Today, wafer-based crystalline silicon solar cells dominate the photovoltaic (PV) market, with a market share of more than 90%. Further cost reductions for this technology can be achieved by developing silicon-based tandem solar cells employing low-cost, abundant, and non-toxic metal oxide materials. Such tandem cells can increase the conversion efficiency of silicon solar cells beyond their conventional limitations with obvious economic and environmental benefits.
Many candidate materials have been proposed for the next generation of solar cells, and metal oxides are considered among the most promising ones. For instance, cuprous oxide (Cu2O) is an attractive material for photovoltaic applications since it’s an earth-abundant and non-toxic p-type semiconductor with high optical absorption and a direct bandgap of approximately 2 eV, yielding a theoretical conversion efficiency limit of 20% . To construct a p-n heterojunction, various n-type oxide materials, such as ZnO and SnO2, can be combined with Cu2O, and accordingly, one can foresee heterojunction solar cells completely based on low-cost metal oxides . In addition, and perhaps even more exciting, a metal oxide heterojunction solar cell can be combined with a silicon based solar cell in a tandem architecture in order to boost the conversion efficiency of crystalline silicon (c-Si) solar cells beyond the conventional limitations of this technology . Figure 1 shows an illustration of a tandem solar cell, combining a conventional c-Si bottom subcell with a ZnO/Cu2O top subcell in a four-terminal configuration, i.e. a mechanical stack of independently connected cells. The ZnO/Cu2O top subcell, which is deposited on a transparent quartz substrate by reactive magnetron sputtering, enables low-energy photons to be transmitted through the top subcell for subsequent absorption in the c-Si bottom subcell. In this way, the solar spectrum can be more efficiently utilized in the wavelength range from ultraviolet to near-infrared.
O and Al-doped ZnO (AZO) thin films were deposited on 10 x 10 x 0.5 mm3
quartz substrates using a direct current/radio frequency (DC/RF) magnetron sputtering system (Semicore Triaxis).
500 nm thick Cu2O film were deposited by reactive sputtering of a 99.999% Cu target in O2/Ar (6/49 sccm) at a substrate temperature of 400°C. The power was fixed at 100 W. As-grown Cu2O films were annealed at 900°C for 3 minutes in vacuum (pressure ~10-1 Torr). 200 nm thick AZO films were deposited by co-sputtering of a 99.99% pure ZnO ceramic target at 50 W and a 99.999% Al target at 3 W in Ar at a substrate temperature of 400°C, yielding an aluminum content of approximately 4 wt% in the deposited layers. During the magnetron sputtering deposition, the base pressure was below 3.0 x 10-7 Torr. The optical properties and surface morphology of the AZO and Cu2O thin films were analyzed using a Horiba Jobin Yvon Uvisel spectroscopic ellipsometer and a Quanta Inspect F 50 scanning electron microscope (SEM), respectively. The optical transmittance spectrum was measured using a setup with spectrophotometers, a deuterium–halogen light source, and an integrating sphere. Also, room temperature Hall effect measurements (LakeShore 7604) were carried out using the van-der Pauw configuration.
Table 1 shows the majority carrier mobility, film resistivity, and carrier concentration for Cu2O (as-grown and annealed) and AZO thin films deposited on quartz. The data suggests that the electrical properties for the Cu2O thin film are enhanced after thermal annealing. The increase in carrier mobility after annealing can, at least partly, be attributed to the increase in grain size and reduced grain-boundary scattering. SEM images of the Cu2O thin film confirm that the average grain size increases from about 70 nm for the as-grown film to about 600 nm for the annealed film . The carrier mobility and concentration are comparable to values previously reported for sputter-deposited Cu2O thin films on quartz  and suggest that the annealed Cu2O thin film is well suited for photovoltaic applications.
The complex refractive index of Cu2O and AZO the thin films were derived from spectroscopic ellipsometry measurements and implemented in a ray tracing model for calculation of the optical characteristics of the ZnO/Cu2O subcell . The absorbed and transmitted spectral intensities for the ZnO/Cu2O subcell are shown in figure 2. The short-wavelength photons below ~550 nm are absorbed in the top subcell, whereas the long-wavelength photons above ~600 nm are transmitted through the top subcell and onto the c-Si bottom subcell.
The electrical performance of the four-terminal tandem solar cell was evaluated based on device modeling in Silvaco Atlas, and both experimental and tabulated materials parameters were adopted in the model . Typical current-voltage (I-V) parameters for the top and bottom subcells are summarized in Table 2. The data suggests an overall power conversion efficiency of 22.5% for the tandem solar cell under 1 sun (100 mW/cm2) illumination, whereas the corresponding efficiency for the c-Si solar cell is 18.2% at 1 sun. This implies that a metal oxide heterojunction solar cell can be used in a tandem configuration in order to boost the conversion efficiency of c-Si solar cells.
In conclusion, the conversion efficiency of crystalline silicon solar cells can be boosted by adopting a tandem solar cell architecture, incorporating low-cost, abundant, and non-toxic metal oxide materials. The sputter-deposited Cu2O and Al-doped ZnO thin films presented in this work show good potential for application in a silicon-based tandem solar cell.
This work was conducted under the research project “High-performance tandem heterojunction solar cells for specific applications (SOLHET)”, financially supported by the Research Council of Norway (RCN) and the Romanian Executive Agency for Higher Education, Research, Development and Innovation Funding (UEFISCDI) through the M-Era.net program. RCN is also acknowledged for the support to the Norwegian Micro- and Nano-Fabrication Facility, NorFab, project number 245963/F50.
Ø. Nordseth1, R. Kumar2, K. Bergum2, L. Fara3, I.la Chilibon4, S. E. Foss1, E. Monakhov2, B. G. Svensson2
1 Institute for Energy Technology, Kjeller, Norway
2 Department of Physics/Center for Materials Science and Nanotechnology (SMN), University of Oslo, Oslo, Norway
3 University Politehnica of Bucharest, Bucharest, Romania
4 National Institute of Research and Development for Optoelectronics (INOE-2000), Magurele, Romania
Department for Solar Energy
Institute for Energy Technology (IFE)
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