Cadmium free high efficiency cu2znsn(s,se)4 solar cell with zn1−xsnxoy buffer layer

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[.]

ORIGINAL ARTICLE

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 2 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 ToolðADEPT 2:1Þ and the device performances of a thin film solar cell based on Cu2ZnSnðS; SeÞ4ðCZTSSeÞ absorber have been measured Initiating with a thin film photovoltaic device structure consisting of n-ZnO: Al=i-ZnO=Zn1
xSnxOy

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 volt-age, Vocwithout deteriorating the short-circuit current density, Jsc The optimized solar cell perfor-mance parameters including Voc, Jsc, fill factorðFFÞ, 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/ ).

1 Introduction

The use of photovoltaic device has been growing so rapidly to

utilize the world’s amplest energy source, incident sunlight

CZTSSe, the promising absorber layer materials, have drawn much attention to the photovoltaic researchers for highly effi-cient 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 effi-ciency 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

* Corresponding author.

E-mail addresses: asaduzzaman.mbstu@gmail.com (Md

Asaduzza-man), 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.

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

http://dx.doi.org/10.1016/j.aej.2016.12.017

104cm
1 [6Ờ10] Moreover, comparing to the expensive and

scarce indiumđInỡ, 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

đZTOỡ 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

inter-faces Besides this, the effects of CBO on Voc, Jsc, FF, andg

have also been analyzed Photovoltaic cell having conduction

band offsets around 0:3 eV provides a better device

perfor-mance[15]

2 Solar cell device modeling

The numerical analysis needed for the solar cell device

model-ing is performed by usmodel-ing the simulator ADEPT 2:1[16] The

steady-state band gap profile, hole and electron carrier

trans-port, recombination profile are estimated by using the

Pois-d

dx
eđxỡdW

dx

Ử q pđxỡ
nđxỡ ợ Nợ

Dđxỡ
N

Ađxỡ ợ Ptđxỡ
Ntđxỡ

đ1ỡ

dnp

dt Ử Gn
np
np

sn ợ nplndn

dxợ lnndnp

dxợ Dn

d2np

dx2 đ2ỡ

dpn

dt Ử Gp
pn
pn

sp ợ pnlpdn

dxợ lpndpn

dxợ Dp

d2pn

dx2 đ3ỡ

wheree is the permittivity, W the electrostatic potential, q the charge of electron, p the free hole, n the free electron, NợDthe donor concentration, N
A the acceptor concentration, n the electric field, Ptthe trapped hole, Ntthe trapped electron, Gn the generation rate for electrons, Gp the generation rate for holes,lnthe electron mobility,lpthe hole mobility, Dnthe dif-fusion coefficient for electrons, and Dpthe diffusion coefficient for holes, and all the parameters are a function of coordinate position x

For bulk defects, the recombination current density is

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 conduc-tion bands to take part in the recombinaconduc-tion process for interfaces

3 Solar cell structure and numerical simulation

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 đISLỡ, CuIn3Se5

is inserted which is commonly known as an ordered vacancy

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 inter-faces The CZTSSe solar cell structure is shown inFig 1

A simulation was conducted in order to interpret the mea-sured current density versus voltage relationship referred to as JỜV characteristic curve An OVC layer with a thickness of

10 cm2V
1s
1, a hole mobility of 40 cm2V
1s
1and a carrier density of 2 1016cm
3have been used[15,18Ờ20] The indi-rect band gap of the ZTO buffer decreases from 3:74 eV at

90C deposition temperature to 3:23 eV at 180C 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

causes lower open circuit voltage and at lower deposition tem-perature 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 increas-ing deposition temperature At 90C deposition temperature the ZTO buffer with aơSn=đơSn ợ ơZnỡ composition of 0:18 results in a band gap of 3:74 eV with a critical thickness of

Figure 1 Schematic diagram of CZTSSe thin film solar cell

structure

of 1 1010

cm
2is placed in the midway of the band gap of the

CZTSSe absorber The most important material properties

used to simulate the CZTSSe solar cell by the ADEPT 2:1 tool

are summarized inTable 1 [2,12,15,18–20]

4 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

Fig 2 In this case, the cell performance was estimated without

using the CuIn3Se5 ISL And the FF and the efficiency of

respectively

4.2 Effect of CuIn3Se5 ISL on the cell performance

While using CuIn3Se5as an inverted surface layer between the

ZTO buffer and CZTSSe absorber layer, the performance

var-ies 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

com-parison with the cell with CdS buffer layer The careful control

of using CuIn3Se5 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

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 cm
2, 731:26 mV, and 73:17% respectively

Fig 3represents the J–V characteristic curve for CZTSSe solar

cell with CuIn3Se5 ISL

The comparison of performance parameters between the

device structures with ZTO buffer and with CdS buffer is

illus-trated in Table 2 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 V¼ Voc whereas the serial resistance has been

calculated from the slope nearby V¼ 0

Fig 4 shows the energy band diagram with a band gap

grading At the ZTO=CZTSSe interface, a barrier was gener- ated because of the difference of electron affinities The CBOgenerates a barrier that acts as a secondary diode similar to

Table 1 Material parameters used in ADEPT 2:1 for CZTSSe solar cell simulation

Figure 2 Current density versus voltage curve of CZTSSe based solar cell

Figure 3 J–V characteristic curve of CZTSSe solar cell with CuIn3Se5ISL

the main diode The CZTSSe absorber absorbs photons

nearby the junction where the band bending produces a well

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

cor-responding to the thickness of the cell are also shown in

Fig 5a and b respectively

5 Conclusions

The ADEPT 2:1 device simulator has been used to conduct a numerical simulation of a thin film solar cell having a device

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 ðVocÞ generated by the cell The Voccan 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 Zn1
xSnxOyðZTOÞ 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 buf-fer layer in the laboratory

Authors’ contributions

MA conducted the device modeling, led the simulation, pre-pared 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

Table 2 Performance parameters of Cu2ZnSnðS; SeÞ4thin film solar cell

Figure 4 Energy band diagram with a band gap grading

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

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