Making Record-efficiency SnS Solar Cells by Thermal Evaporation and Atomic Layer Deposition

Podsumowanie

Tin sulfide (SnS) is a candidate material for Earth-abundant, non-toxic solar cells. Here, we demonstrate the fabrication procedure of the SnS solar cells employing atomic layer deposition, which yields 4.36% certified power conversion efficiency, and thermal evaporation which yields 3.88%.

Streszczenie

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent thermal evaporation, and it absorbs visible light strongly. However, for a long time the record power conversion efficiency of SnS solar cells remained below 2%. Recently we demonstrated new certified record efficiencies of 4.36% using SnS deposited by atomic layer deposition, and 3.88% using thermal evaporation. Here the fabrication procedure for these record solar cells is described, and the statistical distribution of the fabrication process is reported. The standard deviation of efficiency measured on a single substrate is typically over 0.5%. All steps including substrate selection and cleaning, Mo sputtering for the rear contact (cathode), SnS deposition, annealing, surface passivation, Zn(O,S) buffer layer selection and deposition, transparent conductor (anode) deposition, and metallization are described. On each substrate we fabricate 11 individual devices, each with active area 0.25 cm². Further, a system for high throughput measurements of current-voltage curves under simulated solar light, and external quantum efficiency measurement with variable light bias is described. With this system we are able to measure full data sets on all 11 devices in an automated manner and in minimal time. These results illustrate the value of studying large sample sets, rather than focusing narrowly on the highest performing devices. Large data sets help us to distinguish and remedy individual loss mechanisms affecting our devices.

Wprowadzenie

Thin film photovoltaics (PV) continue to attract interest and significant research activity. However, the economics of the PV market are shifting rapidly and developing commercially successful thin film PV has become a more challenging prospect. Manufacturing cost advantages over wafer-based technologies can no longer be taken for granted, and improvements in both efficiency and cost must be sought on an equal footing.^1,2 In light of this reality we have chosen to develop SnS as an absorber material for thin film PV. SnS has intrinsic practical advantages that could translate into low manufacturing cost. If high efficiencies can be demonstrated, it could be considered as a drop-in replacement for CdTe in commercial thin film PV. Here, the fabrication procedure for the recently reported record SnS solar cells is demonstrated. We focus on practical aspects such as substrate selection, deposition conditions, device layout, and measurement protocols.

SnS is composed of non-toxic, Earth-abundant and inexpensive elements (tin and sulfur). SnS is an inert and insoluble semiconducting solid (mineral name Herzenbergite) with an indirect bandgap of 1.1 eV, strong light absorption for photons with energy above 1.4 eV (α > 10⁴ cm^-1), and intrinsic p-type conductivity with carrier concentration in the range 10¹⁵ – 10¹⁷ cm^-3.^3–7 Importantly, SnS evaporates congruently and is phase-stable up to 600 °C.^8,9 This means that SnS can be deposited by thermal evaporation (TE) and its high-speed cousin, closed space sublimation (CSS), as is employed in the manufacture of CdTe solar cells. It also means that SnS phase control is far simpler than for most thin film PV materials, notably including Cu(In,Ga)(S,Se)₂ (CIGS) and Cu₂ZnSnS₄ (CZTS). Therefore, cell efficiency stands as the primary barrier to commercialization of SnS PV, and SnS could be considered a drop-in replacement for CdTe once high efficiencies are demonstrated at the laboratory scale. However this efficiency barrier cannot be overstated. We estimate that the record efficiency must increase by a factor of four, from ~4% to ~15%, in order to stimulate commercial development. Developing SnS as a drop-in replacement for CdTe will also require growth of high quality SnS thin films by CSS, and the development of an n-type partner material on which SnS can be grown directly.

Below is described the step-by-step procedure for fabricating record SnS solar cells using two different deposition techniques, atomic layer deposition (ALD) and TE. ALD is a slow growth method but to-date has yielded the highest efficiency devices. TE is faster and industrially scalable, but lags ALD in efficiency. In addition to the different SnS deposition methods, the TE and ALD solar cells differ slightly in the annealing, surface passivation, and metallization steps. The device fabrication steps are enumerated in Figure 1.

After describing the procedure, test results for the certified record devices and related samples are presented. The record results have been reported previously. Here the focus is on the distribution of results for a typical processing run.

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Protokół

1. Substrate Selection and Cutting

1. Purchase polished Si wafers with a thick thermal oxide. For the devices reported here, use 500 μm thick wafers with a 300 nm or thicker thermal oxide. The substrate selection criteria are discussed in the Discussion section.
2. Spin coat the polished side of wafer with a typical positive photoresist (SPR 700 or PMMA A. 495) and soft bake (30 sec at 100 °C).
   Note: This is a protective layer to prevent damage or contamination during the subsequent cutting step.
3. Use a die saw to cut the wafer into 1” × 1” (25.4 x 25.4 mm²) square substrates.

2. Substrate Cleaning

1. Remove particulates and other residue that result from cutting step using a compressed nitrogen gun, followed by an ultrasonic bath in de-ionized (DI) water for 5 min at 45 – 60 °C.
2. Remove the photoresist layer with an ultrasonic bath in acetone for 5 min at 45 – 60 °C.
3. Clean the exposed substrate with 3 subsequent ultrasonic baths, all for 5 min at 45 – 60 °C: acetone, ethanol, and isopropyl alcohol. Finish by drying with a compressed nitrogen gun, while substrates remain in the quartz carrier.

3. Mo Sputtering

1. Load the clean Si / SiO₂ substrates into a high vacuum sputtering system. Ensure that the substrate plate is unheated and substrate rotation is enabled. For the devices reported here, process in a commercial system with tilted magnetron guns with 2” targets and a throw distance of approximately 4”.
2. Deposit the first layer (the adhesion layer) at relatively high background pressure such as 10 mTorr of Ar. For the devices reported here, process with a sputtering power of 180 W (DC), which gives a growth rate of 2.6 Å/sec, and a first Mo layer that is 360 nm thick.
3. Deposit the second layer (the conductive layer) at a relatively low background pressure such as 2 mTorr of Ar. Use the same sputtering power as the first layer (180 W) and deposit the same thickness.
   Note: The devices reported here had a second Mo layer that was 360 nm thick, same as the first layer.
4. After Mo deposition, store the substrates under vacuum until the SnS deposition step.

4. SnS Deposition

Note: The ALD deposition technique is described in sub-section 4.1, and the TE deposition is described in sub-section 4.2. The ALD deposition system is shown in Figure 2, and TE deposition system is shown in Figure 3.

1. Deposit SnS by ALD
   1. Before loading into the reactor, put Mo substrates in a UV ozone cleaner for 5 min to remove organic particles. Then place the substrates on the substrate holder and insert into the deposition zone.
   2. Stabilize the furnace temperature at 200 °C before starting deposition.
   3. Grow SnS thin films from the reaction of bis(N,N’-diisopropylacetamidinato)-tin(II) [Sn(MeC(N-iPr)₂)₂, referred to here as Sn(amd)₂] and hydrogen sulfide (H₂S)⁴.
      1. Keep the Sn(amd)₂ precursor at a constant temperature of 95 °C. Use pure N₂ gas to assist the delivery of Sn(amd)₂ vapor from the container in the oven to the deposition zone. During each ALD cycle, supply three doses of Sn(amd)₂ precursor for total exposure of 1.1 Torr second.
      2. Use a gas mixture of 4% H₂S in N₂ as the sulfur source. Ensure that the exposure to the hydrogen sulfide vapor is 1.5 Torr second per dose. Ensure that the partial pressure of H₂S and the total pressure of H₂S in N₂ are 0.76 Torr and 19 Torr, respectively.
   4. Set the pumping time between Sn precursor dose and H₂S dose to be only 1 sec (short compared to most other conventional ALD procedures) in order to speed up the deposition.
      Note: Because the Sn precursor is not completely removed by this short pumping time, some residual Sn precursor remains when the H₂S arrives. Thus the process could be described as a pulsed CVD process. The growth rate of SnS film is 0.33 Å/cycle, or 0.04 Å/sec.
2. Deposit SnS by TE
   1. Ensure that the process chamber pressure is 2 x 10^-7 Torr or lower. Load substrates into the chamber through the load lock. Hold the substrates to the plate either with a single clip, or with a custom substrate holder with appropriately sized pockets that is screwed down to the substrate plate.
   2. Ramp the source and substrate heaters to their setpoints. For the device reported here the substrate temperature is 240 °C and the growth rate is 1 Å/sec; to achieve this growth rate set the source temperature in the range 550 – 610 °C (the required source temperature increases with time for a single load of source powder). The target film thickness is 1,000 nm.
   3. Measure deposition rate using the quartz crystal monitor (QCM) before and after the SnS film deposition by moving the QCM arm into the process chamber. For this measurement the substrate is raised so that the QCM can be moved into the substrate growth position.
      Note: The deposition rate remains fairly constant throughout a deposition time of 3 hr (±0.05 Å/sec deviation).
   4. After deposition, transfer the samples back into the load lock before venting to air. Quickly transport the samples through air into storage either in vacuum or in an inert atmosphere glovebox before the next processing step.
      Note: The typical unintentional air exposure time is approximately 3 min. The typical storage time is between a day and a week.

5. SnS Annealing

Note: This step is performed slightly differently for ALD and TE solar cells. The annealing procedure for ALD solar cells is described in sub-section 5.1, and the procedure for TE solar cells is described in sub-section 5.2. The purpose of annealing is discussed in the Discussion section.

1. Anneal the ALD-grown SnS films in H₂S gas.
   Note: This step is performed in the same system used for ALD growth.
   1. Use pure H₂S gas (99.5% pure) at a flow rate of 40 sccm and pressure of 10 Torr.
   2. Heat the SnS film to a temperature of 400 °C and hold for 1 hr in the H₂S gas environment. Ensure that the gas is flowing throughout the whole process, including temperature ramping up and down.
2. Anneal the TE-grown SnS films in H₂S gas. Perform this step in a dedicated tube furnace.
   1. Load the samples onto a clean quartz plate and slide into the hot-zone region of the furnace.
   2. After the furnace is sealed, purge three times with pure N₂ and allow to pump down to base pressure.
   3. Establish gas flow at 100 sccm of 4% H₂S at 28 Torr.
   4. Ramp the temperature to 400 °C over 10 min. Hold at 400 °C for 1 hr, then allow the samples to cool unassisted in the hot-zone. Maintain constant H₂S gas flow and pressure until samples cool below 60 °C. Remove the samples and either proceed immediately to the next step, or place them into storage in an inert gas glove box.

6. SnS Surface Passivation with a Native Oxide

Note: This step is performed slightly differently for ALD and TE solar cells. In sub-section 6.1 the surface passivation procedure for ALD solar cells is described, and the procedure for TE solar cells is described in sub-section 6.2. The function of this step is further discussed in the Discussion section.

1. For the ALD-grown samples, grow a thin layer of SnO₂ by ALD.
   Note: We use a different reactor than that used for SnS growth.
   1. Grow SnO₂ by the reaction of cyclic amide of tin [(1,3-bis(1,1-dimethylethyl)-4,5-dimethyl-(4R,5R)-1,3,2-diazastannolidin-2-ylidene)Sn(II)] and hydrogen peroxide (H₂O₂). Store the cyclic amide tin precursor in an oven at 43 °C, and the H₂O₂ in a bubbler at RT.
   2. Maintain substrate temperature at 120 °C for deposition.
   3. Expose the tin precursor and H₂O₂ using 0.33 and 1.5 Torr second per cycle, respectively, for a total of 5 cycles. Check that the thickness of the resulting SnO₂ is 0.6 0.7 nm, as measured by X-ray photoelectron spectroscopy (XPS) analysis¹⁰.
2. For the TE-grown samples, form a thin layer of SnO₂ by air exposure.
   1. Expose the samples to lab ambient air for 24 hr. Check that the thickness of the resulting SnO₂ is approximately 0.5 nm, as measured by XPS analysis.
      Note: The typical RT is 24 ± 1 °C, and the typical humidity is 45% ± 13% (higher in the summer); for the devices reported here, the values were 24.6 ˚C and < 30%, respectively.

7. Deposition of the Zn(O,S) / ZnO Buffer Layer

Note: This step is performed in the same ALD chamber that is used for SnS growth by ALD.

1. Grow a Zn(O,S):N layer by ALD.
   1. Maintain the substrate temperature at 120 °C.
   2. Grow Zn(O,S):N by ALD from the reaction of diethylzinc (Zn(C₂H₅)₂, DEZ), deionized water (H₂O), 4% H₂S in N₂, and ammonia (NH₃)¹¹. Store the bubbler containing DEZ at RT. Use a cycle sequence of [DEZ-H₂O-DEZ-NH₃]₁₄-[DEZ-H₂S]₁, and repeat this super cycle 12 times. Ensure that the exposure of ammonia is 11 Torr second.
   3. Check that the S/Zn ratio in the resulting film is 0.14, as measured by Rutherford backscattering spectroscopy¹², and that the thickness of the film is approximately 36 nm.
2. Grow a ZnO layer by ALD.
   1. Maintain the substrate temperature at 120 °C.
   2. Grow ZnO with 50 ALD cycles of DEZ-H₂O.
      Note: The thickness of the resulting ZnO film is approximately 18 nm.

8. Deposition of the Transparent Conducting Oxide (TCO), Indium Tin Oxide (ITO)

1. Cut ITO shadow masks from a 0.024” (610 μm) aluminum 6061 sheet using a laboratory laser cutter.
   Note: The masks define 11 rectangular devices that are 0.25 cm² in size plus a larger pad in one corner that is used for optical reflectivity measurements, see Figure 4.
2. Mount the devices and masks in a mask aligner.
   Note: This is an aluminum plate with nested pockets for the substrate and masks and clips to secure the masks in place.
3. Deposit ITO by reactive magnetron sputtering.
   1. Heat the substrate to approximately 80 – 90 ˚C and enable substrate rotation.
   2. Use a 2 inch diameter ITO target (In₂O₃/SnO₂ 90/10 wt.%, 99.99% pure) at 65 W RF sputtering power with 40 / 0.1 sccm Ar / O₂ gas flow at 4 mTorr total pressure.
   3. Grow a 240 nm thick ITO film.
      Note: With these parameters, growth rates of 0.5 Å/sec and sheet resistances in the range 40 – 60 Ω/sq are achieved.

9. Metallization

1. Cut metallization shadow masks from a 127 μm thick austenitic stainless steel sheet.
   Note: These masks are cut with +10/-5 µm tolerance by a commercial company. The metal pattern consists of 2 fingers separated by 1.5 mm, each 7 mm long, and a 1 x 1 mm² contact pad, see Figure 4.
2. Mount the devices and masks in a mask aligner, as in Step 8.2.
3. Deposit Ag (for TE devices) or Ni/Al (for ALD devices) by electron-beam evaporation.
   1. Mount mask aligner onto the substrate plate of an electron beam metals evaporation system. Pump down to a base pressure below 1 x 10^-6 Torr.
   2. Evaporate metal at a rate of 2 Å/sec. Deposit 500 nm total metal thickness.

10. Device Characterization

1. Perform current-voltage (“J-V”) measurements on all devices in the dark and in AM1.5 simulated solar light.
   1. Calibrate the solar simulator by collecting J-V data from a calibrated silicon solar cell and adjusting the solar simulator lamp power and height until reaching the calibrated current value for AM1.5 insolation.
   2. Contact the devices in four-wire mode by using copper beryllium double probe tips to contact to both the top (anode, Ag or Al) and bottom (cathode, Mo) layers. Contact the bottom layer by scratching away the buffer and SnS layers with a scalpel blade.
   3. Measure light and dark J-V data using a source-meter by sourcing voltage and measuring current.
      Note: Devices are typically measured within the range ±0.5 V. The devices are not responsive to the direction or rate of the voltage sweeps. For routine testing an area-defining light aperture is not used, see Discussion section for further details.
2. Perform external quantum efficiency (EQE) measurements on all devices, with variable light and voltage bias.
   1. Calibrate the EQE system by measuring the response of a Si calibration photodiode.
      Note: The software compares this data to measurements performed with a NIST-backed standard to adjust the light level accordingly.
   2. Contact the devices using the four-wire method, as in step 10.1.2.
   3. Measure EQE using a commercial system which illuminates the sample with monochromatic light chopped at 100 Hz over a wavelength range of 270 1,100 nm and measures the resulting current. Perform this measurement according to the manufacturer’s standard operating procedure.
   4. Repeat the EQE measurement with variable voltage and white light bias. Use a sourcemeter to supply the voltage bias, and a halogen lamp to supply the light bias. Measure devices in both forward and reverse voltage bias, and under variable white light intensity up to ~1 Suns.
   5. Measure optical reflectance (%R) of the ITO top surface using a spectrophotometer with an integrating sphere in order to convert external to internal quantum efficiency (IQE). Perform this measurement according to the manufacturer’s standard operating procedure.

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Wyniki

In Figures 6-8 results are shown for two representative “baseline” TE-grown samples as described above. Illuminated J-V data for these two samples is plotted in Figure 6. The first sample (“SnS140203F”) yielded the device with certified efficiency of 3.88% that was reported previously.⁹ Representative J-V distributions are also shown for each sample. For a given bias voltage, these distributions are calculated as

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Dyskusje

Substrate selection cleaning

Oxidized Si wafers are used as substrates. The substrates are the mechanical support for the resulting solar cells, and their electrical properties are not important. Si wafers are preferred to glass because commercially purchased Si wafers are typically cleaner than commercially purchased glass wafers, and this saves time in substrate cleaning. Si substrates also have higher thermal conductivity than glass, which leads to more even heating during ...

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Materiały

Name	Company	Catalog Number	Comments
Quartz wafer carrier	AM Quartz, Gainesville, TX		bespoke design
Sputtering system	PVD Products		High vacuum sputtering system with load lock
4% H2S in N2	Airgas Inc.	X02NI96C33A5626
99.5% H2S	Matheson Trigas	G1540250
SnS powder	Sigma Aldrich	741000-5G
Effusion cell	Veeco	35-LT	Low temperature, single filament effusion cell
diethylzinc (Zn(C2H5)2)	Strem Chemicals	93-3030
Laser cutter	Electrox	Scorpian G2	Used for ITO shadow masks
ITO sputtering target (In2O3/SnO2 90/10 wt.%, 99.99% pure)	Kurt J. Lesker	EJTITOX402A4
Metallization shadow masks	MicroConnex		bespoke design
Electron Beam Evaporator	Denton		High vacuum metals evaporator with load-lock
AM1.5 solar simulator	Newport Oriel	91194	1,300 W Xe-lamp using an AM1.5G filter
Spectrophotometer	Perkin Elmer	Lambda 950 UV-Vis-NIR	150 mm Spectralon-coated integrating sphere
Calibrated Si solar cell	PV Measurements		BK-7 window glass
Double probe tips	Accuprobe	K1C8C1F
Souce-meter	Keithley	2400
Quantum efficiency measurement system	PV Measurements	QEX7
Calibrated Si photodiode	PV Measurements
High-throughput solar cell test station	PV Measurements		bespoke design
Inert pump oil	DuPont	Krytox	PFPE oil, grade 1514; vendor: Eastern Scientific
H2S resistant elastomer o-rings	DuPont	Kalrez	compound 7075; vendor: Marco Rubber
H2S resistant elastomer o-rings	Marco Rubber	Markez	compound Z1028
H2S resistant elastomer o-rings	Seals Eastern, Inc.	Aflas	vendor: Marco Rubber