- Email: [email protected]
Highly efficient CIS solar cells and modules made by the co-evaporation process
Thin Solid Films 517 (2009) 2111–2114
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Highly efficient CIS solar cells and modules made by the co-evaporation process M. Powalla ⁎, G. Voorwinden, D. Hariskos, P. Jackson, R. Kniese Zentrum für Sonnenenergie- und Wasserstoff-Forschung (ZSW), Industriestrasse 6, D-70565 Stuttgart, Germany
a r t i c l e
i n f o
Available online 11 November 2008 Keywords: CuInSe2 Cu(InGa)Se2 Module manufacturing
a b s t r a c t Thin-film photovoltaic modules which use the chalcopyrite Cu(In,Ga)(Se,S)2 (CIGS) as the light-absorbing layer have now entered the decisive industrial phase. Companies located mainly in Germany and Japan will produce more than 100 MWp CIGS modules in 2008, demonstrating that the CIGS technology has already achieved a certain maturity. Whereas key features of the technology are already well-optimized, there are several approaches to further improve the productivity of new lines. The ZSW operates a line for 30 × 30 cm2 modules in which all process steps – from glass cleaning to module encapsulation – are being developed. A major goal of the development is the very fast and efficient transfer of promising new materials and processes from cells to the industrial module level. Therefore, ZSW is focusing on processes like the in-line co-evaporation method for CIS or chemical bath deposition for buffer layers to optimize the junction. We could demonstrate efficiencies close to 18% for small test cells and 14–15% for modules with modified processes. Different cell and material data from optoelectronic measurements and microscopic analysis will be presented in this contribution. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The process technology of Cu(In,Ga)Se2 has reached the maturity for mass production of thin-film photovoltaic modules. The inherent advantages of the direct band gap material Cu(In,Ga)Se2 are based on its high absorption and therewith low layer thickness required for light absorption. The resultant potential for cost reduction, light weight and flexible applications makes this material an all-round candidate for cheap large area module technology as well as special architectural and space applications. Due to this all-round abilities Cu (In,Ga)Se2 is an interesting technology for industrial production. The efficiencies of current solar modules from mass production [1–4] summarized in Table 1 exceed amorphous silicon and CdTe. To further increase the applicability and profitability a further increase of efficiency is necessary. The highest conversion efficiencies of small laboratory cells are close to 20% [5]. If single cells are connected in series by monolithic integration, some principle loss mechanisms arise which may be optimized, but always will reduce the efficiency of large area modules compared to laboratory cells. (i) To allow appropriate current transport through the conducting oxide, this window layer requires a certain thickness and cannot be optimized almost arbitrarily to optical properties like in small area record cells.
⁎ Corresponding author. E-mail address: [email protected] (M. Powalla). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.10.126
(ii) The interconnect between single cells introduces area losses. Since with increasing module size the twisting of the substrate requires larger tolerances for patterning, the area loss due to the interconnects increases with increasing module size. (iii) With increasing module size inhomogeneities will be more pronounced due to limited process stability and homogeneity. Fig. 1 illustrates the decrease of the aperture conversion efficiency with increasing substrate size. The total area efficiency in turn may even increase slightly due to a smaller relative area loss by the frame. The advantage of processing substrate sizes as large as possible lies also in the higher productivity. 2. Technology evolution Increasing the efficiency of solar modules is a process of careful optimisation of each process step in view of optimising the hetero structure and the above mentioned losses related to module technology. Besides this “fine-tuning” a considerable efficiency improvement would be possible by a major change in technology. On the absorber side a fundamental evolution would be the advent of a multi stage process to mass production, which is known to provide the to date highest efficient Cu(In,Ga)Se2 absorber material [5]. Hence one main goal of research at the ZSW is to transfer the efficiency potential of multi stage Cu(In,Ga)Se2 processes to large scale production [6]. A further fundamental advance would be the substitution of the CdS buffer layer by an appropriate high band gap material to reduce absorption losses in the short wavelength range. Among the possible candidates ZnS so far achieves the most promising results in our
2112
M. Powalla et al. / Thin Solid Films 517 (2009) 2111–2114
Table 1 Published module efficiencies from several industrial module fabrications [1–4] Effi./power
Size
(%/Wp)
(cm2)
13.1/64.8 13.1/93.1 13.0/84.6 8.6/63
5400 7128 6500 7381
Company
Published
Avancis (shell solar) Showa shell Würth solar Sulfurcell
2003 2007 2004 2006
Remark
Cd free Sulfide
experiments [7]. This contribution reports on the latest development steps on these two pathways. 3. Multi stage CIGS process To realize a multistage process in an in-line deposition plant, the different stages have to be spatially successively arranged in the system. Therefore we extended our 30 × 30 cm2 deposition chamber and equipped it with additional evaporation sources for the elements copper, indium, gallium and selenium. These allow for the successive co-evaporation of the elements with different rate ratios in one single run. Individually controllable substrate heaters adjust the substrate temperature for each deposition step. The line-shaped evaporation rate profiles of the sources, assure a good homogeneity perpendicular to the transport axis. Finite element simulations helped us to optimize the flux rate distributions [8]. Atomic absorption spectrometers (AAS), coupled into the chamber with optical fibres, measure the copper, indium and gallium evaporation rates [9]. This allows for the control of the evaporation rates to achieve an excellent compositional homogeneity in the substrate movement direction. The selenium flux is a few times the amount necessary for the formation of Cu(In,Ga)Se2. We use soda lime glass coated with DC-sputtered molybdenum as a substrate material. The Cu(In,Ga)Se2 films grow to a thickness of 2 µm. The fabrication of the solar cells is continued with a chemical bath CdS buffer layer, RF-sputtered i-ZnO, and DC-sputtered n-ZnO:Al. On small area single cells Ni/Al grids are applied as front contacts. 4. Film properties The electron micrographs in Fig. 2 depict one major difference between the thin films prepared with our previous process (a) as described in [10], and the ones prepared with the new multi step process (b). It is the different surface morphology. In (a), the grains end up with a pyramidal shape with steep slopes of the surface, whereas in (b) the top surface of the grains is rather flat, but still with large steps between the grains. This change in surface morphology deteriorates the optical properties by increasing the reflection with decreasing
Fig. 2. Scanning electron micrograph cross sections of Cu(In,Ga)Se2 absorbers. (a) Previously developed large area ZSW in-line process [3]. (b) Film prepared with multi stage in-line process.
roughness. On the other hand a flat surface may have advantages for electric properties by reducing the space charge volume. Furthermore a flat surface leads to more conductive and more stable ZnO layers [11]. That is why a flat surface of the CIGS layer is necessary to achieve. The resultant deteriorated optical properties then have to be optimized by other means, including the encapsulation and the front glass. 5. Process stability To achieve a high yield, process stability and reproducibility are major topics for mass production. Figs. 3 and 4 depict the efficiency distribution of 0.5 cm2 cells and of 30 × 30 cm2 modules, respectively. The module statistics in Fig. 4 reveals a similar efficiency scatter as for the small area cells in Fig. 3. This supports the assumption that the monolithic serial interconnections in the modules and their thicker transparent front contact layer does not introduce considerable additional scatter to the results. 6. ZnS buffer The long term goal for alternative buffer processes is, besides the already mentioned increase of the band gap energy, the establishment of a dry vacuum process. However the easiest and fastest way to
Fig. 1. Solar cell and module conversion efficiencies for different glass substrate sizes.
Fig. 3. Process statistics of 0.5 cm2 test cells. Efficiency under AM 1.5 illumination. Cells without anti-reflection coating.
M. Powalla et al. / Thin Solid Films 517 (2009) 2111–2114
2113
Table 2 Device parameters
Device Buffer η [%] VOC [mV] jSC [mA/cm2] FF [%] Eg / Eg − VOC[eV] Fig. 4. Process statistics of monolithic integrated modules 30 cm × 30 cm. Efficiency under AM 1.5 illumination. Modules without anti-reflection coating.
implement a new process into the production line, is to realize a chemical bath deposition (CBD) technique in a similar way as for the CdS buffer layer (Fig. 5). Therefore we concentrated on CBD deposition of ZnS layers by the precursors zinc sulphate, ammonium hydroxide and thiourea. After deposition a rinse with NH4OH removes the superfluous Zn(OH)2 from the surface of the layer. Details on the ZnS deposition process are described in [12]. The final quality of the device for these buffer layers depends strongly on the thin intrinsic ZnO layer adjacent to the buffer. The standard i-ZnO layer in conjunction with ZnS buffers requires annealing in air and light soaking to get a working device as will be shown later. A certain amount of Mg in the ZnO is necessary to overcome these complications [13]. The Zn(1 − x) Mgx O layers are sputter deposited similar to the standard i-ZnO layers. A von Ardenne CS 730S laboratory sputtering system with a base pressure of 2 × 10− 7 mbar served for RF magnetron sputtering from sintered ceramic Zn(1 − x) Mgx O targets with different Mg–contents x = 0.0–0.4. The films were sputtered at a pressure of 1 × 10− 2 mbar with RF power densities of typically 1.0–3.3 W/cm2 at 25 °C substrate temperature. The Zn(1 − x) Mgx O layers have a thickness in the range of 100 nm. The layer sequence was finished with DC sputtered ZnO:Al and a Ni/Algrid in the case of test cells.
Grid shading Anti-reflection coating Measurement carried out by
Old process
New developed multi stage in-line deposition process
1 Cell 0.5 cm2 CdS 16.1 688 30.7 76.2 1.18/ 0.49 7% Yes
2 Cell 0.5 cm2 CdS 17.8 730 31.3 78.1 1.16/ 0.43 7% Yes
3 Cell 0.5 cm2 CdS 18.3 695 34.1 77.3 1.13/ 0.435 3% Yes
4 Module 30 × 30 cm2 CdS 13.8 707/cell 30.0 65.2
ISE⁎
ISE⁎
ZSW
6 Module 10 × 10 cm2 ZnS 15.2 667 31.6 72
No grid No
5 Cell 0.5 cm2 ZnS 17.3 661 35.1 74.9 1.12/ 0.46 3% Yes
ZSW
ZSW
ZSW
No grid Yes
⁎Institut für solare Energiesysteme Freiburg.
substantial increase in VOC with respect to the band gap energy Eg and an increase in short circuit current. Fig. 6 displays external quantum efficiencies of representative devices prepared with (a) the standard in-line CIGS process and (b) the new in-line multistage process, each with CdS/i-ZnO and ZnS/ZnMgO buffer, respectively. The external quantum efficiencies reveal two differences between the two CIGS processes. The slope of the external quantum efficiency EQE at the absorption edge of the CIGS is much steeper for the new multistage process indicating a higher collection length. Furthermore the interferences are more pronounced for the new multistage CIGS, which concurs with the “flat” surface morphology visible in Fig. 2. Regarding the different buffer layers it is striking that, unlike in many cases with alternative buffers, the EQE is not diminished. Especially at the absorption edge of the CIGS the EQE is identical to the
7. Device characterization Table 2 summarizes the device parameters of the cells and modules prepared with the various processes. The highest conversion efficiency reached with our old in-line process was 16.1%. Device 2 with the highest active area efficiency (consider the higher grid shading of this device compared to sample 3) was prepared with the new multi stage process. The higher efficiency results from a
Fig. 5. ZnS-buffer layer on CIGS grains with rather pyramidal surface morphology. The ZnS-film covers the surface exceptionally well.
Fig. 6. External quantum efficiencies of cells with ZnS and CdS based buffers on a) standard inline CIGS and b) multistage inline deposited absorbers.
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M. Powalla et al. / Thin Solid Films 517 (2009) 2111–2114
depending on Mg-content. The fill factor decrease is more pronounced for x = 0.33 and x = 0.4. A 30 min light soak at 80 °C reestablishes the fill factor demonstrating the complete reversibility of this effect. Further light soaking only increases the efficiency of the devices with x = 0.4. Hence these devices have a higher time constant for light soaking and did not reach their maximum fill factor with the first light soak. Storing the samples in the dark for 4 days leads to a decrease in FF which is more pronounced for Mg-contents x = 0.4. The more pronounced efficiency decrease in the dark together with the longer time constant for reestablishing the efficiency under illumination makes the Mg-contents x N 0.3 less suitable, although the maximum efficiency is close to the devices with lower Mg-contents. The influence of the Mg-content on the metastable behavior is not yet understood. The situation is complicated by the fact that the exposure time prior to buffer deposition and the used CIGS also have a strong impact on the metastable behavior of devices with ZnS/ZnMgO buffer. The modules presented in Fig. 7 were prepared with standard in-line CIGS. The metastable behavior of devices with ZnS/ZnMgO buffer deposited on the new multistage CIGS seems less pronounced, which has to be investigated in more detail. 9. Summary
one with CdS buffer, indicating that the collection length is not affected. Differences appear to result from optical effects rather than from changes in electronic properties. Nevertheless differences in electronic properties become obvious from the systematically lower open circuit voltages VOC of the cells with ZnS buffer (device 5) compared to the devices with CdS buffer (device 2 and 3). The reason for this observation is not clear.
In summary we realized an in-line multi stage CIGS deposition process, which can be scaled up for the use in mass production. Efficiencies of 18.3% are reached for small test cells and 13.8% for 30 × 30 cm2 modules. This result is a major step towards highest efficient CIGS mass production and demonstrates the advantage of the co-evaporation for CIGS deposition. Furthermore test cells with ZnS buffer layers deposited by a CBD process reach conversion efficiencies of 17.3% in conjunction with the new multistage CIGS process. Devices with ZnS buffer deposited on standard in-line CIGS suffer from a strong metastability of the fill factor. The metastable behaviour can be controlled by combining the ZnS buffer with Zn(1 − x)MgxO with a proper Mg-content x in the range of x = 0.15–0.26. Nevertheless the metastable effects are not yet controlled in a way that permits the use in mass production.
8. ZnS buffer: annealing and illumination effects
References
In most cases the replacement of the CdS buffer by a high band gap material leads to strong metastable changes in the fill factor by illumination and annealing effects. We also find these effects in our devices with ZnS buffer, depending on the Mg-content x of the Zn(1 − x) MgxO layer deposited on the ZnS. If a CIGS solar cell with ZnS buffer is kept in the dark, mainly the fill factor decreases. Under subsequent illumination it increases again. The sensitivity of a device to annealing in the dark determines the efficiency decrease during the encapsulation process. Fig. 7 displays the change of the open circuit voltage Voc and the fill factor FF of CIGS modules with ZnS/Zn(1 − x)MgxO buffers with different Mg-contents x by various treatments. After completion of the devices a heating step in dry air increases the efficiency of all cells irrespective of the Mg-content, but with the high Mg-contents x = 0.33 and x = 0.4 and the sample without Mg starting at a lower value. Light soaking for 3 min leads to a further increase of the conversion efficiency. Devices with medium Mg-content x = 0.1 and x = 0.26 reach their maximal FF values, whereas devices with higher Mg-content and no Mg do not. During encapsulation the fill factor decreases
[1] V. Probst, W. Stetter, J. Palm, R. Toelle, 3rd WCPEC Osaka, , 2003 20-C7-01. [2] K. Kushiya, 17th PVSEC, Fukuoka, 2007, pp. PL5–2. [3] M. Powalla, B. Dimmler, R. Schäffler, G. Voorwinden, U. Stein, H.-D. Mohring, F. Kessler, D. Hariskos,19th European Photovoltaic Solar Energy Conference, Paris, 2004, p.1663. [4] A. Meeder, A. Neisser, U. Rühle, N. Mayer, Proceedings22nd European Photovoltaic Solar Energy Conference, Milan, 2007, p. 2115. [5] M. Contreras, K. Ramanathan, J.A. Abu Shama, F. Hasoon, D.L. Young, B. Eggas, R. Noufi, Prog. Photovolt. Res. Appl. 13 (2005) 209. [6] G. Voorwinden, R. Kniese, P. Jackson, M. Powalla, Proceedings 22nd European Photovoltaic Solar Energy Conference, Milan, 2007, p. 2115. [7] D. Hariskos, B. Fuchs, R. Menner, M. Powalla, N. Naghavi, D. Lincot, Proceedings 22nd European Photovoltaic Solar Energy Conference, Milan, 2007, p. 1907. [8] G. Voorwinden, M. powalla, 17th European Photovoltaic Solar Energy Conference, Munich, 2001, p. 1203. [9] M. Powalla, G. Voorwinden, B. Dimmler, Proceedings 14th European Photovoltaic Solar Energy Conference, Barcelona, 1997, p. 1270. [10] G. Voorwinden, R. Kniese, M. Powalla, Thin Solid Films 431–432 (2003) 538. [11] J. Klaer, R. Klenk, A. Boden, A. Neisser, C. Kaufmann, R. Scheer, H.W. Schock, 31st IEEE Photovoltaic Spezialist Conference, Orlando, 2005, p. 336. [12] N. Naghavi, C. Hubert, O. Kerrec, D. Lincot, D. Hariskos, R. Menner, M. Powalla, Proceedings 22nd European Photovoltaic Solar Energy Conference, Milan, 2007, p. 2304. [13] A. Ennaoui, M. Bär, M. Rusu, R. Klenk, J. Klaer, T. Kropp, R. Sáez-Aroz, H.-W. Schock, M.C. Lux-Steiner, 21st European Photovoltaic Solar Energy Conference, Dresden, 2006, p. 1835.
Fig. 7. Change of the open circuit voltage VOC and the fill factor FF by different subsequent treatments for different Mg concentrations x in the Zn(1 − x)MgxO layer.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Highly efficient CIS solar cells and modules made by the co-evaporation process M. Powalla ⁎, G. Voorwinden, D. Hariskos, P. Jackson, R. Kniese Zentrum für Sonnenenergie- und Wasserstoff-Forschung (ZSW), Industriestrasse 6, D-70565 Stuttgart, Germany
a r t i c l e
i n f o
Available online 11 November 2008 Keywords: CuInSe2 Cu(InGa)Se2 Module manufacturing
a b s t r a c t Thin-film photovoltaic modules which use the chalcopyrite Cu(In,Ga)(Se,S)2 (CIGS) as the light-absorbing layer have now entered the decisive industrial phase. Companies located mainly in Germany and Japan will produce more than 100 MWp CIGS modules in 2008, demonstrating that the CIGS technology has already achieved a certain maturity. Whereas key features of the technology are already well-optimized, there are several approaches to further improve the productivity of new lines. The ZSW operates a line for 30 × 30 cm2 modules in which all process steps – from glass cleaning to module encapsulation – are being developed. A major goal of the development is the very fast and efficient transfer of promising new materials and processes from cells to the industrial module level. Therefore, ZSW is focusing on processes like the in-line co-evaporation method for CIS or chemical bath deposition for buffer layers to optimize the junction. We could demonstrate efficiencies close to 18% for small test cells and 14–15% for modules with modified processes. Different cell and material data from optoelectronic measurements and microscopic analysis will be presented in this contribution. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The process technology of Cu(In,Ga)Se2 has reached the maturity for mass production of thin-film photovoltaic modules. The inherent advantages of the direct band gap material Cu(In,Ga)Se2 are based on its high absorption and therewith low layer thickness required for light absorption. The resultant potential for cost reduction, light weight and flexible applications makes this material an all-round candidate for cheap large area module technology as well as special architectural and space applications. Due to this all-round abilities Cu (In,Ga)Se2 is an interesting technology for industrial production. The efficiencies of current solar modules from mass production [1–4] summarized in Table 1 exceed amorphous silicon and CdTe. To further increase the applicability and profitability a further increase of efficiency is necessary. The highest conversion efficiencies of small laboratory cells are close to 20% [5]. If single cells are connected in series by monolithic integration, some principle loss mechanisms arise which may be optimized, but always will reduce the efficiency of large area modules compared to laboratory cells. (i) To allow appropriate current transport through the conducting oxide, this window layer requires a certain thickness and cannot be optimized almost arbitrarily to optical properties like in small area record cells.
⁎ Corresponding author. E-mail address: [email protected] (M. Powalla). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.10.126
(ii) The interconnect between single cells introduces area losses. Since with increasing module size the twisting of the substrate requires larger tolerances for patterning, the area loss due to the interconnects increases with increasing module size. (iii) With increasing module size inhomogeneities will be more pronounced due to limited process stability and homogeneity. Fig. 1 illustrates the decrease of the aperture conversion efficiency with increasing substrate size. The total area efficiency in turn may even increase slightly due to a smaller relative area loss by the frame. The advantage of processing substrate sizes as large as possible lies also in the higher productivity. 2. Technology evolution Increasing the efficiency of solar modules is a process of careful optimisation of each process step in view of optimising the hetero structure and the above mentioned losses related to module technology. Besides this “fine-tuning” a considerable efficiency improvement would be possible by a major change in technology. On the absorber side a fundamental evolution would be the advent of a multi stage process to mass production, which is known to provide the to date highest efficient Cu(In,Ga)Se2 absorber material [5]. Hence one main goal of research at the ZSW is to transfer the efficiency potential of multi stage Cu(In,Ga)Se2 processes to large scale production [6]. A further fundamental advance would be the substitution of the CdS buffer layer by an appropriate high band gap material to reduce absorption losses in the short wavelength range. Among the possible candidates ZnS so far achieves the most promising results in our
2112
M. Powalla et al. / Thin Solid Films 517 (2009) 2111–2114
Table 1 Published module efficiencies from several industrial module fabrications [1–4] Effi./power
Size
(%/Wp)
(cm2)
13.1/64.8 13.1/93.1 13.0/84.6 8.6/63
5400 7128 6500 7381
Company
Published
Avancis (shell solar) Showa shell Würth solar Sulfurcell
2003 2007 2004 2006
Remark
Cd free Sulfide
experiments [7]. This contribution reports on the latest development steps on these two pathways. 3. Multi stage CIGS process To realize a multistage process in an in-line deposition plant, the different stages have to be spatially successively arranged in the system. Therefore we extended our 30 × 30 cm2 deposition chamber and equipped it with additional evaporation sources for the elements copper, indium, gallium and selenium. These allow for the successive co-evaporation of the elements with different rate ratios in one single run. Individually controllable substrate heaters adjust the substrate temperature for each deposition step. The line-shaped evaporation rate profiles of the sources, assure a good homogeneity perpendicular to the transport axis. Finite element simulations helped us to optimize the flux rate distributions [8]. Atomic absorption spectrometers (AAS), coupled into the chamber with optical fibres, measure the copper, indium and gallium evaporation rates [9]. This allows for the control of the evaporation rates to achieve an excellent compositional homogeneity in the substrate movement direction. The selenium flux is a few times the amount necessary for the formation of Cu(In,Ga)Se2. We use soda lime glass coated with DC-sputtered molybdenum as a substrate material. The Cu(In,Ga)Se2 films grow to a thickness of 2 µm. The fabrication of the solar cells is continued with a chemical bath CdS buffer layer, RF-sputtered i-ZnO, and DC-sputtered n-ZnO:Al. On small area single cells Ni/Al grids are applied as front contacts. 4. Film properties The electron micrographs in Fig. 2 depict one major difference between the thin films prepared with our previous process (a) as described in [10], and the ones prepared with the new multi step process (b). It is the different surface morphology. In (a), the grains end up with a pyramidal shape with steep slopes of the surface, whereas in (b) the top surface of the grains is rather flat, but still with large steps between the grains. This change in surface morphology deteriorates the optical properties by increasing the reflection with decreasing
Fig. 2. Scanning electron micrograph cross sections of Cu(In,Ga)Se2 absorbers. (a) Previously developed large area ZSW in-line process [3]. (b) Film prepared with multi stage in-line process.
roughness. On the other hand a flat surface may have advantages for electric properties by reducing the space charge volume. Furthermore a flat surface leads to more conductive and more stable ZnO layers [11]. That is why a flat surface of the CIGS layer is necessary to achieve. The resultant deteriorated optical properties then have to be optimized by other means, including the encapsulation and the front glass. 5. Process stability To achieve a high yield, process stability and reproducibility are major topics for mass production. Figs. 3 and 4 depict the efficiency distribution of 0.5 cm2 cells and of 30 × 30 cm2 modules, respectively. The module statistics in Fig. 4 reveals a similar efficiency scatter as for the small area cells in Fig. 3. This supports the assumption that the monolithic serial interconnections in the modules and their thicker transparent front contact layer does not introduce considerable additional scatter to the results. 6. ZnS buffer The long term goal for alternative buffer processes is, besides the already mentioned increase of the band gap energy, the establishment of a dry vacuum process. However the easiest and fastest way to
Fig. 1. Solar cell and module conversion efficiencies for different glass substrate sizes.
Fig. 3. Process statistics of 0.5 cm2 test cells. Efficiency under AM 1.5 illumination. Cells without anti-reflection coating.
M. Powalla et al. / Thin Solid Films 517 (2009) 2111–2114
2113
Table 2 Device parameters
Device Buffer η [%] VOC [mV] jSC [mA/cm2] FF [%] Eg / Eg − VOC[eV] Fig. 4. Process statistics of monolithic integrated modules 30 cm × 30 cm. Efficiency under AM 1.5 illumination. Modules without anti-reflection coating.
implement a new process into the production line, is to realize a chemical bath deposition (CBD) technique in a similar way as for the CdS buffer layer (Fig. 5). Therefore we concentrated on CBD deposition of ZnS layers by the precursors zinc sulphate, ammonium hydroxide and thiourea. After deposition a rinse with NH4OH removes the superfluous Zn(OH)2 from the surface of the layer. Details on the ZnS deposition process are described in [12]. The final quality of the device for these buffer layers depends strongly on the thin intrinsic ZnO layer adjacent to the buffer. The standard i-ZnO layer in conjunction with ZnS buffers requires annealing in air and light soaking to get a working device as will be shown later. A certain amount of Mg in the ZnO is necessary to overcome these complications [13]. The Zn(1 − x) Mgx O layers are sputter deposited similar to the standard i-ZnO layers. A von Ardenne CS 730S laboratory sputtering system with a base pressure of 2 × 10− 7 mbar served for RF magnetron sputtering from sintered ceramic Zn(1 − x) Mgx O targets with different Mg–contents x = 0.0–0.4. The films were sputtered at a pressure of 1 × 10− 2 mbar with RF power densities of typically 1.0–3.3 W/cm2 at 25 °C substrate temperature. The Zn(1 − x) Mgx O layers have a thickness in the range of 100 nm. The layer sequence was finished with DC sputtered ZnO:Al and a Ni/Algrid in the case of test cells.
Grid shading Anti-reflection coating Measurement carried out by
Old process
New developed multi stage in-line deposition process
1 Cell 0.5 cm2 CdS 16.1 688 30.7 76.2 1.18/ 0.49 7% Yes
2 Cell 0.5 cm2 CdS 17.8 730 31.3 78.1 1.16/ 0.43 7% Yes
3 Cell 0.5 cm2 CdS 18.3 695 34.1 77.3 1.13/ 0.435 3% Yes
4 Module 30 × 30 cm2 CdS 13.8 707/cell 30.0 65.2
ISE⁎
ISE⁎
ZSW
6 Module 10 × 10 cm2 ZnS 15.2 667 31.6 72
No grid No
5 Cell 0.5 cm2 ZnS 17.3 661 35.1 74.9 1.12/ 0.46 3% Yes
ZSW
ZSW
ZSW
No grid Yes
⁎Institut für solare Energiesysteme Freiburg.
substantial increase in VOC with respect to the band gap energy Eg and an increase in short circuit current. Fig. 6 displays external quantum efficiencies of representative devices prepared with (a) the standard in-line CIGS process and (b) the new in-line multistage process, each with CdS/i-ZnO and ZnS/ZnMgO buffer, respectively. The external quantum efficiencies reveal two differences between the two CIGS processes. The slope of the external quantum efficiency EQE at the absorption edge of the CIGS is much steeper for the new multistage process indicating a higher collection length. Furthermore the interferences are more pronounced for the new multistage CIGS, which concurs with the “flat” surface morphology visible in Fig. 2. Regarding the different buffer layers it is striking that, unlike in many cases with alternative buffers, the EQE is not diminished. Especially at the absorption edge of the CIGS the EQE is identical to the
7. Device characterization Table 2 summarizes the device parameters of the cells and modules prepared with the various processes. The highest conversion efficiency reached with our old in-line process was 16.1%. Device 2 with the highest active area efficiency (consider the higher grid shading of this device compared to sample 3) was prepared with the new multi stage process. The higher efficiency results from a
Fig. 5. ZnS-buffer layer on CIGS grains with rather pyramidal surface morphology. The ZnS-film covers the surface exceptionally well.
Fig. 6. External quantum efficiencies of cells with ZnS and CdS based buffers on a) standard inline CIGS and b) multistage inline deposited absorbers.
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M. Powalla et al. / Thin Solid Films 517 (2009) 2111–2114
depending on Mg-content. The fill factor decrease is more pronounced for x = 0.33 and x = 0.4. A 30 min light soak at 80 °C reestablishes the fill factor demonstrating the complete reversibility of this effect. Further light soaking only increases the efficiency of the devices with x = 0.4. Hence these devices have a higher time constant for light soaking and did not reach their maximum fill factor with the first light soak. Storing the samples in the dark for 4 days leads to a decrease in FF which is more pronounced for Mg-contents x = 0.4. The more pronounced efficiency decrease in the dark together with the longer time constant for reestablishing the efficiency under illumination makes the Mg-contents x N 0.3 less suitable, although the maximum efficiency is close to the devices with lower Mg-contents. The influence of the Mg-content on the metastable behavior is not yet understood. The situation is complicated by the fact that the exposure time prior to buffer deposition and the used CIGS also have a strong impact on the metastable behavior of devices with ZnS/ZnMgO buffer. The modules presented in Fig. 7 were prepared with standard in-line CIGS. The metastable behavior of devices with ZnS/ZnMgO buffer deposited on the new multistage CIGS seems less pronounced, which has to be investigated in more detail. 9. Summary
one with CdS buffer, indicating that the collection length is not affected. Differences appear to result from optical effects rather than from changes in electronic properties. Nevertheless differences in electronic properties become obvious from the systematically lower open circuit voltages VOC of the cells with ZnS buffer (device 5) compared to the devices with CdS buffer (device 2 and 3). The reason for this observation is not clear.
In summary we realized an in-line multi stage CIGS deposition process, which can be scaled up for the use in mass production. Efficiencies of 18.3% are reached for small test cells and 13.8% for 30 × 30 cm2 modules. This result is a major step towards highest efficient CIGS mass production and demonstrates the advantage of the co-evaporation for CIGS deposition. Furthermore test cells with ZnS buffer layers deposited by a CBD process reach conversion efficiencies of 17.3% in conjunction with the new multistage CIGS process. Devices with ZnS buffer deposited on standard in-line CIGS suffer from a strong metastability of the fill factor. The metastable behaviour can be controlled by combining the ZnS buffer with Zn(1 − x)MgxO with a proper Mg-content x in the range of x = 0.15–0.26. Nevertheless the metastable effects are not yet controlled in a way that permits the use in mass production.
8. ZnS buffer: annealing and illumination effects
References
In most cases the replacement of the CdS buffer by a high band gap material leads to strong metastable changes in the fill factor by illumination and annealing effects. We also find these effects in our devices with ZnS buffer, depending on the Mg-content x of the Zn(1 − x) MgxO layer deposited on the ZnS. If a CIGS solar cell with ZnS buffer is kept in the dark, mainly the fill factor decreases. Under subsequent illumination it increases again. The sensitivity of a device to annealing in the dark determines the efficiency decrease during the encapsulation process. Fig. 7 displays the change of the open circuit voltage Voc and the fill factor FF of CIGS modules with ZnS/Zn(1 − x)MgxO buffers with different Mg-contents x by various treatments. After completion of the devices a heating step in dry air increases the efficiency of all cells irrespective of the Mg-content, but with the high Mg-contents x = 0.33 and x = 0.4 and the sample without Mg starting at a lower value. Light soaking for 3 min leads to a further increase of the conversion efficiency. Devices with medium Mg-content x = 0.1 and x = 0.26 reach their maximal FF values, whereas devices with higher Mg-content and no Mg do not. During encapsulation the fill factor decreases
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Fig. 7. Change of the open circuit voltage VOC and the fill factor FF by different subsequent treatments for different Mg concentrations x in the Zn(1 − x)MgxO layer.