Perovskite solar cells (PSCs) have rapidly emerged as a front-runner in next-generation photovoltaic technologies, boasting a certified power conversion efficiency (PCE) of 26.95% -- now rivaling crystalline silicon and CIGS cells. Yet, a critical bottleneck remains: energy losses stemming from mismatched energy levels between the perovskite absorber and charge transport layers (electron transport layers, ETLs; hole transport layers, HTLs), which hinder charge separation and transport. To address this, a team of researchers from Nanjing Tech University has published a landmark review in Nano-Micro Letters, systematically analyzing strategies to optimize energy-level alignment in PSCs from an energy flow perspective. This work offers a unified framework for minimizing energy loss and guiding the design of high-performance, stable PSCs.
Why Energy-Level Matching Matters for PSCs
At the heart of PSC inefficiency lies poor coordination between the energy levels of the perovskite absorber (ABX structure) and its adjacent transport layers. When energy levels are misaligned, three critical issues arise:
The review emphasizes that resolving these issues requires a holistic understanding of energy flow -- from photon absorption in the perovskite to charge extraction via ETLs/HTLs. By optimizing energy-level alignment, researchers can unlock PSCs' full potential, bridging the gap between theoretical and experimental efficiencies.
Core Strategies for Energy-Level Optimization
The review categorizes energy-level tuning strategies into two key areas: optimizing the perovskite absorber itself and engineering ETL/HTL interfaces. Each approach targets specific energy loss pathways, with real-world case studies validating their effectiveness.
1. Perovskite Absorber Engineering: Tuning Bandgaps and Stability
The perovskite's ABX crystal structure (A: monovalent cation, B: divalent cation, X: halide anion) is highly tunable, enabling precise control of its bandgap and energy levels:
* A-Site Cation Tuning: Replacing methylammonium (MA⁺) with formamidinium (FA⁺) or cesium (Cs⁺) adjusts the perovskite's tolerance factor (T), stabilizing the photoactive cubic phase while modifying the valence band maximum (VBM) and conduction band minimum (CBM). For example, FAPbI (bandgap ~1.48 eV) -- closer to the ideal 1.4 eV for solar absorption -- achieves a record PCE of 26.95% by balancing spectral absorption and Voc.
* Halide Alloying: Mixing I⁻, Br⁻, and Cl⁻ anions fine-tunes the bandgap. For instance, CsPbIBr (bandgap ~1.9 eV) is ideal for tandem cells, while CsPbI (doped with Cl⁻ via additives like CSE) reduces Voc loss by passivating Pb-related defects.
* Phase Heterojunctions: Creating interfaces between different perovskite phases (e.g., γ-CsPbI/β-CsPbI) forms staggered band alignments, enhancing charge separation. This strategy has yielded all-inorganic PSCs with PCEs up to 21.5%.
These modifications not only optimize energy levels but also improve stability -- addressing a long-standing challenge for PSCs. For example, Cs-doped FA-based perovskites resist phase transitions under heat and moisture, extending device lifetimes.
2. ETL Engineering: Reducing Electron Transport Barriers
ETLs play a critical role in extracting electrons from the perovskite's CBM while blocking hole backflow. The review highlights two dominant strategies for ETL optimization:
* Heterojunction Design: Stacking ETLs with stepped energy levels creates "energy ladders" that facilitate electron transfer. For example, inserting a CeO interlayer between SnO and the perovskite reduces the electron barrier, boosting PCE to 24.63%. Other effective heterojunctions include intercalated (TiO/SnO), doped (ZnO:Nb), and quantum dot (CsPbI/PbSe QDs) structures -- all of which minimize recombination at the ETL-perovskite interface.
* Material Selection: Inorganic ETLs (e.g., SnO, TiO) offer high electron mobility and stability, while organic ETLs (e.g., PCBM) provide better energy-level matching with wide-bandgap perovskites. SnO, in particular, outperforms TiO due to its higher electron mobility (~10 cm V s) and lower defect density.
3. HTL Engineering: Optimizing Hole Extraction
HTLs must efficiently extract holes from the perovskite's VBM while preventing electron leakage. The review details how molecular design and interface modification drive HTL performance:
* Molecular Structure Tuning:
* Self-Assembled Monolayers (SAMs): SAMs (e.g., DC-TMPS, 2PACz) form ordered layers at the HTL-perovskite interface, reducing non-radiative recombination. Mixed SAMs (e.g., 2PACz/PyCA-3F) have yielded inverted PSCs with PCEs up to 24.68%.
Future Outlook: Toward System-Level Optimization
While individual strategies have made significant gains, the review argues that future progress will require system-level engineering -- integrating multiple approaches to address efficiency, stability, and scalability simultaneously:
* Tandem Cells: Combining wide-bandgap perovskites (e.g., CsPbIBr) with silicon or low-bandgap perovskites (e.g., CsSnI) bypasses the S-Q limit, with recent tandem cells achieving PCEs >34%.
* Hot Carrier Management: Capturing high-energy electrons before they cool could exceed the S-Q limit. Perovskites' long carrier lifetimes make them ideal for this approach, though practical implementation remains a challenge.
* Stability Engineering: Pairing energy-level optimization with encapsulation and defect passivation (e.g., using hydrophobic SAMs) will extend PSC lifetimes to meet commercial standards (25+ years).
The review also highlights the need for more precise characterization tools -- such as in-situ XPS and transient absorption spectroscopy -- to better understand energy dynamics at interfaces. By merging theoretical modeling with experimental validation, researchers can accelerate the development of next-generation PSCs.
Stay updated on the latest breakthroughs from the Nanjing Tech University team as they continue to push the boundaries of perovskite photovoltaics!