- Email: [email protected]
AZO structure with improved full spectrum performance for high-efficiency thin-film solar cells
Solar Energy Materials & Solar Cells 136 (2015) 11–16
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
HAZO/AZO structure with improved full spectrum performance for high-efficiency thin-film solar cells Qian Huang a,b,c,n, Dekun Zhang a,b,c, Bofei Liu a,b,c, Lisha Bai a,b,c, Jian Ni a,b,c, Ying Zhao a,b,c, Xiaodan Zhang a,b,c,n a
Institute of Photo Electronics Thin Film Devices and Technique of Nankai University Key Laboratory of Photo Electronics Thin Film Devices and Technique of Tianjin c Key Laboratory of Optoelectronic Information Science and Technology, Ministry of Education, Tianjin 300071, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 7 October 2014 Received in revised form 5 December 2014 Accepted 15 December 2014
Advanced light management currently plays an important role in high-performance solar cells. In this paper, a HAZO/AZO structure that was produced using RF-magnetron sputtering with segmented hydrogen mediation was proposed to further simultaneously enhance the transmittance and light trapping capability at full solar spectrum. Compared to the standard AZO front contact, the total transmittance at short wavelength and haze at full spectrum were remarkably enhanced by 5.1% and 20.7%, respectively. Additionally, the resistivity of the HAZO/AZO structure was enhanced by 16.9%. When applied as front electrodes, the spectral response of a-Si:H and μc-Si:H solar cells increased. The quantum efficiency of the a-Si:H solar cell improved by 7.9% (from 59% to 66.9%) at 400 nm. An initial efficiency of 8.69% with Jsc over 27 mA/cm2 was obtained for the μc-Si:H solar cell. Finally, an a-Si:H/ a-SiGe:H/μc-Si:H triple-junction solar cell was obtained with the initial efficiency over 15%, which illustrates a promising and potentially cost-effective alternative structure to further improve the highquality transparent electrodes for high-efficiency thin-film solar cells. & 2014 Elsevier B.V. All rights reserved.
Keywords: Transparent conductive oxides Zinc oxide Double layer Hydrogen mediation Thin-film solar cells
1. Introduction An a-Si:H/a-SiGe:H/μc-Si:H triple-junction structure is currently proposed by United Solar and LG Electronics [1,2] to realize a high conversion efficiency over 16%. This triple-junction structure offers a more efficient usage of the solar spectrum, which suggests a possible approach to exceed the current inadequate efficiency limit of thin-film silicon (TFS) solar cells, and the structure is a promising candidate in terms of the power conversion efficiency. Further development and implementation of efficient light management in high-performance TFS solar cells [3–7] are considered the most important research area. In the p–i–n type solar cell deposited on transparent conductive oxides (TCOs), lights are introduced and trapped by the TCOs with high transmittance and textured-surface morphology. Thus, the performance of TCOs plays a crucial role in the high-efficiency TFS solar cells and forms a significant part of the photovoltaic research and development [1,5–8]. The sputtered aluminum-doped ZnO (AZO) has high electrical conductivity, low cost, thermal stability, and non-toxicity; can be easily textured using a simple wet-etching process; and is an n Corresponding authors at: Institute of Photo Electronics Thin Film Devices and Technique of Nankai University. E-mail addresses: [email protected] (Q. Huang), [email protected] (X. Zhang).
http://dx.doi.org/10.1016/j.solmat.2014.12.025 0927-0248/& 2014 Elsevier B.V. All rights reserved.
important building block in a-Si:H, μc-Si:H and a-Si:H/μc-Si:H solar cells [8–10]. However, when it is applied in the triplejunction structure, both transparent and light-diffusing structures should be further ameliorated to satisfy the rigorous light management requirement over the full solar spectrum [1]. In a triplejunction solar cell, a more suitable spectral splitting is needed to achieve a better current-matching among corresponding sub-cells, thus maximizing the resultant current output. In the meanwhile, a great light trapping over the full solar spectrum will efficiently improve the spectral response of each component, especially for a-SiGe:H middle cell that cannot obtain benefit from the ZnO:B/ Ag/Al back reflector. Benefited from the effective light trapping in the long-wavelength range, the requirements on the thickness and germanium content of a-SiGe:H middle component will be both reduced. Then the electrical performance and stability of corresponding triple-junction solar cells will be efficiently improved. Based on the Burstein–Moss effect and plasmon absorption, a transmittance competition between the short-wavelength region and near-infrared region (NIR) is restricted for AZO films with different carrier concentrations. Improved transmittance near the optical band gap with good NIR response for ZnO films using hydrogen mediation has been obtained by different research groups [11–14], which provides a promising approach to resolve the dilemma. Unfortunately, an effective scattering texture with a root-mean-square (RMS) roughness higher than 100 nm have not
12
Q. Huang et al. / Solar Energy Materials & Solar Cells 136 (2015) 11–16
been obtained for both hydrogen-mediated ZnO films with and without etching [11,14–16], which hinders the application of this highly transparent electrodes in triple-junction solar cells. In the present work, we notice the simple, single step to further improve the light management of AZO films for applications in high-efficiency triple-junction solar cells. An in situ double layer (DL) AZO structure with hydrogenated AZO (HAZO) and AZO are realized using segmented hydrogen mediation. Significant improvements of light transmittance around the optical band gap and light scattering in the NIR are detected using a single deposition and etching process. Each promotion is tested using a-Si:H and μc-Si:H solar cells. Then, an initial efficiency over 15% was obtained to confirm its applicability for high-efficiency triplejunction TFS solar cells. Thus, this HAZO/AZO structure with improved full spectrum performance may provide a promising and potentially cost-effective alternative to further improve the high-quality front contact and the performance of thin-film solar cells.
2. Experiment The films were prepared on Corning eagle XG glass using RF magnetron sputtering from a ceramic ZnO:Al2O3 target (1 wt%) in a KJLC Lab-18 sputtering system. The in situ HAZO/AZO films were performed using segmented hydrogen mediation. At the beginning of the deposition, hydrogen was introduced into the sputtering atmosphere to obtain a 500 nm thick HAZO layer. Then, the hydrogen circuit was cut, and a 500 nm thick AZO layer was obtained in pure Ar plasmon. Thus, a combined HAZO/AZO film with a thickness of approximately 1 μm was obtained. The sputtering chamber was maintained at a base pressure below 1 10 5 Pa before every deposition, and the distance between the target and the substrate was maintained at 190 mm. Low pressure (1 mTorr) and high power density (3.3 W/cm2) were used with the substrate temperature of 325 1C. The flow rate of hydrogen was set at 0.3 sccm, whereas the argon flow rate was maintained at 5 sccm. The sheet resistance, hall mobility and carrier concentration were measured at room temperature using the van der Pauw method in the HL5500 Hall System (Accent, York, UK). The film thicknesses were measured using a Dektak 3030 profilometer (Veeco Instruments Inc., Woodbury, USA). The bonding states of the films were analyzed using an Axis Ultra X-ray photoelectron spectrometer (Kratos Analytical Ltd, Manchester, UK). A SPA 400 atomic force microscope (AFM) (SII Nanotechnology, Inc., Tokyo, Japan) was used to characterize the surface morphology. The
perpendicular specular (Tspe) and total transmittance (Ttot) were measured using a Cary 5000 spectrophotometer (Varian Co., Palo Alto, USA), which was equipped with an integrated sphere in the spectrum range of 300–1100 nm. The transmittance haze (HT) was determined as the fraction of diffuse (scattering angels exceeding 51) transmittance that constituted the total transmittance: HT ¼[Ttot Tspe]/Ttot 100%. The HAZO/AZO and standard AZO films were applied as the front contact for the single-junction a-Si:H, μc-Si:H and a-Si:H/ a-SiGe:H/μc-Si:H triple-junction thin-film solar cells. The intrinsic layer of a-Si:H single-junction solar cells was fabricated using a radio frequency plasma-enhanced chemical-vapor deposition (RF-PECVD) process with a thickness of 140 nm, and an Al back contact was used. The intrinsic layer of the μc-Si:H single-junction solar cell was prepared using the very-high-frequency plasmaenhanced chemical-vapor deposition (VHF-PECVD) technique with a thickness of 2.0 μm, and a ZnO:B/Ag/Al back reflector structure was used in the μc-Si:H single-junction solar cell and triplejunction solar cells to improve the long-wavelength response. The Al back contact with an area of 0.253 cm2 was used to define the area of the solar cells. The current–voltage characteristics (JV) were measured under 1-sun (AM 1.5, 25 1C, 100 mW/cm2) using a Wacom WXs-156s-l2 dual beam solar simulator. An excellent simulation of the AM1.5 spectrum was obtained from the superposition of the simulator’s two filtered light sources with adequate calibration. The external quantum efficiency (EQE) was measured, and Jsc was calculated from the EQE by integrating EQE (λ) with the AM 1.5 spectrum.
3. Results and discussion 3.1. Optical and light-trapping properties Three major critical parameters of the TCO affect the solar cell performance: conductivity, transmittance, and haze. A high transparency in full spectrum normally introduces high Jsc for TFS solar cell by reducing the parasitic light absorption. Fig. 1 shows the spectral transmittance in the wide spectrum range (300–1100 nm) (a) and emphasis in the short-wavelength range (350–500 nm) (b) for different samples. It is observed from Fig. 1(a) that all samples have an absorption tail near 350 nm. Although the tiny hydrogen mediation did not show any broadened optical band gap, which was reported in previous studies [12,13], the transmittance promotion is significantly favorable for a-Si:H solar cells, as shown in Fig. 1(b).
Fig. 1. Spectral transmittance of AZO, HAZO and HAZO/AZO films in the (a) wide spectrum range (300–1100 nm) and (b) short-wavelength range (350–500 nm).
Q. Huang et al. / Solar Energy Materials & Solar Cells 136 (2015) 11–16
The absorption coefficient of a semiconductor thin film is a result of the presence of point defects, dislocations, impurities, grain boundaries, and its strain state. In the current ZnO:Al films, negatively charged oxygen species, which are absorbed on the surface of the grain boundaries, act as trapping sites and form the potential barriers. Hydrogen has been proposed to benefit the removal of such depletion regions [17]. Therefore, efforts have been made to eliminate the depletion regions using various treatments with hydrogen [11,13,18,19]. Among these methods, sputtering with hydrogen mediation appears an attractive method to achieve property enhancement because it is a simple low-cost process. When introduced into the sputtering atmosphere, hydrogen can easily be incorporated during growth. The interstitial hydrogen atom substitutes for oxygen, which is absorbed on the surface of the grain boundaries and equally bonded to all surrounding Zn atoms in a truly multi-coordinated configuration [20]. The removal of the defects, which act as non-radiative recombination centers, affects the optical constants near the band gap. Then, using hydrogen mediation, we effectively reduce the parasitic light absorption near an optical band gap without deterioration at the long-wavelength range. The light trapping of TCO is one of the most important methods to enhance the performance of thin-film solar cells, which has a good relationship with the surface texture morphologies. Changes in the surface morphology of AZO, HAZO and HAZO/AZO films were investigated using an AFM, and the resulting surface images are shown in Fig. 2. After the chemical-etching process, a typical crater-like surface structure was clearly identified for each sample. To the best of our knowledge, the surface morphology and RMS roughness of the obtained etched traditional AZO film, as shown in Fig. 2(a), are on the concurrently comparable level to AZO films in other groups [8,10]. When the single HAZO layer was etched, an unconsolidated surface morphology with plenty of deep and small craters with notably steep edges was detected, as shown in Fig. 2(b). Relatively few scientific studies have been performed because of the etching performance of hydrogenated ZnO films until recently. Surfacetextured HAZO films with deferent etching times were reported by Tark et al., but the optimized RMS value of only 27 nm was obtained for these samples [16]. This small surface texture and resulting insufficient light scattering distinctly cannot match the requirement for high-efficiency TFS solar cells. To further increase the surface texture, Tark et al. developed a multi-layer structure with a multi-step etching process [14]. Although the RMS value was not included in this issue, the light-trapping property with the haze value was 50.8% at 550 nm, which remained incomparable with the traditional AZO films [8,10]. Fortunately, with the HAZO/AZO structure, the surface texture was no longer limited by the HAZO layer. A moderately textured topography was developed with a wide range of crater-like surface features (1–5 μm), large oblique scattering angles and large RMS roughness (231 nm). To further study the light trapping of the texture-etched films, the total transmittance (Ttot) and haze factor
13
(HT) of these samples were illustrated as shown in Fig. 3. When the etching process was performed to obtain the light-trapping property for TFS solar cells, the AZO layer with stronger optical absorption near the band gap on the top was etched out. Because the etched HAZO/AZO structure has a larger scattering cross section, Ttot at a short wavelength further improved compared to that of the HAZO film, as shown in Fig. 3(a). Then, the average Ttot that was extracted from 400 to 500 nm was increased from 79.9% to 85.0%, and Ttot from 400 to 1100 nm (which contained a glass substrate whose average transmittance was approximately 92%) was above 85%. Additionally, this etched HAZO/AZO structure demonstrated an excellent light-trapping property, as shown in Fig. 3(b). The illustrated HT was 83% at a wavelength of 850 nm with an average HT value of 83.7% from 400 to 1100 nm, which showed a remarkable 20.7% improvement compared to the standard AZO films. To the best of our knowledge, this HAZO/AZO structure achieves the best scattering efficiency among all reported sputtered TCOs. We believe that HT is increased because of effective scattering from the wide range of crater-like surface features and must be benefit for light trapping in μc-Si:H solar cells. 3.2. Electrical property The electrical properties of each film must be included to use the film as a front contact in TFS solar cells. Table 1 summarizes the electrical properties of AZO, HAZO and HAZO/AZO films with and without chemical etching. Low resistivity and high carrier concentration were obtained for the as-deposited HAZO sample, which has been theoretically and experimentally presented by different research groups [21–23]. Here, we attributed the increase in carrier concentration to the incorporation of hydrogen atoms into the HAZO thin films as shallow donors, which was suggested by the first-principle calculation [21] and electron paramagnetic resonance (EPR) detection [23]. When the carrier concentration increased from 2.59 1020 to 3.15 1020 cm 3, the carrier mobility increased from 39.5 to 56.3 cm2/V s, which reinforces our conclusion that hydrogen mediation positively affects the improvement in crystal quality. When the wet-chemical-etching step was performed, the conductivity of the AZO films significantly decreased. Considering the decrease in carrier mobility, we attributed this reduction to the increase in scattering boundary, which was introduced by the rough surface morphology. However, the resistance of the etched HAZO was over the range of the Hall system. The binding characteristics of the atoms that composed the standard AZO and HAZO films after wet chemical etching were comparable, which has not been reported previously. Fig. 4 shows the curve fitted O1s peaks for different samples, which significantly changed when hydrogen mediation was introduced. The O1s peak for the standard AZO film was resolved into 2 Gaussian– Lorentzian components. The low binding energy component centered at 530.070.1 eV is attributed to O2 ions that were
Fig. 2. Three-dimensional atomic force microscopy images of the etched films. (a) AZO; (b) HAZO; (c) HAZO/AZO.
14
Q. Huang et al. / Solar Energy Materials & Solar Cells 136 (2015) 11–16
Fig. 3. Optical properties of the etched AZO, HAZO and HAZO/AZO films: (a) total transmittance (Ttot); (b) haze factor (HT). Table 1 Electrical characteristics of as-deposited and etched AZO, HAZO and HAZO/AZO films. Samples AZO HAZO HAZO/AZO
As-deposited Etched As-deposited Etched As-deposited Etched
Thickness (nm)
Sheet resistance (Ω/sq)
Carrier concentration (cm 3)
Mobility (cm2/Vs)
Resistivity (Ω cm)
1166 860 1105 783 1038 819
5.23 11.52 3.18 1 4.73 10.06
2.59 1020 2.63 1020 3.15 1020 – 2.78 1020 2.81 1020
39.5 23.9 56.3 – 45.6 26.9
6.10 10 4 9.91 10 4 3.52 10 4 – 4.92 10 4 8.24 10 4
regions indicates that hydrogen mediation can modify the lattice structure by removing the weakly bound oxygen species on the grain boundaries. The appearance of high-binding-energy oxygen shows that although hydrogen can effectively passivate the films’ surface from the adsorption of oxygen species in the atmospheric environment [20], it tends to bond with other components, such as hydroxyls or carbonates in the etching solution. These oxygen species on the films’ surface, such as –CO3, –OH or adsorbed O2, are expected to increase the electrical resistivity after the etching process. To summarize the resistance results, we conclude that: (i) the carrier concentration and mobility of etched HAZO films significantly deteriorated to an unacceptable level for front-contact usage, (ii) the HAZO/AZO structure can improve the conductivity of the etched film to a certain extent by avoiding exposing the HAZO layer to the etching acid solution, and (iii) the HAZO/AZO structure provides a 16.9% improvement in resistivity compared to the standard etched AZO film and is beneficial to front-contact application. Fig. 4. Core-level XPS spectra of O1s for AZO and HAZO films after wet chemical etching.
surrounded by Zn (or substitutional Al) atoms, which indicates the Zn–O bonds in the ZnO matrix, and the medium binding energy peak of 531.17 0.1 eV is related to O2 ions in oxygen-deficient regions [24,25]. The calculated atomic percentages of oxygen related to Zn–O bonds and the deficient component were 56% and 44%, respectively. When the hydrogen mediation was introduced, the FWHM values of the resolved O1s bonds increased and a component that was 2.3 eV away from the Zn–O bond was identified, which corresponded to the existence of a weakly bound oxygen species on the films’ surface, such as –CO3, –OH or adsorbed O2 [25,26]. For the HAZO film, the atomic percentage of oxygen with low, medium and high binding energy was 50%, 32% and 18%, respectively. The small portion of oxygen in deficient
3.3. Application in TFS solar cells Finally, we emphasize the potential of an improved fullspectrum performance for high-efficiency TFS solar cells. Singlejunction a-Si:H and μc-Si:H cells were co-deposited on traditional AZO and HAZO/AZO films to inspect the improvement in transmittance and light trapping, respectively. To study the electrical performance and wavelength dependence of the cell characteristics, current density voltage (JV) and external quantum efficiency (EQE) measurements were performed. Ten test points were carried out for each cell. Samples in comparison in each group have the same trend of variation. The best performers with deviations less than 3% from the average value are shown in Figs. 5 and 6. As shown in Fig. 5, the enhanced optical transmittance near the ZnO band gap had a decisive effect on the spectral response in a-Si:H solar cells. The HAZO/AZO structure has higher spectral
Q. Huang et al. / Solar Energy Materials & Solar Cells 136 (2015) 11–16
15
Fig. 5. JV and EQE curves of the a-Si:H solar cells that were deposited on the standard AZO and HAZO/AZO front contacts. (a) JV curves of a-Si:H solar cells; (b) EQE curves of a-Si:H solar cells.
Fig. 6. JV and EQE curves of the μc-Si:H solar cells that were deposited on the standard AZO and HAZO/AZO front contacts. (a) JV curves of μc-Si:H solar cells; (b) EQE curves of μc-Si:H solar cells.
transmittance in the short-wavelength region (400–500 nm), which increases the light absorption of the absorber layer (a-Si:H (i)) and promotes the generation of electron–hole pairs. Hence, the quantum efficiency was improved by 7.9% (from 59% to 66.9%) at 400 nm, and the integrated short-circuit current density Jsc, which was calculated from the EQE curve, increased from 10.62 to 11.06 mA/cm2. Although Voc remained almost identical, the FF slightly decreased, which we may attribute to the enlarged surface potential barrier loss between the TCO and the p layer that is related to the large surface contact area [27]. This attribution will be affirmed and improved in future study. As the front contact in μc-Si:H solar cells, the higher EQE in the entire wavelength region resulted in a 5.3% enhancement of the shortcircuit current density, as shown in Fig. 6. This result was benefited by the enhanced light scattering, which increased the optical path length and the absorption. Additionally, the FF was also improved from 64% to 64.9%, which may be attributed to the decrease in ohmic losses at the TCO and TCO/p-contact. Thus, we obtained an initial active-area efficiency of 8.69% using a μc-Si:H single-junction solar cell with a notably good long-wavelength response, where Jsc was over 27 mA/ cm2, which was obtained from the convolution integral of the EQE curve with AM 1.5 solar spectrum, and was the current maximum limit for μc-Si:H solar cells [28]. Finally, we incorporated the HAZO/AZO front contact and component cells into a-Si:H/a-SiGe:H/μc-Si:H triple-junction structures and optimized the cell design. The maximal initial
Fig. 7. JV characteristics of the a-Si:H/a-SiGe:H/μc-Si:H triple-junction solar cell that were deposited on the HAZO/AZO front contact.
active-area cell efficiency of 15.06% was achieved with the JV characteristics as shown in Fig. 7. The detailed preparation and optimization of this triple-junction solar cell will soon be published in another paper [29]. These results demonstrate that the HAZO/AZO structure with improved full-spectrum performance enables the fabrication of high-efficiency TFS solar cells.
16
Q. Huang et al. / Solar Energy Materials & Solar Cells 136 (2015) 11–16
4. Conclusion In summary, a simple and low-cost method to effectively enhance the full-spectrum performance of the front contact for highefficiency TFS solar cells was achieved using an HAZO/AZO structure. The optical transmittance near the ZnO band gap was significantly improved. The average Ttot extracted from 400 nm to 500 nm was increased from 79.9% to 85.0%. A relatively high HT of 83% at 850 nm with an average value of 83.7% from 400 to 1100 nm was developed with remarkable light-trapping improvement. In addition, the resistivity was also increased by 16.9% with the HAZO/AZO structure. Then, these improvements were demonstrated in the usage of top electrodes in the a-Si:H and μc-Si:H solar cells, which resulted in 4.1% and 5.3% enhancement in the short-circuit current density, respectively. Finally, an a-Si:H/a-SiGe:H/μc-Si:H triple-junction solar cell was obtained with an initial efficiency over 15%, which illustrates a promising and potentially cost-effective alternative to further improve the high-quality transparent electrodes for high-efficiency thin-film solar cells. References [1] S. Kim, J.W. Chung, H. Lee, J. Park, Y. Heo, H.M. Lee, Remarkable progress in thin-film silicon solar cells using high-efficiency triple-junction technology, Sol. Energy Mater. Sol. Cells 119 (2013) 26–35. [2] B. Yan, G. Yue, L. Sivec, J. Yang, S. Guha, C. Jiang, Innovative dual function ncSiOx:H layer leading to a 416% efficient multi-junction thin-film solar cell, Appl. Phys. Lett. 99 (2011) 113512. [3] M.M. Islama, S. Ishizukab, A. Yamadab, K. Matsubarab, S. Nikib, T. Sakuraia, K. Akimotoa, Thickness study of Al:ZnO film for application as a window layer in Cu(In1 xGax)Se2 thin film solar cell, Appl. Surf. Sci. 257 (2011) 4026–4030. [4] Y.H. Kim, J.S. Kim, W.M. Kim, T.Y. Seong, J. Lee, L. Müller-Meskamp, K. Leo, Realizing the potential of ZnO with alternative non-metallic co-dopants as electrode materials for small molecule optoelectronic devices, Adv. Funct. Mater. 23 (2013) 3645–3652. [5] O. Isabella, J. Krc, M. Zeman, Modulated surface textures for enhanced light trapping in thin-film silicon solar cells, Appl. Phys. Lett 97 (2010) 1011061–101106-3. [6] V.E. Ferry, M.A. Verschuuren, M.C.V. Lare, R.E.I. Schropp, H.A. Atwater, A. Polman, Optimized spatial correlations for broadband light trapping nanopatterns in high efficiency ultrathin film a-Si:H solar cells, Nano Lett. 11 (2011) 4239–4245. [7] A. Hongsingthong, I.A. Yunaz, S. Miyajima, M. Konagai, Preparation of ZnO thin films using MOCVD technique with D2O/H2O gas mixture for use as TCO in silicon-based thin film solar cells, Sol. Energy Mater. Sol. Cells 95 (2011) 171–174. [8] J. Müller, B. Rech, J. Springer, M. Vanecek, TCO and light trapping in silicon thin film solar cells, Sol. Energy 77 (2004) 917–930. [9] C. Guillén, J. Herrero, Improving conductivity and texture in ZnO:Al sputtered thin films by sequential chemical and thermal treatments, Appl. Surf. Sci., 282, 2013923–929. [10] M. Berginski, J. Hüpkes, M. Schulte, G. Schöpe, H. Stiebig, B. Rech, The effect of front ZnO:Al surface texture and optical transparency on efficient light
[11]
[12]
[13]
[14]
[15] [16]
[17]
[18]
[19]
[20] [21] [22]
[23]
[24] [25]
[26]
[27]
[28]
[29]
trapping in silicon thin-film solar cells, J. Appl. Phys., 101, 20070749031–074903-11. Q. Huang, Y. Liu, S. Yang, Y. Zhao, X. Zhang, Hydrogen mediated self-textured zinc oxide films for silicon thin film solar cells, Sol. Energy Mater. Sol. Cells 103 (2012) 134–139. F. Wang, C. Yang, J. Liou, I. Chen, Effects of hydrogen on the optical and electrical characteristics of the sputter-deposited Al2O3-doped ZnO thin films, J. Nanomater. (2014) 857614-1–857614-7. K. Kim, H. Choia, D. Cho, Y. Chung, K. Lee, N. Park, J. Kim, Electrical and optical properties of radio frequency magnetron-sputtered lightly aluminum-doped zinc oxide thin films deposited in hydrogen–argon gas, Thin Solid Films 540 (2013) 142–145. S.J. Tark, M.G. Kang, S. Park, S.H. Lee, C.S. Son, J.C. Lee, D. Kim, Characterization of hydrogenated Al-doped ZnO films prepared by multi-step texturing for photovoltaic applications, Curr. Appl. Phys. 11 (2011) 362–367. Y. Wang, X. Zhang, Q. Huang, F. Yang, J. Liang, D. Zhang, Y. Zhao, Tailoring of textured ZnO:Al film via hydrogen, Vaccum 107 (2014) 6–9. S.J. Tark, M.G. Kang, S. Park, J.H. Jang, J.C. Lee, W.M. Kim, J.S. Lee, D. Kim, Development of surface-textured hydrogenated ZnO:Al thin-films for μc-Si solar cells, Curr. Appl. Phys., 9, 20091318–1322. W.M. Kim, Y.H. Kim, J.S. Kim, J. Jeong, Y.J. Baik, J.K. Park, K.S. Lee, T.Y. Seong, Hydrogen in polycrystalline ZnO thin films, J. Phys. D: Appl. Phys. 43 (2010) 365406-1–365406-5. I. Lorite, J. Wasik, T. Michalsky, R. Schmidt-Grund, P. Esquinazi, Hydrogen influence on the electrical and optical properties of ZnO thin films grown under different atmospheres, Thin Solid Films 556 (2014) 18–22. M.V. Castro, M.F. Cerqueira, L. Rebouta, P. Alpuim, C.B. Garcia, G.L. Júnior C.J. Tavares, Influence of hydrogen plasma thermal treatment on the properties of ZnO:Al thin films prepared by dc magnetron sputtering, Vacuum 107 (2014) 145–154. A. Janotti, C.G. Van De Walle, Hydrogen multicentre bonds, Nat. Mater. 6 (2007) 44–47. C.G. Van de Walle, Hydrogen as a cause of doping in zinc oxide, Phys. Rev. Lett. 85 (2000) 1012–1015. S.F.J. Cox, E.A. Davis, S.P. Cottrell, P.J.C. King, J.S. Lord, J.M. Gil, H.V. Alberto R.C. Vilão, J. Piroto Duarte, N. Ayres de Campos, A. Weidinger, R.L. Lichti, S.J. C. Irvine, Experimental confirmation of the predicted shallow donor hydrogen state in zinc oxide, Phys. Rev. Lett. 86 (2001) 2601–2604. D.M. Hofmann, A. Hofstaetter, F. Leiter, H. Zhou, F. Henecker, B.K. Meyer S.B. Orlinski, J. Schmidt, P.G. Baraov, Hydrogen: a relevant shallow donor in zinc oxide, Phys. Rev. Lett. 88 (2002) 045504-1–045504-4. B.Y. Oh, M.C. Jeong, J.M. Myoung, Stabilization in electrical characteristics of hydrogen-annealed ZnO:Al films, Appl. Surf. Sci., 253, 20077157–7161. T.G.G. Maffeis, M.W. Penny, A. Castaing, O.J. Guy, S.P. Wilks, XPS investigation of vacuum annealed vertically aligned ultralong ZnO nanowires, Surf. Sci 606 (2012) 99–103. B.J. Coppa, C.C. Fulton, P.J. Hartlieb, R.F. Davis, B.J. Rodriguez, B.J. Shields R.J. Nemanich, In situ cleaning and characterization of oxygen- and zincterminated, n-type, ZnO{0 0 0 1} surfaces, J. Appl. Phys. 95 (2004) 5856–5864. A.N. Corpus-Mendoza, M.M. De Souza, F. Hamelmann, Transport mechanisms and effective Schottky barrier height of ZnO/a-Si:H and ZnO/lc-Si:H heterojunction solar cells, J. Appl. Phys. 114 (2013) 184505-1–184505-6. M. Berginski, J. Hüpkes, M. Schulte, G. Schöpe, H. Stiebig, B. Rech, The effect of front ZnO:Al surface texture and optical transparency on efficient light trapping in silicon thin-film solar cells, J. Appl. Phys. 101 (2007) 0749031–074903-11. B. Liu, L. Bai, Z. Chen, X. Zhang, D. Zhang, J. Ni, Q. Huang, C. Wei, J. Sun, X. Chen, H. Ren, G. Hou, G. Wang, Y. Zhao, High efficiency triple junction thin film silicon solar cells with optimized electrical structure, Prog. Photovoltaics Res. Appl., accepted.
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
HAZO/AZO structure with improved full spectrum performance for high-efficiency thin-film solar cells Qian Huang a,b,c,n, Dekun Zhang a,b,c, Bofei Liu a,b,c, Lisha Bai a,b,c, Jian Ni a,b,c, Ying Zhao a,b,c, Xiaodan Zhang a,b,c,n a
Institute of Photo Electronics Thin Film Devices and Technique of Nankai University Key Laboratory of Photo Electronics Thin Film Devices and Technique of Tianjin c Key Laboratory of Optoelectronic Information Science and Technology, Ministry of Education, Tianjin 300071, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 7 October 2014 Received in revised form 5 December 2014 Accepted 15 December 2014
Advanced light management currently plays an important role in high-performance solar cells. In this paper, a HAZO/AZO structure that was produced using RF-magnetron sputtering with segmented hydrogen mediation was proposed to further simultaneously enhance the transmittance and light trapping capability at full solar spectrum. Compared to the standard AZO front contact, the total transmittance at short wavelength and haze at full spectrum were remarkably enhanced by 5.1% and 20.7%, respectively. Additionally, the resistivity of the HAZO/AZO structure was enhanced by 16.9%. When applied as front electrodes, the spectral response of a-Si:H and μc-Si:H solar cells increased. The quantum efficiency of the a-Si:H solar cell improved by 7.9% (from 59% to 66.9%) at 400 nm. An initial efficiency of 8.69% with Jsc over 27 mA/cm2 was obtained for the μc-Si:H solar cell. Finally, an a-Si:H/ a-SiGe:H/μc-Si:H triple-junction solar cell was obtained with the initial efficiency over 15%, which illustrates a promising and potentially cost-effective alternative structure to further improve the highquality transparent electrodes for high-efficiency thin-film solar cells. & 2014 Elsevier B.V. All rights reserved.
Keywords: Transparent conductive oxides Zinc oxide Double layer Hydrogen mediation Thin-film solar cells
1. Introduction An a-Si:H/a-SiGe:H/μc-Si:H triple-junction structure is currently proposed by United Solar and LG Electronics [1,2] to realize a high conversion efficiency over 16%. This triple-junction structure offers a more efficient usage of the solar spectrum, which suggests a possible approach to exceed the current inadequate efficiency limit of thin-film silicon (TFS) solar cells, and the structure is a promising candidate in terms of the power conversion efficiency. Further development and implementation of efficient light management in high-performance TFS solar cells [3–7] are considered the most important research area. In the p–i–n type solar cell deposited on transparent conductive oxides (TCOs), lights are introduced and trapped by the TCOs with high transmittance and textured-surface morphology. Thus, the performance of TCOs plays a crucial role in the high-efficiency TFS solar cells and forms a significant part of the photovoltaic research and development [1,5–8]. The sputtered aluminum-doped ZnO (AZO) has high electrical conductivity, low cost, thermal stability, and non-toxicity; can be easily textured using a simple wet-etching process; and is an n Corresponding authors at: Institute of Photo Electronics Thin Film Devices and Technique of Nankai University. E-mail addresses: [email protected] (Q. Huang), [email protected] (X. Zhang).
http://dx.doi.org/10.1016/j.solmat.2014.12.025 0927-0248/& 2014 Elsevier B.V. All rights reserved.
important building block in a-Si:H, μc-Si:H and a-Si:H/μc-Si:H solar cells [8–10]. However, when it is applied in the triplejunction structure, both transparent and light-diffusing structures should be further ameliorated to satisfy the rigorous light management requirement over the full solar spectrum [1]. In a triplejunction solar cell, a more suitable spectral splitting is needed to achieve a better current-matching among corresponding sub-cells, thus maximizing the resultant current output. In the meanwhile, a great light trapping over the full solar spectrum will efficiently improve the spectral response of each component, especially for a-SiGe:H middle cell that cannot obtain benefit from the ZnO:B/ Ag/Al back reflector. Benefited from the effective light trapping in the long-wavelength range, the requirements on the thickness and germanium content of a-SiGe:H middle component will be both reduced. Then the electrical performance and stability of corresponding triple-junction solar cells will be efficiently improved. Based on the Burstein–Moss effect and plasmon absorption, a transmittance competition between the short-wavelength region and near-infrared region (NIR) is restricted for AZO films with different carrier concentrations. Improved transmittance near the optical band gap with good NIR response for ZnO films using hydrogen mediation has been obtained by different research groups [11–14], which provides a promising approach to resolve the dilemma. Unfortunately, an effective scattering texture with a root-mean-square (RMS) roughness higher than 100 nm have not
12
Q. Huang et al. / Solar Energy Materials & Solar Cells 136 (2015) 11–16
been obtained for both hydrogen-mediated ZnO films with and without etching [11,14–16], which hinders the application of this highly transparent electrodes in triple-junction solar cells. In the present work, we notice the simple, single step to further improve the light management of AZO films for applications in high-efficiency triple-junction solar cells. An in situ double layer (DL) AZO structure with hydrogenated AZO (HAZO) and AZO are realized using segmented hydrogen mediation. Significant improvements of light transmittance around the optical band gap and light scattering in the NIR are detected using a single deposition and etching process. Each promotion is tested using a-Si:H and μc-Si:H solar cells. Then, an initial efficiency over 15% was obtained to confirm its applicability for high-efficiency triplejunction TFS solar cells. Thus, this HAZO/AZO structure with improved full spectrum performance may provide a promising and potentially cost-effective alternative to further improve the high-quality front contact and the performance of thin-film solar cells.
2. Experiment The films were prepared on Corning eagle XG glass using RF magnetron sputtering from a ceramic ZnO:Al2O3 target (1 wt%) in a KJLC Lab-18 sputtering system. The in situ HAZO/AZO films were performed using segmented hydrogen mediation. At the beginning of the deposition, hydrogen was introduced into the sputtering atmosphere to obtain a 500 nm thick HAZO layer. Then, the hydrogen circuit was cut, and a 500 nm thick AZO layer was obtained in pure Ar plasmon. Thus, a combined HAZO/AZO film with a thickness of approximately 1 μm was obtained. The sputtering chamber was maintained at a base pressure below 1 10 5 Pa before every deposition, and the distance between the target and the substrate was maintained at 190 mm. Low pressure (1 mTorr) and high power density (3.3 W/cm2) were used with the substrate temperature of 325 1C. The flow rate of hydrogen was set at 0.3 sccm, whereas the argon flow rate was maintained at 5 sccm. The sheet resistance, hall mobility and carrier concentration were measured at room temperature using the van der Pauw method in the HL5500 Hall System (Accent, York, UK). The film thicknesses were measured using a Dektak 3030 profilometer (Veeco Instruments Inc., Woodbury, USA). The bonding states of the films were analyzed using an Axis Ultra X-ray photoelectron spectrometer (Kratos Analytical Ltd, Manchester, UK). A SPA 400 atomic force microscope (AFM) (SII Nanotechnology, Inc., Tokyo, Japan) was used to characterize the surface morphology. The
perpendicular specular (Tspe) and total transmittance (Ttot) were measured using a Cary 5000 spectrophotometer (Varian Co., Palo Alto, USA), which was equipped with an integrated sphere in the spectrum range of 300–1100 nm. The transmittance haze (HT) was determined as the fraction of diffuse (scattering angels exceeding 51) transmittance that constituted the total transmittance: HT ¼[Ttot Tspe]/Ttot 100%. The HAZO/AZO and standard AZO films were applied as the front contact for the single-junction a-Si:H, μc-Si:H and a-Si:H/ a-SiGe:H/μc-Si:H triple-junction thin-film solar cells. The intrinsic layer of a-Si:H single-junction solar cells was fabricated using a radio frequency plasma-enhanced chemical-vapor deposition (RF-PECVD) process with a thickness of 140 nm, and an Al back contact was used. The intrinsic layer of the μc-Si:H single-junction solar cell was prepared using the very-high-frequency plasmaenhanced chemical-vapor deposition (VHF-PECVD) technique with a thickness of 2.0 μm, and a ZnO:B/Ag/Al back reflector structure was used in the μc-Si:H single-junction solar cell and triplejunction solar cells to improve the long-wavelength response. The Al back contact with an area of 0.253 cm2 was used to define the area of the solar cells. The current–voltage characteristics (JV) were measured under 1-sun (AM 1.5, 25 1C, 100 mW/cm2) using a Wacom WXs-156s-l2 dual beam solar simulator. An excellent simulation of the AM1.5 spectrum was obtained from the superposition of the simulator’s two filtered light sources with adequate calibration. The external quantum efficiency (EQE) was measured, and Jsc was calculated from the EQE by integrating EQE (λ) with the AM 1.5 spectrum.
3. Results and discussion 3.1. Optical and light-trapping properties Three major critical parameters of the TCO affect the solar cell performance: conductivity, transmittance, and haze. A high transparency in full spectrum normally introduces high Jsc for TFS solar cell by reducing the parasitic light absorption. Fig. 1 shows the spectral transmittance in the wide spectrum range (300–1100 nm) (a) and emphasis in the short-wavelength range (350–500 nm) (b) for different samples. It is observed from Fig. 1(a) that all samples have an absorption tail near 350 nm. Although the tiny hydrogen mediation did not show any broadened optical band gap, which was reported in previous studies [12,13], the transmittance promotion is significantly favorable for a-Si:H solar cells, as shown in Fig. 1(b).
Fig. 1. Spectral transmittance of AZO, HAZO and HAZO/AZO films in the (a) wide spectrum range (300–1100 nm) and (b) short-wavelength range (350–500 nm).
Q. Huang et al. / Solar Energy Materials & Solar Cells 136 (2015) 11–16
The absorption coefficient of a semiconductor thin film is a result of the presence of point defects, dislocations, impurities, grain boundaries, and its strain state. In the current ZnO:Al films, negatively charged oxygen species, which are absorbed on the surface of the grain boundaries, act as trapping sites and form the potential barriers. Hydrogen has been proposed to benefit the removal of such depletion regions [17]. Therefore, efforts have been made to eliminate the depletion regions using various treatments with hydrogen [11,13,18,19]. Among these methods, sputtering with hydrogen mediation appears an attractive method to achieve property enhancement because it is a simple low-cost process. When introduced into the sputtering atmosphere, hydrogen can easily be incorporated during growth. The interstitial hydrogen atom substitutes for oxygen, which is absorbed on the surface of the grain boundaries and equally bonded to all surrounding Zn atoms in a truly multi-coordinated configuration [20]. The removal of the defects, which act as non-radiative recombination centers, affects the optical constants near the band gap. Then, using hydrogen mediation, we effectively reduce the parasitic light absorption near an optical band gap without deterioration at the long-wavelength range. The light trapping of TCO is one of the most important methods to enhance the performance of thin-film solar cells, which has a good relationship with the surface texture morphologies. Changes in the surface morphology of AZO, HAZO and HAZO/AZO films were investigated using an AFM, and the resulting surface images are shown in Fig. 2. After the chemical-etching process, a typical crater-like surface structure was clearly identified for each sample. To the best of our knowledge, the surface morphology and RMS roughness of the obtained etched traditional AZO film, as shown in Fig. 2(a), are on the concurrently comparable level to AZO films in other groups [8,10]. When the single HAZO layer was etched, an unconsolidated surface morphology with plenty of deep and small craters with notably steep edges was detected, as shown in Fig. 2(b). Relatively few scientific studies have been performed because of the etching performance of hydrogenated ZnO films until recently. Surfacetextured HAZO films with deferent etching times were reported by Tark et al., but the optimized RMS value of only 27 nm was obtained for these samples [16]. This small surface texture and resulting insufficient light scattering distinctly cannot match the requirement for high-efficiency TFS solar cells. To further increase the surface texture, Tark et al. developed a multi-layer structure with a multi-step etching process [14]. Although the RMS value was not included in this issue, the light-trapping property with the haze value was 50.8% at 550 nm, which remained incomparable with the traditional AZO films [8,10]. Fortunately, with the HAZO/AZO structure, the surface texture was no longer limited by the HAZO layer. A moderately textured topography was developed with a wide range of crater-like surface features (1–5 μm), large oblique scattering angles and large RMS roughness (231 nm). To further study the light trapping of the texture-etched films, the total transmittance (Ttot) and haze factor
13
(HT) of these samples were illustrated as shown in Fig. 3. When the etching process was performed to obtain the light-trapping property for TFS solar cells, the AZO layer with stronger optical absorption near the band gap on the top was etched out. Because the etched HAZO/AZO structure has a larger scattering cross section, Ttot at a short wavelength further improved compared to that of the HAZO film, as shown in Fig. 3(a). Then, the average Ttot that was extracted from 400 to 500 nm was increased from 79.9% to 85.0%, and Ttot from 400 to 1100 nm (which contained a glass substrate whose average transmittance was approximately 92%) was above 85%. Additionally, this etched HAZO/AZO structure demonstrated an excellent light-trapping property, as shown in Fig. 3(b). The illustrated HT was 83% at a wavelength of 850 nm with an average HT value of 83.7% from 400 to 1100 nm, which showed a remarkable 20.7% improvement compared to the standard AZO films. To the best of our knowledge, this HAZO/AZO structure achieves the best scattering efficiency among all reported sputtered TCOs. We believe that HT is increased because of effective scattering from the wide range of crater-like surface features and must be benefit for light trapping in μc-Si:H solar cells. 3.2. Electrical property The electrical properties of each film must be included to use the film as a front contact in TFS solar cells. Table 1 summarizes the electrical properties of AZO, HAZO and HAZO/AZO films with and without chemical etching. Low resistivity and high carrier concentration were obtained for the as-deposited HAZO sample, which has been theoretically and experimentally presented by different research groups [21–23]. Here, we attributed the increase in carrier concentration to the incorporation of hydrogen atoms into the HAZO thin films as shallow donors, which was suggested by the first-principle calculation [21] and electron paramagnetic resonance (EPR) detection [23]. When the carrier concentration increased from 2.59 1020 to 3.15 1020 cm 3, the carrier mobility increased from 39.5 to 56.3 cm2/V s, which reinforces our conclusion that hydrogen mediation positively affects the improvement in crystal quality. When the wet-chemical-etching step was performed, the conductivity of the AZO films significantly decreased. Considering the decrease in carrier mobility, we attributed this reduction to the increase in scattering boundary, which was introduced by the rough surface morphology. However, the resistance of the etched HAZO was over the range of the Hall system. The binding characteristics of the atoms that composed the standard AZO and HAZO films after wet chemical etching were comparable, which has not been reported previously. Fig. 4 shows the curve fitted O1s peaks for different samples, which significantly changed when hydrogen mediation was introduced. The O1s peak for the standard AZO film was resolved into 2 Gaussian– Lorentzian components. The low binding energy component centered at 530.070.1 eV is attributed to O2 ions that were
Fig. 2. Three-dimensional atomic force microscopy images of the etched films. (a) AZO; (b) HAZO; (c) HAZO/AZO.
14
Q. Huang et al. / Solar Energy Materials & Solar Cells 136 (2015) 11–16
Fig. 3. Optical properties of the etched AZO, HAZO and HAZO/AZO films: (a) total transmittance (Ttot); (b) haze factor (HT). Table 1 Electrical characteristics of as-deposited and etched AZO, HAZO and HAZO/AZO films. Samples AZO HAZO HAZO/AZO
As-deposited Etched As-deposited Etched As-deposited Etched
Thickness (nm)
Sheet resistance (Ω/sq)
Carrier concentration (cm 3)
Mobility (cm2/Vs)
Resistivity (Ω cm)
1166 860 1105 783 1038 819
5.23 11.52 3.18 1 4.73 10.06
2.59 1020 2.63 1020 3.15 1020 – 2.78 1020 2.81 1020
39.5 23.9 56.3 – 45.6 26.9
6.10 10 4 9.91 10 4 3.52 10 4 – 4.92 10 4 8.24 10 4
regions indicates that hydrogen mediation can modify the lattice structure by removing the weakly bound oxygen species on the grain boundaries. The appearance of high-binding-energy oxygen shows that although hydrogen can effectively passivate the films’ surface from the adsorption of oxygen species in the atmospheric environment [20], it tends to bond with other components, such as hydroxyls or carbonates in the etching solution. These oxygen species on the films’ surface, such as –CO3, –OH or adsorbed O2, are expected to increase the electrical resistivity after the etching process. To summarize the resistance results, we conclude that: (i) the carrier concentration and mobility of etched HAZO films significantly deteriorated to an unacceptable level for front-contact usage, (ii) the HAZO/AZO structure can improve the conductivity of the etched film to a certain extent by avoiding exposing the HAZO layer to the etching acid solution, and (iii) the HAZO/AZO structure provides a 16.9% improvement in resistivity compared to the standard etched AZO film and is beneficial to front-contact application. Fig. 4. Core-level XPS spectra of O1s for AZO and HAZO films after wet chemical etching.
surrounded by Zn (or substitutional Al) atoms, which indicates the Zn–O bonds in the ZnO matrix, and the medium binding energy peak of 531.17 0.1 eV is related to O2 ions in oxygen-deficient regions [24,25]. The calculated atomic percentages of oxygen related to Zn–O bonds and the deficient component were 56% and 44%, respectively. When the hydrogen mediation was introduced, the FWHM values of the resolved O1s bonds increased and a component that was 2.3 eV away from the Zn–O bond was identified, which corresponded to the existence of a weakly bound oxygen species on the films’ surface, such as –CO3, –OH or adsorbed O2 [25,26]. For the HAZO film, the atomic percentage of oxygen with low, medium and high binding energy was 50%, 32% and 18%, respectively. The small portion of oxygen in deficient
3.3. Application in TFS solar cells Finally, we emphasize the potential of an improved fullspectrum performance for high-efficiency TFS solar cells. Singlejunction a-Si:H and μc-Si:H cells were co-deposited on traditional AZO and HAZO/AZO films to inspect the improvement in transmittance and light trapping, respectively. To study the electrical performance and wavelength dependence of the cell characteristics, current density voltage (JV) and external quantum efficiency (EQE) measurements were performed. Ten test points were carried out for each cell. Samples in comparison in each group have the same trend of variation. The best performers with deviations less than 3% from the average value are shown in Figs. 5 and 6. As shown in Fig. 5, the enhanced optical transmittance near the ZnO band gap had a decisive effect on the spectral response in a-Si:H solar cells. The HAZO/AZO structure has higher spectral
Q. Huang et al. / Solar Energy Materials & Solar Cells 136 (2015) 11–16
15
Fig. 5. JV and EQE curves of the a-Si:H solar cells that were deposited on the standard AZO and HAZO/AZO front contacts. (a) JV curves of a-Si:H solar cells; (b) EQE curves of a-Si:H solar cells.
Fig. 6. JV and EQE curves of the μc-Si:H solar cells that were deposited on the standard AZO and HAZO/AZO front contacts. (a) JV curves of μc-Si:H solar cells; (b) EQE curves of μc-Si:H solar cells.
transmittance in the short-wavelength region (400–500 nm), which increases the light absorption of the absorber layer (a-Si:H (i)) and promotes the generation of electron–hole pairs. Hence, the quantum efficiency was improved by 7.9% (from 59% to 66.9%) at 400 nm, and the integrated short-circuit current density Jsc, which was calculated from the EQE curve, increased from 10.62 to 11.06 mA/cm2. Although Voc remained almost identical, the FF slightly decreased, which we may attribute to the enlarged surface potential barrier loss between the TCO and the p layer that is related to the large surface contact area [27]. This attribution will be affirmed and improved in future study. As the front contact in μc-Si:H solar cells, the higher EQE in the entire wavelength region resulted in a 5.3% enhancement of the shortcircuit current density, as shown in Fig. 6. This result was benefited by the enhanced light scattering, which increased the optical path length and the absorption. Additionally, the FF was also improved from 64% to 64.9%, which may be attributed to the decrease in ohmic losses at the TCO and TCO/p-contact. Thus, we obtained an initial active-area efficiency of 8.69% using a μc-Si:H single-junction solar cell with a notably good long-wavelength response, where Jsc was over 27 mA/ cm2, which was obtained from the convolution integral of the EQE curve with AM 1.5 solar spectrum, and was the current maximum limit for μc-Si:H solar cells [28]. Finally, we incorporated the HAZO/AZO front contact and component cells into a-Si:H/a-SiGe:H/μc-Si:H triple-junction structures and optimized the cell design. The maximal initial
Fig. 7. JV characteristics of the a-Si:H/a-SiGe:H/μc-Si:H triple-junction solar cell that were deposited on the HAZO/AZO front contact.
active-area cell efficiency of 15.06% was achieved with the JV characteristics as shown in Fig. 7. The detailed preparation and optimization of this triple-junction solar cell will soon be published in another paper [29]. These results demonstrate that the HAZO/AZO structure with improved full-spectrum performance enables the fabrication of high-efficiency TFS solar cells.
16
Q. Huang et al. / Solar Energy Materials & Solar Cells 136 (2015) 11–16
4. Conclusion In summary, a simple and low-cost method to effectively enhance the full-spectrum performance of the front contact for highefficiency TFS solar cells was achieved using an HAZO/AZO structure. The optical transmittance near the ZnO band gap was significantly improved. The average Ttot extracted from 400 nm to 500 nm was increased from 79.9% to 85.0%. A relatively high HT of 83% at 850 nm with an average value of 83.7% from 400 to 1100 nm was developed with remarkable light-trapping improvement. In addition, the resistivity was also increased by 16.9% with the HAZO/AZO structure. Then, these improvements were demonstrated in the usage of top electrodes in the a-Si:H and μc-Si:H solar cells, which resulted in 4.1% and 5.3% enhancement in the short-circuit current density, respectively. Finally, an a-Si:H/a-SiGe:H/μc-Si:H triple-junction solar cell was obtained with an initial efficiency over 15%, which illustrates a promising and potentially cost-effective alternative to further improve the high-quality transparent electrodes for high-efficiency thin-film solar cells. References [1] S. Kim, J.W. Chung, H. Lee, J. Park, Y. Heo, H.M. Lee, Remarkable progress in thin-film silicon solar cells using high-efficiency triple-junction technology, Sol. Energy Mater. Sol. Cells 119 (2013) 26–35. [2] B. Yan, G. Yue, L. Sivec, J. Yang, S. Guha, C. Jiang, Innovative dual function ncSiOx:H layer leading to a 416% efficient multi-junction thin-film solar cell, Appl. Phys. Lett. 99 (2011) 113512. [3] M.M. Islama, S. Ishizukab, A. Yamadab, K. Matsubarab, S. Nikib, T. Sakuraia, K. Akimotoa, Thickness study of Al:ZnO film for application as a window layer in Cu(In1 xGax)Se2 thin film solar cell, Appl. Surf. Sci. 257 (2011) 4026–4030. [4] Y.H. Kim, J.S. Kim, W.M. Kim, T.Y. Seong, J. Lee, L. Müller-Meskamp, K. Leo, Realizing the potential of ZnO with alternative non-metallic co-dopants as electrode materials for small molecule optoelectronic devices, Adv. Funct. Mater. 23 (2013) 3645–3652. [5] O. Isabella, J. Krc, M. Zeman, Modulated surface textures for enhanced light trapping in thin-film silicon solar cells, Appl. Phys. Lett 97 (2010) 1011061–101106-3. [6] V.E. Ferry, M.A. Verschuuren, M.C.V. Lare, R.E.I. Schropp, H.A. Atwater, A. Polman, Optimized spatial correlations for broadband light trapping nanopatterns in high efficiency ultrathin film a-Si:H solar cells, Nano Lett. 11 (2011) 4239–4245. [7] A. Hongsingthong, I.A. Yunaz, S. Miyajima, M. Konagai, Preparation of ZnO thin films using MOCVD technique with D2O/H2O gas mixture for use as TCO in silicon-based thin film solar cells, Sol. Energy Mater. Sol. Cells 95 (2011) 171–174. [8] J. Müller, B. Rech, J. Springer, M. Vanecek, TCO and light trapping in silicon thin film solar cells, Sol. Energy 77 (2004) 917–930. [9] C. Guillén, J. Herrero, Improving conductivity and texture in ZnO:Al sputtered thin films by sequential chemical and thermal treatments, Appl. Surf. Sci., 282, 2013923–929. [10] M. Berginski, J. Hüpkes, M. Schulte, G. Schöpe, H. Stiebig, B. Rech, The effect of front ZnO:Al surface texture and optical transparency on efficient light
[11]
[12]
[13]
[14]
[15] [16]
[17]
[18]
[19]
[20] [21] [22]
[23]
[24] [25]
[26]
[27]
[28]
[29]
trapping in silicon thin-film solar cells, J. Appl. Phys., 101, 20070749031–074903-11. Q. Huang, Y. Liu, S. Yang, Y. Zhao, X. Zhang, Hydrogen mediated self-textured zinc oxide films for silicon thin film solar cells, Sol. Energy Mater. Sol. Cells 103 (2012) 134–139. F. Wang, C. Yang, J. Liou, I. Chen, Effects of hydrogen on the optical and electrical characteristics of the sputter-deposited Al2O3-doped ZnO thin films, J. Nanomater. (2014) 857614-1–857614-7. K. Kim, H. Choia, D. Cho, Y. Chung, K. Lee, N. Park, J. Kim, Electrical and optical properties of radio frequency magnetron-sputtered lightly aluminum-doped zinc oxide thin films deposited in hydrogen–argon gas, Thin Solid Films 540 (2013) 142–145. S.J. Tark, M.G. Kang, S. Park, S.H. Lee, C.S. Son, J.C. Lee, D. Kim, Characterization of hydrogenated Al-doped ZnO films prepared by multi-step texturing for photovoltaic applications, Curr. Appl. Phys. 11 (2011) 362–367. Y. Wang, X. Zhang, Q. Huang, F. Yang, J. Liang, D. Zhang, Y. Zhao, Tailoring of textured ZnO:Al film via hydrogen, Vaccum 107 (2014) 6–9. S.J. Tark, M.G. Kang, S. Park, J.H. Jang, J.C. Lee, W.M. Kim, J.S. Lee, D. Kim, Development of surface-textured hydrogenated ZnO:Al thin-films for μc-Si solar cells, Curr. Appl. Phys., 9, 20091318–1322. W.M. Kim, Y.H. Kim, J.S. Kim, J. Jeong, Y.J. Baik, J.K. Park, K.S. Lee, T.Y. Seong, Hydrogen in polycrystalline ZnO thin films, J. Phys. D: Appl. Phys. 43 (2010) 365406-1–365406-5. I. Lorite, J. Wasik, T. Michalsky, R. Schmidt-Grund, P. Esquinazi, Hydrogen influence on the electrical and optical properties of ZnO thin films grown under different atmospheres, Thin Solid Films 556 (2014) 18–22. M.V. Castro, M.F. Cerqueira, L. Rebouta, P. Alpuim, C.B. Garcia, G.L. Júnior C.J. Tavares, Influence of hydrogen plasma thermal treatment on the properties of ZnO:Al thin films prepared by dc magnetron sputtering, Vacuum 107 (2014) 145–154. A. Janotti, C.G. Van De Walle, Hydrogen multicentre bonds, Nat. Mater. 6 (2007) 44–47. C.G. Van de Walle, Hydrogen as a cause of doping in zinc oxide, Phys. Rev. Lett. 85 (2000) 1012–1015. S.F.J. Cox, E.A. Davis, S.P. Cottrell, P.J.C. King, J.S. Lord, J.M. Gil, H.V. Alberto R.C. Vilão, J. Piroto Duarte, N. Ayres de Campos, A. Weidinger, R.L. Lichti, S.J. C. Irvine, Experimental confirmation of the predicted shallow donor hydrogen state in zinc oxide, Phys. Rev. Lett. 86 (2001) 2601–2604. D.M. Hofmann, A. Hofstaetter, F. Leiter, H. Zhou, F. Henecker, B.K. Meyer S.B. Orlinski, J. Schmidt, P.G. Baraov, Hydrogen: a relevant shallow donor in zinc oxide, Phys. Rev. Lett. 88 (2002) 045504-1–045504-4. B.Y. Oh, M.C. Jeong, J.M. Myoung, Stabilization in electrical characteristics of hydrogen-annealed ZnO:Al films, Appl. Surf. Sci., 253, 20077157–7161. T.G.G. Maffeis, M.W. Penny, A. Castaing, O.J. Guy, S.P. Wilks, XPS investigation of vacuum annealed vertically aligned ultralong ZnO nanowires, Surf. Sci 606 (2012) 99–103. B.J. Coppa, C.C. Fulton, P.J. Hartlieb, R.F. Davis, B.J. Rodriguez, B.J. Shields R.J. Nemanich, In situ cleaning and characterization of oxygen- and zincterminated, n-type, ZnO{0 0 0 1} surfaces, J. Appl. Phys. 95 (2004) 5856–5864. A.N. Corpus-Mendoza, M.M. De Souza, F. Hamelmann, Transport mechanisms and effective Schottky barrier height of ZnO/a-Si:H and ZnO/lc-Si:H heterojunction solar cells, J. Appl. Phys. 114 (2013) 184505-1–184505-6. M. Berginski, J. Hüpkes, M. Schulte, G. Schöpe, H. Stiebig, B. Rech, The effect of front ZnO:Al surface texture and optical transparency on efficient light trapping in silicon thin-film solar cells, J. Appl. Phys. 101 (2007) 0749031–074903-11. B. Liu, L. Bai, Z. Chen, X. Zhang, D. Zhang, J. Ni, Q. Huang, C. Wei, J. Sun, X. Chen, H. Ren, G. Hou, G. Wang, Y. Zhao, High efficiency triple junction thin film silicon solar cells with optimized electrical structure, Prog. Photovoltaics Res. Appl., accepted.