Solar cell technology is changing. New efficiency records are being set. Alta Devices has reached 28.8% efficiency in a thin film single-junction cell at 1-sun, and 30.8% efficiency in a thin-film dual junction cell at 1-sun.
Counter-intuitively, efficient external fluorescence is a necessity for approaching the ultimate limits. A great Solar Cell also needs to be a great Light Emitting Diode. Why would a solar cell, intended to absorb light, benefit from emitting light? Although it is tempting to equate light emission with loss, paradoxically, light emission actually improves the open-circuit voltage, and the efficiency.
The single-crystal thin film technology that achieved these high efficiencies, is created by epitaxial liftoff, and can be produced at cost well below the other less efficient thin film solar technologies. The path is now open to a 30% efficient photovoltaic technology, that can be produced at low cost.

Epitaxial lift off (ELO) of III-V semiconductor materials has often been used to make lightweight, ultrahigh efficiency solar cells. Unfortunately, a major promise - the ability to reuse the expensive substrate material following the removal of the epitaxial active region of the solar cell - has been elusive since the process results in irreversible damage to the parent wafer. Recently, we reported a method to avoid wafer damage caused by ELO using a combination of epitaxial protection layers and non-destructive cleaning procedures of both InP and GaAs substrates [1, 2]. This has resulted, for the first time, in the demonstration of multiple reuse of the parent wafer without incurring changes in either the chemical or morphological status of the original material. This suggests that the wafer itself may be used an unlimited number of times, employing a sequence of epitaxial growth, epitaxial removal using adhesive-free cold weld bonding to a metalized plastic foil, non-destructively cleaning the wafer surface, and then repeat. Indeed, multiple cycles of ELO followed my wafer reuse can be an effective means for achieving very low cost, flexible and extremely lightweight solar cells. In this work, we discuss the progress made in ELO growth of high efficiency p-n junction GaAs solar cells followed by adhesive-free cold-weld bonding to flexible substrates, and then once more reusing the parent wafer. We also discuss other high performance devices (e.g. LEDs, FETs, etc.) that have been grown on original and reused wafers. We will discuss the cost and complexity tradeoffs in this process multiple growths, ELO, and reuse of from a single parent wafer in achieving low cost, high power conversion efficiency III-V solar cells.
[1] K. Lee, K. T. Shiu, J. Zimmerman, and S. R. Forrest, "Multiple growths of epitaxial lift-off solar cells from a single InP substrate," Appl. Phys. Lett., vol. 97, p. 101107, 2010.
[2] K. Lee, J. D. Zimmerman, X. Xiao, K. Sun, and S. R. Forrest, "Reuse of GaAs substrates for epitaxial lift-off by employing protection layers," J. Appl. Phys., vol. 111, p. 033527, 2012.

Monolithically integrated III-V compound semiconductor layers on silicon substrates will enable semiconductor industry to fabricate new interesting device concepts in the near future. The device concepts that have received the most attention include CMOS compatible lasers that could be used in on-chip and chip-to-chip communication, n-channel high mobility transistors and silicon based high-efficiency solar cells. In order to realize these devices, many challenges existing in the polar-on-nonpolar epitaxy (e.g., anti-phase domains, stacking faults, lattice mismatch) needs to be overtaken. Despite the challenges however, it has recently been shown that an atomically smooth low defect density GaP buffer layer can be fabricated on silicon [1]. Therefore, an increasing amount of attention has been addressed on the properties of gallium phosphide based dilute nitride materials (E.g., GaPN, GaAsPN). The benefit of these materials is that the nitrogen incorporation enables strain compensation and the energy band structure engineering.
We have studied the growth of GaP and Ga(As)PN layers on silicon and GaP substrates by metalorganic vapor phase epitaxy. Properties of the grown layers have been examined by Raman scattering, photoreflectance, photoluminescence, IV, atomic force microscopy, Rutherford backscattering and XRD studies[2-5]. We discuss the properties of the fabricated Ga(As)PN layers and focus on the characteristics of the layers that could be utilized to realize a high-efficiency solar cells on silicon substrates. The solar cell device concepts that have drawn our attention include n-GaP heterojunction emitters, GaAsP tandem cells and GaAsPN based intermediate band solar cells (IBSC).
In this work, we present the main experimental results of the fabricated layers. The results show that defects in Ga(As)PN layers on silicon most likely arising from the III-V/Si interface degrade the quality of the layers and that high-quality layers are realized on GaP substrates. Anti-phase domain and stacking fault/threading dislocation type defects have been observed by transverse scan analysis performed by high-resolution XRD setup. Photoreflectance results of Ga(As)PN layers show the conduction band splitting which can then possibly in the future be used to realize IBSC device on silicon substrate. At the moment, we are fabricating our first IBSC devices. In addition, IV measurements performed under 0.54 SUN show that semiconductor devices such as n-GaP/p-Si heterojunction solar cell are realized.
[1]: K. Volz, et al., Journal of Crystal Growth, 315, 37, (2011).
[2]: H. Jussila, et al., J. Appl. Phys. 111, 043518 (2012).
[3]: H. Jussila, et al., Phys. Status Solidi C, 9, 1607 (2012).
[4]: S. Nagarajan, et al., J. Phys. D: Appl. Phys. 46, 165103 (2013).
[5]: H. Jussila, et al., Thin Solid Films, 534, 680-684, (2013).

Multiple batches of thin-film p-n junction GaAs thin-film photovoltaic cells were grown sequentially on a single parent wafer without loss in performance from growth to growth. Through the combination of cold-welding and a modified epitaxial lift-off (ELO) process that eliminates parent wafer damage, the device active region is transferred onto flexible plastic substrates [1]. A combination of alternating arsenide-based and phosphide-based lattice matched epitaxial layers with high etch selectivity grown on both sides of the AlAs sacrificial layer, along with thermal and plasma surface cleaning fully eliminates surface contaminants and damage, creating a pristine surface suitable for subsequent epitaxial layer growth without the use of costly and damaging wafer repolishing methods[2]. Using these methods, three iterations of lightweight and flexible GaAs thin film photovoltaic cells bonded to flexible plastic substrates were fabricated from a single GaAs substrate without any degradation in device performance after regrowth with power conversion efficiencies of ~18%. Wafer recycling without repolishing allows for an indefinite number of growth iterations on a single substrate without damage or loss in the original wafer thickness, providing a path towards cost-effective, high performance, flexible and lightweight thin-film compound semiconductor devices.
[1] K. Lee, K. T. Shiu, J. Zimmerman, and S. R. Forrest, "Multiple growths of epitaxial lift-off solar cells from a single InP substrate," Appl. Phys. Lett., vol. 97, p. 101107, 2010.
[2] K. Lee, J. D. Zimmerman, X. Xiao, K. Sun, and S. R. Forrest, "Reuse of GaAs substrates for epitaxial lift-off by employing protection layers," J. Appl. Phys., vol. 111, p. 033527, 2012

Mono-crystalline germanium (Ge) is the most widely used substrate in concentrated photovoltaics (CPV) high efficiency multijunction solar