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Electron-Selective Epitaxial/Amorphous Germanium Stack Contact for Organic-Crystalline Silicon Hybrid Solar Cells Bingbing Chen, Jianhui Chen, Kunpeng Ge, Linlin Yang, Yanjiao Shen, Wanbing Lu, Li Guan, Lizhi Chu, Qingxun Zhao, Yinglong Wang, Ying Xu, and Yaohua Mai ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00924 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018
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Electron-Selective Epitaxial/Amorphous Germanium Stack Contact for Organic-Crystalline Silicon Hybrid Solar Cells
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Bingbing Chen1, Jianhui Chen*1, Kunpeng Ge1, Linlin Yang1, Yanjiao Shen1, Wanbing Lu1, Li
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Guan1, Lizhi Chu1, Qingxun Zhao1, Yinglong Wang1, Ying Xu1, Yaohua Mai*2
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1 Hebei Key Lab of Optic-electronic Information and Materials, College of Physics Science and
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Technology, Hebei University, Baoding 071002, China
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2 Institute of New Energy Technology, College of Information Science and Technology, Jinan
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University, Guangzhou, 510632, China
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ABSTRACT: Carrier transport properties can be improved through suppressing the charge carrier
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recombination and decreasing the contact resistance using the proper carrier-selective contact. In
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this work, an epitaxial/amorphous germanium (epi/a-Ge) stack thin film is introduced into the rear
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surface of organic-inorganic hybrid (OIH) solar cells as an efficient electron-selective contact.
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This novel electron-selective stack contact simultaneously contributes low recombinative and
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resistive losses through a three-material system composed of n-type silicon (n-Si), epitaxial silicon
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germanium (epi-SiGe), and amorphous germanium (a-Ge), which promotes the tailoring of band
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structures, the passivation of surface dangle bond defects and the formation of Ohmic contact. The
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results result in obvious improvement in the open-circuit voltage (Voc) (from 549.5 mV to 643.0
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mV) and fill factor (FF) (from 70.5% to 75.4%), corresponding to the increase in the power
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conversion efficiency (PCE) of OIH solar cells (from 10.3% to 12.9%). This work indicates
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novel application of the carrier-selective stack contact to achieve high performance OIH solar cells
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with a simple and low temperature process. 1
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KEYWORDS epi/a-Ge stack thin film, tailoring energy band, recombination rate, Ohmic contact, hybrid solar cells
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INTRODUCTION
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Over the past decade photovoltaics (PVs) have been increasing with a tremendous average
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growth rate per year. All-in crystalline silicon (c-Si) solar cells, especially the interdigitated back
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contact (IBC) solar cells with all contacts on the rear side, present promising technique delivering
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high power conversion efficiency (PCE) up to 26.7%, which is very close to the theoretical limit
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(29.4%) of c-Si solar cells [1,2]. This high PCE is mainly achieved by suppressing surface
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recombination and improving contact characteristics, such as the introduction of a decent
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passivation scheme and the formation of a well Ohmic contact between c-Si semiconductor and
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metal electrode. Excellent passivation schemes, e.g., SiO2, hydrogenated amorphous silicon
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(a-Si:H), Al2O3, SiNx and intrinsic hydrogenated amorphous silicon oxide (a-SiOx:H) [3-7], and
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effective contact schemes, e.g., selective emitter (SE) and local contact technologies [8,9], have
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been developed.
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Very recently, the urgent requirement for low-cost roadmap has invigorated research on
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developing the state-of-the-art organic-inorganic hybrid (OIH) solar cells, which is composed of a
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conductive polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)
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coated on a n-type Si wafer [10-12]. The PEDOT:PSS/Si heterojunction at the front side of the
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device is a hybrid Schottky junction thanks to high work function and excellent conductivity of
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PEDOT:PSS thin film [13-15]. In our previous work, PEDOT:PSS is found to be able to passivate
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Si surface well since PSS species in it possessed high-quality passivation effect [16]. This point
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may expand another advantage of the PEDOT:PSS/Si heterojunction solar cells. Although OIH
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solar cells have achieved a relative high PCE in the short several years, the recombination and
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contact resistance loss are still the major issues that limit the device performances. Several studies
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have been done on the rear surface of the OIH solar cells for the improvement of the transport of 3
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carriers. Low work function contact schemes, such as inorganic materials including LiF, Cs2CO3,
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Mg, perylene diimide (PDIN), TiOx, organic materials 8-hydroxyquinolinolato-lithium (Liq) and
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NDI-based
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poly{[N,N0-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,50-(2,20-
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bithiophene)} [P(NDI2OD-T2)); Polyera ActivInk N2200] (N2200) thin films have been exploited
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to improve OIH solar cell performances [17-23]. Very recently, an electron-selective SiO2/Mg
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stack contact is reported to achieve the improvement of OIH solar cell performance because it
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tailors band structures while maintains interface passivation [24]. These improvements are
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attributed to the efficacy of the low work function thin film as back surface field (BSF) by
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polymer
reducing minority carrier recombination and enhancing majority carrier transport.
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Crystalline germanium (c-Ge) is a semiconductor material possessing both metallic and
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non-metallic properties. The electrical conductivity of c-Ge is 104 times higher than c-Si, and the
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band gap is 0.67 eV [25]. In addition, c-Ge has a high mobility, promoting its attractive
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application as the substrate in metal oxide semiconductor field effect transistor (MOSFET) [26,
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27]. Ge is also applied in the bottom cell of tandem solar cells as a form of a hydrogenated
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microcrystalline silicon germanium (µc-SiGe:H) thin film [28]. The amorphous germanium (a-Ge)
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thin film also has been studied for the infrared photo-resistors [29]. Moreover, the heteroepitaxy of
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Ge on Si is a topic research with significant technological importance. Epitaxial SiGe (epi-SiGe)
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layer has already been used in Si-based high performance devices such as heterojunction-bipolar
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transistors and photodetectors [30, 31]. Quite recently, it is reported that an epi-SiGe memory is
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achieved using a single-crystalline SiGe layer epitaxially grown on c-Si as a switching medium
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[32]. 4
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In this work, an epitaxial/amorphous germanium (epi/a-Ge) stack thin layer is introduced as
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electron-selective contact at the rear side of the OIH solar cells. The insertion of the Ge stack thin
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layer between Si and back electrode (e.g., Ag), not only reduces the interface recombination, but
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also obtains good Ohmic contact with Ag electrode, leading to a relative PCE increase of 25%. A
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high open circuit voltage (Voc) up to 643 mV is achieved. After physical insight into the interface
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properties, contact resistance and band lineup, more clear understanding of the improved OIH
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solar cell performance is presented: due to the formation of a Si-Ge epitaxial interface layer, this
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electron-selective stack contact passivated the dangling bonds through a Si/Ge interface obtained
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while tailored energy band structure.
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EXPERIMENTAL
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The OIH solar cells discussed in this work are fabricated on a n-type Czochralski (CZ)-grown
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(100)-oriented single-side-polished Si wafer with a resistivity ranging from 0.1 to 0.3 Ω•cm and a
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thickness of 290 µm. The wafers are first dipped in a hydrofluoric acid (HF) solution (10%, 3 min)
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to remove the naturally grown SiOx layer. PEDOT:PSS (Clevios, PH-1000) mixed with 6 wt%
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ethylene glycol (EG) is spin-coated on the mirrored surface of the Si wafers at 3500 rpm for 40 s
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and subsequently anneals at 130 °C for 10 min in the atmosphere. A Ge thin film is deposited from
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a high pure Ge target on the other side of wafers above at room temperature (RT) by a dual-target
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radiofrequency (rf) magnetron sputtering equipment at a rf power of 50 W in a 1Pa atmosphere of
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Ar after the base pressure of the deposition chamber reached 2×10-4 Pa. Then Ag metal electrode
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is thermally evaporated onto the both sides, and the front side uses a shadow mask. Last, these
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cells with the top electrode are cut into 1×1 cm2 solar cell to remove the conductive edge during
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metal thin film deposition using the sputtering and/or thermal evaporation.
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PV performances of the solar cells are characterized by current density-voltage (J-V) curves at 5
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the standard test condition (AM1.5, 100 mW/cm2 and 25°C) and Suns-Voc measurement module of
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Sinton tool (WCT-120) [33]. The thickness of the Ge thin film is identified by scanning electron
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microscope (SEM) (Figure S1) and high-resolution transmission electron microscope (HRTEM).
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The thicknesses of the Ge thin film including 0 nm, 2 nm, 4 nm, 40 nm, and 120 nm are measured.
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For a clearer description, we define the thickness of Ge thin film with 40 nm as Ge-40 nm, and
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others are defined as the same format. The crystallinity and interface properties of the thin films
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are characterized by X-ray diffraction (XRD) and HRTEM, respectively. The work function of Ge
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stack layer on n-Si is measured by ultraviolet photoelectron spectroscopy (UPS) and the valence
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band structure is demarcated by X-ray photoelectron spectroscopy (XPS). The external quantum
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efficiency (EQE) is measured at a beam size of 1× 1 mm2 using a xenon light source and a
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monochromator in the wavelength range of 400–1100 nm. Electrochemical impedance
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spectroscopy (EIS) is measured in a frequency range of 10 Hz –1 MHz at room temperature.
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RESULTS AND DISCUSSION
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Ge thin layer with the thicknesses of from 0 to 120 nm is inserted between the c-Si substrate
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and the back electrode to examine their applications in the OIH solar cells. Device structure and
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J-V curves of OIH solar cells are presented in Figure 1(a) and (b), and their PV parameters are
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shown in Table 1. The device without Ge thin film has Voc of 550 mV and fill factor (FF) of
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70.5 %. However, for the devices with Ge thin layers, Voc increases to >630 mV and FF to >73%.
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A PCE of 12.9%, with a remarkable Voc of 643 mV, short circuit density (Jsc) of 26.6 mA/cm2, and
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FF of 75.4 %, is achieved when Ge-40 nm thin film is introduced, suggesting that 40 nm is a
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proper thickness for the OIH solar cells. Although Ge has better conductivity than Si, it has lower
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conductivity compared with metal (e.g., Ag). The electrical properties of the Ge thin film is
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measured by Hall measurement, as seen in Table S1. Due to the lower conductivity, FF of the 6
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device decreases with the increase of the thickness of Ge layer. Figure 1 (c) shows EQE
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measurements of OIH solar cells. It can be seen that Jsc did not substantially change at the whole
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wavelenegth region. This reason will be clarified later. In order to understand the improvement in
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device performances such as Voc and FF after inserting the Ge layer between the c-Si substrate and
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the metal electrode, the detailed studies of recombination and contact properties of devices
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without Ge and with Ge-40 nm thin film, are performed in the following sections.
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Figure 1 (a) Schematic of the devices structure, (b) J-V curves of the OIH solar cells, and (c) EQE
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measurements of OIH solar cells.
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Table 1 PV parameters of OIH solar cells
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Ge-0 nm
Ge-2 nm
Ge-4 nm
Ge-40 nm
Ge-120 nm
Voc (mV)
549.8
632.2
636.4
643.0
633.2
Jsc (mA/cm2)
26.5
25.4
25.9
26.6
24.8
FF (%)
70.5
73.5
74.3
75.4
69.2
PCE (%)
10.3
11.8
12.2
12.9
10.9
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Figure 2 (a) shows the Voc vs light intensity (Iph) of the devices without and with the Ge thin
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layer. The Voc is directly proportional to Napierian logarithm of the Iph, i.e., the Voc should increase
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with the increase of Iph, otherwise meaning that a high recombination current (J0) existed in the
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cell, according to the equation (1) :
Voc =
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CI nkT ln( ph ) q J0
(1)
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where n is the ideal factor, k the Boltzmann constant, T the absolute temperature, q the electronic
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charge and C a constant. As shown in the Figure 2 (a), Voc of the device with Ge layer is much
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higher than the one without Ge. At the lower Iph, Voc increases as Iph increases for both of devices,
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Voc- Iph curve remains the linearity. However, with the increases of Iph, the Voc- Iph curve of the
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device without Ge layer gradually departs from its linearity. The appearance of the nonlinearity is
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attributed to Auger recombination of carriers which can’t be effectively transported to electrode.
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The absence of downward band bending at the rear side of the OIH solar cells may be responsible
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for the poor carrier transport, which results in a large reverse saturation current according to the
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equation (1).
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Figure 2 (b) shows the EIS results with a typical equivalent circuit model. The circuit is
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composed of a parallel connected resistance element (Rsh) – capacitance element (C) network and
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a series resistance (Rs) bases on the EIS analysis for OIH solar cell reported by Wang et al. [34],
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and the fitting data is listed in the Table 2.
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Table 2 Impedance characteristics of the investigated OIH solar cells Ge-0 nm
Ge-40 nm
Rsh (Ω)
10384
13360
τ (µs)
413.4
917.9
Rs (Ω)
11.7
6.1
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The results show that the device with Ge layer had higher Rsh，implying a lower carrier
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recombination loss in the device with Ge layer, which is consistent with the result of Suns-Voc
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analysis. The relative minority carrier lifetime (τ) value of the devices is obtained from the
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relationship between Rsh and C, i.e., τ= Rsh×C, τ is 413.4 µs for the device without Ge layer and
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917.9 µs for the device with Ge layer, suggesting that a lower recombination is achieved by
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inserting a Ge contact layer. The Voc decay technique was employed to further confirm this point,
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as shown in Fig. 2 (c). Its principle is as follows: The devices are maintained at open circuit
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conditions under simulated one sun condition, when the light is switched off, Voc decreases due to
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carrier recombination, and the decay rate is an indication of carrier lifetime [35]. It is observed
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that the carrier lifetime of the device becomes longer for the device with Ge layer, indicating that
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the introduction of Ge contact layer in the device reduces the interface recombination rate. Figure
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2 (d) shows the injection level dependent τ curves of the devices without and with the Ge thin
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layer, which was measured using Sinton tool (WCT-120), and the higher τ in the device with the
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Ge layer proved again that Ge addition reduced the interface recombination loss. 9
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It is well known that reducing rear interface recombination should improve EQE in the long
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wavelength region. Here, surprisingly, the improvement is not observed in EQE data in Figure 1
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(c). We speculated that the rear recombination improved EQE could be offset by the optical loss at
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Si/Ge interface. In order to demonstrate this point, a set of samples, including SiNx/n-Si/Ag and
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SiNx/n-Si/Ge/Ag, are designed and fabricated. Here the SiNx is selected to ensure enough incident
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light inside the Si bulk due to its excellent anti-reflection effect. As shown in Figure 2 (e), at the
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same front surface, for the sample with Ge thin layer, the lower reflectance outside at the long
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wavelength means the lower internal reflectance at the rear interface. Further, the Si/Ge interface
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has a higher infrared light transmittance compared to the Si/Ag interace (Figure 2 (f)). These
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results can be explained by the refraction loss from an optically thinner medium (Si, with
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refractive index of 3.5~3.6 @900-1200 nm) into a denser one (Ge, with refractive index of 4.3~4.5
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@900-1200 nm).
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Figure 2 (a) Suns-Voc as a function of light intensity, (b) EIS curves (experimental data are
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represented by dots and fitting data are represented by lines) and the inset is the equivalent circuit
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model, (c) the photovoltage decay curves, (d) the injection level dependent effective carrier
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lifetime curves of the OIH solar cells, (e) the reflectance of the SiNx/n-Si/Ag and SiNx /n-Si/Ge(40 10
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nm)/Ag samples, and (f) the transmittance of the n-Si/Ag (40 nm) and n-Si/Ge (40 nm) samples.
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Similar to Voc, FF is affected by the carrier recombination. In addition, FF is very sensitive to
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the Rs. The Rs is also obtained by combining standard J-V curve and pseudo J-V curve (Suns-Voc
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measurement by Sinton tool), as shown in Figure 3 (a). An accurate value of Rs can be calculated
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according to the following equation [36]:
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Rs =
∆V J mpp
=
VSuns, jmpp - Vmpp J mpp
(2)
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where Vsuns,jmpp is equal to the voltage drop of the pseudo J-V curve at the maximum power point
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current density of the standard J-V curve, and Vmpp and Jmpp are the maximum power point voltage
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and maximum power point current density of the standard J-V curve. The Rs is obtained to be 1.61
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Ω for Ag directly contact with Si. When the Ge layer is inserted, the Rs decreases to 1.24 Ω. The
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lower Rs further explains the improvement of FF for the device with Ge layer.
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Rs not only includes the bulk resistance of various materials but also the contact resistance
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(Rc) at various interfaces. The OIH solar cells in this work adopts the same geometry except for
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the rear electron-selective contact layer, and thus all the variations in cell performance parameters
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should be derived from the modification of the rear contact layer. Here, Rs is mainly determined
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by Rc between Si and back carrier-selective contact layer. Transmission line measurement (TLM)
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is conducted to measure the Rc [37], as shown in Figure 3 (b). The schematic diagram of TLM
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method is shown in Figure 3 (c), and the original current-voltage measurement is shown in Figure
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S2. The Rc is 29.06 Ω•cm2 for the Si/Ag structure and reduces to 0.32 Ω•cm2 when the Ge layer is
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introduced. The lower value of Rc is consistent with the improved Rs for the device with the Ge 11
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contact layer.
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Figure 3 (a) Standard J-V curve and pseudo J-V curve of the OIH solar cell without and with Ge
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layer, (b) resistance vs distance by fitting to obtain the Rc values, (c) the schematic diagram of
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TLM method.
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As mentioned above, the performance parameters, such as Voc and FF, of the OIH solar cell
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devices, have been enhanced by the introduction of a Ge thin layer between the rear side of Si and
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the back electrode. It has been demonstrated that the improvement stemmed from the reduction of
10
interface recombination and the achievement of good contact. However, this explanation is
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incomplete and half way, for example, the reason why Ge addition reduce the interface
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recombination and the contact loss is not unveiled yet. Interestingly, more clear physical insight
13
should be performed. Low work function contact schemes, such as LiF, Cs2CO3, Mg, perylene
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diimide (PDIN), TiOx, Liq and N2200 had been done on the rear surface of the OIH solar cells
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based on the Schottky theory [17-23]. According to the Schottky theory, the barrier height only 12
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depends on the difference of work function between two materials. Taking electron-selective Mg
2
contact as an example, the barrier height is determined by the work function difference
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(Wn-si-WMg) between n-Si and Mg. Duo to the materials of Mg has a lower work function (3.7
4
eV), a downward energy band bending is obtained at the rear surface of the solar cell, which
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suppressed minority carrier recombination and favoured majority carrier transport [19]. The
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optical band gap (Eg) can be determined from the absorption coefficient (α) and photon energy
7
(hν). Figure 4 (a) shows the Eg which is obtained by plotting (αhν)1/2 versus hν and by
8
extrapolating the linear region of the plots to zero absorption. It is found that the band gap of the
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Ge is 0.78 eV. Here, the work function of the Ge is measured to be 4.28 eV, which is higher than
10
the n-type Si. Note that the thicker Ge thin film (120 nm) is selected for UPS measurement to
11
avoid the influence of the signal from the substrate (Figure 4 (b)). The result directly conflicts
12
with the low work function contact which can promote downward energy band bending.
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Figure 4 (a) (αhν)1/2 vs hν to obtain the optical bandgap of Ge thin film and the inset shows the
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sample geometry of the transmission and reflection measurements; (b) UPS spectrum of the Ge
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thin film.
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To unveil this physics, HRTEM is employed to identify interfacial details, as shown in Figure 13
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5 (a). It can be seen that the total thickness of the Ge layer was 4.2 nm, and the Ge layer is proved
2
to be amorphous. The XRD measurement at a small flit angle presented that there are no
3
diffraction peaks of Ge observed from Ge-120 nm film, also implying that the Ge film is
4
amorphous (Figure S3). Moreover, it is surprised that an epitaxial layer with a sharp interface
5
appeared at the Si/Ge interface, and the thickness of the epitaxial layer is only 1 nm. Thus, the Ge
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layer fabricated in this work is not a single phase layer, but an epitaxial/amorphous stack
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functional layer. Because Ge epitaxially grew on Si surface dangling bonds, an ultrathin Si-Ge
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species is embedded at the interface, leading to the passivation of Si surface dangling bonds (see
9
the inset in Figure 5 (a)). From the energy band view, the primary two-material system composed
10
of n-Si and a-Ge was actually a three-material system composed of n-Si, epi-SiGe, and a-Ge
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(n-Si/epi/a-Ge). The epi-SiGe could have energy distributed in the Si energy gap, and a new
12
interfacial layer is formed. Thus, the energy band could be tailored. Figure 5 (b) shows XPS
13
valence band (VB) spectra of the n-Si and the n-Si/epi/a-Ge, and the difference between Fermi
14
level (Ef) and VB level (Ev) are determined by linear extrapolation of the valence edge to zero
15
intensity. Here, note that n-Si is dipped by HF to remove surface oxide layer, and the total
16
thickness of the Ge layer was 4 nm for a reasonable comparison. It can be found that the
17
deposition of the Ge stack layer presented a downward Fermi level moving by 0.32 eV,
18
demonstrating its efficacy for tailoring energy band as shown in from Figure 5 (c) to (d). For the
19
device only with the Ag contact (4.26 eV), an upward band bending was formed on the rear side,
20
and multiple defect energy levels are distributed in the interface energy gap. This case induced the
21
pinning of the Fermi level by the interface states, resulting in that carriers are trapped at interface
22
defect sites during the transport process (Figure 5 (c)). However, the insertion of epi-SiGe layer 14
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achieves: 1) downward band bending at the rear interface due to energy band tailored (Figure 5
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(d)), favoring carrier transport; 2) passivation of Si surface dangling bonds, contributing to the
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unpinning of the Fermi levels, which explained why the recombination loss be improved. In
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addition, the work function of the a-Ge was 4.28 eV being much closed to that of the Ag (4.26 eV),
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so that an Ohmic contact was obtained between a-Ge/Ag contact, which explained the
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improvement of FF.
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Figure 5 (a) Cross-sectional HRTEM image of Ge layer grow on c-Si substrate and the inset shows
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the sketch of the Si-Ge bond contact at the interface, (b) VB XPS spectra of the samples without
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and with epi/a-Ge layer. (c), (d) Band diagrams of OIH solar cells without and with epi/a-Ge layer,
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respectively.
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CONCLUSIONS
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In summary, Ge thin film is fabricated as an electron-selective contact for OIH solar cells by
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magnetron sputtering. The best device, with a 40 nm Ge layer, shows a PCE of 12.9%, with the
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improvement of Voc from 549.8 mV to 643.0 mV and FF from 70.5% to 75.4 % in comparison to
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the reference device. The Ge thin film is demonstrated to be amorphous and an epitaxial layer with
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a sharp interface is found at the Si/Ge interface. Further results suggest that the increases of the
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Voc may be linked to the epi-SiGe. And the efficacy of epi-SiGe layer can be concluded: 1)
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tailoring energy band; 2) passivating Si surface dangling bonds. In addition, the UPS measurement
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shows that work function of a-Ge is close to that of the Ag electrode, which can form an Ohmic
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contact at the rear surface, explaining the improvement of FF. This work presents a promising
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strategy to achieve high-performance OIH solar cell by using a RT-fabricated electron-selective
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epi/a-Ge stack contact: tailoring band structures while maintaining surface passivation and Ohmic
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contact.
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ASSOCIATED CONTENT
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Supporting Information:
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Thickness of the Ge identified by SEM, Current-voltage measurements between Ag fingers with
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and without Ge layer, XRD pattern for Ge thin film on Si substrate, and Carrier density and
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mobility of the Ge.
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AUTHOR INFORMATION
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Corresponding authors
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*1
E-mail:
[email protected] (Jianhui Chen)
22
*3
E-mail:
[email protected] (Y. Mai) 16
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ORCID
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Jianhui Chen: 0000-0002-9875-354X.
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGEMENTS
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This work is supported by NSF of Hebei Province (E2015201203, E2017201034), “Advanced
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Talents Program of Hebei Province (GCC2014013), Top Young Outstanding Innovative Talents
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Program of Hebei Province (BJ2014009), The Midwest universities comprehensive strength
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promotion project (1060001010314), NSF of Hebei Province (F2015201189), “100 Talents
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Program of Hebei Province” (E2014100008) and ISTCP of China (2015DFE62900).
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Figure 1 (a) Schematic of the devices structure, (b) J-V curves of the OIH solar cells, and (c) EQE measurements of OIH solar cells. 125x87mm (300 x 300 DPI)
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Figure 2 (a) Suns-Voc as a function of light intensity, (b) EIS curves (experimental data are represented by dots and fitting data are represented by lines) and the inset is the equivalent circuit model, (c) the photovoltage decay curves, (d) the injection level dependent effective carrier lifetime curves of the OIH solar cells, (e) the reflectance of the SiNx/n-Si/Ag and SiNx /n-Si/Ge(40 nm)/Ag samples, and (f) the transmittance of the n-Si/Ag (40 nm) and n-Si/Ge (40 nm) samples. 101x50mm (300 x 300 DPI)
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Figure 3 (a) Standard J-V curve and pseudo J-V curve of the OIH solar cell without and with Ge layer, (b) resistance vs distance by fitting to obtain the Rc values, (c) the schematic diagram of TLM method. 117x87mm (300 x 300 DPI)
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Figure 4 (a) (αhν)1/2 vs hν to obtain the optical bandgap of Ge thin film and the inset shows the sample geometry of the transmission and reflection measurements; (b) UPS spectrum of the Ge thin film. 83x33mm (300 x 300 DPI)
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Figure 5 (a) Cross-sectional HRTEM image of Ge layer grow on c-Si substrate and the inset shows the sketch of the Si-Ge bond contact at the interface, (b) VB XPS spectra of the samples without and with epi/a-Ge layer. (c), (d) Band diagrams of OIH solar cells without and with epi/a-Ge layer, respectively. 150x103mm (300 x 300 DPI)
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