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Laser lift-off scribing of the CZTSe thin-film solar cells at different pulse durations
Solar Energy 150 (2017) 246–254
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Laser lift-off scribing of the CZTSe thin-film solar cells at different pulse durations Edgaras Markauskas a,⇑, Paulius Gecˇys a, Ingrid Repins b, Carolyn Beall b, Gediminas Racˇiukaitis a a b
Center for Physical Sciences and Technology, Savanoriu Ave. 231, LT-02300 Vilnius, Lithuania National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401, USA
a r t i c l e
i n f o
Article history: Received 14 June 2016 Received in revised form 25 January 2017 Accepted 29 January 2017
Keywords: Kesterite CZTSe Lift-off Laser ablation
a b s t r a c t The transition to fully sized solar modules requires additional three-step laser structuring processes to preserve small-scale cell efficiencies over the large areas. The adjacent cell isolation (the P3 scribe) was found to be the most sensitive process in the case of laser induced damage. The laser induced layer lift-off mechanism seems to be a very attractive process for the P3 patterning, since almost all the laser affected material is removed by mechanical spallation. However, a laser induced layer spallation behavior together with scribe electrical validation under the different laser pulse durations was not investigated extensively in the past. Therefore, we report our novel results on the P2 and P3 laser lift-off processing of the Cu2ZnSn(S, Se4) (CZTSe) thin-film solar cells covering the pulse duration range from 300 fs to 60 ps. Shorter sub-ps pulses enabled us to process smaller P2 and P3 craters, although the lift-off threshold fluences were higher compared to the longer ps pulses. In the case of the layer lift-off, the laser radiation had to penetrate through the layer stack down to the CZTSe/Mo interface. At shorter sub-ps pulses, the nonlinear effects triggered absorption of the laser radiation in the bulk of the material, resulting in increased damage of the CZTSe layer. The Raman measurements confirmed the CZTSe surface stoichiometry changes for shorter pulses. Furthermore, shorter pulses induced higher electrical conductivity of a scribe, resulting in lower photo-electrical efficiency during the mini-module simulation. In the case of the P3 lift-off scribing, the 10 ps pulses were more favorable than shorter femtosecond pulses. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction The Cu2ZnSn(S, Se4) (CZTSe) technology is considered as cheap, earth abundant, and environmentally friendly solar cell technology showing the record efficiency of 12.6% (Wang et al., 2013b). Furthermore, the development is mostly focused on exploiting lowcost deposition processes (Clauwaert et al., 2016; Lin et al., 2015; Tao et al., 2016). Usually, record solar cell efficiencies are set on small-area devices. However, the transition to fully-sized solar panels requires additional production steps to reduce the ohmic losses caused by high generated photocurrents in thin films (Gecys et al., 2012). Patterning of thin-film CIGS (Gecys et al., 2012; Kessler and Rudmann, 2004; Nishiwaki et al., 2015; Westin et al., 2008), CZTSe (Gecys et al., 2014; Gecˇys et al., 2014), GaInP (Weber et al., 2016), a-Si (Bovatsek et al., 2010; García-Ballesteros et al., 2011; Ku et al., 2013), and CdTe (Bosio ⇑ Corresponding author. E-mail addresses: [email protected] (E. Markauskas), [email protected] (P. Gecˇys), [email protected] (I. Repins), [email protected] (C. Beall), [email protected] (G. Racˇiukaitis). http://dx.doi.org/10.1016/j.solener.2017.01.074 0038-092X/Ó 2017 Elsevier Ltd. All rights reserved.
et al., 2014) solar cells was thoroughly investigated in the past. A novel perovskite technology has recently received attention in this field showing the importance of optimization of the serial interconnects (Soo-Jin et al., 2015). The generated photocurrent is proportional to the active area of the solar cell (O’Keefe et al., 2010). Consequently, the photocurrent can be limited by dividing the large area module into smaller cells interconnected in series. Ideally, the voltage is proportionally increased in order to maintain the same generated power. Such approach allows us to bypass the conductivity limitations of thin layers, therefore preserving the solar cell efficiency. Execution of a single serial monolithic interconnect requires 3 high-precision laser scribes. The P1 scribe is needed for the backcontact patterning, the P2 forms the series interconnect while the third P3 scribe is needed for isolation of the neighboring cells. Normally, the width of a patterned cell lies within the range of 5– 10 mm including the dead zone (Dunsky and Colville, 2008). Consequently, it results in 100–200 interconnects per single 1 m2 solar module. Commonly, laser P3 scribing is based on the so-called direct laser ablation process to expose the molybdenum back-contact
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or the removal of the transparent conductive oxide (TCO) only. In both cases laser radiation interaction with thin layers can induce thermal damage to the absorber, such as metallization and formation of secondary phases that can short-circuit the cells (Mitzi et al., 2011; Westin et al., 2008, 2011). These effects can be minimized if only TCO ablation process is used for the P3 patterning (Burn et al., 2013). However, the melting of the exposed absorber cannot be avoided completely. Instead of two previously discussed techniques, we further investigate the entire front-side laser-induced mechanical layer spallation (lift-off) for P2 and P3 processes (Gecys et al., 2014). The method includes the complete removal of constituent layers exposing the molybdenum back-contact. The laser modified layers are removed mostly in the solid phase by a laser induced layer spallation. Such technological approach can be one of the key techniques for sufficient increase of the patterning speeds and scribing quality. However, it is crucial to understand the physics of the liftoff process in order to minimize the induced laser damage to the solar cell material. A lot of effort was applied to study the material layer spallation (lift-off) experimentally (Buzás and Geretovszky, 2012; Domke et al., 2012; Heise et al., 2012; Lee et al., 2013a,b) and numerically (Buzás and Geretovszky, 2012; Sotrop et al., 2013; Wang et al., 2013a) in thin-film solar cell-like structures by different research groups. However, the underlying physics are still not completely clear. Sotrop et al. (2013) performed numerical simulations of femtosecond laser induced back-side ablation of thin molybdenum layers deposited on the transparent glass substrate. The driving force of the layer spallation was the ultra-fast thermal expansion of molybdenum layer caused by the laser radiation absorption. Furthermore, it resulted in a shock wave traveling into the backsurface and reflecting from the glass substrate towards the Mo. Earlier, we simulated a ps-laser radiation absorption in the kesterite solar cell. We showed that most of the laser energy was coupled at the CZTSe/Mo interface (Gecys et al., 2014). In addition, Scragg et al. (2012) reported that the CZTSe film becomes thermodynamically unstable at 550 °C and loses around 50% of its original Se content. As a consequence, the elevated temperatures cause the degradation of the material. It may lead to a reduction of the adhesion forces and minor pressure increase of Se vapor further assisting the layer spallation. That was also confirmed by Buzás and Geretovszky (2012). The authors claim that the melted and vaporized interface after the laser pulse irradiation reduces the adhesion between the layers and, therefore, determines the size of ablated craters in CIGS solar cells. On the other hand, our previous investigations showed that each layer in a solar cell above the backcontact absorbed part of the laser energy (Gecys et al., 2014). The absorption may trigger ultra-fast thermal expansion and may induce bulging and tension in the absorber layer. Considering previous statements, more comprehensive study is needed to determine the physics behind the layer lift-off mechanisms. Furthermore, technological aspects of such processes are also very important in the case of solar module scribing. The laser pulse duration plays a crucial role in the laser-matter interaction, therefore it is necessary to optimize layer spallation in terms of laser pulse duration. Laser-induced lift-off processing with ns (Buzás and Geretovszky, 2012; Lee et al., 2013b) and ps (Heise et al., 2011) pulse durations was shown by several research groups in CIGS samples. On the contrary, the CZTSe samples were investigated only with ps pulses (Gecys et al., 2014). However, no comprehensive study of layer lift-off process and scribe electrical validation was made covering ps and sub-ps pulse durations. Therefore, in this paper, we investigated the role of the laser pulse duration in the case of P2 and P3 laser lift-off processing of the CZTSe thin-film solar cells. Several laser sources were used to cover
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the pulse duration range from 300 fs to 60 ps. We studied the laser pulse duration effects on the threshold fluence required to expose the back-contact layer. Morphology and the structure of ablated craters were investigated by using scanning electron microscope (SEM). The micro-Raman spectroscopy has been employed in order to investigate the structural changes and secondary phase formation at the laser ablated crater edge. Finally, we performed the P3 lift-off scribing followed by the parallel scribe conductivity validation and mini-module simulations. 2. Experimental details Multiple laser sources were utilized in the experiments to cover a range of laser pulse durations from pico- to up to femtoseconds. Two picosecond lasers from Ekspla – Atlantic and Atlantic HE provided 10 and 60 ps pulses at 1064 nm wavelength, respectively. A Pharos laser (from Light Conversion) was utilized to exhibit 300 fs and 1 ps pulses at 1030 nm wavelength. Output pulse duration was tuned by an integrated compressor employing two transmission diffraction gratings. Afterwards, preselected pulse durations were verified with an intensity autocorrelator. The experimental setup was very similar in all used systems: the laser itself, an electro-optical shutter, the beam expander, and the sample/beam positioning system which was realized either by a galvanometer scanner (Scangine14 Scanlab) or by linear XY translation stages (Aerotech). The focusing objective with a focal length of 80 mm was utilized in the experiments resulting in a minimal diffraction-limited spot size of 23 mm. The spot size on the sample surface was controlled by shifting the sample out of the focal plane using Z-axis translation stages (Standa). The sample was always shifted above the focal plane perpendicularly to the beam propagation direction, keeping the focal spot below the processed surface. The quality of the ablated areas was examined with SEM (JSM6490LV JEOL). Characterization of material modifications was carried out by a micro-Raman microscope (nVia Reninshaw) equipped with 633 nm laser. The laser spot size and the spatial resolution were estimated to be 1 mm and 0.5 mm, respectively. Laser power was set to 1 mW and the signal was accumulated for 100 s. Furthermore, a linear laser scribing technique (LLST) (Markauskas et al., 2015) was utilized to extract the laser scribe conductivity. The solar cell’s parallel conductance was measured after each laser scribe by the 4-point probe I-V measurement system (2602A Keithley) connected to a sample. This way, the dependence of the solar cell’s parallel conductance on the laser-scribed length was obtained. Finally, by fitting the results to the proper fitting function provided in Markauskas et al. (2015), we were able to extract laser scribe parallel conductivity. Two types of kesterite thin-film solar cell samples were investigated: (a) a P3 structure consisting of a soda-lime-glass (SLG) substrate, molybdenum (Mo) back-contact, CZTSe absorber, CdS buffer layer, and ZnO front-contact comprised of intrinsic (i-ZnO) and conductive (ZnO:Al) oxide layers; and (b) a P2 structure excluding the front-contact and buffer layers (see Fig. 1). More detailed information on the sample preparation and composition is described in Repins et al. (2012). 3. Results 3.1. Lift-off crater ablation in CZTSe samples Single pulse front-side crater ablation experiments were conducted in P2 and P3 structures of the CZTSe solar cell samples at investigated laser pulse duration range. Typical SEM images of the lift-off craters in CZTSe solar cells are shown in Fig. 2. Our previous investigations have demonstrated that sufficient laser pulse
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Fig. 3. The minimum diameter of exposed molybdenum craters versus the laser pulse duration. Fig. 1. Cross-section schemes of CZTSe thin-film solar cell samples used in the experiments: P3 (a) and P2 (b) structures are presented.
energy must be applied for a given laser spot size to induce layer spallation (Gecys et al., 2013; Gecˇys et al., 2014). However, the irradiated spot cannot be sharply focused since it is crucial to avoid melt pool formation in the case of efficient lift-off ablation (Gecys et al., 2014). This effect limited the smallest size of the exposed crater in the CZTSe structures at the 10 ps pulse duration range (Gecys et al., 2014). For this, we investigated the influence of laser pulse duration on the minimum diameter of exposed molybdenum crater. The laser pulse energy was set to 50 mJ and the spot size was controlled by shifting the sample above the focal plane perpendicularly to the propagation direction of the laser beam. The sample shift was identified as the defocus parameter or focus offset which was described in Gecys et al. (2014). It was noticed that in the case of the P2 craters, the minimal crater size and pulse duration relationship was linear in the logarithmic scale (see Fig. 3). Shorter pulses enabled us to form smaller diameter craters. The minimum P2 crater diameter was 62 mm after the irradiation of 300 fs laser pulse. In order to form such a crater, the laser focal spot was shifted by 0.6 mm out of the focal plane, which corresponded to the fluence of 7.28 J/cm2. A transition to 1 ps range resulted in 74.8 mm crater size at 0.8 mm focus offset (4.79 J/ cm2). 10 ps pulses facilitated the formation of 83.3 mm craters at 1 mm focus offset (2.87 J/cm2). Longest 60 ps pulses produced the biggest craters – the minimum exposed molybdenum area reached a diameter of 117.7 mm at 1.8 mm sample focus offset (1.04 J/cm2).
In the case of a P3 structure, the crater size dependence on the pulse duration was less straightforward (see Fig. 3). Additional ZnO and CdS layers deposited on top of the kesterite absorber increased the structure complexity. Sub-picosecond (300 fs) laser pulses produced 60.2 mm craters at focus offset of 0.9 mm (3.62 J/cm2). Longer 1 ps pulses resulted in the minimal crater size of 48.1 mm (focus offset 0.7 mm, laser fluence of 6.5 J/cm2). A transition to the 10 ps regime produced 56 mm craters at 1 mm defocus and a laser fluence of 3 J/cm2. Finally, the utilization of the longest pulses resulted in a further increase in the crater diameter reaching 96 lm at 2.1 mm focus offset (0.75 J/cm2). Based on the previous statements, we believe that the shorter laser-material interaction (shorter pulses) caused lower thermal laser energy diffusion to the bulk of the material. It resulted in a sharper thermal transition to the cold area of the material and induced higher stresses to the layers. On the contrary, according to Griffith’s criterion (Buzás and Geretovszky, 2012), the lack of sufficient tensile stress required to initiate the spallation can be compensated by a larger delamination area. In the case of long laser pulses (sub-ns), the residual CZTSe/Mo layer adhesion force, together with CZTSe layer fracture strength, can be exceeded only for craters with bigger diameters (see Fig. 3).
3.2. Punching threshold in CZTSe samples The minimum laser fluence required to initiate a clean molybdenum layer exposure is referred as the punching threshold. We observed an inverse relationship between the punching threshold
Fig. 2. SEM images of typical lift-off craters ablated in P2 (a) and P3 (b) structures. Laser fluence of 3.1 J/cm2 and 1 mm defocus was used for 10 ps pulse irradiation in both structures.
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Fig. 6. SEM cross-section images of craters ablated with 10 ps pulses in P2 (a) and P3 (b) structures. Ablation parameters: 2.2 J/cm2 fluence at 1.2 mm defocus.
Fig. 4. Punching threshold fluence versus the laser pulse duration.
and the pulse duration (see Fig. 4). Highest thresholds were obtained for 0.3–1 ps pulses. The peak values were 0.48 and 0.92 J/cm2 for P2 and P3 structures, respectively. The relative difference between P2 and P3 processes reached 52%. Transition to longer pulses reduced both the threshold difference between the two cases and the values itself. The lift-off process was most efficient for 60 ps pulses. Working at these conditions, the smallest threshold value of 0.2 J/cm2 was obtained for both structures. One of the driving mechanisms for the CZTSe lift-off processing is the reduction of the adhesion forces and a minor Se vapor pressure increase at the CZTSe/Mo interface (Gecys et al., 2014). For this, a sufficient amount of laser energy has to be coupled at the CZTSe/Mo interface. However, reduced pulse duration can induce higher nonlinear absorption in the bulk of the removable layers. In such way, a smaller amount of laser energy can reach the interface and, as a result, an increased punching threshold was observed for shorter pulses.
3.3. Lift-off crater morphology SEM was used to evaluate the quality of molybdenum craters fabricated at different pulse durations. Fig. 5a shows front-side views of typical craters formed in a P2 structure. The process quality was very similar despite the different pulse duration. Pulse duration variation did not induce any melt or burr formation at the perimeter of the crater, as well. The use of femtosecond pulses did not provide any noticeable advantage over longer pulses in the picosecond time range. Front-side images of the P3 craters are shown in Fig. 5b. Changes induced by the pulse duration were observed at the surface of the exposed CZTSe, which are evident as a ring-like structure. Transition to longer pulses caused more intense melting of the exposed surface layer. Sub-ps pulses allowed us to preserve the exposed layer topography similar to unaffected areas. However, the transition to 10 ps and 60 ps pulses resulted in visible thermal damage to the exposed kesterite surface. The cross-section views of P2 and P3 crater edges are shown in Fig. 6. A white dotted line separates the exposed molybdenum area
Fig. 5. SEM images of the exposed molybdenum craters in P2 (a) and P3 (b) structures. Laser fluence of 2.2 J/cm2 and 1.2 mm defocus was used for 300 fs, 1 ps and 10 ps pulse durations. In the case of 60 ps pulses, the laser fluence was reduced to 0.83 J/cm2 at 2 mm defocus.
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(to the right of the dotted line) and the surrounding area (to the left of the dotted line). The purple1 area shows the pre-deposited protective platinum (Pt) layer used in a sample preparation by focused ion beam (FIB). In the case of the P2 crater (see Fig. 6a), no damage to the absorber or the back-contact was observed near the crater edge. On the other hand, a thin layer of melted absorber in the exposed kesterite ring area was observed in the P3 structure (see Fig. 6b, marked in yellow1). The thickness of the affected layer was measured to be between 0.25 and 0.3 mm, which was about one-fourth of the CZTSe layer. No direct contact between melted absorber and the molybdenum layer was observed, reducing the probability of short-circuiting the cell. However, the laser process induced minor cracking and delamination of the absorber layer visible in Fig. 6b near the inner edge of the exposed crater. The depth of the defects was limited to 3 mm. Melting of the exposed kesterite ring around the ablated Mo crater was observed in the case of multiple layer ablation by a lift-off process. Earlier, we simulated picosecond laser pulse absorption in CZTSe P3 structure showing high temperature peak at the ZnO top-contact (Gecys et al., 2014). Therefore, this effect can be related to increased laser absorption near the CZTSe surface. This way, the P3 crater ring modifications can follow classical ablation behavior, where shorter 300 fs pulses induced less CZTSe surface melting compared to longer 10 ps pulses. However, we have to consider, that only part of the laser radiation was absorbed at the top-layers, and the other part penetrated through the absorber layer down to the Mo (Gecys et al., 2014).
3.4. Raman analysis We used micro-Raman spectroscopy to indicate the structural changes and secondary phase formation induced by a laser process in CZTSe thin-film solar cell samples. First of all, the laser nonaffected areas were analyzed. Further, Raman measurements were conducted 5 mm away from the laser ablated crater edge in a P2 structure (see Fig. 7a). In the case of a P3 structure, the measurements were taken in the center of the exposed kesterite ring (see Fig. 7b). The distance from the inner edge of the crater was about 5 mm. Laser non-affected spectra are indicated as ‘‘0” and are shown in Fig. 8a and b. Both spectra exhibited typical CZTSe-pronounced peaks appearing at 172 cm1 (Nam et al., 2014) and 196 cm1 (Grossberg et al., 2011). Considerably lower peak intensity was observed at 233 cm1 (Grossberg et al., 2011), which is assigned to CZTSe. In the case of secondary phases, only a low-intensity ZnSe peak was observed at 251–252 cm1 (Salomé et al., 2009; Uday Bhaskar et al., 2013; Venkatachalam et al., 2007). Raman spectra measured in a P2 structure are presented in Fig. 8a. Spectra labeled 1–4 denote laser pulse duration from 60 ps to 300 fs, respectively. An impact of laser pulse duration was clearly visible. The valley between two main CZTSe peaks (172 and 196 cm1) started to rise when the shorter pulses were used. Furthermore, the use of 300 fs pulses caused a fusion into a single asymmetric peak. The effect can be caused by increasing structural disorder and intensive formation of CTSe (Cu2SnSe3) phases at 180 cm1 (Nam et al., 2014). It is a low-band-gap (0.8 eV) secondary phase which can limit the open circuit voltage of the solar cell (Siebentritt, 2013). CZTSe peak broadening at 234 cm1 was observed with a decrease in the pulse duration until it became indistinguishable in 300 fs range (see Fig. 8a). We noticed the appearance of a low1 For interpretation of color in Fig. 6, the reader is referred to the web version of this article.
Fig. 7. SEM images indicating Raman measurement spots in P2 (a) and P3 (b) structures.
intensity peak at 231 cm1 which can be related to CTSe and/or CZTSe phases (Nam et al., 2014). Raman spectra measured at the center of exposed kesterite ring in a P3 structure are shown in Fig. 8b. Significant changes of the CZTSe Raman signal was observed for all inspected laser pulse durations. Such behavior could be related to the increase of structural disorder and formation of the CTSe secondary phase. Furthermore, the characteristic CZTSe low-intensity peak at 234 cm1 was not found. Heat accumulation near the CZTSe surface was responsible for additional modifications in Raman spectra. Therefore, even for the longer pulses material stoichiometry changes were observed. However, we have to consider that only the part of the laser radiation was absorbed at the top layers of the P3 structure. Remaining laser energy penetrated down to the CZTSe/Mo interface removing the absorber layer in a solid state (Gecys et al., 2014). Shorter pulses induced higher nonlinear absorption in the CZTSe layer resulting in significant bulk modifications. This effect was clearly observed in case of a P2 structure, where no additional top-layers were deposited. We are convinced, that both P2 and P3 structures suffered from bulk modifications. However, CZTSe surface modifications were introduced only in the P3 structure.
3.5. P3 scribe conductivity measurements P3 lift-off trench scribing experiments were conducted in a P3 structure comprising of slightly thinner CZTSe absorber layer (0.8 mm). To maintain the experimental conditions as constant as possible, a single ablation regime was optimized for laser pulses of 300 fs, 1 ps, and 10 ps durations. Laser fluence was set to 2.34 J/cm2, scanning speed to 10 mm/s, and the repetition rate to 310 Hz. It corresponded to a pulse overlap of 2.3%. Finally, it allowed us to achieve high scribing quality: the patterns had sharp edges forming a continuous, uninterrupted trench (see Fig. 9). The LLST was utilized to extract parallel conductivity values of the laser scribe (Markauskas et al., 2015). The results showed the inverse dependency of the P3 scribe conductivity on the laser pulse duration (see Fig. 10). Best results were achieved while scribing with longest 10 ps duration pulses - 0.64 S/m. A transition to shorter pulses showed inferior results: the parallel conductivity increased to 0.78 S/m and 0.94 S/m for 1 ps and 300 fs pulses, respectively. In this case, shorter pulses induced more damage which resulted in more significant shunting of the solar cell. According to the Raman measurements, shorter pulses caused more pronounced structural disorder in the bulk of the material and the formation of p-type (CTSe) secondary phase near the ablated craters (Fig. 8b). In case of CIGS solar cells, it was reported that the partial absorber evaporation can lead to the formation of Cu-rich compound, which should be also valid for CZTSe absorber material at elevated temperatures (Redinger and Siebentritt, 2010; Westin et al., 2008). Increasing Cu content in the CZTSe compound
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(a)
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(b)
Fig. 8. Raman spectra of the CZTSe layer measured in P2 (a) and P3 (b) structures 5 mm away from the exposed molybdenum crater edge. The numbers from 1 to 4 correspond to the used laser pulse duration. Non-affected kesterite spectrum is denoted as 0.
gives rise to higher carrier concentration and conductivity values (Olgar et al., 2016). Possibly, this resulted in P3 lift-off scribe conductivity increase for shorter laser pulses. 3.6. Mini-module simulation
Fig. 9. SEM image of the P3 scribe in a P3 structure ablated with 300 fs pulses. Laser fluence of 2.34 J/cm2 at 0.4 mm focus offset was used with translation speed of 10 mm/s.
Further, we conducted a 3-cell CZTSe mini-module simulation under standard test conditions. The purpose of simulations was to evaluate the possible efficiency losses caused by the laser process. The equivalent circuit of the mini-module is shown in Fig. 11. Here, V is the voltage across the output terminals of the minimodule, Rs is the series resistance, Rp is the ohmic and Rp3 is the laser scribe induced shunts. Diode and the photo-generated currents are denoted as ID and Iph, respectively. Generally, a solar cell can be described by a single diode equation:
V þ IRs I ¼ ISC A I0 eqðVþIRs Þ=nkT 1 ; Rp
Fig. 10. Extracted P3 scribe conductivity versus laser pulse duration. Laser lift-off scribing was realized in a P3 structure.
ð1Þ
which is comprised of a diode saturation current I0, ideality factor n, short circuit current Isc measured at a particular solar irradiance, and operating temperature T. Active solar cell area is A, and the output current is I. Elementary charge and Boltzmann constants are described as q and k, respectively. The model considered only the P3 scribe neglecting the influence of P1 and P2 interconnects. Therefore, additional resistors Rp3 must be introduced to include the laser induced P3 scribing damage to solar cells No. 2 and 3. Each cell was described by an individual I-V equation and implemented as a separate subcircuit in PSpice modeling software. Parameters of the 9.7% efficiency DC-sputtered CZTSe solar cell were borrowed from Brammertz et al. (2013). We chose to simulate 1 cm2 area square-shaped solar cells determining the laser scribe length and distance to 1 cm. Finally, the active area loss caused by the width of the serial interconnects was not accounted. Extracted efficiency of 9.76% correlated well with the experimentally measured value in Brammertz et al. (2013). Fig. 12 illustrates the dependence of simulated 3-cell mini module efficiency on P3 scribe conductivity. The scribes performed
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Fig. 11. The equivalent model of the 3-cell CZTSe mini-module.
Fig. 12. Simulated 3-cell CZTSe mini-module efficiency dependence on P3 scribe conductivity.
Table 1 The simulated efficiency of a 3-cell CZTSe mini module based on experimentally measured P3 scribe parallel conductivity. Pulse duration
Scribe conductivity, S/ m
Module efficiency, % (absolute)
Efficiency loss, % (relative change)
300 fs 1 ps 10 ps
0.94 0.78 0.64
9.2 9.32 9.42
6.1 4.7 3.6
with 10 ps duration pulses caused the smallest efficiency losses of just 3.6% relative (see Table 1). The mean value obtained for 1 ps pulses caused efficiency losses of 4.7% relative. Use of the shortest pulses induced the highest losses, reaching the maximum value of 6.1% relative for 300 fs. Overall, the transition from 10 ps pulses to femtosecond pulses (300 fs) decreased the absolute module efficiency from 9.42% to 9.2%. 4. Discussion According to our investigations, shorter pulses induced smaller diameter lift-off craters, however, the threshold fluences were higher. From the ablation experiments and Raman investigations we can speculate that the nonlinear absorption of the laser radiation was more pronounced in the bulk of the CZTSe layer for sub-ps laser pulses. This triggered a more intense thermal expansion of the absorber layer in a confined area. Consequently, the laser induced internal stress facilitated the mechanical rupture of the CZTSe. As a result, the size of the craters was reduced. This effect may become more dominant in layer lift-off processes at even shorter sub-ps pulse durations. At the picosecond pulse duration range, the nonlinear absorption effects may become less pronounced. Longer pulses result in
more prominent thermal diffusion into the material, lowering induced stresses to the layers. On the other hand, the laser beam absorption at the CZTSe/Mo interface may become dominant. Therefore, an increased interface heating will lead to further reduction of the layer adhesion forces and minor Se vapor build-up. Consequently, the laser induced vapor pressure will punch the absorber layer. These effects result in lower punching threshold values at the expense of larger craters. Therefore, the CZTSe layer lift-off mechanism could be the combination of both – the fast expansion of the absorber layer and the laser beam absorption at the CZTSe/Mo interface. Both mechanisms are inseparable, only domination of one or the other may be defined by the laser pulse duration. Together, these effects could induce sufficient forces and strain to initiate laser-induced thermo-mechanical layer spallation and, therefore, the exposure of molybdenum back-contact. Mostly femtosecond laser pulses are associated with low thermal effects, smaller heat affected zone and higher ablation quality. Furthermore, the CIGS material ablation thresholds are lower for shorter pulses due to enhanced nonlinear absorption (Hermann et al., 2006). It applies to the direct layer ablation process. However, our investigation revealed the opposite in the case of laser induced lift-off ablation, since the material was removed mostly in the solid state. No surface melting was observed for P2 craters and only 0.3 mm deep P3 crater edge modification was measured for CZTSe surface. The P3 structure have additional top-contact layers, therefore the laser radiation absorption in such structure is more complex. During the laser exposure, CZTSe surface gets additional modification due to top-contact heat accumulation, which is related to direct absorption. Therefore, CZTSe surface melting is less pronounced in case of shorter pulses. However, we have to consider, that most part of the laser radiation penetrated through the layers down to CZTSe/Mo interface. Due to increasing peak intensities, nonlinear effects become more pronounced, resulting in higher laser absorption in the bulk of the CZTSe absorber (Musazzi and Perini, 2014). Consequently, it results in more significant bulk modification of the absorber layer. Likely, it is the reason why we observed higher lift-off scribe conductivity for shorter pulses. The simulated module efficiencies were lower for short pulse laser lift-off processing. However, best module efficiencies were obtained for the 10 ps laser pulses. Such pulse duration is typical for industrial picosecond lasers. Consequently, implementation of such technology in the industrial environment becomes less complex, since industrially accepted tools for lift-off processing are available. However, the method is sensitive to the absorber material properties variation, since the laser light has to penetrate through the absorber layer. This might be the biggest drawback of this scribing technology, since it is difficult to precisely control the CZTSe deposition processes over the large area. 5. Summary We realized the lift-off ablation process in P2 and P3 structures of thin-film CZTSe solar cells by utilizing the laser pulses of 300 fs, 1 ps, 10 ps, and 60 ps durations. Strong nonlinear laser absorption
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effects in the CZTSe layer were observed for shortest laser pulses. The minimum exposed crater diameter of 48 mm was obtained in a P3 structure with 1 ps duration pulses. In case of a P2 structure, a slightly larger 62 mm diameter crater was exposed with 300 fs pulses. This suggests strong effect of laser light absorption in CZTSe triggering fast expansion of the absorber layer in a confined area. However, the punching threshold of 0.2 J/cm2 for both P2 and P3 structures was lowest for the long 60 ps pulses at the expense of bigger crater sizes. Reduced nonlinear absorption in CZTSe triggered more intense heating at the CZTSe/Mo interface. As a result, the interface adhesion forces were reduced followed by a vapor pressure increase. Therefore, the CZTSe layer lift-off mechanism could be the combination of both – the fast expansion of the absorber layer and the laser beam absorption at the CZTSe/Mo interface. Together, these effects could induce sufficient forces and strain to initiate laser-induced thermo-mechanical layer spallation and, therefore, the exposure of molybdenum back-contact. Further, the Raman measurements revealed significant structural modifications, including the most intensive CTSe phase formation, as well as, the broadening of main CZTSe peaks for shorter sub-ps laser pulses. Electrical measurements were also in favor of longer picosecond laser pulses. The lowest P3 lift-off scribe conductivity of 0.64 S/m was achieved for 10 ps pulses. The utilization of shorter durations notably increased the conductivity to up to 0.94 S/m for 300 fs pulses. The simulation of 3-cell CZTSe mini-module revealed that the device’s efficiency losses could be minimized to 3.46% relative if the P3 lift-off scribes are realized with 10 ps pulses. Overall, the laser-induced layer mechanical spallation is a promising method for thin-film solar cell processing. However, the method is sensitive to the absorber material properties variation. Therefore, comprehensive study is required to validate the complete absorber liftoff process under industrial processing conditions. Acknowledgements The research leading to these results was partially funded by the European Union FP7 Programme under grant agreement no. 609355 (APPOLO). References Bosio, A., Sozzi, M., Menossi, D., Selleri, S., Cucinotta, A., Romeo, N., 2014. Polycrystalline CdTe thin film mini-modules monolithically integrated by fiber laser. Thin Solid Films 562, 638–647. Bovatsek, J., Tamhankar, A., Patel, R.S., Bulgakova, N.M., Bonse, J., 2010. Thin film removal mechanisms in ns-laser processing of photovoltaic materials. Thin Solid Films 518, 2897–2904. Brammertz, G., Buffière, M., Oueslati, S., ElAnzeery, H., Ben Messaoud, K., Sahayaraj, S., Köble, C., Meuris, M., Poortmans, J., 2013. Characterization of defects in 9.7% efficient Cu2ZnSnSe4-CdS-ZnO solar cells. Appl. Phys. Lett. 103, 163904. Burn, A., Muralt, M., Pilz, S., Romano, V., Witte, R., Frei, B., Buecheler, S., Nishiwaki, S., Krainer, L., 2013. All fiber laser scribing of Cu(In, Ga)Se2 thin-film solar modules. Phys. Procedia 41, 713–722. Buzás, A., Geretovszky, Z., 2012. Nanosecond laser-induced selective removal of the active layer of CuInGaSe2 solar cells by stress-assisted ablation. Phys. Rev. B 85, 245304. Clauwaert, K., Binnemans, K., Matthijs, E., Fransaer, J., 2016. Electrochemical studies of the electrodeposition of copper-zinc-tin alloys from pyrophosphate electrolytes followed by selenization for CZTSe photovoltaic cells. Electrochim. Acta 188, 344–355. Domke, M., Rapp, S., Schmidt, M., Huber, H., 2012. Ultra-fast movies of thin-film laser ablation. Appl. Phys. A 109, 409–420. Dunsky, C., Colville, F., 2008. Solid state laser applications in photovoltaics manufacturing. Proc. SPIE 6871, 687129. Heise, G., Domke, M., Konrad, J., Sarrach, S., Sotrop, J., Huber, H.P., 2012. Laser lift-off initiated by direct induced ablation of different metal thin films with ultrashort laser pulses. J. Phys. D Appl. Phys. 45, 315303. García-Ballesteros, J.J., Torres, I., Lauzurica, S., Canteli, D., Gandía, J.J., Molpeceres, C., 2011. Influence of laser scribing in the electrical properties of a-Si: H thin film photovoltaic modules. Sol. Energ. Mater. Sol. Cells 95, 986–991.
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Solar Energy journal homepage: www.elsevier.com/locate/solener
Laser lift-off scribing of the CZTSe thin-film solar cells at different pulse durations Edgaras Markauskas a,⇑, Paulius Gecˇys a, Ingrid Repins b, Carolyn Beall b, Gediminas Racˇiukaitis a a b
Center for Physical Sciences and Technology, Savanoriu Ave. 231, LT-02300 Vilnius, Lithuania National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401, USA
a r t i c l e
i n f o
Article history: Received 14 June 2016 Received in revised form 25 January 2017 Accepted 29 January 2017
Keywords: Kesterite CZTSe Lift-off Laser ablation
a b s t r a c t The transition to fully sized solar modules requires additional three-step laser structuring processes to preserve small-scale cell efficiencies over the large areas. The adjacent cell isolation (the P3 scribe) was found to be the most sensitive process in the case of laser induced damage. The laser induced layer lift-off mechanism seems to be a very attractive process for the P3 patterning, since almost all the laser affected material is removed by mechanical spallation. However, a laser induced layer spallation behavior together with scribe electrical validation under the different laser pulse durations was not investigated extensively in the past. Therefore, we report our novel results on the P2 and P3 laser lift-off processing of the Cu2ZnSn(S, Se4) (CZTSe) thin-film solar cells covering the pulse duration range from 300 fs to 60 ps. Shorter sub-ps pulses enabled us to process smaller P2 and P3 craters, although the lift-off threshold fluences were higher compared to the longer ps pulses. In the case of the layer lift-off, the laser radiation had to penetrate through the layer stack down to the CZTSe/Mo interface. At shorter sub-ps pulses, the nonlinear effects triggered absorption of the laser radiation in the bulk of the material, resulting in increased damage of the CZTSe layer. The Raman measurements confirmed the CZTSe surface stoichiometry changes for shorter pulses. Furthermore, shorter pulses induced higher electrical conductivity of a scribe, resulting in lower photo-electrical efficiency during the mini-module simulation. In the case of the P3 lift-off scribing, the 10 ps pulses were more favorable than shorter femtosecond pulses. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction The Cu2ZnSn(S, Se4) (CZTSe) technology is considered as cheap, earth abundant, and environmentally friendly solar cell technology showing the record efficiency of 12.6% (Wang et al., 2013b). Furthermore, the development is mostly focused on exploiting lowcost deposition processes (Clauwaert et al., 2016; Lin et al., 2015; Tao et al., 2016). Usually, record solar cell efficiencies are set on small-area devices. However, the transition to fully-sized solar panels requires additional production steps to reduce the ohmic losses caused by high generated photocurrents in thin films (Gecys et al., 2012). Patterning of thin-film CIGS (Gecys et al., 2012; Kessler and Rudmann, 2004; Nishiwaki et al., 2015; Westin et al., 2008), CZTSe (Gecys et al., 2014; Gecˇys et al., 2014), GaInP (Weber et al., 2016), a-Si (Bovatsek et al., 2010; García-Ballesteros et al., 2011; Ku et al., 2013), and CdTe (Bosio ⇑ Corresponding author. E-mail addresses: [email protected] (E. Markauskas), [email protected] (P. Gecˇys), [email protected] (I. Repins), [email protected] (C. Beall), [email protected] (G. Racˇiukaitis). http://dx.doi.org/10.1016/j.solener.2017.01.074 0038-092X/Ó 2017 Elsevier Ltd. All rights reserved.
et al., 2014) solar cells was thoroughly investigated in the past. A novel perovskite technology has recently received attention in this field showing the importance of optimization of the serial interconnects (Soo-Jin et al., 2015). The generated photocurrent is proportional to the active area of the solar cell (O’Keefe et al., 2010). Consequently, the photocurrent can be limited by dividing the large area module into smaller cells interconnected in series. Ideally, the voltage is proportionally increased in order to maintain the same generated power. Such approach allows us to bypass the conductivity limitations of thin layers, therefore preserving the solar cell efficiency. Execution of a single serial monolithic interconnect requires 3 high-precision laser scribes. The P1 scribe is needed for the backcontact patterning, the P2 forms the series interconnect while the third P3 scribe is needed for isolation of the neighboring cells. Normally, the width of a patterned cell lies within the range of 5– 10 mm including the dead zone (Dunsky and Colville, 2008). Consequently, it results in 100–200 interconnects per single 1 m2 solar module. Commonly, laser P3 scribing is based on the so-called direct laser ablation process to expose the molybdenum back-contact
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or the removal of the transparent conductive oxide (TCO) only. In both cases laser radiation interaction with thin layers can induce thermal damage to the absorber, such as metallization and formation of secondary phases that can short-circuit the cells (Mitzi et al., 2011; Westin et al., 2008, 2011). These effects can be minimized if only TCO ablation process is used for the P3 patterning (Burn et al., 2013). However, the melting of the exposed absorber cannot be avoided completely. Instead of two previously discussed techniques, we further investigate the entire front-side laser-induced mechanical layer spallation (lift-off) for P2 and P3 processes (Gecys et al., 2014). The method includes the complete removal of constituent layers exposing the molybdenum back-contact. The laser modified layers are removed mostly in the solid phase by a laser induced layer spallation. Such technological approach can be one of the key techniques for sufficient increase of the patterning speeds and scribing quality. However, it is crucial to understand the physics of the liftoff process in order to minimize the induced laser damage to the solar cell material. A lot of effort was applied to study the material layer spallation (lift-off) experimentally (Buzás and Geretovszky, 2012; Domke et al., 2012; Heise et al., 2012; Lee et al., 2013a,b) and numerically (Buzás and Geretovszky, 2012; Sotrop et al., 2013; Wang et al., 2013a) in thin-film solar cell-like structures by different research groups. However, the underlying physics are still not completely clear. Sotrop et al. (2013) performed numerical simulations of femtosecond laser induced back-side ablation of thin molybdenum layers deposited on the transparent glass substrate. The driving force of the layer spallation was the ultra-fast thermal expansion of molybdenum layer caused by the laser radiation absorption. Furthermore, it resulted in a shock wave traveling into the backsurface and reflecting from the glass substrate towards the Mo. Earlier, we simulated a ps-laser radiation absorption in the kesterite solar cell. We showed that most of the laser energy was coupled at the CZTSe/Mo interface (Gecys et al., 2014). In addition, Scragg et al. (2012) reported that the CZTSe film becomes thermodynamically unstable at 550 °C and loses around 50% of its original Se content. As a consequence, the elevated temperatures cause the degradation of the material. It may lead to a reduction of the adhesion forces and minor pressure increase of Se vapor further assisting the layer spallation. That was also confirmed by Buzás and Geretovszky (2012). The authors claim that the melted and vaporized interface after the laser pulse irradiation reduces the adhesion between the layers and, therefore, determines the size of ablated craters in CIGS solar cells. On the other hand, our previous investigations showed that each layer in a solar cell above the backcontact absorbed part of the laser energy (Gecys et al., 2014). The absorption may trigger ultra-fast thermal expansion and may induce bulging and tension in the absorber layer. Considering previous statements, more comprehensive study is needed to determine the physics behind the layer lift-off mechanisms. Furthermore, technological aspects of such processes are also very important in the case of solar module scribing. The laser pulse duration plays a crucial role in the laser-matter interaction, therefore it is necessary to optimize layer spallation in terms of laser pulse duration. Laser-induced lift-off processing with ns (Buzás and Geretovszky, 2012; Lee et al., 2013b) and ps (Heise et al., 2011) pulse durations was shown by several research groups in CIGS samples. On the contrary, the CZTSe samples were investigated only with ps pulses (Gecys et al., 2014). However, no comprehensive study of layer lift-off process and scribe electrical validation was made covering ps and sub-ps pulse durations. Therefore, in this paper, we investigated the role of the laser pulse duration in the case of P2 and P3 laser lift-off processing of the CZTSe thin-film solar cells. Several laser sources were used to cover
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the pulse duration range from 300 fs to 60 ps. We studied the laser pulse duration effects on the threshold fluence required to expose the back-contact layer. Morphology and the structure of ablated craters were investigated by using scanning electron microscope (SEM). The micro-Raman spectroscopy has been employed in order to investigate the structural changes and secondary phase formation at the laser ablated crater edge. Finally, we performed the P3 lift-off scribing followed by the parallel scribe conductivity validation and mini-module simulations. 2. Experimental details Multiple laser sources were utilized in the experiments to cover a range of laser pulse durations from pico- to up to femtoseconds. Two picosecond lasers from Ekspla – Atlantic and Atlantic HE provided 10 and 60 ps pulses at 1064 nm wavelength, respectively. A Pharos laser (from Light Conversion) was utilized to exhibit 300 fs and 1 ps pulses at 1030 nm wavelength. Output pulse duration was tuned by an integrated compressor employing two transmission diffraction gratings. Afterwards, preselected pulse durations were verified with an intensity autocorrelator. The experimental setup was very similar in all used systems: the laser itself, an electro-optical shutter, the beam expander, and the sample/beam positioning system which was realized either by a galvanometer scanner (Scangine14 Scanlab) or by linear XY translation stages (Aerotech). The focusing objective with a focal length of 80 mm was utilized in the experiments resulting in a minimal diffraction-limited spot size of 23 mm. The spot size on the sample surface was controlled by shifting the sample out of the focal plane using Z-axis translation stages (Standa). The sample was always shifted above the focal plane perpendicularly to the beam propagation direction, keeping the focal spot below the processed surface. The quality of the ablated areas was examined with SEM (JSM6490LV JEOL). Characterization of material modifications was carried out by a micro-Raman microscope (nVia Reninshaw) equipped with 633 nm laser. The laser spot size and the spatial resolution were estimated to be 1 mm and 0.5 mm, respectively. Laser power was set to 1 mW and the signal was accumulated for 100 s. Furthermore, a linear laser scribing technique (LLST) (Markauskas et al., 2015) was utilized to extract the laser scribe conductivity. The solar cell’s parallel conductance was measured after each laser scribe by the 4-point probe I-V measurement system (2602A Keithley) connected to a sample. This way, the dependence of the solar cell’s parallel conductance on the laser-scribed length was obtained. Finally, by fitting the results to the proper fitting function provided in Markauskas et al. (2015), we were able to extract laser scribe parallel conductivity. Two types of kesterite thin-film solar cell samples were investigated: (a) a P3 structure consisting of a soda-lime-glass (SLG) substrate, molybdenum (Mo) back-contact, CZTSe absorber, CdS buffer layer, and ZnO front-contact comprised of intrinsic (i-ZnO) and conductive (ZnO:Al) oxide layers; and (b) a P2 structure excluding the front-contact and buffer layers (see Fig. 1). More detailed information on the sample preparation and composition is described in Repins et al. (2012). 3. Results 3.1. Lift-off crater ablation in CZTSe samples Single pulse front-side crater ablation experiments were conducted in P2 and P3 structures of the CZTSe solar cell samples at investigated laser pulse duration range. Typical SEM images of the lift-off craters in CZTSe solar cells are shown in Fig. 2. Our previous investigations have demonstrated that sufficient laser pulse
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Fig. 3. The minimum diameter of exposed molybdenum craters versus the laser pulse duration. Fig. 1. Cross-section schemes of CZTSe thin-film solar cell samples used in the experiments: P3 (a) and P2 (b) structures are presented.
energy must be applied for a given laser spot size to induce layer spallation (Gecys et al., 2013; Gecˇys et al., 2014). However, the irradiated spot cannot be sharply focused since it is crucial to avoid melt pool formation in the case of efficient lift-off ablation (Gecys et al., 2014). This effect limited the smallest size of the exposed crater in the CZTSe structures at the 10 ps pulse duration range (Gecys et al., 2014). For this, we investigated the influence of laser pulse duration on the minimum diameter of exposed molybdenum crater. The laser pulse energy was set to 50 mJ and the spot size was controlled by shifting the sample above the focal plane perpendicularly to the propagation direction of the laser beam. The sample shift was identified as the defocus parameter or focus offset which was described in Gecys et al. (2014). It was noticed that in the case of the P2 craters, the minimal crater size and pulse duration relationship was linear in the logarithmic scale (see Fig. 3). Shorter pulses enabled us to form smaller diameter craters. The minimum P2 crater diameter was 62 mm after the irradiation of 300 fs laser pulse. In order to form such a crater, the laser focal spot was shifted by 0.6 mm out of the focal plane, which corresponded to the fluence of 7.28 J/cm2. A transition to 1 ps range resulted in 74.8 mm crater size at 0.8 mm focus offset (4.79 J/ cm2). 10 ps pulses facilitated the formation of 83.3 mm craters at 1 mm focus offset (2.87 J/cm2). Longest 60 ps pulses produced the biggest craters – the minimum exposed molybdenum area reached a diameter of 117.7 mm at 1.8 mm sample focus offset (1.04 J/cm2).
In the case of a P3 structure, the crater size dependence on the pulse duration was less straightforward (see Fig. 3). Additional ZnO and CdS layers deposited on top of the kesterite absorber increased the structure complexity. Sub-picosecond (300 fs) laser pulses produced 60.2 mm craters at focus offset of 0.9 mm (3.62 J/cm2). Longer 1 ps pulses resulted in the minimal crater size of 48.1 mm (focus offset 0.7 mm, laser fluence of 6.5 J/cm2). A transition to the 10 ps regime produced 56 mm craters at 1 mm defocus and a laser fluence of 3 J/cm2. Finally, the utilization of the longest pulses resulted in a further increase in the crater diameter reaching 96 lm at 2.1 mm focus offset (0.75 J/cm2). Based on the previous statements, we believe that the shorter laser-material interaction (shorter pulses) caused lower thermal laser energy diffusion to the bulk of the material. It resulted in a sharper thermal transition to the cold area of the material and induced higher stresses to the layers. On the contrary, according to Griffith’s criterion (Buzás and Geretovszky, 2012), the lack of sufficient tensile stress required to initiate the spallation can be compensated by a larger delamination area. In the case of long laser pulses (sub-ns), the residual CZTSe/Mo layer adhesion force, together with CZTSe layer fracture strength, can be exceeded only for craters with bigger diameters (see Fig. 3).
3.2. Punching threshold in CZTSe samples The minimum laser fluence required to initiate a clean molybdenum layer exposure is referred as the punching threshold. We observed an inverse relationship between the punching threshold
Fig. 2. SEM images of typical lift-off craters ablated in P2 (a) and P3 (b) structures. Laser fluence of 3.1 J/cm2 and 1 mm defocus was used for 10 ps pulse irradiation in both structures.
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Fig. 6. SEM cross-section images of craters ablated with 10 ps pulses in P2 (a) and P3 (b) structures. Ablation parameters: 2.2 J/cm2 fluence at 1.2 mm defocus.
Fig. 4. Punching threshold fluence versus the laser pulse duration.
and the pulse duration (see Fig. 4). Highest thresholds were obtained for 0.3–1 ps pulses. The peak values were 0.48 and 0.92 J/cm2 for P2 and P3 structures, respectively. The relative difference between P2 and P3 processes reached 52%. Transition to longer pulses reduced both the threshold difference between the two cases and the values itself. The lift-off process was most efficient for 60 ps pulses. Working at these conditions, the smallest threshold value of 0.2 J/cm2 was obtained for both structures. One of the driving mechanisms for the CZTSe lift-off processing is the reduction of the adhesion forces and a minor Se vapor pressure increase at the CZTSe/Mo interface (Gecys et al., 2014). For this, a sufficient amount of laser energy has to be coupled at the CZTSe/Mo interface. However, reduced pulse duration can induce higher nonlinear absorption in the bulk of the removable layers. In such way, a smaller amount of laser energy can reach the interface and, as a result, an increased punching threshold was observed for shorter pulses.
3.3. Lift-off crater morphology SEM was used to evaluate the quality of molybdenum craters fabricated at different pulse durations. Fig. 5a shows front-side views of typical craters formed in a P2 structure. The process quality was very similar despite the different pulse duration. Pulse duration variation did not induce any melt or burr formation at the perimeter of the crater, as well. The use of femtosecond pulses did not provide any noticeable advantage over longer pulses in the picosecond time range. Front-side images of the P3 craters are shown in Fig. 5b. Changes induced by the pulse duration were observed at the surface of the exposed CZTSe, which are evident as a ring-like structure. Transition to longer pulses caused more intense melting of the exposed surface layer. Sub-ps pulses allowed us to preserve the exposed layer topography similar to unaffected areas. However, the transition to 10 ps and 60 ps pulses resulted in visible thermal damage to the exposed kesterite surface. The cross-section views of P2 and P3 crater edges are shown in Fig. 6. A white dotted line separates the exposed molybdenum area
Fig. 5. SEM images of the exposed molybdenum craters in P2 (a) and P3 (b) structures. Laser fluence of 2.2 J/cm2 and 1.2 mm defocus was used for 300 fs, 1 ps and 10 ps pulse durations. In the case of 60 ps pulses, the laser fluence was reduced to 0.83 J/cm2 at 2 mm defocus.
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(to the right of the dotted line) and the surrounding area (to the left of the dotted line). The purple1 area shows the pre-deposited protective platinum (Pt) layer used in a sample preparation by focused ion beam (FIB). In the case of the P2 crater (see Fig. 6a), no damage to the absorber or the back-contact was observed near the crater edge. On the other hand, a thin layer of melted absorber in the exposed kesterite ring area was observed in the P3 structure (see Fig. 6b, marked in yellow1). The thickness of the affected layer was measured to be between 0.25 and 0.3 mm, which was about one-fourth of the CZTSe layer. No direct contact between melted absorber and the molybdenum layer was observed, reducing the probability of short-circuiting the cell. However, the laser process induced minor cracking and delamination of the absorber layer visible in Fig. 6b near the inner edge of the exposed crater. The depth of the defects was limited to 3 mm. Melting of the exposed kesterite ring around the ablated Mo crater was observed in the case of multiple layer ablation by a lift-off process. Earlier, we simulated picosecond laser pulse absorption in CZTSe P3 structure showing high temperature peak at the ZnO top-contact (Gecys et al., 2014). Therefore, this effect can be related to increased laser absorption near the CZTSe surface. This way, the P3 crater ring modifications can follow classical ablation behavior, where shorter 300 fs pulses induced less CZTSe surface melting compared to longer 10 ps pulses. However, we have to consider, that only part of the laser radiation was absorbed at the top-layers, and the other part penetrated through the absorber layer down to the Mo (Gecys et al., 2014).
3.4. Raman analysis We used micro-Raman spectroscopy to indicate the structural changes and secondary phase formation induced by a laser process in CZTSe thin-film solar cell samples. First of all, the laser nonaffected areas were analyzed. Further, Raman measurements were conducted 5 mm away from the laser ablated crater edge in a P2 structure (see Fig. 7a). In the case of a P3 structure, the measurements were taken in the center of the exposed kesterite ring (see Fig. 7b). The distance from the inner edge of the crater was about 5 mm. Laser non-affected spectra are indicated as ‘‘0” and are shown in Fig. 8a and b. Both spectra exhibited typical CZTSe-pronounced peaks appearing at 172 cm1 (Nam et al., 2014) and 196 cm1 (Grossberg et al., 2011). Considerably lower peak intensity was observed at 233 cm1 (Grossberg et al., 2011), which is assigned to CZTSe. In the case of secondary phases, only a low-intensity ZnSe peak was observed at 251–252 cm1 (Salomé et al., 2009; Uday Bhaskar et al., 2013; Venkatachalam et al., 2007). Raman spectra measured in a P2 structure are presented in Fig. 8a. Spectra labeled 1–4 denote laser pulse duration from 60 ps to 300 fs, respectively. An impact of laser pulse duration was clearly visible. The valley between two main CZTSe peaks (172 and 196 cm1) started to rise when the shorter pulses were used. Furthermore, the use of 300 fs pulses caused a fusion into a single asymmetric peak. The effect can be caused by increasing structural disorder and intensive formation of CTSe (Cu2SnSe3) phases at 180 cm1 (Nam et al., 2014). It is a low-band-gap (0.8 eV) secondary phase which can limit the open circuit voltage of the solar cell (Siebentritt, 2013). CZTSe peak broadening at 234 cm1 was observed with a decrease in the pulse duration until it became indistinguishable in 300 fs range (see Fig. 8a). We noticed the appearance of a low1 For interpretation of color in Fig. 6, the reader is referred to the web version of this article.
Fig. 7. SEM images indicating Raman measurement spots in P2 (a) and P3 (b) structures.
intensity peak at 231 cm1 which can be related to CTSe and/or CZTSe phases (Nam et al., 2014). Raman spectra measured at the center of exposed kesterite ring in a P3 structure are shown in Fig. 8b. Significant changes of the CZTSe Raman signal was observed for all inspected laser pulse durations. Such behavior could be related to the increase of structural disorder and formation of the CTSe secondary phase. Furthermore, the characteristic CZTSe low-intensity peak at 234 cm1 was not found. Heat accumulation near the CZTSe surface was responsible for additional modifications in Raman spectra. Therefore, even for the longer pulses material stoichiometry changes were observed. However, we have to consider that only the part of the laser radiation was absorbed at the top layers of the P3 structure. Remaining laser energy penetrated down to the CZTSe/Mo interface removing the absorber layer in a solid state (Gecys et al., 2014). Shorter pulses induced higher nonlinear absorption in the CZTSe layer resulting in significant bulk modifications. This effect was clearly observed in case of a P2 structure, where no additional top-layers were deposited. We are convinced, that both P2 and P3 structures suffered from bulk modifications. However, CZTSe surface modifications were introduced only in the P3 structure.
3.5. P3 scribe conductivity measurements P3 lift-off trench scribing experiments were conducted in a P3 structure comprising of slightly thinner CZTSe absorber layer (0.8 mm). To maintain the experimental conditions as constant as possible, a single ablation regime was optimized for laser pulses of 300 fs, 1 ps, and 10 ps durations. Laser fluence was set to 2.34 J/cm2, scanning speed to 10 mm/s, and the repetition rate to 310 Hz. It corresponded to a pulse overlap of 2.3%. Finally, it allowed us to achieve high scribing quality: the patterns had sharp edges forming a continuous, uninterrupted trench (see Fig. 9). The LLST was utilized to extract parallel conductivity values of the laser scribe (Markauskas et al., 2015). The results showed the inverse dependency of the P3 scribe conductivity on the laser pulse duration (see Fig. 10). Best results were achieved while scribing with longest 10 ps duration pulses - 0.64 S/m. A transition to shorter pulses showed inferior results: the parallel conductivity increased to 0.78 S/m and 0.94 S/m for 1 ps and 300 fs pulses, respectively. In this case, shorter pulses induced more damage which resulted in more significant shunting of the solar cell. According to the Raman measurements, shorter pulses caused more pronounced structural disorder in the bulk of the material and the formation of p-type (CTSe) secondary phase near the ablated craters (Fig. 8b). In case of CIGS solar cells, it was reported that the partial absorber evaporation can lead to the formation of Cu-rich compound, which should be also valid for CZTSe absorber material at elevated temperatures (Redinger and Siebentritt, 2010; Westin et al., 2008). Increasing Cu content in the CZTSe compound
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(a)
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(b)
Fig. 8. Raman spectra of the CZTSe layer measured in P2 (a) and P3 (b) structures 5 mm away from the exposed molybdenum crater edge. The numbers from 1 to 4 correspond to the used laser pulse duration. Non-affected kesterite spectrum is denoted as 0.
gives rise to higher carrier concentration and conductivity values (Olgar et al., 2016). Possibly, this resulted in P3 lift-off scribe conductivity increase for shorter laser pulses. 3.6. Mini-module simulation
Fig. 9. SEM image of the P3 scribe in a P3 structure ablated with 300 fs pulses. Laser fluence of 2.34 J/cm2 at 0.4 mm focus offset was used with translation speed of 10 mm/s.
Further, we conducted a 3-cell CZTSe mini-module simulation under standard test conditions. The purpose of simulations was to evaluate the possible efficiency losses caused by the laser process. The equivalent circuit of the mini-module is shown in Fig. 11. Here, V is the voltage across the output terminals of the minimodule, Rs is the series resistance, Rp is the ohmic and Rp3 is the laser scribe induced shunts. Diode and the photo-generated currents are denoted as ID and Iph, respectively. Generally, a solar cell can be described by a single diode equation:
V þ IRs I ¼ ISC A I0 eqðVþIRs Þ=nkT 1 ; Rp
Fig. 10. Extracted P3 scribe conductivity versus laser pulse duration. Laser lift-off scribing was realized in a P3 structure.
ð1Þ
which is comprised of a diode saturation current I0, ideality factor n, short circuit current Isc measured at a particular solar irradiance, and operating temperature T. Active solar cell area is A, and the output current is I. Elementary charge and Boltzmann constants are described as q and k, respectively. The model considered only the P3 scribe neglecting the influence of P1 and P2 interconnects. Therefore, additional resistors Rp3 must be introduced to include the laser induced P3 scribing damage to solar cells No. 2 and 3. Each cell was described by an individual I-V equation and implemented as a separate subcircuit in PSpice modeling software. Parameters of the 9.7% efficiency DC-sputtered CZTSe solar cell were borrowed from Brammertz et al. (2013). We chose to simulate 1 cm2 area square-shaped solar cells determining the laser scribe length and distance to 1 cm. Finally, the active area loss caused by the width of the serial interconnects was not accounted. Extracted efficiency of 9.76% correlated well with the experimentally measured value in Brammertz et al. (2013). Fig. 12 illustrates the dependence of simulated 3-cell mini module efficiency on P3 scribe conductivity. The scribes performed
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Fig. 11. The equivalent model of the 3-cell CZTSe mini-module.
Fig. 12. Simulated 3-cell CZTSe mini-module efficiency dependence on P3 scribe conductivity.
Table 1 The simulated efficiency of a 3-cell CZTSe mini module based on experimentally measured P3 scribe parallel conductivity. Pulse duration
Scribe conductivity, S/ m
Module efficiency, % (absolute)
Efficiency loss, % (relative change)
300 fs 1 ps 10 ps
0.94 0.78 0.64
9.2 9.32 9.42
6.1 4.7 3.6
with 10 ps duration pulses caused the smallest efficiency losses of just 3.6% relative (see Table 1). The mean value obtained for 1 ps pulses caused efficiency losses of 4.7% relative. Use of the shortest pulses induced the highest losses, reaching the maximum value of 6.1% relative for 300 fs. Overall, the transition from 10 ps pulses to femtosecond pulses (300 fs) decreased the absolute module efficiency from 9.42% to 9.2%. 4. Discussion According to our investigations, shorter pulses induced smaller diameter lift-off craters, however, the threshold fluences were higher. From the ablation experiments and Raman investigations we can speculate that the nonlinear absorption of the laser radiation was more pronounced in the bulk of the CZTSe layer for sub-ps laser pulses. This triggered a more intense thermal expansion of the absorber layer in a confined area. Consequently, the laser induced internal stress facilitated the mechanical rupture of the CZTSe. As a result, the size of the craters was reduced. This effect may become more dominant in layer lift-off processes at even shorter sub-ps pulse durations. At the picosecond pulse duration range, the nonlinear absorption effects may become less pronounced. Longer pulses result in
more prominent thermal diffusion into the material, lowering induced stresses to the layers. On the other hand, the laser beam absorption at the CZTSe/Mo interface may become dominant. Therefore, an increased interface heating will lead to further reduction of the layer adhesion forces and minor Se vapor build-up. Consequently, the laser induced vapor pressure will punch the absorber layer. These effects result in lower punching threshold values at the expense of larger craters. Therefore, the CZTSe layer lift-off mechanism could be the combination of both – the fast expansion of the absorber layer and the laser beam absorption at the CZTSe/Mo interface. Both mechanisms are inseparable, only domination of one or the other may be defined by the laser pulse duration. Together, these effects could induce sufficient forces and strain to initiate laser-induced thermo-mechanical layer spallation and, therefore, the exposure of molybdenum back-contact. Mostly femtosecond laser pulses are associated with low thermal effects, smaller heat affected zone and higher ablation quality. Furthermore, the CIGS material ablation thresholds are lower for shorter pulses due to enhanced nonlinear absorption (Hermann et al., 2006). It applies to the direct layer ablation process. However, our investigation revealed the opposite in the case of laser induced lift-off ablation, since the material was removed mostly in the solid state. No surface melting was observed for P2 craters and only 0.3 mm deep P3 crater edge modification was measured for CZTSe surface. The P3 structure have additional top-contact layers, therefore the laser radiation absorption in such structure is more complex. During the laser exposure, CZTSe surface gets additional modification due to top-contact heat accumulation, which is related to direct absorption. Therefore, CZTSe surface melting is less pronounced in case of shorter pulses. However, we have to consider, that most part of the laser radiation penetrated through the layers down to CZTSe/Mo interface. Due to increasing peak intensities, nonlinear effects become more pronounced, resulting in higher laser absorption in the bulk of the CZTSe absorber (Musazzi and Perini, 2014). Consequently, it results in more significant bulk modification of the absorber layer. Likely, it is the reason why we observed higher lift-off scribe conductivity for shorter pulses. The simulated module efficiencies were lower for short pulse laser lift-off processing. However, best module efficiencies were obtained for the 10 ps laser pulses. Such pulse duration is typical for industrial picosecond lasers. Consequently, implementation of such technology in the industrial environment becomes less complex, since industrially accepted tools for lift-off processing are available. However, the method is sensitive to the absorber material properties variation, since the laser light has to penetrate through the absorber layer. This might be the biggest drawback of this scribing technology, since it is difficult to precisely control the CZTSe deposition processes over the large area. 5. Summary We realized the lift-off ablation process in P2 and P3 structures of thin-film CZTSe solar cells by utilizing the laser pulses of 300 fs, 1 ps, 10 ps, and 60 ps durations. Strong nonlinear laser absorption
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effects in the CZTSe layer were observed for shortest laser pulses. The minimum exposed crater diameter of 48 mm was obtained in a P3 structure with 1 ps duration pulses. In case of a P2 structure, a slightly larger 62 mm diameter crater was exposed with 300 fs pulses. This suggests strong effect of laser light absorption in CZTSe triggering fast expansion of the absorber layer in a confined area. However, the punching threshold of 0.2 J/cm2 for both P2 and P3 structures was lowest for the long 60 ps pulses at the expense of bigger crater sizes. Reduced nonlinear absorption in CZTSe triggered more intense heating at the CZTSe/Mo interface. As a result, the interface adhesion forces were reduced followed by a vapor pressure increase. Therefore, the CZTSe layer lift-off mechanism could be the combination of both – the fast expansion of the absorber layer and the laser beam absorption at the CZTSe/Mo interface. Together, these effects could induce sufficient forces and strain to initiate laser-induced thermo-mechanical layer spallation and, therefore, the exposure of molybdenum back-contact. Further, the Raman measurements revealed significant structural modifications, including the most intensive CTSe phase formation, as well as, the broadening of main CZTSe peaks for shorter sub-ps laser pulses. Electrical measurements were also in favor of longer picosecond laser pulses. The lowest P3 lift-off scribe conductivity of 0.64 S/m was achieved for 10 ps pulses. The utilization of shorter durations notably increased the conductivity to up to 0.94 S/m for 300 fs pulses. The simulation of 3-cell CZTSe mini-module revealed that the device’s efficiency losses could be minimized to 3.46% relative if the P3 lift-off scribes are realized with 10 ps pulses. Overall, the laser-induced layer mechanical spallation is a promising method for thin-film solar cell processing. However, the method is sensitive to the absorber material properties variation. Therefore, comprehensive study is required to validate the complete absorber liftoff process under industrial processing conditions. Acknowledgements The research leading to these results was partially funded by the European Union FP7 Programme under grant agreement no. 609355 (APPOLO). References Bosio, A., Sozzi, M., Menossi, D., Selleri, S., Cucinotta, A., Romeo, N., 2014. Polycrystalline CdTe thin film mini-modules monolithically integrated by fiber laser. Thin Solid Films 562, 638–647. Bovatsek, J., Tamhankar, A., Patel, R.S., Bulgakova, N.M., Bonse, J., 2010. Thin film removal mechanisms in ns-laser processing of photovoltaic materials. Thin Solid Films 518, 2897–2904. Brammertz, G., Buffière, M., Oueslati, S., ElAnzeery, H., Ben Messaoud, K., Sahayaraj, S., Köble, C., Meuris, M., Poortmans, J., 2013. Characterization of defects in 9.7% efficient Cu2ZnSnSe4-CdS-ZnO solar cells. Appl. Phys. Lett. 103, 163904. Burn, A., Muralt, M., Pilz, S., Romano, V., Witte, R., Frei, B., Buecheler, S., Nishiwaki, S., Krainer, L., 2013. All fiber laser scribing of Cu(In, Ga)Se2 thin-film solar modules. Phys. Procedia 41, 713–722. Buzás, A., Geretovszky, Z., 2012. Nanosecond laser-induced selective removal of the active layer of CuInGaSe2 solar cells by stress-assisted ablation. Phys. Rev. B 85, 245304. Clauwaert, K., Binnemans, K., Matthijs, E., Fransaer, J., 2016. Electrochemical studies of the electrodeposition of copper-zinc-tin alloys from pyrophosphate electrolytes followed by selenization for CZTSe photovoltaic cells. Electrochim. Acta 188, 344–355. Domke, M., Rapp, S., Schmidt, M., Huber, H., 2012. Ultra-fast movies of thin-film laser ablation. Appl. Phys. A 109, 409–420. Dunsky, C., Colville, F., 2008. Solid state laser applications in photovoltaics manufacturing. Proc. SPIE 6871, 687129. Heise, G., Domke, M., Konrad, J., Sarrach, S., Sotrop, J., Huber, H.P., 2012. Laser lift-off initiated by direct induced ablation of different metal thin films with ultrashort laser pulses. J. Phys. D Appl. Phys. 45, 315303. García-Ballesteros, J.J., Torres, I., Lauzurica, S., Canteli, D., Gandía, J.J., Molpeceres, C., 2011. Influence of laser scribing in the electrical properties of a-Si: H thin film photovoltaic modules. Sol. Energ. Mater. Sol. Cells 95, 986–991.
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