Investigation of Quantum Dot Solar Cell Device Performance
- PDF / 201,183 Bytes
- 6 Pages / 432 x 648 pts Page_size
- 95 Downloads / 294 Views
Investigation of Quantum Dot Solar Cell Device Performance Neil S. Beattie1, Guillaume Zoppi1, Ian Farrer2, Patrick See3, Robert W. Miles1 and David A. Ritchie2 1
Northumbria Photovoltaics Applications Group, Northumbria University, Newcastle upon Tyne,
NE1 8ST, United Kingdom 2
Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, United Kingdom
3
National Physical Laboratory, Teddington, Middlesex, TW11 0WE, United Kingdom
ABSTRACT The device performance of GaAs p-i-n solar cells containing stacked layers of selfassembled InAs quantum dots is investigated. The solar cells demonstrate enhanced external quantum efficiency below the GaAs band gap relative to a control device without quantum dots. This is attributed to the capture of sub-band gap photons by the quantum dots. Analysis of the current density versus voltage characteristic for the quantum dot solar cell reveals a decrease in the series resistance as the device area is reduce from 0.16 cm2 to 0.01 cm2. This is effect is not observed in control devices and is quantum dot related. Furthermore, low temperature measurements of the open circuit voltage for both quantum dot and control devices provide experimental verification of the conditions required to realise an intermediate band gap solar cell. INTRODUCTION The concept of the intermediate band gap solar cell was theoretically presented as a way to exceed the Shockley-Queisser efficiency limit for single junction solar cells [1]. One of the more promising experimental approaches to realising this prediction is based on incorporating stacked layers of InAs semiconductor quantum dots (QDs) within a GaAs p-i-n structure. The nanometre dimensions of the QDs results in the formation of narrow band of quantum states within the energy band gap. The QDs provide an additional contribution to the photocurrent by capturing photons with energies less than the energy band gap of the solar cell. Experimental verification of this effect has been provided in several reports as an extension of the external quantum efficiency relative to a control sample without QDs [2-4]. Unfortunately however, almost all of the reported devices exhibit degradation in the current-voltage characteristic, stemming from a reduction in the open circuit voltage Voc, caused by thermal escape from the QDs to the conduction band and tunnelling through the barrier regions. Further degradation results from strain which is necessary for the formation of QDs grown in the Stranski-Krastanow self-assembled growth mode. In this context it is noteworthy that Voc 1 V has recently been achieved for an InAs QD solar cell incorporating a strain compensation technique [5]. While many of the reports on this type of solar cell have focused on combating strain, few have examined device performance in terms of conventional photovoltaic parameters. In this work we present the results of experiments on InAs QD solar cells in which strain is uncompensated. Although Voc is degraded relative to a control device we argue that analysing the current-volt
Data Loading...
