Back‐contacted bottom cells with three terminals: Maximizing power extraction from current‐mismatched tandem cells

Abstract

Multi‐junction cells can significantly improve the energy yield of photovoltaic systems over a single‐junction cell. The internal interconnection scheme of the subcells is an important aspect in determining the resulting levelized cost of electricity. For a dual‐junction cell, two approaches are commonly discussed: series‐connected tandem cells with two terminals or independently working subcells in a four‐terminal (4T) tandem device. In this paper, we explore the working principle and the operation modes of a third, rarely discussed option: a three‐terminal (3T) tandem cell using a back‐contacted bottom cell with 3Ts. We use current–voltage measurements of illuminated 3T interdigitated back contact cells and confirm that the front and rear base contacts are at similar quasi‐Fermi level positions, which enables the bottom cell to either efficiently collect surplus carriers, in the case of a current‐limiting or carrier injecting top cell, or inject majority carriers, in the case of a current‐limiting bottom cell. As a result, no current matching is needed. The power output of an idealized 3T bottom cell without resistive effects is independent of the current density applied from the top cell. These characteristics of the 3T bottom cells enable a 3T tandem to operate as efficiently as a 4T tandem, while being compatible with monolithic design and not requiring intermediate grids. We propose a simple equivalent circuit model including additional resistive effects, which describes a real 3T bottom cell and achieves excellent agreement to the experiment. We deduce design guidelines for a 3T bottom cell in different operation regimes.

GaInP/GaAs three-terminal heterojunction bipolar transistor solar cell

“…Cross-talk between sub-cells has been found in other types of three-terminal solar cells, such as interdigitated back contact cells. 17,23,39 Here, conversely, the J-V curves measured while the other junction was in open circuit (OC, dotted lines) are almost indistinguishable from the curves (solid lines) measured when the other junction was biased at a voltage between maximum power point (MPP) and short-circuit (SC). The small difference in short-circuit current density (J SC ) between the two curves of the bottom junction in Figure 2a can be explained by the presence of luminescent coupling, which is frequently observed in conventional MJSCs made of highquality III-V semiconductors when the top junctions are biased close to their V OC .…”

“…21 The latter results from a higher tolerance of 3T and 4T architectures against non-optimal E G values, which facilitates the consideration of economic criteria in the design of MJSC devices. But the positive effect that these factors may have in the nal price of electricity can be jeopardized by an increase in fabrication costs, since most 3T and 4T practical devices reported still involve a structural complexity comparable to current-matched 2T devices 8,[22][23][24][25] (see Figures 1a and b) to which it is added the extra technological complexity introduced by the additional terminals. The HBTSC concept demonstrated here is material-agnostic and its structural simplicity opens the path to new, low-cost fabrication strategies that can compensate the cost of intricate device processing.…”

2D/3D Heterostructure for Semitransparent Perovskite Solar Cells with Engineered Bandgap Enables Efficiencies Exceeding 25% in Four‐Terminal Tandems with Silicon and CIGS

“…[ 4,5,21 ] At the same time, this configuration requires an additional inverter, or voltage‐matching of the subcells to obtain a voltage‐matched 2T‐module, [ 28 ] and it is harder to minimize parasitic absorption and reflectance losses (e.g., caused by additional TCOs, charge transport and optical spacer layers). [ 17 ] A three‐terminal (3T) configuration could combine the advantages of monolithic integration (low parasitic absorption) and independent operation at the MPP (no current matching required), [ 29 ] potentially outperforming the 2T and 4T configurations. [ 30,31 ] A first experimental proof‐of‐concept of 3T perovskite/c‐Si tandem solar cells has only recently been reported.…”

Interdigitated back‐contact double‐heterojunction GaInP/GaAs solar cells

“…The past few decades have seen a rapid development of solar cells with the most advanced state-of-the-art devices being based on interdigitated back-contact (IBC) silicon solar cells, 1,2 efficient singlejunction 3,4 and multi-junction 5,6 III-As compound semiconductor cells, and their combinations. 7 As compared to silicon, the compound semiconductor cells offer several benefits due to their direct and tunable band gap and composition. Consequently, they presently hold the efficiency record for single-junction solar cells, with the the demonstration of 29.1% efficient GaAs-based thin-film devices, 3,4 but even they still fall relatively far behind their theoretical efficiency limit of approximately 33.5%.…”

“…Also, using DDCT structures as the bottom and/or top cell of a multijunction solar cell can prevent current matching problems, as recently suggested. 7,28,29 Furthermore, DDCT solar cells can enable applications in more advanced devices such as thermophotonic heat pumps. 30,31 Indeed, our results suggest that the DDCT structures are very well suited for solar cell operation and can even match the performance of an ideal one-dimensional reference device, under both the AM1.5G solar spectrum (1, 000W/m 2 ) and concentrated sunlight.…”