Improving Optoelectronic Characterisation Methods to Understand Open-Circuit Voltage and Fill Factor Losses in Wide-Bandgap Perovskite Solar Cells

• Applicant: Sandheep Ravishankar, Ph.D.
• Subject Area: Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering ; Experimental Condensed Matter Physics
• Term: from 2021 to 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 462572437

• Final Report Year: 2024

Final Report Abstract

Perovskite solar cells are showing great promise to become a photovoltaic technology that will contribute significantly to the renewable energy transition, with current laboratory efficiencies already comparable to the best-performing silicon solar cells. These devices combine high power conversion efficiencies with ease of fabrication using solution-processed methods such as inkjet printing, thereby allowing for large-scale manufacturing at low cost. In addition to single junction solar cells, the perovskite can also be incorporated in a siliconperovskite or all-perovskite tandem structure that allows a theoretical maximum power conversion efficiency between 42-45%. Further improvements in the performance of state-of-the-art perovskite solar cells are limited by three key factors – 1) Inefficient charge extraction due to the low mobilities of the transport layers that extract electrons and holes from the perovskite layer, 2) Unfavourable energy alignment at the perovskite/transport layer interfaces that reduce the open-circuit voltage and 3) High non-radiative recombination rates due to significant defect densities at the perovskite/transport layer interfaces. Accurate characterization of these loss mechanisms is therefore of vital importance to determine suitable mitigation strategies. However, this task is complicated due to the inability to isolate the response of a specific loss mechanism due to the overlapping and sometimes similar response of other processes. Therefore, the goal of this project is the development of analysis methods and models to accurately determine the fundamental parameters (such as charge extraction velocities and defect densities) governing the loss mechanisms that limit state-of-the-art, triple cation perovskite solar cells. In this regard, we successfully developed an optoelectronic model that discriminates between charge extraction and recombination and extracts their associated parameters from both time domain and frequency domain measurements. We determined charge extraction velocities between 1-100 cm/s and transport layer mobilities between 10^-4 – 3 x 10^-3 cm2 V-1 s-1 for our perovskite solar cells, thus identifying the mobility of the transport layers as a key factor limiting the performance. We further developed an advanced model to interpret different capacitance measurements of perovskite solar cells, identifying that almost all the defect/doping densities and related parameters reported in the literature using these methods are artefacts generated by other processes occurring in the solar cell. We derived resolution limits to avoid such mischaracterization and determined that the defect densities are quite low in state-of-the-art perovskite solar cells. Thus, the capture coefficients of these defect densities are the main factors that dominate the non-radiative recombination losses and hence require suitable mitigation strategies.

Publications

• Intensity Modulated Photocurrent Microspectrosopy for Next Generation Photovoltaics. Small Methods, 6(9).
  Laird, Jamie S.; Ravishankar, Sandheep; Rietwyk, Kevin J.; Mao, Wenxin; Bach, Udo & Smith, Trevor A.
  
• Interpretation of Mott–Schottky plots of photoanodes for water splitting. Chemical Science, 13(17), 4828-4837.
  Ravishankar, Sandheep; Bisquert, Juan & Kirchartz, Thomas
  
• Multilayer Capacitances: How Selective Contacts Affect Capacitance Measurements of Perovskite Solar Cells. PRX Energy, 1(1).
  Ravishankar, Sandheep; Liu, Zhifa; Rau, Uwe & Kirchartz, Thomas
  
• Comparing Methods of Characterizing Energetic Disorder in Organic Solar Cells. Advanced Energy Materials, 13(15).
  Hartnagel, Paula; Ravishankar, Sandheep; Klingebiel, Benjamin; Thimm, Oliver & Kirchartz, Thomas
  
• How Charge Carrier Exchange between Absorber and Contact Influences Time Constants in the Frequency Domain Response of Perovskite Solar Cells. PRX Energy, 2(3).
  Ravishankar, Sandheep; Liu, Zhifa; Wang, Yueming; Kirchartz, Thomas & Rau, Uwe
  
• Discerning rise time constants to quantify charge carrier extraction in perovskite solar cells. Energy & Environmental Science, 17(3), 1229-1243.
  Ravishankar, Sandheep; Kruppa, Lennard; Jenatsch, Sandra; Yan, Genghua & Wang, Yueming