Enhanced Open‐Circuit Voltage in High Performance Polymer/Fullerene Bulk‐Heterojunction Solar Cells by Cathode Modification with a C₆₀ Surfactant

Abstract

Polymer solar cells (PSCs) represent a unique alternative renewable energy source combining the potential benefits of comparatively low-cost fabrication, solution processing, and wide application through the versatility of device design. [1,2] The advantages demonstrated by PSCs over their inorganic counterparts have generated extensive research efforts over the past decade, which have resulted in an impressive body of literature detailing vast improvements in device performance made through molecular design [3][4][5][6][7] and process and device engineering. [3,[8][9][10] Power conversion efficiencies (PCEs) over 7% have recently been reported for bulk-heterojunction (BHJ) type PSCs due to significant strides in polymer design and morphological control of the photoactive blend. [7] Despite these promising advances, there remain a variety of technical issues to be addressed in order for PSCs to mature.The most obvious of these issues is the significantly lower photovoltaic performance as compared to traditional siliconbased and dye-sensitized solar cells. One of the most critical factors determining device performance is the nature of the active layer/electrode interface, which has particularly important implications for the open-circuit voltage (V OC ).[11] In contrast to bilayer solar cells, the photoactive layer of BHJ PSCs typically consists of intimately mixed domains of p-type semiconducting polymer donor and n-type fullerene acceptor such that a bicontinuous pathway for charge transport through the layer to the electrodes is achieved. In the ideal case, the polymer and fullerene domains are sufficiently phase-segregated along the vertical charge-carrier transport axis which results in having only electron-(hole-)transporting material at the cathode (anode). [12] Due to the inherently complicated drying dynamics of the photoactive blend layer upon spincoating, this is often not the case, [13,14] and the device performance suffers accordingly via charge recombination at the electrode interfaces. In addition, proper energy level alignment at these interfaces is critical to reduce contact resistance and ensure a maximum V OC and optimized device performance. To this end, several approaches have been employed to modify the organic/electrode interface. One method involves deliberate inclusion and/ or tuning of a dipole layer to improve alignment between the Fermi level (E F ) of the contact and the appropriate frontier molecular orbital level of the organic, while another entails insertion of a thin layer of polymer surfactant to reduce contact resistance. [15][16][17][18] In conventional BHJ-based PSCs, the conducting polymer poly(ethylenedioxythiophene):poly(styrenesulf onate) (PEDOT:PSS) is used to modify the surface of the hole-collecting anode. With a work function (WF) closely matching the holetransport level of a wide range of donor polymers, of the order of 5 eV below vacuum, PEDOT:PSS is capable of forming excellent electrical contact with the organic active layer of the device at the anode. Therefore, ...

Solution-processed chalcopyrite–perovskite tandem solar cells in bandgap-matched two- and four-terminal architectures

“…50 Bis-C60 surfactant is thereby used to ensure benign band alignment between C60 and the metal cathode. 51 In a tandem device, however, the substitution of metal layers by transparent electrodes is required to allow light transmission to the bottom cell. Promising results have already been obtained with Ag nanowires or thin metal layers but high transmission and conductivities can typically not be obtained at the same time.…”

Efficient Polymer Solar Cells with Alcohol-Soluble Zirconium(IV) Isopropoxide Cathode Buffer Layer

“…The RMS of PBDTBDD:PCBM photoactive layer is increased from 4.62 nm to 4.92 nm after modification by ZrIPO layer, and the Figure 4c suggests weaker interfacial contact. The results elucidate that the interfacial contact with increased contact area and interfacial adhesion can be obtained after introducing ZrIPO buffer layer, which foretells a adhesion can be obtained after introducing ZrIPO buffer layer, which foretells a remarkable reduction in series resistance and therefore contribute to the electron collection ratio [12,15,21].…”

“…Fullerene derivatives have been demonstrated to be promising CBL substitutes due to their strong electron-accepting ability, suitable work function and similar structures with fullerene acceptors [18][19][20]. Recently, the energy level alignment between the photoactive layer and the cathode interface was adjusted efficiently after introducing C 60 bisadduct surfactant in BHJ-PSCs, and consequently realizing an increase in opencircuit voltage (V oc ) [21]. Page et al successfully reduced the WF of Ag, Cu, and Au electrodes to 3.65 eV and obtained a high PCE of exceeding 8.5% by incorporating CBL of fulleropyrrolidines with amine (C 60 -N) or zwitterionic (C 60 -SB) in PSCs [22].…”

High Efficiency Inverted Organic Solar Cells with a Neutral Fulleropyrrolidine Electron-Collecting Interlayer

“…23 Fullerene derivatives functionalized with polar pendents are emerging as novel EC interfacial materials to facilitate electron extraction in OSCs due to the circumvention of the undesirable effects often associated with previous reported interlayers. [24][25][26][27][28] The fullerene based EC interlayer has the following advantages: i) the well-matched energy level with the LUMO level of a commonly utilized acceptor, i.e. [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), ensures the excellent electron extraction ability; ii) the deep highest occupied molecular orbital (HOMO) energy level provides excellent hole blocking ability; iii) the reasonable electron mobility.…”

“…However, most of the fullerene based interfacial materials were demonstrated only in a conventional architecture, i.e., the EC interfacial layer was deposited onto the active layer, followed by thermally evaporating a high work function electrode. [24][25][26][27][28] As far as we are concerned, their application in inverted geometry is rare. One possible reason is that their amphiphilic solubility might prevent multi-layer processing, resulting in the interfacial mixing of different layers.…”