Boosting Energy Match in Perovskite Solar Cells

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 (ABX₃ structure) and its adjacent transport layers. When energy levels are misaligned, three critical issues arise:

1. Charge Recombination: Electrons and holes recombine at interfaces instead of being extracted, wasting energy as heat.
2. Transport Barriers: Mismatched conduction/valence bands create energy barriers that slow charge carrier movement, reducing short-circuit current density (Jsc​) and open-circuit voltage (Voc​).
3. Spectral Underutilization: Improper bandgap tuning limits absorption of the solar spectrum, failing to reach the Shockley-Queisser (S-Q) limit for single-junction cells (~33.7%).

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 ABX₃ 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, FAPbI₃ (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, CsPbI₂Br (bandgap ~1.9 eV) is ideal for tandem cells, while CsPbI₃ (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-CsPbI₃/β-CsPbI₃) 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_x interlayer between SnO₂ and the perovskite reduces the electron barrier, boosting PCE to 24.63%. Other effective heterojunctions include intercalated (TiO₂/SnO₂), doped (ZnO:Nb), and quantum dot (CsPbI₃/PbSe QDs) structures—all of which minimize recombination at the ETL-perovskite interface.
• Material Selection: Inorganic ETLs (e.g., SnO₂, TiO₂) offer high electron mobility and stability, while organic ETLs (e.g., PCBM) provide better energy-level matching with wide-bandgap perovskites. SnO₂, in particular, outperforms TiO₂ due to its higher electron mobility (~10 cm² V^-1 s^-1) 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:
  • Conjugation Length: Extending π-conjugation in organic HTMs (e.g., Spiro-OMeTAD derivatives) lowers the highest occupied molecular orbital (HOMO), aligning it with the perovskite's VBM. For example, linear HTMs with extended thiophene backbones (e.g., SS-6) reduce the HOMO-VBM offset to <0.3 eV, enhancing hole extraction.
  • Donor-Acceptor (D-A) Groups: Incorporating electron-donating (triphenylamine) and electron-withdrawing (cyano) groups in HTMs modulates HOMO/LUMO levels. Star-shaped HTMs like MPTCZ-FNP (with a triphenyl core) achieve PCEs of 20.27% by balancing hole mobility and energy alignment.
  • Anchoring Groups: Phosphonic acid or carboxylic acid groups (e.g., in 2PADBC) bind to perovskite surfaces, passivating defects and inducing energy-level bending. This strategy has increased Voc​ in NiO_x-based PSCs from 0.712 V to 0.825 V.
• 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., CsPbI₂Br) with silicon or low-bandgap perovskites (e.g., CsSnI₃) 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!