Periodic Reporting for period 1 - CLAReTE (Combinatorially Led Advanced Research on Transparent Electrodes)
Reporting period: 2018-07-01 to 2020-06-30
Transparent conductive materials (TCMs) with high p-type conductivity and broadband transparency have remained elusive for years. Despite two decades of research, no p-type material has yet been found to match the performance of n-type TCMs. TCMs of both conduction types are critical to many technologies, including thin film transistors, transparent electronics, flat screen displays, and PV. In PV especially, low transparency and/or poor conductivity of the available p-type TCMs has led to complex engineering workarounds to create sufficient front contacting schemes, and even then these materials reduce cell efficiency. Thus, this area is a perfect “model grand challenge” to which to apply the innovative scientific approach of CLAReTÉ (Combinatorially Led Advanced Research on Transparent Electrodes), wherein fundamental material insights and drop-in testing in state-of-the-art PV cell architectures will guide high-throughput development of a new class of broadly applicable p-type TCMs.
Two solar cell technologies, silicon heterojunction (SHJ) and perovskite solar cells (PSCs), will be used as testing grounds for the new p-type TCMs developed in the proposed work. These technologies were chosen for maximum impact. SHJ cells have reached record efficiencies >25%, while costing less to produce than more complicated interdigitated back contact (IBC) designs. Meanwhile, PSCs have rocketed up the efficiency chart reaching 22.1% in less than five years. However, both technologies suffer from their own bottlenecks. SHJ cells are still not commercially competitive with lower-efficiency homojunction designs, due to complex multi-layer front contacts that drive up capital expenditure and result in optical losses in the amorphous silicon buffer layers. PSCs are notoriously unstable, leading to addition of various protective coatings, but at the expense of parasitic light absorption and increased process complexity. Both problems share the same solution: a single p-type TCM to act as both front current collector and junction partner, thus simplifying cell processing and eliminating parasitic absorption. CLAReTÉ will develop a new class of p-type TCMs to meet this need, guided by design principles for identifying materials with superior p- type electrical transport properties. CLAReTÉ will generate cross-cutting scientific impact by (a) taking an integrated materials science approach in which high-throughput materials screening is coupled to cutting-edge solar cell fabrication, (b) testing novel design principles proposed for p-type TCMs under real-world conditions, and (c) uncovering new high-performance p-type TCMs with applications in modern technologies ranging from PV to consumer electronics.
Conclusion of the Action:
Overall, four materials were selected for investigation during the course of this action. Two materials, p-type microcrystalline silicon (c-Si:H) and nickel oxide (NiOx), are “traditional” p-type contact materials and two others, zirconium oxy-sulfide (ZrOS) and boron phosphide (BP) are novel disperse valence band materials. The rationale behind selecting this set of candidate materials was that each represented an increasing level of risk, in terms of successful synthesis and application in a solar cell contact. PECVD of BP was attempted, but x-ray diffraction showed the wrong polymorph of the material had formed. Attempts to confirm this finding using composition measurements (EDX with a drift detector) were inconclusive, as the B:P ratio was difficult to ascertain. In the end, characterizing boron content was a roadblock to further development of this material, and it was down-selected from the list of transparent contact candidates in the project. NiOx was selected for investigation due to its favorable valence band alignment with crystalline silicon and the reported ability to modulate the valence band position by varying the Ni:O ration during deposition. ZrOS was the focus of an on-going collaboration between the researcher and the University of Twente and was also used as the vehicle for establishing combinatorial capabilities at the host institution (PV-Lab, EPFL). Finally, p-type uc-Si:H was explored as a transparent front contact material in silicon heterojunction (SHJ) solar cells and was the champion material of the project.
Two solar cell technologies, silicon heterojunction (SHJ) and perovskite solar cells (PSCs), will be used as testing grounds for the new p-type TCMs developed in the proposed work. These technologies were chosen for maximum impact. SHJ cells have reached record efficiencies >25%, while costing less to produce than more complicated interdigitated back contact (IBC) designs. Meanwhile, PSCs have rocketed up the efficiency chart reaching 22.1% in less than five years. However, both technologies suffer from their own bottlenecks. SHJ cells are still not commercially competitive with lower-efficiency homojunction designs, due to complex multi-layer front contacts that drive up capital expenditure and result in optical losses in the amorphous silicon buffer layers. PSCs are notoriously unstable, leading to addition of various protective coatings, but at the expense of parasitic light absorption and increased process complexity. Both problems share the same solution: a single p-type TCM to act as both front current collector and junction partner, thus simplifying cell processing and eliminating parasitic absorption. CLAReTÉ will develop a new class of p-type TCMs to meet this need, guided by design principles for identifying materials with superior p- type electrical transport properties. CLAReTÉ will generate cross-cutting scientific impact by (a) taking an integrated materials science approach in which high-throughput materials screening is coupled to cutting-edge solar cell fabrication, (b) testing novel design principles proposed for p-type TCMs under real-world conditions, and (c) uncovering new high-performance p-type TCMs with applications in modern technologies ranging from PV to consumer electronics.
Conclusion of the Action:
Overall, four materials were selected for investigation during the course of this action. Two materials, p-type microcrystalline silicon (c-Si:H) and nickel oxide (NiOx), are “traditional” p-type contact materials and two others, zirconium oxy-sulfide (ZrOS) and boron phosphide (BP) are novel disperse valence band materials. The rationale behind selecting this set of candidate materials was that each represented an increasing level of risk, in terms of successful synthesis and application in a solar cell contact. PECVD of BP was attempted, but x-ray diffraction showed the wrong polymorph of the material had formed. Attempts to confirm this finding using composition measurements (EDX with a drift detector) were inconclusive, as the B:P ratio was difficult to ascertain. In the end, characterizing boron content was a roadblock to further development of this material, and it was down-selected from the list of transparent contact candidates in the project. NiOx was selected for investigation due to its favorable valence band alignment with crystalline silicon and the reported ability to modulate the valence band position by varying the Ni:O ration during deposition. ZrOS was the focus of an on-going collaboration between the researcher and the University of Twente and was also used as the vehicle for establishing combinatorial capabilities at the host institution (PV-Lab, EPFL). Finally, p-type uc-Si:H was explored as a transparent front contact material in silicon heterojunction (SHJ) solar cells and was the champion material of the project.
The champion material studied in the CLAReTE project is p-type c-Si:H applied as a front contact layer in SHJ solar cells. A baseline recipe for depositing this material by plasma-enhanced chemical vapor deposition (PECVD) was used as a starting point for a campaign of varying-temperature depositions to examine the effect of low-temperature deposition (< 200ºC) on the crystalline volume fraction of the microcrystalline layer. It was found that by lowering the growth temperature, crystallinity increased, causing a drop in parasitic absorption in the front contact, as well as a reduction in overall series resistance of the SHJ cells. These findings were published the IEEE Journal of Photovoltaics [DOI:10.1109/JPHOTOV.2019.2917550] and presented at the 2019 Photovoltaic Specialists Conference.
The NiOx work was carried out in collaboration with a PhD student mentored by A. Fioretti who was visiting the host laboratory on a ThinkSwiss research scholarship. The results of this work indicated that band alignment with crystalline silicon is not sufficient for a successful p-type contact; in fact, alignment with the buffer layer (amorphous silicon in this study) was found to be the more critical parameter. This work is in preparation to be submitted as a manuscript to IEEE Journal of Photovoltaics.
Co-sputtering with ZrO2 and ZrS2 ceramic targets in argon and argon + hydrogen atmosphere was implemented on an existing sputtering tool at PV-Lab. Characterization was conducted utilizing spatially-resolved EDX measurements for Zr, O, S ratios, while 25-point x-ray diffraction maps were collected by collaborators in the lab of Dr. Monica Morales-Masis at University of Twente. These efforts resulted in two poster presentations on the synthesis efforts, one presented at the European Materials Research Society (E-MRS) Fall Meeting 2019 and on presented at the Japanese Materials Research Society (MRS-J) meeting in December 2019. Ultimately, synthesis of this material was never achieved, but it was established that careful minimization of oxygen content in the growth environment is critical for future efforts.
The NiOx work was carried out in collaboration with a PhD student mentored by A. Fioretti who was visiting the host laboratory on a ThinkSwiss research scholarship. The results of this work indicated that band alignment with crystalline silicon is not sufficient for a successful p-type contact; in fact, alignment with the buffer layer (amorphous silicon in this study) was found to be the more critical parameter. This work is in preparation to be submitted as a manuscript to IEEE Journal of Photovoltaics.
Co-sputtering with ZrO2 and ZrS2 ceramic targets in argon and argon + hydrogen atmosphere was implemented on an existing sputtering tool at PV-Lab. Characterization was conducted utilizing spatially-resolved EDX measurements for Zr, O, S ratios, while 25-point x-ray diffraction maps were collected by collaborators in the lab of Dr. Monica Morales-Masis at University of Twente. These efforts resulted in two poster presentations on the synthesis efforts, one presented at the European Materials Research Society (E-MRS) Fall Meeting 2019 and on presented at the Japanese Materials Research Society (MRS-J) meeting in December 2019. Ultimately, synthesis of this material was never achieved, but it was established that careful minimization of oxygen content in the growth environment is critical for future efforts.
In the course of this action, we found that thin-film synthesis is critically enabling for electrical characterization and ultimate device integration for candidate p-TCMs. Additionally, focusing research attention on binary and ternary materials with fewer (or less favorable to form) secondary phases, lends itself as a strategy to rapid development and integration of new materials into existing architectures.
In this project, we learned that researcher focusing on computational screening and first principles predictions of p-TCMs must go one step further and translate chemical potential diagrams into practical experimental considerations, to avoid experimentalists to be misled in thinking that a predicted material is more promising than it is.
In this project, we learned that researcher focusing on computational screening and first principles predictions of p-TCMs must go one step further and translate chemical potential diagrams into practical experimental considerations, to avoid experimentalists to be misled in thinking that a predicted material is more promising than it is.