SOLARAWARDS2011 SHORTLISTED
Dipartimento di Ingegneria Elettrica, Universita di Bologna,
A New Approch to Valence and Conduction Band Grading in CIGS Thin Film Solar Cells
Cu (In,Ga)(S,Se2) (CIGS) thin film solar cells have low cost and potentially high efficiency. The record efficiency of about 19.9% has been achieved in laboratory scale . To improve device performance, the electronic and optical properties of the cell have to be optimized.
26
Band gap grading of the cell materials is effective on reducing the recombination losses and amplifying the carrier collection in the cell. In this paper, we briefly review the characteristics of the last proposed graded band-gap profiles and then, due to valence and conduction band widening effects on the performance parameters of the cell, we present a new graded profile in which the band gap widening of the absorber is included both in Conduction Band (CB) and Valence Band (VB).
Widening the band edges at front and back regions of the cell is considered. Furthermore, we discuss the benefits of the CB grading of the window material near the interface region to enhance the carrier passivation and transferring through the cell
Normal Grading
In this case, the bandgap of the absorber linearly increases to the back contact and creates a gradient in the quasi electrical field through the cell. Therefore, at the back contact the recombination rate reduces but the Open-Circuit Voltage (VOC) increases which cause to the small enhancement in efficiency. Unfortunately, the Short-Circuit Current density (Jsc) decreases steadily by linearly increasing the band gap as the absorption coefficient depending on position decreases.
Reverse grading: With gradually decreasing the bandgap of the absorber toward the back contact, the VOC of the cell is high due to widened band gap and
Figure. 1: VB offset effect on cell parameters (left), proposed GB profile (right)
lower recombination rate. In this profile, Jsc increases steadily due to increase in the absorption for smaller band gaps, but it is not significant due to reduced probability of the electron collection affected by a reverse quasi-electrical field through the cell.
Double grading
In this profile, bandgap of the absorber decreases from front surface to an optimum minimum position and then increases to back contact. Front grading repels minority carriers away from the interface and back grading increases the band gap which enhances the carrier collection. Therefore, the internal quantum efficiency and position dependent light absorption is increased and improve the Jsc.
The other possibility to grade the band gap can be grading the VB of the absorber. In this case, we can lower the saturation current in the SCR and enhance the hole transfer in the absorber edges. In these cases, the band gap of the absorber at the surface region of the cell is at least 0.1 eV greater than that of the bulk region. For example, this shift in the VB can be produced by Cu-poor surface phases (i.e. by Cu(In,Ga)3Se5 or by intentional Ga/In/Se/S grading. VB widening is effective on the main parameters of the cell, where the VOC will improve by enlarging the barrier high at the surface.
This is due to the hole concentration which is a limiting parameter for recombination rate on the junction
VOTE @
www.solar-pv-management.com/ solarawards_vote.php
Therefore, based on the above approaches, we present an improved graded profile, (Fig. 1, right) which consider the real changes in VB and CB together. At front region of this profile, VB widening will enlarge the hole depletion as a limiting factor for VOC, CB grading will reduce the recombination rate of the carriers at the interface. At the back region, VB grading will enhance the transfer ability of the majority carriers and CB will improve carrier collection probability of the carriers to contribute to the current.
However, this profile can better define the grading changes on the valence and conduction bands, i.e., for a S-graded absorber material. We also discuss the possibility of the grading the front region of the window layer to enhance the passivation and transferring of the electrons coming from the absorber. This is an effective factor on the performance of the CIGS solar cells where the CB offset between the window layer and absorber layer at the interface can affect on the flow of the electrons from the absorber to the window.
surface and can be controlled by VB grading. When sufficient holes are supplied at the interface, they can limit the VOC. From the curves, we prove that the VB widening will improve the cell parameters by reducing the carrier loss and hole depletion at the surface regions (Fig. 1).
www.solar-pv-management.com Issue VII 2011
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56