Numerical Optimization of Gradient Bandgap ...

energies Article

Numerical Optimization of Gradient Bandgap Structure for CIGS Solar Cell with ZnS Buffer Layer Using Technology Computer-Aided Design Simulation Joonghyun Park 1,2 and Myunghun Shin 3, * 1 2 3

*

School of Electronic Electrical Engineering, College of Information and Communication Engineering, Sungkyunkwan University, Suwon 440-746, Korea; [email protected] Samsung Display Co., Ltd., 1, Samsung-ro, Yongin-si, Gyeonggi-do 17113, Korea School of Electronics and Information Engineering, Korea Aerospace University, Goyang-city, Gyeonggi-do 412-791, Korea Correspondence: [email protected] or [email protected]; Tel.: +82-2-300-0145

Received: 21 June 2018; Accepted: 5 July 2018; Published: 7 July 2018







Abstract: The band structure characteristics of a copper indium gallium sulfur selenide (Cu(In1–x Gax )SeS, CIGS) solar cell incorporating a cadmium-free zinc sulfide (ZnS) buffer layer were investigated using technology computer-aided design simulations. Considering the optical/electrical properties that depend on the Ga content, we numerically demonstrated that the front gradient bandgap enhanced the electron movement over the band-offset of the ZnS interface barrier, and the back gradient bandgap generated a back side field, improving electron transport in the CIGS layer; in addition, the short circuit current density (JSC ) and open circuit voltage (V OC ) improved. The simulation demonstrated that the conversion efficiency of a double graded bandgap cell is higher than with uniform or normal/reverse gradient cells, and V OC strongly correlated with the average bandgap in the space charge region (SCR) of CIGS. After selecting V OC from the SCR, we optimized the band structure of the CIGS cell with a Cd-free ZnS buffer by evaluating JSC and the fill factor. We demonstrated that the cell efficiency of the fabricated cell was more than 15%, which agrees well with the simulated results. Our numerical method can be used to design high-conversion efficiency CIGS cells with a gradient band structure and Cd-free buffer layer. Keywords: CIGS (copper indium gallium selenide); band structure; double graded bandgap; TCAD (technology computer-aided design) simulation; carrier transportation

1. Introduction Copper indium gallium sulfur selenide (Cu(In1−x Gax )SeS, CIGS) solar cells have been widely studied because of their various advantages. For thin film-based solar cells, CIGS solar cells can achieve an efficiency of over 20%, which is as high as that of high-quality crystalline silicon-based solar cells at small sizes (~0.25 cm2 ) [1]. CIGS cells fabricated on flexible substrates such as polyimide or titanium foils can be applied to wider areas than conventional Si solar cells [2]. In order to avoid toxicity to the environment, cadmium-free (Cd-free) zinc sulfide (ZnS) layers can be used as buffer layers for the CIGS solar cells [3]. The bandgap is the key feature of solar cells determining the optical and electrical characteristics of semiconductor devices, and in CIGS solar cells, the bandgap (Eg ) can be engineered by controlling the gallium or sulfur content. Thus, one of the particular merits of CIGS solar cells is that the bandgap structure can be designed with a Ga or S gradient, and a CIGS solar

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cell with an optimized bandgap profile can be fabricated through elemental substitution during the deposition and annealing processes. The Ga/(Ga + In) ratio, the parameter controlling the bandgap of CIGS with the Ga content, and its depth profile in the CIGS layer were studied to increase the efficiency, and the correlation between the CIGS bandgap structure and cell efficiency was analyzed using the numerical simulations in several studies. Gloeckler and Sites, and Song et al. reported that the improvement in back grading in the CIGS solar cell could potentially increase the efficiency when the thickness of the absorber decreased [4–6]. In 2004 and 2010, Song et al. reported various bandgap profiles in the CIGS layer and suggested an optimum bandgap structure, the double graded bandgap (DGB, bandgap gradings on both front and back sides of the CIGS absorption layer) profile using the AMPS-1D and DESSIS 2D simulators [5,6]; the front grading improved the open circuit voltage (V OC ) without reducing the short circuit current density (JSC ) and the back grading improved the V OC . Dullweber et al. reported the effect of back grading on the CIGS layer, which suppressed the carrier recombination and resulted in an enhancement in the V OC by up to 0.09 V [7]. In this work, we demonstrate a numerical procedure to design a gradient band structure for CIGS solar cells with a Cd-free ZnS buffer layer and demonstrate bandgap-optimization to fabricate the CIGS solar cells using a two-stage process. It is well known in CIGS solar cells the band alignment between absorber layer and buffer layer is so important [8]. However, this paper focus on bandgap alignment of absorber layer. Using a technology computer-aided design simulation (TCAD), we investigated the effect of the gradient bandgap near the interface of the ZnS buffer layer on the conversion efficiency parameters of the CIGS cells, which has not been fully proven yet. The conversion efficiency of the CIGS solar cells is also evaluated not only for the front gradient bandgap (FGB) and back gradient bandgap (BGB), but also for various other types of gradient bandgap profiles. In the simulation, in order to reduce the performance difference in the design and fabrication, we use the band structural parameters obtained from the measurements and practical data from the literature, which match well with our device. This work will help us understand the engineering of the bandgap profile and management of the defect density related to the Ga/(Ga + In) ratio, which is key to achieving high efficiencies. 2. Experiment and Simulations In this work, the CIGS solar cells were fabricated by a two-step process, the deposition of a metallic precursor and annealing in a Se and S atmosphere. The general process is as follows. A 300-nm-thick Mo layer is deposited as a back-contact metal by DC magnetron sputtering on a soda lime glass substrate. Cu, In, and Ga alloyed metals are deposited by a DC sputter as a precursor on the Mo layer. The CIGS precursor is annealed in H2 Se and H2 S gas sequentially. By co-sputtering Cu-Ga, and in targets, we controlled the Ga profile in the precursor, and by adjusting the partial pressure of H2 Se and H2 S gases, we controlled Se, S contents in the CIGS cells. CIGS bandgap and profiling the Ga ratio were analyzed using various material analysis methods, and reported in detail in other studies [9,10]. Following these steps, the metallic precursor is reacted and diffused at high temperatures, and the designed gradient bandgap profile in the CIGS layer can only form with the proper precursor and thermal process conditions. After the annealing process, the CIGS absorption layer with a thickness of about 1.8 µm is formed. An approximately 30-nm-thick ZnS buffer layer is deposited by chemical bath deposition (CBD) on the CIGS layer. A 30-nm-thick undoped zinc oxide buffer layer (passivating the vacancy in ZnS layer) and 1.2-µm-thick boron-doped ZnO layer (front transparent conducting oxide, the front contact metal layer) are deposited sequentially by low-pressure chemical vapor deposition without any vacuum break. The device structure, and the scanning electron microscope (SEM) image of fabricated CIGS solar cell are shown in Figure 1a. Statistical approach is important for evaluating the performance of solar cells [11]. When the basic experimental conditions were satisfied, the characteristics and performance parameters of fabricated CIGS cells showed statistically very stable with deviation of 10% or less. More details on the fabrication process are reported elsewhere [9,10].

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The effects of the bandgap gradient on the CIGS solar cell conversion efficiency are investigated the bandgap gradientInc., on the CIGS solar cell conversion efficiency are investigated usingThe a effects TCADofsimulator (Synopsys Mountain View, USA). First, we characterized the using a TCAD simulator (Synopsys Inc., Mountain View, USA). First, we characterized the simulation simulation parameters such as thickness, carrier density, and material properties. Then, the parameters thickness, density, properties. Then, the scanning thickness electron of each thickness ofsuch eachaslayer in the carrier CIGS solar celland wasmaterial measured by cross-sectional layer in the CIGS solar cell was measured by cross-sectional scanning electron microscopy and the microscopy and the carrier density in the CIGS layer was estimated by the capacitive-voltage carrier density in the CIGS layer was estimated by the capacitive-voltage method. The elemental method. The elemental composition in the CIGS layer was measured using secondary ion mass composition CIGS layer was measured using secondary mass spectroscopy calculate the spectroscopyintothe calculate the Ga/(Ga + In) ratio as the gradient ion bandgap parameter. Into order to apply Ga/(Ga + In) ratio as the gradient bandgap parameter. In order to apply the optical characteristics the optical characteristics to the simulation, we modeled the absorption coefficient as a function of to simulation, we by modeled thethe absorption coefficient as a function of the Ga/(Ga + In) ratiocan by thethe Ga/(Ga + In) ratio adopting results from [12]. Because the bandgaps of the CIGS layers adopting the results from [12]. Because the bandgaps of the CIGS layers can be varied from 1.04 eV to be varied from 1.04 eV to 1.68 eV according to the Ga content, their optical parameters were also 1.68 eV according to the Ga content, their optical parameters were also calculated with the bandgap, calculated with the bandgap, which was linearly interpolated. It is well known that the energy level, which was linearly It is states well known that the energy density, and of the carrier defect density, and shapeinterpolated. of the defect are important factorslevel, determining theshape minority states are important factors determining the minority carrier lifetime; particularly, V is strongly lifetime; particularly, VOC is strongly dependent on the minority carrier lifetime. OC Hanna et al. dependent onthe thedefect minority carrier lifetime. Hanna et al.of reported thatthe theacceptor-like defect states states in the of CIGS reported that states in the CIGS layer consist two types; the layer consist of two types; the acceptor-like states of the Cu antisite and donor-like states of the Cu Cu antisite and donor-like states of the Cu interstitial or In antisite [13]. Of these, we primarily interstitial In antisite [13]. states Of these, considered antisite states in the simulation consideredorthe Cu antisite in we theprimarily simulation because the the Cu acceptor-like state in the p-type because the acceptor-like state in the p-type semiconductor is a dominant factor. The defect density semiconductor is a dominant factor. The defect density of the CIGS layer as a function of Ga of the CIGS layer as a applied functiontoofthe Gasimulation. composition also applied to Bthe referred composition was also Wewas referred to model of simulation. the results ofWe Frisk et al. to model B of the results of Frisk et al. [14]. We used a linear DGB structure of the CIGS layer [14]. We used a linear DGB structure of the CIGS layer with a ZnS buffer as [4–6], wherewith the aminimum ZnS buffer as [4–6], where the minimum bandgap of 1.1 eV is located at a depth of 0.25 µm in the bandgap of 1.1 eV is located at a depth of 0.25 μm in the CIGS from the interface of the CIGS from the of the ZnS buffer,isand is contacting 1.5 eV at a depth 1.8 µm ZnS buffer, andinterface the maximum bandgap 1.5the eV maximum at a depthbandgap of 1.8 μm the Mooflayer as contacting the Mo layer as shown in Figure 1b. From reference the structure, wethe investigated the shown in Figure 1 (b). From the reference structure, wethe investigated effects of FGB and BGB effects the solar FGB and on the of CIGS cells.BGB on the CIGS solar cells.

(a)

(b)

Figure 1. 1. Device Devicestructure: structure:(a) (a)scanning scanningelectron electron microscope (SEM) image of fabricated CIGS Figure microscope (SEM) image of fabricated CIGS solarsolar cell, cell, and (b) schematic band structure for TCAD simulation, where FGB, BGB, and SCR are front and (b) schematic band structure for TCAD simulation, where FGB, BGB, and SCR are front gradient gradient bandgap region, back bandgap gradient bandgap region, space charge region, respectively. bandgap region, back gradient region, and spaceand charge region, respectively.

3. Results and Discussion 3. Results and Discussion In our CIGS solar cell, the Ga gradients were built on both the front and back of the CIGS layer. In our CIGS solar cell, the Ga gradients were built on both the front and back of the CIGS layer. In order to apply the Ga gradient effect to the device simulation, we used the results of Wei et al.; the In order to apply the Ga gradient effect to the device simulation, we used the results of Wei et al.; bandgap gradient effect of the CIGS layer by the variation of the Ga content mainly appears as an the bandgap gradient effect of the CIGS layer by the variation of the Ga content mainly appears as an increase in the conduction band derived from first-principles bandgap theory [13]. In the simulation, increase in the conduction band derived from first-principles bandgap theory [13]. In the simulation, the gradient bandgap profiles and electron affinities with various bandgaps were modeled the gradient bandgap profiles and electron affinities with various bandgaps were modeled numerically numerically using Equations (1) and (2): using Equations (1) and (2): (x)==−−0.375 (1) EgEg(x) 0.375 x22 ++ 1.1075 1.1075 xx ++ 0.98 0.98 (1)

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Chi (x) = −0.475 x2 + 0.595 x + 4.48

(2)

where Eg (x) and Chi (x) are the bandgap and electron affinity, respectively, and x is the Ga/(Ga + In) ratio in the CIGS layer. The defect model based on [15] is empirical, but it fits well to our experimental results. The other parameters were selected in a reasonable range or referred from a previous paper [16]. The main simulation parameters used in our simulations are listed in Table 1. Table 1. Primary parameters of the materials in CIGS solar cell. Parameters

Unit

Absorber p-CI1 − x Gx S

Buffer n-ZnS

Window n-ZnO

Thickness Bandgap Electron affinity Carrier density Dielectric constant Electron/Hole mobility

nm eV eV /cm3 cm2 /(V·s)

1800 −0.375 x2 + 1.1075 x + 0.98 −0.475 x2 + 0.595 x + 4.48 1.5 × 1016 13.6 100/25

30 3.0 4.7 1.5 × 1018 9 100/25

1200 3.4 4.5 1.0 × 1018 9 100/25

/cm3 eV eV cm2 cm2

Variable with Ga ratio Mid-gap 0.1 5.0 × 10−13 1.0 × 10 −15

1.0 × 1018 Mid-gap 0.1 1.0 × 10−17 1.0 × 10 −12

1.0 × 1017 Mid-gap 0.1 1.0 × 10−12 1.0 × 10 −15

Defect state parameters Density Peak level Standard deviation Electron capture cross-section Hole capture cross-section

The electron and hole pairs generated by sunlight are separated and transferred to the opposite side. The movement of the carriers is greatly affected by the bandgap profile in the solar cells. The electron and hole pairs can be moved easily in the space charge region (SCR) where a high internal electric field is applied. Especially, the transportation of minority carriers in the CIGS absorption layer, and their movement over the band barrier at the interface of ZnS (due to electron affinity), are important causes for the conversion efficiency of the CIGS solar cells with ZnS buffer layers. Thus, the well-designed FGB of the CIGS layer could become the decisive factor determining the improvement in carrier transportation in the SCR. In particular cases, the FGB could disturb the carrier collection. Therefore, we investigated the conversion efficiency of the CIGS solar cell for the extent of the band gradient of the FGB structures. Figure 2 shows the conduction band and electron concentration profiles in the SCR of the CIGS at the steady state when the bandgap energies at the front edge are 1.1 eV, 1.18 eV, 1.24 eV, and 1.3 eV. The conduction band offset (CBO)—the mismatched alignment in the conduction band between the buffer and CIGS absorber layers—was measured from the J–V distortion curves under red light and dark illuminations [17]. We note that the shapes of the conduction band in Figure 2 were obtained from the J–V distortion measurement and the changes in CBO are very small for the front side bandgaps in our CIGS layers. The simulated results are presented in Figure 3 for the conversion efficiency parameters. As the front side bandgap increases from 1.1 eV to 1.37 eV, V OC gradually improves from 0.614 V to 0.648 V, but JSC and the fill factor (FF) decreases. As a result, the optimal front side bandgap is 1.18 eV in Figure 3. This can be attributed to the fact that the conduction band bending of the FGB helps the electrons to move over the band-offset of the ZnS interface barrier in the SCR for the low front side bandgap. However, for the high front side bandgap, a conduction band barrier forms near the interface of the ZnS buffer layer and disturbs the electron carrier transportation at the forward bias, so JSC and FF decreases gradually, as shown in Figure 3.

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Figure 2.2. 2.Conduction Conductionband band diagram and electron concentration at the position position incell CIGS cell as aa Figure diagram andand electron concentration at theat position in CIGSin as a cell function Conduction band diagram electron concentration the CIGS as of the FGB. function of the the FGB. FGB. function of

Figure 3. 3. Simulated Simulated solar solar cell cellconversion conversionefficiency efficiencyas asaaafunction functionof ofthe thefront frontside sidebandgap: bandgap:JSC SC VOC OC,,, Figure cell conversion efficiency as function of the front side bandgap: JJSC OC ,,,VV FF, Efficiency of CIGS solar cell with ZnS buffer layer as a function of front side bandgap. FF, Efficiency Efficiency of of CIGS CIGS solar solar cell cell with with ZnS ZnS buffer bufferlayer layeras asaafunction function of offront frontside sidebandgap. bandgap. FF,

The BGB BGB for for reducing reducing the the recombination recombination at at the the back back side side was was also also investigated investigated using using the the The BGB for reducing the recombination the back side was also investigated using the The at simulation. While the carriers generated near the front side can be quickly collected by the high simulation. While the carriers generated near the front side can be quickly collected by the high simulation. While the carriers generated near the front side can be quickly collected by the high internal electric electric field inSCR, the the SCR, the generated carriers near generated near the back back long-wavelength side (absorbing (absorbing internal electric field in the SCR, the carriers generated near side internal field in the carriers the back side the (absorbing long-wavelength photons) are slowly slowly transferred andcarrier collected through carrier diffusion. The back long-wavelength photons) are transferred and collected through diffusion. photons) are slowly transferred and collected through diffusion. Thecarrier back side field ofThe the back BGB side field field of of the the BGB BGB can can assist assist in in the the carrier carrier transportation transportation to to increase increase JJSC SC and, and, consequently, consequently, V VOC OC.. side Figure 44 shows shows the the distributions distributions of of the the electric electric field field and and extinction extinction coefficient coefficient near near the the Mo Mo interface interface Figure

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can assist carrier to increase JSC and, consequently, V OC . Figure 4 shows6 of the Energies 2017,in10,the x FOR PEERtransportation REVIEW 10 distributions of the electric field and extinction coefficient near the Mo interface layer as a function of layer as side a function of the side bandgap, and it the numerically showsanthat the BGBback generates an the back bandgap, andback it numerically shows that BGB generates additional side field additional back improving side field the to electron the front side, improving the electron transport. simulated to the front side, transport. The simulated conversion efficiencyThe parameters of conversion efficiency of the5 for BGB are plotted in Mo Figure 5 for the the BGB CIGS cells are parameters plotted in Figure theCIGS back cells side bandgap at the interface. Theback solar side cell bandgap at the Mobyinterface. The solar cell conversion bandgap is conversion efficiency bandgap gradient is combination effects efficiency of back sideby electrical field,gradient extinction combination back side field, extinction coefficient As gradient the back coefficient andeffects defectofdensity. As electrical the back side bandgap increases fromand 1.32defect eV to density. 1.57 eV, the side increases from 1.32 eV to 1.57the eV,electric the gradient in 2027 the conduction band significantly in thebandgap conduction band significantly increases field from V/cm to 3197 V/cm, reducing increases the electric field from 2027 V/cm 3197 V/cm, reducing carrier the probability for carrier recombination at theto back side, so that the solarthe cellprobability conversionfor efficiency recombination at the side, so that cell+conversion efficiency the canextinction be improved. As the can be improved. As back the bandgap (duethe to solar Ga/(Ga In) ratio) increases, coefficient 2 bandgap (due to Ga/(Ga + In) ratio) increases, the extinction coefficient (the absorption coefficient (the absorption coefficient of CIGS) of Figure 4 decreases, however, JSC improves by 0.4 mA/cmof 2 CIGS) ofofFigure 4 decreases, however, 0.4 mA/cm becauseFF of consistently the back side field in because the back side field in the BGB,JSCasimproves shown in by Figure 5. Additionally, degrades the BGB, as shown in bandgap Figure 5.because Additionally, FF consistently degrades the increase the with the increase in the the influence of the defect densitywith also increases. Theinback bandgap because the influence of optimized. the defect density also increases. back side side bandgap grading should be The optimized value The of the BGB is bandgap about 1.5grading eV for should be high optimized. The optimized value of the BGB is about 1.5 eV for achieving high efficiencies. achieving efficiencies. Weevaluated evaluatedthe the conversion efficiency of CIGS the CIGS solarfor cells four different We conversion efficiency of the solar cells fourfor different bandgap bandgap gradient gradientas profiles, in In Figure 6. Into addition to(the the DGB (the optimized oneresults from the results of profiles, shown as in shown Figure 6. addition the DGB optimized one from the of Figures 2 Figure and 4), a(UNI) uniform cellbandgap with no gradient, bandgap normal gradient, normal cell the onlyBGB, withand the and 4), 2a uniform cell(UNI) with no (NOR) cell(NOR) only with BGB, and reverse (REV) cellthe only with thesimulated. FGB wereCapacitance-voltage simulated. Capacitance-voltage profiling (atmV) 100 reverse (REV) cell only with FGB were profiling (at 100 kHz, 50 kHz, mV) for CIGS the fabricated CIGS (DGB) was performed based on approach for the50fabricated cell (DGB) was cell performed based on the approach in the [9,10] acceptorin(N[9,10] a for 16/cm3, and 18 16 3 acceptor (N a for p-CIGS) and donor (N d for n-ZnS) concentrations were about 1.5 × 10 1.5 3×, p-CIGS) and donor (Nd for n-ZnS) concentrations were about 1.5 × 10 /cm , and 1.5 × 10 /cm 18 3 2 2 10 depth /cm , the of SCR was0.3 about 0.3 μm and capacitance was 38.7 aboutnF/cm 38.7 nF/cm at Thus, 0 V. Thus, for the of depth SCR was about µm and capacitance was about at 0 V. for the the comparison, we intentionally setsimulation the simulation parameters theended SCR ended at a of depth of comparison, we intentionally set the parameters so thesoSCR at a depth nearly nearly 0.3 μm in the CIGS, and the average bandgap of the entire CIGS layer is about 1.27 eV for all 0.3 µm in the CIGS, and the average bandgap of the entire CIGS layer is about 1.27 eV for all the the bandgap gradient structures shown in Figure 6 [18]. The conversion efficiencies of these bandgap gradient structures shown in Figure 6 [18]. The conversion efficiencies of these solar solar cells cellssummarized are summarized Tablethough 2. Even though bandgap the average of isthe CIGS layerthe is are in Table in 2. Even the average of thebandgap CIGS layer approximately approximately the same, the conversion efficiency of these solar cells is considerably different; the same, the conversion efficiency of these solar cells is considerably different; the highest efficiency is highest efficiency is obtained with the DGB the lowest value obtained with the REV solar cells. obtained with the DGB and the lowest valueand is obtained with the is REV solar cells.

Figure 4. 4. Electric Electric field field and and extinction extinction coefficient coefficient at at the the position position in in CIGS CIGS cell cell as as aa function function of of the the back back Figure side bandgap. side bandgap.

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Figure Simulatedsolar solarcell cell conversion efficiency at the position in CIGS a function the Figure 5.5.Simulated conversion efficiency at the position in CIGS cell cell as a as function of theofback Figure 5. Simulated solar cell conversion efficiency at the position in CIGS cell as a function of the side J , V , FF, Efficiency of CIGS solar cell with ZnS buffer layer as a function of back backbandgap: side bandgap: J SC , V OC , FF, Efficiency of CIGS solar cell with ZnS buffer layer as a function of SC OCJSC, VOC, FF, Efficiency of CIGS solar cell with ZnS buffer layer as a function of back side bandgap: side backbandgap. side bandgap. back side bandgap.

Figure 6. Bandgap energy at the position in CIGS cell for the different bandgap profiles.

Figure 6. Bandgap energy at the position in CIGS cell for the different bandgap profiles.

Table 2. Performance the experimented and simulated solarfor cells different bandgapprofiles. profiles. Figure 6. Bandgapofenergy at the position in CIGS cell thewith different bandgap Types JSC (mA/cm VOC (V) and simulated FF (%) Efficiency (%)different n bandgap J0 (mA/cm ) Table 2. Performance of the) experimented solar cells with profiles. Table 2. Performance of the experimented and simulated solar cells profiles. −9 DGB 34.38 0.632 70.1 15.23with different 1.54 bandgap 2.0 × 10 2

NOR

31.65

2

0.686

2) 2 ) VV Types TypesJSC (mA/cm OC (V) JSC (mA/cm OC (V) UNI 29.43 0.730 DGBREV 34.38 0.632 27.06 0.757 DGB 34.38 0.632 NOR 0.686 NOR 31.6531.65 0.686 UNI 29.4329.43 0.730 UNI 0.730 REV 27.0627.06 0.757 REV 0.757

68.7

14.90

1.64

1.0 × 10−8

2 )J0 (mA/cm2) FF (%)Efficiency Efficiency (%) n FF (%) (%) n J (mA/cm 68.6 14.74 1.660 1.67 × 10−8 70.1 15.23 1.54 −9 × 10 2.0−8 × 10−9 66.3 70.1 15.2313.57 1.54 1.692.0 × 101.4 −8 1.64 68.768.7 14.90 14.90 1.64 1.0 × 10−8 1.0 × 10 1.66× 10−8 1.67 × 10−8 68.668.6 14.74 14.74 1.66 1.67 −8 1.69 66.366.3 13.57 13.57 1.69 1.4 × 10−8 1.4 × 10

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From OC From the theresults resultsofofTable Table2,2,we wefound foundthat thatVV OC behaves behaves very very interestingly; interestingly; there there isis aa strong strong correlation between V and the average bandgap in the SCR rather than that of an entire OCand the average bandgap in the SCR rather than that of an entire correlation between VOC CIGSCIGS layer. layer. The average SCR bandgaps the DGB, and cells REV are cells are calculated as 1.144 eV, The average SCR bandgaps of theof DGB, NOR,NOR, UNI, UNI, and REV calculated as 1.144 eV, 1.212 1.212 eV, 1.270 eV, and 1.337 eV, respectively, which are strongly proportional to the V of the cells. eV, 1.270 eV, and 1.337 eV, respectively, which are strongly proportional to the VOC OC of the cells. Because density is is modeled modeledasasproportional proportionaltotothe the bandgap a result of the Ga/(Ga Because the the defect defect density bandgap (as(as a result of the Ga/(Ga + In) +ratio), In) ratio), the defect density also increases with the bandgap: for example, thedensities defect densities the defect density also increases with the bandgap: for example, the defect are 1.1 × 14 /cm3 and 14 /cm3 at the bandgap of 1.144 eV and 1.337 eV, respectively [15]. are ×3 10 9.03 × 10141.1 /cm and 9.0 × 1014 /cm at 10 the bandgap of 1.144 eV and 1.337 eV, respectively [15]. However, VOC However, V OC further 0.632 0.7302Vbecause in Tablethe 2 because theof influence of the defectis further increases fromincreases 0.632 V from to 0.730 VV intoTable influence the defect density density small in our(in model (in ourWe cells). note that V OC does not significantly degrade until the small inis our model our cells). noteWe that VOC does not significantly degrade until the defect 15 /cm3 in our simulation. 15 3 defect density of the SCR is much higher than 1.0 × 10 density of the SCR is much higher than 1.0 × 10 /cm in our simulation. As was determined to be gradient profileprofile and V OC was selected within Asthe theDGB DGB was determined tothe be best the bandgap best bandgap gradient and VOC was selected awithin desirable range using the average bandgap in SCR, we further investigated J and FF. We optimized SC a desirable range using the average bandgap in SCR, we further investigated JSC and FF. We the band structure the DGBoftothe obtain best efficiency. To determine the mainthe factor JSC , optimized the bandofstructure DGB the to obtain the best efficiency. To determine mainoffactor we the individual parameters to simulate the DGB solarsolar cell.cell. ForFor a ±a10% of Jvaried SC, we varied the individual parameters to simulate the DGB ±10%variation variationininthe the 2 , and the 2 minimum bandgap with the other parameters fixed, J changes by about ± 0.7 mA/cm minimum bandgap with the other parameters fixed, JSCSCchanges by about ±0.7 mA/cm , and the ±10% 2 . Similarly, for the ±10% 2. Similarly, ±variation 10% variation inside backbandgap side bandgap changes about ±0.2 mA/cm SC by ±0.2 in back changes JSC by Jabout mA/cm for the ±10% variation 2 2 variation in the front side bandgap, J changes by about ± 0.07 mA/cm . The ±10% variation in SC in the front side bandgap, JSC changes by about ±0.07 mA/cm . The ±10% variation in defect density 2 2 defect density in the entire CIGS layer of the DGB structure causes J to change by ± 1.2 mA/cm in the entire CIGS layer of the DGB structure causes JSC to change bySC±1.2 mA/cm . The FF is known. The FFaffected is known bebandgap affected by as well as the defect density. In general,declines FF gradually to be bytothe as the wellbandgap as the defect density. In general, FF gradually when declines when the front or back side bandgap exceeds a certain value, as shown in Figures 3 and the front or back side bandgap exceeds a certain value, as shown in Figures 3 and 5. The reduction 5. of The reduction of FF by theside excessive front side bandgap is mainly the FF by the excessive front bandgap is mainly attributed to the attributed formation to of the formation conductionofband conduction band barrier obstructing electron while theside excessive backis barrier obstructing electron collection, while collection, the reduction bythe thereduction excessiveby back bandgap side bandgap is because the defect traps become deeper as the bandgap increases with the Ga/(Ga because the defect traps become deeper as the bandgap increases with the Ga/(Ga + In) ratio [19,20]. +Finally, In) ratiobased [19,20]. based onresults, the simulation results,the weDGB optimized the for DGB for the on Finally, the simulation we optimized structure thestructure CIGS solar cell CIGS solar cell the incorporating the Cd-free ZnS buffer layer and fabricated with a size of incorporating Cd-free ZnS buffer layer and fabricated the CIGS cells the withCIGS a sizecells of approximately approximately 0.25 cm2 . Figure presents the measured current density-voltage 0.25 cm2. Figure 7 presents the 7simulated andsimulated measuredand current density-voltage curves undercurves 1-sun under 1-sun illumination; thecell fabricated cellisefficiency is about 15.2% and the curve simulated curve the illumination; the fabricated efficiency about 15.2% and the simulated of the CIGSofsolar CIGS solar cell matches well with the experimental curve. cell matches well with the experimental curve.

Figure 7. Light current density-voltage curves of the experimental results (solid symbol) and simulation Figure 7. Light current density-voltage curves of the experimental results (solid symbol) and results (open symbol). simulation results (open symbol).

4. Conclusions We numerically investigated the characteristics of the gradient bandgap profile in the absorption layer to evaluate the conversion efficiency of the CIGS solar cell with a Cd-free ZnS buffer layer using a TCAD simulation. The Ga-related parameters, such as defect density, bandgap,

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4. Conclusions We numerically investigated the characteristics of the gradient bandgap profile in the absorption layer to evaluate the conversion efficiency of the CIGS solar cell with a Cd-free ZnS buffer layer using a TCAD simulation. The Ga-related parameters, such as defect density, bandgap, and electron affinity, can be controlled by the Ga/(Ga + In) ratio in the CIGS and were modeled and presented for the simulation. In the simulation, we considered the FGB, and the BGB formed by spatially varying the Ga content. The FGB enhanced the V OC , but the front side bandgap should be carefully designed to not disturb the electron transport at the conduction band barrier near the ZnS buffer layer. The BGB improved the electron transport at the back surface, but the excessive back side bandgap could induce the defect traps to move deeper in the CIGS, resulting in the degradation of the FF of the CIGS cell. By simulating the various types of band structures, DGB, NOR, REV, and UNI, we demonstrated that the DGB was the most effective structure for achieving the highest cell efficiency. We demonstrated that V OC strongly correlated with the average bandgap in the SCR and could be selected within the desirable range using the DGB; we also numerically evaluated JSC and FF for the band structural parameters of the CIGS cell. In this approach, we designed the optimal band structure of the CIGS cell with a Cd-free ZnS buffer layer and demonstrated a cell efficiency of more than 15% for the fabricated CIGS solar cells. The numerical design procedure of this work will be useful for developing high-conversion efficiency CIGS cells incorporating a gradient bandgap structure and Cd-free buffer layer. Author Contributions: All authors contributed to this work. Conceptualization, simulation and analysis, J. Park.; Investigation, and supervision, M. Shin. Funding: This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20143030011530), and also supported by the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (NRF-2013M3C1A3065040). Acknowledgments: This research was supported by the Solar Energy Group of the Samsung Electronics Inc.; the authors acknowledge the technical support and the assistance offered by Dr. Myeong-Woo Kim at the Samsung SDI. The authors also appreciate Dr. Dong-Seop Kim (Vice President of Samsung SDI) for his advice and leadership. Conflicts of Interest: The authors declare no conflict of interest.

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