Cu(In,Ga)Se2 thin film solar cells with solution processed silver nanowire composite window layers: Buffer/window junctions and their effects
Graphical abstract
Introduction
Most research efforts for the realization of low cost Cu(In,Ga)Se2 (CIGS) thin film solar cells have been dedicated to the development of non-vacuum processing methods for the preparation of CIGS absorber layers [1], [2], [3], [4], [5], [6], [7], [8]. However, recently, research interest on low-cost solution processed silver nanowire (AgNW) based window layers to replace a vacuum processed i-ZnO/ZnO:Al window layer has been also increased to further reduce the cost of producing CIGS photovoltaic modules [9], [10], [11], [12], [13], [14], [15], [16], [17]. Lee et al. first demonstrated AgNW mesh transparent electrodes applicable to thin film solar cells [18]. Chung et al. suggested the necessity of a moderately conductive metal oxide matrix layer filling the lateral gaps present between AgNW networks for the AgNW-based window layer to be properly incorporated into CIGS thin film solar cells [9]. Since the aforementioned reports, additional studies have followed on alternative materials, processing methods and stacking structures for matrix layers embedding AgNWs in CIGS solar cells [10], [11], [12], [13], [14], [15], [16].
Some CIGS devices presented in the aforementioned reports showed a crossover between dark and light current [10] or kinks causing a significant loss in fill factor (FF) [11], [12], [17] for certain materials or processing methods for matrix layers embedding AgNWs. Therefore, for these solution processed AgNW-composites, which are composed of a matrix layer embedding AgNWs, to perfectly replace and/or achieve superior performance to a sputter deposited i-ZnO/ZnO:Al window layer, a deep understanding of the effects of the junction properties between a buffer layer and AgNW-composite window layers on the performance of CIGS solar cells is necessary. Here, we investigate the effects of the junction properties, including interfacial defects at the CdS/AgNW-composite and electrical properties of the AgNW-composite on the energy band alignment and current density-voltage (J-V) characteristics of CIGS solar cells.
Section snippets
Device fabrication and characterization
For the study of the effects of the buffer/AgNW-composite junction on the performance of CIGS thin film solar cells, two type of window layers have been applied the onto the structure of CdS(~50 nm)/CIGS(~2 µm)Mo(~1 µm)/soda-lime glass(~1.1 mm). One is a sputtered i-ZnO/ZnO:Al window layer, the other is an AgNW–ITO-np window layer. The Mo bottom electrodes were prepared by direct-current sputtering, the CIGS absorber layers by hydrazine solution processing, and the CdS buffer by chemical bath
Barrier of photocurrent in CIGS solar cells
S-shaped kinked J-V curves are due to a substantial decrease of photocurrent (Jph) in a certain forward bias region which causes a loss in mainly FF. Extremely strong kink can also cause a loss in short-circuit current density (JSC) and open circuit voltage (VOC). This kink originates from the spike-type energy band alignment of CIGS solar cells in which the electron affinity () of the CIGS layer is greater than that () of an adjacent buffer layer as shown in Fig. 2 [20], [21], [22]
Characteristics of the fabricated CIGS solar cells
AgNW-composite window layers were composed of a network of AgNWs embedded in an ITO-np matrix with a thickness of ~700 nm as shown in Fig. 3. In this AgNW-composite, the AgNW network provides long-range lateral electrical current paths, and the matrix layer is responsible for providing electrical current paths in the spaces present between AgNWs. The sheet resistance of the matrix layer, ITO-np in our case, is required to be less than 1M Ω/sq to 1G Ω/sq, depending on the spacing between each
Conclusion
We have investigated the effect of CdS/AgNW-composite on the performance of CIGS thin film solar cells. The composite window layer in this work was composed of an AgNW network and an ITO-np matrix. Even though both an AgNW network and the ITO-np matrix were sufficiently conductive for lateral charge collection, a significant loss in FF due to the kink in the J-V curve was observed in the CIGS solar cells. The loss of FF is explained as follows. The interfacial defects capture electrons mainly
Acknowledgement
This research was supported by In-House Research and Development Program of the Korea Institute of Energy Research (KIER) (B7-2421) and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant No. NRF-2014R1A1A2059181, and NRF-2016R1D1A1B03934840).
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These authors contributed equally.