Solution-processed germanium nanowire-positioned Schottky solar cells
© Yun et al; licensee Springer. 2011
Received: 12 December 2010
Accepted: 4 April 2011
Published: 4 April 2011
Germanium nanowire (GeNW)-positioned Schottky solar cell was fabricated by a solution process. A GeNW-containing solution was spread out onto asymmetric metal electrodes to produce a rectifying current flow. Under one-sun illumination, the GeNW-positioned Schottky solar cell yields an open-circuit voltage of 177 mV and a short-circuit current of 19.2 nA. Schottky and ohmic contacts between a single GeNW and different metal electrodes were systematically investigated. This solution process may provide a route to the cost-effective nanostructure solar architecture.
Nanostructures, such as carbon nanotubes and nanowires, have emerged as potential building blocks in nanoscale applications of microscopy tips , gas sensors , nanoscale interconnects , and field emitters  mainly because of their tiny size satisfying the needs of down-scale schemes, efficiently and effectively.
Recently, semiconducting nanowires have been intensively investigated for the production of cost-effective solar cells . This type of nanostructure provides advantages of a short length of carrier collection [5, 6] and enhancement of the optical absorption in comparison with the bulk structure . Although the advantages of nanowire-utilizing solar cells have been predicted, their promising potential has not been much achieved because of the difficulty of the junction formation in the tiny nanostructure. The Schottky contact of the metal-semiconductor has attracted huge interests for applications, such as the UV detectors, nanogenerators, and solar cells [8–10] because of the ease of junction formation of the semiconducting nanostructures.
Germanium (Ge) is compatible with Si technology and thus may enhance the performance of Si-based solar cells by modulation of the bandgap to optimize the solar spectrum harvest. Although various nanomaterials have been investigated for solar cell applications [11–13], few researches have been performed on Ge nanowires (GeNWs) for solar cells to date. From the perspective of production, the prescribed solution process is required to achieve cost-effective nanostructure solar cells .
We present here the solution-processed GeNW-positioned Schottky solar cell. A GeNW-containing solution was dropped onto a metal electrode. Asymmetric metals were applied to form a Schottky contact, establishing a rectifying current flow. Under light illumination, the contact system of the GeNW to asymmetric metal electrodes provides Schottky solar cell performance. The junction of metal and GeNW using symmetric and asymmetric metal contacts is also systematically investigated.
The growth of GeNWs was achieved by the thermal vapor transport. A 5-nm-thick Au film coated-Si substrate was placed close to a basket containing Ge powder (Germanium 99.98%, Korea Sigma-Aldrich) inside a quartz tube reactor. The growth temperature was controlled at 800°C under an Ar atmosphere.
To prepare NW-containing solution, the GeNW-grown sample was placed in a methanol-filled vial. An ultrasonication process was carried out for 60 min to separate the grown GeNWs from the substrate and then centrifuged at 10000 rpm for 60 min to remove residuals. The GeNW-containing solution of 2 μL was dropped onto metal electrodes under an ac electric field to align the GeNWs between the electrodes. A field-emission scanning electron microscope (FEI Sirion) was used for observing the GeNWs and their positioning on the metal electrodes. A field-emission transmission electron microscope (TEM) (FEI Tecnai F30 Super-Twin) analysis was performed to verify the nanowire structure. Selected-area electron diffraction and fast Fourier transformation (FFTs, GATAN) revealed a crystalline structure and the growth direction of the GeNW.
Results and discussion
A laser light source (10 mW) was utilized to obtain the photoresponse from the single GeNW-positioned Schottky device and provided an open circuit voltage (V oc) of 0.78 V and a short current (J sc) of 650 pA, corresponding to 2.24 A/cm2 driven from the light-exposed region. Although this high value is induced from the high laser power source, it nevertheless highlights the potential to generate high current from the GeNW comparing to the previous reports on Si NW [12, 13].
In order to evaluate the feasibility of large-scale production of GeNWs as a solar cell, asymmetric metal electrodes were prepatterned by an optical lithography process. This provides a rectifying junction between the metals and the semiconducting NW without an e-beam or a focused ion beam [9, 12]. A unit device has 30 interdigitated fingers with a length of 500 μm and a 20-μm width. A finger electrode has a 1-μm gap to the neighboring ones.
where J s, n, kT, A**, and ϕ B are the saturation current density, ideality factor, thermal energy (eV), Richardson constant, and barrier height, respectively. The ideality factor was obtained to be 2.37, and the barrier height was calculated to be 0.93 eV.
Under the light condition, the multiple GeNW-positioned Schottky device yields a V oc of 177 mV and an I sc of 19.2 nA, as shown in Figure 4d. It is worth noting that the photogenerated voltage is comparable to that of bulk Ge solar cell .
The result indicates that a short-circuit current of 5.8 pA is obtained from a single GeNW, which corresponds to a J sc of 6.69 mA/cm2 considering the light-exposed area. This value is significantly higher than that (5 mA/cm2 of J sc) from a single silicon nanowire , because the Ge has a smaller bandgap (0.67 eV) with a two-order higher light absorption capability than Si. It demonstrates the potential to enhance the photogenerating current using a GeNW light absorber .
In summary, we have fabricated a solution-processed GeNW-positioned Schottky solar cell. A solution containing GeNWs was spread on metal electrodes under an ac electric field, and the GeNWs were positioned onto the metal electrodes. A Schottky contact between metal and a single GeNW was formed using asymmetric metal electrodes, and the photoresponse was demonstrated under laser light. Under one-sun illumination, the GeNW Schottky solar cell provided an open-circuit voltage of 177 mV and a short-circuit current of 19.2 nA. This solution processed-Schottky solar cell is expected to provide a cost-effective nanostructure solar cell architecture.
fast Fourier transformation
scanning electron microscope
transmission electron microscope.
The authors acknowledge the financial support from the Converging Research Center Program through the Ministry of Education, Science and Technology (MEST, 2010K001078) and Institute Pilot Research Program by Korea Institute of Machinery and Materials (KIMM). Professor Wayne A. Anderson is grateful for the support extended by the U.S. Air Force Office of Scientific Research (FA95501010154) with Dr Kitt Reinhardt as monitor.
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