To achieve useful current and voltage, photovoltaic modules have to be “broken up” into small segments that are series-interconnected. The “break lines”, also called “dead area”, should be as small as possible, since they are not available anymore for energy conversion. By applying an optimized laser patterning process, CHEOPS managed to keep this dead area width as small as 400 μm on perovskite solar cells.
To achieve a series-interconnection between cells, three laser lines of different depth are needed (see figure below): The first laser (P1) breaks up the front contact of the cell. The second laser (P2) burns a trench into the perovskite layer. This trench is later filled with back electrode material, serving as the back contact, and forms the series connection between front and back contact of the cell. Finally, the third laser (P3) burns a trench into both back electrode material and perovskite layer and thus makes sure that current flows only between back and front contact.
Perovskite solar technology leader and CHEOPS partner Oxford PV has been named in the Top 50 world most innovative companies of 2017, by the German edition of MIT Technology Review. At the MIT Technology Review’s award ceremony, Oxford PV is also featured in the top three companies in the category “Newcomer of the Year”.
Oxford PV has been recognised for its innovative perovskite solar cell technology that has the potential to dramatically improve the efficiency of silicon solar cells and support the proliferation of solar energy generation globally.
“Oxford PV has made significant progress over the past few years. We have already developed in the lab an advanced perovskite on silicon tandem solar cell and we are in the process of rapidly transferring this to an industrial scale process.” said Frank Averdung, Chief Executive Officer, at Oxford PV.
Yates' and Afzaal's findings show that the black perovskite films have strong absorption and photoluminescence properties. These results confirm that these films are suited to function as light absorbing materials in the production of solar cells.
The CHEOPS project has taken the perovskite technology one step closer towards large area manufacturing: Three CHEOPS partners have developed single-junction perovskite modules on a 5x5 cm² substrate. All modules have reached a power conversion efficiency (PCE) of >12% on active area and a stabilized PCE of more than 10% at maximum power point.
Three of CHEOPS’ partners have tackled the task to optimize the material, processes and encapsulation to achieve the desired high-efficiency 5x5 cm² perovskite modules: CHOSE (Università degli Studi di Roma 'Tor Vergata'), CSEM (Centre Suisse d’Électronique et de Microtechnique) and UOXF (University of Oxford). They have all succeeded to produce such modules with different architectures (mesoscopic and planar), with different techniques (e.g. spin and blade coating) and different materials. The table below shows their interim results.
In the coming year, CHEOPS will select the most promising formulations and techniques to further upscale the modules. The goal is to present a 10x10 cm² demonstrator in January 2018 (other project milestones are documented in our online timeline).
In the past year, CHEOPS partners have developed an encapsulation strategy by testing and comparing several sealing procedures using accelerated life tests on encapsulated cells.
To develop highly efficient perovskite solar cells, applications of hybrid metal halide perovskites (such as MAPbI₃) are currently being extensively studied, but there is a strong concern about long-term stability of devices fabricated with this class of materials. CHEOPS aims to address the stability of the perovskite devices from two angles: first the stability of the different materials and layers and second the stability of the whole device through an optimized encapsulation scheme. One of the critical materials with regard to stability is the HTM (hole transport material). Spiro-OMeTAD has been shown to be temperature sensitive. Therefore CHEOPS will investigate other (organic and inorganic) HTM to overcome this problem. At the same time, encapsulation techniques are being developed and validated on module devices.
Low-temperature fabrication process: A milestone towards high-efficiency perovskite/silicon tandem cells
CHEOPS has developed a low-temperature (i.e. below 200°C) fabrication process yielding perovskite solar cells with highly uniform absorber layers. This is key for the fabrication of perovskite/silicon heterojunction tandem cells, since the perovskite top cell is directly processed onto the bottom cell and temperatures above 200°C would deteriorate the performance of these high-efficiency bottom cells.
Combining a perovskite top-layer with a silicon heterojunction solar cell as bottom cell can provide silicon photovoltaics with an important efficiency boost, which can help overcome their practical efficiency limit of about 27 %. This is possible because the top cell helps to reduce thermalization losses: It absorbs UV and visible light and is transparent to near-infrared (NIR) radiation which is absorbed in the bottom cell. Thanks to the low-cost production potential of perovskite cells, these high-efficiency tandem cells are expected to further reduce the cost of solar energy.
During the consortium meeting in Rome in February, some of the main researchers in the CHEOPS project spontaneously answered two main questions in front of a video camera: What are the main achievements of CHEOPS so far – and what are you going to tackle next? See below for their answers.
High efficiencies achieved, stability to be increased
Next step: Upscaling
Beyond efficiencies, processes and materials