Improved carrier selectivity of diffused silicon wafer solar cells - PowerPoint PPT Presentation

Improved carrier selectivity of diffused silicon wafer solar cells 12 th October 2017 SPREE Seminar Alexander To Supervisor: Dr. Bram Hoex Co-supervisor: Dr. Alison Lennon

Improved carrier selectivity Introduction Basic solar cell operation Figure 1 Schematic representation of a basic solar cell, depicting the basic processes occurring in the device which facilitate power conversion and extraction. Carrier selectivity is engineered to… 1) Reduce recombination, which can quantified by the recombination current density J 0 . 2) Facilitate the extraction of charge carriers at the metal electrodes, which is measured by the contact resistivity ρ c 2

Improved Carrier Selectivity Introduction Industry trends/forecasts Figure 2 Predicted trend for recombination currents J 0bulk , J 0front , Figure 3 Predicted trend for different front side metallisation J 0rear for p -type and n -type solar cell concepts (ITRPV [1]) technologies, (ITRPV [1]) Thesis aim: Investigate how the carrier selectivity of diffused solar cells can be improved for the existing and future diffused silicon wafer based solar cell technologies. 1 International Roadmap for Photovoltaic Results (ITRPV): Results 3 2016 . 2017.

Presentation overview Introduction Improved carrier selectivity of diffused silicon wafer solar cells Majority carrier conductivity at p + and n + metal-silicon interfaces. 1. Exploiting the unintentional consequences of AlO x wrap around on screen printed n + -silicon/Agcontact resistivity. 2. The properties of electroless nickel plated contacts to boron diffused p + -silicon. Carrier selectivity at non-contacted diffused surfaces. 3. Understanding the surface recombination rate of diffused and inverted/depleted surfaces. 4. A novel method of extracting the surface recombination parameters from photoconductance measurements. Applications to diffused homojunction IBC Solar cells 5. Fabrication and simulation solar cells results. 4

Presentation overview Improved carrier selectivity of diffused silicon wafer solar cells Majority carrier conductivity at p + and n + metal-silicon interfaces. 1. Exploiting the unintentional consequences of AlO x wrap around on screen printed n + -silicon/Agcontact resistivity. 2. The properties of electroless nickel plated contacts to boron diffused p + -silicon. Characterising carrier selectivity at non-contacted diffused surfaces. 3. Understanding the surface recombination rate of diffused and inverted/depleted surfaces. 4. A novel method of extracting the surface recombination parameters from photoconductance measurements. Applications to diffused homojunction IBC Solar cells 5. Fabrication and simulation solar cells results. 5

The effect of AlO x wrap-around Research Overview In principle, single sided deposition is actually very hard to achieve and parasitic deposition • onto the front side of the solar cell can occur during fabrication. This has been reported for both PECVD and ALD AlO x deposition processes. • Figure 4 Schematic of a p -PERC solar cell with AlO x wrap-around on the front surface edges studied in this work.. Research questions: 1) What is the effect of AlO x wrap-around on screen-printed contact resistance? 2) Can we model this effect on solar cell performance? 3) What is the effect of AlO x wrap on p -PERC solar cell performance? 6

The effect of AlO x wrap-around Research Methodology: 1. Fabricate TLM test structures with intervening AlO x layers. Vary: Paste, Temperature, Speed. Figure 6 Schematic diagram of the p -type PERC (top) and PERT (bottom) test structures used in this experiment. Figure 5 Processing sequence of the PERC and PERT precursors. 7

The effect of AlO x wrap-around Research Methodology: 1. Fabricate TLM test structures. Vary: Paste, Temperature, Speed. Figure 4 Top view of the equidistant linear TLM structure. Figure 8 Equivalent resistance network for a system of with two interjacent fingers. 𝑆 𝑢𝑝𝑢𝑏𝑚 = 𝑜 + 1 𝑆 𝑡ℎ𝑓𝑓𝑢 𝑒 + 2𝑆 𝑑 + 𝒐𝑺 𝒇𝒓 𝑋 −1 1 1 𝑆 𝑓𝑟 = + 𝑠 𝑡𝑙 2𝑆 𝑑 + 𝑆 𝑛𝑓𝑢𝑏𝑚 Figure 7 Processing sequence of the PERC and PERT precursors. 8

The effect of AlO x wrap-around Research Contact resistivity ρ c – PERC Structures. Key points: 1) A clear ‘U’ -shaped trend, representing a minimum firing temperature. 2) 3 and 5 nm thicknesses appear to improve ρ c 3) A thick (10 nm) AlO x layer adversely affects ρ c Figure 9 Contact resistivity vs. temperature for screen-printed silver fingers fired through an AlO x /SiN x stack. 9 To, A. et. al., IEEE JPV (2017) 99 p. 1-8

The effect of AlO x wrap-around Research Contact resistivity ρ c – PERT Structures, varying paste. Figure 10 Contact resistivity vs. firing temperature for Ag-Si contacts formed with Hereaus (left) and DuPont (right) paste on PERT precursor wafers, for various AlO x thicknesses. 10

The effect of AlO x wrap-around Research Contact resistivity ρ c – PERT Structures, varying speed. Key points: 1) Varying speed does not appear to have significantly improved the 2) The Hereaus paste tested tends to perform better than the Dupont paste tested. Figure 11 Contact resistivity for PERT precursor samples fired at belt speeds S1, S2 and S3 using both Heraeus and DuPont silver pastes, with AlO x capping thicknesses ranging between 0 – 10 nm. 11

The effect of AlO x wrap-around Research Contact resistivity ρ c error analysis. • Scenario 1: Negligible current flows through the doped region under the metal (Low R c ).  Δ R total = n2R c negligible error for low R c • Scenario 2: Negligible current flows in the interjacent finger. (Large R c )  Δ R total = nr sk . Negligible error for low r sk • Scenario 3: Non-negligible current flows through both the interjacent finger and the underlying doped region.  Δ R total = nR eq which will introduce non-negligible error for high R c , and r sk . Figure 8 Equivalent resistance network for a system of with two interjacent fingers. 𝑆 𝑢𝑝𝑢𝑏𝑚 = 𝑜 + 1 𝑆 𝑡ℎ𝑓𝑓𝑢 𝑒 + 2𝑆 𝑑 + 𝒐𝑺 𝒇𝒓 𝑋 Figure 12 Absolute error ΔR total for the Scenarios 1, 2 and 3, is represented −1 1 1 in (a), (b) and (c), respectively, with (d) showing the measured R total as a 𝑆 𝑓𝑟 = + 𝑠 2𝑆 𝑑 + 𝑆 𝑛𝑓𝑢𝑏𝑚 function of n for all samples and AlO x thicknesses. 𝑡𝑙 12 To, A. et. al., E. Procedia (2017) 124 p. 914-921 Schroder, S emiconductor material and device characterization (2006) Wiley & Sons 3 rd Edition.

The effect of AlO x wrap-around Research 2. Modelling the effect of wrap-around using Griddler Methodology: Table 1 Griddler simulated performance characteristics of a p -PERC solar cell without AlO x wrap-around. 1. Simulate a state-of-the-art p - Symbol Parameter Value η Efficiency 21.31 % PERC solar cell in Griddler V oc Open circuit voltage 662 mV using published simulation 39.87 mA.cm -2 J sc Short circuit current density FF Fill factor 80.77 % values. 2. Impose ρ c non-uniformity spatially and simulate solar cell performance for various AlO x thicknesses and wrap- around extents. Figure 13 Screenshot of the Griddler interface in which spatial non-uniformity is simulated. 13 To, A. et. al., IEEE JPV (2017) 99 p. 1-8

The effect of AlO x wrap-around Research Modelling Results 21.5 1) A 3 and 5 nm AlO x does not Efficiency [%] a improve state-of-the-art cells 21.0 AlO x Thickness which are not ρ c limited. 0 nm 20.5 2) A thickness of 10 nm can have 3 nm 5 nm a significant effect on cell FF . 10 nm 20.0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 82 Fill Factor [%] 80 b 0 nm 78 3 nm 5 nm Figure 11 Simulated PL images (at V mpp with current extraction) 10 nm 76 of a p -PERC solar cell with (left) 0 and (right) 10 mm of 10 nm thick AlO x wrap-around deposition. 0 2 4 6 8 10 12 14 16 18 20 22 24 26 AlO x Wrap-around extent [mm] Figure 14 Simulated PL images (at V mpp with current extraction) Figure 15 Simulated effect of parasitic front side AlO x deposition on: of a p -PERC solar cell with (left) 0 and (right) 10 mm of 10 nm (a) efficiency; and (b) FF , of a p -PERC solar cell. thick AlO x wrap-around deposition. 14 To, A. et. al., IEEE JPV (2017) 99 p. 1-8

The effect of AlO x wrap-around Research 3. Effect actual solar cell performance Methodology: 1. Fabricate p -type Al-BSF solar cells with an SiN x /AlO x stack, of varying AlO x thicknesses. Reduce N D, surface = 3x10 19 cm -3 2. Characterisation: 1. Light/Dark-IV 2. Suns-V oc 3. Calculate R series : 𝑆 𝑡𝑓𝑠𝑗𝑓𝑡 = 𝑊 𝑛𝑞,𝑇𝑣𝑜𝑡𝑊𝑝𝑑 − 𝑊 𝑛𝑞,𝑀𝐽𝑊 𝐾 𝑛𝑞,𝑚𝑗𝑕ℎ𝑢 Figure 16 Processing sequence for the p -type Al-BSF cells fabricated in this work. 15 To, A. et. al., IEEE JPV (2017) 99 p. 1-8

The effect of AlO x wrap-around Research 3. Effect of wrap around on solar cell performance Figure 17 Cell characteristics extracted from light- IV , dark- IV and Suns- V OC measurements for Al-BSF solar cells screen-printed with DuPont and Heraeus paste at varying peak temperatures. 16 To, A. et. al., IEEE JPV (2017) 99 p. 1-8

The effect of AlO x wrap-around Research 3. Effect of wrap around on solar cell performance Figure 18 Plots of correlations between Efficiency (a), V oc (b) and J sc (c) vs. efficiency. Figure 19 Plots of correlations between J 01 (a), J 02 (b) and series resistance(c) and shunt resistance (d) vs. efficiency 17