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aPhys. Status Solidi C 8, No. 2, 625–627 (2011) / DOI 10.1002/pssc.201000446

Photovoltaic and photoconductivity characteristics of (a-C:Fe)/Al2O3/Si structure Xinyu Tan*,1,2, Xiaozhong Zhang2, Caihua Wan2, and Hong Lin3

1 School of Science, China Three Gorges University, Yichang 443002, P.R. China 2 Department of Materials Science and Engineering, National Center for Electron Microscopy (Beijing), Tsinghua University,

Beijing 100084, P.R. China 3 China Academic Committee of State Key lab of New Ceramics and Fine Processing, Beijing 100084, P.R. China

Received 13 June 2010, revised 26 July 2010, accepted 27 July 2010 Published online 27 October 2010

Keywords photovoltaic effect, photoconductivity, carbon film, Fe doping * Corresponding author: e-mail: husttanxin@mail.tsinghua.edu.cn, Phone: +86 717 6392618, Fax: +86 717 639 2370

The iron-doped amorphous carbon films (a-C:Fe) and Al2O3 films were deposited on n-type silicon substrates using pulsed laser deposition to form (a-C:Fe)/Al2O3/Si structures. The Fe diffused into a-C films by annealing treatment and formed Fe-doped amorphous homogeneous structure. The a-C:Fe films are disordered graphitized carbon system and are rich in sp2. The results show that these junctions have good rectifying proper-ties and great Photovoltaic (PV) effect and photoconductivity (PC). PV parameters of the cells were studied by varying the iron deposition times in the a-C films and iron content is found

to have a great effect on PV effect. The solar cell with iron de-position time of 5min shows the best PV performance with short-current density of 14.11 mA/cm2 and open-circuit voltage of 436mV. The corresponding fill factor and energy conversion efficiency are 33% and 1.99% respectively. For the reverse bias voltage of -0.5v, the structure shows a photoconductivity of 150 with illumination power of 100mW/cm2. This study shows that the (a-C:Fe)/Al2O3/Si has potential application as PV and other photoelectric devices.

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Carbon is a remarkable element ex-isting in a variety of stable forms ranging from insulator to metal. The various forms of carbon have attracted a great deal of interest in recent years because of their unique structure and properties. Among various application of carbon material, recent study of solar cells [1-3] are quite interesting, such as amorphous carbon (a-C) [2-4], fuller-ences [5], and carbon nanotubes [6] as a versatile material in photovoltaic (PV) materials. Amorphous carbon is the earliest successful carbon-based semiconductor to replace silicon in the solar cell. The interesting feature about PV properties of a-C is that it is able to accept dopants and its properties can be turned over a wide range from that of semimetallic graphite to that of insulating diamond by varying the ratio of sp3 and sp2 hybridized bonds.

Effective doping can modify optoelectronic properties and remodel the conduction type. It is reported that boron (B) is widely used as p-type impurity in Si semiconductor. Recently, it has been reported that B can also be used as p-type dopant of C film, using chemical vapor deposition and

ion implantation techniques [5,6]. But, the boron doping processes often involve borane and other toxic and flam-mable dangerous gases. P-type doping in a-C is difficult and few work has been reported on the other elements as p-type dopants expect boron. Meanwhile, an abrupt change of composition at the C/Si interface may include a large amount of defects and surface recombination, which de-grades the heterojunction performance and solar cell con-version efficiency.

In this paper, we report our recent findings of the PV and PC properties with a structure of (a-C:Fe)/Al2O3/Si by Pulsed Laser Deposition (PLD) method, which uses Fe to dope a-C film and inserts a thin Al2O3 layer between C and Si to suppress the carrier recombination in the heterojunc-tion.

2 Material and methods The iron doped amorphous carbon films (a-C:Fe) and thin Al2O3 layer were deposited on n-type silicon substrates. The resistivity of Si substrates is 0.55~0.8 Ωcm. The substrates were ultrasonically

626 X. Tan et al.: Photovoltaic and photoconductivity characteristics of (a-C:Fe)/Al2O3/Si

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cleaned in ethanol, then acetone, etched in diluted HF solu-tion (10%). Before deposition, the chamber was pumped near 5×10-4 Pa and the substrates were heated and main-tained at 320 oC. The target-to-substrate distance was fixed at about 5 cm. First a crystalline Al2O3 target (99.99%) was ablated by laser with energy of 360 mJ/pulse at frequency of 1 Hz for 4 min to deposit Al2O3 barrier layer. Then iron target (99.99%) and a carbon target (99.9%) were bom-barded by laser with energy of 300 mJ/pulse at frequency of 1 Hz and 6 Hz for x min (x=3,5 and 7 min), respectively. After deposition, the films were annealed in-situ at 320°C for 15 min. The front side (p-C film) and back side (n-Si) contacts of the cells were made with indium (In) electrode. The PV and PC characteristics of the films were measured with a solar simulator under illumination (AM1.5, 100mW/cm2, 25 °C) and dark conditions. The samples were characterized by Transmission electron microscopy (TEM), energy dispersion spectra (EDS) and Raman spec-troscopy to determine the film compositions and micro-structure.

3 Results and discussion 3.1 Cells compositions and microstructure

analysis Figure 1 is the TEM images of the a-C:Fe/Al2O3/Si (Fe deposition time x=5 min) structure which reveals the thickness of Al2O3 layer is 6 nm and the whole thickness of the (a-C: Fe)/Al2O3 layer is 23 nm. The EDS results show that there are C, Fe and Al near the inter-face between the carbon film and the Al2O3 layer. Fe is found in the carbon film everywhere and not exist in the interfae between Si and Al2O3 layer (not shown). So we speculate that the Fe has diffused into a-C films by annea-ling while it is prevented from penetrating into Si substrate by the Al2O3 layer. The corresponding Fast Fourier Trans-formation (FFT) of the high resoltuion TEM (HRTEM) shows only a diffused ring and diffraction pattern of the Si substrate, indicating that the Fe-doped carbon film is amorphous with a homogeneous structure.

Figure 1 Typical HRTEM micrograph of the cross section of a-C:Fe/ Al2O3 /Si. Inset: an FFT pattern of the HRTEM micrograph of the a-C:Fe film.

Raman spectra of the a-C: Fe (x min) samples with x=3, 5 and 7 min were measured. The spectra exhibit peaks at approximately 1590 cm-1 (G-peak) and 1360 cm-1 (D-peak). The variation of the D and G peaks and the ratio of their intensities provide information on sp2/sp3 and the sp2 cluster size in the films [7]. For our three samples, all the spectrum show that the films are disordered graphitized carbon films and contain much more sp2 bonded carbon clusters. Hall measurement confirmed that p-type a-C film was realized by Fe doping in our (a-C:Fe)/glass sample [7]. It is expected that the Fe doping into a-C film in this work also leads to p-type a-C film.

3.2 PV properties analysis The current–voltage characteristics of the a-C:Fe (x=3,5,7 min)/Al2O3/Si solar cells were measured under dark and illumination by simu-lated solar light conditions (AM1.5, 100mW/cm2). The J-V curve of the a-C:Fe (x=5 min)/Al2O3/Si ( sample 1) is shown in Fig. 2 as an example. Without illumination, the solar cell shows rectifying characteristic, indicating the formation of heterojunction. Upon illumination, sample 1 shows the best PV performance in the three samples with open-circuit voltage (VOC) of 436 mV and short-circuit current density (JSC) of 14.11mA/cm2. The fill factor (FF) and energy conversion efficiency (η) is 33% and 1.99% re-spectively. Figure 3 shows the PV parameters, such as the short current density, open circuit voltage and efficiency were significantly improved with increasing iron content in the initial stage and then deteriorates. This is because too much iron impurity will reduce the lifetime of the minority carriers. As a result, the PV performance deteriorates when the iron deposition time is more than 5 min.

Figure 2 Current-voltage characteristics of a-C:Fe(x=5 min)/Al2O3/Si solar cells measured under dark and light illumina-tion (AM1.5, 100 mW/cm2).

Phys. Status Solidi C 8, No. 2 (2011) 627

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Figure 3 PV parameters of solar cells as a function of iron con-tent.

3.3 Photoconductivity characteristics analysis Photoconductivity, defined as the ratio of the current measured on illumination to that measured under dark condition when the source voltage is fixed, (IL/ID)V, is ob-served in the a-C:Fe (x=5 min)/Al2O3/Si film. Here, we choose 0.5 V as the measured reverse bias voltage. When the light was turn on , as seen in Fig. 4, the conductivity immediately increased. This observed photoresponse cor-responds to a nearly 1.55 mA increase in current, which is corresponding to a photoconductivity of 150. After the light was turned off, the current returned to its original val-ue rapidly. This process can repeat many times.

Figure 4 The dependence of photoconductivity on voltage with illumination power of 100 mW/cm2 at the reverse bias voltage of 0.5 V.

4 Conclusion The a-C: Fe/ Al2O3 multilayer films were prepared on n-type silicon using pulsed laser deposi-tion. The Fe diffused into a-C films by annealing and formed Fe-doped amorphous homogeneous structure. The a-C:Fe films are disordered graphitized carbon system and are rich in sp2. With AM1.5 illumination (100 mW/cm2,

25 °C), The Fe-doped sample shows a good PV character-istics with the largest short-current density of 14.1mA/cm2 and open-circuit voltage of 436mV when the deposition time is 5min. The corresponding energy conversion effi-ciency and fill factor of this solar cell are 1.99% and 33% respectively. For the reverse bias voltage of -0.5V, the structure shows a photoconductivity of 150 with AM1.5 il-lumination. We attribute the improvement of cell perform-ance mainly to the effect of iron diffusing into a-C film and achieved p-type doping by annealing. Excess iron content will reduce the minority carriers’ lifetime and deteriorate the photoelectric convert effect. The a-C:Fe /Al2O3/Si structure has great potential in PV and other photoelectric devices. Further research is in progress to improve quality of the films and optimize the cell performance.

Acknowledgements We would like to acknowledge the

financial support by the National Science Foundation of China (Grant Nos.U0734001 and 50772054), The Ministry of Science and Technology of China (2009CB929202.) and the Education Branch of Hubei Province, China (Q20091303).

References [1] V. P. Koch, R. Hezel, and A. Goetzberger, High-Efficient

Low-Cost Photovoltaics: Recent Developments (Springer, New York, 2008), Chap. 1.

[2] H. W. Zhu, J. Q. Wei, K. L. Wang, and D. H. Wu, Sol. En-ergy Mater. Sol. Cells 93, 1461, (2009).

[3] J. C. Han, M. L. Tan, J. Q. Zhu and S. H. Meng, Appl. Phys. Lett. 90, 08350, (2007).

[4] A.M. M. Omer, S. Adhikari, S. Adhikary, H. Uchida, and M. Umeno, Appl. Phys. Lett. 87, 161912, (2005)

[5] G. F. Burkhard, E. T. Hoke, S. R. Scully, and M. D. McGe-hee, Nano. Lett. 9, 4037, (2009).

[6] X. M. Li, H. W. Zhu, K. L. Wang, A. Y. Cao, J. Q. Wei, C. Y. Li, Y. Jia, Z. Li, X. Li, and D. H. Wu, Adv. Mater. 2010, DOI: 10.1002/adma.200904383 (2010).

[7] C. H. Wan, X. Z. Zhang, X. Zhang, X. L. Gao, and X. Y. Tan, Appl. Phys. Lett. 95, 022105 (2009).