Perovskite solar cells (PSCs) have revolutionized the landscape of photovoltaic technology, emerging as one of the most promising candidates to surpass traditional silicon-based solar cells in both efficiency and cost-effectiveness. Achieving an unprecedented certified power conversion efficiency (PCE) of 26.95%, PSCs now rival the performance of crystalline silicon and CIGS solar cells. Despite this rapid advancement, the full potential of PSCs remains hampered by intrinsic energy losses that arise due to mismatched energy levels between the perovskite absorber layer and adjacent electron and hole transport layers. These mismatches manifest as barriers to efficient charge separation and transport, ultimately curbing device performance and stability.
A research team from Nanjing Tech University has published a comprehensive review in Nano-Micro Letters that systematically dissects strategies to fine-tune these energy levels through an energy flow perspective. This framework addresses the core challenge: optimizing the alignment of energy bands so that charge carriers can traverse the device with minimal recombination and resistance. Their work not only offers a cohesive understanding of existing material engineering techniques but also paves the way for unified approaches to enhance efficiency, longevity, and scalability of PSCs.
The fundamental efficiency bottleneck in PSCs is rooted in the electronic energy landscape at the interfaces between the perovskite absorber and its electron transport layers (ETLs) and hole transport layers (HTLs). When energy band edges—specifically the valence band maximum (VBM) and conduction band minimum (CBM)—are misaligned, it triggers a cascade of charge losses. This includes non-radiative recombination, where photogenerated electrons and holes annihilate each other at the interfaces, resulting in wasted energy as heat instead of useful electric current. Additionally, such misalignment introduces transport barriers that impede carrier mobility, detrimentally impacting the short-circuit current density (Jsc) and open-circuit voltage (Voc), two pivotal photovoltaic parameters.
Moreover, spectral utilization efficiency suffers when the perovskite’s bandgap does not optimally match the solar spectrum. Deviations from the ideal bandgap (~1.4 eV) prevent the absorption of the broadest range of photons, constraining the theoretical maximum PCE defined by the Shockley-Queisser limit. This calls for delicate control over perovskite composition and structure, targeting energy-level tuning at an atomic scale to extract maximum photonic energy.
Central to the review is the bifurcation of energy-level tuning approaches into two primary domains: intrinsic perovskite absorber optimization and interface engineering of ETLs and HTLs. Intrinsic modifications mostly revolve around compositional engineering of the ABX₃ perovskite lattice, where ‘A’ is a monovalent cation, ‘B’ a divalent metal ion, and ‘X’ a halide anion. Adjusting these components affects the perovskite’s tolerance factor, crystal phase stability, and electronic band structure, directly influencing the bandgap and energetic alignment with adjacent layers.
For instance, substituting methylammonium (MA⁺) with formamidinium (FA⁺) or cesium (Cs⁺) can shift the VBM and CBM to desired values while reinforcing the cubic phase crucial for efficient absorption. A paradigmatic case is FAPbI₃, which offers an optimal bandgap (~1.48 eV) left-shifted towards the solar spectrum’s peak. This fine-tuning enables a harmonious balance between maximizing Voc and achieving deep light absorption, culminating in PCEs approaching 27%. Halide alloying techniques, where iodide ions (I⁻) are combined with bromide (Br⁻) or chloride (Cl⁻) ions, further enable precise bandgap tailoring appropriate for specialized applications such as tandem solar cells. Addition of Cl⁻, for example, aids in passivating Pb-related defect sites, significantly reducing Voc losses.
Another emergent approach involves engineering phase heterojunctions within the perovskite layer. Constructing interfaces between polymorphs such as γ-CsPbI₃ and β-CsPbI₃ introduces staggered band alignments that promote directional charge separation and inhibit recombination. This architectural nuance has facilitated the production of all-inorganic PSCs with remarkably high PCEs (up to 21.5%) and improved thermal robustness, a key factor in device longevity.
Complementing intrinsic absorber engineering, ETL optimization focuses on mitigating electron transport resistance and blocking undesired hole backflow. Innovative heterojunction designs stack multiple ETLs with carefully tiered energy levels, effectively creating an “energy ladder” that guides electrons downward through the device with minimal loss. One notable advancement is the insertion of a cerium oxide (CeOₓ) interlayer between SnO₂ and the perovskite absorber, which eliminates significant conduction band offsets and elevated electron injection efficiency, driving PCEs above 24.6%. Alternative ETL configurations have employed doped oxides like ZnO:Nb and quantum dot composites (e.g., CsPbI₃/PbSe QDs), all converging on the goal of minimizing trap states and suppressing non-radiative recombination.
Material selection within ETLs is equally critical. Inorganic semiconductors such as SnO₂ and TiO₂ feature prominently given their high electron mobility and intrinsic stability under illumination and heat stress. Among these, SnO₂ is particularly favored due to its exemplary electron transport properties (~10 cm² V⁻¹ s⁻¹) and reduced defect density, which collectively translate to higher overall device performance and operational stability. Organic ETLs, including fullerene derivatives like PCBM, offer complementary advantages in terms of favorable energy-level alignment with wide-bandgap perovskites, preserving spectral coverage while enhancing charge extraction rates.
Hole transport layers also demand meticulous energy-level engineering to expedite hole extraction while simultaneously blocking electron leakage. This dual requirement mandates molecular designs that finely tune highest occupied molecular orbital (HOMO) energies to align seamlessly with the perovskite’s valence band. Extending π-conjugation in organic hole transport materials (HTMs) lowers their HOMO level, optimizing energetic overlap. For example, derivatives of Spiro-OMeTAD functionalized with extended thiophene backbones exhibit HOMO offsets below 0.3 eV relative to the VBM, markedly improving hole extraction efficiency.
Beyond backbone structures, the incorporation of donor-acceptor motifs modulates molecular energy levels through intramolecular charge transfer effects. Star-shaped HTMs with triphenylamine donor cores and cyano acceptor groups, such as MPTCZ-FNP, showcase an excellent balance between hole mobility and energy alignment, pushing PSC efficiencies beyond 20%. Furthermore, anchoring groups like phosphonic and carboxylic acids chemically bond to the perovskite surface, inducing beneficial band bending and passivating interfacial defects responsible for trap-assisted recombination. Such molecular anchoring has elevated Voc in NiOₓ-based devices by over 100 mV, underscoring its critical impact.
A burgeoning frontier within HTL engineering is the deployment of self-assembled monolayers (SAMs) at the perovskite interface. These ultrathin, ordered molecular assemblies reduce interface defect density and suppress non-radiative recombination pathways. Mixed SAM systems, including combinations like 2PACz and PyCA-3F, have demonstrated remarkable improvements in inverted PSC architectures, achieving PCEs exceeding 24.6%. The molecular-level precision and stability these layers afford make them indispensable ingredients for next-generation PSCs.
Looking ahead, the review advocates for a system-level integration of these multifaceted strategies. Isolated improvements in absorber composition, ETL/HTL design, or interface passivation must be harmonized to simultaneously boost efficiency, durability, and manufacturability. Tandem architectures exemplify this integrated ethos by pairing wide-bandgap perovskites (e.g., CsPbI₂Br) with complementary low-bandgap absorbers such as silicon or CsSnI₃, circumventing the Shockley-Queisser limit for single junctions and breaching 34% PCE thresholds in experimental setups.
Beyond spectral management, the capture and utilization of hot carriers—electrons and holes retaining high energy immediately post-photon absorption—promise to unlock efficiency gains exceeding conventional limits. The inherently prolonged carrier lifetimes observed in perovskites render them promising candidates for hot carrier solar cells, though practical realization demands significant advances in carrier extraction dynamics and material design.
Equally critical is the stability challenge. While energy-level optimization reduces intrinsic losses, extrinsic factors such as moisture, oxygen, and thermal fluctuations degrade perovskite layers over time. Pairing optimized electronic structures with robust encapsulation techniques and hydrophobic SAM passivation could extend device lifetimes to commercial viability benchmarks exceeding 25 years, thus cementing PSCs as pragmatic alternatives for large-scale energy generation.
The review also underscores the imperative for advanced characterization methodologies capable of real-time energy-level mapping at interfaces. Techniques such as in-situ X-ray photoelectron spectroscopy (XPS) and transient absorption spectroscopy provide invaluable insights into dynamic charge interactions and recombination mechanisms, informing targeted materials engineering. Coupling these experimental tools with quantum-level theoretical modeling accelerates innovation cycles, enabling iterative refinement of PSC components.
As the Nanjing Tech University team continues to unravel the complex interplay of energy flow within PSCs, their findings fuel optimism that the next generation of solar cells will not only eclipse traditional photovoltaics in performance but also achieve unprecedented stability and scalability. Such progress holds transformative potential for global renewable energy infrastructure, marking an essential step toward a sustainable, low-carbon future.
Subject of Research: Perovskite solar cells; energy-level alignment; photovoltaic efficiency optimization; charge transport engineering.
Article Title: Strategies for Enhancing Energy‑Level Matching in Perovskite Solar Cells: An Energy Flow Perspective
News Publication Date: 24-Jun-2025
Web References: 10.1007/s40820-025-01815-z
Image Credits: Xiaorong Shi, Kui Xu, Yiyue He, Zhaogang Peng, Xiangrui Meng, Fayi Wan, Yu Zhang, Qingxun Guo, Yonghua Chen.
Keywords
Energy; Perovskite solar cells; Energy-level alignment; Charge transport; Photovoltaics; Bandgap engineering; Electron transport layers; Hole transport layers; Tandem solar cells; Stability engineering.
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