An extremely thin and robust interconnecting layer providing 76% fill factor in a tandem polymer solar cell architecture

Abstract

We report a thin and robust interconnecting layer (ICL) for polymer tandem solar cells. This ICL shows low absorption, good electrical contacts, large work function contrast and robustness. Its use yields tandem cells with a very high fill factor of 76%, making this ICL a promising component of future highly efficient multijunction organic solar cells.Bulk heterojunction polymer solar cells represent a promising technology which could be an important player in the future photovoltaic market due to their light weight, exibility, semitransparency, low cost, and rapid energy payback time. 1-6 The power conversion efficiency (PCE) of these polymer solar cells (PSCs) has been improved markedly in recent years 7-10 and has now exceeded the milestone value of 10%, 11 which makes their industrialization to eventually commercialize them very promising. Moreover, by using the tandem concept, 12 in which two or more subcells with complementary absorption spectra are stacked on top of each other and connected in series or parallel conguration, the PCE has been recently boosted to near 11%. The FF characterizes how "rectangular" the J-V curve is and it represents how "difficult" or "easy" the photogenerated carriers can be extracted out of a photovoltaic device. This parameter is crucial to determine the performance of an organic solar cell together with the short circuit current density (J sc ) and the open-circuit voltage (V oc ). There are several factors that can inuence the FF in a complex way, thus making the FF one of the least understood parameters among the ones determining the PCE. 19 Commonly, in the fabrication of PSCs, several actions have shown to be benecial for improving the FF and hence the efficiency like, for instance, improving the morphology 20,21 or choosing appropriate buffer layers that can help diminish the contact resistance (reduce R s ) or the current leakage (increase R sh ). 22 The buffer layers normally used as transporting layers in single junction solar cells also play another important role in multijunction solar cells as they are commonly part of the interconnecting layer (ICL). In tandem cells, the ICL serves as the charge recombination zone between the front and rear subcells; the ICL is vital in realizing high performance in tandem cells. 23 Further requirements of the ICL are transparency and robustness to protect the underlying layers against damage from further solution processing of the rear subcell.In this study, we report a novel and very thin ICL for multijunction congurations which provides FFs exceeding the ones of the single cells forming part of it and reaching values up to 76%, never reported before in organic tandem solar cells to the best of our knowledge. Herein, we employed a MoO 3 /Ag/PFN ICL layer, less than 20 nm thick, connecting two identical, inverted PTB7:PC71BM subcells in series. By using identical polymers in both subcells, one can overcome the decrease in J sc , FF, and ultimately PCE usually observed in single-junction cells when the thickness of the ac...

High‐Performance Polymer Tandem Solar Cells Employing a New n‐Type Conjugated Polymer as an Interconnecting Layer

“…The HTL and ETL should be processed from orthogonal solvents and be able to provide good solvent resistance for the bottom sub-cell to prevent erosion of under layers. [ 11,19 ] The most popular ICL structures reported are p-type metal oxide combined with organic material, such as MoO 3 / PFN, [ 16,20 ] or p-type polymer combined with metal oxide, such as PEDOT:PSS/ZnO, [21][22][23][24][25] in the case of inverted-type tandem OSCs. For conventional tandem OSCs, the ICL structure is composed of a n-type metal oxide combined with p-type poly mer, such as ZnO/PEDOT:PSS [ 26 ] and TiO x /PEDOT:PSS, [ 11,27,28 ] or n-type metal oxide combined with p-type metal oxides, such as ZnO/MoO 3 .…”

“…Sometimes a thin metal fi lm or metal nanoparticle is introduced at the HTL/ETL interface to further improve the recombination effi ciency. [16][17][18] Another important criterion for the selection of the HTL/ETL ICL is its compatibility for building multilayer cells from solution processes. The HTL and ETL should be processed from orthogonal solvents and be able to provide good solvent resistance for the bottom sub-cell to prevent erosion of under layers.…”

“…An ultrathin Ag was also introduced in order to promote better charge recombinations at the ICL. [ 16,45,46 ] The thickness of PFN in our ICL varied from 4 to 18 nm. Theoretically, if effi cient ICL was formed, the V oc of tandem OSC should be equal to the sum of individual V oc of the sub-cells, which is 1.67 V in our system.…”

UV-Induced Oxygen Removal for Photostable, High-Efficiency PTB7-Th:PC71BM Photovoltaic Cells

“…45 PFN interlayer was prepared following the recipe of previous reports. 9,46 The resulting photoactive films were dried under high vacuum (< 5× 10 − 7 mbar) overnight. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 …”

4-Terminal Tandem Photovoltaic Cell Using Two Layers of PTB7:PC71BM for Optimal Light Absorption

“…In other words, the configuration we proposed is optimal to increase light absorption by using active layers with thicknesses below 100 nm which is the limit thickness to reach good electrical performance for the majority of the low band gap polymer cells. 32 We have used the optical simulations as a guideline to fabricate a proof of principle 4-terminal device using PTB7:PC 71 BM blend as the blend material for the active layers of two inverted sub-cells. We Provided that semi-transparent electrodes that can be deposited on top of polymer cells, such as electrodes composed of Ag nanowires 33 or graphene, 34 were to be used, the 4-terminal device performance would be considerably improved and most certainly surpass the one from the single junction device.…”