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Chemistry of Materials
The complex surface chemistry of kesterites: Cu/Zn re-ordering after low temperature post deposition annealing and its role in high performance devices Markus Neuschitzer*,‡,†, Yudania Sanchez‡,† , Tetiana Olar§, Thomas Thersleff¤, Simon LopezMarino†, Florian Oliva†, Moises Espindola-Rodriguez†, Haibing Xie†, Marcel Placidi†, Victor Izquierdo-Roca†, Iver Lauermann§, Klaus Leifer¤, Alejandro Pérez-Rodriguez†,∆, and Edgardo Saucedo† †
Catalonia Institute for Energy Research- IREC, Jadins de les Dones de Negre 1, 08930 Sant Adrià de Besòs (Barcelona), Spain §
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institute for Heterogeneous Materials, Berlin, Germany ¤
The Ångström Laboratory, Department of Engineering Sciences, Uppsala University, Box 534, Uppsala, Sweden
∆
IN2UB, Departament d’Electrònica, Universitat de Barcelona, C. Martí i Franquès 1, 08028 Barcelona, Spain.
ABSTRACT: A detailed study explaining the beneficial effects of low temperature post deposition annealing combined with selective surface etchings for Cu2ZnSnSe4 (CZTSe) based solar cells is presented. After performing a selective oxidizing surface etching to remove ZnSe secondary phases typically formed during the synthesis processes an additional 200ºC annealing step is necessary to increase device performance from below 3% power conversion efficiency up to 8.3% for the best case. This significant increase in efficiency can be explained by changes in the surface chemistry which results in strong improvement of the CdS/CZTSe heterojunction commonly used in this kind of absorber/buffer/window heterojunction solar cells. XPS measurements reveal that the 200ºC annealing promotes a Cu depletion and Zn enrichment of the etched CZTSe absorber surface relative to the CZTSe bulk. Raman measurements confirm a change in Cu/Zn ordering and increase in defect density. Furthermore, TEM microstructural investigations indicate a change of grain boundaries composition by a reduction of their Cu content after the 200ºC annealing treatment. Additionally, insights in the CdS/CZTSe interface are gained showing a significant amount of Cu in the CdS buffer layer which further helps the formation of a Cu-depleted surface and seems to play an important role in the formation of the pn-heterojunction.
1.
INTRODUCTION
Kesterite Cu2ZnSn(S1-xSex)4 (CZTSSe) has attracted much attention in the past years as possible replacement for chalcopyrites (CIGS = CuIn1-xGaxSe2) as absorber layer in thin film solar cells because of its composition of more earth abundant materials. Although there are many similarities between kesterites and CIGS there is still a large gap between record efficiencies of chalcopyrites, which are currently exceeding 21% in comparison to 12.6% for CZTSSe.1,2 Main difference in device performance is a much higher open circuit voltage deficit (Voc-deficit: Eg/qVoc; Eg bandgap of the absorber and q the electron charge) for CZTSSe based solar cells than for CIGS. For record CZTSSe cells the Voc-deficit is around 0.6 eV whereas record CIGS cells obtain Voc deficits of 0.4 eV.1–3 Thus experimentally obtained Voc values for kesterite based solar cells are still much lower than theoretically possible, which is currently the main efficiency limiting factor of this technology. The reasons for this high Voc-deficit are not totally clear yet. Enhanced band tailing, due to elec-
trostatic potential fluctuations induced by charged antisite defects pairs like [CuZn-+ZnCu+] could be one of the reasons.4,5 Furthermore, J. Kim et al. showed that by applying a double In2S3/CdS buffer layer combined with a short post deposition annealing at 250ºC to diffuse In into CdS and CZTSSe layers the Voc deficit could be improved to below 0.6 eV because of an enhanced doping due to In incorporation.6 In general, several groups report beneficial effects of low temperature post deposition annealing either in air or inert atmosphere for kesterite based solar cells.7–9 Already in CIGS, short air annealing is reported to improve solar cell performances due to oxygen related defect passivation and is used in devices with over 20% efficiency.10,11 For kesterite, the aspect of low temperature annealing is especially interesting since recently an orderdisorder transition was observed after low-temperature annealing at around 260ºC or 200ºC for the pure sulfide (CZTS) or selenide (CZTSe) compound, respectively.12,13 Disorder at the Cu and Zn sites increase strongly above this temperature, whereas annealing below it can increase
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Chemistry of Materials
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the order. NMR measurements show that [VCu+ZnCu] complexes control the random disorder created by CuZn and ZnCu interstitial defects.14 However, the relationship of order-disorder with point defects, and their influence on device performance is not yet clearly understood. In this work, we study the complex surface chemistry of Cu2ZnSnSe4 before and after a soft post-growth annealing, and its impact on the optoelectronic parameters of the solar cells. In the first part of this work we present the optimization of different post deposition annealing treatments for either the CZTSe absorber, CZTSe/CdS heterojunction or full CZTS/CdS/iZnO+ITO heterojunction solar cells and its strong impact on device performance. By applying an optimal post deposition treatment, efficiencies could be increased from below 2% to over 8%. In the second part we reveal the influences of this annealing on absorber, CdS buffer layer and especially on the CZTSe absorber surface which can explain the observed beneficial effects. 2.
EXPERIMENTAL SECTION
Pure selenide kesterite absorbers (CZTSe) were grown by a two stage process. First metallic stacks of Cu/Sn/Cu/Zn were sputtered by direct current (DC) magnetron sputtering onto ZnO(10 nm)/Mo(800 nm) coated soda lime glass substrates as described in more detail in S. Lopez-Marino et al.15 These precursor stacks (5x5 cm2) are further reactively annealed in a graphite box containing elemental Se and Sn powders (100 mg Se and 5 mg Sn) inside of a conventional 3 zone tube furnace under argon atmosphere. A two-step temperature profile is used for annealing. First a selenization at 400ºC for 30 minutes under argon flow, keeping the pressure at 1.5mbar, is carried out followed by a second shorter annealing at 550ºC for 15 minutes under static 1000 mbar argon pressure to improve crystallinity. Cooling is allowed naturally to room temperature which normally takes about 1.5 hours. The final metallic composition of the CZTSe absorbers was Cu poor and Zn rich, as reported to yield highest device performance 16, with metallic ratios of [Cu]/([Zn]+[Sn])=0.80±0.02 and [Zn]/[Sn]=1.18±0.07 as measured by X-ray florescence spectroscopy (XRF). To complete solar cells a CdS buffer layer is deposited by chemical bath deposition as described elsewhere.17 Before buffer layer deposition different chemical etchings are employed. To remove possible unwanted ZnSe secondary phases formed on the absorber surface under Zn-rich and Cu-poor growth conditions a combined KMnO4(0.01M)+H2SO4 (0.1M) and Na2S (1M) etching was used for 40 seconds and 60 seconds, respectively.18 Solar cells are finished by depositing DC-pulsed sputtered ZnO (50nm) and In2O3 :SnO2 (ITO; 90/10 wt.%; 350 nm , R□=50 Ωcm-1) window layer. 3x3 mm2 cells are separated by mechanical scribing using a manual microdiamond scriber MR200 OEG and contacted directly on the ITO for electrical characterization, without the use of metal grids and anti-reflective coatings. Illuminated current density voltage (JV) curves are recorded using a Sun 3000 class AAA solar simulator from
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ABET technologies which is calibrated with a Si reference cell to 1 sun. External quantum efficiencies (EQE) are measured using a Bentham PVE300 system. Reversed voltage biased EQE curves were collected by connecting a Keithley 2400 source meter directly to the primary coil of the transformer biasing the device at the desired voltage. Raman scattering measurements were made using a Raman probe developed at IREC coupled with optical fiber to an iHR320 Horiba Jovin Yvon spectrometer. The measurements were made in backscattering configuration focusing the excitation laser spot directly on the surface of the samples (diameter 50 mm, excitation power density
