Researchers fabricate monolithic selenium/silicon tandem solar cell

LAURA LEAY: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on the hot topics in materials research. My name is Laura Leay. Much of the research into improving solar cells has so far concentrated on perovskites and organic materials. A smaller body of research shows promise for one of the oldest studied photvoltaic materials: selenium. Recent work has uncovered a tandem solar cell architecture that can be constructed by depositing the top cell directly onto the bottom cell.

RASMUS NEILSEN: Selenium seems to be very, very easy to get going very quickly without optimizing an awful lot, and I think that holds some really great prospects. It’s a very exciting time to be studying selenium indeed.

LAURA LEAY: That was Dr. Rasmus Neilsen from the Technical University of Denmark. Although previous research into selenium solar cells led to optimization of a single junction architecture, it’s not as simple as layering this on top of silicon. 

RASMUS NEILSEN: The device architecture that has been optimized by IBM researchers in 2017 was actually not transferable to a tandem because as soon as we had to either invert the device architecture or just introduce transparent electrodes instead of gold, the energy band alignment turned out to be quite severe and we got transport issues.

LAURA LEAY: Selenium-based single-junction solar cells have traditionally used fluorine-doped tin oxide. In this work indium-tin oxide was used as transparent conductive layer which is easier to deposit and its use is more widespread. Dr. Neilsen’s team fabricated the entire tandem cell in-house, including the silicon solar cell. This allowed them to alter the device architecture, for example by changing the thickness of the carrier-selective contacts in the silicon solar cell which protects the silicon layer from the processes used to deposit subsequent layers on top. This allows the top cell to be deposited directly, and, since the selenium can be deposited at low temperature, this leads to an efficient monolithic construction process. The selenium forms the top cell of the tandem device, with silicon used as the bottom cell. Initially a zinc-magnesium oxide layer was used at the n-type contact on the bottom of the selenium-based top cell to maximize the voltage, leading to an open circuit voltage for the tandem cell of almost 1.7 Volts. The downside of using zinc-magnesium oxide was that a critical transport barrier restricted the flow of electrons. Replacing this material with titania reduced the barrier height but led to a lower open circuit voltage suggesting there is more work to do. 

RASMUS NEILSEN: What I hoped to be able to get out of actually doing this was to be able to outline the main challenges that need to be overcome. 

LAURA LEAY: Selenium is prone to defects, but despite this, still demonstrates good performance.

RASMUS NEILSEN: Even though we have a lot of defects currently in selenium – and we’re trying to find out exactly whether these are point defects or extended defects – it seems as if our current collection is actually remarkable good. And I think that’s puzzling and – at the same time – very, very interesting.

LAURA LEAY: This research shows that, before efforts focus on defect engineering, attention should be paid to the substrates used to grow the solar cell layers. Consideration should also be given to including surface modification layers. There are few research papers on selenium compared to other candidates for solar cells but despite this, the efficiency of selenium-based solar cells has improved dramatically. Further research could continue to improve their efficiency and demonstrate longevity. This research also demonstrates that it could be easy for other research groups to get involved.

RASMUS NEILSEN: Selenium is quite simple to fabricate so it’s maybe even low capital cost for a research lab. Considering how few groups are doing it right now, I think there’s a lot of low hanging fruits to be picked in terms of improving performance, improving efficiency, and also just to study some of the most fundamental properties of this semiconductor. Even though it’s been around for two hundred years, there are a lot of unanswered questions.

LAURA LEAY: This work was published in a recent issue of PRX Energy. My name is Laura Leay from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on twitter, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.