Shedding light on dark traps

Researchers pinpoint the origin of defects that sap the performance of next-generation solar technology.

We now know what to target to bring up the performances of perovskites.

Samuel Stranks

A multi-institutional collaboration, co-led by scientists at the University of Cambridge and Okinawa Institute of Science and Technology Graduate University (OIST), has identified the source of efficiency-limiting defects in potential materials for next-generation solar cells and LEDs.

In the last decade, perovskites - a diverse range of materials with a specific crystal structure - have emerged as promising alternatives to silicon solar cells, as they are cheaper and greener to manufacture, while achieving a comparable level of efficiency.  

However, perovskites  still  show   significant  performance losses and instabilities , particularly in the specific materials that promise the highest ultimate efficiency.  Most  research to date has focused on ways to remove  these losses , but  their actual physical causes remain unknown.    

Now, i n a paper published in  Nature , researchers from Dr  Sam  Stranks’ s  group at Cambridge ’s  Department of Chemical Engineering and Biotechnology  and Cavendish Laboratory ,  and  Professor  Keshav Dani’s  Femtosecond Spectroscopy Unit  at  OIST  in Japan,  identify the source of the problem. Their discovery could  streamline  efforts to increase  the  efficiency  of perovskites , bringing  them  closer to mass-market production.     

Perovskite materials are much more tolerant of defects in their structure than silicon solar cells, and previous research carried out by  Stranks’ s  group found that to a certain extent,  some heterogeneity in their  composition  actually improves their  performance as solar c ells and light-emitters.   

However, the current limitation of perovskite materials is the presence of a ’ deep trap’ caused by a defect, or minor blemish, in the material.   These are areas in the material where  energised charge  carriers can get stuck and recombine, losing their energy to heat, rather than converting  it  into useful electricity or light. This recombination process can have a significant impact on the efficiency  and stability  of solar panels and LEDs.   

Until now, very little was known about the cause of these traps , in part because they appear to behave differently to traps in traditional solar cell materials.   

In 2015 ,   Stranks   and colleagues  published   a paper in  Science  l ooking  at the luminescence of  perovskites , which  reveals  how good they are  at absorbing or emitting light.  " W e found that  the material  was very heterogeneous ;  y ou had quite  large regions that were bright and  luminescent and other regions that were really dark ,"  said  Stranks.  " These dark regions correspond to power losses in solar cells or LEDs.   But   what was causing th e power loss   was always a mystery ,  especially because   perovskites  are otherwise so defect - tolerant. "  

Due to limitations of standard imaging techniques, the group  couldn’t  tell if the darker areas were caused by one, large  trap site , or many smaller traps, making it difficult to establish why they were forming  only  in certain regions.   

In 2017,  Dani’s group at OIST  made   a  movie of how electrons  behave   in  semiconductors  after absorbing light. " You can learn a lot from being able to see how charges move in a material or device after shining li ght.  For example, you c an  see where they might be getting trapped,"   said Dani.  " However,  these  charges  are hard to visualise as they  move very fast - on the timescale of a millionth of a billionth of a second;  and over very short distances - on the length   scale of a  billionth of a  met r e. "   

On hearing of  Dani’s work ,  Stranks  reached out to see if they could  work together to  address  the problem  visuali s ing  the dark regions in  perovskites.   

The team at OIST used a technique called  photoemission electron microscopy  (PEEM)   for the first time on perovskites ,  where they probed the material with ultraviolet light and built up an image based on how the  emitted  electrons scattered.   

When they looked at the material, t hey   found  that the   dark regions contained  traps ,   around  10-100 nanometers in length,  which  were clusters of smaller atomic-sized trap sites. These trap clusters were spread unevenly throughout the perovskite material, explaining  the heterogeneous luminescence seen in  Stranks’s earlier research.  

When the researchers overlaid images of the trap sites onto images that showed the crystal grains of the perovskite material, they found that the trap clusters only formed at specific places, at the boundaries between certain grains.  

To  understand why this  only  occurred at certain grain boundaries , the group s  worked  together  with Professor Paul Midgley’s team from  Cambridge’s  Department of M a terials Science and Metallurgy  using  a technique called   scanning electron  diffraction  to  create detailed images of the perovskite crystal structure.   The project  team made use  of  the  electron  microscopy setup at the e PSIC   facility  at the Diamond Light Source Synchrotron ,   which has  specialised  equipment for  imaging  beam-sensitive  materials , like perovskites.   

"Because  these materials are   very  beam - sensitive,  typical techniques that you would use  to probe local crystal structure on these length scales   will  quite quickly  change  the  material as you’re looking at it ,  which can make interpreting the data very difficult, " said   Tiarnan  Doherty , a PhD student in   Stranks ’ s  group and  co-l ead author of the study.   " Instead, we were able to use  very low exposure doses and  therefore  prevent  damage.   

" From the  work at OIST , w e knew where the  trap  clusters  w ere  located ,  and at  ePSIC ,  we  scanned   around  those  same area s  to see  the local structure.   W e were  then  able to quickly pinpoint unexpected variations in the crystal  structure  around the  trap  clusters. "  

The  group discovered that the trap clusters  only formed   at junctions where an area of the material with slightly distorted structure met an area with pristine structure.  

"In perovskites ,  we have regular mosaic grains of material and m ost of t he grains are nice and pristine - the structure we would expect,"  said  Stranks.  " But e very now  and again ,  you get a  grain that’s slightly distorted and the chemistry of that  grain  is inhomogeneous. W hat was really interesting and  which initially confused us  was that  it’s not  the distorted grain  that’s the trap  but  whe re  that  grain meets a pristine grain; it’s at that junction that the  traps  cluster. "   

With this understanding of the nature of the traps ,  the team   at OIST  also u sed  the  custom-buil t   PEEM  instrumentation  to   visualise the dynamics of the charge carrier trapping process happening in the perovskite material.   " This was possible as o ne of the unique features of our PEEM setup is  that it can   image   ultra fast processes -  as short as femtoseconds ," said  Andrew Winchester, a PhD student  in Dani ’s  Unit, and  co- lead author of this study. " We   found  that the trapping process was dominated by  charge carriers  diffusing to the trap clusters. "  

The se  discover ies   represent  a breakthrough in the quest to bring perovskites to the solar energy market.   

" We  still  don’t know exactly why  the traps are  clustering there ,   but  we no w  know  that they do form there, a nd  seemingly  only there ,"  said   Stranks.  " T hat’s exciting because it means  we  now  know what to target to bring   up  the performances  of perovskite s. W e need to ta rget those inhomogeneous phases or  get rid of these junctions  in some way. "  

"The fact that charge carriers must first diffuse to the traps could also suggest other strategies to improve these devices," said Dani. "Maybe we  could alter or control the arrangement of the trap clusters, without necessarily changing their average number, such that charge carriers are less likely to reach these defect sites."   

The  team s ’  research focused on one particular perovskite structure.  The scientists  will now be investigating whether the cause of these trapping clusters is universal across  other  perovskite materials.   

"Most of the progress in device performance has been  trial and error  and so far ,   this has been quite an inefficient process ,"  said  Stranks. " To date ,  it really hasn’t been driven by  knowing a specific cause and  systematically  targeting that.  This is one of the first breakthroughs  that  will help us to use the fundamental science to   engineer more efficient devices."  

Tiarnan A.S. Doherty et al. ’ Performance-limiting nanoscale trap clusters at grain junctions in halide perovskites.’ Nature (2020). DOI: 10.1038/s41586-020-2184-1

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