XRD patterns are shown in Figure 1d. Figure 1d(1) is the XRD pattern of the quartz/Al-ZnO film/Al-ZnO nanostructures. All the peaks can be indexed to the hexagonal wurtzite structure of ZnO, and no other detectable phases exist in the Al-ZnO hierarchical nanostructures. After depositing the CdSe layer on the surface of the quartz/Al-ZnO film/nanostructured Al-ZnO, the characteristic (100), (002), and (110) peaks of CdSe were detected beside the peaks of the ZnO in Figure 1d(2), showing that the CdSe layer constituted single hexagonal phases. Figure 1d(3) shows the results of the quartz/Al-ZnO film/nanostructured Al-ZnO/CdSe/Cu2O. It can be found that the characteristic (110), (111), and (200) peaks belong to Cu2O with a cubic structure beside the peaks of ZnO and CdSe.
To summarize, the quartz/Al-ZnO film/nanostructured Al-ZnO/CdSe/Cu2O solar cell was fabricated. The oxide semiconducting materials of the ZnO nanostructure and Cu2O were simultaneously used as n- and p-type window layers; CdSe was used as absorber layer. Although the transmittance of the Al-ZnO nanostructure is only about 60% in the visible region, the solar cell shows a high energy conversion efficiency of 3.16%, indicating that these three materials of ZnO, CdSe, and Cu2O are suitable for application in ETA solar cells. We believe that conversion efficiencies above 5% can be reached by depositing antireflection coating layers onto the back of the glass substrate and reduce the length of the ZnO nanostructures by shortening the growth time, so this kind of ETA solar cell provides a new cheaper alternative to the existing solar cells.
This work was supported by the National Natural Science Foundation of China (contract nos.: 10804071 and 51072113), Shanghai Rising-Star Program (contract no.: 11QA1402700), the Innovation Program of Shanghai Municipal Education Commission (contract no.: 11YZ266), the Natural Science Foundation of Shanghai (contract no.: 09ZR1420500), the Leading Academic Discipline Project of Shanghai Municipal Education Commission (contract no.: J51902), and the Shanghai Municipal Education Commission Project (contract no.: sdj08013).
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