A. Urbaniak

Barriers for current transport in CIGS structures

A potential barrier impeding current flow in the forward direction is observed in some CIGS devices especially at low temperature. In this paper possible origin and location of this barrier is discussed. We have taken into account: a front contact barrier in the window layer, front contact barrier due to the p+ layer, and back contact barrier at CIGS/Mo interface. The discussion is based on current-voltage, capacitance-voltage and admittance spectroscopy data measured for CIGS cells with CdS and In2S3 buffer layers and with Mo and Pt back electrode. Accumulated evidences point towards a front electrode as a source of the barrier in the CdS-buffered cells. We show that the height of this barrier is influenced not only by properties of the buffer but also by sodium supply.

Metastable variations of the fill factor in CIGS thin film solar cells

Faculty of Physics, Warsaw University of Technology, Koszykowa 75, PL 00 662 Warszawa, Poland, ABSTRACT W e investigate the origin of fill factor changes induced by reverse bias treatment. Evolution of current-voltage characteristics have been measured during application of reverse voltage bias. Two different cell behaviors have been identified. At elevated temperatures one kind of the devices strongly deteriorates and exhibit so called double diode behavior. On the other hand, in the same conditions another cells keep their fill factor almost constant. We correlate the fill factor changes with the kinetics of capacitance and show that although increased number of shallow acceptors itself cannot induce severe FF deterioration, it may strongly influence position of the Fermi level at the heterointerface what in a presence of an electron barrier is crucial for the device behavior. INTRODUCTION Bias-induced metastabilities in Cu(In,Ga)Se

Defect levels in Cu(In,Ga)Se2 studied using capacitance and photocurrent techniques

This work contributes to the discussion on defect levels in Cu(In,Ga)Se2 photovoltaic material. CuInSe2- and Cu(In,Ga)Se2- based Schottky junctions, solar cells and thin films were investigated using complementary capacitance and current spectroscopic techniques. Depending on the applied technique and type of investigated structure, six different signals were observed. Out of the signals identified, three were ascribed to responses from bulk defects-two electron and one hole trap. The remainder were discussed in light of available in-literature models including carrier mobility freeze-out and non-ohmic back junction.

Dependence of defect signature on conductivity of polycrystalline Cu(In,Ga)Se2 layers by photocurrent spectroscopy

Photocurrent spectroscopy - a method sensitive only to bulk levels - was used to investigate defect spectrum of CIGS polycrystalline films. Low temperature signal, previously labeled E4, with parameters similar to the N1 was observed in all samples, irrespective of their preparation details. We show that sensitivity to the hole concentration is its characteristic feature. Similar dependence of emission rates on doping changes induced by light soaking has been commonly observed for N1 level in solar cells. Our findings point toward common origin for the N1 level and E4 signal observed by photocurrent methods in the epitaxial and polycrystalline CIGS.