Scalable Pulsed Laser Deposition of Transparent Rear Electrode for Perovskite Solar Cells
Advanced Materials Technologies
Pulsed laser deposition (PLD) is an established technique to deposit thin films with complex stoichiometry that has gained significant research attention after successful fabrication of high-temperature superconductors (HTS) in thin film form.  Since then, PLD was mainly used for applications related to epitaxial growth of multicompound oxides on lattice-matched substrates but has yet to be explored within the photovoltaic (PV) community. Although highly conductive In-based TCOs were
... d TCOs were fabricated by PLD and successfully implemented as front contact in OLEDs [2, 3] in early 2000s, there are still few reports regarding application of PLD-grown contacts in PV devices. Literature reports include doped ZnO films for CIGS  and organic  solar cells and metal oxide transport layers for halide perovskite solar cells.  Moreover, PLD has been proposed for chalcogenide absorber fabrication [7, 8] and, more recently, for halide perovskite absorber layers. [9, 10] Nevertheless, PLD is still considered to be an exotic fabrication method in the PV community due to the common concerns about the scalability of the technique reasoned by limited substrate size for uniform coating and low deposition rates. In fact, significant progress in upscaling  has already allowed fabrication of high-quality piezoelectric devices on 200 mm circular wafers,  as well as annual fabrication of >100 km of HTS tape with deposition rates >750 nm min −1 by PLD.  Here we demonstrate scalable PLD for the fabrication of Zr-doped In 2 O 3 (IZrO) thin films with properties on par with RF-sputtered ones. Furthermore, we apply IZrO films as rear electrodes in proof-of-concept semi-transparent halide perovskite solar cells. Background of PLD Process The principle of PLD processes is illustrated in Figure 1a and is briefly summarized below. The target, placed in a vacuum chamber with base pressure of <10 −7 mbar, is repeatedly ablated by the focused laser beam (typically, an excimer UV laser) with Sputtered transparent conducting oxides (TCOs) are widely accepted transparent electrodes for several types of high-efficiency solar cells. However, the different sputtering yield of atoms makes stoichiometric transfer of target material challenging for multi-compounds. Additionally, the high kinetic energies of the arriving species may damage sensitive functional layers beneath. Conversely, pulsed laser deposition (PLD) is operated at higher deposition pressures promoting thermalization of particles. This leads to stoichiometric transfer and additionally reduces the kinetic energy of ablated species. Despite these advantages, PLD is rarely used within the photovoltaic community due to concerns about low deposition rates and the scalability of the technique. In this study, wafer-scale (4-inch) PLD of high-mobility Zr-doped In 2 O 3 (IZrO) TCO for solar cells is demonstrated. IZrO films are grown at room temperature with deposition rate on par with RF-sputtering (>4 nm min −1 ). As-deposited IZrO films are mostly amorphous and exhibit excellent optoelectronic properties after solid phase crystallization at <200 °C. 100-nm thick films feature a sheet resistance of 21 Ω◻ −1 with electron mobilities ≈70 cm 2 V −1 s −1 . PLD-grown IZrO is applied as rear electrode in efficient semi-transparent halide perovskite solar cells leading to the improved stabilized maximum power point efficiency (15.1%) as compared to the cells with sputtered ITO electrodes (11.9%).