Efficient Heterojunction Solar Cells Research Articles

About the efficiency limits of heterojunction solar cells

We study n-type hydrogenated amorphous silicon/p-type crystalline silicon heterojunction solar cells focusing on the sensitivity of the open-circuit voltage to the presence of an intrinsic layer and to various recombination mechanisms. We describe our fabrication process based on low-cost steps, even for the metallization of the front side. Heterojunctions are characterized by quasi-steady-state photoconductance, capacitance vs. temperature and frequency, capacitance–voltage, quantum efficiency and current–voltage measurements. A new simulation code enables us to model the solar cells. Our best heterojunction solar cell exhibits an efficiency of 15.20% without texturization. We also report a record open-circuit voltage of 677mV for a heterojunction solar cell on p-type substrate and our first results for textured wafers. For our optimized solar cells, the role of the intrinsic layer and the recombination at the a-Si:H/c-Si is negligible. We highlight the importance of conduction band offset in the efficiency of heterojunction solar cells and exhibit a triple paradox in the comprehension of these structures, especially in the comparison between heterojunctions solar cells based on n-type or p-type substrates.

Electronic properties of ZnO/CdS/Cu(In,Ga)Se 2 solar cells — aspects of heterojunction formation

This contribution briefly reviews the present status of our knowledge on the dominant recombination mechanisms in ZnO/CdS/Cu(In,Ga)Se 2 heterojunction devices. Then we discuss the question how the formation and the electronic structure of the heterointerface and the heterojunction partners can influence the electronic properties and even the amount of recombination in the bulk of the absorber material. We examine the role of Cd- and Cu- diffusion during junction formation and air-annealing of the completed device. Furthermore, we explain the role of the intrinsic ZnO layer in the heterostructure routinely used together with a chemical bath deposited CdS layer to produce high efficiency heterojunction solar cells. We propose that these layers prevent electrical inhomogeneities from dominating the open circuit voltage of the entire device. A simple parallel connection model of two diodes demonstrates that 0.1–5% of the cell area with inferior electronic quality could degrade the photovoltaic performance without the buffer layer and have almost no consequences with buffer layer.

The influence of tunneling effects on the efficiency of heterojunction solar cells

A theoretical model is developed which presents the transport properties through the space charge region of ap + n heterojunction solar cell, whereby not only recombination through interface states but also tunneling through potential barriers is taken into account. It is investigated whether tunneling can give rise to optimum heterojunction structures which have better efficiencies that without tunneling. It is found that only if the strongly doped semiconductor has an optimum bandgap and the weakly doped semiconductor a larger bandgap, tunneling can make the structure optimum. In all other cases of optimum structures, tunneling deteriorates the efficiency.

The influence of interface state and energy barriers on the efficiency of heterojunction solar cells

A heterojunction solar cell, in which interface recombination and arbitrary energy barriers at the interface occur, is analysed to find the optimum structure. Thermionic emission theory is used; it is assumed that one type of carriers are majority carriers at the interface and that their concentration can be known a priori. It is shown that these assumptions do not seriously limit the applicability of the theory. The results are discussed in the case when one semiconductor is much more heavily doped than the other, which is a condition that certainly improves the efficiency of the solar cell.