Electrodeposition of Mg doped ZnO thin film for the window layer of CIGS solar cell
Introduction
As promising candidates for next generation photovoltaic modules, chalcopyrite based solar cells has received much attention. Up to now, CuInGaSe2 (CIGS) absorber layer based solar cells have obtained the highest power conversion efficiency of 21.7% in laboratory among the thin film technology [1]. However, the maximum theoretical efficiency of CIGS is about 27% [2]. ZnO, an excellent electron transporter with high mobility and various nanostructures, has been applied to the chalcopyrite based solar cells [3], [4]. However, performance of the ZnO based cell is still limited by serious charge recombination at the ZnO/chalcopyrite absorber interface. One of the important reasons for the degradation is mismatching of conduction band offset (CBO) of window/CIGS layers. Theoretically, if the conduction band level of the window layer is lower than that of CIGS (negative CBO), an interface recombination between window/CIGS layers becomes dominant [5], [6]. A negative CBO acts as a barrier against injected electrons from n-type region at the window/CIGS interface. The recombination between injected carriers and defects at the window/CIGS interface thus increased drastically, resulting in leakage conduction and the decrease of Voc and FF. A positive CBO acting as a barrier for photo-generated electrons in the CIGS layer can result in the light soaking (LS) effect which increases the cell efficiency [7], attributing to the reduction of the window/CIGS interface recombination [8], [9]. Based on the theoretical result, the CBO in the ZnO based chalcopyrite based solar cells can be controlled by introducing Mg into ZnO. ZMO has been investigated to adjust the CBO of the window/CIGS heterointerface to positive by controlling the electron affinity with valence band maximum almost fixed by the incorporation of Mg [10]. With increasing the Mg content, the conduction band level of the ZMO shifts toward the vacuum level. With this interface engineering, charge recombination in this cell can be significantly suppressed. The ZMO has been replaced the ZnO in Cu2O-ZnO solar cell and an increased open circuit voltage and efficiency was obtained [11]. Besides, enhancing the band gap of window layer will be developed as an alternative solution to improve the overall efficiency by decreasing the absorption loss.
In the past decades, numerous approaches have been developed for the fabrication of ZMO including thermal evaporation method [12], pulsed laser deposition [13], microwave annealing [14], ultrasonic spray pyrolysis [15], reactive evaporation [16], sol-gel [17], precipitation [18], solar physical vapor deposition [19], solvothermal method [20] and so on. But the drawbacks including expensive cost, complexity of operation and small-scale layer production have become the obstacles in their potential implementation. Thus an efficient technique requiring non-vacuum, low processing temperature and environmental-friendly process is appropriate for the fabrication of low cost large scale modules. Electrochemical deposition is a potentially suitable preparation method for achieving this goal [21]. Although there are number of studies on the ZnO electrodeposition, the use of this material for photovoltaic applications as window layer or buffer layer is rarely [22], [23], [24]. To the best of our knowledge, using electrodeposited ZnO for photovoltaic application obtaining a record efficiency of 15.8% on CIGS based solar cell [25]. There is no research on ZMO films prepared by electrodeposition technique aiming to be used for the window layer of chalcopyrite solar cells. So one of the important motivations of our work is providing a simple stratege to develop Mg doped ZnO film which shows promise as substitutes for undoped ZnO film.in CIGS solar cell.
Some studies concerning the preparation of ZMO with electrodeposition method show some major problems which have been encountered when using electrodeposition methods to fabricate ZMO films for photovoltaic devices [26], [27], [28]. Aggregates of nanosheets structures with many defects such as pores were obtained in these researches with NO3− ions precursor used for the formation of hydroxide. Theoretically, nanostructured solar cells consisting of nanorod arrays with radial p−n junction geometries can provide several advantages over planar junctions. Well-aligned radial junction geometries can substantially increase the light trapping efficiency by reducing reflection losses due to light scattering among the well-aligned ordered structures [29], [30], [31], [32]. Nanorods can increase the interfacial area compared to the areas obtained from planar geometries, thereby improving carrier extraction. Nanostructuring approaches might not require long minoritycarrier diffusion lengths. For the reasons enumerated above, nanostructuring strategies have received significant attention recently in the context of solar cell and provide good photovoltaic performances [33], [34], [35], [36], [37], [38]. Over the past few years, several reports have described the use of ordered ZnO nanostructures in substrate-type CIGS thin-film photovoltaics [39], [40] and DSSC solar cells [55], [56], [57], [58], [59]. However, Mg doped ZnO nanostructures synthesised by electrodeposition method used for CIGS thin film solar cell has not been reported. Consequently, further work for obtaining the Mg doped ZnO nanostructures synthesised by electrodeposition method to optimize device performance is needed. In this study, the effects of the deposition potentials on morphology, crystalline structure, crystallographic orientation and optical properties of the ZMO thin film were investigated. Our results show that obvious morphology change from nanorod structrue to continuous compact films without secondary phase is obtained. Combining the structural and optical properties analysis, it reveals that the ZMO thin film with the normal hexagonal nanorods morphology deposited at −0.9 V can act as an alternative of the window layer in chalcopyrite solar cells devices which will be investigated in our further work.
Section snippets
Experimental details
In this experiment, the ZMO thin films were electrodeposited potentiostatically in a three-electrode cell in which a ITO-coated (135 nm) soda lime glass (15 Ω/sq, 12mm × 9mm × 0.7 mm) was used as the working electrode, a saturated calomel electrode (SCE, E0 = 244 mV vs SHE) as the reference electrode and a Pt wire as the counter electrode. Prior to the experiment, the substrates were successively cleaned in an ultrasonic bath with acetone, alcohol, deionised water, then dried with flowing N2 atmosphere
Linear sweep voltametry studies
Linear sweep voltametry (LSV) was used to determine the most suitable potential for the deposition [41]. LSV of ITO substrate in blank solution containing 5 mM KCl, zinc acetate solution, the mixture of zinc acetate + hexamine solution and zinc acetate + magnesium acetate + hexamine solution have been presented in Fig. 1. A sweeping potential from 0 V to −1.4 V at a scan rate of 5 mV/s was applied for the working electrode respect to the reference electrode of the three-electrode cell. And the resulted
Conclusions
Mg doped ZnO films with the advantage of adjusting the CBO of the window/chalcopyrite absorber heterointerface to positive to reduce the interface recombination show promise as substitutes for undoped ZnO film.in chalcopyrite based solar cell. A systematic study of the effect of the electrodeposition potential on morphology, crystalline structure, crystallographic orientation and optical properties of ZMO films has been carried out in the present work which has not been reported up to now. An
Acknowledgements
The authors gratefully acknowledge the financial support of the Hunan Provincial Natural Science Foundation of China (No. 2016JJ3122), National Nature Science Foundation (Nos. 51102203, 51172191, 11474244, 51202208), National Basic Research Program of China (Nos. 2012CB921303, 2015CB921103) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT13093).
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2019, Thin Solid FilmsCitation Excerpt :ZnO is a good candidate window material since it has a high bandgap of >3.1 eV [10] and, consequently, will absorb less of the impinging solar radiation. Magnesium-doped ZnO (i.e., MgxZn1-xO or MZO) provides an additional degree of design freedom since the Mg composition (x) permits tuning of the bandgap [11,12]. The lower electron affinity of MZO, relative to ZnO, is also desirable for the front-interface to form a “spike-like” conduction band offset [9] between MZO and CdTe shown in Fig. 1b.