Efficient and reliable encapsulation for perovskite/silicon tandem solar modules

Perovskite/silicon tandem solar cells have a tremendous potential to boost renewable electricity production thanks to their very high performance combined with promising cost structure. However, for actual field deployment, any solar cell technology needs to be assembled into modules, where the associated processes involve several challenges that may affect both the performance and stability of the devices. For instance, due to its hygroscopic nature, ethylene vinyl acetate (EVA) is incompatible with perovskite-based photovoltaics. To circumvent this issue, we investigate here two alternative encapsulant polymers for the packaging of perovskite/silicon tandems into minimodules: a thermoplastic polyurethane (TPU) and a thermoplastic polyolefin (TPO) elastomer. To gauge their impact on tandem-module performance and stability, we performed two internationally established accelerated module stability tests (IEC 61215): damp heat exposure and thermal cycling. Finally, to better understand the thermomechanical properties of the two encapsulants and gain insight into their relation to the thermal cycling of encapsulated tandems, we performed a dynamic mechanical thermal analysis. Our understanding of the packaging process of the tandem module provides useful insights for the development of commercially viable perovskite photovoltaics.

sputtered on the rear side (150nm and 250nm, respectively).At the front, for the recombination junction, 20 nm of indium tin oxide (ITO) was sputtered.
Once the silicon bottom cell is fabricated, the different perovskite top cell layers are deposited to construct the perovskite/silicon tandem.In this work, we opted for the so-called p-i-n configuration (also called inverted structure), resulting in a tandem device where the electron-collecting contact faces sunwards.As hole transport layer (HTL) for the perovskite, 2PACz [2-(9H-Carbazol-9-yl)ethyl]phosphonic acid (>98%, Tokyo chemical industry) has been used, which is a self assembled molecule (SAM), anchored on the top TCO of the silicon bottom cell, via spin coating at 5000 rpm for 50 seconds, followed by a drying step at 100°C for 10 minutes. 3The concentration precursor solution was 1mg/ml in ethanol.A UV-Ozone treatment for 900 seconds was carried out before the deposition.Subsequently, a 1.7M Cs 0.05 FA 0.8 MA 0.15 Pb(I 0.75 Br 0.25 ) 3 perovskite precursor solution was prepared by dissolving a mixture of FAI (Greatcell solar), MABr (Greatcell solar), CsI (99.999%Sigma Aldrich), PbI 2 (99.999%,Alfa Aesar), and PbBr2 (99.999%Alfa Aesar) in a mixed solvent DMF/DMSO 4:1.75μL of perovskite solution was spin-coated at 2,000 rpm for 45 s.After that, the speed was increased to 7,000 rpm for 8 s.Chlorobenzene was dropped on the substrates 12 s before the end of the spin-coating process.After the deposition, the substrates were annealed at 100°C for 15 minutes.The whole process is done in the nitrogen-filled glovebox.On top of the perovskite, 1 nm of lithium fluoride (LiF) (99.85%,Alfa Aesar) and 20nm of C 60 (> 99.95% NanoC) were thermally evaporated (Angstrom Evovac).On top of this contact stack, 20 nm of SnO x were then deposited by atomic layer deposition (ALD) using a Picosun system.The substrate temperature was maintained at 100°C during ALD deposition with tetrakis-dimethylamine tin (TDMA Sn) precursor source at 80°C and H 2 O source at 18°C. 150 cycles were used to achieve the final thickness.For the IZO deposition, a 3-inch IZO ceramic target containing 98 wt% In 2 O 3 and 2 wt% ZnO was used.Through a shadow mask, 75 nm of IZO was sputtered on top of the SnOx.On top of the IZO layer, Ag fingers with a thickness of 350 nm were thermally evaporated using a high-precision magnetic shadow mask.The evaporation rate and thickness of each deposition were monitored by quartz crystal microbalance sensors.
As stated, the minimodule was obtained by sandwiching the tandem solar cell between two encapsulant sheets and two standard module glasses with a dimension of 7 x 7 cm 2 and 3.2 mm in thickness.First, to enable electrical characterization, the contacts need to be extracted from the minimodule, and this has been done via the so-called tabbing process.In particular, two copper ribbons, covered with a Pb/Sn alloy are placed on the top and bottom contacts utilizing a silver paste.To ensure a proper connection and eliminate the solvents present in the silver paste, thermal annealing at 120°C under a mild vacuum for 10 minutes has been carried out.The encapsulations were done using an industrial vacuum laminator (Ecolam 5, Ecoprogetti).To secure the edges of the modules and to avoid moisture ingress through the tabbing we used a double layer of edge sealant (poly isobutylene, PIB, butyl rubber) on the sides of the glass.Finally, the layer stack was placed inside the laminator and encapsulated at 120°C when TPU was adopted or 110°C in the case of TPO.TPU has been purchased from the company Swmintl (https://www.swmintl.com/)while TPO has been purchased from the company Borealis (https://www.borealisgroup.com/).

Tandem Characterization (Solar simulator, EQE, climate chamber)
Once the tandem devices were fabricated, their J-V characteristic curves were collected before and after encapsulation.To do this, a Wavelabs Sinus 220 LED-based solar simulator with AM1.5G irradiance spectrum was used as a light source, coupled with a Keithley 2400 series SourceMeter.The devices were measured with a voltage sweep from -0.1 V to +1.9 V in forward and reverse scan directions with 10 mV of step-voltage and 100 mV/s scan rate, under dark and illumination conditions.The device's active area was defined with a laser-cut shadow mask, with an aperture of 1 cm 2 .The light intensity was calibrated using Fraunhofer ISE CalLab certified c-Si solar cells.
External quantum efficiency (EQE) measurements were collected using a LOANA system (PV-Tools).The chopped monochromatic light beam was focused on the active area of the solar cell, avoiding the silver fingers.Two bias lights were employed.A near-infrared LED (950 nm) and a green LED light (525 nm) were used to saturate the silicon or perovskite sub-cell responses, respectively, to measure the complementary cell's EQE.In addition, 0.6V bias voltage was applied to isolate the current response.

Mechanical, Thermal and Thermomechanical Analyzes Peel Testing
The work of adhesion (WoA) was evaluated using a Yeonjin TXA texture analyzer.The partially encapsulated samples were fixed on glass slides and mounted in a 180° configuration.The dimensions of the sample are 2.1x2.1 cm.The adhesion of the adhesive tape is calibrated on a reference glass slide.The samples are glued on a glass holder during the measurements.