CZTS the range of (1.45-1.6) eV and a high

CZTSbased Solar Cells- A review. VinayTikkiwal, Deeksha Chandola, Priyanka Kwatra Abstract CZTS (Cu2ZnSnS4)has shown great promise in the preparation of absorption layer for low costthin film solar cells because of its ideal band gap, low toxicity and abundanceof the constituents. Several physical and chemical based methods are used tofabricate CZTS thin films.

Sputtering is one of the more prominent methods offabrication employed for preparing these CZTS based films. Sputtering offersbetter control over the film properties as compared to other methods offabrication. Experimental methods using sputtering have led to successfulfabrication of CIGS solar cells, commercially as well. This work summarizes thedevelopmental work of CZTS films via the sputtering route and the performanceof the solar cells based on them. Keywords:CZTS, Sputtering, Thin Films, Solar Cells.  1.IntroductionShortage of non-renewable energy sources and pollution ofthe earth has been the main cause of concern since the 21st century 1. It isestimated that the world’s energy demand will be close to 28 Terawatt by 20502.

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In order to meet the high demand levels renewable energy sources must beexploited. Solar energy is an economic and efficient resource because it isinexhaustible and relatively pollution free. Photovoltaic (PV) systems havebecome popular as they are capable of directly converting sunlight intoelectrical energy. PV systems have long life and very low maintenance costsowing to lack of any mechanical motion. Silicon based PV technology dominatesthe solar market today. The best crystalline silicon solar cells have 27%efficiency while the best thin film solar cells have efficiencies in the 20%range.

Thin film technologies such as CdTe and CuInGaSe2 (CIGS) usedirect band gap compound semiconductors5. Another thin film technology is Cu2SnZnS4(CZTS), it is more environmentally friendly and abundantly available thanCuInGaSe2. In CZTS indium is substituted by tin, gallium by zinc,and selenium by sulfur. CZTS based thin films are considered to be an excellentPV material since it has a band gap in the range of (1.45-1.6) eV and a highabsorption coefficient (> 104 cm-1) 9-10.

The costof raw material for CZTS based solar technology is significantly lower thanthat of the other existing thin film PV technologies 17.SputteringSputtering is used for depositing materials on thesubstrate, by emitting atoms on the substrate and then condensing them in highvacuum environment. It is processed by high acceleration exchange between theions and atoms in the target materials, due to high speed collisions 1. Thetarget is bombarded with very high energy inert gas ions such as Argon.

Vacuumchamber is filled with argon atoms at a pressure of 1 to 10 m Torr. In betweenthe target and substrate a voltage source is introduced, which generatesplasma, hot gas?like phase consisting of ions and electrons, in thechamber.  The Argon ions are charged and are moved at the high speedtowards the target.  Due to this the target atoms travel to thesubstrate and finally settle down. . During the process of Argon ionizationelectrons which are emitted  are moved athigh speed toward the substrate, leading to collision with other Argon atoms,generating more ions and free electrons and this process continues.

 Aslarge numbers of atoms settle down on the substrate, they form a bond with eachother at the level of molecule, leading to formation of tightly bound atomiclayer which further lead to the formation of thin-film structures.Sputtering process is mainly of 4 types: (i) DC, (ii) RF,(iii) DC / RF magnetron and, (iv) Reactive. The system for DC sputteringcontains the planar electrodes, cold cathode and anode.

The cathode is coveredwith target material to be deposited and the anode is covered with thesubstrate. Reactive sputtering is carried out by feeding the chamber withoxygen or nitrogen along with the argon leading to production of oxidic ornitridic films. In ac sputtering, for frequencies below 50 kHz, the targetvoltage is periodically reversed, and highly mobile ions form a dc diode-likedischarge alternatively on each electrode, where the total potential drop isnear the cathode. The substrate chamber walls can be used as the counterelectrode. At frequencies above 50 kHz, the ions are not mobile leading to thefeeling of applied potential throughout the space between electrodes, due towhich they gain enough energy to cause ionizing collisions.

The alternatingpositive-negative potential is generated on the surface, by capacativelycoupling the RF voltage to electrode. In first half-cycle, the ions cause sputtering,by moving at high speed towards the surface and gaining sufficient energy,while in next half-cycle, electrons prevent building up of charge on surface. RFsputtering has the disadvantage that it produces the insulating materialsmostly having worst thermal conductivity, large coefficients of thermalexpansion, and are brittle in nature.

The magnetron sputtering, can be usedwith DC or RF improves the efficiency of sputtering by confining sputteringsource through a magnetron source which causes the electrons to spiral, leadingto ionizing collision thus enabling the plasma to be operated at a higherdensity. It causes low heating of substrate and low radiation damage. Fig. 1shows basic set up of magnetron sputtering.Fig.1 Magnetron sputteringIn1988 Ito and Nakazawa used atom beam sputtering to deposit CZTS films on glasssubstrates heated upto 240°C 3.

The compound target was sputtered by a beamconsisting of mostly neutral particles. Pure Argon was used as the sputteringgas and was maintained at 0.2 Pa in the sputtering chamber during the filmdeposition.

The discharge voltage and the current for the atom beam gun were 7kV and 5mA, respectively. The (112) oriented polycrystalline films wereobtained on  glass substrates heated at90°C and above while the diffraction curve of the film deposited at a substratetemperature lower than 50°C contained (220) peak of the CZTS crystal Fig.2..Figure 2. X-ray diffraction curvesof the films deposited from the CZTS target 3.Allthe deposited films showed p-type conductivity. The resistivity of the filmdecreased with the increase in substrate temperature up to 240°C (Fig.

3) whilethe Hall mobility of the film was 1 cm2/V-s. The films exhibited anabsorption coefficient greater than 104 cm-1. The directoptical band gap of the CZTS film was estimated at 1.45eV.

film exhibited anopen circuit voltage of 165 mV under AM 1.5 illumination.  Figure. 3. Graph showing dependenceof CZTS films resistivity on substrate temperature 3Seol et al.

, in 2003 fabricated CZTS thin films using RFmagnetron sputtering . The constituents of the target were fine mixture of Cu2S,ZnS and SnS2 which were cold pressed at pressure of 250MPa.After this the films were annealed at the temperature of 250-400°C. Mixture ofAr and S2(g) was used as the inert gas atmosphere.  When,RF power greater than 100 W was applied, rapid variation of Cu and Sn contentswith RF power was found which depended on the plasma density Fig. 4.

 However at the RF power from 50 to 100 W, atomic ratio of thin film was fine.At the RF power of 75 W, S/(Cu+Zn+Sn) ratio of thin film was less thanstoichiometry, while Cu/(Zn+Sn) ratio was close to stoichiometry. The thinfilms deposited by as were stochiometric and amorphous in nature and were annealedin the environment of Ar and S2 (g). When the annealing wasdone at the temperature greater than 250°C, thin films became crystallized. Mostof the diffraction peaks of the CZTS thin films were obtained at (1 1 2), (2 00), and (2 2 0), (3 1 2) planes Fig.5 and was stochiometric to all there?ection of a kesterite structure. The optical absorption coef?cient was foundto be more than 104 cm-1 and energy band gapwas found to be 1.

51 eV. The sheet resistance was found to be indirectlyproportion to temperature, and the intensity of the orientation (112) increasedwith annealing temperature Fig.6.

                    Fig.4. Atomic percent vs RFpower curves for CZTS thin films                                                                                                          Fig.5. Atomic percent vs annealingtemperature                                                                              Fig.6.XRD patterns of thin films as a function of annealing temperatureIn2005, Tanaka et al.

used a hybrid sputtering system with two sputter sources tofabricate CZTS films 5.The films were fabricated by sequentially depositing Sn,Zn, and Cu followed by annealing with S flux. The authors proposed that use ofbinary compound ZnS in place of Zn or introduction of S vapor during thedeposition of Zn could help prevent loss of Zn. Increase in the substratetemperature, led to decrease in the film thickness Fig.9. The adhesion of thefilms improved with increase in the substrate temperature.

 Absorption coefficient was larger than 104cm-1, the direct optical band gap was found to be about 1.5 eV. Themain problem of CZTS prescursor films is moisture adsorption on their surface,when they are taken out of deposition chamber before sulfurization process. Inthe year 2007 Jimbo et al.6 used the reaction of N2 and H2S(20%) for processing sulfurization in the inline-type vaccum chamber tofabricate CZTS thin films.  The substratewas heated till the temperature of  800°C. The annealing was carried out by transferring precursor to SiC heater.

Thetesting was done on different samples, and it was found that the constituentswith ratio of  0.87 for Cu/(Zn+Sn) and1.15 for Zn/Sn were best. The author obtained open circuit voltage of 6.62 mV,short circuit current of 15.7 mA/cm2, fill factor of 0.

55 andconversion efficiency of 5.74% for this best Cu-lean and Zn-rich sample. Accordingto the authors annealing in  inline-typevaccum chamber in which moisture did not adhered to the precursor caused theconversion efficiency to improve furtherEricsonet al.7 deposited CZTS thin films using the method of reactive sputtering anddetermined the stability of the constituents of film. Reaction of CuSn alloyalong with Znalloy targets was carried out in H2S.These alloy constituents removed the disadvantage of Sn having low meltingpoint.

The composition of alloy constituents was varied as Cu- 67% Sn -33%(99.99% purity) and Cu-65% Sn-35% (99.999% purity).

Surface sulfurization isaccompanied by preferential removal of Sn which leads to long term changes inthe Sn concentration in the deposited films. It was proved that Cu2Sformation can be curtailed by two methods; one is by decreasing the partialpressure of H2S and other by erasing the surface of targetconsistently. Table 1 describes various CZTS thin films fabricated underdifferent conditions and Table 2 explains various solar cells fabricated andthe results of the parameters achieved by them.

               Table 1. Bandgap achieved for CZTSthin films fabricated by different method. Year Author Temperature Precursor Bandgap 2003 Seol et.al. 250–400°C.

Cu2S, ZnS and SnS2 1.51eV 2005 Tanaka et al 300 – 500°C Cu, Sn, Zn, S 1.5 eV 2013 Wang et. al 750 °C Cu2S-ZnS-SnS2-S 1.

7 eV 2014 Tanaka et al. 530°C Cu, Zn, Sn 1.23,1.35, and 1.48 eV 2014 Singh et al. 80-450° K Cu, Zn, and Sn 1.

49 eV                Table 2. Various parameters achieved for CZTSsolar cells fabricated by different method. Year Author Open circuit voltage (mV) Short circuit current (mA/cm2) Fill factor Efficiency 2007 Jimbo et al. 646 13.

7 0.60 5.33% 2013 He et al. 484 14.

56 50.1% 3.52%, 2014 Pawar et. al 561 18.4 48.2 5%  Tanaka et al. deposited the CZTS films using the method ofsputtering-sulfurization in the year 2014. The Cu to (Zn+Sn) ratio was variedbetween 0.

60 and 4.21 and Zn to Sn ratio was varied between 0.29 and 1.68. Theprecursors were pre heated at 530 during sulfurization process in an atmosphereof H2S with N2. Precursor in which Cu quantity was leastand Zn quantity was maximum was found to have the best efficiency.

The analysisof Power spectrum was performed by varying the dependencies of power on the PLspectrum. The spectrum was found to have 3 emission bands in B1, BT and BBbands, as shown in figure 7. The BI bands were having maximum intensity in thePL spectra it was observed that BI band dominated specifically for samples withCu to Sn ratio less than equal to 2.0.

Authors concluded that with thiscomposition a deep acceptor state can be formed easily which is required toachieve high efficiency in solar cells.Figure 7. PL spectra showing temperature dependence  (a)  PLgroup A and (b)  PL group B measured with24mW excitation power.

8In2014 Pawar et. al 9 investigated the various properties of CZTS absorbersbecause of the effects of sulfurization temperature.  Mo-coated glass substrates were used on whichCZTS absorbers were grown. Rapid thermal processing (RTP) sulfurization techniqueis used in which  metallic precursor ofCZT in stacked forms was deposited using targets Zn , Cu and Sn  in their purest forms ( 99.999%).  The temperature of annealing varying from 500to 580°C was preferred for the sulfurization.

Raman spectra and XRD of theprecursor films Fig. 9 shows that films mainly constituted of  metal elements such as alloys of Zn, Sn andCu. This indicates that Cu atoms diffuse into the Zn and Sn metallic ?lmsresults in binary inter-metallic phases 11. During the process ofsulfurization authors noted that, the CZTS absorber was formed from stackedmetal precursor due to the significant increase of volume which was aroundtwice the previous thickness of CZT film.

The crystallinity of the absorberwas  improved by increasing thetemperature of sulfurization. The sulfurized CZTS absorber ?lms are polycrystallinein nature and has dense morphology. The solar cell which are fabricated usingCZTS absorber films are observed under a temperature range of 500°C to 580°C .They showed highest conversion efficiency of 5% for a 0.44 cm2 areawith Voc to be 561 mV, Jsc=18.4 mA/cm2, andFF=48.

2, at an optimized sulfurized temperature of 580°C.In2014, Singh et al. 10 carried out Raman studies on CZTS thin film grown usinga two-step method; co-sputtering Cu, Zn, and Sn metal targets on cleaned limeglass substrate and sulfurizing it in H2S ambient in the temperaturerange of 80-450° K. SEM micrograph study revealed that the film had dense andcompact morphology and the average grain size was in the range of 500-600 nm.

Raman spectrum of CZTS thin film recorded at different ranges of  temperatures with  wave number in range of 200-450 cm-1 (Fig.9(a)). It was observed that the peak shifts from 337 cm-1- 329 cm-1when the measurements were carried out at 80 K and 450 K, respectively (Fig.8(b)). The shift in the Raman peak position with temperature due to the thermalexpansion as well as an harmonic coupling with other phonons which becomeactive at higher temperature. The experimental values for temperature dependentRaman frequency matched well with the values plotted using Equation (1):?p(T)=?0+??expansion(T)+ ??d(T)                                                                                                                         (1)where?0 is the harmonic frequency , ?? expansion (T) is the frequency dueto thermal expansion and ??d(T) is the frequency due to an  harmonic coupling of different phonons.Therefore, the peak intensity decreases with the increase in measurementtemperature and it also shifts to the lower frequency values.

The line widthchanged from 17 cm-1 to 26 cm-1 when measurements weretaken at 80 K and 450 K, respectively (Fig.9 (b)), leading to the conclusionthat there is a decrease in the intensity of the peaks. Similarly, at highertemperature, the activated phonon interacts with the “A” mode phonon resultingin increase in the FWHM of the peaks. Therefore, for a multi component systemit is necessary to carry out Raman measurements at lower temperature.                                      (a)                                                                                                                          (b)Figure. 8.(a) Raman Spectrum analysis of CZTS thin films observed at differenttemperatures and (b) Mode “A” of Raman spectra at 80 and 450K 8                                      Fig.9.

(a) Raman mode A Spectra analysis at room temperature with different counts(b) Raman “A” mode of CZTS thin film using damped harmonic oscillatormodel, (c) temperature dependence of Raman peak (A mode)  with line width.Recently, in 2015, Cormier et al.successfully synthesized CZTS, which was crystallized in nature by the methodof reactive magnetron sputtering, in which author used two targets- Zinc andCopper (67%)–Sn alloy (37%) 13. During deposition, authors used soda limeglass substrates and varied the substrate temperature in the range of 25°C to600°C. Raman data indicates that there is strong affect on spectra mainly dueto the substrate temperature particularly especially in the range of 240 to 500cm-1. For temperatures less than 400°C, broad peaks are shown byRaman spectra in the range of 160 to 500 cm-1. The disadvantage ofannealing is that it creates the voids because the secondary phases getssublimed and as such is not used in the industrial processes 14.

Emrani etal. 15 studied the structural and optical properties of CZTS thin filmsfabricated at different sulfurization time duration. Sputtering of precursorswas followed by a short duration sulfurization in dilute H2S EDSanalysis revealed that the annealed films were zinc rich and copper poor whichis ideal for the CZTS solar cells 20. It is observed that higher annealingtemperature result in larger grains but more voids Fig.10 (P1) and (P2). Withthe increase in sulfurization time, the grains start to coalesce into largersizes Fig.

10 (bottom).                                                                                 Fig.10.

Top pictures shows surface morphology of the filmsand bottom pictures shows the cross section of the films.                                    Table 3. Averageand RMS values of surface roughness measured for different samples Samples Sulfurization Conditions Average(nm) Root mean square (nm) P1 590oC, 10 min            76                  113 P2 590oC, 60 min           112                  152 P3 525oC, 10 min            86                  198               Table 4Current-Voltage data of the (P1:P5) samples processed through the sulfurizationprocesses   Sample Process temperature and time taken for sulfurization   Efficiency (%)   VOC (mV)   ISC (mA/cm2)   RS (? cm2)   RSH (? cm2)   Fill factor(%) P1 590oC, 10 min 2.

56 441 17.8 13.5 56 36 P2 590oC, 60 min 1.68 434 8.47 18.9 200 42 P3 525oC, 10 min 1.56 446 12.36 34.

2 68 29 P4 580oC, 30 min 3.8 492 22.5 14.9 65 34 P5 550oC, 180 min 5.75 593 20.5 8.55 279 48  ConclusionThis work has focused on the basicaspects of sputtering grown CZTS film. The paper provides a brief overview ofthe various experiments that have been carried out to prepare these films.

Several factors such as elemental composition of the targets, depositiontemperatures, choice of sputtering gas, substrate temperatures etc. have aneffect on the properties of the prepared films and eventually on the solarcells fabricated using them. Improvements in CZTS based solar cells have helped in achieving a highefficiency of 6.77% with the sputtering process. This efficiency is higher ascompared with other techniques such as PLD, sol–gel, electro-deposition etc.

Abetter understanding of the sputtering process and the effect of variousparameters on the film properties is required for the preparation of CZTS basedsolar cells with better performance.  References: 1 R. Behrisch, Sputtering by Particle bombardment, Springer, Berlin.

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