A round robin study of polymer solar cells and small modules across China
Graphical abstract
Introduction
Organic photovoltaics (OPV), using either polymers [1] or oligomers [2] as light absorbing material, have now convincingly peaked beyond 10% power conversion efficiency (PCE) [3], [4], [5]. At the same time roll-to-roll (R2R) fabrication schemes have evolved toward encompassing true scalability, from gadgets to bulk energy production [6], [7], [8], [9]. However, the average laboratory efficiencies, be it for either small area OPV prepared by spin coating or large area OPV prepared via R2R coating and printing methods, are still lacking significantly behind the record numbers. This has been shown neatly by Dang et al., taking a bird's eye view of selected data for the all-time favorite P3HT-PCBM blend system [10]. Thus it might seem a fact of life within the field of OPV that reproducibility is relatively poor. Two distinct factors can be said to contribute to this apparent reproducibility challenge: One is intrinsically inherent to the OPV device, coming from the myriad of parameters entering into the fabrication procedure as well as the synthesis of the materials composing the device. These variations are in a sense hidden variables due to a systematic neglect of statistics when presenting OPV efficiency data, as the current habit is that only the “hero” device is presented. The extent of the spread, however, becomes quite obvious when large PV data sets of similarly prepared devices are studied [11], [12].
Another distinct factor which might be hampering the reproducibility can be said to be extrinsic. This extrinsic factor relates to the variations in the current–voltage (IV) characterization under simulated AM1.5G illumination conditions. Influential parameters on the extrinsic variability includes effects related to masking and defining the device active area [13], while also the type of solar simulator used, especially of course if the spectral mismatch factor is disregarded. But spectral variations might also have other unpredictable effects, depending on materials composition of interfacial layers and electrodes, such as the readily observed UV activation of ZnO [14], [15], [16]. While temporal variations in these extrinsic parameters might occur within each laboratory, the most significant variation must be inter-laboratory.
Perhaps the best way to investigate the inter-laboratory variations is through so-called round robin (RR) studies, where the same devices are measured in many laboratories. Where only a few exists for OPV [15], [17], it is a technique often used within the field of inorganic PV [18], [19], [20].
In this study we employ the RR methodology to investigate the inter-laboratory variations among 14 laboratories in China and one laboratory in Denmark where the devices, a set of all roll-to-roll (R2R) -coated and -printed ITO-free devices, were fabricated [7], [21]. As the number of publications on OPV coming from China today is among the highest for any country, this geographical boundary condition was an obvious choice as the high density of OPV laboratories enabled one operator to travel between each of the participating labs, ensuring that the measurements were conducted as similarly as possible, while keeping the total time of the experiment as low as possible, in order to minimize the effects of device degradation and failure.
Section snippets
PV device preparation
The devices were prepared by R2R following the process earlier reported as “IOne” [7], [21], and were based on a flexible ITO-free substrate (Flextrode [8]), upon which the inverted solar cell stack was completed, so that the entire stack was PET/Ag/PEDOT:PSS/ZnO/P3HT:PCBM/PEDOT:PSS/Ag. As shown in the schematic in Fig. 1, the devices consisted of serially connected stripes each with an active width of 1 cm. The devices were manually cut from the roll of solar cells, in three different sizes
Deviations in the photovoltaic performance
The data treatment to extract the deviations in PV performances for each device, before and after light soaking, was as follows: First the data was filtered for any unphysical anomalies. Then each PV parameter was averaged over all laboratories, from which a new average was calculated including only the data which deviated <10% from the original average. Now, the relative deviation of the raw data to this new average (µ10) is calculated.
We first consider the initial measurement with respect to
Conclusion
By a combined Chinese–Danish collaboration between 15 laboratories, we have conducted a round-robin study of a series of ITO- and vacuum free all R2R coated and printed polymer solar cells and small modules of different sizes. The results show large inter-laboratory variations in photovoltaic parameters obtained in the simulated AM1.5G IV-characterizations. Largest were the variations in Isc (up to 33%) thus accounting for the largest source of observed variations in the PCE (up to 30%). The
Acknowledgments
The Authors from DTU and ZJU acknowledge main support from the Danish National Research Foundation and the National Natural Science Foundation of China (Grant no. 51011130028), while the remaining authors acknowledge support from the NSFC (Grants nos. 51225301, 91233104, 51273193 and 50933003 ), the “100 Talents Program” of Chinese Academy of Sciences, the National High Technology Research and Development Program of China (863 Program), the External Cooperation Program of the Chinese Academy of
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