Close-Space Sublimation-Deposited Ultra-Thin CdSeTe/CdTe Solar Cells for Enhanced Short-Circuit Current Density and Photoluminescence

The addition of cadmium selenium telluride to cadmium telluride absorbers is significant to improvements for photovoltaic efficiencies. Higher efficiencies and material savings with ultra thin layers advance photovoltaic and renewable energy development. The main advantages of automated in-line close-space sublimation deposition or CSS is that the close-space sublimation is fast and the automated deposition ensures reproducibility across devices.

Close-space sublimation addresses one of the challenges in the deposition of film absorbers for solar cells. The challenge is that it can be pretty slow to deposit thin films so close-space sublimation allows us to do that at a faster rate and therefore manufacture more solar cells in one day. This deposition method is also very useful for other thin film depositions in that many times atmospheric exposure is not desirable.

As with any fabrication system, initial oversight from an experienced user is highly advisable. Since the fabrication of these bilayer absorber photovoltaic devices requires many steps, visualization of the sample after each stage is critical to understanding which visual film qualities are required. Begin by using a handheld air blower to gently remove any dust particles from a clean transparent conducting oxide coated substrate marked with permanent marker and use a pair of rubber tipped tweezers to load the clean substrate onto the sample holder transparent conducting oxide side down.

With the load lock door closed, pump the load lock down until the load lock pressure gauge reads below five times 10 to the negative two torr and switch off the load lock pump. Switch open the load lock gate and wait for the pressure to level before manually inserting the transfer arm so that the sample sits above the shuttered cathode. Set the desired deposition time on a timer and begin timing as the shutter is manually opened.

Immediately close the shutter as the timer goes off and completely retract the transfer arm before closing the load lock gate and unloading the sample. To obtain the magnesium zinc oxide deposition rate, use a cotton tipped applicator dipped in methanol to remove the marker from the witness sample and measure the thickness with a profilometer. For close-space sublimation of the absorber layers, set the top and bottom sources in the deposition system with a temperature differential for proper material sublimation.

Use a handheld air blower to gently remove any dust particles from the clean magnesium zinc oxide and transparent conducting oxide coated substrate and load the clean substrate onto the sample holder magnesium zinc oxide side down. After closing the load lock door, turn on the load lock roughing switch to pump down the load lock. While pumping down, set the deposition recipe for the cadmium telluride witness sample to 110 seconds dwell time in the preheat source to raise the glass to approximately 480 degrees Celsius, 110 seconds in the cadmium telluride source for cadmium telluride deposition, 180 seconds in the cadmium chloride source for passivization of the polycrystalline cadmium telluride, 240 seconds in the anneal source to drive the cadmium chloride into the absorber, and 300 seconds in the cooling source.

When the load lock is pumped down below 40 millitorr, open the gate valve and start the deposition recipe in the software. The transfer arm will automatically move into the preset positions returning to the home position upon completion. When the deposition is complete, vent the load lock to atmosphere and open the load lock door.

When the sample is cool enough to handle, remove it with a lint-free cloth. Use deionized water to rinse the visible cadmium chloride residue from the film surface into a graduated beaker and dry the film with compressed nitrogen. Then use a razor blade to scrape a small area of cadmium telluride material off of the substrate and use a surface profilometer to measure the cadmium telluride film thickness to determine the deposition rate.

When the load lock pressure is below 40 millitorr, run a bakeoff recipe in the software. When the bakeoff is complete, set the deposition recipe for the cadmium selenium telluride witness sample to establish thickness. Set 140 seconds dwell time in the preheat source to raise the glass to approximately 540 degrees Celsius, 300 seconds in the cadmium selenium telluride source for cadmium selenium telluride deposition, and 300 seconds in the cooling source.

When the load lock pressure reaches below 40 millitorr, execute the deposition recipe. When the cadmium selenium telluride film deposition is complete, unload the cooled sample with a lint-free cloth and scratch off a small section of material to determine the cadmium selenium telluride deposition rate with a profilometer as previously demonstrated. To fabricate a 1.5 micron single cadmium telluride absorber, set the deposition recipe based on the cadmium telluride deposition rate and a cadmium chloride treatment previously optimized for ultra thin absorbers.

Use a preheat dwell time of 110 seconds, a cadmium telluride dwell time of 60 seconds, a cadmium chloride dwell time of 150 seconds, and anneal time of 240 seconds, and a cooling time of 300 seconds. When the load lock pressure reaches below 40 millitorr, open the load lock gate valve and select start. The program will automatically run the selected deposition recipe returning to the home position upon completion of the cooling step.

To fabricate a 0.5 micron cadmium selenium telluride and 1.0 micron cadmium telluride bilayer absorber, set the deposition recipe based on the absorber deposition rate with a preheat dwell time of 140 seconds, a cadmium selenium telluride dwell time of 231 seconds, a cadmium telluride dwell time of 50 seconds, a cadmium chloride dwell time of 150 seconds, an anneal time of 240 seconds, and a cooling time of 300 seconds. When the load lock pressure reaches below 40 millitorr, execute deposition recipe. Then unload the sample upon recipe completion and sample cooling as demonstrated.

When the top and bottom sources have reached operating temperature, load the sample onto the sample holder and sequentially move the transfer arm into the preheat copper chloride and anneal positions in accordance with a timer set for the deposition time of each position. The copper recipe in this protocol has been optimized for 1.5 micron devices. When the final timer goes off, manually return the transfer arm to the home position and close the load lock gate valve.

For evaporation deposition of thin tellurium, load the sample film side down onto the sample holder and close the chamber top. Manually move the lever into the roughing position. When the pressure drops below 10 millitorr, turn the lever back to the foreline position and wait about 30 seconds for any momentary spike in pressure to be resolved before opening the high vacuum valve.

When the chamber pressure reader has based out, the proper deposition pressure of one times 10 to the negative fifth torr has been reached. Turn on the power switch, open the shutter, and turn up the current control to begin deposition. When the quartz crystal monitor display shows the desired tellurium thickness, quickly and simultaneously turn the current to zero, turn off the power switch and close the shutter.

Before applying the paint back contact, shake the back contact solution to ensure complete mixing. Spray the solution across the sample in a slow lateral motion to apply uniform nickel back contact onto the sample. After allowing the back contact to dry slightly, repeat the application as many times as necessary for complete coverage.

To finish the thin film structure into electrically contactable devices, place the sample loaded into a metal mask into a glove box and use a siphon hose to apply glass beaded medium to the unmasked portions of the sample. Repeat the application with a second mask such that upon completing the delineation, 25 small-area square devices appear in a five by five pattern on the sample. Then clean the film side of the samples with a cotton tipped applicator dipped in deionized water.

To minimize the lateral resistance in electrical measurements of the finished devices, solder a grid pattern between the devices with an indium solder. The addition of cadmium selenium telluride to a thin cadmium telluride absorber improves the device efficiency through superior absorber material quality demonstrated by higher photoluminescence and longer time resolved photoluminescence decay lifetimes. Greater efficiencies are also achieved with higher short circuit current density.

The downward shift in the light current density voltage curve along the current density axis corresponds to an increase in short circuit current density for the best performing bilayer absorber device compared to the best performing single cadmium telluride absorber device. Quantum efficiency measurements of the cadmium telluride and cadmium selenium telluride/cadmium telluride devices show the additional photon conversion in the long wavelength range of the bilayer device and corroborate the increase in the short circuit current density for that device. The importance of optimizing the cadmium selenium telluride to cadmium telluride thickness ratio is demonstrated by a comparison of current density voltage results.

Data for a 0.5 to 1.0 micron ratio and a 1.25 to 0.25 micron ratio show a significant kink in the latter non-optimal device and a resultant decrease in photovoltaic efficiency. The most important thing to remember is that the thickness ratio between the cadmium selenium telluride and the cadmium telluride is imperative for respectable device performance and should be optimized for each bilayer absorber thickness. Following this procedure, an additional material layer can be deposited after the bilayer to act as an electron reflector.

Introducing this layer can minimize the voltage deficit obstacle in cadmium telluride-based devices. The incorporation of a cadmium selenium telluride alloy into the bilayer solar cells has been useful not only for the solar cells, but for developing the properties of that alloy into other photovoltaic applications. Cadmium compounds can be hazardous.

When we use such compounds and when we rinse the residue from the films, it's important to wear gloves, a lab coat, and then to use appropriate procedures to dispose of the hazardous waste.