Demystifying the dominant composite mechanism of thin-film solar cell carriers

At 23:00 on July 21, 2022 Beijing time, Nature Energy, a top international academic journal, published the research results of Professor Hao Xiaojing’s team of Professor Hao Xiaojing of the School of Photovoltaic and Renewable Energy at the University of New South Wales in Australia entitled “Unveiling microscopic carrier loss mechanisms in 12% efficient Cu2ZnSnSe4 solar cells”.

By combining microscopic and macroscopic materials, performance characterization analysis methods and 3D device simulation, the research team established a framework for systematic analysis of the microscopic carrier loss mechanism of polycrystalline thin-film solar cells, which can effectively reveal the key characteristics of the transport and recombination of carriers in grain and grain surface, and diagnose the dominant loss mechanism that limits the performance of solar cells. Using this method, the research team successfully revealed that the current carrier recombination mechanism that limits the efficiency of copper-zinc-tin-sulfur selenium solar cells is grain boundary recombination, rather than bulk phase recombination.

The corresponding authors of the paper are Li Jianjun and Hao Xiaojing; The first authors are Li Jianjun, Huang Jialiang, and Ma Fajun.

Photovoltaic technology is considered to be one of the most attractive clean energy sources to combat global climate change and achieve carbon neutrality. Thin-film photovoltaic technology has attracted wide attention due to its many advantages such as flexibility, low production cost, short energy recovery period, and wide range of application scenarios. Thin-film photovoltaic technology that can be applied on a global scale requires photovoltaic materials that are as rich, stable, environmentally friendly and efficient as silicon. This has spawned extensive worldwide exploration of new inorganic photovoltaic materials. Among them, copper-zinc-tin-thio-selenium (CZTSSe) is considered to be one of the most promising green thin-film photovoltaic materials.

In order to ensure low-cost manufacturing, these thin-film solar cells basically use polycrystalline thin-film materials as light absorbers, and use heterojunction as the PN junction that drives the separation of photogenerated carriers. There are a large number of non-ideal crystal structures such as grain boundaries and heterojunction interfaces in such polycrystalline thin-film solar cells. These regions may have a large number of in-band contained electron states due to periodic disruption, which become carrier traps, scattering and composite centers, which can seriously restrict the performance of such solar cells. Therefore, the performance of polycrystalline thin-film solar cells depends largely on the recombination and transmission mechanism of photogenetic carriers in microscopic regions such as grain boundaries and grain surfaces. However, due to the complex material properties of multiple compounds and the structure of polycrystalline heterojunction devices, it is extremely difficult to systematically analyze the microscopic carrier compound loss mechanism of novel thin-film solar cells such as copper-zinc-tin-thio-selenium (CZTSSe), making it impossible to accurately obtain critical information limiting their photoelectric conversion efficiency. This has also become one of the most important reasons for limiting the rapid improvement in efficiency of new thin-film solar cells such as copper-zinc-tin-sulfur selenium (CZTSSe) in recent years.

Recently, Hao Xiaojing’s team from the University of New South Wales in Australia, together with Professor Mai Yaohua of Jinan University, Professor Li Hui of the Institute of Physics of the Chinese Academy of Sciences, and Professor Thomas Unold of the Helmholtz Berlin Center (HZB) in Germany, developed a systematic framework for the microscopic carrier recombination mechanism of polycrystalline thin-film solar cells, and revealed its dominant composite mechanism with copper-zinc-tin selenium (CZTSe) as an example. The research team used a variety of characterization methods to reveal microscopic information such as material structure, energy band shift, carrier transport and composite in the grain, grain boundaries and anterior and posterior surfaces of the absorbent thin film. Then, by constructing the grain boundary effective fewer-son life, these microscopic characteristics are connected with the information obtained by the macroscopic test, and the key information such as grain boundary composite rate, electrostatic potential fluctuation, and grain boundary band bending are obtained. Enter the above information into the 3D device simulation model constructed according to the CZTSe cell geometry, and further determine the range of electrons and holes in the CZTSe grain by simulating IV and EQE, and determine the key factors that currently limit the photoelectric conversion efficiency, as well as the roadmap to obtaining 20% efficiency. The test and analysis framework can be applied to a variety of complex multi-compound polycrystalline thin film solar cells, and can be used as a dynamic diagnostic analysis method to find out the key performance constraints of various types of thin film solar cells at different stages of development, condense the research strength in the academic field, and promote the rapid development of thin film solar cells.

Figure 1: Device performance, topography, and elemental distribution. (Image source: Nature Energy)

In this work, the CZTSe of copper-zinc-tin-selenium with an efficiency of 12%-12.5% prepared by magnetron sputtering method was used to study. This efficiency is currently the highest efficiency in the world at CZTSe to ensure that the results of the study are representative of the frontier in the field. Through IV, EQE, SEM, TEM-EDS, SIMS and other tests, the basic information such as device performance, morphology, element distribution and possible secondary phase distribution is obtained.

Figure 2: Structure and carrier transport characteristics of the front and rear interfaces. (Image source: Nature Energy)

Surface Raman surface sweep and high resolution TEM analysis showed that the CZTSe surface had an uneven thickness (4-12 nm) epitaxial growth ZnSe phase, which enabled good interface passivation, and the photocurrent could cross the ZnSe barrier through thermal electron emission at room temperature. The results of the variable temperature IV test show that the composite activation of the heterojunction interface is basically the same as that of the CZTSe optical band gap, and the back interface is quasi-ohmic contact.

Figure 3: Microscopic analysis of the Kelvin probe and analysis of electrostatic potential fluctuations. (Image source: Nature Energy)

Cross-sectional KPFM testing shows that band bending at the CZTSe grain boundary is basically negligible. The KPFM test results also show that the fragmented grains and MoSe2 phases at the bottom of the absorbent layer do not form a barrier to block the collection of holes, and the back electrode is basically ohmic contact. Analysis of the photofluorescence spectrum PL and the internal quantum efficiency IQE showed that the Ubach energy of CZTSe was about 20 meV. The device simulation results show that the hyalpsia composite loss is not the main composite mechanism in these batteries.

Figure 4: Electron beam induced current analysis and carrier concentration analysis. (Image source: Nature Energy)

The electron beam induced current results show that the CZTSe grains in direct contact with the heterojunction interface have good carrier separation and collection. Carriers within grains that fail to come into contact with the heterojunction are almost impossible to collect. Cathodic fluorescence (CL) test results indicate that this is caused by severe grain boundary compounding. After fitting calculations, the grain boundary recombination rate of CZTSe is as high as 104-105 cm s-1, which severely limits the effective oligono life of CZTSe. By calculation, the effective oligodon life measured by TRPL is mainly affected by the high grain boundary recombination rate. The CL wire sweep results show that the band gap at the bottom and surface of the CZTSe absorption layer is slightly higher than in vivo. The CL emission peak energy of the absorbing layer is essentially the same as the optical band gap. The free carrier concentration obtained by driving amplitude capacitance DLCP test and admittance spectroscopy test analysis is 1.8×1015 cm-3, of which the heterojunction interface area is calibrated by AFM test.

Figure 5: Cathodic fluorescence (CL) versus photoluminescence (PL) analysis. (Image source: Nature Energy)

Figure 6: CZTSe battery 3D device simulation. (Image source: Nature Energy)

The simulation results show that the electron and hole mobility in CZTSe grain are 80-100 cm2V-1s-1 and 30-50 cm2V-1s-1, respectively, and the lifespans of electrons and holes are 10-30 ns and 0.3-0.7 ns, respectively. The intra-grain electron life is 1 order of magnitude higher than the electron life measured directly from within the film, and the grain boundary recombination rate is currently the most important factor limiting CZTSe cells. When the grain boundary recombination rate of CZTSe can be reduced to the level of 102 cm s-1, the free carrier concentration can be increased to 5×1016 cm-3, and the life of the oligono can be increased to the level of 100 ns, CZTSe can obtain an efficiency of about 20%. The simulation results give the main development path of low bandgap CZTSSe solar cells in the future. (Source: Science Network)

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