The effect of pulsewidth on preparing CuIn1-xGaxSe2 thin film via pulse laser deposition

Shih-Chen Chen,2 Kaung-Hsiung Wu,2 Fang-I Lai,3 Takayoshi Kobayashi,2 and Hao-Chung Kuo.1,*

1Department of Photonics and Institute of Electro-Optical Engineering, National Chiao-Tung University, Hsinchu, Taiwan

2Department of Electrophysics, National Chiao-Tung University, Hsinchu, Taiwan 3Department of Photonics Engineering, Yuan-Ze University, Taoyuan, Taiwan

Email: * hckuo@faculty.nctu.edu.tw

Abstract — We prepared CIGS thin films by pulsed laser

deposition (PLD), the pulsewidth of the laser sources are nanosecond(ns) and femtosecond(fs), respectively. We compared their surface morphologies by scanning electron microscopy images. Following, we analyzed their crystal structure utilizing X-ray diffraction, and Raman spectroscopy. Finally, the ultrafast carrier dynamics measured by optical pump-optical probe (OPOP) system. The results of these measurements reveal the better chalcopyprite structure in fs PLD CIGS. And we obtained lower defect-related non-radiative recombination rate in fs PLD CIGS by using OPOP spectroscopy, reflecting a better quality with higher energy conversion efficiency of them.

Index Terms —charge carrier lifetime, photovoltaic cells.

I. INTRODUCTION

CuIn1-xGaxSe2 (CIGS) has been commonly regarded as one of the most promising materials for low cost and high efficiency solar cell. The superior absorption property (α~105 cm-1) due to that it belongs to the family of direct band gap chalcopyrite semiconductor compound. And another advantage is the band gap can be engineered by the partial substitution of indium by gallium. Recently reported thin film CIGS-based solar cells have achieved efficiencies in excess of 20 % [1]. Currently, CIGS films have been fabricated by non-vacuum processes (electrochemical deposition [2] and ink printing, both with post-selenization) and vacuum processes (co-evaporation, sputtering with/without post-selenization [3,4], pulsed laser deposition[5]).Among these processes, pulsed laser deposition (PLD) provides flash deposition and well stoichiometric transfer from multicomponent targets. In the past, scientists deposited CIGS thin films utilizing excimer laser whose pulsewidth in ns order generally [6]. But rare researches discuss the effect of pulsewidth on preparing CIGS thin film via pulse laser deposition until now.

II. EXPERIMENTS

In this work, KrF excimer laser (λ= 248 nm) and Ti-sapphire femtosecond laser (λ= 800 nm) are employed as light sources during the CIGS thin film deposited process. Their pulsewidth

are 20 ns and 100 fs respectively. The composition of target is CuIn0.7Ga0.3Se2. Substrate is soda lime glass (SLG) with Mo-coating. The optimal growth condition is : laser energy density ~ 5 J/cm2, substrate temperature (Ts) ~500 °C, the distance between substrate and target is 5 cm, the background pressure was kept on 3×106 Torr. In general, the shorter pulsewidth will lead less thermal effect during the target exposed to the laser. When it is faster than ps order, the thermal energy could not be transferred through phonon. The mechanisms of ablating the target will be coulomb explosion rather than evaporation [7,8]. The difference between ns PLD and fs PLD shown in Figure 1. Fig. 1. Mechanisms of (a) ns PLD (b) fs PLD.

III. RESULT AND DISCUSSION

Figure 2 (a) and (b) shows the scanning electron image (top view) of ns PLD and fs PLD CIGS. A post-treatment utilizing KCN solution was performed to remove the Cu segregation and secondary phases in both samples. There is still obvious difference of surface roughness between them. It is attribute to the different mechanism of ablating target. As shown in Fig.1. The ns pulse laser will induce the thermal degradation of target due to the thermal energy transferred by lattice. The reacting period is usually in the order of ps. So there are more clusters in the plume. In contrast, the fs PLD have less thermal effect so that concentrated plume which is consist of nanoparticles. So the roughness of fs PLD CIGS thin film is much lower than

(a) (b)

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ns PLD CIGS. And the smooth surface has low shunt leakage because less surface recombination of carrier.

Fig. 2. SEM image (top view) of (a) ns PLD CIGS (b) fs PLD CIGS.

For analyzing the crystal structure, we measure the X-ray diffraction (Fig.3(a)) and Raman spectroscopy (Fig.3(b)).The XRD pattern exhibits the well polycrystalline structures of both samples. Apparently, the stronger non-polar orientation: (220) was found in PLD CIGS thin film. The past research indicated (220) orientation may improve CIGS-based solar cell performance [9]. And the Raman spectroscopy also reveals the better crystal quality in PLD CIGS thin film. We infer these results all related to the advantage of fs PLD, including: non-thermal ablating (coulomb explosion), nucleating deposition [7,8].

Fig.3. (a)X-ray diffraction pattern and (b)Raman spectroscopy of ns and fs PLD CIGS thin films.

Finally, we measure the ultrafast carrier dynamic utilizing the optical pump-optical probe technique. Femtosecond pump-probe spectroscopy is a powerful tool for investigating the dynamics of nonequilibrium carriers in semiconductors. Ultrafast relaxation processes on the time scales of femtoseconds to picoseconds can be directly measured using this technique. In our previous work, we found this technique provide viable ways for further improving the photovoltaic efficiency of the CIGS-based solar cells as well as optimizing thin film deposition conditions [10]. Fig. 4 shows the result of OPOP measurement. We pump the samples with different fluence for getting different photoexcited carrier density. And then we extract the defect-related non-radiative carrier recombination lifetime (it is independent to carrier density as the photoexcited density larger than density of defect) from the exponential decay. There are ~90 ps and ~190 ps in ns and fs

PLD CIGS, respectively. The longer lifetime reflecting less carrier recombination occurred in the same period, so that it may has better device performance [10]. This result consists with surface morphologies of these films. In addition, the hot phonon relaxation can be found in fs PLD CIGS (fast exponential decay, see Fig. 4 (b) inset).This relaxation usually dominated in OPOP spectrum when the sample has better crystal quality [10].

Fig. 4. Reflectivity transient for the (a) ns PLD and (b) fs PLD CIGS films. The inset of them shows the semi-log plot of the ∆R/R curve for a pump fluence of 0.83 mJ/cm2.

IV. SUMMARY

In this work, we prepared CIGS thin films by pulsed laser deposition, the pulsewidth of the laser sources are nanosecond and femtosecond, respectively. We analyzed their crystal structure utilizing X-ray diffraction, and Raman spectroscopy. Following, we compared their surface morphologies by scanning electron microscopy images. Finally, the ultrafast carrier dynamics measured by optical pump-optical probe system. By XRD patterns, we found the ratio I(220)/(204) /I (112)

are ~1/10 and ~1/100 in fs PLD CIGS and ns PLD CIGS, respectively. Other peaks also enhanced apparently in fs PLD CIGS. The results of RS measurement showed the A1 mode is stronger in fs PLD CIGS than ns PLD CIGS. It all reveal the better chalcopyprite structure in fs PLD CIGS. Lastly, ultrafast carrier dynamics in CIGS thin films are investigated by using OPOP spectroscopy. And we obtained lower defect-related non-radiative recombination rate in fs PLD CIGS, reflecting a better quality with higher energy conversion efficiency of them.

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