Thin Film GaAs Photovoltaics on Metal Substrates

For photovoltaics to be cost-competitive with fossil energy, the cost of solar electricity in terms of $/W has to be reduced. An attractive method to reduce the cost is to improve solar cell efficiency which also reduces the Balance of Systems cost. By far, the highest solar cell efficiencies have been achieved with III-V PV: one-sun efficiencies over 37.9% have been demonstrated [1]. Still, the use of III-V materials in terrestrial applications has been limited to concentrator PV for the utility industry, mainly because of their high cost. The high cost is primarily due to very expensive Ge or GaAs single crystal wafers which could amount up to one-half of the total module cost!

It is in this context that we are developing an innovative, non-mainstream approach:

  1. Replace expensive Ge/GaAs wafers with inexpensive flexible substrates

  2. Demonstrate thin film high efficiency III-V PV using these substrates by roll-to-roll manufacturing

Other researchers have recognized the impact of the high-cost of substrates used for III-V solar cells and are exploring re-use of the expensive substrates [2]. While good efficiency values have been achieved by this method, it is not clear how much of a cost impact would be achieved since it depends on the number of times the wafer substrates can be effectively re-used. Additionally, wafer-based processing of III-V solar cells by MOCVD that is continued to be used by these researchers is a batch process, not very amenable to lower-cost manufacturing.

Our research project is a substantial departure from mainstream PV technology and other investigations in this field and comprises of the following innovations:

  • Extend thin film PV to the realm of high-efficiency III-V compounds by use of epitaxial growth on low-cost, flexible substrates

  • Demonstrations for the very first time of roll-to-roll MOCVD of III-V thin film PV using these substrates

Flexible substrates are needed for roll-to-roll manufacturing and single-crystalline-like surfaces are needed for epitaxial growth of III-V compounds for high-efficiency PV. We have already developed such a substrate technology: single-crystalline-like germanium films on flexible metal substrates using templates made by ion beam assisted deposition (IBAD).

The IBAD templates consist of materials with rock-salt structures such as MgO that are deposited on metal substrates, with simultaneous ion beam bombardment, to achieve a single-crystalline-like film. IBAD templates have been successfully employed for epitaxial growth of superconducting films on inexpensive substrates with current densities as high as those achieved on single crystal substrates [3]. In fact, kilometers of these templates are routinely produced and have been already inserted in the U.S. power grid [3, 4]. We adopted the IBAD technique to synthesize single-crystalline-like germanium films on metal substrates for PV applications [5].

(Top Left) Schematic of a III-V photovoltaic cell structure based on single-crystalline-like germanium thin films on flexible metal substrate, (Bottom Left) photo of single-crystalline-like germanium on flexible metal substrate. (Top Right) X-ray Diffraction theta-2theta scan of single-crystalline-like Ge film on metal substrate showing strong (400) preferential orientation (Bottom right) Mobility of single-crystalline-like Ge films of different thickness on metal substrate

Meter-long tapes of germanium thin films with grains aligned within 1° with respect to each other are routinely fabricated on inexpensive metal substrates. Hall mobility values in excess of 900 cm2/Vs have been achieved with these germanium films [6]. Epitaxial thin film GaAs has been successfully grown on these tapes using metal organic chemical vapor deposition (MOCVD) [7]. The GaAs film on metal substrate shows a strong (00l) preferred orientation with no other orientations, a sharp in-plane crystallographic texture, a strong photoluminescence (PL) peak at room temperature and epitaxial film growth. An electron mobility of 1320 cm2/Vs and hole mobility over 200 cm2/V-s have been achieved in the Si-doped and Zn-doped flexible GaAs films respectively. Controllable doping levels from 1017 – 1019 cm-3 have been demonstrated which is critical for the device structure. Ternary epi Al0.2Ga0.8As has also been grown on IBAD-based metal substrates. We successfully demonstrated promising solar cell device performance characteristics with an open-circuit voltage of 642 mV, short circuit current of 23 mA/cm2, resulting in a conversion efficiency of ~11% [8].

(a) Architecture of single-junction GaAs solar cell made using single-crystalline-like GaAs on flexible metal substrate (b) Current-voltage curve of a 11% efficient flexible GaAs solar cell on metal substrate (c) Photograph of the cell

References :


  2. L. S. Mattos et al. Proc. 35th IEEE Photovoltaic Specialists Conference (PVSC), Seattle, WA, (2011).

  3. V. Selvamanickam, Y. Chen, X. Xiong, Y. Xie, M. Martchevski, A. Rar, Y. Qiao, R. Schmidt, A. Knoll, K. Lenseth, C. Weber, “High performance 2G wires: From R&D to pilot-scale manufacturing”, IEEE Trans. Appl. Supercond., vol. 19, no. 3, pp. 3225–3230, Jun. 2009.


  5. V. Selvamanickam, S. Sambandam, A. Sundaram, S. Lee, A. Rar, X. Xiong, A. Alemu, C. Boney, A. Freundlich, “Germanium films with strong in-plane and out-of-plane texture on flexible, randomly textured metal substrates”, J. Crystal Growth 311, 4553 (2009).

  6. R. Wang, S. Sambandam, G. Majkic, E. Galstyan, V. Selvamanickam, “High mobility single-crystalline-like germanium thin film on flexible, inexpensive substrates”, Thin Solid Films 527, 9–15 (2013).

  7. M. Rathi, P. Dutta, N. Zheng, Y. Yao, D. Khatiwada, A. Khadimallah, Y. Gao, S. Sun, Y. Li, S. Pouladi, P. Ahrenkiel, J.-H. Ryoua, V. Selvamanickama, “High opto-electronic quality n-type single-crystalline-like GaAs thin films on flexible metal substrates”, J. Mater. Chem. C, 5, 7919-7926, (2017).

  8. M. Asadirad, M. Rathi, S. Pouladi, Y. Yao, P. Dutta, S. Shervin, K. Lee, N. Zheng, P. Ahrenkiel, V. Selvamanickam, J.H. Ryou, “High-efficiency flexible III-V photovoltaic solar cells based on single-crystal-like thin films directly grown on metallic tapes”, Prog Photovolt Res Appl. 1–7 (2018)