High-efficiency Cu(In,Ga)Se2 solar cells

    High-efficiency Cu(In,Ga)Se 2solar cells Theresa Magorian Friedlmeier, Philip Jackson, Andreas Bauer, Dimitrios Hariskos, Oliver Kiowski, Richard Menner, Roland Wuerz, Michael Powalla PII: DOI: Reference:

S0040-6090(16)30443-6 doi: 10.1016/j.tsf.2016.08.021 TSF 35394

To appear in:

Thin Solid Films

Received date: Revised date: Accepted date:

30 May 2016 1 August 2016 8 August 2016

Please cite this article as: Theresa Magorian Friedlmeier, Philip Jackson, Andreas Bauer, Dimitrios Hariskos, Oliver Kiowski, Richard Menner, Roland Wuerz, Michael Powalla, High-efficiency Cu(In,Ga)Se2 solar cells, Thin Solid Films (2016), doi: 10.1016/j.tsf.2016.08.021

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ACCEPTED MANUSCRIPT E-MRS 2016, Magorian Friedlmeier

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High-Efficiency Cu(In,Ga)Se2 Solar Cells

Theresa Magorian Friedlmeier, Philip Jackson, Andreas Bauer, Dimitrios Hariskos, Oliver

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Kiowski, Richard Menner, Roland Wuerz, and Michael Powalla

Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg,

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Industriestrasse 6, D-70565 Stuttgart, Germany

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Corresponding Author: Theresa Magorian Friedlmeier

Abstract

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[email protected], Tel: +49-711-7870-293, Fax: +49-711-7870-230

Recent progress in the development of high-efficiency solar cells based on Cu(In,Ga)Se2 at our institute is reviewed. The post-deposition treatment with alkali elements (PDT) has been found to improve device quality, mostly through reduced recombination, with the effect of increasing the open-circuit voltage. At the same time, PDT is shown to improve initial growth properties of the chemical-bath-deposited buffer layer, so that this layer may be made thinner and the corresponding losses are reduced. Further optimization by replacing the non-doped ZnO resistive layer by (Zn,Mg)O enables gains in photocurrent for the ultraviolet region. A certified efficiency of 22.0% with a CdS/(Zn,Mg)O buffer/resistive layer combination is presented here.

ACCEPTED MANUSCRIPT Furthermore, the effect of band gap grading with gallium in the absorber layer is explored with

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respect to the buffer layer properties by means of device simulation.

Highlights

Best Cu(In,Ga)Se2-based solar cell efficiencies have surpassed 22%

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Key developments are briefly reviewed

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Developments in buffer / resistive layer combinations are presented

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Ga gradient profiles are investigated via device simulation with SCAPS

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Keywords

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CIGS solar cells, high efficiency, device simulation, buffer layers, resistive layers

1. Introduction

Decades of research and development in thin-film solar cells based on Cu(In,Ga)Se2 (CIGS) have led to remarkably high efficiencies, currently up to 22.6% and thus exceeding the record value for multi-crystalline Si-based solar cells [1]. Recently, in-house measurements as high as 22.8% have been published by Solar Frontier [2]. Fig.1 provides a summary of efficiency development over time based on the National Renewable Energy Laboratory (NREL) efficiency charts [3], with addition of the certified 22.6% value achieved by the Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW). We see that there have been phases of rapid development followed by phases of more incremental improvement as processes are optimized. Two phases of rapid progress are apparent around the mid 1990’s and current times. The progress

ACCEPTED MANUSCRIPT during the first phase can be attributed to improvements in the CIGS film due to the implementation of the bi-layer process for co-evaporated CIGS with initial Cu-rich growth for

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large grains followed by Cu-poor growth to convert excess Cu-Se to CIGS [4] and the

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incorporation of Na and Ga [5,6]. At the same time, the excellent conformal coating properties of the chemical bath deposition (CBD) process for the CdS buffer layer led to improved fill factors

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and higher efficiencies [7]. The following decade was dominated by the record values presented

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by NREL. They introduced the three-stage process which enables the Ga gradients we typically see in record devices today [8]. Currently we observe record efficiencies being reported by both

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public and industrial research laboratories, by both the co-evaporated and the sequential technique for depositing the CIGS absorber layer, and spanning the globe – from the USA,

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Europe, and Japan. The most recent results are reported to have been achieved by application of a

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post-deposition treatment with alkali elements, similar to that first described by the Swiss Federal

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Laboratories for Materials Science and Technology [9]. A significant insight derived from the latest efficiency developments is that the “high-efficiency” players are building upon experience – both in knowledge and technical processing skills. In the following we will address current focal points of research and development at our institute that are contributing to efficiency improvements, in particular modifications of the buffer/window layer combination and device simulation to investigate effects of the gallium gradient.

2. Experimental

Our standard CIGS solar cell device structure consists of a glass substrate, a sputtered Mo back contact, multi-stage co-evaporated CIGS (static or inline), a CBD buffer layer of CdS or Zn(O,S), and rf-sputtered undoped ZnO or Zn0.75Mg0.25O followed by ZnO:Al for the top contact. The test

ACCEPTED MANUSCRIPT cells are typically 0.5 cm2 and employ a Ni-Al-Ni grid. A MgF2 anti-reflection coating is added to record devices to improve coupling of light into the cell. Efficiencies up to 19.6% have been

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achieved with the inline CIGS coating system [10]. The static system has more flexibility in the

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design and control of the CIGS process and maintains higher specifications on the purity of used materials and procedures. Alkali elements like sodium and potassium are present during CIGS

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growth, as they diffuse from the glass substrate. Some experiments include a subsequent PDT

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process similar to the one described in [9] with alkali fluorides like KF, RbF and CsF and which

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additionally modifies the surface of the film [1].

Process development and optimization is accompanied by standard analysis methods like current-

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voltage (IV, at 25 °C under 1000 W/m² simulated AM1.5G illumination, WACOM WXS-90S-5),

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capacitance-voltage (CV, HP 4192A LF Impedance Analyzer at 100 kHz in the dark), X-ray

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fluorescence spectrometry (XRF, EDAX Eagle XXL) and external quantum efficiency (EQE), as well as destructive methods like scanning electron microscopy on cross-sections (FEI XL30 SFEG) and depth profiling with sputtered neutral mass spectroscopy (SNMS, LEYBOLD LHS 10 system with a secondary ion and neutral mass spectrometer module (SSM 200) using a Balzers 511 quadrupole for mass separation) or glow discharge optical emission spectroscopy (GDOES, Horiba GD-Profiler2) on selected samples. The transmittance of i-ZnO, (Zn,Mg)O, and ZnO:Al is measured on reference samples prepared directly on glass substrates (Perkin Elmer Lambda 900). Device simulation is performed with the SCAPS program [11].

3. Results and Discussion

ACCEPTED MANUSCRIPT Over the past several years the ZSW has been able to steadily increase the best performance of CIGS solar cells by careful optimization and re-optimization of processes, implementation of the

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alkali-based PDT and additional modifications to the layer structure. Table I summarizes these

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results, certified by the CalLab at Fraunhofer ISE, together with information about the buffer layers, application of PDT, and the integral Ga content in the CIGS absorber layer GGI =

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[Ga]/([Ga]+[In]). The first device, 20.3% from 2010, was the result of careful optimization and

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fine tuning of the equipment, materials, process flow and the cell stack itself [12]. The 20.8% device in 2013 resulted from a first round of experiments with the PDT processing step [13].

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Reduction of the CBD-CdS processing time and corresponding CdS film thickness, together with PDT and a modified GGI gradient led to the significant increase in efficiency up to 21.7%

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[14,15]. The 21.0% and the 22.0% devices will be described in more detail in the following. The

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IV characteristic for the 22.0% device is presented here in Figure 2. The corresponding external

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quantum efficiency (EQE) curve is included in comparison with other devices in Figure 3. A wide range of processing conditions for CIGS and PDT has been explored experimentally. The reverse saturation current density J0 and diode factor A are both generally lower for devices processed with PDT. Both trends indicate reduced recombination which is in good agreement with the observed higher open-circuit voltages VOC. The actual mechanisms involved are a subject of current scientific investigation and are still open to discussion. Typical features already described in the literature include a reduced Cu content at the CIGS surface [9,16] and a wider band gap due to formation of In2Se3 or KInSe2 compounds at the surface [17]. Parallel to research regarding the effects of the PDT process, we have proceeded with the experimental optimization of the CIGS solar cell stack. In the following we will focus on two aspects of device development involved in the latest results listed in Table I: 1) optical properties of the window and buffer layers, and 2) the GGI gradient within the CIGS absorber layer.

ACCEPTED MANUSCRIPT 3.1 Optical Advances The EQE curves for three devices from Table I are compared in Figure 3. The Mo, CIGS, and

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ZnO:Al processes were similar for these devices, but they employ different buffer/resistive layer

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combinations. The 21.7% device is represented by the blue line. The shoulder between 400 and

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500 nm is typical for absorption by the CdS buffer layer and is already less pronounced due to the thinner CdS layer as enabled by the PDT process. The red and green curves in Figure 3 illustrate

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the gains possible by employing different materials for the buffer/resistive layers. The red curve

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represents the 21.0% device which employed a Zn(O,S) buffer and (Zn,Mg)O resistive layer. Since Zn(O,S) has a much higher band gap than CdS, the parasitic absorption in this layer is

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eliminated. (Zn,Mg)O has a better band alignment with Zn(O,S) than undoped ZnO, so we

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typically pair these materials [18]. CIGS solar cells with the Zn(O,S) buffer typically have lower VOC’s and FF’s compared to those with CdS buffers, so that the efficiencies are lower despite the

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gains in photocurrent [19]. However, the VOC in excess of 700 mV achieved with PDT-CIGS are among the highest reported so far for the Zn(O,S) buffer. This device also demonstrates that the CBD-Zn(O,S) buffer is compatible with the PDT-modified CIGS absorber. While the EQE of the Zn(O,S)-buffered device shows gains in the 300 – 500 nm region, we also observe pronounced interference effects which indicate that the anti-reflection coating (ARC) is not optimal. The third, green curve in Figure 3 is the EQE of the latest ZSW high-efficiency device with a certified efficiency of 22.0%. This device employs a CBD-CdS buffer layer paired with a sputtered (Zn,Mg)O resistive layer which has a slightly higher band gap than undoped ZnO. The gain in the wavelength region between 300 and 400 nm is apparent and the short-circuit current density (JSC) is slightly improved. Absorption by CdS still leads to losses in the photocurrent. (Zn,Mg)O has the potential to also replace the CdS layer, as demonstrated by atomic layer

ACCEPTED MANUSCRIPT deposition [20]. Sputtered (Zn,Mg)O directly on the PDT-CIGS layer has so far led to poor results in our laboratory. Future optimization goals include suitable buffer/resistive layer

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materials and deposition techniques as well as improved anti-reflection measurements.

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3.2 Effect of GGI Gradient

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The GGI gradient, which reflects a band gap variation that predominantly changes the conduction

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band position, typically has a minimum (“notch”) slightly behind the CdS interface in multi-stage coevaporated CIGS. The GGI then increases both towards the front (CdS) and the back (Mo)

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sides. Figure 4 illustrates these features in measured GGI gradients for three devices from Table I. The double-graded GGI configuration has the advantage of reduced recombination at the back

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contact due to the higher band gap and the gradient also directs minority charge carriers

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(electrons) towards the space-charge region for collection. At the same time, the lower band gap

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at the notch allows the absorption of photons at lower energy. The spatially limited region with reduced band gap compensates the related increase in Shockley-Read-Hall recombination. The role of an increased band gap towards the CdS is not conclusive. High efficiencies have been achieved with steeper and with flatter GGI gradients towards the buffer interface. In order to better understand the effect of GGI gradients on solar cell performance, we established a CIGS device model in SCAPS [11] that matches very well experimental data acquired by IV, CV, EQE, GDOES (GGI profile), and XRF [15]. Special features of this model are the non-uniform effective doping of the CIGS layer, a strongly doped p+-layer covering the CIGS surface, and an interface defect layer between p+ - and buffer layer [21,22]. The nonuniform CIGS doping profile and the p+-layer are necessary to model CV measurement data. Figure 5 illustrates some selected simulation results for fixed notch positions and variations of GGI slopes towards both CIGS layer interfaces, as illustrated by the schematic diagram in the

ACCEPTED MANUSCRIPT lower right of the figure. Positive slopes mean increasing GGI towards CIGS interfaces with respect to the notch’s GGI. Within the SCAPS device model, we systematically investigated a

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variety of possible GGI profiles to create contour maps of efficiency: GGI variation at the CIGS

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back side (0-0.65, Mo contact), at the CIGS front side (0.2-0.4, at the CdS interface), at the notch position (0-0.4), as well as the notch position itself (0.02-1.42 µm from the CdS interface, total

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CIGS layer thickness is 2.5 µm. Fig. 5 illustrates results for a notch position of 0.42 µm). The

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dashed lines in the contour maps highlight cases with zero GGI slope (constant GGI) either towards the back contact (horizontal) or towards the buffer layer (vertical).

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One key result of our simulations is that the efficiency maximum is clearly located in the quadrant with positive slopes to both interfaces (Fig. 5). The optimum back-side slope is in the

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range of 0-0.15 µm-1 or even up to 0.2 µm-1, depending on the front-side GGI. The front-side

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slope can be much higher, in the range of 0.1 to 0.5 µm-1. Therefore, the notch GGI must be

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lower than the interface GGI’s, but not too low. Following these guidelines, a wide range of GGI profiles can enable efficiencies above 21.5% but still less than 22.0% (within the present model). There may be some advantage to increasing the GGI at the CdS interface from 0.3 to 0.4. Since the initial device’s simulated efficiency is 21.6%, we see that experimental optimization has already accessed the full potential of GGI profile engineering. Furthermore, both experimental and simulated results agree that high efficiencies can be generated with a wide range of GGI profiles. We observe a variety of GGI profiles measured in devices with efficiencies close to or above 21%. A common feature for all high-efficiency cells is a positive GGI slope towards the back side while the slope towards the buffer side ranges from rather small (e.g. 22% device) to quite pronounced (e.g. 21.7%), as illustrated in Fig. 4 and in agreement with the simulated efficiency maps.

ACCEPTED MANUSCRIPT 4. Conclusion Highest efficiencies for CIGS solar cells already exceed 22% and devices in this range have been

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reported by several groups around the world. The current fast development in record efficiencies

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is mostly attributed to the PDT process, but also to the high level of expertise attained over time

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and practical experience. There is still room for fine-tuning, in particular regarding optical losses in the window and buffer layers, as well as with anti-reflection coatings. A certified efficiency of

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22.0% is presented here, as attained by applying a higher band gap resistive layer (Zn,Mg)O

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together with the thin CdS buffer layer. SCAPS simulations demonstrate that the GGI profile optimized experimentally is already within the expected optimal range and that this range is wide

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enough to allow flexibility in CIGS processing. As the level of understanding and experience

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increases, especially regarding the effects of the PDT process, continued efficiency

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improvements in CIGS-based photovoltaic devices are to be expected in the near future. Acknowledgements

We gratefully acknowledge the support of the ZSW MAT team, in particular Dieter Richter for sample preparation and Wolfram Hempel for depth profile measurements. We also thankfully acknowledge the funding by the German Federal Ministry of Economics and Technology (BMWi) under contracts no. 0329585G (CISEffTec) and 0325715 (CISProTec), as well as the Ministry of Finance and Economics Baden-Württemberg (KaliTan). References [1]

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average GGI and cell parameters.

η [%]

Buffer

VOC

GGI

Ref

35.4

77.5

0.34

[12]

34.8

79.1

0.32

[13]

[mA/cm ]

no

CdS / i-ZnO

20.3

2013

KF

CdS / i-ZnO

20.8

2014

RbF

CdS / i-ZnO

21.7

748

36.5

79.4

0.32

[14]

2015

RbF

Zn(O,S) / Zn,Mg)O

21.0

717

37.2

78.6

0.31

[15]

21.9

739

36.9

80.5

0.29

22.0

746

36.5

80.6

0.31

CdS / ZMO

CdS / ZMO

757

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2016

RbF

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2010

2016

740

FF [%]

2

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[mV]

JSC

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PDT

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Year

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Table I: List of most recent certified ZSW record cells with buffer layer, application of PDT,

this work 22.0

744

36.7

80.5

0.31

22.6

741

37.8

80.6

0.31

[1]

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Figure 1: Summary of reported record conversion efficiencies for CIGS-based solar cells, based

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on the NREL efficiency chart [3]. ZSW results are highlighted in blue.

Figure 2: Current-voltage characteristic of the 22.0% CIGS solar cell produced by ZSW with a

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CBD-CdS buffer and (Zn,Mg)O resistive layer (with ARC, designated area, certified by

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Fraunhofer ISE CalLab PV Cells).

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Figure 3: External quantum efficiency (EQE) for the devices certified at 21.7%, 21.0%, and

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22.0% (with ARC) illustrating influence of buffer/resistive layers.

Figure 4: Measured GGI profiles for three devices from Table I.

Figure 5: Simulation of device efficiency for various GGI profiles and their corresponding slopes towards the CIGS interfaces, mapped for several GGI’s at the buffer interface (0.2, 0.3 and 0.4). The notch position is fixed at 420 nm from the CdS interface in each case. The dashed lines indicate no slope, i.e., constant GGI towards the interface (horizontal = Mo, vertical = CdS). The white point in the upper right figure designates the device model of the 21.7% experimental device [14].

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ACCEPTED MANUSCRIPT Table I: List of most recent certified ZSW record cells with buffer layer, application of PDT,

η [%]

Buffer

JSC

[mV]

[mA/cm2]

FF [%]

GGI

Ref

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PDT

VOC

35.4

77.5

0.34

[12]

34.8

79.1

0.32

[13]

36.5

79.4

0.32

[14] [15]

no

CdS / i-ZnO

20.3

740

2013

KF

CdS / i-ZnO

20.8

757

2014

RbF

CdS / i-ZnO

21.7

2015

RbF

Zn(O,S) / Zn,Mg)O

21.0

717

37.2

78.6

0.31

739

36.9

80.5

0.29

22.0

746

36.5

80.6

0.31

2016

RbF

RbF

CdS / ZMO

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2016

CdS / ZMO

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21.9

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2010

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Year

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average GGI and cell parameters.

this work

22.0

744

36.7

80.5

0.31

22.6

741

37.8

80.6

0.31

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