Microcrystalline n-i-p tunnel junction in a-Si:H/a-Si:H tandem cells

Abstract

The kinetics controlling the electrical transport inside the c-Si tunnel-recombination junction ͑TRJ͒ of a-Si:H/a-Si:H tandem solar cells was studied in detail with computer simulations. Trap assisted recombination tunneling and Poole-Frenkel mechanisms were included in our analysis. Three different c-Si tunnel junctions were investigated: ͑a͒ n-p, ͑b͒ n-oxide-p and ͑c͒ n-i-p. The highest theoretical efficiencies in a-Si:H/a-Si:H tandem cells were achieved with the n-i-p tunnel junction structure. The impact of the c-Si effective masses, mobility gap, and mobilities in the tandem solar cell efficiency is also studied in this article. Several a-Si:H/a-Si:H tandem solar cells were made with the c-Si tunnel configurations of types ͑b͒ and ͑c͒. In all of these samples one extra oxide layer was needed at the i-a-Si:H/n-c-Si interface. Both tunnel junctions lead us to comparable experimental tandem solar cell efficiencies. When the n-i-p structure is implemented as TRJ in the a-Si:H/a-Si:H tandem solar cell, efficiencies sensitively depend upon the tunnel junction intrinsic layer thickness. The optimization of this thickness provides a more controlled way of maximizing the tandem solar cell efficiency. Illuminated J-V and QE characteristics were successfully fitted using computer modeling.

Exploring dark current voltage characteristics of micromorph silicon tandem cells with computer simulations

“…This process recognized as "good" recombination sustains the photocurrent present in the tandem solar cell. 16,18 One of the tasks of an efficient TRJ is to provide enough good recombination to allow for the full recombination of the free carriers photogenerated in both the subcells. It is well known that the classical Shockley-Read-Hall ͑SRH͒ formalism is not able to reproduce the experimental tandem solar cell open circuit voltage ͑Voc͒ and fill factor ͑FF͒.…”

“…This result was also obtained for the recombination of e-h pairs taking place in the TRJ of tandem solar cells under illumination conditions. 16,18 Figure 3 shows the dependence of the dark J-V with respect to the parameter F o predicted by D-AMPS for isolated n-i-p c-Si: H TRJ. The basic parameters used in these simulations are listed in Table II.…”

“…2͒, i.e., exactly the same regions where the good recombination reaches its maximum values when the same tandem solar cell is subject to AM1.5 illumination. 16,18 The position of the good recombination peak is dictated by the asymmetry of the ͑n͒ c-Si and ͑p͒ c-Si activation energies. The high recombination rates are found inside the doped region with the highest activation energy and in the TRJ intrinsic layer.…”

“…16 The letter D stands for new developments that were implemented by the last author of this manuscript. They refer to some additional physics integrated in order to improve the characterization and design of single and multiple solar cells of amorphous, microcrystalline, and crystalline silicon.…”

“…Tails host single donorlike and acceptorlike states and Gaussian host bivalent states. 14,16,17,19,20 Since there is not yet a clear picture available for the distribution of energy offsets they were equally split between the conduction and the valence bands. 19,20 Contact effects will not be discussed in this paper.…”