Carrier loss mechanisms in textured crystalline Si-based solar cells

Carrier loss mechanisms in textured crystalline Si-based solar cells

Akihiro Nakane, Shohei Fujimoto, and Hiroyuki Fujiwaraa)

Department of Electrical, Electronic and Computer Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan

Abstract

A quite general device analysis method that allows the direct evaluation of optical and recombination losses in crystalline silicon (c-Si)-based solar cells has been developed. By applying this technique, the optical and physical limiting factors of the state-of-the-art solar cells with ~20% efficiencies have been revealed. In the established method, the carrier loss mechanisms are characterized from the external quantum efficiency (EQE) analysis with very low computational cost. In particular, the EQE analyses of textured c-Si solar cells are implemented by employing the experimental reflectance spectra obtained directly from the actual devices while using flat optical models without any fitting parameters. We find that the developed method provides almost perfect fitting to EQE spectra reported for various textured c-Si solar cells, including c-Si heterojunction solar cells, a dopant-free c-Si solar cell with a MoOx layer, and an n-type passivated emitter with rear locally diffused (PERL) solar cell. The modeling of the recombination loss further allows the extraction of the minority carrier diffusion length and surface recombination velocity from the EQE analysis. Based on the EQE analysis results, the carrier loss mechanisms in different types of c-Si solar cells are discussed.

2 I. INTRODUCTION

Pyramid-shaped textures with sizes of 5~10 µm are generally incorporated into crystalline silicon (c-Si) solar cells to suppress the front light reflection and thus to enhance the light absorption in the devices.1,2 The large textures formed on the front and rear surfaces of the c-Si, however, complicate the optical analysis significantly, making the determination of the carrier loss mechanisms within the devices quite challenging. The difficulty of performing the explicit optical characterization arises particularly from the randomness of the pyramid textures, formed generally by alkaline wet etching of c-Si (100) wafers.1-3 So far, to characterize the light trapping properties of various c-Si textures, a computer-intensive ray tracing technique has been applied,1,4-11 but these studies provide limited success, as the full optical analysis of the multilayered c-Si device has been rather difficult due to the large calculation cost of this approach. Thus, there is still a strong need for the development of a novel optical simulator that can be employed for the practical characterization of textured c-Si devices on a routine basis. Such a technique is critical for the efficient optimization of solar cells. In particular, in c-Si heterojunction solar cells, hydrogenated amorphous silicon (a-Si:H) and transparent conductive oxide (TCO) layers exhibit large unfavorable parasitic absorption, reducing the external quantum efficiency (EQE) in the short and long wavelength (λ) regions notably.9-13 For the heterojunction solar cells formed on flat c-Si substrates, detailed EQE characterization has been performed to determine the optical losses in the component layers;11 however, for c-Si solar cells with random textures, the complete optical-loss analysis has not been reported yet. On the other hand, the effect of the carrier recombination appears clearly in the EQE spectra of c-Si solar cells, and the intensive carrier recombination observed at a c-Si/Al rear interface reduces the EQE response in the longer λ region remarkably.14 Accordingly, all the optical and recombination losses in the solar cells can be assessed quantitatively based on the EQE analysis if the proper analysis method is established. In our previous study,15-17 we have established an EQE analysis technique for thin-film solar cells with submicron textures. In this method, to determine the light absorption in solar cells accurately, reflectance spectra obtained experimentally have been applied assuming flat optical models within the framework of the optical admittance method.18 This method provides excellent fittings to numerous EQE spectra reported for Cu(In,Ga)Se2 (Ref. 15), Cu2ZnSn(S,Se)4 (Ref. 17), and hybrid perovskite16,19 solar cells, enabling the accurate characterization of the carrier loss mechanisms in these devices.

3 In this study, to reveal the optical and physical limiting factors of various c-Si-based solar cells, we have developed a global EQE analysis method in which the light absorption in the c-Si with a random texture is assessed using the experimental reflectance spectrum while assuming a perfectly flat optical model. By this procedure, the EQE calculations of the textured structures are simplified drastically. To reproduce the incoherent light absorption

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