Cadmium free high efficiency Cu2ZnSn(S,Se)4 solar cell with Zn1−xSnxOy buffer layer Alexandria Engineering Journal (2017) xxx, xxx–xxx HO ST E D BY Alexandria University Alexandria Engineering Journ[.]
Alexandria Engineering Journal (2017) xxx, xxx–xxx H O S T E D BY Alexandria University Alexandria Engineering Journal www.elsevier.com/locate/aej www.sciencedirect.com ORIGINAL ARTICLE Cadmium free high efficiency Cu2ZnSn(S,Se)4 solar cell with Zn1xSnxOy buffer layer Md Asaduzzaman a,*, Ali Newaz Bahar a, Md Mohiuddin Masum a, Md Mahmodul Hasan b a Department of Information and Communication Technology (ICT), Mawlana Bhashani Science and Technology University (MBSTU), Santosh, Tangail 1902, Bangladesh b Department of Computer Science and Engineering (CSE), Mawlana Bhashani Science and Technology University (MBSTU), Santosh, Tangail 1902, Bangladesh Received 30 September 2016; revised December 2016; accepted 21 December 2016 KEYWORDS CZTSSe solar cell; Cd free; ZTO buffer; Efficiency; Conduction band offset Abstract We have investigated the simulation approach of a one-dimensional online simulator named A Device Emulation Program and Tool ðADEPT 2:1Þ and the device performances of a thin film solar cell based on Cu2 ZnSnðS; SeÞ4 ðCZTSSeÞ absorber have been measured Initiating with a thin film photovoltaic device structure consisting of n-ZnO : Al=i-ZnO=Zn1x Snx Oy ðZTOÞ=CZTSSe=Mo=SLG stack, a graded space charge region ðSCRÞ and an inverted surface layer ðISLÞ were inserted between the buffer and the absorber The cadmium ðCdÞ free ZTO buffer, a competitive substitute to the CdS buffer, significantly contributes to improve the open-circuit voltage, Voc without deteriorating the short-circuit current density, Jsc The optimized solar cell performance parameters including Voc , Jsc , fill factor ðFFÞ, and efficiency ðgÞ were calculated from the current density-voltage curve, also known as J–V characteristic curve The FF was determined as 73:17%, which in turns, yields a higher energy conversion efficiency of 14:09% Ó 2016 Faculty of Engineering, Alexandria University Production and hosting by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Introduction The use of photovoltaic device has been growing so rapidly to utilize the world’s amplest energy source, incident sunlight And in recent years the Cu2 ZnSnS4 ðCZTSÞ and the * Corresponding author E-mail addresses: asaduzzaman.mbstu@gmail.com (Md Asaduzzaman), bahar_mitdu@yahoo.com (A.N Bahar), masum.mbstu@gmail com (M.M Masum), hasan.cse.mbstu@gmail.com (M.M Hasan) Peer review under responsibility of Faculty of Engineering, Alexandria University CZTSSe, the promising absorber layer materials, have drawn much attention to the photovoltaic researchers for highly efficient and low-cost thin film solar cells [1–4] Besides, the CZTSSe absorber-based solar cells expose more radiation severity, excellent stability and higher energy conversion efficiency of 12:6% [2] Despite having lower energy conversion efficiency than the most common absorbers, CIGS and CdTe based thin-film solar cells having the recorded efficiencies of 22:3% and 22:1% respectively [5], the CZTSSe solar cell has become an emerged photovoltaic absorber to the researchers because of its p-type conductivity and tunable direct band gap of 1:50 eV with a higher absorption coefficient of http://dx.doi.org/10.1016/j.aej.2016.12.017 1110-0168 Ó 2016 Faculty of Engineering, Alexandria University Production and hosting by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Please cite this article in press as: M Asaduzzaman et al., Cadmium free high efficiency Cu2ZnSn(S,Se)4 solar cell with Zn1xSnxOy buffer layer2ZnSn(S,Se)4 solar cell – >, Alexandria Eng J (2017), http://dx.doi.org/10.1016/j.aej.2016.12.017 104 cm1 [6–10] Moreover, comparing to the expensive and scarce indium ðInÞ, the global annual production of Zn and Sn is about 20 and 340 times more, and the availability is 500 times and 14 times higher [11] And therefore, the mixed chalcogenide, CZTSSe has become a more emergent choice and a potential alternative to the CIGS and CdTe absorbers However, an environment-friendly non-toxic zinc-tin-oxide ðZTOÞ material was also introduced as an alternative buffer layer to the conventional toxic CdS buffer layer material [12] Another reason beyond using ZTO buffer in CZTSSe thin film solar cell is that it has a wider energy band gap ranges from 3:20 to 3:74 eV [13,14], permitting the photons having a lower wavelength into the absorber and thus increasing the conversion efficiency The theoretic knowledge of solar cells anticipates that for all voltages the light current density ought to be fixed But CZTSSe photovoltaic devices often show deviances from this ideal behavior It will be shown by using numerical simulation that this effect can be demonstrated introducing a conduction band offset ðCBOÞ at the ZnO=ZTO and ZTO=CZTSSe interfaces Besides this, the effects of CBO on Voc , Jsc , FF, and g have also been analyzed Photovoltaic cell having conduction band offsets around 0:3 eV provides a better device performance [15] Md Asaduzzaman et al d dW eðxÞ ẳ q pxị nxị ỵ Nỵ D xị NA xị ỵ Pt xị Nt xị dx dx 1ị dnp np np0 dn dnp d2 np ẳ Gn ỵ Dn ỵ np ln ỵ ln n dx dt sn dx dx dpn pn pn0 dn dpn d2 pn ẳ Gp ỵ Dp þ pn lp þ lp n dx dt sp dx dx ð2Þ ð3Þ where e is the permittivity, W the electrostatic potential, q the charge of electron, p the free hole, n the free electron, Nỵ D the donor concentration, N A the acceptor concentration, n the electric field, Pt the trapped hole, Nt the trapped electron, Gn the generation rate for electrons, Gp the generation rate for holes, ln the electron mobility, lp the hole mobility, Dn the diffusion coefficient for electrons, and Dp the diffusion coefficient for holes, and all the parameters are a function of coordinate position x For bulk defects, the recombination current density is determined by the Shockley-Read-Hall ðSRHÞ modeling approach and for interface defects, an extension of the SRH modeling approach is used The SRH model for interface defects permits carriers from both the valence and the conduction bands to take part in the recombination process for interfaces Solar cell structure and numerical simulation Figure Schematic diagram of CZTSSe thin film solar cell structure Solar cell device modeling The numerical analysis needed for the solar cell device modeling is performed by using the simulator ADEPT 2:1 [16] The steady-state band gap profile, hole and electron carrier transport, recombination profile are estimated by using the Poisson’s equation and the electron and hole continuity equations given by the followings [17]: The CZTSSe solar cell structure is considered to consist of the material layers including n-type Al-doped ZnO, intrinsic-ZnO, n-type ZTO buffer, p-type CZTSSe absorber, and Mo on soda-lime glass substrate Between the ZTO buffer and CZTSSe absorber, an inverted surface layer ðISLÞ, CuIn3 Se5 is inserted which is commonly known as an ordered vacancy compound ðOVCÞ layer [18] The inverted surface layer reduces the recombination rate and hence improves the cell performances by shifting away the electrical junction from the higher-recombination interface to the ZTO=CZTSSe interfaces The CZTSSe solar cell structure is shown in Fig A simulation was conducted in order to interpret the measured current density versus voltage relationship referred to as J–V characteristic curve An OVC layer with a thickness of 60 nm, a band gap of 1:37 eV, an electron mobility of 10 cm2 V1 s1 , a hole mobility of 40 cm2 V1 s1 and a carrier density of 1016 cm3 have been used [15,18–20] The indirect band gap of the ZTO buffer decreases from 3:74 eV at 90 C deposition temperature to 3:23 eV at 180 C deposition temperature [13,14] The main reasons behind differing the deposition temperature are to change the conduction band energy level and to affect the size of the grain of crystalized materials which in turn contribute to improve the performance of the photovoltaic cells [12] It is proven that the higher atomic layer deposition ðALDÞ temperature of ZTO buffer causes lower open circuit voltage and at lower deposition temperature the efficiency of the solar cell is limited by the lower fill factor [12] So, the lower deposition temperature is more preferable as the band gap becomes narrower with the increasing deposition temperature At 90 C deposition temperature the ZTO buffer with a ẵSn=ẵSn ỵ ẵZnị composition of 0:18 results in a band gap of 3:74 eV with a critical thickness of around 50 nm [12] For modeling an effective recombination rate, a deep level defect with an interface defect concentration Please cite this article in press as: M Asaduzzaman et al., Cadmium free high efficiency Cu2ZnSn(S,Se)4 solar cell with Zn1xSnxOy buffer layer2ZnSn(S,Se)4 solar cell – >, Alexandria Eng J (2017), http://dx.doi.org/10.1016/j.aej.2016.12.017 Cadmium free high efficiency Cu2ZnSn(S,Se)4 solar cell of 1010 cm2 is placed in the midway of the band gap of the ZTO, the OVC and the space charge region ðSCRÞ of the CZTSSe absorber The most important material properties used to simulate the CZTSSe solar cell by the ADEPT 2:1 tool are summarized in Table [2,12,15,18–20] Results and discussions 4.1 Effect of ZTO buffer layer on the cell performance The light J–V characteristic curve obtained after conducting the simulation of CZTSSe solar cell with ZTO buffer under the global AM1:5G illumination condition is shown in Fig In this case, the cell performance was estimated without using the CuIn3 Se5 ISL And the FF and the efficiency of CZTSSe solar cell were calculated as 70:23% and 12:98% respectively 4.2 Effect of CuIn3 Se5 ISL on the cell performance While using CuIn3 Se5 as an inverted surface layer between the ZTO buffer and CZTSSe absorber layer, the performance varies This layer is used to inhibit the interface recombination processes and to reduce the defect density at the interface [21] However, the short-circuit current density, Jsc reduces owing to the spike barrier for recombination at the ZTO=CZTSSe interface and photo-generated electrons in comparison with the cell with CdS buffer layer The careful control of using CuIn3 Se5 between the ZTO buffer and the CZTSSe absorber is necessary as the cell performance is greatly affected by the thickness of this inverted surface layer [22] It is observed that the use of 60 nm thick ISL at the ZTO=CZTSSe interface in CZTSSe solar cell yields a higher energy conversion efficiency of 14:09% with Jsc , Voc , and FF of 33:74 mA cm2 , 731:26 mV, and 73:17% respectively Fig represents the J–V characteristic curve for CZTSSe solar cell with CuIn3 Se5 ISL The comparison of performance parameters between the device structures with ZTO buffer and with CdS buffer is illustrated in Table From the observation, it is clear that the ZTO should be a promising option as a substitute buffer layer to CdS The shunt resistance has been computed from the slope close to V ¼ Voc whereas the serial resistance has been calculated from the slope nearby V ¼ Fig shows the energy band diagram with a band gap grading At the ZTO=CZTSSe interface, a barrier was gener- Table Figure Current density versus voltage curve of CZTSSe based solar cell Figure J–V characteristic curve of CZTSSe solar cell with CuIn3 Se5 ISL ated because of the difference of electron affinities The CBO generates a barrier that acts as a secondary diode similar to Material parameters used in ADEPT 2:1 for CZTSSe solar cell simulation Properties Cu2 ZnSn ðS; SeÞ4 Zn1x Snx Oy i-ZnO ZnO : Al Thickness, s ½nm Band gap, Eg ½eV Electron affinity, ve ½eV Donor concentration, Nd ½cm3 Acceptor concentration, Na ½cm3 Hole mobility, lp ½cm2 V1 s1 1600 1:50 4:63 – 1016 25 50 3:74 4:06 1017 – 40 20 3:30 4:60 1016 1016 20 350 3:40 4:60 1017 – 20 Electron mobility, ln ½cm2 V1 s1 100 160 80 80 Please cite this article in press as: M Asaduzzaman et al., Cadmium free high efficiency Cu2ZnSn(S,Se)4 solar cell with Zn1xSnxOy buffer layer2ZnSn(S,Se)4 solar cell – >, Alexandria Eng J (2017), http://dx.doi.org/10.1016/j.aej.2016.12.017 Md Asaduzzaman et al Table Performance parameters of Cu2 ZnSnðS; SeÞ4 thin film solar cell Description Voc ðmVÞ Jsc ðmA cm2 Þ FF ð%Þ g ð%Þ CZTSSe cell with CdS buffer [1] CZTSSe cell with ZTO buffer CZTSSe cell with ZTO buffer and CuIn3 Se5 ISL 513:40 656:31 731:26 35:21 36:04 33:74 69:80 70:23 73:17 12:60 12:98 14:09 Conclusions Figure Energy band diagram with a band gap grading the main diode The CZTSSe absorber absorbs photons nearby the junction where the band bending produces a well This well amasses electrons and recombines at the ZTO=CZTSSe interface or defeat the barrier The barrier can halt the current flow if this barrier is bigger than we supposed The electric field response and the recombination rate corresponding to the thickness of the cell are also shown in Fig 5a and b respectively Figure The ADEPT 2:1 device simulator has been used to conduct a numerical simulation of a thin film solar cell having a device structure n-ZnO : Al/i-ZnO/n-ZTO/p-CZTSSe with an ordered vacancy compound OVC layer, also known as inverted surface layer, sandwiched between the ZTO buffer and the CZTSSe absorber The recombination rate in the space charge region ðSCRÞ controls the open circuit voltage ðVoc Þ generated by the cell The Voc can be decreased by enhancing the SCR band gap followed by the increase in the barrier height By using the ZTO buffer in the CZTSSe solar cell, a 14:09% power conversion efficiency ðPCEÞ was achieved under the global illumination condition AM1:5G with an operating temperature of 300:15K and a shadowing factor of 0:10 It can be concluded that the Zn1x Snx Oy ðZTOÞ material can be used as a buffer layer substitute to the toxic and carcinogenic CdS material as the photovoltaic parameters of the cell with ZTO buffer substantiate a better performance than the existing cell with conventional CdS buffer And thus it supports strongly to fabricate an environment-friendly, cost-effective and highly efficient CZTSSe thin film solar cell with ZTO buffer layer in the laboratory Authors’ contributions MA conducted the device modeling, led the simulation, prepared and drafted the manuscript MMM and MMH helped to analyze the results and to prepare the manuscript ANB supervised the research and aided to submit the manuscript All authors read and approved the final manuscript (a) Electric field response; (b) recombination profile Please cite this article in press as: M Asaduzzaman et al., Cadmium free high efficiency Cu2ZnSn(S,Se)4 solar cell with Zn1xSnxOy buffer layer2ZnSn(S,Se)4 solar cell – >, Alexandria Eng J (2017), http://dx.doi.org/10.1016/j.aej.2016.12.017 Cadmium free high efficiency Cu2ZnSn(S,Se)4 solar cell Acknowledgment The authors would like to express their gratitude for the use of ADEPT 2:1, an online based one-dimensional simulation tool developed by the research group of Purdue University, USA References [1] S Tajima, R Asahi, D Isheim, D.N Seidman, T Itoh, M Hasegawa, K Ohishi, Atom-probe tomographic study of interfaces of Cu2ZnSnS4 photovoltaic cells, Appl Phys Lett 105 (2014) 093901, http://dx.doi.org/10.1063/1.4894858 [2] W Wang, M.T Winkler, O Gunawan, T Gokmen, T.K Todorov, Y Zhu, D.B Mitzi, Device characteristics of CZTSSe thin-film solar cells with 12.6% efficiency, Adv Energy Mater (2014), http://dx.doi.org/10.1002/aenm.201301465, n/a–n/a [3] W Wu, N.G Tassi, Y Cao, J.V Caspar, K Roy-Choudhury, L Zhang, Optoelectronic characteristics of >9% efficient bilayered CuZnSn(S,Se)4 photovoltaic device, Phys Status Solidi RRL (2015) 236–240, http://dx.doi.org/10.1002/ pssr.201510048 [4] S.W Seo, J.-O Jeon, J.W Seo, Y.Y Yu, J Jeong, D.-K Lee, H Kim, M.J Ko, H.J Son, H.W Jang, J.Y Kim, Compositional and interfacial modification of Cu2ZnSn(S,Se)4 thin-film solar cells prepared by electrochemical deposition, ChemSusChem (2016) 439–444, http://dx.doi.org/10.1002/cssc.201501256 [5] M.A Green, K Emery, Y Hishikawa, W Warta, E.D Dunlop, Solar cell efficiency tables (version 48), Prog Photovolt.: Res Appl 24 (2016) 905–913, http://dx.doi.org/10.1002/pip.2788 [6] S Bag, O Gunawan, T Gokmen, Y Zhu, T.K Todorov, D.B Mitzi, Low band gap liquid-processed CZTSe solar cell with 10.1% efficiency, Energy Environ Sci (2012) 7060–7065, http://dx.doi.org/10.1039/C2EE00056C [7] Q Guo, G.M Ford, W.-C Yang, B.C Walker, E.A Stach, H W Hillhouse, R Agrawal, Fabrication of 7.2% efficient CZTSSe solar cells using CZTS nanocrystals, J Am Chem Soc 132 (2010) 17384–17386, http://dx.doi.org/ 10.1021/ja108427b [8] I.D Olekseyuk, L.D Gulay, I.V Dydchak, L.V Piskach, O.V Parasyuk, O.V Marchuk, Single crystal preparation and crystal structure of the Cu2Zn/Cd, Hg/SnSe4 compounds, J Alloys Compd 340 (2002) 141–145, http://dx.doi.org/10.1016/S09258388(02)00006-3 [9] I Repins, C Beall, N Vora, C DeHart, D Kuciauskas, P Dippo, B To, J Mann, W.-C Hsu, A Goodrich, R Noufi, Coevaporated Cu2ZnSnSe4 films and devices, Sol Energy Mater Sol Cells 101 (2012) 154–159, http://dx.doi.org/10.1016/ j.solmat.2012.01.008 [10] K Wang, O Gunawan, T Todorov, B Shin, S.J Chey, N.A Bojarczuk, D Mitzi, S Guha, Thermally evaporated Cu2ZnSnS4 solar cells, Appl Phys Lett 97 (2010) 143508, http://dx.doi.org/10.1063/1.3499284 [11] USGS Minerals Information: Commodity Statistics and Information, (n.d.) (accessed November 17, 2016) [12] J Lindahl, J Keller, O Donzel-Gargand, P Szaniawski, M Edoff, T Toărndahl, Deposition temperature induced conduction band changes in zinc tin oxide buffer layers for Cu (In,Ga)Se2 solar cells, Sol Energy Mater Sol Cells 144 (2016) 684690, http://dx.doi.org/10.1016/j.solmat.2015.09.048 [13] C Platzer-Bjoărkman, C Frisk, J.K Larsen, T Ericson, S.-Y Li, J.J.S Scragg, J Keller, F Larsson, T Toărndahl, Reduced interface recombination in Cu2ZnSnS4 solar cells with atomic layer deposition Zn1xSnxOy buffer layers, Appl Phys Lett 107 (2015) 243904, http://dx.doi.org/10.1063/1.4937998 [14] J Lindahl, C Haăgglund, J.T Waătjen, M Edoff, T Toărndahl, The effect of substrate temperature on atomic layer deposited zinc tin oxide, Thin Solid Films 586 (2015) 82–87, http://dx.doi org/10.1016/j.tsf.2015.04.029 [15] T Minemoto, T Matsui, H Takakura, Y Hamakawa, T Negami, Y Hashimoto, T Uenoyama, M Kitagawa, Theoretical analysis of the effect of conduction band offset of window/CIS layers on performance of CIS solar cells using device simulation, Sol Energy Mater Sol Cells 67 (2001) 83–88, http://dx.doi.org/10.1016/S0927-0248(00)00266-X [16] J Gray, X Wang, R.V.K Chavali, X Sun, A Kanti, J.R Wilcox, ADEPT 2.1, 2015 doi:D39S1KM3S [17] S.M Sze, K.K Ng, Physics of Semiconductor Devices, third ed., John Wiley & Sons Inc., n.d (accessed September 13, 2016) [18] S.M Wasim, C Rinco´n, G Marı´ n, J.M Delgado, J Contreras, Effect of ordered defects on the crystal structure of In-rich ternary compounds of the Cu–In–Se system, J Phys D Appl Phys 37 (2004) 479, http://dx.doi.org/10.1088/0022-3727/37/3/ 028 [19] S Ahn, S Jung, J Gwak, A Cho, K Shin, K Yoon, D Park, H Cheong, J.H Yun, Determination of band gap energy (Eg) of Cu2ZnSnSe4 thin films: on the discrepancies of reported band gap values, Appl Phys Lett 97 (2010) 021905, http://dx.doi org/10.1063/1.3457172 [20] M Asaduzzaman, M Hasan, A.N Bahar, An investigation into the effects of band gap and doping concentration on Cu(In,Ga) Se2 solar cell efficiency, SpringerPlus (2016) 578, http://dx.doi org/10.1186/s40064-016-2256-8 [21] Y Cho, D.-W Kim, S Ahn, D Nam, H Cheong, G.Y Jeong, J Gwak, J.H Yun, Recombination in Cu(In,Ga)Se2 thin-film solar cells containing ordered vacancy compound phases, Thin Solid Films 546 (2013) 358–361, http://dx.doi.org/10.1016/j tsf.2013.04.078 [22] S.H Kwon, S.C Park, B.T Ahn, K.H Yoon, J Song, Effect of CuIn3Se5 layer thickness on CuInSe2 thin films and devices, Sol Energy 64 (1998) 55–60, http://dx.doi.org/10.1016/S0038-092X (98)00024-3 Please cite this article in press as: M Asaduzzaman et al., Cadmium free high efficiency Cu2ZnSn(S,Se)4 solar cell with Zn1xSnxOy buffer layer2ZnSn(S,Se)4 solar cell – >, Alexandria Eng J (2017), http://dx.doi.org/10.1016/j.aej.2016.12.017 ... al., Cadmium free high efficiency Cu2ZnSn(S,Se)4 solar cell with Zn1xSnxOy buffer layer2 ZnSn(S,Se)4 solar cell – >, Alexandria Eng J (2017), http://dx.doi.org/10.1016/j.aej.2016.12.017 Cadmium free. .. this article in press as: M Asaduzzaman et al., Cadmium free high efficiency Cu2ZnSn(S,Se)4 solar cell with Zn1xSnxOy buffer layer2 ZnSn(S,Se)4 solar cell – >, Alexandria Eng J (2017), http://dx.doi.org/10.1016/j.aej.2016.12.017... level defect with an interface defect concentration Please cite this article in press as: M Asaduzzaman et al., Cadmium free high efficiency Cu2ZnSn(S,Se)4 solar cell with Zn1xSnxOy buffer layer2 ZnSn(S,Se)4