© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim COMMUNICATION 1 wileyonlinelibrary.com www.MaterialsViews.com www.advenergymat.de Ultralight High-Efficiency Flexible InGaP/(In)GaAs Tandem Solar Cells on Plastic Davood Shahrjerdi,* Stephen W. Bedell, Can Bayram, Cristina C. Lubguban, Keith Fogel, Paul Lauro, John A. Ott, Marinus Hopstaken, Michael Gayness, and Devendra Sadana D. Shahrjerdi, S. W. Bedell, C. Bayram, C. C. Lubguban, K. Fogel, P. Lauro, J. A. Ott, M. Hopstaken, M. Gayness, D. Sadana IBM T. J. Watson Research Center Yorktown Heights, NY 10598 E-mail: davood@us.ibm.com DOI: 10.1002/aenm.201200827 Flexible solar cells are envisioned to open up a myriad of pos- sibilities for enabling new applications in consumer electronics and space satellites. [1–3] Organic and amorphous semiconduc- tors hold a great promise for realizing bendable and light-weight solar cells, largely due to their fairly strong light absorption properties, process temperature compatibility with flexible sub- strates and potentially inexpensive processing cost. [4–6] However, the poor minority carrier lifetime in these materials, inherent to their highly disordered and defective crystalline structure, inhibit their use for making high efficiency and reliable solar cells. This limitation becomes more pronounced in applica- tions with stringent specifications in terms of the total area and weight of the photovoltaic (PV) module. Conversely, the exquisite optical and electrical properties of III–V semiconductors permit the fabrication of extremely high-efficiency solar cells exploiting thin III–V layers. [7–9] For example, multijunction III–V solar cells have currently reached ≥36% conversion efficiency at one sun intensity, for which the total thickness of the solar cell structure is ≤ 10 μm. [10] How- ever, III–V solar cells are conventionally grown on mechanically rigid gallium arsenide (GaAs) and germanium (Ge) substrates that serve as an epitaxial template. Therefore, the release of thin III–V layers from the growth substrate is essential for rendering the solar cell structure flexible. Furthermore, there has been a growing interest in exploiting inverted metamorphic structures to attain higher conversion efficiency, in which the removal of the solar cell structure from the growth substrate is necessary for the proper function of the device. [7,10] It is also important to consider that the use of a viable layer transfer scheme will ide- ally lead to substantial reduction in material cost by enabling (i) substrate reuse and (ii) thinner solar cell structures with poten- tially higher conversion efficiency utilizing back reflectors. [11,12] In order for the widespread adoption of a layer transfer tech- nology, it should offer process simplicity and compatibility with an incumbent solar cell technology, while making it more cost-effective. Recently, there has been an enormous effort to revive the epitaxial layer lift-off (ELO) technique for separating III-V solar cell layers from a GaAs host substrate. [12–17] This technique, in principle, relies on the selective lateral etch of an embedded sacrificial layer – usually an Al-rich AlGaAs layer. [18] From a practical standpoint, an additional apparatus is required to progressively pull the lifted layer away from the host wafer while the sample is immersed in the etch solution – generally concentrated hydrofluoric acid. [14] This is to enhance the inher- ently slow lateral etch rate of the embedded sacrificial layer and avoid the sudden halt of the etch process. Hence, these practical pitfalls combined with difficulties in handling the free-standing thin layers – particularly film cracking issues during the release process – severely hamper the simplicity and applicability of this method when larger size wafers are used. We have previously demonstrated the use of our novel layer transfer technique, called controlled spalling for realizing thin- film tandem junction InGaP/(In)GaAs/Ge solar cells rigidly bonded on silicon (Si) handle substrates. [19] This technique works based on the propagation of a spalling mode fracture inside the substrate parallel to the surface, wherein the frac- ture front is mechanically guided using a flexible handle layer in a controllable manner. [20,21] The equilibrium fracture depth inside the substrate and the final residual strain in the trans- ferred film is engineered by adjusting the intrinsic proper- ties of the stressor layer, which is generally nickel (Ni) owing to its superb fracture toughness. Most notably, the Ni stressor in conjunction with the flexible handle layer provides a robust mechanical support that remarkably facilitates manipulation of very thin layers. The details of the controlled spalling have been described elsewhere. [20,21] We report here ultralight flexible dual-junction InGaP/(In) GaAs solar cells on plastic with conversion efficiency ≥28% employing the controlled spalling technique. Our solar cells exhibit remarkably high specific power and excellent stability under different bending conditions, thus demonstrating their suitability for applications requiring light-weight and high- efficiency flexible PV. Finally, we demonstrate that the integrity of the entire device structure is maintained during the layer transfer process. An inverted dual-junction InGaP/(In)GaAs solar cell struc- ture, schematically illustrated in Figure 1a, was devised and grown on germanium substrates. In this structure, Ge wafer serves only as an epitaxial template and is not part of the solar cell. However, the growth of an (In)GaAs buffer layer on Ge is necessary to terminate anti-phase boundary defects prior to the growth of the solar cell structure. Because of slight lattice mismatch between GaAs and Ge, it is imperative to incorporate precisely 1% indium in GaAs while growing the buffer layer and the bottom cell in order to avoid the formation of misfit dis- locations. It has been previously reported that the misfit dislo- cations result in the degradation of the open circuit voltage ( V oc ) without causing an apparent change in the spectral response Adv. Energy Mater. 2012, DOI: 10.1002/aenm.201200827