Final Report Summary - PERCIGS (PercIGS)
The intention of PercIGS is to advance thin-film (TF) solar cell design by the use of state-of-the-art (SoA) Si solar cell know-how. Previously, most TF solar cell R&D has been successfully focused on enhancing semiconductor material quality. However, as the material quality of typical TF semiconductors (copper indium gallium selenide (CIGS), cadmium telluride (CdTe), copper zinc tin sulfide (CZTS) …) upgraded, other solar cell parts became the new bottlenecks to increase cell efficiency even further. Such a novel area of attention is the rear-contact/semiconductor rear interface, where recombination of charge carriers typically is rather high. Indeed, as the TF semiconductor material enhancements also resulted in longer diffusion lengths, this highly recombinative rear interface starts to limit further efficiency increases. Therefore, in PercIGS the introduction of a rear surface passivation layer with nano-sized contacts is suggested and developed as innovative approach to reduce recombination of charge carriers at the rear CIGS semiconductor interface. This idea stems from the Si solar cell industry, where at the rear of advanced cell concepts rear surface passivation layers are combined with micron-sized point openings. This PercIGS project has been focused on CIGS solar cells – as they offer the highest efficiencies of all TF technologies – but the concept can be generalized to other high quality TF solar cell technologies as well.
Within this project, various new technologies to address key challenges of the TF solar cell industry have been developed. During PercIGS, innovative two-dimensional SoA Si solar cell concepts have been developed, studied and integrated in SoA CIGS cells. More specifically, (i) new methods to develop nano-sized contacting points, (ii) novel CIGS surfaces passivation layers, and (iii) advanced device structures have been developed and integrated in CIGS solar cells, resulting in increased cell efficiency and improved understanding. Hence, these unique cell concepts increase the stability and the scientific & engineering knowledge of CIGS solar cells, and have potential to be low-cost, high-throughput and could be rapidly scaled-up.
Surface passivation layers and nano-sized contacting techniques have been studied and developed, resulting in three different methodologies to fabricate innovative two-dimensional rear surface passivated CIGS solar cells with higher cell efficiency compared to matching reference solar cells. Two industrially viable rear surface passivation approaches have been developed as proof-of-principles, resulting in rear surface passivated CIGS solar cells with sub-optimized grids of local rear contacts. In both processes, aluminium oxide (Al2O3) is used as CIGS surface passivation layer because of similar arguments made as for its use as Si surface passivation layer. The processes differ in contacting: (i) local point contacts are used, as generated by the formation of nano-sphere shaped precipitates in chemical bath deposition of CdS [1], or (ii) Mo nano-particles as contacts are used, as grown in a highly ionized pulsed plasma [2]. Both approaches have been integrated in CIGS solar cells with ultra-thin absorber layers, showing an increase in solar cell efficiency caused by an improvement in rear surface passivation and optical confinement. Additionally, lab-scale rear surface passivated CIGS solar cells with well-controlled grids of nano-sized local rear point contacts are developed using electron beam lithography technology. Also these cells show a similar increase in efficiency and are outstanding study devices [3]. A standard one-dimensional CIGS solar cell cross-section and analogous pictures of the three novel two-dimensional cell designs are shown in Fig. 1. These new technologies enable fabrication of highly efficient ultra-thin CIGS solar cells, but also hold potential to increase the world record efficiency of CIGS solar cells with standard CIGS absorber layer thickness.
Fig. 1. Transmission electron microscopy cross-section images of (a) a standard state-of-the-art CIGS solar cell, (b) a rear surface passivated CIGS solar cell with randomized nano-sized local rear point contacts, (c) a rear surface passivated CIGS solar cell with Mo nano-particles (NP) as rear contacts, and (d) a rear surface passivated cell with a well-controlled grid of nano-sized local rear point contacts, taken from [1-3]. Samples (b), (c) and (d) have been cut by focused ion beam. See attachment.
[1] B. Vermang, J. T. Wätjen, V. Fjällström, F. Rostvall, M. Edoff, R. Kotipalli, F. Henry, and D. Flandre, Prog. Photovoltaics: Res. Appl., DOI: 10.1002/pip.2527 (2014).
[2] B. Vermang, J. T. Wätjen, V. Fjällström, F. Rostvall, M. Edoff, R. Gunnarsson, I. Pilch, U. Helmersson, R. Kotipalli, F. Henry, and D. Flandre, Thin Solid Films, submitted for publication.
[3] B. Vermang, J. T. Wätjen, C. Frisk, V. Fjällström, F. Rostvall, M. Edoff, P. Salomé, J. Borme, N. Nicoara, and S. Sadewasser, IEEE J. Photovoltaics, to be published.
Last 2 years, this pioneering work resulted in several invited oral presentations at international conferences and numerous articles in peer-reviewed scientific journals. As a result the future of the newly developed solar cell back contact technique seems to be solid. The results of this study have been presented in invited presentations at IEEE PVSC 2013 (Tampa, FL) and at ALD4PV 2014 (Eindhoven, The Netherlands), as an extended presentation at IEEE PVSC 2014 (Denver, CO), and as a standard oral presentation at e-MRS spring meeting 2014 (Lille, France). Additionally, the work has been published in three journal articles (Solar Energy Materials & Solar Cells, IEEE Journal of Photovoltaics, and Progress in Photovoltaics: Research and Applications) and is under review in three more journals (IEEE Journal of Photovoltaics, Thin Solid Films, and physica status solidi Rapid Research Letters). It was also presented in two MSc. theses (F. Rostvall and J. Joel, University of Uppsala 2014), and has/will be presented during seminars at imec in Belgium and at the University of New South Wales in Australia. Thanks a to all these dissemination activities several new contacts have been made (imec, INL, Linköping University, UCL, Solibro…), which will be involved in the search for future funding of this work (Solar-era.net Horizon 2020…).
About the University of Uppsala:
The University of Uppsala is located in Uppsala (and Visby), Sweden. At this University, the thin film solar cell group at the Solid State Electronics division did host this project. This group holds own equipment for complete fabrication and in-depth characterization of thin film solar cells based on co-evaporation of CIGS or sputtering of CZTS. At present, the group has base funding provided by the Swedish Energy Agency, the Swedish strategic research program STandUp, Sweden's Innovation Agency Vinnova, and the Swedish Research Council. Previously, the group has been project leader for numerous EU projects and its research also resulted in a spin-off company: Solibro, which has grown to a commercial manufacturer of CIGS modules and currently holds the world record efficiencies of 18.7 and 21.0 % for CIGS thin-film solar modules and cells, respectively.
Contact details:
Bart Vermang, PercIGS project leader, University of Uppsala, +46(0)18-471 7238, Bart.Vermang@angstrom.uu.se
Marika Edoff, thin film solar cell group leader, University of Uppsala, +46(0)18-471 7249, Marika.Edoff@angstrom.uu.se
Executive summary
The intention of PercIGS is to advance thin-film (TF) solar cell design by the use of state-of-the-art (SoA) Si solar cell know-how. Previously, most TF solar cell R&D has been successfully focused on enhancing semiconductor material quality. However, as the material quality of typical TF semiconductors (copper indium gallium selenide (CIGS), cadmium telluride (CdTe), copper zinc tin sulfide (CZTS) …) upgraded, other solar cell parts became the new bottlenecks to increase cell efficiency even further. Such a novel area of attention is the rear-contact/semiconductor rear interface, where recombination of charge carriers typically is rather high. Indeed, as the TF semiconductor material enhancements also resulted in longer diffusion lengths, this highly recombinative rear interface starts to limit further efficiency increases. Therefore, in PercIGS the introduction of a rear surface passivation layer with nano-sized contacts is suggested and developed as innovative approach to reduce recombination of charge carriers at the rear CIGS semiconductor interface. This PercIGS project has been focused on CIGS solar cells but the concept can be generalized to other high quality TF solar cell technologies as well.
Within PercIGS, innovative SoA Si solar cell concepts have been developed, studied and integrated in SoA CIGS cells. More specifically, (i) new methods to develop nano-sized contacting points, (ii) novel CIGS surfaces passivation layers, and (iii) advanced device structures have been developed and integrated in CIGS solar cells, resulting in increased cell efficiency and improved understanding. Surface passivation layers and nano-sized contacting techniques have been studied and developed, resulting in three different methodologies to fabricate innovative two-dimensional rear surface passivated CIGS solar cells with higher cell efficiency compared to matching reference solar cells. Two industrially viable rear surface passivation nano-particle-based approaches have been developed as proof-of-principles, resulting in rear surface passivated CIGS solar cells with sub-optimized grids of local rear contacts. Additionally, lab-scale rear surface passivated CIGS solar cells with well-controlled grids of nano-sized local rear point contacts are developed using electron beam lithography technology. Cross-section pictures of these three novel cell designs are shown in Fig. 1.
Fig. 1. Transmission electron microscopy cross-section images of a rear surface passivated CIGS solar cell
(a) with randomized nano-sized local rear point contacts, (b) with Mo nano-particles (NP) as rear contacts, and (c) with a well-controlled grid of nano-sized local rear point contacts. See attachment.
The new technologies enable fabrication of highly efficient ultra-thin CIGS solar cells, but also hold potential to increase the world record efficiency of CIGS solar cells with standard CIGS absorber layer thickness. Additionally, these unique cell concepts increase the stability and the scientific & engineering knowledge of CIGS solar cells, and have potential to be low-cost, high-throughput and could be rapidly scaled-up.
Within this project, various new technologies to address key challenges of the TF solar cell industry have been developed. During PercIGS, innovative two-dimensional SoA Si solar cell concepts have been developed, studied and integrated in SoA CIGS cells. More specifically, (i) new methods to develop nano-sized contacting points, (ii) novel CIGS surfaces passivation layers, and (iii) advanced device structures have been developed and integrated in CIGS solar cells, resulting in increased cell efficiency and improved understanding. Hence, these unique cell concepts increase the stability and the scientific & engineering knowledge of CIGS solar cells, and have potential to be low-cost, high-throughput and could be rapidly scaled-up.
Surface passivation layers and nano-sized contacting techniques have been studied and developed, resulting in three different methodologies to fabricate innovative two-dimensional rear surface passivated CIGS solar cells with higher cell efficiency compared to matching reference solar cells. Two industrially viable rear surface passivation approaches have been developed as proof-of-principles, resulting in rear surface passivated CIGS solar cells with sub-optimized grids of local rear contacts. In both processes, aluminium oxide (Al2O3) is used as CIGS surface passivation layer because of similar arguments made as for its use as Si surface passivation layer. The processes differ in contacting: (i) local point contacts are used, as generated by the formation of nano-sphere shaped precipitates in chemical bath deposition of CdS [1], or (ii) Mo nano-particles as contacts are used, as grown in a highly ionized pulsed plasma [2]. Both approaches have been integrated in CIGS solar cells with ultra-thin absorber layers, showing an increase in solar cell efficiency caused by an improvement in rear surface passivation and optical confinement. Additionally, lab-scale rear surface passivated CIGS solar cells with well-controlled grids of nano-sized local rear point contacts are developed using electron beam lithography technology. Also these cells show a similar increase in efficiency and are outstanding study devices [3]. A standard one-dimensional CIGS solar cell cross-section and analogous pictures of the three novel two-dimensional cell designs are shown in Fig. 1. These new technologies enable fabrication of highly efficient ultra-thin CIGS solar cells, but also hold potential to increase the world record efficiency of CIGS solar cells with standard CIGS absorber layer thickness.
Fig. 1. Transmission electron microscopy cross-section images of (a) a standard state-of-the-art CIGS solar cell, (b) a rear surface passivated CIGS solar cell with randomized nano-sized local rear point contacts, (c) a rear surface passivated CIGS solar cell with Mo nano-particles (NP) as rear contacts, and (d) a rear surface passivated cell with a well-controlled grid of nano-sized local rear point contacts, taken from [1-3]. Samples (b), (c) and (d) have been cut by focused ion beam. See attachment.
[1] B. Vermang, J. T. Wätjen, V. Fjällström, F. Rostvall, M. Edoff, R. Kotipalli, F. Henry, and D. Flandre, Prog. Photovoltaics: Res. Appl., DOI: 10.1002/pip.2527 (2014).
[2] B. Vermang, J. T. Wätjen, V. Fjällström, F. Rostvall, M. Edoff, R. Gunnarsson, I. Pilch, U. Helmersson, R. Kotipalli, F. Henry, and D. Flandre, Thin Solid Films, submitted for publication.
[3] B. Vermang, J. T. Wätjen, C. Frisk, V. Fjällström, F. Rostvall, M. Edoff, P. Salomé, J. Borme, N. Nicoara, and S. Sadewasser, IEEE J. Photovoltaics, to be published.
Last 2 years, this pioneering work resulted in several invited oral presentations at international conferences and numerous articles in peer-reviewed scientific journals. As a result the future of the newly developed solar cell back contact technique seems to be solid. The results of this study have been presented in invited presentations at IEEE PVSC 2013 (Tampa, FL) and at ALD4PV 2014 (Eindhoven, The Netherlands), as an extended presentation at IEEE PVSC 2014 (Denver, CO), and as a standard oral presentation at e-MRS spring meeting 2014 (Lille, France). Additionally, the work has been published in three journal articles (Solar Energy Materials & Solar Cells, IEEE Journal of Photovoltaics, and Progress in Photovoltaics: Research and Applications) and is under review in three more journals (IEEE Journal of Photovoltaics, Thin Solid Films, and physica status solidi Rapid Research Letters). It was also presented in two MSc. theses (F. Rostvall and J. Joel, University of Uppsala 2014), and has/will be presented during seminars at imec in Belgium and at the University of New South Wales in Australia. Thanks a to all these dissemination activities several new contacts have been made (imec, INL, Linköping University, UCL, Solibro…), which will be involved in the search for future funding of this work (Solar-era.net Horizon 2020…).
About the University of Uppsala:
The University of Uppsala is located in Uppsala (and Visby), Sweden. At this University, the thin film solar cell group at the Solid State Electronics division did host this project. This group holds own equipment for complete fabrication and in-depth characterization of thin film solar cells based on co-evaporation of CIGS or sputtering of CZTS. At present, the group has base funding provided by the Swedish Energy Agency, the Swedish strategic research program STandUp, Sweden's Innovation Agency Vinnova, and the Swedish Research Council. Previously, the group has been project leader for numerous EU projects and its research also resulted in a spin-off company: Solibro, which has grown to a commercial manufacturer of CIGS modules and currently holds the world record efficiencies of 18.7 and 21.0 % for CIGS thin-film solar modules and cells, respectively.
Contact details:
Bart Vermang, PercIGS project leader, University of Uppsala, +46(0)18-471 7238, Bart.Vermang@angstrom.uu.se
Marika Edoff, thin film solar cell group leader, University of Uppsala, +46(0)18-471 7249, Marika.Edoff@angstrom.uu.se
Executive summary
The intention of PercIGS is to advance thin-film (TF) solar cell design by the use of state-of-the-art (SoA) Si solar cell know-how. Previously, most TF solar cell R&D has been successfully focused on enhancing semiconductor material quality. However, as the material quality of typical TF semiconductors (copper indium gallium selenide (CIGS), cadmium telluride (CdTe), copper zinc tin sulfide (CZTS) …) upgraded, other solar cell parts became the new bottlenecks to increase cell efficiency even further. Such a novel area of attention is the rear-contact/semiconductor rear interface, where recombination of charge carriers typically is rather high. Indeed, as the TF semiconductor material enhancements also resulted in longer diffusion lengths, this highly recombinative rear interface starts to limit further efficiency increases. Therefore, in PercIGS the introduction of a rear surface passivation layer with nano-sized contacts is suggested and developed as innovative approach to reduce recombination of charge carriers at the rear CIGS semiconductor interface. This PercIGS project has been focused on CIGS solar cells but the concept can be generalized to other high quality TF solar cell technologies as well.
Within PercIGS, innovative SoA Si solar cell concepts have been developed, studied and integrated in SoA CIGS cells. More specifically, (i) new methods to develop nano-sized contacting points, (ii) novel CIGS surfaces passivation layers, and (iii) advanced device structures have been developed and integrated in CIGS solar cells, resulting in increased cell efficiency and improved understanding. Surface passivation layers and nano-sized contacting techniques have been studied and developed, resulting in three different methodologies to fabricate innovative two-dimensional rear surface passivated CIGS solar cells with higher cell efficiency compared to matching reference solar cells. Two industrially viable rear surface passivation nano-particle-based approaches have been developed as proof-of-principles, resulting in rear surface passivated CIGS solar cells with sub-optimized grids of local rear contacts. Additionally, lab-scale rear surface passivated CIGS solar cells with well-controlled grids of nano-sized local rear point contacts are developed using electron beam lithography technology. Cross-section pictures of these three novel cell designs are shown in Fig. 1.
Fig. 1. Transmission electron microscopy cross-section images of a rear surface passivated CIGS solar cell
(a) with randomized nano-sized local rear point contacts, (b) with Mo nano-particles (NP) as rear contacts, and (c) with a well-controlled grid of nano-sized local rear point contacts. See attachment.
The new technologies enable fabrication of highly efficient ultra-thin CIGS solar cells, but also hold potential to increase the world record efficiency of CIGS solar cells with standard CIGS absorber layer thickness. Additionally, these unique cell concepts increase the stability and the scientific & engineering knowledge of CIGS solar cells, and have potential to be low-cost, high-throughput and could be rapidly scaled-up.