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Stable CdTe solar cell with V2O5 as a back contact buffer layer
Solar Energy Materials & Solar Cells 144 (2016) 500–508
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Stable CdTe solar cell with V2O5 as a back contact buffer layer Kai Shen a, Ruilong Yang a, Dezhao Wang a, Mingjer Jeng c, Sumit Chaudhary d, Kaiming Ho a, Deliang Wang a,b,e,n a Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China b CAS Key Laboratory of Energy Conversion Materials, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China c Department of Electronic Engineering and the Green Technology Research Center, Chang Gung University, Taoyuan 333, Taiwan d Department of Electrical and Computer Engineering, Iowa State University, Ames, IA 50011, USA e National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 28 February 2015 Received in revised form 2 July 2015 Accepted 24 September 2015
A low electric resistive and stable back contact on p-type CdTe semiconductor is crucial for the commercial employment of high efficiency CdTe thin film solar cell. In this study, V2O5 was deposited as a buffer layer between CdTe and metal electrode in the back contact of CdTe solar cells. Different back contact structures were fabricated on CdTe to study the effect of a V2O5 buffer layer on cell device performance. Both the quantitative band alignment and the device performance of the CdTe solar cells with a V2O5 buffer layer demonstrated that a much lower Schottky barrier was formed compared to the cells with an Au-only back contact. The defect states related to oxygen vacancy within the band gap of the V2O5 played a crucial role in reducing the energy barrier for hole carrier transport. Employing a back contact structure of Cu/V2O5/Cu/Au, a CdTe solar cell with an efficiency as high as 14.0% was fabricated. Long term device stressing test demonstrated that, compared to the CdTe cells with a Cu/Au back electrode, solar cells with the insertion of a V2O5 buffer layer showed much enhanced device stability. & 2015 Elsevier B.V. All rights reserved.
Keywords: CdTe V2O5 Interfacial layer Back contact Solar cell
1. Introduction CdTe is a good photovoltaic material with a nearly ideal direct band gap of 1.45 eV and an absorption coefficient as high as 105 cm 1 [1]. In the last three years, the world-record efficiency for the CdTe solar cell has been dramatically increased from 16.5% to 21.5% [2]. This makes the CdTe solar cell a good candidate which can compete with the market-dominated Si-based photovoltaic product in regarding to the large-scale commercialization. Even though CdTe solar cell has been commercialized, it still has some key issues to be solved or understood to improve the device performance. Compared to the Sibased solar cell, which has been intensively studied regarding both to the material and the device structures, CdTe solar cell has been fabricated mainly based on the empirically acquired knowledge and/or technique. The recent quick progress in the increase of the CdTe solar cell efficiency is mainly attributed to the effort of the companies, namely, First Solar and General Electric. Due to the relatively short history and rather small scientific community devoted to the CdTe n Corresponding author at: Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China. E-mail address: [email protected] (D. Wang).
http://dx.doi.org/10.1016/j.solmat.2015.09.036 0927-0248/& 2015 Elsevier B.V. All rights reserved.
solar cell research, many fundamental problems, both material and device, are remained to be investigated. One difficulty is to realize a low electric resistive and stable back contact to the p-type CdTe material. CdTe has a high work function of 5.7 eV, which is higher than that of all the common metals used as metal contacts [3]. So far, almost all the CdTe solar cell fabrication employs Cu-containing material or Cu metal film as the crucial material to obtain a lowresistive back contact on the CdTe absorber [4,5]. However, Cu is a fast diffuser in CdTe and it can diffuse along the CdTe grain boundary into the cell junction CdS/CdTe [6]. Cu has been reported to have an diffusion coefficient of 10 12 cm2/s at room temperature, which is rather large and therefore the presence of Cu in the CdTe solar cell structure present a threat to cell stability when large-scale CdTe solar cell is employed outdoor for electricity production, where an environment temperature can be as high as 80 °C. At such a high temperature, Cu can easily diffuse through the CdTe absorber into the CdS/CdTe junction and even to the window layer CdS. At the cell junction Cu was proposed to form recombination centers and shunt paths, limiting the cell performance [7]. Cu is a multi-configuration impurity in the CdTe crystal. It can be a deep-acceptor in the form of CuCd, or it can be a shallow donor in the form of an interstitial impurity [8]. In our laboratory we intentionally doped Cu in the CdS window layer. We found that the presence of Cu in CdS dramatically decreased the fill
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factor of the current–voltage curve. Therefore Cu-free material is desirable for the fabrication of a stable CdTe solar cell. Transition metal oxides have recently been paid much attention for the use as a buffer/interfacial layer in organic semiconductor devices [9,10]. Some of the transition metal oxides can be used as selective charge-injection layers to decrease carrier injection energy barrier. Depending on the electronic band structure, insertion of an oxide layer can efficiently enhance hole or electron injection [9]. Ohmic contact can be formed through band alignment engineering between oxide layer and organic semiconductor. Transition metal oxides possess a wide range of work function from 3.5 eV for defective ZrO2 to an extreme high value of 7.0 eV for the stoichiometric V2O5. Highwork-function transition metal oxides are often used as hole-injecting buffer layer at a device anode and low-work-function metal oxides as electron-injecting buffer layer at a device cathode [9]. Recently, molybdenum oxide MoO3 back contact buffer was employed as a buffer layer at the back contact of a CdS/CdTe solar cell. A nearly ohmic contact was achieved at the cell back contact and the cell performance was significantly improved [11,12]. V2O5 possesses similar electrical, structural and optical properties to those of MoO3. In this study, V2O5 thin film was deposited as an interfacial layer at the back contact of CdTe solar cells. Vanadium oxides have been employed as the key materials in electrochemical devices, gas sensors and catalysis reaction system [13,14]. Moreover, several VnOm phases can be relatively easily formed by controlling the experimental parameters or by losing oxygen during the heat treatment of V2O5 [15,16]. Nonstoichiometry of V2O5 can introduce defect states or even intermediate band within the band gap, which may be beneficial to the hole transport at the CdTe back contact [9,17]. In this study we demonstrated that the insertion of a V2O5 buffer layer could dramatically reduce the Schottky barrier at the back contact and at the same time improve the cell performance, including the device stability. The defect states within the band gap of the V2O5 played a crucial role in reducing the energy barrier for hole carrier transport.
2. Experimental The CdTe solar cells fabricated in this study had a structure of glass/SnO2:F(FTO)/n-CdS/p-CdTe/back contact/electrode [18]. The CdS window layers with a thickness of 100 nm were prepared on glass/ FTO substrates by chemical bath deposition technique (CBD) from a solution composed of de-ionized water, cadmium acetate, ammonium acetate, and thiourea. After heat treated in a CdCl2 atmosphere, high crystalline CdS thin film with a mono-grain layer was fabricated [19]. The CdTe absorber layers, which had a thickness of 4 μm, were deposited by the close-spaced sublimation technique (CSS) in a homemade film-deposition system [18]. Subsequently, the CdS/CdTe stacked structures were activated with the presence of CdCl2 in the air atmosphere. In order to study the effect of a V2O5 buffer layer on both the cell efficiency and the cell stability, solar cells were fabricated with different back contact structures, namely, Au-only, Cu/Au bi-metal layer, V2O5/Au, V2O5/Cu/Au, Cu/V2O5/Au, and Cu/V2O5/Cu/Au composite contact were fabricated as the back contacts. These different back contact structures allowed us to study the individual impact of Cu and V2O5 on the improved efficiency and stability of the cell devices. The total Cu material in the back contacts were divided into two parts, namely, directly applied Cu on the surface of CdTe and indirectly applied Cu on the buffer V2O5. Also the Cu thickness was varied in the directly and indirectly applied Cu layers to investigate the factors which induced the cell instability. Stress testings for the CdTe solar cells with different back contact structures were carried out for different times at 85 °C. No chemical surface etching was applied to the CdTe surface before back-contact film deposition for the Au-only and the V2O5/Au back contact structures. For the solar cells using Cu/Au bimetal layer, V2O5/Cu/Au, Cu/V2O5/Au or Cu/V2O5/Cu/Au as the back
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contact, the CdTe surface was chemically etched in a nitric–phosphoric (NP) solution. The V2O5 films were prepared by the RF magnetron-sputtering technique. A 99.99% vanadium target was sputtered in a reactive gas mixture of O2 and Ar with a pressure ratio O2/Ar of 2/3. The working pressure was 0.6 Pa and the RF power was 60 W. The metal layers, namely, Cu, Au, and bilayer Cu/Au were prepared by thermal evaporation in a vacuum chamber. The CdTe solar cell size was 4 4 mm2. The film morphological microstructures were characterized by using a field emission scanning electron microscope (SEM, Sirion 200). The solar cell current–voltage (J–V) curves were measured under standard AM1.5 illumination (1 kW/m2, 25 °C) using a solar simulator (Oriel Sol 3A, USA). XPS measurements were performed on a Thermo VG Scientific ESCALAB 250 instrument with Al Kα as the X-ray source. All the XPS spectra were calibrated by using the carbon 1s peak (284.6 eV). A Kelvin probe force microscope (KPFM) system (Veeco Multimode V) was employed to perform characterization on both the film surface potential and the surface topographic microstructure. The tip used was a highly doped silicon tip coated with Pt/Ir.
3. Results and discussions The microstructures of CdS, CdTe films and a solar cell are shown in Fig. 1. Fig. 1(a) shows the SEM cross-sectional morphology of a CdS film composed of one mono-grained CdS layer, which was obtained by a CdCl2 heat treatment in a chamber filled with a CdCl2 vapor and the air atmosphere [19]. The CdS grains had an in-plane size of 100– 200 nm and the grains were densely packed along the CdS/FTO interface. The cross-sectional microstructure of a CdS/CdTe p–n junction is shown in Fig. 1(b). It can be seen that the CdS/CdTe interface demonstrated a very good junction with a uniform CdS window layer beneath the CdTe absorber layer. The cross-sectional and the surface microstructures of a CdTe film are shown in Fig. 1 (c) and (d). The CdTe film had an excellent crystallinity with preferential vertical growth along the normal direction of the film. The film is composed of columnar CdTe grains with a threading growth from the CdS/CdTe interface straight to the film surface. This preferential vertical growth for the CdTe absorber was induced by a liquid-phase-assisted film growth mechanism. A detailed study on this growth technique will be published elsewhere. The CdTe layer was composed of densely-packed CdTe grains with diameters of 1– 3 μm. Such a range of grain size is the optimum crystalline size for high efficient CdTe solar cell fabrication. All the SEM images of the CdTe films and the CdS/CdTe interface were taken before the CdCl2 heat treatment. After the CdCl2 heat treatment the CdTe layer and the CdS/CdTe interface structure was almost unchanged for high efficiency CdTe solar cells. The V2O5 thin films were nearly transparent and looked yellowish by naked eyes. The as-deposited V2O5 films were uniform and smooth as shown in Fig. 2(a). The root-mean-square surface roughness measured by AFM was 0.26 nm. Fig. 2(b) is a plot of (αhν)1/2 versus photon energy, which was derived from the film light absorption measurement, where α, h, and ν are the absorption coefficient, Planck constant and photon frequency, respectively. The band gap energy obtained from Fig. 2(b) is 2.18 eV, which is consistent with the reported values of 2.2–2.8 eV [20,21]. Fig. 2(c) shows the X-ray spectrum of an asdeposited film, showing the peaks that coincided with the V2O5 (001) and V2O5 (002) [22]. The full width at half maximum (FWHM) for the V2O5 (001) is relatively large with a value of 1.1°. This was due to the small V2O5 nanocrystalline grains with a size of 7 nm. The X-ray data revealed that V2O5 was the dominant phase in the film. The Raman scattering spectrum of the V2O5 film is shown in Fig. 2(d). All the bands centered at 101, 146, 284, 404, 529, 705 and 997 cm 1
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Fig. 1. (a) SEM cross-sectional microstructure of a mono-grained CdS film on Glass/SiO2/FTO; (b) cross-sectional microstructure of a CdS/CdTe interface; (c) and (d) SEM cross-sectional and surface morphology of a 4-μm-thick CdTe film.
Fig. 2. (a) AFM surface morphology of a 15-nm-thick V2O5 film on glass substrate; (b) film absorption measurement, (αhν)1/2 versus photon energy hν of a V2O5 film on glass; (c) X-ray spectrum and (d) Raman spectrum of an as-deposited V2O5 film.
corresponded to the phonon modes of the V2O5 phase, and these values are consistent with the reported data [22]. The contact energy barrier at the interface of a semiconductor/ metal heterostructure depends critically on the work functions of the metal and the semiconductor. Some metal oxide thin films have been employed as a buffer layer to modify electrode work function [9]. In this study in order to characterize the variation of the barrier height at the back contact upon inserting a V2O5 buffer layer, a Kelvin probe force microscope (KPFM) was employed to directly map the surface potentials of a Au film and a V2O5/Au composite bilayer. KPFM can provide surface potential mapping information in parallel to the acquisition of surface topography.
Quantitative work function measurement has been successfully obtained [23]. In our experiment design, in order to get reliable work function data, a 200-nm-thick Au film was first deposited on a glass substrate and then part of the Au film was covered and a 15-nm-thick V2O5 film was deposited on the uncovered part of the Au film. Such an experimental design allowed us to obtain work function mappings of both the Au and the V2O5 on Au film on one sample. This avoided data acquisition uncertainty, which was induced by changing different samples. The potential mappings obtained on Au and V2O5/Au are shown in Fig. 3. A surface potential difference of 315 mV was obtained between the Au and the V2O5/Au. This represents a work function increase of 315 mV
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Fig. 3. (a) Kelvin probe force microscopy surface potential mappings of Au/glass and V2O5 on Au/glass; (b) potential profiles obtained along the two lines indicated in (a); and (c) schematic energy band diagram of the CdTe, Au, and V2O5/Au structure.
Fig. 4. The (a) V 2p3/2, (b) O 1s and (c) valence-band photoemission spectra of a V2O5 film; and (d) schematic energy-band diagram with defect state in the gap, whose energy level was clearly shown in (c).
for the composite V2O5/Au film compared to the Au metal. Take the Au work function of 5.1 eV as the reference, the work function of V2O5 was therefore estimated to be about 5.4 eV. This relatively high work function led to a reduced Fermi level mismatch from 0.6 to 0.3 eV between CdTe and the electrodes of Au only and V2O5/Au. The direct surface potential mapping data were quantitatively consistent with the observation of the roll-over behavior of the cell light J–V curve to be discussed in the following paragraph (see Fig. 7). Transition metal oxides exhibit a relatively large range of stoichiometries because they are prone to form many different types of defect, such as oxygen vacancies, oxygen interstitials, and metal point defects [9]. Vanadia V2O5 possesses a layered structure built up of VO5 square pyramids sharing edges and corners. It is well known that oxygen is easily to be stochiometric deficient or taken up into the V2O5 lattice. Therefore different vanadia phases derived from non-stoichiometric V2O5 through the loss of oxygen, such as V4O9, could be formed as nanocrytalline in a nominal V2O5 film [24,25]. Barrierless hole injection has been achieved by using a substoichiometric MoOx thin film as an anode interfacial layer in
a hole only device. This was ascribed to the formation of an efficient ohmic contact via the excellent band alignment through occupied gap states at the device interface [17]. It is highly possible that band gap states induced by the point defects presented in the V2O5 film may play a crucial role in hole transport at the back contact of a CdTe solar cell. Eventhough our XRD and Raman data shown in Fig. 2 indicated that the vanadium oxide was nominally made of the V2O5 phase, however, non-stochiometric vanadium phases and oxygen vacancy could unavoidably be formed in these films, which were fabricated by reactive magnetron sputtering, due to the oxygen deficient environment or incomplete reaction during the relatively fast material deposition [24]. In this study high resolution XPS experiments were carried out to study the degree of oxidation and defect states of the prepared vanadium oxide films. Fig. 4 shows the V 2p and the O 1s XPS spectra, which were fitted with mixed Lorentzian–Gaussian curves. For the V 2p XPS spectrum, the peaks located at 517.4 and 516.0 eV correspond to the V5 þ and the V4 þ states. Both values are consistent with the reported binding energies for these states in the literatures [26,27]. For the O 1s XPS spectrum, the main peak located at
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530.3 eV corresponds to the lattice oxygen of V2O5. The shoulder peak located at 531.7 eV is related to the oxygen vacancy or surface contamination [28,29]. The presence of oxygen vacancy was also evidenced by the reduced work function of V2O5 obtained in this study compared to the stoichiometric V2O5, which had a work function as high as 7.0 eV [9]. Oxygen vacancy in oxides usually acts as n-type dopant and raises the Fermi level, leading to a decreased value of work function [30]. Theoretically, for the transition metal oxides, such as V2O5, MoO3, TiO2, WO3, the conduction band minima are primarily composed of the empty metal d states, and valence-band maxima are primarily composed of the O 2p states. Therefore stoichiometric transition-metal oxides tend to be insulators. However, oxygen vacancy is almost unavoidably formed in these oxides during material fabrication. With a high density of oxygen vacancy, an intermediate band would be formed due to the electron wave function overlapping of the occupied states within the band gap [9,31]. These defect states were clearly observed in the V2O5 valence band spectrum, shown in the inset in Fig. 4(c). Because of the relatively small energy gap between the gap states and the conduction band, oxygen deficient V2O5 is an ntype semiconductor. Fig. 4(d) shows schematically the band diagram of V2O5 with the consideration of defect states induced by oxygen deficiency [31]. When a transition metal oxide is employed as a buffer layer between a semiconductor and a metal electrode, it has been reported that gap states play an important role in assisting the hole injection [17,30,32]. The defect-state peak observed in the valence-band photoemission XPS spectrum shown in Fig. 4 (c) demonstrated that a large density of occupied sub-band gap states centered around 0.4 eV below the Fermi level had been formed in the V2O5 buffer layers [32]. The band alignment at the interface of CdTe/back contact is crucial for the hole transport. In order to have a better understanding of the hole transport at the back contact, the band alignment of the V2O5/ CdTe heterostructure was quantitatively characterized by the XPS technique. The XPS technique has been proved to be a direct and powerful tool for measuring the valence band offset (VBO) at a heterostructural interface [33,34]. The valence band offset EVBO at the V2O5/ CdTe junction can be calculated by using XPS data according to the following equation, EVBO ¼ EVBM Cd3d5=2 bulk CdTe EVBM V2p3=2 V2p3=2 Cd3d5=2 bulk V2 O5
The corresponding values of EVBM and V 2p3/2 measured on a 100-nmthick V2O5 film are 2.48 and 517.42 eV, respectively, as shown in Fig. 5 (b). ΔECL ¼(V 2p3/2 Cd 3d5/2)interface is the energy difference between the two core levels of the V 2p3/2 and the Cd 3d5/2, which were measured at the V2O5/CdTe interface, where the V2O5 coating thickness was 3 nm. The value of ΔECL was 111.98 eV. The band alignment at the V2O5/CdTe interface obtained based on the XPS data is shown in Fig. 6. The valence band offset between V2O5 and CdTe is 1.49 eV. The quantitative band alignment obtained by the XPS data, including the gap states, for the back contact structure CdTe/V2O5/Au is show in Fig. 6. As can be seen in Fig. 6, the V2O5 gap states lie very close to the CdTe valence band maximum. These band gap sates acted as efficient hole injection routes to the Au electrode. The valence band maximum of the CdTe, which had a hole carrier concentration of 1013– 1014 cm 3 (measured by capacitance–voltage measurement), was about 0.3–0.4 eV below its Fermi level. The effect of inserting a V2O5 buffer layer at the back contact on the CdTe solar cell performance is demonstrated in Fig. 7. CdTe solar cells with two kinds of back contact structures, namely, one with Au metal only and the other with a V2O5/Au composite back contact, were fabricated. Au has a work function of 5.1 eV, this value is lower than that of CdTe, whose work function is as high as 5.7 eV. As can be seen in Fig. 7(a), the cell with the Au-only back contact exhibits a roll-over behavior, which led to rather low values of fill factor (FF) and open-circuit (Voc). The roll-over behavior at high voltage is indicative of a non-ohmic contact, which was caused by the large work-function mismatch between Au and p-type CdTe [35]. With a V2O5/Au back contact, where the V2O5 buffer layer was 15 nm thick, the roll-over phenomenon disappeared and consequently the FF and the Voc were increased by 6.3% and 89 mV, respectively. The increased Voc for a CdTe solar
interface
where (EVBM Cd 3d5/2)bulk CdTe is the energy difference between the valence band maximum and the Cd 3d5/2 core level measured using a thick CdTe film, shown in Fig. 5(a). The valence band maximum EVBM was obtained by linear fittings to the XPS data. The intersect of the two linear extrapolations corresponds to the valence band maximum, shown in the inset of Fig. 5(a). The values of EVBM and Cd 3d5/2 measured on a 4-μm-thick CdTe are 0.44 and 404.89 eV, respectively.
Fig. 6. Band alignment at the CdTe/V2O5/Au contact interface obtained based on the XPS data.
Fig. 5. (a) Core-level Cd 3d and valence band XPS spectra of bulk CdTe; (b) core-level V 2p and valence band XPS spectra of bulk V2O5. The insets shows detailed XPS spectrum near the valence band maxima; (c) the two core levels of the V 2p3/2 and the Cd 3d5/2, which was measured at a V2O5/CdTe interface with a V2O5 coating thickness of 3 nm.
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Fig. 7. (a) Light and (b) dark J–V curves of two CdTe solar cells with different back contact structures. (c) dV/dJ curve versus the (J þJSC) 1, derived from the dark J–V curves shown in (b).
cell is usually ascribed either to the carrier recombination reduction at the CdS/CdTe junction and within the CdTe absorber or to a much reduced back contact barrier [36]. The barriers estimated from the two J–V curves, shown in Fig. 7(a), were 0.50 and 0.35 eV for the cells with Au only and V2O5/Au back contacts. The barrier heights were calculated based on the thermionic emission theory [37,38]. The Voc increase for the cell with a V2O5/Au contact in this study was due to the reduction of barrier height as evidenced by the band alignment characterization discussed above. Fig. 7 (b) shows the corresponding dark J–V curves for the two cells. The device with the composite V2O5/Au back contact demonstrated a nearly ideal diode J–V curve. At a forward voltage larger than 0.5 V, the current of the cell with the Au metal contact was significantly less than that of the cell diode with the V2O5/Au back contact. This was caused by the relatively large resistance at the back contact, which was in series with the main diode. The lumped series resistances, which were obtained by the dark J–V curves and shown in Fig. 7(c), were 6.21 and 1.21 Ω cm2 for the two cells with the Au and the V2O5/Au back contact, respectively. It can be clearly seen that the incorporation of a V2O5 interfacial layer led to the formation of a much decreased Schottky barrier at the back contact. The diode ideal factors were 1.86 and 1.71 for the two cells, indicating that the junction current was dominated by the Shockley–Read–Hall recombination for the two cells. With the discussions and the experimental results presented above, we believe that, for the CdTe/V2O5/Au composite electrode, two beneficial factors contributed to the low back contact barrier observed in our CdTe solar cell. First, the high V2O5 work function reduced the Schottky barrier which was caused by the work function mismatch between Au and CdTe. Second, the oxygen deficiency related gap states in the V2O5 buffer layer efficiently assisted the hole injection. The use of a V2O5 buffer layer at the back contact makes it possible not only to fabricate high efficient CdTe solar cells but also to fabricate cells with high device stability. In a CdTe solar cell, Cu has been routinely adopted as a material to reduce the back contact barrier with CdTe and improve the cell device performance [4,5]. Cu dopes the CdTe film surface to form a p þ region and reacts with Te to form CuxTe compound to reduce the contact barrier [4,18]. However, Cu is the main concern which induces instability for a CdTe cell employed for long-term use to generate electricity in a harsh outdoor environment. Therefore in principle the amount of Cu deposited for low-resistance contact fabrication should be as less as possible. We found that without any use of Cu, even though we could fabricate relatively high efficient CdTe solar cells, if we deposited a very thin layer of Cu and together with a thin V2O5 buffer layer, CdTe solar cells with an efficiency as high as 14% could be routinely fabricated. In order to study the effect of a V2O5 buffer layer on cell efficiency and device stability, solar cells with different contact
Fig. 8. Light J–V curves of four CdTe solar cells with different back contact structures, which had different directly and indirectly applied Cu materials in the back contact.
structures, namely, Cu/Au, V2O5/Cu/Au, Cu/V2O5/Au and Cu/V2O5/ Cu/Au, were fabricated. These different structure combinations allowed us to explore the respective roles of the Cu and the V2O5 layer in the contact. Fig. 8 shows the device performance of cells with four different back contact structures. The two cells with contact structures of 2 nm Cu/V2O5/Au and 2 nm Cu/V2O5/3 nm Cu/Au demonstrated almost the same energy conversion efficiency, indicating that the directly applied Cu(2-nm-thick) was the most influencing material for the device performance. For the cell with a V2O5/3 nm Cu/Au contact, the device performance was similar to the cell with a V2O5/Au contact shown in Fig. 7(a). It can be seen that the indirectly applied Cu layer between V2O5 and Au had little effect on the cell performance. Compared to the cell with V2O5/3 nm Cu/Au as the back contact, the efficiency of the cell with directly applied Cu on the CdTe surface, having a back contact structure of 2 nm Cu/V2O5/3 nm Cu/Au, was much improved. We call the first Cu layer, which was directly contacted with the CdTe surface and effected the cell performance apparently, the primary Cu layer. The metal bilayer of Cu/Au has been widely employed for fabrication of high efficient CdTe solar cells. The optimal Cu thickness was found to be 5 nm, which is consistent with the results of other research groups [18,39]. Compared to the cell with a 5 nm Cu/Au contact, the J–V curve of the cell with a 2 nm Cu/Au contact showed a slight roll-over at high bias, indicating the existence of a Schottky barrier at the back contact. With the insertion of a V2O5 buffer layer, much less amount of primary Cu material, only 2-nm-thick, was needed to obtain high efficient solar cells. The V2O5 buffer layer makes it possible to obtain a lowbarrier back contact with less or even no Cu. In order to study the effect of a V2O5 buffer layer on cell stability, stress testings of CdTe solar cells with different back contact structures were conducted both at room temperature and under thermal condition. The room-temperature-aging tests for the two cells with back contact structures of 5 nm Cu/Au and 2 nm Cu/
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Fig. 9. (a) and (c) Current–voltage characteristics of two CdTe solar cells with different contact structures aged at room temperature and in the air atmosphere for different times; (b) and (d) the corresponding dV/dJ curves versus the (Jþ JSC) 1 derived from the J–V curves shown in (a) and (c).
Fig. 10. Long term stress testings for four CdTe solar cells with different back contact structures. (a) open-circuit voltage, (b) short-circuit current, (c) fill factor, and (d) cell efficiency, versus the stress time.
V2O5/3 nm Cu/Au are shown in Fig. 9. These two back contact structures were chosen to keep the total Cu layer thickness the same. For the cell with a Cu/V2O5/Cu/Au contact, after aged at room temperature and in the air atmosphere for 266 days, the efficiency and the fill factor decreased slightly from 14.0% and 70.0% to 13.8% and 69.7%, respectively. After more than one year aging test, namely 413 days, the cell efficiency remained a value of
13.8%, the same as that after 266 days aging. But for the cell with a Cu/Au contact, after aged for only 146 days, the efficiency and the fill factor were decreased quickly from 14.3% and 70.2% to 12.7% and 65.3%, respectively. After one year aging test (368 days) the cell efficiency was decreased further to a value of 11.9%. These data clearly demonstrated that the insertion of a V2O5 buffer layer and the rather thin thickness of a 2-nm-thick primary Cu layer greatly
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slowed the device degradation compared to the cell with a 5-nmthick Cu contact layer. It can be seen that Cu is indeed the main source for cell instability even for cells aged at room temperature. For the cell with a Cu/Au back contact, after aged for 146 and 368 days, the ideality factor and series resistance increased from 1.96 and 1.43 Ω cm2, to 2.49 and 2.57 Ω cm2, and 2.51 and 3.73 Ω cm2, see Fig. 9(d). The much increased ideality factor indicates that Cu, which diffused to the CdS/CdTe junction, acted as traps assisting carrier tunneling in the junction and in the CdTe film [40]. For the cell with a 2 nm Cu/V2O5/3 nm Cu/Au back contact, the ideality factor and the series resistance kept almost unchanged after aged for more than one year, see Fig. 9(b). With much less amount material of a primary Cu layer and the insertion of a V2O5 buffer layer, the cell degradation was successfully suppressed. The comparison study also demonstrated that the V2O5 oxide layer can act as an effective barrier for Cu diffusion from the back contact structure into the CdTe. The long term device stability of CdTe solar cells, which had back contact composite structures of 2 nm Cu/Au, 5 nm Cu/Au, V2O5/3 nm Cu/Au, and 2 nm Cu/V2O5/3 nm Cu/Au, have been evaluated by employing thermal stress testing. The thermal stress testings were carried out at a temperature of 85 °C in air in an electric thermal oven. The solar cell device parameters measured after different stressing times are shown in Fig. 10. Except the cell with 5 nm Cu/Au bi-metal layer as the back contact, all the other cells showed rather stable device performance. For the cell with 5 nm Cu/Au back contact, all the cell parameters, namely, the open-circuit voltage, the short-circuit current, and especially the fill factor, were constantly decreased with the stressing time. After 387 h stressing, the fill factor was decreased from 70.4% to 67.3%, and the efficiency was decreased from 14.1% to 13.2%. With the insertion of a V2O5 buffer layer, the stability of the cell with 2 nm Cu/V2O5/3 nm Cu/Au back contact was much improved. The stability improvement mainly came from the reduced thickness of the primary Cu layer. This was also confirmed by the stressing test of the cell with reduced thickness of the primary Cu in the contact, namely, a 2 nm Cu/Au bimetal layer contact. The V2O5 buffer layer blocked the Cu material, which was located between V2O5 and Au, to diffuse into the CdTe and further into the CdS/CdTe interface area, which is the most essential part in a solar cell to improve the device stability. The blocking effect was also confirmed by the stressing test of the cell with a V2O5/Cu(3-nm-thick)/Au back contact.
4. Conclusions In summary, V2O5 thin film was employed as a buffer layer to form low-barrier back contact for CdTe solar cell fabrication. Different back contact structures were fabricated to study the effect of a V2O5 buffer layer on CdTe solar cell device performance. Both the quantitative band alignment and the device performance of the CdTe solar cells with a V2O5/Au back contact demonstrated that a much lower Schottky barrier was formed compared to the cells with an Au-only back contact. The defect states related to oxygen vacancy within the band gap of the V2O5 played a crucial role in reducing the energy barrier for hole carrier transport. With much less Cu directly deposited on the surface of CdTe in a Cu/ V2O5/Cu/Au back contact, CdTe solar cell with an efficiency as high as 14.0% was obtained. Compared with the CdTe solar cells having a traditional Cu/Au back electrode, the device stability of the cells with the insertion of a V2O5 buffer layer in the back contact was significantly enhanced.
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Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 51272247, 61474103) and Natural Science Foundation of Anhui Province (No. 1408085MF119).
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Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Stable CdTe solar cell with V2O5 as a back contact buffer layer Kai Shen a, Ruilong Yang a, Dezhao Wang a, Mingjer Jeng c, Sumit Chaudhary d, Kaiming Ho a, Deliang Wang a,b,e,n a Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China b CAS Key Laboratory of Energy Conversion Materials, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China c Department of Electronic Engineering and the Green Technology Research Center, Chang Gung University, Taoyuan 333, Taiwan d Department of Electrical and Computer Engineering, Iowa State University, Ames, IA 50011, USA e National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 28 February 2015 Received in revised form 2 July 2015 Accepted 24 September 2015
A low electric resistive and stable back contact on p-type CdTe semiconductor is crucial for the commercial employment of high efficiency CdTe thin film solar cell. In this study, V2O5 was deposited as a buffer layer between CdTe and metal electrode in the back contact of CdTe solar cells. Different back contact structures were fabricated on CdTe to study the effect of a V2O5 buffer layer on cell device performance. Both the quantitative band alignment and the device performance of the CdTe solar cells with a V2O5 buffer layer demonstrated that a much lower Schottky barrier was formed compared to the cells with an Au-only back contact. The defect states related to oxygen vacancy within the band gap of the V2O5 played a crucial role in reducing the energy barrier for hole carrier transport. Employing a back contact structure of Cu/V2O5/Cu/Au, a CdTe solar cell with an efficiency as high as 14.0% was fabricated. Long term device stressing test demonstrated that, compared to the CdTe cells with a Cu/Au back electrode, solar cells with the insertion of a V2O5 buffer layer showed much enhanced device stability. & 2015 Elsevier B.V. All rights reserved.
Keywords: CdTe V2O5 Interfacial layer Back contact Solar cell
1. Introduction CdTe is a good photovoltaic material with a nearly ideal direct band gap of 1.45 eV and an absorption coefficient as high as 105 cm 1 [1]. In the last three years, the world-record efficiency for the CdTe solar cell has been dramatically increased from 16.5% to 21.5% [2]. This makes the CdTe solar cell a good candidate which can compete with the market-dominated Si-based photovoltaic product in regarding to the large-scale commercialization. Even though CdTe solar cell has been commercialized, it still has some key issues to be solved or understood to improve the device performance. Compared to the Sibased solar cell, which has been intensively studied regarding both to the material and the device structures, CdTe solar cell has been fabricated mainly based on the empirically acquired knowledge and/or technique. The recent quick progress in the increase of the CdTe solar cell efficiency is mainly attributed to the effort of the companies, namely, First Solar and General Electric. Due to the relatively short history and rather small scientific community devoted to the CdTe n Corresponding author at: Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China. E-mail address: [email protected] (D. Wang).
http://dx.doi.org/10.1016/j.solmat.2015.09.036 0927-0248/& 2015 Elsevier B.V. All rights reserved.
solar cell research, many fundamental problems, both material and device, are remained to be investigated. One difficulty is to realize a low electric resistive and stable back contact to the p-type CdTe material. CdTe has a high work function of 5.7 eV, which is higher than that of all the common metals used as metal contacts [3]. So far, almost all the CdTe solar cell fabrication employs Cu-containing material or Cu metal film as the crucial material to obtain a lowresistive back contact on the CdTe absorber [4,5]. However, Cu is a fast diffuser in CdTe and it can diffuse along the CdTe grain boundary into the cell junction CdS/CdTe [6]. Cu has been reported to have an diffusion coefficient of 10 12 cm2/s at room temperature, which is rather large and therefore the presence of Cu in the CdTe solar cell structure present a threat to cell stability when large-scale CdTe solar cell is employed outdoor for electricity production, where an environment temperature can be as high as 80 °C. At such a high temperature, Cu can easily diffuse through the CdTe absorber into the CdS/CdTe junction and even to the window layer CdS. At the cell junction Cu was proposed to form recombination centers and shunt paths, limiting the cell performance [7]. Cu is a multi-configuration impurity in the CdTe crystal. It can be a deep-acceptor in the form of CuCd, or it can be a shallow donor in the form of an interstitial impurity [8]. In our laboratory we intentionally doped Cu in the CdS window layer. We found that the presence of Cu in CdS dramatically decreased the fill
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factor of the current–voltage curve. Therefore Cu-free material is desirable for the fabrication of a stable CdTe solar cell. Transition metal oxides have recently been paid much attention for the use as a buffer/interfacial layer in organic semiconductor devices [9,10]. Some of the transition metal oxides can be used as selective charge-injection layers to decrease carrier injection energy barrier. Depending on the electronic band structure, insertion of an oxide layer can efficiently enhance hole or electron injection [9]. Ohmic contact can be formed through band alignment engineering between oxide layer and organic semiconductor. Transition metal oxides possess a wide range of work function from 3.5 eV for defective ZrO2 to an extreme high value of 7.0 eV for the stoichiometric V2O5. Highwork-function transition metal oxides are often used as hole-injecting buffer layer at a device anode and low-work-function metal oxides as electron-injecting buffer layer at a device cathode [9]. Recently, molybdenum oxide MoO3 back contact buffer was employed as a buffer layer at the back contact of a CdS/CdTe solar cell. A nearly ohmic contact was achieved at the cell back contact and the cell performance was significantly improved [11,12]. V2O5 possesses similar electrical, structural and optical properties to those of MoO3. In this study, V2O5 thin film was deposited as an interfacial layer at the back contact of CdTe solar cells. Vanadium oxides have been employed as the key materials in electrochemical devices, gas sensors and catalysis reaction system [13,14]. Moreover, several VnOm phases can be relatively easily formed by controlling the experimental parameters or by losing oxygen during the heat treatment of V2O5 [15,16]. Nonstoichiometry of V2O5 can introduce defect states or even intermediate band within the band gap, which may be beneficial to the hole transport at the CdTe back contact [9,17]. In this study we demonstrated that the insertion of a V2O5 buffer layer could dramatically reduce the Schottky barrier at the back contact and at the same time improve the cell performance, including the device stability. The defect states within the band gap of the V2O5 played a crucial role in reducing the energy barrier for hole carrier transport.
2. Experimental The CdTe solar cells fabricated in this study had a structure of glass/SnO2:F(FTO)/n-CdS/p-CdTe/back contact/electrode [18]. The CdS window layers with a thickness of 100 nm were prepared on glass/ FTO substrates by chemical bath deposition technique (CBD) from a solution composed of de-ionized water, cadmium acetate, ammonium acetate, and thiourea. After heat treated in a CdCl2 atmosphere, high crystalline CdS thin film with a mono-grain layer was fabricated [19]. The CdTe absorber layers, which had a thickness of 4 μm, were deposited by the close-spaced sublimation technique (CSS) in a homemade film-deposition system [18]. Subsequently, the CdS/CdTe stacked structures were activated with the presence of CdCl2 in the air atmosphere. In order to study the effect of a V2O5 buffer layer on both the cell efficiency and the cell stability, solar cells were fabricated with different back contact structures, namely, Au-only, Cu/Au bi-metal layer, V2O5/Au, V2O5/Cu/Au, Cu/V2O5/Au, and Cu/V2O5/Cu/Au composite contact were fabricated as the back contacts. These different back contact structures allowed us to study the individual impact of Cu and V2O5 on the improved efficiency and stability of the cell devices. The total Cu material in the back contacts were divided into two parts, namely, directly applied Cu on the surface of CdTe and indirectly applied Cu on the buffer V2O5. Also the Cu thickness was varied in the directly and indirectly applied Cu layers to investigate the factors which induced the cell instability. Stress testings for the CdTe solar cells with different back contact structures were carried out for different times at 85 °C. No chemical surface etching was applied to the CdTe surface before back-contact film deposition for the Au-only and the V2O5/Au back contact structures. For the solar cells using Cu/Au bimetal layer, V2O5/Cu/Au, Cu/V2O5/Au or Cu/V2O5/Cu/Au as the back
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contact, the CdTe surface was chemically etched in a nitric–phosphoric (NP) solution. The V2O5 films were prepared by the RF magnetron-sputtering technique. A 99.99% vanadium target was sputtered in a reactive gas mixture of O2 and Ar with a pressure ratio O2/Ar of 2/3. The working pressure was 0.6 Pa and the RF power was 60 W. The metal layers, namely, Cu, Au, and bilayer Cu/Au were prepared by thermal evaporation in a vacuum chamber. The CdTe solar cell size was 4 4 mm2. The film morphological microstructures were characterized by using a field emission scanning electron microscope (SEM, Sirion 200). The solar cell current–voltage (J–V) curves were measured under standard AM1.5 illumination (1 kW/m2, 25 °C) using a solar simulator (Oriel Sol 3A, USA). XPS measurements were performed on a Thermo VG Scientific ESCALAB 250 instrument with Al Kα as the X-ray source. All the XPS spectra were calibrated by using the carbon 1s peak (284.6 eV). A Kelvin probe force microscope (KPFM) system (Veeco Multimode V) was employed to perform characterization on both the film surface potential and the surface topographic microstructure. The tip used was a highly doped silicon tip coated with Pt/Ir.
3. Results and discussions The microstructures of CdS, CdTe films and a solar cell are shown in Fig. 1. Fig. 1(a) shows the SEM cross-sectional morphology of a CdS film composed of one mono-grained CdS layer, which was obtained by a CdCl2 heat treatment in a chamber filled with a CdCl2 vapor and the air atmosphere [19]. The CdS grains had an in-plane size of 100– 200 nm and the grains were densely packed along the CdS/FTO interface. The cross-sectional microstructure of a CdS/CdTe p–n junction is shown in Fig. 1(b). It can be seen that the CdS/CdTe interface demonstrated a very good junction with a uniform CdS window layer beneath the CdTe absorber layer. The cross-sectional and the surface microstructures of a CdTe film are shown in Fig. 1 (c) and (d). The CdTe film had an excellent crystallinity with preferential vertical growth along the normal direction of the film. The film is composed of columnar CdTe grains with a threading growth from the CdS/CdTe interface straight to the film surface. This preferential vertical growth for the CdTe absorber was induced by a liquid-phase-assisted film growth mechanism. A detailed study on this growth technique will be published elsewhere. The CdTe layer was composed of densely-packed CdTe grains with diameters of 1– 3 μm. Such a range of grain size is the optimum crystalline size for high efficient CdTe solar cell fabrication. All the SEM images of the CdTe films and the CdS/CdTe interface were taken before the CdCl2 heat treatment. After the CdCl2 heat treatment the CdTe layer and the CdS/CdTe interface structure was almost unchanged for high efficiency CdTe solar cells. The V2O5 thin films were nearly transparent and looked yellowish by naked eyes. The as-deposited V2O5 films were uniform and smooth as shown in Fig. 2(a). The root-mean-square surface roughness measured by AFM was 0.26 nm. Fig. 2(b) is a plot of (αhν)1/2 versus photon energy, which was derived from the film light absorption measurement, where α, h, and ν are the absorption coefficient, Planck constant and photon frequency, respectively. The band gap energy obtained from Fig. 2(b) is 2.18 eV, which is consistent with the reported values of 2.2–2.8 eV [20,21]. Fig. 2(c) shows the X-ray spectrum of an asdeposited film, showing the peaks that coincided with the V2O5 (001) and V2O5 (002) [22]. The full width at half maximum (FWHM) for the V2O5 (001) is relatively large with a value of 1.1°. This was due to the small V2O5 nanocrystalline grains with a size of 7 nm. The X-ray data revealed that V2O5 was the dominant phase in the film. The Raman scattering spectrum of the V2O5 film is shown in Fig. 2(d). All the bands centered at 101, 146, 284, 404, 529, 705 and 997 cm 1
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Fig. 1. (a) SEM cross-sectional microstructure of a mono-grained CdS film on Glass/SiO2/FTO; (b) cross-sectional microstructure of a CdS/CdTe interface; (c) and (d) SEM cross-sectional and surface morphology of a 4-μm-thick CdTe film.
Fig. 2. (a) AFM surface morphology of a 15-nm-thick V2O5 film on glass substrate; (b) film absorption measurement, (αhν)1/2 versus photon energy hν of a V2O5 film on glass; (c) X-ray spectrum and (d) Raman spectrum of an as-deposited V2O5 film.
corresponded to the phonon modes of the V2O5 phase, and these values are consistent with the reported data [22]. The contact energy barrier at the interface of a semiconductor/ metal heterostructure depends critically on the work functions of the metal and the semiconductor. Some metal oxide thin films have been employed as a buffer layer to modify electrode work function [9]. In this study in order to characterize the variation of the barrier height at the back contact upon inserting a V2O5 buffer layer, a Kelvin probe force microscope (KPFM) was employed to directly map the surface potentials of a Au film and a V2O5/Au composite bilayer. KPFM can provide surface potential mapping information in parallel to the acquisition of surface topography.
Quantitative work function measurement has been successfully obtained [23]. In our experiment design, in order to get reliable work function data, a 200-nm-thick Au film was first deposited on a glass substrate and then part of the Au film was covered and a 15-nm-thick V2O5 film was deposited on the uncovered part of the Au film. Such an experimental design allowed us to obtain work function mappings of both the Au and the V2O5 on Au film on one sample. This avoided data acquisition uncertainty, which was induced by changing different samples. The potential mappings obtained on Au and V2O5/Au are shown in Fig. 3. A surface potential difference of 315 mV was obtained between the Au and the V2O5/Au. This represents a work function increase of 315 mV
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Fig. 3. (a) Kelvin probe force microscopy surface potential mappings of Au/glass and V2O5 on Au/glass; (b) potential profiles obtained along the two lines indicated in (a); and (c) schematic energy band diagram of the CdTe, Au, and V2O5/Au structure.
Fig. 4. The (a) V 2p3/2, (b) O 1s and (c) valence-band photoemission spectra of a V2O5 film; and (d) schematic energy-band diagram with defect state in the gap, whose energy level was clearly shown in (c).
for the composite V2O5/Au film compared to the Au metal. Take the Au work function of 5.1 eV as the reference, the work function of V2O5 was therefore estimated to be about 5.4 eV. This relatively high work function led to a reduced Fermi level mismatch from 0.6 to 0.3 eV between CdTe and the electrodes of Au only and V2O5/Au. The direct surface potential mapping data were quantitatively consistent with the observation of the roll-over behavior of the cell light J–V curve to be discussed in the following paragraph (see Fig. 7). Transition metal oxides exhibit a relatively large range of stoichiometries because they are prone to form many different types of defect, such as oxygen vacancies, oxygen interstitials, and metal point defects [9]. Vanadia V2O5 possesses a layered structure built up of VO5 square pyramids sharing edges and corners. It is well known that oxygen is easily to be stochiometric deficient or taken up into the V2O5 lattice. Therefore different vanadia phases derived from non-stoichiometric V2O5 through the loss of oxygen, such as V4O9, could be formed as nanocrytalline in a nominal V2O5 film [24,25]. Barrierless hole injection has been achieved by using a substoichiometric MoOx thin film as an anode interfacial layer in
a hole only device. This was ascribed to the formation of an efficient ohmic contact via the excellent band alignment through occupied gap states at the device interface [17]. It is highly possible that band gap states induced by the point defects presented in the V2O5 film may play a crucial role in hole transport at the back contact of a CdTe solar cell. Eventhough our XRD and Raman data shown in Fig. 2 indicated that the vanadium oxide was nominally made of the V2O5 phase, however, non-stochiometric vanadium phases and oxygen vacancy could unavoidably be formed in these films, which were fabricated by reactive magnetron sputtering, due to the oxygen deficient environment or incomplete reaction during the relatively fast material deposition [24]. In this study high resolution XPS experiments were carried out to study the degree of oxidation and defect states of the prepared vanadium oxide films. Fig. 4 shows the V 2p and the O 1s XPS spectra, which were fitted with mixed Lorentzian–Gaussian curves. For the V 2p XPS spectrum, the peaks located at 517.4 and 516.0 eV correspond to the V5 þ and the V4 þ states. Both values are consistent with the reported binding energies for these states in the literatures [26,27]. For the O 1s XPS spectrum, the main peak located at
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530.3 eV corresponds to the lattice oxygen of V2O5. The shoulder peak located at 531.7 eV is related to the oxygen vacancy or surface contamination [28,29]. The presence of oxygen vacancy was also evidenced by the reduced work function of V2O5 obtained in this study compared to the stoichiometric V2O5, which had a work function as high as 7.0 eV [9]. Oxygen vacancy in oxides usually acts as n-type dopant and raises the Fermi level, leading to a decreased value of work function [30]. Theoretically, for the transition metal oxides, such as V2O5, MoO3, TiO2, WO3, the conduction band minima are primarily composed of the empty metal d states, and valence-band maxima are primarily composed of the O 2p states. Therefore stoichiometric transition-metal oxides tend to be insulators. However, oxygen vacancy is almost unavoidably formed in these oxides during material fabrication. With a high density of oxygen vacancy, an intermediate band would be formed due to the electron wave function overlapping of the occupied states within the band gap [9,31]. These defect states were clearly observed in the V2O5 valence band spectrum, shown in the inset in Fig. 4(c). Because of the relatively small energy gap between the gap states and the conduction band, oxygen deficient V2O5 is an ntype semiconductor. Fig. 4(d) shows schematically the band diagram of V2O5 with the consideration of defect states induced by oxygen deficiency [31]. When a transition metal oxide is employed as a buffer layer between a semiconductor and a metal electrode, it has been reported that gap states play an important role in assisting the hole injection [17,30,32]. The defect-state peak observed in the valence-band photoemission XPS spectrum shown in Fig. 4 (c) demonstrated that a large density of occupied sub-band gap states centered around 0.4 eV below the Fermi level had been formed in the V2O5 buffer layers [32]. The band alignment at the interface of CdTe/back contact is crucial for the hole transport. In order to have a better understanding of the hole transport at the back contact, the band alignment of the V2O5/ CdTe heterostructure was quantitatively characterized by the XPS technique. The XPS technique has been proved to be a direct and powerful tool for measuring the valence band offset (VBO) at a heterostructural interface [33,34]. The valence band offset EVBO at the V2O5/ CdTe junction can be calculated by using XPS data according to the following equation, EVBO ¼ EVBM Cd3d5=2 bulk CdTe EVBM V2p3=2 V2p3=2 Cd3d5=2 bulk V2 O5
The corresponding values of EVBM and V 2p3/2 measured on a 100-nmthick V2O5 film are 2.48 and 517.42 eV, respectively, as shown in Fig. 5 (b). ΔECL ¼(V 2p3/2 Cd 3d5/2)interface is the energy difference between the two core levels of the V 2p3/2 and the Cd 3d5/2, which were measured at the V2O5/CdTe interface, where the V2O5 coating thickness was 3 nm. The value of ΔECL was 111.98 eV. The band alignment at the V2O5/CdTe interface obtained based on the XPS data is shown in Fig. 6. The valence band offset between V2O5 and CdTe is 1.49 eV. The quantitative band alignment obtained by the XPS data, including the gap states, for the back contact structure CdTe/V2O5/Au is show in Fig. 6. As can be seen in Fig. 6, the V2O5 gap states lie very close to the CdTe valence band maximum. These band gap sates acted as efficient hole injection routes to the Au electrode. The valence band maximum of the CdTe, which had a hole carrier concentration of 1013– 1014 cm 3 (measured by capacitance–voltage measurement), was about 0.3–0.4 eV below its Fermi level. The effect of inserting a V2O5 buffer layer at the back contact on the CdTe solar cell performance is demonstrated in Fig. 7. CdTe solar cells with two kinds of back contact structures, namely, one with Au metal only and the other with a V2O5/Au composite back contact, were fabricated. Au has a work function of 5.1 eV, this value is lower than that of CdTe, whose work function is as high as 5.7 eV. As can be seen in Fig. 7(a), the cell with the Au-only back contact exhibits a roll-over behavior, which led to rather low values of fill factor (FF) and open-circuit (Voc). The roll-over behavior at high voltage is indicative of a non-ohmic contact, which was caused by the large work-function mismatch between Au and p-type CdTe [35]. With a V2O5/Au back contact, where the V2O5 buffer layer was 15 nm thick, the roll-over phenomenon disappeared and consequently the FF and the Voc were increased by 6.3% and 89 mV, respectively. The increased Voc for a CdTe solar
interface
where (EVBM Cd 3d5/2)bulk CdTe is the energy difference between the valence band maximum and the Cd 3d5/2 core level measured using a thick CdTe film, shown in Fig. 5(a). The valence band maximum EVBM was obtained by linear fittings to the XPS data. The intersect of the two linear extrapolations corresponds to the valence band maximum, shown in the inset of Fig. 5(a). The values of EVBM and Cd 3d5/2 measured on a 4-μm-thick CdTe are 0.44 and 404.89 eV, respectively.
Fig. 6. Band alignment at the CdTe/V2O5/Au contact interface obtained based on the XPS data.
Fig. 5. (a) Core-level Cd 3d and valence band XPS spectra of bulk CdTe; (b) core-level V 2p and valence band XPS spectra of bulk V2O5. The insets shows detailed XPS spectrum near the valence band maxima; (c) the two core levels of the V 2p3/2 and the Cd 3d5/2, which was measured at a V2O5/CdTe interface with a V2O5 coating thickness of 3 nm.
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Fig. 7. (a) Light and (b) dark J–V curves of two CdTe solar cells with different back contact structures. (c) dV/dJ curve versus the (J þJSC) 1, derived from the dark J–V curves shown in (b).
cell is usually ascribed either to the carrier recombination reduction at the CdS/CdTe junction and within the CdTe absorber or to a much reduced back contact barrier [36]. The barriers estimated from the two J–V curves, shown in Fig. 7(a), were 0.50 and 0.35 eV for the cells with Au only and V2O5/Au back contacts. The barrier heights were calculated based on the thermionic emission theory [37,38]. The Voc increase for the cell with a V2O5/Au contact in this study was due to the reduction of barrier height as evidenced by the band alignment characterization discussed above. Fig. 7 (b) shows the corresponding dark J–V curves for the two cells. The device with the composite V2O5/Au back contact demonstrated a nearly ideal diode J–V curve. At a forward voltage larger than 0.5 V, the current of the cell with the Au metal contact was significantly less than that of the cell diode with the V2O5/Au back contact. This was caused by the relatively large resistance at the back contact, which was in series with the main diode. The lumped series resistances, which were obtained by the dark J–V curves and shown in Fig. 7(c), were 6.21 and 1.21 Ω cm2 for the two cells with the Au and the V2O5/Au back contact, respectively. It can be clearly seen that the incorporation of a V2O5 interfacial layer led to the formation of a much decreased Schottky barrier at the back contact. The diode ideal factors were 1.86 and 1.71 for the two cells, indicating that the junction current was dominated by the Shockley–Read–Hall recombination for the two cells. With the discussions and the experimental results presented above, we believe that, for the CdTe/V2O5/Au composite electrode, two beneficial factors contributed to the low back contact barrier observed in our CdTe solar cell. First, the high V2O5 work function reduced the Schottky barrier which was caused by the work function mismatch between Au and CdTe. Second, the oxygen deficiency related gap states in the V2O5 buffer layer efficiently assisted the hole injection. The use of a V2O5 buffer layer at the back contact makes it possible not only to fabricate high efficient CdTe solar cells but also to fabricate cells with high device stability. In a CdTe solar cell, Cu has been routinely adopted as a material to reduce the back contact barrier with CdTe and improve the cell device performance [4,5]. Cu dopes the CdTe film surface to form a p þ region and reacts with Te to form CuxTe compound to reduce the contact barrier [4,18]. However, Cu is the main concern which induces instability for a CdTe cell employed for long-term use to generate electricity in a harsh outdoor environment. Therefore in principle the amount of Cu deposited for low-resistance contact fabrication should be as less as possible. We found that without any use of Cu, even though we could fabricate relatively high efficient CdTe solar cells, if we deposited a very thin layer of Cu and together with a thin V2O5 buffer layer, CdTe solar cells with an efficiency as high as 14% could be routinely fabricated. In order to study the effect of a V2O5 buffer layer on cell efficiency and device stability, solar cells with different contact
Fig. 8. Light J–V curves of four CdTe solar cells with different back contact structures, which had different directly and indirectly applied Cu materials in the back contact.
structures, namely, Cu/Au, V2O5/Cu/Au, Cu/V2O5/Au and Cu/V2O5/ Cu/Au, were fabricated. These different structure combinations allowed us to explore the respective roles of the Cu and the V2O5 layer in the contact. Fig. 8 shows the device performance of cells with four different back contact structures. The two cells with contact structures of 2 nm Cu/V2O5/Au and 2 nm Cu/V2O5/3 nm Cu/Au demonstrated almost the same energy conversion efficiency, indicating that the directly applied Cu(2-nm-thick) was the most influencing material for the device performance. For the cell with a V2O5/3 nm Cu/Au contact, the device performance was similar to the cell with a V2O5/Au contact shown in Fig. 7(a). It can be seen that the indirectly applied Cu layer between V2O5 and Au had little effect on the cell performance. Compared to the cell with V2O5/3 nm Cu/Au as the back contact, the efficiency of the cell with directly applied Cu on the CdTe surface, having a back contact structure of 2 nm Cu/V2O5/3 nm Cu/Au, was much improved. We call the first Cu layer, which was directly contacted with the CdTe surface and effected the cell performance apparently, the primary Cu layer. The metal bilayer of Cu/Au has been widely employed for fabrication of high efficient CdTe solar cells. The optimal Cu thickness was found to be 5 nm, which is consistent with the results of other research groups [18,39]. Compared to the cell with a 5 nm Cu/Au contact, the J–V curve of the cell with a 2 nm Cu/Au contact showed a slight roll-over at high bias, indicating the existence of a Schottky barrier at the back contact. With the insertion of a V2O5 buffer layer, much less amount of primary Cu material, only 2-nm-thick, was needed to obtain high efficient solar cells. The V2O5 buffer layer makes it possible to obtain a lowbarrier back contact with less or even no Cu. In order to study the effect of a V2O5 buffer layer on cell stability, stress testings of CdTe solar cells with different back contact structures were conducted both at room temperature and under thermal condition. The room-temperature-aging tests for the two cells with back contact structures of 5 nm Cu/Au and 2 nm Cu/
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Fig. 9. (a) and (c) Current–voltage characteristics of two CdTe solar cells with different contact structures aged at room temperature and in the air atmosphere for different times; (b) and (d) the corresponding dV/dJ curves versus the (Jþ JSC) 1 derived from the J–V curves shown in (a) and (c).
Fig. 10. Long term stress testings for four CdTe solar cells with different back contact structures. (a) open-circuit voltage, (b) short-circuit current, (c) fill factor, and (d) cell efficiency, versus the stress time.
V2O5/3 nm Cu/Au are shown in Fig. 9. These two back contact structures were chosen to keep the total Cu layer thickness the same. For the cell with a Cu/V2O5/Cu/Au contact, after aged at room temperature and in the air atmosphere for 266 days, the efficiency and the fill factor decreased slightly from 14.0% and 70.0% to 13.8% and 69.7%, respectively. After more than one year aging test, namely 413 days, the cell efficiency remained a value of
13.8%, the same as that after 266 days aging. But for the cell with a Cu/Au contact, after aged for only 146 days, the efficiency and the fill factor were decreased quickly from 14.3% and 70.2% to 12.7% and 65.3%, respectively. After one year aging test (368 days) the cell efficiency was decreased further to a value of 11.9%. These data clearly demonstrated that the insertion of a V2O5 buffer layer and the rather thin thickness of a 2-nm-thick primary Cu layer greatly
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slowed the device degradation compared to the cell with a 5-nmthick Cu contact layer. It can be seen that Cu is indeed the main source for cell instability even for cells aged at room temperature. For the cell with a Cu/Au back contact, after aged for 146 and 368 days, the ideality factor and series resistance increased from 1.96 and 1.43 Ω cm2, to 2.49 and 2.57 Ω cm2, and 2.51 and 3.73 Ω cm2, see Fig. 9(d). The much increased ideality factor indicates that Cu, which diffused to the CdS/CdTe junction, acted as traps assisting carrier tunneling in the junction and in the CdTe film [40]. For the cell with a 2 nm Cu/V2O5/3 nm Cu/Au back contact, the ideality factor and the series resistance kept almost unchanged after aged for more than one year, see Fig. 9(b). With much less amount material of a primary Cu layer and the insertion of a V2O5 buffer layer, the cell degradation was successfully suppressed. The comparison study also demonstrated that the V2O5 oxide layer can act as an effective barrier for Cu diffusion from the back contact structure into the CdTe. The long term device stability of CdTe solar cells, which had back contact composite structures of 2 nm Cu/Au, 5 nm Cu/Au, V2O5/3 nm Cu/Au, and 2 nm Cu/V2O5/3 nm Cu/Au, have been evaluated by employing thermal stress testing. The thermal stress testings were carried out at a temperature of 85 °C in air in an electric thermal oven. The solar cell device parameters measured after different stressing times are shown in Fig. 10. Except the cell with 5 nm Cu/Au bi-metal layer as the back contact, all the other cells showed rather stable device performance. For the cell with 5 nm Cu/Au back contact, all the cell parameters, namely, the open-circuit voltage, the short-circuit current, and especially the fill factor, were constantly decreased with the stressing time. After 387 h stressing, the fill factor was decreased from 70.4% to 67.3%, and the efficiency was decreased from 14.1% to 13.2%. With the insertion of a V2O5 buffer layer, the stability of the cell with 2 nm Cu/V2O5/3 nm Cu/Au back contact was much improved. The stability improvement mainly came from the reduced thickness of the primary Cu layer. This was also confirmed by the stressing test of the cell with reduced thickness of the primary Cu in the contact, namely, a 2 nm Cu/Au bimetal layer contact. The V2O5 buffer layer blocked the Cu material, which was located between V2O5 and Au, to diffuse into the CdTe and further into the CdS/CdTe interface area, which is the most essential part in a solar cell to improve the device stability. The blocking effect was also confirmed by the stressing test of the cell with a V2O5/Cu(3-nm-thick)/Au back contact.
4. Conclusions In summary, V2O5 thin film was employed as a buffer layer to form low-barrier back contact for CdTe solar cell fabrication. Different back contact structures were fabricated to study the effect of a V2O5 buffer layer on CdTe solar cell device performance. Both the quantitative band alignment and the device performance of the CdTe solar cells with a V2O5/Au back contact demonstrated that a much lower Schottky barrier was formed compared to the cells with an Au-only back contact. The defect states related to oxygen vacancy within the band gap of the V2O5 played a crucial role in reducing the energy barrier for hole carrier transport. With much less Cu directly deposited on the surface of CdTe in a Cu/ V2O5/Cu/Au back contact, CdTe solar cell with an efficiency as high as 14.0% was obtained. Compared with the CdTe solar cells having a traditional Cu/Au back electrode, the device stability of the cells with the insertion of a V2O5 buffer layer in the back contact was significantly enhanced.
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Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 51272247, 61474103) and Natural Science Foundation of Anhui Province (No. 1408085MF119).
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