IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 62, NO. 4, APRIL 2015
1113
Ultrathin Crystalline Silicon Heterojunction Solar Cell Integrated on Silicon-on-Insulator Substrate Weiyuan Duan, Jiantao Bian, Jian Yu, Jianhua Shi, and Zhengxin Liu
Abstract— To achieve power generation on IC chips, a hydrogenated amorphous silicon (a-Si:H)/crystalline silicon (c-Si) heterojunction solar cell was designed and fabricated on silicon-on-insulator substrate, where a 9-µm epitaxial p-type c-Si layer served as light absorption layer and the buried SiO2 as back surface passivation layer. It was found that a 1-µm heavily doped thin p+ layer was vital for improving the cell performances. Efficiency up to 12.7% with an open-circuit voltage of 679.7 mV was achieved on a 1.0-cm2 square cell. The device performance was also investigated by annealing at different temperatures. The results suggested that a relatively large thickness of a-Si:H and transparent conductive oxide layers could improve thermal stability of the solar cells at temperature above 300 °C. Index Terms— Power generation, silicon heterojunction (SHJ) solar cell, silicon on insulator (SOI), thermal stability.
I. I NTRODUCTION
S
OLAR cells integrated on silicon-on-insulator (SOI) substrate are an efficient energy source for IC on the same chip, such as distributed wireless sensor networks and ultralow power (ULP) autonomous circuits that are with transparent packages [1], [2]. It can provide sole power at milliwatt level, which is an optional choice for ULP circuit design. From the view point of fabrication processing, solar cells have the advantages of simple structure and easy preparation. Beyond that, the sunlight needed for power generation is inexhaustible. Aiming at integration of solar cells with IC technology on SOI wafer, different approaches toward high efficiency and simple structure have been studied. Murcia et al. [3] proposed a patterned emitter design that has an advantage on passivation properties and reduction of junction area. Ok et al. [6] realized an interdigitated front grid structure and prepared cells through a set of photolithography processes. By optimizing the base and shadowing fraction, conversion efficiency up to 11.5%
Manuscript received October 24, 2014; revised February 5, 2015; accepted February 5, 2015. Date of publication February 24, 2015; date of current version March 20, 2015. This work was supported in part by the Main Direction Program of Knowledge Innovation through the Chinese Academy of Sciences, Beijing, China, under Grant KGCX2-YW-399+11 and in part by the National Natural Science Foundation of China under Grant 61204005. The review of this paper was arranged by Editor A. G. Aberle. W. Duan is with the Research Center for New Energy Technology, Shanghai Institute of Microsystem and Information Technology, Shanghai 200050, China, and also with the University of the Chinese Academy of Sciences, Beijing 100049, China (e-mail:
[email protected]). J. Bian, J. Yu, J. Shi, and Z. Liu are with the Research Center for New Energy Technology, Shanghai Institute of Microsystem and Information Technology, Shanghai 200050, China (e-mail:
[email protected];
[email protected];
[email protected];
[email protected]). 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/TED.2015.2402175
Fig. 1.
Cross-sectional schematics of the ultrathin SHJ solar cells on SOI.
was achieved on bonded SOI wafer with a cell thickness of 50 μm [4]. Kerst et al. [5] designed a solar cell on 4-in SOI wafer with a 15-μm monocrystalline active layer. This system was consisted of 20 interconnected solar cells and reached an open-circuit voltage (Voc ) of 7.5 V with a short-circuit current density (Jsc ) of 17 mA/cm2 . In this paper, we introduce an ultrathin silicon heterojunction (SHJ) solar cell, which consisted of a 10-μm epitaxial mono-crystalline silicon (c-Si) layer and hydrogenated amorphous silicon (a-Si:H) layers on SOI substrate. The buried SiO2 (BOX) in SOI does not only provide good passivation for the back surface to improve Voc but also optical reflection for long-wavelength solar light to improve Jsc . In addition to the above-mentioned approaches, the introduction of a-Si:H/c-Si heterojunction has the advantages of simple fabrication process, high performance, and low process temperature (
