Introduction

This research delves into how chemical reactions between PEDOT:PSS as a hole transport layer and the perovskite active layer lead to device performance loss, proposing a new iodine reducer strategy to address this issue.

By incorporating an iodine reducer (BHC) into lead-tin mixed perovskite solar cells, a breakthrough power conversion efficiency (PCE) of 23.2% was achieved, ranking among the highest efficiencies for lead-tin perovskite solar cells.

The study found that while thiocyanate addition can improve device efficiency, it forms toxic cyanide in humid environments, accelerating device aging.

This research reveals that controlling iodine content is crucial for improving lead-tin perovskite device efficiency and stability under environmental conditions.

Research Team

This research was conducted through collaboration among researchers from multiple institutions, including the University of Surrey, Imperial College London, Sichuan University, and the National Renewable Energy Laboratory. Key authors include:

• First Authors:
  • W. Hashini K. Perera (University of Surrey)
  • Thomas Webb (Imperial College London)
• Corresponding Authors:
  • K. D. G. Imalka Jayawardena (University of Surrey)
  • Saif A. Haque (Imperial College London)
  • S. Ravi P. Silva (University of Surrey)
• Contributing Authors:
  • Dewei Zhao, Yuliang Xu, Jingwei Zhu (Sichuan University)
  • Yundong Zhou, Gustavo F. Trindade, Yunlong Zhao (National Physical Laboratory, NPL)
  • Mateus G. Masteghin, Wei Zhang, Steven J. Hinder (Advanced Technology Institute, University of Surrey)
  • Linjie Dai, Samuel D. Stranks, Sanjayan Sathasivam (University of Cambridge)
  • Steven P. Harvey (National Renewable Energy Laboratory, NREL)
  • Sandra Jenatsch (Fluxim AG)
  • Thomas J. Macdonald (Department of Chemistry, Imperial College London)

Research Background

• Overview: Perovskite solar cells (PSCs) have attracted significant attention due to their high efficiency and low cost. However, device stability remains a key bottleneck limiting their commercial application.
• Current Achievements: Traditional lead-based perovskite solar cells have achieved efficiencies exceeding 26.7%, while narrow bandgap lead-tin mixed perovskite solar cells have reached 23.6%, crucial for achieving higher efficiency all-perovskite tandem cells.
• Technical Challenges: While thiocyanate additives in the perovskite absorption layer can enhance device stability in inert environments, these devices remain susceptible to degradation under environmental conditions, particularly in high humidity.
• Material Limitations: PEDOT:PSS, a commonly used hole transport material, undergoes unfavorable chemical reactions when in contact with perovskite materials, leading to device performance loss.

Solution

• Comprehensive Analysis: Through chemical, electronic, and computational methods, the research investigated PEDOT:PSS-perovskite interface reactions and the role of thiocyanate additives in device performance and stability.
• Key Findings: The study revealed that organic amines from the perovskite active layer diffuse into the PEDOT:PSS layer, causing dedoping and producing corrosive iodine and triiodide (I3-), affecting device performance and stability.
• Thiocyanate Effects: While thiocyanate additives effectively suppress amine diffusion and reduce PEDOT:PSS interface dedoping, they form cyanide under high humidity conditions, accelerating perovskite degradation.
• Novel Strategy: Based on these findings, the research proposes incorporating iodine reducers into the perovskite absorption layer to suppress cyanide formation, thereby improving device efficiency and stability.

Experimental Process and Steps

This research employed the following experimental steps to verify the proposed solutions:

• Device Fabrication: Researchers prepared inverted p-i-n structure perovskite solar cells with a device structure of ITO/PEDOT:PSS/perovskite/C60/BCP/Ag, using ITO/PTAA/perovskite/C60/BCP/Ag as reference devices. To eliminate complex factors such as tin precursor composition changes, phase separation, and instability, methylammonium lead iodide (MAPI) was initially used as the absorption layer material.
• Additive Introduction: Building upon the PEDOT:PSS foundation, researchers added lead thiocyanate (Pb(SCN)2) or guanidinium thiocyanate (GASCN) respectively, and optimized the additive concentrations.
• Iodine Reducer Introduction: In lead-tin mixed perovskite devices, researchers added the iodine reducer – benzhydrazine chloride (BHC) to the Pb-Sn precursor solution.
• Stability Testing: To evaluate the stability of different device structures, researchers conducted tests under ISOS-D-1 (dark, 65% relative humidity) and ISOS-D-1I (dark, nitrogen environment) conditions.
• Interface Reaction Mechanism Study: To gain a deeper understanding of the chemical reaction mechanisms at the PEDOT:PSS-perovskite interface, researchers employed a series of characterization techniques, including absorption spectroscopy, mass spectrometry, X-ray photoelectron spectroscopy, grazing incidence X-ray diffraction, and time-of-flight secondary ion mass spectrometry.
• Theoretical Calculations: Researchers utilized density functional theory (DFT) to calculate the thermodynamic parameters of relevant chemical reactions to gain a deeper understanding of experimental phenomena.

Research Characterization

This research utilized multiple characterization techniques to investigate the chemical reactions at the PEDOT:PSS-perovskite interface and the effects of iodine reducers on device performance.

A. Current-Voltage Characteristics (J-V) and Solar Simulator

Researchers first used Enlitech SS-X AM1.5G standard spectrum solar simulator and current-voltage (J-V) measurement system to evaluate the photovoltaic performance of different device structures, focusing on key parameters including open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and power conversion efficiency (PCE).

• As shown in Figure 2c and Figure 2d, PTAA-based devices achieved 19% PCE, while PEDOT:PSS-based devices showed lower PCE at 11.4%, indicating high recombination losses at the PEDOT:PSS interface.
• Adding thiocyanate to PEDOT:PSS devices significantly improved PCE, with enhancement in all device parameters.
• In lead-tin mixed perovskite solar cells, BHC addition increased PCE from 21.86% to 23.18%, with Jsc reaching 31.84 mA cm-2, Voc of 0.875 V, and FF of 83.23% (Figure 5a and Table 2).
  
• Steady-state efficiency measurements showed higher steady-state output power for BHC devices at 22.87%, compared to 21.64% for control devices (Figure 5b).
  
• Dark J-V curve measurements indicated reduced dark saturation current density (J0) in BHC devices, suggesting decreased leakage current and smaller charge transfer barriers (Figure 5d).
  

Enlitech SS-X AM1.5G standard spectrum solar simulator was used for testing

B. External Quantum Efficiency (EQE) and QE-R PV/Solar Cell Quantum Efficiency Optical System

External quantum efficiency (EQE) measurement is a crucial method for evaluating the efficiency of solar cells in generating charge carriers under different wavelengths of light. Researchers used a quantum efficiency measurement system to obtain EQE spectra of different device structures and calculated the integrated current density (JEQE) corresponding to Jsc through integration of the EQE spectra. 

• As shown in Figure 2d, PEDOT:PSS devices exhibited significant photocurrent loss across the entire absorption range, further confirming high recombination losses at the PEDOT:PSS interface.
• Thiocyanate addition significantly improved EQE spectra and JEQE of PEDOT:PSS devices, indicating effective suppression of interface recombination losses.
• In lead-tin mixed perovskite devices, BHC addition significantly enhanced EQE response in the 415-600 nm wavelength range, increasing JEQE from 30.98 mA cm-2 to 31.56 mA cm-2, consistent with J-V measurement results (Figure 5c).
  

C. Other Characterizations

In addition to J-V and EQE measurements, this study employed the following characterization techniques to investigate the mechanisms of device performance enhancement:

• • Transient Absorption Spectroscopy (TAS): TAS measurements of carrier dynamics showed that thiocyanate addition reduced trap state density at the PEDOT:PSS/MAPI interface, suppressing non-radiative recombination losses (Figure S5).
    
  • Transient Photocurrent (TPC) and Intensity-Modulated Photocurrent Spectroscopy (IMPS): Results showed improved charge extraction efficiency and suppressed ion migration in PEDOT:PSS devices with thiocyanate addition (Figure S6).
    
  • Electrochemical Impedance Spectroscopy (EIS): Measurements showed increased recombination resistance in BHC-treated lead-tin mixed perovskite devices, indicating effective suppression of non-radiative recombination (Figure 5e).
    
  • X-ray Photoelectron Spectroscopy (XPS): Results revealed metallic lead (Pb0) formation at the PEDOT:PSS/MAPI interface without thiocyanate addition, indicating iodide ion (I-) loss (Figure S11 and Figure S12).
    
  • Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS): Measurements confirmed organic amine diffusion into the PEDOT:PSS layer, which was suppressed by thiocyanate addition (Figure 3g-i).
    
  • Density Functional Theory (DFT) Calculations: Results showed thermodynamically favorable reactions between organic amines and PSS, explaining PEDOT:PSS dedoping phenomena (Figure 4c).
    
  • Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS): GIWAXS is a technique used to characterize the crystal structure and orientation of thin film materials. Researchers utilized GIWAXS to analyze the crystal structure of perovskite thin films with different device architectures, discovering that thiocyanate addition promotes perovskite grain growth, thereby enhancing device performance (Figure S22). GIWAXS data analysis revealed high PbI2 concentration at the PEDOT:PSS/MAPI interface without thiocyanate addition (Figure 3f). This indicates that organic amines react with PSS, leading to PbI2 accumulation at the interface.
    
    GIWAXS data analysis revealed high PbI2 concentration at the PEDOT:PSS/MAPI interface without thiocyanate addition (Figure 3f). This indicates that organic amines react with PSS, leading to PbI2 accumulation at the interface.
    
• Scanning Electron Microscopy (SEM): Observations showed improved perovskite film morphology and reduced defects with thiocyanate addition (Figure S9).
  
• Nuclear Magnetic Resonance (NMR): Analysis of thiocyanate-iodine reaction products revealed cyanide formation in humid environments, explaining accelerated device aging with thiocyanate additives (Figure S41).
  

Research Results

The main achievements of this research can be summarized as follows:

• Interface Reaction Mechanism: The study revealed the chemical reaction mechanism at the PEDOT:PSS-perovskite interface. Research found that organic amines diffuse into the PEDOT:PSS layer, leading to PEDOT:PSS dedoping and producing corrosive iodine and triiodide (I3-), thereby affecting device performance and stability.
• Thiocyanate Additive Mechanism: The research clarified that thiocyanate additives can effectively suppress amine diffusion, reduce PEDOT:PSS interface dedoping, and remove corrosive iodine. However, under high humidity conditions, thiocyanate forms cyanide, accelerating perovskite degradation.
• Novel Iodine Reducer Strategy: By incorporating the iodine reducer BHC into the lead-tin mixed perovskite absorption layer, cyanide formation can be suppressed, thereby improving device efficiency and stability.

The research results demonstrate that adding benzhydrazine chloride (BHC) as an iodine reducer in Pb-Sn perovskite solar cells can effectively enhance device efficiency and stability.

After adding BHC, the device achieved a maximum power conversion efficiency (PCE) of 23.18% and steady-state efficiency of 22.87%, representing increases of 1.32% and 1.23% respectively compared to the control devices without BHC.

This improvement is attributed to BHC’s ability to reduce iodine (I2) to iodide ions (I-), thereby reducing carrier recombination and enhancing device performance. Furthermore, after adding BHC, the device’s T80 lifetime (time required for efficiency to decay to 80% of its initial value) increased by approximately 66%, reaching 83 hours. This demonstrates that using BHC as an iodine reducer can effectively enhance the long-term stability of Pb-Sn mixed perovskite solar cells.

The study also found that while thiocyanate additives can improve device performance by reducing PEDOT:PSS dedoping, they hydrolyze to form corrosive cyanide in humid environments, accelerating device degradation.

Therefore, using BHC as an iodine reducer can effectively avoid stability issues associated with thiocyanate additives, providing a new strategy for the development of Pb-Sn mixed perovskite solar cells.