Influence of Defects on Solar Cell Characteristics

Otwin Breitenstein,Pietro P Altermatt,Klaus Ramspeck,Jan Bauer

doi:10.4028/www.scientific.net/ssp.156-158.1

Abstract

The current-voltage (I-V) characteristics of most industrial silicon solar cells deviate rather strongly from the exponential behavior expected from textbook knowledge. Thus, the recombination current may be orders of magnitude larger than expected for the given material quality and often shows an ideality factor larger than 2 in a wide bias-range, which cannot be explained by classical theory either. Sometimes, the cells contain ohmic shunts although the cell’s edges have been perfectly insolated. Even in the absence of such shunts, the characteristics are linear or super-linear under reverse bias, while a saturation would be classically expected. Especially in multicrystalline cells the breakdown does not tend to occur at -50 V reverse bias, as expected, but already at about -15 V or even below. These deviations are typically caused by extended defects in the cells. This paper reviews the present knowledge of the origin of such non-ideal I-V characteristics of silicon solar cells and introduces new results on recombination involving coupled defect levels.

Highlights

The classical theory of p-n junction diodes was established by Shockley [1] and was later extended by recombination in the p-n junction depletion region by Sah, Noyce, and Shockley [2]
This paper reviews the present knowledge of the origin of such nonideal I-V characteristics of silicon solar cells and introduces new results on recombination involving coupled defect levels
Advanced theory predicts avalanche breakdown above a certain reverse bias, which is below -50 V for typical solar cells having a base doping concentration of 1016 cm-3 [4]

Summary

Introduction

The classical theory of p-n junction diodes was established by Shockley [1] and was later extended by recombination in the p-n junction depletion region by Sah, Noyce, and Shockley [2] According to this theory, the dark current-voltage (I-V) characteristics are described by the so-called "two diode model": I. According to [2] and to recent, more realistic numerical simulations [3], J02 should be in the order of 10-10 A/cm for relatively poor material with a excess carrier lifetime of = 10 μs, and the ideality factor should be maximally n = 2 Modern textbooks such as Sah [4] describe the current-voltage (I-V) characteristic of solar cells on these theoretical grounds, which predicts that the reverse current-density saturates for a reverse bias below -100 mV to a constant value of -(J01 + J02). It is common practice to include these entities in a one- or two-diode model via an equivalent circuit [5]: exp e(V

IRs kT

Recombination current

Reverse characteristics

Ohmic shunts

Conclusions