Author's personal copy Investigation of photoconductivity of silicon solar cells by a near-field scanning microwave microscope Jongchel Kim a , Arsen Babajanyan a , Tigran Sargsyan a , Harutyun Melikyan a , Seungwan Kim a , Barry Friedman b , Kiejin Lee a,Ã a Department of Physics and Interdisciplinary Program of Integrated Biotechnology, Sogang University, Seoul 121-742, Republic of Korea b Department of Physics, Sam Houston State University, Huntsville, TX 77341, USA article info PACS: 07.50.Qx 87.50.S 07.79.v 42.79.Ek 82.47.Jk Keywords: Solar cells Photoconductivity Near-field Microwave microprobe abstract The photovoltaic effect in silicon solar cells were investigated by using a near-field scanning microwave microscope (NSMM) technique by measuring the microwave reflection coefficient at an operating frequency near 4 GHz. As the photoconductivity in the solar cells was varied due to the incident light intensities and the wavelength, we could observe the photoconductivity changes at heterojunction interfaces inside the solar cells by measuring the change of reflection coefficient S 11 of the NSMM. By measuring the change of reflection coefficient, we also directly imaged the photoconductivity changes at heterojunction interfaces inside the solar cells. & 2009 Elsevier B.V. All rights reserved. 1. Introduction Recent developments of solar cells suggest that photovoltaic technology may soon be playing a much larger role in all our lives [1,2]. Many studies have investigated the improvement of solar cells properties to enhance device performance. The knowledge of the kinetics of mobile charge carriers at heterojunction interfaces is essential for the function of solar cell devices. For this purpose, various evaluation techniques for photovoltaic heterojunction interfaces in solar cells have been developed [3,4]. Among these techniques, a contactless method is particularly attractive because it can avoid the influence of contacts. Recently, near-field scanning microwave microscope (NSMM) techniques have been developed for the direct evaluation of conductivity of materials and imaging patterns [5–8]. An important ability of the NSMM is noncontact characterization of multilayer structures. Compared to the usual electrical measurement the main advantages of NSMM are the direct imaging of the carrier conductivity at heterojunction interfaces of solar cells with high sensitivity and the evaluation of the carrier photoconductivity under an external light intensity with varying wavelengths [9–12]. In this paper, we imaged the changes of photoconductivity of solar cells depending on the incident light intensity and wavelengths using a NSMM technique. The operation principle of the measurement is the change of the microwave reflection coefficient S 11 of the resonator due to the changes of the photoconductivity of the solar cell. In order to demonstrate the direct imaging ability of NSMM, we scanned over the solar cells surface for differing incident light intensities and wavelengths and compared the results with electrical measurements. 2. Experiment The experimental setup of our NSMM is presented in Fig. 1(a) and was described in detail in Ref. [6]. We designed a NSMM system with a tuning fork distance control system to keep a constant distance between the sample and the tip using a piezo electric tube (PZT) that supports the sample stage. The sample was mounted onto an x–y–z-translation stage for coarse adjustment, which was driven by a computer-controlled microstepping motor with a resolution of 0.01 mm, whereas fine movement of the sample was controlled by a PZT tube. The probe tip to sample distance was kept at about 10nm. All NSMM measurements were done at the same sample-tip distance. The probe tip was made of tungsten wire with a diameter of 50 mm and tapered end size of 5 mm. The probe tip was oriented perpendicular to the sample surface and the other end of the tip was directly connected to a coupling loop in the dielectric ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ultramic Ultramicroscopy 0304-3991/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2009.03.038 Ã Corresponding author. Tel.: +82 2 705 8429; fax: +82 2 715 8429. E-mail address: klee@sogang.ac.kr (K. Lee). Ultramicroscopy 109 (2009) 958–962