Process Engineering for Low-Temperature Carbon-Based Perovskite Solar Modules ^†

Abstract

In less than a decade, Perovskite solar cell (PSC) technology has gained high efficiency and broad attention because of its key enabling physical and morphological features. One of the main obstacles to PSC industrialization and commercialization is managed with the demonstration of stable devices by adopting low-cost, reliable materials and fabrication process methods. Here, we report a Perovskite solar module based on a low-temperature carbon electrode. The full process was performed in ambient air and engineered by printing techniques.

1. Introduction

The photovoltaic (PV) sector has a fundamental role in covering the energy demand caused by the constant and never-ending technology progress [1]. To date, the PV market is based mainly on the “first generation” technology, i.e., monocrystalline and polycrystalline silicon. Despite that, the expensive manufacturing processes of silicon and its decreasing availability in nature have accelerated the “second generation” thin-film technologies exploitation based on materials such as cadmium telluride (CdTe) and amorphous silicon (a-Si) [2]. The “third generation” PV based on hybrid-organic materials was born to maintain high efficiency by reducing process fabrication costs [3,4,5,6]. Natural dyes such as anthocyanins [7], polymers [8], or fullerenes [9] are just a few examples of molecules used for this purpose. The main issues concerning these new technologies are the stability and upscaling from lab scale cells (area ˂ 1 cm²) to modules (interconnection of cells) [10].

In the recent years, Perovskite (PVSK) gained attention from scientists and investors because of its optical and electronic properties allowing a continuous development reaching record efficiencies (close to 26%) [11,12,13,14]. PVSK is a chemical compound with ABX₃ formula, where A and B are cations with different atomic radii and X represents an anion. PVSK-based materials organometallic compounds of halogens have gathered particular interest in the PV field thanks to the easy processability by solution-based methods [15].

Each layer forming a PVSK solar cell (PSC) has a well-defined function affecting the performance and the stability of the device. The PVSK is sandwiched between one electron transport layer (ETL) and a hole transport layer (HTL). The ETL and HTL are both connected to an external circuit by gold or silver contacts. In the case of n-i-p architecture, ETL is generally composed by c-TiO₂ (compact TiO₂) and mp-TiO₂ (mesoporous TiO₂) or SnO₂ and guarantees good conduction, low charge recombination, and high transparency. The most widespread HTM (hole transporting material) is the 2,2′,7,7′-Tetrakis(N,Ndi-p-methoxyphenylamine)-9,9′- spirobifluorene. Spiro-MeOTAD ensures good energy levels of band gap, quick charge transfer, and low recombination, however, it has low stability, high cost (about 200 euro/g), complex synthesis, and low yield [16]. Moreover, spiro-OMeTAD is doped to increase the mobility of the holes with highly hygroscopic salts (lithiumbis(trifluoromethylsulfonyl)imide (Li-TFSI) or 4-tert-butylpyridine (t-BP)) leading to PSC degradation and deterioration. In addition, the gold metal counter-electrode diffuses into the structure of the device when exposed to continuous lighting, causing huge PVSK degradation. Both the organic HTL and the metal electrode can be replaced with one low temperature conductive carbon layer. Carbon materials are cheap if compared to HTMs and gold, resulting in a reduction in cells cost and an increase in stability. Carbon is a key material widely used in the PV field due to its abundance, low cost, and appropriate energy level. In the PVSK field, low temperature carbon pastes, unlike the porous high temperature inks [17,18], can be processed at temperatures below 130 °C, exhibiting features of high conductivity, low cost, good stability, and high throughput process [18,19,20,21,22,23].

In this paper, we demonstrate a simple process based on scalable printing techniques out of the glove-box to fabricate a gold-free perovskite solar module (PSM) based on low temperature carbon counter-electrode.

2. Materials and Methods

The 31.36 cm² module (active area 6.25 cm²) is fabricated by interconnecting in series four n-i-p cells. The stack of each cell and the device process fabrication steps are depicted in Figure 1a,b, respectively.

A nanosecond raster Nd:YVO₄ pulsed scanning laser patterned (P1, fluence = 10.5 J/cm²) the FTO (fluorinated tin oxide) glasses (NSG, 7 Ω/sq) to form four series connected cells. Then, we cleaned the substrates in an ultrasonic system by using subsequent solvents (soap/water, acetone, ethanol, and isopropanol) for 5 min each. We deposited 40 nm thick c-TiO₂ as reported in [24] and 250 nm thick mp-TiO₂ paste (Great Cell Solar 18 nrt, diluted with ethanol 1:4 w/w) by a blade coating method, followed by 30 min@500 °C sintering in oven. The substrates are exposed to a UV lamp and then we deposited the triple cation PVSK (Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃ in DMF/DMSO) (PbI₂ from TCI Co., Ltd., Tokyo, Japan; CsI, FAI and MABr from Great Cell Solar, Queanbeyan, Australia) by a double step blade-coating method in ambient air according to Vesce et al. [24]. We reduced the recombination by depositing PEAI (phenethyl ammonium iodide) passivating agent [25,26]. A stable HTM suitable for carbon was deposited by a blade-coating technique. Then, the ETL/PVSK/PEAI/HTL stack is removed from the vertical connection areas by laser (P2, fluence = 200 J/cm²) to series connect two adjacent cells by the counter-electrode. Following this, the carbon counter-electrode (Dyenamo, Stockholm, Sweden) is blade-coated on the cells composing the module and annealed at 120 °C in air for 15 min. The IV curves are reported by a Keithley source meter/LabVIEW under a class A sun simulator (Sun 2000, Abet, Milford, Connecticut) at AM 1.5 1000 W/m² calibrated by a Skye Instruments sensor Ltd (Llandrindod Wells, UK).

3. Results and Discussion

In the upscaling process from lab-scale cell to module there are different issues to be considered: the high front contact sheet resistance, the interconnection dead area and resistance, and the layer inhomogeneity. In this work, we apply reliable mitigation action to reduce the losses. The sheet resistance is faced by using a low sheet resistance substrate, by adopting the series interconnection strategy, and by optimizing the cell width (i.e., reducing the recombination path). The interconnection dead area is reduced by narrowing the interconnection and separation areas. The laser process optimization is useful in limiting the interconnection resistance. The layer homogeneity is achieved by combining the right coating technique and the material composition according to the deposition environment. Moreover, we worked in ambient air to simulate a real plant condition.

In Figure 1c, the fabricated PVSK carbon-based module exhibits 8.36% and 8.25% efficiency in reverse and forward scan, respectively. The Voc is about 4 V, meaning about 1 V per cell, because the cells are series connected in Z configuration.

4. Conclusions

PV exploitation should avoid high cost and high-CO₂ footprint materials and fabrication processes. The adoption of low-temperature carbon-based counter-electrodes permits the avoidance of expensive and unstable organic HTM and metal counter-electrodes, such as gold or silver. In addition, the full fabrication process must be based on scalable printing techniques out of the glove box to be transferred to industry. In this direction, the carbon inks can be deposited by large area printing techniques, such as screen-printing and blade-coating. Since the carbon layer can be annealed at temperature less than 120 °C, the impact from the LCA (life cycle assessment) point of view is low. Here, we demonstrated a printed PSM based on a low temperature carbon counter-electrode.