Microstructure Control of Absorber Sb2S3 and p-Type Semiconductor CuSCN for Semiconductor-Sensitized Solar Cells (TiO2/Sb2S3/CuSCN)

INTRODUCTION

Semiconductor-sensitized solar cells (SSSCs) have greater stability of the semiconductor compared to organic dyes. Antimony sulfide (Sb₂S₃) is considered as a remarkable absorber for SSSCs due to its high absorption coefficient[1], low toxicity, abundance of its elements, and direct band gap with absorption in the visible range (approximately 1.8eV[2]). Sb₂S₃-based extremely thin absorber (ETA) solar cells reportedly have a 3.7% efficiency when the structure is porous TiO₂ as n-type semiconductor and CuSCN as p-type semiconductor[3]. Photo-generated carriers are produced in Sb₂S₃ films, and a fast charge separation occurs due to the “thinness” of the absorber. However, the relation between the microstructure of absorber Sb₂S₃and the power generation characteristics of this type of ETA solar cells remains unclear.

In this study, two deposition methods were introduced, namely, sputtering, which allows easy control of Sb₂S₃ deposition on thin TiO₂ films by adjusting the deposition time_, and inkjet-printing, which promises no physical contact with Sb₂S₃ when depositing CuSCN. Compared with Sb₂S₃ and CuSCN’s previous deposition methods, chemical bath deposition (CBD) and wiping, new deposition methods have their advantages. Improved cell efficiency is expected by controlling the microstructure of absorber Sb₂S₃and p-type semiconductor CuSCN with these two deposition methods.

EXPERIMENTAL

A thin TiO₂ film was deposited on transparent conductive oxide (TCO) glass by spray pyrolysis. Sb₂S₃ was then deposited on this film by CBD from a solution of SbCl₃ and Na₂S₂O₃ (Aldrich)_. Alternatively, Sb₂S₃ film was deposited by sputtering using a Sb₂S₃ target of 99.9% purity (Kojundo Chemical) in an argon atmosphere with a base pressure of 5×10⁻⁴ Pa and with a launch power of 50 W. Both of  these amorphous Sb₂S₃ films deposited by CBD and sputtering were then annealed in nitrogen at 320°C for 20 min to yield dark-brown crystalline stibnite. CuSCN was deposited by wiping or by inkjet-printing in an ambient atmosphere using a slightly under-saturated solution of CuSCN in dipropyl sulfide heated to 80°C. ETA solar cells were completed by depositing a gold film as the electrode by vacuum evaporation.

I-V curves were measured using a solar simulator (CEP-25MLH) under simulated AM1.5, 100mW/cm² sunlight and in the dark. Photovoltaic parameters, i.e. short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF) and cell efficiency (η) were determined from the I-V curves. For the field emission scanning electron microscopy (FE-SEM) images, samples were prepared with a cross-section polisher (CP) and by deposition of a platinum coating.

RESULTS AND DISCUSSION

Figure 1 shows the dependence of Jsc of ETA solar cells upon absorbance of sputter-deposited Sb₂S₃ at 500 nm. Absorbance depends on the thickness of Sb₂S₃. By controlling the deposition time of Sb₂S₃, the film thickness can easily be controlled by sputtering. Sb₂S₃ absorbance around 0.6 yielded the highest Jsc. With thinner films, the number of absorbed photons is not sufficient to yield a high amount of photo-generated carriers, whereas thicker films increase the amount of recombination of photo-generated carriers. Cell efficiency of ETA solar cells with sputter-deposited Sb₂S₃ is clearly improved compared with CBD-deposited Sb₂S₃(Table 1).

Figure 2 compares cross-sectional FE-SEM images of typical cells with wiping-deposited and inkjet-printed CuSCN films. The inkjet-printed CuSCN film exhibited large holes and was approximately two times thicker than the wiping-deposited CuSCN film. Table 2 shows the photovoltaic parameters of ETA solar cells with wiping-deposited and inkjet-printed CuSCN films. In this study, ETA solar cells with wiping-deposited CuSCN films exhibited better power generation characteristic compared with inkjet-printed CuSCN films. However, the deposition conditions for inkjet-printing still need optimization, and cell efficiency is expected to improve for this method.

CONCLUSION

In summary, with the new deposition methods introduced in this study, namely sputtering and inkjet-printing, absorber Sb₂S₃ and p-type semiconductor CuSCN were deposited successfully. By controlling the microstructure of Sb₂S_3, ETA solar cell efficiency was improved. The inkjet-printing conditions still need optimization to yield a higher cell efficiency.

REFERENCE

[1] Versavel et al.,Thin Solid Films,515,7171-7176(2007)

[2]Mane et al.,Master.Chem.Phys,141,2871-2877(1994)

[3]S.Nezu et al., J.Phys.Chem.C, 114, 6854-6859(2010)