## INTRODUCTION

Due to the nearly unlimited abundance of solar energy, photovoltaic cells that convert sunlight directly into electricity represent the most promising alternative energy source. However, cost-efficient solar-to-electrical energy harvesting remains a major hurdle that must be fully surmounted if we are to expect its eventual widespread deployment. Considerable efforts in developing photovoltaics have therefore focused on achieving low cost while increasing their power conversion efficiency (PCE).1,2,3,4 One recent achievement has been the demonstration of thin-film GaAs solar cells approaching their thermodynamic efficiency limit.5,6,7,8 However, the cost reduction long promised by the epitaxial lift-off (ELO) process has primarily been limited by the inability to fully recover the original wafer surface quality after each growth, leading to a limited number of times that the substrate can be recycled due to the accumulation of defects and to wafer thinning incurred by chemo-mechanical polishing.9,10,11,12,13 Furthermore, high PCE alone does not necessarily translate into low-cost solar energy production when expensive active materials and fabrication processes are used in their manufacture. As an alternative to simply improving PCE, solar concentrators have been demonstrated as a means for reducing the use of costly active solar cell materials.14,15 However, most concentrators suffer from a significant roll-off in efficiency at large light incident angles and can also result in high cell operating temperatures, thereby necessitating expensive active solar tracking and solar cell cooling systems.16

Here, we demonstrate that thin-film GaAs solar cells produced by an accelerated non-destructive ELO (ND-ELO) fabrication process that are integrated with simple thermoformed mini-concentrators can lead to a dramatic reduction in the cost of the production of electricity via solar energy harvesting. This approach reduces cell material and fabrication costs to only 3% that of analogous substrate-based GaAs cells, and only 11% that of ELO-processed GaAs solar cells, while the optical system maximizes the annual energy output using highly truncated two-dimensional mini-compound parabolic concentrators (CPCs). This low-profile concentrator provides a very thin and lightweight module with improved off-angle sunlight absorption compared to conventional concentrators both in direct and in diffuse sunlight with only minor losses. Our approach, therefore, eliminates the need for high concentration factor optics that require expensive and heavy solar tracking paraphernalia. Furthermore, the unique geometry of thin-film GaAs solar cells mounted on a heat-sinking metal layer enables operation at or near room temperature without active cooling, even for concentration factors approaching 4×, representing a reduction of over 40 °C compared to substrate-based GaAs solar cells.

## MATERIALS AND METHODS

### Epitaxial growth

The solar cell epitaxial layer structures are grown by gas-source molecular beam epitaxy (GSMBE) on Zn-doped (100) p-GaAs substrates. The growth starts with a GaAs buffer layer (0.2 µm thick) followed by InGaP/GaAs (100 nm/100 nm) protection layers and an AlAs (20 nm) sacrificial layer. Next, an inverted active device region is grown as follows: 5×1018 cm−3 Be-doped GaAs (0.15 µm) contact layer, 2×1018 cm−3 Be-doped Al0.20In0.49Ga0.31P (0.025 µm) window, 1×1018 cm−3 Be-doped p-GaAs (0.15 µm) emitter layer, 2×1017 cm−3 Si-doped n-GaAs (3.0 µm) base layer, 6×1017 cm−3 Si-doped In0.49Ga0.51P (0.05 µm) back surface field (BSF) layer and 5×1018 cm−3 Si-doped n-GaAs (0.1 µm) contact layer. The GaAs/AlAs layers are grown at 600 °C, and the Al0.20In0.49Ga0.31P/In0.49Ga0.51P layers are grown at 480 °C.

### Pre-mesa patterning, cold weld bonding and epitaxial lift-off

Figure 1a shows the schematic illustration of the process flow for pre-mesa patterning, cold welding and ND-ELO. Mesas of 2.5 mm×6.5 mm Cr/Au (4 nm/350 nm) are patterned by photolithography using a LOR 3A and S-1827 (Microchem, Newtown, MA, USA) bi-layer photoresist as a mask and H3PO4:H2O2:deionized H2O (3125) and HCl:H3PO4 (31) as etchants for GaAs and InGaP, respectively. The patterned Au on the epitaxial GaAs wafer is bonded to the Au-coated 25 µm thick Kapton® sheet using an EVG 520 wafer bonder at 10−5 torr. Then, a pressure of 4 MPa with an 80 N s−1 ramp rate is applied to the 2-inch-diameter substrate to establish a bond between the Au films. The temperature is increased to 230 °C at 25 °C min−1 and held at that temperature for 8 min. The substrate is then rapidly cooled. To apply uniform pressure, a soft graphite sheet is inserted between the sample and the press head. Once the GaAs substrate fully adheres to the Kapton® sheet, the thin active device region is removed from its parent substrate using ND-ELO.9 The sample is immersed in a 20% HF:H2O solution maintained at 60 °C while agitating the solution with a stir bar at 900 r.p.m. The total lift-off time is 30 min.

### Solar cell fabrication

Following lift-off, the thin-film active region and flexible plastic host are fixed to a rigid substrate using Kapton® tape. The front surface contact grid is photolithographically patterned using the LOR 3A and S-1827 (Microchem) bi-layer photoresist; then, a Pd(5 nm)/Zn(20 nm)/Pd(20 nm)/Au(700 nm) metal contact is deposited by e-beam evaporation. The widths of the grid and bus bar are 20 µm and 150 µm, respectively, and the spacing between the grid fingers is 300 µm. The total coverage of the solar cell active area by the metallization is 4%. After the metal layer is lifted off, the highly doped 100 nm p++ GaAs contact layer is selectively removed by plasma etching. The thin-film solar cells are annealed in air for 1 h at 200 °C to form ohmic contacts. An anti-reflection coating bilayer composed of 49 nm thick TiO2 and 81 nm thick MgF2 is deposited by e-beam evaporation. The solar cells on the plastic sheet are covered by a plastic film to protect them from debris generated during dicing along the etched trench using a CO2 laser cutter (50 W; Universal Laser Systems, Scottsdale, AZ USA) with 2.5 W power and 500 pulses per inch (see Supplementary Information SI 1 and Supplementary Movie, laser cutting, for details).

### Vacuum-assisted thermoforming of the CPCs

Figure 1b shows the schematic illustration of the vacuum-assisted thermoforming process for CPC fabrication. The polyethylene terephthalate glycol-modified (PETG) sheet is fixed with Kapton® tape across the top of a metal mold containing holes at its base. While vacuum is applied through the holes, the assembly is placed in an oven at 60 °C. The PETG is drawn into the mold as the oven temperature is raised to 96 °C for 15 min, forming a compound parabolic shape. The CPC is then cooled, after which the CPC is detached from the metal mold.

### Characterization of the concentrated GaAs photovoltaic

An Oriel solar simulator (model: 91191) with a Xe arc lamp and an AM 1.5 Global filter is used for I-V measurements obtained with an Agilent 4155B parameter analyzer. The simulator intensity is calibrated using a National Renewable Energy Laboratory (NREL)-certified Si reference cell with a diameter of 5 mm. The light incident angle is adjusted using an optical fiber and rotation stage (Newport, Irvine, CA USA, 481-A). The concentration factor under diffuse illumination (N-BK7 ground glass diffusers, 220 grit polish, Thorlabs, Newton, NJ USA) is measured with an identical setup. The solar cell operating temperature is measured by a thermal imaging camera (A325, FLIR, Wilsonville, OR USA).

## CONCLUSIONS

In summary, we demonstrated thin-film GaAs solar cells integrated with low-cost, thermoformed, lightweight and wide acceptance angle mini-CPCs. The fabrication combines rapid ND-ELO thin-film cells that are cold-welded to a foil substrate and are subsequently attached to the CPCs in an adhesive-free transfer printing process. The combination of the low-temperature operation of the thin-film solar cells with the highly truncated low-profile plastic CPCs provides 2.8× enhanced energy harvesting throughout the year without the need for active solar tracking while eliminating losses incurred at the high operating temperatures characteristically encountered in concentration systems. Additionally, the combination of the potentially low cost fabrication and lightweight materials enables significant reductions in the balance of the system costs. This demonstration represents a significant step toward removing the cost barriers to the widespread deployment of lightweight and high performance thin-film GaAs solar cells in terrestrial and commercial solar electricity generation applications.