Mobility lifetime product—A tool for correlating <i>a</i>-Si:H film properties and solar cell performances

The missing correlation between film characteristics and <i>α</i>-Si:H-based <i>p-i-n</i> solar cells is still a controversial subject. The authors present a new parameter <i>µ</i>⁰<i>τ</i>⁰, evaluated from steady-state transport measurements on <i>a</i>-Si:H layers, which can indeed relate film quality and cell performance as far as the latter is limited by the quality of the intrinsic ; <<i>i</i>> layer. Thereby, two specific features of the evaluated <i>µ</i>⁰<i>τ</i>⁰ product can explain its successful role as a quality parameter for <i>a</i>-Si:H: First, the computation of <i>µ</i>⁰<i>τ</i>⁰ takes into account the effects of the prevailing dangling bond occupation, which is very different in uniform films as compared to the occupation profile prevailing through the <i>i</i> layer of a <i>p-i-n</i> solar cell; second, the evaluated <i>µ</i>⁰<i>τ</i>⁰ product combines information about band mobility and defect density; furthermore it avoids some of the well-known pitfalls of usual deep defect density measurements such as constant photocurrent method and photothermal deflection spectroscopy. Experimental data on a series of layers and <i>p-i-n</i> solar cells illustrates the determination of <i>µ</i>⁰<i>τ</i>⁰ in a given practical case and its successful correlation with cell efficiency. In this context, an estimation for the ratio of charged to neutral capture cross sections <i>σ</i>^±/<i>σ</i>⁰ of around 50 is found.