Influence of deposition conditions for bottom cell on micromorph tandem device performance Paola Delli Veneri a, * , Lucia V. Mercaldo a , Domenico Caputo b , Iurie Usatii a , Carlo Privato a a Enea, Portici Research Center, Localita ` Granatello, 80055 Portici (Napoli), Italy b Department of Electronic Engineering, University of Rome ‘‘La Sapienza’’, via Eudossiana, 18, 00184 Rome, Italy Available online 20 February 2008 Abstract We have realized micromorph tandem solar cells on Asahi U-type TCO-covered glass substrates. The intrinsic layers of both amor- phous top cell and microcrystalline bottom cell are grown by very high frequency plasma enhanced chemical vapour deposition (VHF- PECVD) at 100 MHz at low substrate temperature (150 °C). For the bottom cell different growth regimes have been explored by changing both chamber pressure and plasma power. The effect of the structural composition of the microcrystalline absorber layer on the electrical parameters of the device has been investigated. High short circuit current density and constant FF in a wide silane concentration range are obtained when using large power to pressure ratio (0.5 W/Pa). However, low open circuit voltage is generally found in this regime. The largest V OC values are found at 67 Pa and power to pressure ratio of 0.3 W/Pa, where the highest efficiency (11.1%) is reached. An eval- uation of device stability has been done by exposing the tandem solar cells to white light (AM 1.5-like spectrum) for 200 h. Ó 2008 Elsevier B.V. All rights reserved. PACS: 84.60.jt; 81.15.z; 85.30.z Keywords: Silicon; Solar cells; Photovoltaics; Microcrystallinity 1. Introduction ‘Micromorph’ solar cells, combining a high band gap (1.7 eV) amorphous silicon top cell and a low band gap (1.1 eV) microcrystalline silicon bottom cell in a stacked tandem structure, represent one of the most promising thin-film solar cell concepts [1–4]. Like other multijunction concepts they allow for a reduction of the light-induced degradation in the device. Moreover, a better utilization of the solar spectrum is obtained, thus opening up to larger efficiencies. The pronounced difference in energy gap between top and bottom cell asks for a careful design of the two component cells [5]. From this point of view micro- crystalline silicon is a highly complex and versatile mate- rial: The structural composition can be changed by varying the deposition conditions, allowing for a tuning of several device parameters. The use of microcrystalline material in the bottom cell is considered particularly useful to improve the tandem device stability under light soaking [6,7]. However, since high efficiency microcrystalline cells are usually deposited around the transition from crystalline to amorphous growth [8], both top and bottom cell might in principle be involved in the degradation effect. Therefore, it is impor- tant to study the behavior of the micromorph devices under light soaking. In this paper VHF-grown micromorph tandem solar cells with different bottom cells have been investigated. A low deposition temperature (150 °C), compatible with future use of low cost plastic substrates, has been selected. For the microcrystalline bottom cell, two different growth regimes, characterized by two values of plasma power to chamber pressure ratios, have been explored. Moreover, 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.09.029 * Corresponding author. Tel.: +39 081 7723259; fax: +39 081 7723344. E-mail address: paola.delliveneri@portici.enea.it (P. Delli Veneri). www.elsevier.com/locate/jnoncrysol Available online at www.sciencedirect.com Journal of Non-Crystalline Solids 354 (2008) 2478–2482