Elsevier

Current Applied Physics

Volume 20, Issue 11, November 2020, Pages 1237-1243
Current Applied Physics

Characteristic material parameters of CIGS solar cell related with device performance

https://doi.org/10.1016/j.cap.2020.08.003Get rights and content

Highlights

  • CIGS solar cells with PCE ranging from 1.82% to 12.3% were analyzed systematically with various spectroscopic tools.

  • The correlation of device parameters with the PCE was probed deeply.

  • Lower PCE cells are affected by defect states both in bulk of absorber and at pn interface.

  • Charge recombination in pn interface is weak for the higher PCE cell.

  • Some material parameters can be used as a quick indicator to evaluate PCE of solar cell.

Abstract

CIGS solar cells with power conversion efficiency (PCE) in the range of 1.82%–12.30% were obtained by using two-step process, and were further analyzed through various measurement techniques. Material parameters showed diverse values and some trends depending on the device performance. The lower performance device showed small integrated PL intensity, short minority life time, larger defect density and lower activation energy, whereas the higher performance device showed opposite values. We investigated relationship between material parameters and PCE of solar cells, and found that some physical parameters such as integrated PL intensity, minority life time, defect density, and difference between band gap and activation energy (Eg-Ea), which all reflect defect states in bulk and at pn interface, are strongly related with PCE and would be used as a good indicator to evaluate device performance quickly.

Introduction

CuIn1-xGax (S,Se)2 (CIGS) based thin film solar cells are exhibiting over 23% power conversion efficiencies (PCE) in lab scale production and it's quite well competing with the conventional crystalline silicon (c-Si) solar cells [1,2]. However, CIGS solar cells facing difficulty to overcome the market share of c-Si solar cells, due to its large difference of PCE between the lab scale and module scale productions [3,4]. Commercial CIGS modules are showing average PCEs of 15–16% which is nearly 7–8% far away from the champion CIGS device. The key factors affecting on intensively decreased PCE for large scale productions are generally viewed as thin film inhomogeneities during scale up process and resistive losses through the connection of sub modules [5,6].

CIGS consists of quaternary elements and such wide range of elemental composition results in various intrinsic defects named as selenium vacancies (Vse), copper vacancies (VCu), gallium copper antisite (GaCu), indium copper antiste (InCu) and their combinations (Vse + VCu), (VCu + InCu) [[7], [8], [9]]. As reported in earlier, the VCu and InCu antisite lead to deteriorated open circuit voltage (Voc) and VSe together with GaCu antisite cause band tailing issues and further affect the device efficiency [[10], [11], [12]]. Therefore, achieving minimized inhomogeneity and reduced defect density in the module fabrication will be the main step for narrowing the difference between module scale and lab scale PCE.

In this regard, here we systematically studied scattered PCE of CIGS solar cells and tried to clarify the main reasons behind the diversity. We performed several types of analysis methods such as XRD, Raman spectroscopy, admittance spectroscopy (AS), temperature dependent current-voltage characteristic (J-V-T) and time-resolved photoluminescence (TRPL), and investigated how much strongly those characteristic material parameters are related with PCE. We will explain the interconnection between the material parameters and device performance with device PCE. Furthermore, if the material parameters have proportional relation with PCE of solar cell, we can use those parameters for quick evaluation of device performance, i.e. alternative indicators showing device performances.

Section snippets

Experimental section

Commercial SLG/Mo/CuInGa precursor films were obtained from Avancis, Korea. Then the precursor films were post annealed with large graphite box which containing Se pellets and the annealing recipe followed two steps: 1) Se soaking at 350 °C for 10 min and 2) selenization at 550 °C for 10 min [13]. After that, around 30-nm-thick Zn(O,S) buffer layer or ~80-nm-thick CdS buffer layer was deposited on the CIGS absorber films individually. Conventional CBD method was used for coating of CdS and ALD

Results and discussion

Fig. 1(a) and (b) show J-V curves and EQE spectra of fabricated CIGS devices with different PCEs, where C5-5, C5-6, C6-5 and C6-6 solar cells are buffered with conventional CdS whereas C5-7, C5-8, C6-7 and C6-8 solar cells are buffered with Zn(O,S). CIGS/CdS devices show better Voc and high fill factor (FF) compared with CIGS/Zn(O,S) devices as details are summarized in Table 1. However, CIGS/Zn(O,S) devices exhibit slightly higher short circuit current (Jsc) due to its improved EQE spectrum in

Conclusions

In this work, we systematically studied CIGS devices with diverse PCEs and further analyzed its correspondence of material and device parameters to overall performances. By using various characterization methods, we deeply probed the structural and optical characteristics of CIGS solar cells with different device performances. As the result, low performance devices showed much shifted Bragg angles, lower integrated PL intensity, shorter minority life time, higher defect densities and smaller Ea

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) funded by the Korean Government (NRF-2016M1A2A2937010) and the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20173010012980 and No.2018302001097).

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