Cu(In,Ga)Se2 thin film solar cells with solution processed silver nanowire composite window layers: Buffer/window junctions and their effects

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Highlights

  • Silver nanowire (AgNW) composites window layers are employed to CIGS solar cells.

  • CdS/AgNW-composite window interfacial defects could cause significant loss in FF.

  • The value of NdDi2 has to be greater than ~4×10−6 cm to avoid loss in FF.

  • Nd is approximately free electron concentration in a matrix layer embedding AgNWs.

  • Di is the negatively charged defect density at the CdS/AgNW-composite interface

Abstract

We quantitatively and analytically investigate the properties of buffer/window junctions and their effects on the energy band alignment and the current-voltage characteristics of Cu(In,Ga)Se2 (CIGS) thin film solar cells with solution processed silver nanowire (AgNW) composite window layers. AgNWs are generally embedded in a moderately conductive matrix layer to ensure lateral collection efficiency of charge carriers photogenerated in the lateral gaps present between AgNWs. Studies on the junctions between a buffer and AgNW-composite window layers and their effects on the performances of CIGS thin film solar cells have seldom been addressed. Here, we show that solution processed AgNW-composite window layers could induce defect states at the buffer/window interface, resulting in poor energy band alignment impeding carrier transport in the solar cells. On the basis of our analysis, we suggest an analytical expression of nmatrixDi23.46×105ε cm to avoid losses in the power conversion efficiency of the solar cells. nmatrix is the carrier concentration in a matrix layer embedding AgNWs, Di is the negative defect density at the buffer/window interface, and ε is the relative dielectric constant of the matrix layer embedding AgNWs.

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 (χCIGS) of the CIGS layer is greater than that (χbuffer) 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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