Demonstrating the GaInP/GaAs Three-Terminal Heterojunction Bipolar Transistor Solar Cell

Abstract

The three-terminal heterojunction bipolar transistor solar cell (HBTSC) concept enables the realization of a monolithic double-junction device with individual current extraction. We present an HBTSC realized by a heterojunction of GaInP and GaAs. The one-sun open-circuit voltage (VOC ) of the top and bottom junctions are 1.33 V and 0.99 V, respectively, while fill factors (FF) are above 80%. At one-sun illumination, reducing one junction's bias from VOC to maximum power point degrades the performance of the other junction only slightly (< 0.5% efficiency loss). These results demonstrate the potential of the HBTSC concept to produce high-efficiency independently connected double-junction solar cells.

GaInP/GaAs three-terminal heterojunction bipolar transistor solar cell

“…The design of the base layer is key for the operation of the HBTSC. 11,32 In this case the base is 565 nm thick and has a concentration of p-dopant atoms (Zn) of 6×10 18 cm -3 , notably higher than the ndopant concentration in the emitter and collector (4×10 17 and 5×10 17 cm -3 ; all values determined from secondary ion mass spectroscopy (SIMS) measurements). Further details are given in the Methods section and the Supplementary Information (SI, section S1).…”

“…This can be avoided in future prototypes by inverting the growth and contacting the base from below with interdigitated base and collector contacts. 34 Following the recently proposed taxonomy for three-terminal tandem solar cells, 35 this cell is denoted GaInP/r/GaAs(p/n). The cell has a MgF 2 /ZnS anti-re ection coating (ARC) deposited by e-beam evaporation.…”

“…10,11 A signi cant injection of carriers (electrons in the case of our npn structure) from the emitter into the base would imply that the current density associated with the bent minority carrier QFL is not negligible, and consequently, neither is the power loss. A second possible origin for cross-talk between junctions is a high resistance that opposes the ow of current through the base, 32 which would involve a gradient of the majority carrier QFL, with similar consequences on the power production of the HBTSC (this gradient would take place along a lateral dimension not represented in Figure 2c). This second possibility is discussed in more detail towards the end of the paper.The difference between the V OC s of the top and bottom junctions in Figure 2a can be understood with the help of the ideal band diagrams depicted in Figure 2b (HBTSC in equilibrium) and 2c (HBTSC under illumination and forward voltage bias on both junctions).…”

“…More discussion can be found in the SI, Section S3. We have shown elsewhere 34 that HBTSC devices can be fabricated using an inverted epitaxial structure, which makes it possible to deposit the metal ngers of the base contact underneath the metal ngers of the emitter contact and to avoid un ltered areas of the bottom junction. Therefore, in optimized HBTSC devices the shadowing factor is expected to be similar to that of conventional MJSCs and the e ciency to be a more straightforward determination.…”

“…It has been shown that the effect of lateral series resistance in HBTSCs can be minimized not only by reducing the current path, as we have done in the prototype presented here, but also through other approaches, such as increasing the base thickness. 32 The most effective strategy to eliminate this problem in practical devices is to use an inverted epitaxial structure that enables the fabrication of base contact ngers located underneath the emitter contact ngers. 34 Those ngers can be separated by a distance comparable to the width of our proof-of-concept HBTSC prototype, and the series resistance problem in the base can be treated like it is for conventional emitters.…”