Tin oxide thin films for photovoltaic devices

This thesis outlines the results of a four-year research project into the properties of Tin (Sn) based thin film materials prepared with magnetron sputtering, and how their properties can be optimised to improve the efficiency of photovoltaic (PV) systems. It consists of two projects: the first is an examination of how different heat treatments affect the optical and electrical properties of Indium Tin Oxide (ITO), a popular transparent conducting oxide (TCO) material which is widely used in PV. The second is an extensive research project carried out in collaboration with industry-leading labs (NREL, First Solar Inc. and CTF GmbH.) on how to improve the efficiency of Cadmium Telluride (CdTe) solar cells by optimising the properties of the buffer layer using innovative combinations of Tin Oxide (SnO2) and Zinc Oxide (ZnO).

The first chapter outlines how optical and electrical properties of 500 nm thick ITO films prepared by RF magnetron sputtering were affected by two different heat treatments: annealing and heating the substrate, testing temperatures of 300°C and 500°C. A direct comparison of these two concluded that heating the substrate during sputtering resulted in better ITO properties by almost every metric. Heating the substrate during deposition produced lower resistivity films with higher carrier concentrations and higher transmission over visible wavelengths. Microstructural analysis revealed that both heat treatments produced crystalline ITO films, but the preferred crystal orientations were different. Annealing resulted in a preferred cubic (222) orientation, and high substrate temperatures yielded a mixed phase crystal structure (cubic and rhombohedral) with a preferred cubic (211) orientation. Scanning Transmission Electron Microscopy (STEM) analysis showed that sputtering at room temperature with no added heat produced films with a mostly amorphous structure which were converted into equiaxed grains by annealing. High substrate temperatures resulted in a columnar grain structure. These results have useful implications for the deposition of ITO films for various opto-electronic applications.

Subsequent chapters centered around optimising the properties of SnO2 and ZnO based buffer layers to achieve high CdTe efficiencies. These materials are well-established, widely available semiconductor materials which are well-liked by industry, so any performance breakthroughs can be quickly adopted by industry to lower the cost of CdTe PV and address the urgent threats of climate change. All chapters pertaining to buffer layer optimisation are iterative, building on previous findings to refine the deposition process for high cell efficiencies.

Initial experiments focused on finding what thickness and oxygen content was optimal for intrinsic SnO2 buffer layers prepared with RF sputtering. Copper and arsenic-doped CdTe cells were prepared and examined with J-V, EQE and cross-sectional microscopy. The first set of devices examined the effects of 50 and 100 nm buffer layers subjected to different CdCl2 treatment lengths, finding that thinner buffer layers improved Voc and yielded higher efficiencies. The second set, based on the findings of the first, further analysed thickness and found that the optimum lies somewhere around 70 nm. The highest efficiency was of 19.79%, achieved with a 70 nm SnO2 buffer layer prepared in a 25% oxygen environment, using a cell fabricated by First Solar. Tests were also carried out using different oxygen contents (10, 15 and 25%) on SnO2 buffer layers, resulting in films which were all made from intrinsic SnO2 with no sub-oxides or Sn metal.

The next investigation examined the effects of alloying zinc with a SnO2 buffer layer on device efficiency. Co-sputtering was used to finely tune the composition and bandgap of the ZnO-SnO2 films and was found to be very effective at tailoring film properties. Film bandgap decreased as zinc content was increased. Two sets of CdSeTe/CdTe solar cells using these buffer layers were made with separate labs: the first, made at NREL, explored a wide range of zinc values (0-25 At% Zn) and the second, made with CTF GmbH. explored a narrower range (0-5 At% Zn). Both investigations independently found that buffer layers using around 5 At% Zn achieved higher efficiencies than those using pure SnO2, indicating a potentially revolutionary improvement in CdTe efficiency through effective use of widespread materials. These ZnO-SnO2 films were highly transparent and highly conductive, with the best buffer layer having a carrier concentration of 9.25 x 1018 cm-3. Buffer layers using 5% zinc showed a mixture of SnO2 and zinc stannate (ZnSnO3) peaks in their XRD spectra with a low surface roughness which translated into higher efficiencies. The best performing cell from this investigation had an efficiency of 18.2%, short-circuit current of 27.4 mA/cm2, open-circuit voltage of 842 mV and fill factor of 79.0%.

The next method tested was depositing a ZnO-SnO2 buffer layer using a pre-alloyed target, custom-made to have the zinc content of around 5 At%, found to be optimal in previous sections. CdSeTe/CdTe solar cells made using a single target had comparatively low efficiencies, reaching a maximum of only 14.4% due to low Voc. ZnO-SnO2 films made from the pre-alloyed target were much less zinc-rich than the target itself, meaning the buffer layers were not using the optimal composition. The effects of substrate heating and oxygen content were tested on the single-target films, neither having a significant effect on film composition. Zinc stannate peaks were found in all films even with the lower zinc contents. Effects of substrate temperature on pure SnO2 buffer layers was also tested in this chapter, finding that 300 and 500°C yield high efficiency cells, but temperatures in between these do not.

Lastly, ZnO and SnO2 thin films were combined in a bilayer configuration and used as buffer layers in CdSeTe/CdTe cells made by First Solar. This method achieved the highest efficiencies out of the entire thesis: a buffer layer consisting of 70 nm SnO2 with 10 nm ZnO deposited above it had an efficiency of 20.4%, short-circuit current of 30.86 mA/cm2, open-circuit voltage of 841 mV and a fill factor of 77.8%. Different SnO2 and ZnO thicknesses were tested, all of them outperforming a single SnO2 buffer layer and once more indicating that combining these materials can lead to a potentially groundbreaking improvement in CdTe efficiency. Due to the limitations of the sputtering system used, the vacuum had to be broken between the deposition of the first and second bilayer in the stack. In a manufacturing setting more inert conditions could be preserved which could improve efficiency even further.

To our knowledge, this is the first time these methods are used to prepare buffer layers for a CdSeTe/CdTe cells. High efficiencies were achieved with both the co-sputtering and bilayer methods, nearing the current world-record efficiency with the latter of these. High efficiencies were attained without the use of an antireflective coating (ARC) which could improve device efficiency even further.

This combined with further optimisations to buffer layer thickness, temperature and composition have the potential to be scaled industrially and help address upcoming energy and climate challenges.