April 24, 2019

Video: Analysis of techniques for measuring carrier recombination lifetime


Author:  Dr. Richard K. Ahrenkiel is a Research Professor of Metallurgical and Materials Engineering at the Colorado School of Mines in Golden, Colorado.

Abstract

Rapid, accurate and contactless measurement of the recombination lifetime is a very important activity in photovoltaics. The excess carrier lifetime (Δn(t)) is the most critical and variable parameter in the development of photovoltaic materials. Device performance can be accurately predicted from the lifetime measurement of the starting material. However, there is no single measurement that directly measures the bulk lifetime as all measurements are based on a device model.

A primary issue is that the lifetime is a function of excess carrier lifetime, and measurements must be linked to an injection level. The most common measurements are based on either photoconductive (PCD) or photoluminescence (TRPL) decay. PC decay senses the product of excess carrier concentration (Δn) and mobility (μ (Δn)). This mobility variation must be included in order to extract the true excess carrier lifetime. TRPL works best for direct band gap materials and therefore is not applicable to silicon. For polycrystalline materials, shallow traps distort the measurement and must be included in the analysis of the data. Finally, surface and interface recombination have a profound influence on most measurements and must be minimized for accurate measurement of the true bulk lifetime.


Both techniques and analysis methods will be discussed in this seminar. Typical sample measurements will be shown, including representative thin film and wafer materials that are currently popular in the photovoltaic community.

Richard Keith Ahrenkiel (2013), "Analysis of Techniques for Measuring Carrier Recombination Lifetime," https://nanohub.org/resources/19884

April 23, 2019

Paper: Complex Interplay between Absorber Composition and Alkali Doping in High-Efficiency Kesterite Solar Cells

Title: Complex Interplay between Absorber Composition and Alkali Doping in High-Efficiency Kesterite Solar Cells
Authors: Stefan G. Haass,* Christian Andres, Renato Figi, Claudia Schreiner, Melanie Bürki, Yaroslav E. Romanyuk, and Ayodhya N. Tiwari
Link (Open Acess): Adv. Energy Mater. 2018, 8, 1701760

Abstract:
Sodium treatment of kesterite layers is a widely used and efficient method to boost solar cell efficiency. However, first experiments employing other alkali elements cause confusion as reported results contradict each other. In this comprehensive investigation, the effects of absorber composition, alkali element, and concentration on optoelectronic properties and device performance are investigated. Experimental results show that in the row Li–Na–K–Rb–Cs the nominal Sn content should be reduced by more than 20% (relative) to achieve the highest conversion efficiency. The alkali concentration resulting in highest device efficiencies is lower by an order of magnitude for the heavy alkali elements (Rb, Cs) compared to the lighter ones (Li, Na, K). Utilization of a wide range of characterization techniques helps to unveil the complex interplay between absorber composition and alkali doping. A ranking of alkali for best device performances, when employing alkali treatment, resulted in the order of Li > Na > K > Rb > Cs based on the statistics of more than 700 individual cells. Finally, a champion device with 11.5% efficiency (12.3% active area) is achieved using a high Li concentration with an optimized Sn content.

Highlights:
  • Best published solar cell CZTSe: 12.6 % by IBM and DGIST (0.4 - 0.5 cm2 active area)
  • Solution process deposition technique [14]
  • The secondary phase Sn(S,Se)2 can be identify from XRD when Sn nominal content is > 33.3% 
  • The formation of the second phase tin selenide is influenced by the type of concentration of alkali elements 
  • Minority carriers trapping, surface effects and energetic relaxation of carriers has been identified to severely affect the PL transition times. Thus the measurement of transition decay times does not represent the real minority carriers lifetime in the kesterite absorber layer. 
    • The champion solar cell has high Li content (3.3%) and 33.3% of Sn nominal concentration of 33.3 %.  11.55 % with metal electrodes and 12.3 without a metal grid. Area = 0.29 cm2. 
  • A ranking of best device performances employing alkali treatment resulted in the order of Li > Na > K > Rb > Cs based on the statis- tics of more than 700 individual cells. 



Characterization techniques: 

 Material 
  • ICP-MS (Inductively coupled plasma-mass spectroscopy), detect alkali content in the absorber layer.
  • SEM (Scanning electron microscopy)
  • XRD (X-ray diffraction): To understand the device performance reduction at high Sn content
  • XRF (X-ray fluorescence)
Solar cell 
  • JV (current-voltage)
  • C-V (capacitance-voltage): Apparent carrier concentration and depletion region width.
  • TRPL (Time-resolved photoluminescence): 639 nm pulse diode laser , 90ps pulse width and 10 MHz 
  • EQE (External quantum efficiency)
Device fabrication:
  1. Substrate: Soda lime glass
  2. Barrier: SiOx alkali diffusion barrier layer sputtered onto a 1 mm thick glass substrate
  3. Back contact: 1 µm molybdenum
  4. Absorber: CZTSe precursor solution was spin-coated onto the back contact and dried n a hot plate at 320 ºC. (12 times to obtain 1.5µm thickness) 
  5. Selenization: RTP (Rapid thermal annealing) inside a close graphite box with selenium pellets (800 mg)
  6. Etching: After selenization, the Mo/CZTSe has been immersed in 10% KCN solution  (30 seconds) in order to remove copper-rich phases and clean the surface from contamination and oxides. 
  7. Buffer: 50-70 nm of CdS, (Chemical deposition)
  8. Front contact: 70/250 nm of i-ZnO/Al:ZnO bilayer (Sputtering process)  
  9. Grid contact: Ni/Al grid contact (evaporation PVD?)
  10. Anti-reflective: MgF2 anti-reflective coating (e-beam evaporation)

Notes: 
  • Alcaly High doping ratio of 3.3% (100mM)  (related to all elements present in the solution of reaction).  Doping is necessary for increased efficiency but it depends on Sn content as seen in the above image. 


Disclaimer: The intention of this post is to bring some personal notes of the literature review. Open access, follow the link to fin the PDF. 


April 22, 2019

Paper: How small amounts of Ge modify the formation pathways and crystallization of kesterites

Ge incorporation on Kesterites

Title:
How small amounts of Ge modify the formation pathways and crystallization of kesterites
Authors: S. Giraldo, E. Saucedo, M. Neuschitzer, F. Oliva, M. Placidi, X. Alcobe´, V. Izquierdo-Roca, S. Kim, H. Tampo, H. Shibata, A. Pérez-Rodríguez and P. Pistor

Link: Energy Environ. Sci., 2018, 11, 582-593

Abstract: 


The inclusion of Ge into the synthesis of Cu2ZnSn(S,Se)4 absorbers for kesterite solar cells has been proven to be a very efficient way to boost the device efficiency in a couple of recent publications. This highlights the importance to elucidate the mechanisms by which Ge improves the kesterite solar cells properties to such a large extent. In this contribution, we first show how controlling the position and thickness of a very thin (10–15 nm) layer of Ge greatly influences the crystallization of kesterite thin films prepared in a sequential process. Typically, Cu2ZnSnSe4 (CZTSe) films form in a bi-layer structure with large grains in the upper region and small grains at the back. By introducing Ge nanolayers below our precursors, we observe that large CZTSe grains extending over the whole absorber thickness are formed. Additionally, we observe that Ge induces fundamental changes in the formation mechanism of the kesterite absorber. In a detailed analysis of the phase evolution with and without Ge, we combine the results of X-ray fluorescence, X-ray diffraction, and Raman spectroscopy to demonstrate how the Ge influences the preferred reaction scheme during the selenization. We reveal that the presence of Ge causes a large change in the in-depth elemental distribution, induces a stabilizing Cu–Sn intermixing, and thus prevents drastic compositional fluctuations during the annealing process. This finally leads to a change from a tri-molecular towards, mainly, a bi-molecular CZTSe formation mechanism. Kesterite thin films with surprisingly large crystals of several microns in diameter can be fabricated using this approach. The results are related to the increase in device performance, where power conversion efficiencies of up to 11.8% were obtained. Finally, the consequences of the disclosed crystallization pathways and the extension to other chalcogenide technologies are discussed

Highlights:

  • Kesterite solar cell record efficiency of 12.6% (2018) [1]
  • Advantages like earth abundant and non-toxic materials of CZTSe will success if the technology reaches 20% efficiency and be ready for industrial manufacturing. 
  • Disadvantages: Low Voc (Open circuit voltage is an indirect measurement of the recombination process of the solar cell, following Shockley design). 
    • Potential fluctuations
    • Band tailing
    • Disorder defects 
    • Interface recombination 
    • Secondary phases 
    • Compositional inhomogeneities
  • The presence of Ge drastically modifies the reaction pathway in which the kesterite is formed
  • The beneficial effects of Ge incorporation are not limited to some surface modifications it affects the whole bulk of the absorber. 
  • The observed improvement should be located on the absorber bulk due to an increase in the charge carrier lifetime. 

Characterization Techniques:

  • Materials
    • SEM (5 keV) 
    • Thickness by SEM (2µm of the absorber layer)
    • EDX (20 kV)
    • XRS (Brag-Brentano configuration, 4-145º, step 0.017º, )
    • Raman (excitation wavelengths: 633 nm, 532 nm, 488 nm )
    • XRF (X-ray fluorescence) to determine overall composition and thickness
  • Solar cell
    • JV curve (Standard parameters)
    • EQE (Increase due optimization at the bulk of the absorber)
    • Voc vs T  (Activation Energy of recombination process)

Relevant information:

  • Heat treatment to induce grain growth is crucial for a better solar cell. Then Ge assisted crystallization process affect the whole bulk absorber. 
  • Goal: Increase Voc on the device is the challenge to increase efficiency > 12%. 
  • Goal: Detect the dominant recombination mechanism.


Disclaimer: The intention of this post is to bring some personal notes of the literature review. I'm not sharing the PDF files. For that purpose, please ask the authors or follow the link to the journal.