Modulated electroluminescence technique for determination of the minority carrier lifetime of solar cells Sanchit Khatavkar 1 , Kulasekaran M 1 , C V Kannan 2 , Vijay Kumar 2 , Pradeep Nair 1 and B M Arora 1 1 Indian Institute of Technology, Powai, Mumbai-400076, India; 2 Moser Baer Photovoltaic Pvt.Ltd., Greater Noida, U.P.-201306, India                                  !   "#    $ $     $! %    $       $             #   &       $     $       #     & $ &   $!&  & %# I. INTRODUCTION HIT solar cells are of great interest due to their potential of reaching the Shockley-Queisser limit [1]. Conversion efficiency of 23.9±0.6% on an area of 101.8 cm 2 has already been demonstrated by Panasonic [2]. It is well known that the overall performance of these cells depends critically on the quality of heterojunction interfaces between the bulk crystalline silicon and intrinsic hydrogenated amorphous silicon. Effective lifetime of the excess carriers generated by light excitation is an important metric to characterize the interfaces and thus get an idea about the parameters restricting the performance of the solar cell. Luminescence techniques are finding increasing use in the area of photovoltaic materials and devices [3] because of the high sensitivity of emission to radiative and non-radiative recombination. Time resolved photoluminescence and modulated photoluminescence techniques have been used to obtain lifetimes directly in solar cells [4, 5]. Photoluminescence (PL) as well as Electroluminescence (EL) imaging has been commonly used for selection of high quality wafers (PL), process optimization (PL) and defect diagnostics in the finished high efficiency silicon solar cells (EL) [6]. In this work we explore the frequency dependence of modulated electroluminescence (MEL) for the characterization of solar cells, in particular, its effective lifetime or the minority carrier lifetime. Results are applied to the study of n type a- Si:H/c-Si heterojunction (HIT) solar cells fabricated at Moser Baer Photovoltaic Pvt.Ltd. II. EXPERIMENTAL HIT cells investigated in this work were fabricated from CZ n type monocrystalline silicon substrates of resistivity of about 4 Ω- cm. After the saw damage removal, wafers were textured, and surfaces were passivated. This was followed by deposition of about 3-5 nm i- layer a-Si:H on both sides and subsequent 6-12 nm p layer a-Si:H deposition on the front side and about 50 nm n layer a-Si:H on the back side by PECVD. Deposition of about 100 nm thick TCO layers on both, the front and the back sides, followed by the deposition of front and back contacts completes the device fabrication. Dark and lighted current-voltage characteristics of the devices were measured by using Sun 3000 Solar Simulator of ABET Technologies. Spectral response of the EL is measured by using a 750 mm focal length Acton monochromator, fitted with a 600 g/mm grating blazed at 1 µm and liquid nitrogen cooled 1024x1Indium Gallium Arsenide (InGaAs) detector array. The spectral response of both the grating as well as the photodetector was relatively flat over the wavelength range of the measurements, which is 0.95 µm to 1.3 µm Modulated electroluminescence (MEL) was measured by using the set up shown in Fig. 1. The excitation frequency was varied over the range 100 Hz to 10000 Hz using a signal generator. The integrated electroluminescence from the devices has been measured using a 6 inch diameter Labsphere integrating sphere fitted with a Judson Germanium (Ge) photodiode. The Ge diode was terminated with a 50 ohm resistance to measure the inphase and quadrature components of EL emission using a Stanford Research Instruments lock-in amplifier. The frequency response of the measurement system was tested by using a Roithner 1060 nm infrared LED. The frequency response of the LED was flat over the range of frequencies used in the present experiments. Both the magnitude and the phase of the EL signal of the HIT solar cell was frequency dependent in this frequency range.