IEEE JOURNAL OF PHOTOVOLTAICS, VOL. 3, NO. 1, JANUARY 2013 295 Novel Hybrid Amorphous/Organic Tandem Junction Solar Cell Sambit Pattnaik, Teng Xiao, R. Shinar, J. Shinar, and V. L. Dalal Abstract—We report on a novel hybrid amorphous Si-organic series-connected tandem junction solar cell. The solar cell is fabri- cated on indium tin oxide (ITO)-coated glass and uses an a-(Si,C):H as the first cell and a P3HT/PCBM organic cell as the second cell. An intermediate ITO layer is used as an ohmic layer which pro- vides an excellent contact to both the first and the second cells. By adjusting the bandgap and thickness of the first a-(Si,C):H cell, we achieve an almost complete matching of currents produced by the first and the second cells. The first cell produces ∼0.95–1.0-V open- circuit voltage, and the second cell produces ∼0.6-V open-circuit voltage. The combined cell produces 1.5-V open-circuit voltage and had a fill factor of 77%, showing the effectiveness of the interme- diate ITO layer to act as an excellent connecting layer between the two cells. When such an ITO layer is not used, the fill factor is very poor. The solar conversion efficiency of the organic cell was 4.3%, whereas the efficiency of the tandem cell was 5.7%. We also mea- sured the stability of the organic cell with and without an inorganic cell acting as a filter in front. It is shown that the degradation of the organic cell is much higher when it is subjected to a full solar spectrum, as compared with when it is subjected to light passing through an inorganic cell first, which filters out ultraviolet (UV) and blue photons. Thus, we show that this new cell combination has the potential to significantly increase the efficiency of organic cells while also decreasing the instability. We also discuss the potential of achieving much higher efficiencies, that is approaching 20%, by using an appropriate combination of amorphous and organic cells. An example is shown next. Index Terms—Amorphous semiconductors, degradation, or- ganic semiconductors, photovoltaic (PV) cells, silicon. I. INTRODUCTION A MORPHOUS Si (a-Si:H) solar cells are widely used in the solar cell industry. In particular, tandem junction solar cells based on a-Si:H/nanocrystalline Si, or a-Si:H/a-(Si,Ge):H alloys represent a major industrial product [1]–[4] and have Manuscript received June 4, 2012; revised July 27, 2012; accepted August 5, 2012. Date of publication September 17, 2012; date of current version Decem- ber 19, 2012. This work was supported by the National Science Foundation and Iowa Power Fund for some of the work. This work was also supported in part by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division, under Contract DE-AC 02-07CH11358. S. Pattnaik, R. Shinar, and V. L. Dalal are with the Microelectronics Re- search Center and the Department of Electrical and Computer Engineering, Iowa State University, Ames, IA 50011 USA (e-mail: pattnaik@iastate.edu; rshinar@iastate.edu; vdalal@iastate.edu). T. Xiao and J. Shinar are with the Department of Physics and Astronomy and Ames Laboratory, Iowa State University, Ames, IA 50011 USA (e-mail: txiao@iastate.edu; shinar@ameslab.gov). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JPHOTOV.2012.2212700 reached efficiencies of ∼16% in a triple-junction configura- tion [5]. The virtues of a-Si:H alloys for use in multiple-junction configuration include the ability to vary the bandgap widely be- tween ∼2 eV, by alloying with C, and ∼1.1 eV by using a-Ge:H. This ability provides significant flexibility to a device designer for optimizing the efficiency of multiple-junction cell configu- ration by selecting the best combination of bandgaps and thick- nesses. However, amorphous materials suffer from relatively poor electronic properties, with hole diffusion lengths being rather small, i.e., ∼0.3–0.4 μm in a-Si:H, and even poorer when the materials such as a-(Si,Ge):H alloys. This small diffusion length leads to the need for a field-assisted transport, using a p + - i-n + device rather than a p + -n-n + device so that the junction field can extend across the entire middle i-layer. The intrinsic defect densities are also quite high in the 10 16 /cm 3 range. This high defect density means that the thickness of the intrinsic layer must be small, i.e., ∼0.3 μm for the best a-Si:H cells [6], and even smaller for a-(Si,Ge):H alloy cells. These small thick- nesses, in turn, mean that light is incompletely absorbed in the a-(Si,Ge):H, requiring techniques to enhance light absorption in these cells [7]–[9]. An alternative design for enhancing the efficiency of thin- film amorphous cells is to use a nanocrystalline (nano) Si as the second cell, which can absorb lower energy photons. How- ever, nano-Si is a poor absorber of light, since Si is an indirect bandgap material. Nano-Si also has a limited diffusion length, in the range of a few micrometers [10], [11]. This means that the thickness of nano-Si cannot be much more than a few mi- crometers. This fact limits light absorption in such thin films and again requires complicated optical absorption enhancement techniques to generate enough current in a thin nano-Si cell. In this paper, we suggest and implement an alternative design that overcomes both these problems by using an organic solar cell as a lower gap cell. The commonly used organic photo- voltaic (PV) materials have a very high absorption coefficient (3–4 × 10 5 /cm), albeit over a limited wavelength range [12]. This means that the organic material need only be very thin, ∼200 nm, to absorb a significant amount of photons without us- ing any optical enhancement techniques. With optical enhance- ment, one can expect almost 100% absorption of photons within the optical range of the material. It is also useful to note that the organic cells can be deposited at low temperatures (<150 ◦ C) and thus are compatible with the deposition temperatures of amorphous cells, which range from 200 ◦ C to 300 ◦ C. In Fig. 1, we show the typical absorption curve for the P3HT material, showing how it absorbs strongly in the 450–625-nm range, with absorption falling off strongly for shorter and longer wavelengths. 2156-3381/$31.00 © 2012 IEEE