Abstract
Multi-junction solar cells show the highest photovoltaic energy conversion efficiencies, but the current technologies based on wafers and epitaxial growth of multiple layers are very costly. Therefore, there is a high interest in realizing multi-junction tandem devices based on cost-effective thin film technologies. While the efficiency of such devices has been limited so far because of the rather low efficiency of semitransparent wide bandgap top cells, the recent rise of wide bandgap perovskite solar cells has inspired the development of new thin film tandem solar devices. In order to realize monolithic, and therefore current-matched thin film tandem solar cells, a bottom cell with narrow bandgap (~1 eV) and high efficiency is necessary. In this work, we present Cu(In,Ga)Se2 with a bandgap of 1.00 eV and a maximum power conversion efficiency of 16.1%. This is achieved by implementing a gallium grading towards the back contact into a CuInSe2 base material. We show that this modification significantly improves the open circuit voltage but does not reduce the spectral response range of these devices. Therefore, efficient cells with narrow bandgap absorbers are obtained, yielding the high current density necessary for thin film multi-junction solar cells.
Highlights
With continuously declining prices of solar modules, the improvement of solar cell efficiency has become a very important subject to further reduce the cost of the full photovoltaic (PV) system
With single-junction solar cells getting closer to their maximal efficiency limit [2], the alternative concept of multi-junction solar cells is an interesting option for higher efficiencies surpassing the Shockley–Queisser limit [3,4,5]
This paper presents our results with this approach and their relevance for perovskite/CIGS tandem solar cells
Summary
With continuously declining prices of solar modules, the improvement of solar cell efficiency has become a very important subject to further reduce the cost of the full photovoltaic (PV) system. Recent developments of high efficiency perovskite solar cells, especially semitransparent ones, open the opportunities for all thin film tandem solar modules on large and inexpensive substrates. While most of the development work has focused on the high efficiency CIGS with a bandgap suitable for single junction cells, narrow bandgap CIS has been lagging behind, keeping record efficiency at 15.0% [7]. The CIGS record solar cell is considerably closer to the theoretical maximum In such high performing CIGS cells, the absorber layers consists of a material with varying gallium to indium ratio across the thickness of the absorber, resulting in a complex bandgap grading profile. This paper presents our results with this approach and their relevance for perovskite/CIGS tandem solar cells
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