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Optimization of laser-patterning process and module design for transparent amorphous silicon thin-film module using thin OMO back electrode
Solar Energy 201 (2020) 75–83
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Optimization of laser-patterning process and module design for transparent amorphous silicon thin-film module using thin OMO back electrode Jaeho Parka,b, Soo-Won Choia,d, Sangah Leec, Jaesung Leec, Myunhun Shinc, , Jung-Dae Kwona,b, ⁎
T
⁎
a
Materials Center for Energy Convergence, Korea Institute of Materials Science, Changwon, Gyeongnam 51508, Republic of Korea Department of Advance Materials Engineering, University of Science and Technology (UST), 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea c School of Electronics and Information Engineering, Korea Aerospace University, Goyang, Gyeonggi 10540, Republic of Korea d Department of Materials Science and Engineering, Pusan National University, Busan, 46241, Republic of Korea b
ARTICLE INFO
ABSTRACT
Keywords: Transparent amorphous silicon photovoltaic Oxide-metal-oxide electrode Laser patterning Cell geometry Building integrated photovoltaic Equivalent circuit
Transparent hydrogenated amorphous silicon thin-film solar modules are fabricated using oxide-metal-oxide (OMO) electrodes as the back electrode for building-integrated photovoltaic applications. The outer aluminumdoped zinc oxide and inner silver layers constitute a thin OMO electrode (~110 nm thick), exhibiting a sheet resistance of 6.8 Ω/□ and an average transmittance of ~88% in the visible range of 400–800 nm. The external quantum efficiency and average transmittance of the cell were investigated for the absorber-layer thickness using the finite-difference time-domain method, and it was found that the optical loss in the cell was mainly due to the absorption of the front electrode in the ultra-violet region and free-carrier absorption of the OMO in the infrared region. Fabrication issues are introduced for a 532 nm short-pulse high-power laser patterning process for transparent modules with thin OMO electrodes. Optimization of the laser power for the P2 and P3 laser processes is demonstrated by observing the profiles and measuring the shunt resistance of the laser-patterned edges. Furthermore, the cell width is optimized based on an equivalent circuit model using PSpice simulation. The highest module efficiency and average transparency achieved in the range of 500–800 nm were 5.6% and 15.2%, respectively. The short-circuit current density, fill factor, and open-circuit voltage per cell of the module were found to be 10.8 mA/cm2, 62.7%, and 0.830 V, respectively.
1. Introduction The use of fossil fuels is one of the most direct causes of greenhouse gas emissions that contribute to global warming. In addition, volatile organic compounds, i.e., the by-products of burning fossil fuels, such as oxides of carbon, sulfides, and nitrides (COx, SOx, and NOx), generate fine dust, which is a major cause of urban air pollution, and pose a serious threat to the health of urban residents (Perera, 2017). Various renewable energies are essential as a city’s alternative energy source. Thus, wind, geothermal, bio-, wave, and solar energies have been attracting significant attention (Bernardino et al., 2017; Ghosh, 2016; Karthikeya et al., 2016; Nathan, 2016; Melikoglu, 2017). In cities and urban areas, electricity is the most commonly used energy source and it can be directly provided by solar cells. Therefore, solar cells are an important technology for realizing net zero-energy buildings (Eshraghi et al., 2014).
Many solar cells in practical use have focused on the characteristics of high power conversion efficiency (PCE) and low production cost, such as with multi- or mono-crystalline silicon (Si) solar cells. In innercity areas, solar cells can also be integrated into buildings without the need for additional installation space. Building-integrated photovoltaic (BIPV) devices can be constructed as roofs, facades, building-walls, and even windows (Lee et al., 2019). Modern buildings have started using glass exteriors as walls, in which power generation using solar cells along with other architectural characteristics, such as a variety of colors, flexibility for rounded areas, transparency, and durable stability during the building’s life cycle, are important features. Solar windows using dye-sensitized solar cells and organic photovoltaics feature flexibility and transparency, but their long-term stability for commercial usage has not been verified owing to electrolyte leakage, photo-thermal degradation, and thermal instability (Asghar et al., 2010). In contrast, thin film solar cells composed of hydrogenated amorphous silicon (a-
Abbreviations: AZO, aluminum-doped zinc oxide; BIPV, building-integrated photovoltaic; DPSS, diode pumped solid state; OMO, oxide-metal-oxide; PCE, power conversion efficiency ⁎ Corresponding authors at: School of Electronics and Information Engineering, Korea Aerospace University, Goyang, Gyeonggi 10540, Republic of Korea (M. Shin), and Materials Center for Energy Convergence, Korea Institute of Materials Science, Changwon, Gyeongnam 51508, Republic of Korea (J.-D. Kwon). E-mail addresses: [email protected] (M. Shin), [email protected] (J.-D. Kwon). https://doi.org/10.1016/j.solener.2020.02.092 Received 10 January 2020; Received in revised form 24 February 2020; Accepted 26 February 2020 Available online 03 March 2020 0038-092X/ © 2020 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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thereby separating each individual cell. When using very thin OMO electrodes for a transparent solar module, the laser power of the P2 and P3 processes must be carefully optimized. The laser power of the P2 and P3 processes needs to be sufficient to remove the a-Si:H absorber layers, but not excessive, to mitigate the damage of the front TCO underneath and to reduce thermal defects in the a-Si:H layers close to the laser scribed patterns. In addition, the deposition of the OMO layers is affected by the P2 process and the transmittance or reflectance of the back electrodes affects the P3 process. In this work, we fabricated a transparent a-Si:H thin film module using OMO electrodes as the back electrode to present the optical and electrical optimization of the module design and optimize the laser patterning process for transparent modules. Using the finite-difference time-domain (FDTD) method, we analyzed the optical loss in the transparent a-Si:H solar cells and optimized the cell width based on an equivalent circuit model using the PSpice simulator. We also introduced fabrication problems in the nano-second laser (532 nm) patterning processes of transparent solar modules when using very thin OMO electrodes. The P2 and P3 laser processes were examined and optimized using focused ion beam-scanning electron microscopy (FIB-SEM), optical microscopy (OM), and measuring the current density and voltage (J-V) characteristics of the fabricated modules.
ZnO:Al (50 nm) Ag (8 nm) ZnO:Al (50 nm) n-uc-SiOx:H (30 nm) i-a-Si:H (200, 300, 400 nm) p-SiOx:H (15 nm) SnO2:F (600 nm) Glass Fig. 1. Schematic cross-section of transparent a-Si:H solar cell employing textured FTO as a front electrode and OMO as a back electrode. The thickness of ilayer was varied (200, 300, and 400 nm).
2. Module fabrication, simulation, and characterization 2.1. Fabrication of cell and module
Si:H) have already proven their stability by being installed and operated in the field for more than 20 years (King et al., 2000). In addition, the a-Si:H thin film solar cells use Si, which is abundant on earth; hence, their material cost is low. It has also been demonstrated in the display industry that the manufacturing processes using large-area glass substrates are feasible, which is essential for the large windows of modern buildings. Recently, we implemented an oxide-metal-oxide (OMO) structure as the back electrode of a transparent a-Si:H thin film solar cell for a BIPV window (Choi et al., 2019; Jo et al., 2018, 2019; Yang et al., 2018). In general, OMOs have the advantage of simultaneously increasing the electrical conductivity and optical transmittance by including a very thin metal layer with high electrical conductivity between the highly transparent conductive oxide (TCO) films. When silver (Ag) is used as the interstitial metal layer between TCO films at the percolation thickness, the OMO film exhibits optimized performance considering the optical transmittance and PCE of the transparent a-Si:H solar cell. Monolithic fabrication of a thin film module on a glass substrate commonly uses a three-step patterning process with a short-pulse high power laser (Jung et al., 2014). The first pattern (P1) removes the front TCO along a line isolating the front electrodes. The second pattern (P2) ablates the absorber semiconductor layer, thereby creating contact lines that connect the front TCO electrodes in series with the back electrodes. The third pattern (P3) removes both the absorber and back electrode,
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2.1.1. Fabrication of transparent a-Si:H solar cell Fig. 1 depicts the structure of the transparent a-Si:H solar cell. To fabricate the solar cells, we used a commercial glass substrate with a deposited textured fluorine-doped tin oxide (FTO, 600 nm) layer (Pilkington Ltd., 8 Ω/□, 50 mm × 50 mm). After the substrate was cleaned with a neutral detergent, p-, i- and n-type (p-/i-/n-) layers were sequentially deposited on the substrate by radio frequency (RF) plasmaenhanced chemical vapor deposition (PECVD) at 250 °C. The i-layer of a-Si:H was obtained from a silane (SiH4) source gas diluted by hydrogen (H2) at high-frequency RF conditions of 40 MHz and 30 W. The i-layer with thicknesses of 200, 300, and 400 nm was used to observe the dependence of the characteristics of cells on i-layer thickness. The ptype hydrogenated amorphous silicon oxide (p-a-SiOx:H) and n-type hydrogenated microcrystalline silicon oxide (n-μc-SiOx:H) layers were prepared using mixed gases of diborane (B2H6, 1% diluted in H2), phosphine (PH3, 1% diluted in H2), and carbon dioxide (CO2) of oxygen dopant under the RF conditions of 13 MHz and 50 W. An OMO structure was applied as the back electrode. Aluminum-doped zinc oxide (AZO) with a thickness of 50 nm was used in the upper and lower oxide layers and Ag with a thickness of 8 nm was used in the middle metal layer. Under these conditions, the OMO electrodes exhibited a sheet resistance of 6.8 Ω/□ and average transmittance of 88% in the visible range of 400–800 nm. The oxide and metal layers were deposited using a magnetron sputtering system at DC powers of 500 and 50 W,
P2
Deposit OMO by DC sputtering
Fig. 2. Schematic of the laser scribing process for fabricating amorphous silicon solar cell module.
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Fig. 3. 2D equivalent circuit model of transparent a-Si:H solar cell.
respectively. The sputtering process was performed at room temperature in a high vacuum (2 × 10−6 Torr) and a shadow mask was used to define a cell active area of 0.25 cm2 during the OMO deposition.
measured optical parameters of the individual layers of the OMO electrode (AZO and Ag) and the p-/i-/n-layers of a-Si:H cells to determine the structural parameters. Layer thicknesses (as shown in Fig. 1) were also used in the simulations. The transmittance and absorbance of the cell were estimated by FDTD simulation for varying values of the absorber layer thickness. To design the module based on the performance parameters of a single transparent a-Si:H cell and the sheet resistances of the front TCO and back OMO, we constructed a distributed equivalent circuit model of the module, as shown in Fig. 3. The equivalent circuit model can incorporate the dead area loss (the observed area loss between P1–P3) and resistance losses in the front TCO and back OMO to estimate the performance of the module (Kim et al., 2017). In addition to the area loss in P1–P3, the resistance loss in the thin OMO back electrodes is a major cause of transparent module degradation. Each of these effects depends on the unit cell size of the module. Thus, the width of the cell is carefully designed based on the simulation results.
2.1.2. Fabrication of transparent a-Si:H solar module To fabricate the module, a diode-pumped solid state (DPSS) laser delivering a short-pulse high-power output was used for the patterning process. The DPSS laser employed a neodymium-doped yttrium orthovanadate (Nd:YVO4) gain medium, operated at a wavelength of 532 nm to emit optical pulses with a variable pulse duration of up to 41 ns. The overlap ratio of the scribed patterns was controlled by the frequency and scan speed of the laser. In addition, the laser beam was focused to be as flat as the laser equipment. Thus, in our experiments, the laser output power was adjusted by changing the pump diode current of the laser and fine-tuned with an external attenuator. The laser beam was delivered to samples by a scanner head, which processed the high-speed patterning. The three-step scribing process (P1, P2, and P3) was used for the monolithic interconnection of solar cells in a module, as shown in Fig. 2, where P1, P2, and P3 were performed with the laser incident on the glass substrate side. The P1 process patterned the isolation lines in the FTO; then, the p-/i-/n-layers of the a-Si:H cells were deposited on the patterned FTO glass by PECVD. The P2 process was performed to selectively remove the p-/i-/n-layers from the front FTO electrode layer. After OMO electrode deposition, P3 ablated the p-/i-/n-layers, thereby removing the OMO back electrode. It is generally known that the absorber layers are instantaneously ablated by absorbing the laser power in the P2 and P3 processes, and the back electrodes are mechanically removed by P3 when the absorber layers are ablated by a short-pulse laser. After P3, the cells were separated and connected in series. In this study, modules with various cell widths ranging from 2 to 8 mm were fabricated to compare the performance parameters of modules as a function of cell width.
2.2.2. Characterization OM and FIB-SEM (Nova 200, FEI) were used to observe the surface and cross-section of the cells after the layer patterning process, respectively. A spectroscopic ellipsometer (SE MG-1000, Nano-View Co.) was used to determine the refractive indices, extinction coefficients, and thickness of the absorber layer on the glass substrates. The transmittance and reflectance of the cells were obtained via ultraviolet–visible (UV–VIS) spectrophotometry (Cary 5000, Varian). The JV characteristics of the solar cells and modules were measured using a solar simulator (Oriel 300, Newport Co.) under standard 1-sun illumination (100 mW/cm2 of AM1.5G). When measuring the performance of a cell or module, solar irradiance masks were used and aligned to the apertures of the devices to avoid potential overestimation of the photocurrents of the devices. The external quantum efficiency (EQE) (QuantX 300, ORIEL) of the solar cells was measured to observe the spectral response.
2.2. Simulation and characterization 2.2.1. Simulation of transparent a-Si:H solar module The optical transmittance and absorbance were evaluated using the FDTD method. For the FDTD simulation, we used the ellipsometrically
3. Results and discussion Fig. 4 shows a comparison between the simulated EQE and
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Fig. 4. Characteristics of a-Si:H solar cells (the area of ~0.25 cm2) for different i-layer thicknesses (200, 300, 400 nm). (a) Simulated external quantum efficiency (EQE), (b) simulated total transmittance, (c) measured EQE, (d) measured total transmittance, (e) measured and simulated reflectance of cells, and (f) simulated absorbance of each layer for an i-layer thickness of 300 nm.
transmittance spectra and the measured ones that depend on the thickness of the i-layer. Overall, the EQE increased with the increasing thickness of the i-layer whereas the total transmittance decreased. In the simulation shown in Fig. 4a and b, the short wavelength light was expected to be absorbed in the front TCO (under ~ 350 nm) and the player (350–400 nm); however, the spectral EQE response in the visible light in the range of 450–700 nm was expected to increase with the thickness of i-layer. The simulation results were significantly consistent
with the measured EQE response and transmittance shown in Fig. 4c and d. The transmittance was first observed at wavelengths over 500 nm; it increased with the wavelength. In contrast, the overall transmittance decreased with the thickness of the i-layer. In Fig. 4e, neither the simulated nor measured reflectance changed significantly with the thickness of the i-layer. In thin film solar cells, it is difficult to directly measure the optical losses occurring in each layer. Thus, we simulated the optical losses (absorbance) in each layer, as shown in
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demonstrates the schematic cross-sectional structure of the laser patterning of the module, where the cells are connected monolithically in series. Fig. 6b shows an OM image taken from the film side of the module. In Fig. 6b, the line widths of P1–P3 are 34, 27, and 30 µm, respectively, and the spaces between P1–P2 and P2–P3 are 100 µm each. Fig. 6a indicates the active and dead areas; the width of the unit cell is the sum of these areas. The area between P1 and P3 (~200 µm) is called the dead area because it cannot generate power effectively (Brecl et al., 2005). Fig. 6c shows the cross-sectional FIB-SEM images at the edges of the laser patterns of P1–P3. In Fig. 6c, it can be observed that the FTO layer forms a gentle slope at the pattern edge of P1 but the aSi:H layers are sharply cut at the pattern edges of P2 and P3. The edge of P2 should have a smooth profile to reliably connect the step coverage of the thin OMO layers. Besides the edge profile, there are more difficulties in the P2 process associated with the OMO layers. The P2 process does not damage the underlying layer of the front electrode (FTO) but also needs to selectively remove the absorber layers (a-Si:H). When P2 was performed at an average laser power lower than 1.2 W, the absorber was not completely removed and the remaining absorber formed edge ridges, as shown in Fig. 6d. Incomplete removal of the absorber layer and unevenly formed contact between the front and back electrode led to an increase in the series resistance of the module (Ku et al., 2011). When P2 was performed at an average power higher than 2.0 W, damages were observed on the front TCO; even for powers below 2.0 W, it was often observed that the deposited OMO formed island-like growths, as evident in Fig. 6e. We observed significant loss in the series resistance of the module with the P2 edge of Fig. 6e. We believe that some a-Si:H residue, which was scattered by the excess laser power, acted as nucleation centers for the growth of nanoscale islands of OMO during the deposition due to the surface interaction described in the work of Henley et al. (2005). Opaque solar cells generally use a reflective back metal as the back electrodes. When P3 is performed with a reflective back metal electrode in an opaque solar cell module, the metal electrodes reflect the laser beam back into the absorber layer, which enables P3 at reduced laser power, thereby mitigating thermal damage to the cells. However, for transparent cells, the OMO electrodes are designed to be transparent in the visible range. The OMO is very transparent to the P3 laser output at 532 nm. Moreover, the effective refractive index of the OMO is between those of the absorber and air, and the reflection at the interface of the OMO and absorber layers is lower than without the OMO layer during the P3 process. Thus, more power is required for the P3 process than the P2 process. Nevertheless, we used a short-pulse laser with a pulse duration of less than 41 ns. The heat generated during P3 can be transferred to the adjacent active region of the a-Si:H layers or the OMO electrodes near the edge of the patterns. Such rapid heating causes unwanted phase transitions and deformations in the materials. In addition, the thin OMO electrodes can form burs or give rise to debris under unoptimized laser process conditions. Fig. 7a shows that parasitic shunt resistance due to P3 and the PCE of the module are closely related and both are very sensitive to the average laser power of P3. If the average power is insufficient to completely remove the OMO layer, the OMO layer partially remains. The partially remaining OMO layer electrically connects the adjacent cells serving as shunt paths; thus, the shunt resistance increases with the average power of P3 in Fig. 7a. Interestingly, the shunt resistance becomes worse, thereby deteriorating the cell performance at average powers of P3 exceeding a certain value (1.35 W), as shown in Fig. 7a. We expect that there are several reasons for the shunt resistance behavior demonstrated in Fig. 7a. First, the OMO edge undergoes plastic
Fig. 5. Dependency of J-V characteristics of a-Si:H solar cells on thickness of the absorption layer (the cell area is ~0.25 cm2). Table 1 Trade-off between the efficiency and transmittance of a-Si:H solar cell depends on the thickness of the absorption layer. i-layer thickness (nm)
Efficiency (%)
T500–800 (%)
200 300 400
5.42 5.90 6.22
18.2 15.2 12.8
T500–800: average transmittance over 500–800 nm.
Fig. 4f. In this figure, the absorbance in the front TCO contributes to lowering the EQEs of the i-layers and accounts for the nonlinear inflections observed in the EQE curves of the i-layers in the 350–400 nm range in Fig. 4a and c. Fig. 4f reveals the absorbance still present in the thin back OMO in the 650–800 nm range, and the small absorbance due to thin p-type and n-type layers can also contribute to reduce the cell efficiency. Thus, the results shown in Fig. 4f suggest that the high bandgap materials in the front TCO and p-type layer can be modified to improve the EQE response of the cell by reducing the optical loss in a short wavelength range. In addition, considering the free carrier absorptions in metals in the long wavelength range, appropriately selecting the middle metal and amount of dopant in the rear TCO can reduce the optical loss in the cell and improve the transmittance in the visible range of 500–800 nm. It should be noted that both the simulated and measured reflectance and the optical losses in each layer did not significantly change with the change in the thickness of the i-layer. Fig. 5 presents the measured J-V characteristics of the fabricated cells for each thickness of the i-layer. When the i-layer thickness increases, no significant change is observed in the open circuit voltage (VOC) and fill factor (FF) of the cells; however, the short circuit current density (JSC) of the cell increases, which results in an increase in the cell efficiency. These results are consistent with the observed behaviors in Fig. 4 and are summarized in Table 1. Therefore, to optimize the a-Si:H solar cell for BIPV applications, it is necessary to select the appropriate i-layer thickness because there is a trade-off between the conversion efficiency. As introduced, the P2 and P3 processes can affect the fabrication process and final performance of transparent solar cell modules employing the thin OMO electrodes (thickness = 108 nm). Fig. 6a
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Fig. 6. (a) Cross-sectional structure of the a-Si:H module for monolithic serial inter-connection; (b) optical microscopy image of P1, P2, and P3 scribes; and (c) FIBSEM images of P1, P2, and P3. Result of performing P2 with an average power of (d) 0.59 W and (e) 2.0 W.
deformation (partial melting) during the P3 process due to the excessive energy forming a structure shown in Fig. 7b. The thin OMO layer used in this work (total thickness is less than 110 nm) can be deformed very easily. Second, a conductive sidewall is formed by the evaporation and re-deposition of the front FTO as a result of the excess power of P3, as depicted in Fig. 7c. (Ku et al., 2013; García-Ballesteros et al., 2011). At the higher average powers of P3, damages to the front FTO were confirmed in the OM image. The OM image (average power = 2.0 W) in Fig. 7a exhibits bright areas of damaged FTO. Finally, thermal defects or recrystallization of a-Si:H layers by the excess laser power can reduce the shunt resistance near the P3 edges. As shown in Fig. 7a, P3 must be carefully optimized and stabilized while processing the transparent modules with thin OMO electrodes. Using the measured J-V characteristics of a small transparent a-Si:H cell (0.25 cm2) and the sheet resistances of the front TCO and back OMOs, we configured the distributed equivalent circuits of the modules for various widths of the unit cells. In the modules, the unit cells were
4 cm long, and 2, 4, 6, and 8 mm wide; the number of cells was fixed to 5 to exclude problems induced by the number of laser scribes. Fig. 8a shows the simulated and measured J-V characteristics of the modules where the current density of a module is considered to be a module current divided by the area of a unit cell and the voltage per cell is the module voltage divided by the number of unit cells in a module. All performance parameters of the module are compared in Fig. 8b for varying widths of the unit cells (JSC and VOC are the current density of the module at short circuit condition and voltage per cell at open circuit condition, respectively). As the unit cell width increases from 2 to 8 mm, the JSC increases from 10.5 to 11.2 mA/cm2, and FF deteriorates from 63.7 to 56%, but VOC exhibits minimal changes from 0.825 V. Fig. 8b implies that the major loss in the module for increasing the unit cell width is the resistive loss. When the unit cell width increases, the relative area loss (dead area for the unit cell) decreases such that JSC increases. However, the resistance in the front TCO or the back OMO increases with the unit cell width; thus, the FF decreases without
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Fig. 7. (a) Influence of the average power of P3 on fabricated modules. The modules consist of 10 cells (4-mm width and 4-cm length) so that the total area is ~16 cm2. Left y-axis shows normalized shunt resistance (RSH), right y-axis shows normalized power conversion efficiency (PCE). Schematics reducing shunt resistance (b) plastic deformation of OMO or (c) conductive sidewall of FTO.
serious degradation of the VOC of the module. Therefore, as shown in Fig. 8a, the optimized transparent module with a unit cell width of approximately 4 mm exhibits the highest module efficiency (η) of 5.6% with JSC, FF, and VOC of 10.9 mA/cm2, 62.7%, and 0.830 V, respectively. In this work, we used the 300-nm thick i-layer to fabricate the modules and optimize the module with the unit cell width, because at the thickness of 300 nm, the EQE was better than that of 200-nm thick ilayer and the transmittance was better than that of 400-nm thick ilayer, respectively. However, it is noteworthy that the device structure can be further optimized by considering both optical and electrical characteristics of the overall module performance, such as the best figure of merit (the product of the transparency and PCE in a module) (Jo et al., 2019). The thickness of the i-layer, front TCO and each layer of the OMO electrode affects not only the generated current (closely related to the EQE response) but also the transmittance in the visible range, so normally a trade-off between the PCE and transmittance must be made. The thickness and electrical conductivity of each of these layers is related to the unit cell width, which determines the optimum efficiency of the module. Therefore, each layer thickness should be optimized in consideration of the change in the transmittance (as shown in Fig. 4) and efficiency of the module (as shown in Fig. 8) through in-depth iterations of experiments and simulations, which can be explored in further works.
modules for BIPV applications where OMO electrodes consisting of an AZO sandwiched Ag layer were used as the back electrode of the devices. The ~110 nm-thick OMO electrode exhibited a sheet resistance of as low as 6.8 Ω/□ and the average transmittance was as high as 88% in the visible range of 400–800 nm. We investigated the external quantum efficiency and average transmittance of the cell for varying thicknesses of the i-layer by measurements and FDTD simulation. An efficiency of 5.42–6.22% and T500-800 of 12.8–18.2% were obtained for i-layer thicknesses of 200–400 nm with a trade-off between the efficiency and T500-800. The FDTD simulation showed that the band-toband absorption of the front TCO in the ultra-violet region and freecarrier absorption of the back OMO in the infrared region are the main causes of the optical loss in the transparent cell. To fabricate the transparent modules, we used the monolithic patterning method with a 532 nm short-pulse high-power laser. We demonstrated that an unoptimized laser power for P2 could not only damage the front TCO or cause incomplete removal of a-Si:H layers, but also create unwanted OMO nucleation even after the P2 process. We also showed that the module PCE is sensitive to the average power of P3 because parasitic shunt paths at the edge of the P3 patterns were easily formed when using thin OMO electrodes in the transparent module. We demonstrated that the P2 and P3 processes can be optimized by observing the edge profiles of the patterns and measuring the shunt resistance of the modules. We also optimized the module cell width using an equivalent circuit model in the PSpice simulator; the highest module efficiency and average transparency in the range of 500–800 nm were found to be 5.6% and 15.2%, respectively. Additionally, the short-circuit current
4. Conclusions We fabricated transparent Si (a-Si:H) thin film solar cells and
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Fig. 8. Characteristics of a-Si:H modules according to the width of the unit cells. Each unit cell has the same length of 4 cm, but differs in width, having widths of 2, 4, 6, and 8 mm. Each module consists of five same unit cells with a 300-nm thick absorber layer, so the module sizes are 4, 8, 12, and 16 cm2 for the width of the different unit cells. Simulated and measured (a) J-V curves and (b) solar cell parameters. Solid lines denote the measured values and dashed lines denote the simulated values.
density, fill factor, and open circuit voltage per cell of the module were 10.9 mA/cm2, 62.7%, and 0.830 V, respectively. The optimization of the laser patterning process and the evaluation and design methods used for the transparent modules can help in the development of advanced transparent a-Si:H thin film solar modules using thin and highly transparent OMO electrodes.
Eshraghi, J., Narjabadifam, N., Mirkhani, N., Sadoughi Khosroshahi, S., Ashjaee, M., 2014. A comprehensive feasibility study of applying solar energy to design a zero energy building for a typical home in Tehran. Energy Build. 72, 329–339. 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. Energy Mater. Sol. Cells 95, 986–991. Ghosh, S.K., 2016. Biomass & bio-waste supply chain sustainability for bio-energy and bio-fuel production. Proc. Environ. Sci. 31, 31–39. Henley, S.J., Carey, J.D., Silva, S.R.P., 2005. Pulsed-laser-induced nanoscale island formation in thin metal-on-oxide films. Phys. Rev. B 72, 1–10. Jo, H., Yang, J.H., Choi, S.W., Park, J., Song, E.J., Shin, M., Ahn, J.H., Kwon, J.D., 2019. Highly transparent and conductive oxide-metal-oxide electrodes optimized at the percolation thickness of AgOx for transparent silicon thin-film solar cells. Sol. Energy Mater. Sol. Cells 202, 110131. Jo, H., Yang, J.H., Lee, Ji hoon, Lim, J.W., Lee, Jaesung, Shin, M., Ahn, J.H., Kwon, J.D., 2018. Transparent bifacial a-Si:H solar cells employing silver oxide embedded transparent rear electrodes for improved transparency. Sol. Energy, vol. 170, pp. 940–946. Jung, S.J., Kim, B.J., Shin, M., 2014. Low-refractive-index and high-transmittance silicon oxide with a mixed phase of n-type microcrystalline silicon as intermediate reflector layers for tandem solar cells. Sol. Energy Mater. Sol. Cells 121, 1–7. Karthikeya, B.R., Negi, P.S., Srikanth, N., 2016. Wind resource assessment for urban renewable energy application in Singapore. Renew. Energy 87, 403–414. Kim, H., Jo, J., Lee, G., Shin, M., Lee, J.C., 2017. Design and analysis of a highly reliable large-area Z-type transparent module for dye-sensitized solar cells. Sol. Energy 155, 585–592. King, D.L., Quintana, M.A., Kratochvil, J.A., Ellibee, D.E., Hansen, B.R., 2000. Photovoltaic module performance and durability following long-term field exposure. Prog. Photovolt. Res. Appl. 8, 241–256. Ku, S., Haas, S., Pieters, B.E., Zastrow, U., Besmehn, A., Ye, Q., Rau, U., 2011. Investigation of laser scribing of a-Si: H from the film side for solar modules using a UV laser with ns pulses. Appl. Phys. A Mater. Sci. Process. 105, 355–362. Ku, S., Pieters, B.E., Haas, S., Bauer, A., Ye, Q., Rau, U., 2013. Electrical characterization of P3 isolation lines patterned with a UV laser incident from the film side on thin-film silicon solar cells. Sol. Energy Mater. Sol. Cells 108, 87–92. Lee, Ganghoo, Shin, M., Lee, Gi yong, Ko, H., 2019. High-efficiency white-light solar window using waveguide glass plate. Energy Build., vol. 202, pp. 109341.
Declaration of Competing Interest None. Acknowledgments Funding: This work was supported by the Energy Technology Development Program of the Korean Institute of Energy Technology Evaluation and Planning (KETEP) [grant numbers 20172010104940 and 20192010107400]. References Asghar, M.I., Miettunen, K., Halme, J., Vahermaa, P., Toivola, M., Aitola, K., Lund, P., 2010. Review of stability for advanced dye solar cells. Energy Environ. Sci. 3, 418–426. Bernardino, M., Rusu, L., Guedes Soares, C., 2017. Evaluation of the wave energy resources in the Cape Verde Islands. Renew. Energy 101, 316–326. Brecl, K., Topič, M., Smole, F., 2005. A detailed study of monolithic contacts and electrical losses in a large-area thin-film module. Prog. Photovolt. Res. Appl. 13, 297–310. Choi, S.W., Yang, J., Park, J.H., Han, S.J., Song, P., Kang, D.W., Kwon, J.D., 2019. P/i interfacial engineering in semi-transparent silicon thin film solar cells for fabrication at a low temperature of 150 °C. Curr. Appl. Phys. 19, 1120–1126.
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Perera, F.P., 2017. Multiple threats to child health from fossil fuel combustion: impacts of air pollution and climate change. Environ. Health Perspect. 125, 141–148. Yang, J., Jo, H., Choi, S.W., Kang, D.W., Kwon, J.D., 2018. All p-i-n hydrogenated amorphous silicon oxide thin film solar cells for semi-transparent solar cells. Thin Solid Films 662, 97–102.
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Solar Energy journal homepage: www.elsevier.com/locate/solener
Optimization of laser-patterning process and module design for transparent amorphous silicon thin-film module using thin OMO back electrode Jaeho Parka,b, Soo-Won Choia,d, Sangah Leec, Jaesung Leec, Myunhun Shinc, , Jung-Dae Kwona,b, ⁎
T
⁎
a
Materials Center for Energy Convergence, Korea Institute of Materials Science, Changwon, Gyeongnam 51508, Republic of Korea Department of Advance Materials Engineering, University of Science and Technology (UST), 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea c School of Electronics and Information Engineering, Korea Aerospace University, Goyang, Gyeonggi 10540, Republic of Korea d Department of Materials Science and Engineering, Pusan National University, Busan, 46241, Republic of Korea b
ARTICLE INFO
ABSTRACT
Keywords: Transparent amorphous silicon photovoltaic Oxide-metal-oxide electrode Laser patterning Cell geometry Building integrated photovoltaic Equivalent circuit
Transparent hydrogenated amorphous silicon thin-film solar modules are fabricated using oxide-metal-oxide (OMO) electrodes as the back electrode for building-integrated photovoltaic applications. The outer aluminumdoped zinc oxide and inner silver layers constitute a thin OMO electrode (~110 nm thick), exhibiting a sheet resistance of 6.8 Ω/□ and an average transmittance of ~88% in the visible range of 400–800 nm. The external quantum efficiency and average transmittance of the cell were investigated for the absorber-layer thickness using the finite-difference time-domain method, and it was found that the optical loss in the cell was mainly due to the absorption of the front electrode in the ultra-violet region and free-carrier absorption of the OMO in the infrared region. Fabrication issues are introduced for a 532 nm short-pulse high-power laser patterning process for transparent modules with thin OMO electrodes. Optimization of the laser power for the P2 and P3 laser processes is demonstrated by observing the profiles and measuring the shunt resistance of the laser-patterned edges. Furthermore, the cell width is optimized based on an equivalent circuit model using PSpice simulation. The highest module efficiency and average transparency achieved in the range of 500–800 nm were 5.6% and 15.2%, respectively. The short-circuit current density, fill factor, and open-circuit voltage per cell of the module were found to be 10.8 mA/cm2, 62.7%, and 0.830 V, respectively.
1. Introduction The use of fossil fuels is one of the most direct causes of greenhouse gas emissions that contribute to global warming. In addition, volatile organic compounds, i.e., the by-products of burning fossil fuels, such as oxides of carbon, sulfides, and nitrides (COx, SOx, and NOx), generate fine dust, which is a major cause of urban air pollution, and pose a serious threat to the health of urban residents (Perera, 2017). Various renewable energies are essential as a city’s alternative energy source. Thus, wind, geothermal, bio-, wave, and solar energies have been attracting significant attention (Bernardino et al., 2017; Ghosh, 2016; Karthikeya et al., 2016; Nathan, 2016; Melikoglu, 2017). In cities and urban areas, electricity is the most commonly used energy source and it can be directly provided by solar cells. Therefore, solar cells are an important technology for realizing net zero-energy buildings (Eshraghi et al., 2014).
Many solar cells in practical use have focused on the characteristics of high power conversion efficiency (PCE) and low production cost, such as with multi- or mono-crystalline silicon (Si) solar cells. In innercity areas, solar cells can also be integrated into buildings without the need for additional installation space. Building-integrated photovoltaic (BIPV) devices can be constructed as roofs, facades, building-walls, and even windows (Lee et al., 2019). Modern buildings have started using glass exteriors as walls, in which power generation using solar cells along with other architectural characteristics, such as a variety of colors, flexibility for rounded areas, transparency, and durable stability during the building’s life cycle, are important features. Solar windows using dye-sensitized solar cells and organic photovoltaics feature flexibility and transparency, but their long-term stability for commercial usage has not been verified owing to electrolyte leakage, photo-thermal degradation, and thermal instability (Asghar et al., 2010). In contrast, thin film solar cells composed of hydrogenated amorphous silicon (a-
Abbreviations: AZO, aluminum-doped zinc oxide; BIPV, building-integrated photovoltaic; DPSS, diode pumped solid state; OMO, oxide-metal-oxide; PCE, power conversion efficiency ⁎ Corresponding authors at: School of Electronics and Information Engineering, Korea Aerospace University, Goyang, Gyeonggi 10540, Republic of Korea (M. Shin), and Materials Center for Energy Convergence, Korea Institute of Materials Science, Changwon, Gyeongnam 51508, Republic of Korea (J.-D. Kwon). E-mail addresses: [email protected] (M. Shin), [email protected] (J.-D. Kwon). https://doi.org/10.1016/j.solener.2020.02.092 Received 10 January 2020; Received in revised form 24 February 2020; Accepted 26 February 2020 Available online 03 March 2020 0038-092X/ © 2020 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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thereby separating each individual cell. When using very thin OMO electrodes for a transparent solar module, the laser power of the P2 and P3 processes must be carefully optimized. The laser power of the P2 and P3 processes needs to be sufficient to remove the a-Si:H absorber layers, but not excessive, to mitigate the damage of the front TCO underneath and to reduce thermal defects in the a-Si:H layers close to the laser scribed patterns. In addition, the deposition of the OMO layers is affected by the P2 process and the transmittance or reflectance of the back electrodes affects the P3 process. In this work, we fabricated a transparent a-Si:H thin film module using OMO electrodes as the back electrode to present the optical and electrical optimization of the module design and optimize the laser patterning process for transparent modules. Using the finite-difference time-domain (FDTD) method, we analyzed the optical loss in the transparent a-Si:H solar cells and optimized the cell width based on an equivalent circuit model using the PSpice simulator. We also introduced fabrication problems in the nano-second laser (532 nm) patterning processes of transparent solar modules when using very thin OMO electrodes. The P2 and P3 laser processes were examined and optimized using focused ion beam-scanning electron microscopy (FIB-SEM), optical microscopy (OM), and measuring the current density and voltage (J-V) characteristics of the fabricated modules.
ZnO:Al (50 nm) Ag (8 nm) ZnO:Al (50 nm) n-uc-SiOx:H (30 nm) i-a-Si:H (200, 300, 400 nm) p-SiOx:H (15 nm) SnO2:F (600 nm) Glass Fig. 1. Schematic cross-section of transparent a-Si:H solar cell employing textured FTO as a front electrode and OMO as a back electrode. The thickness of ilayer was varied (200, 300, and 400 nm).
2. Module fabrication, simulation, and characterization 2.1. Fabrication of cell and module
Si:H) have already proven their stability by being installed and operated in the field for more than 20 years (King et al., 2000). In addition, the a-Si:H thin film solar cells use Si, which is abundant on earth; hence, their material cost is low. It has also been demonstrated in the display industry that the manufacturing processes using large-area glass substrates are feasible, which is essential for the large windows of modern buildings. Recently, we implemented an oxide-metal-oxide (OMO) structure as the back electrode of a transparent a-Si:H thin film solar cell for a BIPV window (Choi et al., 2019; Jo et al., 2018, 2019; Yang et al., 2018). In general, OMOs have the advantage of simultaneously increasing the electrical conductivity and optical transmittance by including a very thin metal layer with high electrical conductivity between the highly transparent conductive oxide (TCO) films. When silver (Ag) is used as the interstitial metal layer between TCO films at the percolation thickness, the OMO film exhibits optimized performance considering the optical transmittance and PCE of the transparent a-Si:H solar cell. Monolithic fabrication of a thin film module on a glass substrate commonly uses a three-step patterning process with a short-pulse high power laser (Jung et al., 2014). The first pattern (P1) removes the front TCO along a line isolating the front electrodes. The second pattern (P2) ablates the absorber semiconductor layer, thereby creating contact lines that connect the front TCO electrodes in series with the back electrodes. The third pattern (P3) removes both the absorber and back electrode,
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2.1.1. Fabrication of transparent a-Si:H solar cell Fig. 1 depicts the structure of the transparent a-Si:H solar cell. To fabricate the solar cells, we used a commercial glass substrate with a deposited textured fluorine-doped tin oxide (FTO, 600 nm) layer (Pilkington Ltd., 8 Ω/□, 50 mm × 50 mm). After the substrate was cleaned with a neutral detergent, p-, i- and n-type (p-/i-/n-) layers were sequentially deposited on the substrate by radio frequency (RF) plasmaenhanced chemical vapor deposition (PECVD) at 250 °C. The i-layer of a-Si:H was obtained from a silane (SiH4) source gas diluted by hydrogen (H2) at high-frequency RF conditions of 40 MHz and 30 W. The i-layer with thicknesses of 200, 300, and 400 nm was used to observe the dependence of the characteristics of cells on i-layer thickness. The ptype hydrogenated amorphous silicon oxide (p-a-SiOx:H) and n-type hydrogenated microcrystalline silicon oxide (n-μc-SiOx:H) layers were prepared using mixed gases of diborane (B2H6, 1% diluted in H2), phosphine (PH3, 1% diluted in H2), and carbon dioxide (CO2) of oxygen dopant under the RF conditions of 13 MHz and 50 W. An OMO structure was applied as the back electrode. Aluminum-doped zinc oxide (AZO) with a thickness of 50 nm was used in the upper and lower oxide layers and Ag with a thickness of 8 nm was used in the middle metal layer. Under these conditions, the OMO electrodes exhibited a sheet resistance of 6.8 Ω/□ and average transmittance of 88% in the visible range of 400–800 nm. The oxide and metal layers were deposited using a magnetron sputtering system at DC powers of 500 and 50 W,
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Fig. 2. Schematic of the laser scribing process for fabricating amorphous silicon solar cell module.
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Fig. 3. 2D equivalent circuit model of transparent a-Si:H solar cell.
respectively. The sputtering process was performed at room temperature in a high vacuum (2 × 10−6 Torr) and a shadow mask was used to define a cell active area of 0.25 cm2 during the OMO deposition.
measured optical parameters of the individual layers of the OMO electrode (AZO and Ag) and the p-/i-/n-layers of a-Si:H cells to determine the structural parameters. Layer thicknesses (as shown in Fig. 1) were also used in the simulations. The transmittance and absorbance of the cell were estimated by FDTD simulation for varying values of the absorber layer thickness. To design the module based on the performance parameters of a single transparent a-Si:H cell and the sheet resistances of the front TCO and back OMO, we constructed a distributed equivalent circuit model of the module, as shown in Fig. 3. The equivalent circuit model can incorporate the dead area loss (the observed area loss between P1–P3) and resistance losses in the front TCO and back OMO to estimate the performance of the module (Kim et al., 2017). In addition to the area loss in P1–P3, the resistance loss in the thin OMO back electrodes is a major cause of transparent module degradation. Each of these effects depends on the unit cell size of the module. Thus, the width of the cell is carefully designed based on the simulation results.
2.1.2. Fabrication of transparent a-Si:H solar module To fabricate the module, a diode-pumped solid state (DPSS) laser delivering a short-pulse high-power output was used for the patterning process. The DPSS laser employed a neodymium-doped yttrium orthovanadate (Nd:YVO4) gain medium, operated at a wavelength of 532 nm to emit optical pulses with a variable pulse duration of up to 41 ns. The overlap ratio of the scribed patterns was controlled by the frequency and scan speed of the laser. In addition, the laser beam was focused to be as flat as the laser equipment. Thus, in our experiments, the laser output power was adjusted by changing the pump diode current of the laser and fine-tuned with an external attenuator. The laser beam was delivered to samples by a scanner head, which processed the high-speed patterning. The three-step scribing process (P1, P2, and P3) was used for the monolithic interconnection of solar cells in a module, as shown in Fig. 2, where P1, P2, and P3 were performed with the laser incident on the glass substrate side. The P1 process patterned the isolation lines in the FTO; then, the p-/i-/n-layers of the a-Si:H cells were deposited on the patterned FTO glass by PECVD. The P2 process was performed to selectively remove the p-/i-/n-layers from the front FTO electrode layer. After OMO electrode deposition, P3 ablated the p-/i-/n-layers, thereby removing the OMO back electrode. It is generally known that the absorber layers are instantaneously ablated by absorbing the laser power in the P2 and P3 processes, and the back electrodes are mechanically removed by P3 when the absorber layers are ablated by a short-pulse laser. After P3, the cells were separated and connected in series. In this study, modules with various cell widths ranging from 2 to 8 mm were fabricated to compare the performance parameters of modules as a function of cell width.
2.2.2. Characterization OM and FIB-SEM (Nova 200, FEI) were used to observe the surface and cross-section of the cells after the layer patterning process, respectively. A spectroscopic ellipsometer (SE MG-1000, Nano-View Co.) was used to determine the refractive indices, extinction coefficients, and thickness of the absorber layer on the glass substrates. The transmittance and reflectance of the cells were obtained via ultraviolet–visible (UV–VIS) spectrophotometry (Cary 5000, Varian). The JV characteristics of the solar cells and modules were measured using a solar simulator (Oriel 300, Newport Co.) under standard 1-sun illumination (100 mW/cm2 of AM1.5G). When measuring the performance of a cell or module, solar irradiance masks were used and aligned to the apertures of the devices to avoid potential overestimation of the photocurrents of the devices. The external quantum efficiency (EQE) (QuantX 300, ORIEL) of the solar cells was measured to observe the spectral response.
2.2. Simulation and characterization 2.2.1. Simulation of transparent a-Si:H solar module The optical transmittance and absorbance were evaluated using the FDTD method. For the FDTD simulation, we used the ellipsometrically
3. Results and discussion Fig. 4 shows a comparison between the simulated EQE and
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Fig. 4. Characteristics of a-Si:H solar cells (the area of ~0.25 cm2) for different i-layer thicknesses (200, 300, 400 nm). (a) Simulated external quantum efficiency (EQE), (b) simulated total transmittance, (c) measured EQE, (d) measured total transmittance, (e) measured and simulated reflectance of cells, and (f) simulated absorbance of each layer for an i-layer thickness of 300 nm.
transmittance spectra and the measured ones that depend on the thickness of the i-layer. Overall, the EQE increased with the increasing thickness of the i-layer whereas the total transmittance decreased. In the simulation shown in Fig. 4a and b, the short wavelength light was expected to be absorbed in the front TCO (under ~ 350 nm) and the player (350–400 nm); however, the spectral EQE response in the visible light in the range of 450–700 nm was expected to increase with the thickness of i-layer. The simulation results were significantly consistent
with the measured EQE response and transmittance shown in Fig. 4c and d. The transmittance was first observed at wavelengths over 500 nm; it increased with the wavelength. In contrast, the overall transmittance decreased with the thickness of the i-layer. In Fig. 4e, neither the simulated nor measured reflectance changed significantly with the thickness of the i-layer. In thin film solar cells, it is difficult to directly measure the optical losses occurring in each layer. Thus, we simulated the optical losses (absorbance) in each layer, as shown in
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demonstrates the schematic cross-sectional structure of the laser patterning of the module, where the cells are connected monolithically in series. Fig. 6b shows an OM image taken from the film side of the module. In Fig. 6b, the line widths of P1–P3 are 34, 27, and 30 µm, respectively, and the spaces between P1–P2 and P2–P3 are 100 µm each. Fig. 6a indicates the active and dead areas; the width of the unit cell is the sum of these areas. The area between P1 and P3 (~200 µm) is called the dead area because it cannot generate power effectively (Brecl et al., 2005). Fig. 6c shows the cross-sectional FIB-SEM images at the edges of the laser patterns of P1–P3. In Fig. 6c, it can be observed that the FTO layer forms a gentle slope at the pattern edge of P1 but the aSi:H layers are sharply cut at the pattern edges of P2 and P3. The edge of P2 should have a smooth profile to reliably connect the step coverage of the thin OMO layers. Besides the edge profile, there are more difficulties in the P2 process associated with the OMO layers. The P2 process does not damage the underlying layer of the front electrode (FTO) but also needs to selectively remove the absorber layers (a-Si:H). When P2 was performed at an average laser power lower than 1.2 W, the absorber was not completely removed and the remaining absorber formed edge ridges, as shown in Fig. 6d. Incomplete removal of the absorber layer and unevenly formed contact between the front and back electrode led to an increase in the series resistance of the module (Ku et al., 2011). When P2 was performed at an average power higher than 2.0 W, damages were observed on the front TCO; even for powers below 2.0 W, it was often observed that the deposited OMO formed island-like growths, as evident in Fig. 6e. We observed significant loss in the series resistance of the module with the P2 edge of Fig. 6e. We believe that some a-Si:H residue, which was scattered by the excess laser power, acted as nucleation centers for the growth of nanoscale islands of OMO during the deposition due to the surface interaction described in the work of Henley et al. (2005). Opaque solar cells generally use a reflective back metal as the back electrodes. When P3 is performed with a reflective back metal electrode in an opaque solar cell module, the metal electrodes reflect the laser beam back into the absorber layer, which enables P3 at reduced laser power, thereby mitigating thermal damage to the cells. However, for transparent cells, the OMO electrodes are designed to be transparent in the visible range. The OMO is very transparent to the P3 laser output at 532 nm. Moreover, the effective refractive index of the OMO is between those of the absorber and air, and the reflection at the interface of the OMO and absorber layers is lower than without the OMO layer during the P3 process. Thus, more power is required for the P3 process than the P2 process. Nevertheless, we used a short-pulse laser with a pulse duration of less than 41 ns. The heat generated during P3 can be transferred to the adjacent active region of the a-Si:H layers or the OMO electrodes near the edge of the patterns. Such rapid heating causes unwanted phase transitions and deformations in the materials. In addition, the thin OMO electrodes can form burs or give rise to debris under unoptimized laser process conditions. Fig. 7a shows that parasitic shunt resistance due to P3 and the PCE of the module are closely related and both are very sensitive to the average laser power of P3. If the average power is insufficient to completely remove the OMO layer, the OMO layer partially remains. The partially remaining OMO layer electrically connects the adjacent cells serving as shunt paths; thus, the shunt resistance increases with the average power of P3 in Fig. 7a. Interestingly, the shunt resistance becomes worse, thereby deteriorating the cell performance at average powers of P3 exceeding a certain value (1.35 W), as shown in Fig. 7a. We expect that there are several reasons for the shunt resistance behavior demonstrated in Fig. 7a. First, the OMO edge undergoes plastic
Fig. 5. Dependency of J-V characteristics of a-Si:H solar cells on thickness of the absorption layer (the cell area is ~0.25 cm2). Table 1 Trade-off between the efficiency and transmittance of a-Si:H solar cell depends on the thickness of the absorption layer. i-layer thickness (nm)
Efficiency (%)
T500–800 (%)
200 300 400
5.42 5.90 6.22
18.2 15.2 12.8
T500–800: average transmittance over 500–800 nm.
Fig. 4f. In this figure, the absorbance in the front TCO contributes to lowering the EQEs of the i-layers and accounts for the nonlinear inflections observed in the EQE curves of the i-layers in the 350–400 nm range in Fig. 4a and c. Fig. 4f reveals the absorbance still present in the thin back OMO in the 650–800 nm range, and the small absorbance due to thin p-type and n-type layers can also contribute to reduce the cell efficiency. Thus, the results shown in Fig. 4f suggest that the high bandgap materials in the front TCO and p-type layer can be modified to improve the EQE response of the cell by reducing the optical loss in a short wavelength range. In addition, considering the free carrier absorptions in metals in the long wavelength range, appropriately selecting the middle metal and amount of dopant in the rear TCO can reduce the optical loss in the cell and improve the transmittance in the visible range of 500–800 nm. It should be noted that both the simulated and measured reflectance and the optical losses in each layer did not significantly change with the change in the thickness of the i-layer. Fig. 5 presents the measured J-V characteristics of the fabricated cells for each thickness of the i-layer. When the i-layer thickness increases, no significant change is observed in the open circuit voltage (VOC) and fill factor (FF) of the cells; however, the short circuit current density (JSC) of the cell increases, which results in an increase in the cell efficiency. These results are consistent with the observed behaviors in Fig. 4 and are summarized in Table 1. Therefore, to optimize the a-Si:H solar cell for BIPV applications, it is necessary to select the appropriate i-layer thickness because there is a trade-off between the conversion efficiency. As introduced, the P2 and P3 processes can affect the fabrication process and final performance of transparent solar cell modules employing the thin OMO electrodes (thickness = 108 nm). Fig. 6a
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Fig. 6. (a) Cross-sectional structure of the a-Si:H module for monolithic serial inter-connection; (b) optical microscopy image of P1, P2, and P3 scribes; and (c) FIBSEM images of P1, P2, and P3. Result of performing P2 with an average power of (d) 0.59 W and (e) 2.0 W.
deformation (partial melting) during the P3 process due to the excessive energy forming a structure shown in Fig. 7b. The thin OMO layer used in this work (total thickness is less than 110 nm) can be deformed very easily. Second, a conductive sidewall is formed by the evaporation and re-deposition of the front FTO as a result of the excess power of P3, as depicted in Fig. 7c. (Ku et al., 2013; García-Ballesteros et al., 2011). At the higher average powers of P3, damages to the front FTO were confirmed in the OM image. The OM image (average power = 2.0 W) in Fig. 7a exhibits bright areas of damaged FTO. Finally, thermal defects or recrystallization of a-Si:H layers by the excess laser power can reduce the shunt resistance near the P3 edges. As shown in Fig. 7a, P3 must be carefully optimized and stabilized while processing the transparent modules with thin OMO electrodes. Using the measured J-V characteristics of a small transparent a-Si:H cell (0.25 cm2) and the sheet resistances of the front TCO and back OMOs, we configured the distributed equivalent circuits of the modules for various widths of the unit cells. In the modules, the unit cells were
4 cm long, and 2, 4, 6, and 8 mm wide; the number of cells was fixed to 5 to exclude problems induced by the number of laser scribes. Fig. 8a shows the simulated and measured J-V characteristics of the modules where the current density of a module is considered to be a module current divided by the area of a unit cell and the voltage per cell is the module voltage divided by the number of unit cells in a module. All performance parameters of the module are compared in Fig. 8b for varying widths of the unit cells (JSC and VOC are the current density of the module at short circuit condition and voltage per cell at open circuit condition, respectively). As the unit cell width increases from 2 to 8 mm, the JSC increases from 10.5 to 11.2 mA/cm2, and FF deteriorates from 63.7 to 56%, but VOC exhibits minimal changes from 0.825 V. Fig. 8b implies that the major loss in the module for increasing the unit cell width is the resistive loss. When the unit cell width increases, the relative area loss (dead area for the unit cell) decreases such that JSC increases. However, the resistance in the front TCO or the back OMO increases with the unit cell width; thus, the FF decreases without
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Fig. 7. (a) Influence of the average power of P3 on fabricated modules. The modules consist of 10 cells (4-mm width and 4-cm length) so that the total area is ~16 cm2. Left y-axis shows normalized shunt resistance (RSH), right y-axis shows normalized power conversion efficiency (PCE). Schematics reducing shunt resistance (b) plastic deformation of OMO or (c) conductive sidewall of FTO.
serious degradation of the VOC of the module. Therefore, as shown in Fig. 8a, the optimized transparent module with a unit cell width of approximately 4 mm exhibits the highest module efficiency (η) of 5.6% with JSC, FF, and VOC of 10.9 mA/cm2, 62.7%, and 0.830 V, respectively. In this work, we used the 300-nm thick i-layer to fabricate the modules and optimize the module with the unit cell width, because at the thickness of 300 nm, the EQE was better than that of 200-nm thick ilayer and the transmittance was better than that of 400-nm thick ilayer, respectively. However, it is noteworthy that the device structure can be further optimized by considering both optical and electrical characteristics of the overall module performance, such as the best figure of merit (the product of the transparency and PCE in a module) (Jo et al., 2019). The thickness of the i-layer, front TCO and each layer of the OMO electrode affects not only the generated current (closely related to the EQE response) but also the transmittance in the visible range, so normally a trade-off between the PCE and transmittance must be made. The thickness and electrical conductivity of each of these layers is related to the unit cell width, which determines the optimum efficiency of the module. Therefore, each layer thickness should be optimized in consideration of the change in the transmittance (as shown in Fig. 4) and efficiency of the module (as shown in Fig. 8) through in-depth iterations of experiments and simulations, which can be explored in further works.
modules for BIPV applications where OMO electrodes consisting of an AZO sandwiched Ag layer were used as the back electrode of the devices. The ~110 nm-thick OMO electrode exhibited a sheet resistance of as low as 6.8 Ω/□ and the average transmittance was as high as 88% in the visible range of 400–800 nm. We investigated the external quantum efficiency and average transmittance of the cell for varying thicknesses of the i-layer by measurements and FDTD simulation. An efficiency of 5.42–6.22% and T500-800 of 12.8–18.2% were obtained for i-layer thicknesses of 200–400 nm with a trade-off between the efficiency and T500-800. The FDTD simulation showed that the band-toband absorption of the front TCO in the ultra-violet region and freecarrier absorption of the back OMO in the infrared region are the main causes of the optical loss in the transparent cell. To fabricate the transparent modules, we used the monolithic patterning method with a 532 nm short-pulse high-power laser. We demonstrated that an unoptimized laser power for P2 could not only damage the front TCO or cause incomplete removal of a-Si:H layers, but also create unwanted OMO nucleation even after the P2 process. We also showed that the module PCE is sensitive to the average power of P3 because parasitic shunt paths at the edge of the P3 patterns were easily formed when using thin OMO electrodes in the transparent module. We demonstrated that the P2 and P3 processes can be optimized by observing the edge profiles of the patterns and measuring the shunt resistance of the modules. We also optimized the module cell width using an equivalent circuit model in the PSpice simulator; the highest module efficiency and average transparency in the range of 500–800 nm were found to be 5.6% and 15.2%, respectively. Additionally, the short-circuit current
4. Conclusions We fabricated transparent Si (a-Si:H) thin film solar cells and
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Fig. 8. Characteristics of a-Si:H modules according to the width of the unit cells. Each unit cell has the same length of 4 cm, but differs in width, having widths of 2, 4, 6, and 8 mm. Each module consists of five same unit cells with a 300-nm thick absorber layer, so the module sizes are 4, 8, 12, and 16 cm2 for the width of the different unit cells. Simulated and measured (a) J-V curves and (b) solar cell parameters. Solid lines denote the measured values and dashed lines denote the simulated values.
density, fill factor, and open circuit voltage per cell of the module were 10.9 mA/cm2, 62.7%, and 0.830 V, respectively. The optimization of the laser patterning process and the evaluation and design methods used for the transparent modules can help in the development of advanced transparent a-Si:H thin film solar modules using thin and highly transparent OMO electrodes.
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