The conversion of solar energy into valuable products and fuels using abundant and low-cost feedstocks like water is crucial for decarbonization and clean energy storage. Water splitting at 25°C requires a minimum of 1.23 V thermodynamically, but kinetic limitations of oxygen and hydrogen evolution reactions raise the practical minimum to 1.6 V. This necessitates careful selection of semiconductor and photovoltaic materials. Several approaches exist for sunlight-driven, electrochemically mediated hydrogen generation, with stability and STH efficiency as key metrics. Integrated photoelectrochemical (PEC) devices with buried junctions offer high lifetimes and cost minimization. Coupling photo-absorbers directly to catalysts and improving solar energy utilization through thermal integration enhances reaction efficiency and device durability. State-of-the-art PEC devices using III-V semiconductors have achieved STH efficiencies exceeding 19% but suffer from high cost and photo-corrosion. Halide perovskites (HaPs) offer a low-cost, solution-processed alternative with desirable properties like high absorption coefficients and tunable bandgaps, enabling power conversion efficiencies exceeding 25% in single-junction perovskite solar cells (PSCs). However, HaPs' ionic nature necessitates preventing their dissolution in aqueous environments. Previous HaP-PEC attempts involved surface modifications or physical barriers, but these compromised performance, resulting in limited STH efficiencies. This work addresses this challenge by introducing a lossless anticorrosion barrier to enable efficient and stable HaP-PEC devices.

The literature review highlights the challenges and progress in solar-driven water splitting. High STH efficiencies have been demonstrated using III-V semiconductors, exceeding 19% but with limited stability and high costs. Low-cost alternatives like metal oxides have shown significantly lower efficiencies and stabilities. Halide perovskites have emerged as promising low-cost photo-absorbers due to their exceptional photovoltaic performance, but their integration into PECs has been hampered by instability in aqueous environments. Previous strategies for protecting perovskites in PECs, such as surface modifications and physical barriers, have resulted in significant performance losses. The authors demonstrate a thorough understanding of the limitations of prior work, underscoring the need for a lossless anticorrosion barrier.

The researchers designed and fabricated a conductive adhesive-barrier (CAB) to address the challenges of integrating halide perovskites into efficient and stable PEC devices. The CAB is a bilayer structure consisting of a conductive, inert, and impermeable barrier material (graphite) bonded to the photovoltaic component using a conductive adhesive. This adhesive is formulated with a polymer matrix and conductive fillers (carbons or metals). The performance of the CAB was first evaluated using a standard three-electrode cell with individual PEC devices (photocathode: p-i-n PSC|CAB|Pt catalyst; photoanode: n-i-p PSC|CAB|IrOx catalyst). Unassisted water-splitting measurements were performed using two distinct architectures: (1) a co-planar photocathode-photoanode connected in series, and (2) a monolithic stacked silicon-perovskite tandem photoanode with a Pt foil cathode. Detailed descriptions of perovskite solar cell device fabrication (p-i-n and n-i-p architectures), characterization (J-V curves, EQE), pressure-sensitive adhesive synthesis, CAB fabrication, 1-junction PEC fabrication, silicon-perovskite PEC fabrication, iridium electrocatalyst synthesis, electrochemical measurements (3-electrode and 2-electrode), Faradaic efficiency measurements, and 3D-printed polypropylene reactor design are provided. Specific materials and their purity are listed in Table 1. The study includes photovoltaic stability testing (MPP tracking under AM 1.5G illumination) and various characterization techniques like SEM and XPS.

The CAB demonstrated >99% translation of halide perovskite photovoltaic power to chemical reactions. In the co-planar architecture, the integrated PEC achieved an STH efficiency of 13.4% with 16.3 h of operation, demonstrating record efficiency for this design. The stability limitation was attributed to the n-i-p device, not the CAB. The monolithic silicon-perovskite tandem system achieved a peak STH efficiency of 20.8% and operated continuously for 102 h before reaching t60 (time to 60% of initial efficiency). This represents a state-of-the-art efficiency for integrated water-splitting PEC devices. Analysis of post-reaction solar cell J-V curves revealed significant degradation in the n-i-p devices, explaining the limitation in the co-planar system's long-term stability. The tandem device's performance was analyzed through J-V curves, showing a slight decrease in current density compared to the photovoltaic maximum power point due to catalyst overpotential losses. Faradaic efficiency measurements for both architectures confirmed near-unity efficiency. SEM and XPS analysis of the IrOx catalyst in the tandem after the reaction showed delamination and oxidation of carbon species, contributing to long-term degradation. A comparison with other reported integrated PEC devices showed that the current work achieved the highest STH efficiency exceeding 20% for a non-concentrator PEC, showcasing the significance of the CAB approach.

The results demonstrate the effectiveness of the CAB in creating a robust barrier that protects the halide perovskite photo-absorber without compromising the device performance. The achievement of a 20.8% STH efficiency and 102h of continuous operation in the monolithic tandem device is a significant breakthrough in the field, surpassing previously reported results for both efficiency and stability. The observed degradation in the tandem device was partially attributed to catalyst degradation and contact resistance issues that need to be addressed in future studies. The results highlight the versatility of the CAB platform, applicable to various photo-absorbers and electrochemical reactions. The improved performance and stability compared to previous HaP-PEC systems highlight the success of the CAB approach.

This study successfully demonstrated a novel conductive adhesive barrier (CAB) for integrating halide perovskites into high-efficiency and durable photoelectrochemical cells. The achieved STH efficiencies (13.4% for co-planar and 20.8% for tandem) and operational lifetimes significantly advance solar-driven water splitting technology. Future research should focus on improving the long-term stability of perovskite photoelectrodes, employing lower-cost catalysts, and optimizing reactor designs for enhanced performance and economic feasibility. The CAB platform holds promise for various solar-to-fuel conversion applications.

The study identified limitations in long-term stability, primarily due to degradation in the n-i-p perovskite solar cells and partial catalyst degradation in the tandem system. The use of a 3D printed reactor is an important advancement but is subject to further development and scalability. Atmospheric exposure during CAB and epoxy encapsulation might have contributed to accelerated degradation of the hole transport layer. Further optimization of the catalyst and improved catalyst-barrier contact are areas of ongoing research.