Three-junction monolithic interconnected modules for concentrator photovoltaics

A core issue in concentrator photovoltaic technology (CPV) is the resistive losses in cells that usually limits the maximum photoconversion efficiency under high concentration. We propose the use of three-junction monolithic interconnected modules (MIM) to mitigate resistive losses by providing high-voltage low-current power. First, we present the fabrication of InGaP/InGaAs/Ge front-contacted microcells with various designs and dimensions. Front-contacted cells are the key enabler for the MIM fabrication and demonstrate good electrical characteristics under one sun, similar to standard-contacted cells. The base front contact size is minimized to limit the unutilized area on the wafer. Second, fabrication techniques for interconnecting cells in MIM are described. Finally, electrical measurements show a record conversion efficiency of 35.1% under 798 suns for the first three-junction MIM reported (17.8% when considering the entire device area). Versatility and further optimization of the devices are discussed to enlarge their field of application.


| INTRODUCTION
Concentrator photovoltaic (CPV) cells and modules under high-power illumination are typically limited in efficiency because of resistive losses. Two major alternative approaches, based on unconventional cell/module assemblies, were proposed to mitigate these losses.
The micro-CPV approach was presented in recent years to mitigate resistive losses among others. 1 In this technology, sub-mm 2 cells are used to form compact modules. Miniaturizing cells leads to less current to handle-as the photocurrent is proportional to the illuminated area of the cell-and a lower series resistance. Micro-CPV approach requires automated high-speed assembly techniques, such as the ones used for consumer electronics, to maintain the cost competitiveness. Both micrometre-scale high-efficiency solar cells and cells with both contacts on the rear surface or on the front surface are required and were investigated. [2][3][4] Another approach to limit resistive losses relies on the use of monolithic interconnected modules (MIM). In MIM, subcells are series connected at a wafer level to provide high-voltage low-current minimodules when the irradiance typically exceeds several tens of watt per square centimeter. 5 MIM were first introduced with silicon structures in the 1970s, and numerous developments allowed CPV MIM made with GaInP/InGaAs structure to reach a record efficiency of 26.0% under 496 suns. 6 Whereas methods were proposed for threejunction MIM, no demonstrators were presented yet. 7 One of the challenges is that both emitter and base contacts must be on the same side of the wafer, which is not straightforward for the state-of-the-art InGaP/InGaAs/Ge structure. Wiesenfarth et al. proposed frontcontacted InGaP/InGaAs/Ge cells that use a back-surface lateral conductive layer to minimize the bulk Ge contribution to resistive losses due to the large dimensions of the cells. 8 However, these cells were interconnected using the conventional wirebonding technique (i.e., nonmonolithic assembly). The use of Ge-On-Insulator (GOI) wafers and the growth of Ge on semi-insulating GaAs substrates were also proposed to form MIM but not proven experimentally. 5,9,10 Combining sub-mm 2  with standard contacts (base contact is on the back surface of the wafer) was presented in Albert et al. 12 Based on that, Figure 1 shows the major steps used for the fabrication of front-contacted cells.
F I G U R E 1 Major steps for front-contacted cells fabrication F I G U R E 2 Scanning electron microscope tilted view of a plasma-etched cell mesa. The image shows the smoothness of the Ge-substrate surface surrounding the mesa structure. Image: M. de Lafontaine The first step consists in depositing the emitter contact by evaporating Pd/Ge/Ti/Pd/Al (50 nm/100 nm/50 nm/50 nm/1000 nm) ( Figure 1A). 13 Then, cells are electrically isolated from one another with a SiCl 4 /Cl 2 /H 2 plasma etch process to form 10-μm-high mesas ( Figure 1B). 12,14 The etching step isolates the three subcells (1 μm deep in the Ge) and is followed by the contact layer wet etching. The plane surface on the Ge substrate, resulting from the plasma isolation, can be seen on the scanning electron microscope (SEM) in Figure 2.
The low topography (lower than the subsequent metallization thickness) is the key enabler for depositing the base contact on the front surface ( Figure 1C) using photolithography, instead of the regular backside of the wafer. Ti/Al (50 nm/500 nm) base contacts are evaporated on the etched front surface of the Ge substrate nearby the mesa. To do so, an extra photolithographic step is used, as ohmic contacts were not optimized for both the emitter and the base contacts.
No extra conducting layer is added on the back surface. The process is also compatible with the conventional base contact on the wafer backside. In this case, the base contact deposition process on the front side would be skipped and a blank sheet of Ti/Al (50 nm/500 nm) would be evaporated on the wafer rear surface. After the base contact is deposited, an antireflective coating (ARC) that consists in a SiN x H y /SiO x H y bi-layer (66 nm/69 nm) is deposited by plasma-enhanced chemical vapour deposition (PECVD) and is known to provide cell passivation. 15 As the ARC deposition/passivation is performed at 300 C during approximately 9 min, no additional contact annealing was added. Electrical contacts are revealed by etching the ARC on top of them using CF 4 reactive ion etching (RIE) ( Figure 1D).
Finally, cells are singulated from the wafer. For this purpose, 500 nm of SiO x is first deposited on the active wafer rear surface (Ge face) by PECVD before bonding on a handling wafer ( Figure 1E). The cell singulation occurs by means of plasma etching through the 170-μm active wafer ( Figure 1F). The etching is performed using a Bosch process that consists in alternating etching steps (13 s, SF 6 /O 2 ) and passivation steps (7 s, C 4 F 8 ) for 45 min (average etch rate is >3.8 μm/min for 20-μm-wide trenches). 16 The SiO x layer acts as an etch-stop layer and can be removed by etching after the cell singulation occurs in case the front-contacted cells are used as stand-alone devices.

b. Front-contacted cells designs
Leveraging the versatility of lithography and plasma etching, we designed and fabricated solar cells with rectangular, circular and hexagonal active areas. The cell active area dimensions varied between 11.55 and 0.047 mm 2 (defined as the mesa area to which busbars area is deducted). In rectangular cells, the emitter electrode of square cells is composed of 80-μm-wide busbars and equally spa-    Figure 6D) and 73.0% (circular MIM; Figure 6C). The surface utilization of the MIM is discussed in Section 4.  The 200-μm-wide O-shape base contacts use a large amount of the expensive wafer compared with the cell size and should be optimized to maximize wafer usage while maintaining a low-enough series resistance to minimize resistive losses. The series resistance of conventional CPV cells results from many parameters (e.g., layers conductivity or electrode geometry). Apart from the bulk germanium resistance and the base contact resistance, one can expect the series resistance of front-contacted cells to be the same as the one of conventional cells.

| Characterization methods
• Bulk Ge contribution to series resistance In conventional cells, considering the bulk Ge is uniform, we can evaluate the series resistance R back S_Ge due to the substrate in a backcontacted cell with a square mesa having a width w M as with ρ Ge the germanium resistivity and t Ge the thickness of the substrate.
In front-contacted cells, the electrons flow laterally in the germanium between their point of entry in the base and the front base contact, which is expected to increase the bulk germanium resistance contribution (longer average distance than the germanium thickness t Ge of 170 μm). Assuming a square mesa with a width w M , the longest distance electrons have to cross is w M for I-and L-shape contacts (i.e., with base contact adjacent to one or two sides of the cell; see Supporting Information) or WM 2 for U-and O-shape contacts (i.e., with base contact adjacent to three or four sides of the cell; see Supporting Information).
As a first approximation, one can estimate the series resistance due to bulk Ge in cells with I-or L-shape front contacts R front_IL which is R front_IL S_Ge < ρ Ge tGe and in the case of U-and O-shape contacts, R front_UO S_Ge can be estimated as which gives R front_UO S_Ge < ρ Ge 2 × tGe . Consequently, the increase in bulk Ge contribution to series resistance, due to I-or L-shape front contact, can be expected as In the case of U-and O-shape front contacts, the increase of series resistance due to bulk Ge contribution can be estimated as As an example, for cells with w M = 500 μm and t Ge = 170 μm, the series resistance due to bulk Ge is predicted to increase by a maximum factor of 8.65 in cells with I-or L-shape contacts and 4.33 in Uor O-shape contacts compared with back-contacted cells.
In conventional cells, bulk germanium contribution to series resistance is considered negligible. 17 Therefore, given the calcula- • Contact contribution to series resistance In front-contacted cells, the contact area is smaller than the cell surface, which is also anticipated to increase the contact resistance.
To evaluate the contact contribution, TLM structures were fabricated on the germanium substrate simultaneously as the solar cells fabrication. A specific contact resistivity ρ BC of 8.02 × 10 −5 Ω.cm 2 and a transfer length L T of 49.5 μm were found for the used ohmic contact.
In a standard back-contacted cell, one would expect the contact resistance R BC to be whereas for a contact width w C , one would expect a contact resis- with n BC = 1,2,3 or 4 for I-, L-, U-or O-shape front base contacts. The term min(w BC , L T ) indicates that R front BC is not further lowered by contacts for which w BC is larger than L T .
Therefore, the increase in contact resistance due front contact can be anticipated as As an example, for a cell with w M = 500 μm and an I-shape contact with w BC = 10 μm, the contact resistance is expected to increase by  Figure 8, the FF as a function of the front base contact width w BC reaches a plateau (82.5% < FF < 82.8%) when w BC is larger than 50 μm (i.e., contact resistance R front BC ≈ 82 mΩ). This confirms that both bulk Ge and base contact resistance are of minor impact for w BC ≥ 50 μm and for w M = 500 μm. However, the FF declines when w BC decreases below 50 μm, indicating that the contact resistance cannot be neglected anymore when the contact width is smaller than the transfer length L T .
Nonetheless, the FF for w BC = 10 μm (smallest contact width considered here) remains larger than 81% under 723 suns, which indicates that such small contacts could be envisioned for lower concentration applications.

| Monolithic interconnected modules a. Characterization under one-sun illumination
Fabricated MIM were electrically characterized under one sun and under high-intensity light. Figure 9 shows their one-sun currentdensity-vs-voltage characteristics. In order to validate the MIM performance, the electrical parameters V OC , FF and J SC are compared with those of a single cell.
Considering the interconnections schemes, one can expect with J cell SC the short-circuit current density of a single cell and the fill factor of the MIM FF MIM is anticipated as where  Figure 6A,B) and 0.343 mm 2 (2-cell circular MIM, Figure 6C). The active area of a cell in the MIM is considered as the mesa area to which busbars area and interconnectioninduced shading on the mesa were deducted. Figure 9 and Table 1 show that the first prototypes of MIM show remarkable perfor-   b. Characterization under high-intensity illumination Figure 10 shows the V OC , FF and the efficiency η of the fourcell square MIM (shown in Figure 6A) as a function of concentration, for which a total active area of 1.829 mm 2 was measured. The We have shown that, using the presented metallization and cells, a base contact width w BC of 49.5 μm was necessary for high performance under high concentration. This value corresponds to the transfer length L T and therefore, as confirmed experimentally in Section 3.1 [b]), a larger contact would not result in a lower series resistance. Only the series resistance due to bulk Ge may affect the largest devices performance, depending on the targeted application and the generated current, and for which a lateral conduction layer may be required, as developed in. 8 We can easily consider lowering photolithographic margin width to w PL = 5 μm between the structures defined by photolithographic processes and the plasma dicing trench to w D = 10 μm.
Therefore, the minimal distance between two adjacent square mesas is 129 μm in the case of O-shape base front contacts and 74.5 μm in the case of L-shape contacts (see details in the Supporting Information and Figure 11). Thanks to the use of plasma etching as the dicing technique, the lost area between cells is kept low, close to that offered by saw dicing technique in standard-contacted cells (i.e., 50-120 μm). 19 Figure 12 illustrates the Amesa Adevice of a single front-contacted cell and MIM of 4, 9 and 16 cells with 49.5 μm I-shape base contacts, which is the most optimized base contact shape. The A mesa is the mesa area that corresponds to the device area A device minus the lost area due to front base contacts, photolithography margins and dicing trenches in the case of MIM.
As an example, it is shown that a cell with 49.5 μm I-shape base Therefore, depending on the targeted module design, a trade-off must be found between the usable active area ( Amesa A device ) and the resistive losses reduction.  Figure 6D). The Amesa A device was not optimized in these cases. It is also important to note that in the case of front-contacted cells or MIM, large busbars on the cell active surface may not be necessary. Indeed, in the case of front-contacted cells, alternative assembly schemes can be proposed (e.g., flip-chip-like technique) instead of wirebonding. 3,4 In the case of MIM, the cells are interconnected by means of microfabrication techniques, providing very tight features, obtainable by photolithographic processes. One could also envision interconnections to be as large as the cell mesas to further reduce their resistive effect. Moreover, the absence of large busbars reduces the dark current generation due to the shading, which may be limiting for small-dimension cells. 20

| Applications and limitations
We have shown that MIM are particularly well suited for highconcentration applications, when the high current of conventional cells would reduce the efficiency because of resistive losses.
High concentration is always associated with a reduction of the system acceptance angle. Moreover, due to their series arrangement, MIM are expected to be sensitive to nonuniformity.
It is therefore anticipated that MIM integration in point focus concentrator photovoltaic systems would be associated with a secondary optical element. Such element could be co-optimized with the MIM to favour light redistribution outside of the unused area between the MIM subcells, mitigating the negative impact of surface loss, as proposed in Norman et al. 21 for example. In addition, the process presented in this paper allows the fabrication of densely packed cells, mitigating the cells tilting, typically induced by the shingling technique used for dense-arrays assembly. Finally, MIM could offer the possibility to monolithically integrate by-pass diodes to simplify the complete module assembly, as developed in Loeckenhoff et al. 22

| CONCLUSION
We proposed InGaP/InGaAs/Ge cells with both contacts on the front side to allow fabrication of MIM. Plasma etching isolation is the key enabler technology for front-contacted cells fabrication. We demonstrated front-contacted cells with various shapes and dimensions (rectangular, circular and hexagonal and sub-mm 2 active areas) having the same electrical performance as standard-contacted cells. We found that 50-μm-wide base contacts were necessary for the cells to F I G U R E 1 2 Mesa-to-device area ratio A mesa /A device as a function of device area A device for a single front-contacted square cell, a 4-cell, a 9-cell and a 16-cell square MIM with I-shape base contacts. The graph also includes the MIM fabricated in this work and the dualjunction MIM demonstrated by Helmers et al. 6